late oligocene tibetan plateau warming and humidity evidence from a sporopollen record dramatic warming in the late oligocene occurred after 8 ma characterized by ocean warming and a collapse of the antarctic ice sheet the late oligocene warming event lowe however few terrestrial records have yet been employed to verify the lowe and investigate its associated driving forces the nima basin located in the central tibetan plateau contains thick oligocene sediments allowing its ecological record to be used as verification of the lowe here we used the reconstructed vegetation record from the nima basin in the 1 100 m thick dzc section aided with zircon u pb dating of the interbedded tuffite layers two tuffite layers yielded ages of about 23 4 and 24 8 ma thus the sedimentary age of the whole section is predicted to be 27 5 22 4 ma sporopollen analysis revealed that the dominant taxa throughout the whole section were conifers over 85 including piceapollis pinuspollenites and abietpollenites one notable feature is that the broad leaved trees e g quercoidites and meliaceoidites increased obviously averaging from 0 9 to 6 6 max 41 2 after 25 6 ma this change could be interpreted as a response to late oligocene global warming which changed the forest line and corresponding plant species composition the mean average precipitation of 400 850 and 800 1 000 mm respectively inferred from the above vegetation types in the nima basin both exceed the present 150 mm this indicates that the south asian monsoon had already developed in the oligocene and that the low event accompanied the global warming

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

tectonic evolution of nw iberia during the paleozoic inferred from the geochemical record of detrital rocks in the cantabrian zone the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges in age from ediacaran to early permian its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana ii an active continental margin or iii the margin of a drifting ribbon continent and iv a combination of the three aforementioned possibilities in this paper we present geochemical data from 21 samples taken in detrital rocks in the cantabrian zone from ediacaran to early permian age the results together with previously published detrital zircon ages from these strata allow a comprehensive analysis of changing tectonic scenarios and provenance through time collectively these data indicate that this portion of iberia was an active margin during the ediacaran that evolved to become part of the passive margin of gondwana at least from ordovician to late devonian times changes in geochemistry zircon age distribution and sm nd isotopes during the carboniferous are attributed to the far field effects of the onset of the collision between gondwana and laurussia and related processes such as recycling of older sedimentary sequences as well as the involvement of the rocks formed during the variscan orogeny and oroclinal buckling latest carboniferous and permian show a more juvenile sm nd and higher values of illite cristallinity that may support the hypothesis of lithospheric foundering after oroclinal buckling

acquisition of chemical remanent magnetization during experimental ferrihydrite hematite conversion in earth like magnetic field implications for paleomagnetic studies of red beds abstract hematite bearing red beds are renowned for their chemical remanent magnetization crm if the crm was acquired substantially later than the sediment was formed this severely compromises paleomagnetic records to improve our interpretation of the natural remanent magnetization the intricacies of the crm acquisition process must be understood here we contribute to this issue by synthesizing hematite under controlled earth like field conditions 100 t crm was imparted in 90 oriented samples with varying inclinations the final synthesis product appeared to be dominated by hematite with traces of ferrimagnetic iron oxides when the magnetic field intensity is 40 t the crm records the field direction faithfully however for field intensities 40 t the crm direction may deviate considerably from that of the applied field during synthesis the crm intensity normalized by the isothermal remanent magnetization crm irm 2 5 t increases linearly with the intensity of growth field implying that crm could potentially be useful for relative paleointensity studies if hematite particles of chemical origins have consistent properties crm in hematite has a distributed unblocking temperature spectrum from 200 to 650 c while hematite with a depositional remanent magnetization drm has a more confined spectrum from 600 to 680 c because it is usually coarser grained and more stoichiometric therefore the thermal decay curves of crm with their concave shape are notably different from their drm counterparts which are convex these differences together are suggested to be a potential discriminator of crm from drm carried by hematite in natural red beds and of significance for the interpretation of paleomagnetic studies on red beds

tectonic evolution of the iberian margin of gondwana and of correlative regions a celebration of the career of cecilio quesada

paleomagnetism of paleocene maastrichtian 60 70 ma lava flows from tian shan central asia directional analysis and paleointensities

late cretaceous to middle tertiary basin evolution in the central tibetan plateau changing environments in response to tectonic partitioning aridification and regional elevation gain located in the bangong suture zone between the lhasa and qiangtang terranes of central tibet the nima basin records cretaceous late miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation the bangong suture zone originally formed during late jurassic early cretaceous time as the lhasa terrane collided with the qiang tang terrane during early to middle cretaceous time the region lay at northern near equatorial paleolatitudes near sea level by aptian time the nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated bangong suture zone in the southern nima basin an 50 m y late cretaceous eocene depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the lhasa terrane in the northern nima basin deposition continued during latest cretaceous time recording arid paleoclimate in evaporitic lacustrine and eolian dunefield deposits by oligocene time the nima basin comprised two independent depocenters that accumulated coarse grained alluvial fluvial lacustrine evaporitic and fan delta deposits in close association with reactivated thrusts in the bangong suture zone carbon and oxygen isotope data from oligocene paleosol carbonate reported elsewhere indicate that regional paleoelevation during the late oligocene was 4 6 km as it is today overall the depositional record of the nima basin combined with ongoing structural and geochronologic studies demonstrates that the bangong suture zone was reactivated during middle cretaceous and middle tertiary time that the intervening 50 m y interval was a time of regional upper crustal shortening in the lhasa terrane followed by regional ignimbrite eruptions and that arid paleoclimate and high paleoelevation were established by the late cretaceous and late oligocene respectively within the context of other data sets from the lhasa terrane the record of deposition in the nima basin is consistent with low angle subduction of neotethyan oceanic lithosphere and reactivation of the bangong suture zone during the early cretaceous followed by shortening within the gangdese retroarc and northern lhasa terrane thrust belts during the late cretaceous lithospheric delamination dripping and regional magmatic flare up during latest cretaceous through early tertiary time and underthrusting of indian lower crust and lithosphere as far north as the bangong suture zone during late oligocene time

paleomagnetic constraints on the paleolatitude of the lhasa block during the early cretaceous implications for the onset of india asia collision and latitudinal shortening estimates across tibet and stable asia abstract interbedded volcano sedimentary sequences are well exposed in the northern part of the lhasa block in southern tibet zircon u pb dating results from two samples indicate that the emplacement age of the duoni formation volcanic flows is 120 2 0 5 ma paleomagnetic results from 235 progressively demagnetized volcanic rock samples 25 sites and 41 sandstone samples 5 sites indicate that the dominant remanence carriers are ti poor titanomagnetite and ti poor titanohematite in the volcanic samples and ti rich titanomagnetite in the sandstone samples rock magnetic investigations systematic demagnetization behavior positive fold test results and direct petrographic identifications all indicate that the paleodirections recorded by the chemically stable magnetic particles are primary thermal remanent magnetization in the volcanic flows and primary detrital remanent magnetization in the sandstones the tilt corrected chrm mean direction is d i 356 4 16 4 with 95 6 3 n 19 corresponding to a paleopole position of p 66 9 n p 281 2 e with a95 6 1 combined with previously published results the geochronological dating and paleomagnetic analysis indicate a paleolatitude of 13 1 2 7 n for the southern margin of the lhasa block during the early cretaceous therefore the southernmost margin of the eurasian continent likely remained at the low middle paleolatitude of 13 1 2 7 n between the early cretaceous and the paleocene based on comparisons to results from the tethyan himalayan block and the reference poles from stable india and eurasia the low middle paleoposition of the lhasa block during the early cretaceous through early paleocene suggests that the initial contact between india and asia occurred at ca 59 3 ma under the assumption of a rigid eurasian plate this timing implies that a total collision related latitudinal convergence of 1450 400 km 13 1 3 7 n has been accommodated by folding thrust faulting normal faulting crustal thickening intracontinental subduction in tibet and central asia and southeastward continental extrusion of the indo china block from the eastern syntaxis between the lhasa block and stable asia

the importance of along margin terrane transport in northern gondwana insights from detrital zircon parentage in neoproterozoic rocks from iberia and brittany abstract detrital zircons from late neoproterozoic rocks of the peri gondwanan cadomian belt of sw iberia and north armorican domain of brittany record neoproterozoic ca 860 550 ma palaeoproterozoic ca 2300 1800 and archaean ca 3300 2600 ma u pb ages the absence of mesoproterozoic zircons suggests that these terranes evolved in a peri w african realm this is in contrast to other western european terranes that preserve mesoproterozoic zircons and are likely to have evolved in a peri amazonian realm such a contrast in detrital zircon populations coupled with the presence of mesoproterozoic zircons in the ordovician armorican quartzite deposited in a peri african platform is interpreted to record along margin terrane transport the change in provenance suggests that subduction was replaced by transform faults that juxtaposed amazonia derived terranes against w africa derived terranes to form the avalonia and armorica microcontinents subsequent extension along the margin resulted in the birth of the rheic ocean and the outboard drift of avalonia

reduced convergence within the tibetan plateau by 26 ma understanding the dynamics of double thickening and uplifting of the tibetan crust requires constraints on the magnitude and timing of crustal shortening new elongation inclination e i corrected paleomagnetic data from similar to 26 22 ma sediments indicate that the latitude of southern tibet in the early miocene was 31 1 6 8 5 2 degrees n not significantly different from today this implies that the southern margin of asia which was at 21 24 degrees n latitude from the late cretaceous to the early eocene advanced 8 10 degrees northward between the early eocene and the latest oligocene our results therefore suggest that at least 900 1100 km of continental shortening and significant regional uplift of the plateau occurred between the early eocene and late oligocene our results suggest that n s intra asian convergence was considerably reduced around 26 ma corresponding to a transition from compression to extension within the tibetan plateau plain language summary two critical questions involving the geodynamic evolution of the tibetan plateau are i when did a compression dominated tectonic regime change to an extension dominated tectonic regime on the plateau and ii how does this timing correlate with double thickening of the crust here we provide paleomagnetic evidence for the timing magnitude and partitioning of intra asian convergence our results imply a first order constraint of 900 1100 km north south shortening concentrated within the interval from 53 to 26 ma this suggests that a fundamental change at similar to 26 ma occurred in the geodynamics of the tibetan plateau marked by a reduction in large scale convergence and compressive deformation the implication is that significant regional uplift of the proto tibetan plateau occurred within the similar to 55 26 ma interval the paleolatitude paleoclimate and topography of south central tibet seen at present were likely established around 26 ma

paleomagnetism of the central iberian curve s putative hinge too many oroclines in the iberian variscides abstract the variscan mountain belt in iberia defines a large s shape with the cantabrian orocline in the north and the central iberian curve an alleged orocline belt of opposite curvature to the south the cantabrian orocline is kinematically well constrained but the geometry and kinematics of the central iberian curve are still controversial here we investigate the kinematics of the central iberian curve which plays an important role in the amalgamation of pangea since it may have accommodated much of the post collisional deformation we have performed a paleomagnetic study on carboniferous granitoids and cambrian limestones within the hinge of the curve our paleomagnetic and rock magnetic results show a primary magnetization in the granitoids and a widespread carboniferous remagnetization of the limestones syn kinematic granitoids show ca 70 counter clockwise rotations consistent with the southern limb of the cantabrian orocline post kinematic granitoids and cambrian limestones show consistent inclinations but very scattered declinations suggesting that they were magnetized coevally to and after the 70 rotation our results show no differential rotations between northern southern limb and the hinge zone therefore we discard a late carboniferous oroclinal origin for the central iberian curve

first evidence for ultrahigh pressure garnet peridotite in the north american cordillera constraints on the thickness of mantle lithosphere involved in collisional orogenesis are fundamental for understanding the geodynamics of mountain building and the overall growth of continents by accretionary tectonics garnet peridotite and ultrahigh pressure uhp crustal rocks provide such a constraint in many collisional orogens but have hitherto been unrecognized in western north america s cordillera here we show the first evidence for exhumation of uhp 2 8 gpa garnet peridotite and eclogite and for deposition of these rocks as detritus in an early jurassic forearc basin laberge group yukon territory and british columbia our results suggest that collision in this part of the north american cordillera must have been thick skinned involving a proterozoic continental mass with a lithosphere 100 km and possibly to 150 km thick our discovery also provides insight into the vigor of uplift and erosion of deep seated rocks in a nascent continental arc

kinematic evolution of a continental collision constraining the himalayan tibetan orogen via bulk strain rates abstract we investigated temporal variations of the bulk strain rate across the himalayan tibetan orogen which impact the magnitudes rates and distribution of deformation across the orogen the present day strain rate estimated from geodesy is 5 6 10 16 s 1 and estimation of past rates depends on the orogenic shortening rate and effective orogen width plate circuit reconstructions provide constraints on time varying india asia convergence rates showing a marked deceleration since initial collision at ca 58 ma geologic evidence suggests that most of the tibetan crust started deforming shortly after initial collision which simplifies estimation of the initial orogen width we examined several kinematic models 1 greater indian and tibetan crust started deforming immediately after collision 2 only tibetan crust deformed initially 3 plate convergence was decoupled from crustal shortening with tibetan crust deforming slower than plate rates or 4 a hard india asia collision at ca 45 ma following closure of the xigaze backarc models 1 3 and 4 yield early cenozoic bulk strain rates that match present day rates 5 6 10 16 s 1 whereas model 2 yields faster rates 1 2 10 15 s 1 that must have decreased through time to avoid shortening exceeding total plate convergence using these bounds we constrain crustal shortening across the entire orogen and along its southern and northern margins defined by the himalaya and qilian shan respectively application of these models to tibetan crustal thickening suggests that the plateau reached its present day value 70 km by ca 15 ma which corresponds to the onset of lateral deformation e g strike slip and normal faulting lateral deformation in the himalayan tibetan orogen may have resulted from progressive crustal thickening and reorientation of the intermediate principal stress axis at the thrust belt scale our modeling yields shortening rates and magnitudes that match geologic observations this approach provides testable external kinematic constraints to guide future geological and geophysical investigations

ediacaran palaeozoic tectonic evolution of the ossa morena and central iberian zones sw iberia as revealed by sm nd isotope systematics abstract sm nd isotopic analyses of palaeozoic sedimentary and igneous rocks in the southwest iberian massif western end of the european variscan belt are presented in order to unravel its complex poly orogenic evolution during the closure of the rheic ocean and the amalgamation of pangea the gondwanan margin in southwest iberia sw iberia is subdivided into the ossa morena and central iberian zones separated by the badajoz cordoba shear zone which represents a cryptic suture zone between these terranes the relationships between these terranes and between units preserved within the suture zone e g the sierra albarrana group during the palaeozoic and neoproterozoic are controversial sm nd isotopic studies of representative sedimentary sequences covering the entire pre variscan record of the ossa morena and central iberian zones show very similar characteristics from the uppermost ediacaran onwards these data indicate that their accretion to one another must have been completed by the late neoproterozoic ediacarin that time an event assigned to cadomian orogeny and that they never separated substantially from each other since that time the sm nd isotopic composition of the sierra albarrana group metasedimentary rocks is similar to that of the pre cadomian sequences of the ossa morena zone serie negra suggesting derivation from a common source the common provenance of the palaeozoic sequences in the two zones is identical to that of the pre cadomian serie negra of the ossa morena zone which in accordance with the data presented herein and published u pb zircon data indicates a west african affinity

paleomagnetic and geochronologic results of latest cretaceous lava flows from the lhasa terrane and their tectonic implications to position the asian southern margin before the india asia collision paleomagnetic and geochronologic studies were performed on the dianzhong formation lava flows from the shiquanhe area of the westernmost lhasa terrane lt zircon u pb analyses dated the lava flows to 69 5 2 5 ma the characteristic remanent magnetization chrm directions contain antipodal polarities and pass fold tests implying that they are primary magnetizations this interpretation is supported by rock magnetic analyses and petrographic observations forty four site mean directions were divided into 17 statistically independent direction groups the group mean direction after tilt correction is ds 43 3 is 30 3 k 28 0 95 6 9 the corresponding paleopole at 47 8 n 181 4 e a95 6 4 yields a paleolatitude of 16 6 6 4 n for the shiquanhe area of westernmost tibet 32 34 n 80 12 e consistent paleolatitudes for the southern margin of the lt calculated from the western and central part of the lt indicate that the leading edge of the lt was aligned relatively w e when compared with the reference pole at 70 ma for eurasia this new paleopole suggests that crustal shortening between the shiquanhe area and stable asia was 1500 800 km this is supported by the crustal shortening 600 1000 km absorbed by cenozoic thrust and fold belts within this area indicating that the magnitude of crustal shortening within asia north of the india asia suture zone was similar in the central and western part of the plateau

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

crustal melting and recycling geochronology and sources of variscan syn kinematic anatectic granitoids of the tormes dome central iberian zone a u pb la icp ms study in this study we report u pb laser ablation icp ms zircon and id tims monazite ages for peraluminous granitoid plutons biotite muscovite cordierite sillimanite in the tormes dome one of the gneiss cored domes located in the central iberian zone of the variscan belt of northern spain textural domains in zircon interpreted to represent the magmatic crystallization of the granitoids and one monazite fraction in the ledesma pluton yielded ages around 320 ma in agreement with other geochronological studies in the region this age is interpreted to date the timing of decompression crustal melting driven by the extensional collapse of the orogenic belt in this domain of the variscan chain of western europe in addition there are several populations of inherited xenocrystic zircon 1 carboniferous zircon crystals ca 345 ma as well as one of the monazite fractions in the coarse grained facies of the ledesma pluton that also yielded an age of ca 343 ma 2 devonian silurian zircon xenocrysts with scattered ages between ca 390 and 432 ma 3 middle cambrian ordovician ca 450 511 ma 4 ediacaran cryogenian zircon ages ca 540 840 ma 5 mesoproterozoic to archaean zircon 900 2700 ma the abundance of carboniferous inherited zircon shows that crustal recycling cannibalization may often happen at a fast pace in orogenic scenarios with only short lapses of quiescence in our case study it seems plausible that a crustal layer of ca 340 ma granitoids migmatites was recycled partially or totally only 15 20 my after its emplacement

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

east central asian climate evolved with the northward migration of the high proto tibetan plateau the evolution of cenozoic climate patterns in asia has been linked to uplift of the tibetan plateau tp retreat of the paratethys sea and global cooling however less attention has been placed on the latitudinal change of the tp here we report new climate modeling to explore how modern climate changes as a function of topographic growth and spatial migration of the tp our results show that the northward displacement of the uplifted proto tp within the subtropics can significantly affect the wind and precipitation pattern over east central asia by compiling proxy based climatic records paleolatitudinal and paleoelevational evolution models of the proto tp and in comparison with previous modeling under a global paleogeography we suggest that the northward migration of the proto tp in the paleogene could have intensified the aridity in central asia but its influence on east asian precipitation and monsoonal circulation could be dependent on the paleogeography and other boundary conditions plain language summary the evolution of the asian monsoons and aridification of asia s interior are thought to be driven by tibetan himalayan uplift recent geological observations suggest that the uplifted proto tibetan plateau proto tp migrated northward in the subtropics since the early eocene and reached its present position by the latest oligocene we therefore simulated the effects of this process on east central asian climate our climate modeling shows the northward migration of the high proto tp from 21 29 degrees n to 29 37 degrees n can significantly change the present day wind and precipitation patterns over east central asia especially central asia become drier because of the displacement of the anomalous dry belt caused by large scale atmospheric subsidence we further suggest that forcing by the paleolatitudinal migration of the high proto tp within the subtropics from the eocene to the earliest miocene could have drove the central asian paleoclimate to dryer conditions but migration might not be the sole parameter that governed the east asian monsoon during the paleogene

tectonic significance of a llanvirn age for the dunn point volcanic rocks avalon terrane nova scotia canada implications for the evolution of the iapetus and rheic oceans abstract the paleozoic evolution of avalonia is crucial to the understanding of the development of the appalachian orogen and the iapetus and rheic oceans paleomagnetic faunal structural and isotopic data indicate that avalonia had accreted to laurentia by the early silurian in the avalon terrane of nova scotia ordovician early devonian rocks consist of bimodal volcanic rocks at the base dunn point formation disconformably overlain by a thick sequence of fossiliferous siliciclastics arisaig group which contain llandoverian to lochkovian fossils until now the only published age data for the volcanic rocks yielded an imprecise rb sr whole rock isochron age of 421 15 ma and the volcanism has been interpreted to reflect local extension and basin development related to the oblique collision between avalonia and laurentia after closure of the iapetus ocean we present u pb zircon data from a rhyolite which indicate an age of 460 0 3 4 ma for the dunn point formation an age that requires a re interpretation of its tectonic setting the dunn point volcanism probably developed on the avalonian microcontinent outboard from both laurentia and gondwana possibly in a rifted arc setting analogous to the modern taupo zone in northern new zealand a llanvirn age for these felsic rocks also reconciles apparently conflicting paleomagnetic data and inferred paleolatitude for avalonia in the ordovician silurian and implies 10 northward movement of avalonia between 460 and 440 ma and about 5 5 cm year for the latitudinal component of the convergence between avalonia and laurentia i e the destruction of the iapetus ocean and divergence between gondwana and avalonia development of the rheic ocean during this interval

the origin of the variscan upper allochthons in the ortegal complex northwestern iberia sm nd isotopic constraints on the closure of the rheic ocean northwestern iberia preserves a stack of allochthons in which the vestiges of a suture zone generated during the variscan orogeny by the late paleozoic collision between laurussia and gondwana are exposed lower allochthons contain ordovician ophiolite known as the lower ophiolite and are structurally overlain by devonian ophiolite upper ophiolite which are in turn structurally overlain by allochthons containing high grade metamorphic rocks with continental affinities and late cambrian early ordovician protolith ages known as the upper units geochemical and sm nd isotopic data from the upper ophiolite and the structurally overlying upper units exposed in the ortegal complex of galicia show that these allochthons are derived from a variety of mantle and crustal sources and indicate that the suture zone juxtaposes a variety of oceanic assemblages the general isotopic characteristics of each assemblage are similar to allochthons in other variscan complexes in nw iberia suggesting that the allocht

paleomagnetism of early paleogene marine sediments in southern tibet china implications to onset of the india asia collision and size of greater india article i nfo we report a paleomagnetic study of paleocene marine sediments in the gamba area of the tethyan himalayan terrane southern tibet which aims to accurately locate the position of the northern margin of greater india and further constrain timing of initial contact between india and asia following detailed rock magnetic and paleomagnetic experiments on a total of 675 drill core samples collected from the zongpu and upper jidula formations characteristic remanent magnetizations chrms were successfully isolated from most samples following alternating field af or integrated thermal and af demagnetization the chrms are of dual polarity and pass fold and reversal tests indicating a pre folding origin together with detailed biostratigraphic investigation of the sampling section a magnetic polarity sequence is constructed from data at 167 sampled horizons and satisfactorily correlates with polarity chrons c24r to c27r dating the zongpu formation between 56 2 and 61 8 ma on the geomagnetic polarity time scale the chrm directions from the zongpu formation are grouped stratigraphically into 33 sites and yield two paleopoles of 71 6 n 277 8 e a95 2 5 and 67 3 n 266 3 e a95 3 5 for the time intervals comprising 56 to 59 ma and 59 to 62 ma respectively these new paleopoles imply that the tethyan himalayan terrane was sited at low latitudes in the northern hemisphere during the interval 62 56 ma suggestingthatinitial contact between thetethyanhimalayaand lhasa terranes was established before60 5 1 5 ma and verylikelyoccurred nearthecretaceous tertiary boundary at least in the central partof the suturezone the results alsoindicate that atleast 1500 480 kmof post collisionalcrustal shortening occurred within the himalayas from the analysis of available paleomagnetic data obtained on both sides of the suture zone we propose a conceptional collision model for the india asia collision

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

propagation of the deformation and growth of the tibetan himalayan orogen a review abstract long standing problems in the geological evolution of the tibetan himalayan orogen include where the india asia convergence was accommodated and how the plateau grew to clarify these problems we review the deformations and their role in the plateau s growth our results show that 1630 km of shortening occurred across the tibetan himalayan orogen since 55 ma with more than 1400 km accommodated by large scale thrust belts these thrust belts display an outward expansion from central tibet and couple with the surficial uplift the development of the tibetan plateau involved three significant steps primitive plateau 90 55 ma proto plateau 55 40 ma and neoteric plateau 40 0 ma several processes have collaborated to produce the proto plateau including the pre existing primitive plateau the india asia collision and subductions of greater india and songpan ganzi beneath the lhasa qiangtang terrane since 40 ma the proto plateau which was dominated by a topographic gradient lower crustal flow and continuous india asia convergence experienced three periods of rapid outward growth 40 23 23 10 and 10 0 ma in general the n s trending rifts were caused by the eastward growth of the plateau dominated by thrusting and crust flow in central tibet while they were the results of intense n s shortening in himalaya

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

cambrian ensialic rift related magmatism in the ossa morena zone evora aracena metamorphic belt sw iberian massif sm nd isotopes and shrimp zircon u th pb geochronology abstract the late ediacaran c 560 550 ma serie negra sediments of the evora aracena metamorphic belt ossa morena zone sw iberian massif preserve a record of the erosion of an avalonian cadomian magmatic arc and subsequent related turbiditic sedimentation detrital zircon from the serie negra is characterized by predominantly ediacaran and cryogenian ages with few paleoproterozoic and archean cores and a marked lack of grenvillian ages these features when combined with the metasediments enrichment in lree la yb 14 negative eu anomalies low 147 sm 144 nd values 0 121 and negative e nd 550 5 5 indicate that the protolith serie negra sediments were derived from a continental magmatic arc a period of late cadomian ca 560 540 ma tectonism was followed by an extended episode of widespread bimodal magmatism related to cambrian ca 540 500 ma rifting this tectonic inversion is expressed in the geological record by a regional early cambrian unconformity shrimp zircon u th pb ages from four felsic orthogneisses from the evora massif record cambrian 527 10 ma 522 5 ma 517 6 ma and 505 5 ma crystallization ages for their igneous protoliths this confirms the existence of widespread lower paleozoic igneous activity in the ossa morena zone i a lower cambrian ca 535 515 ma igneous felsic dominated sedimentary complex with calc alkaline signature and associated carbonate and siliciclastic deposition and ii a middle cambrian ordovician ca 515 490 ma igneous bimodal sedimentary complex with calc alkaline and tholeiitic signatures and associated dominant siliciclastic deposition but also carbonate sediments the cambrian felsic magmatism was characterized by negative eu anomalies la lu n 0 8 11 147 sm 144 nd 0 1289 0 1447 and e nd 500 ranging from 1 5 to 0 8 a tendency towards peraluminous compositions suggests late fractionation low degrees of partial melting or the mixing of crustal and mantle derived material in the magma source region some felsic rocks possibly represent the last residual melts of high temperature zircon undersaturated mafic magmas later affected by crustal contamination while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids the transition from early felsic dominated to later more mafic magmatism suggests the gradual opening of the system to tholeiitic n e morb products th n ta n 147 sm 144 nd 0 1478 0 1797 and e nd 500 values ranging from 6 4 to 7 3 while the n morb amphibolites have 147 sm 144 nd 0 1818 0 1979 and e nd 500 values of 5 8 and 7 0 reaching a maximum of 9 1 in contrast other amphibolites have a negative ta anomaly 1 35 n ta n 147 sm 144 nd values ranging from 0 1639 to 0 1946 and e nd 500 values of 3 5 to 5 2 suggesting derivation by crustal assimilation processes the sub alkaline igneous precursors of the amphibolites were most likely generated in a rift setting by asthenospheric upwelling these results strengthen the proposed geodynamic scenarios for the sw iberian massif by which cadomian accretion gave rise to an ensialic rift that developed into a proto oceanic basin and incipient spreading opening of the rheic ocean a similar transition from a convergent to a divergent plate boundary during the ediacaran to cambrian ordovician has also been reported in other segments of the northern gondwana margin

birth and demise of the rheic ocean magmatic arc s combined u pb and hf isotope analyses in detrital zircon from sw iberia siliciclastic strata paleozoic continental reconstructions indicate that subduction of rheic oceanic lithosphere led to collision between laurussia and gondwana which was a major event in the formation of the ouachita appalachian variscan orogenic belt and the amalgamation of pangea however arc systems which record rheic ocean subduction are poorly preserved the preservation of devonian detrital zircon in late devonian early carboniferous siliciclastic rocks of sw iberia rather than arc related igneous rocks indicates that direct evidence of the arc system may have been largely destroyed by erosion here we report in situ detrital zircon u pb isotopic analyses of late devonian early carboniferous siliciclastic rocks from the pulo do lobo zone which is a reworked late paleozoic suture zone located between laurussia and gondwana detrital zircon age spectra from the pulo do lobo zone frasnian formations show striking similarities revealing a wide range of ages dominated by neoproterozoic and paleoproterozoic grains sourced from rocks typical of peri gondwanan terranes such as avalonia the meguma terrane and the ossa morena zone pulo do lobo rocks also include representative populations of mesoproterozoic and early silurian zircons that are typical of avalonia and the meguma terrane which are absent in the ossa morena zone the famennian tournaisian formations from the pulo do lobo zone however contain more abundant middle late devonian zircon indicating the contribution from a previously unrecognized source probably related to the rheic ocean magmatic arc s the middle late devonian to early carboniferous zircon ages from the siliciclastic rocks of sw iberia south portuguese pulo do lobo and ossa morena zones have a wide range in ehft values 8 2 to 8 3 indicating the likely crystallization from magmas formed in a convergent setting the missing rheic ocean arc was probably built on a meguma avalonia type basement we propose for the pulo do lobo zone that the frasnian sedimentation occurred through the opening of a back arc basin formed along the laurussian active margin during rheic ocean subduction as has been recently proposed for the rhenohercynian zone in central europe detrital zircon ages in the frasnian siliciclastic rocks indicate provenance in the meguma terrane avalonia and devonian rheic ocean arc s as a result of back arc basin inversion the frasnian formations underwent deformation metamorphism and denudation and were unconformably overlain by famennian to visean siliciclastic strata including the phyllite quartzite formation of the south portuguese zone the latest devonian early carboniferous detritus were probably shed to the pulo do lobo zone represa and santa iria formations by recycling of devonian siliciclastic rocks from the south portuguese zone meguma terrane and from a new distinct source with baltica laurentia derivation preserved in the horta da torre formation and alajar melange

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

preliminary early cretaceous paleomagnetic results from the gansu corridor china we report results from our paleomagnetic study of lower cretaceous redbeds from the gansu corridor northwestern china the characteristic remanent magnetization chrm resides in hematite often at very high unblocking temperatures 660 c the directions associated with this component exhibit only reversed polarities from locality a sunan area but the samples from locality b lanzhou area 480 km to the southeast show roughly antipodal normal and reversed polarities the combined sample directional data from both localities pass a fold test at the 99 confidence level the mean paleomagnetic pole is located at 48 7 n 199 7 e with a 95 4 1 which is discordant with poles of similar age elsewhere from neighboring regions in china although represented by relatively few samples n 21 this pole suggests that significant post cretaceous motion may have occurred between the gansu corridor and adjacent blocks relative to eurasia or north china the discordance corresponds to 28 1 5 2 or 35 6 9 7 clockwise rotation and 9 5 4 5 or 9 8 8 2 northward displacement respectively the rotations support but do not yet distinguish between several neotectonic models assumed to have acted over the past 15 40 m y the displacement is not predicted by any of these models if real it may have occurred early in the history of the india asia collision or even before

late oligocene early miocene evolution of the lunpola basin central tibetan plateau evidences from successive lacustrine records abstract widespread cenozoic sediments in and around the tibetan plateau tp are thought to have played an important role in explaining the process of the india asia collision as well as its interactions with global and regional paleoclimate however high resolution temporal frameworks of sedimentary sequences and controls on geological and climatic events are still absent to study the abovementioned issues we investigate the oligocene miocene lacustrine sequences the dingqinghu formation of the lunpola basin central tp in this work cyclostratigraphic analyses are conducted with gamma ray log and pollen data to establish a high resolution temporal framework ranging from ca 25 4 to 18 0 ma for the sections along these sections sediment accumulation rates are calculated with orbital signals to monitor clastic input of the lake basin elemental palynological and isotopic data are summarized to depict the paleoclimate and paleoelevation evolution of this drainage system integrating all these clues together we sort out a chronological list of events including lake basin tectonics and paleoclimate regional uplift took place at 23 7 ma simultaneously a distinct lake basin transition characterized by accelerated sediment accumulation rate is recognized about 0 2 ma later at 23 5 ma catchment scale drought occurred and maintained to the end of the sections our results demonstrate that paleoclimate did not impose decisive influence on the late oligocene early miocene evolution of the lake basin instead regional uplift and its associated accelerated exhumation of the source area resulted in the lake basin transition and paleoclimatic drought after reviewing the oligocene miocene sedimentary records distributed in and around the tp we argue that the 23 7 ma geological event of the lunpola basin is probably not a single case but a regional effect of a dramatic tectonic transition of the plateau

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

greater india basin hypothesis and a two stage cenozoic collision between india and asia cenozoic convergence between the indian and asian plates pro duced the archetypical continental collision zone comprising the himalaya mountain belt and the tibetan plateau how and where india asia convergence was accommodated after collision at or be fore 52 ma remains a long standing controversy since 52 ma the two plates have converged up to 3 600 35 km yet the upper crustal shortening documented from the geological record of asia and the himalaya is up to approximately 2 350 km less here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic indian lithosphere within the himalaya be tween approximately 50 and 25 ma paleomagnetic data show that this extended continental and oceanic greater india promontory resulted from 2 675 700 km of north south extension between 120 and 70 ma accommodated between the tibetan himalaya and cratonic india we suggest that the approximately 50 ma india asia collision was a collision of a tibetan himalayan microconti nent with asia followed by subduction of the largely oceanic greater india basin along a subduction zone at the location of the greater himalaya the hard india asia collision with thicker and contiguous indian continental lithosphere occurred around 25 20 ma this hard collision is coincident with far field deforma tion in central asia and rapid exhumation of greater himalaya crys talline rocks and may be linked to intensification of the asian monsoon system this two stage collision between india and asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography

high relief topography of the nima basin in central tibetan plateau during the mid cenozoic time abstract there remains inconsistence of uplift history of the tibetan plateau here we estimated paleoelevation of mid cenozoic nima basin located in central tibet using multi proxies they included meteoric water and vegetation isotope proxies based on hydrogen and carbon isotopic compositions d and 13c respectively of terrestrial plant wax lipid of n alkanes and temperature proxy derived from microbial membrane lipid of branched glycerol dialkyl glycerol tetraethers brgdgts lipid analysis on sedimentary rocks in the nima basin yielded 222 12 and 30 0 0 7 for d and 13c of n c29 alkane respectively and brgdgt derived mean annual air temperature maat of 21 5 3 1 c for the paleo basin using the sea level meteoric water 18o and miocene siwalik paleovegetation 13c as low elevation references values of d and 13c of n c29 alkane in this study suggested paleoelevations of 4546 m and 2800 m respectively whereas by using a subtropical sea surface temperature as a reference brgdgt derived maats suggested a paleoelevation of 893 m we believed that the three estimates reflect a high relief topography in the study basin during the mid cenozoic representing elevations of orographic barrier of southerly oceanic moisture basin catchment and basin lake level respectively this scenario implies a low elevation of the basin relative to the lhasa terrane located to the south during the mid cenozoic hence challenging the classical crustal thickening models assuming a simple crustal thickening uplift relationship

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

provenance analysis of the paleozoic sequences of the northern gondwana margin in nw iberia passive margin to variscan collision and orocline development abstract the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges from neoproterozoic to early permian in age its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana or ii an active continental margin or iii a drifting ribbon continent in this paper we present detrital zircon u pb laser ablation age data from 13 samples taken in detrital rocks from the cantabrian zone sequence ranging from early silurian to early permian in depositional age the obtained results together with previously published detrital zircon ages from ediacaran ordovician strata allow a comprehensive analysis of changing provenance through time collectively these data indicate that this portion of iberia was part of the passive margin of gondwana at least from ordovician to late devonian times zircon populations in all samples show strong similarities with the sahara craton and with zircons found in libya suggesting that nw iberia occupied a paleoposition close to those regions of present day northern africa during this time interval changes in provenance in the late devonian are attributed to the onset of the collision between gondwana and laurussia additionally the middle carboniferous to permian samples record populations consistent with the recycling of older sedimentary sequences and exhumation of the igneous rocks formed before and during the variscan orogeny late devonian to permian samples yield zircon populations that reflect topographic changes produced during the variscan orogeny and development of the lithospheric scale oroclinal buckling

amazonian mesoproterozoic basement in the core of the ibero armorican arc 40ar 39ar detrital mica ages complement the zircon s tale the 40 ar 39 ar age data on single detrital muscovite grains complement u pb zircon ages in provenance studies as micas are mostly derived from proximal sources and record low temperature processes ediacaran and cambrian sedimentary rocks from northwest iberia contain unmetamorphosed detrital micas whose 40 ar 39 ar age spectra suggest an amazonian middle american provenance the ediacaran sample contained only neoproterozoic micas 590 783 ma whereas the cambrian sample contained three age groups neoproterozoic 550 640 ma avalonian cadomian pan african mesoproterozoicneoproterozoic boundary ca 920 1060 ma grenvillian sunsas and late paleoproterozoic ca 1580 1780 ma rio negro comparison of 40 ar 39 ar muscovite ages with published detrital zircon age data from the same formations supports the hypothesis that the neoproterozoic basins of northwest iberia were located in a peri amazonian realm where the sedimentary input was dominated by local periarc sources tectonic slivering and strike slip transport along the northern gondwanan margin affected both the basins and fragments of basement that were transferred from amazonian to northern african realms during the latest neoproterozoic earliest cambrian exhumation and erosion of these basement sources caused shedding of detritus to the cambrian basins in addition to detritus sourced in the continental mainland the apparent dominance of rio negro aged micas in the cambrian sandstone suggests the presence of unexposed basement of that age beneath the core of the ibero armorican arc

geomagnetic secular variation and the statistics of palaeomagnetic directions in this study we examine the role of palaeosecular variation psv in the use of statistics for palaeomagnetic studies and we provide new reliability criteria for palaeomagnetic poles or directions we first conclude that fisher statistics should not be applied to average palaeomagnetic directions but to virtual geomagnetic pole vgp distributions instead secondly we strongly advocate that typical properties of geomagnetic field behaviour are taken into account in the assessment of palaeomagnetic data sets the latitude dependent properties e s k provide useful guidelines for the reliability of a palaeomagnetic data set a reliable assessment of these properties depends on the sufficient number of palaeomagnetic samples being taken therefore as an additional instrument of assessing data sets we provide a n dependent a95 envelope bounded by an upper limit a95max and a lower limit a95min that helps to ascertain whether or not a distribution has sufficiently well sampled psv and therefore geomagnetic field behaviour applying these criteria is indispensable for studies of geomagnetic behaviour or for studies aiming at using tk03 gad for inclination error correction through the elongation inclination e i method for palaeomagnetic studies aimed at geological reconstructions they form helpful guidelines and increase the confidence in the rocks having faithfully recorded the field an analysis of published eastern mediterranean data shows that the vast majority of studies do not conform to the van der voo criteria in particular with respect to n and a95 we have provided criteria that are on the one hand more lenient lower n may still provide relevant information and on the other hand more strict for high n the criterion of a95 16 should be adapted to a requirement of lower a95 e g a95 80

geochemistry and u pb protolith ages of eclogitic rocks of the asis lithodeme piaxtla suite acatlan complex southern mexico tectonothermal activity along the southern margin of the rheic ocean recent data indicating that the piaxtla suite acatlan complex southern mexico underwent eclogite facies metamorphism and exhumation during the devono carboniferous suggest an origin within the rheic ocean rather than the iapetus ocean the asis lithodeme piaxtla suite consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks u pb zircon data indicate that the metasediments were deposited after c 700 ma and before intrusion of c 470 420 ma quartz augen granite the metasedimentary rocks contain abundant mesoproterozoic detrital zircons c 1050 1250 ma and a few zircons in the range of c 900 992 and c 1330 1662 ma their geochemical and sm nd isotopic signature is typical of rift related passive margin sediments derived from an ancient cratonic source which is interpreted to be the adjacent mesoproterozoic oaxacan complex megacrystic granites were derived by partial melting of a c 1 ga crustal source similar to the oaxacan complex amphibolitic layers exhibit a continental tholeiitic geochemistry with a c 0 8 1 1 ga source t dm age and are inferred to have originated in a rift related environment by melting of lithospheric mantle in the ordovician this rifting may be related to the early ordovician drift of peri gondwanan terranes e g avalonia from gondwana and the origin of the rheic ocean

oligocene miocene source rocks of the zhongcang basin implications for hydrocarbon potential differentiation between lake basins in central tibet abstract the oligocene miocene dingqinghu formation is widely distributed in lake basins of the bangong nujiang suture zone bnsz central tibet and has been reported as an important source rock layer however limited works have been conducted in the zhongcang basin to gain knowledge on hydrocarbon potential of this area and further investigate the organic matter om accumulation mechanisms of source rocks in the bnsz basins samples collected along two measured sections from the zhongcang basin were subjected to sedimentological organic geochemical and petrographic analyses sedimentological analyses indicate that all samples were deposited in prodeltic to deep lake environments total organic carbon toc values and pyrolysis data suggest a poor to fair hydrocarbon source potential of the middle dingqinghu formation with the exception of om rich samples locate in a marker bed of the upper part vitrinite reflectance ro values of the upper part and middle lower part samples range from 0 41 to 0 50 and 0 52 to 0 61 indicating immature and marginal mature stages respectively these maturity levels were also supported by specific maturity indices of sample extracts such as ratios of c31 22s 22s 22r homohopanes and c29 steranes bulk geochemistry maceral composition of kerogen and extract biomarkers demonstrate that om rich samples were deposited in fresh to brackish waters with type ii iii kerogen mainly derived from algae and higher plants while poor to fair samples have less higher plant contribution but relatively high bacterial content that probably originated from more saline environments the paleoclimate induced environmental reorganization is likely responsible for the differential om accumulation in the zhongcang basin relatively wet climates would facilitate primary productivity of the lake as well as development of swamps on the delta resulting in deposition of mixed terrestrial and aquatic om with higher toc whilst dry conditions would restrict the development of river systems and promote the establishment of saline environments causing decreased total om input but increased proportion of halophilic bacterial om in addition we summarized published data of the dingqinghu formation in the bnsz basins and preliminarily reconstructed paleogeography of central tibet two types of drainage patterns were confirmed in this area a long lived hierarchical drainage system is developed in the lunpola area while single lake systems were prevalent in other basins including the zhongcang basin the hierarchical drainage pattern could minimize clastic dilution effects during tectonic active and or relatively wet periods and prevent the establishment of hypersaline environments during the dry period the drainage center in this kind of hierarchical system has lower sedimentation accumulation rate and could receive much more aquatic om at any climatic conditions we argue that tectonic induced drainage organization primarily controlled the hydrocarbon potential differentiation between lake basins of central tibet

u pb geochronology and hf nd isotope compositions of the oldest neoproterozoic crust within the cadomian orogen new evidence for a unique juvenile terrane new u pb dates combined with nd and hf isotopic data from rocks within the port morvan area of the baie de st brieuc region of brittany identify a unique portion of the neoproterozoic cadomia terrane two gneisses near port morvan yielded u pb dates of 754 6 0 8 ma and 746 0 0 9 ma ages that are more than 130 myr older than the oldest units formed during the main phase of early cadomian magmatism two trondhjemite boulders from the monogenetic facies of the cesson conglomerate yielded identical ages of 665 2 0 5 ma and 665 5 0 7 ma and a cobble from the polygenetic facies yields a 207pb 206pb date of 637 2 ma individual detrital zircons from a sandstone associated with the cesson conglomerates yield concordant u pb dates ranging from 650 3 ma to 624 1 0 6 ma initial nd values for the rocks in this region range from 5 0 to 6 6 indicative of a substantial input from depleted mantle initial hf values determined on zircons from these neoproterozoic rocks including the detrital zircons range from 6 7 to 14 5 consistent with the nd isotopic results maximum initial hf values for two 2 ga icartian gneisses considered basement to cadomia average 8 4 and 8 7 in contrast to the results of the port morvan rocks 616 608 ma syn tectonic intrusions from normandy and the british channel islands all have negative initial nd values 10 4 to 8 3 consistent with significant contamination by ancient crust such as the 2 ga gneisses the oldest arc related magmas should have interacted most extensively with cadomian basement buffering younger mantle derived magmas that were generated in subsequent magmatic episodes the rocks within the port morvan region are thus inconsistent as examples of the earliest cadomian intrusions as they show no evidence of interaction with 2 ga basement instead the older ages and mantle like isotopic composition of these rocks suggest they are part of an independent terrane that formed prior to and independently from the cadomian arc possible terrane scale structural boundaries have recently been identified including the newly recognized port morvan thrust fault and the nw dipping main cadomian thrust

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

cambro ordovician paleogeography of the southeastern new england avalon zone implications for gondwana breakup paleomagnetic measurements and u pb geochronology of alkalic igneous rocks intruding the ediacaran cambridge argillite and the lower cambrian weymouth formation north of boston at nahant massachusetts yield a well constrained paleopole the oldest intrusive phases are a syenodiorite sill and a granite dike with 206 pb 238 u crystallization ages of 490 19 0 90 ma and 489 03 0 82 ma respectively this sequence along with undated mafic sills penetrating both the argillite and weymouth strata are cut by 488 53 0 81 ma gabbro locally associated with 488 48 0 79 ma syenite forming a steep sided intrusive plug ten paleomagnetic sampling sites within the mafic sills and the gabbro contain a stable a magnetic component with a mean direction of d 278 5 i 76 7 a 95 3 9 n 10 this is considered to be the primary magnetization direction because it passes both fold and reversals tests the corresponding paleopole at 320le 34 n a 95 7 2 n 10 places the southeastern new england avalon zone at a southerly paleolatitude of 65 slightly removed from the african margin of gondwana and suggests that bimodal nahant magmatism is linked with latest cambrian early ordovician rifting of west avalonia to form the rheic ocean stable b and c magnetic components in nahant syenodiorite and syenite give rise to virtual geomagnetic poles that track late silurian through carboniferous segments of the north american apparent polar wander path indicating docking of west avalonia by mid paleozoic time

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

hydrogeochemistry of the chhota shigri glacier meltwater chandra basin himachal pradesh india solute acquisition processes dissolved load and chemical weathering rates the chemical compositions of various ions in meltwater of the chhota shigri glacier were analysed during the observation period from may to october 2010 total 164 samples of meltwater were collected in two times a day at 8 0 a m and 6 0 p m bicarbonate and calcium were the dominant anion and cation in meltwater of the investigation area high ratios of ca mg versus tz and ca mg versus na k demonstrate that hydrogeochemistry of the study area was mainly governed by carbonate type weathering the average sulphate mass fraction of meltwater was computed to be 0 31 0 12 showing dominancy of bicarbonate over sulphate this excess bicarbonate would be generated by an alternative proton supply mechanism due to biological activities in the bed rock lithology and sulphide oxidation may be about the same as carbonation as a driver of chemical weathering at chhota shigri glacier in general major ions and total dissolved solid concentrations showed decreasing trend with increasing discharge from the study area the average daily mean dissolved load of this glacier was much lower than that of the gangotri glacier which may be due to low meltwater runoff and lithology of the investigation area the cation weathering rates of study area for early peak and late melt seasons were computed to be 694 1631 and 481 meq m 2 a 1 respectively the average value of chemical denudation rates of the chhota shigri glacier meltwater was found lower than that of the dokriani glacier

deciphering the origin of the cenozoic intracontinental rifting and volcanism in eastern china using integrated evidence from the jianghan basin abstract intracontinental rifting and low volume volcanism are a globally common phenomenon yet the underlying driving mechanisms and whether they can be explained through classic plate tectonic concepts remain hotly debated a prominent example is the cenozoic rift and volcanic province in eastern china using an integration of geological geophysical and geochemical data we unravel the spatial and temporal variations of the rifting and volcanism in the jianghan basin both rifting and volcanism in the jianghan basin show two intense to weak cycles 65 50 ma and 50 26 ma respectively with significant enhancement in activity during the late rift phase moreover rifting and depocentres progressively migrated eastward the jianghan basalts all share an asthenospheric origin while the source of the late phase basalts is slightly more enriched and heterogenous in nd hf isotopes than that of the early phase basalts the late phase basalts also display a smaller extent of partial melting even under a thinner lithosphere likely indicating a significant decrease of volatile content in the mantle source based on regional tectonic correlations the main stages of tectonic evolution of the jianghan basin and eastern china are not synchronous with changes in pacific plate motion while they are coincident with india asia collision processes these observations lead us to propose that the asthenospheric flow driven by india asia collision rather than the rollback of the subducted pacific slab has caused the widespread rifting and volcanism in eastern china the variations of rifting and volcanism in the jianghan basin suggest a multiphase and eastward asthenospheric flow beneath eastern china driven by india asia collision with an intense upwelling when passing through the north south gravity lineament nsgl the much more intense rifting and volcanism during the late rift phase may indicate a much larger scale of volatile poor asthenospheric flow than the early rift phase which could result in a more intense erosion of ancient enriched lithospheric mantle and the volatile content in the mantle source dropping sharply this study provides an improved model based on our multidisciplinary observations for asthenospheric flow which may be an alternative driving mechanism for intracontinental rifting and low volume volcanism in the regions where there are step changes in lithospheric thickness globally

iberian late variscan granitoids some considerations on crustal sources and the significance of mantle extraction ages a suite of post tectonic granitoids mostly peraluminous broadly i type granodiorites and monzogranites and mafic rocks from nwiberia with crystallization ages between ca 309 and 290 ma has been investigated for sm nd isotopes and inherited zircon content in order to constrain the nature of their source rocks end values at 300 ma vary from 0 2 to 5 9 and tdm values range from 1 01 to 1 58 ga inherited xenocrystic zircons yielded ages ranging from 458 to 676 ma with 90 of data between 490 and 646 ma corresponding to neoproterozoic mostly ediacaran cambrian andordovician ages only three highlydiscordant analyses yielded ages older than 650 ma based on the data reported herein and relevant data fromthe literaturewe contend that post tectonic granitoids of the iberian variscan belt with exception of the scarce anatectic s type granitoids were derived mostly from metaigneous lower crustal sources which in turn were ultimately derived from a subcontinental lithospheric mantle enriched between ca 0 9 and 1 1 ga i type granitoids and mantle derived mafic rocks both underwent varying degrees of contamination by ametasedimentary lower crust depleted in pre 650 ma zircon through previousmelting episodes with a time integrated sm nd evolution different to that of the metaigneous lower crust participation of this metasedimentary crust in the genesis of these granitoids may account for nd isotopic variability and nd model ages well in excess of 1 2 ga

late neoproterozoic to carboniferous genesis of a type magmas in avalonia of northern nova scotia repeated partial melting of anhydrous lower crust in contrasting tectonic environments avalonian rocks in northern mainland nova scotia are characterized by voluminous 640 600 ma calc alkalic to tholeiitic mafic to felsic magmas produced in a volcanic arc however after the cessation of arc activity repeated episodes of felsic magmatism between ca 580 ma and 350 ma are dominated by a type geochemical characteristics sm nd isotopic data combined with zircon saturation temperature estimates indicate that these magmas were formed by high temperature 800 1050 c melting of the same anhydrous crustal source regional tectonic considerations indicate that a type felsic magmatism was produced 1 at 580 ma in a san andreas type strike slip setting 2 at 495 ma as avalonia rifted off gondwana 3 at 465 and 455 in an ensialic island arc environment and 4 at 360 350 ma during post collisional intra continental strike slip activity as avalonia was translated dextrally along the laurentian margin these results attest to the importance of crustal source rather than tectonic setting in the generation of these a type magmas and are an example of how additional insights are provided by comparing the geochemical and isotopic characteristics of igneous suites of different ages within the same terrane they also suggest that the shallow crustal rocks in northern mainland nova scotia were not significantly detached from their lower crustal source between ca 620 ma and 350 ma a time interval that includes the separation of avalonia from gondwana its drift and accretion to laurentia as well as post accretionary strike slip displacement

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

late cretaceous cenozoic intraplate extension and tectonic transitions in eastern china implications for intraplate geodynamic origin abstract the geodynamic origin of intraplate extension remains poorly understood for the case of the late cretaceous cenozoic intraplate extension in eastern china its driving mechanism remains highly controversial due to the absence of a comprehensive understanding of significant tectonic transitions this study uses the well documented cretaceous tectonic evolution as a background and integrates extensive multidisciplinary data in the jianghan basin bohai bay basin and zhejiang area to develop an integrated view of the late cretaceous cenozoic geodynamic origin for the first time during the late cretaceous rifting and depocentres were distributed in the jianghan basin and were located in the central bohai bay basin during the cenozoic rifting and depocentres were initially focused in the western basins and then progressively migrated eastward rather than continuing migrating eastward the rifting and volcanism during the middle cenozoic 50 23 ma significantly increased and all rift basins in eastern china were abandoned at the end of the middle cenozoic 26 23 ma the volcanism in the zhejiang area was markedly enhanced during the late cenozoic 23 0 ma the asthenospheric mantle sources beneath the two rift basins were isotopically more depleted with an increased extent of melting from the late cretaceous 100 65 ma to early cenozoic 65 50 ma however they became isotopically more enriched and heterogenous with a decreased melting extent significantly reduced volatile content from the early cenozoic to middle cenozoic the mantle source beneath the zhejiang area during the late cenozoic was isotopically more depleted and heterogeneous than during the middle cenozoic with an increased extent of melting these three phase tectonic transitions together with tectonic correlations directly constrain the causal relationships between the intraplate extension in eastern china and the motions of the adjacent plates consequently we propose that the late cretaceous cenozoic intraplate extension and tectonic transitions in eastern china were successively driven by the paleo pacific slab rollback during the late cretaceous the india asia collision during the early middle cenozoic and by the combination of the subduction of the pacific plate and india asia collision during the late cenozoic our study provides an integrated perspective for unraveling the geodynamic origin of intraplate extension

zircon u pb geochronology of paragneisses and biotite granites from the sw iberian massif portugal evidence for a palaeogeographical link between the ossa morena ediacaran basins and the west african craton

palaeolatitude and age of the indo asia collision palaeomagnetic constraints summary ongoing controversies on the timing and kinematics of the indo asia collision can be solved by palaeomagnetically determined palaeolatitudes of terranes bounding the indo asia suture zone we show here based on new palaeomagnetic data from the linzizong volcanic rocks 54 47 ma near the city of lhasa that the latitude of the southern margin of asia was 22 8 4 2 n when these rocks were deposited this result combined with revised palaeomagnetic results from the northernmost sedimentary units of greater india and with apparent polar wander paths of india and eurasia palaeomagnetically constrain the collision to have occurred at 46 8 ma 95 per cent confidence interval these palaeomagnetic results are consistent with tomographic anomalies at 15 25 n that are interpreted to locate the tethyan oceanic slab that detached following collision and with independent 56 46 ma collision age estimates inferred from the timing of slowing down of india high pressure metamorphism the end of marine sedimentation and the first occurrence of suture zone and arc detritus on the greater indian margin when compared with apparent polar wander paths of india and eurasia the 46 ma onset of collision at 22 8 4 2 n implies 2900 600 km subsequent latitudinal convergence between india and asia divided into 1100 500 km within asia and 1800 700 km within india

the pangea conundrum geodynamic models for supercontinent assembly whereby the dispersing continental fragments of a supercontinent break up and migrate from geoid highs to reassemble at geoid lows fail to account for the amalgamation of pangea such models would predict that the oceans created by continental breakup in the early paleozoic e g iapetus rheic would have continued to expand as the continents migrated toward sites of mantle downwelling in the paleo pacific reassembling an extroverted supercontinent as this ocean closed instead pangea assembled as a result of the closure of the younger iapetus and rheic oceans geodynamic linkages between these three oceans preserved in the rock record suggest that the reversal in continental motion may have coincided with the ordovician emergence of a super plume that produced a geoid high in the paleo pacific if so the top down geodynamics used to account for the breakup and dispersal of a supercontinent at ca 600 540 ma may have been overpowered by bottom up geodynamics during the amalgamation of pangea

chronological link between deep seated processes in magma chambers and eruptions permo carboniferous magmatism in the core of pangaea southern pyrenees abstract in the southern pyrenees there are upper carboniferous lower permian sedimentary basins with a significant volume of volcanic material derived from explosive eruptions rhyolitic ignimbrites and andesitic flows these basins are spatially associated with granodiorites and dacitic dykes emplaced in variscan basement rocks u pb shrimp dating of zircons extracted from three granodiorites an andesitic flow a dacitic dyke and six ignimbrites revealed that magmatism occurred over an extended period of thirty eight million years from ca 304 ma to ca 266 ma upper carboniferous middle permian a scattering of zircon ages in each sample shows that the history of melt crystallization was complex with more than one zircon forming event in each magma chamber the prolonged crystallization history was transferred to the product of the eruptions a chronological link between the deep seated magma chambers and processes in eruptions was identified on the basis of four overlapping intervals at ca 309 307 ma upper carboniferous ca 304 296 ma upper carboniferous lower permian ca 294 282 ma lower permian and ca 276 ma lower permian the variation of zircon u th ratios exposes a tendency for an increase in mafic sources as crystallization advances in the permian zircons probably crystallized from melt phases related to both a felsic intermediate metaluminous source from ca 310 293 ma mostly 0 1

early paleozoic avalon gondwana unity an obituary response to palaeontological evidence bearing on global ordovician silurian continental reconstructions by r a fortey and l r m cocks

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

paleomagnetism and microtextures reveal neohimalayan deformation pattern in the northwestern tethys himalaya abstract the india asia collision produced the himalayan orogenic belt the kinematics of crustal blocks within the orogenic belt can shed insights into the tectonic processes related with the continental collision we report a combined study of paleomagnetism and microtextures from the zhongba microterrane northwestern tethys himalaya the zhongba microterrane is located in the southwestern part of tibet a total of 71 samples collected from permian formations yield a distinct remanent component carried by pyrrhotite which failed fold tests compared with the apparent polar wander path of india it is suggested that this overprint component is acquired recently 10 0 ma and a 16 clockwise rotation is also indicated microtextures from several localities show e w directed shear together with reported gps observations and clockwise rotations a bookshelf style deformation pattern associated with strike slip faulting is proposed for the neohimalayan tectonics in the tethys himalaya bookshelf style rotation of small blocks coordinated by strike slip faults is not only a practical model to absorb the crustal shortening caused by the convergence of the indian eurasian plate but also an important way for the escape of the tibetan plateau to the southeast

a stable southern margin of asia during the cretaceous paleomagnetic constraints on the lhasa qiangtang collision and the maximum width of the neo tethys

u pb geochronology and sm nd isotopic composition of proterozoic gneisses channel islands uk gneissic units of the channel islands united kingdom have traditionally been considered to be palaeoproterozoic basement to the late neoproterozoic cadomia terrane based on lithological correlations and imprecise or ambiguous isotopic data a new precise u pb date of 2061 2 ma based on analyses of single zircons for the icart granitic orthogneiss of guernsey confirms a palaeoproterozoic age of its igneous protolith the nd depleted mantle model age t dm of this gneiss is 2220 ma only slightly older than its crystallization age indicating that it represents juvenile crust other gneisses from guernsey have similar t dm ages ranging from 2210 to 2370 ma suggesting that they are also exposures of juvenile palaeoproterozoic crust a component from a penetratively deformed orthogneiss from sark previously correlated with icartian gneisses on guernsey yields a u pb zircon date of 616 4 2 ma this crystallization age demonstrates that the protolith was a cadomian intrusion and that penetrative deformation and amphibolite facies metamorphism in the northern channel islands were cadomian in age the use of gneissic features as a means of invoking the antiquity of many other undated gneissic units in the region is thus considered unreliable the presence of 2170 7 ma zircon xenocrysts within the sark orthogneiss combined with its initial e nd 615 9 5 are strong evidence that pre cadomian basement was a significant source component and that documenting the extent and age of pre cadomian basement may be possible by indirect geochemical methods

rifting during separation of eastern avalonia from gondwana evidence from subsidence analysis subsidence curves for cambrian ordovician sequences from the anglo welsh segment of the paleocontinent of avalonia reveal two periods of regionally enhanced basement subsidence early cambrian 545 518 ma and late cambrian to early tremadocian 505 490 ma the earlier event may record transtension following the avalonian cadomian orogeny the second event may be a transtensional precursor to the late tremadocian volcanic arc on eastern avalonia however paleomagnetic faunal volcanic and sedimentary evidence suggests that the main separation of eastern avalonia from gondwana occurred after middle arenigian time rifting during separation is probably recorded by localized middle arenigian to llanvirnian 480 462 ma subsidence along the welsh basin margin but rifting must have occurred mainly on the now obscured southern margin of the avalonian continent pronounced caradocian 462 449 ma subsidence is associated with back arc rifting after separation from gondwana

proto avalonia a 1 2 1 0 ga tectonothermal event and constraints for the evolution of rodinia the neoproterozoic evolution of avalonia is thought to have been geodynamically linked to the amalgamation and dispersal of rodinia similar sm nd isotopic signatures for different periods of arc activity suggest that avalonian basement or proto avalonia was generated in a series of primitive oceanic island arcs between 1 2 and 1 0 ga because this interval coincides with the amalgamation of rodinia proto avalonia is inferred to have been located in a panthalassa like peri rodinian ocean an early 760 660 ma phase of avalonian arc activity is attributed to renewed subduction in the peri rodinian ocean following the breakup of rodinia which caused the accretion of avalonian terranes to the gondwanan margin by ca 650 ma further subduction along the margin occurred outboard of these terranes and resulted in the onset of main phase avalonian volcanism at 630 ma the diachronous cessation of arc magmatism is attributed to ridge trench collision and the generation of a continental transform the geodynamic link between avalonia and rodinia is analogous to that between the mesozoic dispersal of pangea and the tectonothermal evolution of western north america this event also resulted in the accretion of outboard terranes and in arc related magmatism that is currently being terminated in a diachronous manner by ridge collision and the generation of the san andreas transform the model implies that the neoproterozoic evolution of avalonia and other peri gondwanan terranes provide important constraints on the tectonic history of a large portion of the rodinian continental margin

isotope geochemistry and revised geochronology of the purrido ophiolite cabo ortegal complex nw iberian massif devonian magmatism with mixed sources and involved mesoproterozoic basement abstract in the purrido ophiolite cabo ortegal complex new u pb zircon dating of the amphibolite g03 8 by laser ablation inductively coupled plasma mass spectrometry confirms the existence of a dominant mesoproterozoic zircon population with a refined age of 1155 14 ma however the u pb zircon dating of two more amphibolite samples by sensitive high resolution ion microprobe has provided new ages of 395 3 ma and 395 2 ma respectively interpreted as the crystallization age hf isotope data for zircon show that most of the devonian zircons crystallized from a juvenile depleted mantle source the mesoproterozoic zircons have relatively juvenile hf isotopic composition reflecting some influence of an older component a few devonian zircon crystals show evidence of mixing with an older component represented by the mesoproterozoic zircons the whole rock sm nd isotope data indicate an important heterogeneity in the composition of the purrido amphibolites only compatible with the generation of their protoliths from two sources we interpret these puzzling data as resulting from the mixing of a devonian mantle derived magma with a mesoproterozoic basement these new data provide new perspectives in the interpretation of the most common ophiolites across the variscan suture in europe supplementary material analytical methods zircon u th pb shrimp analytical data zircon u th pb la icp ms analytical data zircon lu hf la mc icpms analytical data and whole rock sm nd isotope data are available at http www geolsoc org uk sup18449

quantifying the rise of the himalaya orogen and implications for the south asian monsoon we reconstruct the rise of a segment of the southern flank of the himalaya tibet orogen to the south of the lhasa terrane using a paleoaltimeter based on paleoenthalpy encoded in fossil leaves from two new assemblages in southern tibet liuqu and qiabulin and four previously known floras from the himalaya foreland basin u pb dating of zircons constrains the liuqu flora to the latest paleocene ca 56 ma and the qiabulin flora to the earliest miocene 21 19 ma the proto himalaya grew slowly against a high similar to 4 km proto tibetan plateau from similar to 1 km in the late paleocene to similar to 2 3 km at the beginning of the miocene and achieved at least similar to 5 5 km by ca 15 ma contrasting precipitation patterns between the himalaya tibet edifice and the himalaya foreland basin for the past similar to 56 m y show progressive drying across southern tibet seemingly linked to the uplift of the himalaya orogen

the andean type gangdese mountains paleoelevation record from the paleocene eocene linzhou basin abstract paleoelevation reconstruction using oxygen isotopes is making a significant contribution to understanding the cenozoic uplift of the himalayas and the tibetan plateau this paper presents new oxygen and carbon isotopic compositions from well dated tertiary paleosols lacustrine calcareous carbonates and marls from the nianbo 60 54 ma and upper pana formations 51 48 ma of the linzizong group in the linzhou penbo basin the sediments of the nianbo formation which are 180 m thick were deposited in alluvial fans braided rivers fan deltas and on nearshore to offshore lacustrine settings whereas those of the upper pana formation are 100 m thick and are comprised predominantly of proximal alluvial fan and braided river deposits correlations between the lithofacies and stable isotopic compositions suggest that the basin was mainly a hydrologically open environment it is confirmed that the 18 o c and 13 c c values from nianbo and pana formations have not yet been reset by late stage diagenesis based on petrographic examination oxygen isotope of the fossil ostracodes and tectonic deformation of strata the paleoelevations are reconstructed using the corrected most negative paleosurface water 18 o psw values these imply that the linzhou area had attained an elevation of 4500 400 m during the period of the indo asian collision i e achieved a near present elevation and may form an andean type mountain range stretching the gangdese arc before collision the gangdese mountains probably maintained high elevations since at least the paleocene and could play a crucial role in the climate change in the interior of the tibetan plateau during the early cenozoic the paleogeomorphic scenario of the eocene tibet is proposed to exist at two high mountains in excess of 4500 m that sandwiched a low elevation basin

defining the limits of greater india greater india comprises a part of the indian plate that subducted under asia to help form the tibetan plateau defining the size of the greater india is thus a key constraint to model the india asia collision growth of the plateau and the tectonic evolution of the neo tethyan realm we report early cretaceous paleomagnetic data from the central and eastern tethyan himalaya that yield paleolatitudes consistent with previous early cretaceous paleogeographic reconstructions these data suggest greater india extended at least 2 675 720 and 1 950 970 km farther north from the present northern margin of india at 83 6 degrees e and 92 4 degrees e respectively an area of lithosphere 4 7 x 10 6 km 2 was consumed through subduction thereby placing a strict limit on the minimum amount of indian lithosphere consumed since the breakup of gondwanaland plain language summary greater india is part of the indian plate subsequently subducted under asia that helped create the tibetan plateau the amount of greater indian crust therefore plays a critical role to address key problems in continental geodynamics to what extent can continental crust be subducted how much crust was derived from horizontal shortening of existing crust how much of tibet was created by subducted buoyant continental crust we provide paleomagnetic evidence that defines the minimum size of greater india our data show that a lithospheric area of 4 7 x 10 6 km 2 was subducted which supports the notion that the growth of tibetan plateau in the cenozoic occurred by adding buoyant material to its base

nd isotopic evidence for juvenile crust in the carolina terrane southern appalachians nd isotopic analyses of whole rock samples from the older portion of the carolina terrane one of the largest terranes in the appalachian orogen demonstrate that part of this terrane is composed of juvenile mantle derived crust these data suggest that the terrane may not have originally been built upon old evolved basement material but rather may have been built upon oceanic crust a recent study by other workers demonstrates a more crustally evolved nd isotopic signature for younger components of the carolina terrane these data may indicate that the terrane interacted with evolved crust at a later time possibly by amalgamation with a more evolved crustal fragment before final accretion to laurentia rather than indicating a primary old basement a juvenile nature for the older portion of the terrane contrasts with models that suggest it is an evolved crustal fragment that formed in a continental margin setting a scenario proposed to explain the high proportion of felsic volcanic rocks within the terrane it is herein suggested that carolina is a chemically evolved but isotopically juvenile crustal fragment because it remained in an oceanic setting for an unusually long time in this regard the carolina terrane is similar to some of the large accreted terranes in the canadian cordillera such as wrangellia and alexander the presence of juvenile crust in the carolina terrane documents that at least part of the southern appalachian orogen is not composed solely of reactivated pre existing continental crust the importance of this part of the orogen in terms of the volume of juvenile phanerozoic crustal material in north america may be larger than previously thought however until additional major appalachian terranes have been isotopically characterized the volume of juvenile crust in the whole orogen remains unknown the isotopic make up of a terrane can be an important aspect of terrane analysis as different terranes may have significantly different isotopic compositions while even widespread pieces of a single terrane should have very similar isotopic characteristics the nd isotopic data for the carolina terrane form the beginning of an isotope database for terranes in the southern appalachians

present day crustal deformation in china constrained by global positioning system measurements global positioning system gps measurements in china indicate that crustal shortening accommodates most of india s penetration into eurasia deformation within the tibetan plateau and its margins the himalaya the altyn tagh and the qilian shan absorbs more than 90 of the relative motion between the indian and eurasian plates internal shortening of the tibetan plateau itself accounts for more than one third of the total convergence however the tibetan plateau south of the kunlun and ganzi mani faults is moving eastward relative to both india and eurasia this movement is accommodated through rotation of material around the eastern syntaxis the north china and south china blocks east of the tibetan plateau move coherently east southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year respectively with respect to the stable eurasia

comment on comparing paleomagnetic study means with apparent wander paths a case study and paleomagnetic test of the greater india versus greater indian basin hypotheses by david b rowley

u pb geochronological constraints on precambrian stratified units in the avalon composite terrane of nova scotia canada tectonic implications concordant u pb ages of detrital zircons in several proterozoic units in the avalon composite terrane of nova scotia along with previously published data indicate that deposition of low and high grade stratified units in the cobequid highlands gamble brook formation and central cape breton island variously assigned to the george river group or george river metamorphic suite and the bras d or gneiss started after 977 ma extended through a period of volcanic activity between 694 and 637 ma and ended before the 629 612 600 ma volcanic activity associated with deposition of the georgeville and jeffers groups in the antigonish and cobequid highlands and the east bay hills and coxheath groups in southern cape breton island these age data along with u pb analyses of metamorphic and igneous zircon and monazite also constrain three late proterozoic tectonothermal events i oblique sinistral ductile shear deformation and greenschist amphibolite facies metamorphism probably occurred between 694 an

re interpreting the devonian ophiolites involved in the variscan suture u pb and lu hf zircon data of the moeche ophiolite cabo ortegal complex nw iberia new u pb zircon data of a mylonitic greenschist from the moeche ophiolite one of the mafic units involved in the variscan suture in the cabo ortegal complex nw of the iberian massif yielded an age of 400 3 ma consequently this unit can be considered one of the devonian ophiolites the most extended group of oceanic units in the variscan belt the mafic rocks show transitional compositions between n morb and island arc tholeiites although lu hf isotope signatures of its zircons clearly indicate contribution from an old continental source ehf values in the analysed zircons are negative generally below ehf 5 and thence they are not compatible with their generation from a juvenile mantle source accordingly the igneous protoliths were generated in a setting where juvenile mafic magmas interacted with an old continental crust the devonian ophiolites from the variscan suture have been repeatedly interpreted as remnants of the rheic ocean however the presence of a continental source in the origin of the mafic rocks of the moeche ophiolite allows discarding an intraoceanic setting for their generation at least for the nw iberian counterparts the tectonic setting for the devonian ophiolites of nw iberia is very likely represented by an ephemeral oceanic basin opened within a continental realm herein the real rheic ocean suture could only be located west of the terrane represented by the upper units of the allochthonous complexes apparently that suture is not represented in nw iberia

tectonic significance of late ordovician silicic magmatism avalon terrane northern antigonish highlands nova scotia12 1 this article is one of a series of papers published in cjes special issue in honour of ward neale on the theme of appalachian and grenvillian geology 2 contribution to international geological correlation programme igcp project 497 avalonia was a microcontinent during most of the ordovician separating the iapetus ocean to the north from the rheic ocean to the south in the northern antigonish highlands nova scotia volcanic rocks dunn point and mcgillivray brook formations underlie early silurian early devonian strata arisaig group and were thought to represent extension that heralded the development of the basin into which arisaig group strata were deposited however recent u pb zircon thermal ionization mass spectrometry data from rhyolite in the dunn point formation dpf yielded an age of 460 0 3 4 ma and here we report a concordant age of 454 5 0 7 ma for an ignimbrite in the overlying mcgillivray brook formation mbf these data confirm a ca 10 million year gap between volcanism and onset of arisaig group deposition which occurred after accretion of avalonia to baltica the dpf and mbf both resemble a type sio2 rich magmas the mbf has very high concentrations of zr 745 1965 ppm y 65 213 ppm nb 57

the impact of rayleigh number on assessing the significance of supercontinent insulation several processes unfold during the supercontinent cycle more than one of which might result in an elevation in subcontinental mantle temperatures thus multiple interpretations of the concept of continental insulation exist although a consensus seems to have formed that subcontinental mantle upwellings appear below large continents extensively ringed by subduction zones there are differing views on what role continental insulation plays in the production of elevated mantle temperatures here we investigate how the heating mode of the mantle can change the influence of the thermal blanket effect we present 2 d and 3 d cartesian geometry mantle convection simulations with thermally and mechanically distinct oceanic and continental plates the evolution of mantle thermal structure is examined after continental accretion at subduction zones e g the formation of pangea for a variety of different mantle heating modes our results show that in low rayleigh number models the impact of the role of continental insulation on subcontinental temperatures increases when compared to models with higher convective vigor broad hot upper mantle features generated in low rayleigh number models due in part to the thermal blanket effect are absent at higher rayleigh numbers we find that subcontinental heating in a high rayleigh number flow occurs almost entirely as a consequence of the influence of subduction initiation at the continental margin rather than the influence of continental insulation in our models featuring earth like convective vigor we find that it is difficult to obtain subcontinental temperatures in significant excess of suboceanic temperatures over timescales relevant to supercontinent aggregation

global palaeobiogeography of brachiopod faunas during the early katian late ordovician greenhouse episode the early katian late ordovician was marked by a major greenhouse episode and the early stage of a first order marine transgression that led to the flooding of much of north america during the middle late katian multivariate analyses of 33 brachiopod faunas of early katian trentonian chatfieldian late caradoc age including 252 rhynchonelliform genera demonstrate that the trentonian brachiopod faunas of laurentia were already clearly differentiated into a scoto appalachian fauna and an epicontinental fauna globally four distinct faunal clusters are identified kazakhstan avalonia epicontinental laurentia and scoto appalachia the scoto appalachian fauna which originated during the sandbian persisted into the early katian and became most closely related to the brachiopod faunas along the western margins of laurentia the early katian epicontinental faunas of laurentia were more closely related to the coeval brachiopods of the lithuanian north estonian confacies belts of baltica than to the scoto appalachian fauna palaeoecological control on the faunal endemism within laurentia was attributable to isolation of appalachian foreland basin by faulting and peripheral bulging during the early taconic orogeny palaeolatitudinal faunal gradient from the subtropical appalachia to mid tropical epicontinental laurentia as well as different water depth water temperature and substrate types between the two palaeogeographical settings of laurentia

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

careon ophiolite nw spain suprasubduction zone setting for the youngest rheic ocean floor the careon ophiolite galicia nw iberian massif shows lithological and geochemical features suggestive of an origin in a suprasubduction zone setting as with other devonian ophiolites in the european variscan belt it was generated within a contracting rheic ocean this setting and the general absence of large silurian devonian volcanic arcs on both of the rheic ocean margins strongly suggest that this ocean was closed by intraoceanic subduction directed to the north this subduction removed the older normal n mid oceanic ridge basalt morb oceanic lithosphere and gave rise to a limited volume of new suprasubduction zone oceanic lithosphere the careon ophiolite is a key element in understanding the evolution of the rheic ocean which was the main oceanic domain that closed during the paleozoic convergence of gondwana and laurussia preceding the assembly of pangea

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

conjugate strike slip faulting along the bangong nujiang suture zone accommodates coeval east west extension and north south shortening in the interior of the tibetan plateau this is the published version copyright 2002 american geophysical union all rights reserved

high and dry in central tibet during the late oligocene the time at which the tibetan plateau rose to its present high elevation remains controversial with estimates ranging from 40 ma to more recent than 7 ma new stable isotope analyses of modern and accurately dated ancient paleosol carbonate in the nima basin of central tibet point to an arid climate and high paleoelevation 4 5 5 km comparable to today s setting by 26 ma oxygen isotope values of ancient 26 ma soil carbonate are both very negative and indistinguishable after modest corrections for changes in global climate from the lowest least evaporated oxygen isotope values of modern soil carbonates in the area substantial enrichments in oxygen 18 in paleolacustrine carbonates as well as high carbon isotope values from paleosol carbonates indicate considerable lake evaporation and low soil respiration rates respectively and both are consistent with the present arid climate of the nima area blockage of tropical moisture by the himalaya and perhaps the gangdese shan probably has contributed strongly to the aridity and very negative oxygen isotope values of soil carbonate and surface water in the nima area since at least the late oligocene the maintenance of high elevation since at least 26 ma suggests that any flow of lower crust from beneath central tibet must have been balanced by coeval northward insertion of indian crust beneath the plateau

position of the lhasa terrane prior to india asia collision derived from palaeomagnetic inclinations of 53 ma old dykes of the linzhou basin constraints on the age of collision and post collisional shortening within the tibetan plateau the lhasa terrane comprises the southern eurasian margin since at least mid cretaceous therefore paleomagnetic data from the lhasa terrane are crucial to reconstruct the pre collisional southern margin of eurasia previous paleomagnetic data from the cretaceous takena formation provide a rough constraint on the pre collisional paleolatitudinal position of the southern eurasian margin however paleogene paleomagnetic data from the lhasa terrane which can provide more direct constraints are very limited in addition reliability of the existing data is ambiguous because of a possible later overprint in our new study 17 sites were drilled from mafic dykes intruded in the t1 unit of the linzizong formation of the linzhou basin near lhasa for paleomagnetic studies figure 1 the dykes were ar 40 ar 39 dated to c 53 ma by yue and ding 2006 from ten sites a higher coercivity component demagnetized between 20 and 100 mt could be isolated detailed rockmagnetic analyses reveal ti rich titanomagnetite as the remanence carrier which suggests that the rock is not much altered and the remanent magnetization is likely of primary origin this is supported by a positive fold test the occurrence of normal and reverse polarity directions within single sites the subvolcanic character of the dykes and the within site scatter as well as the scatter between site mean directions indicate that paleosecular variation is recorded within sites tilt correction was performed using the bedding of overlying fluvial lacustrine sediments and tilt angles of ignimbrite columns postdating the dyke emplacement bedding corrected directions give an overall mean direction of d i 12 3 27 2 95 10 6 k 21 7 n 10 corresponding to a paleolatitude of 13 2 5 8 n at a reference location on the suture zone 20 n 90 e comparison with previous cretaceous data mainly from the takena formation yields a stable position of the lhasa terrane during cretaceous and early eocene this indicates that no large north south crustal shortening occurred along the southern eurasian margin between late cretaceous and early eocene the difference between expected paleolatitudes determined from the apwp of eurasia and observed paleolatitudes reveals relative northward movement of the lhasa block of ca 1847 763 km since early eocene this is attributed to indentation of india into asia and implies a considerable amount of north south crustal shortening two models of greater india were used to determine the extended northern margin of india ali and aitchison 2005 proposed a greater india based on bathymetric features in the eastern indian ocean and patzelt and others 1996 determined the paleolatitude of northern india using paleomagnetic investigations on paleocene sediments of the zongpu formation at gamba and duela intersection of the pre collisional southern eurasian margin and the northern margin of india yields a collision age of the indian and eurasian continents of c 5349 ma an initial collision within this time range is in accordance with previous studies based on paleomagnetic or geologic data e g patriat and achache 1984 klootwijk and others 1992 guillot and others 2003 a significantly later collision age at the eocene oligocene boundary as proposed by aitchison and others 2007 is not in agreement with our new and previous paleomagnetic data from the lhasa block