shrimp u pb ages and depth of emplacement of ladakh batholith eastern ladakh india abstract the ladakh batholith represents subduction related andean type magmatism intruding an island arc setting the age and emplacement of the batholith was examined along the kharu chang la section having a width of 25 km the shrimp ii u pb zircon age from a granodiorite near its northern boundary with the shyok suture zone ssz is 60 1 0 9 ma whereas a diorite from its southern boundary close to the indus tsangpo suture zone itsz yields an age of 58 4 1 0 ma al in hornblende barometry indicates the depth of crystallization for the diorite to be 16 1 km and granodiorite to be around 8 1 km the data indicate that diorite and granodiorite were emplaced with a depth difference of about 8 km and are now juxtaposed at the present level of erosion exposed with a 2 km elevation difference this can be attributed to tectonic removal of material along the kharu chang la section the depth of emplacement data also indicates northward tilting of the ladakh batholith after crystallization

growth and deformation of the ladakh batholith northwest himalayas implications for timing of continental collision and origin of calc alkaline batholiths abstract the calc alkaline ladakh batholith nw himalayas was dated to constrain the timing of continental collision and subsequent deformation batholith growth ended when collision disrupted subduction of the tethyan oceanic lithosphere and thus the youngest magmatic pulse indirectly dates the collision both u pb ages on zircons from three samples of the ladakh batholith and k ar from one subvolcanic dike sample were determined magmatic activity near leh the capital of ladakh occurred between 70 and 50 ma with the last major magmatic pulse crystallizing at ca 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 ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand tex

rupture area and displacement of past cascadia great earthquakes from coastal coseismic subsidence coastal marshes record a 6500 yr history of coseismic displacements in great earthquakes at the cascadia subduction zone we compiled estimates of coseismic displacement for past megathrust events based on correlations with megathrust triggered turbidites and estimated megathrust slip based on comparisons of marsh displacements with dislocation model predictions age correlated marsh data are compatible with event rupture extents defi ned by the published turbidite record and a 6500 yr mean recurrence interval that increases northward from 230 to 480 yr within the constraints of the marsh data the width of the coseismic rupture zone generally agrees with the downdip width of the interseismic locked zone inferred from geodetic and thermal data in southernmost cascadia where the model does not include the complex deformation near the mendocino triple junction the coastal data may be better fi t by a model with an 25 narrower rupture than that inferred from regional geophysical data at each coastal marsh site coseismic displacements are roughly similar from event to event independent of the time since the previous event slip in the a d 1700 earthquake was consistent with the preceding interval of strain accumulation 200 yr only at the northern and southern ends of the margin but it was apparently much higher in southern washington and northern oregon possibly indicating postseismic contamination and or catch up coseismic slip to make up for a defi cit in the preceding event overall agreement between the dislocation models and the marsh data for most of the margin implies that such models can be usefully applied to rupture and ground shaking predictions

the 1978 brooks peninsula vancouver island earthquakes a pair of mw 5 5 earthquakes occurred beneath the brooks peninsula on the northwest coast of vancouver island on 2 june and 25 july 1978 these are the largest and best recorded earthquakes to date in the vicinity of northern vancouver island and the adjacent coast margin a detailed study of these earthquakes was undertaken to examine the contemporary tectonics of this region specifically the poorly understood interaction between the explorer plate and the north american plate at the northern end of the cascadia subduction zone both earthquakes were followed by well defined aftershock sequences a four station temporary seismograph array deployed on the brooks peninsula following the 2 june mainshock allowed for accurate aftershock locations this earthquake was located at 50 15 n 127 84 w based on the center of a 9 km diameter circular region of aftershocks the 25 july earthquake was located 4 to 7 km to the northeast of the june epicenter based on waveform comparison of the two events both earthquakes occurred at 15 to 16 km depth the focal mechanisms as determined from body wave modeling are nearly identical and show left lateral strike slip motion along a shallow north dipping east west striking fault the focal mechanism and depth of these two earthquakes indicates that they were not megathrust events on the explorer north america plate boundary but rather that they occurred within the north american plate 5 to 10 km above the megathrust the northeast directed pressure axes for these earthquakes suggests coupling across the explorer north america segment of the cascadia subduction zone consistent with contemporary convergence of the explorer plate with the north american plate in a northeast southwest direction

tectonic history of the north portion of the san andreas fault system california inferred from gravity and magnetic anomalies geologic and geophysical data for the san andreas fault system north of san francisco suggest that the eastern boundary of the pacific plate migrated eastward from its presumed original position at the base of the continental slope to its present position along the san andreas transform fault by means of a series of eastward jumps of the mendocino triple junction these eastward jumps total a distance of about 150 km since 29 ma correlation of right laterally displaced gravity and magnetic anomalies that now have components at san francisco and on the shelf north of point arena indicates that the presently active strand of the san andreas fault north of the san francisco peninsula formed recently at about 5 ma when the triple junction jumped eastward a minimum of 100 km to its present location at the north end of the san andreas fault prior to 5 ma the triple junction was located at the north end of a proposed northwesterly extension of the pilarcitos fault this jump took place at what is now about latitude 38 20 n on the north american plate and may have occurred during part of the time when the relative motion between the north american and pacific plates rotated 20 clockwise resulting in transpression along the earlier transcurrent san andreas fault system the proposed 150 km eastward movement of the triple junction explains the submarine topography near cape mendocino where the continental shelf south of the mendocino fault extends about 130 km farther west than does the shelf directly north of the fault

provenance of cenozoic sedimentary rocks from the sulaiman fold and thrust belt pakistan implications for the palaeogeography of the indus drainage system abstract the provenance of middle eocene to early miocene sedimentary rocks cropping out in the sulaiman fold and thrust belt has been determined examining the mineralogy bulk rock major and trace elements and nd sr isotopes the older 50 30 ma deposits are characterized by a mixed orogenic provenance with a major contribution from the karakorum and the tethyan belt c 80 as the 50 30 ma deposits have a provenance distinct from that of coeval subathu khojak and ghazij shallow marine formations of india and pakistan we propose that they were deposited as a distinct delta system that once fed the palaeo indus fan we document a major change in provenance that occurred before the early late oligocene transition at c 30 ma this change in provenance is marked by the appearance of chlorite and monazite and a shift toward more radiogenic nd sr isotopic compositions we interpret this change as the result of the exhumation and erosion of the proto higher himalaya the 30 15 ma sampled rocks are characterized by a major contribution from the tethyan belt and the higher himalayan crystallines 70 90 and a subordinate contribution 10 30 from the karakorum ophiolitic suture and trans himalaya as the end 0 values of our 30 15 ma samples are similar to those of the palaeo indus fan deposits we suggest that the 30 15 ma sedimentary rocks of the sulaiman fold and thrust belt were the fluvial onshore record of the indus fan other coeval deposits of india and pakistan recorded similar increasing exhumation of the higher himalaya range so that we postulate that these sedimentary rocks all derived from the palaeo indus drainage basin this would suggest that the modern indus drainage basin is no younger than 30 ma

structure of the makran subduction zone from wide angle and reflection seismic data abstract makran is one of the largest accretionary wedges on the globe formed by the convergence between the eurasian and the arabian plates it is characterised by an extremely high sediment input of 7 km and a shallow subduction angle we present seismic velocity models from four wide angle seismic lines that image the wedge sediments and the subducted oceanic crust three strike lines show complex but rather one dimensional structures where the observed seismic phases have been studied with the reflectivity method a 160 km long dip line was surveyed coincident with a mcs line collected by cambridge university in 1986 prestack depth migration of this data with the new wide angle velocities shows improved images compared to earlier results interpretation of the wide angle and the mcs data indicates that a decollement is developed along a bright reflector within the turbiditic sequence more than 3 km of sediment bypasses the first accretionary ridges underthrusting and is transported to greater depth this might help to explain the sparse earthquake activity associated with subduction here in the strike lines low velocity layers occur landward of the deformation front in zones where active thrusting or underplating takes place the oceanic crust is characterised by a strong velocity contrast between layer ii and layer iii

multiple monsoon controlled breakdown of oxygen minimum conditions during the past 30 000 years documented in laminated sediments off pakistan abstract late holocene laminated sediments from a core transect centred in the oxygen minimum zone omz impinging at the continental slope off pakistan indicate stable oxygen minimum conditions for the past 7000 calendar years high sw monsoon controlled biological productivity and enhanced organic matter preservation during this period is reflected in high contents of total organic carbon toc and redox sensitive elements ni v as well as by a low diversity high abundance benthic foraminiferal buliminacea association and high abundance of the planktonic species globigerina bulloides indicative of upwelling conditions surface water productivity was strongest during sw monsoon maxima stable omz conditions reflected by laminated sediments were found also during warm interstadial events preboreal bolling allerod and dansgaard oeschger events as well as during peak glacial times 17 22 5 ka all ages in calendar years sediment mass accumulation rates were at a maximum during the preboreal and younger dryas periods due to strong riverine input and mobilisation of fine grained sediment coinciding with rapid deglacial sea level rise whereas eolian input generally decreased from glacial to interglacial times in contrast the occurrence of bioturbated intervals from 7 to 10 5 ka early holocene in the younger dryas 11 7 13 ka from 15 to 17 ka heinrich event 1 and from 22 5 to 25 ka heinrich event 2 suggests completely different conditions of oxygen rich bottom waters extremely low mass and organic carbon accumulation rates a high diversity benthic fauna all indicating lowered surface water productivity during these intervals the omz was very poorly developed or absent and a sharp fall of the aragonite compensation depth favoured the preservation of pteropods the abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding with the north atlantic heinrich events and dust peaks in the tibetan loess records the correlation of the monsoon driven omz variability in the arabian sea with the rapid climatic fluctuations in the high northern latitudes suggests a close coupling between the climates of the high and low latitudes at a global scale

episodic unroofing of the kohistan batholith pakistan implications from k feldspar thermochronology new 40ar 39ar and u pb mineral ages from plutonic rocks help constrain the thermal and tectonic evolution of the kohistan island arc terrane following its collision with india in early tertiary time kohistan has experienced a prolonged tectonomagmatic history extending from the early cretaceous through the tertiary thermal histories derived from multi diffusion domain analyses of k feldspar within the kohistan batholith reveal rapid cooling events 70 140 c m y distributed through space and time the cooling histories show a systematic variation along the length of the batholith suggesting that kohistan experienced differential unroofing an episode of rapid cooling in the middle eocene is recognized in western kohistan whereas rapid cooling occurs substantially later during the middle and late miocene in eastern kohistan rapid cooling in western kohistan might have been caused by postemplacement cooling of hot magma against cold country rocks at relatively shallow crustal levels within the nw region of eastern kohistan rapid cooling at 13 12 ma is recorded in k feldspars 60 km from the contact with the nanga parbat haramosh massif nphm whereas cooling does not occur until 11 ma only 2 5 km from the contact this temporal and spatial variation in cooling histories may record differential unroofing in response to the development and propagation of the nphm structure beneath kohistan the nphm has experienced rapid cooling and unroofing over the last 10 m y and our results are consistent with this mid late miocene event

confirmation of arabia plate slow motion by new gps data in yemen 1 during the last 10 years a network of about 30 gps sites was measured in djibouti east africa additional points were also measured in yemen oman ethiopia iran and on la reunion island merged with data from the available international gps service permanent stations scattered on the different plates in the area eurasia anatolia africa arabia somalia this unique data set provides new insight on the current deformation in the africa somalia arabia triple junction area and on the arabian plate motion here we show that coherent motions of points in yemen bahrain oman and iran allow us to estimate a geodetically constrained angular velocity for the arabian plate 52 59 n 15 74 w 0 461 myr in itrf2000 this result differs significantly from earlier determinations and is based upon our vectors in yemen they provide new additional data and better geometry for angular velocity determination combined with the african and somalian motions this new angular velocity results in predicted spreading rates in the red sea and the gulf of aden which are 15 20 lower than those measured from oceanic magnetic anomalies and thus averaged over the last 3 myr with respect to eurasia the geodetic motion of arabia is also about 30 slower than predicted by nuvel 1a on the basis of the kinematic results presented here and on other evidence for a similar slower geodetic rate of the indian plate we suggest that the whole collision zone between africa arabia india on one hand and eurasia on the other hand has slowed down in the last 3 myr

the taiwan ryukyu subduction collision complex folding of a viscoelastic slab and the double seismic zone 1 the termination of the ryukyu trench against eurasia and the oblique subduction of the philippine sea plate create a subduction collision complex offshore taiwan which has not previously been elucidated in detail we combine traveltime data from the seismic networks in taiwan and japan to better illuminate how the subducting ryukyu slab deforms in this subduction collision zone more than 5000 events recorded by both networks were relocated with the double difference method using an optimal regional one dimensional velocity model the offshore seismicity indicates that the double seismic zone with a gap of 15 20 km exists in the subducting slab in the depth range of 40 80 km focal mechanisms suggest that the double seismic zone is caused by east west compression resulting from oblique convergence the improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the ryukyu trench in the depth range 50 100 km the anomalous curvature together with the focal mechanisms suggests that the slab folds against the eurasian lithosphere and that this deformation cannot be fully elastic we model this deformation mode as the developing instability of a viscoelastic maxwell layer embedded in a viscous medium the characteristic wavelength of the instability i e 250 km is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along strike elastic membrane strain as small as 0 01 or more than 3 orders of magnitude higher if 5 elastic strain is allowed

rapid crustal accretion and magma assimilation in the oman u a e ophiolite high precision u pb zircon geochronology of the gabbroic crust 1 new high precision u pb zircon geochronology from the oman united arab emirates u a e ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust the new data come from a well preserved and exposed crustal section in the center of the wadi tayin massif single grain and grain fragment 206pb 238u dates from upper level gabbros tonalites trondhjemites and gabbroic pegmatites corrected for initial th exclusion range from 112 55 0 21 to 95 50 0 17 ma with most data clustered between 96 40 0 17 to 95 50 0 17 ma zircon dates from upper level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half rates of 50 100 km ma dates from tonalites trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper level gabbros suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates three of the dated upper level gabbros and a single gabbroic pegmatite from the base of the crust have 1 ma intrasample variability in single grain dates suggesting assimilation of older crust during the formation or crystallization of the magmas whole rock end t of seven samples including the upper level gabbros with variable zircon dates have tightly clustered initial values ranging from end 96 ma 7 59 0 23 to 8 28 0 31 the end values are similar to those from other gabbros within the ophiolite suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas the new dates suggest that the studied section formed at a fast spreading mid ocean ridge between 96 4 95 5 ma the large intrasample variability in zircon dates in some samples is unexpected in this setting and may be related to propagation of a younger ridge into older oceanic lithosphere

locating earliest records of orogenesis in western himalaya evidence from paleogene sediments in the iranian makran region and pakistan katawaz basin a combination of sediment petrography detrital zircon u pb and fission track dating is used to show that provenance of the paleogene sedimentary rocks exposed in the makran region of southern iran and the katawaz basin of pakistan is consistent with a source from the nascent western himalaya and associated magmatic arc results from this important archive show that paleogene erosion was focused mainly on the arc and northern indian margin and in contrast to the east central himalaya we have not detected widespread rapid exhumation indicative of strong forcing by climate driven erosion at that time

great earthquakes on canada s west coast a review this article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent geological survey of canada contribution 20090196 the first lithoprobe transect in 1984 across vancouver island had primary objectives to define the structure associated with subduction and constraints on the potential for great thrust earthquakes

large scale sediment underplating in the makran accretionary prism southwest pakistan field relations in the emergent part of the makran accretionary prism show that a mid miocene to early pliocene slope and shelf sedimentary sequence was deposited directly on abyssal plain turbidites without any detectable stratigraphic or structural discordance sedimentological evidence for rapid shoaling however indicates that the underlying sediment column was tectonically thickened by a factor of between two and three during this period this can be explained by large scale underthrusting and underplating of sediment a mode of accretion that is also favored by mass balance considerations the visible deformation in the coastal makran occurred when the region was 70 100 km north of the contemporary prism front

quantitative provenance analysis of sediments review and outlook provenance analysis of sediments is aimed at reconstructing the parent rock assemblages of sediments and the climaticphysiographic conditions under which sediments formed inferring sediment provenance from the final product a basin fill is anything but straightforward because the detrital spectrum evolves as the sediment is transported along the pathway from source to basin successful provenance analysis requires that the nature and extent of compositional and textural modifications to the detrital spectrum be recognised if not quantified the history of quantification in sediment provenance studies is summarised and illustrated by tracking two fundamental ideas the concept of the sediment petrological province or petrofacies and the relation between sandstone composition and plate tectonic environment progress in sedimentary provenance analysis has been closely linked with advancements in measurement technology a brief survey of modern data acquisition tools illustrates the possibilities and limitations of modern provenance research an operational definition of quantitative provenance analysis qpa is presented in which the central role of mass balance is acknowledged extension of this definition to include quantitative predictions obtained by forward modelling computer simulation of sediment production as well as methodological improvements in data acquisition and processing is needed to cover likely future developments in qpa the contributions to the special issue quantitative provenance analysis of sediments illustrate the intrinsic multidisciplinarity and rapid expansion of qpa d 2004 elsevier b v all rights reserved

detrital zircon provenance of the late triassic songpan ganzi complex sedimentary record of collision of the north and south china blocks the majority of the songpan ganzi triassic flysch sequence is believed to have derived from denudation of the dabie and sulu ultrahigh pressure uhp metamorphic belt in eastern china e g nie et al 1994 1 zhou and graham 1996 2 however intense debate still exists on the sources of

tectonics of the southern asian plate margin along the karakoram shear zone constraints from field observations and u pb shrimp ages abstract new geological observations recent published data and u pb shrimp zircon dating from the karakoram mountains along the nubra and shyok rivers reveal that the initial subduction of the tethyan oceanic lithosphere took place 110 ma beneath the paleozoic mesozoic platform of the southern edge of the asian plate this has produced the i type plutons within the karakoram batholith complex well before the juxtaposition of the asian plate along the karakoram shear zone within this shear zone u pb zircon crystallisation ages of 75 ma from mylonitised granitoids and 68 ma from undeformed tirit granodiorite constrain the timing of suturing of the karakoram terrain with the trans himalaya between 75 and 68 ma post shearing leucogranite was episodically generated within frontal migmatised karakoram metamorphic belt and emplaced between 20 and 13 ma within the shear zone presence of a low resistivity zone as a possible indication of mid crustal partial molten crust underneath the higher himalaya ladakh karakoram terrains manifests the impingement of the indian plate along the main himalayan thrust at depth physical continuity of the baltoro granite belt into the karakoram batholith is established as well as the continuity of the shyok suture as the shiquanhe suture zone in western tibet through the chushul dungti sector the karakoram shear zone therefore displays a complex geological history of movements since 75 ma and plays a very significant role in the overall india asia convergence rather than merely being a strike slip fault for eastward extrusion of a segment of asia in tibet

aseismic transform fault slip at the mendocino triple junction from characteristically repeating earthquakes author s materna k taira t burgmann r abstract 2018 american geophysical union all rights reserved the mendocino triple junction mtj at the northern terminus of the san andreas fault system is an actively deforming plate boundary region with poorly constrained estimates of seismic coupling on most offshore fault surfaces characteristically repeating earthquakes provide spatial and temporal descriptions of aseismic creep at the mtj including on the oceanic transform mendocino fault zone mfz as it subducts beneath north america using a dataset of earthquakes from 2008 to 2017 we find that the easternmost segment of the mfz displays creep during this period at about 65 of the long term slip rate we also find creep at slower rates on the shallower strike slip interface between the pacific plate and the north american accretionary wedge as well as on a fault that accommodates gorda subplate internal deformation after a nearby m5 7 earthquake in 2015 we observe a possible decrease in aseismic slip on the near shore mfz that lasts from 2015 to at least early 2017

hydrocarbon evaporative loss from shale core samples as revealed by rock eval and thermal desorption gas chromatography analysis its geochemical and geological implications abstract the s1 peak generated by rock eval analysis of source rock and reservoir interval core and cuttings samples has been routinely employed for in place hydrocarbon resources assessment by petroleum geochemists geologists and even engineers significant evaporative loss of gaseous and light hydrocarbons up to c10 can occur during core and cuttings collection and storage and subsequent sample preparation in the laboratory before instrumental analysis consequently a correction to the s1 values is needed in order to obtain a more accurate estimate of the resources to investigate the effect of evaporative loss on both the amount and the composition of the hydrocarbons retained in shale a time series of rock eval and thermal desorption gas chromatography td gc analyses have been carried out on an organic rich devonian duvernay formation shale core sample from the western canada sedimentary basin wcsb and an organic lean ordovician lotbiniere formation shale core sample from quebec canada the results suggest that the content of total organic carbon toc may play an important role in retaining and preserving light hydrocarbons in sedimentary rocks a total loss of gaseous and light liquid hydrocarbons up to c9 is shown to take place within 21 h of sample preparation for the low toc 11 devonian duvernay shale core sample after 360 h of exposure to the open air the powdered duvernay shale sample shows only 15 decline in its rock eval s1 peak and still contains high relative concentrations of c7 c9 hydrocarbons in addition the evaporative loss of some isomers of c5 c7 hydrocarbons from the organic rich shale is not fully in agreement with their gc retention elution behavior but appears to be partially controlled by their adsorption desorption on the organic matter and mineral matrix this suggests that the c5 c8 gasoline range hydrocarbon parameters should be used with caution for oil oil correlation for unconventional shale and tight petroleum systems while the findings in this study validate the current industrial practice that targets organic rich and mature shale intervals for unconventional shale gas and oil exploration because of their large hydrocarbon storage capacity the results also have an important implication that high toc content may adversely affect the production efficiency of shale and tight hydrocarbon reservoirs due to the strong adsorption of hydrocarbons by the high maturity organic matter

internal deformation and compaction of the makran accretionary wedge the makran accretionary wedge is one of the largest on earth a 7 km thick column of sands and quartzolithic turbidites are incorporated into this wedge in a series of deformed thrust sheets we present the results of prestack depth migration and focusing error analysis migration velocity analysis performed on a profile across the makran wedge the depth section shows the deformation style of the accreted sediments and the migration velocities allow us to estimate porosity variations in the sediments the thrust sheets show evidence of fault propagation folding with a long wavelength of deformation 12 km and secondary thrusting in the kink bands of the folds such that the central part of each thrust sheet is elevated to form an additional ridge this deformation style and the 15 steep surface slope of the first ridge suggest a high degree of consolidation porosities were calculated from the seismic migration velocities and the ratio of fluid pressure to lithostatic pressure was estimated for 5 locations along the profile rather than being undercompacted and overpressured as in most accretionary wedges the sedimentary input is normally compacted exponential porosity decay throughout almost the whole wedge however a slight increase in porosity and at depth with respect to the normal compaction curve indicates that the turbiditic sequence might be overpressured landward of the deformation front

the cape mendocino california earthquakes of april 1992 subduction at the triple junction the 25 april 1992 magnitude 7 1 cape mendocino thrust earthquake demonstrated that the north america gorda plate boundary is seismogenic and illustrated hazards that could result from much larger earthquakes forecast for the cascadia region the shock occurred just north of the mendocino triple junction and caused strong ground motion and moderate damage in the immediate area rupture initiated onshore at a depth of 10 5 kilometers and propagated up dip and seaward slip on steep faults in the gorda plate generated two magnitude 6 6 aftershocks on 26 april the main shock did not produce surface rupture on land but caused coastal uplift and a tsunami the emerging picture of seismicity and faulting at the triple junction suggests that the region is likely to continue experiencing significant seismicity

detailed structures of the subducted philippine sea plate beneath northeast taiwan a new type of double seismic zone we studied the detailed structure of the subducted philippine sea plate beneath northeast taiwan where oblique subduction regional collision and back arc opening are all actively occurring simultaneous inversion for velocity structure and earthquake hypocenters are performed using the vast high quality data recorded by the taiwan seismic network we further supplement the inversion results with earthquake source parameters determined from inversion of teleseismic p and sh waveforms a critical step to define the position of plate interface and the state of strain within the subducted slab the most interesting feature is that relocated hypocenters tend to occur along a two layered structure the upper layer is located immediately below the plate interface and extends down to 70 80 km at a dip of 40 50 below approximately 100 km the dip increases dramatically to 70 80 the lower layer commences at 45 50 km and stays approximately parallel to the upper layer with a separation of 15 5 km in between down to 70 80 km below that the separation decreases and the two layers seem to gradually merge into one wadati benioff zone we propose to term the classic double seismic zones observed beneath japan and kuril as type i and that we observed as type ii respectively a global survey indicates that type ii double seismic zones are also observed in new zealand near the southernmost north island cascadia just north of the mendocino triple junction and the cook inlet area of alaska all of them are located near the termini of subducted slabs in a tectonic setting of oblique subduction we interpret the seismogenesis of type ii double seismic zones as reflecting the lateral compressive stress between the subducted plate and the adjacent lithosphere originating from oblique subduction and the downdip extension from slab pulling force the upper seismic layer represents seismicity occurring in the upper crust of a subducted plate and or along the plate interface whereas the lower layer is associated with events in the uppermost mantle

a double seismic zone in the subducting juan fernandez ridge of the nazca plate 32 s central chile 1 the region of central chile offers a unique opportunity to study the links between the subducting juan fernandez ridge the flat slab the double seismic zone dsz and the absence of modern volcanism here we report the presence and characteristics of the first observed dsz within the intermediate depth nazca slab using two temporary seismic catalogs ovalle 1999 and chile argentina seismological measurement experiment the lower plane of seismicity lp is located 20 25 km below the upper plane begins at 50 km depth and merges with the lower plane at 120 km depth where the slab becomes horizontal focal mechanism analysis and stress tensor calculations indicate that the slab s state of stress is dominantly controlled by plate convergence and overriding crust thickness above 60 70 km depth the slab is in horizontal compression and below it is in horizontal extension parallel to plate convergence which can be accounted for by vertical loading of the overriding lithosphere focal mechanisms below 60 70 km depth are strongly correlated with offshore outer rise bend faults suggesting the reactivation of preexisting faults below this depth the large interplane distances for all nazca dszs can be related to the slab s unusually cold thermal structure with respect to its age since lps globally seem to mimic mantle mineral dehydration paths we suggest that fluid migration and dehydration embrittlement provide the mechanism necessary to weaken the rock and that the stress field determines the direction of rupture

intraslab stresses in the cascadia subduction zone from inversion of earthquake focal mechanisms at the cascadia subduction zone intraslab earthquakes occur mostly in the northern part of the margin and its southern end the mendocino triple junction mtj we determine intraslab stress orientations by inverting earthquake focal me chanisms and develop working hypotheses to explain the inferred intraslab stresses and observed seismicity our inversion results show that the subducting juan de fuca jdf slab in northern cascadia is primarily under compression normal to the slab surface and tension in the downdip direction most likely controlled by the net slab pull an exception is a northernmost shallow region near the nootka fault zone where the state of stress is dominated by nearly east west tension we hypothesize that the shear force on the nootka fault zone and margin parallel mantle resistance to slab motion induce the east west tension in this region near the mtj stresses in the jdf plate are dominated by north south compression down to about 20 km depth consistent with a strong push by the pacific plate from south of the mendocino trans form fault but the deeper part of the slab shows downdip tension similar to northern cascadia deviatoric stresses in the jdf slab appear to be very low resulting in very low intraslab seismicity in comparison with northern cascadia the stresses in most of southern cascadia are even lower resulting in nearly no intraslab seismicity

structure and seismotectonics of the mendocino triple junction california 1 we analyze and interpret two marine multichannel seismic reflection onshore offshore ocean bottom seismometer obs seismic profiles that cross in the region of the mendocino triple junction over the fault plane of the 1992 petrolia california ms 7 1 earthquake from the offshore reflection data and the wide angle constraints provided by the onshore offshore and obs data we have determined a rough three dimensional view of crustal and uppermost mantle structure beneath the triple junction region this structure is also consistent with gravity data the upper and mid crust has seismic velocities and densities consistent with accretionary complex rocks of the franciscan terrane a ubiquitous lower crustal layer with velocities of 6 5 7 1 km s 1 is 5 7 km thick throughout most of the region over an area extending at least 25 km to the south and 60 km to the west of the triple junction the lower crustal layer is up to 12 km thick this area is overlain by undisturbed pre 11 ma sediments indicating little deformation since that date the thick lower crust extends beneath the coast suggesting that the deep boundary between pacific and north american terranes lies onshore the northeastern edge of the thick lower crustal layer is marked by concentrated seismicity with the hypocenter of the 1992 petrolia earthquake lying close to the ne corner we speculate that at depth the pacific lower crust acts an asperity acting to concentrate stress from which the petrolia earthquake ruptured

crustal velocity structure north of the mendocino triple junction a 140 km long refraction wide angle reflection profile recently recorded by stanford university and the u s geological survey imaged the subducting gorda slab beneath northern california the profile which is subparallel to the coast from cape mendocino northward indicates that the north american plate is 13 to 14 km thick along the coast north of cape mendocino the crust is characterized by relatively uniform low velocities of 6 km s interpreted as franciscan rocks two strong reflections define the upper and lower boundaries of the subducting gorda crust our data indicate that the subducting gorda crust thickens northward from cape mendocino from 7 km thick just north of cape mendocino to 10 km thick 120 km to the north this change in thickness is coincident with a change in velocity from 6 7 km s south to 6 2 km s north mantle velocities of 7 7 km s are observed for offsets greater than c 80 km we interpret our model to indicate that the gorda slab is not imbricated on a crustal scale beneath our profile that sediments and or a tectonically thickened oceanic layer 2 are present to the north but not in the vicinity of cape mendocino and that proximity to the mendocino triple junction affects the way sediments are subducted

latest cretaceous himalayan tectonics obduction collision or deccan related uplift abstract diverging interpretations and incompatible scenarios have been proposed for the early stages of himalayan history numerous researchers have postulated that northern india was involved in ophiolite obduction arc continent or continent continent collision during the late cretaceous or early paleocene but firm geological evidence was never produced in this article we argue against orogenic events predating the late paleocene when the neotethys ocean was still open the tethys himalayan sedimentary record testifies to anorogenic evolution primarily controlled by dynamic uplift of the passive margin prior to the massive outburst of deccan lavas and eventually followed by thermal subsidence major stratigraphic gaps in pelagic sediments suggest that such tectono magmatic episode started to affect the base of the indian plate in the campanian or possibly even in the santonian 10 to 20 ma before the climax of deccan flood basalt eruptions the abrupt increase in siliciclastic supply and accumulation rates recorded in sedimentary basins all around the indian subcontinent during the maastrichtian was followed by progradation of coastal quartzarenites along the northern indian margin in the early paleocene sandstones derived from the rejuvenated craton and uplifted inner continental margin in the south are dominantly but not exclusively quartzose felsitic volcanic rock fragments and cr spinels many of which with the same geochemical fingerprint as deccan spinels and newly found throughout the maastrichtian to danian succession resisted the combined effect of subequatorial weathering and subsequent diagenesis and testify that detritus from deccan basalts reached the indian passive margin as far as south tibet at the close of the early paleocene india drifted away from the seychelles block and thermal subsidence led to widespread carbonate deposition along the tethys himalaya this article illustrates how mega events of magmatic upwelling followed by lithospheric cooling may control passive margin sedimentation and stratigraphic patterns as occurred in northern india first in the early cretaceous and next in the latest cretaceous paleocene

the transhimalaya gangdese plutonism in the ladakh region a upb and rbsr study abstract the age and origin of the transhimalaya gangdese plutonic belt in the ladakh area has been studied by high resolution u pb analyses of accessory minerals zircon and monazite allanite and rb sr measurements on whole rock samples the ages determined of 101 2m y for a granodiorite and of 60 7 0 4m y for a granite substantiate that the transhimalaya plutonism was active at least in mid cretaceous albian and earliest tertiary palaeocene times a rb sr isochron of 73 4 2 4m y might be fortuitous because the rb sr systematics shows important heterogeneities on the whole rock scale the inherited radiogenic lead in zircon demonstrates that anatexis of continental crust was involved in magma genesis the thus recycled continental material was heterogeneous with respect to its primary ages and or metamorphic history the approximate minimum ages of the continental sources range from 350 to 590 m y the 101 2m y old transhimalaya granodiorite which intrudes a series of mainly basaltic island arc rocks dras series shows that this island arc was attached to the continental margin at that time this is consistent with the hypothesis that the ladakh tethys closed through two subduction regimes 1 an early cretaceous subduction which formed the dras island arc and 2 a late cretaceous to palaeocene subduction at the continental margin which caused the emplacement of the transhimalaya plutonic belt

sampling the oxygen minimum zone off pakistan glacial interglacial variations of anoxia and productivity preliminary results sonne 90 cruise during cruise sonne 90 the well developed oxygen minimum zone omz in the northeastern arabian sea was sampled in great detail to study surface ocean productivity and organic carbon accumulation preservation during the last hundred to hundred thousands of years nearly anoxic conditions on and below the seafloor are indicated by the lack of macrobenthos and nekton absence of bioturbation high marine organic carbon accumulation and preservation of laminated sediments between about 300 and 1000 m water depth on the steep continental slope this pattern approximately coincides with the zone of extremely low o2 concentrations presently observed in the water column accumulation of organic material on the deeper slope near the lower boundary of the omz is enhanced by downslope transport downcore we observe a complex alternation of dark colored laminated and light colored homogeneous facies in general laminated organic carbon rich intervals were deposited under suboxic bottom water and enhanced productivity conditions mainly during warm stages e g bolling allerod and holocene from 8000 yrs b p to recent the homogeneous bioturbated intervals indicate normal bottom water oxygen levels and reduced productivity during or just after cold stages e g terminations ia and b and younger dryas the striking facies similarities of the sedimentary record in cores collected from different areas and water depths suggest a unique pattern of late pleistocene corg accumulation in the northeastern arabian sea which depends mainly on regional surface ocean productivity and intermediate water oxygenation however the sedimentary record is also influenced by variations in sediment supply from the makran and karachi shelf and from indus river discharge during rapid sea level changes

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

u pb age of the baltoro granite northwest himalaya and implications for monazite u pb systematics the baltoro granite is a major late to post tectonic plutonic phase of the karakoram batholith of the northwest himalaya in northern pakistan u pb zircon analyses indicate both emplacement at 21 0 0 5 ma and significant precambrian zircon inheritance dates on monazite are 17 19 ma and are interpreted to have remained near their closure temperature of about 700 c for several million years after emplacement resulting in pb loss by diffusion we suggest that the granite was emplaced into rocks which were at high temperature and that they remained so until late miocene northeast tilting rapid uplift and or tectonic denudation and cooling

case for very low coupling stress on the cascadia ssubduction fault a fundamental problem in plate tectonics is the shear strength of major plate boundary faults this translates to the question whether the generally observed small earthquake stress drops of 3 10 mpa on major faults release most of the accumulated stress or only a small fraction of it there is strong evidence that the san andreas fault a major transform plate boundary is weak 20 mpa shear resistance it is not yet clear whether subduction thrust faults are also weak we present two types of evidence from the northern cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault comparable to that estimated for the san andreas fault first the well defined surface heat flow and heat generation allow negligible frictional heating on the plate interface the average shear stress on the fault must thus be very low over a time scale of a few million years second focal mechanism solutions for small crustal earthquakes in the southern vancouver island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress this inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low one explanation for the weakness of the fault is the presence of near lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence the conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked accumulating strain energy toward a future great earthquake the surface deformation responds to the small 20 mpa temporal changes of the stress field associated with the subduction earthquake cycle this transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin the weakness of the cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards for example a subduction earthquake may induce large strike slip earthquakes in the forearc that affect a large area

stress pattern in the double seismic zone beneath cook inlet south central alaska abstract relocation of south central alaskan earthquakes recorded by the regional seismograph network using the joint hypocenter determination method revealed a double seismic zone in the underthrusting pacific plate beneath the cook inlet area the lower zone is separated from the upper one by a decrease in seismic activity within a 5 10 km wide band and extends from a depth of 50 km to about 90 km stress tensor inversion for earthquakes in the upper and lower zones shows different stress regimes in the upper zone minimum principal stress is oriented in the down dip direction and maximum principal stress is normal to the wadati benioff zone wbz in the lower zone both maximum and minimum principal stresses are horizontal and rotated by 40 60 from the down dip direction of the wbz below 80 90 km depth stress tensor inversion results show down dip orientation of the minimum principal stress and the maximum stress aligned in the direction of convergence of pacific plate

a giant catastrophic mud and debris flow in the miocene makran we challenge the former interpretation of the sedimentary me lange of the makran accretionary complex as a tectonic me lange diapirically emplaced from below and provide evidence for its sedimentary gravitational emplacement from the north during tortonianmessinian times between 11 8 and 5 8 ma it is an olistostrome that includes blocks of ophiolites and oceanic sediments derived from the coloured me lange to the north and reworked chunks of the turbidites on which it rests with an erosional truncation the chaotic scattering of blocks of any size and lithology and the weak soft sediment deformation of the matrix argue against a tectonic emplacement of the chaotic formation its size and internal structure together with the size of the individual blocks make the olistostrome a fossil equivalent of the large gravitationally emplaced debris flows observed along present day continental margins and unstable volcanic edifices

magmatism and metamorphism in the ladakh himalayas the indus tsangpo suture zone abstract ladakh india provides a complete geological section through the northwestern part of the himalayas from kashmir to tibet within this section the magmatic metamorphic and geotectonic evolution of the northern himalayan orogeny has been studied using petrographic geochemical and isotope analytical techniques the beginning of the himalayan cycle was marked by large basaltic extrusions panjal trap of permian to lower triassic age at the northern margin of the gondwana continent indian shield these continental type tholeiitic basalts were followed by a more alkaline volcanism within the triassic to jurassic lamayuru unit of the gondwana continental margin lower jurassic to cretaceous oceanic crust and sediments ophiolitic melange s s accompany the triassic to cretaceous flysch deposits within the indus tsangpo suture zone the major structural divide between the indian shield high himalaya and the tibetan platform so far no relic of paleozoic oceanic crust has been found subduction of the tethyan oceanic crust during upper jurassic and cretaceous time produced an island arc represented by tholeiitic and calc alkaline volcanic rock series dras volcanics and related intrusives accompanied by volcaniclastic flysch deposits towards the tibetan continental margin subsequent to the subduction of oceanic crust large volumes of calc alkaline plutons trans himalayan or kangdese plutons intruded the tibetan continental margin over a distance of 2000 km and partly the dras island arc in the ladakh region the collision of the indian shield and tibetan platform started during the middle to upper eocene and caused large scale still active intracrustal thrusting as well as the piling up of the himalayan nappes the tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle spongtang klippe in the high himalayas the tectonic activity was accompanied and outlasted by a barrovian type metamorphism that affected triassic sediments of the kashmir nun kun synclinorium up to kyanite staurolite grade and the deeper seated units up to sillimanite grade cooling ages of micas are around 20 m y muscovite and 13 m y biotite towards the indus tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist prehnite pumpellyite to zeolite grade remnants of possibly an eo himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

a kinematic model for the gorda deformation zone as a diffuse southern boundary of the juan de fuca plate several lines of evidence indicate that the space problem caused by southeastward motion of the juan de fuca plate toward the east west mendocino transform is resolved by distributed deformation of the oceanic crust north of the transform a kinematic model for this deformation based on the assumption of nondivergent flow conservation of area within the area known as the gorda plate and on the assumption of no subduction along the mendocino transform successfully matches the isochron pattern inferred from magnetic anomalies the model assumes strain rate decreasing to zero approaching the rigid juan de fuca plate with no sharp boundary or separate rigid area that would imply a discrete gorda plate this interpretation allows magnetic anomalies from south of the blanco fracture zone to be used to constrain the kinematics of the juan de fuca plate these anomalies imply a substantial change in the distance to the rotation pole at roughly 3 ma in addition to the clockwise shift in motion direction at 5 ma

jurassic rifting at the eurasian tethys margin geochemical and geochronological constraints from granitoids of north makran southeastern iran this study focuses on an east west trending belt of granitic to intermediate intrusions and their volcanic cover in the northern dur kan complex a continental slice outcropping to the north of the exposed makran accretionary wedge in southeastern iran field observations petrographic descriptions trace element and isotope analyses combined with u pb zircon geochronology are presented to determine the time frame of magmatism and tectonic setting during the formation of these rocks results document three magmatic episodes with different melt sources for 1 granites 2 a diorite trondhjemite plagiogranite sequence and 3 diabases and lavas granites dated at 170 175 ma represent crystallized melt with a strong continental isotopic contribution the diorite trondhjemite plagiogranite sequence is 165 153 ma old and derives from a mantle magma source with minor continental contribution east west trending diabase dikes and bodies intruded the granitoids which were eroded and then covered by valanginian 140 133 ma alkaline lavas and sediments alkaline dikes and lavas have a mantle isotopic composition temporal correlation with plutonites of the sanandaj sirjan zone to the northwest defines a narrow nw se striking and nearly 2000 km long belt of jurassic intrusions the increasing mantle influence in the magma sources is explained by thinning of continental lithosphere and related mantle upwelling decompression melting accordingly the formation of the studied igneous rocks is related to the extension of the iranian continental margin which ultimately led to the formation of the tethys related north makran ophiolites

seismicity of the gorda plate structure of the continental margin and an eastward jump of the mendocino triple junction analysis of 9 years of data from the humboldt bay seismic network sheds new light on the structure and evolution of the gorda plate and mendocino triple junction significant findings include the buttressing effect of the pacific plate which demonstrates that there is no underthrusting of the gorda plate along the mendocino fault and the pattern of left lateral northeast trending faults which demonstrates how the gorda plate accommodates n s shortening without such underthrusting focal mechanisms are consistent with northward compression of the gorda plate by the pacific plate until the gorda plate passes the triple junction beyond which the n s compressive stress is effectively removed and the focal mechanisms show a change from strike slip to normal faulting with downslab tension another significant feature is the shallow benioff zone 10 40 km which shows a double seismogenic layer unlike most other double seismic layers in deep subduction zones this one appears to be due to underplating with a new subduction zone being developed east of the former one a corner of continental margin material has been partially subducted we attribute this double seismic zone to reactivation of both the old and new subduction boundaries under n s compression produced by pacific gorda plate interaction at the triple junction because the geometry of this triple junction indicates that it is unstable and because there is evidence that as part of its evolution the san andreas fault has migrated eastward we have constructed a model that accounts for both the eastward migration of the san andreas fault and the doubling of the seismic zone in the gorda plate this can be done by assuming that at the time of the last eastward jump of the fault the overriding continental margin north of the triple junction was broken and underthrust producing a new subduction zone that is collinear with the san andreas fault other scenarios for the evolution of the mendocino triple junction do not require that the subduction zone migrate eastward however the position and dip of the double zone indicate that an eastward jump of 100 km some 5 my ago could have taken place this is consistent with other evidence for an eastward jump of the san andreas fault

u pb zircon ages from the spontang ophiolite ladakh himalaya the spontang ophiolite in the ladakh himalaya consist of a thrust sheet of upper mantle and oceanic crustal rocks obducted onto the northern passive continental margin of india during the late cretaceous precise isotopic ages of himalayan ophiolites are presently unavailable u pb dating of zircons from a dioritic segregation in the high level gabbros of the spontang ophiolite shows that the complex formed at 177 1 ma mid jurassic and we suggest that it represents a fragment of neo tethyan oceanic crust this jurassic age contrasts with late cretaceous ages 91 95 ma previously reported for the formation of the troodos and oman ophiolite complexes further west along the neo tethyan suture zone and also predates the early to mid cretaceous masirah ophiolite c 150 ma and its alkaline volcanic seamounts 115 125 ma an andesitic rock from an arc sequence overlying the spontang ophiolite was also dated using as 88 5 ma this late cretaceous age constrains the minimum age of initiation of subduction beneath the spontang ophiolite the complex was emplaced during the turonian maastrictian some 80 ma after its formation when the indian continental margin collided with a cretaceous arc about 25 ma before the india asia collision the lack of a sub ophiolite metamorphic sheet beneath the spontang mantle sequence may be explained by the fact that the ophiolite was old and cold at the time of obduction

constraints on the collision and the pre collision tectonic configuration between india and asia from detrital geochronology thermochronology and geochemistry studies in the lower indus basin pakistan abstract knowledge of the timing of india asia collision is a fundamental prerequisite for understanding the evolution of the himalayan tibetan orogen and its role in global climate oceanic chemistry and ecological evolution despite much active research the basic pre collision tectonic configuration and the timing of terminal india asia suturing remain debated for example debates regarding when and how the intervening kohistan ladakh arc was sutured with india and asia still remain elusive some models propose the arc collided with asia at about 100 ma with india asia collision at ca 55 ma whilst a newer model proposed the arc s collision with india at 50 ma and subsequently with asia at 40 ma another example is the recent proposition that an oceanic greater india basin separated the tethyan himalaya microcontinent from the remaining indian plate until 20 25 ma with the consumption of this oceanic basin marking the final collision at this time these controversies relate to whether the commonly documented 50 ma contact represents the terminal india asia suturing or the amalgamation between various arcs or microcontinents with india or asia here we present an integrated provenance study of geochronology thermochronology and geochemistry on the late cretaceous pleistocene sediments from the lower indus basin on the indian plate the detrital zircon u pb and fission track data show a reversal in sediment source from a pure indian signature to increasing inputs from the suture zone and the asian plate between the middle paleocene and early oligocene the nd and sr isotopes narrow down this change to 50 ma by revealing input of asian detritus and the establishment of a nd sr isotopic pattern similar to the present day indus fan by 50 ma with no significant variations up section contrary to what might be expected if later major collisions had occurred our isotopic data indicate that greater india was occupied by a fluvial deltaic system analogous to the present day indus and named as the paleo indus which has been transporting asian detritus southward across the suture zone and kohistan ladakh arc since 50 ma suggesting no other ocean basins intervened between india and asia after this time in this region our data require that in the west the india asia collision were accomplished by 50 ma

morphotectonics and mechanics of the central makran accretionary wedge off pakistan abstract swath bathymetry images and 3 5 khz data acquired during rv sonne cruise 123 in autumn 1997 are the base for analysing the morphology and deformation styles of the huge makran accretionary wedge the continental slope off pakistan is characterised by three morphological provinces a the lower slope is built of a sequence of imbricate thrust slices which form long and narrow accretionary ridges with flanks locally as steep as 20 and between 10 and 20 on average while the regional slope is only 1 2 b the nearly flat mid slope terrace is present in the central east part of the margin narrows significantly towards the east and is absent east of 63 45 and c the upper slope up to the shelf has a regionally uniform slope as steep as about 8 and comprises a very rough topography with many gullies and canyons and evidence for mass wasting two meandering canyons crossing the entire margin down to the abyssal plain at about 63 15 63 30 can be correlated with onshore rivers they have been formed by episodic erosion through turbidity flows and are characterised by erosional and depositional portions sinuosity of the canyons is exclusively caused by the accretionary ridges functioning as obstacles for flow to be directly downhill application of coulomb rheology to the frontal part of the makran wedge implies that the mid level decollement is intrinsically extraordinary weak with a great strength contrast to the overlying sediments comparison with nankai cascadia and the western mediterranean ridge reveals new insights in the parameters controlling the shape of an accretionary wedge and the role of a mid level decollement

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

confidence intervals for motion and deformation of the juan de fuca plate finite rotation parameters for 17 isochrons from anomalies 1 4a for the juan de fuca pacific plate pair are reported comparison of confidence intervals for plate rotations derived by different techniques indicates that a single plate misfit criterion significantly underestimates the confidence region defining the confidence interval by a threshold level of misfit determined by an f statistic gives similar results to the technique of bootstrap resampling for internally consistent data bootstrap resampling offers far greater robustness in the case of inconsistent data precision of about 1 km in the direction normal to isochrons is available for the younger anomalies this precision allows confident recognition of 2 5 km of eastward deformation of the pacific plate 50 100 km south of the active sovanco transform slight deformation of the northern juan de fuca plate adjacent to the nootka fault zone is also apparent east of the northern gorda ridge younger 3 ma anomalies do not show measurable deformation near the blanco fracture zone but older anomalies are up to 30 km east of their position predicted by rigid behavior of the juan de fuca plate reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km even with offset of 75 km in one case and 140 km in another new techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99 confidence a clockwise change in motion direction at about 5 ma and an increase in the gradient of spreading rate since 2 ma two other changes in gradient since 5 ma are required at 95 confidence

evidence for both crustal and mantle earthquakes in the subducting juan de fuca plate 1 we investigate the relationship between high precision hypocentres in the subducting juan de fuca plate beneath southwest british columbia and the velocity structure from receiver function analysis and reflection seismic data 108 earthquakes at depths of 40 to 65 km were relocated using a double difference algorithm to obtain precise earthquake locations correlation of the relocated seismicity with structural information shows a concentration of earthquakes near the top of the subducting oceanic crust and a deeper layer of seismicity in the uppermost mantle of the subducting juan de fuca plate the strong correlation of the seismicity with the structure of the subducting plate suggests that seismic failure is caused by changes in mechanical strength within the plate supporting the hypotheses that phase transformations and thermal petrological conditions play an important role in the seismogenesis of double seismic zones index terms 7209 seismology earthquake dynamics and mechanics 7218 seismology lithosphere and upper mantle 7230 seismology seismicity and seismotectonics 7220 seismology oceanic crust citation cassidy j f and f waldhauser evidence for both crustal and mantle earthquakes in the subducting juan de fuca plate geophys res lett 30 2 1095 doi 10 1029 2002gl015511 2003

slip vectors and fault mechanics in the makran accretionary wedge southwest pakistan the makran is a broadly east west trending arcuate accretionary wedge currently forming where the oceanic floor of the gulf of oman is being subducted north northeastward beneath the south asian margin two traverses across the makran coast ranges extending about 80 km inland from the coastline which lies about 100 km north of the wedge front show that the structure of this region is dominated by large back rotated south directed thrust faults and associated folds related structures include duplexes fault gouge containing reidel shears and melange along fault zones these structures are cut by later north directed backthrusts out of sequence south directed thrusts and apparently conjugate sets of nne trending sinistral and nw trending dextral strike slip faults slip vectors were determined from slickenlines fiber lineations gouge fabrics and riedel shears vectors from early thrusts and bedding parallel slip surfaces corrected for tectonic rotation form a broad south directed concentration lying between the normal to the regional strike varying from 160 to 180 and the present plate convergence vector 200 locally the concentration is bimodal this pattern may represent the interaction between plate forces and body forces in the accretionary wedge the late thrusts have widely variable slip vectors and stair step off the early thrusts they overlap in orientation and slip vector with gently dipping sinistral strike slip faults suggesting that they also stair step in plan off the latter the steep strike slip faults represent the final tectonic stage involving minor n s shortening and longitudinal extension these three stages probably occurred in sequence in any one area but migrated diachronously southward as the wedge grew

deformation of the so called gorda plate a new compilation of magnetic anomaly identifications recent side scan sonar data and other data provide constraints on the mechanism of deformation of the area commonly known as the gorda plate a sharp boundary between the rigid juan de fuca plate and deformed material trends southeast from the northern part of the gorda ridge in material older than 2 3 ma the boundary is gradational and trends more to the east previous observations that the anomalies east of the ridge are shorter than their counterparts on the pacific plate are confirmed there is no evidence for subduction or obduction along the mendocino transform adjacent to the ridge the deformation is interpreted to be distributed nw se right lateral simple shear possibly controlled by ductile deformation of the lower crust deformation of this sense is capable of producing the observed shortening the anomalies away from the ridge apparently in crust of age 2 3 ma there must be a transition to deformation on observed ne sw left lateral faults parallel to the original spreading fabric

sandstone detrital modes in the makran accretionary wedge southwest pakistan implications for tectonic setting and long distance turbidite transportation detrital modes of early miocene to early pliocene sandstones from the makran accretionary wedge in southwest pakistan show a mainly quartzolithic composition with an evolution from the transitional recycled to quartzose recycled the lithic types however indicate two distinct petrofacies accreted abyssal plain turbidites have qp11lvm27lsm62 and lm39lv27ls34 showing a predominant supply from sedimentary and metasedimentary source terranes whereas slope and shelf facies sediments deposited on the accretionary wedge have qp7lvm47lsm47 and lm22lv48ls30 due to an increase of volcanic detritus the detrital modes of the abyssal plain sediments suggest a recycled orogenic source probably the himalayan collision zone the facies and longitudinal dispersal pattern suggest deposition in an oligo miocene analogue of the present indus fan the sediment must have been transported across strike parallel to the transform structure linking the makran wedge to the himalayas chaman ornach nal fault system and fed into the fan at the western end of the subduction zone the detrital modes also show an increase in volcanic detritus with time lv l 0 27 for the early miocene abyssal plain sediments to 0 47 for the slope sequences this may have been derived from late mesozoic volcanic terrains in northern baluchistan or the ladakh himalayas or more probably from the early to middle miocene andesitic volcanic centre in the northern makran

himalayan detrital chromian spinels and timing of indus yarlung ophiolite erosion abstract the geochemistry of detrital chromian spinels is commonly used to discriminate provenance from different tectonic settings of mafic and ultramafic igneous rocks detrital spinels in cenozoic foreland basin successions fed from the himalaya orogen were assertively interpreted as sourced from the ophiolitic rocks of the indus yarlung suture zone this study compares the geochemistry of detrital cr spinels from the tethys himalaya passive margin and cretaceous xigaze forearc successions with those from the indus yarlung ophiolites cr spinels in the indus yarlung ophiolites have low tio2 mostly 0 2 and low al2o3 mainly 6 23 indicating a rift related basaltic origin detrital cr spinels from the xigaze forearc basin have either low tio2 mostly 1 0 indicating intra plate basaltic origin compositional fingerprints of detrital cr spinels from lower eocene foreland basin strata in the central eastern himalaya indicate provenance from the lhasa block without input from the indus yarlung ophiolites only cr spinels from the lower eocene foreland basin strata in the north western himalaya and the upper eocene lower miocene remnant ocean turbidites of the bengal basin are mostly ophiolite derived the indus yarlung ophiolites were thus emplaced and exposed to erosion since the early eocene 50 ma in the nw himalaya but only subsequently 50 38 ma in the eastern himalaya

a 5000 yr record of climate change in varved sediments from the oxygen minimum zone off pakistan northeastern arabian sea the upper holocene marine section from a kasten core taken from the oxygen minimum zone off karachi pakistan at water depth 700 m contains continuously laminated sediments with a sedimentation rate of 1 2 mm yr and a unique record of monsoonal climatic variability covering the past 5000 years our chronostratigraphy is based on varve counts verified by conventional and ams 14 c dating individual hemipelagic varve couplets are about 0 8 1 5 mm thick with light colored terrigenous laminae a deposited mainly during the winter monsoon alternating with dark colored laminae b rich in marine organic matter coccoliths and fish debris that reflect deposition during the high productivity season of the late summer monsoon august october precipitation and river runoff appear to control varve thickness and turbidite frequency we infer that precipitation decreased in the river watershed indicated by thinning varves after 3500 4000 yr b p this is about the time of increasing aridification in the near east and middle east as documented by decreasing nile river runoff data and lake level lowstands between turkey and northwestern india this precipitation pattern continued until today with precipitation minima about 2200 1900 yr b p 1000 yr b p and in the late middle ages 700 400 yr b p and precipitation maxima in the intervening periods as documented by spectral analysis the thickness of varve couplets responds to the average length of a 250 yr cycle a 125 yr cycle the gleissberg cycle of solar activity 95 yr and a 56 yr cycle of unknown origin higher frequency cycles are also present at 45 39 29 31 and 14 yr the sedimentary gray value also shows strong variability in the 55 yr band plus a 31 yr cycle because high frequency cyclicity in the enso band ca 3 5 and 5 yr is only weakly expressed our data do not support a straightforward interaction of the pacific enso with the monsoon driven climate system of the arabian sea

newly discovered paleogene deltaic sequence in katawaz basin pakistan and its tectonic implications in the katawaz basin pakistan the deltaic and turbidite facies of the khojak formation are the paleogene analogue of the modern indus river emptying into the arabian sea to form the indus delta fan system facies identified include upper continental slope to prodelta distal to proximal distributary mouth bar distributary channel interdistributary bay estuary fluvial channel natural levee and flood plains of lower and upper delta plains the proposed model is a fluvial dominated wave modified delta that axially fed khojak submarine fan turbidites exposed in the southwestern part of the basin these sediments were eroded from the early himalayan orogenic highlands and transported southwestward down the axis of the katawaz remnant ocean basin

three dimensional dislocation model for great earthquakes of the cascadia subduction zone there have been no historical cascadia great subduction thrust earthquakes but there is good recent evidence that very large earthquakes have occurred in the past and that strain is building up toward a future great event geodetic measurements in the coastal region from northern california to southern british columbia show vertical and horizontal deformation as expected for the strain accumulation of a locked thrust fault the segment of the subduction thrust that is locked and may rupture in future great events has previously been estimated through two dimensional 2 d elastic dislocation modeling of interseismic deformation geodetic data in this study a general 3 d dislocation model for thrust faults has been developed that accommodates curved fault geometry and nonuniform interseismic locking or coseismic rupture the model is based on die surface deformation due to shear faulting in an elastic half space the 3 d model of the cascadia subduction zone calculates the surface deformation for a locked zone or a rupture zone of variable width along the margin the bend in the margin trend and subducting slab end effects are included there is a downdip transition zone between interseismic completely locked and free slip portions of the fault or between coseismic full rupture and no displacement an initial 3 d model based upon 2 d dislocation models and upon thermal constraints was adjusted to optimize the fit of the predicted interseismic surface deformation to current deformation geodetic data the best fit model has the thrust locked along the whole margin with an average locked zone width of 60 km and a transition zone width of 60 km the two zones lie mainly offshore beneath the continental shelf and slope the locked and transition zone widths vary smoothly along the margin being greater off northern washington where the thrust dip is shallow and narrower off central oregon assuming that the locked plus transition zones approximate the maximum coseismic rupture area these widths permit amw 9 earthquake

importance of the lu hf isotopic system in studies of planetary chronology and chemical evolution the 176lu 176hf isotope method and its applications in earth sciences are discussed greater fractionation of lu hf than sm nd in planetary magmatic processes makes 176hf177hf a powerful geochemical tracer in general proportional variations of 176hf177hf exceed those of 143ndl44nd by factors of 1 5 3 in terrestrial and lunar materials lu hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time and this is the principal importance of lu hf new data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in 176hf177hf 87sr86sr and 143ndl44nd 87sr86sr diagrams 176hf177hf and 143nd144nd display a single linear isotopic variation in the suboceanic mantle these discordant 87sr86sr relationships may allow with the acquisition of further hf nd sr isotopic data a distinction between processes such as mantle metasomatism influence of seawater altered material in the magma source or recycling of sediments into the mantle in order to evaluate the hf nd isotopic correlation in terms of mantle fractionation history there is a need for measurements of hf distribution coefficients between silicate minerals and liquids and specifically for a knowledge of hf behavior in relation to rareearth elements for studying ancient terrestrial hf isotopic variations the best quality hf isotope data are obtained from granitoid rocks or zircons new data show that very u pb discordant zircons may have upwardly biased 176hf177hf but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial 176hf177hf until the controls on addition of radiogenic hf to zircon are understood combined zircon whole rock studies are recommended lu hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples but because it offers little advantage over existing methods is unlikely to find wide application in pure chronological studies

a tectonic pulse in the makran accretionary prism recorded in iranian coastal sediments many recent studies of syndeformational sediments in a wide range of tectonic environments have improved our understanding of the development of individual or regional deformation structures here we report studies of growth strata that imply an unexpected tectonic history within part of the makran accretionary prism currently the largest on earth we interpret the lithostratigraphy and ichnofacies of plio pleistocene marine deposits logged along part of the makran coast in terms of their palaeoenvironments lateral trends in thickness and depositional facies of rock units and erosional surfaces among them are correlated and then related to displacements along syndepositional faults that are distinguishable from changes in global sea level past studies of accretionary prisms have emphasized thrusting at the deformation front offshore however at the shoreline we find episodes of deposition during subsidence as a result of oblique slip normal faulting alternating with episodes of erosion during uplift driven by oblique slip thrusting the absolute timing of these displacements is not yet known however faults above a deep zone of subcretion or underplating well behind the surficial deformation front have clearly undergone episodic reversals in throw that imply alternations in lateral shortening and extension

temperatures heat flux and frictional stress near major thrust faults we examine the two dimensional advective and conductive transport of heat in a region of thrust faulting both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor s 1 b z vsin below what they would be with the same heat sources but in the absence of advection in this expression z is the depth to the fault v is the slip rate is the component of the dip of the fault in the direction of underthrusting is thermal diffusivity and b is a dimensionless factor that is essentially equal to one for most forms of heating initial changes in temperatures near the fault are given by t z t t z 0 12vsin t z 0 z these simple formulae are successful because of the neglible influence of lateral conduction of heat at least for slip rates of a few mm yr or more two time constants govern the transition from the initial change in temperature to steady state t1 u v where u is the distance along the fault in the direction of underthrusting from the surface to the depth in question and t2 z 2 2 where z is the depth to the fault when elapsed times exceed the sum of these two time constants temperatures differ from their steady state values by only about 10 percent the numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted putain an application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 mpa and are greater than 30 mpa calculations of temperatures appropriate for the himalaya suggest that shear stresses of 100 mpa on the main central thrust probably are required to account for the tertiary granites of the region if melting took place after slip began on the thrust similarly the cut off in seismicity at a depth of about 15 km in the himalaya if due to temperatures exceeding 350 to 450 c implies a deviatoric stress close to 100 mpa

zircon u pb and hf isotopic constraints on petrogenesis of the cretaceous tertiary granites in eastern karakoram and ladakh india abstract multiple subduction events occurred preceding the collision of greater india with the karakoram terrane that consumed the neotethyan ocean between early cretaceous and eocene time zircon u pb dating and hf isotope analyses of granitic rocks collected from the karakoram terrane and ladakh batholith have been conducted to constrain these geological processes undeformed ladakh batholith granodiorite yielded zircon u pb ages of 68 and 64 ma with ehf t values of 6 4 and 10 3 and three diorites near leh and darbuk gave zircon u pb ages of 50 4 51 0 ma and ehf t values of 7 4 to 8 9 indicating that the paleocene eocene was the most important period of granitic magmatism that derived from a juvenile source three tonalite granodiorite enclaves at skyangpoche within the karakoram batholith have zircon u pb ages of 101 103 ma with ehf t values of 2 3 to 3 7 indicating generation of granitic rocks by partial melting of the karakoram basement three samples of the miocene karakoram batholith from around darbuk yielded ages of 16 6 19 1 ma with ehf t values of 8 5 and 10 5 that indicate an input of ancient crust from the indian plate granodiorite within the karakoram metasedimentary sequence zircon u pb age 74 ma ehf t of 5 4 from the tangtse gorge and granite around muglib zircon u pb age of 63 and 56 ma ehf t of 6 8 and 6 0 indicate similar geochronological and isotopic characteristics to those of the ladakh batholith thus the time resolved hf isotopic compositions of magmatic zircons from the granites in the karakoram and ladakh batholith show a large variation that indicate their derivation from both ancient and juvenile continental crusts in active continental margin since at least the mid cretaceous

age dependence of oceanic intraplate seismicity and implications for lithospheric evolution we have determined depths for 16 oceanic intraplate earthquakes using body wave modeling a data set composed of these and 11 other well constrained depths shows that the maximum depth of seismicity deepens with increasing lithospheric age and appears to be bounded by a 700 800 c isotherm the maximum depth to which intraplate earthquakes occur is approximately equal to the flexural elastic thickness but much less than the seismic thickness from surface wave dispersion the maximum faulting depth is consistent with the predictions of rapid weakening at high temperatures for dry olivine rheologies determined by experimental rock mechanics oceanic intraplate earthquakes can occur at greater depths and temperatures than has previously been reported for continental crustal events presumably because oceanic lithosphere is stronger at high temperatures both the number of earthquakes and the cumulative seismic moment decrease with lithospheric age the cause of this trend is unclear possibilities include nonrandom location of preexisting weak zones with respect to the age provinces fault strengthening with age or a decrease in the stress level a decrease in intraplate stress with age is opposite that expected from ridge push stresses which increase with age but may result either from thermoelastic stresses which are greatest in young lithosphere or near ridge stress concentrations due to local heterogeneities in the spreading process

detrital zircon and provenance analysis of late cretaceous miocene onshore iranian makran strata implications for the tectonic setting a multidisciplinary provenance study including sandstone framework heavy mineral analysis in situ u pb dating of detrital zircon and hf isotopic ratio analysis of dated zircons was undertaken on late cretaceous miocene deep marine turbiditic and deltaic sandstones of makran accretionary wedge se iran to determine their sedimentary provenance and tectonic setting sandstone framework modes reveal both magmatic arc rocks as a source of late cretaceous oligocene detritus and recycling of miocene sandstones heavy mineral assemblages cr spinel and blue amphibole indicate ophiolite and high pressure low temperature metamorphic rocks blueschists as a supplementary provenance in total 2931 laser ablation inductively coupled plasma mass spectrometry icp ms u pb detrital zircon ages on 21 sandstone samples yielded three major age peaks at ca 167 ma 88 7 ma and 48 9 ma also 241 in situ hf isotope analyses of dated zircons provide evidence for dominantly igneous source rocks two main detrital zircon ages are identified 1 abundant middle jurassic grains with hf isotopic compositions of continental crust suggesting a rifting related magmatic provenance and 2 late cretaceous eocene grains with hf isotopic compositions of continental crust and nondepleted mantle suggesting a continental magmatic arc provenance this change in provenance is attributed to the late cretaceous convergence between arabia and eurasia

juan de fuca slab geometry and its relation to wadati benioff zone seismicity 1 a new model of the subducted juan de fuca plate beneath western north america allows first order correlations between the occurrence of wadati benioff zone earthquakes and slab geometry temperature and hydration state the geo referenced 3d model constructed from weighted control points integrates depth information from earthquake locations and regional seismic velocity studies we use the model to separate earthquakes that occur in the cascadia forearc from those that occur within the underlying juan de fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes seismicity within the slab is most prevalent where the slab is warped beneath northwestern california and western washington suggesting that slab flexure in addition to expected metamorphic dehydration processes promotes earthquake occurrence within the subducted oceanic plate earthquake patterns beneath western vancouver island are consistent with slab dehydration processes conversely the lack of slab earthquakes beneath western oregon is consistent with an anhydrous slab double differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern california that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 m7 1 mendocino earthquake we infer that this earthquake ruptured a surface within the cascadia accretionary margin above the juan de fuca plate we further speculate that this earthquake is associated with a detached fragment of former farallon plate other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes

multiple mantle sources during island arc magmatism u pb and hf isotopic evidence from the kohistan arc complex pakistan this paper presents high precision u pb ages and initial hf isotopic compositions of zircon from mafic to felsic rocks of the kohistan arc complex pakistan three magmatic pulses tapping geochemically different reservoirs are distinguished partial melting of mantle with morb type isotopic characteristics generated 99 92 ma old magmas plutonism around 85 ma tapped a more fertile mantle source most likely consisting of a 600 ma old metasomatically enriched mantle or of mantle contaminated by an old sedimentary component 82 ma old felsic peraluminous dykes have morb type isotopic compositions considered to be inherited from remelting earlier magmas in the deep base of the arc the isotopic results demonstrate several and rather rapid changes in melt source region during arc development they also show that there was subordinate continental influence and negligible importance of slab components for the hf budget during the generation of the kohistan arc complex

rate of crustal shortening and non coulomb behaviour of an active accretionary wedge the folded fluvial terraces in makran se iran abstract we surveyed fluvial terraces to decipher the quaternary increment of crustal shortening and shortening rate in the on shore makran accretionary wedge we focused on three major catchment basins and associated fold systems terrace profiles reconstructed from differential gps measurements combined with dem revealed two regional dominant wavelengths about 5 km in the northern part of the study area and about 15 km to the south these two wavelengths suggest the existence of two active decollement layers at two rooting depths the average shortening rate due to folding is estimated at 0 8 1 2 mm a over the last 130 ka this accounts for 10 15 of the shortening rate 8 mm a given by kinematic gps measurements between chabahar and bazman and 3 of the convergence between arabia and eurasia across the makran subduction zone despite active deformation and a relatively high shortening rate the geophysical record shows nearly absent seismic activity in makran we propose that strain accumulated in folds over intermediate decollement levels within a thick incompletely lithified sedimentary cover explains the essentially aseismic recent tectonics in this region the importance of folds points to imperfect coulomb behaviour of the wedge

granites of western karakorum and northern kohistan pakistan a composite mid cretaceous to upper cenozoic magmatism abstract the petrography the chemical mineralogical typology and the ages of six plutonic units four from the karakorum axial batholith darkot pass ghamu bar batura and hunza units and two of northern kohistan gindai and nomal plutons are defined and compared based on field data and analyses of up to 78 samples major elements ree rb s sbnd sr and k ar isotopes the karakorum axial batholith is a composite body three major intrusive stages occurred 1 around mid cretaceous times ca 110 95 ma with the emplacement of subalkaline i e monzonitic darkot pass and calc alkaline hunza ghamu bar units 2 during palaeogene maybe up to 43 ma batura subalkaline unit a strong tectonommetamorphic event recorded in the gneissification of the cretaceous intrusives occurred between these two stages it may be of palaeocene age p t estimates of the highest metamorphic grade rocks of the hunza unit have yielded values of 580 640 c and 5 0 5 kbar and 3 during upper miocene ca 9 ma baltoro subalkaline unit debon et al 1986c in addition a conspicuous network of aplo pegmatitic dykes emplaced into the hunza area possibly from the eocene up to the upper cenozoic with a maximum during the middle miocene ca 15 ma most of these major magmatic stages are met again among the acidic intrusives and dykes of northern kohistan the first one as blastomylonitic tholeiitic plutons nomal ca 102 ma petterson and windley 1985 the second one as subalkaline plutons gindai ca 59 ma and a third one as leucocratic dykes of oligocene age ca 30 ma petterson and windley 1985 these data may be related to the geodynamic evolution of the nw part of the india eurasia suture zone thus allowing better constraints on the major steps of this evolution the partly synchronous closures by n dipping subduction of the two tethys branches which are assumed to have encircled the kohistan arc in upper mesozoic times may have generated both the karakorum and the kohistan intrusives of cretaceous and palaeogene ages the northern branch very likely closed before the southern one at the time of the second intrusive stage palaeogene a kohistan and karakorum had already collided were welded and had suffered the same major tectonometamorphic event b subduction of the southern tethys floor beneath the welded kohistan karakorum was still active c however collision between india and kohistan karakorum may have already begun particularly at the level of the nanga parbat promontory finally it is emphasized that the intrusive processes continued in the karakorum long after the collision e g baltoro granite

correlation of fluvial terraces and temporal steady state incision on the onshore makran accretionary wedge in southeastern iran insight from channel profiles and 10be exposure dating of strath terraces we describe and summarize the geomorphology of fluvial terraces along the four major rivers draining the central onshore makran accretionary wedge in iran and describe uplifted marine terraces on the coast of this region thirty five strath terraces at different sites were dated using in situ produced cosmogenic 10 be concentrations from surfaces and depth profiles these new measurements reveal abandonment ages between ca 15 and 380 ka the age distribution allows determining the chronology of terrace levels and establishing regional correlations between two major regional levels the geographically widespread correspondence suggests that these two levels result from a regional climatically driven force systematic dating also provides evidence for time and spatial variations in incision rate which enables distinguishing between a regionally uniform incision rate of 0 3 0 4 mm a and higher local incision and or uplift 0 8 mm a rates these spatial changes are consistent with where localized tectonic activity is recognized in the field we also dated using 14 c in shells four uplifted marine terraces on the coast of the study area comparing incision rates derived from strath terraces 0 3 mm a with published uplift estimates from marine terraces 0 2 mm a reveals that fluvial rivers responded to a regional long term incision and surface uplift pattern this rate reflects the tectonically steady state of the wedge on a regional scale locally high incision rates delineate active faults and folds indicating that perfect steady state is unlikely on short length scales

the northern limit of the subducted juan de fuca plate system analysis of data recorded at an array of three component broadband seismograph stations deployed on northern vancouver island and the adjacent british columbia mainland at the northern end of the cascadia subduction zone provides the first constraints on the s wave velocity structure of this region and permits us to define the northern limit of the subducted juan de fuca plate system during a 2 year period more than 80 teleseisms were recorded at our five stations the method of receiver function analysis was used to constrain the s velocity structure to upper mantle depths beneath the northern three stations a relatively simple continental crust is interpreted with a well defined moho near 37 39 km depth an upper crustal s velocity discontinuity at these stations is interpreted as the top of the high velocity rocks of the wrangellia terrane in contrast more complicated structure dominated by pronounced low velocity zones dipping to the ne are interpreted beneath our southern two stations the shallower low velocity zone is 6 8 km thick has an s velocity contrast of 0 6 1 1 km s and lies within the continental crust this feature is similar to a pronounced low velocity layer the e zone imaged beneath southern vancouver island the deeper low velocity zone is interpreted as the subducted oceanic crust we interpret the pronounced change in s velocity structure that we observe as the northern limit of the subducted oceanic plate beneath vancouver island this change coincides with significant changes in topography heat flow gravity and geochemistry

stratal extension in thrust footwalls makran accretionary prism implications for thrust tectonics small scale stratal extension is commonly associated with thrust faults in the makran accretionary prism of southwest pakistan low angle normal faults dipping in the direction of transport relative to the thrust are common in gouge zones and duplexes and may also cut the footwall and hanging wall where they cause stratal extension these faults are probably riedel shears we have studied a broad zone of more complex extensional deformation beneath one bedding parallel thrust the zone disrupts earlier compressional structures and affects a significant volume of the footwall normal faults initiating in a rearward sequence branch off the thrust and cut down into the footwall merging at a lower stratigraphic level part of the thrust displacement was thereby transfe red to that level and fragments of the footwall were detached and transferred southward beneath the hanging wall the latter remained undeformed and accommodation structures are confined to the footwall these effects may have been caused by variations in sliding resistance along the fault if a thrust transfers part or all of its displacement down section along normal faults in this way before eventually cutting back up toward the surface it will detach a rootless fragment of footwall rocks from a position forward of the initial ramp through that horizon this process can be referred to as footwall plucking it can explain rootless basement slices along thrusts for example and it has important implications for section restoration techniques and the interpretation of thrust belts

age constraints on ductile deformation and long term slip rates along the karakoram fault zone ladakh the dextral karakoram fault zone that bounds the southwestern margin of tibet is widely regarded as one of the major strike slip faults that accommodates the eastward extrusion of the thickened crust of tibet in northern ladakh the karakoram fault bounds the pangong transpressional zone with the active fault trace following the eastern margin of the range exhumation within this zone reveals mylonites that have absorbed much of the deformation along the fault two sets of dikes are evident within the main fault strand that bounds the transpressional zone whilst one dike set is concordant with the dominant foliation and shows mylonitic fabric the youngest dike set are less deformed and cross cut the shear zone fabric using u pb id tims data from five shear zone samples deformed by differing degrees we infer that fault initiation occurred between 15 68f0 52 and 13 73f0 28 ma since absolute offset is difficult to determine we couple these age constraints with suggested minimum and maximum offsets of the baltoro type granites the offset range of 40 150 km reveals that the karakoram fault has a longterm average slip rate in the range 2 7 10 2 mm year the lower rate being compatible with gps insar and cosmogenic data for the fault the small range of offset and low slip rates do not support rapid large scale extrusion of tibet along lithosphericscale faults and strengthens the argument that tibet does not behave in a rigid plate like manner d 2004 elsevier b v all rights reserved

an explanation for the double seismic layers north of the mendocino triple junction we propose that the gently eastward dipping double planed seismic zone observed at 15 25 km depths in the southern cascadia subduction zone just north of the mendocino triple junction is a direct consequence of the thermally controlled rheology as the oceanic lithosphere subducts to a depth of about 15 km the temperature regime causes a brittle plastic transition to occur within the oceanic crust thus a ductile layer forms in the lower oceanic crust sandwiched between the brittle upper crust and brittle upper mantle the very high strain rates near the triple junction caused by the northward push of the pacific plate on the gorda plate increase the seismicity and thus accentuate the double seismic zone in this region this model explains the focal mechanisms observed in the seismic zone and their spatial change the double seismic layers clearly define the position of the subducting gorda plate previously uncertain in the cape mendocino region

transform push oblique subduction resistance and intraplate stress of the juan de fuca plate the juan de fuca plate is a small oceanic plate between the pacific and north america plates in the southernmost region referred to as the gorda deformation zone the maximum compressive stress 1 constrained by earthquake focal mechanisms is n s off oregon and possibly off washington nw trending left lateral faults cutting the juan de fuca plate indicate a 1 in a ne sw to e w direction the magnitude of differential stress increases from north to south this is inferred from the plastic yielding and distribution of earthquakes throughout the gorda deformation zone to understand how tectonic forces determine the stress field of the juan de fuca plate we have modeled the intraplate stress using both elastic and elastic perfectly plastic plane stress finite element models we conclude that the right lateral shear motion of the pacific and north america plates is primarily responsible for the stress pattern of the juan de fuca plate the most important roles are played by a compressional force normal to the mendocino transform fault a result of the northward push by the pacific plate and a horizontal resistance operating against the northward or margin parallel component of oblique subduction margin parallel subduction resistance results in large n s compression in the gorda deformation zone because the force is integrated over the full length of the cascadia subduction zone the mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike normal compressive stresses internal failure of the gorda deformation zone potentially places limits on the magnitude of the fault normal stresses being transmitted and correspondingly on the magnitude of strike parallel subduction resistance transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner

deformation microstructures of glaucophane and lawsonite in experimentally deformed blueschists implications for intermediate depth intraplate earthquakes a series of simple shear experiments on blueschist was performed at 400 500 c and 1 2 5 gpa to understand the deformation of seismically active subducting oceanic crust the experiments show that brittle microstructures are mainly found at pressures of 1 2 gpa whereas ductile microstructures form at 2 5 gpa j indices a measure of fabric intensity of glaucophane crystal preferred orientations change systematically with changing shear strain and confining pressure and the angle between the slip plane and the shear direction of samples deformed at 2 gpa is similar to that of a strain ellipsoid these results together with the variable orientations of fine grains in a selected area electron diffraction image at 2 gpa indicate that the brittle ductile transition for glaucophane occurs at 2 gpa in contrast to this lawsonite in the experiments show abundant fracturing in most specimens and a poor correlation between the j index shear strain and confining pressure this demonstrates that lawsonite deformed by brittle failure under all experimental conditions in the case of a starting material that has a strong fabric and deformed under dry experimental conditions the brittle ductile transition zone of glaucophane will be much shallower than 2 gpa therefore our initial experimental results on the deformation behavior of blueschist indirectly support the dehydration embrittlement of subducting oceanic crust glaucophane as an important factor in the origin of intraplate earthquakes