late cenozoic sedimentation of nihewan basin central north china and its tectonic significance abstract late cenozoic sediments of the nihewan basin in the northeastern part of the shanxi graben north china craton include buried late neogene rocks and exposures of the pliocene yuxian formation the pleistocene nihewan formation and xiaodukou formation with the age at least from 3 6 to 0 11 ma the sedimentary succession records the tectonic processes of the graben in the eastern nihewan basin on the basis of sedimentary facies and provenance analysis we have identified three main evolution stages during the late cenozoic the first stage was rapid expansion of the basin in the neogene which is thought to have commenced at least from 3 6 ma and lasted until 2 6 ma this stage is characterized by lacustrine facies dark colored mud and silt in depocenter of the basin with a potential source area from the northeast of the basin the following stage was a period of contraction at 2 6 1 8 ma characterized by deltaic deposits in depocenter of the basin and braided fluvial deposits in northeast margin of the basin and the deposition of cross bedded sands that were derived from erosion of archean proterozoic paleozoic and jurassic rocks from the north northwest and south of the basin the last stage was one era of gradually extinction of the basin from 1 8 to 0 11 ma followed by dried up of the basin and replaced by loess sediments were deposited in deltaic in depocenter of the basin and alluvial fan in northeast margin of the basin these coarse grained sands and pebbles were derived mainly from north and north northeast of the basin and generally from archean proterozoic and paleozoic rocks these three stages and the conversion ages 2 6 1 80 and 0 11 ma are largely consistent with three episodes of tibetan plateau rapid uplift therefore we infer that the evolution of the nihewan basin was predominantly influenced by the far field effect of the progressive uplift and northeastward expansion of the tibetan plateau

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

age and sr isotopic signature of the catoctin volcanic province implications for subcrustal mantle evolution variations in chemical and sr isotopic signatures of a series of basaltic flows have been correlated with newly mapped stratigraphic sections in the upper precambrian catoctin flood basalts in the central appalachians element mobility through fluid interaction is commonly observed in flows with a well developed fabric but some samples with more massive textures commonly in the cores of thicker flows still retain relict clinopyroxene and igneous textures and show only minor element redistribution five whole rock samples with relict igneous textures and a clinopyroxene mineral separate yield an isochron with an age of 570 36 ma 2sigma and a strontium initial ratio of 0 7035 1 this age is consistent with recent paleontological evidence for a latest precambrian time for catoctin volcanism and along with isotopic age determinations of 680 710 ma for other rift related magmatism suggests either a two stage or a very long protracted episode of continental rifting paleogeographic reconstruction indicates that the original site of eruption for the lavas must have been at least 56 km to the east in the same geographic location from which the mesozoic appalachian tholeiite terrane evolved variations in sr isotopic signatures and differences in major element abundances between these two sequences suggest mantle heterogeneitiesmore that must have developed beneath the eastern north american craton during the interim 400 m y less

the role of radiogenic heat production in the thermal evolution of a proterozoic granulite facies orogenic belt eastern ghats indian shield abstract the eastern ghats belt egb is a deeply eroded proterozoic orogenic belt juxtaposed against the archaean dharwar bastar singhbhum cratons in the indian shield the tectono metamorphic history of this belt is broadly similar to the grenvillian orogenic belts the egb exposes dominantly the granulite facies rocks such as charnockites khondalites and migmatitic gneisses with less abundant intermediate to mafic granulites the granulites are intruded by small plutons of granites syenites and anorthosites the northern and southern parts of the belt negb and segb have distinct crustal histories but 1600 1000 550 ma old tectono metamorphic events are common to both the belts in this research we attempt to understand the present day and past thermal state of this orogen on the basis of our radioelement k u and th measurements at 562 sites using in situ gamma ray spectrometry with other geological and geophysical constraints the dominant rock types of the negb and segb have similar radioelement abundances 3 k 3 ppm u and 30 ppm th and heat production heat production of charnockites gneisses khondalites and intermediate granulites of the negb is 2 9 2 8 2 9 and 1 1 w m 3 respectively in the segb they are 2 7 2 5 2 8 and 0 6 w m 3 respectively and the mafic granulites are the lowest in heat production 0 3 w m 3 on the basis of the heat production data and crustal petrologic models we show that the crustal contribution of the negb and segb is 54 and 45 mw m 2 respectively which are broadly in good agreement with the available surface heat flow data thermal models of the present day egb crust indicate that the temperature at the moho is 550 c radioelements and heat production of the eastern ghats granulites are higher than the other granulite belts in the indian shield the crustal radiogenic heat contribution and moho temperatures of the egb are higher than the adjoining archaean cratons this could also be responsible for the development of inverted metamorphic isograds in the tectonic boundary between the egb and the adjoining cratons during a collisional orogenic event at about 550 ma ago

paleogene clockwise tectonic rotation of the xining lanzhou region northeastern tibetan plateau 1 to help understand the deformational history of the northeastern tibetan plateau paleomagnetic samples were collected from 177 sites and two magnetostratigraphic sections at 16 localities distributed among upper jurassic lower cretaceous to pliocene sedimentary and subordinate volcanic rocks within the xining lanzhou region 34 37 n 101 105 e a total of 127 sites at 12 localities yielded primary magnetizations confirmed by fold reversal and conglomerate tests age control on sedimentary rocks is provided by regional synthesis of chronostratigraphic data and our own biostratigraphic and magnetostratigraphic analysis presented in the companion paper by horton et al 2004 analysis of paleomagnetic declination combined with results from previous studies yield a remarkably consistent trend of vertical axis tectonic rotations across the studied region whereas 19 0 7 2 to 37 8 10 6 clockwise rotations are recorded consistently in all paleomagnetic localities in lower cretaceous to eocene rocks all paleomagnetic localities in oligocene to pliocene rocks have recorded minor to insignificant rotations indicating that the xining lanzhou region has undergone a wholesale regional clockwise rotation during late paleogene time consistent with regional chronostratigraphic and thermochronologic results this late paleogene tectonic rotation confirms that deformation reached regions of the northern tibetan plateau shortly after the initial collision of india with asia when compared to rotational paleomagnetic results from adjacent regions several mechanisms can be proposed to explain the clockwise rotation on the basis of consistency with geologic data we prefer a model involving clockwise rotation of the xining lanzhou region through right lateral shear and associated shortening related to northward indentation of the qaidam basin index terms 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 8102 tectonophysics continental contractional orogenic belts 9320 information related to geographic region asia 9604 information related to geologic time cenozoic keywords tectonics paleomagnetism tibetan plateau

paleomagnetic constraints from the baoshan area on the deformation of the qiangtang sibumasu terrane around the eastern himalayan syntaxis the sibumasu block in se asia represents the eastward continuation of the qiangtang block here we report a detailed rock magnetic and paleomagnetic study on the middle jurassic and paleocene rocks from northern sibumasu to document the crustal deformation during the india asia collision since the paleocene and reconstruct the overall strike of the qiangtang sibumasu elements before the india asia collision although the fold test is inconclusive based solely on our data a positive reversal test a positive regional fold test with previous paleomagnetic results and a detrital origin of hematite in the red beds as indicated by scanning electron microscopy suggest that the magnetizations obtained from the jurassic and paleocene rocks are most likely primary showing an 80 clockwise rotation since paleocene these results together with previously published paleomagnetic data suggest that the northern sibumasu and northern simao elements experienced a 60 80 clockwise rotation since paleocene this large clockwise rotation is also consistent with the surface gps velocity field and ne sw fault networks suggesting a rotational motion of crustal material from southeastern tibet during late cenozoic we infer that the large clockwise rotation is a sum of rotation in the eocene to middle miocene time associated with indochina extrusion and rotation after the middle miocene associated with the e w extension in central tibet this suggests that the eastward motion of tibetan crustal material along the xianshuihe xiaojiang fault after middle miocene is transmitted to the southwest toward myanmar jurassic and cretaceous paleomagnetic results suggest that the qiangtang northern sibumasu was originally a curved structure with an orientation of n60 w in tibet and changes to n10 w in southern sibumasu

paleomagnetic evidence for 25 15 ma crust fragmentation of north indochina 23 26 n consequence of collision with greater india ne corner

a late miocene early pleistocene palynological record and its climatic and tectonic implications for the yunnan plateau china abstract in this paper we provide a late miocene tortonian to early pleistocene gelasian palynological record from the eryuan basin yunnan plateau china to investigate plant diversity over this time between 7 6 and 4 3 ma the palynological assemblage was dominated by coniferous trees 70 2 on average with subordinate amounts of broadleaved trees 8 8 on average this suggests that the vegetation mainly comprised mixed forests and that the climate was relatively warm and humid after 4 3 ma the proportion of conifer pollen increased overall and amount of cold temperate coniferous forest picea and abies pollen in particular increased significantly from 19 1 to 41 9 concurrent with decreases in the pollen of broadleaved trees shrubs herbs and other plant taxa we suggest that these plant variations and overall reduced diversity may be responses to the tectonic uplift of the yunnan plateau and global cooling

700 ma rift event in the blue ridge province of virginia a unique time constraint on pre iapetan rifting of laurentia discovery of compositionally distinctive rhyolites produced from a peralkaline volcanic center within the late neoproterozoic robertson river batholith and which are intercalated with terrestrial deposits of the previously undated mechum river formation indicates that rift related sedimentation and magmatism were ongoing at 702 705 ma about 130 m y before the breakup of laurentia recognition of late neoproterozoic age rhyolites in central virginia indicates that rift related magmatism was more widespread along the incipient eastern present coordinates margin of laurentia than previously thought neoproterozoic rifting in the blue ridge province of virginia appears to have been part of a broad scale process that affected most if not all of the preserved margins of laurentia the association of similar extension related terrestrial deposits and compositionally distinctive felsic rocks elsewhere may provide a unique marker for testing proposed predrift configurations of the late neoproterozoic supercontinent rodinia

geochemistry of the late mesoproterozoic mount eve granite suite implications for late to post ottawan tectonics in the new jersey hudson highlands

paleoenvironmental implications of molecular and carbon isotopic distributions of n alkanes in the hominoid bearing upper miocene sediments from the zhaotong basin southwestern china abstract it is crucial to understand the relationship between hominoid primates and their living environment located at the southeastern margin of the tibetan plateau the shuitangba stb outcrop from the zhaotong basin is famous for bearing the remains of the youngest known ape fossil in the terminal miocene 6 2 ma in this study the molecular and carbon isotope compositions of n alkanes together with the loss on ignition and the bulk carbon isotope composition of total organic matter were investigated in the stb outcrop to infer the paleoenvironmental conditions in this uppermost miocene sedimentary sequence with the emphasis on the habitat of the stb hominoid in the hominoid bearing peaty clay layer and the surrounding lignite layers the n alkane molecular and carbon isotope compositions support a primary contribution from terrestrial higher plants in contrast n alkanes in the upper shallow lake sediments are derived from multiple origins mainly from aquatic plants and bacteria both carbon preference index cpi and average chain length acl vary closely with the lithology but the cpi record shows the larger amplitude of change than the acl and varies sharply during the lithological boundary it is notable that the changes of the molecular and carbon isotope compositions of long chain n alkanes do not parallel with the abrupt increase of poaceae pollen percentage during the stb hominoid appearing stage such an inconsistency between herb pollen percentage and n alkane ratios may result from the different sources our results clearly support that these organic matter based ratios have the potential to record the paleoenvironmental changes in ancient lacustrine sediments

timing of the nihewan formation and faunas abstract magnetostratigraphic dating of the fluvio lacustrine sequence in the nihewan basin north china has permitted the precise timing of the basin infilling and associated nihewan mammalian faunas the combined evidence of new paleomagnetic findings from the hongya and huabaogou sections of the eastern nihewan basin and previously published magnetochronological data suggests that the nihewan formation records the tectono sedimentary processes of the plio pleistocene nihewan basin and that the nihewan faunas can be placed between the matuyama brunhes geomagnetic reversal and the onset of the olduvai subchron 0 78 1 95 ma the onset and termination of the basin deposition occurred just prior to the gauss matuyama geomagnetic reversal and during the period from the last interglaciation to the late last glaciation respectively suggesting that the nihewan formation is of late pliocene to late pleistocene age the nihewan faunas comprising a series of mammalian faunas such as maliang donggutuo xiaochangliang banshan majuangou huabaogou xiashagou danangou and dongyaozitou are suggested to span a time range of about 0 8 2 0 ma the combination of our new and previously published magnetostratigraphy has significantly refined the chronology of the terrestrial nihewan formation and faunas

geochronology of the mesoproterozoic state farm gneiss and associated neoproterozoic granitoids goochland terrane virginia the goochland terrane is an isolated block of mesoproterozoic crust in the piedmont province of central virginia we report u pb zircon dates and whole rock major and trace element data for the state farm gneiss one of the main units in the terrane and additional results for a newly recognized suite of neoproterozoic granitoids that intrude the gneiss the state farm gneiss ranges in bulk composition from quartz monzodiorite to granite and three samples yield u pb zircon dates of 1046 7 6 ma 1039 18 11 ma and 1023 10 ma we interpret these dates as igneous crystallization ages which indicate a maximum emplacement interval of ca 1057 1013 ma for these samples neoproterozoic granitoids including the fine creek mills and flat rock granites are more alkaline than the state farm gneiss and display all compositional characteristics of a type granites e g high fe mg ga ga al nb zn y zr u pb zircon analyses from five separate bodies indicate crystallization ages from ca 654 to 588 ma but all results are complicated by mesoproterozoic ca 1 ga inheritance coupled in some cases with secondary pb loss we interpret the ages and compositions of the younger rocks to reflect neoproterozoic rifting of the goochland terrane but its location during rifting is uncertain the terrane may have been separated from laurentia during the neoproterozoic breakup of rodinia and later reattached

the structural geometry and evolution of foreland thrust systems northern virginia seismic reflection data reveal that the structural geometry of the central appalachians of northern virginia consists of three distinct thrust systems each thrust system is characterized by a unique internal geometry the blue ridge thrust sheet is a composite thrust sheet composed primarily of imbricated precambrian crystalline rocks it over rode cambrian ordovician carbonates and formed a sheared basement cored fault bend fold thrusts within the sheet may be taconic and earliest alleghanian whereas final thrusting and emplacement of the sheet were probably slightly younger but still early alleghanian the north mountain thrust sheet is characterized by imbricated cambrian ordovician carbonates that are deformed into large amplitude mode ii fault bend folds and fault propagation folds rocks of this sheet were transported more than 60 km across a similar section of carbonates the leading edge of the north mountain thrust sheet was deformed into a fold with a mode ii fault bend fold geometry and was juxtaposed against middle paleozoic rocks the middle paleozoic rocks occur in a ramp across which displacement along the north mountain thrust was transferred to a higher detachment more than 60 km of cover rocks displaced during the emplacement of the north mountain thrust sheet either were transported across this ramp and thrust over a similar section in the western valley and ridge province or were backthrust above the sheet the timing for imbrication and emplacement of the north mountain thrust sheet is probably main phase alleghanian the lower carbonate duplex extends from beneath the blue ridge and north mountain thrust sheets westward across the western valley and ridge province the lower carbonate duplex is characterized by imbricated cambrian ordovician carbonates that form low amplitude fault bend folds this thrust system also probably formed during main phase alleghanian deformation

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

geodetic measurement of crustal motion in southwest china global positioning system measurements performed over a 2 4 yr period confirm the xianshuihe xiaojiang fault system as one of the primary active structures in southwest china stations southwest of these faults show southerly motions of 5 15 mm yr relative to the western sichuan basin and stations in a 200 km geodetic network located northwest of the basin move at only 0 5 mm yr these results are consistent with clockwise rotation of southwestern sichuan and western yunnan about the eastern tibetan syntaxis accommodated by left lateral slip of 12 4 mm yr on the xianshuihe xiaojiang fault system the results imply that if eastward extrusion of crustal material from the plateau occurs at present it is not accommodated by east west shortening along the margin of the plateau and must involve wholesale eastward motion of low lying regions to the east

paleomagnetic study of cretaceous rocks from pu er western yunnan china evidence of internal deformation of the indochina block abstract cretaceous red sandstones and siltstones were collected at 31 sties for paleomagnetic investigation from the jingxing and nanxin formation around pu er 23 0 n 101 0 e western part of yunnan province china in order to examine quantitative amount of tectonic deformation of the simao terrane due to collision of india with asia after stepwise thermal demagnetization the high temperature component magnetization with unblocking temperature of around 680 c is isolated positive field tests show that the high temperature component magnetization from 25 sites is primary for the cretaceous formations a tilt corrected mean direction is d 59 9 i 45 2 with 95 5 1 corresponding to a paleopole at 35 8 n 173 1 e with a 95 5 6 comparison with cretaceous paleomagnetic pole of the whole china block indicates that amount of clockwise rotation in the pu er area is evaluated to be 46 9 with respect to the whole china block which is consistent with large amount of rotation in the mengla and jinggu areas 47 and 66 in the central part of the simao terrane clockwise rotation of the central part is larger by more than 20 compared to the yunlong and yongping areas of northern part of the simao terrane suggesting that the simao terrane experienced internal deformation the southward displacement of the pu er area expected from inclination data is 7 5 6 6 with respect to the chuan dian fragment which is a similar estimate to other areas in the simao terrane and the indochina block while the simao terrane displaced southward by 800 km together with clockwise rotation of 30 as a whole further clockwise rotation occurred in the central part

palaeomagnetic study of eastern tibet deformation of the three rivers region summary a palaeomagnetic study was undertaken for cretaceous red beds and granitic rocks of three geological provinces the lhasa the qiangtang and the sichuan along a road between chengdu and lhasa in eastern tibet in an attempt to determine aspects of deformation of se asia high temperature characteristic directions were isolated from 20 sites the tilt corrected data from the qiangtang terrane at markam 29 7 n 98 6 e exhibit largely clockwise deflected declinations with steep inclinations d 48 2 i 49 0 a95 8 8 whereas the lhasa terrane at basu 3o l0n 96 9 e and the sichuan province at ya an 3o l0n 103 0 e have northerly directions with shallow inclinations d 3 9 i 23 4 a95 19 2 and d 2 lo z 30 7 ag5 11 3 respectively the palaeomagnetic data indicate that the qiangtang terrane was subjected to clockwise rotation through 50 with respect to the neighbouring provinces of the lhasa and sichuan since the cretaceous tectonic deformation occurred later than the cretaceous in the eastern part of the qiangtang terrane and formed the arcuate shape of the orogenic belt in the three rivers region

origin of mg metatholeiites of the schirmacher region east antarctica constraints from trace elements and nd sr isotopic systematics abstract the mafic granulites of schirmacher region east antarctica the rocks under study occur more or less as concordant sills or as lenses or as boudinage structures within the felsic rocks charnockites or metapelites of the region they show variation from garnet bearing two pyroxene granulites and garnet free pyroxene granulites to transitional amphibolite pyroxene granulites their major trace ree and isotopic chemistry are not distinct from each other and they represent mg basalts with mgo 7 and al 2 o 3 the majority of the analyzed samples plot in the tholeiitic field or show tholeiitic trends suggesting their metatholeiitic nature as well as general preservation of original composition the rocks are characterized by enriched large ion lithophile elemental concentrations than that of mid oceanic ridge basalts their high field strength elements and heavy rare earth elemental concentrations however are as that of mid oceanic ridge basalts a feature which is also reflected in the ratios of their large ion lithophile elements against high field strength elements and heavy rare earth elements wherein we find these ratios are higher than n type morb further the rocks show negative nb anomaly high th ta ratio and low la nb ratio which are also characteristics of subduction related magmatism the isotopic studies carried out on these samples show that the sm nd and rb sr dating did not yield much spread but suggested a sm nd metamorphic age of 960 ma rb sr dating gave ages 886 ma suggesting the reworking of the rb and sr elements during subsequent tectonothermal overprinting the nd model ages t dm nd of these rocks show a relatively restricted range of 1120 to 1357 ma suggesting mafic magmatism 1200 ma the positive end values 4 22 to 6 07 shown by these rocks represent a juvenile crustal fragment derived from melting of mantle precursors without significant reworking of older crustal material it is proposed that these rocks were produced by partial melting of a mantle source characterized by lile enrichment related principally to dehydration of subducted oceanic crust

sedimentary provenance constraints on drainage evolution models for se tibet evidence from detrital k feldspar the unusual drainage pattern of the jinsha river upper yangtze is closely linked to the uplift of the tibetan plateau but when this pattern became established is controversial in this study we compare the pb isotopic composition of detrital k feldspars from the modern jinsha river with those from the plio pleistocene xigeda formation a sequence of fluvial and lacustrine deposits widely distributed along the major rivers draining the tibetan plateau to investigate their relationship and to constrain the drainage evolution of the region our results demonstrate that the jinsha river had achieved its current sediment transport pattern by the late pliocene indicating a drainage character similar to that operating at present our findings not only suggest an upper age limit for formation of the jinsha river but also shed new light on the evolution of the whole yangtze system k feldspar grains from the jinsha river have less radiogenic pb than those in the red mekong and salween rivers providing a key test as to whether the jinsha river was previously connected to the paleo red river comparison with published pb isotopic data shows that these characteristically unradiogenic feldspars are absent in the onshore cenozoic archive from the hanoi basin suggesting that there has been no connection between the lower jinsha river and the red river since the eocene

origin of granulite terranes and the formation of the lowermost continental crust differences in composition and pressures of equilibration between exposed regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust as had been thought but rather middle and lower middle crustal levels application of well calibrated barometers indicate that exposed granulites record equilibration pressures of 0 6 to 0 8 gigapascal 20 to 30 kilometers depth of burial whereas granulite xenoliths which also tend to be more mafic record pressures of at least 1 0 to 1 5 gigapascals 35 to 50 kilometers depth of burial thickening of the crust by the crystallization of mafic magmas at the crust mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep seated mafic crust represented by many xenolith suites

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

partial melting of apatite bearing charnockite granulite and diorite melt compositions restite mineralogy and petrologic implications melting experiments p 6 9 kbar t 850 950 c nno 02 hm were done on mafic to felsic charnockites a dioritic gneiss and a felsic garnet granulite all common rock types in the grenville basement of eastern north america a graphite bearing granulite gneiss did not melt water h2o 0 60 to 2 0 wt is bound in low grade retrograde metamorphic minerals and is consumed during the earliest stages of melting most melts are water undersaturated melt compositions range from metaluminous silicic granodiorite diorite starting composition to peraluminous or weakly metaluminous granites all others in general liquids become more feldspathic less silicic and less peraluminous and are enriched in feo mgo and tio2 with increasing temperature residual feldspar mineralogy controls the cao k2o and na2o contents of the partial melts and the behavior of these elements can be used particularly if the degree of source melting can be ascertained to infer some aspects of the feldspar mineralogy of the source k feldspar a common restite phase in the charnockite and granulite but not the diorite should control the behavior of ba and possibly eu in these systems and yield signatures of these elements that can distinguish source regions and in some cases bulk versus melt assimilation apatite a common restite phase is enriched in rare earth elements ree especially middle ree retention of apatite in the restite will result in steep light ree enriched patterns for melts derived from the diorite and charnockites

a high resolution late neogene geochemical record of the southeastern margin of the tibetan plateau constraints on dali fault system activity and uplift of the diancang shan massif abstract the dali basin located northeast of the diancang shan massif is tectonically controlled by the dali fault system located along the southeastern margin of the tibetan plateau the sediments in the dali basin thus record the activity of the dali fault system and uplift of the diancang shan massif during the late neogene period in this study we present the chemical compositions and bulk mineralogical characteristics of fine grained sediment samples collected from the dasongping section in the dali basin spanning an interval from 7 6 to 1 8 ma mineral assemblages and geochemical data indicate that the sediment provenance in dali basin was a distally sourced weathered material from 7 6 to 4 2 ma and changed to a locally sourced bedrock after 4 2 ma this suggests that the dali basin was an open lake from 7 6 to 4 2 ma and changed to a closed lake at 4 2 ma the main reason for this change was intensified normal fault activity in the dali fault system at 4 2 ma that resulted in the isolation of the dali basin and the infilling of local bedrock around the diancang shan massif within the dali basin late pliocene global cooling marked by the expansion of northern hemisphere glaciation further intensified the uplift of the diancang shan massif at 3 6 ma which exposed paleogene conglomerates east of the dali basin since 2 7 ma the dali fault system has been dominated by normal extensional processes the enhanced reactivation resulted in the rapid uplift of the diancang shan massif and the rapid infill of the dali basin by coarse grained sediments

new paleomagnetic results from the beiya porphyry skarn gold polymetallic deposit at the western dali faulted block implications for the cenozoic tectonic rotation of the chuan dian fragment southeastern tibetan plateau abstract the chuan dian fragment cdf in the southeastern tibetan plateau is crucially important for understanding the intra continental deformation behavior of the blocks between the stable yangtze craton and the indian plate after the indian asian collision the beiya porphyry skarn gold polymetallic deposit at the northwestern part of the fragment may provide an important paleomagnetic result for understanding the deformation behavior of the cdf the paleomagnetic study of mineralized and intrusive rocks from the 35 ma beiya deposit reveals a high temperature component with site mean direction of d 337 6 i 32 6 k 74 8 with 95 4 4 our results show that the beiya deposit experienced 21 5 3 1 of anticlockwise rotation relative to the stable part of the south china block since 35 ma the stable part of the dali faulted block is deduced to have experienced 20 clockwise rotation based on a comparison of the fold axis strike of triassic strata between the beiya deposit and surrounding stable area a best fit linear regression of the fold axis strike direction and the tectonic rotation at each sampling site reveals that differential tectonic rotation occurred accompanied by the horizontal deflection of strata within the cdf

uplift of the hengduan mountains on the southeastern margin of the tibetan plateau in the late miocene and its paleoenvironmental impact on hominoid diversity abstract the southeastern margin of the tibetan plateau has a number of well known late miocene localities that have yielded fossil hominoids belonging to lufengpithecus and khoratpithecus these localities provide critical evidence about the evolutionary history of hominoids including their response to environmental changes associated with uplift of the tibetan plateau here we present magnetostratigraphic dating of the baoshan hominoid locality in yunnan southwestern china which based on preliminary biochronological evidence was previously assumed to be the youngest known miocene hominoid locality in eurasia paleomagnetic investigations yield three normal and three reversed magnetozones which can be best correlated in conjunction with biostratigraphic constraints to chrons c3n 4n to c3ar the baoshan hominoid is derived from a horizon correlated with the lowest part of c3r having an estimated age of 6 0 ma this indicates that the baoshan hominoid is slightly younger than l cf lufengensis from zhaotong dated to 6 2 ma and l lufengensis from lufeng dated to 6 9 6 2 ma our results coupled with previously published magnetochronology indicate that hominoids occurred in southeast asia from the latest middle miocene to the terminal miocene 13 6 ma the period after 6 ma coincides with the uplift of the hengduan mountains especially the gaoligong mountain we propose that the late miocene uplift of these n s oriented mountains on the southeastern margin of the tibetan plateau created a barrier to the warm and moist indian monsoon from the west and this led to a cooler and drier climate in the southeastern margin of the tibetan plateau this paleoenvironmental change potentially had a profound impact on the diversity of the hominoid community in southwestern china and southeast asia

partition coefficients of hf zr and ree between zircon apatite and liquid concentration ratios of hf zr and ree between zircon apatite and liquid were determined for three igneous compositions two andesites and a diorite the concentration ratios of these elements between zircon and corresponding liquid can approximate the partition coefficient although the concentration ratios between apatite and andesite groundmass can be considered as partition coefficients those for the apatite in the diorite may deviate from the partition coefficients the hree partition coefficients between zircon and liquid are very large 100 for er to 500 for lu and the hf partition coefficient is even larger the ree partition coefficients between apatite and liquid are convex upward and large d 10 100 whereas the hf and zr partition coefficients are less than 1 the large differences between partition coefficients of lu and hf for zircon liquid and for apatite liquid are confirmed these partition coefficients are useful for petrogenetic models involving zircon and apatite

abrupt shifts in the indian monsoon during the pliocene marked by high resolution terrestrial records from the yuanmou basin in southwest china abstract a 650 m thick sequence of fluvio lacustrine sediments from the yuanmou basin in southwest china was analyzed at 20 cm intervals for grain size distribution to provide a high resolution terrestrial record of indian summer monsoon variations during the pliocene the concentrations of the clay and clay plus fine silt fractions are inferred to reflect the water level status of the lake basin related to the intensity of the indian summer monsoon and high concentrations reflect high lake levels resulting from the intensified summer monsoon the frequency of individual lacustrine mud beds is considered to reveal the frequency of the lakes developed in the basin associated with the variability of the indian summer monsoon and an increased frequency of the lakes reveals an increased variability of the summer monsoon the proxy data indicate that the indian summer monsoon experienced two major shifts at 3 57 and 2 78 ma and two secondary shifts at 3 09 and 2 39 ma during the pliocene the summer monsoon displayed a general trend of gradual intensification during the period of 3 57 2 78 ma coeval with an accelerated uplift of the tibetan plateau implying a close link between the monsoon intensification and the plateau uplift at 2 78 ma the summer monsoon was markedly weakened synchronous with the formation of extensive northern hemisphere ice sheets denoting a quick response of the monsoon regime to the northern hemisphere glaciation the variability of the summer monsoon decreased at 3 09 ma and increased at 2 39 ma presumably suggesting that variations of the indian monsoon would be modulated by the initiation and periodic fluctuations of ice sheet covers in northern hemisphere high latitudes

magnetostratigraphy of the dali basin in yunnan and implications for late neogene rotation of the southeast margin of the tibetan plateau 1 the rotation pattern and fault activity in the southeast margin of the tibetan plateau semtp provide meaningful constraints on the geodynamic evolution of the plateau however the lack of cenozoic paleomagnetic studies and accurate age constraints on neogene sediments prevents a better understanding of the late cenozoic tectonic activity in this area to clarify the tectonic rotation pattern and deformation history of the semtp we report new magnetostratigraphic data from a late neogene sedimentary sequence in the dali basin northwestern yunnan province china rock magnetic analyses indicate that both magnetite and hematite are the main carriers of the characteristic remanent magnetizations chrms magnetostratigraphic results show that the sedimentary profile spans from chron c4n 1r to chron c2n the age of the sedimentary sequence in the dali basin can thus be paleomagnetically constrained to an interval from late miocene to early pleistocene the basal age of the sediments is 7 6 ma which indicates that the unroofing of diancang shan and activation of the dali fault system were initiated at this time the appearance of conglomerates and syntectonic sediments suggests the reactivation of the dali fault system at 2 5 ma moreover the overall mean chrm direction suggests that the dali basin experienced significant 4 4 2 5 but minor post late miocene rotation this indicates that most of the clockwise rotation demonstrated by previous paleomagnetic studies in the semtp occurred prior to late miocene and may be concentrated between eocene and miocene which is contemporaneous with the sinistral slip of the ailao shan red river fault

variation in the indian summer monsoon intensity during the bolling allerod and holocene variations in the indian summer monsoon ism intensity during the last 16 7 ka have been studied using organic carbon corg 15 n of sedimentary organic matter caco3 sediment texture 18 oc and mg ca derived sea surface temperature 18 o of sea water and sea surface salinity in a 14 c dated sediment core from the eastern arabian sea the 18 o in water and planktonic foraminifera shells off the central west coast of india may be controlled by the ism intensity as this area receives high precipitation and land runoff also the corg and caco3 contents of sediments and 15 n of sedimentary organic matter may be linked to ism induced productivity and denitrification the results of the present study reveal that between 16 and 15 2 ka bp the ism was weak with minor fluctuations and started intensifying around 15 2 ka bp at the onset of the bolling allerod b a event the b a event is characterized by higher water column denitrification rates comparable to the present day the ism signatures observed in the 18 oc record of b a event compare well with those from timta cave of the western himalayas and also the asian summer monsoon signatures from the hulu caves in china and warming signatures in greenland ice sheet project 2 gisp2 suggesting atmospheric tele connections through intertropical convergence zone itcz the boundary between the younger dryas and the holocene is discernible with small episodes of abrupt events of increased ism intensity this decrease in 18 oc values at 11 8 ka bp is contemporary with june solar insolation maximum at 30 north and the increase in methane in the gisp2 ice core supporting episodes of warmer climate and increase in ism intensity the ism seems to have been most stable between 7 and 5 6 ka bp the core exhibits periodicity of 500 that is comparable to the atlantic water formation and the chinese monsoon

paleoseismological events in the seismic gap between the 2008 wenchuan and the 2013 lushan earthquakes and implications for future seismic potential abstract the aftershocks of the 2008 wenchuan and 2013 lushan earthquakes delineated a prominent 60 km long seismic gap the uncertainty regarding the near future seismic potential of this seismic gap has become of increasing importance to both the scientific community and the public using paleoseismology this study investigated the occurrence of the last major event in the seismic gap and then considered the future seismic potential based on image interpretation trench excavation radiocarbon and optically stimulated luminescence dating and comparisons with historical earthquake rupture lengths the most recent paleoearthquake estimated magnitude m6 5 7 0 along the shuangshi dachuan fault sdf which is the southern segment of the longmen shan fault was constrained to 640 1330 ce very possibly 876 ce according to the most recent geological and geophysical findings we suggest that the seismic gap i e the sdf could be capable of generating m6 0 7 0 earthquakes but that the likelihood of such an occurrence in the near future is not very high

accessory phase controls on the geochemistry of crustal melts and restites produced during water undersaturated partial melting the profound geochemical conseqences of accessory phase behaviour during partial melting of highgrade metapelites are demonstrated with reference to two geochemically distinct crustal melts produced by biotite dehydration melting reactions under granulite facies kbar 860 cc conditions these two leucogneiss suites from the brattstrand bluffs coastline eastern antarctica have similar field relations transport distances 10 100 s of metres and major element chemistry type 1 leucogneisses have low zr th and lree positive eu anomalies and zr zr and lreet lreet values less than 1 0 i e less than required to saturate the melt mass balance constraints suggest that these melts have been extracted before equilibration with host melanosomes the dry peraluminous nature of vapour undersaturated melts inhibits monazite and zircon solubility and results in concentration of these phases in the residue melts are consequently depleted in lree and hree melanosomes show complementary enrichment in lree while hree patterns are dominated by residual garnet type 2 leucogneisses in contrast have strongly enriched zr th and lree abundances negative eu anomalies and zr zr and lreet lreet 1 resulting from accessory phase entrainment vapour absent partial melting under moderate 6 8 kbar pressure granulite facies conditions of a pelitic source containing monazite is likely to give disequilibrium melts depleted in lree and hree as monazite and garnet are concentrated in the residue if temperatures are high enough 850 870 c to permit relatively large degrees of partial melting then the feldspar component of the source will be removed almost completely giving melts with large positive eu anomalies melts formed under vapour present conditions are unlikely to show such extreme lree and hree depletion or positive eu anomalies even at high degrees of partial melting disequilibrium melting coupled with source entrainment could fortuitously produce ree and trace element signatures similar to those typical of s type granites and usually ascribed to equilibrium melting conditions

new paleomagnetic constraints on central asian kinematics displacement along the altyn tagh fault and rotation of the qaidam basin received 23 july 2001 revised 9 january 2002 accepted 26 february 2002 published 5 october 2002 1 in order to better understand the tectonic evolution of central asia under the influence of the india asia collision we carried out a paleomagnetic study of 1500 cores from 106 sites along the altyn tagh fault in the qaidam and tarim basins and on the tibetan plateau samples were mainly collected from jurassic to neogene siltstones and sandstones in most cases stepwise thermal demagnetization unblocks low and high temperature components carried by magnetite and hematite low temperature components are north and down directed and lie close to the recent geomagnetic field high temperature components from 10 of 13 age locality groups pass fold and or reversal tests and likely represent primary remanent magnetizations the ten overall mean directions display a complex pattern of vertical axis block rotations that are compatible with a tectonic model of clockwise rotation of the qaidam basin and concomitant left lateral slip on the altyn tagh fault two of the ten localities are rotated significantly counterclockwise they lie adjacent to the altyn tagh fault zone consistent with the idea that left lateral strike slip motion occurred along it the age of counterclockwise rotation near the eastern extremity of the fault was dated as younger than 19 ma three widely spread areas within the qaidam basin exhibit similar and significant clockwise rotations on the order of 20 with respect to the north china block tarim and eurasia the mean of the three values is thought to represent the total rotation of qaidam because the youngest rocks displaying clockwise rotations are oligocene the main phase of qaidam basin rotation and hence shear on the altyn tagh fault took place after or near the end of the oligocene 24 ma upper neogene strata located on the qaidam basin are not significantly rotated thus tectonic deformation acting since the upper neogene 5 ma is not resolvable by paleomagnetic methods given a 20 5 clockwise rotation of the qaidam basin with respect to the tarim basin the maximum left lateral displacement on the altyn tagh fault since 24 ma is 500 130 km index terms 1525 geomagnetism and paleomagnetism paleomagnetism applied to tectonics regional global 1527 geomagnetism and paleomagnetism paleomagnetism applied to geologic processes 8105 tectonophysics continental margins and sedimentary basins

late cenozoic eolian deposition in the north pacific asian drying tibetan uplift and cooling of the northern hemisphere a newly constructed record of eolian dust accumulation from the central north pacific shows that dust deposition increased by an order of magnitude quite rapidly at 3 6 ma we associate this sudden drying with the uplift of at least the northern portion of the tibetan plateau shutting off the indian ocean moisture source to central and western china this ten fold increase in atmospheric dust loading appears to be associated with the 1 m y long shift toward heavy 18o values that occurred at 3 6 2 6 ma the dust grain size record of wind intensity begins its late cenozoic coarsening a million years before the drying event at 4 5 ma the northern hemisphere cooling that results in intensification of the subpolar westerly winds may have as its ultimate cause the drawdown of atmospheric co2 in the latest miocene and the early pliocene closing of the panamanian seaway

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

magnetostratigraphy of the xiaolongtan formation bearing lufengpithecus keiyuanensis in yunnan southwestern china constraint on the initiation time of the southern segment of the xianshuihe xiaojiang fault the late cenozoic extensional basins in yunnan province southwestern china which are kinematically linked with the regional strike slip faults can provide meaningful constraints on the fault activity history and tectonic evolution of the southeast margin of the tibetan plateau semtp and further on the geodynamic evolution of the tibetan plateau however this has been severely impeded by the lack of precise age constraints on the timing of fault activity to better constrain the timing of fault activity and the tectonic rotation of semtp we undertook a high resolution magnetostratigraphic study on the xiaolongtan formation in the xiaolongtan basin which is located at the southern tip of the xianshuihe xiaojiang fault and is well known by the presence of hominoid lufengpithecus keiyuanensis rock magnetic experiments indicate that magnetite is the main remanence carrier correlation to the geomagnetic polarity timescale was achieved by combining magnetostratigraphic and biostratigraphic data our correlation suggests that the xiaolongtan formation sedimentary sequence spans from chron c5ar 1r to chron c5n 2n which indicates that the age of the xiaolongtan formation ranges from 10 ma to 12 7 ma and that the ages of the two sedimentary layers possibly bearing the hominoid l keiyuanensis are 11 6 ma or 12 5 ma the basal age of the sediments is 12 7 ma which indicates that the activation of the southern xianshuihe xiaojiang fault was initiated at this time the overall mean paleomagnetic direction d 353 2 i 34 2 95 2 1 n 166 documents a counter clockwise vertical axis rotation of 8 3 with respect to eurasia which is the response to the activity of the left lateral xianshuihe xiaojiang fault

extensional and contractional deformation in the blue ridge province virginia basement rocks of the blue ridge anticlinorium in central and northern virginia were involved in two temporally distinct deformation episodes the older event is characterized by a locally pervasive low strain fabric and high strain shear zones a preferred grain shape orientation in recovered quartz feldspar and biotite combined with a poorly developed symmetric quartz c axis pattern defines the low strain fabric kinematic indicators are typically symmetric but where asymmetric they consistently indicate top down to the southeast extensional motion discrete shear zones are within and close to late proterozoic plutons kinematic evidence fron these zones records large noncoaxial strains also with a down to the southeast extensional shear sense feldspar microstructures range from cataclastic fractures to complete recovery suggesting that deformation occurred at greenschist to amphibolite grade both low strain foliations and high strain shear zones are cut by paleozoic thrust related tectonites on the basis of kinematic evidence and spatial association with late proterozoic plutons cross cutting relations with thrust related structures and regional geologic relations we suggest that extensional deformation is related to late proterozoic rifting of the laurentian margin the kinematic histories of the rockfish valley fault zone and related shear zones appear to be relatively simple and consistent with paleozoic west to northwest directed thrusting the rockfish valley fault zone does not separate distinctly different basement terranes and thus its displacement is thought to be small mylonitic rocks from this fault zone display crystal plastic deformation in quartz and cataclastic deformation in feldspars under lower to middle greenschist facies conditions solution transfer processes were an important deformation mechanism in these tectonites thrust related mylonitic rocks are overprinted by related cataclasite and semibrittle shear bands suggestive of a transition toward more brittle deformation as the thrust system cut higher in the crust the rockfish valley fault zone appears to predate the formation of the south mountain cleavage in the valley and ridge province but it may be part of a lengthy progressive thrust related deformation

cretaceous and tertiary paleomagnetic results from southeast china and their tectonic implications abstract we report cretaceous and tertiary paleomagnetic data from fujian guangxi and guangdong provinces in south china in guangxi province we sampled early cretaceous redbeds of the xinlong formation at eight localities separated by up to 100 km the high temperature component obtained by thermal demagnetization is of dual polarity and passes the fold test and implies 18 8 counterclockwise rotation and insignificant poleward displacement with respect to cratonic localities in sichuan province a tertiary basin lying in a southeast trending fault zone is also rotated counterclockwise by 40 8 both results can be explained very well by a simple block rotation model for the cenozoic tectonic evolution of western guangxi province that is driven by left lateral shear on southeast trending faults the timing and sense of shear are the same as for india asia collision models involving the extrusion of indochina along the red river fault zone which lies several hundred kilometers to the southwest but the magnitude of fault displacement in guangxi is much less in the coastal provinces of fujian and guangdong we find late cretaceous directions that are rotated clockwise with respect to sichuan province combined with similar clockwise rotations found by two other studies of coeval rocks from the same region the mean rotation for the coastal provinces is 12 8 this could indicate a coherent block rotation of the region driven by extrusion tectonics however the limited number of sampling localities also allows the possibility that the directions reflect local rotations due to dextral shearing driven by circum pacific tectonics

paleomagnetism of miocene sediments from the turfan basin northwest china no significant vertical axis rotation during neotectonic compression within the tian shan range central asia abstract paleomagnetic results are reported from 377 samples of miocene red sediments in the turfan basin of east tian shan northwest china thermal demagnetization for most specimens isolates a low temperature component that groups around the present geomagnetic field a high temperature characteristic component is isolated from four localities and interpreted as primary remanent magnetization from apparent pre folding origin and positive reversal test as for previous results obtained from tertiary rocks in central asia the magnetic inclination is significantly shallower than the reference field computed from the eurasian apw path at 10 and 20 ma magnetic susceptibility measured in 324 specimens exhibits low anisotropy of less than 1 05 for 80 of the sample the low anisotropy degrees ams shape and orientations suggest that the studied rocks are unlikely to have undergone significant strain during compaction or tectonism we argue that the shallow inclination is likely to result from an initially shallow detrital remanent magnetization comparison of the paleomagnetic declination from eurasia north china and tarim suggests that the turfan basin has been subjected to little or no vertical axis rotation since the miocene together with paleomagnetic declinations on both sides of the talas ferghana fault we propose that the tian shan to the east of the talas ferghana fault has remained relatively stationary with respect to adjacent blocks since the miocene

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

paleomagnetism of the yuanmou basin near the southeastern margin of the tibetan plateau and its constraints on late neogene sedimentation and tectonic rotation abstract new paleomagnetic investigation was carried out on the late neogene fluviolacustrine sequence of the yuanmou basin located near the southeastern margin of the tibetan plateau magnetostratigraphic results indicate nine reverse magnetozones r1 to r9 and eight normal magnetozones n1 to n8 in the sedimentary profile which can be correlated to the geomagnetic polarity timescale from c3n 3r to c1r 1r the age of the sedimentary sequence of the yuanmou basin can thus be paleomagnetically constrained to an interval from early pliocene to pleistocene with sedimentation rates varying from 12 5 to 55 cm kyr in addition to its highly resolved magnetostratigraphic sequence the yuanmou basin provides a record of plio pleistocene tectono and climato sedimentary processes the mean declinations of the seventeen polarity units excluding samples with transitional directions can be grouped into three distinct directional intervals group i 2 58 1 37 ma group ii 4 29 2 58 ma and group iii 4 91 4 29 ma these directions indicate that the yuanmou basin has probably experienced vertical axis clockwise rotation of about 12 from 1 4 ma to 4 9 ma which may be related to slip activity of the red river fault to the southwest and the xianshuihe xiaojiang fault to the east

chemical diversity and origin of precambrian charnockitic rocks of the central pedlar massif grenvillian blue ridge terrane virginia abstract middle proterozoic rocks of the pedlar massif are comprised of varied charnockitic and granulitic lithologies representing 1150 1000 ma grenvillian metamorphic and igneous events in the central appalachians of virginia chemically and mineralogically diverse units include metaluminous to peraluminous charnockites enderbites jotunites and granulite gneisses ranging in sio2 from 47 to 79 wt relative to bulk continental crust typical charnockites are enriched by 3 6 in rb k ba lree zr and hf less enriched in ta sr p and hree and depleted in cs th ti and sc although low sio2 members have relatively high ti and p and some high sio2 units are depleted in k rb and ba the pedlar river charnockite suite prcs vesuvius megacrystic charnockite vmc nellysford granulite gneiss ngg and the lady slipper granulite gneiss lsgg exhibit common chemical signatures and trends notably evident in uniform lree enriched patterns with variable eu anomalies which reflect their derivation from a mixed protolith of volcanics and reworked volcaniclastic sediments dated at 1130 1150 ma pedlar river charnockites represent portions of deep seated granulite gneisses that were mobilized by granulite facies dehydration melting and emplaced en masse as plutons into overlying and surrounding gneisses during the grenville episode divergent ree patterns in the prcs are attributed to crystallization of quartz feldspar garnet or hree compatible mafic and accessory phases leading to complementary chemical signatures in mafic and felsic layers depletion of cs throughout the pedlar massif is attributed to its incompatibility with major mineral phases during either protolith evolution or granulite facies dehydration while dispersions in k rb sr ba and eu are due to the relative proportions of the major phases quartz k feldspar plagioclase and biotite mobility of insoluble trace elements zr hf ta and th require partial melting and crystallization of accessory mineral phases whereas sc cr and ree mobility depends on the proportions of pyroxene and garnet several leucocharnockites and other felsic rocks of the prcs show markedly elevated th values ranging from 10 to 83 ppm which complement depleted th in typical prcs units these charnockitic rocks represent low degree partial melts 1 5 from normal prcs or ngg protoliths to produce high silica magmas which separated from the main charnockitic body

paleomagnetism of cretaceous rocks from eastern qiangtang terrane of tibet thermal demagnetization has successfully isolated high unblocking temperature characteristic directions from paleomagnetic samples of lower and middle cretaceous redbed formations from eastern qiangtang terrane tibet the tilt corrected mean direction for the lower cretaceous is d i 57 7 50 2 with 95 10 9 and n 12 sites and that for the middle cretaceous is d i 39 7 51 6 with 95 8 5 and n 11 sites both formations pass the fold test and the latter possesses two polarities the magnetization predates the eocene and was probably acquired during cretaceous time comparison of the mean directions with expected directions from the stable interior of eurasia indicates significant clockwise rotation but little paleolatitudinal change for the sampling area deformation and rotation of these rocks are presumably related to the collision of india with asia and its subsequent northward indentation into asia existing paleomagnetic data from tibet and its environs seems to be compatible with a crustal shortening thickening model dewey et al 1988 in that the continental crust directly north of the indian indenter was substantially shortened north south whereas in the vicinity of the western and eastern syntaxes latitudinal displacement was minimal but tectonic rotation was very important more data are needed to further differentiate between this model and the lateral eastward crustal extrusion model molnar and tapponier 1975

paleomagnetism of the jianshui basin in yunnan sw china and geomorphological evolution of the yunnan plateau since the neogene abstract there are still a lot of uncertainty on the process of the tibetan plateau uplift and the response of the yunnan plateau in the cenozoic in order to have a further understanding on the yunnan plateau uplift process we combine the studies on the sedimentary history of the jianshui basin and other basins and faults in the yunnan plateau we established magnetostratigraphy for the wanyao wy and qingyunshuiku qs fluviolacustrine sections in the northern and middle parts of the jianshui basin which indicates eight reverse magnetozones r1 r8 and seven normal magnetozones n1 n7 correlating to chrons c1r 3r through c3n 2r the strata are constrained to a span from 4 77 to 1 73 ma representing an early pliocene to early pleistocene age the accumulation rate of the wy section has accelerated trend stopping deposit earlier than the qs section the stratigraphic characteristics of the two sections and their nearby regions indicate the shiping jianshui sj and xiaojiang faults have already existed and affected the accumulation of the basin at least since the early pliocene according to the history of faults activities and the sedimentary strata of basins we established the geomorphological evolution process of the yunnan plateau since the neogene

petrology and isotopic composition of a grenvillian basement fragment in the northern appalachian orogen blair river inlier nova scotia canada mesoproterozoic metaplutonic rocks in northern cape breton island introduction nova scotia occur in a tectonic inlier within the appalachian in eastern north america mesoproterozoic laurentian orogen although they have been multiply metamorphosed and basement rocks and their neoproterozoic to mid paleovariably deformed the petrology and geochemistry of these rocks zoic volcanic and sedimentary cover preserve a record provide insight into the tectonomagmatic evolution of easternmost of the proterozoic to mid paleozoic geologic history of laurentia anorthosite syenite and granitoid plutons eastern laurentia the record includes mesoproterozoic 1100 980 ma intruded the sailor brook and polletts cove river orogeny on the ancient north american craton breakgneisses new nd isotopic data are presented from a biotite rich up of a neoproterozoic supercontinent development of part of the sailor brook gneiss ndi 0 7 two anorthosite a passive continental margin and mid paleozoic orogeny samples ndi 2 1 and 2 8 and a charnockite unit exposures of mesoproterozoic laurentian basement in ndi 0 4 new u pb zircon data from the anorthosite yield easternmost canada and the usa occur in a dispb pb ages between 975 and 1095 ma disrupted u pb continuous band of inliers in the appalachian orogen systematics preclude a unique age interpretation but the 1095 ma from newfoundland to alabama fig 1 they consist single grain date is a minimum age for the anorthosite field relations mostly of rocks formed or deformed during major and trace element geochemistry and isotopic characteristics 1190 980 ma grenvillian tectonic and magmatic indicate that the anorthosite and charnockite were probably coexisting events and are distinct from neoproterozoic and melts but not differentiates of a single parent magma the lithological paleozoic units of accreted appalachian terranes in that and chemical characteristics of the lowland brook syenite are they include granulite facies metamorphic rocks and similar to those of a contemporaneous belt of ultrapotassic plutons in many cases anorthosite charnockite mangerite suites in the southern grenville province and both have the chemical bartholomew 1984 and papers therein pettingill et al characteristics of certain modern continental margin arc magmas 1984 rankin et al 1989 1993 owen erdmer 1990 these data indicate that the mesoproterozoic units in the blair hughes et al 1997 river inlier were juvenile crustal additions to eastern laurentia the sinuous trend of the band of basement inliers during grenville related orogenic events broadly reflects promontories and re entrants that formed

pollen evidence of the palaeoenvironments of lufengpithecus lufengensis in the zhaotong basin southeastern margin of the tibetan plateau evolutionary processes in hominoid primates were closely related to global and or regional environmental changes and therefore palaeoenvironmental reconstruction is fundamental for understanding how environmental changes shaped their evolution here we present pollen data from the 16 m thick shuitangba stb section yunnan province southwestern china bearing remains of the hominoid lufengpithecus lufengensis of the terminal miocene and use principal component analysis to reconstruct the palaeovegetation and palaeodimate during the key period when the hominoid lived our results show that before the stb hominoid appeared zone a the vegetation was dominated by subtropical evergreen broad leaved taxa with a few temperate deciduous taxa e g quercus castanea castanopsis alnus the development of aquatic plants commencing at the similar to 12 m depth is a prominent feature indicating a warm and humid climate during the time when the hominoid lived zone b evergreen broad leaved forests with evergreen quercus were predominant while grasses including poaceae began to expand and simultaneously conifers decreased indicating a warm climate the significant presence of aquatic pollen taxa in subzone a 2 and zone b suggests the occurrence of lacustrine or swampy environments in contrast in zone c the vegetation changed to coniferous forest indicating cooler and drier conditions these results provide substantive evidence of the vegetation conditions when the hominoid lived suggesting that the greater diversity of vegetation and the warm humid climate compared to the present day would have favoured its survival c 2015 elsevier b v all rights reserved

age and origin of anorthosites charnockites and granulites in the central virginia blue ridge nd and sr isotopic evidence rb sr isotopic data for anorthosites charnockites ferrodioritic to quartz monzonitic plutons and high grade gneisses of the blue ridge of central virginia show evidence of post emplacement metamorphism but in some cases retain grenville ages the pedlar river charnockite suite yields an isochron age of 1021 36 ma initial 87sr 86sr ratio of 0 7047 6 which agrees with published u pb zircon ages five samples of that unit which contain paleozoic mylonitic fabrics define a regression line of 683 ma interpreted as a mixing line with no age significance samples of the roseland anorthosite complex show excessive scatter on a rb sr evolution diagram probably due to paleozoic 475 m y metamorphism data from the ferrodioritic to quartz monzonitic plutons of the area yield an age of 1009 26 ma inital ratio 0 7058 4 which is in the range of the u pb zircon ages of 1000 1100 ma the stage road layered gneiss yields an age of 1147 34 ma initial ratio of 0 7047 5 sm nd data for the pedlar river charnockite suite reflect a pre grenville age of 1489 118 ma nd 6 7 1 2 data for the roseland anorthosite complex and the ferrodioritic to quartz monzonitic plutons yield grenville isochron ages of 1045 44 ma nd 1 0 0 3 and 1027 101 ma nd 1 4 1 0 respectively two roseland anorthosite samples plot far above the isochron demonstrating the effects of post emplacement disturbance of sm nd systematics while mylonitized pedlar river charnockite suite samples show no evidence of sm nd redistribution the disparity of the sm nd age and other isotopic ages for the pedlar river charnockite suite probably reflects a sm nd source age suggesting the presence of an older crust within this portion of the ca 1 ga old basement

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

paleomagnetism indicates no neogene rotation of the qaidam basin in northern tibet during indo asian collision paleomagnetic data were obtained from tertiary red sedimentary rocks at two locations separated by several hundred kilometers within the qaidam basin in the east central part of the basin 30 sites from the lower pliocene youshashan formation yielded characteristic remanent magnetization chrm directions with intermediate unblocking temperatures 100 600 8c chrm with high unblocking temperatures to 680 8c was isolated from 14 sites in the same area chrm directions were obtained from six sites within the oligocene lower gancaigou formation characteristic magnetization was also determined from 16 sites within the lower gancaigou formation exposed in the e bo liang range of the north central qaidam basin when compared with equivalentage expected directions for eurasia the mean paleomagnetic directions indicate no neogene vertical axis rotation of the qaidam basin or the altyn tagh fault the qaidam basin may act as an indentor translating without rotation toward the north china block and driving clockwise vertical axis rotations by differential shortening within the nan shan fold and thrust belt

the jinhe qinghe fault an inactive branch of the xianshuihe xiaojiang fault zone eastern tibet abstract the xianshuihe xiaojiang fault zone xfz forms a prominent linear nw se trending tectonic feature along the southeastern margin of the tibetan plateau the spatial and temporal evolution of the predominantly strike slip movement along the fault remains unclear as estimates for the initiation age of its different segments range from 5 to 13 ma the jinhe qinghe fault jqf the south part of the longmen shan thrust belt provides a key for understanding the tectonic role and deformation history of the xianshuihe xiaojiang fault zone in terms of timing magnitude of displacement and spatial temporal evolution this paper presents research on the structure and chronology of the jqf the results of a geometric and kinematic study of the jinhe qinghe fault show it to be a thrust fault with a left lateral strike slip component the strike slip offset extends at least 13 18 km to southwest based on the shortening of its folds and faulting and based on stream deflection ten apatite fission track apparent ages show a pronounced change in age elevation gradient at 17 2 ma consistent with an abrupt increase of exhumation due to uplift of the hanging wall of the jqf at that time combined with the age of deformed tertiary strata in the footwall of the jqf and the geometric relationships of the jqf with the chenghai fault and the xfz this indicates that the jqf was active between 17 and 5 ma based on the fault chronology geometry and movement we suggest that the fault was a transpressional structure at the south end of the xfz during the period from 17 to 5 ma

magnetostratigraphic study of the potash bearing strata from drilling core zk2893 in the sakhon nakhon basin eastern khorat plateau abstract the khorat plateau holds one of the world s largest potash deposits although many attempts have been made the age of the potash bearing strata remains poorly determined here we report a high resolution magnetostratigraphic study of a 595 4 m deep potash bearing borehole zk2893 in the southeast sakhon nakhon basin in central laos this borehole penetrated the whole potash bearing nong boua formation and reached the underlying sandstone beds of the nam noy formation a total of 500 383 level cubic paleomagnetic samples were collected k t curves and hysteresis loops revealed that hematite and magnetite were the main magnetic carriers and were likely of detrital origin as indicated by microscopic and electron microprobe analysis empa examinations progressive thermal and alternating field af demagnetization isolated 229 valid characteristic remnant magnetizations chrms which showed a normal and antipodal distribution of the inclination data both the microscopic and empa examinations and inclination only analysis suggest primary magnetizations of the chrms yielding nine normal and seven reversed polarity zones combined with regional palynological isotopic and sedimentologic evidence the obtained polarity zones were best correlated to the geomagnetic polarity time scale of gradstein et al 2012 gpts2012 yielding magnetostratigraphic ages of 63 5 ma to 92 ma for the potash bearing nong boua formation and 85 ma for the potash bed in its lower salt unit

oligocene deformation of the chuandian terrane in the se margin of the tibetan plateau related to the extrusion of indochina mechanisms driving the tectonic evolution of the southeast se margin of tibet include the paleogene extrusion of the coherent indochina lithospheric block and the continuous deformation caused by lower crustal flow since the middle miocene the timing and style of regional deformations are key to determining the role of each mechanism fault bounded and controlled cenozoic basins within the se margin of tibet record regional deformation surface uplift and variations in paleoclimate but often are poorly dated new magnetostratigraphy and 40ar 39ar dating of volcanic ashes constrain precisely the timing of sedimentation within the luhe basin to between 35 and 26 5 ma the basin is located in the chuandian terrane along the chuxiong fault which lies 70 km north of and parallel to the ailao shan red river fault the asymmetric syncline of the luhe basin suggests syn contractional sedimentation and the basal age of the basin represents the initiation of the chuxiong fault and crustal shortening at 35 ma this is coincident with the onset of the ailao shan red river fault and supports a kinematic link between them our study suggests that like the ailao shan red river fault the chuxiong fault is a paleogene transpressional structure that developed during the extrusion and clockwise rotation of indochina around the eastern himalayan syntaxis which caused the late paleogene deformation and surface uplift of the chuandian terrane and indochina our revised chronostratigraphy of the luhe basin provides further evidence that many of the neogene sedimentary basins in the se margin of tibet may be much older than previously thought

alleghanian episode of k bentonite illitization in the southern appalachian basin mixed layer illite smectite i s from middle ordovician potassium k bentonites was found to be uniformly illitic throughout the southern appalachian basin regardless of variable depths of burial of the k bentonites the k ar ages of illitization are narrowly confined between 272 and 303 ma late pennsylvanian to early permian this suggests that illitization was a short lived episode coincident with and prompted by the alleghanian orogeny the illitization is explainable by the fluid expulsion hypothesis recently proposed by others e g oliver hot saline fluids were flushed to the basin edges from the deeply buried part of the foreland basin during the orogeny these fluids are thought to be effective agents of illitization some of our k bentonite samples are stratigraphically and geographically close to mississippi valley type lead zinc deposits suggesting a similar mode of origin rapid illitization of shales in a fashion similar to that observed for the bentonites should have led to high pore water pressures that enabled them to act as ideal decollements during alleghanian thin skinned deformation

tectonic evolution of the grenville orogen in the central appalachians abstract remnants of the grenville orogeny a globally pervasive tectonothermal event resulting from the amalgamation of the supercontinent rodinia between ca 1 3 and 1 0 ga are exposed throughout eastern north america for ca 3000 km from labrador to alabama although tectonic models for the development of the grenville have historically been based on observations from the large and well exposed canadian grenville province recent investigations suggest that initial continental collision occurred proximal to the appalachian blue ridge mesoproterozoic rocks within this portion of the orogen are exposed within a series of allochthonous terranes that were extensively reworked during paleozoic orogenesis to better understand the tectonic history of this critical region we utilized garnet and monazite geochronology to reconstruct the metamorphic history of eleven samples of ortho and paragneiss from the appalachian blue ridge province the resultant dates together with new results from trace element thermobarometry define a prolonged record of high temperature metamorphism coincident with the timing of continent continent collision as recognized in the northern portion of the orogen initial garnet and monazite growth commenced between 1100 and 1065 ma with peak granulite facies conditions 750 900 c 0 55 0 85 gpa occurring from 1050 to 1010 ma burial and heating of the paragneisses progressed rapidly with peak metamorphism occurring 20 30 myr after deposition of the sedimentary protoliths local hydrothermal alteration followed peak metamorphism and occurred at 1000 990 ma with later cooling through 660 c between 980 and 935 ma 3 2 c myr the metamorphic history outlined here together with insights from previous studies is consistent with formation of the blue ridge rocks within a continental suture zone that had an orogenic architecture comparable to that of the himalayan tibetan orogen

paleomagnetism indicates no neogene vertical axis rotations of the northeastern tibetan plateau 1 paleomagnetic data were obtained from 108 paleomagnetic sites collected in cretaceous to tertiary red beds from seven localities distributed in three general regions adjacent to the altyn tagh fault at the northern edge of the tibetan plateau in the hexi corridor 12 sites in oligocene strata at yaoquanzi 39 97 n 97 68 e yield a mean paleomagnetic direction i 33 0 d 8 6 95 6 0 with concordant declination and 36 sites in early cretaceous mudstones of the longshou shan 39 09 n 100 50 e provide a concordant paleomagnetic direction i 53 4 d 6 8 95 3 8 across the nan shan fold thrust belt miocene paleomagnetic directions from 39 sites distributed among four localities have a concordant mean declination i 40 6 d 7 2 95 5 8 in the altyn tagh range 21 sites in oligocene strata at xorkoli 38 93 n 91 43 e yield a paleomagnetic direction with concordant declination i 49 7 d 5 0 95 5 4 these results combined with existing regional paleomagnetic data indicate that 1 the hexi corridor along with the north china block has not undergone tectonic vertical axis rotation since at least early cretaceous time and is separated from the xining lanzhou basin and eastern tibet by an important post early cretaceous tectonic boundary 2 the nan shan fold thrust belt and the qaidam basin have not experienced wholesale vertical axis rotation during neogene time and 3 absence of vertical axis rotations in areas adjacent to the altyn tagh fault indicates that sinistral shear strain between the tarim basin and the northern tibetan plateau is concentrated on the fault these results are consistent with asian tectonic models that combine distributed lithospheric deformation and thickening with narrow and weak shear zones

pliocene to present morphotectonics of the dien bien phu fault in northwest vietnam abstract the north to northeast trending dien bien phu fault dbpf zone appears to the south of the red river fault rrf zone sharing the spatial alignment of the xianshuihe xiaojiang fault xxf and extending over a distance of 150 km from yunnan china through northwest vietnam into laos although the dbpf is one of the most conspicuous active fault systems in indochina it is less studied than the rrf and xxf and its quantified kinematics remain mostly unknown our detailed modern fault trace mapping compiled from topographic maps stereographic aerial photos aster satellite imageries and field reconnaissance reveals new information on the fault geometry the slip magnitude and distribution along the fault and the relationship between river channel offset and fault activity the geometry of the modern dbpf is complex consisting of single strands and stepovers abundant geomorphic expressions along the dbpf illustrate that the modern fault is dominated by sinistral motion and the present left lateral component of motion is also clearly demonstrated by the existence of numerous rivers offsets multiple offsets of geomorphic features along the fault are recognized and reconstructed and the largest sinistral displacement on the dbpf is ca 12 5 km based on drainage network restoration because sinistral motion likely initiated approximately 5 ma the most probable pliocene to present average slip rate on the dbpf is on the order of 2 5 mm yr based on the slip rate the dbpf could generate an earthquake with a magnitude greater than m w 7 and a recurrence interval of 500 to 1000 yr the combined gps velocity fields observed from northwest vietnam and south china reveal approximately 2 to 3 mm yr left lateral slip across the dbpf and significant east west extension exists in the western crustal block of dbpf although the boundary fault system of the active crust rotation around the eastern himalayan syntaxis i e xxf does not cut the rrf the pliocene to present activity along the dbpf favors the possibility that the tectonic shear has been transmitted across the rrf and taken up on the dbpf the results of this study suggest that the modern dbpf zone acts as a reactivated fault with a different slip sense from its previous phase and plays a role as an eastern boundary of the crustal deformation in northern indochina

right lateral shear and rotation as the explanation for strike slip faulting in eastern tibet the convergence of two continents can be accommodated by crustal thickening and by lateral transport of crust out of the path of the converging continents1 3 the principal features suggesting that lateral transport is important are major strike slip faults striking roughly orthogonally to the orientation of convergence between the continents displacement on such faults is imagined to allow lateral transport of material with respect to the converging continents possibly accounting for a large fraction of the convergence between the two continents the most spectacular example of such strike slip faulting is in the eastern part of the tibetan plateau where three major left lateral faults with slip rates of 10 mm yr 1 and several smaller roughly parallel faults dominate the regional strain field2 3 cobbold and davy4 suggested that these faults may be rotating in a clockwise fashion here we put bounds on that rate of rotation and conclude that the image of lateral transport on such faults2 known also as continental escape extrusion or expulsion is an illusion and that instead the left lateral slip on east striking planes in eastern tibet is a manifestation of north striking right lateral simple shear if this conclusion is correct the east striking left lateral faults and the crustal blocks between them are rotating clockwise at 1 2 myr 1 the east west dimension of eastern tibet is shortening at 10 20 mm yr 1 and little material is moving eastward out of india s path into eurasia by left lateral simple shear

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

the cenozoic rotational extrusion of the chuan dian fragment new paleomagnetic results from paleogene red beds on the southeastern edge of the tibetan plateau abstract paleomagnetic studies were conducted on the eocene and oligocene strata at the western part of the chuan dian fragment in order to describe the crustal deformation induced by continuous penetration of the indian plate into eurasia during the late cenozoic high temperature magnetic components with unblocking temperatures of 680 c were isolated and positive fold and or reversal tests reveal the primary nature of the magnetization the tilt corrected site mean directions obtained from the oligocene and middle early eocene strata are respectively ds 200 9 is 31 3 k 52 8 95 7 7 and ds 29 7 is 32 0 k 44 9 95 5 6 comparison of these results with previous paleomagnetic data from the chuan dian fragment shows that the western and central parts of the chuan dian fragment experienced 20 integral clockwise rotation relative to east asia since the middle miocene however the eastern part of the chuan dian fragment has experienced different rotational deformation relative to east asia since the pliocene because of the intense regional crustal deformation and activity on fault systems the eastern boundary of the chuan dian fragment was bounded by the yuanmou luezhijiang left lateral strike slip fault prior to the pliocene and then substituted by the xiaojiang left lateral strike slip fault since the pliocene due to the eastwards spreading of the clockwise rotational movement of the chuan dian fragment the evolutionary characteristics of the ailaoshan red river and xianshuihe xiaojiang strike slip faults were controlled by the difference between the clockwise rotational extrusion velocities of the chuan dian fragment and the indochina block

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

late cenozoic to holocene deformation in southwestern sichuan and adjacent yunnan china and its role in formation of the southeastern part of the tibetan plateau from at least 2 4 ma to present crust in the southeastern part of the tibetan plateau west of the convex east xianshuihe xiaojiang fault system has deformed internally and rotated clockwise around the eastern himalayan syntaxis the northwest striking ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78 100 km of which 60 km is transferred to the xianshuihe fault zone across a diffuse transfer zone and 22 40 km is absorbed by bending of older structures and crustal shortening crustal shortening is expressed along and east of the eastern end of the ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the tibetan plateau a similar transfer of left slip into shortening occurs farther south across the xianshuihe fault in the high mountains around and east of gongga shan 7556 m the northwest striking convex east left lateral litang fault zone lies southwest of the ganzi xianshuihe xiaojiang fault zone and appears to be less well developed but otherwise similar to the ganzi fault zone the batang chenzhi and other northeast striking right lateral faults of small displacement occur within the rotating crustal fragment together with the left slip faults they accommodate east west shortening northeast of the eastern himalayan syntaxis south of this region of shortening the crust is extending to form grabens within the dali and southern xiaojiang fault systems and in the tengchong volcanic province the progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern himalayan syntaxis the assemblage of structures in southwestern sichuan geometrically resembles structures of eocene to miocene age in southern yunnan that were positioned northeast of the eastern himalayan syntaxis similar to present day southwestern sichuan at the time of their development the similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past 50 m y structures in sichuan are less fully developed than older structures in southwestern yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis deformation in these areas indicates that plateau formation has been complex inhomogeneous and diachronous at scales from 1000 km to less than 100 km

paleomagnetic age of the tangxian planation surface northwestern zhongtiao shan of the shanxi graben system north china abstract a numerical formation age for the tangxian planation surface txps in north china is still controversial the txps beveled across paleoarchean basement rocks is overlain by late cenozoic conglomerate aeolian red clay and loess the magnetostratigraphy of two late cenozoic deposition sections is established and two other sections are recorded and measured in this work the magnetostratigraphy indicates six normal magnetozones n1 to n6 and six reverse magnetozones r1 to r6 these magnetozones are correlated to chron c1n through c2an 1r indicating that the txps was formed at 3 12 3 03 ma the characteristics of the zhongtiao shan tectonic uplift and its relationship with the tibetan plateau and ordos block are discussed in terms of tectonic uplift the txps formation and accumulation of the late cenozoic sediments are mainly controlled by tectonic activities the formation and uplift of the txps with a lag in the shanxi graben system is consistent with the activities of the tibetan plateau and ordos block implying that the ne extension of the tibetan plateau epeirogenic uplift advances successively

u pb geochronology of the grenville orogen of ontario and new york constraints on ancient crustal tectonics based on lithological structural and geophysi cal characteristics the proterozoic grenville orogen of southern ontario and new york has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta morphism u pb ages were determined on metamorphic and igneous sphenes from marbles calc silicate gneisses amphibolites granitoids skarns and pegmatites in addi tion u pb ages were obtained for monazites from meta pelites and for a rutile from an amphibolite these mineral ages constrain the timing of mineral growth the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed grenville orogen based on the ages from metamorphic minerals regional and contact metamorphism occurred in the following intervals central granulite terrane