palaeomagnetism of late cretaceous sediments from southern tibet evidence for the consistent palaeolatitudes of the southern margin of eurasia prior to the collision with india abstract twenty sites were drilled in the late cretaceous shexing formation for palaeomagnetic studies in the lhasa terrane near the locality of maxiang 29 9 n 90 7 e the stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions the tilt corrected mean direction is d i 350 8 32 1 with 95 8 1 and n 20 sites corresponding to a paleopole at 75 0 n 306 7 e with a 95 6 8 positive fold tests indicate a primary origin for the characteristic remanence based on previous cretaceous data mainly from the takena formation and paleocene data from the linzizong volcanic rocks near the city of lhasa the latitude of the southern margin of asia is located at about 15 n and yields a stable position of the lhasa terrane during cretaceous and paleocene compared with expected paleomagnetic directions from the stable india and eurasia blocks the collision palaeolatitude further implies the total latitudinal convergence was accommodated by 1700 800 km 16 2 7 6 between southern tibet and stable eurasia and 1500 830 km 14 4 7 9 between southern tibet and stable india since the collision of india and eurasia a collision age between c 54 and 47 ma was determined using the results for the southern margin of eurasia according to our new data and the extent of greater india

paleomagnetism of cretaceous rocks in the jiaodong peninsula eastern china insight into block rotations and neotectonic deformation in eastern asia 1 although first order agreement of paleomagnetic directions between south china and siberia shows that terranes between them were sutured by cretaceous times deformation by ongoing tectonism has continued and been especially focused near the continental margin of eastern asia to help evaluate differential rotations and relate them to regional strike slip and extensional tectonic regimes we have conducted paleomagnetic study on cretaceous sediments and volcanic rocks from the jiaodong peninsula south of the bohai sea in eastern china bordering the tan lu fold zone tlfz paleomagnetic results from 73 sites in the early cretaceous laiyang and qingshan groups and late cretaceous wangshi group yield magnetizations of primary or synfolding mid cretaceous origins large differential block rotations of early cretaceous age are identified near the muping jimo fracture zone mjfz dissecting the peninsula although late cretaceous declinations are regionally systematic and show post late cretaceous counterclockwise rotation of 14 in the jiaolai basin relative to the reference north and south china blocks block boundaries are identified as the tlfz to the west and the mjfz in the southeast the eastern jiaodong peninsula is rotated clockwise relative to the north and south china blocks by a comparable amount 20 to the southern korean peninsula to the east and to the liaodong peninsula and yanji basin to the north although these blocks are unlikely to be tightly coupled their collective clockwise rotation mirrors counterclockwise rotations north of the japan sea and probably resulted from regional extension focused above uprise of asthenosphere beneath eastern asia during cretaceous times

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

deep water density current deposits of delaware mountain group permian delaware basin texas and new mexico the guadalupian delaware mountain group is a 1 000 1 600 m 3 281 5 250 ft thick section of siltstone and sandstone deposited in a deep water density stratified basin surrounded by carbonate banks or reefs and broad shallow evaporite clastic shelves the most prevalent style of basinal deposition was suspension settling of silt laminated siltstone beds are laterally extensive and cover basin floor topographic irregularities and flat floored channels as much as 30 m 99 ft deep and 1 km or more wide channels can be observed in outcrop at the basin margin and can be inferred from closely spaced wells in the basin the channels are straight to slightly sinuous trend at high angles to the basin margin and extend at least 70 km 43 mi into the basin sandstone beds con ined to channels form numerous stratigraphic traps hydrocarbon sealing beds are provided by laminated organic siltstone which laterally can form the erosional margin where channels are cut into siltstone beds thick beds of very fine grained sandstones fill the channels these sandstones contain abundant large and small scale traction current produced stratification these sandy channel deposits generally lack texturally graded sedimentation units and show no regular vertical sequence of stratification types or bed thickness outcrop and subsurface evidence indicates delaware mountain group sediments were deposited by saline density currents dense saline water originated on evaporitic shelves and spilled across the carbonate rim down steep marginal slopes and into the basin basinal waters were density stratified denser flows moved along the basin floor cutting channels or depositing sand in existing channels less dense flows moved along density interfaces in the water column and carried silt size material far into the basin where it settled to the floor as thin alternating layers of detrital silt and organic debris little proximal to distal change occurred in the size or nature of the channels exploration predictions based on submarine fan models formed by turbidity currents would anticipate very d fferent proximal distal changes in sandstone geometry and facies

palaeomagnetic constraints on the collision and rotation of north and south china the eastern half of china is dominated by the north china block ncb and south china block scb separated by a complex fold system that trends roughly east west interpretation of the geology of this fold belt has led to a variety of estimates of the age of amalgamation of these two blocks1 7 palaeomagnetic results are few and have also been subject to conflicting interpretations8 11 here we present our own palaeomagnetic results for late permian rocks for both the scb and ncb and summarize other relevant palaeomagnetic data by reinterpreting the polarity given in some previous studies we are able to synthesize the results for the widespread emeishan basalts of the scb and to suggest a new model for the collision and relative rotation of the two blocks

new paleomagnetic results of paleocene volcanic rocks from the lhasa block tectonic implications for the collision of india and asia abstract a combined geochronological and paleomagnetic investigation has been performed on paleocene volcanic sequences in the lhasa block near the localities of mendui 30 1 n 90 9 e a total of 15 sites have been sampled from rhyolitic tuffs stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions the tilt corrected mean direction is d i 359 0 26 1 with 95 9 2 and n 14 sites corresponding to a paleopole at 73 6 n 274 3 e with a 95 7 3 positive fold tests suggest a primary origin for the characteristic remanence in order to provide a more accurate pole we propose to combine site mean directions from this study and achache et al s 1984 the combined average palaeomagnetic direction from early paleocene volcanic rocks is d 356 6 i 25 9 21 7 95 6 8 after tilt correction n 22 sites corresponding to a pole at 73 2 n 282 4 e with a 95 5 4 the paleomagnetic results yield a paleolatitude of 13 6 5 4 n for the southern margin of eurasia at 55 ma compared with expected paleomagnetic directions from the stable india and eurasia blocks significant crustal shortening of 1400 600 km and 2000 550 km respectively may have occurred between the southern margin of eurasia and the stable india and within eurasia since the collision of india and eurasia

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

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

persistently low asian paleolatitudes implications for the india asia collision history 1 paleomagnetism provides independent paleolatitude constraints on the india asia convergence however implied cenozoic latitudinal convergence within asia thousands of km largely exceeds geologic estimates of tectonic shortening hundreds of km this discrepancy may result from a notoriously low bias in paleomagnetically determined cenozoic paleolatitudes in asia we provide here new paleomagnetic data from cenozoic mongolian volcanic rocks and from chinese paleogene sediments corrected from the depositional bias of inclination shallowing these results combined with similar asian data sets confirm that paleolatitudes are still 5 10 lower than predicted by the paleomagnetic apparent polar wander path apwp for asia between 50 and 20 ma inclination shallowing being excluded from the selected data sets we investigate the likeliness of other proposed mechanisms for this discrepancy 1 more southerly positions of asia than expected by the apwp due to apwp inaccuracies rather than eurasian non rigidity or 2 non dipolar geomagnetic field contributions fully explaining this discrepancy by only one of these mechanisms would imply either unrealistically large 10 apwp inaccuracies or unrealistically large octupolar field contributions up to 16 a combination of these mechanisms is found more likely to have produced the observed latitudinal discrepancy but their respective contributions cannot be quantified given the still relatively low amount and poor quality of cenozoic paleomagnetic data from stable cratons of asia india and europe by allowing for reasonable time dependant non dipolar contributions and a slight 5 apwp bias the latitudinal discrepancy can be resolved and the excessive amounts of intra asian shortening decrease to values in line with tectonic shortening from structural studies

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

reservoir characterization of a permian deep water sandstone east ford field delaware basin texas deep water sandstones of the delaware mountain group in west texas and southeast new mexico contained an estimated 1 8 billion bbl of original oil in place but primary recovery from these fields is commonly less than 20 east ford field in reeves county texas which produces from the ramsey sandstone in the upper bell canyon formation went directly from primary production to tertiary recovery by co2 flooding field production has increased from 30 to more than 185 bopd oil recovery has been improved by the co2 flood but not as much as expected geologic heterogeneities such as interbedded siltstones are apparently influencing reservoir displacement operations in the east ford unit a depositional model of the east ford unit was developed using data from bell canyon outcrops and subsurface data the ramsey sandstones were deposited by turbidity currents in a basin floor setting the sandstones are interpreted as having been deposited in a channel levee system that terminated in broad lobes overbank splays filled topographically low interchannel areas injection wells located in splay sandstones apparently have poor communication with wells in channel sandstones perhaps because communication is restricted through levee and channel margin deposits the south part of the unit is responding well to the flood because the injection and production wells are in the same interconnected lobe depositional environment

paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present paleomagnetic study of china and its environs has been the center of a major international effort for the last 10 years in this paper we critically review all available paleomagnetic poles of upper permian to tertiary age from the main blocks of china with the goal of placing constraints on models of the formation and the subsequent deformation of the region after selecting reliable poles by applying objective criteria we divide our analysis into first order motions of blocks and second order deformation within blocks for first order analysis apparent polar wander paths are constructed for the major blocks we discuss the compatibilities and contradictions between the geological and paleomagnetic records a sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented in general the major blocks were probably in contact throughout the permian and triassic but the jurassic was the key age during which most of the movement toward china s present configuration took place our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events during the cretaceous chinese poles agree with poles from other continents transferred onto eurasia at the second order we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region indicating that much of china has been affected by small 20 differential rotations this we interpret as deformation caused in part by the extrusion of the chinese blocks away from the indian collision the complete annotated list of poles is given as an appendix

mid paleozoic age of the roberts thrust unsettled by new data from northern nevada the roberts thrust is a major thrust in nevada on which ordovician to devonian siliceous facies rocks were carried more than 80 km eastward over contemporaneous carbonate facies for more than two decades a mid paleozoic age for this structure has been widely accepted the bases for dating the thrust are 1 the assumption that the roberts thrust formed at the time of the antler orogeny a reliably dated mid paleozoic event and 2 the concept of an overlapping assemblage consisting of sedimentary strata deposited on both the allochthon and autochthon the first assumption is unproved and the second relies on another assumption that the overlap assemblage was not cut and telescoped by the roberts thrust in the years since a mid paleozoic age for the roberts thrust became widely accepted much new information on the stratigraphy and facies relations of mississippian rocks in northern nevada has accumulated one interpretation of these new data suggests that in the pinon range where a mid paleozoic age for the thrust was most convincingly displayed the overlap assemblage actually may have been cut by a major thrust that juxtaposed contrasting facies of mississippian rocks if so the principal evidence used to date the roberts thrust is compromised and the time has come for an agonizing reappraisal of all evidence bearing on the question a mid paleozoic age can no longer be taken for granted and a post paleozoic age cannot be ruled out

a palaeomagnetic study from the mongol okhotsk region rotated early cretaceous volcanics and remagnetized mesozoic sediments we collected 47 sites of upper triassic to middle jurassic siltstones and sandstones and two sites of lower cretaceous andesites in a large basin located south of the mongol okhotsk suture the suture separates the siberian craton to the north from the mongolian and chinese blocks to the south laboratory treatment and analyses identify the same post folding direction in all rocks the mean direction with a corresponding palaeopole at 76 8on 152 2oe a95d 4 2o of the overprint component nd 49 sites is significantly different at the 95 confidence level from the expected time averaged brunhes or present day field directions the lower cretaceous andesites possess a stable remanent direction at high temperatures that is distinct at 95 confidence limits from the overprint direction the corresponding pole 58 3on 51 0oe dp dm d 3 8o 4 6o based on only fourteen samples is significantly rotated 78 4o 5 3o counterclockwise and insignificantly displaced 2 7o 3 4o north with respect to the early cretaceous reference pole for siberia we argue that the rotation is likely tectonic in nature and not due to a chance reading of the palaeosecular variation of the earth s magnetic field both the palaeomagnetic data and the folding patterns we observed in the field suggest that deformation associated with the suture continued after the early cretaceous and involved sinistral shear 1998 elsevier science b v all rights reserved

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

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

jurassic paleomagnetic constraints on the collision of the north and south china blocks this paper reports a paleomagnetic study of jurassic rocks from the ordos basin in the north china block ncb a recent remagnetization and a high temperature and or high coercivity component with dual polarities have been isolated the middle jurassic pole 74 n 233 e a95 5 is roughly consistent with previous results and the lower jurassic pole 82 n 286 e a95 7 is located between the late triassic and middle jurassic poles these results combined with paleomagnetic results from adjacent blocks suggest that 1 the pole for the ncb suffered an angular displacement of 47 8 between the late triassic and middle jurassic 2 the scb underwent final accretion to the ncb in the middle jurassic and 3 accretion of the ncb to siberia was not complete until the late jurassic and possibly even until the early cretaceous

northeastward growth of the tibet plateau deduced from balanced reconstruction of two depositional areas the qaidam and hexi corridor basins china we address the problem of late cenozoic uplift erosion and growth of northeastern tibet by reconstructing from isopach maps and drill holes the volumes and masses of sediments deposited in the qaidam and hexi corridor basins since 35 myr ago the mass budget is based on simple geometrical assumptions such as regional similarity of the thickness ratios between strata of different ages in the qaidam where our record extends back to the oligocene the budget shows a huge rise of the accumulation rates after the beginning of the pliocene 5 3 ma the early pliocene seems to be the period of maximum deposition with accumulation rates in excess of 1 mm yr 1 2 7 kg m 2 yr 1 of compacted rocks throughout the basin there also seems to be a southeastward shift of the largest depocenters between the upper pliocene 3 4 1 6 ma and the quaternary in the hexi corridor sedimentation is confined to small foreland flexural depressions associated with the frontal thrusts of the qilian shan and occurs at an average rate one order of magnitude smaller than in the qaidam basin the accumulation rate is maximum in the quaternary the sedimentation history appears to support a plateau building mechanism resulting from the combination of two geologicallly common processes crustal scale thrusting and sedimentary basin infilling the time needed to completely fill the qaidam basin and make its catchment a plateau closely resembling that of the highest part of tibet qangtang is of the order of 9 myr the mechanism now at work north of the kunlun which involves rapid infilling of broad flat areas separated by relatively narrow mountain ranges has thus probably been important in producing the high smooth topography that characterizes much of central tibet

paleomagnetism of jurassic rocks from southwestern sichuan and the timing of the closure of the qinling suture a paleomagnetic study has been carried out on the jurassic redbeds from southwestern sichuan province on the south china block thermal demagnetization has isolated a high temperature component from 20 of 24 sites from the upper jurassic which passes the fold and reversal tests the tilt corrected mean direction computed using principal component analysis in combination with great circle intersection techniques is d i 17 9 45 6 with 95 3 7 corresponding to a paleopole at 74 2 n 185 7 e with a95 3 8 most of the mid lower jurassic samples studied are unusually weak in magnetization and failed to give stable directions at high temperatures however a tilt corrected mean direction of d i 30 3 44 6 95 13 6 was determined utilizing the techniques given above from 8 sites of the lower jurassic which yields a paleopole at 63 8 n 188 2 e with a95 16 1 this mean direction marginally passes the fold and reversal tests a comparison with the coeval poles for the north china block and eurasia indicates that during the early jurassic these plates had not coalesced together but that they had sutured together by the late jurassic minor but noticeable differences still exist between the late jurassic poles for these plates probably due to uncertainties associated with the age of magnetization and the apparent polar wander path for eurasia

paleomagnetic study of cretaceous rocks from the yangtze block central yunnan china implications for the india asia collision abstract more than 200 samples for this study were collected at 30 sites from the cretaceous matoushan and puchanghe formations around the city of chuxiong 25 n 101 5 e twenty four sites have characteristic directions with a high temperature component above 350 c paleomagnetic directions from 21 sites of the cretaceous formations reveal clockwise deflection in declination d 44 6 i 41 3 and 95 10 7 the reliability of the direction is ascertained through a positive fold test and the presence of normal and reversed polarities comparison with paleomagnetic data from other areas indicates that the chuxiong area was rotated clockwise through26 17 with respect to the eastern part of the yangtze block whereas it was subjected to more than 44 of clockwise rotation with respect to the neighboring blocks of lhasa 56 21 and western sichuan 44 16 since cretaceous time the clockwise rotation of chuxiong is a reflection of rotational motion of the large region including the southeastern qiangtang terrane and the jinsha suture red river fault paleomagnetic data and tectonic features suggest that the rotation occurred prior to the folding in the central yunnan province probably during the paleogene this large rotation is a spectacular example of the deformation in the southeastern part of the asian continent due to the collision of india

preliminary fission track results from the southern urals sterlitamak to magnitogorsk abstract results are reported from a pilot fission track study of a west east traverse across the southern urals from sterlitamak through to the dzhabyk granite east of magnitogorsk which was undertaken in order to study the low temperature 300 c thermotectonic post uralian history of the mountain belt the transect crosses the major tectonic units including the main uralian fault the suture zone which separates the east european craton from palaeozoic oceanic island arc and continental terranes of asia zircon fission track ages range from 226 ma to 604 ma in the foreland thrust and fold belt with a single age of 262 ma in the magnitogorsk zone an age grouping of about 250 ma for zircons on either side of the main uralian fault suggests that there has been no differential movement identifiable through fission track analysis along this line since the triassic the similarity of apatite ages ranging from 180 to 210 ma further verifies little differential movement within the footwall to the suture since the jurassic in the central part of the magnitogorsk zone apatite ages are older they are slightly annealed crystallisation cooling ages but further confirm that there has been only little tectonic activity since the triassic except in the east uralian zone apatite fission track analysis reveals variable denudation rates the highest rates occurred during the permo triassic ending at about 210 ma just prior to the final development of the jurassic peneplain this was followed possibly by a period of burial before final cooling exhumation from approximately middle cretaceous onwards

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

lower middle jurassic paleomagnetic data from the mae sot area thailand paleogeographic evolution and deformation history of southeastern asia we have carried out a paleomagnetic study 12 sites 85 samples of early middle jurassic limestones and sandstones from the mae sot area of western thailand this area is part of the shan thai malay stm block and its geological characteristics have led some authors to suggest a late jurassic accretion of this region against the rest of indochina along the changning menglian zone the latter sometimes being interpreted as a mesozoic suture the high temperature or high coercivity component isolated yields a paleodirection at d 359 8 i 31 4 95 5 0 the primary nature of the magnetization acquisition is ascertained at a site with reversed polarity and a positive fold test at the 95 confidence level comparison of the mae sot paleolatitude and another one from the stm with those recently published for the simao and khorat blocks show no significant difference at the 95 level showing that the stm was situated close to or had already accreted with the simao or khorat blocks in the early middle jurassic comparison of the latitudes from these blocks with those from china indicates a relative southward motion of 8 4 of indochina as a single entity relative to china most rotations of these regions relative to china are found to be clockwise between 14 and 75 these rotations and most prominently the 1200 500 km post cretaceous left lateral motion inferred for the red river fault provide quantitative estimates of the large amount of extrusion of indochina with respect to the rest of asia

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

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

new early cretaceous paleomagnetic data from volcanic and red beds of the eastern qaidam block and its implications for tectonics of central asia abstract in order to better understand anomalously low paleomagnetic inclinations in the cretaceous rocks of central asia a combined geochronologic and paleomagnetic investigation has been performed on cretaceous basalt sequences intercalated with red beds from zeku area in the eastern qaidam block northeastern tibetan plateau potassium argon k ar dating indicates that the basalt sequences are of early cretaceous age 89 9 2 8 to 112 1 2 4 ma isothermal remanent magnetization irm thermal demagnetization of a three component irm and curie point experiment suggest that magnetite and hematite dominate in red beds magnetite in basalt flows stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization chrm from 7 lava flows and 12 red beds sites the chrms of red bed and basalt sites pass fold test and likely represent primary remanent magnetization the tilt corrected mean direction from red bed sites basalts plus baked red bed site is d s 18 4 i s 55 3 s 69 1 95 5 5 n 11 sites d s 16 1 i s 52 0 s 42 6 95 8 6 n 8 corresponding to a pole at 75 2 n 182 3 e with a 95 7 07 76 9 n 194 9 e with a 95 10 0 respectively the consistent inclination recorded in red beds and basalts implies no significant inclination shallowing caused by deposition compaction in red beds from the studied area compared with the early cretaceous poles from the north china block ncb or eurasia insignificant post early cretaceous northward motion may have occurred between the qaidam block and the ncb on the basis of the existing regional paleomagnetic data the qaidam block has not experienced wholescale vertical axis rotation since the early cretaceous with respect to ncb

the configuration of asia prior to the collision of india cretaceous paleomagnetic constraints paleomagnetic data from central asia show that 1700 610 km of shortening of southern asia since cretaceous time have been absorbed by distributed deformation between southern tibet and the siberia craton this result is based on a compilation of cretaceous poles from the junggar tarim tibet indochina south china north china and mongolia blocks complementing the recent compilation of enkin et al 1992a we propose a paleogeographic reconstruction of asia in the cretaceous in which the position of siberia is derived from the synthetic apparent polar wander path of besse and courtillot 1991 the resulting map which likely represents asia as it remained throughout the cretaceous until the collision with india began features an unbent tibet with an east west trending andean margin at tropical latitudes and a rather continuous belt of continental red bed basins extending from sichuan to tarim through tibet the map allows one to estimate continental shortening and rotations between the blocks which are attributed to the collision despite large uncertainties these have amounts and senses which are in all cases compatible with some recent kinematic models such as that of avouac 1991 appendix is available with entire paper on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b93 007 2 50 payment must accompany order

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

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

a paleomagnetic study of mesozoic sediments from the junggar and turfan basins northwestern china mesozoic sedimentary rocks have been collected from the karamay area northwestern junggar block and in the turfan basin northeastern tarim block for a paleomagnetic study with the aim of testing the rigid nature of these blocks and to better constrain their paleogeographic relationships within central asia prior to the collision with india the results from the karamay area are disappointing lower cretaceous grey sandstones having apparently been recently remagnetized this remagnetization is carried by magnetite and yields highly clustered normal polarity directions both a medium mtc and a high htc unblocking temperature component are isolated from flat lying or weakly tilted jurassic red sandstones the mtc could be carried by maghemite and the htc is carried by hematite the two components are statistically identical and seem to correspond to an ancient magnetization however there is no fold test and comparison with previous data from junggar and the reference apwp of siberia does not allow us to decide between a middle jurassic primary magnetization and an upper jurassic or cretaceous remagnetization stronger constraints are obtained from upper jurassic to lower eocene fine grained red sandstones from anticlines that have been thrust southwards in the turfan area of south tien shan after the elimination of a recent overprint that is probably related to weathering a high coercivity high unblocking temperature component carried essentially by hematite is isolated all directions from the jurassic to the lower eocene which yield positive fold and reversal tests are statistically indistinguishable this is compatible with a primary origin for these magnetizations similar to what has previously been found in junggar tarim and western tibet the corresponding turfan pole is not statistically distinct from that of tarim for the same period in accordance with the notion of a large quasi rigid tarim block remaining attached to eurasia in a rather constant position and at a constant latitude throughout the second half of the mesozoic this new datum can therefore be included to derive a new mean pole for the tarim that is valid for the period from the middle jurassic to the end of the cretaceous 68 4 n 224 5 e a95 3 1

late cretaceous cenozoic deformation of northeast asia abstract the plate tectonic paradigm implies rigid plates and narrow plate boundaries in contrast diffuse plate boundaries are common both in the oceans and continents r g gordon annu rev earth planet sci 26 1998 615 642 and their history is difficult to constrain especially in remote tectonically complex areas such as northeast asia m e chapman s c solomon j geophys res 81 1976 921 930 here we show how extensive north atlantic marine magnetic r macnab et al eos 76 1995 449 458 and gravity data d t sandwell w h f smith j geophys res 102 1997 10039 10054 can be used to unravel with tight confidence limits successive periods of deformation over 80 million years along the diffuse continental eurasian north american plate boundary a period of compression in the late cretaceous 14 mm yr in the laptev sea to 20 mm yr in kamchatka led to thrusting in the verkhoyansk mountains and was followed by extension from 68 to 40 ma when 400 km of extension was accommodated by the formation of a series of grabens including the moma rift system since 40 ma time varying compression and transpression along the moma rift system created strike slip faults thrusts and folds at rates up to 6 3 mm yr in the laptev sea region 600 km of extension from latest late cretaceous to present created the laptev sea and lena rift systems the deformation predicted by our model fits most geological features formed in the laptev sea and central northeast asia during late cretaceous cenozoic times the most recent deformation late miocene pliocene is not very well constrained since our model lacks data younger than 11 ma the deformation that occurred in kamchatka reflects a complex tectonic setting and our model s predictions are only tentative

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

delaware mountain group west texas a case of refound opportunity part 2 cherry canyon formation recent drilling activity in the war wink area of west texas has established a new play in submarine channel and levee overbank sandstones of the middle and lower cherry canyon formation delaware mountain group guadalupian previous exploration and development spanning the 1960s 1970s and early 1980s had focused on the upper part of the formation the new play located a few miles west of the central basin platform suggests the existence of multiple channel trends not yet explored in the delaware basin entrapment in the war wink area is related to both structural and stratigraphic factors lower cherry canyon reservoirs have porosities ranging from 18 to 24 and permeabilities ranging from 10 to 60 md and thus are higher in quality than reservoirs of the underlying brushy canyon formation core data indicate that permeabilities increase rapidly above a threshold porosity value of about 18 lower cherry canyon wells are completed by fracture stimulation in one or two main reservoir zones with several secondary zones commonly behind pipe wells usually flow at rates of 100 200 bbl oil per day for up to 1 yr before being put on pump main producing zones are capable of recoveries ranging from 50 000 to 120 000 bbl attempts at using horizontal drilling in place of fracture stimulation have proved highly successful in certain cases use of this approach for cherry canyon development guided by good geologic control may provide an excellent technique applicable to many portions of the delaware basin

from trash to treasure three dimensional basement imaging with excess data from oil and gas explorations modern oil and gas seismic surveys commonly use areal arrays that record continuously and thus routinely collect excess data that are not needed for the conventional common reflection point imaging that is the primary goal of exploration these excess data have recently been recognized to have utility not only in resource exploration but also for addressing a diverse range of scientific issues here we report processing of such discarded data from recent exploration surveys carried out in southeastern new mexico these have been used to produce new three dimensional 3 d seismic reflection imagery of a layered complex within the crystalline basement as well as elements of the underlying crust this enigmatic basement layering is similar to that found on industry and academic seismic reflection surveys at many sites in the central united states correlation of these reflectors with similar features encountered by drilling in northwestern texas suggest that they may be part of an extensive continental scale network of tabular mafic intrusions linked to keweenawan rifting of the igneous eastcentral unites states during the late proterozoic more importantly this analysis clearly demonstrates that the new generation of continuously recorded 3 d exploration datasets represent a valuable source of fresh information on basement structure that should be examined rather than discarded such basement information is not only important to understanding crustal evolution it is directly relevant to assessing risks associated with fossil fuel extractions such as induced seismicity related to waste water injection

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

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

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

comparison of linear regression and a probabilistic neural network to predict porosity from 3 d seismic attributes in lower brushy canyon channeled sandstones southeast new mexico the lower brushy canyon formation of the delaware basin new mexico consists of a series of overlying sand filled channels and associated fans separated by laterally extensive organic siltstone and carbonate interbeds this laterally and vertically complex geology creates the need for precise interwell estimation of reservoir properties in this paper we integrate wireline log and 3 d seismic data to directly predict porosity in the area of an existing oil field in southeast new mexico the 3 d seismic data were used to interpret the location of major stratigraphic markers between wells and these seismic horizons were used to constrain a time window for a volume based attribute analysis stepwise regression and crossvalidation were used to combine seismic attributes to predict porosity in wells where the porosity was known from the well logs the results of a linear regression porosity model showed good correlation r2 0 74 between seven seismic attributes and the observed porosity logs at 11 wells in

a new late cretaceous to present apwp for asia and its implications for paleomagnetic shallow inclinations in central asia and cenozoic eurasian plate deformation based on a compilation of 533 cretaceous to present day palaeomagnetic poles obtained from both sedimentary and igneous rocks we present a new analysis of the so called asian inclination anomaly and demonstrated the anomaly to be twofold a 2nd order anomaly characterized by high palaeolatitudes in indochina and low palaeolatitudes over tibet and central asia is superimposed on a 1st order anomaly characterized by cenozoic low palaeolatitudes found all over northeastern asian stable blocks the analysis herein convincingly shows that the europe apparent polar wandering path apwp can no longer be used to interpret palaeomagnetic data east of the urals including interpretation of asian tertiary deformation related to the india asia collision we thus construct a new apwp for east asia based on palaeopoles from blocks assumed to be stable this new apwp is consistent with and reinforces previous analyses of asian tectonics such as the age 55 ma and locus 5 10 n of the indo asian collision the lateral extrusion of se asian continental blocks and the intracontinental shortening in central asia possible origins of the 1st order palaeolatitude anomaly are 1 a geomagnetic origin due to long lasting non dipolar contribution to the magnetic field and 2 a tectonic hypothesis in which a newly defined east asia plate was located 10 farther south than expected from the current europe apwp based on a set of six new reconstructions from 90 ma to present we show that our tectonic model reconciles geophysical geological and tectonic observations throughout eurasia from siberia to europe including kinematics in the arctic ocean up to northwestern arctic alaska beyond possible occurrences of non dipolar field contribution and or local inclination flattening in the sedimentary data our model leads us to conclude that cenozoic tectonics is the dominant contributor to the observed 1st order 10 low palaeolatitude anomaly over asia during the tertiary

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

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

new constraints to the onset of the india asia collision paleomagnetic reconnaissance on the linzizong group in the lhasa block china abstract to better understand the onset of the india asia collision a paleomagnetic study has been conducted on paleogene volcanic rocks and sediments of the linzizong group 64 44 ma from the linzhou and namling basins of the lhasa block at the southern margin of the asian continent following detailed rock magnetic investigations and progressive thermal or hybrid thermal and alternating field demagnetization stable characteristic remanent magnetizations chrms were successfully defined from the dianzhong 64 60 ma nianbo and pana 60 44 ma formations these chrms are of dual polarity and pass a fold test with some of them in volcanic rocks carried by oxidation induced hematite we thus interpret them as primary remanences paleomagnetic results from the bulk of the collection imply that the southern leading edge of asia i e the lhasa block remained essentially stationary in low latitudes at 10 n without paleomagnetically detectable tectonic rotation and latitudinal variation during the formation of the linzizong group consequently a large scale post collisional northward convergence in the tibetan plateau and central asia is very likely to have occurred after formation of the linzizong group and the definitive paleoposition of the lhasa block during the period embracing the interval between 64 and 44 ma provides further constraints on the latitudinal position of the initial contact between india and asia this evidence rules out the model for a younger 35 ma india asia collision and implies that the initial india asia collision occurred between 65 and 50 ma given that greater india comprised the present indian continent together with a 950 km extension beyond its northern margin prior to the rifting of gondwana in the middle jurassic we conclude that the initial india asia collision most probably occurred between 55 and 60 ma

delaware mountain group west texas and southeastern new mexico a case of refound opportunity part 1 brushy canyon exploration in permian guadalupian deep water sandstones of the delaware mountain group west texas and southeast new mexico represents a success story of the 1990s derived from reevaluation of reservoirs previously deemed uneconomical recent discoveries have concentrated on the brushy canyon in new mexico and to a lesser extent the cherry canyon in texas brushy canyon reservoirs in particular previously were overlooked due to indications of poor reservoir quality from log and well test data however oil shows observed on mud logs across the northern delaware basin led to new completion efforts in the late 1980s and 1990s using gel sand fracture stimulations productive reservoirs are very fine to fine grained arkosic to subarkosic sandstones with porosities of 12 25 and permeabilities typically of 1 5 md better reservoir quality is concentrated in massive channel sandstones variably interpreted as deposited by turbidity or saline density currents significant clay content lamination and close interbedding between oil and water bearing units make log analysis and reserve estimates problematic as a result the mud log remains the cheapest most practical indicator of pay reservoir sandstones can be divided into a series of major productive trends related to proximal slope and more distal basin floor depositional settings well productivity is variable within each trend but primary recovery rarely exceeds 10 options for enhanced recovery include pressure maintenance waterflooding and carbon dioxide flooding early indications suggest that carbon dioxide flooding may be most appropriate in these low permeability clay bearing reservoirs

paleontological view of the ages of the deccan traps the cretaceous tertiary boundary and the india asia collision volcanism in the deccan traps of india occurred over at most three magnetic chrons centered on a main reversed chron paleontological data indicate that this reversed chron must coincide with 29r which contains the cretaceous tertiary k t boundary recent 40 ar 39 ar data may therefore provide one of the best estimates of the absolute age of the k t boundary 65 7 2 0 ma moreover the eurasian character of k t boundary terrestrial faunas in india suggests that this was also close to the time of india asia collision this is significantly earlier than generally recognized and implies about 4000 km of continental shortening in the collision zone probably absorbed jointly by crustal thickening and strike slip extrusion

comments on sequence stratigraphy of an argillaceous deepwater basin plain succession vischkuil formation permian karoo basin south africa from van der merwe flint and hodgson marine and petroleum geology 2010 27 321 333

new tertiary paleomagnetic poles from mongolia and siberia at 40 30 20 and 13 ma clues on the inclination shallowing problem in central asia we report results of a paleomagnetic study of 490 cores from 59 sites corresponding to 52 distinct basaltic flows from mongolia and siberia khaton sudal 39 4 ma 44 5 n 101 4 e taatsyn gol 1 31 5 ma 45 4 n 101 3 e 2 28 0 ma 45 5 n 101 1 e ust bokson 19 9 ma 52 1 n 100 3 e and taatsyn gol 3 12 7 ma 45 5 n 101 0 e stepwise thermal and alternating field demagnetizations isolated a stable high temperature component htc of magnetization in most specimens which we interpret as the primary magnetization of these basaltic lava flows the four corresponding paleopoles appear consistent with coeval paleopoles from other asian effusive formations however except for the 12 7 ma paleopole the paleopoles are systematically far sided from the european apparent polar wander path apwp with respect to site locations corresponding to anomalously shallow inclinations in tertiary asian effusive formations in the hypothesis of a dipolar magnetic field in the tertiary this indicates a 1000 1500 km position of the siberia craton and amuria block farther south than expected at 40 and 30 ma tectonically this interpretation implies decoupling and relative rotations between the western and eastern parts of eurasia between the cretaceous and present we show that if siberia were located more to the south the 15 20 paleolatitude anomaly generally observed in sedimentary formations from central asia reduces to a more reasonable average value of 7 which could result from the superimposition of shallowing mechanisms due to sedimentary processes and northward motion of asian blocks under the effect of ongoing penetration of india into eurasia in the tertiary

new cretaceous paleomagnetic poles from the tarim basin northwestern china most paleomagnetic studies of the tarim have been focused on the paleozoic with little data published for mesozoic rocks however the ancient configuration of this part of asia has been affected by the indian eurasian collision during the second french chinese karakorum expedition from kashkar to kunjerab in the autumn of 1990 we sampled about 500 cores from 53 sites of upper and lower cretaceous red sandstones from three areas near uytak yingjisha and wuqia 38 5 39 5 n 75 76 5 8 isothermal remanent magnetization acquisition indicates the presence of minerals with high magnetic coercivity stepwise thermal demagnetization confirms that hematite is the dominant carrier and allowed the separation of low temperature components ltc 600 c in most samples the mean direction of ltcs is aligned with that of the present earth field normal and reversed polarities have been found for the htcs of each age group htcs show positive fold tests for at least one sub study where the attitudes of bedding were sufficiently different to allow the test to be performed more important the four sub studies from yingjisha and wuqia give fully compatible results results from the uytak area show a wide dispersion in declinations as could be expected from the rather severe deformation and proximity to major thrust and strike slip faults data obtained in the kuche area from northern tarim by li and co workers and in southern junggar by chen and co workers are statistically identical to the new data from yingjisha and wuqia data from paicheng in northern tarim appear to have been subject to local rotation excluding those sites which appear from paleomagnetic study to have been significantly rotated we calculated a mean cretaceous pole for the tarim basin at 69 1 n 223 88 ass 3 5 n 5 this result indicates that the total post cretaceous rotation of tarim relative to junggar and to eurasia has been negligible 2 6 6 1 and 0 4 4 6 respectively continental shortening and rotation between tarim and junggar within the tien shan are not resolved by paleomagnetism although the values obtained 400 t 600 km 3 6 are compatible with tectonic models of the range a major discrepancy warranting further geological investigation is that between the tarim and north china block poles which would imply some 1400 1060 km of north south relative displacement

a regional look at morrow formation gas in light of conflicting resource development in southeast new mexico several factors have led to reservoir underdevelopment in the morrow formation in southeast new mexico including conflicting interests with potash development companies government regulations protecting potash resources lack of completions within successful wells and lack of exploration over the past 70 yr the secretary of the interior has set aside 777 mi2 2012 km2 of southeastern new mexico referred to as the secretary of the interior s potash area spa to preserve potash resources oil and gas drilling are severely limited within the spa where leasing is managed by the bureau of land management the morrow formation reservoirs have significant undiscovered reserves in the spa morrow formation production well density is roughly half that of the surrounding area which may also be underdeveloped operators have drilled a few directional wells to target morrow formation reservoirs while avoiding potash resources near the boundaries of the protected area and they used horizontal wells to develop the shallower brushy canyon formation in one area older abandoned wells within the spa may be used in the future for drilling islands but this practice has not yet begun the purpose of this article is to provide structural and stratigraphic maps of morrow formation reservoir horizons to present areas with future potential and to suggest future development scenarios the article also provides a regional morrow formation depositional picture and correlates this regional description to field scale interpretations the morrow formation is typically divided into two intervals the morrow clastics and the overlying morrow limestones the fluvial deltaic to shallow marine morrow clastic interval currently provides more than 40 of the natural gas production within the spa depth to morrow clastic production ranges from about 11 000 to approximately 14 000 ft 3352 to approximately 4267 m in the spa this study divides the morrow clastic interval into three informal subunits based on log character and uses flooding surfaces marked by correlatable bounding shales the resulting net sand maps for these subunits indicate an overall transgression from deltaic to distributary mouth bar or reworked deltaic to deposition of overlying carbonates and marine shales trends of producing wells along depositional axes that are interpreted in this report indicate potential areas for future exploration and development future drilling will target depositionally complex reservoirs the author recommends directional and horizontal drilling aided by three dimensional seismic imaging to explore and develop spa resources

comment on a paper by zou et al 2017 entitled an integrated chemo and sequence stratigraphic framework of the early pennsylvanian deepwater outcrops near kirby arkansas usa and its implications on remnant basin tectonics