palustrine carbonates and the florida everglades towards an exposure index for the fresh water environment abstract palustrine carbonates are shallow fresh water deposits showing evidence of subaqueous deposition and subaerial exposure these facies are common in the geological record the intensity of modification is highly variable depending on the climate and the length of emergence palustrine limestones have previously been interpreted as marginal lacustrine deposits from fluctuating low salinity carbonate lakes but several problems remain with existing facies models 1 palustrine carbonates possess a lacustrine biota but commonly display fabrics similar to those of calcretes and peritidal carbonates 2 the co occurrence of calcrete horizons and karst like cavities is somewhat unusual and appears to indicate contemporaneous carbonate precipitation and dissolution in the vadose zone 3 the dominance of gray colors indicates water saturation apparently inconsistent with the evidence for strong desiccation overprint 4 profundal lake deposits are generally absent from palustrine sequences and sublittoral facies commonly make up only a small proportion of total thicknesses 5 no good modern analogue has been identified for the palustrine environment analogy with the florida everglades suggests a re interpretation of palustrine limestones not as pedogenically modified lake margin facies but as the deposits of extensive very shallow carbonate marshes the distribution of environments in the everglades is determined by the local hydrology reflecting the control of seasonal water level fluctuations and topography climate and topography were the main controls on deposition of ancient palustrine carbonates as in peritidal sequences aggradational cycles are capped by a range of lithologies evaporites desiccation and microkarst breccias calcretes lignite or coal horizons etc permitting interpretation of the climate careful analysis of lateral facies variations may permit reconstruction of subtle topography consideration of the florida everglades as a modern analogue for the palustrine environment has suggested the development of an exposure index for fresh water carbonates

crustal shortening during the paleoproterozoic can it be accommodated by paleomagnetic data abstract in the last 130 million years perhaps as much as 2000 km of intra continental shortening has taken place within asia during 10 000 km of convergence between siberia and india with 8000 km being accommodated by oceanic subduction given that earth was hotter during the paleoproterozoic crustal shortening at this time may have been equally if not more significant compared to today warranting a search of the paleomagnetic data base for any supporting evidence paleomagnetic data from units on either side of the paleoproterozoic trans hudson orogen in canada have been chosen because this orogen bears close similarity both in size and history to the india siberia one the data do not prove but can accommodate continental crustal shortening between the archean slave and superior provinces of about 3000 1000 km the results allow crustal shortening to have been a significant component of continental collision in the paleoproterozoic an important issue in any re assembly of precambrian continents if true the present dimensions of precambrian shields may be significantly different compared to those existing in earlier times

active thrusting and folding along the northern tien shan and late cenozoic rotation of the tarim relative to dzungaria and kazakhstan we have studied geometries and rates of late cenozoic thrust faulting and folding along the northern piedmont of the tien shan mountain belt west of urumqi where the m 8 3 manas earthquake occurred on december 23 1906 the northern range of the tien shan rising above 5000 m overthrusts a flexural foredeep filled with up to 11 000 m of sediment of the dzungarian basement our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front within a 200 km long zone of neogene quaternary anticlines fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range in all the valleys the scarps offset vertically the highest terrace surface by the same amount 10 2 0 7 m inferring an early holocene age 10 2 kyr for this terrace which is continuous with the largest recent fans of the piedmont yields a rate of vertical throw of 1 0 0 3mm yr on the main active thrust at the surface a quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5 5 2 5 m 2 kyr a rather fresh surface scarp 0 8 0 15 m high that is unlikely to result from shallow earthquakes with 6 m 7 in the last 230 years is visible at the extremities of the main fold zone we associate this scarp with the 1906 manas earthquake and infer that a structure comprising a deep basement ramp under the range gently dipping flats in the foreland and shallow ramps responsible for the formation of the active fault propagation anticlines could have been activated by that earthquake if so the return period of a 1906 type event would be 850 380 years the small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth 2 3 is taken up by incremental folding near the surface comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising quaternary ranges such as the san gabriel mountains in california or the mont blanc aar massifs in the alps we estimate the finite cenozoic shortening of the folded dzungarian sediments to be of the order of 30 km and the cenozoic shortening rate to have been 3 1 5 mm yr assuming comparable shortening along the tarim piedmont and minor additional active thrusting within the mountain belt we infer the rate of shortening across the tien shan to be at least 6 3 mm yr at the longitude of manas 85 5 e a total shortening of 125 30 km is estimated from crustal thickening assuming local airy isostatic equilibrium under the same assumption serial n s sections imply that cenozoic shortening across the belt increases westwards to 203 50 km at the longitude of kashgar 76 e as reflected by the westward increase of the width of the belt this strain gradient implies a clockwise rotation of tarim relative to dzungaria and kazakhstan of 7 2 5 around a pole located near the eastern extremity of the tien shan west of hami 96 e 43 5 n comparable to that revealed by paleomagnetism between tarim and dzungaria 8 6 8 7 a 6 mm yr rate of shortening at the longitude of manas would imply a rate of rotation of 0 45 m y and would be consistent with a shortening rate of 12 mm yr north of kashgar taking such values to be representative of late cenozoic rates would place the onset of reactivation of the tien shan by the india asia collision in the early to middle miocene 16 22 9 m y in accord with the existence of particularly thick late neogene and quaternary deposits such reactivation would thus have started much later than the collision roughly at the time of the great mid miocene changes in tectonic regimes denudation and sedimentation rates observed in southeast asia the himalayas and the bay of bengal and of the correlative rapid change in seawater sr isotopic ratio 20 to 15 ma like these other changes the rise of the tien shan might be a distant consequence of the end of indochina s escape

magmatism and metamorphism in the lhasa block tibet a geochronological study u pb systematics in small fractions of zircon monazite and sphene 5 to 20 grains each have been analyzed in order to decipher the age and origin of magmatism in the lhasa block and to date the tectono metamorphic events additionally we analyzed a few minerals for rb sr the results show that the granites and an ignimbrite were formed essentially through anatexis of continental crust the magma source regions of which consisted of palaeozoic to precambrian materials up to 2 b y in age the following episodes can be distinguished 1 an early tertiary plutonism volcanism in the southern lhasa block yangbajing region strongly related to intracontinental tectono metamorphism 2 a late jurassic to early cretaceous plutonism in the central and northern lhasa block and 3 a middle jurassic low grade metamorphism affecting the basement at the northern margin 171 6 m y ago the early tertiary magmatism and metamorphism in the south are probably a direct result of tethys subduction indian plate

paleomagnetic and geochronological constraints on the post collisional northward convergence of the southwest tian shan nw china a combined paleomagnetic and geochronological study is reported of paleogene basalt lavas and an intercalated red bed succession comprising a minimum of 14 basalt flows and 10 red bed horizons in the tuoyun basin of the southwest tian shan range china two basalt matrix samples yield 40 ar 39 ar isochron ages of 58 5f1 3 ma 2r mswd 0 9 and 60 4f1 3 ma 2r mswd 1 7 these compare well with a previously published k ar dilution age of 61 7f2 3 ma for comparable paleogene basalts and confirm that the younger pulse of magmatism in this basin is represented by both intrusive and extrusive activity demagnetization and component analysis identify a stable characteristic remanence chrm with predominantly reversed polarity following removal of secondary remanence by peak demagnetization steps below 250 350 8c or 5 mt rock magnetic analysis identifies pseudo single domain magnetite or titanomagnetite as carriers the stable chrm passes a fold test it was probably acquired at the time of lava emplacement results from the bulk of the collection imply that paleomagnetic data from the upper and lower 115 ma basalt series in the tuoyun basin are not distinguishable at the 95 significance level and indicate that this tectonic domain remained essentially stationary with respect to the earth s spin axis for 50 ma prior to onset of the india asia collision in early eocene times it is therefore probable that no paleomagnetically detectable crustal shortening occurred in the southwest tian shan prior to collision paleomagnetic data sets from the tuoyun basin also show that little or no paleolatitude difference is present between the tian shan and the reference latitude of eurasia at 60 ma this supports previous evidence suggesting that central asian blocks in the vicinity of the tian shan are unlikely to have experienced appreciable northward convergence relative to eurasia since onset of the india asia collision and initiation of the himalaya d 2005 elsevier b v all rights reserved

what was the paleogene latitude of the lhasa terrane a reassessment of the geochronology and paleomagnetism of linzizong volcanic rocks linzhou basin tibet the paleogene latitude of the lhasa terrane southern tibet can constrain the age of the onset of the india asia collision estimates for this latitude however vary from 5 n to 30 n and thus here we reassess the geochronology and paleomagnetism of paleogene volcanic rocks from the linzizong group in the linzhou basin the lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes we report consistent 40ar 39ar and u pb zircon dates of 52 ma for the upper linzizong and 40ar 39ar dates 51 ma from the lower linzizong are significantly younger than u pb zircon dates 64 63 ma suggesting that the lower linzizong was thermally and or chemically reset paleomagnetic results from 24 sites in lower linzizong confirm a low apparent paleolatitude of 5 n compared to the upper part 20 n and to underlying cretaceous strata 20 n detailed rock magnetic analyses end member modeling of magnetic components and petrography from the lower and upper linzizong indicate widespread secondary hematite in the lower linzizong whereas hematite is rare in upper linzizong volcanic rocks of the lower linzizong have been hydrothermally chemically remagnetized whereas the upper linzizong retains a primary remanence we suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower linzizong we estimate that the paleogene latitude of lhasa terrane was 20 4 n consistent with previous results suggesting that india asia collision likely took place by 52 ma at 20 n

palaeomagnetic data about southern tibet xizang i the cretaceous formations of the lhasa block summary a palaeomagnetic study of middle to late cretaceous redbeds from linzhou basin lhasa block north of the yarlung zangbo suture zone gives a stable palaeomagnetic direction of magnetization with a positive fold test six sites 57 samples d 333 i 38 k 78 95 8 pole 64 n 348 e we discuss the problem of a possible remagnetization but consider that this direction of magnetization gives a good approximation for the palaeolatitude of the lhasa block during middle to late cretaceous time results from more recent tibetan formations are also presented late cretaceous to palaeocene sediments and volcanics give a lower palaeolatitude of 10 n and but more recent andesites have emplaced about 30 n close to the present day latitude an interpretation is proposed whereby the lhasa block which was a part of asia in the early cretaceous has undergone first a southward motion accompanied by an anticlockwise rotation and then after the palaeocene a northward motion under the constraint of the colliding india

palaeomagnetism and 40ar 39ar geochronology of upper palaeogene volcanic rocks from central tibet implications for the central asia inclination anomaly the palaeolatitude of tibet and post 50 ma shortening within asia summary palaeomagnetic results from upper palaeogene volcanics from three localities in the qiangtang terrane of central tibet record a maximum likelihood palaeolatitude of 28 7 3 7 n these are the first volcanic based palaeomagnetic results from the plateau interior that appear to average secular variation well enough to yield a reliable palaeolatitude estimate this palaeolatitude estimate is significantly lower than the 36 3 n predicted for central tibet by late palaeogene eurasia reference poles consistent with the so called central asia inclination anomaly but it is consistent with values predicted from upper palaeogene reference poles for mongolia the mean inclination of coeval sedimentary rocks from central tibet is 15 5 shallower than our volcanic results similar to the magnitude of sedimentary inclination shallowing estimated from elongation inclination analysis of remanent directions in clastic rocks from the same region we conclude that most of the inclination anomaly observed in volcanic rocks and that which remains after removal of sedimentary flattening in clastic rocks probably is due to the combined effect of reference poles that are not representative of stable asia and by cenozoic intracontinental shortening within central asia on the order of several hundred not more than 1000 kilometers our compilation of palaeomagnetic data from the qiangtang terrane further suggest that the southern margin of the lhasa terrane the southern margin of pre india asia was located as far south as 20 n throughout eocene time these palaeolatitudes in conjunction with the upper crustal shortening record for asia indicate that only approximately one third of the post 50 ma india asia convergence is partitioned in asian lithosphere low latitudes also imply that much of the proto tibetan plateau occupied southerly tropical to subtropical latitudes which has important implications for the weathering and erosion of the tibetan plateau palaeoaltimetry proxies and early cenozoic climate dynamics

palaeomagnetism of cretaceous to tertiary sediments from southern tibet evidence for the extent of the northern margin of india prior to the collision with eurasia abstract palaeomagnetic studies have been carried out on mid cretaceous to latest palaeocene sediments from the tethyan himalaya on the northern margin of the indian plate near the localities of gamba 28 3 n 88 5 e and duela 28 0 n 89 2 e in southern tibet a total of 61 sites more than 700 cores have been sampled mainly from limestones zongshan formation jidula formation member ii zongpu formation sandstones jidula formation member iii and graywackes gamba group member ii with ages well defined from detailed lithological and biostratigraphic analyses carriers of characteristic remanences are magnetite in limestones and graywackes and hematite in sandstones reliable palaeodirections were obtained from 35 sites 356 specimens positive fold tests antiparallel normal and reverse polarities a northward trend of the palaeolatitudes with decreasing age and absence of any metamorphic overprint indicate a primary origin for the characteristic remanences the results have been compared with expected palaeodirections according to available data for the indian subcontinent based on its present day geometry the late maastrichtian sites fourteen sites 156 specimens and the middle late palaeocene sites fourteen sites 113 specimens yield palaeolatitudes of 5 7 s and 4 0 n for the sampling localities in southern tibet respectively these palaeolatitudes are much further north than expected and imply that crustal shortening between the tethyan himalaya and the indian shield since the time of india eurasia collision has occurred on the order of 1500 km at the longitude of gamba duela comparison of our observed palaeolatitudes with palaeolatitudes for the lhasa block during cretaceous and palaeocene times indicate that initial contact of the northern indian margin with the lhasa block was already established at 65 60 ma with suturing completed at about 55 50 ma

thermal evolution of the gangdese batholith southern tibet a history of episodic unroofing the gangdese batholith southern tibet was part of an andean type arc at the southern margin of asia prior to the collision of india and asia at approximately 50 to 40 ma fission track and 40ar 39ar analyses of 28 rocks from 10 gangdese granitoid plutons along an 250 km length of the batholith in the lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks these data suggest a range of ages for these plutons of 94 to 42 ma with the majority being of tertiary age the postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities we conclude that most of these discontinuities and all of them after 40 ma reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time in addition to the initial cooling of hot magma against cold country rock all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling dropping many tens of degrees in a few million years conversely these plutons all experienced intervals during which they cooled very slowly or not at all these slow cooling intervals lasted from 5 to 50 million years our data indicate that since the collision between india and asia began response to continued convergence has been quite variable in even this relatively small area the data reported here are consistent with a recently proposed model of oligo miocene crustal shortening along the gangdese thrust system in this area

magnetic inclination shallowing problem and the issue of eurasia s rigidity insights following a palaeomagnetic study of upper cretaceous basalts and redbeds from se china

paleomagnetic results of cretaceous cherts from zhongba southern tibet new constraints on the india asia collision abstract an oceanic greater indian basin was hypothesized to have formed between 120 ma and 65 ma by regional extension that separated the cratonic india from its northern margin of tethyan himalaya th however the lack of paleomagnetic data from this critical time interval in the th marks an important gap in our knowledge about the tectonic evolution of the greater india we have conducted a paleomagnetic study of a cretaceous marine succession in zhongba southern tibet that consists of predominantly purple reddish cherts and siliceous limestone paleomagnetic samples from 12 sites were subjected to stepwise thermal demagnetization that revealed either two or three component magnetizations with the high temperature component htc typically unblocked at 650 c the htcs exhibit dual polarities that yield a positive reversed test at 95 confidence level suggesting a primary origin of the remanence with chronologic constraints from the combined analysis of the magnetostratigraphic and radiolarian data the htcs yield a mean of ds 336 9 is 50 7 ks 18 0 95 5 3 for the 120 8 93 9 ma interval of the th which corresponds to a paleopole at 25 0 n 285 7 e dp dm 4 8 7 1 the apparent polar wander path apwp of the th overall resembles that of cratonic india suggesting no polarward separation that is sufficiently large to merit an oceanic basin between the th and cratonic india therefore our new reliable paleomagnetic data from the critical time interval do not support the greater indian basin hypothesis and its implied dual collisional processes between india and asia on the other hand the apwps exhibit relative shifts for the post 120 ma segment and the pre 130 ma segment indicating relative motions between the th and cratonic india that most likely resulted from shearing along boundary parallel faults during the india asia collision

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

position of the lhasa block south tibet during the late cretaceous the latitude of the lhasa block xizang south tibet during the late cretaceous early tertiary is of prime importance for understanding the collision of india and eurasia and the mechanism of the formation of the qinghai xizang plateau during the first field expedition of the french chinese collaboration several geological formations have been sampled in southern tibet between lhasa 29 40 n 91 09 e and dingri 28 35 n 86 38 e we present here the first results obtained for middle cretaceous to palaeocene red beds from the lhasa block north of the yarlung zang bo suture zone recognized as the main suture between the indian and the eurasian plates they indicate a palaeolatitude of the lhasa block of 20 n during the late cretaceous while india was situated 30 s

new paleomagnetic results from the lhasa block revised estimation of latitudinal shortening across tibet and implications for dating the india asia collision knowing how and when the tibetan plateau formed is of global concern as high topography modifies climate circulation patterns and alters species migration paths previous paleomagnetic work suggested that intracontinental shortening from deformation related to the india asia collision played a major role in building the tibetan plateau however these studies were derived from red bed facies which are often afflicted by inclination shallowing leading to overestimates of continental shortening we report new paleomagnetic data from the lhasa block the southern continental margin of asia before the india asia collision a high temperature characteristic remanent magnetization chrm was isolated from 43 sites of late cretaceous red beds 32 sites of late cretaceous lava flows and nine sites of eocene tuffs all the chrm directions are significantly different from the present day geomagnetic field and the mean directions of red beds lava flows and tuffs pass the fold test the distribution patterns of chrm directions derived from volcanic rocks are consistent with a secular variation model suggesting that secular variation was likely averaged out in contrast the red bed chrm directions exhibit an east west elongated distribution pattern indicative of inclination flattening the mean chrm inclination of the red beds 23 5 degrees 2 5 degrees is consistent with previous results but significantly shallower than that of 41 9 degrees 4 4 degrees carried by the coeval lava flows an elongation inclination analysis of the red bed chrm directions yielded an unflattened mean inclination of 39 7 degrees with 95 confidence limits between 37 7 degrees and 41 9 degrees the paleolatitudes of the late cretaceous volcanic rocks and inclination corrected sediments suggest about 10 degrees less shortening occurred between lhasa and siberia than previous estimates with total shortening lying between 3 3 degrees 2 6 degrees and 7 3 degrees 5 4 degrees since the late cretaceous the mean chrm inclination derived from the eocene tuffs 51 8 degrees 5 2 degrees is indistinguishable from the expected reference direction our new results imply that india asia convergence was accommodated mainly through consumption of the india plate and yield a much younger age ca 43 ma for the initial india asia collision than previously estimated by paleomagnetic data c 2010 elsevier b v all rights reserved

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

positioning paleogene eurasia problem solution for 60 50 ma and broader tectonic implications abstract recently published paleomagnetic data from the faroe islands and se england have enabled a hybrid pole 72 0 n 177 9 e a95 7 9 to be calculated for stable eurasia 55 ma it is somewhat different to previous proposals being a further 8 9 from the present day north pole a strong positive test of the new pole is provided by 2002 published paleomagnetic data from basaltic rocks in the tien shan range in kyrgyzstan the paleolatitude derived from the inclination angle matches the predicted value to within 0 2 an unfortunate drawback with kyrgyzstan pole is its large age error 15 m y for rocks estimated to have formed 50 ma fortuitously an alternative test is now available using paleomagnetic data from paleocene basalts in the tien shan range of western china for which a robust radiometric age date 59 1 ma based on two ar ar results also exists although the locality has experienced a large vertical axis rotation the mean declination being 54 5 the inclination angle appears undisturbed and the derived paleolatitude matches the value predicted by hybrid pole to within 4 0 thus it is contended the faroe sheppey pole provides one of the most reliable means of fixing eurasia s position for the interval 60 50 ma it also impacts on various model proposals for the india asia collision and subsequent crustal shortening and or extrusion between southern tibet and stable eurasia north of the tien shan

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

modification of wave cut and faulting controlled landforms from a casual observation that the form of degraded fault scarps resembles the error function this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms the morphologic observations can be analyzed either in the form of one or more cross strike elevation profiles or in the form of the slope offset plot a point plot of maximum scarp slope versus scarp offset working with either or both of these data representations for nine geologic structures which range in age from 3 to 400 ka b p and in offset from 1 to 50 m we apply analytical solutions for the vertical initial value scarp the vertical continuous offset scarp and the finite slope initial value scarp the model calculations are intrinsically ambiguous yielding as the final answer only the product t in the case of the initial value problem or the product a 1 in the case of the repeated faulting problem here t is the age of a single scarp forming event 2a is the vertical slip rate and is the mass diffusivity a single profile across three sea cliffs along the santa cruz california coast is analyzed as three separate initial value problems a reasonably constrained age for the sea cliff standing above the highway 1 platform returns 11 gkg 1 gkg 1 m2 ka with this we can date the two older sea cliffs in fact we do the converse age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same as for the lower sea cliff we treat a single profile of the raymond fault in pasadena san marino in terms of the repeated faulting problem for it the uplift rate of r crook and others yields 16 gkg the very substantial preexisting offset across the raymond fault must have been buried leveled some 230 ka b p when the modern topography began to form our analysis of the lake bonneville shoreline scarps reveals a dependence of t on 2a suggestive of nonlinear modification processes this appearance is treated with the finite slope initial value scarp model to determine 1 1 gkg for the lake bonneville shoreline scarps the suggestion of m n machette that approximately 100 000 year old meter high scarps are unobservable in weakly consolidated alluvial terranes of the basin and range and rio grande rift valley provinces can be formulated as 1 gkg the coincidence between this inequality and the lake bonneville shoreline is striking and it suggests that the value of 1 gkg may be generally applicable as a good first approximation to the modification of alluvial terranes within the semiarid regions of the western united states the lake bonneville shoreline is the basis for dating four sets of fault scarps in west central utah the drum mountains fault scarps can be modeled in several different circumstances but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3 6 to 5 7 ka b p the younger age is associated with quite low initial slope angles 25 the other three sets of fault scarps show no evidence for finite initial value slopes fault scarps along the eastern base of the fish springs range are very young 3 ka b p we estimate the age of fault scarps along the western flank of the oquirrh mountains to be 32 ka b p which meets the weak geologic constraint that they be older than the lake bonneville shoreline fault scarps along the northeastern margin of the sheeprock mountains are even older 53 ka b p an intriguing consequence of our single event analysis of these scarps is that an 11 5 m offset occurred in a single earthquake

late pleistocene and holocene paleoearthquake activity of the olinghouse fault zone nevada paleoseismic trenching and fault surface trace mapping indicate that the olinghouse fault zone a northeast trending left lateral strike slip fault located in the northern walker lane nevada has been the source of multiple latest pleistocene and holocene surface rupturing earthquakes a trench exposure near the eastern end of the fault records two and possibly three earthquakes after 3360 190 cal yr b p and two trenches at the western end of the fault contain evidence for two earthquakes after 19 800 630 cal yr b p with the most recent earthquake occurring after 1935 70 cal yr b p the apparent higher frequency of recent earthquakes at the eastern end of the fault may reflect triggered slip on the olinghouse fault zone due to earthquakes on the conjugate northwest trending right lateral pyramid lake fault repeated late pleistocene and holocene earthquakes on the olinghouse fault zone demonstrate that northwest directed right lateral shear of the northern walker lane is accommodated in part by northeast trending left lateral faults

morphologic dating of fluvial terrace scarps and fault scarps near west yellowstone montana analysis of fault and fluvial terrace scarps in the west yellowstone basin south of hebgen lake montana indicates that during degradation the gradient of the scarp face and the curvatures of the basal concavity and crestal convexity of the scarp face decrease become more rounded with time the pattern of degradation is identical with that predicted by a simple analytical model for degradation by creep in which the change in elevation at a point on a hillslope is proportional to the curvature of the profile at that point the model therefore provides the basis for morphologic dating a technique for determining the age of a scarp by the extent to which its initial morphology has been degraded in order to date a scarp the coefficient of proportionality c must be known the technique is probably not suitable for use on hillslopes degraded by processes other than creep both the potential and the limitations of morphologic dating are demonstrated via measurements of a sequence of terrace scarps along the madison river the calculated relative ages are in agreement with the known relative ages despite the fact that the gradient of the youngest scarp is significantly less than that of the older scarps to calculate an absolute morphologic age c must be derived from analysis of a scarp of known age having the same aspect must be underlain by the same material and must be within the same climatic zone as the scarp to be dated in the west yellowstone basin a value of c 2 00 0 38 10 3 m 2 yr was calculated from a minimum date of 7100 50 yr b p determined by radiocarbon dating for one of the terrace scarps scarps formed by normal faulting in the west yellowstone basin are much more difficult to date than are terrace scarps because of the possible complexity of their initial morphology fault scarps formed during the 1959 yellowstone earthquake commonly consist of an assemblage of two or more smaller scarps or are superimposed on preexisting scarps formed by one or more previous faulting events by using a selective sampling procedure however one can screen out these fault scarps in this way it was possible to determine an age of 2 800 1 100 yr for a pre 1959 fault scarp in the west yellowstone basin because the fault and terrace scarps in the west yellowstone area do not vary significantly in height the morphologic dating technique proposed earlier by nash 1980a cannot be applied to them the substantially modified procedure developed for the present study does not require a wide range of heights for each scarp to be dated it can be performed using a relatively small number of scarp profiles if c is known it is easy to use and when used with considerable care it is reasonably accurate 36 59 in this study it is potentially applicable to scarps in other areas

dem based morphometry of range front escarpments in attica central greece and its relation to fault slip rates in this paper we apply current geological knowledge on faulting processes to digital processing of digital elevation models dem in order to pinpoint locations of active faults the analysis is based on semiautomatic interpretation of 20 and 60 m dem and their products slope shaded relief in northern eastern attica five normal fault segments were recognized on the 20 m dem all faults strike wnw ese the faults are from west to east thriassion thfs fili fifs afidnai affs avlon avfs and pendeli pefs and range in length from 10 to 20 km all of them show geomorphic evidence for recent activity such as prominent range front escarpments v shaped valleys triangular facets and tilted footwall areas however escarpment morphometry and footwall geometry reveal systematic differences between the bexternalq segments pefs thfs and avfs and the binternalq segments affs and fifs which may be due to mechanical interaction among segments and or preexisting topography in addition transects across all five escarpments show mean scarp slope angles of 22 18f0 78 for both carbonate and metamorphic bedrock the slope angle equation for the external segments shows asymptotic behaviour with increasing height we make an empirical suggestion that slope angle is a function of the long term fault slip rate which ranges between 0 13 and 0 3 mm yr the identified faults may rupture up to magnitude 6 4 6 6 earthquakes the analysis of the 60 m dem shows a difference in fault patterns between western and northern attica which is related to crustal rheology variations d 2004 elsevier b v all rights reserved

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

preliminary conclusions of the royal society and academia sinica 1985 geotraverse of tibet the 1985 chinese british expedition to the tibetan plateau attempted to solve the question of the origin of the very thick crustal rocks in this region continuing northwards movement of the indian plate over the past 38 myr has given rise to severe folding and thrust faulting causing crustal thickening by internal deformation previous collisions of microplate terranes derived from gondwanaland occurred during mesozoic times but the kun lun terrane of northern tibet was already part of laurasia by the carboniferous

application of uav photography to refining the slip rate on the pyramid lake fault zone nevada the pyramid lake fault zone plfz is a 50 km long active northwest trending right lateral fault in the northern walker lane located 30 km east of reno nevada previous paleoearthquake and slip rate studies report that the pyramid lake fault has produced four surface rupturing paleoearthquakes since 15 475 720 cal b p three of which occurred after 8980 260 cal b p and has had an average minimum slip rate of 2 6 0 3 mm yr since the late pleistocene these observations imply that coseismic offset for each paleoearthquake averaged 7 9 m larger than expected from empirical scaling relationships for a strike slip fault of its length to reconcile this discrepancy we used a small camera mounted unmanned aerial vehicle to develop high resolution digital elevation models and interpret previously unreported right laterally offset geomorphic features along the northern section of the plfz offset measurements at seven sites range from 8 to 21 m the ages of displaced features are interpreted from previous lake level and mapping studies of lake lahontan from these observations slip rate estimates at the sites range between 0 5 and 1 6 mm yr these lower values of slip rate require that coseismic displacements associated with previously reported paleoearthquakes average only 3 5 m within the range that would be predicted from empirical scaling relationships of rupture length and coseismic displacement

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

ages estimated from a diffusion equation model for scarp degradation the diffusion equation derived from the continuity equation for hillslopes is applied to scarp erosion in unconsolidated materials solutions to this equation allow direct calculation of the product of the rate coefficient and the age of the scarp from measurements of scarp morphology where the rate coefficient can be estimated or can be derived from scarps of known age this method allows direct calculation of unknown ages of scarps

neotethys and the india asia collision insights from a palaeomagnetic study of the dazhuqu ophiolite southern tibet since 1999 independently derived geophysical and geological models have been published arguing for an intra oceanic subduction system along essentially the entire width of the india eurasia collision belt this idea conflicts with earlier proposals where in the eastern part of the convergence zone neotethyan mid ocean ridge generated lithosphere directly north of the indian craton was consumed beneath eurasia in tibet in an andean type plate configuration new palaeomagnetic data are reported from three barremian aptian 120 ma sequences of chert siliceous mudstones and volcaniclastic rocks these rocks form the uppermost part of the dazhuqu supra subduction zone ophiolite terrane which crop out along substantial portions of the india eurasia yarlung tsangpo suture zone in southern tibet the declination data provide little regionally useful tectonic information they are dominated by local rotations presumably related to the dazhuqu terrane s initial obduction onto the india plate in the palaeocene and subsequent movement s as india later collided and indented into eurasia the inclination data are however useful because they yield consistent sub equatorial formation sites 2 7 s 3 0 7 9 n 2 7 1 4 n 5 7 which correspond with the location of the neotethyan intra oceanic subduction system inferred from the seismic tomographic data at the time these dazhuqu terrane rocks formed the intra tethyan subduction zone would have been about 2500 km south of eurasia

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

a quasi linear structure of the southern margin of eurasia prior to the india asia collision first paleomagnetic constraints from upper cretaceous volcanic rocks near the western syntaxis of tibet we report the first combined geochronologic and paleomagnetic study of volcanic rocks from the shiquanhe and yare basins at the westernmost lhasa terrane which aims to provide an accurate constraint on the shape and paleoposition of the southern margin of asia prior to the india asia collision three new 40ar 39ar ages of 92 5 2 9 ma 92 4 0 9 ma and 79 6 0 7 ma determined by fresh matrix or feldspar from lava flows suggest a late cretaceous age for the investigated units characteristic remanent magnetizations have been successfully isolated from 38 sites which pass positive fold and or reversal conglomerate tests and are hence interpreted as primary in origin the two paleopoles obtained from yare and shiquanhe yield consistent paleolatitudes of 13 6 n 9 6 n and 14 2 n 2 7 n respectively for a reference site of 31 5 n 80 e indicating that the southern margin of asia near the western syntaxis was located far south during the late cretaceous time a reconstruction of the lhasa terrane in the frame of eurasia with paleomagnetic data obtained from its western and eastern parts indicates that the southern margin of eurasia probably had a quasi linear orientation prior to the collision formerly trending approximately 315 e this is compatible with the shape of the neo tethys slab observed from seismic tomographic studies our findings provide a solid basis for evaluating cenozoic crustal shortening in the asian interior and the size of greater india near the western syntaxis

paleomagnetism of eocene red beds in the eastern part of the qiangtang terrane and its implications for uplift and southward crustal extrusion in the southeastern edge of the tibetan plateau abstract a primary magnetic component with the acquisition time of 56 0 43 2 ma was isolated between 580 c and 685 c from the eocene gonjo and ranmugou formations in the eastern part of the qiangtang terrane tibetan plateau the tilt corrected site mean direction is d s 35 5 i s 29 3 k 45 9 95 3 2 the site mean inclination increased from 29 3 to 41 6 after multiple inclination shallowing corrections giving a paleopole of 57 9 n 192 1 e a 95 2 9 comparison of the paleogene paleomagnetic data for the qiangtang terrane and lhasa terrane reveals that both terranes experienced latitudinal crustal shortening before 54 43 ma indicating the uplift of southern and central tibet in the early eocene subsequently since 35 4 2 4 ma the north of the qiangtang terrane experienced 1300 410 km of crustal shortening indicating the uplift of the northern tibetan plateau the lhasa terrane and qiangtang terrane have not experienced further crustal shortening since the late eocene and the southeastern part of tibet cannot have provided abundant crustal material to accommodate the significant crustal southward extrusion in the southeastern edge of the tibetan plateau the crust of the tengchong terrane and shan thai block did not experience significant southward extrusion since the late oligocene early miocene the indochina block was situated in the north of the qiangtang terrane before the oligocene and since the early oligocene the indochina block began to experience southward extrusion from the north of the qiangtang terrane which absorbed part of the crustal shortening in the north of the qiangtang terrane

geomorphic evidence for holocene earthquakes in the olinghouse fault zone western nevada mapping and morphologic analysis of young fault scarps in the olinghouse fault zone of western nevada yields information about the ages and magnitudes of holocene late pleistocene surface rupturing earthquakes twenty profiles of single event scarps in the eastern third of the fault zone yield ages that average 4 5 2 0 ka ages were determined assuming linear diffusion of scarp material with the diffusion coefficient as a function of scarp offset determined from profiles of lake lahontan high stand shorelines located nearby the mid holocene age is consistent with the observation that the scarps are cutting alluvial material that buries the 15 4 ka shorelines the minimum length of faulting during this earthquake was 8 km and the maximum normal fault offset was about 1 m suggesting an earthquake of about m 6 7 several older scarp segments have cumulative normal offsets of 2 5 to 4 8 m and one or two crest bevels above the most recent scarp slope suggesting that two to three earthquakes have formed these scarps the average diffusion age of five multiple event scarp profiles is about 13 ka if this represents a weighted average of the ages of the earthquakes that formed the scarps then two or three large earthquakes have occurred since about 21 ka which gives a speculative average recurrence period of 8 to 16 kyr historical accounts and geomorphic evidence of young left slip offsets of up to 3 6 m m 7 1 at the western end of the olinghouse fault zone suggest that it may have ruptured historically thus the potential exists for large earthquake generation all along the 20 km length of the fault zone

cretaceous paleomagnetic results from western tibet and tectonic implications we present paleomagnetic results obtained from samples collected in the summer of 1989 along a 1000 km traverse between yecheng xinjiang and shiquanhe tibet in the western qinghai xizang plateau along 80 e longitude during this field trip 480 paleomagnetic cores in jurassic to tertiary limestones and sandstones were drilled at 49 sites 400 were measured isothermal remanent magnetization analysis indicates the existence of high coercivity minerals and thermal rather than alternating field demagnetization was found to be more efficient in isolating paleomagnetic directions the characteristic magnetization has rather low unblocking temperatures 250 560 c in most limestone samples and higher unblocking temperatures 400 c 680 c in the sandstone samples about half of the measured samples show the present geomagnetic field or aberrant directions the remainder display stable multicomponent magnetizations the stable characteristic magnetizations indicate 1 a probable jurassic remagnetization 2 a cretaceous 120 80 ma magnetization which passes a fold test yielding a mean pole at 66 2 n 245 0 e a95 5 1 n 14 sites and 3 a large scatter of tertiary directions which may be due to tectonic problems and or remagnetization in the sampling zone the cretaceous paleodirections from aksaichin and longmuco 11 sites on the qiangtang north tibet block are very similar to those from shiquanhe three sites on the lhasa south tibet block separated by 300 km this suggests that the north and south tibet blocks formed a single unit at least as early as the cretaceous comparing our results with previous paleomagnetic work on the tibetan plateau it is found that the paleolatitude of 10 6 5 1 obtained from this study for a reference at domar 33 75 n 80 4 e is close to that 9 6 2 8 obtained by achache et al 1984 and lin and watts 1988 from the takena formation 110 100 ma in the lhasa region 30 n 91 e in addition it appears that the southern margin of the tibetan blocks i e eurasia occupied an e w trending position at about 7 6 n latitude paleodeclination differences between the western and eastern tibetan blocks suggest internal deformations of the plateau at different scales from 1 to 1000 km according to differences in paleomagnetic declinations the arcuate shape of the blocks would have been partly acquired after the collision

paleomagnetism of paleogene basalts from the tien shan kyrgyzstan rigid eurasia and dipole geomagnetic field several conflicting explanations invoking persistent non dipole fields or tectonic motion on various scales were suggested to account for anomalously low paleomagnetic inclinations in cenozoic rocks of central asia in order to better understand this problem we undertook a paleomagnetic study of paleogene basalts from a part of the tien shan close to the china kyrgyzstan border stepwise thermal demagnetization showed that only a single component remanence of reversed polarity is present in these rocks above 200 260 c the overall mean direction of this remanence d 194 6 i 354 0 k95 3 8 n 18 sites agrees well with the eurasian reference directions for the paleogene which is in sharp contrast with most earlier published cenozoic inclinations from central asia which are 20 30 shallower than the expected values good agreement of the reference inclinations and those from basalts rules out models which invoke non dipole fields and or large scale tectonic motions for explanation of the cenozoic inclination anomaly instead a mechanism related to natural remanent magnetization acquisition in redbeds is strongly indicated the above conclusion also implies that the differences between the reference data and measured inclinations in cretaceous redbeds from central asia are also of non tectonic origin and a revision of our views on evolution of this region is required fl 2002 elsevier science b v all rights reserved

remnants of a cretaceous intra oceanic subduction system within the yarlung zangbo suture southern tibet extensive field investigations along the yarlung zangbo suture zone in southern tibet reveal the presence of now fragmented remnants of a south facing intra oceanic subduction system this system developed within tethys during the cretaceous the associated arc forearc ophiolite and subduction complex were emplaced onto the leading edge of india at the end of the cretaceous rapid sedimentation in oblique slip basins and disruption of water saturated sediments into melange was widespread and concomitant with ophiolite emplacement we describe the tectonic entities that developed during this previously unrecognized phase of tethys tibet evolution and present a new model for the evolution of this portion of tibet

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

new cretaceous and early tertiary paleomagnetic results from xining lanzhou basin kunlun and qiangtang blocks china implications on the geodynamic evolution of asia we present the results of a paleomagnetic study of 360 cores drilled at three different areas from the tibetan plateau 13 sites from lower cretaceous red beds in the xining lanzhou basin around xining and lanzhou cities southwest of the qilian mountains 36 2 n 103 5 e 13 sites from cretaceous red beds in the kunlun block near maqin 34 5 n 100 1 and 9 sites from paleocene lower eocene red beds in the qiangtang block near fenghuoshan 34 5 n 92 8 e thermal demagnetization of the samples allowed us to isolate a high temperature component which passes both positive reversal and fold tests for all formations the corresponding paleopoles lie at 50 3 n 195 5 e a95 4 6 for xining lanzhou 80 1 n 281 2 e dp 7 8 dm 12 7 for maqin and 62 6 n 210 5 e dp 3 9 dm 6 8 for fenghuoshan we discuss these in the frame of a new paleogeographic reconstruction of the cretaceous paleoposition of the blocks forming the asian mosaic we conclude that the xining lanzhou area could not be part of the north china block but rather was associated with the tarim qaidam assemblage paleomagnetic data argue in favor of a qaidam kunlun tarim junggar assemblage in the cretaceous significantly to the south of its current position with respect to the asian continent siberia mongolia and north china the large n s convergence 800 500 km implied since the cretaceous appears to be far larger than could be absorbed in the altay range to the north and qilian shan to the east of the order of 300 km part of this motion could have occurred along a large left lateral strike slip fault system which may connect with the mongol okhotsk suture to the northeast verifying this hypothesis will require new geologic and paleomagnetic data from these remote regions

a successful rock magnetic technique for correctng paleomagnetic inclination shallowing case study of the nacimiento formation new mexico the accuracy of an inclination shallowing correction technique was tested on the remanence of the paleocene nacimiento formation the correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy the remanence anisotropy is also a function of the magnetic anisotropy of the individual magnetic grains the nacimiento formation was selected for this test because its inclination is shallow by 7 to 8 and previous paleomagnetic studies indicate that it has a primary remanence with a well constrained age and that magnetic overprints are minimal tectonics are an unlikely explanation for its shallow inclination these conditions allow the inclination corrected remanence to be compared to a well defined paleomagnetic pole to determine the accuracy of the technique the characteristic remanence from 20 nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site individual particle anisotropy was determined by drying a magnetic separate epoxy mixture in a 35 50 mt magnetic field laboratory compaction of disaggregated nacimiento material was also used to yield an effective individual magnetic particle anisotropy the two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections both corrections were successful and the inclination corrected formation mean direction was indistinguishable from the direction predicted by north america s paleocene paleopole the inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes e g compaction

paleogeographic and tectonic evolution of southern tibet since middle cretaceous time new paleomagnetic data and synthesis during the second french chinese paleomagnetic campaign in tibet in the summer of 1981 we sampled the middle cretaceous red beds of the takena formation and the cenozoic volcanics of the lingzizong formation a detailed analysis of about 400 specimens of red beds reveals a stable component of normal magnetization n in the north of the lhasa block north of the nyainqentanglha range and two stable components in the south lt ht where the red beds are more strongly folded and are unconformably overlain by the lingzizong volcanics all three components are carried by hematite blocking temperatures range from 400 to 600 c for lt and up to the neel temperature of hematite for both n and ht lt is shown to postdate part of the folding of the takena whereas n and ht both indicate a positive fold test at the 99 probability level in addition a small less than 10 of the natural remanent magnetization antiparallel component is superimposed in most samples and may be consistent with a model of long term acquisition of a chemical remanent magnetization for the red beds analysis of the volcanics reveals a single stable component with mixed polarities carried either by hematite or magnetite the paleomagnetic directions indicate that 1 the lhasa block stood at 12 5 3 n by upper cretaceous and at 13 5 6 5 n in the paleocene eocene thus confirming the limited displacement of the margin of eurasia between albian aptian deposition of the red beds and eocene onset of the india asia collision these values imply crustal shortening by as much as 1900 850 km north of the block since the upper cretaceous and by 1400 1200 km south of the block since the onset of the collision 2 southern tibet formed a single tectonic unit by middle cretaceous time and 3 differential rotations are observed within the block and are related to the collision these rotations are discussed in the frame of a recent propagating extrusion tectonics model this study confirms the close paleogeographic relationship between indochina khorat plateau and southern tibet prior to the india asia collision

can a primary remanence be retrieved from partially remagnetized eocence volcanic rocks in the nanmulin basin southern tibet to date the india asia collision paleomagnetic dating of the india asia collision hinges on determining the paleogene latitude of the lhasa terrane southern tibet reported latitudes range from 5 n to 30 n however leading to contrasting paleogeographic interpretations here we report new data from the eocene linzizong volcanic rocks in the nanmulin basin which previously yielded data suggesting a low paleolatitude 10 n new zircon u pb dates indicate an age of 52 ma negative fold tests however demonstrate that the isolated characteristic remanent magnetizations with notably varying inclinations are not primary rock magnetic analyses end member modeling of isothermal remanent magnetization acquisition curves and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously previously reported paleomagnetic data from the nanmulin basin implying low paleolatitude should thus not be used to estimate the time and latitude of the india asia collision we show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization we tentatively propose a new method to recover a primary remanence with inclination of 38 1 35 7 40 5 95 significance and a secondary remanence with inclination of 42 9 41 5 44 4 95 significance the paleolatitude defined by the modeled primary remanence 21 n 19 8 n 23 1 n is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper linzizong group corrected for inclination shallowing the start of the tibetan himalaya asia collision was situated at 20 n and took place by 50 ma

holocene slip rate and earthquake recurrence on the honey lake fault zone northeastern california the honey lake fault zone a major right lateral fault in the basin and range province of eastern california is one of a broad system of faults that accommodate some of the relative motion between the pacific and north american plates these right lateral faults may have significantly higher slip rates and thus greater earthquake hazards than the normal faults for which the basin and range is more commonly known in the honey lake valley the fault forms a 50 km long zone of landforms typical of active strike slip faults right lateral offset of an incised creek channel is used to estimate a holocene slip rate of between 1 1 and 2 6 mm yr a fault exposure in holocene alluvium shows evidence for at least four late holocene surface faulting earthquakes

gondwana to asia plate tectonics paleogeography and the biological connectivity of the indian sub continent from the middle jurassic through latest eocene 166 35 ma abstract using the most up to the date information available we present a considerably revised plate tectonic and paleogeographic model for the indian ocean bordering continents from gondwana s middle jurassic break up through to india s collision with asia in the middle cenozoic the landmass framework is then used to explore the sometimes complex and occasionally counter intuitive patterns that have been observed in the fossil and extant biological records of india madagascar africa and eastern eurasia as well those of the more distal continents although the paleogeographic model confirms the traditional view that india became progressively more isolated from the major landmasses during the cretaceous and paleocene it is likely that at various times minor physiographic features principally ocean islands provided causeways and or stepping stone trails along which land animals could have migrated to from the sub continent aside from a likely link albeit broken by several marine gaps to africa for much of this time it is notable that the present day recent biota of madagascar indicates that the ancestors of five land mammal orders plus bats crossed the 400 km wide mozambique channel at different times in the cenozoic it is possible that the kerguelen plateau connected india and australia antarctica in the mid cretaceous approximately 115 90 ma later the seychelles mascarene plateau and nearby elevated sea floor areas could have allowed faunas to pass between southern india and madagascar in the late cretaceous from around 85 65 ma with an early cenozoic extension to this path forming as a result of the reunion hot spot trace islands growing on the ocean floor to the ssw of india the modelling also suggests that india s northward passage towards asia with eventual collision at 35 ma involved the ne corner of the sub continent making a glancing contact with sumatra followed by burma from 57 ma late paleocene onwards a scenario which is compatible with the fossil record indicating that india asia faunal exchanges began occurring at about this time finally we contend that a number of biologically based direct terrestrial migration routes that have been proposed for last 15 m y of the cretaceous asia to india antarctica to madagascar and or india can probably be dismissed because the marine barriers likely varying from 1000 up to 2500 km were simply too wide

strain accumulation and rotation in western nevada 1993 2000 1 the positions of 44 gps monuments in an array extending from the sierra nevada at the latitude of reno to near austin nevada have been measured several times in the 1993 2000 interval the western half of the array spans the walker lane belt whereas the eastern half spans the central nevada seismic zone cnsz the principal strain rates in the walker lane belt are 29 6 5 3 nstrain yr 1 n88 4 e 5 4 and 12 8 6 0 nanostrain yr 1 n01 6 w 5 4 extension reckoned positive and the clockwise as seen from above the earth rotation rate about a vertical axis is 13 6 4 0 nrad yr 1 the quoted uncertainties are standard deviations the motion in the walker lane belt can then be represented by a zone striking n35 w subject to 16 8 4 9 nstrain yr 1 extension perpendicular to it and 19 5 4 0 nstrain yr 1 right lateral simple shear across it the n35 w strike of the zone is the same as the direction of the local tangent to the small circle drawn about the pacific north america pole of rotation the principal strain rates for the cnsz are 46 2 11 0 nstrain yr 1 n49 9 w 6 0 and 13 6 6 1 nstrain yr 1 n40 1 e 6 0 and the clockwise rotation rate about a vertical axis is 20 3 6 3 nrad yr 1 the motion across the cnsz can then be represented by a zone striking n12 e subject to 32 6 11 0 nstrain yr 1 extension perpendicular to it and 25 1 6 3 nstrain yr 1 right lateral simple shear across it the n12 e strike of the zone is similar to the strikes of the faults rainbow mountain fairview peak and dixie valley within it

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

paleomagnetism of the oligocene kangtuo formation red beds central tibet inclination shallowing and tectonic implications abstract a paleomagnetic study on the red beds of the oligocene kangtuo formation fm was carried out in the gerze basin of the lhasa terrane a total of 700 samples were collected from 37 sites stepwise thermal demagnetization revealed that the main magnetic carrier is hematite the natural remnant magnetization nrm consists of two components a low temperature component ltc is identified below 300 c and is interpreted to be a recent viscous overprint whereas a high temperature component htc unblocks at 665 690 c and is interpreted to be the primary magnetization the htc distributions show a clear east west elongated distribution which is considered as reflecting inclination flattening of deposited magnetic remanence carriers after inclination calibration using the e i method the htc could pass both a reversal test and a fold test at 95 confidence level showing the mean direction at ds 340 3 is 44 2 with k 63 0 and 95 3 1 corresponding to a paleopole at 71 7 n 339 3 e a95 3 1 and the paleolatitude of the sampling site at 25 9 3 1 n the paleolatitude is consistent with that expected from the coeval pole of the qiangtang terrane obtained from volcanic rocks suggesting that there has been no paleomagnetically discernable latitudinal motion between the qiantang and lhasa terranes since 30 ma comparing our new data with the apparent polar wander paths apwps of east asian blocks cogne et al 2013 europe and india besse and courtillot 2002 we have reached the following conclusions 1 there is no significant paleolatitudinal difference observed between the lhasa terrane and other central and northern asian terranes at 30 ma 2 the observed paleolatitude of the kangtuo fm is 8 0 4 9 lower than the expected paleolatitude deduced from the data of stable europe highlighting the asian inclination anomaly phenomenon but is 4 6 5 1 higher than that deduced from the data of the india plate likely presenting a 506 561 km shortening between the india and lhasa terrane since the oligocene it is inferred that this amount of shortening has been absorbed mainly by the himalaya orogenic belt

effect of height and orientation microclimate on geomorphic degradation rates and processes late glacial terrace scarps in central idaho terrace scarps can serve as a nearly ideal natural laboratory for the study of the evolution of slopes this paper examines the effects of scarp size height and orientation microclimate by keeping constant variables such as age lithology and regional climate if a scarp degrades as a closed system and downslope movement is directly proportional to surface gradient the evolution of the scarp is modeled by the diffusion equation for a group of scarps of same age and known starting angle the diffusion equation model predicts the relation between maximum scarp angle and scarp height h late pleistocene terrace scarps now as steep as 33 25 as well as measured angles of repose for sand and gravel require a starting angle as steep as 33 5 for latest pleistocene idaho and utah scarps as h increases is gentler more degraded than modeled by the diffusion equation with a constant rate coefficient the degradation rate coefficient c increases tenfold with scarp height it should not change with scarp height if downslope movement is solely determined by surface gradient to the first power soil wash appears to be responsible for this departure from the diffusion equation model for transport rate by soil wash is a function of scarp size height south facing scarps are less vegetated and more degraded than north facing scarps for scarps 2 m high the degradation rate c on s facing scarps is 2 times that on n facing scarps for 10 m scarps it is 5 times the observed dependence of the rate coefficient c on scarp height can be removed by normalizing c to values for west facing scarps of the same height the residual c values calculated by this method correlate well with differences in incident solar radiation resulting from the different scarp orientations and maximum gradients this correlation demonstrates the importance of orientation on slope processes and their rates through the differences in freeze thaw cycles soil moisture and vegetative cover scarp morphology may be used to estimate age if one accounts for the effects of climate and for scarp height orientation and lithology for example using the dated bonneville shoreline scarps for calibration and comparing only scarps of equal height we estimate the drum mountains fault scarps to be 9 000 yr old this age is about twice that produced by previous diffusion equation calculations that have not accounted for the height as we have here but it is the same as independent geologic estimates of their age

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

the relationship between the instantaneous velocity field and the rate of moment release in the lithosphere summary instantaneous velocity gradients within the continental lithosphere are often related to the tectonic driving forces this relationship is direct if the forces are secular as for the case of loading of a locked section of a subduction interface by the downgoing plate if the forces are static as for the case of lateral variations in gravitational potential energy then velocity gradients can be produced only if the lithosphere has on average zero strength the static force model may be related to the long term velocity field but not the instantaneous velocity field typically measured geodetically over a period of several years because over short time intervals the upper lithosphere behaves elastically in order to describe both the short and long term behaviour of an elastic lithosphere viscoelastic asthenosphere system in a selfconsistent manner i construct a deformation model termed the expected interseismic velocity eiv model assuming that the lithosphere is populated with faults that rupture continually each with a definite mean recurrence time and that the earth is well approximated as a linear elastic viscoelastic coupled system i derive a simple relationship between the instantaneous velocity field and the average rate of moment release in the lithosphere examples with synthetic fault networks demonstrate that velocity gradients in actively deforming regions may to a large extent be the product of compounded viscoelastic relaxation from past earthquakes on hundreds of faults distributed over large 10 6 km 2 areas

oligo miocene magnetostratigraphy and rock magnetism of the xishuigou section subei gansu province western china and implications for shallow inclinations in central asia magnetostratigraphy of 222 remanent directions together with late oligocene to early miocene mammal and charophyte paleontology suggest that 2179 m of the xishuigou section subei gansu province china were deposited from 26 to 19 ma stratigraphic patterns of bulk susceptibility anisotropy of magnetic susceptibility parameters and natural and anhysteretic remanent magnetization intensities demonstrate that 1 faulting does not significantly affect the record 2 sediment deposition was relatively continuous 3 sediment source changed around 23 ma and 4 rapid uplift near subei occurred at 21 ma subei rotated 27 5 counterclockwise with respect to the 20 ma pole from the eurasian synthetic apparent polar wander path folding and rotation of the section took place after 19 ma the paleolatitude of subei is 14 less than at present and 19 3 less than predicted from the reference pole both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing a phenomenon which has likely affected other paleomagnetic data from central asia

restoration of cenozoic deformation in asia and the size of greater india a long standing problem in the geological evolution of the india asia collision zone is how and where convergence between india and asia was accommodated since collision proposed collision ages vary from 65 to 35 ma although most data sets are consistent with collision being underway by 50 ma plate reconstructions show that since 50 ma 2400 3200 km west to east of india asia convergence occurred much more than the 450 900 km of documented himalayan shortening current models therefore suggest that most post 50 ma convergence was accommodated north of the indus yarlung suture zone we review kinematic data and construct an updated restoration of cenozoic asian deformation to test this assumption we show that geologic studies have documented 600 750 km of n s cenozoic shortening across and north of the tibetan plateau the pamir hindu kush region accommodated 1050 km of n s convergence geological evidence from tibet is inconsistent with models that propose 750 1250 km of eastward extrusion of indochina approximately 250 km of indochinese extrusion from 30 to 20 ma of indochina suggested by se asia reconstructions can be reconciled by dextral transpression in eastern tibet we use our reconstruction to calculate the required size of greater india as a function of collision age even with a 35 ma collision age the size of greater india is 2 3 times larger than himalayan shortening for a 50 ma collision the size of greater india from west to east is 1350 2600 km consistent with robust paleomagnetic data from upper cretaceous paleocene tethyan himalayan strata these estimates for the size of greater india far exceed documented shortening in the himalaya we conclude that most of greater india was consumed by subduction or underthrusting without leaving a geological record that has been recognized at the surface

scarp degraded by linear diffusion inverse solution for age under the assumption that landforms unaffected by drainage channels are degraded according to the linear diffusion equation a procedure is developed to invert a scarp profile to find its diffusion age diffusion age having dimension length 2 is the product of diffusivity times chronological age the second moment of scarp slope grows linearly with age this fact together with an assumption about initial scarp shape allows the inverse determination of diffusion age age found assuming a vertical initial scarp is termed apparent age any nonvertical initial scarp profile has a nonzero initial apparent age true diffusion age differs from apparent age by a fraction of scarp offset squared the inverse procedure applied to synthetic data yields the following rules of thumb evidence of initial scarp shape has been lost when apparent age reaches twice its initial value if a scarp is formed by two events the inversion gives their offset weighted mean age with an error that is a fraction of offset squared a scarp that appears to have been formed by one event may have been formed by two with an interval between them as large as apparent age after scarps of two fault traces have diffused to appear as one the error in inferred age may be as large as half the apparent age variation of apparent age along strike would indicate multiple fault traces the simplicity of scarp profile measurement and this inversion makes profile analysis attractive if linearity of the flow law time for a free face to be reduced to the angle of repose and variation of diffusivity with climate and material could be established profile analysis would become a reliable dating technique

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

cretaceous tertiary geology of the gangdese arc in the linzhou area southern tibet abstract knowledge of the cretaceous tertiary history of upper crustal shortening and magmatism in tibet is fundamental to placing constraints on when and how the tibetan plateau formed in the lhasa terrane of southern tibet the widely exposed angular unconformity beneath uppermost cretaceous lower tertiary volcanic bearing strata of the linzizong formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs during the indo asian collision in the linzhou area located 30 km north of the city of lhasa a 3 km thick section of the linzizong formation lies unconformably on cretaceous and older rocks that were shortened by both northward and southward verging structures during the late cretaceous the linzizong formation dips northward in the footwall of a north dipping thrust system that involves triassic jurassic strata and a granite intrusion in the hanging wall u pb zircon geochronologic studies show that the linzizong formation ranges in age from 69 ma to at least 47 ma and that the hanging wall granite intrusion crystallized at 52 ma coeval with dike emplacement into footwall cretaceous strata 40ar 39ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 ma followed by an episode of accelerated cooling to upper crustal levels beginning at 42 ma the onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern tibet and is interpreted be a consequence of either 1 tertiary thrusting in this region or 2 regional rock uplift and erosion following removal of overthickened gangdese arc lower crust and upper mantle or break off of the neo tethyan oceanic slab

anomalous low palaeomagnetic inclinations from oligocene lower miocene red beds of the south west tien shan central asia summary we studied oligocene to lower miocene red beds from the western margins of the south west ghissar range the westernmost part of the tien shan fold belt central asia a total of 19 sites were sampled on both limbs of a gentle syncline after removal of an unstable component at 200 to 300 c the natural remanent magnetization in these red beds from 18 sites is accounted for by one characteristic component chrm which persists up to 680 c intermediate directions of the geomagnetic field were recorded at five sites after tilt correction the overall mean direction for the remaining 13 sites is d 3 i 30 k 19 95 8 9 both the reversal and the fold tests are positive haematite is found to be the main remanence carrier in these red beds the calculated mean declination fits well with the reference data besse courtillot 1991 and no rotation with respect to eurasia is detected in accord with geological data in contrast the mean inclination is about 30 shallower than expected we argue that this result is not due to large scale northward transport of the area mean inclinations showing a similar amount of shallowing were also reported from the tadzhik and fergana basins and kirghiz tien shan from various rock types thomas et al 1993 1994 and we are of the opinion that neither inclination error nor compaction could have led to such a consistent pattern the analysis of all reliable early tertiary palaeomagnetic results extracted from the global paleomagnetic data base for the northern part of the eurasian plate reveals that inclination anomalies are very common in this data set whatever the rock type the general tendency is a progressive increase in the anomaly amplitude from the atlantic ocean to central asia the size and localization of this tertiary field anomaly is comparable with the present day non dipole field anomaly this suggests that there may have been a long term standing non dipole anomaly in the tertiary earth magnetic field perhaps correlated to major features of the contemporaneous plate tectonics for example the india asia collision

effect of far field slope on morphologic dating of scarplike landforms the principal finding of this paper is that the far field slope has a first order effect on model age determinations of scarplike landforms in weakly consolidated terrains observationally this can be demonstrated in two ways using the lake bonneville and lahontan shoreline scarps as separate and combined data sets use of the reduced scarp slope tan theta sub s minus b where theta sub s is the maximum scarp angle and b is the far field or fan slope instead of tan theta sub s alone as the measure of scarp slope measurably reduces separation between the two data sets induced by different average fan slopes for the two data sets and significantly reduces scatter in the slope offset plot for both the separate and combined data sets theoretically the argument can be put even more strongly at least within the range of linear and nonlinear diffusion models that the authors consider here together with a mathematical transformation of the empirical approach of r c bucknam and r e anderson when one correctly takes into account the far field slope one will basically get the same age determination no matter which of these models one uses conversely without accounting properly for the effect of far field slope one is virtuallymore guaranteed to get an erroneous age determination no matter which model is used less

too low magnetic inclinations in central asia an indication of a long term tertiary non dipole field observed tertiary palaeolatitudes in central asia are more southerly by about 1600 km on average than those predicted from the eurasian reference palaeopoles subsequent northward displacements of the central asian terranes are unlikely to have been this large in this study we analyse to what extent non dipole fields especially octupole fields can explain this phenomenon a global zonal octupole field manifests itself in two ways 1 because the reference apwp is based mostly on results from the uk and north america its palaeopoles will be far sided as seen from the north atlantic but near sided as seen from eastern asia giving predicted palaeolatitudes that are too high 2 an octupole field contribution produces observed palaeolatitudes as calculated with the dipole formula for central asia that are too low both effects therefore increase the palaeolatitude anomalies in asia we find that an octupole dipole field ratio g3 of 0 06 or greater will reduce the palaeolatitude discrepancies significantly and is of the same magnitude as the g3 estimate of a recent analysis of early tertiary european and north american data

recurrent holocene strike slip faulting pyramid lake fault zone western nevada trenches excavated at two sites on scarps of the pyramid lake fault zone revealed evidence of recurrent holocene strike slip faulting control for estimating the ages of faulting events is provided by soil stratigraphy c 14 ages on organic sediments and the mazama ash 7000 6700 yr which is present at both sites the first site displayed evidence of at least three probably four and possibly five displacements within the past 7500 yr evidence of at least three and probably four post mazama ash faulting events was found at the second trench site the juxtaposition of lithologically contrasting deposits and units with different thicknesses on opposite sides of the faults indicates strike slip faulting the estimated average recurrence of surface faulting events m s 6 5 is about 1800 yr assuming four post mazama ash events and about 2400 yr for three events the most recent event occurred within the past few hundred years the rate of recurrence of surface faulting events on the pyramid lake fault zone is three to five times higher than for most faults in the western basin and range province

palaeomagnetic results from the tibetan plateau palaeomagnetic measurements were carried out on 1325 oriented samples collected from 246 sites on a traverse of the tibetan plateau from lhasa to golmud in 1985 crossing the lhasa terrane qiangtang terrane and kunlun terrane high blocking temperature high coercivity statistically grouped magnetizations were isolated from the following units lhasa terrane cretaceous takena formation mid cretaceous nagqu volcanics mid cretaceous qelico volcanics qiangtang terrane norian batang group volcanics kimmeridgian yanshiping group paleocene to eocene fenghuoshan group kunlun terrane visean to namurian dagangou formation dykes of the triassic igneous province the triassic data from the kunlun terrane triassic and lower tertiary data from the qiangtang terrane and the cretaceous data from the lhasa terrane indicate palaeolatitudes ca 20 s of their present position within the eurasian frame of reference a possible interpretation is that the terranes successively accreted to eurasia and remained in the southern position until the convergence of india drove them northward via a process of tectonic shortening and or displacement of continental crust the carboniferous data from the kunlun terrane are consistent with moderate southern hemisphere latitude well separated from eurasia which was in the northern hemisphere at this time implying the existence of ocean crust between these blocks during the carboniferous

active faulting in the walker lane 1 deformation across the san andreas and walker lane fault systems accounts for most relative pacific north american transform plate motion the walker lane is composed of discontinuous sets of right slip faults that are located to the east and strike approximately parallel to the san andreas fault system mapping of active faults in the central walker lane shows that right lateral shear is locally accommodated by rotation of crustal blocks bounded by steep dipping east striking left slip faults the left slip and clockwise rotation of crustal blocks bounded by the east striking faults has produced major basins in the area including rattlesnake and garfield flats teels columbus and rhodes salt marshes and queen valley the benton springs and petrified springs faults are the major northwest striking structures currently accommodating transform motion in the central walker lane right lateral offsets of late pleistocene surfaces along the two faults point to slip rates of at least 1 mm yr the northern limit of northwest trending strike slip faults in the central walker lane is abrupt and reflects transfer of strike slip to dip slip deformation in the western basin and range and transformation of right slip into rotation of crustal blocks to the north the transfer of strike slip in the central walker lane to dip slip in the western basin and range correlates to a northward broadening of the modern strain field suggested by geodesy and appears to be a long lived feature of the deformation field the complexity of faulting and apparent rotation of crustal blocks within the walker lane is consistent with the concept of a partially detached and elastic brittle crust that is being transported on a continuously deforming layer below the regional pattern of faulting within the walker lane is more complex than observed along the san andreas fault system to the west the difference is attributed to the relatively less cumulative slip that has occurred across the walker lane and that oblique components of displacement are of opposite sense along the walker lane extension and san andreas contraction respectively despite the gross differences in fault pattern the walker lane and san andreas also share similarities in deformation style including clockwise rotations of crustal blocks leading to development of structural basins and the partitioning of oblique components of slip onto subparallel strike slip and dip slip faults

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

volcanic rocks of the 1985 tibet geotraverse lhasa to golmud volcanic rocks encountered during the tibet geotraverse have been studied in the field in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment rocks from a total of eleven distinct volcanic provinces were studied in this way they provide evidence for an active continental margin or post collision province of probable devonian early carboniferous age in the northern kunlun mountains an active continental margin of late carboniferous age in the southern lhasa terrane permian continental rifts in the central qiangtang and central kunlun terranes triassic volcanic arcs in the southern lhasa and northern qiangtang terranes a triassic active continental margin dyke swarm in the northern kunlun mountains a jurassic post collision or back arc rifting province in the southern qiangtang terrane a jurassic island arc in the northern lhasa terrane a cretaceous post collision province in the northern lhasa terrane possibly extending into the southern qiangtang terrane and a palaeogene active continental margin in the southern lhasa terrane an oligocene trachyte plug in the northern qiangtang terrane was the only evidence encountered during the geotraverse of volcanism post dating the palaeogene india eurasia collision however the composition of this plug coupled with new and published analyses from miocene volcanics in the southern lhasa terrane and from the pliocene recent volcanic province of northwest tibet places important constraints on models for post collision underplating of tibet by continental lithosphere any underplating is likely to have been a much later than the start of collision b directed beneath tibet from the north as well as the south and c limited in extent

resolving the problem of shallow magnetizations of tertiary age in asia insights from paleomagnetic data from the qiangtang kunlun and qaidam blocks tibet china and a new hypothesis we present new paleomagnetic results obtained at 39 sampling sites from five sections of tertiary red bed formations two eocene formations from the qiangtang block of tibet xialaxiu locality 32 8 n 96 6 e and the xining basin of qaidam xining locality 36 5 n 102 0 e and three neogene formations from the xining basin jungong locality 34 7 n 100 7 e and the kunlun block tuoluo lake and west yushu localities 35 3 n 98 6 e and 33 2 n 96 7 e respectively thermal demagnetization of the rocks isolated a high temperature component that we interpret as the primary magnetization in four localities the paleopoles lie at 52 6 n 352 e dp dm 6 0 10 7 for xialaxiu 61 6 n 211 3 e dp dm 9 7 16 1 for xining 66 0 n 228 6 e dp dm 3 6 6 9 for jungong and 53 9 n 205 4 e dp dm 5 6 10 0 for west yushu as in previous studies of tertiary formations from asia the inclinations we obtained are shallower by 18 to 26 than the magnetic field computed from the eurasian apparent polar wander path apwp at 10 and 20 ma for neogene rocks and at 40 and 60 ma for eocene rocks on the basis of a compilation of eocene data from the south china block tibet central asia and kyrgyzstan we conclude that this inclination anomaly reflects erroneous predictions of positions of the siberian craton when based on the apwp of eurasia the main reason for this discrepancy might be nonrigid behavior of the eurasian plate in the tertiary combination of this with intracontinental shortening of asia under the penetration of india provides a full explanation for the anomaly verification of this new interpretation of the inclination anomaly will require new geologic and paleomagnetic data from the northern parts of these remote regions in mongolia and siberia

did a large departure from the geocentric axial dipole hypothesis occur during the eocene evidence from the magnetic polar wander path of eurasia abstract low paleolatitudes are very frequent in eocene to oligocene formations particularly in the tethyan mobile belt these anomalies and their possible sources are discussed they are neither due to unreliable results nor to inclination errors in sedimentary rocks neither can large tectonic movements explain these discrepancies the most probable causes having been rejected therefore the best explanation consists in an alternative magnetic pole position for eurasia located at 69 n 215 e it is proposed that the mean magnetic field of the earth was dipolar but that its axis has swung from one stable position to another during the middle tertiary and was therefore sometimes different from the rotation axis

when and where did india and asia collide timing of the collision between india and asia is the key boundary condition in all models for the evolution of the himalaya tibetan orogenic system thus it profoundly affects the interpretation of the rates of a multitude of associated geological processes ranging from tibetan plateau uplift through continental extrusion across eastern asia as well as our understanding of global climate change during the cenozoic although an abrupt slowdown in the rate of convergence between india and asia around 55 ma is widely regarded as indicating the beginning of the collision most of the effects attributed to this major tectonic episode do not occur until more than 20 ma later refined estimates of the relative positions of india and asia indicate that they were not close enough to one another to have collided at 55 ma on the basis of new field evidence from tibet and a reassessment of published data we suggest that continent continent collision began around the eocene oligocene boundary 34 ma and propose an alternative explanation for events at 55 ma

a new paleogeographic configuration of the eurasian landmass resolves a paleomagnetic paradox of the tarim basin china 1 new paleomagnetic data from permian red beds and middle jurassic limestones from the tarim basin pose a paradox their declinations are similar to upper carboniferous to neogene rocks collected from the same sections and their inclinations parallel present values when assuming that lower than expected inclinations in continental sedimentary rocks arise from inclination shallowing effects then the paleolatitudes of all upper carboniferous to present rocks from tarim are indistinguishable local vertical axis block rotations occurring in the last 20 million years explain why declinations vary at different localities in the basin our middle jurassic data positions tarim 23 6 8 4 farther south than that predicted from the coeval reference pole for eurasia however no geologic argument exists to support the closure of a large ocean basin between tarim and siberia since the middle jurassic thus the paradox are the rocks from tarim totally overprinted or is the middle mesozoic part of the reference eurasian apparent polar wander path erroneous several lines of evidence suggest the tarim rocks are not remagnetized we conclude that tarim has experienced little or no apparent polar wander since the carboniferous moreover our middle mesozoic reconstruction of eurasia using the new middle jurassic pole from tarim results in a more geologically compatible solution for the eastern asian blocks over previous reconstructions

combined paleomagnetic and geochronological study on cretaceous strata of the qiangtang terrane central tibet abstract a combined paleomagnetic and geochronological investigation has been performed on cretaceous rocks in southern qiangtang terrane 32 5 n 84 3 e near gerze central tibetan plateau a total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled our new u pb geochronologic study of zircons dates the volcanic rocks at 103 8 0 46 ma early cretaceous while the red beds belong to the late cretaceous rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers after removing a low temperature component of viscous magnetic remanence stable characteristic remanent magnetization chrm was isolated successfully from all the sites by stepwise thermal demagnetization the tilt corrected mean direction from the 14 lava sites is d 348 0 i 47 3 k 51 0 95 5 6 corresponding to a paleopole at 79 3 n 339 8 e a 95 5 7 and yielding a paleolatitude of 29 3 5 7 n for the study area the chrm directions isolated from the volcanic rocks pass a fold test at 95 confidence suggesting a primary origin the volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole vgp scatter the mean inclination from the late cretaceous red beds however is 13 1 shallower than that of the 100 ma volcanic rocks after performing an elongation inclination analysis on 174 samples of the red beds a mean inclination of 47 9 with 95 confidence limits between 41 9 and 54 3 is obtained which is consistent with the mean inclination of the volcanic rocks the site mean direction of the late cretaceous red beds after tilt correction and inclination shallowing correction is d 312 6 i 47 7 k 109 7 95 3 0 n 22 sites corresponding to a paleopole at 49 2 n 1 9 e a 95 3 2 yielding a paleolatitude of 28 7 3 2 n for the study area the chrm of the red beds also passes a fold test at 99 confidence indicating a primary origin comparing the paleolatitude of the qiangtang terrane with the stable asia there is no significant difference between our sampling location in the southern qiangtang terrane and the stable asia during 100 ma and late cretaceous our results together with the high quality data previously published suggest that an 550 km n s convergence between the qiangtang and lhasa terranes happened after 100 ma comparison of the mean directions with expected directions from the stable asia indicates that the gerze area had experienced a significant counterclockwise rotation after 100 ma which is most likely caused by the india asia collision

reply to comment by eduardo garzanti on when and where did india and asia collide we thank eduardo garzanti for his comment garzanti 2008 on our recent paper aitchison et al 2007 a traditional approach is to apply occam s razorand view our planet s most significant orogenic zone as aresponse to closure of a one ocean two continent conver gence system however a vast body of new data has beenobtained in the decades following widespread acceptance ofthis conservative model by the scientific community wecontend that significant details are lost with this conven tional view critically it is now clear that the northernmargin of india experienced not one but two quite discretecollisions during the paleogene

paleomagnetism of the upper cretaceous red beds from the eastern edge of the lhasa terrane new constraints on the onset of the india eurasia collision and latitudinal crustal shortening in southern eurasia abstract the late cretaceous location of the lhasa terrane is important for constraining the onset of india eurasia collision however the late cretaceous paleolatitude of the lhasa terrane is controversial a primary magnetic component was isolated between 580 c and 695 c from upper cretaceous jingzhushan formation red beds in the dingqing area in the northeastern edge of the lhasa terrane tibetan plateau the tilt corrected site mean direction is d s i s 0 9 24 3 k 46 8 95 5 6 corresponding to a pole of plat plon 71 4 273 1 with a 95 5 2 the anisotropy based inclination shallowing test of hodych and buchan 1994 demonstrates that inclination bias is not present in the jingzhushan formation the cretaceous and paleogene poles of the lhasa terrane were filtered strictly based on the inclination shallowing test of red beds and potential remagnetization of volcanic rocks the summarized poles show that the lhasa terrane was situated at a paleolatitude of 13 2 8 6 n in the early cretaceous 10 8 6 7 n in the late cretaceous and 15 2 5 0 n in the paleogene reference point 29 0 n 87 5 e the late cretaceous paleolatitude of the lhasa terrane 10 8 6 7 n represented the southern margin of eurasia prior to the collision of india eurasia comparisons with the late cretaceous to paleogene poles of the tethyan himalaya and the 60 ma reference pole of east asia indicate that the initial collision of india eurasia occurred at the paleolatitude of 10 8 6 7 n since 60 5 1 5 ma reference point 29 0 n 87 5 e and subsequently 1300 910 km post collision latitudinal crustal convergence occurred across the tibet the vast majority of post collision crustal convergence was accommodated by the cenozoic folding and thrust faulting across south eurasia

the direction correction tilt test an all purpose tilt fold test for paleomagnetic studies abstract the tilt or fold test in paleomagnetism is used to infer whether paleomagnetic remanence was acquired before or after tectonic tilting while several tilt test formulations have been proposed none fully satisfies the requirements of statistical validity applicability to all bedding geometries and ease of use this paper introduces the direction correction dc test which examines the relationship between the paleomagnetic site mean directions and their corresponding bedding tilt corrections the dc test is similar to mcfadden s correlation test because both test whether or not the site directions contain information about the bedding tilts however the dc test gives greater weight to sites with greater counter bedding the dc test is similar to watson s numerical tilt tests because they determine the degree of untilting which gives optimal concentration of site directions however the dc test uses analytical rather than numerical methods graphical output aids the researcher in recognizing problem sites using both real and simulated data the dc test is demonstrated to be more discriminating than other tilt test formulations for all bedding geometries the simulations show that the power of the tilt test is inversely proportional to the 95 confidence interval 95 of the overall mean as a rule of thumb paleomagnetists should attempt to sample sufficient sites to obtain an 95 less than 1 6 of the bedding attitude difference