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

denudation and cooling of the lake teletskoye region in the altai mountains south siberia as revealed by apatite fission track thermochronology abstract lake teletskoye occupies a narrow graben located in the northwestern sector of the altai fold belt in south siberia the lake basin is thought to have formed during the pleistocene as a distant result of the cenozoic collision of india and eurasia that caused a tectonic reactivation of the palaeozoic gorny altai ga and west sayan ws blocks the present work reports of a pilot fission track study performed on 13 apatite separates collected from rocks that were sampled along two profiles in close proximity of the lake the age length data and aft thermochronological modelling reveal two important phases of cooling in the altai mountains a first one during the late jurassic early cretaceous and a second one that started in the miocene pliocene and that persists until today the first event is interpreted to result from uplift induced denudation probably related to the closure of the mongol okhotsk ocean the second event can be linked to the young cenozoic movements that lie at the origin of the formation of the lake teletskoye basin

fault reactivation in the junggar region northwest china the role of basement structures during mesozoic cenozoic compression basement structures exposed at the margins of the junggar basin were created during the altaid orogeny in the late palaeozoic the most prominent structures are backstops to subduction accretion complexes north tien shan fault or major thrusts within these complexes dalabute and kelameili faults both types of basement structure are far more common in accretionary turkic type orogens such as the altaids than true sutures probably the only suture sensu stricto in the junggar area is cryptic and lies under the junggar basin s thick mesozoic cenozoic cover the exposed palaeozoic fault zones have been reactivated by mesozoic cenozoic compressional events which are the long distance expression of orogenies at the southern margin of asia latest palaeozoic and mesozoic events reactivated a larger number of fault zones than have been affected by the cenozoic india asia collision possibly because of an increase in the strength of the junggar basement over time following late permian rifting cenozoic strain is partitioned between strike slip motion on basement structures within the palaeozoic orogenic belts around the junggar basin and numerous thrusts and transpressive faults in regions marginal to and within the basin itself most major strike slip faults are reactivated structures and occupy narrower zones than their palaeozoic precursors thrust zones follow the palaeozoic basement grain but active faults have propagated into the mesozoic cenozoic cover

early cretaceous denudation related to convergent tectonics in the baikal region se siberia we present 24 new apatite fission track aft ages and 18 track length measurements from the baikal region se siberia most samples have aft ages between 140 and 100 ma with relatively high mean track lengths c 13 2 m the relationship between aft ages elevation and mean track lengths indicate that the samples record rapid cooling during the early cretaceous 140 120 ma as also shown by thermal history inversion of track length distributions cooling took place during a late jurassic early cretaceous orogenic phase related to closure of the mongol okhotsk ocean and reflected in the exhumation of metamorphic core complexes followed by thrusting and reverse faulting basin inversion and large vertical motions the variation in aft ages throughout the study area can be partly explained by differences in geothermal structure but differential denudation also played a role minimum amounts of early cretaceous denudation are estimated at 2 3 km

mongolian summits an uplifted flat old but still preserved erosion surface in gobi altay and altay mongolia several flat surfaces worn through basement rocks and uplifted during the ongoing tectonic episode to a similar altitude of 4000 m suggests disruption of a single large scale surface new thermochronology and field data show that the plateau surfaces represent uplifted parts of an ancient peneplain that formed during jurassic time the gobi altay and altay flattopped massifs are tectonically and geomorphologically unique their preservation for 150 m y implies that no further tectonic movements occurred before the onset of the last deformation episode 5 3 m y ago it also suggests that very low erosion rates were maintained by a dry climate over millions of years

neoproterozoic to paleozoic geology of the altai orogen nw china new zircon age data and tectonic evolution abstract we present a synthesis and a new account of the geological and tectonic history of the terranes of the chinese paleozoic altai orogen together with new single zircon ages for granitic and rhyodacitic rocks a central terrane consists of neoproterozoic to silurian amphibolite facies metasedimentary rocks and abundant devonian carboniferous granites the presence of precambrian basement is indicated by sinian fossils our xenocryst ages and published nd mean crustal residence ages of granites felsic arc type lavas on the southern margin of the terrane have a mean 207pb 206pb zircon age of 505 ma reflecting the time of arc volcanism and the presence of xenocysts with ages between 614 and 921 ma suggests derivation by intracrustal melting accordingly we suggest that a cambro ordovician continental magmatic arc was built on the southern margin of the central terrane by northward subduction a low grade ordovician andean type arc with a continental basement is situated above a normal fault on

fault interaction and stress triggering of twentieth century earthquakes in mongolia 1 a cluster of exceptionally large earthquakes in the interior of asia occurred from 1905 to 1967 the 1905 m7 9 tsetserleg and m8 4 bolnai earthquakes the 1931 m8 0 fu yun earthquake the 1957 m8 1 gobi altai earthquake and the 1967 m7 1 mogod earthquake sequence each of the larger m 8 earthquakes involved strike slip faulting averaging more than 5 m and rupture lengths of several hundred kilometers available geologic data indicate that recurrence intervals on the major source faults are several thousands of years and distances of about 400 km separate the respective rupture areas we propose that the occurrences of these and many smaller earthquakes are related and controlled to a large extent by stress changes generated by the compounded static deformation of the preceding earthquakes and subsequent viscoelastic relaxation of the lower crust and upper mantle beneath mongolia we employ a spherically layered viscoelastic model constrained by the 1994 2002 gps velocity field in western mongolia vergnolle et al 2003 using the succession of twentieth century earthquakes as sources of deformation we then analyze the time dependent change in coulomb failure stress f at remote interaction distances static f values are small however modeled postseismic stress changes typically accumulate to several tenths of a bar over time intervals of decades almost all significant twentieth century regional earthquakes m 6 with well constrained fault geometry lie in positive f lobes of magnitude about 0 5 bar our results suggest that significant stress transfer is possible among continental faults separated by hundreds of kilometers and on timescales of decades

evolution of the west siberian basin the west siberian basin is one of the largest intracratonic basins of the world and an important hydrocarbon province of russia perhaps the most important geologic event in siberia was the emplacement of basalts around 250 ma i e permo triassic boundary covering an area of about 5 10 6 km 2 this volcanism may be responsible for a mass extinction that occurred around permian triassic time the prebasaltic rifting event was limited to the north northeastern sector of the basin initial basin wide subsidence took place in the jurassic as a result of which the western part of siberia became the west siberian basin bounded by uplifts to the east and to the west one of the surprising aspects of the west siberian basin is the abundance of subvertical faults believed to be result of strike slip movement while intraplate inversions and fault reactivation structures have been observed in many cratons subvertical faults observed in the west siberian basin are unique because of their geometries and abundance the differentiation between the effects of tectonics and eustasy in cratonic basins is simple the global eustatic signal is basin wide with regional and local tectonics playing an overprinting role thus the middle jurassic turonian first second and third order cycles in the west siberian basin were primarily driven by eustasy the middle jurassic turonian series can be subdivided into two second order and 16 third order transgressive regressive cycles within dataset extent fourth order cycles appear to be controlled by delta shifting although extensively studied a number of fundamental questions regarding the origin and evolution of the west siberian basin remain unresolved or poorly documented in the literature

paleomagnetic study of cenozoic sediments from the zaisan basin se kazakhstan and the chuya depression siberian altai tectonic implications for central asia abstract this paper presents new paleomagnetic results on cenozoic rocks from northern central asia eighteen sites were sampled in pliocene to miocene clays and sandy clays of the zaisan basin southeastern kazakhstan and 12 sites in the upper oligocene to pleistocene clays and sandstones of the chuya depression siberian altai thermal demagnetization of isothermal remanent magnetization irm showed that hematite and magnetite are the main ferromagnetic minerals in the deposits of the zaisan basin stepwise thermal demagnetization up to 640 660 c isolated a characteristic chrm component of either normal or reverse polarity at nine sites at two other sites the great circles convergence method yielded a definite direction measurements of the anisotropy of magnetic susceptibility showed that the hematite bearing sediments preserved their depositional fabric these results suggest a primary origin of the chrm and were substantiated by positive fold and reversal tests the mean paleomagnetic direction for the zaisan basin d 9 i 59 k 19 95 11 is close to the expected direction derived from the apw path of eurasia j geophys res 96 1991 4029 and shows that the basin did not rotated relative to stable asia during the tertiary in the upper pliocene pleistocene sandstones of the chuya depression a very stable chrm carried by hematite was found its mean direction d 9 i 46 k 25 95 7 is characterized by declination close to the one excepted for early quaternary whereas inclination is lower in the middle miocene to lower pliocene clays and sandstones a stable chrm of both normal and reverse polarities carried by magnetite was isolated its mean direction d 332 i 63 k 31 95 4 is deviated with respect to the reference direction and implies a neogene 39 8 counterclockwise rotation of the chuya depression relative to stable asia these results and those from the literature suggest that the different amount of rotation found in the two basins is related to a sharp variation in their tectonic style predominantly compressive in the zaisan basin and transpressive in the siberian altai at a larger scale the pattern of vertical axis rotations deduced from paleomagnetic data in northern central asia is consistent with the hypothesis of a large left lateral shear zone running from the pamirs to the baikal heterogeneous rotations however indicate changes in style of faulting along the shear zone and local effect for the domains with the largest rotations

formation of band ogives and associated structures at bas glacier d arolla valais switzerland hambrey m j glasser n f goodsell becky 2002 formation of band ogives and associated structures at bas glacier d arolla valais switzerland journal of glaciology 48 161 pp 287 300 keywords europe switzerland geometrical structure three dimensional structure radar observation glacier crevasse models debris interface basal ice glacial tongue stratification glacier

the south siberia central mongolia transect abstract studies of geological and geophysical data on the south siberia central mongolia geoscience transect which runs approximately along 100 e show that the asian continent was formed in the phanerozoic by accretion of terranes most of which were microcontinents with precambrian basement suture zones separating the terranes are deformed and deeply eroded magmatic arcs of different widths likewise classified as specific terranes in these sutures magmatic arc rocks are locally associated with fore and back arc basin series most of the oceanic crust has been subducted the observed ophiolite belts are as a rule attributed to the base of oceanic island arcs the fragment of the asian continent traversed by the transect was formed through early middle and late paleozoic stages of accretion

variation in apatite fission track length measurement implications for thermal history modelling abstract predictive thermal history modelling using apatite fission track ft data is dependent on an algorithm to describe the time and temperature dependency of ft annealing which in turn relies on the empirical determination of ft length as a measure of the annealing process assessment of variation in ft length measurement is poorly described with few comparisons between analysts and little interlaboratory standardisation using apatites of various compositions containing induced tracks annealed to differing degrees this study has assessed variation in horizontal confined track length measurement for a variety of procedural conditions replicate analysis by a single observer is typically within 3 but increases inversely with track length comparison between observers on the same samples shows significant generally nonsystematic variation between observers for a complex length distribution variation is 12 sources of variation are identified as a variation from track revelation including etching track in track tint vs track in cleavage tincle measurement and use of 252 cf irradiation to produce additional etching channels b bias in measurement including equipment analytical procedures and sample size and c observer bias principally differences in and consistency of personal technique 5 m hno 3 is preferred to weaker etchants although more anisotropic tracks are better defined permitting more rigorous measurement while c axis parallel sections where 2 4 geometry is better defined are more easily identified for all but the longest length distributions tincles are significantly longer than tints with few short tincles at high angles measurement of tincles effectively masks the anisotropy of annealing 252 cf irradiation is effective in increasing the number of tints sampled and measured variation between values measured for unirradiated and cf irradiated aliquots does not exceed that found for a single analyst although a slight systematic shift to longer lengths for cf irradiated samples is seen as reported by other workers track length distributions are anisotropic anisotropy increasing with annealing level track angle exerts a major influence on measured length summing affects from annealing and etching anisotropies with observer bias track angle should be accommodated within the annealing algorithm it is recommended that similar track revelation observation and measurement conditions are used for the analysis of field samples as are used in annealing experiments and subsequently employed in numerical models to predict thermal history a parallel argument can be advanced for using samples of similar composition further we recommend that the ft community should seek as a matter of some urgency a programme of interlaboratory comparison of track length measurement using standard apatite samples containing artificial length distributions typifying various levels of complexity such comparisons would provide a more rigorous baseline for thermal history prediction in geological case studies

multi method chronometry of the teletskoye graben and its basement siberian altai mountains new insights on its thermo tectonic evolution

evolution of the mongol okhotsk ocean as constrained by new palaeomagnetic data from the mongol okhotsk suture zone siberia summary this paper presents new data from palaeomagnetic investigations on the upper palaeozoic and mesozoic geological units from the siberian platform and the mongol okhotsk suture zone within the southern portion of the mongol okhotsk suture we collected palaeomagnetic samples from the late permian belektuy formation borzya region 50 7un 116 9ue and the middle late jurassic shadaron formation unda daya 51 5un 117 5ue we sampled the late permian alentuy formation khilok region 50 8un 107 2ue the early to middle jurassic irkutsk sedimentary basin isb 52 0un 104 0ue the late jurassic badin formation mogzon region 51 8un 112 0ue and the early cretaceous gusinoozesk formation gusinoe lake region 51 2un 106 5ue additionally in the northern region of the mongol okhotsk suture apart from the results of the isb and gusinoozersk formations which show very large ellipses of confidence and might be the present day geomagnetic field overprint our results allow us to constrain the evolution of the mongol okhotsk ocean palaeomagnetically from the late permian to the middle late jurassic they confirm that this large permian ocean closed during the jurassic ending up in the late jurassic or the beginning of the cretaceous in the eastern end of the suture zone as suspected on geological grounds however although geological data suggest a middle jurassic closure of the mongol okhotsk ocean in the west trans baikal region our data show evidence of a still large palaeolatitude difference between the amuria and siberia blocks this is interpreted as a result of the quite fast closure of the ocean after the middle jurassic finally our new palaeomagnetic results exhibit very large tectonic rotations around local vertical axes which we interpret as probably arising both from collision processes and from a left lateral shear movement along the suture zone due to the eastward extrusion of mongolia under the effect of the collision of india into asia

siluro devonian paleomagnetic results from the tuva terrane southern siberia russia implications for the paleogeography of siberia new paleomagnetic data from seven sections of siluro devonian age sedimentary sequences in the tuva terrane south siberia russia reveal the presence of up to three different components of magnetization component a is of recent or late mesozoic cenozoic origin and component b identified unambiguously in 25 samples from one section yields a sample mean direction declination inclination of 300 6 54 7 k 42 2 95 4 5 in geographic and 283 9 63 5 k 45 4 95 4 3 in stratigraphic coordinates however since the results of the fold test are inconclusive no further attempt has been made to assign an age to this magnetization component c with unblocking temperatures of 500 680 c is identified in all seven sections and with dual polarity after bedding correction the section mean directions all plot along a common small circle in stereographic projection with a mean inclination of 42 1 n 7 sites k 184 2 95 2 9 it passes the inclination only fold test the within site fold test both on the 95 significance level and the reversal test and is thus interpreted as being primary in origin and siluro devonian in age adopting the normal polarity option for component c yields a paleolatitude of 24 n 2 for the tuva terrane on the basis of geological evidence the tuva terrane was clearly accreted to siberia by early silurian times the results obtained in this study therefore indicate lower paleolatitudes for siberia than those predicted from previously published apparent polar wander paths for the siberian platform and demonstrate that siberia was positioned between the equator and 30 n during latest silurian earliest devonian times these new results are also supported by paleoecological and paleoclimatological evidence the dispersion of declinations along a small circle is indicative of clockwise block rotations within the tuva terrane of up to 72 with respect to siberia

age of palaeozoic granites and metamorphism in the tuvino mongolian massif of the central asian mobile belt loss of a precambrian microcontinent abstract the tuvino mongolian massif tmm was previously interpreted as a precambrian block within the central asian mobile belt according to this idea it consists of tectonic slices composed of metamorphic rocks of pre mesoproterozoic basement that experienced two episodes of regional metamorphism and mesoproterozoic cover rocks that were reworked together with the basement during high grade metamorphism zircon u pb dating of granitoids from all metamorphic complexes demonstrates that the earliest metamorphic event occurred at 536 6 ma significantly later than the deposition of the cover rocks regional upper amphibolite facies metamorphism which affected all metasedimentary units of the tmm occurred in the short time interval of 497 4 to 489 3 ma we propose that there is no simple basement cover relationship in the tuvino mongolian massif instead the massif consists of tectonic domains composed of heterogeneous metasedimentary successions with distinct pre metamorphic and pre tectonic histories that were juxtaposed by thrusting prior to 497 ma and then metamorphosed under upper amphibolite facies conditions we suggest that this thrusting event was associated with early palaeozoic collisional processes related to amalgamation of the central asian mobile belt our new model for the evolution of the tmm implies derivation of neoproterozoic terrigeneous metasediments from 800 900 ma granitoids formed in an andean type active continental margin setting

late jurassic early cretaceous closure of the mongol okhotsk ocean demonstrated by new mesozoic palaeomagnetic results from the trans baikal area se siberia summary we present new palaeomagnetic results from the transba ikal area se siberia from the mongol okhotsk suture zone the boundary between the amuria and siberia blocks in order to better constrain the time of closure of the mongol okhotsk ocean in the mesozoic we col lected 532 rock samples at 68 sites in six localities of basalts trachy basalts and andesites from both sides of the mongol okhotsk suture at unda river j 3 51 7 n 117 4 e kremljevka peak k1 51 8 n 117 5 e and torey lakes k 1 50 1 n 115 9 e on the southern side of the suture and at monostoy river j 1 51 1 n 106 8 e ingoda river k 1 51 2 n 112 2 e and bichura town k 1 50 6 n 107 6 e on the northern side progressive thermal demagnetization enabled us to resolve low ltc and high htc temperature components of magnetization at most sites jurassic palaeopoles computed from the htcs show a large discrepancy with respect to the apparent polar wander path of eurasia which we interpret in terms of 1700 2700 km of post late jurassic northward movement of amuria with respect to siberia although ge ological data suggest a middle jurassic closure of the mongol okhotsk ocean in the west trans baikal region our data give evidence of a large remaining palaeolatitude difference between the amuria and siberia blocks in contrast early cretaceous sites cluster remarkably well along a small circle which is centred on the average site location this implies the absence of post early cretaceous northward motion of amuria relative to siberia and demonstrates the pre early cretaceous closure of the mongol okhotsk ocean finally we interpret the very large tectonic rotations about local vertical axes evidenced by the small circle distribution of poles as arising both from collision processes and from left lateral shear movement along the suture zone due to the eastward extrusion of amuria under the effect of the collision of india into asia

late paleozoic faults of the altai region central asia tectonic pattern and model of formation abstract the present kinematic and dynamic analysis of large scale strike slip faults which enabled the formation of a collage of altai terranes as a result of two collisional events the late devonian early carboniferous collision of the gondwana derived altai mongolian terrane and the siberian continent resulted in the formation of the charysh terekta system of dextral strike slip faults and later the kurai and kuznetsk teletsk bashkauss sinistral strike slip faults the late carboniferous permian collision of the siberian and kazakhstan continents resulted in the formation of the chara irtysh and north east sinistral strike slip zones the age of deformation of both collisional events becomes younger toward the inner areas of the siberian continent in the same direction the amount of displacement of strike slip faulting decreases from several thousand to several hundred kilometers the width of the late paleozoic zone of deformation reaches 1500 km these events deformed the accretion collision continental margins and their primary paleogeographic pattern

lithospheric thermal regimes in europe abstract the heat flow pattern of europe derived from more than 8000 observations is dominated by a general northeast to southwest increase in geothermal activity which is the consequence of the tectonic evolution of the whole continent this paper reviews the history of the geothermal mapping of europe and describes the recently compiled set of geothermal maps of europe surface heat flow is correlated with the age of the last tectonothermal event with the near surface heat production and with the crustal thickness deep temperature distributions within the crust lithosphere are calculated for 1 d 2 d and simple 3 d models characteristic temperature depth curves are given for some of the major tectonic provinces and the results of various techniques are compared the value of the crustal heat contribution is assessed and a regional moho heat flow pattern is proposed the mohorovicic discontinuity is clearly not an isothermal surface neither is the heat flow from below the crust constant

accretionary orogenesis of the chinese altai insights from paleozoic granitoids zircon u pb dating and whole rock major oxide trace element and nd sr isotope compositions have been determined for four representative granitic intrusions in the sw chinese altai in order to understand the orogenesis and history of crustal growth in the central asian orogenic belt caob the ashile and halatas bodiesare relatively small intrusions formed in the late carboniferous 318 6ma andpermian 267 4ma respectively thelargertarlang tb andkeketuohaibatholiths kb havemagmaticagesof 359 to 412 ma the granitoids consist chiefly of tonalite granodiorite and granite and are metaluminous to weakly peraluminous in composition asi 0 8 1 1 the ashile pluton is characterized by relatively high mgo 2 73 3 54 wt and sr 395 456 ppm contents with low k2o na2o ratios 0 31 0 42 and mantle like nd sr isotopic compositions endt 2 85 to 3 26 initial 87 sr 86 sr 0 7047 0 7051 it probably formed by fractional crystallization of a mantle derived magma with limited assimilation of oldcrustalmaterial theotherintrusionshavenear zeroornegative endtvalues 0 67to 4 41 andhighersrisotopiccompositions initial 87 sr 86 sr 0 7067 0 7092 indicating a mixture of juvenile and old components in their genesis these intrusions are characterized by relatively low k rb cs contents and low rb sr ratios mostly b0 6 and generally exhibit moderate to high heavy rare earth element hree concentrations ybn 8 26 and weak hree fractionation dy ybnb1 5 with or without negative europium anomalies eu eu 0 5 1 0 these characteristics are consistent with an origin by dehydration melting of a hornblendebearing mid crustal source above the garnet stability field dominated by mafic to intermediate rocks this suggests that the deep crust of the chinese altai may contain a considerable proportion of juvenile material mantle derived magmatism probably in an active extensional continental margin setting played an important role in the formation of the chinese altai acting not only as a heat sourceforcrustalmeltingbutasasourceofjuvenilecomponents therelativelysmallvolumesanddistinctgeochemicalcompositions of the ashile and halatas plutons may imply a substantial change in the geodynamic setting in the late paleozoic 2007 elsevier b v all rights reserved

uplift age and rates of the gurvan bogd system gobi altay by apatite fission track analysis abstract the dating of the uplift onset of the mongolian mountain ranges the northernmost relief associated with the india eurasia convergence is a fundamental issue to better understand the mechanisms of propagation of the cenozoic transpressive deformation in central asia using apatite fission tracks we determined the timing and strain rates of the tectonics affecting the gurvan bogd system in the gobi altay since the middle mesozoic to the late cenozoic the region was firstly affected by a lower middle jurassic tectonic phase characterized by a vertical crustal movement larger than 2 km then followed a protracted period without major crustal vertical movements until the last uplift phase the peneplanation of the jurassic relief produced an erosional surface that has undergone negligible denudation or sedimentation for more than 100 ma this same surface corresponds to the present summit plateaux of the massifs standing about 2000 m above the surrounding region which corresponds to the vertical movement produced by the ongoing uplift modelling of fission track data from the massifs of ih bogd and baga bogd shows that this uplift phase probably started at 5 3 ma the gobi altay mountain range appears therefore as one of the youngest mountain ranges in central asia which is consistent with the idea of a northward propagation of the transpressional deformation from the himalayan front to the siberian craton the cenozoic uplift rate of the massifs is estimated to be between 0 25 and 1 mm yr which is slightly higher than the upper pleistocene vertical slip rates of the bordering faults this suggests that thrust faults observed within the massifs would increase the uplift rate inside the massifs compared to the uplift rate determined at their boundaries

the central main ethiopian rift is younger than 8 ma confirmation through apatite fission track thermochronology an isolated block of precambrian basement rocks and mesozoic sediments is exposed at kella along the western margin of the central main ethiopian rift mer surrounded by tertiary to quaternary volcanic rocks apatite fission track thermochronology on two basement samples yielded ages of 7 2 1 0 ma and 6 7 3 0 ma and a long mean track length 14 5 lm rapid late miocene cooling is attributed to denudation related to rifting despite the paucity of data due to the absence of suitable lithologies in the area our data confirm that the central mer is younger than 8 ma as recently proposed on the basis of field evidence and radiometric dating of volcanics this implies that the central mer formed after the northern mer indicating a diachronous development of this third arm of the red sea gulf of aden ethiopian rift system

geodynamics of the western part of the mongolia okhotsk collisional belt trans baikal region russia and mongolia abstract after the western edge of the mongolian microcontinent joined the siberian continent in the region of central mongolia in the earliest permian these two continental blocks remained turned at an angle of about 120 with respect to each other and separated on greater extent of their present day boundary by an enormous gulf of the paleopacific called the mongolia okhotsk ocean closure of this ocean at the early middle jurassic boundary led to the complete collision of siberia and mongolia which by then had already become part of the mongolia north china continent this main collisional episode which lasted through the middle and late jurassic involved thrusting folding and magmatism and produced the mongolia okhotsk belt the onon island arc which was located in the mongolia okhotsk ocean was squeezed between the two major continents inasmuch as the third element the island arc was involved in the collision it is reasonable to distinguish two branches of the mongolia okhotsk suture these branches control the spatial distribution of gold mineralization in the trans baikal region on the southeastern periphery of siberia the crust thickened considerably after the collision and a plateau like uplift formed in the early cretaceous when compression ceased the collisional uplift collapsed and the thrusts were transformed into low angle normal faults the motions on which were responsible for the formation of rift basins and exhumation of metamorphic core complexes

mesozoic tertiary exhumation history of the altai mountains northern xinjiang china new constraints from apatite fission track data abstract this study uses apatite fission track ft analysis to constrain the exhumation history of bedrock samples collected from the altai mountains in northern xinjiang china samples were collected as transects across the main structures related to palaeozoic crustal accretion events ft results and modeling identify three stages in sample cooling history spanning the mesozoic and tertiary stage one records rapid cooling to the low temperature part of the fission track partial annealing zone circa 70 10 c stage two records a period of relative stability with little if any cooling taking place between 75 and 25 20 ma suggesting the altai region had been reduced to an area of low relief support for this can be found in the adjacent junngar basin that received little if any sediment during this interval final stage cooling took place in the miocene at an accelerated rate bringing the sampled rocks to the earth s surface this last stage linked to the far field effects of the himalayan collision most likely generated the surface uplift and relief that define the present day altai mountains

pseudotachylytes in the southern border fault of the cenozoic intracontinental teletsk basin altai russia abstract the cenozoic intracontinental teletsk basin in the central asian altai mountains is composed of a complexly structured northern and a more simple southern sub basin these sub basins formed in two distinct kinematic stages when first the nnw striking teletsk and then the ne striking west sayan shear zones became reactivated in the cenozoic under dominant ns oriented horizontal compression although the entire teletsk basin strikes roughly ns the southern sub basin is parallel to the nnw trending amphibolite facies teletsk ductile shear zone while the northern sub basin is ns striking and flanked by differently structured greenschist facies basement basement reactivation closely controlled the southern sub basin formation but this is less clear for the northern sub basin contrasts between northern and southern basement and the exclusive occurrence of pseudotachylytes along the margins of the southern basin are explored for their contribution to the formation of the teletsk basin with two distinct sub basins in the ductile shear fabric of the basement flanking the southern sub basin concordantly interleaved pseudotachylytes and isolated breccia lenses reflect local brittle deformation along the ductile fabric the genetic link between breccia lenses and pseudotachylyte occurrences was defined by microstructural investigation it allows to explore their possible development in a dextral strike slip zone these rocks occur in a large fault bounded segment of the basement the geometry of the structures in the segment is comparable with a dextral strike slip sidewall ripout structure along the teletsk shear zone seismic slip related to pseudotachylytes is attributed to the sudden stress release on the nnw striking teletsk shear zone when the latter became unconstrained by reactivation of the ne trending west sayan fault zone at its northern boundary the boundary of the sidewall ripout structure was reactivated as a large listric fault in a later stage the northern sub basins roughly develop along an ns strike and are assumed to reflect reactivation of the ductile shear zone underneath the variably structured greenschist facies basement outcropping along the flanks of the sub basin

a vendian cambrian island arc system of the siberian continent in gorny altai russia central asia an extended vendian cambrian island arc system similar to the izu bonin mariana type is described in the gorny altai terrane at the margin of the siberian continent three different tectonic stages in the terrane are recognized 1 a set of ensimatic active margins including subducted oceanic crust of the paleo asian ocean the uimen lebed primitive island arc oceanic islands and seamounts the set of rocks is assumed to be formed in the vendian 2 a more evolved island arc comprising calc alkaline volcanics and granites a fore arc trough in middle late cambrian time was filled with disrupted products of pre middle cambrian accretionary wedges and island arcs 3 collision of the more evolved island arc with the siberian continent folding metamorphism and intrusion of granites occurred in late cambrian early ordovician time in the late paleozoic the above mentioned caledonian accretion collision structure of the siberian continent was broken by large scale strike slip faults into several segments this resulted in the formation of a typical mosaic block structure

constraints on the viscosity of the continental crust and mantle from gps measurements and postseismic deformation models in western mongolia 1 we use gps measurements and models of postseismic deformation caused by seven m6 8 to 8 4 earthquakes that occurred in the past 100 years in mongolia to assess the viscosity of the lower crust and upper mantle we find an upper mantle viscosity between 1 1018 and 4 1018 pa s the presence of such a weak mantle is consistent with results from independent seismological and petrological studies that show an abnormally hot upper mantle beneath mongolia the viscosity of the lower crust is less well constrained but a weak lower crust 3 1016 to 2 1017 pa s is preferred by the data using our best fit upper mantle and lower crust viscosities we find that the postseismic effects of viscoelastic relaxation on present day horizontal gps velocities are small 2 mm yr 1 but still persist 100 years after the 1905 m8 4 bolnay earthquake this study shows that the gps velocity field in the baikal mongolia area can be modeled as the sum of 1 a rigid translation and rotation of the whole network 2 a 3 5 mm yr 1 simple shear velocity gradient between the siberian platform to the north and northern china to the south and 3 the contribution of postseismic deformation mostly caused by the 1905 bolnay tsetserleg sequence and by the smaller but more recent 1957 bogd earthquake

upper mantle anisotropy beneath the altai sayan region of central asia abstract the altai sayan region of central asia is actively deforming despite its being located several thousands of kilometers away from the collisional front between india and eurasia to add constraints to the role of the upper mantle in this type of tectonic environment we analyzed intermediate period analogue records of sks and skks to determine the state of mantle anisotropy in this region we find that for all stations located in actively deforming areas the fast direction of anisotropy correlates well with indicators of cenozoic to the present day deformation and correlates with older structures only when they happen to align with younger ones the general east northeast trend of the fast direction of anisotropy in active regions also aligns with the principle extensional strain axis determined from earthquake moment tensors at one station nvs located outside of any actively deforming region the n s fast direction may correspond to the long wavelength strain field of the mantle beneath much of siberia these correlations along with the size of the delay times 1 2 s suggest that the anisotropy in the upper mantle beneath the altai sayan is responding to recent tectonic activity

gigantic paleolandslide associated with active faulting along the bogd fault gobi altay mongolia on the basis of analyses of satellite imagery aerial photographs and field observations we describe the occurrence of one of the largest paleolandslides 50 km3 ever recognized in an intracontinental domain the slide occurred along the active bogd fault in the gobi altay mountain range in mongolia morphological and structural analyses of the relationships between the landslide and the area affected by active tectonics suggest that this gigantic mass movement was associated with surface faulting during a strong earthquake

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting