crustal deformation along the altyn tagh fault system western china from gps we collected gps data from the southern tarim basin the qaidam basin and the western kunlun shan region between 1993 and 1998 to determine crustal deformation along the altyn tagh fault system at the northern margin of the tibetan plateau we conclude from these data that the altyn tagh is a left lateral strike slip fault with a current slip rate of 9 mm yr in sharp contrast with geological estimates of 20 30 mm yr this contrast poses an enigma because the gps data cover a wider region than the geologic data they might be expected to reveal somewhat more slip we also find that the tarim and qaidam basins behave as rigid blocks within the uncertainty of our measurements rotating clockwise at a rate of 11 and 4 5 nrad yr respectively with respect to the eurasia plate the rotation of the tarim basin causes convergence across the tian shan increasing progressively westward from 6 mm yr at 87 e to 18 mm yr at 77 e our data and other gps data suggest that the indo asia collision is mainly accommodated by crustal shortening along the main himalayan thrust system 53 and the tian shan contractional belt 19 eastward extrusion of the tibetan plateau along the altyn tagh and kunlun faults accommodates only 23 of the indo asia convergence

late cretaceous early palaeogene tectonic development of se asia abstract the late cretaceous early palaeogene history of the continental core of se asia sundaland marks the time prior to collision of india with asia when se asia from the tethys in the west to the palaeo pacific in the east lay in the upper plate of subduction zones in myanmar and sumatra subduction was interrupted in the aptian albian by a phase of arc accretion woyla and mawgyi arcs and in java eastern borneo and western sulawesi by collision of continental fragments rifted from northern australia subsequent resumption of subduction in the myanmar thailand sector explains 1 early creation of oceanic crust in the andaman sea in a supra subduction zone setting 95 ma 2 the belt of granite plutons of late cretaceous early palaeogene age starting 88 ma in western thailand and central myanmar and 3 amphibolite grade metamorphism between 70 and 80 ma seen in gneissic outcrops in western and central thailand and 4 accretionary prism development in the western belt of myanmar until glancing collision with the ne corner of greater india promoted ophiolite obduction deformation and exhumation of marine sediments in the early palaeogene the ranong strike slip fault and other less well documented faults were episodically active during the late cretaceous palaeogene time n to nw directed subduction of the palaeo pacific ocean below southern china vietnam and borneo created a major magmatic arc associated with rift basins metamorphic core complexes and strike slip deformation which continued into the late cretaceous the origin and timing of termination of subduction has recently been explained by collision of a large luconia continental fragment either during the late cretaceous or palaeogene evidence for such a collision is absent from the south china sea well and seismic reflection record and here collision is discounted instead relocation of the subducting margin further west possibly in response of back arc extension which created the proto south china sea is preferred lying between the two subduction related arcs the khorat basin is of predominantly late jurassic early cretaceous age but stratigraphic and apatite fission track data also indicates deposition of 1 2 km of late cretaceous sediments the synformal basin geometry probably arose due to the dynamic topography created by converging tethyan and palaeo pacific subduction zones the aptian albian slowing of basin subsidence and onset of evaporite deposition coincides with collision of the mawgyi and woyla island arcs extensive palaeogene deformation and exhumation 3 km in places affected all margins of the khorat plateau deformation includes folds of the phu phan uplift and strike slip faults thrusts and folds on the southern and eastern margins south of the khorat plateau outcrop and seismic reflection data from the ton le sap basin cambodia and the gulf of thailand indicate syn depositional fault controlled subsidence was important during cretaceous deposition the hot thickened crust developed during the late cretaceous palaeogene events follows the weak indosinian crustal scale inthanon and sukhothai zones which persistently guided the location of later structures including cenozoic extensional and post rift basins and influenced the widespread occurrence of low angle normal faults metamorphic core complexes and eastern gulf of thailand super deep post rift basins

crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

intraplate extension prior to continental extrusion along the ailao shan red river shear zone left lateral movement of the ailao shan red river shear zone lends support to the hypothesis of continental extrusion resulting from the collision of india with asia our new observations from northwestern yunnan china and northwestern vietnam on different sides of the shear zone demonstrate that the sinistral offset was 600 km according to correlations of permian triassic flood basalt successions and late paleogene highly potassic mafic magmas we conclude that the shear was propagating on the south china continental margin and does not correspond to a suture between south china and indochina furthermore the highly potassic magmas were emplaced from ca 40 to 30 ma before the shear movement which was caused by the late oligocene to early miocene ca 27 22 ma extrusion activity this suggests that a late eocene to early oligocene intraplate extension possibly induced by delamination of thickened continental lithosphere took place in northwestern yunnan or eastern tibet as a response to the india asia collision this extension and sea floor spreading of the south china sea that began ca 30 ma could have accounted for the initiation of the ailao shan red river shear zone

miocene jiali faulting and its implications for tibetan tectonic evolution the karakoram jiali fault zone kjfz comprises a series of right lateral shear zones that southerly bound the eastward extrusion of northern tibet relative to india and stable eurasia here we present new 40 ar 39 ar age data from the puqu and parlung faults two easternmost branches of the jiali fault zone which indicate a main phase of the kjfz shearing from v18 to 12 ma thus the tibetan eastward extrusion bounded by principal strike slip fault zones started and was probably most active around the middle miocene an interval marked also by active east west extension in southern tibet the coincidence of these two tectonic events strongly suggests a common causal mechanism which is best explained as oblique convergence between india and asia under the framework of this mechanism the extension in southern tibet is not a proxy for the plateau uplift the kjfz activity was furthermore coincident with right lateral displacements along the gaoligong and sagaing faults in southeast asia this defines a miocene deformation record for the regional dextral accommodation zone that in response to the continuing india asia collision may have accounted for the initiation and prolonged history of clockwise rotation of the tibetan extrusion around the eastern himalayan syntaxis 2002 elsevier science b v all rights reserved

the western ailaoshan volcanic belts and their se asia connection a new tectonic model for the eastern indochina block the western ailaoshan wal volcanic belts mark the western boundary of the south china indochina suture zone along the ailaoshan fold belt geochronological and geochemical evidence suggests that the wal volcanic belts vary in ages and origins and includes 1 l carboniferous earliest m permian ca 300 270 ma incipient backarc basin magmatism 2 m permian ca 265 ma island arc mature backarc basin magmatism 3 early l permian ca 260 255 ma wal indochina arc continent collision and the associated syn collisional granitic magmatism as well as subsequent 4 e triassic ca 250 245 ma post collisional magmatism regional geological comparison indicates that the post l carboniferous wal tectonic evolution was closely related to the eastern indochina block we present a new g plate geodynamic evolution model for the ailaoshan fold belt and its surrounding south china indochina region based on new and published u pb zircon ages together with whole rock and pb isotope geochemical data

the red river fault system in the tonkin gulf vietnam abstract the red river fault system in the tonkin gulf offshore haiphong was studied using seismic profiles calibrated by deep wells well characterised left lateral strike slip occurred continuously within a narrow 30 km wide zone southwest of the vinh minh fault between 30 ma and 5 5 ma however the corresponding amount of offset probably does not exceed a few tens of kilometres no sign of post 5 5 ma right lateral motion can be detected prior to 30 ma there is a widespread extension in a wider 100 km wide zone which could be related to a significant amount of left lateral motion the motion of the fault splays to the northeast of the vinh minh fault prior to 30 ma was absorbed in the rifting of the gulf of beibu a 15 5 ma unconformity separates the transtensional regime from a later transpresional regime this 15 5 ma date coincides with the cessation of sea floor spreading in the south china sea

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

tectonics of the western margin of the shan plateau central myanmar implication for the india indochina oblique convergence since the oligocene abstract we present new data and interpretations on the neogene tectonics of the shan scarp area central myanmar and its relationship with the india indochina oblique convergence we describe ductile and brittle fabrics associated with the major features in this area the mogok metamorphic belt mmb the shan scarp and the sagaing fault from these data we identify a succession of two tectonic regimes first a dominant nnw sse trending extension marked by ductile stretching that characterizes the mmb and associated n70e brittle normal faults later from middle or upper miocene to the present these structures were cross cut by brittle right lateral faults among which the most important are the n20w transpressive shan scarp fault zone and the n s sagaing fault to explain this transition from a dominant transtensive to a transpressive stress regime that occurred during miocene we place our data within a larger geodynamic context we suggest that like the intraplate deformation in the indian ocean the end of spreading in the south china sea the opening of the andaman basin or the end of subduction within the indo burma range the change in the tectonic regime in central myanmar could be in response to a major miocene regional plate kinematic reorganization

structural petrological and thermal evolution of a tertiary ductile strike slip shear zone diancang shan yunnan the diancang shan a horst massif within the red river fault zone in yunnan people s republic of china preserves a structural petrological and thermal record of two distinct phases of tectonic activity a left lateral ductile shear that terminated between 20 and 17 ma and a ductile to brittle phase of normal faulting which began at 4 7 ma and remains active mylonitic rocks in the core of the range display an early steep high temperature ht schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt kinematic indicators indicate that shear was left lateral the complex shape of the ht schistosity at the southern termination of the massif likely results from a large scale oblique left lateral c shear plane that dismembered the shear zone and separated the ailao shan and the diancang shan as left lateral deformation terminated thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event along the eastern edge of the diancang shan the ht fabrics were overprinted by low temperature structures during activation of east dipping normal faults cooling associated with this normal right lateral faulting along the diancang shan and perhaps activation of the right lateral normal movement on the range front fault farther south along the ailao shan began at 4 7 0 1 ma these results tend to support the view that extrusion of indochina occurred along the left lateral red river shear zone between 35 and 19 17 ma initiation of right lateral normal slip during the late miocene may relate to eastward extensional collapse of the thickened tibetan crust or more probably to initiation of the second phase of extrusion

structural evolution of the day nui con voi metamorphic complex implications on the development of the red river shear zone northern vietnam the day nui con voi dncv metamorphic complex in north vietnam is the southernmost high grade metamorphic zone along the nw se trending red river shear zone rrsz in indochina the rrsz was considered as a classical large scale continental strike slip fault that had played a significant role in the continental extrusion of southeast asia since the collision of india and eurasia earlier ideas determined the rrsz as a steep shear zone that penetrated the entire lithosphere both metamorphism and structures within rocks along the dncv metamorphic complex have been previously thought to be formed syn tectonically by left lateral shearing of the rrsz during the oligocene miocene continental escape tectonics however our meso and microstructural re examination of this region shows that these metamorphic rocks were formed during earlier tectonic episodes unrelated to strike slip shearing high angle to near orthogonal overprinting fabrics indicated that this region recorded three episodes of ductile deformation followed by brittle faulting events with different intensity spanning from the triassic to the tertiary d1 is preserved as nw se striking upright folds under garnet grade regional metamorphism during the triassic indosinian orogeny as south china block amalgamated with the indochina block the large scale horizontal d2 folds with a dominant top to n nw bottom to s sw sense of shear and sub horizontal fold axial planes suggest that the dncv metamorphic complex remained at midcrustal depths since the indosinian orogeny the youngest ductile deformation event d3 refolded d2 recumbent folds into a dome and uplifted the dncv as lower temperature fabrics s3 indicated steep mylonite zones with left lateral kinematic indicators and brittle faulting were developed on both limbs of the dome along the steep song hong and song chay faults during left lateral movement of the rrsz our new spatial temporal and kinematic correlations of metamorphic fabrics within the dncv metamorphic complex support the suggestion that the rrsz developed after regional metamorphism and remained purely a crustal fault the complicated deformation history recorded within the dncv metamorphic complex provides an alternative interpretation and suggests that crustal scale strike slip faults such as the rrsz need not root from the mantle

thermodynamics of folding in the middle to lower crust we show that dissipative processes involving thermal mechanical coupling produce fold systems with characteristics observed in the middle to lower crust these comprise folds in layers with little strength contrast scale invariance no strict periodicity small wavelength thickness ratios nonconcentric fold shapes and axial plane crenulations this contrasts with classical folding theory that emphasizes large mechanical contrasts between a weak matrix and strong embedded layers leading to periodic concentric fold systems with no formal link to axial plane structures

synkinematic magmatism heterogeneous deformation and progressive strain localization in a strike slip shear zone the case of the right lateral karakorum fault 1 new structural observations coupled with 15 u pb and 24 ar ar new ages from the karakorum shear zone ksz constrain the timing and slip rate of the right lateral karakorum fault zone kfz one of the great continental asian strike slip faults in the tangtse darbuk area the tangtse sw and muglib ne mylonitic strands of the ksz frame the less deformed pangong range inherited u pb ages show that granitic protoliths are mostly from the karakorum and ladakh batoliths with a major miocene melting event lasting from 21 5 to 13 5 ma some of the miocene granitic bodies show structural evidence for intrusion synkinematic to the ksz the oldest of these granitoids is 18 8 0 4 ma old implying that deformation started prior to 19 ma microstructural data show that right lateral deformation pursued during cooling ar ar data show that ductile deformation stopped earlier in the tangtse 11 ma than in the muglib strand 7 ma deformation ended at 11 ma in the tangtse strand while it is still active in the muglib strand suggesting a progressive localization of deformation when merged with published observations along the kfz these data suggest that the kfz nucleated in the north ayilari range at least 22 ma ago the long term fault rate is 0 84 to 1 3 cm yr considering a total offset of 200 to 240 km the ksz collected magma produced within the shear zone and or deeper in crust for which the producing mechanism stays unclear but was not the lower crustal channel flow

exhumation of the diancang shan metamorphic complex along the ailao shan red river belt southwestern yunnan china evidence from 40ar 39ar thermochronology abstract exhumation of the high grade metamorphic rocks from deep to shallow crustal levels in the diancang shan dcs massif is evident by the temporal transition from shearing and mylonitization at the amphibolite facies in the lower to middle crust through retrograde ductile brittle faulting at greenschist facies in the middle crust and brittle faulting near the surface new 40ar 39ar ages of amphibole muscovite biotite and k feldspar mostly from mylonitic rocks help constrain the thermal evolution of the massif during its exhumation the thermochronologic studies have shown at least three stages of exhumation and cooling from late oligocene to pliocene e g 28 13 ma 13 4 ma and 4 0 ma respectively the initiation of the unroofing history of the dcs massif resulted from ductile left lateral shearing along the ailao shan red river shear zone asrr since the late oligocene diachronous onset and subsequent parallel cooling and exhumation characterize the early thermal history of the massif the first stage of exhumation might have stopped due to the cessation of the left lateral shearing at ca 21 ma although cooling continued until 13 ma ago to relatively low temperature conditions the second and third stages of cooling started from 13 ma and lasted until the recent active faulting the diversity of cooling rates during the second stage from 13 to 4 ma suggests differences in exhumation rates at various localities ductile to brittle shearing along the eastern flank retrograde normal shear zone is the best explanation for the exhumation during this stage the inhomogeneous cooling and exhumation of the lower plate of the normal fault zone possibly resulted from the tilting of the lower plate or differential exhumation of slabs in the lower plate during the third stage cooling and exhumation of the dcs massif since 4 ma ago is recognized when higher temperature cooling paths are extended and plotted at a lower temperature of about 16 c the present day average surface temperature therefore the diachronous cooling and exhumation continuously spanned to the surface the tectonic exhumation of the dcs massif constitutes a part of the tectono thermal evolution and diachronous exhumation of the metamorphic complexes in the fan shaped area bounded broadly in se asia by the red river fault zone and sagaing fault zone since 36 ma extrusion induced strike slip shearing and rotation associated normal faulting played important roles in the diachronous cooling and exhumation of the dcs massif and other metamorphic complexes in se asia

variations in the kinematics of deformation along the zagros inclined transpression zone iran implications for defining a curved inclined transpression zone abstract the combination of inclined collision and plate boundary shape can control the nature of deformation and the sense of shear along a transpression zone the present study investigated the effects of a boundary zone with curvilinear shape along a transpression zone on the kinematics of deformation the kinematics of the zagros transpression zone varies with the orientation of the zone boundary detailed structural and microstructural studies showed sinistral sense of shear on the southeastern part of the zagros inclined transpression zone fars arc but dextral sense of shear on the northwestern part of the zone it is inferred that the both senses of shear were developed coevally under a bulk general shear regional scale deformation along a curved inclined transpression miming the shape of the fras arc of the zagros and the reentrant of the bandar abbas syntaxis the zagros transpression zone formed by inclined continental collision between the afro arabian continent and iranian microcontinent

40ar 39ar dating of the jiali and gaoligong shear zones implications for crustal deformation around the eastern himalayan syntaxis abstract we conducted a comprehensive 40ar 39ar geochronological study of the jiali and gaoligong shear zones to obtain a better understanding of crustal deformation and tectonic evolution around the eastern himalayan syntaxis ehs the new age data reveal that the main phase of deformation in the jiali and gaoligong shear zones occurred from 22 to 11 ma and from 18 to 13 ma respectively structural data collected during this study indicate that the jiali shear zone underwent a change in shear sense from sinistral to dextral during its movement history based on a comparison with the deformation histories of other major shear zones in the region we argue that the initial sinistral motion recorded by the jiali shear zone was coincident with that of the ailao shan red river shear zone which marked the northern boundary of the southeastward extrusion of the indochina block during the early miocene from the middle miocene 18 ma the jiali shear zone changed to dextral displacement becoming linked with the dextral gaoligong shear zone that developed as a consequence of continued northward indentation of the indian continent into asia since this time the jiali and gaoligong shear zones have been united defining the southwestern boundary of the ehs during clockwise rotation of the eastward extruding tibetan block as revealed by recent gps data the temporal change in regional deformation pattern from southeastward block extrusion to clockwise rotation of crustal fragments may have played an important role in the development of the eastern himalayan drainage system around the ehs

kinematics and ar 40 ar 39 geochronology of the gaoligong and chongshan shear systems western yunnan china implications for early oligocene tectonic extrusion of se asia the gaoligong and chongshan shear systems glss and csss in western yunnan china have similar tectonic significance to the ailaoshan red river shear system asrrss during the cenozoic tectonic development of the southeastern tibetan syntaxis to better understand their kinematics and the cenozoic tectonic evolution of se asia this paper presents new kinematic and 40 ar 39 ar geochronological data for these shear systems all the structural and microstructural evidence indicate that the glss is a dextral strike slip shear system while the csss is a sinistral strike slip shear system and both were developed under amphibolite to greenschist grade conditions the 40 ar 39 ar dating of synkinematic minerals revealed that the strike slip shearing on the glss and csss at least began at 32ma possibly coeval with the onset of other major shear systems in se asia the latestage shearing on the glss and csss is dated at 27 29ma by the biotite 40 ar 39 ar ages consistent with that of the wang chao shear zone wcsz but 10ma earlier than that of the asrrss the dextral gaoligong shear zone within the glss may have separated the india plate from the indochina block during early oligocene combined with other data in western yunnan we propose that the baoshan southern indochina block escaped faster southeastward along the csss to the east and the glss to the west than the northern indochina block along the asrrss accompanying with the obliquely northward motion of the india plate during early oligocene 28 36ma during 28 17ma the northern indochina block was rotationally extruded along the asrrss relative to the south china block as a result of continuously impinging of the india plate

structure kinematics and ages of transpression during strain partitioning in the chongshan shear zone western yunnan china abstract the chongshan shear zone extends from the eastern himalayan syntaxis to the lincang granitic pluton in yunnan province china the structure and kinematics show that the shear zone comprises mainly of mylonitic gneiss migmatite and schist with a dextral dominated strike slip motion in an n s trending northern segment and a sinistral strike slip shear in nw se trending middle and southern segments both were developed under a bulk regional scale sinistral transpression shrimp and la icpms u pb and 40 ar 39 ar dating reveal two tertiary magmatic events along the zone followed by younger sinistral strike slip shear the eocene magmatic event c 55 38 ma followed by metamorphism at c 36 ma happened before the strike slip motion the strike slip shear along the zone began c 32 ma which generated shear heating from 32 to 22 ma the 40 ar 39 ar ages of syn kinematic micas range from 19 to 14 ma and indicate that the strike slip shear continued to this time with coeval transpressional exhumation and uplift of the metamorphic rocks along the biluoxueshan chongshan chain the chongshan zone is thus a cenozoic shear zone which was contemporaneous with motion on the left lateral ailao shan red river shear zone and the right lateral gaoligong shear zone and should be important in accommodating the northwards movement of india during collision during oligocene to miocene times the continental block that was extruded between the ailao shan red river and gaoligong shear zones was dismembered into at least two major fragments by the chongshan shear zone

using along strike variation in strain and kinematics to define the movement direction of curved transpressional shear zones an example from northwestern superior province manitoba a review of current understanding of structures in shear zones reveals that correlating the orientation of stretching lineations with movement direction once a common practice based on the simple shear model is not generally applicable to transpressional zones it is difficult to determine the movement direction of a transpressional zone based on internal structures we show that the movement direction can be better defined by considering the variation in strain geometry and kinematics of differently oriented and kinematically related shear zones or different segments of a curved shear zone application of this method to a shear zone in the northwestern superior province of manitoba shows that the movement direction there is approximately north south in contrast to a commonly held view for similarly oriented shear zones in the superior province that the movement direction is approximately east west

the nature of the ailao shan red river asrr shear zone constraints from structural microstructural and fabric analyses of metamorphic rocks from the diancang shan ailao shan and day nui con voi massifs abstract the structural geology timing of shearing and tectonic implications of the asrr shear zone one of the most striking lineaments in southeast asia have been the topics of extensive studies over the past few decades the xuelong shan xls diancang shan dcs ailao shan als and day nui con voi dncv metamorphic massifs along the shear zone have preserved important information on its structural and tectonic evolution our field structural analysis detailed microstructural and fabric analysis as well as the quartz sillimanite and garnet fabric studies of the sheared rocks from the massifs demonstrate the dominant roles of three deformation episodes during cenozoic tectonic evolution in the shear zone among the contrasting structural and microstructural associations in the shear zone d2 structures which were formed at the brittle to ductile transition during large scale left lateral shearing in the second deformation episode predominate over the structural styles of the other two deformation episodes discrete micro shear zones with intensive grain size reduction compose the characteristic structural style of d2 deformation in addition several types of folds early shearing folds f 2 1 and late shearing folds f 2 2 were formed in the sheared rocks including discrete to distributed mylonitic foliation stretching lineation and shear fabrics e g mica fish domino structures as well as sigma and delta fabrics a sequence of microstructures from syn kinematic magmatic flow high temperature solid state deformation to brittle ductile shearing is well preserved in the syn kinematic leucocratic intrusions deformation structures from the first episode d1 are characterized by f1 folds and distributed foliations s1 in rocks due to pure shearing at high temperatures they are preserved in weakly sheared d2 rocks along the eastern margin of the als belt or in certain low strain tectonic enclaves within the shear zone furthermore semi brittle deformation structures such as hot striae and discrete retrogression zones are attributed to normal slip shearing in the third deformation episode d3 which was probably locally active along the eastern flank of the dcs range for example there are four quartz c axis fabric patterns in the mylonitic rocks including type a point maxima type b y point maxima with crossed girdles superimposition type c quadrant maxima as well as type d point and quadrant maxima combination they are consistent with microscopic observations of microstructures of high temperature pure shearing low temperature simple shearing and their superimposition integrated microstructural analysis and fabric thermometer studies provide information on both high temperature up to 750 c and dominant low temperature 300 600 c deformations of quartz grains in different rock types sillimanite and garnet fabrics especially the latter were primarily formed at the peak metamorphism during high temperature pure shearing the above structural microstructural and fabric associations were generated in the tectonic framework of the indian eurasian collision the low temperature microstructures and fabrics are attributed to left lateral shearing along the asrr shear zone from 27 to 21 ma during the southeastward extrusion of the indochina block which postdated high temperature deformation at the peak metamorphism during the collision

northwest trending middle cenozoic left lateral faults in southern yunnan china and their tectonic significance abstract the northwest striking jianshui qujiang and chuxiong faults and probably the zhongdian fault south china show evidence for different amounts of middle cenozoic pre pliocene and post early paleogene left lateral displacement that range from 6 to 25 km the age and orientation of the left lateral faults suggest that the faults are related to a regional deformational event associated with important left lateral shear on the ailao shan shear zone which lies 50 70 km to the south identification of these faults extends left lateral shear during this deformational event farther to the northeast than previously recognized unlike the ailao shan shear zone these faults do not appear to be part of a through going nw trending shear zone these faults are interpreted to be the northernmost part of a nw trending boundary whose main structure the left lateral ailao shan shear zone forms a broad zone of mylonitic rocks bounding a crustal fragment that rotated clockwise around the eastern himalayan syntaxis following middle cenozoic left lateral displacement only the jianshui and qujiang faults exhibited a reversal of displacement in post miocene time this reversal is interpreted to be the result of counterclockwise rotation within the broad n s striking left lateral xiaojiang fault zone none of the post miocene displacement on these two faults appears to be related to post miocene right lateral displacement on the red river fault system the chuxiong fault may have reversed its early left lateral displacement but evidence from surface geology for young and active right lateral displacement is not clear the zhongdian fault appears to have undergone only left lateral displacement some of which may be middle cenozoic in age and some post miocene in age active displacement on the zhongdian fault is interpreted to mark the eastern boundary for a small crustal fragment that rotates clockwise around the eastern himalayan syntaxis

age and origin of magmatism along the cenozoic red river shear belt china to decipher the geodynamic significance of cenozoic magmatism along the red river shear belt geochemical analyses u pb and rb sr dating and pb sr nd isotope tracing were undertaken zircon monazite titanite and a ti u oxide from foliated granitoid intrusions in the shear belt gneisses yield u pb emplacement ages of 33 1 0 2 2 31 9 0 3 25 8 0 2 and 24 7 0 2 ma and an age of 35 0 0 3 ma was obtained for the roughly 100 km long adjacent jinping phan si pang alkali granite together with our previous data the new ages suggest that magmatism and left lateral strike slip movements occurred coevally during latest eocene oligocene times from 33 to 22 ma the rb sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500 c occurred at 52 6 1 1 ma pre dating the onset of magmatism whereas further cooling to 300 c took place at 28 9 0 6 this shows that unroofing in the north took place almost 9 million years earlier than in the central gneiss segments of the shear zone geochemical data substantiate two types of magmas 1 amphibole bearing intrusions of alkaline trend which are derived from sources with isr 0 7065 0 7089 and i nd 3 7 to 6 6 2 leucogranitic layers and bodies having isr 0 7084 0 7354 and i nd 3 3 to 13 4 the former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks whereas leucogranites are restricted to the shear belt gneisses source signatures of the alkaline intrusions lie adjacent to the those of oib plotting at the lower end of the mantle array contamination of these melts by continental material seems to be very limited on the other hand the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures requiring melting of crust different from the actually exposed gneisses magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains most likely present in the lithosphere underneath the region since lithospheric thickening or subduction can be ruled out to produce both types of magmas the presence of an important thermal anomaly is required which is coevally active with left lateral strike slip shear adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas which successively migrate through the crust to induce anatectic melting at 20 15 km crustal depth alkaline magmas largely dominate the volume of magmatism along the belt being continuously present in the shear zone for millions of years such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the red river belt possibly also playing a role for where and when zones of plate scale lateral extrusion develop

evolution of north himalayan gneiss domes structural and metamorphic studies in mabja dome southern tibet field structural and metamorphic petrology investigations of mabja gneiss dome southern tibet suggest that contractional extensional and diapiric processes contributed to the structural evolution and formation of the domal geometry the dome is cored by migmatites overlain by sillimanite zone metasedimentary rocks and orthogneiss metamorphic grade diminishes upsection and is defined by a series of concentric isograds evidence for three major deformational events two older penetrative contractional and extensional events and a younger doming event is preserved metamorphism migmitization and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid crustal levels metamorphic temperatures and pressures range from 500 8c and 150 450 mpa in chloritoid zone rocks to 705 65 8c and 820 100 mpa in sillimanite zone rocks we suggest that adiabatic decompression during extensional collapse contributed to development of migmatites diapiric rise of low density migmatites was the driving force at least in part for the development of the domal geometry the structural and metamorphic histories documented in mabja dome are similar to kangmar dome suggesting widespread occurrence of these events throughout southern tibet q 2004 elsevier ltd all rights reserved

polystage deformation of the gaoligong metamorphic zone structures 40ar 39ar mica ages and tectonic implications abstract the gaoligong metamorphic zone is located southeast of the eastern himalayan syntaxis in western yunnan china the zone is characterized by four stages of deformation d1 d4 d1 structures record early compressive deformation during the indosinian orogeny which formed tight to isoclinal f1 folds of bedding with a penetrative s1 foliation developed parallel to fold axial planes mid crustal horizontal shearing during d2 resulted in overprinting of d1 structures d1 and d2 structures are associated with granulite facies metamorphism d3 doming resulted in late crustal thickening and the development of a regional nw se trending f3 antiform synchronous with or slightly subsequent to d3 deformation the zone experienced d4 ductile strike slip shearing resulting in its exhumation to shallow crustal levels and retrograde metamorphism granitic d4 mylonites predominantly yield 40 ar 39 ar mica ages of 15 16 ma indicating that d4 dextral strike slip shearing occurred in the miocene weakly deformed leucogranite and protomylonite yield 40 ar 39 ar ages of 10 11 ma suggesting that ductile strike slip shearing continued to the late miocene the new 40 ar 39 ar data indicate that escape related deformation along the gaoligong strike slip shear zone occurred in the miocene in association with recent geophysical studies and on the basis of the structural crystal preferred orientation cpo and geochronological data presented in this paper we suggest that the gaoligong metamorphic zone formed in response to intracontinental transpression in the southeast of tibet characterized as intense deformation and metamorphism at middle upper crustal levels

lateral extrusion in transpression zones the importance of boundary conditions the homogeneous transpression strain model formulated by sanderson and marchini journal of structural geology 6 449 458 1984 has proved to be a useful tool in the analysis of complex three dimensional deformation patterns however some of the boundary conditions introduced in the mathematical model may be unrealistic and unnecessarily restrictive in this paper a strain matrix for unconfined transpression and transtension is derived in which material is allowed to move not only vertically but also horizontally in and out of the deforming zone parallel to its length lateral extrusion three end member plane strain components are defined wrench simple shear pure shear in xy lateral stretch and pure shear in yz vertical stretch these biaxial strains can be viewed as the apices of a strain triangle for unconfined transpression or transtension the edges of the triangle correspond to triaxial pure shear non coaxial biaxial lateral extrusion and the triaxial confined transpressional or transtensional strain of sanderson and marchini during unconfined transpression the orientation and in particular the geometry k value of the finite strain ellipsoid depends upon not only the amount of shortening across the zone and the amount of strike slip parallel to the zone but also upon the ratio of vertical to lateral stretch this can present serious difficulties when attempting to use finite strains to infer the direction and magnitude of zone boundary displacements examples of transpression zones in which there is evidence of a component of lateral extrusion are described from sw cyprus and central scotland these examples illustrate that antithetic strike slip shearing is a kinematic requirement of laterally unconfined transpression implying that synchronous shear sense indicators may give opposite senses of movement in shear zones specific geometric and mechanical boundary conditions together with internal fault zone rheologies may favour the lateral extrusion of material kinematic partitioning can occur to form fault bounded domains in which end member biaxial and or non coaxial strains are developed analysis of such domains can give a clearer understanding of regional scale triaxial deformations these findings illustrate the importance of establishing displacement boundary conditions when qualitatively or quantitatively analysing crustal deformation zones

combined effects of eurasia sunda oblique convergence and east tibetan crustal flow on the active tectonics of burma it is widely accepted that deformation of the india sunda plate is the result of partitioned hyper oblique convergence presently sub meridian dextral strike slip faulting accommodates this india sunda motion in a buffer zone the burma platelet this wide dextral strike slip shear zone is complicated by the side effect of the tibet plateau collapse that can be described in term of crustal flow and gravity tectonics the loss of potential energy related to this plateau collapse affects most of the burmese platelet particularly in its northernmost part interaction of these two distinct geodynamic processes is recorded in the gps based regional strain field the analysis of seismic focal mechanism but also from direct geologic observations both onshore and offshore myanmar and bangladesh we propose the apparent e w shortening component of this so called partitioned hyper oblique subduction is only the effect of regional gravitational forces related to the tibet plateau collapse whereas the ns strike slip faulting accommodates the india sunda motion

internal structures of zircons from archaean granites from the darling range batholith implications for zircon stability and the interpretation of zircon u pb ages internal structures in zircons from granitoids from the late archaean darling range batholith show secondary features revealed by hf etching which record reconstitution of the zircons and modification of the distribution of trace elements during post crystallisation cooling of the granitoid zircons from the granites commonly contain unzoned to weakly zoned cores surrounded by rims showing oscillatory zoning which has been modified by recrystallisation the most striking feature is the development of high trace element concentration areas found in zircons from a number of granites these structures range from enhanced trace element concentrations in primary zones to a single accumulation of most trace elements in one band about half way between the outer edge and the centre of the zircon in any zircon the extent of the concentration of trace elements towards the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory zones in the outer rim this suggests that the trace element bands formed by migration of trace elements from the outer primary zones to new concentration sites on an inner set of primary zones this explanation is supported by the formation of multiple curved trace element bands that transgress primary zoning and the determination of younger shrimp ages on depleted zircon outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres also observed in some granite zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs throughout the zircon this structure is explained in terms of secondary migration and reconcentration of trace elements in curved bands all structures can be transgressed by generally rounded lobes and patches of low u weakly nebulously zoned zircon this is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation of the granitoid resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements from the zircon

first local seismic tomography for red river shear zone northern vietnam stepwise inversion employing crustal p and pn waves abstract the 900 km long red river shear zone rrsz lends a compelling support to the continental extrusion model for the tectonic evolution of southeastern asia but has been challenged by many of views as some new records mainly from northern vietnam suspecting the dimensions of rrsz neither in depth nor in displacement are as large as we expected before however compared to the northwestern half of the rrsz in yunnan province better studied by many fields the southeastern half in northern vietnam is relatively poorly constrained by seismic study due to insufficient stations and data in the past this study using a newly deployed portable broadband seismic network obtained the first local seismic tomography with a stepwise inversion using p and p n phases surface geology major structures and rock properties are well correlated and identified in our model suggesting the rrsz is a lithospheric structure at least penetrating to the uppermost mantle with mantle thermal anomalies in general the crust of northern vietnam appears to be weak and sits on a relatively hot uppermost mantle showing a long and complex thermo tectonic history a mid lower crustal segmentation of rrsz is also proposed to compromise the discrepancies recently observed between yunnan province and northern vietnam

kinematics and dynamics of the namche barwa syntaxis eastern himalaya constraints from deformation fabrics and geochronology abstract field observations deformation and fabric analyses and precise age data acquired by zircon shrimp la icp ms u pb and 40 ar 39 ar dating methods have yielded new constraints on the kinematics and dynamics of the namche barwa syntaxis nbs which is the eastern corner of the himalaya a two stage model has been established to explain the formation and evolution of the nbs the northward indentation of the indian plate beneath the lhasa terrane began at 55 40 ma and crustal materials at this corner were subducted to depths 70 km where they experienced hp uhp metamorphism since 40 ma large scale right lateral strike slip along the sagaing fault has accommodated the rapid northward movement of the eastern indian plate corner with respect to the indochina block this caused significant and progressive bending of the indus yarlung suture zone iysz such that it became the dongjiu milin left lateral strike slip shear zone dmsz in the west and the aniqiao motuo right lateral strike slip shear zone amsz in the east both zones underwent strong mylonitization meanwhile the hp uhp metamorphic rocks were rapidly exhumed first into the deep crust at 22 18 ma and then to the shallow crust to form an antiformal dome at 6 2 ma our model provides new insight into the processes of post collisional crustal thickening related to the formation of the himalayan orogenic belt

cenozoic tectonic evolution of asia a preliminary synthesis abstract asia has been a major testing ground for various competing models of continental deformation due to its relatively well understood plate boundary conditions in the cenozoic exceptional exposure of active structures and strain distribution and widespread syn collisional igneous activity as a proxy for the thermal state of the mantle and crust two cenozoic orogens dominate the continent the himalayan tibetan orogen in the east induced by the india asia collision and the turkish iranian caucasus orogen in the west induced by the arabia asia collision the development of the two orogens was accomplished by shortening in the early stage followed by strike slip faulting and extension in the late stage in the himalayan tibetan orogen shortening across two discrete thrust belts at 55 30 ma in southern and northern tibet created a large intracontinental basin the paleo qaidam basin in between subsequent crustal thickening and a possible thermal event in the mantle e g convective removal of central tibetan mantle lithosphere may have raised the elevation of this early intra plateau basin up to 2 3 km to its current height collision between india and asia also caused lateral extrusion of southeast asia between 32 ma and 17 ma the latest stage of the india asia collision was expressed by north trending rifting and the development of trench facing v shaped conjugate strike slip faults in central mongolia central tibet eastern afghanistan and southeast asia in the turkish iranian caucasus orogen early crustal thickening in the orogenic interior began at or prior to 30 20 ma this style of deformation was replaced by strike slip faulting at 15 5 ma associated with further northward penetration of arabia into asia westward extrusion of the anatolia turkey block and rapid extension across the sea of crete and sea of aegean the late stage extension in both orogens was locally related to extensional core complex development the continental margin extension of east asia was developed in two stages initially in a widely distributed zone that has an east west width of 500 800 km during 65 35 ma which was followed by localized extension and opening of back arc basins associated with the development of spreading centers at 32 17 ma e g japan sea or east korea sea bohai bay and south china sea opening of the back arc basins could be induced by 1 rapid eastward migration of the western pacific trench system or 2 oblique subduction of pacific plate beneath asia that had produced a series of en echelon right slip primary shear zones linking with back arc spreading centers oriented obliquely to the strike of the nearby trench since 15 ma the eastern margin of asia became contractional in the east west direction as indicated by the collapse of back arc basins in the western pacific and the development of fold thrust belts along the eastern continental margin coeval with the contraction is widespread east west extension in siberia north china and the tibetan plateau the above observations can be explained by a change in boundary condition along the eastern margin of asia that allowed the thickened asian continent to spread eastward causing east west extension in its trailing edge and east west compression in its leading edge in west asia continental margin extension started at about 25 20 ma in the aegean and cretan regions which was associated with a rapid southward retreat of the hellenic arc the complex evolution of cenozoic deformation in asia may be explained by a combined effect of temporal changes in plate boundary conditions thermal evolution of the upper mantle perturbed by collisional tectonics and the built up of gravitational energy through crustal thickening and thermal heating although the past research in asia has treated the india asia and arabia asia convergence as separate collisional processes their interaction may have controlled the far field cenozoic deformation in asia the most pronounced result of this interaction is the creation of a northeast trending 300 400 km wide and 1500 km long zone of northwest striking right slip faults which extends from the zagros thrust belt in the south to western mongolia in the north and links with the active tian shan and altai shan intracontinental orogens cenozoic deformation and coeval igneous activity spatially overlap with one another in the himalayan tibetan and turkish iranian caucasus orogens a large cenozoic magmatic gap exists between tibet in the south and mongolia in the north where cenozoic deformation has not been associated with any coeval igneous activity finally cenozoic igneous activity is always associated with jurassic cretaceous magmatic arcs suggesting a causal relationship between the early arc magmatism and later syn collisional magmatism

late cenozoic right lateral strike slip faulting in southern tibet field observations collected during the sino french himalaya tibet cooperation project 1980 1982 show that right lateral faulting between southern and northern tibet roughly along the chord joining the eastern and western syntaxes of the himalayan arc is a major component of the active tectonics north of the himalayas just north of the well developed rifts of southern tibet we studied three right lateral 100 200 km long strike slip faults forming an en echelon array along that chord evidence for earthquake surface breaks was found on each of these faults for the m 8 beng co or damxung strike slip earthquake of november 18 1951 rupture parameters could be constrained to be 8 2 m and l 90 km offset glacial and postglacial morphological markers 105 years and 104 years b p imply displacement rates of the order of 1 2 cm yr on the three faults the en echelon fault zone appears to extend westward 2000 km across the plateau to the karakorum fault and is thus referred to as the karakorum jiali fault zone kjfz fast rates of slip on it and on faults along the northern rim of tibet appear to contrast with the slower rates of deformations in the interior of the plateau our estimates of slip rates on the strike slip and normal faults of southeastern tibet suggest that eastward extrusion of north tibet between the right lateral kjfz and the left lateral altyn tagh and kun lun faults absorbs at least 30 of the present convergence between india and asia we propose an instantaneous velocity model of the active tectonics of asia compatible with such faulting rates and geometries evidence for tertiary right lateral strike slip movements in southern tibet and around the eastern syntaxis of the himalayas is also found in the structural record offsets of various large scale geological features imply that these movements have been large

large river offsets and plio quaternary dextral slip rate on the red river fault yunnan china using multispectral spot images and 1 100 000 topographic data we present an improved map of the active red river fault zone between midu yunnan china and hanoi vietnam the fault zone is composed of parallel strands one of which the yuanjiang fault was previously undetected there also appears to be a component of extension all along the fault zone such extension increases toward the se from yunnan to the south china sea coast and the vector describing the motion of south china relative to indochina points within the n45 135 e quadrant we attempt to assess the plio quaternary dextral slip rate on the red river fault rrf by restoring large river offsets and searching for the largest plausible one across much of yunnan the fault is perpendicular to local catchments that drain into the red river from precise mapping of the river courses on spot satellite images and on 1 100 000 topographic maps numerous multiple offsets along the fault can be detected and reconstructed the lack of correlation between the apparent offsets and the lengths of the rivers upstream from the fault suggests either that the drainage system was in large part established prior to the onset of dextral slip along the fault or that frequent captures have occurred we thus try to find the best fit between series of river channels upstream and downstream from the fault by progressively restoring the dextral displacement in increments of 500 m up to an offset of 50 km for each increment we measure the misfits root mean squares rms between the upstream and downstream channels the best fit and smallest rms are obtained for an offset of 25 0 5 km that we interpret to represent the clearest large right lateral displacement recorded in the geomorphology along the active red river fault since dextral motion is likely to have started around 5 myr the most probable average plio quaternary slip rate on the fault is of order of 5 mm yr we attribute the apparent lack of seismic activity on a large stretch of the fault to millennial recurrence times between great earthquakes our study shows that relatively small drainage systems can keep a good record of fairly large cumulative fault offsets

gneiss domes and orogeny many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression the decreasing buoyancy associated with increasing melt fraction drives further decompression at near isothermal conditions as the partially molten crust rises diapirically this combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep seated migmatite terranes and inferred for active orogens in exhumed orogens the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite granite the large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis

fluid flow and water rock interaction in the east rift zone of kilauea volcano hawaii the east rift zone of kilauea volcano in hawaii represents a major area of geothermal activity fluid inclusion and stable isotope analyses of secondary hydrothermal minerals in core samples from three scientific observation holes soh drilled into the rift zone indicate that the geothermal system is dominated by meteoric waters to depths of as much as 1500 m below sea level calculated 18o and d values for fluids on the north side of the rift zone indicate that the deep meteoric fluids may be derived from precipitation on the upper slopes of mauna loa volcano in the interior of the rift zone recharge is dominated by seawater mixed with local meteoric water water rock ratios in the rift area are approximately 2 but strongly 18o enriched fluids in the deeper parts of the soh 2 and soh 4 drill holes on the north side of the rift indicate that the fluids underwent extensive interaction with rocks prior to reaching this part of the rift zone marine carbonates at the subaerial to submarine transition between 1700 and 1780 m depth in soh 4 have not fully equilibrated with the fluids suggesting that the onset of hydrothermal activity in this area was relatively recent 2000 years this may represent increased volcanic activity along the rift after the end of the ai la au phase of eruptive activity at the kilauea summit approximately 1000 years ago or it may reflect progressive evolution of the hydrothermal system in response to southward migration of intrusive activity within the rift

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

zircon u pb and hf isotope constraints from the ailao shan red river shear zone on the tectonic and crustal evolution of southwestern china abstract the ailao shan red river asrr shear zone one of the most prominent geologic strike slip shear zones in southeast asia consists of high grade metamorphic complexes that provide a rare opportunity to sample the mid crustal rocks along the western margin of the yangtze block of south china here we report combined in situ analyses of zircon u pb and lu hf isotopes of eight gneisses from the diancang shan and ailao shan segments of the asrr shear zone our zircon u pb data indicate that the rocks contain abundant magmatic zircons ranging in age from 1785 to 25 ma with peak ages at ca 770 350 and 240 ma suggesting three major periods of protolith formation the 350 ma zircons observed only in an orthogneiss from the diancang shan show uniform initial hf isotopic ratios marked by high and positive e hf t values from 16 to 10 this is in contrast with the other two zircon populations that are more common occurring in both the diancang shan and ailao shan and overall delineate very heterogeneous hf isotopic compositions with e hf t values ranging from 15 to 16 many zircons reveal distinctive core rim age variations zircon rims formed between ca 34 and 26 ma show significant variations in th u ratios 5 0 01 and e hf t values 14 to 10 that suggest complicated magmatic and metamorphic zircon overgrowth during the oligocene the presence of both metamorphic and magmatic overgrowths on zircons suggests that the metamorphism reached upper amphibolite facies corresponding to mid crustal level p t conditions as also suggested by structural and petrologic data our data furthermore suggest that the asrr gneisses were not produced solely by shear heating during the tertiary strike slip faulting but are uplifted mid crustal basement rocks that formed essentially during two major stages of magmatism represented by the neoproterozoic kangding complex and late permian emeishan large igneous province as a result of the india asia collision these basement rocks underwent regional magmatic and metamorphic overprinting in the oligocene that based on our relevant work searle et al 2010 geosphere 6 1 23 predated the initial left lateral movement along the asrr shear zone the 350 ma protolith which requires a dominant depleted mantle input in the petrogenesis cannot be linked to any major magmatic events in south china but may be interpreted as part of the paleotethys remnants that were later incorporated into the asrr shear zone

maximum effective moment criterion and the origin of low angle normal faults abstract the origin of the low angle normal faults of metamorphic core complexes has been debated for over two decades proponents of andersonian fault mechanics have long argued that it is mechanically unfeasible for slip to occur along shallowly dipping normal faults a new theory named the maximum effective moment criterion is proposed here for their origin using an effective moment approach formulated as m eff fh where f is the force tangentially acting on the unit shear boundaries and h represents its arm the maximum value appears at angles of 54 7 with the 1 since extensional crenulation cleavages eccs and the contractional crenulation cleavages cccs occur in conjugate pairs with 110 angle between them it is suggested that they tend to be oriented in the directions of maximum effective moment the differential stress for formation of the ecc or ccc are less than that for fracturing the orientations of conjugate eccs depending on relative magnitudes of simple shear versus coaxial strain components the synthetic ecc set is much better developed than the antithetic set due to anisotropy upward propagation of the synthetic ecc set from mid crustal domains of mylonitization through strain localization and strain softening is considered an effective mechanism for the formation of the low angle normal faults of metamorphic core complexes

spatial controls on erosion in the three rivers region southeastern tibet and southwestern china abstract global data suggest that erosion rates variously scale with steepness or climate forcing precipitation or glacial excavation but the relative influence of these factors has proven difficult to assess without comparisons from a single location a new suite of detrital 10be data from the three rivers region se tibet is used to examine the relative importance of rainfall and relief in predicting patterns of erosion rates across a region with a strong gradient in exhumation the data reveal millennial erosion rates vary by two orders of magnitude from 0 01 to 8 mm yr across a regional gradient in exhumation rates inferred from previous thermochronology and cosmogenic nuclide data to the west and east of the study region the new millennial erosion rates mirror the pattern of decreasing exhumation rates from west to east across the region with the highest rates in the lower salween river drainage and the lowest rates in the yangtze river drainage erosion rates in the mekong and salween river drainages are correlated with mean local relief whereas in the yangtze river drainage they are correlated most strongly with mean annual rainfall the tectonic setting of this region with a strong west to east gradient in exhumation rates which we infer to mirror a gradient in rock uplift seems to exert a stronger control on erosion rate patterns than rainfall or relief

discussion on the role of the red river shear zone yunnan and vietnam in the continental extrusion of se asia journal vol 163 2006 1025 1036 p h leloup p tapponnier r lacassin write in his recent paper searle 2006 acknowledges that the 1000 km long ailao shan red river shear zone is a large miocene left lateral shear zone but speculates that left lateral slip started after 21 ma and claims that the total finite offset remains unknown from this he concludes that continental extrusion was only a relatively minor tectonic factor during the india asia collision as long argued by other workers e g england houseman 1986 cobbold davy 1988 dewey et al 1989 houseman england 1993 we summarize below the field and geochronological evidence that makes us maintain a viewpoint in better accordance with facts timing of left lateral shear along the ailao shan red river shear zone the ailao shan red river shear zone is composed mostly of high grade metamorphic rocks and deformed granitoids with ubiquitous evidence for left lateral shear parallel to the belt e g tapponnier et al 1986 1990 leloup et al 1993 1995 2001 the crystallization of the granitoids has been dated between 22 and 35 ma fig 1d f for example by scharer et al 1990 1994 and zhang scharer 1999 leading those workers to propose that left lateral shear started at least c 35 ma ago in contrast searle 2006 claims that all the deformed granitoids found within the shear zone predate left lateral shear and that their crystallization ages should thus be interpreted to provide an upper limit for the onset of deformation rather than a lower limit thus suggesting a maximum age of 21 ma for this deformation a clear understanding of the p t history and in situ deformation history of the intrusions as well as of their relationships with surrounding paragneisses is fundamental for interpreting correctly the geochronological

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

the xuelongshan high strain zone cenozoic structural evolution and implications for fault linkages and deformation along the ailao shan red river shear zone abstract new structural geometric petrographic and u pb zircon data constrain the kinematics of the xuelongshan high strain zone northermost parts of the ailao shan red river shear zone in the xuelongshan range mylonitic foliation mainly trends nw se on steep foliation and flatted foliation mineral stretching lineation is various from gentle to steep and ductile shear senses indicate a left lateral top to the northwest shear movement on the eastern limb and a left lateral top to the southeast shear on the western limb of the range these dome related ductile geometries and kinematics namely d1 deformation represent regional high temperature metamorphism due to crustal thickening the superimposed sinistral strike slip shearing of d2 deformation occurred locally along the earlier domal fabrics metamorphism migmatization and the emplacement of leucogranitic veins associated with partial melting were syntectonic with the regional scale upright antiform la icp ms u pb dating of zircons reveals the late permian triassic and oligocene thermal events the latter event ca 33 26 ma or possibly earlier from the leucogranitic veins corresponds to the regional metamorphism during crustal thickening along the xuelongshan these data indicate that the jiali xuelongshan diancangshan ailao shan high strain zones can belong to a system around the eastern himalayan syntaxis ehs and that the xuelongshan zone played a role in linking other segments between the tibet to indochina in cenozoic

high temperature metamorphism in a major strike slip shear zone the ailao shan red river people s republic of china abstract petrographic and thermobarometric analysis provides constraints on thept path of the mylonitic gneisses of the left lateral ailao shan red river shear zone which has accommodated the lateral extrusion of indochina during the tertiary two different paragenesis p1 and p2 are coeval with this deformation and correspond respectively to the amphibolite and greenschist facies microprobe analysis reveals that p1 garnets bear a chemical zonation from core to rim this zonation indicates a temperature increase during garnet growth conditions of formation of garnet rims p1b which are estimated using biotite garnet and plagioclase garnet thermobarometers are close to the granitic solidus 710 70 c and4 5 1 5 kbar p2 conditions are estimated to be approximately 500 c and

role of the red river shear zone yunnan and vietnam in the continental extrusion of se asia the 1000 km long nw se striking left lateral ailao shan red river shear zone runs from the southeastern corner of tibet to the gulf of tonkin and the south china sea it has been used as the prime example of a lithospheric scale strike slip fault that has accommodated between 500 and 1000 km of southeastwards extrusion of indo china away from the indian plate indentor central to the model of continental extrusion is that such faults cut through the entire lithosphere that shear heating resulted in high grade metamorphism and local anatexis and that the ages of sheared granites along the fault also date the timing of strike slip shearing however structural data from the red river shear zone clearly show that vertical strike slip faulting post dated metamorphism and granite emplacement most granites along the shear zone are mantle related granodiorites or within plate alkali granites formed prior to shearing along the red river shear zone left lateral kinematic indicators are ubiquitous within the red river mylonites but they are always lower temperature fabrics formed after peak sillimanite metamorphism and after granite crystallization it is suggested that left lateral strike slip shearing along the red river shear zone started after 21 ma not at 35 ma as previously thought and the fault was purely a crustal structure none of the geological features used to propose the 500 1000 km offsets are robust and the total finite offset remains unknown

a provenance study of the paleozoic ventania system argentina transient complex sources from western and eastern gondwana abstract the u pb and lu hf isotopic analyses of the different sedimentary sequences of the ventania system an old paleozoic orogenic belt exposed in the southern region of the rio de la plata craton in the province of buenos aires argentina provide new evidence for the understanding of the tectonic evolution of the western sector of the gondwanides mountain belt these ranges formed as result of the late paleozoic collision of the patagonia terrane against the continental margin of gondwana the provenance analysis together with the sedimentary paleocurrents confirm a dominant source from the tandilia system a paleoproterozoic mountain belt formed during the amalgamation of the rio de la plata craton at about 1800 2200 ma and incorporated to western gondwana during the brasiliano orogeny at 550 530 ma the local dominant source at the base of the early paleozoic changed to more distant supplies toward the top of the sequences when an increasing participation of detritus from first cambrian 560 520 ma zircons from the pampean orogen and later on ordovician 480 460 ma zircons from the famatinian orogen is recorded the detrital zircon patterns and the maximum age of the units shed light on some previous discrepancies in the early paleozoic stratigraphy the balcarce formation an early paleozoic sedimentary cover of the tandilia metamorphic and igneous basement shows striking differences when compared with the new data from the ventania system the two data sets reveal different sources for the two regions the late paleozoic foreland basin deposits mark an abrupt change of 180 in the paleocurrent directions in the petrographic composition of the sediments and in the provenance of detrital zircons these data indicate a southern provenance with the first evidence of carboniferous and permian magmatic zircons the oldest archean zircons together with the finding of clasts with archeocyathids support the provenance from patagonia which was derived from eastern gondwana the u pb ages of the ash fall tuffs in the tunas formation confirm the early permian age of the eurydesma fauna in the ventania system the u pb data together with the lu hf isotopic data enhance the comprehension of the tectonic evolution of the ventania system as part of the larger gondwanides belt that amalgamated to western gondwana during late permian times with some independent pieces derived from eastern gondwana

spatial variations in damage zone width along strike slip faults an example from active faults in southwest japan abstract field investigations reveal spatial variations in fault zone width along strike slip active faults of the arima takatsuki tectonic line attl and the rokko awaji fault zone rafz of southwest japan which together form a left stepping geometric pattern the fault zones are composed of damage zones dominated by fractured host rocks non foliated and foliated cataclasites and a fault core zone that consists of cataclastic rocks including fault gouge and fault breccia the fault damage zones of the attl are characterized by subsidiary faults and fractures that are asymmetrically developed on each side of the main fault the width of the damage zone varies along faults developed within granitic rocks of the attl and rafz from 50 to 1000 m in contrast the width of the damage zone within rhyolitic tuff on the northwestern side of the attl varies from 30 to 100 m the fault core zone is generally concentrated in a narrow zone of 0 5 5 m in width consisting mainly of pulverized cataclastic rocks that lack the primary cohesion of the host rocks including a narrow zone of fault gouge

shear heating in continental strike slip shear zones model and field examples summary a two layer crust and upper mantle finite diverence steady state thermomechanical model of a long lived several million years lithospheric strike slip fault is presented and its predictions compared with field observations from various major fault zones in order to estimate the maximum amount of shear heating all mechanical energy is assumed to be dissipated in heat in ductile as well as in brittle layers deformation follows a friction law in the brittle layer s and a power flow law in the ductile one s variations of several independent parameters and their influence on the thermomechanical state of the fault zone and on shear heating are systematically explored shear heating is found to be more important in fault zones avecting an initially cold lithosphere and increases with slip rate friction coeycient and stivness of materials in extreme cases slip rate of 10 cm yr 1 stiv lithosphere shear heating could lead to temperature increases close to 590 c at the moho and 475 c at 20 km depth for more common cases shear heating leads to smaller temperature increases but can still explain high grade metamorphic conditions encountered in strike slip shear zones however modelled temperature conditions often fall short of those observed this could be due to heat transport by mechanisms more eycient than conduction common syntectonic emplacement of granitic melts in ductile strike slip shear zones can be explained by lower crust partial melting induced by shear heating in the upper mantle besides slip rate the possibility of such melting depends mostly on the upper mantle rheology and on the fertility of the lower crust for hard upper mantle and highly fertile lower crust partial melting could occur at rates of 1 cm yr 1 while in most cases it would result from the breakdown of micas for slip rates over 3 cm yr 1 as a result of shear heating partial melting of the upper mantle could occur in the presence of small amounts of fluids rise of magmas and or hot fluids in the shear zone will further enhance the temperature increase in shallower parts of the fault zone in nature shear heating would inevitably cause strain localization in the deeper parts of strike slip faults as is often observed in the field for crustal shear zones