crustal deformation along the altyn tagh fault system western china from gps we collected gps data from the southern tarim basin the qaidam basin and the western kunlun shan region between 1993 and 1998 to determine crustal deformation along the altyn tagh fault system at the northern margin of the tibetan plateau we conclude from these data that the altyn tagh is a left lateral strike slip fault with a current slip rate of 9 mm yr in sharp contrast with geological estimates of 20 30 mm yr this contrast poses an enigma because the gps data cover a wider region than the geologic data they might be expected to reveal somewhat more slip we also find that the tarim and qaidam basins behave as rigid blocks within the uncertainty of our measurements rotating clockwise at a rate of 11 and 4 5 nrad yr respectively with respect to the eurasia plate the rotation of the tarim basin causes convergence across the tian shan increasing progressively westward from 6 mm yr at 87 e to 18 mm yr at 77 e our data and other gps data suggest that the indo asia collision is mainly accommodated by crustal shortening along the main himalayan thrust system 53 and the tian shan contractional belt 19 eastward extrusion of the tibetan plateau along the altyn tagh and kunlun faults accommodates only 23 of the indo asia convergence

magnitude of post middle jurassic bajocian displacement on the central altyn tagh fault system northwest china the altyn tagh fault system a key structural feature in the tectonic collage of central asia is an active left lateral strike slip fault system however its age of initiation kinematic history and total magnitude of displacement are not well known middle jurassic nonmarine sedimentary rocks that crop out along the central segment of the altyn tagh fault were deposited in a contiguous tarim qaidam foreland style basin that was geodynamically linked to mesozoic contraction in the tian shan bei shan qilian shan and kunlun shan this study documents the best evidence of a piercing point to date consisting of an offset lacustrine shoreline demarcated by aalenian bajocian open lacustrine strata to the west and fluvial and alluvial strata of comparable age to the east detrital compositions palynology paleocurrents and lithostratigraphy support correlation of these sections restoration of this jurassic facies boundary indicates 400 60 km of post bajocian left lateral separation on the altyn tagh fault consistent with this result restoration of the jurassic shoreline aligns felsic plutons across the western segment of the altyn tagh fault system that may have been left laterally offset by 360 km these two pairs of offset features suggest that the current best estimate for net magnitude of post bajocian left lateral separation on the altyn tagh fault is about 360 km

kinematic model of active deformation in central asia the velocity field of present day deformation in central asia is modelled using a set of four rotating blocks siberia tarim tibet india on a spherical earth a best fit is inverted on the basis of estimated shortening rates across the main thrust zones himalayas tien shan and measured slip rates along the principal strike slip faults altyn tagh and karakorum separating those blocks the fit to the data implies that nearly all the present convergence between india and asia can be accounted for by slip partitioning on these four zones with as much as 50 absorbed by northeastwards extrusion of tibet this suggests that localised deformation governs the present mechanical behaviour of the central asian lithosphere

magnitude of late quaternary left lateral displacements along the north edge of tibet images taken by the earth observation satellite spot of the quaternary morphology at 18 sites on the 2000 kilometer long altyn tagh fault at the north edge of tibet demonstrate that it is outstandingly active long term left lateral strike slip offsets of stream channels alluvial terrace edges and glacial moraines along the fault cluster between 100 and 400 meters the high elevation of the sites mostly above 4000 meters in the periglacial zone suggests that most offsets resulted from slip on the fault since the beginning of the holocene these data imply that slip rates are 2 to 3 centimeters per year along much of the fault length and support the hypothesis that the continuing penetration of india into asia forces tibet rapidly toward the east

two stage evolution model for the altyn tagh fault china the northeastern extension of the altyn tagh fault china beyond the apparently truncated qilian shan has been controversial since the recognition of this large left slip fault we propose that the altyn tagh fault may have been contiguous with the east northeast to northeast trending currently inactive but apparently active during the cenozoic alxa east mongolia fault the latter truncated the beishan orogen to the northwest and the inner mongolia orogen to the southeast the beishan orogen can be correlated with the inner mongolia orogen on the basis of the recognition of three offset petrotectonic units and two offset sutures this correlation requires 400 50 km of left lateral offset along the alxa east mongolia fault identical to the slip along the northern segment of the altyn tagh fault inferred by p molnar and p tapponnier we suggest a two stage model for fault evolution during the first stage 400 km of displacement separated the inner mongolia orogen from the originally contiguous beishan orogen along a continuous altyn tagh alxa east mongolia fault during the second stage the alxa east mongolia fault became inactive and additional offset along the altyn tagh fault has been mainly accommodated by shortening of the qilian shan and the qaidam basin we infer that the first stage of movement along the altyn tagh alxa east mongolia fault system may have begun around oligocene time the region transformed to the second stage of fault movement ca 13 16 ma probably because of thinning of the lithospheric mantle beneath northern tibet the proposed model suggests that the derbur fault in northernmost china and beyond is an extension of the alxa east mongolia fault and has a history of strike slip movement the nearly 400 km of offset on the alxa east mongolia fault may have been accommodated mainly by subduction in the sea of okhotsk

geodetic evidence for a low slip rate in the altyn tagh fault system the collision between india and asia has been simulated with a variety of computational models that describe or predict the motions of the main faults of east asia geological slip rate estimates of 20 30 mm yr 1 suggest that the largest of these faults the 2 000 km long altyn tagh fault system on the northern edge of the tibetan plateau absorbs as much of the indo asian convergence signal as do the himalayas partly by oblique slip and partly by contraction and mountain growth however the predictions of dynamic models for asian deformation and the lower bounds of some geological slip rates estimates 3 9 mm yr 1 refs 7 8 suggest that the altyn tagh system is less active here we report geodetic data from 89 91 degrees e that indicate left lateral shear of 9 5 mm yr 1 and contraction of 3 1 mm yr 1 across the altyn tagh system this result combined with our finding that at 90 degrees e tibet contracts north south at 9 1 mm yr 1 supports the predictions of dynamic models of asian deformation

miocene cooling in the northern qilian shan northeastern margin of the tibetan plateau revealed by apatite fission track and vitrinite reflectance analysis apatite fission track and vitrinite reflectance data from phanerozoic rocks of the northern qilian shan and jiuxi basin at the northeastern margin of the tibetan plateau show a complex thermal history with multiple paleothermal events peak paleotemperatures reached in the middle cretaceous suggest that hydrocarbon generation occurred in the basin at that time paleozoic mesozoic samples show evidence of a middle tertiary cooling episode between 20 and 10 ma this thermal fingerprint is significant because stratigraphic evidence for miocene exhumation is overwhelmed by spectacular pliocene pleistocene deformation and erosion we interpret the miocene cooling as recording initial exhumation of the northern qilian shan earlier than other models have proposed i e after 6 5 ma miocene cooling is broadly coincident with cenozoic uplift and exhumation recorded elsewhere in the tibetan tarim region

kinematics topography shortening and extrusion in the india eurasia collision we examine the problem of partitioning between shortening and extrusion in the india asia collision since 45 ma we compute the amount of shortening expected from the kinematics of india s motion with respect to eurasia using the reconstruction at collision time to put bounds on the possible amounts of surface loss within greater india and within eurasia we then compute the amounts of surface loss corresponding to the thickened crust of tibet and of the himalayas assuming conservation of continental crust the spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it this distribution indicates an important eastward crustal mass transfer however the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis the deficit may be accounted either by loss of lower crust into the mantle for example through massive eclogitization or by lateral extrusion of nonthickened crust a mass budget of the crust of the himalayas indicates that lower crust has not been conserved there but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit alternatively following zhao and morgan 1985 lower crust may have been transferred below the tibetan crust we conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between india and asia since 45 ma this conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m y history of the collision

active deformation of asia from kinematics to dynamics using estimated strain rates across the actively deforming region of asia to infer variations in viscous stress it is shown that gradients of stress point uphill toward the center of tibet where gravitational potential energy per unit surface area reaches a maximum thus the dynamics of the deformation seem to obey the equation of creeping flow which expresses a balance between gradients in stress and the gravitational body force this balance in the region of the tibetan plateau yields an estimate of 10 22 pascal second for the average viscosity of the tibetan lithosphere which is only about 10 to 100 times greater than the viscosity of the convecting upper mantle

geology of the haiyuan fault zone ningxia hui autonomous region china and its relation to the evolution of the northeastern margin of the tibetan plateau the haiyuan area located along the northeastern margin of the tibetan plateau in north central china provides a laboratory for studying how the plateau has grown in late cenozoic time rocks in the area range from pre silurian precambrian to recent the pre silurian and cenozoic rocks form the most extensive outcrops the pre silurian rocks consist of amphibolite and greenschist grade metasedimentary and metaigneous rocks unconformably overlain by silurian and devonian red beds all of these rocks are intruded by granodiorite of unknown age cenozoic rocks consist of 2 6 3 0 km of eocene to miocene red beds that were deposited over an extensive area in this part of china pliocene conglomerate contains clasts from all older formations and is interpreted to have been derived from highlands developed during the beginning of cenozoic deformation in the haiyuan area except for the widespread loess deposits quaternary rocks reflect deposition in local tectonic environments the oldest cenozoic structures in the haiyuan area are folds and small thrust faults that generally strike n30 45 w and involve mostly pre quaternary rocks these structures and all the quaternary rocks are cut by the haiyuan left lateral strike slip left slip fault zone that generally trends n60 65 w and is nearly vertical at the western end of the mapped area a fault zone which strikes n75 90 w forms a left stepping transfer zone that connects with another segment of the haiyuan fault zone which continues n60 65 w west into gansu province a small basin the salt lake basin is marked by active faults in the area of the transfer zone and is interpreted as a pull apart basin along the left slip haiyuan fault zone at its eastern end the haiyuan fault zone has an irregular surface trace east of luzigou an active fault striking n35 45 w branches to the south this southern branch appears to be a younger fault and now accommodates most of the left slip deformation that formerly occurred on the easternmost part of the haiyuan fault zone this younger fault connects through a left stepping transfer zone to a parallel fault the xiaokou fault that can be traced into the liupan shan about 60 km to the southeast the laohuyaoxian basin is interpreted as a very young pull apart basin in the area of the transfer zone matching different geological features across the haiyuan fault zone yields a total left slip offset of between 10 5 and 15 5 km and the best constrained offsets yield 12 9 14 8 km if left slip began near the end of the pliocene time or earliest pleistocene time it indicates an average slip rate between 5 and 10 mm yr progressively smaller offsets can be determined on progressively younger geological features but dates for these younger features are too imprecise to constrain slip rates through time surface ruptures that formed during the 1920 haiyuan earthquake m 8 7 show mostly left slip displacement with magnitudes of more than 10 m in some places active faulting in the region suggests the tibetan plateau may be extending to the northeast in time in the haiyuan area deformation probably began in pliocene time compared with a likely earlier initiation to the southwest thus deformation began about 40 45 m y after collision between india and asia formation of the low ranges to the northeast of the haiyuan area however may have developed at different times and deformation may not have propagated regularly to the northeast a total displacement of 10 5 15 5 km on the haiyuan fault zone indicates that this fault zone does not accommodate large scale eastward lateral transfer of continental fragments in the northern tibetan plateau

petrology and geochronology of eclogites from the western segment of the altyn tagh northwestern china abstract the eclogites from the western segment of the altyn tagh occur as lens or boundins within quartzofeldspathic gneisses or pelitic gneisses characterized by amphibolite facies parageneses the petrographic features and reaction textures testify to three main metamorphic stages 1 the peak eclogite facies stage p 15 kbar t 665 880 c 2 the granulitic facies stage p 11 14 kbar t 670 800 c and 3 the amphibolite facies stage p 6 3 9 5 kbar t 619 738 c during decompression the trace element rare earth element and sm nd isotopic data suggest that most of the eclogites have protolith features resembling transitional type t type mid ocean ridge basalt morb the well preserved eclogite was selected for sm nd and u pb isotopic dating the sm nd isotopic data yield a whole rock garnet omphacite isochron of 500 10 ma age the u pb isotopic measurement of zircons from the same eclogite shows that the four grain populations are near concordant and are well plotted on concordia giving a weighted mean age of 503 9 5 3 ma the two different geochronological methods yield a similar age which is interpreted as the peak metamorphic age of eclogites and reflects the existence of a mountain root related to continental subduction during early paleozoic moreover based on the comparison with similar rocks in the northern margin of the qaidam basin a displacement of 350 400 km for the altyn tagh fault have been estimated

jurassic to cenozoic exhumation history of the altyn tagh range northwest china constrained by 40ar 39ar and apatite fission track thermochronology

displacement and timing along the northern strand of the altyn tagh fault zone northern tibet abstract the northern strand of the altyn tagh fault has a late cenozoic left slip offset of 69 90 km near its northeastern end deformation began in this area as early as the oligocene but left slip displacement may be mainly of middle miocene to recent age because the altyn tagh fault zone transfers strike slip displacement into shortening in the qilian shan the total offset changes along the fault but the magnitude of displacement along this part of the fault zone suggests that only small scale eastward movement of crustal rocks occurred in this part of the tibetan plateau during the late cenozoic

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

low slip rates and long term preservation of geomorphic features in central asia in order to understand the dynamics of the india asia collision zone it is important to know the strain distribution in central asia whose determination relies on the slip rates for active faults1 2 3 4 5 many previous slip rate estimates of faults in central asia were based on the assumption that offset landforms are younger than the last glacial maximum 20 kyr ago 6 7 8 9 10 11 in contrast here we present surface exposure ages of 40 to 170 kyr obtained using cosmogenic nuclide dating for a series of terraces near a thrust at the northern margin of the tibetan plateau combined with the tectonic offset the ages imply a long term slip rate of only about 0 35 mm yr 1 for the active thrust an order of magnitude lower than rates obtained from the assumption that the terraces formed after the last glacial maximum our data demonstrate that the preservation potential of geomorphic features in central asia is higher than commonly assumed

paleozoic to cenozoic deformation along the altyn tagh fault in the altun shan massif area eastern qilian shan northeastern tibet china

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

fluvial dissection isostatic uplift and geomorphological evolution of volcanic islands gran canaria canary islands spain abstract digital analysis of torrential gullies barrancos deeply incised into the volcanic island of gran canaria canary islands allows us to extract the longitudinal profiles and pre incision surfaces for individual basins from which morphometric parameters length elevation area slope have been calculated other derived parameters such as ridgeline profiles maximum incision values volume removed by fluvial erosion geophysical relief and isostatic uplift have also been computed based on k ar ages for the island well constrained incision uplift rates have been calculated by means of the combination of different methodological approaches commonly used in orogens and large mountain ranges the geomorphological and morphometric analyses reveal that the island is clearly divided into four environmental quadrants determined by the combination of a couple of key factors the age of the volcanic surfaces and the climatic conditions these factors determine a young sector covered with plio quaternary platform forming lavas finished at 1 9 1 5 ma evolving under contrasting wet ne to dry se climates and an older sector conserving the residual surfaces of the miocene shield building 14 5 8 7 ma at the ridgelines also subjected to wet nw and dry sw climates incision is related to the age zonation of the island maximum incisions

slowing extrusion tectonics lowered estimate of post early miocene slip rate for the altyn tagh fault abstract determination of long term slip rate for the altyn tagh fault is essential for testing whether asian tectonics is dominated by lateral extrusion or distributed crustal shortening previous slip history studies focused on either quaternary slip rate measurements or pre early miocene total offset estimates and do not allow a clear distinction between rates based on the two the magmatic and metamorphic history revealed by shrimp zircon dating of clasts from miocene conglomerate in the xorkol basin north of the altyn tagh fault strikingly matches that of basement in the southern qilian shan and northern qaidam regions south of the fault this match requires that the post early miocene long term slip rate along the altyn tagh fault cannot exceed 10 mm year supporting the hypothesis of distributed crustal thickening for post early miocene times this low long term slip rate and recently documented large pre early miocene cumulative offset across the fault support a two stage evolution wherein asian tectonics was dominated by lateral extrusion before the end of early miocene and since then has been dominated by distributed crustal thickening and rapid plateau uplift

late holocene earthquake history of the central altyn tagh fault china the altyn tagh fault accommodates sinistral motion between the tibetan plateau and the tarim block within the india eurasia collision zone we used well preserved evidence for surface rupturing earthquakes to reconstruct the earthquake history for the central altyn tagh fault we identified three geometric fault segments bounded by left steps and a bend geomorphic offsets indicate that the most recent event had maximum surface displacement of 5 5 m in the west 38 58n 90 08e 7 m in the central part of our study area and 4 m in the east 38 88n 91 58e the 14 c dates and trench logs of disrupted sediments indicate that these offsets occurred either in a single earthquake with a surface rupture length 240 km dated as 680 6 108 yr b p or as two events if there were two events the westernmost recent event occurred 518 6 268 yr ago whereas the eastern event occurred 650 6 80 yr ago and had a surface rupture length 155 km we find two events in the past 0 8 2 2 k y in the west and two or three events in the east yielding recurrence intervals of 0 7 6 0 4 k y and 1 1 6 0 3 k y respectively these recurrence rates for major earthquakes are lower than expected if the long term fault slip rate is 20 mm yr explanations for the discrepancy include an overdue major earthquake or accelerated deformation elsewhere in the india eurasia orogen

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

detrital zircon geochronology of the northeastern tibetan plateau u pb geochronologic analyses have been conducted on 413 detrital zircon grains collected from 16 samples in the altun shan nan shan and qilian shan the samples come primarily from quartz arenites and metaturbidites of middle to late proterozoic age and from feldspathic and volcanic clast rich sandstones of early paleozoic age zircon grains in proterozoic strata resting on tarim basement yielded mainly 2 0 1 9 ga ages whereas proterozoic strata of the qaidam and qilian terranes yielded mainly ca 930 820 ma and ca 1 9 1 1 ga ages the younger grains were apparently shed from local igneous rocks whereas the grains older than 1 1 ga were shed from an undetermined continental source grains in the lower paleozoic strata are mainly ca 500 430 ma and were shed from nearby plutonic and possibly volcanic rocks that formed in a magmatic arc setting our detrital zircon ages are consistent with a model first proposed by e r sobel and n arnaud in which early paleozoic magmatism occurred within a single northeast facing magmatic arc that was constructed across an assemblage of middle to late proterozoic accretionary complexes remnants of magmatic arcs and shallow marine strata this arc system was accreted to the tarim and sino korean cratons during silurian devonian time the resulting suture has been reactivated as tertiary thrust faults that currently define the structural and topographic margin of the tibetan plateau our data also provide two new estimates for the offset along the eastern altyn tagh fault a belt of middle proterozoic shallow marine strata is offset by 400 km whereas a belt of 490 480 ma magmatic arc rocks is offset by 370 km these values are generally similar to the 350 400 km offset reported in most previous studies

oligocene miocene tectonics and sedimentation along the altyn tagh fault northern tibetan plateau analysis of the xorkol subei and aksay basins abstract tertiary basins adjacent to the northern edge of the tibetan plateau include both giant intracontinental sedimentary basins such as tarim and qaidam and smaller basins directly related to the evolution of the altyn tagh strike slip fault system and nearby fold thrust belts three examples of these smaller basins that formed during early evolution of the altyn tagh fault system are the xorkol aksay and subei basins the xorkol basin was the site of nonmarine siliciclastic sedimentation in alluvial fan playa and fluvial environments from at least the mid oligocene through the miocene sediment was delivered to the basin from source areas uplifted by basin bounding thrust faults and from the opposite side of the altyn tagh fault the age of the aksay basin is more poorly constrained but it also contains oligocene miocene nonmarine clastic strata that were derived from the southern side of the altyn tagh fault and adjacent thrust fault bounded uplifts the subei basin is characterized by s

volatile sulphur and chlorine major and trace element geochemistry of mafic to intermediate tephras from the chilean southern volcanic zone 33 43 s here we present the first systematic investigation of volatile geochemistry along the southern volcanic zone svz of chile holocene olivine hosted melt inclusions in the most mafic tephras sampled from 16 volcanoes along the volcanic front of the svz between 33 s and 43 s were analysed for pre eruptive sulphur chlorine and major element contents these results are combined with trace element compositions of the host whole rocks the highest fractionation corrected gas contents occur in the least degassed melt inclusions from small monogenetic cones of los hornitos cabeza de vaca and apagado from both the transitional and the southern central svz reaching 3 000 g g s and 1 400 g g cl while the lowest abundances of 1 100 g g s and 600 g g cl were found in the central svz at volcan lonquimay volcan llaima and volcan villarrica chlorine co varies with trace element indicators for the degree of melting and or source enrichment such that the lowest cl contents are found in high degree melts from the most depleted mantle sources the size of the volcanic edifices correlates inversely with cl abundances in the melt this could reflect more extensive degassing during ascent through the complex magma plumbing systems beneath the stratovolcanoes or greater dilution during larger degrees of melting of more depleted sources or a combination of these factors compared to other subduction zones the svz melt inclusions exhibit cl and s abundances in the same range as most of those from the central american and those from the marianas arcs