tracking the late jurassic apparent or true polar shift in u pb dated kimberlites from cratonic north america superior province of canada different versions of a composite apparent polar wander apw path of variably selected global poles assembled and averaged in north american coordinates using plate reconstructions show either a smooth progression or a large 30 gap in mean paleopoles in the late jurassic between about 160 and 145 ma in an effort to further examine this issue we sampled accessible outcrops subcrops of kimberlites associated with high precision u pb perovskite ages in the timiskaming area of ontario canada the 154 9 1 1 ma peddie kimberlite yields a stable normal polarity magnetization that is coaxial within less than 5 of the reverse polarity magnetization of the 157 5 1 2 ma triple b kimberlite the combined 156 ma triple b and peddie pole 75 5 n 189 5 e a95 2 8 lies about midway between igneous poles from north america nearest in age 169 ma moat volcanics and the 146 ma ithaca kimberlites showing that the polar motion was at a relatively steady yet rapid 1 5 myr pace a similar large rapid polar swing has been recognized in the middle to late jurassic apw path for adria africa and iran eurasia suggesting a major mass redistribution one possibility is that slab breakoff and subduction reversal along the western margin of the americas triggered an episode of true polar wander

palaeomagnetism and 40ar 39ar age from a cretaceous volcanic sequence inner mongolia china implications for the field variation during the cretaceous normal superchron abstract an integrated palaeomagnetic and 40ar 39ar dating study was carried out on an early cretaceous volcanic lava sequence from the suhongtu section inner mongolia to determine the field behavior within the cretaceous normal superchron cns 40ar 39ar ages were obtained from 12 lava flows indicating that the studied lava was formed around 114 1 0 3 ma for the lower interval and 110 6 0 1 ma for the upper interval rock magnetic experiments and electron microprobe analyses indicate that the primary fe ti oxides are the main magnetic carriers all lava flows carry normal palaeomagnetic directions which can be grouped into 31 units by an f test with a fisher mean characteristic remanent magnetization chrm of d i 12 8 58 6 95 2 3 the corresponding palaeomagnetic pole is located at 80 3 n and 200 3 e a95 3 2 k 64 4 which is indistinguishable at the 95 confidence level from the eurasia pole derived from the apparent polar wander path for the early cretaceous using a modified thellier palaeointensity method with stringent acceptance criteria we obtained two time series of palaeointensity records from 15 independent palaeomagnetic units total 136 samples the virtual dipole moment vdm values varied from 2 53 1022 am2 to 9 92 1022 am2 the mean vdms for the upper and lower intervals are 5 38 2 06 1022 am2 and 4 61 2 67 1022 am2 respectively the observed time series of palaeointensity together with the previously available data for the cns suggest that magnetic field strength during the cns fluctuated significantly with time

tracking magma dynamics by laser ablation la icpms trace element analysis of glass in volcanic ash the 1995 activity of mt etna 1 glass fragments in tephra erupted at mt etna from may to december 1995 have been analyzed by laser ablation icpms the trace element compositional variability of ashes deposited during this interval reveals the presence of discrete magma batches with different crystallization degrees in the shallow plumbing system from may to october a highly crystalline magma is predominant within the conduit with only minor sporadic input of fresh and more primitive magma batches after october new and less evolved magma batches become more prevalent and become progressively homogenized within more evolved resident magma in december ashes closely match the chemistry of the volcanics subsequently erupted till february 1996 this study demonstrates that the trace element characterization of ashes has important implications for volcanic monitoring and is a useful tool for the forecasting of paroxysmal events at mt etna

a tale of amalgamation of three permo triassic collage systems in central asia oroclines sutures and terminal accretion the central asian orogenic belt records the accretion and convergence of three collage systems that were finally rotated into two major oroclines the mongolia collage system was a long n s oriented composite ribbon that was rotated to its current orientation when the mongol okhotsk orocline was formed the components of the kazakhstan collage system were welded together into a long single composite arc that was bent to form the kazakhstan orocline the cratons of tarim and north china were united and sutured by the beishan orogen which terminated with formation of the solonker suture in northern china all components of the three collage systems were generated by the neoproterozoic and were amalgamated in the permo triassic the central asian orogenic belt evolved by multiple convergence and accretion of many orogenic components during multiple phases of amalgamation followed by two phases of orocline rotation

new late jurassic to early cretaceous paleomagnetic results from north china and southern mongolia and their implications for the evolution of the mongol okhotsk suture

the drift history of adria and africa from 280 ma to present jurassic true polar wander and zonal climate control on tethyan sedimentary facies article i nfo the orogenic belts surrounding the undeformed adriatic sea represent the margins of an area known as adria the african promontory we have undertaken a critical appraisal of paleomagnetic data from regions of adria considered parautochthonous relative to africa obtained either from biostratigraphically dated sed imentary rocks corrected for inclination shallowing or from igneous rocks that are regarded as free from any inclination shallowing bias paleomagnetic directions were used to calculate paleomagnetic poles for compar ison with coeval and inclination flattening free paleomagnetic poles from stable africa visual coherence of paleopoles for several time slices from the early permian to the eocene supports the construction of a com posite apparent polar wander path apwp valid for parautochthonous adria and stable africa this compos ite apwp is compared to previous apwps finding good agreement with the global apwp of kent and irving 2010 both apwps show a remarkable and rapid polar shift of 40 in the jurassic that other apwps tend to underestimate we interpret this shift to represent a major episode of true polar wander tpw from 183 ma in the early jurassic to 151 ma in the late jurassic using a simple zonal climate model the drift motion of adria attached to africa appears to be consistent with the distribution of permian cretaceous sed imentary facies on adria

anomalous late jurassic motion of the pacific plate with implications for true polar wander abstract true polar wander or tpw is the rotation of the entire mantle crust system about an equatorial axis that results in a coherent velocity contribution for all lithospheric plates one of the most recent candidate tpw events consists of a 30 rotation during late jurassic time 160 145 ma however existing paleomagnetic documentation of this event derives exclusively from continents which compose less than 50 of the earth s surface area and may not reflect motion of the entire mantle crust system additional paleopositional information from the pacific basin would significantly enhance coverage of the earth s surface and allow more rigorous testing for the occurrence of tpw we perform paleomagnetic analyses on core samples from ocean drilling program odp site 801b which were taken from the oldest available pacific crust to determine its paleolatitude during the late jurassic and early cretaceous 167 133 ma we find that the pacific plate underwent a steady southward drift of 0 49 0 74 my 1 except for an interval between kimmeridgian and tithonian time 157 147 ma during which it underwent northward motion at 1 45 0 76 my 1 1 this trajectory indicates that the plates of the pacific basin participated in the same large amplitude 30 rotation as continental lithosphere in the 160 145 ma interval such coherent motion of a large majority of the earth s surface strongly supports the occurrence of tpw suggesting that a combination of subducting slabs and rising mantle plumes was sufficient to significantly perturb the earth s inertia tensor in the late jurassic

testing the occurrence of late jurassic true polar wander using the la negra volcanics of northern chile abstract true polar wander tpw is the reorientation of the crust mantle system driven by the redistribution of masses in the mantle and on the earth s surface in the ideal case characterization of tpw requires paleomagnetic constraints on the motion of all major plates and independent reconstructions of relative plate positions while such complete datasets are absent for pre mesozoic tpw inferences due to the absence of oceanic plates they are available for the late jurassic 165 145 ma monster shift a 30 amplitude proposed tpw event here we perform paleomagnetic sampling and ar ar geochronology on the la negra volcanics of northern chile producing two new paleomagnetic poles with ages 165 8 1 8 ma 1 84 3 n 0 9 e 95 7 6 n 28 and 152 8 0 8 ma 84 5 n 256 4 e 95 10 8 n 18 by combining these data with other recently published results we compute a net lithospheric rotation of 25 3 7 3 1 at a mean rate of 1 21 0 35 my 1 between 170 and 145 ma with a peak rate of 1 46 0 65 my 1 between 160 and 145 ma these rates are consistent with inferences from the pacific plate implying true whole lithosphere rotation given coherent motion involving the entire lithosphere we conclude that the earth underwent rapid tpw between approximately 165 and 145 ma potentially driven by the cessation of subduction along the western north american margin

paleomagnetism of paleozoic and mesozoic sediments from the southern margin of mongol okhotsk ocean far eastern russia 1 this paper presents new data from paleomagnetic investigations of middle paleozoic early middle late devonian early carboniferous and middle jurassic geological units from the amuria block south of the mongol okhotsk suture zone with fold tests for all localities and two polarities for the devonian our new results constrain the evolution of the mongol okhotsk ocean from the devonian to the middle jurassic the corresponding paleopoles lie at 21 6 n 6 3 e dp dm 5 3 10 6 for early devonian 26 3 n 345 3 e dp dm 6 4 12 8 for early middle devonian 24 6 n 12 9 e dp dm 8 7 16 9 for middle late devonian 40 5 n 352 4 e dp dm 9 16 7 for late devonian 39 8 n 31 6 e dp dm 10 15 5 for early carboniferous and 46 n 37 9 e dp dm 9 4 13 4 for middle jurassic the poles confirm that the large paleozoic mongol okhotsk ocean closed during the jurassic ending up in the late jurassic or early cretaceous at the eastern end of the suture zone as originally proposed on geological grounds the new paleomagnetic results exhibit large tectonic rotations about local vertical axes which we interpret as probably arising from both the collision process and left lateral shear movement along the suture zone related to incipient extrusion of amuria due to indentation of india into asia

further paleomagnetic results from the 155 ma tiaojishan formation yanshan belt north china and their implications for the tectonic evolution of the mongol okhotsk suture abstract a new paleomagnetic study on well dated 155 ma volcanic rocks of the tiaojishan formation fm in the northern margin of the north china block ncb has been carried out a total of 194 samples were collected from 26 sites in the yanshan belt areas of luanping beipiao and shouwangfen all samples were subjected to stepwise thermal demagnetization after removal of a recent geomagnetic field viscous component a stable high temperature component htc was isolated the inclinations of our new data are significantly steeper than those previously published from the tiaojishan fm in the chengde area pei et al 2011 tectonophysics 510 370 380 our analyses demonstrate that the paleomagnetic directions obtained from each sampled area were strongly biased by paleosecular variation psv but the psv can be averaged out by combining all the virtual geomagnetic poles vgps from the tiaojishan fm in the region the mean pole at 69 6 n 203 0 e a95 5 6 passes a reversal test and regional tilting test at 95 confidence and is thus considered as a primary paleomagnetic record this newly determined pole of the tiaojishan fm is consistent with available late jurassic poles from red beds in the southern part of the ncb but they are incompatible with coeval poles of siberia and the reference pole of eurasia indicating that convergence between siberia and the ncb had not yet ended by 155 ma our calculation shows a 1600 km latitudinal plate movement and crustal shortening between the siberia and ncb after 155 ma in addition no significant vertical axis rotation was found either between our sampled areas or between the yanshan belt and the major part of the ncb after 155 ma

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

jurassic cretaceous low paleolatitudes from the circum black sea region crimea and pontides due to true polar wander article in a recent study paleomagnetic and paleoenvironmental data from adria as part of the african plate suggest a trend toward much lower 15 latitudes from early jurassic to earliest cretaceous at the position of adria than suggested by the apparent polar wander apw paths the smoothing of existing apw paths has most likely caused this low latitude episode to be overlooked in this study we test if the low paleolatitudes in the jurassic to early cretaceous can also be found in eurasia i e crimea ukraine and the pontides turkey that are situated in the present day black sea region our eurasian data suggest the same low late jurassic to early cretaceous paleolatitudes as shown for africa the jurassic to lower cretaceous time span is characterized by tethys subduction between the african and eurasian continents and these subduction zones likely functioned as an anchor in the mantle therefore we regard it unlikely that both the african and eurasian plates moved by n1500 km south and subsequently north with respect to the mantle as suggested by the paleomagnetic results true polar wander tpw provides a mechanism that rotates the earth s crust and mantle with respect to its core and it was recently quantified the period from 195 135 ma early jurassic to earliest cretaceous is subject to clockwise tpw which could well explain our results we conclude that tpw rather than plate tectonics is the cause of low late jurassic to early cretaceous african and eurasian paleolatitudes in the eastern mediterranean area

two phase southward subduction of the mongol okhotsk oceanic plate constrained by permian jurassic granitoids in the erguna and xing an massifs ne china abstract geodynamics of the mongol okhotsk oceanic plate southward subduction are still pending problems this paper presents new zircon la icp ms u pb age and whole rock geochemical data for the middle permian to middle jurassic granitoids in the western erguna and central xing an massifs 267 264 ma 241 ma and 173 ma i type granites and 216 ma a type granites were identified in the erguna and xing an massifs ne china the i type granites were produced by partial melting of the lower mafic crust the 216 ma a type granites were derived from partial melting of crustal materials with tonalitic to granodioritic compositions the 267 264 ma and 241 ma i type granites were generated in an andean type arc setting wheras the 216 ma a type and 173 ma granites were formed in supra subduction extensional setting we summarized previous age data of the middle permian to middle jurassic magmtaic rocks in the erguna and xing an massifs and identified two isolated phases of magmatic activity including the ca 267 225 ma and ca 215 165 ma periods with a significant magmatic gap at ca 225 215 ma these middle permian to middle jurassic magmatic rocks are closely related to the southward subduction of the mongol okhotsk ocean a two stage tectonic evolutionary model was proposed to account for these geological observations in the erguna and xing an massifs involving permian to middle triassic continuous southward subduction of the mongol okhotsk oceanic plate and late triassic to jurassic slab rollback and supra subduction extension

timing and evolution of jurassic cretaceous granitoid magmatisms in the mongol okhotsk belt and adjacent areas ne asia implications for transition from contractional crustal thickening to extensional thinning and geodynamic settings abstract the mongol okhotsk belt and adjacent areas are key areas to study the relationship between the okhotsk and paleo pacific tectonic regimes and their superposition on the older paleo asian regimes during late mesozoic times this paper summarizes the spatial temporal evolution of late mesozoic jurassic cretaceous granitoids and related intrusions in these areas and interprets the magmatic evolution in terms of a transition from contractional crustal thickening to extensional thinning according to 407 published zircon ages these granitoids were mainly emplaced during the intervals 200 180 ma 180 165 ma 165 145 ma 145 135 ma and 135 100 ma jurassic granitoids 200 145 ma predominately occur in the baikal ne mongolia bnem and great xing an range early cretaceous 145 100 ma granitoids are mainly occur in the great xing an range and display a southward younging migration significantly early cretaceous granitoids also extend into the trans baikal area across the mongol okhotsk suture far away from the paleo pacific plate margin in ne china thus they were more plausibly related to post orogenic collapse of the mongol okhotsk orogen from the late jurassic to early cretaceous the granitoids evolved compositionally from calc alkaline and high k calc alkaline i type with some adakite like features to high k calc alkaline and shoshonitic highly fractionated i transitional i a or a type characterized by a significant decrease in their sr y ratios this evolution coincided with a tectonic transition from contractional crustal thickening to extensional thinning combined with regional geology we speculate that the jurassic granitoids were likely derived from melting of the deep seated thickened lower continental crustal lcc sources whereas the cretaceous granitoids produced through crustal melting from an extensional thinning setting our results provide a case study demonstrating that the petrogenesis of granitic magmatism was closely associated with crustal tectonics early jurassic granitoids along the okhotsk belt formed in a subduction collision setting related to closure of the mongol okhotsk ocean whereas late jurassic granitoids in the great xing an range and in the northern north china craton may have formed in a syn or post collisional setting superposed by far field affects of subduction of the paleo pacific plate early cretaceous granitoids in these areas formed in response to post orogenic extensional collapse of the mongol okhotsk belt coupled with back arc extension related to paleo pacific plate subduction

timing of the north south china collision from new middle to late mesozoic paleomagnetic data from the north china block we collected paleomagnetic samples from 43 sites of middle and upper jurassic and cretaceous rocks along a 250 km wide east west trending belt located north of the suture between the north and south china blocks in each case laboratory treatment isolates dual polarity antipodal magnetizations that are demonstrably pre folding the paleomagnetic poles show very good coherence with coeval poles from other localities from the north china block ncb suggesting the apparent polar wander path apwp from the ncb is not distorted by post middle jurassic local vertical axis block rotations a comparison of the ncb apwp with that from the south china block suggests that collision continued after the early jurassic and ended no later than the late jurassic a cusp in the ncb apwp coincides with a marked angular unconformity that separates middle jurassic from upper jurassic and younger sediments together these observations indicate that the scissor like collision and associated deformation ceased at the middle to late jurassic boundary 159 ma the time of rapid 1 ma relative angular velocity between the two plates 225 to 190 ma coincides with a peak in u pb and ar ar dates obtained from metamorphic rocks in the suture the combined late jurassic and cretaceous paleomagnetic data from both blocks suggest that the united china plate traveled 2000 km north relative to eurasia during the late jurassic before docking in the cretaceous this younger collision event is also marked by a cusp on the cretaceous part of the eurasian apwp

a record of the jurassic massive plate shift from the garedu formation of central iran modern generations of apparent polar wander paths apwps show the occurrence in north american and african coordinates of a major and rapid shift in pole position plate shift during the middle to late jurassic 175 145 ma that alternative curves from the literature tend to underestimate this jurassic massive polar shift jmps of vast and as yet unexplored paleogeographic implications is also predicted for eurasia from the north atlantic plate circuit but jurassic data from this continent are scanty and problematic here we present paleomagnetic data from the kimmeridgian tithonian upper jurassic garedu formation of iran which was part of eurasia since the triassic paleomagnetic component directions of primary pre folding age indicate a paleolatitude of deposition that is in excellent agreement with the latitude drop predicted for iran from apwps incorporating the jmps moreover we show that paleolatitudes calculated from these apwps used in conjunction with simple zonal climate belts better explain the overall stratigraphic evolution of iran during the mesozoic as iran drifted from the tropical arid belt to the mid latitude humid belt in the late triassic carbonate platform productivity stopped while widespread coal bearing sedimentation started whereas as iran returned to arid tropical latitudes during the jmps carbonate platform productivity and evaporitic sedimentation resumed these results illustrate 1 the potent but often neglected control that plate motion continental drift and or true polar wander across zonal climate belts exerts on the genesis of sedimentary facies and 2 the importance of precisely controlled paleogeographic reconstructions for tectonic interpretations especially during times of fast plate motion like the jurassic as a suggestion for future research we predict that the adoption of eurasian reference paleopoles incorporating the jmps may lead to a reconciliation or reinterpretation of existing geologic and paleomagnetic data regarding the deformation history of central asia

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

inclination flattening and the geocentric axial dipole hypothesis abstract william gilbert first articulated what has come to be known as the geocentric axial dipole hypothesis the gad hypothesis is the principle on which paleogeographic reconstructions rely to constrain paleolatitude for decades there have been calls for permanent non dipole contributions to the time averaged field recently these have demanded large contributions of the axial octupole which if valid would call into question the general utility of the gad hypothesis in the process of geological recording of the geomagnetic field earth filters distort the directions many processes for example sedimentary inclination flattening and random tilting can lead to a net shallowing of the observed direction therefore inclinations that are shallower than expected from gad can be explained by recording biases northward transport or non dipole geomagnetic fields using paleomagnetic data from the last 5 million years from well constrained lava flow data allows the construction of a statistical geomagnetic field model such a model can predict not only the average expected direction for a given latitude but also the shape of the distribution of directions produced by secular variation the elongation of predicted directions varies as a function of latitude from significantly elongate in the up down direction at the equator to circularly symmetric at the poles sedimentary inclination flattening also works in a predictable manner producing elongations that are stretched side to side and the degree of flattening depending on the inclination of the applied field and a flattening factor f the twin tools of the predicted elongation inclination relationship characteristic of the geomagnetic field for the past 5 million years and the distortion of the directions predicted from sedimentary inclination flattening allows us to find the flattening factor that yields corrected directions with an elongation and average inclination consistent with the statistical field model the method can be tested using sediments deposited in a known field application of the elongation inclination correction method to two magnetostratigraphic data sets from red beds in asia and pakistan brings the inclinations into agreement with those predicted from modern gps measurements and from global paleomagnetic data there appears to be no compelling reason at this time to abandon the geocentric dipole hypothesis which has provided such an excellent working model for so long

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

deep mantle structure as a reference frame for movements in and on the earth earth s residual geoid is dominated by a degree 2 mode with elevated regions above large low shear wave velocity provinces on the core mantle boundary beneath africa and the pacific the edges of these deep mantle bodies when projected radially to the earth s surface correlate with the reconstructed positions of large igneous provinces and kimberlites since pangea formed about 320 million years ago using this surface to core mantle boundary correlation to locate continents in longitude and a novel iterative approach for defining a paleomagnetic reference frame corrected for true polar wander we have developed a model for absolute plate motion back to earliest paleozoic time 540 ma for the paleozoic we have identified six phases of slow oscillatory true polar wander during which the earth s axis of minimum moment of inertia was similar to that of mesozoic times the rates of paleozoic true polar wander 1 my are compatible with those in the mesozoic but absolute plate velocities are on average twice as high our reconstructions generate geologically plausible scenarios with large igneous provinces and kimberlites sourced from the margins of the large low shear wave velocity provinces as in mesozoic and cenozoic times this absolute kinematic model suggests that a degree 2 convection mode within the earth s mantle may have operated throughout the entire phanerozoic

a paleomagnetic study from the late jurassic volcanics 155 ma north china implications for the width of mongol okhotsk ocean abstract a new paleomagnetic study has been carried out on the tiaojishan formation effusives in the chengde area north china block ncb the age of the tiaojishan formation volcanics was recently dated from 153 ma to 161 ma by u pb and 40 ar 39 ar a total of 154 samples from 16 sites within the tiaojishan formation were collected detailed stepwise thermal demagnetization allows us to isolate two components from 77 oriented drill core samples a low temperature component identified in most samples falls close to the present local earth field direction characteristic remanent directions obtained from high temperature pass reversal test yield a pole position at 240 3 e 59 9 n with 95 6 8 combined with coeval paleopoles obtained from the siberia block sib these results show a significant paleolatitude displacement between the ncb and sib which indicates that the mongol okhotsk ocean was still open with 3000 km width in its eastern part at ca 155 ma

new paleomagnetic results from northern china collision and suturing with siberia and kazakhstan abstract we report new results from our paleomagnetic studies of the inner mongolia autonomous region in northeastern china and the xinjiang autonomous region in northwestern china primary magnetizations are isolated in tertiary cretaceous late jurassic and late permian rocks and secondary components of magnetization of late jurassic and recent ages are seen in strata of inner mongolia western junggar basin and part of the tarim block suffered widespread complete tectono thermal remagnetization at about 280 ma permian the paleomagnetic evidence suggests that the sampled areas in inner mongolia have been part of the north china block at least since late permian time and the junggar block collided with the tarim block by late permian time our own paleomagnetic results together with available data from other parts of eastern eurasia suggest a tectonic model in which kazakhstan europe and siberia collided with each other and formed the europe siberia kazakhstan plate eur sib kaz in late permian time to the east mongolia outer mongolia inner mongolia and the north china block were also wedged together forming the mongolia north china plate mon nc by this time these two composite continental plates were not a stable unit in the late permian the geometrical relationship of the paleomagnetic poles for these two plates suggests that collision initially occurred in the late permian near the western end 50 n 75 e of the southern siberian fold belt and progressed eastward as the mon nc plate rotated and clockwise about 117 relative to the eur sib kaz plate during the early mesozoic suturing and rotation were probably complete by the late jurassic significant relative motion between the tarim and siberia blocks continued from early triassic to late jurassic as tarim moved northeastward 1400 km relative to siberia

paleomagnetic constraints on the paleogeography of the east asian blocks during late paleozoic and early mesozoic times abstract although the axial and dominant geocentric character of the earth s magnetic field means that paleolongitude is indeterminate paleomagnetism is otherwise the only truly quantitative method available to the earth scientist for constraining paleogeography and reconstructing the kinematic evolution of continental blocks during the past half century numerous paleomagnetic results have provided substantial quantitative constraints on the origin and tectonic evolution of the major tectonic divisions now comprising eastern asian in this paper we first assess the available early paleozoic to early cretaceous paleomagnetic results from the south china north china and tarim blocks using internationally recognized reliability criteria we then construct a running mean through a 20 ma window by weighting the poles according to their q factors and fit a spherical spline with smoothing factor of 300 to derive apparent polar wander apw paths for these three major blocks during this early paleozoic to late mesozoic time interval together with late paleozoic to early mesozoic paleomagnetic poles from the qaidam qiangtang lhasa sibumasu indochina and some other smaller blocks terranes of east asia we produce a series of paleogeographic reconstructions for these major blocks and lesser terranes of east asia between mid ordovician and late jurassic times 460 160 ma which although based primarily upon paleomagnetic evidence aim to embrace the geological constraints finally we discuss the current evidence for closure times of the paleo asian mianlue and east paleo tethyan oceans which incorporate fundamental issues concerning the formation of the east asian continental collage and collision with the northern main body of the pangean supercontinent we use the collective evidence to argue that these major paleo oceans had closed by the late triassic and that the east asian sector of the supercontinent had united to become an integral part of pangea by that time 220 ma

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

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

the effect of variable thermal diffusivity on kinematic models of subduction 1 we examine the influence of variable thermal properties on the thermal state of a subducting slab in the upper mantle and the transition zone by combining a kinematic slab model with models of thermal conductivity thermal diffusivity and conductivity models for major mantle minerals in the mgo sio2 system are developed based on experimental measurements on these minerals at high pressure and temperature the models show significantly higher thermal conductivity for stishovite and clinopyroxene compared to the mg2sio4 polymorphs and majorite garnet in our subduction model we consider scenarios with a differentiated slab basalt harzburgite pyrolite in a pyrolite mantle and uniform composition for both the slab and the mantle pyrolite or a pure mg2sio4 based system the role of highly conductive pyroxene is examined by taking it into account in some models and replacing it in others with majorite garnet this choice has a strong influence on the thermal state of the slab shifting the depth of the 1000 k temperature anomaly by as much as 100 km this is caused by faster cooling of the plate at the surface and thermal insulating effects once subducted temperature differences between models with variable thermal diffusivity and those with constant parameters can reach 125 k i e 10 of total thermal anomalies of the slab relative to an average geotherm taking into account the stable mineral phases we evaluate density variations between different models and find that variable thermal diffusivity results in a modest increase of negative buoyancy of the slab