hierarchical segmentation of the malawi rift the influence of inherited lithospheric heterogeneity and kinematics in the evolution of continental rifts we used detailed analysis of shuttle radar topography mission digital elevation model and observations from aeromagnetic data to examine the influence of inherited lithospheric heterogeneity and kinematics in the segmentation of largely amagmatic continental rifts we focused on the cenozoic malawi rift which represents the southern extension of the western branch of the east african rift system this north trending rift traverses precambrian and paleozoic mesozoic structures of different orientations we found that the rift can be hierarchically divided into first order and second order segments in the first order segmentation we divided the rift into northern central and southern sections in its northern section the rift follows paleoproterozoic and neoproterozoic terrains with structural grain that favored the localization of extension within well developed border faults the central section occurs within mesoproterozoic neoproterozoic terrain with regional structures oblique to the rift extent we propose that the lack of inherited lithospheric heterogeneity favoring extension localization resulted in the development of the rift in this section as a shallow graben with undeveloped border faults in the southern section mesoproterozoic neoproterozoic rocks were reactivated and developed the border faults in the second order segmentation only observed in the northern section we divided the section into five segments that approximate four half grabens asymmetrical grabens with alternating polarities the change of polarity coincides with flip over full grabens occurring within overlap zones associated with 150 km long alternating border faults segments the inherited lithospheric heterogeneity played the major role in facilitating the segmentation of the malawi rift during its opening resulting from extension

late cenozoic evolution of epithermal gold metallogenic provinces in kyushu japan kyushu island japan is located at the junction of the southwest japan arc and the ryukyu arc there are two major late cenozoic epithermal gold silver provinces in kyushu which are termed the northern and southern provinces the provinces are characterized by 1 pliocene volcanism dominated by calc alkaline andesite followed by quaternary volcanism including extrusion of both calc alkaline and tholeiitic magmas 2 formation of extensional grabens 3 pliocene to pleistocene mineralization which was dominated by abundant low sulfidation ls epithermal deposits with a few high sulfidation hs examples the two epithermal gold silver provinces have evolved differently since about 5 ma the northern province has exhibited diminished hydrothermal activity from the pliocene to pleistocene whereas the southern province has witnessed increased hydrothermal activity mainly in easterly and northerly directions changes of tectonic setting from the pliocene to pleistocene account for the variable trends in epithermal gold deposit formation westward oblique subduction of the philippine sea plate beneath the southwest japan arc caused development of the hohi graben and arc related volcanism at about 6 ma this was associated with widespread ls mineralization in and surrounding the hohi graben as is represented by the bajo and taio deposits the subduction of the relatively buoyant kyushu palau ridge during the early pliocene strengthened the coupling between the slab and overriding ryukyu arc leading to polygenetic andesite volcanism with associated hs kasuga iwato and akeshi and ls kushikino mineral deposits forming in the southern province a change of the subduction direction of the philippine sea plate from west to north northwest in the early pliocene increased the orthogonal convergence rate between the southwest japan arc and the philippine sea plate resulting in a decrease of volcanic and hydrothermal activity in the hohi graben of the northern province the more northerly subduction of the philippine sea plate shifted the locus of the kyushu palau ridge subduction northward resulting in underplating of the older 85 60 ma negatively buoyant amami basin oceanic slab in the southern province rather than continued subduction of the young 27 15 ma buoyant shikoku basin slab this replacement caused steepening of the slab angle and slab rollback in the southern province which was associated with regional extension an eastward shift of the ryukyu volcanic front and development of the kagoshima and shimabara grabens as well as the okinawa trough rhyolite and basalt volcanism in addition to andesite volcanism have occurred since 2 ma in the area of the ryukyu back arc coincident ls mineralization at hishikari and ohkuchi was affiliated with the rhyolite volcanism another change of the subduction direction of the philippine sea plate to the northwest occurred at 2 1 ma the forearc sliver of the southwest japan arc shifted westward in association with right lateral strike slip faulting along the median tectonic line due to the increase of the westward convergence rate this shift resulted in shortening and cessation of graben development in the hohi area restricting the subsequent volcanism and related hydrothermal activity to the central part of the graben

exhumation of a cretaceous rift complex within a late cenozoic restraining bend southern mongolia implications for the crustal evolution of the gobi altai region abstract the gobi altai region of southern mongolia contains intramontane basins dominated by jurassic cretaceous clastic and volcanic deposits the origin of the basins is poorly documented because late cenozoic transpressional mountain building and associated alluvial sedimentation have overprinted and obscured the older mesozoic history in this study we report the discovery of cretaceous high angle normal faults and a major low angle extensional detachment fault bordering the altan uul range in southern mongolia which indicates that nw se crustal stretching was responsible for creating the accommodation space for thick jurassic cretaceous sedimentary accumulations in southern mongolia the detachment fault separates chlorite grade metasedimentary rocks in the footwall from intensely sheared and flattened unmetamorphosed conglomerates and breccias in the hanging wall the significant metamorphic break across the detachment fault associated extensional structures in the hanging wall and domal form of the footwall block indicate that nw altan uul has some structural and topographic characteristics typically associated with metamorphic core complexes however nw altan uul lacks ductile extensional fabrics along the detachment fault and in the adjacent footwall and limited stretching in the upper plate suggests less total extensional slip than is reported from typical metamorphic core complexes unconformable relations in hanging wall strata and palynological data indicate that extensional detachment faulting at nw altan uul occurred in the aptian 120 112 ma similar in time to metamorphic core complex development previously reported in mongolia china border areas and the daqing shan of northern china post detachment fault cretaceous sedimentation buried nw altan uul which is now exposed only as a result of erosional denudation associated with uplift of the recent nemegt altan uul restraining bend the discovery of significant aptian crustal extension in the southern gobi altai suggests that early cretaceous diffuse rifting encompassed an even wider region than was previously recognized including areas of the eastern altai central southern and eastern mongolia and adjacent areas of northern and northeastern china and transbaikalia constituting one of earth9s largest continental interior extensional provinces workers investigating the neotectonic development of the gobi altai should consider the extent to which the pre existing rift basin architecture may have influenced the recent range development and network of seismically active faults in the region

detection of mantle earthquakes beneath the east african rift summary recordings from a temporary seismic network in the rwenzori region of the east african rift show seven earthquakes at a depth range between 53 and 60 km evidence from receiver functions indicates a crustal thickness of about 32 km in this region suggesting that the earthquakes are located well within the mantle beneath the rift the occurrence of faulting at this depth is of relevance for models of continental rift initiation and related deep magmatic processes

oxygen isotopic constraints on the geneses of the miocene outer zone granitoids in japan abstract miocene ilmenite series granitoids of the outer zone of southwest japan have been classified mineralogically into the southern s type and the northern i type oxygen isotopic ratios 18 o smow were determined on 45 representative samples from 17 granitic plutons the results indicate that the outer zone granitoids have much higher 18 o values than magnetite series granitoids of similar ages confirming a contribution from crustal sediments to the genesis of the ilmenite series granitoids and that s type granitoids are higher than i type with separation at 10 5 which is slightly higher than the 9 7 boundary in the lachlan fold belt of australia assuming the granitoids originated from mixtures of basaltic magmas from the upper mantle and sedimentary rocks of the accretionary complex from the fore arc basin mixing ratios were calculated the s type granitoids have sedimentary contributions of 64 45 wt with one exception while i type granitoids have that of 43 30 wt two trends are typically observed in the 18 o sio 2 diagram i granitoids having large variation in 18 o values but little variation in sio2 contents and ii vice versa the former granitoids solidified from heterogeneous magmas containing restite fragments but the latter crystallized from a completely homogenized magma with magmatic fractionation this type is associated with mineralization

thermoluminescence and compositional zoning in the mesostasis of a semarkona group a1 chondrule and new insights into the chondrule forming process a large group a1 porphyritic olivine chondrule in the semarkona ll3 0 chondrite with induced thermoluminescence tl and compositional zoning in its mesostasis has been discovered the chondrule has ca rich and fe poor olivine cao 0 36 0 40 wt fa0 3 0 5 and its mesostasis is highly anorthite normative 52 5 wt the chondrule shows an intense induced tl peak at 300 c with a half width of 180 c the induced tl in the 40 440 c range increases monotonically by a factor of 6 from center to rim while sio2 na2o and mno increase by factors of 1 1 3 6 and 6 respectively the spectrum of the induced tl over the 200 350 c range i e a dominant peak at 570 nm with a half width of about 100 nm and the mn tl correlation suggest mn activated plagioclase is an important constituent of the refractory mesostases in group a1 chondrules the zoning may reflect fractional crystallization soret diffusion transport of volatiles into the chondrule by aqueous alteration a zoned precursor reduction of precursor dust aggregate or recondensation of volatiles lost during chondrule formation the first four possibilities seem unlikely explanations for the zoning of the mesostasis but both reduction of precursor dust aggregate and recondensation of volatiles seem to have played significant roles to the formation of the zoning reducing conditions at high temperatures are suggested to have prevailed during the formation of group a1 chondrules

volcano tectonic interactions during rapid plate boundary evolution in the kyushu region sw japan evolution of the local plate tectonic and volcanic system relationship at kyushu island is defined by major changes in tectonics and volcanic style at ca 15 10 6 and 2 ma plate reconstructions presented here suggest that prior to 15 ma the pacific plate subduction dominated kyushu tectonics from 15 to 6 ma the evolving relative plate motions shifted the triple junction between the pacific plate philippine sea plate and southwest japan northwards so that the philippine sea plate was subducted beneath kyushu we suggest that a lack of subduction related volcanism from 10 to 6 ma is due to shallow subduction of the young shikoku basin lithosphere by 6 5 ma changes in the philippine sea plate motion led to more rapid nearly trench normal subduction of the eocene west philippine basin crust beneath kyushu this model is supported by an increase in arc like geochemistry of lavas since ca 6 5 ma subduction of fluid rich features such as the kyushu palau ridge introduced large volumes of fluids into the kyushu arc system leading to voluminous volcanism across kyushu focused particularly in areas where the ridge subduction occurs in tandem with local extensional tectonics key issues such as the timing of izu arc collision with central japan and the history of motion of the philippine sea plate are reassessed here resulting in a model that favors izu arc central japan collision at ca 8 6 ma rather than the more widely accepted date of ca 15 ma

shear wave velocity structure of the southern african upper mantle with implications for the uplift of southern africa summary broad band seismic data from the southern african seismic experiment and the africaarray network are used to investigate the seismic velocity structure of the upper mantle beneath southern africa and in particular beneath the kaapvaal craton a two plane approximation method that includes a finite frequency sensitivity kernel is employed to measure rayleigh wave phase velocities which are inverted to obtain a quasi 3 d shear wave velocity model of the upper mantle we find phase velocities for the kaapvaal craton and surrounding mobile belts that are comparable to those reported by previous studies and we find little evidence for variation from east to west across the namaqua natal belt a region not well imaged in previous studies a high velocity upper mantle lid is found beneath the kaapvaal craton and most of southern africa for the kaapvaal craton the thickness of the lid 150 200 km is consistent with the lid thicknesses reported in many previous studies the cratonic lid is underlain by a 100 km thick low velocity zone with a 3 9 per cent maximum velocity reduction by comparing the velocity model to those published for other archean cratons we find few differences and therefore conclude that there is little evidence in the shear wave velocity structure of the mantle to indicate that the southern african plateau is supported by an upper mantle thermal anomaly

fault segmentation deep rift earthquakes and crustal rheology insights from the 2009 karonga sequence and seismicity in the rukwa malawi rift zone the rukwa malawi rift zone has a record of seismic events down to depths in excess of 30 km deep for a zone of active continental extension this deep seismicity as well as the presence of long 100 km border faults has previously been explained by the long term bulk rheology of intact old cold anhydrous strong crust in east africa or the presence of mafic material in the lower crust the karonga sequence of 2009 showed a style of faulting different from continuous slip along long border faults and is interpreted as segmented failure of hanging wall faults coulomb stress transfer in this sequence is calculated and found to be consistent with segmented slip on a fault system synthetic to a nearby border fault and restricted to depths 12 km the inferred thermal structure of the malawi rift indicates that slip at depths in excess of 30 km occurs at temperatures greater than the 350 450 c commonly inferred at the base of the seismogenic zone crustal strength calculations indicate that long border faults and deep seismicity require the presence of a weak zone of localized deformation with increased strain rate or fluid pressure within a strong lower crust a hypothesis is proposed where shallow segmented frictional failure occurs in regions of relatively strong intact crust e g the karonga sequence whereas long border faults and deep earthquakes are representative of zones of weakness within strong crust this hypothesis if correct implies that seismogenic thickness can vary within thick elastic lithosphere such that localized weak zones of the crust enable nucleation of larger seismic events whereas strong intact crust favors smaller segmented events and a shallower seismogenic zone

finite element modelling of crustal deformation in the baikal rift zone new insights into the active passive rifting debate abstract the origin of the forces responsible for crustal extension in the baikal rift zone siberia is the object of a debate between passive rifting models where crustal extension is primarily caused by horizontal forces related to the kinematics of asia india eurasia collision and active rifting models where crustal extension is primarily caused by a diapiric mantle upwelling in this work we used a two dimensional visco elastic finite element model in order to determine whether horizontal forces alone can account for the present day deformation in the baikal rift zone we tested a number of kinematic boundary conditions and compared predictions of various models against the observed stress and strain field deduced from seismotectonic data earthquake focal mechanisms and microtectonic analyses by adjusting the kinematic boundary conditions and using a three plate model with a differential displacement between the mongolian and amurian plates we found a best fit model that correctly accounts for the observed strain and stress field over the entire baikal rift zone the fact that our model does not take into account vertical forces but still explains most of the observed deformation suggests that the present day opening of the baikal rift is essentially controlled by horizontal forces related to the regional kinematics these forces could have their origin in the india eurasia collision zone further south this result does not imply that the asthenosphere played no role in the rifting process in particular before the fast rifting stage of the baikal rift evolution 3 4 ma but might suggest a recent plio quaternary triggering effect of the india asia collision on the deformation in the baikal rift

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

peculiarities of surface wave propagation across central eurasia abstract the recent installation of six broadband digital iris ida seismic stations in the ussr has provided new opportunities for studying surface wave propagation across eurasia group velocities of fundamental rayleigh and love modes between epicenters and these stations were determined for 35 events that occurred since april 1989 to the middle of july 1990 near eurasia differential phase velocities were found for the same arrivals along paths between several pairs of stations group and phase velocities were obtained in the period range from 15 to 300 sec frequency time polarization analysis was used for studying polarization properties of surface waves in some cases significant anomalies in the particle motion for periods up to 100 sec were observed they are attributed to surface wave refraction and scattering due to lateral inhomogeneities at the boundaries and inside the eurasia continent

correlated mineralogy chemical compositions oxygen isotopic compositions and size of chondrules we report results of a study into the size chemical composition mineralogy and oxygen isotopic compositions of 40 chondrules in the size range 1 9 mm diameter separated from chainpur ll3 4 and parnallee ll3 6 these 4 parameters were found to be interdependent the main control on the bulk compositions of chondrules is the relative volatilities of the elements the major elements mg al ti ca show strong correlations among one other and most of the remaining more volatile elements form another grouping increasing proportions of refractory to volatile elements correlate with chondrule size smaller chondrules tend to have more glassy sio2 rich mesostases than larger chondrules which have a higher proportion of mesostasis with feldspathic compositions chondrules with high sio2 glassy mesostases have exchanged with 16o poor gases to a greater degree than low sio2 mesostases containing crystalline feldspar the state of mesostases of the chondrules is the most important control on their oxygen isotopic compositions although there also seems to be a size effect in chainpur smaller chondrules in chainpur have more 16o poor compositions suggesting that the relatively large surface areas of smaller chondrules facilitated gas solid exchange this correlation is obscured in parnallee because it has undergone more extensive devitrification than chainpur leading to an increase in the proportion of crystalline mesostasis the model we propose to explain the interdependence of bulk compositions size and oxygen isotopes is in two stages chondrules which were still partially molten experienced an influx of relatively volatile elements from ambient gases smaller chondrules experienced a relatively greater influx than larger chondrules because of their greater surface area volume ratio the second stage following crystallisation of the chondrules and limited devitrification of the mesostases is the influx of an 16o poor gas which led to the isotopic exchange we believe the most likely setting for the formation of these chondrules and the associated volatile rich and 16o poor gas influxes is a planetary regolith setting

seismic images of magmatic rifting beneath the western branch of the east african rift 1 we have performed a tomographic study using a joint data set that includes local and teleseismic events recorded by a temporary network in the western branch of the east african rift system from the travel time residuals we derive a three dimensional model of seismic p wave velocity anomalies for the crust and upper mantle down to a depth of 80 km particular attention is paid to the verification of the inversion results by various resolution tests the results show that the eastern rift shoulder is characterized by relatively high seismic velocities lower velocities are obtained beneath the entire length of the rift valley and the rwenzori mountains a prominent feature is observed north east of the mountain range here we detected a vertically oriented cylindrical low velocity anomaly with maximum amplitudes in the middle crust and the upper mantle lithosphere we suggest that this anomaly indicates reservoirs of molten material related to the ongoing rifting process within this segment of the rift just above this anomaly at depths between 5 and 16 km earthquake swarms exist the observed reduction in p wave velocity is used to provide constraints on the possible melt content and temperature anomaly in the uppermost mantle the observed 3 5 p velocity decrease can be explained by melt fraction up to 2 3 3 or alternatively by a temperature increase of at least 248 to 376 k and even higher temperature anomalies are possible if lower ambient temperatures in the reference mantle are assumed probably the two effects act in combination

a crustal thickness map of africa derived from a global gravity field model using euler deconvolution summary we develop a new continental scale crustal model for africa by modelling the free air gravity anomaly eigen gl04c which was developed from 30 months of grace level 1b data covering the period from 2003 february to 2005 july and surface gravity data from seven different sources from this gravity model crustal thickness is estimated using 3 d euler deconvolution a method that does not rely on ap rioridepth and density constraints the results are in good agreement i e within 5km of seismically determined moho depth estimates from across the continent except for narrow tectonic regions such as rift valleys and areas where seismic velocity models of the crust indicate a gradational moho the results show that crustal thickness is fairly homogeneous with an average crustal thickness for the whole continent of 39 2 sd km the average moho depth for most terrains is within 5km of the continental average and there is little variability between terrains of different age the average thickness for archean proterozoic and palaeozoic crust is 39 39 and 41km respectively crustal thickness in sedimentary basins across northern and central africa varies between 33 and 36km through comparison with global averages for similar aged terrains we find that african crustal thickness does not deviate significantly from the thickness of crust in other parts of the world

metacraton nature genesis and behavior article i nfo in this paper we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events leading to the transformation of their margins in some cases craton interiors can also be shaped by intense collisional processes leading to the generation of intra cratonic orogenic belts we propose to call these events metacratonization and the resulting lithospheric tract metacraton metacratons can appear similar to typical orogenic belts i e active margin transformed by collisional processes but are actually sharply different their main distinctive characteristics not all are present in each metacraton are 1 absence of pre collisional events 2 absence of lithospheric thickening high pressure metamorphism being generated by subduction leading to high gradient in strain and meta morphic intensity 3 preservation of allochthonous pre collisional oceanic terranes 4 abundant post collisional magmatism associated with shear zones but not with lithospheric thickening 5 presence of high temperature low pressure metamorphism associated with post collisional magmatism 6 intraconti nental orogenic belts unrelated to subduction and oceanic basin closures reactivation of the rigid but fractured metacratonic lithosphere will cause doming asthenospheric volcanism emplacement and mineral izations due to repetitive mineral enrichments this paper provides several geological cases exemplifying these different metacratonic features in scandinavia sahara central africa and elsewhere a special focus is given to the saharan metacraton because it is where the term metacraton originated and it is a vastly ex panded tract of continental crust 5 000 000 km 2

earthquake distribution patterns in africa their relationship to variations in lithospheric and geological structure and their rheological implications we use teleseismic waveform inversion along with depth phase analysis to constrain the centroid depths and source parameters of large african earthquakes the majority of seismic activity is concentrated along the east african rift system with additional active regions along stretches of the continental margins in north and east africa and in the congo basin we examine variations in the seismogenic thickness across africa based on a total of 227 well determined earthquake depths 112 of which are new to this study seismogenic thickness varies in correspondence with lithospheric thickness as determined from surface wave tomography with regions of thick lithosphere being associated with seismogenic thicknesses of up to 40 km in regions of thin lithosphere the seismogenic thickness is typically limited to 20 km larger seismogenic thicknesses also correlate with regions that have dominant tectonothermal ages of 1500 ma where the east african rift passes around the archean cratons of africa through the older proterozoic mobile belts these correlations are likely to be related to the production affected by method and age of basement formation and preservation affected by lithospheric thickness of a strong anhydrous lower crust the congo basin contains the only compressional earthquakes in the continental interior simple modelling of the forces induced by convective support of the african plate based on long wavelength free air gravity anomalies indicates that epeirogenic effects are sufficient to account for the localization and occurrence of both extensional and compressional deformation in africa seismicity along the margins of africa reflects a mixture between oceanic and continental seismogenic characteristics with earthquakes in places extending to 40 km depth

fluid inclusion and stable isotope constraints on ore genesis of the zajkan epithermal base metal deposit tarom hashtjin metallogenic belt nw iran in the zajkan pb zn cu au ag deposit of the tarom hashtjin metallogenic belt thmb northwest iran mineralization occurs as quartz base metal sulfide veins hosted by eocene volcanic and volcaniclastic rocks of the karaj formation four stages of ore formation are identified at zajkan 1 quartz chalcopyrite pyrite veins 2 quartz galena sphalerite chalcopyrite pyrite veins and breccias 3 quartz hematite veins and breccias and 4 barren quartz carbonate veinlets the primary metallic minerals are pyrite chalcopyrite sphalerite galena and trace amounts of hematite gangue minerals are mainly quartz sericite chlorite and calcite the main wall rock alteration is silicification and intermediate argillic and propylitic alteration the coexistence of different types of fluid inclusions lv vl and v in individual fluid inclusion assemblages in quartz and sphalerite suggests boiling and heterogeneous trapping excluding the heterogeneously entrapped fluid inclusions the microthermometric measurements of primary lv fluid inclusion assemblages in quartz and sphalerite indicate that the veins were formed at temperatures between 217 and 273 degrees c from fluids with salinities between 1 7 and 6 4 wt nacl equiv the oxygen isotopic compositions delta o 18 water vsmow of 3 7 to 1 1 parts per thousand indicate that the ore forming fluids were mainly derived from magmatic water with the input of meteoric water in the later ore stages sulfur isotopic values of sulfide minerals vary over a narrow range from 5 0 to 0 9 parts per thousand vcdt averaging 2 2 parts per thousand vcdt suggesting that sulfur was derived from a homogeneous magmatic source the fluid inclusion and stable isotope data indicate that fluid boiling and mixing facilitated hydrothermal alteration and mineralization at zajkan our data suggest that the zajkan deposit is an intermediate sulficiation style epithermal mineralization the estimated ore formation depth of zajkan indicates minor denudation in the region and may imply a great prospecting potential for other epithermal mineralization in the thmb

extension across the laptev sea continental rifts constrained by gravity modeling the laptev shelf is the area where the gakkel ridge an active oceanic spreading axis approaches a continental edge causing a specific structural style dominated by extensive rift structures from the latest cretaceous to the pliocene extension exerted on the laptev shelf created there several deep subsided rifts and high standing basement blocks to understand syn rift basin geometries and sediment supply relationships across the laptev shelf accurate extension estimates are essential therefore we used 2 d gravity modeling and 3 d gravity inversion to constrain the amount of crustal stretching across the north america eurasia plate boundary in the laptev shelf the latest cretaceous cenozoic extension in that area is partitioned among two rift zones the laptev rift system and the new siberian rift these rifts were both overprinted on the eurasian margin that had been stretched by 190 250 km before the late cretaceous while the laptev rift system connected to the gakkel ridge reveals increasing extension toward the shelf edge 190 380 km the new siberian rift is characterized by approximately uniform stretching along strike 110 125 km the architecture of the laptev rift system shows that the finite extension of about 500 km is sufficient to entirely eliminate crystalline continental crust in the most stretched rift segment continental mantle is exhumed at the base of the late mesozoic basement the example of the laptev rift system shows that extension driven by divergent plate movement is a sufficient cause to produce almost complete continental breakup without an increased heat input from the asthenospheric mantle

seismic anisotropy and mantle dynamics beneath the malawi rift zone east africa sks skks and pks splitting parameters measured at 34 seismic stations that we deployed in the vicinity of the cenozoic malawi rift zone mrz of the east african rift system demonstrate systematic spatial variations with an average splitting time of 1 0 0 3 s the overall ne sw fast orientations are consistent with absolute plate motion apm models of the african plate constructed under the assumption of no net rotation of the global lithosphere and are inconsistent with predicted apm directions from models employing a fixed hotspot reference frame they also depart considerably from the trend of most of the major tectonic features these observations together with the results of anisotropy depth estimation using the spatial coherency of the splitting parameters suggest a mostly asthenospheric origin of the observed azimuthal anisotropy the single layered anisotropy observed at 30 and two layered anisotropy observed at four of the 34 stations can be explained by apm related simple shear within the rheologically transitional layer between the lithosphere and asthenosphere as well as by the horizontal deflection of asthenospheric flow along the southern and western edges of a continental block with relatively thick lithosphere revealed by previous seismic tomography and receiver function investigations this first regional scale shear wave splitting investigation of the mrz suggests the absence of rifting related active mantle upwelling or small scale mantle convection and supports a passive rifting process for the mrz

surface wave imaging of the weakly extended malawi rift from ambient noise and teleseismic rayleigh waves from onshore and lake bottom seismometers located at the southernmost sector of the western branch of the east african rift system the malawi rift exemplifies an active magma poor weakly extended continental rift to investigate the controls on rifting we image crustal and uppermost mantle structure beneath the region using ambient noise and teleseismic rayleigh wave phase velocities between 9 and 100 s period our study includes six lake bottom seismometers located in lake malawi nyasa the first time seismometers have been deployed in any of the african rift lakes noise levels in the lake are lower than that of shallow oceanic environments and allow successful application of compliance corrections and instrument orientation determination resulting phase velocity maps reveal slow velocities primarily confined to lake malawi at short periods t 25 s a prominent low velocity anomaly exists beneath the rungwe volcanic province at the northern terminus of the rift basin estimates of phase velocity sensitivity indicates these low velocities occur within the lithospheric mantle and potentially uppermost asthenosphere suggesting that mantle processes may control the association of volcanic centres and the localization of magmatism beneath the main portion of the malawi rift a modest reduction in velocity is also observed at periods sensitive to the crust and upper mantle but these velocities are much higher than those observed beneath rungwe

the kago low sulfidation gold and silver deposit a peripheral mineralisation to the nansatsu high sulfidation system southern kyushu japan abstract the kago deposit is a small deposit located at the southern tip of the satsuma peninsula of southern kyushu japan it lies proximal to the well known nansatsu type mineralisation province dominated by high sulfidation type epithermal deposits the deposit was heavily mined in the 18 th century largely for its relatively higher gold compared to that of surrounding and regional deposits the kago deposit is a typical low sulfidation deposit characterised by adularia quartz veins composed of electrum ag tetrahedrite polybasite chalcopyrite and pyrite based on mine records the grade ranged from 4 1 to 13 3 g t au and 2 6 6 6 g t ag alteration grades from low to high temperature argillic into a propylitic zone at the extremes of the vein exposures carbonate is absent fluid inclusion microthermometry reveals a typical temperature range of 220 240 c with salinity of 0 7 2 6 nacl eq wt electrum from high grade ore ranges from 66 to 69 wt au 40ar 39ar age dating of adularia bearing colloform crustiform and brecciated veins suggests a mineralisation event from 4 23 to 4 0 ma 18o of veined and silicified quartz ranges from 4 0 to 18 4 18oh2o of fluids in equilibrium with quartz in the dominant range of measured fluid inclusion temperatures ranges from 6 5 to 0 2 34s of pyrite has a narrow range from 1 8 to 2 7 the deposit lies at the northern extent of the classic nansatsu high sulfidation epithermal area in which a number of silicified bodies punctuate the region in a roughly semi circular shape the kago deposit lies within the principle mineralisation age range of the high sulfidation deposits which range from 5 5 to 3 7 ma the structural displacement of the kago deposit from the nansatsu mineralisation and the differing host rocks has greatly influenced alteration ore and rock water interaction of the ore depositing fluids here we seek to establish the relationship that this extended mineral province has between the differing styles of mineralisation

origin and evolution of hydrothermal fluids in the marshoun epithermal pb zn cu ag deposit tarom hashtjin metallogenic belt nw iran abstract the marshoun epithermal pb zn cu ag deposit is located in the tarom hashtjin metallogenic belt thmb northwest iran the epithermal base metal veins are mainly hosted in eocene volcanic and volcaniclastic rocks of the karaj formation and late eocene pyroxene quartz monzodiorite the mineralization process can be divided into five stages 1 quartz chalcopyrite pyrite veins and breccias 2 quartz calcite sphalerite galena chalcopyrite pyrite veins and breccias 3 barren post ore quartz veinlets 4 barren post ore calcite veinlets and 5 supergene mineral assemblages the primary metallic minerals are pyrite chalcopyrite sphalerite and galena gangue minerals are mainly quartz calcite sericite and chlorite the main wall rock alteration is silicification intermediate argillic carbonate and propylitic alteration microthermometric measurements of primary lv fluid inclusion assemblages in quartz and sphalerite indicate that the veins were mainly formed at temperatures between 173 and 285 c from fluids with salinities between 3 4 and 6 3 wt nacl equiv coexisting lv vl and v inclusions in quartz and sphalerite provide evidence for boiling in ore stage breccia and veins additionally the occurrence of bladed calcite as well as plumose and colloform crustiform banded quartz in the ore zones and the presence of hydrothermal breccias are consistent with boiling obtained oxygen and sulfur isotope compositions 18owater 9 2 to 1 5 34sh2s 10 0 to 5 7 indicate there was a major contribution from a magmatic component to metallogenesis the fluid inclusion and stable isotope data indicate that fluid boiling and mixing facilitated hydrothermal alteration and mineralization at marshoun the marshoun deposit is interpreted as an intermediate sulfidation style of epithermal mineralization our data suggest that the epithermal mineralization in the thmb are related to late eocene ca 36 37 ma shallow magmatic hydrothermal activity the altered eocene volcanic subvolcanic rocks particularly at the intersection of subvolcanic rocks with faults were the most favorable locus for exploration targeting of epithermal ores at thmb

lithospheric boundaries and upper mantle structure beneath southern africa imaged by p and s wave velocity models we report new p and s wave velocity models of the upper mantle beneath southern africa using data recorded on seismic stations spanning the entire subcontinent beneath most of the damara belt including the okavango rift our models show lower than average velocities 0 8 vp 1 2 vs with an abrupt increase in velocities along the terrane s southern margin we attribute the lower than average velocities to thinner lithosphere 130 km thick compared to thicker lithosphere 200 km thick immediately to the south under the kalahari craton beneath the etendeka flood basalt province higher than average velocities 0 25 vp 0 75 vs indicate thicker and or compositionally distinct lithosphere compared to other parts of the damara belt in the rehoboth province higher than average velocities 0 3 vp 0 5 vs suggest the presence of a microcraton as do higher than average velocities 1 0 vp 1 5 vs under the southern irumide belt lower than average velocities 0 4 vp 0 7 vs beneath the bushveld complex and parts of the mgondi and okwa terranes are consistent with previous studies which attributed them to compositionally modified lithosphere resulting from precambrian magmatic events there is little evidence for thermally modified upper mantle beneath any of these terranes which could provide a source of uplift for the southern african plateau in contrast beneath parts of the irumide belt in southern and central zambia and the mozambique belt in central mozambique deep seated low velocity anomalies 0 7 vp 0 8 vs can be attributed to upper mantle extensions of the african superplume structure

evidence for primitive nebular components in chondrules from the chainpur chondrite abstract the least equilibrated ordinary chrondrites contain chondrules which have experienced little change since the time of their formation in the early solar system these chondrules are excellent indicators of the physical and chemical nature of the solar nebula we separated 36 chondrules from the chainpur ll3 4 chondrite and analyzed each for 20 elements and petrographic properties sampling biases were minimized as far as possible chondrules seem to have formed through the melting of random mixtures of grains comprising a limited number of nebular components the identity of these components can be deduced from chondrule compositions the dominant components appear to be 1 a mixture of metal and sulfide with composition similar to whole rock metal and sulfide 2 refractory ir rich metal 3 refractory olivine rich silicates 4 low temperature pyroxene rich silicates and possibly 5 a component containing the more volatile lithophiles most of the textural types of chondrules formed from the same set of precursor components in some cases chondrules having different textures are almost identical in composition a few unusual chondrule types seem to mainly consist of uncommon nebular components possibly indicating different modes of formation etching experiments confirm that chondrule rims are enriched in metal troilite and moderately volatile elements relative to the bulk chondrules however a large fraction of the volatiles remains in the unetched interior

ar 40 ar 39 hornblende geochronology from the forsmark area in central sweden constraints on late svecofennian cooling ductile deformation and exhumation at forsmark ca 120 km north of stockholm in central sweden ductile high strain belts with wnw to nw trend anastomose around tectonic lenses with an inferred lower degree of ductile strain previous studies of ductile deformation zones with wnw to nw trend elsewhere in the western part of the svecofennian orogen in central sweden have yielded estimates for the timing of at least one phase of discrete ductile deformation that fall in the time interval 1 82 1 78 ga most of these ages were determined by the u pb dating of titanite and for this reason provide no information on the thermal evolution in this paper we make use of ar 40 ar 39 hornblende geochronology to address late svecofennian cooling ductile deformation and exhumation the data demonstrate the presence of three ar 40 ar 39 hornblende age generations all ages have been adjusted to take account of ca 1 systematic bias between ar 40 ar 39 and u pb ages recently reported in the literature the oldest age ca 1 87 ga and the intermediate age generation 1 85 1 84 ga are spatially restricted to the tectonic lenses by contrast the youngest age generation 1 83 1 81 ga occurs both within the tectonic lenses and the enveloping high strain belts one explanation for the structurally controlled age distribution involves regional cooling beneath the closure temperature for argon isotopic mobility around or above 500 degrees c by 1 84 ga as represented in the oldest and intermediate age generations followed by resetting of the argon isotope system in hornblende between 1 83 and 1 81 ga as represented in the youngest age generation this resetting occurred in response to retrograde lower amphibolite to upper greenschist facies deformation along discrete high strain zones within the broader high strain belts and was associated with regional exhumation an alternative explanation involves no resetting of the ages instead it is suggested that a period of slow cooling of hornblendes with slightly different closure temperatures from ca 1 87 to 1 82 ga may have caused the age variation observed within the tectonic lenses whereas locally maintained higher temperatures due to activity along the discrete high strain zones can explain the consistently younger ages in the broad enveloping high strain belts in this explanation an increase in cooling rate in response to regional exhumation finally closed the argon isotope system in hornblende throughout the area at 1 83 1 81 ga it is suggested that the regional exhumation at 1 83 1 81 ga which is included in both explanations is related to far field effects of the deformation that ended an accretionary tectonic cycle in adjacent tectonic domains c 2008 elsevier b v all rights reserved less

geology geochemistry and origin of high sulfidation cu au mineralization in the nansatsu district japan the nansatsu district of southern kyushu has been the site of calc alkaline volcanism for the last 10 m y shifting eastward with time associated hydrothermal activity followed deposition of the volcanic host rocks by about 0 5 m y and was characterized by interaction of magmatic fluids with meteoric water under epithermal conditions resulting in the formation of high sulfidation cu au deposits at kasuga iwato and akeshi the orebodies consist of 95 wt percent sio 2 and result from leaching of the original andesite lava and pyroclastic flows by acid chloride sulfate waters these are inferred to have formed when magmatic vapors containing hcl and so 2 condensed into meteoric water the residual silica now quartz orebodies are best developed where the host was initially permeable the margins of the quartz bodies are abrupt with narrow 1 2 m halos representing the reaction front of acid waters isochemically dissolving the host rock the halo comprises alunite strongly zoned in na and k with p rich cores dickite and or kaolinite or pyrophyllite grading out into illite and interlayered illite smectite clays and finally propylitic alteration this pattern is characteristic of deposits of this type throughout the world for example at summitville colorado and lepanto philippines mineralization occurred after initial leaching by the vapor condensates with metals transported by a dense magmatic fluid mixing with meteoric water and the subsequent temperature decrease caused the general decrease in grade toward the margin of the quartz bodies ore grades are restricted to the quartz bodies gold is most closely associated with enargite and pyrite later minerals include covellite and then sulfur the last stage of activity was steam heated with descending waters oxidizing sulfides to goethite and locally remobilizing au into fractures this varies in degree between deposits erosion exposed the orebodies to supergene weathering continuing the sulfide oxidation and au remobilization stable isotope results indicate that hypogene alunite formed from a mixture of magmatic fluid delta 18 o 7 or 2 per thousand delta d 25 or 5 per thousand similar to nearby active volcanic discharges with local meteoric water in contrast the clays in the marginal halo have isotopic compositions indicating a delta 18 o shift of 6 to 8 per mil from local meteoric water values probably due to water rock interaction and the delta 18 o values of residual silica quartz may also be due to meteoric water domination fluid inclusion study of postmineralization quartz crystals indicates that the fluids had a salinity of about 1 wt percent nacl equiv during late quartz growth though there is evidence in one sample for higher salinity fluid having been present up to 30 wt percent nacl equiv some inclusions contain daughter minerals of halite and sulfur the t h values of over 1 000 measurements on late quartz from the ore zones indicate that the mean temperature during that stage ranged from 200 degrees c at akeshi to about 200 degrees c at iwato and 230 degrees c at kasuga the presence of vapor rich inclusions some with t hv similar to t hl indicate the presence at times of a two phase fluid in the center of the ore zones with depths of about 150 to 300 m below the paleowater table the mineralizing fluid was relatively oxidized sulfide sulfate ratio about 3 1 close to pyrite alunite coexistence under these redox conditions a ph of 3 and over a temperature range of 200 degrees to 300 degrees c aucl 2 super complexing may dominate over hau sh 2 at salinities above about 2 wt percent nacl several conditions are conducive for high sulfidation mineralization to occur 1 a crystallizing magma exsolves a fluid with lower pressure conditions favoring metal fractionation from melt to fluid 2 the exsolved fluid separates into vapor and saline liquid phases due to immiscibility with the latter being metal rich 3 the gas rich hcl and so 2 h 2 s vapor ascends to the surface with at least a portion condensed into meteoric water forming an acid fluid which leaches the host rock to create permeable zones for later mixing and 4 the dense metal bearing fluid also ascends into this leached zoned and precipitates cu sulfosalts sulfides and au upon mixing with meteoric water if the saline liquid is not released from its source adjacent to the magma due to lack of fracturing or if there is a strong hydraulic gradient caused by high relief only the vapor related stage may occur this will leave leached barren rock which is characteristic of many eroded volcanic terranes

the history of vein formation determined by 40ar 39ar dating of adularia in the hosen 1 vein at the hishikari epithermal gold deposit japan the hosen 1 vein has a symmetrical banded structure within the hishikari epithermal gold deposit in japan the vein can be divided into four macroscopic bands we used 40ar 39ar dating of adularia by incremental step heating to determine the duration of mineralization in the individual bands of the vein and the history of vein formation plateau ages were obtained from all adularia samples and no significant excess 40ar is recognized from inverse isochrons the derived age spectra suggest that thermal overprinting of the epithermal system did not affect the plateau ages although early bands must have been reheated by the thermal pulses associated with later bands the 40ar 39ar ages of adularia indicate that vein formation occurred episodically in distinct periods separated by intervals of about 30 000 to 110 000 yr the hosen 1 vein grew in at least six increments at the 20 m level and at least four increments at the 62 5 m level the total duration of mineralization from the margin to the center of the hosen 1 vein was about 260 000 yr during which time the temperature of the hydrothermal fluid decreased from 190 to 210 c to less than 170 c

rapid erosion of the southern african plateau as it climbs over a mantle superswell we present new sedimentary flux data confirming that a large pulse of erosion affected the southern african plateau in the late cretaceous and is likely to be related to a major uplift episode of the plateau this short phase of erosion i e less than 30 myr in duration has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift given its size the rise of the african superplume is likely to have lasted much longer here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent wide drainage reorganization leading to substantial denudation in a relatively short amount of time the amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the karoo sedimentary and volcanic rocks and the underlying basement we also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse suggesting that climate may have also played a role in triggering the denudation the diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are however better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling