along arc segmentation and interaction of subducting ridges with the lesser antilles subduction forearc crust revealed by mcs imaging highlights we image the deep structure of the lesser antilles subduction zone by mcs profiles the complex deformation of the outer forearc crust is induced by subducting ridges we discuss also the effect of the subducting compressive nam sam plate boundary along strike variations of the seaward edge of the outer forearc crust are discovered the updip limit proxy of the seismogenic part reaches 20 km trenchwards than believed abstract we present the results from a new grid of deep penetration multichannel seismic mcs profiles over the 280 km long north central segment of the lesser antilles subduction zone the 14 dip lines and 7 strike lines image the topographical variations of i the subduction interplate decollement ii the top of the arcward subducting atlantic oceanic crust toc under the huge accretionary wedge up to 7 km thick and iii the trenchward dipping basement of the deeply buried forearc backstop of the caribbean upper plate the four northernmost long dip lines of this new mcs grid reveal several kilometre high topographic variations of the toc beneath the accretionary wedge offshore guadeloupe and antigua islands they are located in the prolongation of those mapped on the atlantic seafloor entering subduction such as the barracuda ridge this mcs grid also provides evidences on unexpected huge along strike topographical variation of the backstop basement and of the deformation style affecting the outer forearc crust and sediments their mapping clearly indicates two principal areas of active deformation in the prolongation of the major barracuda and tiburon ridges and also other forearc basement highs that correspond to the prolongation of smaller oceanic basement highs recently mapped on the atlantic seafloor although different in detail the two main deforming forearc domains share similarities in style the imaged deformation of the sedimentary stratification reveals a time and space dependent faulting by successive warping and unwarping which deformation can be readily attributed to the forearc backstop sweeping over the two obliquely oriented elongated and localized topographical ridges the induced faulting producing vertical scarps in this transport does not require a regional arc parallel extensional regime as proposed for the inner forearc domain and may support a partitioned tectonic deformation such as in the case of an outer forearc sliver a contrasted reflectivity of the sedimentary layering at the transition between the outer forearc and accretionary domains was resolved and used to define the seaward edge of the outer forearc basement interpreted as being possibly a proxy to the updip limit of the interplate seismogenic zone its mapping documents along arc variations of some tens of kilometres of the subduction backstop with respect to the negative gravity anomaly commonly taken as marking the subduction trench with the exception of the southernmost part the newly mapped updip limit reaches 25 km closer to the trench thus indicating a possible wider seismogenic zone over almost the whole length of the study area

discovery of lower cretaceous synmetamorphic thrust tectonics in french lesser antilles la desirade island guadeloupe implications for caribbean geodynamics located east of guadeloupe the island of la desirade exhibits the oldest rocks of the lesser antilles arc and the eastern caribbean plate an old magmatic basement is composed of 1 late jurassic ophiolitic complex with meta basaltic pillow lavas and interbedded radiolarites 2 acid igneous complex comprising meta quartz diorite and meta rhyolitic lavas flows and 3 meta diabasic microdioritic dyke swarm complex we present and discuss the discovery of synmetamorphic thrust tectonics in this island based on detailed structural analysis we evidenced two main compressive events the first event d1 is characterized by pervasive folding associated with thrust development in the northeastern part of the island a major thrust fault the grand abaque thrust has been discovered which displaces for several kilometers the acid igneous complex onto the ophiolitic unit the second deformation event d2 is featured by a main pervasive cleavage s2 observed at the regional scale associated with upright folding f2 and by the development of two sets of conjugated steeply dipping strike slip shear zones dextral 130 n and sinistral 20 n striking respectively microstructural observations indicate that greenschist facies metamorphism is clearly contemporaneous with the development of d1 and d2 superimposed tectonic structures ar ar geochronology was performed on two bulk of adularia minerals sampled in the northeast volcanic complex within a deformed zone related to a d2 event these analyses yield well defined and concordant plateau ages at 106 2 1 7 ma and 107 2 1 8 ma thus shortening and thickening tectonics occurred during lower cretaceous albian in the available framework of caribbean geodynamics the synmetamorphic thrust tectonics we discovered in the french lesser antilles is related to the collision between an aptian albian oceanic plateau and the andean cordilleran east dipping subduction zone our results support subduction polarity reversal during lower cretaceous

archean gold mineralization synchronous with the final stages of cratonization yilgarn craton western australia sm nd ages of pegmatite dikes that crosscut gold bearing structures in the southern yilgarn craton western australia provide minimum age constraints of 2640 6 11 ma 2628 6 10 ma and 2620 6 36 ma for gold mineralization at the westonia and nevoria yellowdine terrane and scotia norseman terrane gold deposits respectively similarly a post gold mineralization dike at westonia has a u pb zircon age of 2637 68 ma theseconstraintsareconsistentwith andprovidefurthersupportfor suggestionsthatthe majority of gold deposits in the yilgarn craton formed during a regionally extensive gold mineralization event at ca 2630 ma i e 2630 6 10 ma in combination with recent geochronological results these data also provide further evidence that although mineralization appears to significantly postdate the majority of magmatic and metamorphic activity at higher crustal levels widespread thermal reworking of the lower middle crust involvingpartialmelting amphibolitetogranulitefaciesmetamorphism andlowercrustal granitoid intrusion occurred concurrently with gold mineralization at ca 2630 ma it is probable that the large scale hydrothermalfluidflow that produced widespread gold deposition was also part of this tectonothermal event models developed for other archean terranes whereby gold mineralization postdates formation and cratonization of host granite greenstone terranes and is therefore unrelated to these processes are not required to explain the timing of the majority of gold deposits in the yilgarn craton

package fluids 1 computer programs for analysis of fluid inclusion data and for modelling bulk fluid properties the computer package fluids contains five sets of computer programs written in c for the calculation of fluid properties both model fluids and data from fluid inclusions can be analysed in the programs bulk density molar volume and composition of fluid inclusions are calculated with the program bulk which uses microthermometric data micro raman spectroscopic data cation ratios and volume fraction estimations as input the program offers a broad variety of equations of state which are selected according to the fluid system or the temperature and pressure conditions of interest the isochores that correspond to the output or test v x properties of fluid inclusions are calculated by the program isoc in calculating the isochores this program takes account of the compressibility and expansion of selected host minerals the program test allows different equations of state to be compared directly with experimental data thereby facilitating selection of the most adequate model in the programs bulk and isoc loner is a group of programs that handle individual equations of state eos for all fluid systems presented in the programs bulk and isoc aqso is a group of programs that allows purely empirical modelling of electrolyte bearing aqueous solutions henry s law for dilute gas bearing aqueous solutions and osmotic coefficients for dilute electrolyte bearing aqueous solutions the programs are available on the university of leoben web site at http www unileoben ac at buero62 minpet ronald

crustal deformation at the sumatran subduction zone revealed by coral rings analyses of coral rings grown in the interval 1970 1997 reveal a geographically distinct pattern of interseismic uplift off sumatra s western coast at distances less than 110 km from the sumatran trench coral reefs are submerging as fast as 5 mm y at 130 and 180 km distance from the trench they are emerging at similar rates we suggest that a locked or partially locked patch located above 30 km depth on the upper surface of the subducting oceanic plate generates this pattern

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

high rates of arc consumption by subduction processes some consequences new estimates of long term consumption of japanese and peruvian continental fore arc margins together with recent drilling data ocean drilling program on some other erosional island arcs such as izu bonin mariana or tonga allow the calculation of a mean rate of 7 3 km m y landward migration of the arc trench system on both east and west sides of the pacific basin during cenozoic time such relative arcward retreat counterbalances seaward migration of the slab hinge line to accommodate opening of the atlantic ocean retreat can thus be interpreted as a delay in seaward or retrograde migration due to upper mantle resistance to contraction of the pacific basin mass transfers in subduction zones must be reconsidered in the light of these new estimates especially the balance between net growth and loss of continental crust in recent geologic time a 50 m y period is long enough to erode the initial width of the volcanic arc and fore arc massif of any subduction zone assuming a mean trench volcanic arc landward migration of 5 km m y and a mean trench volcanic arc distance of 250 km this circumstance thus raises the question of whether the izu bonin mariana and tonga arcs are older than their reported eocene age

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

heterogeneous coupling of the sumatran megathrust constrained by geodetic and paleogeodetic measurements geodetic and paleogeodetic measurements of interseismic strain above the sumatran portion of the sunda subduction zone reveal a heterogeneous pattern of coupling annual banding in corals provides vertical rates of deformation spanning the last half of the 20th century and repeated gps surveys between 1991 and 2001 and continuous measurements at gps stations operated since 2002 provide horizontal velocities near the equator the megathrust is locked over a narrow width of only a few tens of kilometers in contrast the locked fault zone is up to about 175 km wide in areas where great interplate earthquakes have occurred in the past formal inversion of the data reveals that these strongly coupled patches are roughly coincident with asperities that ruptured during these events the correlation is most spectacular for rupture of the m w 8 7 nias simeulue earthquake of 2005 which released half of the moment deficit that had accumulated since its previous rupture in 1861 suggesting that this earthquake was overdue beneath the mentawai islands strong coupling is observed within the overlapping rupture areas of the great earthquakes of 1797 and 1833 the accumulated slip deficit since these events is slowly reaching the amount of slip that occurred during the 1833 earthquake but already exceeds the slip that occurred during the 1797 earthquake thus rerupture of part of the mentawai patch in september 2007 was not a surprise in contrast coupling is low below the batu islands near the equator and around enggano island at about 5 s where only moderate earthquakes m w 8 0 have occurred in the past two centuries the correlation of large seismic asperities with patches that are locked during the interseismic period suggests that they are persistent features this interpretation is reinforced by the fact that the large locked patches and great ruptures occur beneath persistent geomorphologic features the largest outer arc islands depth and convergence rate dependent temperature might influence the pattern of coupling through its effect on the rheology of the plate interface but other influences are required to account for the observed along strike heterogeneity of coupling in particular subduction of the investigator fracture zone could be the cause for the low coupling near the equator

giant mesozoic gold provinces related to the destruction of the north china craton abstract lode gold deposits in precambrian cratons represent the world s major gold source and were mostly generated during formation and stabilization of the cratons however there is an extraordinary exception in the north china craton ncc where lode gold deposits formed after prolonged stabilization of the craton molybdenite re os and hydrothermal sericite and biotite 40 ar 39 ar dating of major gold deposits from the xiaoqinling district southern ncc bracket their emplacement in the range of 154 1 1 1 to 118 9 1 2 ma n 23 postdating formation of the craton by more than 1 7 billion years fluid inclusions extracted from gold bearing pyrite have elevated 3 he 4 he ratios 1 52 0 22 ra and mantle like ne isotopes 20 ne 22 ne 10 02 9 22 and 21 ne 22 ne 0 033 0 027 indicating presence of mantle derived fluids in the ore system measured 34 s of pyrite and d and 18 o of hydrothermal micas and fluid inclusion waters in auriferous quartz further confirm a magmatic mantle source for sulfur and ore fluids gold deposits of similar ages also widely occur in the eastern and northern margins of the ncc which together with those in the xiaoqinling district have a total reserve of 2500 t gold forming the only known giant late mesozoic gold province in the world s precambrian cratons these deposits formed coevally with extensive felsic to mafic magmatism development of intracontinental rift basins and exhumation of metamorphic core complexes across the eastern ncc events interpreted as indicating thinning and destruction of the lithosphere beneath the craton rising of asthenosphere coupled with destruction of the lithosphere has generated voluminous mafic and felsic magmas that provided sufficient fluids sulfur and by inference other ore components to form the giant gold provinces

deep structure of an island arc backstop lesser antilles subduction zone 1 we present the results from a coincident seismic reflection refraction grid conducted at the lesser antilles subduction zone near 16 n this paper focuses on the seismic refraction data and constraints these data place on the three dimensional structure of the island arc backstop we find that the backstop in this region contains considerable topography in both the strike and dip directions two ridges each 25 35 km in length and 10 km in width rise 1 6 km above the adjacent basement the eastern edge of one of the ridges deepens by 4 6 km over a horizontal distance of 10 km and forms the eastern edge of the backstop in contrast to the complex nature of the backstop the adjacent accretionary wedge displays little lateral variability at large scales this may be a consequence of the spatial scales involved the backstop topography is 10 35 km in width while the accretionary wedge extends 125 km from the deformation front to the backstop the top of the subducting oceanic crust as identified by an increase in velocities to 6 6 5 km s intersects the backstop at a depth of 14 15 km the updip limit of plate boundary seismicity is located 75 100 km west and downdip of the backstop however two earthquake clusters are observed at the intersection of the subducted barracuda ridge and tiburon ridge with the backstop suggesting active deformation associated with the backstop edge at these locations

the archaean mount gibson gold deposits yilgarn craton western australia products of combined synvolcanic and syntectonic alteration and mineralisation abstract the mount gibson gold deposits are sited at the southern tip of the archaean yalgoo singleton greenstone belt in the southern portion of the murchison province yilgarn craton western australia primary gold mineralisation is hosted within an eastward facing dominantly tholeiitic metabasalt sequence which has been metamorphosed to amphibolite facies conditions alteration and mineralisation occurs within the north to north northeast striking steeply east dipping mount gibson shear zone paralleling primary layering within the tholeiitic mine sequence two main styles of wallrock alteration and associated sulphide mineralisation are present the first of these is represented by garnet spessartine almandine gahnite and cordierite muscovite bearing schists these lithologies are unusual for lode gold mineralisation and despite their apparently pelitic nature have mafic precursors and reflect pre metamorphic alteration of tholeiitic basalts the garnet bearing schists form a coherent stratiform horizon which overlies the main ore zone at the orion two deposit and are associated with recrystallised metamorphosed polymetallic sphalerite galena pyrite pyrrhotite chalcopyrite tetrahedrite mineralisation a more typical for archaean lode gold pervasive syn peak metamorphic syn shearing quartz biotite sulphide alteration envelope is associated with pyrrhotite pyrite chalcopyrite mineralisation at mount gibson although free gold is rare there is a strong empirical relationship between enhanced gold grades and the pyrrhotite pyrite chalcopyrite assemblage suggesting that gold was introduced as part of this event mass balance calculations at orion two show enrichment of au ag k rb ba and w and depletion of na ca and sr over mineralised zones which is typical of lode gold mineralisation however the deposit also exhibits strong base metal enrichment particularly within the garnet bearing schists and zonation from the au cu enrichment of the main ore zone to the overlying stratiform pb zn ag mn enrichment which is dominant in the garnet bearing schist horizon these features are typical of vhms mineralisation hornet exhibits similar overall elemental enrichment and depletion to orion two lead isotope data for sulphides from orion two and hornet provide evidence of two discrete mineralising events at mount gibson separated by approximately 300 m y a two stage model best explains the features of the mount gibson gold deposits metamorphism of the aluminous alteration and weak mineralisation associated with an early synvolcanic event resulted in the formation of cordierite muscovite and garnet spessartine almandine gahnite bearing assemblages respectively which were less competent than the surrounding tholeiitic metabasalts and consequently tended to localise shearing shear hosted lode gold mineralisation was synchronous with peak metamorphism and shearing like the shearing hydrothermal alteration was mostly concentrated in and around the less competent rocks which were produced by metamorphism of synvolcanically altered precursors resulting in a fundamental stratigraphic control on gold mineralisation at mount gibson

differential uplift and tilt of pleistocene reef platforms and quaternary slip rate on the morne piton normal fault guadeloupe french west indies 1 the guadeloupe islands are cut by normal faults that accommodate oblique convergence between the north american and caribbean plates such faults are responsible for part of the shallow seismicity and have produced m 5 damaging earthquakes to better assess the seismic hazard in guadeloupe we quantify the slip rate on one of the largest fault morne piton this roughly e w fault crosses the island of marie galante and uplifts a flight of reef terraces from geomorphic analysis we mapped three main terraces new u th datings show that they formed during the latest interglacials 120 and 240 kyr ago correlation with specmap isotopic records implies that the marie galante plateau emerged during the 330 ka highstand topographic profiles show that the terraces and the plateau are deformed by faulting elastic modeling of their shape constrains the geometry of the fault 70 80 dip 5 km depth and its slip rate 0 5 0 2 mm yr given its length 50 km depth and slip rate this fault might produce maximum m 6 5 earthquakes with a recurrence time of 1400 to 3300 years or more likely smaller events such as the m 5 5 16 may 1851 and 3 august 1992 shocks that might recur every 400 to 1000 years we also show that all the islands and terraces are tilted westward perpendicularly to the trench la desirade closest to the trench is uplifted by 276 m whereas subsidence 70 m is observed 10 km east of the volcanic arc this tilt probably resulted from a transient deformation episode at the subduction interface that predated the late pleistocene

middle to late jurassic felsic and mafic magmatism in southern hunan province southeast china implications for a continental arc to rifting abstract extensive middle to late jurassic felsic and mafic magmatism occurred in the southern hunan province southeast china shrimp zircon u pb dating mineral chemical element geochemical and sr nd hf isotopic data have been determined for these rocks the results indicate that the middle jurassic 178 170 ma changchengling and ningyuan basaltic rocks belong to tholeiitic series and alkaline series respectively and were formed by fractional crystallization of the parent magmas that were derived from partial melting of asthenospheric mantle triggered by the slab released fluids of the subducted paleo pacific plate the late jurassic 152 146 ma daoxian basalts and guiyang lamprophyre dikes are low ti high mg potassic rocks they were derived directly by partial melting of shallower 60 100 km lithospheric mantle composed of amphibole and phlogopite bearing lherzolite in response to thermal perturbations associated with the rifting of continental arc the middle jurassic 164 ma tongshanling granitic rocks belong to calc alkaline series and are strongly peraluminous they were formed by partial melting of palaeoproterozoic metasedimentary basement in the lower crust plus additional input from coeval basaltic magma the late jurassic 156 ma jinjiling and xishan granitic rocks show a2 subtype affinity they were generated by partial melting of granulitized palaeoproterozoic metamorphic basement in the lower crust in response to injection of coeval low ti high mg potassic magmas a moderate degree 56 58 of fractionation of these pure crustal melts could account for more felsic end member of granitic rocks detailed petrologic and geochemical data for the middle to late jurassic felsic and mafic rocks from the southern hunan imply that during the middle jurassic time se china on the southeast of the shi hang zone was a continental arc coupled with the subduction of the palaeo pacific plate and since the beginning of late jurassic time an intra arc rift has been formed along the shi hang zone as a consequence of slab roll back

an early cretaceous carbonate replacement origin for the xinqiao stratabound massive sulfide deposit middle lower yangtze metallogenic belt china stratabound massive sulfide deposits are widespread along the middle lower yangtze metallogenic belt mlymb and serve as an important copper producer in china two contrasting genetic models have been proposed interpreting the stratabound massive sulfide deposits as a carboniferous sedex protore overprinted by cretaceous magmatic hydrothermal system or an early cretaceous carbonate replacement deposit these two contrasting models have been applied to the xinqiao stratabound cu au sulfide deposit which is dominated by massive sulfide ores hosted in marine carbonates of the carboniferous chuanshan and huanglong formations with minor cu au skarn ores localized in the contact zone between the cretaceous diorite jitou stock and the carboniferous carbonate rocks new sims zircon u pb dating suggests that the jitou stock formed at 138 5 1 1 ma 2 mswd 0 6 pyrite re os dating yields an imprecise date of 142 47 ma 2 mswd 7 8 the geochronological data thus constrain the mineralization of the xinqiao deposit at early cretaceous fluid inclusions in prograde skarn diopside have homogenization temperatures of 450 600 c and calculated salinities of 13 58 wt nacl equiv quartz from the stratabound ores and pyrite quartz vein networks beneath the stratabound ores have homogenization temperatures of 290 360 and 200 300 c with calculated salinities of 5 12 and 2 10 wt nacl equiv respectively quartz from the skarn ores and veins beneath the stratabound ores have 18o values of 12 32 0 55 2 sd n 22 and 15 57 1 92 2 sd n 60 respectively corresponding to calculated 18o values of 6 22 1 59 2 and 6 81 2 76 2 for the equilibrated ore forming fluids the fluid inclusion and oxygen isotope data thus support a magmatic hydrothermal origin rather than a sedex system for the stratabound ores with the hydrothermal fluids most likely being derived from the jitou stock or associated concealed intrusion results from this study have broad implications for the genesis and exploration of other stratabound massive sulfide deposits along the mlymb

minerals in bituminous coals of the sydney basin australia and the illinois basin u s a although other forms of inorganic components may be significant in lower rank coals the mineral matter in most bituminous coals is dominated by silicates carbonates sulphides phosphates and other crystalline mineral groups these may be present in various forms including thin bands and laminae intimately associated with the organic matter as well as lenticles nodules crystal aggregates and cell infillings all of which indicate approximately contemporaneous formation with the peat bed other minerals however occur as veins or cleat and fracture fillings indicating precipitation after most of the compaction and presumably after most of the rank advance ome of the penecontemporaneous mineral matter is of detrital origin representing epiclastic or pyroclastic particles washed or blown into the peat swamp other mineral matter however including much of the pyrite quartz and siderite as well as abundant well crystallized kaolinite appears to have been formed by precipitation processes either within the swamp waters or in the pores of the peat deposit pyrite largely formed by bacterial reduction of dissolved sulphate ions seems to be related in abundance to marine transgressions which allowed permeation of sea water into the swamp and through the peat bed beneath siderite appears to form in the absence of sulphate rich sea water probably from co2 released by organic decay the clay fraction of the noncoal rocks in many coal bearing sequences is typically dominated by illite or interstratified clay minerals with kaolinite chlorite and possibly montmorillonite also present to a variable though typically minor extent although perhaps modified by chemical reactions or ion exchange mechanisms in the peat swamp these clay minerals may also be present in the coal as well especially near the base of top of the seam the kaolinite in the noncoal rocks is usually poorly crystallized and typically a minor constituent but well crystallized kaolinite is characteristically present and often the dominant component in the mineral fraction of bituminous coal beds as well as in some of the lutites including underclays intimately associated with the seams a progressive upward gradation to a particularly kaolinite dominated mineral assemblage occurs in some seams of the sydney basin australia this gradation is thought to be due to either increased blockage of detrital contaminants as the swamp developed or to in situ chemical or biological leaching of the mineral fraction and reconstitution of the residues in the upper parts of the peat bed superimposition of such cycles is also noted within the seam in some instances suggesting repetition of several phases of this type during major peat accumulations

formation of the campbell red lake gold deposit by h 2 o poor co 2 dominated fluids the campbell red lake gold deposit in the red lake greenstone belt with a total of approximately 840 t of gold past production reserves and an average grade of 21 g t au is one of the largest and richest archean gold deposits in canada gold mineralization is mainly associated with silicification and arsenopyrite that replace carbonate veins breccias and wallrock selvages the carbonate veins and breccias which are composed of ankerite quartz and characterized by crustiform cockade textures were formed before and or in the early stage of penetrative ductile deformation whereas silicification arsenopyrite replacement and gold mineralization were coeval with deformation microthermometry and laser raman spectroscopy indicate that fluid inclusions in ankerite and associated quartz q1 and main ore stage quartz q2 are predominantly carbonic composed mainly of co2 with minor ch4 and n2 aqueous and aqueous carbonic inclusions are extremely rare in both ankerite and quartz h2o was not detected by laser raman spectroscopic analyses of individual carbonic inclusions and by gas chromatographic analyses of bulk samples of ankerite and main ore stage quartz q2 fluid inclusions in post mineralization quartz q3 are also mainly carbonic but proportions of aqueous and aqueous carbonic inclusions are present trace amounts of h2s were detected by laser raman spectroscopy in some carbonic inclusions in q2 and q3 and by gas chromatographic analyses of bulk samples of ankerite and q2 3he 4he ratios of bulk fluid inclusions range from 0 008 to 0 016 ra in samples of arsenopyrite and gold homogenization temperatures th co2 of carbonic inclusions are highly variable from 4 1 to 30 4 c mostly to liquid some to vapor but the spreads within individual fluid inclusion assemblages fias are relatively small within 0 5 to 10 3 c carbonic inclusions occur both in fias with narrow th co2 ranges and in those with relatively large th co2 variations the predominance of carbonic fluid inclusions has been previously reported in a few other gold deposits and its significance for gold metallogeny has been debated some authors have proposed that formation of the carbonic fluid inclusions and their predominance is due to post trapping leakage of water from aqueous carbonic inclusions h2o leakage model whereas others have proposed that they reflect preferential trapping of the co2 dominated vapor in an immiscible aqueous carbonic mixture fluid unmixing model or represent an unusually h2o poor co2 dominated fluid single carbonic fluid model based on the fia analysis reported in this study we argue that although post trapping modifications and host mineral deformation may have altered the fluid inclusions in varying degrees these processes were not solely responsible for the formation of the carbonic inclusions the single carbonic fluid model best explains the extreme rarity of aqueous inclusions but lacks the support of experimental data that might indicate the viability of significant transport of silica and gold in a carbonic fluid in contrast the weakness of the unmixing model is that it lacks unequivocal petrographic evidence of phase separation if the unmixing model were to be applied the fluid prior to unmixing would have to be much more enriched in carbonic species and poorer in water than in most orogenic gold deposits in order to explain the predominance of carbonic inclusions the h2o poor co2 dominated fluid may have been the product of high grade metamorphism or early degassing of magmatic intrusions or could have resulted from the accumulation of vapor produced by phase separation external to the site of mineralization

ore forming fluids in archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec canada archean gold bearing quartz veins at the sigma mine occur in deformed metavolcanics intruded by two generations of preore porphyries the veins were emplaced during a late compressional tectonic event and they consist primarily of quartz tourmaline carbonates pyrite and scheelite vein minerals have been deformed and are also cut by abundant healed fractures outlined by fluid inclusions in the veins gold typically occurs in late fractures and in recrystallized quartz microthermometric sem eda leachate analysis and gas chromatographic techniques were used to study fluid inclusions in quartz most fluid inclusions at sigma occur in late fractures and are secondary the fluid inclusions are commonly found in fractures parallel to or less commonly within gold bearing fractures and are genetically related to the gold three types of fluid inclusions are identified 1 high salinity na ca cl aqueous inclusions 25 34 wt salt with small amounts of co 2 some containing halite calcite and nahcolite as daughter minerals 2 co 2 rich inclusions containing small amounts of h 2 o and ch 4 and 3 low salinity h 2 o co 2 inclusions 10 equiv wt nacl with co 2 contents which are variable but commonly in the range of 15 to 30 mole percent co 2 leachate analyses show that in addition to na and ca the aqueous fluids contain minor amounts of k fe mg and li homogenization temperatures of h 2 o co 2 inclusions with constant phase ratios range from 285 degrees to 395 degrees c those of aqueous inclusions range from 60 degrees to 295 degrees c the aqueous and co 2 rich inclusions are interpreted to have trapped two coexisting fluids resulting from the unmixing of a homogeneous h 2 o co 2 parent fluid itself occasionally trapped in late fractures the ore bearing fluid is interpreted to be a low salinity 10 equiv wt nacl fluid containing 15 to 30 mole percent co 2 at minimum temperatures in the range of 285 degrees to 395 degrees c the relatively low homogenization temperatures of aqueous inclusions are interpreted to be the result of co 2 effervescence from a co 2 saturated aqueous fluid due to pressure fluctuations at the time of entrapment it is also proposed that the onset of gold deposition is related to the onset of fluid unmixing

arsenopyrite compositional variation over variable temperatures of mineralization otago schist new zealand compositions of arsenopyrite coexisting with pyrite from eight different mesothermal gold bearing systems in the otago schist fall mainly into a narrow band of 30 or 1 at percent as intragrain and intergrain compositional variations combined with analytical uncertainty preclude resolution of differences in arsenopyrite compositions between samples of less than 1 at percent as only one of the vein systems has an arsenopyrite compositional range that extends below 28 at percent as arsenopyrite from a pyrrhotite bearing assemblage has compositional range extending above 32 at percent as the vein systems have formed over a wide range of temperatures between about 200 degrees and 400 degrees c it is not possible to resolve the influence of temperature on arsenopyrite compositions for most samples sulfur and oxygen activity of the hydrothermal fluid may be responsible for the low degree of arsenopyrite compositional variation

arc parallel extension and localization of volcanic complexes in guadeloupe lesser antilles 1 subduction of atlantic seafloor under the caribbean plate causes shallow earthquakes within the lesser antilles volcanic arc such earthquakes above the subduction interface show strike slip or normal fault plane solutions the latter with e w striking nodal planes to better assess seismic hazard and the coupling between volcanism and tectonics we investigated faulting related to overriding plate deformation in the guadeloupe archipelago using aerial photographs satellite spot images and topographic maps 1 25000 scale we mapped active and middle to late pleistocene fissures and normal fault systems that cut the uplifted coral platforms grande terre and marie galante and the volcanic rocks of basse terre the available marine geophysical data show that the faults extend offshore to bound submarine rifts the e w striking 1500 m deep v shaped marie galante rift separates the two islands of marie galante and grande terre normal faults in the north of grande terre appear to mark the similarly v shaped western termination of the 5000 m deep n 50e to n130 e striking desirade graben three shallow m 5 5 earthquakes 6 may 1851 29 april 1897 3 august 1992 appear to have ruptured segments of the marie galante rift boundary faults the young la grande decouverte volcanic complex of basse terre including the 1440 a d soufriere dome lies within the western termination of the marie galante rift the ancient volcanic shoulders of the rift buttress the active dome to the north and south which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the caribbean sea or southeastward into the atlantic ocean the marie galante rift is typical of other troughs transverse to the northeastern edge of the caribbean plate we interpret such troughs which are roughly orthogonal to the arc to result from slip partitioning and extension perpendicular to plate convergence that they disappear southward implies that they result from interaction between the caribbean and north american plates

copper lead zinc and cobalt mineralization in the english lake district classification conditions of formation and genesis the copper lead zinc and minor cobalt mineralization of the english lake district has been classified on the basis of mineralogy spatial distribution and geological setting of the veins and age of mineralization the two major types of mineralization are 1 chalcopyrite pyrite arsenopyrite of lower devonian caledonian age and 2 galena sphalerite of early carboniferous age when the former is closely associated with granite intrusions more complex mineral assemblages are found involving tungsten or molybdenum bearing phases also distinguished from these two major types are later assemblages which probably result from their alteration a later upper carboniferous permian baryte mineralization is also present minor graphite and stibnite deposits and the major haematite mineralization found in the area are incorporated into the classification but are not discussed since they have been dealt with by other authors using data from fluid inclusions and experimental data on particular mineral assemblages occurring in the major types of mineralization estimates have been made of the temperatures and compositions of the ore forming fluids the lower devonian veins formed at temperatures in the range of 350 200 c with typical a s 2 values of 10 10 l0 16 atm and a o 2 of 10 25 10 45 atm or even less the solutions were probably brines of low to moderate salinity 5 10 equiv wt nacl the lower carboniferous veins formed at temperatures in the region of 110 130 c from highly saline brines 23 equiv wt nacl theories regarding the origins of the mineralization are also discussed the distribution of lower devonian veins shows a clear relationship to the underlying composite granite batholith the lower carboniferous galena sphalerite veins show no such relationship but tensional tectonic activity in the early carboniferous may have provided a mechanism for the opening of fissures the source of the mineralizing fluids forming the lower devonian veins may have been the borrowdale volcanic group with contributions from fluids associated with the granites which are also likely to have provided a source of heat the high salinities of the fluids producing the lower carboniferous veins suggest that seawater may have been involved in leaching the metals from basement rocks

a fluid inclusion and stable isotope study at the loulo mining district mali west africa implications for multifluid sources in the generation of orogenic gold deposits the world class loulo mining district 15 5 moz resource in the birimian terrane of western mali contains a range of mineralogically diverse styles of orogenic gold mineralization the district is distinguished by as rich orebodies as at yalea as well as tourmaline bearing fe rich orebodies as at gara new fluid inclusion and stable isotope data presented here constrain the nature of the hydrothermal fluids responsible for these different types of mineralization and point towards the role of multifluid sources metamorphic and magmatic in the formation of orogenic gold deposits microthermometric and laser raman studies from yalea main and two other similar orebodies loulo 3 and baboto reveal the dominance of coexisting co 2 n 2 ch 4 type 1 and h 2 o nacl type 2 fluid inclusions these inclusions show evidence of fluid unmixing from reduced quartz faylite magnetite qfm buffered low salinity 10 wt nacl equiv co 2 rich h 2 o nacl n 2 ch 4 primary ore fluids the combination of microthermometric data and geothermometry based on ore and alteration assemblages indicate trapping temperatures and pressures of 270 to 340 c at 1 4 to 1 8 kbar the p t x state of the yalea style mineralizing fluids along with the 18 o fluid compositions of 8 8 to 10 7 is consistent with the derivation of auriferous fluids during greenschist facies regional metamorphism of the host terrane similar fluid compositions were previously reported elsewhere in the birimian crust of west africa and in other orogenic gold districts worldwide the precipitation of gold from the h 2 s rich metamorphic fluid is primarily linked to phase separation of the ore fluid which is controlled by p t fluctuations and or fluid rock interaction with carbonaceous host sedimentary rocks confirmed by low 13 c values of 21 7 to 15 8 fluid inclusion investigations from gara and a similar style orebody yalea north indicate the presence of coexisting co 2 n 2 ch 4 type 1 and mixed salinity 5 21 wt nacl equiv co 2 rich h 2 o nacl inclusions type 3 inclusion assemblages also contain common oxidized hematite magnetite hm buffered high temperature 400 c hypersaline 35 50 wt total dissolved solids metalliferous na fe ca cu ni w pb zn multiphase h 2 o rich co 2 nacl fecl 2 inclusions type 4 this inclusion type has not been previously reported in other birimian terranes the composition of the brines along with carbon isotope data 13 c of 14 4 to 4 5 suggests a magmatic input to the gara style hydrothermal system the coexistence in the fluid inclusion assemblages of the magmatic brines with yalea style co 2 rich metamorphic fluids and the positive correlation between salinity and homogenization temperatures suggest mineralization was locally controlled by fluid mixing the interaction of these two chemically contrasting fluids explains the distinctive petrographic characteristics of the gara style orebodies this includes the growth of widespread multi stage fe 3 rich tourmaline b rich granite source and sodic alteration and ore assemblages consisting of abundant nickeloan pyrite ree phosphates ni co pb zn minor trace sulfides and scheelite gold deposition in the gara style hydrothermal system is related to physical and chemical changes of the two fluids during mixing e g decreases in f o2 and t in the brines and retrograde boiling of the co 2 component in the metamorphic fluids a salting out effect

actively evolving microplate formation by oblique collision and sideways motion along strike slip faults an example from the northeastern caribbean plate margin abstract the pattern of folding faulting and late quaternary coral reef uplift rates in western and central hispaniola haiti and dominican republic suggest that the elongate gonave microplate a 190 000 km2 area of the northeastern caribbean plate is in the process of shearing off the caribbean plate and accreting to the north american plate late cenozoic transpression between the southeastern bahama platform and the caribbean plate in hispaniola has inhibited the eastward motion of the northeastern corner of the plate transpression is manifested in western and central hispaniola by the formation of regional scale folds that correspond to present day anticlinal topographic mountain chains continuous with offshore anticlinal ridges areas of most rapid quaternary uplift determined from onland coral reefs 125 ka and younger coincide with the axial traces of these folds offshore data suggest recent folding and faulting of the seafloor onshore reef data do not conclusively require late quaternary folding but demonstrate that tectonic uplift rates of the axial areas of the anticlines decrease from the northwest peninsula of haiti 0 37 mm yr to to the central part of the coast of western haiti 0 19 mm yr to the south central part of western haiti 0 mm yr formation of the 1200 km long enriquillo plantain garden walton fault zone as a bypass strike slip fault has isolated the southern edge of the gonave microplate and is allowing continued unimpeded eastward motion of a smaller caribbean plate past the zone of late neogene convergence and quaternary uplift of coral reefs in hispaniola offshore seismic reflection data from the jamaica passage the marine strait separating jamaica and haiti show that the enriquillo plantain garden fault zone forms a narrow but deep active fault bounded trough beneath the passage the active fault is continuous with active faults mapped onshore in western haiti and eastern jamaica the bathymetric deep is present because the jamaica passage fault segment represents a 50 km wide transtensional left step of the fault trace between haiti and jamaica onshore satellite imagery and field observations suggest that the enriquillo plantain garden fault forms a continuous trace extending from central hispaniola east of lake enriquillo dominican republic to the westernmost end of the southern peninsula of haiti the regional lineament corresponds to a recent fault scarp in quaternary alluvium of the clonard pull apart basin in the central part of the southern peninsula of haiti and suggests that at least this part of the lineament has undergone recent slip calmus 1983 has suggested a total offset of 30 50 km of the enriquillo plantain garden fault zone in the southern peninsula of haiti using the apparent offset of lithologic units seismic reflection data from lake enriquillo document recent deformation of quaternary lake sediments where the lineament crosses the lake leveling of the crest of a late holocene coral reef and associated algal tufa around lake enriquillo demonstrate late holocene vertical movement and tilting in a 1500 m wide zone parallel to the fault trend lateral offset is difficult to show in the enriquillo valley area because of rapid recent sedimentation into the valley the pattern of inactive strike slip faults and fold belts of cenozoic age in cuba and the yucatan basin suggest that two elongate microplates were sheared off the proto caribbean plate and accreted to the north american plate by a similar process in paleocene and eocene times age of terminal deformation in western central and eastern cuba is consistent with southeastward younging and migration of arc collision the similarity of the size and sequence of events in cuba suggests that the process of oblique collision and sideways motion of the plate along a new strike slip fault towards a free face may be an important process of microplate formation and interplate transfer in other areas

mesozoic large scale magmatism and mineralization in south china preface abstract south china is famous for its polymetallic deposits with more than 50 of the world s w and sb reserves and 20 of the sn reserves as well as abundant cu mo pb zn au ag ta nb hree and u this special issue reports recent progress on the tectonic evolution magmatism and ore deposits of south china which leads to a more comprehensive understanding of the mesozoic large scale magmatism and mineralization events 1 south china has been located in backarc settings during multiple subductions since the paleozoic which may have contributed to pre enrichments of the ore forming materials 2 the pacific plate subduction was a principle factor that controlled the late mesozoic large scale magmatism and mineralization in south china through flat subduction ridge subduction slab tearing and slab rollbacks

relative motion between the caribbean and north american plates and related boundary zone deformation from a decade of gps observations global positioning system gps measurements in 1986 1994 and 1995 at sites in dominican republic puerto rico cuba and grand turk define the velocity of the caribbean plate relative to north america the data show eastward motion of the caribbean plate at a rate of 21 1 mm yr 1 standard error in the vicinity of southern dominican republic a factor of 2 higher than the nuvel 1a plate motion model prediction of 11 3 mm yr independent measurements on san andres island and an euler vector derived from these data also suggest a rate that is much higher than the nuvel 1a model available data combined with simple elastic strain models give the following slip rate estimates for major left lateral faults in hispaniola 1 the north hispaniola fault offshore the north coast of hispaniola 4 3 mm yr 2 the septentrional fault in northern dominican republic 8 3 mm yr and 3 the enriquillo fault in southern dominican republic and haiti 8 4 mm yr the relatively high plate motion rate and fault slip rates suggested by our study combined with evidence for strain accumulation and historical seismicity imply that seismic risk in the region may be higher than previous estimates based on low plate rate low fault slip rate models and the relatively low rate of seismicity over the last century

tectonic and sedimentary architecture of the karukera spur a record of the lesser antilles fore arc deformations since the neogene the karukera spur is an easternmost submerged rise of the lesser antilles fore arc in the guadeloupe archipelago culminating about 4000 m above the fore arc basin 150 km west of the deformation front the analysis of 3500 km 2d high resolution multichannel seismic reflection and multibeam bathymetric data and the study of 14 core samples allow for the first time to reconstruct its sedimentary anatomy and tectonic evolution seven seismic units us1 to us7 are evidenced organised into four major depositional sequences separated by erosional surfaces units us1 to us4 thicken to the south southeast on the spur the sedimentary deposits rest upon a deeply eroded metamorphic basement of the caribbean plate as in la desirade island sequence 1 units us1 and us2 remains unknown in its lower part because it does not outcrop its upper part comprises early miocene shallow water carbonates sequence s2 unit us3 only comprises an upper miocene transgressive systems tract deposited into basinal environment it is bounded by major erosional surfaces sb2 and sb3 sequence 3 us4 to us6 is zanclean to pleistocene it is dominantly composed of shallow water reefal deposits sequence 4 corresponds to recent deposits the karukera spur experienced three main extensional episodes with alternations between uplift and subsidence periods uplifts are responsible for emersions and erosions subsidence severely drowned shallow water deposits the first extensional episode led to the formation of horsts and grabens in the basement prior to early miocene deposits the second extensional episode occurred during upper miocene in accordance with data in surrounding islands the third tectonic episode developed since calabrian noticeable is the constant reactivation of n130 n150 n150 n180 and n40 trending faults since early miocene and the neoformation of e w trending faults since calabrian our results suggest that at the latitude of guadeloupean archipelago the lesser antilles subduction zone is an erosional margin because of the successive sweepings of the santa lucia then tiburon ridges

coastal staircase sequences reflecting sea level oscillations and tectonic uplift during the quaternary and neogene many coasts feature sequences of quaternary and neogene shorelines that are shaped by a combination of sea level oscillations and tectonics we compiled a global synthesis of sea level changes for the following highstands mis 1 mis 3 mis 5e and mis 11 also we date the apparent onset of sequences of paleoshorelines either from published data or tentatively extrapolating an age for the uppermost purported oldest shoreline in each sequence including the most documented mis 5e benchmark we identify 926 sequences out of which 185 also feature holocene shorelines six areas are identified where elevations of the mis 3 shorelines are known and 31 feature elevation data for mis 11 shorelines genetic relationships to regional geodynamics are further explored based on the elevations of the mis 5e benchmark mean apparent uplift rates range from 0 01 0 01 mm yr hotspots to 1 47 0 08 mm yr continental collision passive margins appear as ubiquitously uplifting while tectonic segmentation is more important on active margins from the literature and our extrapolations we infer ages for the onset of formation for 180 coastal sequences sea level fingerprinting on coastal sequences started at least during mid miocene and locally as early as eocene whether due to the changes in the bulk volume of seawater or to the temporal variations in the shape of ocean basins estimates of eustasy fail to explain the magnitude of the apparent sea level drop thus vertical ground motion is invoked and we interpret the long lasting development of those paleoshore sequences as the imprint of glacial cycles on globally uplifted margins in response to continental compression the geomorphological expression of the sequences matches the amplitude and frequency of glacial cyclicity from middle pleistocene to present day moderately fast 100 000 yrs oscillating sea levels favor the development of well identified strandlines that are distinct from one another pliocene and lower pleistocene strandlines associated with faster cyclicity 40 000 yrs are more compact and easily merge into rasas whereas older cenozoic low frequency eustatic changes generally led to widespread flat lying coastal plains

mesoproterozoic neoproterozoic transition geochemistry provenance and tectonic setting of clastic sedimentary rocks on the se margin of the yangtze block south china abstract the se margin of the yangtze block south china is composed of the mesoproterozoic lengjiaxi group and the neoproterozoic banxi group with sinian and post sinian cover a geochemical study was undertaken on the mesoproterozoic neoproterozoic clastic sediments in order to delineate the characteristics of the sediment source and to constrain the tectonic development and crustal evolution of south china our results show that the mesoproterozoic clastic sediments have a dominant component derived from a metavolcanic plutonic terrane with a large of mafic component there is a minor contribution of mafic rocks and older upper crustal rocks to the provenance strong chemical weathering in the source area occurred before transport and deposition the provenance for the neoproterozoic clastic sediments was most likely old upper continental crust composed of tonalite granodiorite dominated tonalite granodiorite granite source rocks which had undergone strong weathering and or recycling a minor component of older k rich granitic plutonic rocks and younger volcanogenic bimodal rocks is also indicated based on the regional geology the geochemical data and the inferred provenance the mesoproterozoic group is interpreted as a successive sedimentary sequence deposited in an extensional rifting back arc basin adjacent to a 1 80 ga continental margin arc terrane the progressive extension rifting of the back arc basin was followed by increasing subsidence and regional uplift during continental marginal arc continent the cathaysian block collision at 1 0 ga caused the deposition of the neoproterozoic group into back arc to retro arc foreland basin therefore the depositional setting of the proterozoic clastic sediments and associated volcanic rocks within the back arc basin reflected basin development from an active continental margin back arc basin with extension or rifting of the back arc basin to a passive continental margin

caledonian early paleozoic veins overprinted by yanshanian late mesozoic gold mineralization in the jiangnan orogen a case study on gold deposits in northeastern hunan south china abstract the jiangnan orogen situated between the yangtze and cathaysia blocks in south china is an important gold polymetallic metallogenic belt in china like many gold deposits hosted in precambrian rocks in eastern china the timing of gold mineralization in the jiangnan orogen mostly hosted by neoproterozoic rocks has been controversial a number of previous geochronological studies have suggested that mineralization ages range from neoproterozoic to cretaceous with caledonian early paleozoic and yanshanian jurassic cretaceous being considered most important as caledonian is the most important period of regional metamorphism and yanshanian is the most important period of granitic magmatism the timing of mineralization is closely related to the genetic nature orogenic versus post orogenic of the gold mineralization this paper addresses these problems by studying the ages and characteristics of ore forming fluids of two major gold deposits wangu and huangjindong in the central part of the jiangnan orogen in northeastern hunan muscovite ar ar dating suggests that the bulk of the quartz veins in the wangu and huangjindong deposits were formed at ca 399 397 ma i e during the caledonian orogeny whereas gold mineralization and associated veining took place at ca 142 130 ma i e during the yanshanian orogeny fluid inclusions in the barren caledonian veins are characterized by the h2o nacl co2 system homogenization temperatures of 266 276 c and salinities of 5 5 4 wt nacl equivalent similar to those found in many orogenic type gold deposits whereas the fluid inclusions in the gold bearing yanshanian veins with abundant sulfides overprinting the barren veins have a h2o nacl composition system homogenization temperatures of 169 to 256 c and salinities from 8 7 to 15 2 wt nacl equivalent it is proposed that both the caledonian and yanshanian veining events are controlled by the same fracture systems with the former being formed in a relatively deep orogenic deformation metamorphic environment and the latter in a relatively shallow extensional environment accompanied by extensive granitic magmatism reactivation of the caledonian structures during the yanshanian orogeny together with availability of sulfur and gold possibly associated with granitic intrusions played a key role in the formation of the gold deposits in northeastern hunan and possibly elsewhere in the jiangnan orogen

evidence for quaternary convergence across the north america south america plate boundary zone east of the lesser antilles new seismic and bathymetric data reveal that the plate boundary zone between the north american and south american plates east of the caribbean has occupied a 200 km wide zone of localized basement uplift and folding and of faulting of the sedimentary cover during the early pleistocene to holocene this zone which includes the barracuda ridge and tiburon rise exhibits north south compressional structures and continues eastward up to 500 km from the boundary of the caribbean plate a sequence of young turbiditedominated sediments fi lls the basin between these two ridges and shows synsedimentary deformation the base of the sequence is of early pleistocene age dated by biostratigraphic correlation with deep sea drilling project site 27 recent deformation in the turbidite fi lled trough north of the barracuda ridge is shown by uplift of the seafl oor at the crests of anticlines across which the shallowest sediment layers thin or pinch out

subduction of aseismic ridges beneath the caribbean plate implications for the tectonics and seismic potential of the northeastern caribbean normal seafloor entering the puerto rico and northern lesser antillean trenches in the northeastern caribbean is interrupted by a series of aseismic ridges on the north and south american plates these topographic features lie close to the expected trend of fracture zones created about 80 110 m y ago when this seafloor was formed at the mid atlantic ridge the northernmost of the ridges that interact with the lesser antillean subduction zone the barracuda ridge intersects the arc in a region of high seismic activity some of this seismicity including a large shock in 1974 occurs within the overthrust plate and may be related to the deformation of the caribbean plate as it overrides the ridge a large bathymetric high the main ridge is oriented obliquely to the puerto rico trench and intersects the subduction zone north of the virgin islands in another cluster of seismic activity along the inner wall of the trench data from a seismic network in the northeastern caribbean indicate that this intersection is also characterized by both interpolate and intraplate seismic activity magnetic anomalies bathymetric trends and the pattern of deformed sediments on the inner wall of the trench strongly suggest that the main and barracuda ridges are parts of a formerly continuous aseismic ridge a segment of which has recently been overridden by the caribbean plate reconstruction of mid miocene to recent plate motions also suggest that at least two aseismic ridges and possibly fragments of the bahama platform have interacted with the subduction zone in the northeastern caribbean the introduction of these narrow segments of anomalous seafloor into the subduction zone has segmented the arc into elements about 200 km long these ridges may act as tectonic barriers or asperities during the rupture processes involved in large earthquakes they also leave a geologic imprint on segments of the arc with which they have interacted a 50 km landward jump of the locus of island arc volcanism occurred in late miocene time along the northern half of the lesser antilles we postulate that the subduction of a segment of seafloor of anomolously thick crust being more buoyant than adjacent seafloor resulted in a marked shoaling in the dip of the descending slab and therefore a shift of the locus of volcanism in the region near western puerto rico and eastern hispanolia plio pleistocene interaction with a similar feature in this case a part of the bahama platform about 3 4 m y ago led to a jump in the locus of subduction as evidenced by a gap in the downgoing seismic zone that segment of the bahama platform interferred with the subduction process and was subsequently sutured onto the caribbean plate when the boundary jumped about 60 km to the northeast the maximum size of historic shallow earthquakes along the lesser antillean arc varies from about 7 0 7 5 in the center of the arc where the dip of the shallow part of the plate boundary is steep to 8 0 8 5 along the northern part of the arc where the dip is shallow the interaction of anomalous seafloor as along the northern lesser antilles can lead to the development of a wider than normal zone of interplate contact and hence to earthquakes that are larger than those associated with more typical seafloor entering subduction zones major seismic gaps and regions of high seismic potential currently exist along the northern lesser antilles and to the north of puerto rico both gaps are bounded by anomalous features on the downgoing plate the intersection of these features with the plate boundary created large asperities that may be good places to search for precursors to future large earthquakes a great shock in 1787 may have ruptured an existing seismic gap north of puerto rico between 65 and 67 w thus that gap can be expected to eventually rupture again in a great shock and not to accommodate plate motion by totally aseismic processes

decreasing uplift rates and pleistocene marine terraces settlement in the central lesser antilles fore arc la desirade island 16 n this study investigates the lesser antilles fore arc at the latitude of guadeloupe archipelago and evidences that la desirade island the eastermost island of the forearc displays a staircase coastal sequence including four uplifted marine terraces and an upper reefal platform with mean shoreline angle elevations ranging from 10 to 210 m above sea level asl the platform paleobathymetry is constraint by a detailed analysis of the sediments we propose a revised morphostratigraphy for this coastal sequence including 5 paleo shorelines based on six u th dating from aragonitic corals from the three lowest terraces combined with paleobathymetric analysis of the fossil corals present in the upper platform terrace and upper platform carving of construction periods occurred during marine isotopic stages mis 5e mis 9 and during the intervals mis 15 17 mis 19 25 and mis 31 49 upper coral reef platform our results evidence a bulk decreasing uplift rate since early calabrian to present day clearly documented since 310 ka mis 9 from 0 14 to 0 19 to ca 0 mm y our data are consistent with first the transient influence of the subducting oceanic tiburon ridge during calabrian then with other parametres of the subduction zone since late calabrian to present day dip of the slab basal erosion of the upper plate inherited structures

non steady long term uplift rates and pleistocene marine terrace development along the andean margin of chile 31 s inferred from 10be dating pleistocene uplift of the chilean coast is recorded by the formation of wave cut platforms resulting from marine erosion during sea level highstands in the altos de talinay area 31 s we have identified a sequence of 5 wave cut platforms using in situ produced 10be exposure ages we show that these platforms were formed during interglacial periods at 6 122 232 321 and 690 ka these ages correspond to marine isotopic stages mis or substages miss 1 5e 7e 9c and 17 shoreline angle elevations used in conjunction with our chronology of wave cut platform formation illustrate that surface uplift rates vary from 103 69 mm ka between 122 and 6 ka to 1158 416 mm ka between 321 and 232 ka the absence of preserved platforms related to the mis 11 13 and 15 highstands likely reflects slow uplift rates during these times we suggest that since 700 ka the altos de talinay area was predominantly uplifted during 2 short periods following mis 17 and miss 9c this episodic uplift of the chilean coast in the pleistocene may result from subduction related processes such as pulses of tectonic accretion at the base of the forearc wedge

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent

relative sea level fall since the last interglacial stage are coasts uplifting worldwide the growing interest in quantification of vertical ground motion stems from the need to understand in detail how the earth s crust behaves for both scientific and social reasons however only recently has the refinement of dating techniques made possible the use of paleoshorelines as reliable tools for tectonic studies although there are many local studies of quaternary vertical motions of coastlines we know of no comprehensive worldwide synthesis here we provide a compilation of 890 records of paleoshoreline sequences with particular emphasis on the last interglacial stage marine isotopic stage mis 5e 122 ka the quality of dating mis 5e makes it a reliable marker to evaluate vertical ground motion rates during the late quaternary on a global scale the results show that most coastal segments have risen relative to sea level with a mean uplift rate higher than 0 2 mm yr i e more than four times faster than the estimated eustatic drop in sea level the results also reveal that the uplift rate is faster on average for active margins than for passive margins neither dynamic topography nor glacio hydro isostasy may explain sustained uplift of all continental margins as revealed by the wide distribution of uplifted sequences of paleoshorelines instead we suggest that only plate tectonic processes reconcile all observations of quaternary coastal uplift we propose that long term continental accretion has led to compression of continental plates and uplift of their margins therefore this study concludes that plate tectonics processes impact all margins and emphasizes the fact that the notion of a stable platform is unrealistic these results therefore seriously challenge the evaluation of past sea levels from the fossil shoreline record

the mokrsko west gold deposit bohemian massif czech republic mineralogy deposit setting and classification abstract the mokrsko west deposit is unique among european variscan gold deposits from the points of view of both the structure an approx 200 m thick complex of sheeted several mm thick densely spaced quartz veins and the economic viability gold reserves of about 100 t the deposit is hosted mainly by tonalite of the calc alkaline sazava tonalite suite ca 354 ma of the central bohemian plutonic complex mineralization is characterized by quartz dominated gangue no visible hydrothermal alteration low sulfide content high fineness native gold accompanied by maldonite aurostibite native bismuth and numerous bi te s phases five mineralogical stages are described in great detail arsenopyrite and chlorite thermometers mineral phase stabilities and published isotope and fluid inclusion data are used to reconstruct the temperature and compositional evolution of the system the role of liquid bismuth in the sequestration of gold is also discussed the deposit shares the features of both orogenic gold org and intrusion related gold irg deposits the irg model is advocated by close spatial association between the ore zone and the tonalite host intrusion by the absence scarcity of hydrothermal alteration by the au bi te as elemental association and by marked thermal gradients from the early to late mineralization stages the org model is advocated by an approx 15 10 ma gap between the intrusion of the tonalite host and the ore formation by isotope and geochemical evidence for a key role of metamorphic fluids in the mobilization and transfer of many elements species inclusive s and au the apparently ambiguous classification of the deposit can most probably be attributed to deposit formation at a depth of 9 km and to setting of the deposit at inside a large scale plutonic complex with multiple and prolonged tectonic and intrusive activity

revisiting tectonic corrections applied to pleistocene sea level highstands tectonic displacement contaminates estimates of peak eustatic sea level and equivalently minimum continental ice volumes determined from the elevation of quaternary interglacial highstand markers for sites at which a stratigraphic or geomorphic marker of peak marine isotope stage mis 5e sea level exists the standard approach for estimating local tectonic uplift or subsidence rates takes the difference between the elevation of the local highstand marker and a reference mis 5e eustatic value commonly chosen as 6 m and divides by the age of the marker the resulting rate is then applied to correct the elevation of all other local observed sea level markers for tectonic displacement including peak highstands of different ages e g mis 5a mis 5c and mis 11 under the assumption that the tectonic rate remained constant over those periods this approach introduces two potentially significant errors first the peak eustatic value adopted for mis 5e in most previous studies i e 6 m is likely incorrect second local peak sea level during mis 5e is characterized by significant departures from eustasy due to glacial isostatic adjustment in response to both successive glacial interglacial cycles and excess polar ice sheet melt relative to present day values we use numerical models of glacial isostatic adjustment that incorporate both of these effects to quantify the plausible range of the combined error and show that even at sites far from melting ice sheets local peak sea level during mis 5e may depart from eustasy by 2 4 m or more we also demonstrate that the associated error in the estimated tectonic rates can significantly alter previous estimates of peak eustatic sea level during quaternary highstands notably those associated with earlier interglacials e g mis 11

structure of the lesser antilles subduction forearc and backstop from 3d seismic refraction tomography in 2007 the sismantilles ii experiment was conducted to constrain structure and seismicity in the central lesser antilles subduction zone the seismic refraction data recorded by a network of 27 obss over an area of 65 km 95 km provide new insights on the crustal structure of the forearc offshore martinique and dominica islands the tomographic inversion of first arrival travel times provides a 3d p wave velocity model down to 15 km basement velocity gradients depict that the forearc is made up of two distinct units a high velocity gradient domain named the inner forearc in comparison to a lower velocity gradient domain located further trenchward named the outer forearc whereas the inner forearc appears as a rigid block uplifted and possibly tilted as a whole to the south short wavelength deformations of the outer forearc basement are observed beneath a 3 to 6 km thick sedimentary pile in relation with the subduction of the tiburon ridge and associated seafloor reliefs north offshore dominica island the outer forearc is 70 km wide it extends as far as 180 km to the east of the volcanic front where it acts as a backstop on which the accretionary wedge developed its width decreases strongly to the south to terminate offshore martinique where the inner forearc acts as the backstop the inner forearc is likely the extension at depth of themesozoicmagmatic crust outcropping to the north in la desirade island and along the scarp of the karukera spur the outer forearc could be either the eastern prolongation of the inner forearc but the crust was thinned and fractured during the past tectonic history of the area or by recent subduction processes or an oceanic terrane more recently accreted to the island arc

pliocene to pleistocene vertical movements in the forearc of the lesser antilles subduction insights from chronostratigraphy of shallow water carbonate platforms guadeloupe archipelago an integrated stratigraphic study was conducted on the shallow water carbonate platforms of the guadeloupe archipelago to refine the tectonic evolution of the lesser antilles forearc the carbonate platforms are now dated to the zanclean calabrian interval and their demise occurred between 1 5 and 1 07 ma the precise chronostratigraphy allows dating of the main extensional tectonic events since the late miocene an initial episode occurred during the late miocene related to the reactivation of inherited n130 e trending shear zones and led to the emergence of most parts of the forearc subsequently zanclean to early piacenzian carbonate platforms developed in association with a general subsidence of the forearc during the late piacenzan a second extensional episode occurred at this time la desirade underwent major uplift and emergence whereas most of the forearc remained submerged prior to 1 07 ma a third north south extensional episode occurred and led to the final demise of the carbonate platforms thus the forearc was characterized by general subsidence since the early pliocene interrupted by three main extensional episodes and related differential uplifts this suggests that the lesser antilles subduction is probably erosive north of latitude 15 n since c 5 ma related to aseismic ridge subduction supplementary data planktonic foraminiferal assemblages calcareous nannofossil taxa associations 40 ar 39 ar geochronological data and palaeomagnetic data are available at www geolsoc org uk sup18724

east asian gold deciphering the anomaly of phanerozoic gold in precambrian cratons early cretaceous orogenic gold deposits in eastern asia are globally unique in that large phanerozoic lode gold deposits occur in archean paleoproterozoic cratons in the northern pacific region ca 125 ma orogenic gold deposits in the north china yangzte and siberian craton margins as well as in young terranes in california may ultimately relate to the giant cretaceous mantle plume in the southern pacific basin and the relatively rapid tectonic consequences along both continental margins from resulting pacific plate reconfigurations in eastern asia such consequences include reactivation of and fluid flow along major fault systems with fluid focusing into simultaneously forming isolated core complexes of uncertain genesis deposition of gold ores in previously devolatilized high grade precambrian metamorphic rocks requires an exotic source of ore fluid most likely subducted mesozoic oceanic crust and or overlying sediment an implication is that phanerozoic metamorphic core complexes in other destabilized craton margins could host large gold resources

nature age and tectonic setting of granitoid hosted orogenic gold deposits of the jiaodong peninsula eastern north china craton china the jiaodong gold province along the southeastern margin of the north china craton has combined production and resources of 900 tonnes of gold more than 95 of the gold in this province is hosted in mesozoic granitoids which intruded late archean metamorphic basement rocks episodically at ca 160 150 and 130 126 ma the deposits of the jiaodong peninsula form one of the largest provinces of granitoid hosted lode gold deposits in the world the jiaodong gold deposits occur as massive auriferous quartz veins hosted in subsidiary second or third order faults and as disseminated and stockwork style mineralization hosted in alteration zones along major regional scale faults mineralization took place during brittle reactivation d3 of pre existing d2 ductile shear zones wall rock alteration is dominated by k feldspar and sericite with also sulfide minerals and extensive silicification closest to the orebodies the alteration was accompanied by the introduction of volatiles fe mg k rb ag as au bi and sb enrichment of base metals in the gold lodes is generally low although locally significant gold deposition took place at temperatures of 250 to 350 c for most gold deposits and the ore fluids are co2 bearing and have calculated salinities of 6 13 wt nacl equiv deposits from the zhao ye xixia and muping rushan belts have similar if not identical structural controls alteration patterns ore mineral assemblages and stable isotope signatures lead isotope data indicate that the ore stage leads were buffered by their host rocks the granitoid hosted gold deposits in the zhao ye belt are most radiogenic the greenstone hosted deposits in the xixia belt is the least radiogenic and the granitoid hosted deposits in the muping rushan belt have intermediate compositions gold mineralization in the zhao ye xixia and muping rushan belts in the jiaodong gold province appears to be broadly contemporaneously at ca 130 120 ma as constrained by shrimp u pb zircon dating on host rocks and a post gold dike as well as by ar ar rb sr and k ar dating of hydrothermal alteration minerals and ores this early cretaceous age is about 100 million years younger than the peak of ultra high pressure metamorphism in the qinling dabie sulu orogen between the north china and yangtze cratons and about 500 km south of the gold province however it overlaps with the extension and exhumation stage of the collisional orogeny and the subduction of the pacific plate beneath the eurasian continental crust whereas the majority of the jiaodong gold deposits occur on the margin of the north china craton some also occur in the suture zone between the two cratons they are significantly landward relative to the subduction zone of the pacific plate compared with typical orogenic lode gold deposits globally lode gold deposits of ca 130 120 ma occurred on both sides of the tan lu fault zone available data favor a temporal link of gold mineralization with pacific plate subduction

late quaternary folding of coral reef terraces barbados uplifted late ouaternary coral reefs on the island of barbados record folding of the emergent crest of the lesser antilles accretionary prism barbados ridge complex since ca 1 ma three northeast striking folds are defined by systematic changes in altitudes in the crest of first high cliff a mostly constructional reef terrace about 125 ka and second high cliff a partially erosional reef terrace about 500 ka the folds have wavelengths of 6 to 8 km and fold axes extend about 10 km the largest anticline rises to the northeast where it has been breached by erosion exposing highly deformed eocene to lower miocene rocks of the scotland district uplift rates based on heights of the last interglacial first high cliff range from 0 07 to 0 44 mm yr quaternary folding on barbados indicates that the crest of the accretionary prism continues to be an active fold belt undergoing northwestsoutheast shortening

the polyphased tectonic evolution of the anegada passage in the northern lesser antilles subduction zone the influence of the highly oblique plate convergence at the northern lesser antilles onto the margin strain partitioning and deformation pattern although frequently invoked has never been clearly imaged the anegada passage is a set of basins and deep valleys regularly related to the southern boundary of the puerto rico virgin islands prvi microplate despite the publications of various tectonic models mostly based on bathymetric data the tectonic origin and deformation of this passage remains unconstrained in the absence of deep structure imaging during cruises antithesis 1 and 3 2013 2016 we recorded the first deep multichannel seismic images and new multibeam data in the northern lesser antilles margin segment in order to shed a new light on the structure and tectonic pattern of the anegada passage we image the northeastern extent of the anegada passage from the sombrero basin to the lesser antilles margin front our results reveal that this northeastern segment is an ew trending left stepping en echelon strike slip system that consists of the sombrero and malliwana pull apart basins the malliwana and anguilla left lateral faults and the ne sw compressional restraining bend at the malliwana hill reviewing the structure of the anegada passage from the south of puerto rico to the lesser antilles margin front reveals a polyphased tectonic history the anegada passage is formed by a nw se extension possibly related to the rotation or escape of prvi block due to collision of the bahamas bank currently it is deformed by an active wnw ese strike slip deformation associated to the shear component of the strain partitioning resulting from the subduction obliquity

seismic activity offshore martinique and dominica islands central lesser antilles subduction zone from temporary onshore and offshore seismic networks this work focuses on the analysis of a unique set of seismological data recorded by two temporary networks of seismometers deployed onshore and offshore in the central lesser antilles island arc from martinique to guadeloupe islands during the whole recording period extending from january to the end of august 2007 more than 1300 local seismic events were detected in this area a subset of 769 earthquakes was located precisely by using hypoellipse we also computed focal mechanisms using p wave polarities of the best azimuthally constrained earthquakes we detected earthquakes beneath the caribbean forearc and in the atlantic oceanic plate as well at depth seismicity delineates the wadati benioff zone down to 170 km depth the main seismic activity is concentrated in the lower crust and in the mantle wedge close to the island arc beneath an inner forearc domain in comparison to an outer forearc domain where little seismicity is observed we propose that the difference of the seismicity beneath the inner and the outer forearc is related to a difference of crustal structure between the inner forearc interpreted as a dense thick and rigid crustal block and the lighter and more flexible outer forearc seismicity is enhanced beneath the inner forearc because it likely increases the vertical stress applied to the subducting plate

an early paleozoic orogenic gold belt along the jiang shao fault south china evidence from fluid inclusions and rb sr dating of quartz in the huangshan and pingshui deposits abstract there are several gold deposits in the eastern section of the regional jiang shao fault between the yangtze and cathaysia blocks in south china auriferous quartz veins in these deposits are strictly hosted in second order ne trending ductile shear zones the ores generally contain low amounts of sulfide minerals

intracontinental subduction a possible mechanism for the early palaeozoic orogen of se china the early palaeozoic orogen of se china consists of three lithotectonic elements from top to bottom a sedimentary upper unit a metamorphic lower unit and a gneissic basement the boundaries between these units are flat lying south directed ductile decollements the lower one is coeval with an amphibolite facies metamorphism m1 the belt is reworked by migmatite granite domes high temperature metamorphism m2 and granitic plutons related to post orogenic crustal melting we date here the syn m1 ductile shearing at 453 7 ma by u th pb method on monazite previous ages and our new 40 ar 39 ar ages of biotites and muscovites show that the metamorphic rocks experienced syn m2 exhumation from 440 to 400 ma the early palaeozoic orogen of se china is an intracontinental belt in which decollements accommodated the north directed subduction of the cathaysian continent this orogen is an example of intracontinental subduction that was not preceded by oceanic subduction

the cenozoic tectonostratigraphic evolution of the barracuda ridge and tiburon rise at the western end of the north america south america plate boundary zone the barracuda ridge and the tiburon rise two major oceanic basement ridges lie at the western end of the diffuse north america south america plate boundary zone where they enter the subduction zone beneath the lesser antilles island arc there is a large degree of uncertainty in the motion between the north american and south american plates predicted by kinematic models of plate motion for the region of these two ridges during the cenozoic and quaternary from the analysis of new multibeam and seismic reflection profiles acquired in 2007 together with older geophysical and geological data we provide new information on the geological history of this area including the timing of the formation of the barracuda ridge and tiburon rise in their present day configurations the timing of the deformation in this region is now much better constrained through the correlation of several key seismic horizons with existing dsdp and odp holes the seafloor topography inherited from the process of formation of the crust at the mid oceanic ridge was buried by distal turbidites by the end of the paleogene beginning in the middle late miocene and then the pleistocene the tiburon rise and barracuda ridge respectively were uplifted and acquired their present day forms and elevation which is much more recent than previously believed in the quaternary the uplift was accompanied by the deposition of very large mass transport deposits the causes of uplift and deformation of the ridges have been convergence between the north american and south american plates and the flexure of these plates as they enter the lesser antilles subduction zone the zone of uplift and deformation migrated northward during the neogene and quaternary

a revised morpho and chronostratigraphy of the late and middle pleistocene coral reef terraces on southern barbados west indies the barbados coral reef terraces are one of the few type localities worldwide that provide insights into interglacial sea level change during the late and middle pleistocene several sea level estimates have been established since the late 1960s and each has contributed to the barbados model of sea level change this paper presents new morpho and chonostratigraphic data from the barbados coral reef terraces developed over the last 12 years the work is based on significant advances in electron spin resonance esr dating of fossil coral air photo interpretation and a greatly improved geomorphic map of preserved fossil beach formations and reef terraces above present sea level the need for a revision of past published morpho and chronostratigraphies is best demonstrated on the southern part of the island the morpho and chronostratigraphic sequence in this region is both more complex and diverse than has been assumed so far the revised morphostratigraphy presented here includes a differentiation of coral reef terraces wave cut platforms and other erosive features such as notches and cliffs our study of these geomorphic features combined with new numeric dating results esr u th enable a revised estimate of the spatial and temporal variation in tectonic uplift rate within south barbados these new rates are an essential requirement for more precise glacio eustatic sea level reconstructions during the late and middle pleistocene from this region

infrared microthermometric and noble gas isotope study of fluid inclusions in ore minerals at the woxi orogenic au sb w deposit western hunan south china abstract the woxi au sb w deposit hosted by the neoproterozoic low grade metamorphic clastic rocks is located in a brittle ductile shear zone within the xuefengshan range south china orebodies are predominantly banded quartz veins which are strictly controlled by bedding faults and display significant vertical extents up to 2 km without obvious vertical metal zoning fluid inclusions hosted in quartz scheelite and stibnite from quartz scheelite and quartz sulfide gold veins have been studied using conventional and infrared microscopy respectively four types of fluid inclusions were identified based on petrography including type i two phase liquid rich aqueous inclusions type ii two or three phase co 2 rich inclusions type iii two phase vapour rich aqueous inclusions and type iv single phase aqueous inclusions the fluid inclusions in ore minerals scheelite and stibnite and their coexisting quartz largely share similar characteristics in terms of their types homogenization temperatures and salinities this is consistent with the fact that these ore minerals are always intergrown with quartz microthermometric and laser raman data indicate a low to moderate temperature 140 240 c low salinity 2 rich n 2 bearing aqueous ore forming fluid such fluid is further identified as a deeply non magmatic crustal fluid rather than a mantle source fluid by the significantly low 3 he 4 he ratios 0 002 0 281 ra and a small amount of meteoric water or host rock buffered fluid could be involved w ore precipitation was probably associated with mixing between a deeply originated crustal fluid and host rock buffered fluid based on the fluid inclusion features in scheelite and quartz i however au and sb ore deposition probably resulted from boiling which was caused by the marked pressure drop geological features such as banded structure and crack sealing structure also indicate that fluid pressure fluctuation induced by fault valve mechanism occurred during ore precipitation these characteristics of the ore forming fluids in the woxi deposit are in good agreement with the definition of orogenic gold deposits and the woxi au sb w deposit is probably an atypical orogenic gold deposit for its unique ore forming element association

plio pleistocene palaeogeography of the llanura costera del caribe in eastern hispaniola dominican republic interplay of geomorphic evolution and sedimentation this paper aims to reconstruct the palaeogeographic evolution of the llanura costera del caribe lcc in eastern hispaniola dominican republic during the pleistocene adding new insights to published information on pliocene early pleistocene deposits the lcc is a generally flat region comprising the unfolded sedimentary cover of the cordilleras central and oriental within this cover the pliocene early pleistocene yanigua formation mainly consisting of marl changes seawards to the mainly limestone los haitises formation both units formed in a shallow water platform rimmed by a reef barrier at least in the latest depositional stages the overlying pleistocene la isabela formation consists of two major offlapping reef terraces in which a reef core enclosed a lagoon and prograded over forereef bioclastic debris two belts in lcc s morphostructure directly reflect its sedimentary evolution the inner belt extends over the marly substrate of the yanigua formation and the coastal belt comprises three major surfaces corresponding to the depositional top of the los haitises formation upper surface and to the upper and lower terraces of the la isabela formation intermediate and lower surfaces respectively the mis 5e age of the 10 20 m high lower terrace implies a low uplift rate of 0 033 0 068 mm yr for the lower surface the pliocene early pleistocene platform was emergent in the early middle pleistocene the early pleistocene reef barrier separated endorheic watersheds extending over the former shelf lagoon from the open ocean reefs built the upper terrace of the la isabela formation during one or several middle pleistocene highstands and the lower terrace during mis 5e and previous highstands siliciclastic deposits in this terrace record the opening to the caribbean sea of watersheds in the eastern lcc the lower terrace emersion and opening of the large drainage systems of the western and central lcc took place after mis 5e relative sea level fall and emersion of a large area did not imply increased terrigenous sedimentation in the adjacent marine basin development of endorheic watersheds over most of the emergent surface delayed the diachronic arrival of siliciclastics into the marine basin for hundreds of thousands of years

ore forming epoch of gold deposits in northeastern hunan gold deposits in northeastern hunan are mainly hosted in epimetamorphic clastic rocks of middle proterozoic lengjiaxi group it was thought by some earlier researchers that gold mineralization took place in caledonian or yanshanian epoch which however was not supported by high quality age data in this paper the rb sr isochron of fluid inclusions trapped in quartz was used to date the ore forming epochs of gold deposits the huangjindong wangu and tuanshanbei gold deposits are dated at 462 18 ma mswd 1 6 425 33 ma mswd 0 14 and 222 4 9 4 ma mswd 2 6 respectively with 87sr 86sr i being 0 7589 0 0005 0 7517 0 0004 and 0 7335 0 0009 dating results show that gold mineralization in northeastern hunan mainly occurred in caledonian with indosinian probably being the other metallogenic epoch in addition gold ore forming epochs of western hunan and northeastern hunan were likely concentrated in caledonian and indosinian implying that caledonian and indosinian were the two main ore forming epochs in jiangnan terrain

rapid pulses of uplift subsidence and subduction erosion offshore central america implications for building the rock record of convergent margins integrated ocean drilling program iodp expedition 334 to southern costa rica central america documented unprecedented subduction erosion in an area of active seismic slip widespread subduction erosion of the upper plate initiated when the cocos ridge an overthickened aseismic ridge arrived at the middle america trench subduction erosion was coeval with the rapid formation of deposition centers on the forearc of the upper plate the completely recovered shelf sequence constrains a short 2 m y interval of extreme subsidence 1200 m with a rapid pulse occurring during the first 0 3 m y this event removed an estimated 1 2 106 km3 of forearc material at a rate of 1690 km3 m y km of trench during a time of rapid 1035 m m y shelf sediment accumulation at this erosive margin a sediment starved trench persisted in spite of abundant sediment supply because subduction erosion led to the creation of forearc basins similar rapid pulses of subduction erosion may punctuate the evolution of many margins contributing disproportionately to crustal recycling at subduction zones with implications for the evolution of continental crust and mountain belts and recycling of continental material into the mantle

properties of a lacustrine subbituminous k1 seam with special reference to the contact metamorphism soma turkey abstract the subbituminous k1 seam in the middle miocene soma formation of western turkey reaches a thickness of 24 m typical samples on an air dried basis have 9 5 14 2 moisture 4 3 13 8 ash and 0 57 2 54 total sulphur the coal rank parameters and non agglomerating behaviour show that the coals are of subbituminous a rank the k1 seam is huminite rich coal with a mean random ulminite reflectance rr between 0 46 and 0 49 fluorescence emission measurements show that the sporinite cutinite resinite and alginite present have typical fluorescence spectra between 460 and 700 nm the soma formation was intruded by an olivine basalt during the pliocene pleistocene which caused local contact metamorphism at the top of the k1 seam on the basis of proximate ultimate and petrographic analyses the intruded seam can be divided into normal coal transition zone and natural coke intervals each differing in their degree of metamorphism moisture volatile matter hydrogen and oxygen contents very rapidly decrease close to the contact point with the intrusion whilst calorific value carbon content and mean random reflectance value increase a diagram ofo c versus h c also shows a distinct increase in rank caused by the intrusion minor illite siderite and plagioclase feldspar are present in the natural coke samples butpyrrhotite is not observed mineralogical geochemical and reflectance studies indicate that the general temperature during formation of natural coke was probably relatively low

dating open systems by the 238 u 234 u 230 th method application to quaternary reef terraces abstract the sedimentation conditions and diagenetic processes of biogenic and inorganic carbonates are such that the assumptions of a closed system evolution and the absence of initial th invoked in classical 238 u 234 u 230 th dating methods are very often not fulfilled for such material this problem is of interest for all studies requiring accurate dating of corals or aragonite rich sediments as in palaeoclimatology or neotectonics extending the approach of henderson and slowey nature 404 2000 61 66 we propose a consistent model that takes into account possible initial 230 th excess and where continuous selective redistribution gain or loss of 234 u 234 th and 230 th is controlled by recoil processes this model applied to quaternary marine terraces in barbados marie galante lesser antilles arc and pacific islands is able to explain the large scatter in isotopic compositions observed in the series of cogenetic corals or sediments we show that recoil processes generally tend to increase the 234 u 238 u and 230 th 234 u ratios with time leading to an overestimation of the apparent ages calculated using classical methods on the basis of simple assumptions identical marine initial 234 u 238 u 0 and 230 th 232 th 0 ratios we propose a method for calculating a best estimate of the age of cogenetic series our model offers an alternative way to date old reef terraces where corals are almost systematically altered and where classical dating methods do not apply it may also explain the apparent variations of the 234 u 238 u seawater initial ratio estimated from corals formed in the last 300 ka and the apparent uncoupling between sea level highstands and high latitude northern hemisphere summer insolation our set of equations can be applied to all weathering processes where the mobility of u series isotopes is controlled by recoil

sea level and deep sea temperature variability over the past 5 3 million years ice volume and hence sea level and deep sea temperature are key measures of global climate change sea level has been documented using several independent methods over the past 0 5 million years myr older periods however lack such independent validation all existing records are related to deep sea oxygen isotope 18o data that are influenced by processes unrelated to sea level for deep sea temperature only one continuous high resolution mg ca based record exists with related sea level estimates spanning the past 1 5 myr here we present a novel sea level reconstruction with associated estimates of deep sea temperature which independently validates the previous 0 1 5 myr reconstruction and extends it back to 5 3 myr ago we find that deep sea temperature and sea level generally decreased through time but distinctly out of synchrony which is remarkable given the importance of ice albedo feedbacks on the radiative forcing of climate in particular we observe a large temporal offset during the onset of plio pleistocene ice ages between a marked cooling step at 2 73 myr ago and the first major glaciation at 2 15 myr ago last we tentatively infer that ice sheets may have grown largest during glacials with more modest reductions in deep sea temperature a novel approach to the estimation of sea level and deep sea temperature has been used to determine these quantities over the past 5 3 million years this approach based on oxygen isotope records from the eastern mediterranean shows that temperature and sea level histories are broadly correlated but also show intriguing temporal offsets most records of past sea level variation and associated changes in ocean temperature rely on variations in oxygen isotopes recorded in deep sea marine microorganisms these signals unfortunately are confounded by other influences making it difficult to extract a clear climate related signal eelco rohling and colleagues now present a novel approach to estimating sea level and deep sea temperature variations for the past 5 3 million years the record based on oxygen isotope variations from the eastern mediterranean avoids the issues of the deep sea approach and generally corresponds to an independent record of the past 1 6 million years as expected temperature and sea level are broadly correlated but also show intriguing temporal discontinuities as at the initiation of plio pleistocene ice ages when a major sea level drop lagged a sharp temperature decline by several hundred thousand years

synchronous vertical and horizontal tectonism at late stages of archean cratonization and genesis of hemlo gold deposit superior craton ontario canada the hemlo gold deposit 700 t au northwestern ontario canada is hosted in a shear zone in the hemlo greenstone belt which is a synclinal keel between two granitoid domes the dome and keel structure formed by diapirism and sagduction vertical tectonism that was synchronous with regional horizontal shearing horizontal tectonism mineralization occurred during shearing and mineralizing fluids probably had a magmatic source that is interpreted to be related to syntectonic mantle derived granodiorite sanukitoid intrusions the gold deposit the intrusions and the hosting shear zone all developed within a regime characterized by synchronous vertical and horizontal tectonism they were all possibly linked to a range of processes associated with the accretionary growth and stabilization of the craton in particular slab breakoff and the associated extensional orogenic collapse following terrane accretion

neoproterozoic to early paleozoic polyorogenic deformation in the southeastern margin of the yangtze block constraints from structural analysis and 40ar 39ar geochronology abstract the southeastern margin of the yangtze block is a suitable region to decipher neoproterozoic to early paleozoic polyorogenic deformation of the south china block based on 1 50 000 scale geological mapping four deformational phases were identified through structural analysis and 40 ar 39 ar dating the earliest event d 1 corresponds to nearly e w striking folding and top to the south thrusting developed during neoproterozoic orogeny caused by the collision between the yangtze block and the cathaysia block the d 2 event is characterized by the nearly e w striking top to the north thrusting the d 3 deformation is marked by the nne striking folding thrusting and dextral shearing the d 3 structures constitute a positive flower pattern with opposing kinematics rooted in the jiangwan dextral shear zone the d 4 event is the nearly e w striking ductile dextral shearing 40 ar 39 ar geochronology shows that the nne trending dextral shearing and the nearly e w striking dextral shearing occurred at 449 4 ma and 429 3 ma respectively combining with previous geochronology all the d 2 d 3 d 4 events are related to the early paleozoic orogeny both the d 2 top to the north thrusts and d 3 nne striking positive flower structures are syn orogenic products these incompatible syn orogenic structures indicate that the orientation of the principal stress changed from the n s striking to e w striking at 450 ma the nearly e w striking ductile dextral shearing d 4 is the outcome of post orogeny

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt

the gold carbonate association source of co2 and co2 fixation reactions in archaean lode deposits abstract abundant carbonate is a ubiquitous feature of archaean lode gold deposits both in domains of alteration enveloping veins and in rocks enclosing auriferous sediments detailed studies of chemical mass balance in metabasalts progressively altered towards gold bearing quartz carbonate veins disposed within shear zones at yellowknife reveal massive additions of co2 k si and fe accompanying mineralisation with concomitant depletions of na coherent behaviour of al sc zr also v and nb provides a reference for constraining the volume relations during hydrothermal alteration in the peripheral regions of alteration depletions of ca and mg result in overall volume reduction but these elements are added to veins and their immediate alteration envelopes where there is a large positive volume change precious metals together with cr ni cu zn pb rb cd and ba have been added to the veins and altered wall rocks whereas quartz noble metals and other trace elements have been precipitated from hydrothermal fluids solution of ca mg and some fe indigenous to peripheral alteration regions combining with co2 from the mineralising reservoir appears to be the process for forming the abundant ferri dolomite gangue observations of many archaean au deposits reveals that carbonate chemistry reflects the nature of wall rocks with wall rocks donating the bivalent metal cations and hydrothermal fluids the co2 alteration reactions at yellowknife involved hydrolysis of albite accompanied by fixation of kaqueous to produce muscovite with na loss and hydrolysis of chlorite epidote with co2 fixation to form ferri dolomite studies of oxygen isotopes fe 2 fe 2 fe 3 and structure reveal that au quartz and carbonate were precipitated in the presence of fluids of probable metamorphic origin 18 o 8 9 at low redox potential and at ambient temperatures of 400 450 c during episodes of hydrofracturing the abundant co2 and k required for extensive carbonate muscovite replacement alteration could be supplied by fluids released during prograde metamorphism under greenschist or greenschist amphibolite facies conditions where the relative proportion of co 2 h 2 o is in the order of 0 2 to 0 5 and k na 1 given high co2 co in the hydrothermal reservoir these molecules may act as complexing agents for transport of au and other rare elements e g w pd ni cr

formation of forearc basins by collision between seamounts and accretionary wedges an example from the new hebrides subduction zone seabeam data reveal two deep subcircular reentrants in the lower are slope of the new hebrides island arc that may illustrate two stages in the development of a novel type of forearc basin the malekula reentrant lies just south of the partly subducted bougainville seamount this proximity as well as the similarity in morphology between the reentrant and an indentation in the lower arc slope off japan suggests that the malekula reentrant formed by the collision of a seamount with the arc an arcuate fold thrust belt has formed across the mouth of the reentrant forming the toe of a new accretionary wedge the efate reentrant may show the next stage in basin development this reentrant lies landward of a lower slope ridge that may have begun to form as an arcuate fold thrust belt across the mouth of a reentrant this belt may have grown by continued accretion at the toe of the wedge by underplating beneath the reentrant and by trapping of sediment shed from the island arc these processes could result in a roughly circular forearc basin basins that may have formed by seamount collision lie within the accretionary wedge adjacent to the aleutian trenches

sedimentology palaeoenvironments and biostratigraphy of the pliocene pleistocene carbonate platform of grande terre guadeloupe lesser antilles forearc pliocene and pleistocene deposits from grande terre guadeloupe archipelago french lesser antilles provide a remarkable example of an isolated carbonate system built in an active margin setting with sedimentation both controlled by rapid sea level changes and tectonic movements based on new field sedimentological and palaeontological analyses these deposits have been organized into four sedimentary sequences s1 to s4 separated by three subaerial erosion surfaces sb0 sb1 and sb2 sequences s1 and s2 calcaires inferieurs a rhodolithes deposited during the late zanclean to early gelasian planktonic foraminiferal zones pl2 to pl5 in low subsidence conditions on a distally steepened ramp dipping eastward red algal rich deposits which dominate the western part of grande terre change to planktonic foraminifer rich deposits eastward vertical movements of tens of metres were responsible for the formation of sb0 and sb1 sequence s3 formation volcano sedimentaire calcaires superieurs a rhodolithes and calcaires a agaricia was deposited during the late piacenzian to early calabrian zones pl5 to pt1a on a distally steepened red algal dominated ramp that changes upward into a homoclinal coral dominated ramp deposition of sequence s3 occurred during a eustatic cycle in quiet tectonic conditions its uppermost boundary the major erosion surface sb2 is related to the cala1 eustatic sea level fall finally sequence s4 calcaires a acropora probably formed during the calabrian developing as a coral dominated platform during a eustatic cycle in quiet tectonic conditions the final emergence of the island could then have occurred in late calabrian times

forearc basins types geometries and relationships to subduction zone dynamics forearc basins are one of the fundamental sedimentary basins and they preserve various geological phenomena arising from dynamic interactions between subducting and overriding plates along convergent plate boundaries previous studies have proposed descriptive classification schemes for such basins and performed analog experiments and numerical simulations examining basin formation and evolution however the great variability of forearc basins means that a comprehensive understanding of these features remains elusive this paper proposes a new classification scheme for forearc basins from the viewpoints of material transfer between the two plates accretionary or nonaccretionary and the long term strain field in the basin compressional or extensional by using characteristics of 41 modern forearc basins geometric characteristics were also analyzed in order to reveal the relationship to subduction zone dynamics including material transfer at the plate boundary accretion erosion plate convergence configurations and kinematics and the mechanics of strain and its partitioning compressional accretionary type basins which are characterized by landward tilting strata and the landward migration of depocenters show a constant width thickness ratio indicating self similar growth of the basin as the outer wedge builds up extensional accretionary type basins develop normal faults in the basin probably due to a gravitational instability of the outer wedge an increase of sediment flux at the trench may grow the accretionary prism and thicken the subduction channel faster leading to a reduction of basal and internal shear strengths of the outer wedge subsidence caused by crustal thinning characterizes the nonaccretionary type basins which have thin trench fill sediments large forearc slopes steady widths of the outer wedge and variable width thickness ratios of the basins from these results the sediment flux at the trench and the plate boundary can be considered as a dominant factor that determines accretion or erosion of the outer wedge and uplift or subsidence of the outer arc high and that is also responsible for the style of deposition in the forearc basin an increase or decrease of the sediment flux may change the type geometry and deformation manner of the forearc basin the results of this study provide important clues to further our understanding of dynamic interactions between forearc basins and subduction zone processes

a one million year long sequence of marine terraces on sumba island indonesia abstract an exceptional sequence of raised coral reef terraces is visible at cape laundi on the north coast of the island of sumba in the savu sea in which at least eleven terraces wider than 100 m six of them even being over half a kilometer wide are geomorphologically distinguishable between an ancient patch reef now at 475 m and the present sea level esr date estimations of almost unrecrystallized corals most of them in situ make possible a tentative identification of the terraces corresponding to isotope stages 15 ca 600 ka ago and 9 ca 330 ka ago when the uplift trend deduced from the present altitude of dated terraces 0 49 0 01 mm yr is extrapolated to the whole raised section most geomorphological features appear to correspond to interglacial stages of the astronomically calibrated oxygen isotope record up to stage 27 ca 0 99 ma beyond the matuyama jaramillo geomagnetic boundary the upper terraces would in this case be twice as old as previously estimated ca 0 5 ma main terraces seem to correspond to high sea levels at certain interglacial stages and substages but in some cases the action of low sea levels at glacial stages cannot be ruled out for terraces between 30 and 70 m in altitude th u and esr dates reveal that aragonitic corals at similar elevations may differ in age as much as 100 ka this can be explained by taking into account a polycyclic origin for several terraces reconstructions available of changes in the eustatic sea level during the last 250 kyr and the fact that coral reef development may have been scattered at some periods only without totally covering subjacent older formations the sumba sequence of marine terraces at cape laundi is among the longest and most complete for the past million years reported in the literature its relatively good state of preservation will probably give impetus to a number of new studies on the chemical physical isotopic morphological palaeoclimatic pedological ecological and diagenetic changes which have occurred during the past one million years

the early cretaceous yangzhaiyu lode gold deposit north china craton a link between craton reactivation and gold veining the yangzhaiyu gold deposit is one of numerous lode gold deposits in the xiaoqinling district southern margin of the north china craton gold mineralization is hosted in neoarchean to early paleoproterozoic amphibolite facies metamorphic rocks and consists of auriferous quartz veins and subordinate disseminated ores in the vein proximal alteration zone ore related hydrothermal alteration is dominated by sericite quartz sulfide assemblages close to gold veins and biotite quartz pyrite chlorite epidote alteration generally distal from mineralization pyrite is the predominant sulfide mineral locally coexisting with minor amounts of chalcopyrite sphalerite and galena gold occurs mostly as free gold enclosed in or filling microfractures of pyrite and quartz and is also present in equilibrium with au bearing tellurides mainly petzite and calaverite coexisting with hessite tellurobismuthite and altaite fluid inclusion studies suggest that gold veins were deposited at intermediate temperatures 175 313 c from aqueous or aqueous carbonic fluids with moderate salinity 5 14 wt nacl equiv 34s values of sulfide minerals range mainly from 2 0 to 4 4 whereas auriferous quartz vein samples have 18o values of 12 4 to 9 6 with calculated 18oh2o values of 6 0 to 3 2 gold related pyrite grains yield elevated 3he 4he ratios 1 51 0 32 ra relative to crustal reservoirs and mantle like 20ne 22ne and 21ne 22ne ratios 9 90 9 68 and 0 029 respectively the stable and noble gas isotopes thus suggest deep seated most likely magmatic and mantle derived sources for the ore fluids sulfur and by inference other components in the ore system 40ar 39ar dating of ore related sericite and biotite separates indicates two episodes of gold genesis at 134 5 to 132 3 and 124 3 to 123 7 ma the mineralization ages overlap zircon u pb ages of 141 0 1 6 to 125 8 1 4 ma 2 for the wenyu and niangniangshan monzogranite plutons and a number of mafic to intermediate dikes intruding these plutons all being proximal to the yangzhaiyu gold deposit the synchronism of gold genesis and magmatism provides additional weights of evidence for a magmatic derivation of ore fluids and sulfur the geochronologic data also suggest that gold veining took place billions of years after the stabilization of the north china craton and associated metamorphism in the late archean to early paleoproterozoic this contrasts sharply to lode gold deposits in other precambrian cratons that formed predominantly in late archean to paleoproterozoic temporarily and genetically related to regional high grade metamorphism and compressional or transpressional tectonism available data have demonstrated that the north china craton was reactivated in the late mesozoic as marked by voluminous igneous rocks faulted basin formation high crustal heat flow and widespread metamorphic core complexes in the eastern part of the craton it is thus suggested that the yangzhaiyu gold deposit together with other deposits of similar ages in the xiaoqinling district were products of this craton reactivation event lithospheric extension and extensive magmatism related to the craton reactivation may have provided sufficient heat energy fluid and sulfur required for the formation of the gold deposits

trench bending initiation upper plate strain pattern and volcanism insights from the lesser antilles arc st barthelemy island french west indies the upper plate deformation pattern reflects the mechanical behavior of subduction zones here we focus on the consequences of the entrance of a buoyant bank into the caribbean subduction zone during the eocene by studying the oldest exposed rocks belonging to the lesser antilles volcanic arc using a novel geochronological data set we show that the volcanic arc activity on the island of st barthelemy spanned over the mid eocene to early miocene with a westward migration of the tectono volcanic activity which is comparable to what has already been observed on other volcanic islands in the lesser antilles the kinematics analysis allows us to identify a switch in the stress field from pure to radial extension at the oligo miocene hinge with a subhorizontal 3 that has a mean trend of n20 a three step restoration of the regional deformation indicates that this switch from pure parallel to the trench extension to radial extension may reflect a strain partitioning initiation affecting the upper caribbean plate in response to trench bending that followed the entrance of the bahamas bank into the subduction zone we show that the northern end of the lesser antilles arc shows a tectono volcanic evolution which is similar to the southern one the north south dichotomy in the perpendicular to the trench extension 15 in the north versus 30 in the south may reflect different slab ends that are highly curved to the north restraining the extension in the upper plate versus a tear to the south allowing a larger amount of extension within the upper plate

evolution of marine terraces and sea level in the last interglacial cave hill barbados the uplifted marine terrace of last interglacial stage 5e age at cave hill barbados has been investigated with views toward terrace architecture developmental processes and sea level history methods include stratigraphic analysis of new deep exposures precise 230th geochronology and geomorphic mapping cave hill was central to earlier studies that led to important ideas on reef and terrace evolution but with which we find significant disagreement we present a new model of terrace evolution that emphasizes the role of marine erosion deposition of carbonate cover during the full eustatic cycle rather than only at highstand and principal reef development during transgression rather than at highstand by keep up rather than catch up growth the new model and geochronology contribute to an improved understanding of surficial processes during emergence of uplifting coral coasts and of global sea level changes in the last interglacial transgression in stage 5e at cave hill was accompanied by progressive marine erosion of a terrace floor and receding seacliff and deposition of a seaward thickening reefal wedge on the floor and above limestones of stage 6 and 7 ages the wedge contains a diachronous basal acropora palmata fringe reef this is overlain by an in place a cervicornis reef that aggraded progressively during sea level rise the transgressive phase took place between 136 ka or earlier and ca 128 ka during highstand between ca 127 and 120 ka the floor and seacliff continued to recede landward re gression began at or after ca 120 ka and sea level fell 37 m below the highstand level by ca 115 ka in stage 5c the seaward front of the last interglacial terrace was eroded landward an uncertain distance the sea level record at cave hill has implications for timing and constituent events of the penultimate glacial the last interglacial highstand and the fall in sea level at the transition from stage 5e to stage 5d shoreline angles which are isochronous linear geomorphic features are the most accurate markers of sustained highstand levels highstand levels and uplift rates interpreted from a palmata in barbados are less accurate and lower because the coral grew mainly in transgression the coral derived data therefore include uncertainties of depths of growth and collapse

tectonic erosion at the front of the japan trench convergent margin abstract the imaging of a multichannel seismic record was improved by reprocessing using pre stack techniques the reprocessed record shows structures that indicate tectonic erosion and gravity collapse at the front of the japan trench margin much of the lower slope appears to be underlain by a detached coherent block of continental crust the lower slope has failed by mass wasting and the resulting apron of slump debris at the base of the slope has become involved in thrust faulting at the front of the subduction zone slumping continues as long as debris is removed from the front of the margin by subduction and the apron cannot build up sufficiently to stabilize the failing lower slope truncated beds at the base of the upper plate indicate subcrustal erosion as well this probably being the main cause of massive subsidence of the margin subsidence was the cause of oversteepening destabilization and subsequent gravity collapse of the leading edge of the upper plate

andean coastal uplift and active tectonics in southern peru 10be surface exposure dating of differentially uplifted marine terrace sequences san juan de marcona 15 4 s article i nfo along the san juan de marcona bay of southern peru two spectacular sequences of preserved marine terraces record net quaternary uplift previous geomorphic analysis of these paleo shorelines has revealed evidence of upper plate deformation and regional uplift however in the absence of a robust absolute dating method these studies contain substantial uncertainties concerning the numerical dating of these marine markers and thus the corresponding calculated surface uplift rates however field mapping surveying the neotectonic features and 10 be dating of abraded surfaces contained within two sequences of marine terraces along this margin should allow for the robust calculation of pleistocene uplift rates the san juan de marcona bay lies on the southern flank of the subducting and south migrating aseismic nazca ridge where the maximum rates of coastal uplift are expected in this locality we measure high uplift rates ranging from 0 4 m ka to 0 9 m ka during the pleistocene margin parallel normal faults displace several marine terraces and influence the development of bays thereby contributing to the configuration of paleo and present day coastlines the faults have relatively low slip rates b0 1 m ka over 400 ka and have been inactive for the last 80 ka the presence and activity of these normal faults can be directly linked to subduction zone processes with the release and accommodation of short term coseismic compression during megathrust subduction zone events in contrast the regional permanent uplift is probably controlled by post seismic and or interseismic strain accumulation over longer time scales due to inelastic behavior of the upper plate since at least the latest pliocene the san juan de marcona area has experienced long term regional tectonic uplift that has increased since about 800 ka due to the southward migration of the subducting nazca ridge based on migration velocity and geometry of the ridge the influence of the nazca ridge on the uplift of the forearc should account for 0 4 m ka within 145 km south of the ridge axis hence most of the post 400 ka mean uplift rate of the san juan de marcona area could be explained by the nazca ridge subduction

application of arsenopyrite geothermometry and sphalerite geobarometry to the taebaek pb zn ag deposit at yeonhwa i mine republic of korea the taebaek pb zn ag deposit of the yeonhwa i mine republic of korea occurs in a broadly folded and reverse faulted terrain of paleozoic sedimentary rocks the taebaeksan basin the orebodies consist of several thin tabular orebodies of hydrothermal replacement type where they are hosted by carbonate rocks the pb zn ag mineralization can be divided into four distinct stages based upon the mode of occurrence of ore minerals ore textural relationships and their composition based on temperatures inferred from arsenopyrite compositions by means of electron microprobe and fluid inclusions the estimated temperatures for the stages i ii iii and iv reach 330 to 350 c 270 to 340 c 230 to 250 c and 220 c respectively the sulphur activity atm of ore formation at the taebaek deposit was estimated for each stage as 10 11 to 10 11 5 10 9 5 to 10 13 10 13 5 to 10 15 and 10 15 respectively even though application of sphalerite geobarometry is problematic because of the absence of good mineral assemblages sphalerite coexisting with pyrite but not with pyrrhotite was used to estimate the minimum mineralization pressure about 1 kbar

from the seismic cycle to long term deformation linking seismic coupling and quaternary coastal geomorphology along the andean megathrust measurement of interseismic strain along subduction zones reveals the location of both locked asperities which might rupture during megathrust earthquakes and creeping zones which tend to arrest such seismic ruptures the heterogeneous pattern of interseismic coupling presumably relates to spatial variations of frictional properties along the subduction interface and may also show up in the forearc morphology to investigate this hypothesis we compiled information on the extent of earthquake ruptures for the last 500 years and uplift rates derived from dated marine terraces along the south american coastline from central peru to southern chile we additionally calculated a new interseismic coupling model for that same area based on a compilation of gps data we show that the coastline geometry characterized by the distance between the coast and the trench the latitudinal variations of long term uplift rates and the spatial pattern of interseismic coupling are correlated zones of faster and long term permanent coastal uplift evidenced by uplifted marine terraces coincide with peninsulas and also with areas of creep on the megathrust where slip is mostly aseismic and tend to arrest seismic ruptures we conclude that spatial variations of frictional properties along the megathrust dictate the tectono geomorphological evolution of the coastal zone and the extent of seismic ruptures along strike

phanerozoic tectonothermal events of the xuefengshan belt central south china implications from upb age and luhf determinations of granites the xuefengshan belt characterized by large scale fold and thrust structures and widespread granites is a key area to decipher the tectonic evolution of the south china block in this belt two magmatic episodes are recorded by early paleozoic and early mesozoic granites in this paper we carried out precise sims zircon usingle bondpb dating and in situ lusingle bondhf isotopes measurements on these granitic plutons our study indicates that the early paleozoic and the early mesozoic granites are late orogenic products of the early paleozoic orogen and the middle triassic xuefengshan orogen respectively in the xuefengshan belt the early paleozoic event is poorly registered since this area corresponds to the outer zone of the early paleozoic orogen the silurian early devonian granites are late orogenic plutons emplaced after the main tectonic and metamorphic stage in the wuyi baiyun yunkai belt coeval with a widespread subsequent extensional event in eastern and southern south china on the other hand triassic granites are formed in an intracontinental environment with weakly to strongly peraluminous signatures zircon usingle bondpb ages presented here associated with a summary of newly acquired data in the same region suggest that the emplacement of anatectic granites occurred around 225 215 ma not in a rather wide range of ca 245 200 ma in situ zircon ehf t values indicate a crust derived source without a mantle derived input for the two generations of granites combining our data with recent studies we infer that the central area of the south china block has experienced two tectonothermal events the early paleozoic magmatism developed as a result of the collapse of the early paleozoic orogen while these early mesozoic granites can be the late orogenic products of the intracontinental xuefengshan orogen most likely manifesting the far field effect by the

episodic imbricate thrusting and underthrusting analog experiments and mechanical analysis applied to the alaskan accretionary wedge seismic reflection profiles from the sediment rich alaska subduction zone image short frontally accreted imbricate thrust slices and repeated sequences of long underthrust sheets rapid landward increases in wedge thickness backthrusting and uplift of the forearc are observed suggesting underthrusting beneath the wedge these features and a widely varying frontal wedge morphology are interpreted to be caused by different modes of accretion active concurrently along the trench at different locations episodic wedge growth is observed in high basal friction experiments using sand as an analog material two phases of an accretionary cycle can be distinguished frontal accretion of short imbricate thrust slices alternating with underthrusting of long undeformed sheets the phase is shown experimentally to depend upon the surface slope of the wedge mechanical analysis of the forces at work predicts these two modes of deformation due to the varying frictional forces and yield strengths for a temporally varying wedge geometry maximum length of thrust slices is calculated for experimental conditions and confirmed by the observations for a steep frontal slope at the upper limit of the mohr coulomb taper stability field the overburden is too great to permit underthrusting and failure occurs repeatedly at the wedge front producing short imbricate slices the wedge grows forward lowering the surface angle to the minimum critical taper for a shallow frontal slope the reduced overburden along an active roof thrust permits sustained underthrusting causing frontal erosion and backthrusting steepening the wedge and thus completing the cycle

structure of the lesser antilles subduction zone backstop and its role in a large accretionary system 1 the role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies however few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction a seismic reflection and refraction survey acquired in 1998 with the r v maurice ewing reveals the geometry of the forearc igneous crust accretionary wedge and forearc basin structure of the northern guadeloupe area of the lesser antilles forearc an accreted block of buoyant crust accreted in the late miocene forms the toe of the overriding arc crust and forms the backstop we imaged the top of this surface beneath the forearc basin to its seaward edge where it meets the subducting oceanic crust the toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge the steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop and a narrow 10 km transition between accreted and forearc basin sediment seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe separating the decollement and the subducting crust is an interval usually between 500 and 750 m of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop we speculate that the upturned geometry of the toe of the backstop and a weak fluid rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle

upper plate deformation associated with seamount subduction in many active margins severe deformation is observed at the front of the overriding plate where seamounts or aseismic ridges subduct such deformation appears to be a main tectonic feature of these areas which influences the morphology and the seismicity of the margin to better understand the different stages of seamount subduction we have performed sandbox experiments to study in detail the evolution of deformation both in space and time and thus complement seismic images and bathymetry interpretation we focus in this paper on the surface deformation directly comparable with seafloor morphology two types of subducting seamounts were modelled relatively small conical seamounts and larger flat topped seamounts the indentation of the margin by the seamount inhibits frontal accretion and produces a re entrant the margin uplift includes displacement along backthrusts which propagate from the base of the seamount and out of sequence forethrusts which define a shadow zone located on the landward flank of the seamount when the seamount is totally buried beneath the margin this landward shielded zone disappears and a larger one is created in the wake of the asperity due to

late neoproterozoic early paleozoic evolution of the south china block as a retroarc thrust wedge foreland basin system the early paleozoic wuyi yunkai fold belt of south china represents a major orogenic belt in east asia however its formation is not well understood this paper evaluates the origins of the neoproterozoic early paleozoic basin and the southern margin of the south china block scb based on a compilation of the spatiotemporal evolution of the late neoproterozoic early paleozoic strata and their depositional environments these data are combined with high resolution structural geochronologic metamorphic and petrologic data that are published from the region a new model of a retroarc thrust wedge foreland basin system is proposed for the succession based on the following 1 the nature of the basal unconformity 2 the wedge shaped basin fill geometry bounded by the mountain belt to the south 3 basinward propagation of the thrust front with time and 4 the presence of a typical underfilled stratigraphic trinity of the foreland basin the proposed model suggests two major cycles of evolution of the retroarc thrust wedge foreland basin system 725 635 and ca 564 446 ma which record two periods of northwestward subduction of the oceanic slab beneath the southern margin of the scb terrane accretion is inferred to have taken place during the late stage of these cycles

smectite and illite smectite mixed layer clay minerals in the ashibetsu coals abstract smectite k smectite and illite smectite i s mixed layer minerals are found as expandable clay minerals in the paleogene coal seams of the ashibetsu coal mine in the ishikari coal field hokkaido japan as k smectite often exists in tuff tuffaceous shale and coal near tuff and only occasionally in shale the k smectite is simply thought to represent detrital input from tuff smectite often exists in the coals near intercalated tuff roof tuff and tuffaceous shale beds but is not found in these sediments although many tuffs contain both k smectite and i s mixed layer clay minerals the smectite coexists with only k smectite in many coals and the coal in which smectite occurs never has both of the other minerals from these relationships it may be inferred that smectite was formed in the coal mainly from the i s mixed layer and only partly from k smectite the i s mixed layer clay found in the tuff and tuffaceous shale is never accompanied by illite but the i s mixed layer clay in the shale is almost always accompanied by illite i s mixed layer clay without illite exists in some coals near tuff and i s mixed layer clay with illite is often found in the other coals these relationships mean that the i s mixed layer clay in the coal is also of detrital origin derived from the same sources as both the tuff and the shale beds the i s mixed layer clays plotted using watanabe s 1981 1988 identification graph show a pattern consistent with illitization by diagenesis the illitization appears to be related to the burial depth and or coal rank it is also shown that the mixed layer clays in the shales have higher illite layer ratios than those in the tuffs the tuffaceous shales and the coals without illite from this evidence it appears that the composition of i s mixed layer clay depends mainly on processes associated with the source of the original sediment input

formation of an active thrust triangle zone associated with structural inversion in a subduction setting eastern new zealand 1 we analyze a thrust triangle zone which underlies the continental shelf of hawke bay eastern new zealand within the hikurangi subduction margin this triangle zone differs from many other examples in that it is active 90 km from the leading edge of the overriding plate and formed due to polyphase deformation involving opposed dipping thrust duplex and backthrust with the later structure forming in response to inversion of an extensional graben the component structures of the zone mainly developed sequentially rather than synchronously high quality marine seismic reflection lines tied to well and seabed samples reveal the three dimensional structure of the zone together with its 25 myr evolution and late quaternary activity the triangle zone occurs in the lateral overlap between a stack of nw dipping blind thrusts and a principal backthrust the kidnappers fault the nw dipping thrusts initiated in the early middle miocene during the early stages of subduction with subsequent thrust duplex formation producing major uplift and erosion in the late miocene early pliocene the active backthrust formed during the late miocene to early pliocene as a thin skinned listric extensional fault confined to the cover sequence structural inversion of the extensional fault commenced in the early middle pliocene produced the backthrust and marks the formation of the thrust triangle zone the thrust duplex and backthrust accrued strain following inversion however the later structure accommodated most of the surface deformation in the quaternary section balancing of the triangle zone together with a detailed analysis of reverse displacements along the backthrust reveal spatial and temporal variations of strain accumulation on the two principal components of the zone although the formation of the triangle zone is strongly influenced by regional tectonics of the subduction system these variations may also in part reflect local fault interaction for example high quaternary displacement rates on the backthrust accounts for 70 of the displacement loss that occurs on the southern segments of the overlapping lachlan fault understanding the tectonic evolution of such complex polyphase thrust triangle zones requires the preservation of growth strata that record sequential deformation history in the absence of such data synchroneity of opposed dipping thrusts in triangle zones cannot be assumed