evidence for postseismic viscoelastic relaxation following the 1959 m 7 5 hebgen lake montana earthquake a 1983 releveling survey conducted by the national geodetic survey combined with previous surveys in 1923 1960 1967 and 1975 provides evidence for ongoing vertical deformation of a broad region diameter of 150 km surrounding the site of the 1959 hebgen lake earthquake deformation consists of relative uplift centered roughly on the coseismic fault and a smaller amplitude zone of relative subsidence south of the fault maximum observed elevation change during the postseismic period 1960 1983 exceeds 30 cm assuming that the entire uplift i e 1923 1983 occurred following the 1959 earthquake i e uplift measured between 1923 and 1960 was entirely postseismic the rate of vertical deformation appears to have decreased exponentially with a characteristic decay time of about 10 years the spatial pattern and time behavior of the observed movements are consistent with simple models of postseismic deformation following normal faulting in an elastic layer thickness of 30 40 km overlying a viscoelastic half space viscosity of 1019 pa s this model may also account for the anomalously large horizontal strain around the hebgen lake region measured by repeated trilateration surveys during the period 1973 1984 the releveling measurements in the hebgen lake region appear to provide the first observations of viscoelastic relaxation in the asthenosphere following an intraplate earthquake in the united states

new age constraints on the lan sang gneiss complex thailand and the timing of activity of the mae ping shear zone from in situ and depth profile zircon and monazite u th pb geochronology abstract the mae ping shear zone mpsz is one of four major strike slip systems in thailand and a key structural element in some models of the cenozoic tectonic evolution of southeast asia in northwestern thailand the mpsz exposes a 5 km wide core of intensely deformed high grade metamorphic and minor undeformed magmatic rocks the lan sang gneiss complex lsgc despite previous efforts to determine the chronology of the lsgc and ultimately the mpsz the evolution of this gneiss complex remains incompletely resolved here we present new zircon and monazite u th pb la mc icp ms ages and whole rock geochemical analyses from three peraluminous high k meta igneous granitic rocks augen gneiss sheared granodiorite undeformed subvolcanic dike of the lsgc our zircon u pb dating suggests that the granitic protolith of the augen gneiss intruded metasedimentary basement of sibumasu as early as 235 220 ma zircon rim and monazite u th pb ages from the same sample attest to periods of metamorphic recrystallization and overgrowth in the late triassic early jurassic early cretaceous and eocene the latter event had started by 45 ma and culminated in partial melting of the basement gneisses and the intrusion of granodiorite magma into the lsgc at 35 ma contemporaneous with ductile deformation along the mpsz a late undeformed subvolcanic dike that crosscuts the mylonitic fabric of the calc silicate and marble core of the mpsz dated in this study at 22 ma may indicate that ductile shearing had ceased by that time thus our study provides the first evidence for late eocene early miocene magmatism in the lsgc

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

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

temporal gravity and height changes of the yellowstone caldera 1977 1994 this paper describes the longest record of gravity measurements in the area of the yellowstone caldera wyoming the temporal gravity changes at the 12 gal 10 8 ms 2 precision level are compared with changes in heights from leveling and gps the gravity field decreased across the caldera from 1977 to 1983 during the uplift and attained a maximum decrease of up to 60 2 gal along the caldera axis the gravity field then reversed polarity to increasing values of up to 60 12 gal between 1986 and 1993 the ratio between height and gravity changes varied during the entire time but converged over the latter period following the free air gravity gradient general ground deformation deduced from leveling showed caldera wide uplift of 15 mm a during the period of gravity decrease then from leveling and gps subsidence of 25 mm a during the gravity increase

lake sediment evidence for the date of deglaciation of the hidden lake area kenai peninsula alaska an abrupt environmental change is reflected in a core from hidden lake alaska by differences in sediment type chlorite crystallinity and content of organic carbon and water of the sediments this abrupt change in the sedimentary record occurred about 14 500 14c yr ago and probably marks the time of recession of the glacier from the hidden lake drainage basin deglaciation of the area was then underway and rock flour was being deposited in the lake after recession of the glacier from the hidden lake drainage basin rock flour was no longer introduced and organic matter content of the sediment increased by the dating of these changes in sediment type we show that retreat of glaciers in this area took place significantly earlier than previously estimated this agrees with the timing of retreat of alpine glaciers elsewhere in western north america

plutonism in three orogenic pulses eastern blue ridge province southern appalachians department of geological sciences university of north carolina mitchell hall chapel hill nc 27599 3315

evolution from an oblique subduction back arc mobile belt to a highly oblique collisional margin the cenozoic tectonic development of thailand and eastern myanmar

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

petrogenesis of malaysian granitoids in the southeast asian tin belt part 1 geochemical and sr nd isotopic characteristics the malaysian granitoids of the southeast asian tin belt have been traditionally divided into a permian to late triassic i type dominated arc related eastern province indochina terrane and a late triassic s type dominated collision related main range province sibumasu terrane separated by the bentong raub paleo tethyan suture that closed in the late triassic the present study however shows that this model is oversimplified and that the direct application of chappell and white s 1974 i and s type classification cannot account for many of the characteristics shared by malaysian granitoids despite being commonly hornblende bearing as is typical for i type granites the roof zones of the eastern province granites are hornblende free in addition the main range province granitoids contain insignificant primary muscovite and are dominated by biotite granites mineralogically similar to many of the plutons of the eastern province in general the malaysian granitoids from both provinces are more enriched in high field strength elements than typical cordilleran i and s type granitoids the mineralogy and geochemistry of the eastern province granitoids and their relationship with contemporaneous volcanics confirm their i type nature the bulk liquid lines of descent of both granitic provinces largely overlap with one another sr nd isotopic data further demonstrate that the malaysian granitoids especially those of the main range were hybridized melts derived from two end member source regions one of which is isotopically similar to the kontum orthoamphibolites and the other akin to the kontum paragneisses of the indochina block however there are differences in the source rocks for the two provinces and it is suggested in this paper that these are related to differing proportions of igneous and sedimentary protoliths the incorporation of sedimentary sourced melts in the eastern province is insignificant which allowed the granites in this belt to maintain their i type nature the presence of minor primary tin mineralization in the eastern province compared to the much more significant tin endowment in the main range is considered to reflect the incorporation of a smaller proportion of sedimentary protolith in the melt products of the former

understanding mesozoic accretion in southeast asia significance of triassic thermotectonism indosinian orogeny in vietnam results from a zircon u pb study of the metamorphic basement of vietnam reveal that a large part of the continental crust was affected by a short lived episode of ductile deformation and high temperature metamorphism between 258 6 ma and 243 5 ma although coincident with final stages of north south china collision qinling orogenesis the thermotectonism in vietnam was caused by accretion of sibumasu to indochina south china this accretion event indosinian orogeny has regional significance because it contributed to the final stages of north south china collision an aspect not explicitly recognized in qinling orogenic models

geological model and development of the cenozoic wiang pa pao basin chiang rai province northern thailand based on gravity data modelling and surface structural interpretation abstract 3 d inverse gravity modelling constrained by surface geological data across the neogene wiang pa pao basin wpb northern thailand provides new subsurface models of the basin geometry and its structural development the intermontaine wpb is seismically active and exhibits many geothermal sites both are indicative of active fault zones six hundred thirty six terrestrial gravity stations with 500 m spacing were acquired the residual complete bouguer anomaly map cba reveals an n s elongate basin characterized by gravity lows bounded by gravity highs on the eastern margin of the basin metasedimentary basement and moderate gravity anomalies on the western margin granitic basement structural edge detection revealed n s trending faults along the eastern and western margins linked by ne sw and nw se trending lineaments the 3 d gravity model indicates an asymmetric graben with a maximum depocenter depth of 1300 m surface structure and seismicity confirm the gravity interpretation that the wpb is a dextral transtensional basin characterized by wsw ene trending r shears and nne ssw r shears distributed in duplex like geometries along the n s trending principal displacement zone the wpb displays unusually well developed r shears where palaeozoic metasedimentary rocks in the basement and cenozoic basin sediments are present these faults disappear to the south where the main n s fault resides entirely within the granite the wpb is associated with cenozoic multi phase rifting in the late oligocene miocene major rift basins formed under w e extension strongly influenced by wsw ene to ne sw and nw se pre existing trends in the latest miocene recent the younger basins including the wpb underwent dextral strike slip along new and existing n s dextral fault trends

structure of the mae on depression chiang mai province based on gravity modelling and geological field observation implications for tectonic evolution of the chiang mai chiang rai suture zone northern thailand abstract we propose a structural model for the tectonic evolution of the mae on depression area along the chiang mai chiang rai suture zone cmcrsz the study location is the central mae on district where a topographic depression is present that is continuous with the eastern margin of chiang mai rift basin yet volcanic exposures mapped in topographic lows indicate the depression has not developed as part of the syn rift or post rift basin with sediment fill terrestrial gravity was acquired to visualize the subsurface extent of the volcanic exposures which are part of a melange complex associated with the sibumasu sukhothai terrane collision during the permo triassic indosinian orogeny gravity anomaly highs correspond to the asymmetrical trapezoid geometry of a relatively dense volcanic block with a thickness of around 750 900 m overlain by lower density units comprising carbonate and pelagic formations interpreted as typical ocean plate stratigraphy associated with a volcanic seamount gravity gradient analysis fits with the geological interpretation of a klippe of mae tha formation derived from the western margin of the sukhothai terrane in thrust contact with the cmcrsz melange the structural interpretation of the gravity modelling provides important constraints on the tectonic contacts along the cmcrsz that fits the eastward subduction model of the palaeo tethys beneath the sukhothai terrane the mae on depression is one of the best localities in northern thailand for demonstrating the thickness and extent of blocks of palaeo tethys volcanics associated with carbonates which are interpreted as representing former seamounts

jurassic to miocene magmatism and metamorphism in the mogok metamorphic belt and the india eurasia collision in myanmar situated south of the eastern himalayan syntaxis at the western margin of the shan thai terrane the highgrade mogok metamorphic belt mmb in myanmar occupies a key position in the tectonic evolution of southeast asia the first sensitive high resolution ion microprobe u pb in zircon geochronology for the mmb shows that strongly deformed granitic orthogneisses near mandalay contain jurassic 170 ma zircons that have partly recrystallized during 43 ma high grade metamorphism a hornblende syenite from mandalay hill also contains jurassic zircons with evidence of eocene metamorphic recrystallization rimmed by thin zones of 30 9 plus or minus 0 7 ma magmatic zircon the relative abundance of jurassic zircons in these rocks is consistent with suggestions that southern eurasia had an andean type margin at that time mid cretaceous to earliest eocene 120 to 50 ma i type granitoids in the mmb myeik archipelago and western myanmar confirm that prior to the collision of india an up to 200 km wide magmatic belt extended along the eurasian margin from pakistan to sumatra metamorphic overgrowths to zircons in the orthogneiss near mandalay date a period of eocene 43 ma high grade metamorphism possibly during crustal thickening related to the initial collision between india and eurasia at 65 to 55 ma this was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 ma similar syntectonic syenites and leucogranites intruded the ailao shan red river shear belt in southern china and vietnam during the eocene oligocene to miocene and the wang chao and three pagodas faults in northern thailand that most likely link with the mmb were also active at this time the complex history of eocene to early miocene metamorphism deformation and magmatism in the mmb provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of india and resulting extrusion or rotation of indochina

u pb zircon and chime monazite dating of granitoids and high grade metamorphic rocks from the eastern and peninsular thailand a new report of early paleozoic granite abstract in order to understand the age and tectonic framework of eastern to peninsular thailand from the viewpoint of basement metamorphic and plutonic geology the la icp ms u pb zircon dating and the chemical th u total pb isochron method chime monazite dating were performed in the khao chao hub kapong to pran buri and khanom areas in eastern to peninsular thailand the la icp ms u pb zircon dating of the garnet hornblende gneiss from the khao chao area gave 229 3 ma representing the crystallization age of the gabbro and that of the garnet biotite gneisses gave 193 4 ma representing the timing of an upper amphibolite facies metamorphism the chime monazite dating of pelitic gneiss from the khao chao gneiss gave scattered result of 68 22 ma due to low pbo content and rejuvenation of older monazite grains during another metamorphism in the late cretaceous to tertiary time the u pb ages of zircon from the hua hin gneissic granite in the hub kapong to pran buri area scatter from 250 ma to 170 ma on the concordia granite crystallization was at 219 2 ma followed by the sillimanite grade regional metamorphism at 185 2 ma monazite in the pelitic gneiss from this area also preserves early to middle jurassic metamorphism and rejuvenation by later contact metamorphism by non foliated granite or by another fluid infiltration event in the late cretaceous to tertiary time the khao dat fa granite from the khanom area of peninsular thailand gave a u pb zircon age of 477 7 ma this is the second oldest granite pluton ever reported from thailand and is a clear evidence for the sibumasu block having a crystalline basement that was formed during the pan african orogeny the khao pret granite gives u pb zircon concordia age of 67 5 1 3 ma which represents the timing of zircon crystallization from the granitic melt and accompanied sillimanite grade contact metamorphism against surrounding metapelites and gneisses metamorphic rocks in the doi inthanon area also share the similar plutono metamorphic history with the khanom and the hub kapong to pran buri areas this suggests that these three areas belong to the sibumasu block and the sibumasu block records similar plutono metamorphic history from northern to peninsular thailand relative abundance of oceanic components in the khao chao gneiss their late triassic magmatic ages and the early jurassic metamorphic ages prefer the interpretation that the khao chao gneiss belongs to the sukhothai arc

postglacial history of subalpine forests olympic peninsula washington usa to investigate subalpine vegetation history on the olympic peninsula washington usa two pol len macrofossil and charcoal records were studied in climatically distinct zones martins lake 1415 m in the moist tsuga mertensianazone and moose lake 1508 m in the drier abies lasiocarpa zone the interpret ation of fossil pollen assemblages was aided by comparisons with 308 modern assemblages from the olympic peninsula and western north america both pollen records show a cold dry period following deglaciation 10 000 radiocarbon years bp with sparse tundra and little similarity to any modern pollen assemblage in the early holocene when summer temperatures are thought to have been higher than present high percentages of alnus sinuata type pollen at both lakes suggest increased avalanche activity at martins lake warmer summers were not accompanied by forest establishment possibly because persistent spring snow pack snow avalanches and or edaphic constraints limited tree establishment at this site the martins lake record shows a steplike shift in vegetation to modern tsuga mertensiana abies amabilis parkland across the mazama tephra 6730 bp in contrast to martins lake abies lasiocarpa forest quickly established at moose lake in the early holocene though forests were probably initially open and fires may have been frequent from 7800 to 5100 bp forests near moose lake shifted gradually to cooler and moister species composition with the addition of chamaecyparis nootkatensis tsuga mertensianaand pinus though abies lasiocarpa remained dominant for est cover was probably greatest during this transition with parklike conditions at moose lake beginning at c 5100 bp the major differences in the records between the two sites may be due to differences in the local expression of regional climatic change and or differences in soil development and stabilization

structure and evolution of the ban pong basin chiang mai province thailand abstract the ban pong basin bpb is a small intermontane basin of neogene age on the western margin of the chiang mai basin in northern thailand the bpb was discovered in 2012 because of new quarrying activity the basin complex formed in the hanging wall of a major cenozoic low angle normal fault lanf called the inthanon detachment and provides a unique record of the neogene deformation related to the detachment the bpb s preservation is probably due to its location in a synform within the rising western ranges metamorphic complex which forms the suthep inthanon mountain range initially extensional faults in the bpb were oriented n s but then rotated to nnw sse nw se and ene wsw to ne sw directions which we interpret as arising due to local stress rotation related to the adjacent rising metamorphic complex folds thrusts and inverted normal faults both within neogene sediments of the mae rim formation and adjacent palaeozoic metasedimentary rocks indicate a phase of compression that affected the basin late in its history probably latest miocene pliocene based on the regional timing of inversion complete bouguer anomaly interpretations of 235 reduced gravity stations reveal two en echelon sub basins bounded by gravity highs related to shallow lying palaeozoic rocks and underlying high grade metamorphic basement rocks two gravity density cross section models indicate the basin fill comprises up to 250 m of weakly lithified syn rift sediments the absence of a classic half graben morphology is probably related to inversion and erosion that significantly modified the extensional basin geometry and original thickness

direct dating of left lateral deformation along the red river shear zone china and vietnam 1 exposures of high grade midcrustal rocks within the red river shear zone rrsz which separates the indochina and south china blocks exhibit clear evidence of left lateral ductile deformation assuming that the south china sea represents a pull apart basin formed at the southeastern termination of the rrsz it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the rrsz between 32 and 17 ma at a rate of 4 cm yr while 40ar 39ar thermochronometry indicates that left lateral slip occurred along the rrsz between 25 and 17 ma the timing of earlier high temperature deformation has not been directly constrained in situ th pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite grade metamorphism and synchronous left lateral shearing results from northern segments of the rrsz in yunnan china indicate that synkinematic garnet growth occurred between 34 and 21 ma and are the first to document late oligocene metamorphism and left lateral shearing data from the southern rrsz within vietnam are complicated by tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the indosinian orogeny 220 ma the period during which sinistral deformation is now constrained to have occurred along the rrsz i e 34 17 ma is essentially coincident with spreading of the south china seafloor 32 17 ma this temporal and kinematic link between left lateral shearing along the rrsz and opening of the south china sea supports the view that indochina was extruded from asia as a block along lithospheric scale strike slip faults

new constraints on the structure thermochronology and timing of the ailao shan red river shear zone se asia new structural petrographic and 40ar 39ar data constrain the kinematics of the asrr ailao shan red river shear zone in the xuelong shan xls geochronological data reveal triassic early tertiary and oligo miocene thermal events the latter event 33 26 ma corresponds to cooling during left lateral shear in the fansipan fsp range thrusting of the sapa nappe linked to left lateral deformation and cooling of the fsp granite occurred at 35 ma rapid cooling resumed at 25 29 ma as a result of uplift within the transtensive asrr in the daynuiconvoi dncv foliation trends nw se but is deflected near large scale shear planes stretching lineation is nearly horizontal on steep foliations shear criteria indicate left lateral shear sense zones with flatter foliations show compatible shear senses petrographic data indicate decompression from 6 5 kbar during left lateral shear temperatures 700 c 40ar 39ar data imply rapid cooling from above 350 c to below 150 c between 25 and 22 ma without diachronism along strike along the whole asrr cooling histories show two main episodes 1 rapid cooling from peak metamorphism during left lateral shear 2 rapid cooling from greenschist conditions during right lateral reactivation of the asrr in the nw part of the asrr xls diancang shan we link rapid cooling 1 to local denudations in a transpressive environment in the sw part ailao shan and dncv cooling 1 resulted from regional denudation by zipper like tectonics in a transtensive regime the induced cooling diachronism observed in the ailao shan suggests left lateral rates of 4 to 5 cm yr from 27 ma until 17 ma dncv rocks always stayed in a transtensive regime and do not show cooling diachronism the similarities of deformation kinematics along the asrr and in the south china sea confirms the causal link between continental strike slip faulting and marginal basin opening

crustal deformation and source models of the yellowstone volcanic field from geodetic data 1 geodetic observations comprising interferometric synthetic aperture radar insar global positioning system gps and precision leveling measurements are used to infer volume change in the subsurface associated with the dynamics of the yellowstone volcanic system we focus primarily on the yellowstone caldera and its related magmatic hydrothermal and fault systems it appears that known faults play a significant role in controlling crustal volume increases and decreases due to the migration of volcanic and hydrothermal fluids for example over 5 cm of subsidence from 1992 to 1995 is associated with source volume changes 6 10 km beneath the nw trending elephant back fault zone and a north trending fault cutting across the caldera furthermore we are able to image an episode of fluid intrusion near the northern edge of the caldera the intrusion is elongated in the north south direction and is parallel to the north trending volume decrease the primary intrusion and related hydrothermal activity occurred between 1996 and 2000 though the volume changes appear to have continued shallowed and changed shape between 2000 and 2002 there is evidence that the intrusive activity influenced extensional faults to the north of the caldera

ar ar and fission track ages in the song chay massif early triassic and cenozoic tectonics in northern vietnam the song chay massif is the northeasternmost metamorphic complex in vietnam to the east of the red river shear zone it shows a large antiformal structure involving orthogneisses and migmatites overlain on its northern flank by muscovite bearing marbles an e w striking fault bounds the dome to the south kinematic indicators along a s n section reveal top to the n shear sense along the interface between the orthogneissic core and the overlying metasediments radiometric ages were obtained by the 40ar 39ar method using purified mica separates across the dome ages range from 236 ma at the southern edge to 160 ma in the core attesting to a strong imprint in the early triassic time a clear difference is seen between these mesozoic ages and the eocene to miocene ages from 40 to 24 ma that obtained in the nearby red river shear zone using the same method these data show that the song chay massif was already high in the crust when the high temperature deformation of the red river shear zone took place the final exhumation of the song chay orthogneiss constrained by fission track analysis on samples along the same transect occurred during the early miocene and could be interpreted as the consequence of a first normal sense of motion along the fault which bounds the massif to the south timing is similar to that of exhumation in the red river shear zone

east flank of the sibumasu block in nw thailand and myanmar and its possible northward continuation into yunnan a review and suggested tectono stratigraphic interpretation abstract the east flank of the sibumasu block was a passive continental margin and in nw thailand is marked by the absence of the autochthonous middle permian triassic platform carbonates which are widespread across the rest of sibumasu further west instead the carbonates are represented by hemipelagic cherts mudstones and sandstones including turbidites during the northward drift of sibumasu following its early permian rifting from gondwana an accretionary complex was present where palaeotethyan pelagic rocks as old as devonian were subducted beneath the sukhothai volcanic arc at the time of sibumasu s collision with the sukhothai arc beginning in the middle triassic the accretionary complex was thrust westwards across the east flank of sibumasu it is suggested that in the late triassic the thrust pile which had been the accretionary complex underwent erosion and was the source of terrigenous clastic rocks deposited further west in a foredeep basin the boundary of sibumasu s east flank with the permo triassic carbonate platform further west is the arcuate mae ping nam teng fault system notwithstanding later cenozoic strike slip displacement those faults as well as the mae yuam fault are interpreted to have had an earlier history of westward directed indosinian thrusting northwards in myanmar and yunnan the sibumasu permo triassic carbonate shelf continues as the shan plateau and baoshan block the east flank is represented by the changning menglian belt and the palaeotethys cryptic suture in thailand possibly joins with the lancangjiang suture

tectonics and metallogeny of mainland southeast asia a review and contribution abstract the southeast asia region is endowed with a diversity of mineral resources notably porphyry related skarn epithermal and sediment hosted orogenic gold deposits formation of these deposits was associated with a long and complex tectonic history of gondwana supercontinent break up arc magmatism backarc basin development together with arc continent and continent continent collisions that created the present day mainland se asia this paper summarizes historical and current se asian geological research and ore deposit studies here we present a new tectonic and metallogenic model for phanerozoic mainland se asia from this model we conclude that incipient arc backarc basin magmatism is the key to the formation of many important ore deposits in the truong son and loei fold belts the two major metallogenic belts in mainland se asia in addition triassic to cenozoic arc continent and continent continent collisions have led to the formation of many sediment hosted orogenic gold deposits in the sukhothai and the sibumasu terranes oblique cretaceous to recent subduction along the andaman sunda trench was responsible for gold and copper gold molybdenum porphyry and epithermal mineralization along the kawlin wutho mt popa arc in myanmar in the north and the sumatran volcanic arc in the south we anticipate that the present emphasis on exploring for shallower level epithermal deposits will increasingly lead to exploration for deeper level porphyry and porphyry related skarn systems in the coming decades

late paleogene rifting along the malay peninsula thickened crust abstract sedimentary basins often develop above internal zones of former orogenic belts we hereafter consider the malay peninsula western sunda as a crustal high separating two regions of stretched continental crust the andaman malacca basins in the western side and the thai malay basins in the east several stages of rifting have been documented thanks to extensive geophysical exploration however little is known on the correlation between offshore rifted basins and the onshore continental core in this paper we explore through mapping and seismic data how these structures reactivate pre existing mesozoic basement heterogeneities the continental core appears to be relatively undeformed after the triassic indosinian orogeny the thick crustal mega horst is bounded by complex shear zones ranong klong marui and main range batholith fault zones initiated during the late cretaceous early paleogene during a thick skin transpressional deformation and later reactivated in the late paleogene the extension is localized on the sides of this crustal backbone along a strip where earlier late cretaceous deformation is well expressed to the west the continental shelf is underlain by three major crustal steps which correspond to wide crustal scale tilted blocks bounded by deep rooted counter regional normal faults mergui basin to the east some pronounced rift systems are also present with large tilted blocks western thai songkhla and chumphon basins which may reflect large crustal boudins in the central domain the extension is limited to isolated narrow n s half grabens developed on a thick continental crust controlled by shallow rooted normal faults which develop often at the contact between granitoids and the host rocks the outer limits of the areas affected by the crustal boudinage mark the boundary towards the large and deeper andaman basin in the west and the malay and pattani basins in the east at a regional scale the rifted basins resemble n s en echelon structures along large nw se shear bands the rifting is accommodated by large low angle normal faults lanf running along crustal morphostructures such as broad folds and mesozoic batholiths the deep andaman malay and pattani basins seem to sit on weaker crust inherited from gondwana derived continental blocks burma sibumasu and indochina the set of narrow elongated basins in the core of the region khien sa krabi and malacca basins suffered from a relatively lesser extension

did oligocene crustal thickening precede basin development in northern thailand a geochronological reassessment of doi inthanon and doi suthep the doi inthanon and doi suthep metamorphic core complexes in northern thailand are comprised of amphibolite grade migmatitic gneisses mantled by lower grade mylonites and metasedimentary sequences thought to represent cordilleran style core complexes exhumed through the mobilization of a low angle detachment fault previous studies have interpreted two metamorphic events late triassic and late cretaceous followed by ductile extension between the late eocene and late oligocene a model which infers movement on the detachment at ca 40 ma and which culminates in a rapid unroofing of the complexes in the early miocene the chiang mai basin the largest such cenozoic basin in the region lies immediately to the east its development is related to the extension observed at doi inthanon and doi suthep however it is not definitively dated and models for its development have difficulty reconciling miocene cooling ages with eocene detachment movement here we present new in situ la icp ms and sims u pb age data of zircon and monazite grains from gneiss and leucogranite samples taken from doi inthanon and doi suthep our new zircon data exhibit an older age range of 221 210 ma with younger ages of ca 72 ma and 32 26 ma our monazite data imply an older age cluster at 83 67 ma and a younger age cluster of 34 24 ma while our data support the view of indosinian basement being reworked in the cretaceous they also indicate a late eocene oligocene tectonothermal event resulting in prograde metamorphism and anatexis we suggest that this later event is related to localized transpressional thickening associated with sinistral movement on the mae ping fault coupled with thickening at the restraining bend of the mae yuan fault to the immediate west of doi inthanon further this upper oligocene age limit from our zircon and monazite data would imply a younger miocene constraint on movement of the detachment which when combined with the previously recorded miocene cooling ages has implications for a model for the onset of extension and subsequent development of the chiang mai basin in the early mid miocene

understanding sibumasu in the context of ribbon continents abstract well described ribbon continent collision belts and modern examples with 2d seismic coverage particularly zealandia provide analogues for the permo triassic development of sibumasu two key parameters for the ribbon continents are their width and how rifting developed which in turn impact the structural metamorphic igneous and stratigraphic development of ribbon continents in ways that make them significantly different from the collisional development of major continents key characteristics of the tectonics of sibumasu are 1 a tendency towards basin inversion during the late triassic and possibly early jurassic and mixed thick skinned and thin skinned style deformation 2 extensional collapse around 215 ma is an important feature of sibumasu particularly in northern thailand and probably other areas too 3 slab delamination and extension related decompression melting were probably important mechanisms for generating granites in addition to crustal thickening 4 metamorphism is relatively high temperature lower pressure

p t t constraints on the development of the doi inthanon metamorphic core complex domain and implications for the evolution of the western gneiss belt northern thailand abstract the western gneiss belt in northern thailand is exposed within two overlapping cenozoic structural domains the extensional doi inthanon metamorphic core complex domain located west of the chiang mai basin and the mae ping strike slip fault domain located west of the tak batholith new p t estimates and u pb and 40ar 39ar age determinations from the doi inthanon domain show that the gneiss there records a complex multi stage history that can be represented by a clockwise p t t path u pb zircon and titanite dating of mylonitic calc silicate gneiss from the mae wang area of the complex indicates that the paragneissic sequence experienced high grade medium pressure metamorphism m1 in the late triassic early jurassic ca 210 ma in good agreement with previously determined zircon ages from the underlying core orthogneiss exposed on doi inthanon late cretaceous monazite ages of 84 and 72 ma reported previously from the core orthogneiss are attributed to a thermal overprint m2 to upper amphibolite facies in the sillimanite field u pb zircon and monazite dating of granitic mylonite from the doi suthep area of the complex provides an upper age limit of 40 ma late eocene for the early stage s of development of the actual core complex by initially ductile low angle extensional shearing under lower amphibolite facies conditions m3 accompanied by near isothermal diapiric rise and decompression melting 40ar 39ar laserprobe dating of muscovite from both doi suthep and doi inthanon provided miocene ages of ca 26 15 ma representing cooling through the ca 350 c isotherm and marking late stage development of the core complex by detachment faulting of the cover rocks and isostatic uplift of the sheared core zone and mantling gneisses in the footwall similarities in the thermochronology of high grade gneisses exposed in the core complex and shear zone domains in the western gneiss belt of northern thailand and also in northern vietnam laos yunnan and central myanmar suggest a complex regional response to indentation of southeast asia by india

a 12 000 year record of vertical deformation across the yellowstone caldera margin the shorelines of yellowstone lake the 600 ka yellowstone caldera exhibits several signs of unrest the most evident of which is historic ground deformation including both uplift and subsidence we document deformation in the area of the southeastern caldera across 12 000 years using the postglacial shoreline terraces of yellowstone lake raised shoreline elevations were interpreted from 230 leveling profiles surveyed across flights of terraces with an accuracy of 0 5 m of about 11 recognizable terraces the five most continuous raised shorelines were correlated around the lake basin to reveal deformation patterns shoreline ages are based on minimum and maximum limiting radiocarbon and obsidian hydration dates each terrace is interpreted as representing an episode of uplift 1 kyr 1 of the caldera interior and subsequent subsidence with little net volume change this cyclic behavior may result from magma emplacement and subsequent withdrawal or cooling and crystallization and or episodic trapping and release of magmatic fluids evolved during cooling and crystallization in a self sealing reservoir as hypothesized by dzurisin et al 1990 early postglacial shoreline deformation reveals substantial intracaldera subsidence possibly reflecting cooling and loss of trapped hydrothermal fluids extension along north south trending normal faults and related structures is also apparent in early to middle postglacial deformation both inside and outside of the caldera margin including downwarping and local faulting of hot weak intracaldera crust net deformation over the past 3 kyr has been dominantly up within the caldera interior and slightly down along the caldera rim relative to the extracaldera region this uplift is roughly similar to the historic pattern and may largely represent the effects of the most recent inflation episode subtraction of the total estimated magnitude of inflation in this episode suggests that the overall trend of postglacial deformation has been subsidence the cause of this trend is undetermined but is most likely related to the effects of regional extension and long term cooling within the yellowstone caldera

dating multiple generation of zircons from granites and gneiss from thailand implication for the crustal evolution of the sibumasu terrane abstract the widespread granitic rocks of the sibumasu terrane were emplaced during the collision of the sibumasu and indochina in the triassic previous studies have suggested that each granite had a simple single step magmatic evolution in situ la mc icp ms u pb zircon dating of granites and gneiss from thailand revealed the evolution of these rocks inherited zircon core ages 515 130 ma from a granite of the chonburi pluton in the central granitoid belt are coeval with an orthogneiss protolith age 513 6 9 2 ma from hua hin area similar cambrian ages were also reported from the khao tao area in southern thailand moreover there are a series of inherited zircon core ages from neo proterozoic 1017 43 and 984 56 ma ordovician 449 26 ma devonian 385 15 ma carboniferous 316 12 ma to permian 276 31 and 266 12 ma taken together these neo proterozoic to permian protolith ages are interpreted to represent the basement of the sibumasu terrane in thailand two magmatic zircon growth events can be distinguished at approximately a 241 8 8 2 to 235 12 ma and b 230 6 3 4 to 213 4 3 0 ma we concluded that the sibumasu and indochina collision during the triassic caused a two step magmatic evolution of the granites of the sibumasu terrane the first magmatic crystallization during the middle triassic corresponded to the sibumasu and indochina collision and the second magmatic crystallization during the late triassic was related to the late collisional stage

combined escape tectonics and subduction rollback back arc extension a model for the evolution of tertiary rift basins in thailand malaysia and laos the tertiary rift basins of thailand and adjacent countries show considerable variability in the timing of rift initiation termination the timing and magnitude of thermal subsidence and the timing and intensity of inversion episodes the rift basins developed on continental blocks that were extruded southeastwards hence their development must be tied into himalayan extrusion tectonics current tectonic models propose that the tertiary basins opened up as pull apart basins associated with strike slip faults published geochronology of strike slip fault zone rocks mapping of fault patterns in the tertiary basins and mapped releasing restraining bend geometries all indicate that in thailand major sinistral strike slip motion ceased at about 30 ma prior to the formation of most rift basins the thailand the effects of later dextral slip were minor and probably a result of reactivation during episodes of inversion during nw se to ne sw himalayan compression dextral slip was not responsible for opening most of the rift basins in thailand an alternative mechanism to open the rift basins is subduction rollback of the indian plate to the west of thailand it is proposed that subuction rollback can help explain some of the characteristics of the rift basins such as non uniform lithospheric extension deep sag basins and the diachronous onset and termination of rifting

tectonics of strike slip restraining and releasing bends restraining and releasing bends are common but enigmatic features of strike slip fault systems occurring in all crustal environments and at regional to microscopic scales of observation regional scale restraining bends are sites of mountain building transpressional deformation and basement exhumation whereas releasing bends are sites of topographic subsidence transtensional deformation basin sedimentation and possible volcanism and economic mineralization because restraining and releasing bends often occur as singular self contained domains of complex deformation they are appealing natural laboratories for earth scientists to study fault processes earthquake seismology active faulting and sedimentation fault and fluid flow relationships links between tectonics and topography tectonic and erosional controls on exhumation and tectonic geomorphology this volume addresses the tectonic complexity and diversity of strike slip restraining and releasing bends with 18 contributions divided into four thematic sections 1 a topical review of fault bends and their global distribution 2 bends sedimentary basins and earthquake hazards 3 restraining bends transpressional deformation and basement controls on development 4 releasing bends transtensional deformation and fluid flow

lower permian conodonts from palaeo tethys ocean plate stratigraphy in the chiang mai chiang rai suture zone northern thailand abstract lower permian lower sakmarian conodonts are reported from a coherent section of ocean plate stratigraphy and from a limestone block in the palaeo tethys suture zone between lamphun and lampang south of chiang mai northern thailand conodont species from both the pelagic limestones of the ops section and the limestone block are deep water forms with distinctive biogeographic affinities comparable to faunas of the urals and north american mid continent a new name chiang mai chiang rai suture zone is here proposed for the palaeo tethys suture in northern thailand that forms the boundary between the sibumasu terrane and the sukhothai arc terrane the inthanon zone of northern thailand is interpreted as representing a fold and thrust belt west of the chiang mai chiang rai suture zone which comprises sibumasu terrane continental margin rocks and remnant klippen of chiang mai chiang rai suture zone rocks thrust as a nappe westward during the triassic collision between sibumasu and the sukhothai arc indochina terrane

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

exhumation of the ailao shan shear zone recorded by cenozoic sedimentary rocks yunnan province china 1 the role of strike slip faults such as the ailao shan shear zone in southwestern china in accommodating the india eurasia collision remains controversial cenozoic sedimentary rocks preserved along the northeast side of the ailao shan shear zone however record the nature and timing of exhumation of shear zone rocks field mapping shows that the lower strata consist of early oligocene lacustrine red mudstone and gypsum and the upper of late oligocene to miocene fluvial conglomerate with rare fine grained fossil leaf bearing intervals sediment derived from shear zone gneiss is present only in the upper part of the section our observations indicate deformation began in oligocene time with the onset of rapid exhumation by early miocene time consistent with existing geochronologic results exhumation must have ended before the formation of a regional low relief erosion surface and the onset of brittle deformation along the red river fault probably in early pliocene time rapid exhumation of the ailao shan shear zone has been interpreted to have occurred in transtension accommodated by a normal fault on the northeast margin of the shear zone however we observe consistent and pervasive shortening structures within the cenozoic strata including overthrusting and folding of the basin along faults defined by thick gouge zones within weak cenozoic lithologies deformation is likely syndepositional these structures are distinct from subvertical deformational features associated with the active red river fault our field data thus support mid cenozoic transpressional exhumation of the ailao shan shear zone this is consistent with regional evidence for widespread left lateral and compressional deformation in the region to the east of the eastern himalayan syntaxis during this time focused erosion along the red river established by late oligocene time may have contributed to deeper exhumation of the northeast part of the shear zone

two dimensional inverse modeling of sedimentary basin subsidence recently developed mathematical models of subsidence in extensional basins provide a good basis for forward modeling however the large number of parameters to be specified by the user makes it difficult not only to do the modeling itself but also to judge the meaning of the results we present a new method for automatic searching of the best fitting parameter set of a 2d basin formation model weighted goal functions are used in the minimization process by the inverse problem solver algorithm the nominal parameter set in the present case includes profiles of the crustal and subcrustal thinning factors and the level of lithosphere necking the method was tested on synthetic data with parameters that were perturbed beforehand with some restrictions the algorithms are capable of resolving such perturbations the inversion technique has been applied to two profiles crossing the dnieper donets basin ukraine the crustal thinning factors obtained argue for a scissors like style of basin opening high subcrustal thinning values are necessary in order to explain the abnormally thick carboniferous section in the basin these values should be treated as cumulative ones due to the currently unresolvable influence of other rifting related processes particularly phase transitions

long and complex thermal history of the song chay metamorphic dome northern vietnam by multi system geochronology multi system geochronology was used to unravel the long and complex thermal history of the song chay range northern vietnam a high grade granitic and metamorphic dome in the vicinity of the cenozoic ailao shan red river fault zone it was considered to be proterozoic south china basement but its geological history was basically unknown scattered field observations suggest three episodes of high temperature deformation firstly at the time of granite emplacement secondly a decollement with top to the north shear and thirdly anticlinal doming of the foliations formed during the two first stages p t estimates suggest that metamorphism coeval with the second deformation phase culminated at 580 ca nd 4 5 kbar 16 km depth multi system geochronology is applied to a two micas granite sample slightly deformed within the decollement u pb dating of zircon yields an age of 428 5m a 2s interpreted as the time of granite crystallization within the south china caledonian belt rb sr on white micas and biotite yields ages of 206 10 ma and 176 5 3 ma respectively 2s whereas 39ar 40 ar ages of the same minerals are 210 9a nd 190 8m a 2s these ages suggest an upper triassic episode of rapid cooling interpreted as due to doming a few million years after the end of movement on the decollement the k feldspar irregular 39ar 40ar age spectrum can to the first order be explained by a cooling history with two episodes of rapid cooling one at 140 ma and a second around 41 to 25 ma apatite fission tracks central age 33 6 3 6 ma 1s confirms a tertiary rapid cooling event interpreted as the final exhumation of the song chay dome 2000 elsevier science b v all rights reserved

tertiary diachronic extrusion and deformation of western indochina structural and 40ar 39ar evidence from nw thailand the wang chao and three pagodas fault zones cut the western part of the indochina block and run parallel to the red river fault evidence of intense ductile left lateral shear is found in the lansang gneisses which form a 5 km wide elongated core along the wang chao fault zone dating by 40 ar 39 ar shows that such deformation probably terminated around 30 5 ma the wang chao and three pagodas faults offset the north striking lower mesozoic metamorphic and magmatic belt of northern thailand 40 ar 39 ar results suggest that this belt suffered rapid cooling in the tertiary probably around 23 ma these results imply that the extrusion of the southwestern part of indochina occurred in the upper eocene lower oligocene it probably induced rifting in some basins of the gulf of thailand and in the malay and mekong basins in the oligo miocene the continuing penetration of india into asia culminated with the extrusion of all of indochina along the ailao shan red river fault this occurred concurrently with the onset of e w extension more to the south plotting in a geographical reference frame the diachronic time spans of movement on left lateral faults east and southeast of tibet implies that the northward movement of the indian indenter successively initiated new strike slip faults located farther and farther north along its path

postglacial vegetation and climate of the cascade range central oregon abstract pollen data from two sites provide information on the postglacial vegetation and climate history of the cascade range indian prairie in the western cascade range was colonized by subalpine forests of pinus picea and tsuga and open meadows prior to ca 12 400 14 c yr b p the treeline lay 500 to 1000 m below its modern elevation and conditions were cooler than at present from ca 12 400 to ca 9950 14 c yr b p abies became important and the forest resembled that presently found at middle elevations in the western cascade range the pollen record implies a rise in treeline and warmer conditions than before from ca 10 000 to 4000 4500 14 c yr b p conditions that were warmer and effectively drier than today led to the establishment of a closed forest composed of pseudotsuga abies and at lower elevations quercus and corylus during this period gold lake bog in the high cascades was surrounded by closed forest of pinus and abies the early holocene pollen assemblages at both indian prairie and gold lake bog lack modern analogues and it is likely that greater than present summer radiation fostered unique climatic conditions and vegetation associations at middle and high elevations in the late holocene beginning ca 4000 4500 14 c yr b p cooler and more humid conditions prevailed and the modern vegetation was established a comparison of these sites with others in the pacific northwest suggests that major patterns of vegetational change at individual sites were a response to large scale changes in the climate system that affected the entire region

assessment of global positioning system measurements for studies of crustal deformation the u s geological survey has been using the global positioning system gps to perform repeated surveys of networks of different sizes since january 1986 the intersite distances from these various networks are 223 km for a single line connecting the palos verdes and vandenberg sites in southern california 30 50 km for a network of three sites south of the san francisco bay near loma prieta 9 30 km for a network located near hebgen lake montana 7 12 km for a network of sites in the vicinity of parkfield california and 240 m for a line located at the goldstone antenna complex in the mojave desert the lengths quoted do not include the intersite distances for the fiducial sites included in the solutions for orbit determination which can be up to 8000 km we have attempted to assess the long term precision and accuracy of determinations of relative site position for a range of line lengths this assessment has consisted of several different studies in assessing precision we have generally investigated repeatability usually about a best fit linear temporal trend the smallest fractional repeatability was yielded by the measurements on the palos verdes to vandenberg line the root mean square rms residual about the best fit line for these determinations was 6 mm 0 03 parts per million or ppm for the north component 11 mm 0 05 ppm for the east component and 40 mm 0 18 ppm for the vertical component for these determinations gps data from 3 4 consecutive days were combined to produce a single estimate of the vector separation we also used the day to day scatter from these measurements to perform an analysis of variance and found that these multiday determinations can be significantly more precise than estimates determined from a single observing session the worst results were obtained for the loma prieta network where we obtained rms residuals of 11 12 mm 0 3 0 4 ppm for north 14 18 mm 0 3 0 6 ppm for east and 18 21 mm 0 4 0 7 ppm for vertical we cannot definitely explain these poor results but we have evidence to indicate that this poor scatter is due to the short observing session 4 5 hours compared to 6 5 hours for the palos verdes to vandenberg results in assessing accuracy we compared the gps results to those from independent techniques at parkfield we compared the gps estimates of fault parallel motion to those from creep meters and alignment arrays and found excellent agreement within 1 2 mm yr 1 not only for the slip but also for the variation of slip along the san andreas fault we compared the loma prieta and hebgen lake line length estimates to those obtained from a geodolite a high precision laser distance measurement device that the u s geological survey has used for 20 years for studies of crustal deformation if we model the geodolite minus gps line length differences l as l l where l is the line length then we find 0 6 0 5 mm and 0 1 0 1 mm km 1 the uncertainties quoted are one standard deviation from this comparison we conclude that at this level of significance there is no detectable difference between lengths determined from geodolite and gps data finally we compared the gps estimates of the palos verdes to vandenberg line length to those obtained from very long baseline interferometry vlbi the mean difference between the six gps estimates of line length and a best fit line through the vlbi estimates was 1 mm and the rms difference was 12 mm 0 05 ppm close to the standard deviation of the gps measurements for this predominantly east west line

structural development of the mid tertiary doi suthep metamorphic complex and western chiang mai basin northern thailand abstract the northern western ranges of thailand contain mylonitic gneisses of the doi suthep metamorphic complex dsmc near chiang mai mylonitic foliations outline a southeast trending dome and stretching lineations trend n80 w low grade metasedimentary rocks and tertiary fanglomerates flank the dome on the east and widespread chloritic breccia marks the intervening doi suthep detachment fault we interpret that the metamorphic complex developed between triassic and early miocene with detachment faulting mylonitization and uplift during oligocene to miocene within the dsmc dikes of mutually cross cutting hence coeval granitic pegmatite and aplite cut mylonitic orthogneiss although the contacts of the dikes are sharp the dikes contain a mylonitic fabric that parallels the fabric in the surrounding mylonitic orthogneiss thus we infer that intrusion occurred during mylonitization east of doi suthep tertiary fanglomerates of the mae rim formation overlie the doi suthep detachment fault variable strikes with dips of 10 50 and several outcrop scale fold hinges suggest that the mae rim formation is deformed into a set of non cylindrical folds numerous variably striking high angle normal faults suggest either multiple deformations or deformation in a three dimensional strain field the discovery of an intraformational angular unconformity confirms that the mae rim formation accumulated during folding

age tectonic setting and regional implications of the chiang khong volcanic suite northern thailand the chiang khong volcanic suite forms the northeastern part of the tak chiang khong volcanic belt in the sukhothai terrane of northern thailand it consists mainly of subaerial andesitic to rhyolitic tuffaceous rocks a rhyolitic tuff yielded a middle triassic u pb zircon age of 232 9 0 4 ma the suite displays calc alkalic chemical trends and nb p and ti depletion characteristic of subduction related magmatism based on petrological similarities the suite is correlated with the c 240 ma lampang volcanic rocks located farther to the sw in the tak chiang khong belt and with volcanic rocks of similar age in the lincang jinghong belt in yunnan in the western part of the simao block these correlations support the placement of the palaeo tethyan suture to the west of the sukhothai terrane

contemporary vertical surface displacements in yellowstone national park relative vertical displacements and average relative vertical velocities have been determined for bench marks located throughout yellowstone national park ynp by comparing geodetic leveling measurements made in 1923 second order 1936 third order 1941 third order 1955 third order 1960 first order and 1975 1977 first order the 1923 1975 data indicate uplift greater than 400 mm 7 7 mm yr of an area approximately 15 20 km wide and at least 40 km long which is centered along the northeast trending axis of the 600 000 year old yellowstone caldera the maximum 1923 1975 vertical displacement in this area is 726 mm 14 0 mm yr the existence of a well defined and untilted yellowstone lake terrace implies that the 1923 1975 average vertical velocities cannot have been maintained for more than a few hundred years the 1955 1977 data from central ynp indicate that the northeastern caldera half has risen relative to the southwestern half at an average velocity of about 8 mm yr a similar bias is present in the 1923 1975 data but it is less pronounced the 1923 1960 and 1960 1975 data from northwestern ynp and a 200 mm depression in the 1923 1975 data near norris junction may be interpreted in terms of deformation associated with the 1959 hebgen lake earthquake and the 1975 yellowstone park earthquake a composite profile of the 1936 1977 and 1941 1977 average vertical velocity data which skirts the edge of the yellowstone caldera in northeastern ynp is most easily explained as the result of random leveling error and or bench mark instabilities the apparent youth of the 1923 1975 yellowstone uplift and its close association with the yellowstone caldera suggest that the most likely cause of the uplift is a recent not more than 500 years ago influx of molten material into the upper crust beneath the caldera

closure of the east paleotethyan ocean and amalgamation of the eastern cimmerian and southeast asia continental fragments abstract the phanerozoic record of southeast asia preserves the history of opening and subsequent consumption of the paleotethyan ocean and the assembly of continental fragments into asia as part of the broader scale reconstruction of pangea however uncertainty remains as to which of the many suture zones in southeast asia represents the relict of the main ocean when final ocean closure occurred and the assembly history of the eastern cimmerian and southeast asia continental fragments our overview of the geological features of the suture zones the bounding continental fragments and their magmatic metamorphic and sedimentary records resolves many of these key issues the sedimentary biogeographical structural lithological geochemical and geochronological data from the changning menglian inthanon and bentong raub suture zones argue for their linkage with the longmu co shuanghu suture zone in central tibet and together constitute the main east paleotethyan ocean relict the eastward subduction of the ocean resulted in the development of a series of magmatic arc back arc basin and continental fragments in southeast asia including from west to east the lincang sukhothai east malaya arc the jinghong nan sa kaeo back arc basin the simao west indochina fragment the luang prabang loei back arc basin the south indochina fragment the wusu and truong son back arc basins the north indochina fragment the jinshajiang ailaoshan song ma branch back arc basin and the south china block assembly of these fragments resulted in indosinian high temperature and high pressure metamorphism and related tectonothermal event available data indicate a switch from subduction of the main east paleotethyan ocean to the collision of the sibumasu with simao indochina blocks at 237 ma with subsequent syn and post collisional events at 237 230 ma and 230 200 ma respectively along the changning menglian inthanon and bentong raub suture zones the timing of initial syn and post collision events along the jinshajiang ailaoshan song ma suture zone with its record of back arc basin closure is at 247 ma 247 237 ma and 237 200 ma generally 10 ma older than that along the changning menglian inthanon and bentong raub suture zones our synthesis of all available data enables establishment of a comprehensive geodynamic model for the east paleotethyan evolution this model links the spatial temporal pattern across southeast asia into a series of tectonic events including ocean back arc basin opening subduction closure subsequent assemblage and orogenic collapse along with associated igneous metamorphic and sedimentary activities

reconstruction of holocene changes in alpine vegetation and climate in the maligne range jasper national park alberta abstract pollen macrofossil logs and needles and oxygen isotope data from tree rings are presented for three alpine bog sites in the maligne range of the canadian rockies organic sedimentation at the watchtower site began prior to ca 9500 14 c yr b p and by 8770 yr b p timberlines were at least 100 m above present levels the two pollen records are dominated by pinus and subsequent changes in timberlines are inferred from pollen ratio data abies pinus and from macrofossils the recovered records indicate two periods of high hypsithermal timberlines ca 8800 7500 and 7200 5200 yr b p separated by a short interval ca 7300 7400 yr b p when timberline approached modern levels after ca 4500 yr b p timberlines have been similar to or lower than present reaching minimum values in the last 500 yr 18 o determinations from the cellulose of samples of 5 or 10 annual rings cut from contemporary picea englemannii at timberline show strong correlations with mean annual temperatures at jasper isotopic determinations from hypsithermal age logs recovered from sites above present timberline indicate mean annual temperatures were at least 0 5 c warmer ca 8060 and 8770 yr b p than at present at the watchtower site data from logs at the maligne pass site suggest that temperatures were about 1 2 and 1 6 c warmer ca 6000 and 5300 yr b p estimates of temperature from the pollen ratio data are more conservative and suggest maximum differences of ca 1 0 c in july temperatures over the last 8000 yr

aseismic growth of durmid hill southeasternmost san andreas fault california our 2 27 km leveling line across and normal to the san andreas fault at durmid hill domed upward by 9 mm between september 1985 and december 1991 the flanks of the line tilted more than 1 rad yr implying that the uplift extends beyond the ends of the line and that absolute uplift of the crest of the hill may greatly exceed the measured rate of 1 5 mm yr trilateration measurements sampling 5 km of the fault zone within the hill from 1978 to 1992 yield a mean dextral slip rate of 2 4 0 3 mm yr and fault normal extension of 1 0 3 mm yr the inferred uplift rate during the 5 5 year leveling period as well as from geomorphic samples at 330 years and 35 000 years and from erosion of sedimentary strata younger than 738 000 years in monoclines abutting the fault at durmid hill is remarkably consistent at 1 to 4 mm yr and implies that the current processes may have persisted for millennia we investigate two mechanisms for the deformation that are defined by the symmetry of the observed doming transpression on a vertical or near vertical subsurface san andreas fault or slip on a shallow flat fault or detachment that terminates beneath durmid hill the resultant models suggest that uplift rates of 1 mm yr will result from 6 mm yr dextral shear along a vertical fault together with 1 mm yr horizontal convergence across that fault and this strain field is consistent with the paleoseismic record and our leveling data we conclude that interseismic processes account for most of the growth of durmid hill because the interseismic vertical deformation rates equal or exceed the long term vertical slip rate measured across the fault because dextral shear of 6 mm yr at durmid hill represents a small fraction of the 25 mm yr measured across the entire san andreas fault zone in coachella valley and because the san andreas fault at durmid hill may represent an earthquake nucleation zone the observed deformation processes may be of significant importance for understanding future earthquake rupture initiation or termination

evolution of deformation styles at a major restraining bend constraints from cooling histories mae ping fault zone western thailand the c 500 km long mae ping fault zone trends nw se across thailand into eastern myanmar and has probably undergone in excess of 150 km sinistral motion during the cenozoic a large c 150 km long restraining bend in this fault zone lies on the western margin of the chainat duplex the duplex is a low lying region dominated by north south trending ridges of mesozoic and palaeozoic sedimentary metamorphic and igneous rocks flanked by flat post rift basins of pliocene recent age to the north and south a review of published cooling age data plus new apatite and zircon fission track results indicates that significant changes in patterns of exhumation occurred along the fault zone with time oldest uplift and erosion eocene occurred in the umphang gneiss region west of an inferred thrust dominated restraining bend setting from 36 ma to 30 ma exhumation was strongest north of the duplex along the nw se trending segment of the fault zone at the northern exiting bend of the chainat duplex this region of the fault zone is characterized by a mid crustal level shear zone 5 6 km wide lan sang gneisses that passes to the nw into an apparent strike slip duplex geometry the deformation is interpreted to have occurred during passage around the northern restraining bend which resulted in vertical thickening uplift erosion and extensional collapse of the northern side of the shear zone this concentration of deformation at the bends at the ends of the restraining bend is thought to be a characteristic of strike slip dominated restraining bends following late oligocene early miocene extension there is apatite fission track evidence for 22 18 ma exhumation in the chainat duplex that coincides with a phase of inversion in the phitsanulok basin to the north the miocene recent history of the chainat duplex is one of minor sinistral and dextral displacements related to a rapidly evolving stress field influenced by the numerous tectonic reorganizations that affected se asia during that time

new accelerator mass spectrometry radiocarbon ages for the mazama tephra layer from kootenay national park british columbia canada charcoal fragments recovered from the mazama air fall tephra layer in cores from dog and cobb lakes kootenay national park british columbia yielded accelerator mass spectrometry ages of 6720 70 and 6760 70 14c years bp respectively these two new ages together with other previously published radiocarbon ages on charcoal and twig fragments from mazama air fall deposits indicate that the climactic eruption of mount mazama occurred 6730 40 14c years bp

la icpms zircons u pb dating of permo triassic and cretaceous magmatisms in northern vietnam geodynamical implications abstract in northeastern vietnam the major tectonic episode responsible for nappes emplacement is triassic these allochtonous structures are intruded by granitic melts two post tectonic massifs showing no sign of deformation have been dated by the la icpms zircon u pb techniques dating reveals a multiphase history with zircon cores showing evidence of proterozoic magmatism the emplacement of the phia bioc granite intrusive in allochtonous units is 248 245 ma an age which assesses a younger limit for the major nappes tectonic this tectonic could be synchronous of the tectonometamorphic strike slip faulting events 250 245 ma defined in the truong son belt as the indosinian orogen the phia bioc intrusion is probably linked with the intra plate magmatism of the emeishan large igneous province or with magmatism associated with the paleotethys closure the age of the phia oac granite intrusion in displaced units is much younger at 87 3 1 2 ma this granite is probably linked to the magmatic activity produced during the paleo pacific plate subduction under the se asia continental plate during the mesozoic although the cenozoic red river fault system is close to these two plutons this last thermotectonic episode has not been strong enough to disturb the u pb system zircons rims do not show any tertiary magmatic or metamorphic overprint

the basins of sundaland se asia evolution and boundary conditions abstract most of the basins developed in the continental core of se asia sundaland evolved since the late cretaceous in a manner that may be correlated to the conditions of the subduction in the sunda trench by the end of mesozoic times sundaland was an elevated area composed of granite and metamorphic basement on the rims which suffered collapse and incipient extension whereas the central part was stable this promontory was surrounded by a large subduction zone except in the north and was a free boundary in the early cenozoic starting from the palaeogene and following fractures initiated during the india eurasia collision rifting began along large faults mostly n s and nnw sse strike slip which crosscut the whole region the basins remained in a continental fluvio lacustrine or shallow marine environment for a long time and some are marked by extremely stretched crust phu khanh natuna n makassar or even reached the ocean floor spreading stage celebes flores western sundaland was a combination of basin opening and strike slip transpressional deformation the configuration suggests a free boundary particularly to the east trench pull associated with the proto south china sea subduction java sulawesi trench subduction rollback in the early miocene australian blocks reached the sunda subduction zone and imposed local shortening in the south and southeast whereas the western part was free from compression after the indian continent had moved away to the north this suggests an important coupling of the sunda plate with the indo australian plate both to se and nw possibly further west rollback had ceased in the java sumatra subduction zone and compressional stress was being transferred northwards across the plate boundary the internal compression is expressed to the south by shortening which is transmitted as far as the malay basin in the late miocene most of the sunda plate was under compression except the tectonically isolated andaman sea and the damar basins in the pliocene collision north of australia propagated toward the north and west causing subduction reversal and compression in the short lived damar basin docking of the philippine plate confined the eastern side of sundaland and created local compression and uplift such as in nw borneo palawan and taiwan transpressional deformation created extensive folding strike slip faulting and uplift of the central basin and arakan yoma in myanmar minor inversion affected many thailand rift basins all the other basins record subsidence the uplift is responsible for gravity tectonics where thick sediments were accumulated sarawak ne luconia bangladesh wedge

tectonic evolution of the mogok metamorphic belt burma myanmar constrained by u th pb dating of metamorphic and magmatic rocks 1 the mogok metamorphic belt mmb extends for over 1500 km along the western margin of the shan thai block from the andaman sea north to the eastern himalayan syntaxis previous geochronology has suggested that a long lasting jurassic early cretaceous subduction related event resulted in emplacement of granodiorites and orthogneisses 171 120 ma and a poorly constrained tertiary metamorphic event on the basis of new u pb isotope dilution thermal ionization mass spectrometry and u th pb laser ablation multicollector inductively coupled plasma mass spectrometer geochronology presented here we propose two tertiary metamorphic events affected the mmb in burma the first was a paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at 59 ma a second metamorphic event spanned late eocene to oligocene at least from 37 possibly 47 to 29 ma this resulted in the growth of metamorphic monazite at sillimanite grade growth of zircon rims at 47 43 ma sillimanite muscovite replacing older andalusite and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45 5 0 6 ma and 24 5 0 7 ma these data imply high temperature sillimanite muscovite metamorphism peaking at 680 c and 4 9 kbar between 45 and 33 ma to around 606 656 c and 4 4 4 8 kbar at 29 3 0 5 ma the later metamorphic event is older than 24 5 0 3 ma the age of leucogranites that crosscut all earlier fabrics our structural and geochronological data suggest that the mmb links north to the unexposed middle or lower crust rocks of the lhasa terrane south tibet and east to high grade metamorphic core complexes in northwest thailand

tilt observations using borehole tiltmeters 2 analysis of data from yellowstone national park we have installed borehole tiltmeters at five sites in yellowstone national park wyoming and have used these instruments to measure the spatial variation of the amplitude and phase of the principal semidiurnal tide the measured tides vary both with position and azimuth and differ from the sum of the body tide and the ocean load by up to 50 the difference predicted by a finite element model constructed from seismic refraction and gravity data has a maximum value of only 12 although the discrep ancy between our observations and the model is only marginally significant at some sites the disagreement between the model and our observa tions is much larger than we observed using the same instruments at other sites and cannot be attributed to an instrumental effect we have been unable to modify the model to explain our results while keeping it consistent with the previous observations

crustal deformation of the yellowstone caldera from first gps measurements 1987 1989 a global positioning system gps network was established in 1987 in order to measure crustal deformation across the yellowstone plateau centered on the yellowstone caldera and adjacent teton wyoming and hebgen lake montana fault zones forty four gps stations in the yellow stone network were re surveyed in 1989 and station coordinates using broadcast orbits were obtained these results were used to assess the precision and accuracy of the gps surveys and to identify areas of rapid crustal deformation on the yellowstone plateau the gps measured displacements between 1987 and 1989 were calculated relative to a station at the eastern edge of the 100 km wide network the relative horizontal displacements have an average magnitude of 15 10 mm the relative vertical displacements are generally less than 25 mm about an average near zero and with an uncertainty of 20 mm comparisons with leveling and trilateration measurements taken nearly simultaneously suggest that the accuracy of these gps measurements is at the level of the above uncertainties gps measured vertical displacements across the northern part of the yellowstone caldera and 1st order leveling results show up to 20 mm of subsidence between 1987 and 1989 this subsidence is part of a six year period of caldera deflation that began in 1985 in addition up to 66 mm of uplift is observed with gps at the northwest edge of the yellowstone caldera and may be associated with processes following an intense earthquake swarm in 1985 86 and with caldera subsidence

palaeocurrents and provenance of the mae rim formation northern thailand implications for tectonic evolution of the chiang mai basin the chiang mai basin is the largest of a zone of rift basins that define the northern thailand basin and range province this province lies within the shan thai block of nw indochina that extruded out of the india asia collision the mae rim formation crops out along the western side of the chiang mai basin detachment faults border the mae rim formation on the west and north and juxtapose it against the high grade western ranges metamorphic complex and its low grade cover of palaeozoic shan thai rocks understanding the age and tectonic relationships between rifting detachment faulting ductile shear uplift and exhumation of the western ranges metamorphic complex is critical to understanding the underlying deformational mechanisms that accompanied indochina9s extrusion to better understand the palaeogeography of the chiang mai basin and its relationship to the western ranges metamorphic complex several partial sections were measured and palaeocurrent and clast composition data from the mae rim formation were collected poorly organized immature conglomerates of the mae rim formation probably represent alternating debris flows sheet flood and braided channel deposits the mae rim formation probably accumulated in range front alluvial fans and graded rapidly into lacustrine deposits within the buried central part of the chiang mai basin except for the youngest strata where gneissic clasts predominate virtually all clasts in the mae rim formation came from the low grade metasedimentary rocks that form the cover of the western ranges metamorphic complex imbricated clasts indicate eastward palaeocurrents away from the metamorphic complex most of the mae rim formation accumulated during the uplift of the western ranges but before erosion had breached the detachment fault these new data suggest a tectonic history for the chiang mai basin that involved distinct phases of extension from late oligocene to quaternary with at least two intervening periods of basin inversion

earthquake geodesy and hazard monitoring catastrophic events such as major earthquakes and volcanic eruptions result when the earth s crust fails in response to accumulated deformation the accumulation of deformation results from ongoing processes such as aseismic deformation of subcrustal rock associated with relative plate motions or the ascent of magma through a volcanic plumbing system geodetic measurements document the crustal deformation leading to these failures and the deformation resulting from them providing unique insight into the physical processes involved by virtue of the space and time domains of ground deformation detected with geodetic measurements geodesy is a useful tool in monitoring active tectonic and volcanic regimes for a wide range of natural events including earthquakes aseismic fault motions and volcanic events geodetic measurements constrain physical models of the processes that cause such events because these geodetic studies probe the underlying processes leading to and culminating in catastrophic natural events many such studies are directly applicable to earthquake and volcano hazard assessment

timing of metamorphism of the lansang gneiss and implications for left lateral motion along the mae ping wang chao strike slip fault thailand the mae ping fault mpf western thailand exhibits dominantly left lateral strike slip motion and stretches for 600 km reportedly branching off the right lateral sagaing fault in myanmar and extending southeast towards cambodia previous studies have suggested that the fault assisted the large scale extrusion of sundaland that occurred during the late eocene early oligocene with a geological offset of 120 150 km estimated from displaced high grade gneisses and granites of the chiang mai lincang belt exposures of high grade orthogneiss in the lansang national park part of this belt locally contain strong mylonitic textures and are bounded by strike slip ductile shear zones and brittle faults geochronological analysis of monazite from a sample of sheared biotite k feldspar orthogneiss suggests two episodes of crystallization with core regions documenting th pb ages between c 123 and c 114 ma and rim regions documenting a significantly younger age range between c 45 37 ma these data are interpreted to represent possible magmatic protolith emplacement for the lansang orthogneiss during the early cretaceous with a later episode of metamorphism occurring during the eocene textural relationships provided by in situ analysis suggest that ductile shearing along the mpf occurred during the latter stages of or after this metamorphic event in addition monazite analyzed from an undeformed garnet two mica granite dyke intruding metamorphic units at bhumipol lake outside of the mae ping shear zone produced a th pb age of 66 2 1 6 ma this age is interpreted to date the timing of dyke emplacement implying that the mpf cuts through earlier formed magmatic and high grade metamorphic rocks these new data when combined with regional mapping and earlier geochronological work show that neither metamorphism nor regional cooling was directly related to strike slip motion

zircon and monazite upb dating of the doi inthanon core complex northern thailand implications for extension within the indosinian orogen abstract the doi inthanon metamorphic core complex occupies a prominent central position within the northwestern thai gneiss belt which has been inferred to be as old as precambrian and as young as permo triassic u pb dating of zircon from core orthogneisses of the complex suggests that the orthogneisses were derived from granitic protoliths with essentially the same late triassic early jurassic age as the mae cham pluton 205 3 4 ma to the west of the doi inthanon complex monazite ages from the orthogneisses indicate that low pressure and high temperature metamorphism and migmatitization occurred in the late cretaceous 84 72 ma limited published radiometric age data from elsewhere in the migmatitic gneiss belt suggest that widespread minor plutonism accompanied this thermal event a younger u pb data of 26 8 0 5 ma for zircon and monazite from the mae klang granite which intruded mylonitic gneisses on the eastern side of the doi inthanon complex indicates that there was also significant plutonism in the late oligocene these u pb ages constrain the development of the complex and hence of the migmatitic gneiss belt as a whole to an interval between late cretaceous and early miocene ductile shearing uplift and tectonic denudation via detachment faulting chronologically overlap the initiation of extensional basin development to the east and southeast of the gneiss belt and imply a genetic connection thus oligocene early miocene thermal rifting from the gulf of thailand northward into mainland thailand may have been the direct product of crustal extension rather than the transtensional product of regional strike slip faulting

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

deformation history and u pb zircon geochronology of the high grade metamorphic rocks within the klaeng fault zone eastern thailand abstract in eastern thailand the klaeng fault zone includes a high grade metamorphic rock assemblage named nong yai gneiss which extends about 30 km in a nw se direction along the fault zone the rocks of this brittle fault strand consist of amphibolite to granulite grade gneissic rocks structural analysis indicates that the rocks in this area experienced three distinct episodes of deformation d1 d3 the first d1 formed large scale nw se trending isoclinal folds f1 that were reworked by small scale tight to open folds f2 during the second deformation d2 d1 and d2 resulted from ne sw shortening during the triassic indosinian orogeny before being cross cut by leucogranites d1 and d2 fabrics were then reworked by d3 sinistral shearing including shear planes s3 and mineral stretching lineations l3 la mc icp ms u pb zircon dating suggested that the leucogranite intrusion and the magmatic crystallization took place at 78 6 0 7 ma followed by a second crystallization at 67 1 to 72 1 0 6 ma both crystallizations occurred in the late cretaceous and it is suggested were tectonically influenced by se asian region effects of the west burma and shan thai sibumasu collision or development of an andean type margin the sinistral ductile movement of d3 was coeval with the peak metamorphism that occurred in the eocene during the early phases of the india asia collision

intracrustal detachment within zones of continental deformation geologic mapping of active to recently active geologic structures in panamint valley western united states and in the haiyuan region and northern tibetan plateau china suggests detachment in the middle and lower crust on a scale of tens of kilometres to at least several hundred kilometres detachment occurs similarly in predominantly extensional panamint valley and in predominantly compressional china environments it involves structures with displacements of more than 10 km and displacement rates of more than 3 mm yr perhaps more than 10 mm yr the steeply dipping strike slip faults present in all three areas hunter mountain fault haiyuan fault and altyn tagh fault zone terminate in zones of extension or compression and geometric relations indicate that all structures including strike slip faults are thin skinned and restricted to the upper crust in panamint valley and in the haiyuan region deformation within these systems can be reconstructed in three dimensions displacement on the strike slip faults is absorbed by extension or compression occurring at the termination of the faults so strike slip displacement is roughly equal to and in the same direction as shortening or extension we propose that left slip on the altyn tagh fault zone in northern tibet is similarly absorbed by shortening southeast of the fault zone within the qaidam basin and the nan shan

influence of phosphate on mobility and adsorption of rees during weathering of granites in thailand abstract the permo jurassic north thai nt granites and the late cretaceous to paleogene western province wp granites in thailand are contrasting in terms of tectonic settings and chemical compositions the nt granites which are dominated by s type features are characterized by lower sio2 contents and higher p2o5 contents than the wp granites in this study in order to compare the mobility and adsorption of rare earth elements rees during weathering of the two granite suites geochemical analyses were conducted on the granite and weathered granites the weathered wp granites show wider ranges of rees y rey contents percentages of ion exchangeable rey and ce anomalies than the weathered nt granites these results indicate that rees were less mobile during weathering of the nt granites than those of the wp granites the low mobility of rees can be explained by the occurrences of residual monazite and secondary ree phosphates which immobilize rees during weathering therefore in the weathered nt granites rees are mostly contained in the phosphate minerals in contrast the weathered wp granites are dominated by ion exchangeable rees adsorbed rees which are likely to exist on the surface of clays previous studies and our study results suggest that the ion exchangeable rees in the weathered granites were probably sourced from weatherable allanite titanite apatite and or ree fluorocarbonate and rarely from monazite and zircon which are resistant to weathering the weathered granites of low phosphate contents potentially show high percentages of ion exchangeable rey although they can be influenced by the degree of hydrothermal alteration or weathering of granites

oligo miocene granitic magmatism in central vietnam and implications for continental deformation in indochina two granitoid intrusions within the bu khang extensional complex in central vietnam have been dated by u pb and rb sr geochronology a monazite u pb age of 26 0 0 2 2 myr was obtained for the bu khang pluton and 23 7 1 6 1 7 myr for monazite allanite and zircon from the dai loc intrusion these ages date crystallization of magmas previously assigned precambrian to devonian rb sr analyses of k feldspar and biotite fractions from the samples yield ages of 19 8 0 6 2 myr and 19 6 0 5 myr respectively the thermal history recorded by the different geochronometers implies an average exhumation rate of 2 mm yr 1 corresponding to 9 km of unroofing magmatism was either i induced passively by lithospheric thinning driven by changes in regional tectonic stresses or ii triggered actively by an ascending plume tertiary exhumation and magmatism documented elsewhere in indochina e g ailao shan red river and wang chao shear zones favours a regional tectonic cause for extension and granitoid magmatism in the bu khang complex on the other hand the presence of an upwelling thermal anomaly since at least 35 ma causing mantle melting below indochina is supported by shear wave velocity variations in the mantle and source geochemistry of both the bu khang plutons and the red river belt intrusions in either case tertiary exhumation of the bu khang complex can account for previously undocumented ne sw directed extension which is required in northern vietnam to account for structural changes related to the opening of the south china sea

oligo miocene midcrustal subhorizontal shear zone in indochina new structural observations of oligo miocene deformation in north vietnam along the red river shear zone suggest that left lateral strike slip shear was restricted to the upper and middle crust above a horizontal shear zone left lateral shear deformation is associated with low pressure low temperature parageneses high temperature deformation is restricted to zones where the foliation has a low dip in the core of the dai nui con voi antiformal dome these observations complete those made earlier in the bu khang extensional dome farther south above the horizontal shear zone left lateral transpression was active during the first stage and changed to transtension some 33 myr ago purely extensional metamorphic domes bu khang or transtensional domes dai nui con voi were exhumed during this younger stage our observations plead for caution when interpreting strike slip structures at lithospheric scale

exhumation of west sundaland a record of the path of india abstract the indian plate commenced its northward migration towards eurasia in the early cretaceous the lateral effect of this migration on the western edge of the sunda plate in southeast asia still remains equivocal in order to assess this effect we evaluate several key sectors characterized by deep crustal exhumation along a n s transect from the southern malay peninsula to the east himalayan syntaxis the evaluation is aided by a structural analysis of vertical movements and basin development five major metamorphic domes with similar geodynamic evolution maximum p t burial conditions and exhumation are studied exhumation of these domes migrated with time between late cretaceous in the stong complex north malaysia in the south and late miocene in the gaoligong shear zone south china in the north as documented by published work and our new fission track data presented herein deformation is characterized by a n s oriented extension that followed the more regional e w oriented plate tectonic shortening creating local core complexes and syn kinematic magmatism in the footwall of crustal scale detachments which displays a consistent temporal northward migration the n s extension was associated with the onset of hanging wall deposition in the sedimentary basins of western sundaland e g malay sumatra and thai basins during continuous exhumation of the footwall to upper brittle levels our multifaceted analysis of structural and thermochronological data shows a similar succession of tectonic thermal and sedimentary events in west sundaland that was driven by the gradual northward migration of india starting from cretaceous times we infer that the principal mechanism was driven by the subduction of an excess topography of greater india rifted continental margin during its underplating resulting in uplift thermal anomalies extensional exhumation and associated subsidence