late cenozoic transpressional ductile deformation north of the nazca south america antarctica triple junction abstract the southern andes plate boundary zone records a protracted history of bulk transpressional deformation during the cenozoic which has been causally related to either oblique subduction or ridge collision however few structural and chronological studies of regional deformation are available to support one hypothesis or the other we address along and across strike variations in the nature and timing of plate boundary deformation to better understand the cenozoic tectonics of the southern andes two east west structural transects were mapped at puyuhuapi and aysen immediately north of the nazca south america antarctica triple junction at puyuhuapi 44 s north south striking high angle contractional and strike slip ductile shear zones developed from plutons coexist with moderately dipping dextral oblique shear zones in the wallrocks in aysen 45 46 top to the southwest oblique thrusting predominates to the west of the cenozoic magmatic arc whereas dextral strike slip shear zones develop within it new 40ar 39ar data from mylonites and undeformed rocks from the two transects suggest that dextral strike slip oblique slip and contractional deformation occurred at nearly the same time but within different structural domains along and across the orogen similar ages were obtained on both high strain pelitic schists with dextral strike slip kinematics 4 4 0 3 ma laser on muscovite biotite aggregates aysen transect 45 s and on mylonitic plutonic rocks with contractional deformation 3 8 0 2 to 4 2 0 2 ma fine grained recrystallized biotite puyuhuapi transect oblique slip dextral reverse kinematics of uncertain age is documented at the canal costa shear zone 45 s and at the queulat shear zone at 44 s published dates for the undeformed protholiths suggest both shear zones are likely late miocene or pliocene coeval with contractional and strike slip shear zones farther north coeval strike slip oblique slip and contractional deformation on ductile shear zones of the southern andes suggest different degrees of along and across strike deformation partitioning of bulk transpressional deformation the long term dextral transpressional regime appears to be driven by oblique subduction the short term deformation is in turn controlled by ridge collision from 6 ma to present day this is indicated by most deformation ages and by a southward increase in the contractional component of deformation oblique slip to contractional shear zones at both western and eastern margins of the miocene belt of the patagonian batholith define a large scale pop up structure by which deeper levels of the crust have been differentially exhumed since the pliocene at a rate in excess of 1 7 mm year

origin of the early cretaceous liucheng oib type dolerite sill in zhejiang province coastal region of se china implications for recycling of the paleo pacific slab abstract both early cretaceous ocean island basalt oib type and subduction related arc type mafic magmatism occurred in coastal region of se china however whether there existed a genetic relationship between the oib type magmatism and contemporary subduction of the paleo pacific ocean remains unclear here we present a comprehensive geochemical dataset of the liucheng dolerite sill 108 ma from coastal se china with aims to investigate the possible genetic link between them the dolerite displays oib like trace element features and contains moderately radiogenic sr nd pb hf and light ca isotopic compositions the nearly identical sr pb isotopes between plagioclase and bulk rock indicate a minor role of crustal assimilation during magmatic evolution the remarkable positive ba anomalies similar sr and pb isotopic compositions to the contemporary arc type mafic rocks and light ca isotopes suggest a source modified by recycled sediments additionally the low la nb and zr nb ratios indicate the involvement of an oceanic crust component that experienced dehydration and metamorphism to retain rutile further element isotopic modeling suggests derivation of the parental magma from the asthenospheric mantle source that had been metasomatized by melts from both sediments and mafic oceanic crust considering that se china was mainly influenced by subduction of the paleo pacific ocean during late mesozoic the subducting paleo pacific slab was therefore the most likely candidate for such recycled components interaction of the melts from the recycled oceanic slab with the asthenospheric mantle possibly as a consequence of rollback and tearing of the subducting paleo pacific slab formed the oib type source of the liucheng dolerite sill

mesozoic volcanism in the middle east geochemical isotopic and petrogenetic evolution of extension related alkali basalts from central lebanon mesozoic picritic and alkali basalts from central lebanon represent a significant part of an extension related upper jurassic to upper cretaceous discontinuous volcanic belt which occurs throughout the middle east volcanism was associated with an episode of intraplate extension that followed a period of continental break up where mesozoic micro continental blocks separated from gondwana as the neotethys ocean opened in jurassic times this volcanic episode produced mafic lava flows ranging in thickness from 5 to 20 m along with some minor pyroclastic flows these flows are stratigraphically intercalated with thick carbonate platform deposits the basalts are made up of about 15 20 olivine fo 78 91 30 35 clinopyroxene salite 40 50 plagioclase an 56 71 and opaque fe ti oxides 5 geochemically the rocks exhibit a relatively wide range of sio 2 40 4 to 50 5 wt and mgo 5 1 to 15 5 wt contents are relatively enriched in tio 2 1 7 to 3 7 wt and vary in composition from alkali picrite and basanite to alkali basalt the mg numbers range from 0 56 to 0 70 with an average of 0 63 the rocks are enriched in incompatible trace elements such as zr 86 247 ppm nb 16 66 ppm and y 19 30 ppm such compositions are typical of those of himu oib and plume related magmas the ree patterns are fractionated la yb n 11 lree enriched and are generally parallel to subparallel elemental ratios such as k p 1 1 4 7 la ta 11 13 la nb 0 57 0 70 nb y 0 68 1 55 and th nb 0 20 0 36 suggest that crustal contamination was minor or absent this may be related to a rapid ascent of the magma in agreement with the nature mafic oceanic like and the small thickness about 12 km of the mesozoic crust of the eastern mediterranean region the 143 nd 144 nd isotopic compositions of the lavas range from 0 512826 to 0 512886 and 87 sr 86 sr from 0 702971 to 0 703669 suggesting a himu like mantle source trace element compositions indicate a melt segregation depth of 100 110 km well within the garnet lherzolite stability field the geochemical characteristics of the rocks are typical of within plate alkali basalts and suggest that the magmas were derived from a fertile possibly plume related enriched mantle source petrogenetic modelling indicates that the magmas were produced by very small degrees of batch partial melting f 1 5 of a primitive garnet bearing mantle source garnet lherzolite

evolution of a metamorphic fluid and its role in lode gold mineralisation in the central lberian zone the central iberian zone of spain and portugal contains numerous gold deposits of varying size age and host lithology previous work concentrated on single deposits leading to a need to apply a range of techniques to the region as a whole to form a model for metallogenesis throughout the region with this aim a number of occurrences of gold mineralisation in the south western central iberian zone were studied the central iberian zone consists essentially of greenschist facies metasedimentary units intruded by hercynian granitoids hercynian deformation occurred in three main stages the earliest d1 being the most intense quartz veining is common throughout the region and a vein chronology was established relative to these deformation events v1 veins are pre hercynian and may predate the precambrian sardic deformation v2 veins are syndeformational d1 and associated with peak greenschist metamorphism v3 veins are late kinematic syn to post d2 v4 veins are syn to post d3 occurring in the final stages of the hercynian and often associated with or post dating granites reactivation of vein sets by later fluids was common fluid chemistry studies of samples of known chronology indicate three main sources of fluid the dominant fluid throughout the region from v2 to v4 is a dilute aqueo carbonic fluid of varying composition the combination of fluid inclusion analyses and geochemical studies of wallrock alteration indicates that the fluid was the result of devolatilisation and dehydration during prograde metamorphism at varying depths the resulting aqueo carbonic fluids flowed upwards mixing with each other and with meteoric waters contact metamorphism associated with granite intrusions during the d3 deformation released further aqueo carbonic fluids which in some cases unmixed to form separate carbonic and aqueous inclusions circulation of meteoric waters was important during later stages d3 prolonged low temperature fluid rock interaction produced highly saline aqueous fluids which post date all major quartz veining no single set of characteristics exists for gold mineralisation in the central iberian zone deposits cover a wide spectrum but three main types have been identified 1 early d1 v2 syn peak metamorphic gold quartz veins associated with an aqueo carbonic fluid with high n2 and ch4 2 late kinematic d2 v3 veins often associated with late brittle faults again the fluid present at the time of formation was aqueo carbonic but with lower n2 and ch4 3 late v4 low temperature veins formed by a mainly meteoric fluid which was also responsible for remobilisation and concentration of gold in earlier deposits the gold mineralisation in the central iberian zone supports the continuum model for lode gold deposits gold was leached from metasediments during prograde metamorphism transported to shallower crustal levels and deposited wherever suitable traps occurred channelling of fluid from deeper levels resulted in the localisation of deposits close to major shear zones depositional sites were usually dilational zones in faults and more rarely folds but locally lithological controls were important h2o co2 unmixing was common but was not necessarily related to gold deposition

the evolution of water in the contact metamorphic aureole of the duluth complex northeastern minnesota contact metamorphism of the rove formation by the duluth complex northeastern minnesota resulted from the discontinuous reaction chlorite muscovite quartz cordierite biotite h 2 o and the continuous reaction muscovite biotite quartz cordierite k feldspar h 2 o bulk compositions of hornfelses and mass balance calculations indicate that metamorphism occurred isochemically except for the loss of 0 6 to 2 7 wt h 2 o calculated values of temperature and f h 2 o indicate that during progressive metamorphism h 2 o evolved from the rocks progressively diluted a co 2 rich fluid phase the relations among temperature x h 2 o and progress of the continuous reaction in an average bulk composition suggest that at t 500 c and p 1 500 bars chlorite muscovite quartz in 1 m 3 of rock reacted with at least 0 5 m 3 of a co 2 rich fluid producing a fluid with x h 2 o 0 15 the continuous reaction buffered the composition of this fluid to a value of x h 2 o 1 0 at t 600 c the relation between x h 2 o and the progress of the continuous reaction suggests that the volumetric fluid rock ratio decreased from a value of 0 55 to essentially 0 0 as temperature increased from 500 c to 600 c the study indicates that multivariate devolatilization reactions can be capable buffers of fluid composition the dynamic variables rate of temperature change rate of h 2 o evolution and rate of reaction were determined by a heat conduction model for contact metamorphism the calculated width of the aureole 180 m and temperature distribution within the aureole 500 to 620 c are consistent with observations the calculated reaction rates for the continuous reactions ranging from 1 2 10 4 to 3 10 2 moles m 3 yr are much lower than reported rates for other reactions minimum permeabilities implied by the rate of h 2 o evolution ranging from 10 12 to 5 10 15 cm 3 sec cm 3 rock are 4 to 7 orders of magnitude lower than measured permeabilities of shales these results strongly support the assumption of chemical equilibrium during metamorphism even in contact metamorphic aureoles where the rate of temperature change is great

geology and geochemistry of the clear creek intrusion related gold occurrences tintina gold province yukon canada the clear creek gold occurrences lie within deformed lower greenschist facies rocks of the western selwyn basin they consist of auriferous sheeted quartz veins that cut six cretaceous stocks and their hornfels the veins contain 1 2 combined pyrite and arsenopyrite with lesser pyrrhotite bismuthinite and scheelite as well as 2 5 g t au new 40ar 39ar geochronology of hydrothermal micas indicates that the veins formed within 1 2 million years of granitoid emplacement fluid inclusion microthermometry defines a parent ore fluid of 81 mol h2o 14 mol co2 4 mol nacl kcl and 1 mol ch4 which unmixed into a low and high salinity immiscible pair this suggests a more saline parent fluid and a greater degree of fluid unmixing relative to the other occurrences in the eastern tintina gold province inclusions trapped in as and bi rich high gold grade veins have homogenization temperatures of 300 350 c whereas inclusions found in more ag and pb rich veins are characterized by temperatures of

ridge subduction kinematics and implications for the nature of mantle upwelling discussion the authors e farrar and j m dixon in a recent paper on ridge subduction discuss the magmatic and tectonic effects of overridden sea floor spreading centres their paper provides a useful compilation of information on several ridge trench encounters and elucidates a certain type of plate interaction called eduction however much of their analysis is flawed because they 1 do not consider the geometry of subducted slabs and intervening slab windows 2 misrepresent the importance of slab windows to magmatism and tectonism in the overriding plate and to mantle currents and 3 incorrectly link the concept of slab window formation to a model of passive mantle upflow consequently their paper overlooks the controlling influence that slab windows have on asthenosphere lithosphere interactions when diverging plates are subducted since the authors present their material in the style of a comprehensive review their omissions and misrepresentations warrant critical response numerous authors have established that ridge trench encounters tend to affect the overriding plate in three main ways 1 tectonic erosion of forearc and related uplift of forearc and arc 2 change in magmatic regime from arc type volcanism to riftor plume type volcanism extending from forearc to retroarc and 3 broad thermally supported uplift high heat flow and extension stacey 1974 fox et al 1985 merritts and vincent 1989 thorkelson and taylor 1989 hole 1990 hole et al 1991 babcock et al 1992 hole and larter 1993 references cited in the authors paper the authors concur and appeal to active asthenospheric upwelling e g hess 1962 gough 1984 wilson 1988 beneath the overriding plate as a principal cause although the authors depiction of active mantle dynamics is plausible the geological context of their arguments is unacceptable specifically they appeal to ridge subduction and mantle upwelling without providing a framework of subducted slab geometry without full appreciation of slab geometry and the constraints imposed by slab windows their arguments are limited to a vague precept of mantle convection

subduction of the south chile active spreading ridge a 17 ma to 3 ma magmatic record in central patagonia western edge of meseta del lago buenos aires argentina abstract the chile triple junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent the mlba plateau meseta del lago buenos aires is one of the neogene alkali basaltic plateaus located in the back arc region of the andean cordillera at the latitude of the current chile triple junction the genesis of mlba can be related with successive opening of slabs windows beneath patagonia within the subducting nazca plate itself and between the nazca and antarctic plates detailed 40ar 39ar dating and geochemical analysis of bimodal magmatism from the western flank of the mlba show major changes in the back arc magmatism which occurred between 14 5 ma and 12 5 ma with the transition from calc alkaline lavas cerro plomo to alkaline lavas mlba in relation with slab window opening in a second step at 4 3 ma alkaline felsic intrusions were emplaced in the western flank of the mlba coevally with the mlba basalts with which they are genetically related these late oib like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting nazca plate during the opening of the south chile spreading ridge related slab window these basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones the close association of bimodal magmatism with extensional tectonic features in the western mlba is a strong support to the model of patagonian collapse event proposed to have taken place between 5 and 3 ma as a consequence of the presence of the asthenospheric window scr 1 segment of south chile ridge below the mlba area

genesis of the vein type tungsten mineralization at nyakabingo rwanda in the karagwe ankole belt central africa the vein type tungsten deposit at nyakabingo in the central tungsten belt of rwanda is located in the eastern flank of the complex bumbogo anticlinal structure the host rock is composed of alternating sequences of sandstones quartzites and black pyritiferous metapelitic rocks two types of w mineralized quartz veins have been observed bedding parallel and quartz veins that are at high angle to the bedding which are termed crosscutting veins both vein types have been interpreted to have been formed in a late stage of a compressional deformation event both vein types are associated with small alteration zones comprising silicification tourmalinization and muscovitization dating of muscovite crystals at the border of the veins resulted in a maximum age of 992 4 1 5 ma this age is within error similar to the ages obtained for the specialized g4 granites i e 986 10 ma the w bearing minerals formed during two different phases the first phase is characterized by scheelite and massive wolframite while the second phase is formed by ferberite pseudomorphs after scheelite these minerals occur late in the evolution of the massive quartz veins sometimes even in fractures that crosscut the veins the ore minerals precipitated from a h2o co2 ch4 n2 nacl kcl fluid with low to moderate salinity 0 6 13 8 eq wt nacl and minimal trapping temperatures between 247 and 344 c the quartz veins have been crosscut by sulfide rich veins based on the similar setting mineralogy stable isotope and fluid composition it is considered that both types of w mineralized quartz veins formed during the same mineralizing event given the overlap in age between the g4 granites and the mineralized quartz veins and the typical association of the w deposits in rwanda but also worldwide with granite intrusions w originated from the geochemically specialized g4 granites intense water rock interaction and mixing with metamorphic fluids largely overprinted the original magmatic hydrothermal signature

source side shear wave splitting and upper mantle flow in the chile ridge subduction region the actively spreading chile ridge has been subducting beneath patagonian chile since the middle miocene after subduction continued separation of the faster nazca plate from the slow antarctic plate has opened up a gap a slab window between the subducted oceanic lithospheres beneath south america we examined the form of the asthenospheric mantle flow in the vicinity of this slab window using s waves from six isolated unusual 2007 earthquakes that occurred in the generally low seismicity region just north of the ridge subduction region the s waves from these earthquakes were recorded at distant seismic stations but were split into fast and slow orthogonally polarized waves at upper mantle depths during their passage through the slab window and environs we isolated the directions of fast split shear waves near the slab window by correcting for upper mantle seismic anisotropy at the distant stations the results show that the generally trench parallel upper mantle flow beneath the nazca plate rotates to an ene trend in the neighborhood of the slab gap consistent with upper mantle flow from west to east through the slab window

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

seismic structure of the upper mantle beneath the northern canadian cordillera from teleseismic travel time inversion abstract teleseismic tomography is used to image upper mantle structure beneath the northern canadian cordillera with the objective of determining the physical state of the upper mantle along the western half of the snorcle lithoprobe transect the resulting three dimensional p wave velocity model resolves structure beneath the southern yukon and northernmost british columbia between 100 and 600 km depth two significant anomalies are identified the first a relatively shallow high velocity feature located at the western edge of the model is interpreted as being the edge of the pacific slab from the southern alaska subduction zone the second is a large tabular low velocity anomaly centred at 60 n by 136 w elongate northwest southeast dipping southwest and reaching a depth of 450 500 km this low velocity anomaly is judged to reflect a thermal anomaly of 100 200 c with a possible compositional component its northeastern boundary is particularly sharp and is interpreted to represent the boundary between thin cordilleran lithosphere and a colder cratonic mantle root our preferred explanation for the low velocity feature is a thermal anomaly resulting from the advective upflow produced by the opening of a slab window beneath the northern cordillera a possible alternative is dense downward percolating partial melt generated by sub orogenic heating as past work e g stolper et al j geophys res 86 1981 6261 6271 suggests that basic partial melts may be denser than peridotite below 200 km depth

extensional tectonics during late cretaceous evolution of the southern central andes evidence from the chilean main range at 35 s abstract the study of late cretaceous earliest paleogene series in the chilean andes at 35 s provides new constraints on the coeval evolution of the area and the southern central andes these correspond to the continental series of brcu and the overlying plan de los yeuques formation pyf zircon age determinations reveal that these represent an almost continuous record of deposition respectively from the cenomanian early campanian to the campanian danian according to paleogeographic reconstructions the brcu series corresponds to the coeval foreland basin deposits in a near arc position which was westerly bounded by the earliest late cretaceous orogen detrital zircon age spectra for this reveal only a cretaceous component thus indicating a depocenter disconnected to that represented by coeval synorogenic deposits in argentina the volcanic rocks of the pyf record an eastern migration of the magmatic loci into the area at least since 80 ma its outcrops display a series of syn sedimentary normal faults indicating extensional conditions throughout its deposition its volcanic rocks conform an apparent bimodal suite characterized by arc like affinities low pressure conditions for the corresponding magmas and juvenile isotopic signatures for all the compositional spectra comparison and modeling of such compositional characteristics indicate a derivation from parental magmas similar to those currently sourcing the southern segment of the andean southern volcanic zone 37 42 s altogether these characteristics suggest an extensional intra arc basin setting for the deposition of this unit probably framed by a normal to thin crust of 30 35 km the extensional conditions observed for this unit are also recorded in numerous late cretaceous arc deposits along the andean margin thus indicating that this corresponds to a regional scale feature this evidence records a change from late cretaceous from compressional to extensional conditions in the arc area which agrees well with major plate tectonic configuration proposed for this time span in the andean margin

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

field geology geochronology and geochemistry of mafic ultramafic rocks from alxa china implications for late permian accretionary tectonics in the southern altaids abstract the time of termination of orogenesis for the southern altaids has been controversial systematic investigations of field geology geochronology and geochemistry on newly discriminated mafic ultramafic rocks from northern alxa in the southern altaids were conducted to address the termination problem the mafic ultramafic rocks are located in the bijiertai honggueryulin and qinggele areas stretching from west to east for about 100 km all rocks occur high grade gneisses as tectonic lenses that are composed of peridotite pyroxenite gabbro and serpentinite most of which have undergone pronounced alteration i e serpentinization and chloritization geochemically the rocks are characterized by uniform compositional trends i e with low sio2 contents 42 51 52 21 wt and alkalinity na2o k2o 0 01 5 45 wt mostly less than 0 8 wt and enrichments in mgo 7 37 43 36 wt with mg 52 75 91 87 as the rocks have been strongly altered and have a wide range of loss on ignition loi 0 44 14 07 wt values they may have been subjected to considerable alteration by either seawater or metamorphic fluids the ree and trace element patterns show a relatively fractionated trend with lile enrichment and hfse depletion similar to that of t morb between n morb and e morb indicating that the parental melt resulted from the partial melting of oceanic lithospheric mantle overprinted by fluid alteration of island arc origin the ultramafic rocks are relics derived from the magma after a large degree of partial melting of oceanic lithospheric mantle with superposed island arc processes under the influence of mid ocean ridge magmatism la icp ms u pb zircon ages of gabbros from three spots are 274 3 ma mswd 0 35 306 3 ma mswd 0 49 262 5 ma mswd 1 2 respectively representing the formation ages of the mafic ultramafic rocks therefore considering other previously published data we suggest that the mafic ultramafic rocks were products of south dipping subduction most probably with a slab window caused by ridge subduction of the paleo asian ocean plate beneath the alxa block in the late carboniferous to late permian before the ocean completely closed this sheds light on the controversial tectonic history of the southern altaids and supports the concept that the termination of orogenesis was in the end permian to triassic

ridge subduction kinematics and implications for the nature of mantle upwelling ridge subduction follows the approach of an oceanic spreading centre towards a trench and subduction of the leading oceanic plate beneath the overriding plate there are four possible kinematic scenarios 1 welding of the trailing and overriding plates e g aluk antarctic ridge beneath antarctica 2 slower subduction of the trailing plate e g nazca antarctic ridge beneath chile and pacific izanagi ridge beneath japan 3 transform motion between the trailing and overriding plates e g san andreas transform or 4 divergence between the overriding and trailing plates e g pacific north america in case 4 the divergence may be accommodated in two ways the overriding plate may be stretched e g basin and range province extension which has brought the continental margin into collinearity and therefore transform motion with the pacific north america relative motion or divergence may occur at the continental margin and be manifest as a change in rate and direction of sea floor

power laws and snow avalanches 1 this paper presents evidence of frequency size power laws in several groups of snow avalanche paths other natural hazards such as earthquakes and forest fires exhibit similar power law relationships in addition an analysis of the response of one group of snow avalanche paths to storms through time demonstrates a power law between the response of the system and the binned frequency of those responses our results as well as our experience with these complex non linear systems are consistent with self organized criticality the practical implication of this work is that the frequency size relationship for small and medium sized avalanches may be useful for quantifying the risk of large snow avalanches within a group of avalanche paths

igneous history of the andean cordillera and patagonian plateau around latitude 46 degrees s from the middle jurassic onwards persistent igneous activity in the southern andes around 46 s was controlled by easterly dipping subduction along the pacific margin cogenetic plutonic rocks belonging to the patagonian batholith and calc alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences for example in the cretaceous lower tertiary divisadero formation the coyhaique region marks the northerly extension of a narrow back arc basin in which the marine neocomian successions accumulated volcaniclastics from the island arc which presumably lay to the west are intercalated with the sediments although the marine basin was short lived a mildly extensional back arc regime may have existed through much of mesozoic recent times widespread basalt rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment remnants of the patagonian basalt plateau at latitude 45 47 s extend from the argentine chile frontier to lago colhue huapi four principal age and compositional groups have been distinguished in the lavas i the oldest which are about 80 ma occur in sections at senguerr and morro negro they are almost exclusively tholeiitic but show some calc alkaline affinities and resemble in other respects basalts from marginal basins ii the second group 57 43 ma occur in the lower part of the chile chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite iii the upper part of the main plateau sequence where the flows are in the range 25 9 ma are dominantly of alkali basalt composition iv post plateau flows from small cinder cones on the surface of the plateau range in age from ca 4 ma to 0 2 ma or less they are mostly highly undersaturated basanites with occasional leucite basanites enriched in incompatible elements a few of the earlier tholeiites with calc alkali traits may have been closely associated with subduction or marginal basin processes the younger lavas are more alkalic intraplate types generated in the remote back arc extensional zone

fluid inclusion study in different types of tin deposits associated with the hercynian granites of western spain most of the spanish sn deposits are closely associated with intrusions of syn and late kinematic hercynian granites a fluid inclusion study using microthermometric raman and crushing test analyses has been made of quartz cassiterite scheelite and calcite from greisens and quartz veins in the following types of deposits a disseminated cassiterite in apogranites golpejas and penouta b stanniferous pegmatites la fregeneda and c quartz veins and stockworks with cassiterite teba cassiterite and wolframite san finx and cassiterite and scheelite la parrilla the homogenization temperatures and physico chemical characteristics of the inclusions indicate three stages of fluid circulation the first stage is characterized by the trapping of complex co2 aqueous h2o nacl co2 ch4 n2 h2s complex co2 co2 ch4 n2 h2s and low salinity aqueous h2o nacl inclusions 10 eq wt nacl at temperatures ranging between 500 and 250 c and pressures below 2000 bar the second stage is represented by low salinity aqueous inclusions 10 eq wt nacl trapped at homogenization temperatures between 300 and 60 c and pressures of 200 bar the third stage is characterized by mixed salt aqueous inclusions h2o nacl kcl cacl2 mgcl2 trapped at homogenization temperatures between 170 and 60 c and low pressures the results obtained by this study indicate that in general with descending temperature the chemical evolution of the mineralizing fluids is such that the fluid inclusions are characterized by a progressive increase in the total density and a progressive decrease in the salinity and the co2 and volatile contents however for the fluids trapped at lower temperatures 100 c the salinity is much higher which is presumably due to the sudden increase in ca2 mg2 na and k salt contents

a model for the tectonic evolution of south cornwall a stratigraphy for south cornwall is combined with an analysis of the chronology of thrust nappe emplacement the resulting model suggests that northward overthrusting of nappes of flysch and olistostrome ophiolite and continental basement is related to the closure of an oceanic basin in south cornwall involving southward subduction throughout the devonian the subsequent continental collision between the normannian high and part of laurentia occurred in the upper devonian with deformatian continuing until late carboniferous times

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

mesozoic cenozoic denudation history of the patagonian andes southern chile and its correlation to different subduction processes fission track ft analysis is applied to assess the mesozoic and cenozoic thermal and denudational history of the patagonian andes between 44 and 51 s and the geologic and geomorphic response of late cenozoic subduction of the active chile rise mid oceanic spreading center on the overriding plate seventy two ft ages from 43 samples are presented zircon ft ages indicate fast post intrusion cooling of cretaceous parts of the patagonian batholith and previously unreported miocene magmatic rocks south of 48 s metamorphic basement rocks to the east of the batholith are constrained as having been deposited and metamorphosed in the early carboniferous and late permian apatite ft data reveal initiation of accelerated cooling and denudation at ca 30 ma at the western margin of southern continental south america followed by an up to 200 km eastward migration of the locus of maximum denudation that ceased at ca 12 8 ma at the position of the present day main topographic divide this migration is proposed to be related to either coeval eastward migration of the retroarc deformation the effects of subduction erosion in the overriding plate at the peru chile trench or less likely shallowing of the angle of subduction east of the divide 3 km of denudation has occurred since the late cretaceous enhanced denudation is interpreted to be the result of increased tectonic uplift driven by a large increase in convergence rates at ca 28 26 ma that triggered orographically enhanced precipitation on the west side of the patagonian andes allowing increased erosion by fluvial incision and mass transport processes the actual process of spreading center subduction had remarkably little influence on denudation in the upper plate and indeed coincides with a slowdown in denudation

pliocene extensional tectonics in the eastern central patagonian cordillera geochronological constraints and new field evidence recent field work and review of radiometric data obtained from neogene lavas and plutonic rocks exposed in the eastern central patagonian cordillera 46 48os which overlie subducted segments of the south chile ridge suggest important late miocene to pleistocene morphological changes in relation to base level variations and or tectonic events we present new field observations from a region south of lago general carrerabuenos aires between the main cordillera and the meseta del lago buenos aires demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5 4 ma and after 3 ma we also show that the 12 ma basaltic flows of the meseta del lago buenos aires 2000 m a s l have been subjected to deep incision with younger lavas dated at 1 2 ma partially filling the valleys these incisions are thought to reflect progressive eastward tilting of the entire meseta our new observations together with additional features from central patagonia strongly suggest that tectonic events led to a regional widespread morphological change after 5 ma the coincidence in time and space between the subduction of segments of the south chile ridge at 6 and 3 ma causing opening of a slab window and strong base level variations in the studied area suggests a cause and effect relationship in central patagonia compressional tectonics ended well before extensional events reported here causes of uplift and further extension are probably completely disconnected the uplift is purely tectonic in origin and occurred prior to the subduction of the south chile ridge extension should be a consequence of this subduction

very shallow melting of oceanic crust during spreading ridge subduction origin of near trench quaternary volcanism at the chile triple junction 2nd 2 9 to 3 8 and d 18 o values 6 4 to 6 9 respectively consistent with mixed magma sources that include a morb type component and sediments we propose that the high si dacites are derived from the hydrous melting of a mixture of morb and sediments at high temperatures 800 900 c under low pressures 0 8 gpa the low si dacites originate from the melting of a similar source under higher pressures consistent with depths of 25 45 km two scenarios accounting for the near trench position of these latter rocks are envisioned the first invokes rapid tectonic erosion and changes in sedimentary wedge geometry the second one postulates that parts of the slab are subducted rapidly to depths of 20 30 km right under the trench index terms 3640 mineralogy and petrology igneous petrology 8120 tectonophysics dynamics of lithosphere and mantle general 9360 information related to geographic region south america keywords slab melting chile trench ridge subduction forearc magmatism geochemistry calc alkaline magmatism

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

tectonics in the eastern central patagonian cordillera 45 30 47 30 s a subhorizontal angular erosional unconformity that separates subhorizontal upper palaeocene lower eocene fluvial deposits of the ligorio marquez formation from folded and faulted lower cretaceous tuffs and sedimentary rocks in the area south of chile chico in the northern patagonian fold and thrust belt of southern chile 46 45 s indicates tectonism prior to the previously recognized post early miocene contractional event similar observations between latitudes 45 30 and 47 30 s in argentina support the existence of a pre palaeocene eocene contractional tectonic phase and locally of a late early cretaceous event this indicates that the northern patagonian fold and thrust belt includes structures earlier than previously thought probably masked by the later neogene ones the timing of this earlier tectonism is not precisely known beyond being pre upper palaeocene lower eocene and post barremian one possibility is that it may be the earlier stages of a progressive deformation that in its later stages is represented by the post lower miocene tectonism previously documented in the region in this case it may represent the latest cretaceous palaeocene major contractional tectonism and uplift well documented along the southernmost patagonian cordillera south of latitude 51 s the upper palaeocene lower eocene ligorio marquez formation may represent the first accumulation in a foreland wedge top basin developed east of the orogenic front subsequently and probably during the middle miocene these beds were folded and faulted during the tectonism that generated the foreland basin where the fluvial deposits of the galera formation accumulated different speculative scenarios for the origin of this compression can be invoked depending on its age

mineralogical and physiochemical evolution of the los santos scheelite skarn salamanca nw spain the los santos scheelite skarn deposit located in the province of salamanca northwest spain occurs in meter thick strata bound but irregular lenses of 1 km in length and contains a current total resource of 3 09 million metric tons mt with an average grade of 0 54 percent wo3 it was produced by the metasomatic alteration of upper vendian lower cambrian metasedimentary rocks and granitic rocks located in the western part of the avila batholith plutonic complex magmatic activity in the los santos zone comprises two intrusions including biotite monzogranite 281 280 6 ma granite i and porphyritic biotite granodiorite monzogranite 270 269 6 ma granite ii the mineralized skarn distributed close to the intrusions occurred in two main stages i prograde skarn and ii retrograde alteration of the stage i skarn the prograde exoskarn is made up of a thick almost monomineral mass of hedenbergite hd82 87 accompanied by grossular grs53 73 and scheelite rich in mo sch i early mineralization is associated with granite i thus it is possible to recognize an endoskarn of calcic plagioclase and clinopyroxene developed over aplitic granite the stage i skarn formed from h2o rich fluids xco2 0 1 with an upper temperature limit of 630 c at 0 18 to 0 2 gpa a log f o2 of 21 and log f s2 ranging from 9 7 to 5 0 after the emplacement of granite ii there was a late event characterized by pegmatites dike swarms breccia pipes and intense fracturing associated with the retrograde alteration which was superimposed on massive skarn the retrograde skarn has a complex paragenesis that can be divided into two substages in the first subcalcic garnet formed associated with the leaching of early scheelite and the precipitation of late scheelite poor in mo sch ii there is also ferroactinolite together with important amounts of anorthite zoisite apatite titanite quartz and sulfides mainly pyrrhotite the first substage skarn formed from a complex h2o co2 ch4 n2 c3h8 nacl cacl2 mixture with f o2 values of 23 and log f s2 ranging from 10 to 7 the data on fluid inclusions indicate that fluid boiling and immiscibility at temperatures between 439 and 405 c and pressures ranging from 0 1 to 0 042 gpa was a possible mechanism in the sch ii and sulfide deposition during the first substage of the retrograde alteration of the skarn the second substage is characterized by the formation of clinozoisite prehnite quartz calcite chlorite white mica zeolite and mineral phases belonging to the bi te ag s au system the mineral assemblages indicate that the f o2 values are between 25 9 and 33 4 while log f s2 ranges from 10 6 to 6 3 for t 300 c all the data suggest that the development of the los santos skarn represents the evolution of a magmatic hydrothermal system in this model carbon aqueous fluids would have been exsolved from the plutonic complex during its emplacement and crystallization the gradual interaction of magmatic fluids at high temperatures with the graphitic rocks of the schist greywacke complex resulted in a reduction in ore fluids and higher ch4 co2 ratios the fluid associated with the last substage of the retrograde alteration of the deposit belongs to the h2o nacl system and suggests an influx of a surface derived fluid

structural control magmatic hydrothermal evolution and formation of hornfels hosted intrusion related gold deposits insight from the thaghassa deposit in eastern anti atlas morocco in the moroccan eastern anti atlas the thaghassa intrusion related gold deposit is hosted in hornfelsed metasedimentary rocks that lie adjacent to the ikniwn granodiorite fields studies reveal three tectono magmatic stages controlling the formation of the deposit i the first stage refers to the top to the south asymmetry and the syn kinematic ikniwn pluton emplacement controlled by a compressional or transpressional strain regime ii the second stage is characterized from older to younger and further away from the intrusion by metatexite with leucocratic stromatic bands aplo pegmatite sills intermediate veinlets composed of quartz k feldspar and muscovite and then gold bearing striped foliation veins all these features are assumed to have been emplaced during a large scale ene wsw dextral shearing process that results from an ese wnw shortening direction during transtensive tectonics we suggest that the progressive and continuous shearing was initiated since the aplo pegmatite stage and achieved during the hydrothermal phase the existence of intermediate veins characterized by quartz rich core and apatite muscovite feldspar rich rims demonstrates the progressive evolution from the magmatic to the hydrothermal stage and evidence for the persistence of the magmatic character at least until the onset of the hydrothermal process iii the late stage developed large volcanic dyke swarm and brittle faulting zircon u pb la icp ms dating yields a concordia age of 563 5 6 3 ma for the ikniwn granodiorite intrusion the fluid inclusions data besides the mineral thermometry indicate that two main types of fluids can been highlighted i a hot aquo carbonic h2o nacl co2 fluid with n2 and ch4 evolving from vapour rich n2 and ch4 inclusions for the magmatic stage 550 c to ch4 co2 biphased inclusions for the ongoing hydrothermal stage 450 300 c and ii an always secondary low t 200 c saline aqueous type probably nacl free from volatiles with very variable salinity eventually we highlight that in the hornfels hosted irg deposits fluid sources may originate from both magmatic processes and devolatization of the metamorphic host rocks

reconstructing the ca 100 year flood in northern thailand the key utilities of palaeohydrological studies are the potential to extend the flood record of a catchment and improve estimates of flood frequency and magnitude in this study we use a hydraulic model to produce estimates of discharge based on psi palaeo stage indicator evidence from large floods for a study reach of the mae chaem river thailand careful calibration of this model especially manning s n was possible because of the availability of psis from several modern gauged flows organic psi evidence contained in a cave permitted dating of the largest flood for which evidence was available which is close to the 100 year flood the magnitude of this flood is twice that predicted from an extrapolation of the 49 year gauging record employing the gumbel ev1 flood frequency model officially endorsed by water management agencies in thailand this magnitude of flood is however very similar to that predicted from a power law model and suggests that extreme flood magnitude and frequency in this catchment conform to power law behaviour these results significantly increase the perception of flood risk for the area and this palaeohydrological case study represents the first for the region

evolution of the cornubian ore field southwest england part i batholith modeling and ore distribution the cornubian ore field of southwest england is associated with an s type ilmenite series high heat production hhp two mica tourmaline bearing monzogranite batholith which formed between 270 to 295 ma base metal deposits are mainly in the form of steeply dipping lodes emplaced in the batholith roof and a hornfelsic sequence of upper paleozoic mud rocks sandstones and mafic volcanics the steep sided batholith has a gently sloping base 10 to 15 km below the present surface three dimensional modeling of gravity data indicates that it has a total volume of about 68 000 km 3 and is divisible into distinct western and eastern parts computer aided evaluation of mineral production statistics shows that most of the sn w cu and as production is located within the axial trace of the batholith with the western end enriched in sn cu and zn relative to the eastern end by contrast most of the fe as sb f ba and china clay production is located in the eastern end while pb and ag production is concentrated in the country rock filled south facing embayments on the flank of the batholith all significant wolframite production is from within the granite or within a 700 m distance of an observed or projected granite contact whereas 90 to 100 percent of the sn cu and as production occurs within 1 500 m of a contact by contrast 30 percent of the zn production and over 80 percent of the pb and ag production is from distances of more than 1 500 m from a granite contact pre syn and postbatholith faults and extensional fractures exert a fundamental control on ore distribution with most of the sn w cu as zn ores hosted by fractures trending parallel to the axial trace of the batholith and most of the pb sb ag ba and fluorite mineralization hosted by faults trending approximately normal to the axial trace of the batholith the nature of the sedimentary sequence also influenced metal distribution with most of the sn cu as production coming from areas underlain by devonian argillaceous rocks an important feature of the granite which has a bearing on its metallogenic potential is its high internal heat production due to its high total content of u th and k modeling of the cooling history of the granites reveals that temperatures in the deeper parts of the batholith were maintained close to or above the solidus for 10 to 20 ma after initial emplacement at 280 to 295 ma residual volatile and metal enriched magmas trapped within the batholith provided source materials for younger magmatism 270 280 ma and the main episode of lode formation which was related to the late magmatic events continued high heat flow throughout the mesozoic and cainozoic promoted thermally driven ground water convection and epithermal mineralization

late cenozoic tectonics of the southern chile trench marine geophysical data from off shore of southern chile are used to define the interaction between the chile ridge and chile trench during late cenozoic time we identify three distinct ridge trench collision events between 14 and 10 ma a 700 km long nearly continuous section of the chile ridge was sub ducted between 55 s and 48 s shorter sections of the ridge offset by large transform faults were subducted at 6 and 3 ma between 48 s and 47 s at the present day triple junction the subduction of the ridge has a strong influence on the chile trench in this region the landward trench slope has undergone a recent episode of subduction driven tectonic erosion the trench slope is narrower and steeper than along other sectors of the margin and the trench axis has migrated shoreward evidence exists for late neogene and quaternary uplift and plutonism on the adjacent continental margin south of 48 s where collision took place between 10 and 14 ma the effects of collision are much less pronounced in particular the landward trench slope does not appear to have been subjected to extensive tectonic erosion we conclude that the configuration of the spreading centers and transform faults on the chile rise is the chief factor controlling ridge trench tectonic interaction tectonic erosion of the landward trench slope and tectonic activity in the adjacent continental margin are much greater when short sections of ridge offset by large transform faults are subducted a major sub marine channel leads along the trench axis south ward from the triple junction this channel cuts across the outer trench rise and carries sediment westward to the mornington abyssal plain the paleogene tectonic history of the southern chile trench includes a southward migrating ridge trench collision involving the farallon aluk spreading center in eocene time

jurassic to miocene k ar dates from eastern central patagonian cordillera plutons chile 45 48 s thirty nine k ar and one ar ar radiometric dates from the eastern central meso cenozoic patagonian batholith and eastern satellite plutons of the aysen region of southern chile between latitudes 45 and 48 s combined with previous dating of seven plutons have yielded six age groups 1 middle to late jurassic 2 early cretaceous 3 mid cretaceous 4 late cretaceous 5 oligocene and 6 miocene in general the cretaceous and younger ages correspond to previous reported ages for other parts of the main batholith but for the satellite plutons to the east show a wider age spectrum than the previously accepted late miocene dates these results indicate a relatively continuous late jurassic to mid cretaceous plutonism known to have been coeval with volcanic activity followed by intermittent magmatism biotite k ar dates of c 143 151 and 106 109 ma from cataclastic granitoids may be marking the time of deformation a review of all radiometric data on magmatic rocks from the region between 45 and 48 s in chile shows a gap in palaeocene ages that may correlate with a period of low angle flat slab subduction between 65 50 ma

andean foothills structures in northern magallanes basin argentina the foothills of the northern segment of the magallanes basin between 47 and 49 s latitudes record the development of a complex fold and thrust belt in the patagonian cordillera during the late miocene upper paleozoic sedimentary and metasedimentary rocks and thick sequences of lower cretaceous deposits with well defined source and reservoir rocks are deformed by the andean events the structure is characterized by a triangle zone and a series of related underthrusts which define a nonemergent backthrust system at the mountain front differences in the degree of deformation from south to north reveal the evolution of the triangle zone the stratigraphy of the molasse deposits constrains the beginning of the deformation at least to the early tertiary although the final structure was developed during the late miocene these deformed deposits are covered by extensive late miocene horizontal retro arc basalts analysis of the evolving subduction zone located west of and beneath the study area shows that the formation of a subsequent volcanic gap in the magmatic arc and the development of extensive retro arc basalts are related to the collision of a segment of the chile ridge during the late miocene the final deformation and uplift of the patagonian cordillera fold and thrust belt at these latitudes are closely linked to this collision

late quaternary tephrostratigraphy of southern chile and argentina abstract the southern and austral volcanic zones of the andes comprise 74 volcanic centres with known post glacial activity at least 21 of these have had one or more large explosive eruptions in the late quaternary dispersing tephra over vast areas these tephra layers therefore have great potential as tephrochronological marker horizons in palaeoenvironmental studies in southern chile and argentina a region that is particularly useful to study climate dynamics of the southern hemisphere however to date tephrochronology has rarely been fully utilised in this region as a correlation and dating tool here we review the existing post glacial tephrostratigraphic record of the southern and austral volcanic zones and compile a database of known occurrences of tephra from these volcanoes in ice and lacustrine marine peat and cave sediment records we address the inconsistencies in and revisions of the tephrostratigraphies presented in prior literature and discuss the challenges in correlating tephras and the limitations of the tephrostratigraphic record in this area this study highlights the many gaps that still exist in our knowledge of the eruptive histories of these volcanoes but also reveals the largely under utilised potential of tephra as a correlation tool in this region this is exemplified by the severe lack of adequate geochemical analysis of tephra layers preserved in many lacustrine and peat sediment sections which are particularly important tephrostratigraphic records in southern chile and argentina due to the paucity of surface preservation

neogene kinematic history of nazca antarctic phoenix slab windows beneath patagonia and the antarctic peninsula article i nfo the patagonian slab window is a subsurface tectonic feature resulting from subduction of the nazca antarctic spreading ridge system chile rise beneath southern south america the geometry of the slab window had not been rigorously defined in part because of the complex nature of the history of ridge subduction in the southeast pacific region which includes four interrelated spreading ridge systems since 20 ma first the nazca phoenix ridge beneath south america then simultaneous subduction of the nazca antarctic and the northern phoenix antarctic spreading ridge systems beneath south america and the southern phoenix antarctic spreading ridge system beneath antarctica spreading ridge paleo geographies and rotation poles for all relevant plate pairs nazca phoenix antarctic south america are available from 20 ma onward and form the mathematical basis of our kinematic reconstruction of the geometry of the patagonia and antarctic slab windows through neogene time at approximately 18 ma the nazca phoenix antarctic oceanic ridge ridge ridge triple junction enters the south american trench we recognize this condition as an unstable quadruple junction heat flow at this junction and for some distance beneath the forearc would be considerably higher than is generally recognized in cases of ridge subduction from 16 ma onward the geometry of the patagonia slab window developed from the subduction of the trailing arms of the former oceanic triple junction the majority of the slab window s areal extent and geometry is controlled by the highly oblique near parallel subduction angle of the nazca antarctic ridge system and by the high contrast in relative convergence rates between these two plates relative to south america the very slow convergence rate of the antarctic slab is manifested by the shallow levels achieved by the slab edge b45 km thus no point on the antarctic slab is sufficiently deep to generate normal mantle derived arc type magmas upwelling beneath the region may have contributed to uplift and eastward transfer of extension in the scotia sea

geochemical and geochronological study of early carboniferous volcanic rocks from the west junggar petrogenesis and tectonic implications abstract a series of early carboniferous volcanic rocks i e basalt andesite and dacite occur in the west junggar zircon u pb isotopic data indicate that these rocks coevally erupted at 331 344 ma these volcanic rocks have comparable trace element patterns characterized by the enrichment of large ion lithophile elements lile and the depletion of high field strength elements hfse isotopically they display consistently depleted sr nd isotopic compositions initial 87sr 86sr ratios 0 7034 0 7054 end t 4 2 7 7 these rocks are interpreted to be derived from a slightly metasomatized depleted mantle in a normal subduction environment in the context of a previously proposed ridge subduction model this study constrains that ridge subduction if it indeed occurred commenced between 331 and 310 ma in the central west junggar

subduction of a spreading ridge within the bangong co nujiang tethys ocean evidence from early cretaceous mafic dykes in the duolong porphyry cu au deposit western tibet abstract the intrusion of mafic dykes into a near trench accretionary prism and continental margin magmatism with characteristics that differ from those of adjacent arc magmatism are direct manifestations of the subduction of a spreading ocean ridge and the formation of a slab window in this paper we investigated mafic dykes intruded into the accretionary prism that hosts the duolong porphyry cu au deposit dcad of western tibet la icp ms analysis of u pb in zircon indicates that the dykes formed during the early cretaceous 126 127 ma the dykes are characterized by ehf t values from 2 44 to 11 8 twenty nine mafic dyke samples were divided into three groups based on their locations and geochemical compositions group i has nb 8 31 10 2 ppm nb la 0 71 1 20 and nb u 21 4 37 9 group ii has nb 40 5 52 6 ppm nb la 0 84 1 58 and nb u 18 8 47 8 and group iii has nb 65 7 105 ppm nb la 1 35 2 08 and nb u 36 5 73 8 group i is classified as nb enriched basalts enbs whereas groups ii and iii are classified as high nb basalts hnbs both the enbs and hnbs were derived from an adakite metasomatized mantle wedge that subsequently underwent crystallization of olivine and clinopyroxene the compositional variations of the studied dykes resulted mainly from mantle source heterogeneity the volume of the slab melts gradually increases from group i enbs to group ii hnbs and group iii hnbs leading to gradually increasing incompatible element concentrations considering their geochemical characteristics and field relationships as well as the unique characteristics of continental margin magmatism in the dcad we propose that the dykes emplaced in an extensional accretionary prism were derived from the northward subduction of a spreading ridge in the bangong co nujiang tethys ocean during the early cretaceous the ridge subduction event was also responsible for the generation of coeval adakites intermediate felsic intrusions the maierze bimodal volcanic rocks as well as the metallogenesis of the dcad

a neoproterozoic seamount in the paleoasian ocean evidence from zircon u pb geochronology and geochemistry of the mayile ophiolitic melange in west junggar nw china abstract the mayile ophiolitic melange mom is located in the southwestern part of the west junggar nw china and forms part of the southern altaids the mom comprises ultramafic rocks gabbro pillow and massive lavas abyssal radiolarian cherts and volcaniclastic rocks zircons with magmatic crystallization features including oscillatory zoning and high th u values from the isotropic gabbro within the mom yield la icp ms u pb age of 572 9 ma mswd 1 0 marking the timing of crystallization of these rocks as late neoproterozoic geochemically the basalts of the corresponding gabbros from mom display oib type alkali basalt and e morb type tholeiitic basalt features both of these groups are characterized by lile and lree enrichment and hree depletion very weak or no eu anomalies eu eu 0 9 1 and no obvious nb ta and ti negative anomalies suggesting a typical oib affinity we propose that these volcanic rocks were derived from a mantle plume related magmatism associated with the evolution of the paleoasian oceanic system with the mantle source containing 2 5 garnet 2 spinel and 2 amphibole the basalts show within plate affinity marked geochemical similarities with those from hawaii and xigaze seamount suggestive of their intra oceanic setting subduction of the oceanic lithosphere commenced during late cambrian to early ordovician with the eventual accretion of the seamounts in the fore arc together with oceanic fragments forming the mayile ophiolitic melange

geochronological and geochemical study of the darbut ophiolitic complex in the west junggar nw china implications for petrogenesis and tectonic evolution abstract the darbut ophiolitic complex consists of ultramafic rocks cumulates pillow lavas and abyssal radiolarian cherts and occurs in the sartuohai suluqiaoke area which is situated in the eastern part of the west junggar nw china southern altaids in the present study we report new geochronological and geochemical data obtained for gabbro and basalt blocks embedded in the darbut ophiolitic complex zircon u pb analyses from one representative gabbro sample by la icp ms yielded a weighted mean age of 391 6 ma showing that it was emplaced in the middle devonian the samples bear n morb e morb and arc like geochemical signatures they are tholeiitic in composition with low concentrations of na 2 o k 2 o varying from 1 36 to 4 09 wt mg and ti depleted large ion lithophile elements liles enriched with insignificant negative eu they also show negative nb ta anomalies and high positive e nd t values with relative high la nb ba nb ba la ba th and zr nb ratios we interpret that these volcanic rocks were derived from a slightly metasomatized depleted mantle source in a back arc basin setting which corresponds to 10 30 partial melting of a depleted spinel lherzolite mantle the data presented here in combination with previously published data suggest a northwestward subduction model for the west junggar during the period of the early devonian to early carboniferous the west junggar was dominated by normal northwestward subduction and the ridge subduction commenced in the late carboniferous

a morphotectonic analysis of central patagonian cordillera negative inversion of the andean belt over a buried spreading center 1 we use digital elevation models and satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the central cordillera of patagonia between 46 s and 48 s in a region overlying subducted segments of the chile spreading ridge this region shows a number of peculiar features not expected in a normal subduction related belt located away from a buried spreading center first it is characterized by overall short wavelength topography and highly contrasted reliefs typical of deeply incised young mountain belts but surprisingly also by an average very low elevation of 900 m second the cordillera lacks central high reliefs instead there is a central depressed domain with average low slope gradients including the deepest point of the study area 300 m below sea level in the bottom of lake general carrera buenos aires this depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips to the east the central patagonian cordillera is now separated from the flat plains of the pampean domain representing its former piedmont and several elevated basaltic plateaus mesetas by 160 n 180 n trending morphotectonic corridors and the western border scarps of the mesetas these scarps correspond to neotectonic features controlled by recent vertical faults mostly active after 3 ma remnants of a former peneplain the avellanos surface are widespread within the entire cordilleran region from geological evidence this peneplain formed between 15 and 4 ma and was connected originally with the mesetas domain and now is dissected by these corridors it is shown that a negative inversion related to extensional and strike slip faulting occurred in the frontal region of the cordillera at 3 ma synchronously with the emplacement of magmatic products timing of the extensional deformation in central patagonia is concomitant with the migration at depth of different segments of the south chile ridge which entered the trench at 6 and 3 ma respectively and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions finally migration of segments of the chile spreading ridge since 6 ma triggered regional uplift of the cordillera followed by limited extension in relation with increased thermal weakening of the crust

geochemistry of the cornubian tin province the exposed crust of the cornubian massif in s w england has been systematically sampled and the rocks analysed for a wide range of major and trace elements geochemically this region is highly anomalous with three elements boron lithium tin occurring at levels several times higher than the average for other parts of the continental crust the greatest degree of enrichment is found in the rocks of the cornubian granite batholith but it is also present in rocks pre dating the formation of the batholith the mineral deposits of cornubia derive their trace elements from different sources b and sn from the granite and cu and s from the country rocks

seismic noise tomography in the chile ridge subduction region summary we used cross correlation of ambient seismic noise recorded in the chile triple junction ctj region to estimate interstation surface wave time domain green s functions and then inverted traveltimes to obtain crustal surface wave velocity models interstation distances within the chile ridge subduction project crsp temporary seismic network ranged from 40 to 100 km we selected 365 d and cross correlated and stacked 24 hr of vertical component data at 38 stations pairs resulting in nominally 703 traveltimes along assumed straight interstation paths velocities in 2 d cells of 30 km 30 km were calculated using a linear least squares inversion of the rayleigh wave group velocity traveltimes furthermore we performed a rayleigh wave group velocity dispersion analysis to estimate the sensitivity of different period waves at depth and to calculate a 3 d shear velocity model of the patagonian crust the process was applied to cross correlation pairs determined in two period bands 5 10 s corresponding to shallow crustal velocities down to approximately 10 km depth and 10 20 s for velocities down to around 20 km our results show that cell velocities correlate well with known geological features we find high crustal velocities where the patagonian batholith outcrops or is likely present at depth and low velocities correlate with the active volcanic arc of the southern volcanic zone and the subducted chile ridge in taitao peninsula where thermal activity of hot springs is present high velocities in the mountainous portions of the southeastern study area appear to correlate with outcropping older metamorphic units low velocity in the east correlate with sequences of volcaniclastic deposits

the mari rosa late hercynian sb au deposit western spain geology and geochemistry of the mineralizing processes the central iberian zone of the hesperian massif hosts a series of late hercynian vein type sb deposits one of them is the mari rosa mineralization hosted by metagreywackes and slates of the schist greywacke complex upper precambrian the mineralization is characterized by a complex paragenesis comprising three hydrothermal stages stage h1 arsenopyrite pyrite stage h2 stibnite gold and stage h3 pyrite pyrrhotite galena sphalerite chalcopyrite tetrahedrite boul angerite stibnite of these only the second episode was of importance and gave rise to the main mineralized bodies of the deposit hydrothermal alteration consists of a mild sericitization chloritization and carbonatization of the metasedimentary rocks around the veins chemical changes in the hydrothermal halos include a remarkable increase in the ratio k2o na2o and a decrease in the ratio sio2 volatiles together with a sharp increase in sb mo au and n fluids associated with ore deposition lie in the h2o nacl co2 ch4 n2 compositional system these fluids evolved progressively cooling from initial circulaion temperatures close to 400 c in the early stage h1 to temperatures of approximately 150 c in the late one h3 fluid composition evolution was characterized by a progressive increase in the bulk water content of the fluids and with an increase in the relative proportion of n2 with respect to ch4 and co2 in the volatile fraction massive stibnite deposition resulted from a boiling process developed at 300 c and 0 9 1 kb at a depth of 4 5 km geological geochemical and fluid inclusion evidence suggest that the intrusion of the alburquerque batholith late hercynian s type granitoids triggered hydrothermal activity leading to the transport and deposition of sb and au in mari rosa

the distribution of ammonium in granites from south west england the ammonium contents of granites pegmatites and hydrothermally altered rocks from sw england have been measured ammonium levels in the granites are generally high compared with those from other regions averaging 36ppm and they differ markedly between intrusions the pegmatites show higher ammonium contents than any other igneous rocks which have yet been investigated ammonium contents are strongly enriched in the hydrothermally altered rocks including greisens and kaolinized granites there is a good correlation between the average ammonium content of the intrusions in sw england and their initial 87 sr 86 sr ratios and peraluminosity this relationship supports the hypothesis that the ammonium in the granites is derived from a sedimentary source either in the magmatic source region or via contamination of the magma

petrogenesis of the eocene and mio pliocene alkaline basaltic magmatism in meseta chile chico southern patagonia chile evidence for the participation of two slab windows the meseta chile chico mcc 46 48s is the westernmost exposure of eocene lower basaltic sequence lbs 55 40 ma k ar ages and mio pliocene upper basaltic sequence ubs 16 4 ma k ar ages flood basalt volcanism in patagonia the mcc is located south of the lago general carrera buenos aires lgcba southeast from the present day chile triple junction ctj east of the actual volcanic gap between southern south volcanic zone and austral volcanic zone ssvz and avz respectively and just above the inferred location of the south chile ridge segment subducted at 6 ma scr 1 erupted products consist of mainly ne normative olivine basalt with minor hy normative tholeiites basalt trachybasalt and basanite mcc lavas are alkaline 42 7 53 1 wt sio2 3 8 wt na2o k2o and relatively primitive ni 133 360 ppm cr 161 193 ppm co 35 72 ppm 4 16 5 mgo wt they have a marked oib like signature as shown by their isotopic compositions 87 sr 86 sro 0 70311 0 70414 and qnd 4 7 5 1 and their incompatible trace elements ratios ba la 10 20 la nb 0 46 1 09 ce pb 15 52 27 5 sr lab25 reflecting deep mantle origin ubs primitive lavas have characteristics similar to those of the eocene lbs basalts while ubs intermediate lavas show geochemical imprints la nbn1 sr lan25 low ce pb nb u compatible with contamination by arc slab derived and or crustal components we propose that the genesis and extrusion of

subduction of diverging plates and the principles of slab window formation consumption of an ocean basin by subduction commonly brings a sea floor spreading ridge toward a deep sea trench if plate divergence and convergence continue after the ridge intersects the subduction zone a slab window forms between the subducted parts of the diverging oceanic plates producing anomalous thermal physical and chemical effects in the surrounding asthenospheric mantle in turn these conditions alter the tectonic and magmatic evolution of the overriding plate usually disturbing ordinary fore arc and arc regimes differential lithospheric stresses on opposite sides of the triple junction contribute to disturbances in the overriding plate anomalous magmatism from fore arc to back arc may be accompanied by fore arc metamorphism strike slip faulting uplift extension and in extreme cases rifting the shape and size of the window are controlled mainly by the pre subduction ridge transform trench configuration slab dip angles and vectors of plate convergence subducted ridge segments expand into windows whose margins approximately parallel the motion vectors between the triple junction and the subducting plates subducted transform faults continue to be active usually as oblique slip faults until the plates separate as transform faults subduct they become longer on the plate which occupies the acute angle between ridge and trench and shorter on the other plate trains of isolated windows produced by subduction of a segmented ridge transform system progressively expand during descent commonly merging together to form a composite slab window oblique subduction of a highly segmented ridge is likely to produce two or more fraternal slab windows one at each site of ridge trench intersection above a slab window arc volcanism diminishes and may be replaced by volcanism of mid ocean ridge or rift affinity the change in chemical character reflects various processes including elevated heat flow decreasing hydration of the upper mantle juxtaposition of supra and sub slab mantle reservoris asthenospheric upwelling and melting of the trailing plate edges if the slab window migrates the anomalous magmatic regime may be replaced by renewed arc volcanism identifying the effects of slab windows in ancient convergent margin assemblages requires an understanding of slab window principles and implications

stratigraphic and structural evolution of the tertiary cosmelli basin and its relationship to the chile triple junction at the taitao peninsula in southern chile 46 5 s an active mid ocean ridge is being subducted under the south american continent continental crust south of the current triple junction has experienced subduction of three ridge segments since the mid miocene a series of mid late tertiary sedimentary basins lies inboard of the triple junction the cosmelli basin lies in the region which has experienced the earlier ridge subduction events within the cosmelli basin fill abrupt facies dislocations give clear evidence of marked base level changes four sequence boundaries are delineated with an upward increase in degree of facies dislocation which is believed to reflect increasing magnitudes of base level fall the lower part of the basin fill is folded and then thrust eastward as a series of imbricate slices while the overlying greater thickness of fluvial sediments is only gently tilted westwards this geometry indicates that the early basin fill was deforming due to contractional tectonics while the later basin fill was being deposited these processes controlled accommodation space and thus stratigraphic architecture during much of the basin life this complex basin history plus the presence of basaltic sheet intrusions may reflect the underlying slab history as successive ridge segments were subducted

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

power law extreme flood frequency conventional flood frequency analysis ffa has been criticized both for its questionable theoretical basis and for its failure in extreme event prediction an important research issue for ffa is the exploration of models that have theoretical explanatory value as the first step towards more accurate predictive attempts self similar approaches offer one such alternative with a plausible theoretical basis in complexity theory that has demonstrable wide applicability across the geophysical sciences this paper explores a simple self similar approach to the prediction of extreme floods fifty river gauging records from the usa exhibiting an outlier event were studied fitting a simple power law pl relation to events with return period of 10 years or greater resulted in more accurate discharge and return period estimates for outlier events relative to the log pearson iii model similar success in predicting record events is reported for 12 long term rainfall records from the uk this empirical success is interpreted as evidence that self similarity may well represent the underlying physical processes generating hydrological variables these findings have important consequences for the prediction of extreme flood events the pl model produces return period estimates that are far more conservative than conventional distributions

new geochemical constraints for the origin of ridge subduction related plutonic and volcanic suites from the chile triple junction taitao peninsula and site 862 leg odp141 on the taitao ridge abstract several features of neogene and quaternary magmatism in the region south of the present day chile triple junction ctj at 46 12 s are directly related to the migration of the triple junction due to the obliquity of the ridge orientation with respect to the subduction front the triple junction migrated from south to north during the last 14 ma the taitao peninsula the westernmost promontory of the chile coast and the taitao ridge a submarine promontory north of the taitao peninsula provide the most complete collection of ridge subduction related magmatic products in the region the emplacement of near trench volcanics the intrusion of a variety of plutonic rocks and the related hydrothermal activity at these two sites have been interpreted as resulting from magma interactions between subducted ridge segments of the chile spreading centre and the continental crust we present new field observations and geochemical data that help to better constrain the problem of the sources and evolution of the taitao magmas the new geochemical data were obtained on samples collected from the taitao peninsula during a field expedition in 1995 and from samples of the taitao ridge during leg odp141 site 862 which have been re sampled in 1996 by one of us selected major and trace element compositions of 20 volcanic rocks from the taitao ridge are discussed together with 53 analyses from different rock types from the taitao peninsula including 24 unpublished analyses nd and sr isotopic compositions were obtained from 5 whole rocks and separated minerals of the taitao peninsula together with the oxygen isotope composition of four separated clinopyroxenes six main magmatic types are identified 1 n type morb 2 e type morb 3 lree depleted morb showing some trace element features typical of arc basalts 4 moderately nb depleted e morb 5 calc alkaline andesites dacites and rhyolites and 6 andesites and dacites with adakitic signature chemical similarities exist between some forearc magmas of the taitao ridge and the taitao peninsula and magmas emplaced at the chile active spreading ridge one important result based on isotope data is that the lavas emplaced over the continental crust taitao peninsula did not originate from melting of continental crust nor from extensive assimilation of such a crust by mantle derived magmas the likely source of these basalts could be the hot convective oceanic mantle of the southern chile spreading ridge buried at moderate depth 10 30 km

scarping of artificially nourished mixed sand and gravel beaches sedimentological characteristics of hayling island beach southern england abstract beach cliffing is a wide spread characteristic of artificially replenished beaches which has many undesirable engineering environmental and economic consequences a sedimentological study undertaken on the replenished mixed gravel and sand beach of hayling island southern england has shown that the persisting beach cliff consists typically of three distinct layers the textural geochemical and mineralogical analyses of the deposit showed that the development of the cliff and particularly the formation of the intermediate densely packed layer could be attributed to a series of processes firstly the techniques used to emplace the recharge material on the beach i e placement by heavy dumper trucks bulldozers result in a compaction and grain fracture of the recharge material and therefore a denser packing arrangement than that expected by its textural characteristics alone secondly percolating water transfers medium and fine grained material to deeper parts of the deposit resulting in the clogging of the interstices between the gravels and the formation of a densely packed poorly sorted layer the above processes promote the interactions between the clay minerals of the deposit with ca2 cations resulting in the formation of particular cementing materials such as calcium silicate hydrates csh

the evolution of pegmatite hosted sn w mineralization at nong sua thailand evidence from fluid inclusions and stable isotopes abstract the nong sua aplite pergmatite complex contains two dominant styles of sn w ta nb mineralization cassiterite nb ta ti oxide minerals are disseminated in the pegmatite and cassiterite and wolframite are hosted by quartz tourmaline veins which are contained solely within aplite the orthomagmatic fluid at nong sua is preserved as primary fluid inclusions in the cores of magmatic garnet crystals that have high tin concentrations garnet cores without fluid inclusions do not contain elevated tin concentrations these fluid inclusions have a composition of 3 wt nacl eq the low salinity suggests that at vapor saturation tin was partitioned in favour of the melt which allowed cassiterite to initially crystallize directly from the melt primary pseudosecondary and secondary fluid inclusions in cassiterite tourmaline and quartz record three component mixing of the orthomagmatic fluid with high salinity aqueous and with co2 rich fluids the orthomagmatic water is interpreted to have had 18 o value of 8 7 to 9 9 per mil and a d value of 72 to 78 per mil from 18o analyses of muscovite and quartz and d of muscovite the 18o composition of muscovite decreased from 10 1 to 8 0 per mil and d increased from 106 to 85 per mil from the magmatic to the hydrothermal stages of pegmatite evolution these changes are consistent with an influx of metamorphic fluids or evolved meteoric waters we consider that the saturation of the melt with vapor caused the pressure in the pegmatite to rise to approximately 3 8 kbar at a temperature of 650 c fluid overpressure caused the aplite to fracture and veins to form from fluids which migrated into the fracture induced low pressure zones this event can be modeled by an isothermal decompression to 2 7 kbar cassiterite deposition was probably controlled by increasing fo2 whereas wolframite deposition resulted from the mixing of w rich with fe mn rich fluids in both cases decompression cooling and addition of volatiles may have also contributed to mineralization

overview tectonic implications of cenozoic volcanism in coastal california cenozoic volcanic rocks of coastal california were erupted west of the magmatic arc trend related to subduction along the continental margin two assemblages representing discrete pulses of mid tertiary oligocene miocene and mid miocene volcanism occupied relatively compact tracts as restored palinspastically prior to disruption and dispersal by tectonic displacements within the neogene san andreas transform system a third assemblage records migratory post mid miocene volcanism at centers located progressively farther north the distribution and petrologic character of all three assemblages reflect slab window magmatism triggered by decompression melting of upwelling mantle paleogeographic reconstructions based on analysis of magnetic anomaly patterns offshore and restoration of on land features prior to san andreas transform slip and associated transrotational deformation indicate the paleotectonic positions of the cenozoic volcanic fields before structural disruption the pulses of mid tertiary and mid miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise trench encounters associated with subduction of the vancouver farallon and monterey arguello plates respectively along the continental margin off southern california migratory post mid miocene volcanism in central california accompanied the incremental expansion of a growing slab window as the mendocino triple junction migrated northward along the continental margin mid miocene columbia river basalt volcanism in the pacific northwest was coeval with the mid miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the san andreas transform as a coherent plate boundary columbia river volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the pacific plate

geochemical constraints on the genesis of the scheelite dome intrusion related gold deposit tombstone gold belt yukon canada the scheelite dome intrusion related gold deposit western selwyn basin yukon is hosted in hornfelsed metasedimentary strata that lie adjacent to the exposed apices of a monzogranite to quartz monzonite plutonic complex of the mid cretaceous tombstone tungsten magmatic belt tintina gold province alaska and yukon a variety of mineralization styles occur throughout a 10 3 km east trending corridor and include reduced au and w rich skarns au w and ag pb zn sb rich quartz tension vein arrays and multiphase fault veins and isolated zones of au rich sericite carbonate altered rock integrated u pb shrimp data for magmatic zircon and ar ar data for magmatic and hydrothermal biotite indicate that gold mineralization occurred within 1 to 2 m y of magma emplacement fluid inclusion oxygen isotope and arsenopyrite geothermometry data indicate that hydrothermal minerals formed at depths of 6 to 9 km over a temperature range from 550 c high temperature au rich skarns formed at 400 c whereas vein hosted mineralization formed at 280 to 380 c in skarns au is strongly associated with enrichments of bi te w and as whereas a variety of au rich veins occur with as rich type 1 and te and w rich type 2 end members silver pb zn sb veins are typically au poor and represent the latest and lowest temperature phase in the hydrothermal paragenesis the fluid inclusion data indicate that all mineralization styles were formed from low salinity 4 wt nacl equiv aqueous carbonic fluids consistent with the composition of fluid inclusions within infilled miarolitic cavities in the intrusive rocks however the nonaqueous fluid was predominantly ch 4 in skarn co 2 in au te and au w veins and a fluid with roughly equal amounts of co 2 ch 4 and n 2 in au as and ag pb zn sb veins oxygen isotope data are consistent with a mineralizing fluid of predominantly magmatic origin that was variably modified to more positive 18 o values during interaction with 18 o enriched metasedimentary strata sulfur isotope data suggest two possible sources of sulfur a magmatic source characterized by 34 s values of approximately 5 to 0 per mil and sulfur from the metasedimentary country rocks characterized by more negative 34 s values of approximately 15 to 10 per mil collectively the data indicate that gold at scheelite dome was deposited from a magmatic hydrothermal system interaction of magmatic fluids with graphitic hornfels rocks resulted in reduction of the ore fluids higher ch 4 co 2 ratios and modification of the oxygen and sulfur isotope values of the ore fluids toward those of the metasedimentary hornfels progressive reduction and cooling of hydrothermal fluids in addition to phase separation in vein hosted mineralization were the mechanisms for gold deposition compared to other intrusion related gold deposits associated with the tombstone tungsten magmatic belt magmatism exposed mineralization at scheelite dome is predominantly hosted by hornfelsed metasedimentary rocks this results in more diverse mineralization styles and a greater spread of isotope and fluid inclusion data

short paper palynological evidence from the porthleven area south cornwall implications for devonian stratigraphy and hercynian structural evolution detailed structural and lithological mapping combined with a paly nological study of devonian sediments in the porthleven area have revealed that previous structural and stratigraphic interpretations are seriously in error palynomorphs recovered from sediments classically assigned to the gramscatho group and generally regarded as pre famennian indicate a late famennian age a reinterpretation of the section is proposed which emphasizes the importance of imbricate thrusting and local facies variations interpretations of the devonian geology of south cornwall have been hindered by the lack of palaeontological control on the ages of the main lithostratigraphic units this paper details new palynomorph assemblages which necessitate a reinterpretation of the structure and stratigraphy of the south cornish nappe and thrust terrain rattey sanderson 1984 barnes andrews 1986 holder leveridge 1986 palynology a suite of 14 samples was collected from the parautochthonous mylor slate formation and allochthonous portscatho formation holder leveridge 1986 on the west coast of the lizard peninsula fig 1 palynomorphs were recovered from three samples representative of the sequence exposed immediately southeast of loe bar sw646237 sw647234 within this section traditionally referred to the gramscatho group sediments of both mylor and gramscatho affinity are intercalated the finer grained mylor type lithologies were preferentially sampled and yielded comparatively diverse rich and remarkably well preserved although opaque palynomorph assemblages yields range from 22 146 specimens per gramme of sediment the kerogens were oxidized using potassium chlorate and fuming nitric acid and in transmitted light were found to be composed almost exclusively of miospores with occasional acritarchs

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

avalanche crown depth distributions 1 the literature disagrees about the statistical distribution of snow avalanche crown depths large datasets from mammoth mountain california and the westwide avalanche network show that the three parameter generalized extreme value distribution provides the most robust fit followed by a two parameter variation the frechet distribution the most parsimonious explanation is neither self organized criticality nor other complex cascades but the maximum domain of attraction implying that distributions of individual avalanche crown depths are scaling we also show that crown depths do not have a universal tail index rather they range from 2 8 to 4 6 over different avalanche paths consistent with other geophysical phenomena such as wildfires which show similar variability

supergene oxidation of epithermal gold silver mineralization in the deseado massif patagonia argentina response to subduction of the chile ridge radiometric ages for supergene alunite and jarosite effectively date the oxidation of former concentrations of pyrite and any associated sulfide minerals these k bearing sulfate minerals formed under low ph conditions are uncommon supergene products in low sulfidation epithermal deposits because of the general paucity of pyrite for acid generation for this reason the age of supergene oxidation locally to depths of 200 m or more in the epithermal au ag deposits of the deseado massif located in the extra andean foreland of patagonia southern argentina has remained unknown although theoretically the oxidation could have taken place anytime between the late jurassic when the au ag mineralization formed and pleistocene k ar ages for alunite and jarosite from two widely separated and unusually pyritic ag bearing hydrothermal breccias lejano and libanesa show it to have been mid miocene 13 8 1 8 ma this is the time when the deseado massif underwent appreciable regional scale tectonic uplift and valley incision following 140 myr during which the region was topographically subdued and the site of either fluvio lacustrine or shallow marine sedimentation the uplift combined with increasing aridity due to the orographic rain shadow caused by growth of the patagonian andes to the west and enhanced by global cooling would have depressed regional groundwater tables thereby promoting the supergene sulfide oxidation the mid miocene uplift appears to have been triggered by development of a slab tear and slab window beneath the deseado massif during early stages of subduction of the chile oceanic ridge spreading center at the pacific margin supergene sulfide oxidation in both the deseado massif and atacama desert of northern chile was the result of cenozoic uplift during progressive aridification although the causes of these phenomena were radically different however when the supergene oxidation was taking place in the deseado massif up to 30 myr of supergene activity in the atacama desert were coming to an end because of the onset of hyperaridity

k ar dating young rhyolitic rocks a case study of the sierra la primavera jalisco mexico this study demonstrates the feasibility of dating by the k ar method an eruptive sequence of silicic volcanic rocks with ages close to the limits of conventional 14 c dating forty six conventional k ar analyses of sanidine and glass from rhyolites of the sierra la primavera mexico yielded dates of 5 to 150 ka thousand years one sigma errors range from 3 to 39 ka although the samples contain small amounts of extraneous argon most of the dates are consistent with relative ages determined in the field the exceptions are dates on immediately postcaldera eruptive units which are older than the dates for the caldera producing ash flow tuff a similar result has been obtained on dating tuffs and lavas of some other pleistocene calderas this effect is attributed to incomplete outgassing of the larger amount of extraneous argon dissolved in magma that occupied a position deeper in the chamber than the ash flow tuff magma and that erupted immediately after caldera collapse we present two new error equations one a simple extension of the commonly used equation demonstrates the importance of including a term for the uncertainty in the spectrometer mass discrimination factor in the error equation the second equation is a more rigorous formulation of the propagation of errors in the calculation of the age it yields meaningful error estimates for all proportions of radiogenic 40 ar and because it expresses the error in absolute rather than relative terms it can be used to put an upper limit on the age of a sample when no radiogenic 40 ar has been resolved

criticality in earthquakes good or bad for prediction abstract through the analysis of the correlation functions in simulations of an earthquake model the critical properties of the system are studied simulations are performed in a more realistic modification of the olami feder christensen model of earthquakes and result in uncorrelated avalanches distributed following a power law with weak signs of foreshocks and aftershocks the spatial autocorrelation function of the system and other structural variables are computed in every step of the simulation the spatial autocorrelation between points separated from each other by a constant distance equal to 1 4 and 1 8 of the linear size of the system shows large variations temporally correlated with the time series of avalanche size i e spatial correlation values are in average very high before a large earthquake very small after a large earthquake and they evolve between these two states however the temporal average of the spatial autocorrelation over the whole simulation shows values close to zero result that is in contradiction with the idea that the correlation length is of the same order as the linear size of the system diverging in an infinite system which is the main signature of a critical scenario by averaging the autocorrelation in smaller time windows the critical properties of temporal states can be used as an indication of upcoming catastrophic events the structural variables are also correlated with the occurrence of large avalanches suggesting the possibility of monitoring these variables in order to achieve prediction

a geodynamic model linking cretaceous orogeny arc migration foreland dynamic subsidence and marine ingression in southern south america abstract this study synthesizes the tectonomagmatic evolution of the andes between 35 30 s to 48 s with the aim to spotlight early contractional phases on andean orogenic building and to analyze their potential driving processes we examine early tectonic stages of the different fold thrust belts that compose this andean segment additionally we analyzed the spatio temporal magmatic arc evolution as a proxy of dynamic changes in andean subduction during critical tectonic stages of orogenic construction this revision proposes a hypothesis related the existence of a continuous large scale flat subduction setting in cretaceous times with a similar size to the present largest flat slab setting on earth this potential process would have initiated diachronically in the late early cretaceous and achieved full development in late cretaceous to earliest paleocene times constructing a series of fold thrust belts on the retro arc zone from 35 30 s to 48 s moreover we assess major paleogeographic changes that took place during flat slab full development in maastrichtian danian times at this moment an enigmatic atlantic derived marine flooding covered the patagonian foreland reaching as far as the andean foothills based on flexural and dynamic topography analyses we suggest that focused dynamic subsidence at the edge of the flat slab may explain sudden marine ingression previously linked to continental tilting and orogenic loading during a high sea level global stage finally flat subduction destabilization could have triggered massive outpouring of synextensional intraplate volcanic rocks in southern south america and the arc retraction in late paleogene to early neogene times

neogene uplift of central eastern patagonia dynamic response to active spreading ridge subduction the chile triple junction ctj episodically migrated northward during the past 14 ma from 54 s to its present day position at 46 30 s as different almost trench parallel spreading segments entered in subduction this migration resulted in the opening of an asthenospheric window below patagonia inducing a disturbance in the regional mantle convection on the overriding plate the middle miocene time corresponds to a major change in the central patagonian basin dynamics with a transition from subsidence to generalized uplift the detailed mapping and the morphological study of post middle miocene terraces evidence regional scale trench parallel tilt histories that differ depending on latitude south of 46 30 s the slopes of the fluvial terraces indicate a change from northward to southward tilt during late miocene terraces younger than the early pliocene do not show any trench parallel tilt north of 46 30 s in contrast only northward tilt active until the pleistocene is recorded we use a semianalytical model of dynamic topography which suggests that the northward migration of the chile triple junction should be accompanied by a dynamic uplift of the central patagonian basin uplift would cause both north directed and south directed tilt north and south of the triple junction respectively with values of 0 1 0 2 500 km east of the trench tilt measured in the southeastern part of the central patagonian basin 0 1 is comparable to values predicted using the semianalytical model the dynamic topography associated to the chile ridge subduction may have exerted a major control on the neogene dynamics of patagonia

volatile production during contact metamorphism the role of organic matter in pelites elemental analyses of palaeozoic mudrocks from south cornwall indicate the presence of 0 04 0 74 wt carbon primarily in an elemental and or organic form in addition to 0 04 0 10 wt nitrogen as nh 4 in micas and in organic material both elements are depleted in samples from the lithologically homogeneous mylor slate formation next to the tregonning granite an offshoot of the cornubian batholith the depletions are due to an h 2 o co 2 n 2 ch 4 fluid formed during contact metamorphic devolatilization of the pelitic country rocks fluids of this type have been reported from a number of hercynian sn w provinces and may play an important role in mobilizing metals in pelitic rocks during granite related contact metamorphism and hence in the metallogenic evolution of such regions

nitrogen isotope record of fluid rock interactions in the skiddaw aureole and granite english lake district the skiddaw granite and its contact metamorphic aureole in the english lake district provide an excellent opportunity to test the capability of the n isotope system to trace devolatilization and large scale transfer of crustal fluids in the aureole skiddaw group metasedimentary rocks with relatively uniform lithology and major element compositions show a dramatic decrease in n content toward the granite contact from or 800 ppm at distances 2 5 km from the contact to 1 5 km these rocks have extremely uniform delta 15 n air near 3 7 per mil whereas closer to the contact or 1 km delta 15 n is shifted to higher values up to 8 7 per mil the coupled decreases in n content and increases in delta 15 n are compatible with the removal of n having low delta 15 n in fluids during continuous prograde devolatilization reactions involving the break down of white mica and the stabilization of biotite cordierite and andalusite bearing assemblages in the same metasedimentary rocks the lack of obvious trends in major element concentrations including sio 2 tio 2 sio 2 al 2 o 3 and the ratios of other major oxides to tio 2 and al 2 o 3 with distance from the granitic contact is consistent with minimal change in major element composition during the contact metamorphism ratios of whole rock n b rb and ba concentrations to whole rock k 2 o content are believed to reflect the differing fluid mica partitioning and involving varying relative proportions of white mica and biotite of these trace elements during devolatilization reactions greisenized skiddaw granite from a borehole is enriched in n range of 17 225 ppm for whole rocks and white mica separates relative to the unaltered granite whole rock 30 ppm and has delta 15 n of 1 0 to 4 8 per mil the n concentrations and delta 15 n of the wall rocks and greisenized granites combined with c isotopic data carbonate and carbonaceous matter for the same rocks are consistent with the mobilization of fluids having low delta 15 n and delta 13 c pdb values from the devolatilized aureole into the cooling intrusive body such transport is consistent with the predictions of recent theoretical models of late stage hydrothermal evolution in cooling intrusive systems

thermal model for approach of a spreading ridge to subduction zones and its implications for high p high t metamorphism importance of subduction versus ridge approach ratio 1 the approach of oceanic ridges to subduction zones finally resulting in ridge subduction is observed on the present earth s surface the thermal effects of the ridge approach event are studied using a two dimensional numerical model around subduction zones with particular emphasis on the implications for the associated high p high t metamorphism the ridge approach occurs when the subduction rate v is larger than the half spreading rate at the ridge the difference between the two rates represents the ridge approach rate to the trench u which is newly incorporated in the present thermal model the calculated thermal structure along the slab surface when a ridge reaches the trench generally depends only on u v when shear heating or mantle flow is not taken into account if u v is small or the ridge approach is slow compared to the subduction rate the thermal effect of the ridge approach is large enough to explain the hot general p t conditions of high p high t metamorphism relative to the steady state conditions predicted by numerical simulations in addition when the ridge is close to the trench the subduction stage p t paths experienced by high p high t metamorphic rocks show a strongly curved trajectory with the dp dt progressively increasing with pressure this suggests that the slope of petrologically determined subduction p t paths may indicate if ridge approach contributed to the formation of the metamorphic rocks

the transition from granite to banded aplite pegmatite sheet complexes an example from megiliggar rocks tregonning topaz granite cornwall abstract the genetic relationship between a granite pluton and adjacent complex of rare metal pegmatite aplite banded sheets megiliggar sheet complex msc has been studied at the border of the tregonning topaz granite at megiliggar rocks cornwall sw england similarities in whole rock chemical and mineralogical compositions together with a gradual change in textures away from the granite margin provide strong evidence for a genetic link between the tregonning granite and msc the sheets are likely to represent apophyses of residual melt which escaped from the largely crystallized roof of the granite pluton the escaping melt was peraluminous had a composition near the f b li slightly enriched granite minimum and in comparison with other cornish granites was enriched in f li rb cs sn w nb ta and u and depleted in fe mg ca sr th zr and ree with increasing distance from the tregonning granite the silicate melt crystallized as homogeneous leucogranite sheets and banded complex sheets i e combinations of bands with granitic aplitic and pegmatitic textures then layered aplite pegmatites this sequence becoming progressively more depleted in the fluxing and volatile elements f li rb and cs but showing no change in zr hf ratios the fixed zr hf ratio is interpreted as indicating a direct genetic link parental melt between all rock types however the melt progressively lost fluxing and volatile elements with distance from the granite pluton probably due to wall rock reaction or fluid exsolution and migration via fractures differentiation of the primary melt into na li f rich and separate k b rich domains was the dominant chemical process responsible for the textural and mineral diversity of the msc on a large cliff section scale the proximal na li f rich leucogranite passes through complex sheets into k b rich aplite pegmatites whilst at a smaller

the chon aike province of patagonia and related rocks in west antarctica a silicic large igneous province the field occurrence age classification and geochemistry of the mesozoic volcanic rocks of patagonia and west antarctica are reviewed using published and new information dominated by rhyolitic ignimbrites which form a bimodal association with minor mafic and intermediate lavas these constitute one of the largest silicic igneous provinces known equivalent in size to many mafic lips diachronism is recognized between the early middle jurassic volcanism of eastern patagonia marifil and chon aike formations and the middle jurassic earliest cretaceous volcanism of the andean cordillera el quemado ibanez and tobifera formations this is accompanied by a change in geochemical characteristics from relatively high zr and nb types in the east to subalkaline arc related rocks in the west although the predominance of rhyolites remains a constant factor all of the associated mafic rocks are well fractionated compared to direct mantle derivatives petrogenetic models favour partial melting of immature lower crust as a result of the intrusion of basaltic magmas possibly with some hybridisation of the liquids and subsequent fractionation by crystal settling or solidification and remelting the formation of large amounts of intracrustal silicic melt acted as a density barrier against the further rise of mafic magmas which are thus rare in the province

geochemical diversity of late miocene volcanism in southern baja california mexico implication of mantle and crustal sources during the opening of an asthenospheric window abstract five main petrologic and geochemical groups can be identified among the middle to late miocene lavas from the western part of southern baja california 1 calc alkaline and k rich andesites emplaced between 15 5 and 11 7 ma 2 adakites and 3 associated niobium rich basalts erupted between 11 7 and 8 5 ma in the santa clara volcanic field vizcaino peninsula 4 10 6 9 2 ma tholeiitic basalts and basaltic andesites that form large tabular plateaus near san ignacio and 5 magnesian and basaltic andesites of adakitic affinity whose emplacement started at 11 7 ma south of san ignacio and between 9 7 and 8 8 ma near la purisima these lavas although spatially and temporally related display very different geochemical signatures their trace elements and isotopic characteristics suggest that three different magma sources were involved in their genesis partial melts of subducting altered oceanic crust produced the adakites when erupted directly at the surface these magmas were eventually trap

cocos nazca slab window beneath central america abstract integration of petrologic and tectonic data favours a model of slab window formation beneath central america in the pliocene pleistocene central america has been the site of voluminous cenozoic arc volcanism the cocos and nazca plates which are subducting beneath central america are diverging along the east trending cocos nazca spreading ridge since 25 ma the americas have advanced about 1800 km west over the ridge transform system since at least 8 ma plate integrity and the ridge transform configuration have been preserved during convergence resulting in subduction of the spreading ridge and development of a slab window the panama fracture zone an active transform fault is the part of the ridge transform system currently being subducted the ridge transform system formerly adjoining the northern end of the panama fracture zone is likely to have been left stepping we use present day plate motions to design a slab window to fit known variations in igneous composition hypocentre distribution and mantle anisotropy the modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 ma cessation of ridge subduction occurred between 1 and 3 ma when subduction of the panama fracture zone is considered to have begun the slab window is continuing to expand and migrate northeastward below the central american volcanic arc the absence of a wadati benioff zone from southeastern costa rica through panama corresponds to the position of the slab window within this region dacitic and rhyolitic volcanic rocks have adakitic compositions and are thought to result from anatexis of the young buoyant crust which forms the trailing edges of the slabs bounding the window basalts in this area were derived from an enriched ocean island type mantle source whereas basalts from the rest of the arc in nicaragua el salvador and guatemala are mainly derived from slab modified depleted mantle characteristic of volcanic arcs the presence of ocean island type mantle beneath southern costa rica and panama is explained by eastward flow of enriched asthenosphere from the galapagos plume head through the slab window and into the volcano source region eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the nazca and caribbean plates the flow of peridotite is a consequence of progressive shrinkage of the pacific mantle reservoir and concurrent expansion of the atlantic mantle reservoir

glacial interglacial trench supply variation spreading ridge subduction and feedback controls on the andean margin development at the chile triple junction area 45 48 s during the chile triple junction ctj cruise march april 1997 em12 bathymetry and seismic reflection data were collected in the vicinity of the chile triple junction 45 480s where an active spreading ridge is being subducted beneath the andean continental margin results show a continental margin development shaped by tectonic processes spanning a spectrum from subduction erosion to subduction accretion the andean continental margin and the chile trench exhibit a strong segmentation which reflects the slab segmentation and the chile triple junction migration three segments were identified along the andean continental margin the presubduction the synsubduction and the postsubduction segments from north to south both climate induced variations of the sediment supply to the trench and the tectonic reorganization at the nazca antarctica plate boundary involving postsubduction ridge jump are the two main factors that control the tectonic regime of this continental margin along the survey area we infer the succession of two different periods during the last glacial interglacial cycle a glacial period with ice rafted detrital discharges restricted to the shoreline area and low river output and a warmer period during which the andean ice cap retreat allowed the andes to be drained off during these warm periods rapid increase in trench deposition caused the margin to switch from subductionerosion or nonaccretion to subduction accretion 1 along the presubduction segment after the last deglaciation and 2 along the postsubduction segment after the interglacial episode at 130 117 ka conversely a nonaccretion or subduction erosion mode characterized the presubduction and postsubduction segments during glacial maximums the major effects of subduction of the buoyant chile ridge include a shallow trench which diverts trench sediment supply and tectonic instabilities at the nazca antarctica plate boundary we suggest that a postsubduction westward jump of the chile ridge occurred during the past 780 kyr it produced slab fragmentation and individualization of an ephemeral microplate north of the taitao fracture zone the chonos microplate in 780 kyr two episodes of subduction accretion separated by an episode of subduction erosion occurred in relation with the chonos microplate individualization and subduction the current northward migration of the triple junction along the chonos microplate south america plate boundary introduces a sharp change in the tectonic mode from subduction erosion to the north to subduction accretion to the south the data collected along the taitao ridge have revealed the complex three dimensional structure of an accretionary wedge which includes a midslope thrust sheet exhibiting the characteristics of an ophiolite the taitao ridge ophiolite no connection exists between the taitao ridge ophiolite and the bahia barrientos ophiolite cropping out onland in the taitao peninsula

miocene to late quaternary patagonian basalts 46 47 s geochronometric and geochemical evidence for slab tearing due to active spreading ridge subduction miocene to quaternary large basaltic plateaus occur in the back arc domain of the andean chain in patagonia they are thought to result from the ascent of subslab asthenospheric magmas through slab windows generated from subducted segments of the south chile ridge scr we have investigated three volcanic centres from the lago general carrera buenos aires area 46 47 s located above the inferred position of the slab window corresponding to a segment subducted 6 ma ago 1 the quaternary rio murta transitional basalts display major trace elements and sr and nd isotopic features similar to those of oceanic basalts from the scr and from the chile triple junction near taitao peninsula e g 87sr 86sr o 0 70396 0 70346 and end 5 5 3 0 we consider them as derived from the melting of a chile ridge asthenospheric mantle source containing a weak subduction component 2 the plio quaternary 1 and tio2 2 wt these intermediate basalts differ from their strictly alkalic equivalents by having lower high field strength element hfse and higher end up to 5 4 these features are consistent with their derivation from an enriched mantle source contaminated by ca 10 rutile bearing restite of altered oceanic crust the petrogenesis of the studied mio pliocene basalts from mlba and mcc is consistent with contributions of the subslab asthenosphere the south american subcontinental lithospheric mantle and the subducted pacific oceanic crust to their sources however their chronology of emplacement is not consistent with an ascent through an asthenospheric window opened as a consequence of the subduction of segment scr 1 which entered the trench at 6 ma indeed magmatic activity was already important between 12 and 8 ma in mlba and mcc as well as in southernmost plateaus i e 6 ma before the subduction of the scr 1 segment we propose a geodynamic model in which oib and intermediate magmas derived from deep subslab asthenospheric mantle did uprise through a tear in the slab which formed when the southernmost segments of the scr collided with the chile trench around 15 ma during their ascent they interacted with the patagonian supraslab mantle and locally with slivers of subducted pacific oceanic crust that contributed to the geochemical signature of the intermediate basalts

neogene to quaternary tectonic evolution of the patagonian andes at the latitude of the chile triple junction abstract we conducted a preliminary structural and tectonic analysis of the region north and south of the lake general carrera buenos aires lgcba southern patagonian andes chile and argentina in order to study the effects of the subduction of the active south chile spreading ridge scr on the development of the tectonic front of the patagonian andes at the latitude of the chile triple junction ctj here one segment of the scr entered the trench at 6 ma and the corresponding slab window is now located below the study area we emphasize the segmentation of the tectonic front which can be divided into four successive oblique segments a to d trending mainly n170 and right laterally offset this geometrical pattern is consistent with a transpressive stress regime in relation to oblique subduction the four segments of the tectonic front are separated and locally deformed by n170 oriented more recent sinistral strike slip fault zones kinematic analysis of minor fault and striaes confirms the abundance of strike slip faults and suggests rotation of the stress tensor after at least 9 ma a consequence of the subduction of the scr beneath the south american plate at the latitude of the lgcba is a two step history 1 pre ridge subduction oblique to the western margin of the south america plate showing contractional regime with its main axis trending ne sw responsible for dextral transpressive deformation and 2 post ridge subduction more orthogonal convergent regime recent uplift occurred later starting at 3 5 ma and could still be active neotectonic activity has been evaluated in the region from preliminary analysis of river patterns and fluvial lacustrine terraces anomalies in stream development can be found although evidences for recent to active faulting can be shown effects of syn glacial and post glacial related processes cannot be ruled out the evolution of this region during the late cenozoic can be divided into three distinct periods 1 20 15 ma deposition of foreland continental detrital sediments rio zeballos group followed by a major contractional phase leading to the development of the main thrust front sometimes between 15 and 10 9 ma 2 9 4 ma erosion peneplanation and emplacement of voluminous plateau basalts coeval with fluvial glacial sedimentation and 3 3 0 ma renewed orogenic activity uplift and disruption of former erosional surfaces in relation to activation of strike slip fault zones and north trending thrusts onset of the major contractional event in the region of study can be correlated with the initiation of the subduction of the chile ridge at 14 15 ma in southernmost patagonia inducing a different stress regime in the frontal arc region