from orogenic hinterlands to mediterranean style back arc basins a comparative analysis hinterland plateaux and mediterranean style back arc basins both form behind active subduction zones or collisional megathrusts and share many characteristics 1 early crustal thickening to about twice normal continental thickness 2 thin lithospheric mantle 3 mixed magmatism including asthenospheric lithospheric and crustal melts 4 late stage horizontal extension accompanied by vertical shortening horizontal extension and vertical shortening are driven by high gravitational potential energy gpe contrasts between the hinterland and surrounding lithosphere which may reach 7 10 12 n m 1 equivalent to about 2 5 times the ridge push force if extension is rapid relative to the rate of lithospheric cooling gpe may remain positive even as extension continues declining to the mid ocean ridge value as crustal thickness approaches zero this suggests that hinterland plateaux could ultimately evolve into oceanic back arc basins the rate direction and amount of extension and the rate of vertical shortening depend on the plate boundary conditions and the gpe of the surrounding lithosphere vertical shortening in tibet is limited by work required to deform the surrounding asian lithosphere whereas mediterranean back arc basins can extend at the expense of regions of thin continental or oceanic crust

extensional collapse of the tibetan plateau results of three dimensional finite element modeling 1 following their initial collision 50 70 myr ago the indian and eurasian plates have been continuously converging toward each other whereas the regional stress field is predominately compressive the late cenozoic tectonics within the tibetan plateau features widespread crustal extension numerous causes of the extension have been proposed but their relative roles remain in debate we have investigated the major factors contributing to the tibetan extension in a three dimensional viscoelastic model that includes both lateral and vertical variations of lithospheric rheology and relevant boundary conditions constrained by the present topography and gps velocity field the model predicted predominately extensional stress states within the plateau crust resulting from mechanical balance between the gravitational buoyancy force of the plateau and the tectonic compressive stresses the predicted stress pattern is consistent with the earthquake data that indicate roughly e w extension in most of tibet and nearly n s extension near the eastern margin of the tibetan plateau we explored the parameter space and boundary conditions to examine the stress evolution during the uplift of the tibetan plateau when the plateau was lower than 50 of its present elevation strike slip and reverse faults were predominate over the entire plateau and no e w crustal extension was predicted significant crustal extension occurs only when the plateau has reached 75 of its present elevation basal shear associated with underthrusting of the indian plate beneath tibet may have enhanced crustal extension in southern tibet and the himalayas and a stronger basal shear during the miocene may help to explain the development of the south tibetan detachment system

gravitational collapse of orogenic crust a preliminary three dimensional finite element study although gravitational collapse has been widely accepted as a viable explanation for both synorogenic and postorogenic extension its mechanisms have not been well understood previous models of continental deformation that assumed a viscous or viscoplastic rheology are not well suited for simulating stress states and brittle deformation within the upper crust where most extensional collapse occurs here we present preliminary results of a three dimensional 3 d finite element modeling of stress and faulting patterns within the orogenic crust using a viscoelastic rheology major parameters controlling orogenic extension including topographic loading tectonic compression basal shear thermal structure and 3 d tectonic boundary conditions are systematically explored in the model for typical tectonic compression 60 100 mpa and 2 3 km elevation the model predicts synorogenic extension in high plateaus and concomitant thrusting in the lowlands near the foothills viscous relaxation within the ductile crust may amplify deviatoric stresses within the brittle crust by a few times and stress amplification is greater when the crust is hotter and therefore the brittle crust is thinner most synorogenic extension occurs in the direction orthogonal to regional compression while postorogenic extension is more likely to occur normal to the trend of mountain belts we apply the model results to the tibetan plateau and suggest that the age of the north trending grabens is not a reliable proxy for the time when the plateau had reached it present mean high elevation conversely development of the east trending south tibetan detachment system in the miocene by gravitational collapse was more difficult and may require a higher elevation of the himalayas than present or much of the tibetan plateau to have remained low till late miocene

provenance evolution of collapse graben fill in the himalaya the miocene to quaternary thakkhola mustang graben nepal abstract this study focuses on the detailed provenance evolution of young syn to post orogenic extensional grabens in orogens like the himalaya to trace the tectonic history of such late stage basins using the neogene thakkhola mustang graben as a case study the graben is situated within the tibetan tethys zone and is filled with 870 m of continental deposits of miocene to holocene age based on logged sections within the predominantly alluvial to coarse grained fluvial fill of the graben we investigated paleocurrent data and the petrology of sandstones and conglomerates including heavy minerals studies to interpret provenance and source areas in detail significant changes are recorded by slight differences in heavy mineral and pebble compositions the sandstones can be classified as lithic greywackes lithic arkoses and feldspathic litharenites sandstone mudstone quartzite and some granite clasts are dominant in conglomerates of the central part of the graben tetang formation conglomerates of miocene age comprise mostly clasts of mesozoic rocks with an eastern provenance consistent with measured paleocurrent directions all paleocurrent data and compositional analyses of imbricated conglomerates of the miocene pliocene thakkhola formation in the northeast of the graben suggest that clasts were derived from eastern source areas comprising mainly mesozoic rocks whereas paleozoic clasts of a western to northern source area predominate in the centre of the graben heavy mineral analysis indicates that tourmaline staurolite zircon garnet and apatite constitute a significant proportion of the assemblages of all formations through time whereas epidote andalusite kyanite chloritoid hornblende chrome spinel rutile and amphiboles are less common these assemblages reflect in general stable minerals and low to high grade metamorphic source rocks and are principally controlled by reworking of older passive margin sediments of the tibetan tethys zone as indicated by provenance discrimination diagrams three successive stages in provenance evolution were recognized 1 the miocene tetang formation characterized by higher kyanite values corresponding to the himalayan foreland evolution 2 the thakkhola formation characterized by granite clasts and significantly higher amounts of andalusite indicating source area expansion and erosion of the mustang mugu granites to the northwest 3 the upper pleistocene holocene kaligandaki formation bearing higher amounts of epidote klinozoisite and ophiolite and high pressure low temperature detritus as indicated by chrome spinel and blue amphiboles derived from the north lying indus tsangpo suture zone the change in source areas from the miocene pliocene to the late pleistocene holocene is interpreted as a result of the evolution from an initial stage of high angle normal faulting and collapse basin formation to a low angle extensional detachment basin system

development of extension faults on the oblique thrust ramp hanging wall example from the tethys himalaya abstract inhomogeneous strain at late stages of deformation of modelling clay models with frontal and oblique thrust ramp geometries results in formation of extension faults on the hanging wall the extension faults trend nearly parallel to the axis of maximum compression and orthogonal to the trend of the frontal ramp faults with similar geometries occur in the upper satluj river basin of the tethys himachal himalaya the experimental results help in understanding the present day normal faulting and related active tectonics of the area

palaeomagnetic remanences in high grade metamorphic rocks of the everest region indication for late miocene crustal doming summary high grade metamorphic rocks of the upper amphibolite to granulite facies in the higher himalayan crystalline hhc were studied for their appropriateness for palaeomagnetic investigations sampling and fieldwork was accomplished on a south north transect at 87 ei n the khumbu region south of everest paragenesis of iron sulphides and oxides was found to be variableduetodifferentsourcerocksandmetamorphicconditionsinthisarea unblockingtemperature ranges isothermal remanence acquisition low temperature magnetic properties and microscopic observations confirm the presence of pyrrhotite and magnetite two remanence componentswereseparatedwithunblockingtemperatureintervalsof250 330 cand370 550 c which can be attributed to characteristic remanent magnetizations chrm of monoclinic pyrrhotite chrm pyr and magnetite chrm mag respectively mainly the chrm pyr showed a consistent demagnetization behaviour probably residing in pyrrhotite particles within the single domain to pseudo single domain range normal and reverse polarities were observed partly co existing within the unblocking spectrum of single specimens this i supports a thermoremanent trm origin of the chrm pyr and ii indicates that the time span of remanence acquisition was long enough to average out palaeosecular variation the age of the chrm pyr can be related tothe last metamorphic cooling event at the early to late miocene boundary 16 ma the chrm mag is probably a secondary chemical or thermochemical remanence acquired byhydrothermalprocessesduringretrogression chrm pyr andchrm mag directionsaresimilar suggesting that pyrrhotite and magnetite were magnetized approximately contemporaneously separation of the sampling area into three sectors reveals a south north trend with i variable block rotations in the southernmost part close to the main central thrust five sites ii no rotation in the area between lukla and namche 10 sites and iii a significant clockwise rotation n 5 sites d i 42 3 29 4 k 20 7 95 17 2 along the imja khola north of namche we interpret the clockwise rotation as an apparent rotation due to tilting by crustal doming in the everest region this doming obviously occurred north of a fault zone located at around namche and was not affecting the region south of it the clockwise sense of the apparent vertical axis rotation indicates that the area north of namche is located at the western side of the dome structure where westward tilting around a subhorizontal axis occurred best fit tilt direction 277 tilt angle 51

the role of extension during brittle deformation within the nw indian himalaya 1 synorogenic extension has been recognized as an integral structural constituent of mountain belts and high elevation plateaus during their evolution in the himalaya both orogen parallel and orogen normal extension has been recognized however the underlying driving forces for extension and their timing are still a matter of debate here we present new fault kinematic data based on systematic measurements of hundreds of outcrop scale brittle fault planes in the nw indian himalaya this new data set as well as field observations including crosscutting relationships mineral fibers on fault planes and correlations with deformation structures in lake sediments allows us to distinguish different deformation styles the overall strain pattern derived from our data reflects the large regional contractional deformation pattern very well but also reveals significant extensional deformation in a region which is dominated by shortening in total we were able to identify six deformation styles most of which are temporally and spatially linked representing protracted shortening our observations also furnish the basis for a detailed overview of the younger deformation history in the nw himalaya which has been characterized by extension overprinting previously generated structures related to shortening the four dominant deformation styles are 1 shortening parallel to the regional convergence direction 2 arc normal extension 3 arc parallel extension and finally 4 e w extension this is the first data set where a succession of both arc normal and e w extension has been documented in the himalaya importantly our observations help differentiate e w extension triggered by processes within the tibetan plateau from arc parallel and arc normal extension originating from the curvature of the himalayan orogen

full interseismic locking of the nankai and japan west kurile subduction zones an analysis of uniform elastic strain accumulation in japan constrained by permanent gps we analyze permanent global positioning system gps data obtained over japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the pacific and philippine sea plates below the japanese islands we first derive the gps strain rate fields that characterize the crustal deformation of southern and northern japan and invert them to determine the effective subduction velocity along the central nankai trough on one side and the japan west kurile trench on the other these reference free velocities are close to those predicted by plate motion models with respect to eurasia we conclude that the eurasian reference frame gives a good approximation to the subduction motion and that to first order both subduction zones were fully locked during the period of measurements we then test whether the coupling ratio shows local variations within the seismogenic zones to do this we divide the subduction interface into 35 km 30 km elements that we model by point source groups and we invert the gps velocity field referenced to eurasia to derive the coupling ratio between 0 and 1 on each fault element the results are coherent over the 3 years and confirm that both the central nankai and the japan west kurile seismogenic zones are homogeneously fully locked most of the coupling ratios are close to 1 and a few are close to 0 intermediate values are rare the zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 sanriku oki interplate earthquake japan trench or to a small overestimation of the actual lower limit of the locked zone we conclude that within the resolution of the gps data and the model 1 partial coupling did not exist during these 3 years along the nankai and japan west kurile trenches 2 the small seismic coupling ratio previously derived from earthquakes analysis for the japan and kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip and 3 the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading finally we discuss the nonelastic residual deformation within the frame of the long term deformation of the japanese islands

imaging magma transport during the 1997 seismic swarm off the izu peninsula japan the spatio temporal evolution of a propagating magma filled crack was estimated from inversion of global positioning system gps data tiltmeters and leveling the dike opened at a maximum rate of 50 millimeters per day and had a peak magma flux of 2 x 10 6 cubic meters per day although the spatial resolution was limited slow upward propagation was resolved during the 9 day long intrusion in contrast the earthquakes migrated rapidly upward during the first 12 hours of the swarm and nearly all of the seismic energy was released in the first 2 days comparison of inversion results with accurate hypocenter locations will lead to improved understanding of magma transport through the brittle crust and of the causes of volcanic seismicity

a slow earthquake sequence on the san andreas fault earthquakes typically release stored strain energy on timescales of the order of seconds limited by the velocity of sound in rock over the past 20 years observations1 13 and laboratory experiments14 have indicated that rupture can also occur more slowly with durations up to hours such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones the detection of events with larger timescales requires near field deformation measurements in december 1992 two borehole strainmeters close to the san andreas fault in california recorded a slow strain event of about a week in duration and we show here that the strain changes were produced by a slow earthquake sequence equivalent magnitude 4 8 with complexity similar to that of regular earthquakes the largest earthquakes associated with these slow events were small local magnitude 3 7 and contributed negligible strain release the importance of slow earthquakes in the seismogenic process remains an open question but these observations extend the observed timescale for slow events by two orders of magnitude

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

seismic properties of rocks from an exposure of extended continental crust new laboratory measurements from the ivrea zone abstract compressional wave velocities for new samples from the ivrea verbano zone ivz and strona ceneri zone scz have been measured to 600 mpa in order to better understand the petrologic controls on velocity and to provide an expanded base for seismological models of the crust as represented by these terrains these data coupled with previous measurements show that preferred orientation of mica controls both the velocity hysteresis with pressure and seismic anisotropy of scz and ivz schists and gneisses ivz paragneisses form the basis of a linear velocity density relationship for metagraywackes of various metamorphic grade this relationship falls close to that expected for mafic rocks and overlaps velocity density data for ivz mafic gneisses one dimensional near vertical reflection models based on these data and recent structural reconstructions of the terrain show that lower crustal reflectivity can be related to high reflection coefficients associated with interlayered sequences of upper amphibolite granulite facies paragneiss mafic gneiss and ultramafic rocks transparent upper crustal levels in the model result from the overall similarity of velocities for schists gneisses and granites from the structurally and lithologically complex scz a mean lower crustal velocity of 7 1 km s is calculated for the part of the ivz composed of interlayered mafic gneiss and paragneiss and for another area dominated by a mafic layered igneous complex the velocity density analysis coupled with this observation emphasize the ambiguity inherent in interpretation of crustal composition from p wave velocity data in the absence of other constraints

a slow thrust slip event following the two 1996 hyuganada earthquakes beneath the bungo channel southwest japan we report a slow thrust slip event that occurred beneath the bungo channel region southwestern japan on oct and dec 1996 two hyuganada earthquakes both mw 6 7 followed by afterslips occurred in addition a crustal movement characterized by an extremely slow rise was observed around the bungo channel about 200 km north from the epicenters and continued for about 300 days long assuming a slow slip on the plate boundary we estimate its duration and surface displacements from gps time series data by curve fitting and then determine the fault slip distribution we found that a slow slip without any earthquakes continued for nearly one year and released the seismic moment comparable to that of the hyuganada earthquakes occurrence of the slow thrust slip event suggests that this kind of event may be a characteristic mode of stress release at a transition region of interplate coupling

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

imaging of aseismic fault slip transients recorded by dense geodetic networks summary the large scale continuous gps networks that have emerged in the last decade have documented a wide range of aseismic deformation transients that resulted from physical processes such as aseismic fault slip and magma intrusion in particular a new class of m 7 slow earthquakes with durations ranging from days to months have been observed with gps arrays located above the downdip portion of subduction zone thrust interfaces interpretation of the displacement time series resulting from these events is not straightforward owing to the contaminating effects of multiple contributing signals such as fault slip local benchmark motion seasonal effects and reference frame errors we have developed a time dependent inversion algorithm based on the extended kalman filter which can separate the various signals and allow the space time evolution of these slow slip transients to be studied in detail we applied the inversion algorithm to the 1999 cascadia slow earthquake this event had two primary episodes of moment release separated by a two week period in which relatively little moment release occurred the cascadia event and other slow earthquakes share numerous similarities with both ordinary earthquakes and afterslip transients suggesting that they may represent fault slip under a conditionally stable regime

extension in south central british columbia mechanical and thermal controls abstract geological data indicate that the omineca belt in the canadian cordillera experienced a rapid episode of extension between 58 and 48 ma following a compressional event and crustal thickening in the early tertiary restored crustal sections have been constructed using listric fault geometries to infer that the amount of eocene extension affecting the southern omineca belt exceeded 50 at the end of compression in the early tertiary the omineca and intermontane belts of southern british columbia reached a crustal thickness of 60 km isostatic compensation implies that these belts formed elevated ranges this is consistent with sediment deposition west of these ranges and the absence of in situ sedimentary basins the stress induced by topography and crustal thickening is calculated the calculations show that the stress in homogeneously thickened lithosphere is small and that tensile stress in the crust is compensated by compression in the mantle removal of the mantle lithosphere induces a stress field that is tensile throughout the lithosphere this local stress field combined with stress induced at the plate boundary by the oblique impingement of the kula farallon ridge controlled the position and direction of crustal extension in south central british columbia the present high heat flow on the west flank of the omineca belt 80 mw m 2 is interpreted as the relict of eocene extension following removal of the lithospheric mantle

dome formation and extension in the tethyan himalaya leo pargil northwest india metamorphic dome complexes occur within the internal structures of the northern himalaya and southern tibet their origin deformation and fault displacement patterns are poorly constrained we report new fi eld mapping structural data and cooling ages from the western fl ank of the leo pargil dome in the northwestern himalaya in an attempt to characterize its post middle miocene structural development the western fl ank of the dome is characterized by shallow west dipping pervasive foliation and wnw ese mineral lineation shear sense indicators demonstrate that it is affected by east west normal faulting that facilitated exhumation of highgrade metamorphic rocks in a contractional setting sustained top to northwest normal faulting during exhumation is observed in a progressive transition from ductile to brittle deformation garnet and kyanite indicate that the leo pargil dome was exhumed from the mid crust 40 ar 39 ar mica and apatite fi ssion track aft ages constrain cooling and exhumation pathways from 350 to 60 c and suggest that the dome cooled in three stages since the middle miocene 40 ar 39 ar white mica ages of 16 14 ma suggest a fi rst phase of rapid cooling and provide minimum estimates for the onset of dome exhumation aft ages between 10 and 8 ma suggest that ductile fault displacement had ceased by then and aft track length data from high elevation samples indicate that the rate of cooling had decreased signifi cantly we interpret this to indicate decreased fault displacement along the leo pargil shear zone and possibly a transition to the kaurik chango normal fault system between 10 and 6 ma aft ages from lower elevations indicate accelerated cooling since the pliocene that cannot be related to pure fault displacement and therefore may refl ect more pronounced regionally distributed and erosion driven exhumation

exhumation of the sanbagawa blueschist belt sw japan by lateral flow and extrusion evidence from structural kinematics and retrograde p t t paths

metamorphic history of the south tibetan detachment system mt everest region revealed by rscm thermometry and phase equilibria modelling this study combines microstructural observations with raman spectroscopy on carbonaceous material rscm phase equilibria modelling and u pb dating of titanite to delineate the metamorphic history of a well exposed section through the south tibetan detachment system stds in the dzakaa chu valley of southern tibet in the hanging wall of the stds undeformed tibetan sedimentary series rocks consistently record peak metamorphic temperatures of 340 c temperatures increase down section reaching 650 c at the base of the shear zone defining an apparent metamorphic field gradient of 310 c km 1 across the entire structure u th pb geochronological data indicate that metamorphism and deformation at high temperatures occurred over a protracted period from at least 20 to 13 ma deformation within this 1 km thick zone of distributed top down to the northeast ductile shear included a strong component of vertical shortening and was responsible for significant condensing of palaeo isotherms along the upper margin of the greater himalayan series ghs we interpret the preservation of such a high metamorphic gradient to be the result of a progressive up section migration in the locus of deformation within the zone this segment of the stds provides a detailed thermal and kinematic record of the exhumation of footwall ghs rocks from beneath the southern margin of the tibetan plateau

thermal constraints on the emplacement rate of a large intrusive complex the manaslu leucogranite nepal himalaya the emplacement of the manaslu leucogranite body nepal himalaya has been modelled as the accretion of successive sills the leucogranite is characterized by isotopic heterogeneities suggesting limited magma convection and by a thin 100 m upper thermal aureole these characteristics were used to constrain the maximum magma emplacement rate models were tested with sills injected regularly over the whole duration of emplacement and with two emplacement sequences separated by a repose period additionally the hypothesis of a tectonic top contact with unroofing limiting heat transfer during magma emplacement was evaluated in this latter case the upper limit for the emplacement rate was estimated at 3 4 mm year or 1 5 myr for 5 km of granite geological and thermobarometric data however argue against a major role of fault activity in magma cooling during the leucogranite emplacement the best model in agreement with available geochronological data suggests an emplacement rate of 1 mm year for a relatively shallow level of emplacement granite top at 10 km uninterrupted by a long repose period the thermal aureole temperature and thickness and the isotopic heterogeneities within the leucogranite can be explained by the accretion of 20 60 m thick sills intruded every 20 000 60 000 years over a period of 5 myr under such conditions the thermal effects of granite intrusion on the underlying rocks appear limited and cannot be invoked as a cause for the formation of migmatites

late cenozoic extension and crustal doming in the india eurasia collision zone new thermochronologic constraints from the ne chinese pamir 1 the northward motion of the pamir indenter with respect to eurasia has resulted in coeval thrusting strike slip faulting and normal faulting the eastern pamir is currently deformed by east west oriented extension accompanied by uplift and exhumation of the kongur shan 7719 m and muztagh ata 7546 m gneiss domes both domes are an integral part of the footwall of the kongur shan extensional fault system kes a 250 km long north south oriented graben why active normal faulting within the pamir is primarily localized along the kes and not distributed more widely throughout the orogen has remained unclear in addition relatively little is known about how deformation has evolved throughout the cenozoic despite refined estimates on present day crustal deformation rates and microseismicity which indicate where crustal deformation is presently being accommodated to better constrain the spatiotemporal evolution of faulting along the kes we present 39 new apatite fission track zircon u th sm he and 40ar 39ar cooling ages from a series of footwall transects along the kes graben shoulder combining these data with present day topographic relief 1 d thermokinematic and exhumational modeling documents successive stages rather than synchronous deformation and gneiss dome exhumation while the exhumation of the kongur shan commenced during the late miocene extensional processes in the muztagh ata massif began earlier and have slowed down since the late miocene we present a new model of synorogenic extension suggesting that thermal and density effects associated with a lithospheric tear fault along the eastern margin of the subducting alai slab localize extensional upper plate deformation along the kes and decouple crustal motion between the central western pamir and eastern pamir tarim basin

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

rapid aseismic moment release following the 5 december 1997 kronotsky kamchatka earthquake model inversions of displacements of continuously operating gps stations in kamchatka show that aseismic afterslip during 2 months following the mw 7 8 5 december 1997 kronotsky subduction earthquake released as much moment as the earthquake itself the rapidly decaying transient slip on the subduction underthrust occurred near the downdip edge of the coseismic rupture and extended laterally away from it including a region of vigorous foreshock activity a logarithmic decay function fit to the cumulative afterslip curve has a relaxation time of about 3 days rapid afterslip can contribute significantly to the moment release of partially coupled subduction zones

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

crustal structure of the middle urals based on reprocessing of russian seismic reflection data summary reprocessed russian seismic reflection data combined with existing russian geological maps and seismic interpretations provide the basis for a revised tectonic interpretation of the upper crust in the middle urals the main uralian fault muf the principal suture of the urals separating the east european craton to the west from accreted terranes to the east is found to penetrate to depths of at least 18 km and perhaps much deeper with a uniform eastward dip of 35 after migration as the muf appears to truncate reflections in both the foreland and hinterland of the urals it is arguably one of the youngest structural features of the orogen basement rocks of the east european platform underthrust the suture and comprise the deep crust a minimum of 30 km east of the surface expression of the muf the tagil volcanic arc complex greater than 1000 km in length along strike comprises at least the upper 10 km of the crust east of the muf this terrane shallows eastwards where it appears to be truncated by the west dipping serov mauk fault based on correlation with surface geology the hinterland of the urals east of the muf is characterized by a previously unrecognized zone of discontinuous mid crustal reflectivity at depths of 14 16 km that is discordant with upper crustal reflections this mid crustal zone of reflectivity is interpreted to represent either a major subhorizontal structural detachment or a zone of magmatic intrusion

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

the structure and tectonic evolution of the aguilon fold nappe sierra alhamilla betic cordilleras se spain abstract detailed structural work in the sierra alhamilla se spain shows that the aguilon nappe comprising triassic and older metasedimentary rocks is a fold nappe the most prominent set of small scale folds changes from dominantly n vergent in the upper right way up limb to s vergent in the greatly thinned lower limb the nappe closes to the north and must have been emplaced in this direction nappe formation was accompanied by small scale folding and extensive solution transfer producing a pronounced differentiated crenulation cleavage these structures overprint an earlier set of folds and cleavage and are overprinted in turn by late n vergent structures the lower limb of the nappe was thinned and disrupted during continued nappe transport pre triassic schist in the core of the nappe was affected by medium grade metamorphism of probable post triassic age the contact with low grade permo triassic sediments above and below the schist coincides with a distinct change in metamorphic grade this contact may be a post metamorphic extensional fault that is now folded around the nappe which suggests that nappe formation was preceded by extensional faulting this is consistent with gravity spreading as the driving process for nappe emplacement

correction to a thin viscous sheet model for continental deformation summary the governing equation derived by england mckenzie for the deformation of a thin viscous shell contains an error we give the correct derivation here and correct those figures where the change makes a visible difference the correct results differ quantitatively from those of england mckenzie but their conclusions are not affected

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

cretaceous paleogeography and paleoclimate and the setting of ski borehole sites in songliao basin northeast china article i nfo as a paradigm of greenhouse climate in earth s history the cretaceous provides significant rock records of global climate changes under conditions of greenhouse climate the songliao basin among the longest dura tion 85 90 m y of continental sedimentary basins provides an excellent opportunity to recover a nearly complete cretaceous terrestrial sedimentary record extensive lake deposits ten kilometers deep and cover ing an area of 260 000 km 2 of the songliao basin provide unique detailed records that can be tied to the glob al stratigraphic time scale thereby improving our understanding of the continental paleoclimate and ecological system the two coreholes at skis and skin sites were drilled into this basin and completed with a total length of 2485 89 m of recovered core that spanned the complete middle to upper cretaceous strata in the basin the unique geological setting of long term continuous subsidence within the largest cretaceous landmass in the world makes the cretaceous songliao basin of northeastern china an ideal place to study cretaceous climate change on the continent this paper reviews the literature on the paleogeography and paleoclimate of the northern east asia and the songliao basin during the cretaceous based on the climatolog ically sensitive deposits oxygen isotope studies and paleontology the climate during the cretaceous in the songliao basin was temperate and humid with relatively abundant rainfall during the period significant changes four cooling three warming and three semiarid events are generally consistent with the oxygen isotope data from east asia and the four cooling events in berriasian valanginian aptian albian early santonian and campanian maastrichtian may be related to potential glaciations in cretaceous

tectonic mode switches and the nature of orogenesis abstract the birth and death of many mountain belts occurs in lithosphere that over rides major subduction zones here the tectonic mode shortening versus extension can abruptly switch even during continuous and otherwise smooth convergence if the hinge line of the foundering slab rapidly retreats i e rolls back the foundering slab creates a gravitational potential well into which the orogen collapses this motion coupled with stress guides can pull the orogen apart a slowing of roll back or of hinge retreat means that the subduction flexure may subsequently begin to be pushed back or be pushed over by the advancing orogen the consequence of such changes in relative motion is that orogenic belts are affected by abrupt tectonic mode switches the change from push to pull leads to a sudden change from horizontal extension to horizontal shortening potentially throughout the entire mass of the orogenic lithosphere that over rides the subducting slab the sequencing of these tectonic mode switches affects the thermal evolution of the orogen and thus fundamentally determines the nature of orogenesis this insight led to us to our quite different views as to how orogens work it is evident that orogens affected by abrupt push pull mode switches are characterized by high pressure metamorphism whereas orogens affected by abrupt pull push mode switches are characterized by high temperature metamorphism magmatism and anatexis

role of inversion tectonics in structural development of the himalaya abstract analysis of surface and subsurface structures variation of shortening amounts obtained by restoration of deformed cross sections and occurrence of younger hangingwall rocks over the older footwall rocks across the vaikrita thrust in the higher himalaya suggests reactivation of early normal faults as thrusts based on this an inversion tectonics model is proposed for structural development of the himalaya the model explains the geometrical shape of the himalaya as primary arcuation and helps in resolving superimposed deformation in the region

40ar 39ar thermochronology of the kampa dome southern tibet implications for tectonic evolution of the north himalayan gneiss domes structural and thermochronological studies of the kampa dome provide constraints on timing and mechanisms of gneiss dome formation in southern tibet the core of kampa dome contains the kampa granite a cambrian orthogneiss that was deformed under high temperature sub solidus conditions during himalayan orogenesis the kampa granite is intruded by syn tectonic leucogranite dikes and sills of probable oligocene to miocene age overlying paleozoic to mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite staurolite grade at the base of the sequence to unmetamorphosed at the top the kampa shear zone traverses the kampa granite metasediment contact and contains evidence for high temperature to lowtemperature ductile deformation and brittle faulting the shear zone is interpreted to represent an exhumed portion of the south tibetan detachment system biotite and muscovite 40 ar 39 ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14 22 0 18 to 15 54 0 39 ma cooling ages and concordant spectra with 14 64 0 15 to 14 68 0 07 ma cooling ages petrographic investigations suggest disturbed samples are associated with excess argon intracrystalline deformation mineral and fluid inclusions and or chloritization that led to variations in argon systematics we conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at 14 6 ma the kampa granite yields the youngest biotite 40 ar 39 ar ages of 13 7 ma immediately below the granite metasediment contact we suggest that this age variation reflects either varying mica closure temperatures re heating of the kampa granite biotites above closure temperatures between 14 6 ma and 13 7 ma or juxtaposition of rocks with different thermal histories our data do not corroborate the inverse mica cooling gradient observed in adjacent north himalayan gneiss domes instead we infer that mica cooling occurred in response to exhumation and conduction related to top to north normal faulting in the overlying sequence top to south thrusting at depth and coeval surface denudation

metamorphism and crustal stacking in the north indian plate north pakistan abstract the northern part of the indian plate in north pakistan is composed of a number of large scale crustal nappes each of which are stratigraphically distinct and which were stacked late in the main phase of southeasterly directed thrusting associated with the himalayan event the major nappes recognised in the swat to kaghan area of north pakistan are the besham swat hazara banna lower kaghan and upper kaghan nappes metamorphism was synchronous with early stages of deformation within each nappe the metamorphic grade increases upwards an overall inversion that represents post metamorphic imbrication within individual nappes synchronous with the main phase of nappe stacking rather than a hot iron type inversion as described under the mct in nepal and india as a result of this within nappe imbrication each thrust slice within any particular nappe contains rocks of a higher metamorphic grade than those in the slice below with sharp metamorphic breaks across the imbricating thrusts as well as across the major shears that bound the crustal scale nappes uplift along these imbricating thrusts initiated cooling of the stack k ar mica and hornblende cooling ages imply that much of this uplift was completed by 30 ma or within 20 ma of the collision

variation of the regional stress tensor at the western great basin boundary from the inversion of earthquake focal mechanisms the orientation of the principal axes of the regional stress tensor in and near the western great basin boundary is calculated from the inversion of earthquake focal mechanism data data were taken from three regions along the western great basin sierra nevada boundary mammoth lakes round valley and chalfant valley and at two regions within the great basin interior mono walker lake and west central nevada the depth range is similar for all data earthquakes are essentially restricted to the uppermost 16 km of the crust the stress tensor changes from the great basin boundary to the basin interior for example the extension direction changes by about 50 from roughly e w at the boundary in the west to roughly nw se inside of the basin in the boundary region the mean t axis inferred from focal mechanisms rotates clockwise by 27 from n64 e in the mammoth lakes area to n91 e in the chalfant valley about 50 km southeast of mammoth lakes however the components of the best fitting stress tensors in the two regions vary only by 12 so that a common stress tensor with a greatest principal stress azimuth of n10 w and a least principal stress azimuth of n80 e both components nearly horizontal favoring strike slip describes the seismic deformation of the boundary region quite well the stress tensors for the mono region and west central nevada have similar greatest and least principal stress azimuths northeasterly and northwesterly respectively however the plunges of the greatest principal stress are different nearly horizontal for the mono area and about 45 for central nevada reflecting the strong contribution of normal faulting in the central nevada earthquake data the resulting values which describe the relative sizes of the three stress components vary from 0 5 to 0 65 which indicates that the magnitudes of the three principal stresses are of similar size

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

crustal scale geometry and kinematics of late orogenic extension in the central aegean cyclades and ewia island abstract the aegean continental domain is known to be the site of widespread back arc extension since at least the middle miocene overprinting structures related to the mesozoic cenozoic hellenic orogeny features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered hp lt metamorphism across the aegean this study summarizes the results of a structural study in the central aegean area cyclades and evvia island examining in particular the relationship between ductile and brittle deformation on a regional scale extension appears to be responsible for most of the ductile deformation within hp rock units that have experienced penetrative greenschist facies and higher grade metamorphic overprinting on each island studied progressive extensional deformation has occurred through the development of a major normal sense detachment zone down to depths of about 18 25 km the geometry of the extensional system on the scale of the central aegean is described taking into account the characteristic features of the regional scale ductile strain field at least two probably three subparallel major north dipping detachment zones are identified trending nw se in the northwestern cyclades and e w in the southeastern cyclades both with initial dips estimated at around 30 45 the present geometry of the extensional system implies that these detachment zones and related metamorphic core complexes interfere with each other two main evolutionary models are considered which can account for such a kinematic link between parallel detachment zones structural data favour the sequential development of synthetic major detachments zones in a direction opposite to the sense of dip of the detachments early oligocene lower miocene detachment type extension in the area studied is related to a late orogenic post thickening back arc setting

periodic slow earthquakes from the cascadia subduction zone continuous geodetic measurements from convergent margins have shown that deep transient creep events can release large amounts of strain energy without detectable seismic shaking and they are thus known as slow or silent earthquakes because subduction zones generate the largest earthquakes

time dependent distributed afterslip on and deep below the izmit earthquake rupture surface deformation transients measured with the global positioning system during the 87 days between the 17 august 1999 izmit earthquake and the 12 november 1999 duzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture elastic model inversions for time dependent distributed fault slip using a network inversion filter approach show that afterslip was highest between and below the regions of maximum coseismic slip and propa gated downward to or even below the base of the crust maximum afterslip rates decayed from greater than 2 m yr immediately after the i zmit earthquake to about 1 2 m yr just prior to the duzce earthquake maximum afterslip occurred below the eastern karadere rupture segment and near the i zmit hypocenter afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone these observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby mw 7 2 12 november duzce earthquake

kinematics and vorticity in kangmar dome southern tibet testing midcrustal channel flow models for the himalaya 1 kinematic kinematic vorticity wm and deformation temperature analyses were completed to test the hypothesis that midcrustal rocks exposed in the core of the kangmar gneiss dome southern tibet record ductile deformation patterns of a frozen segment of a southward flowing midcrustal channel microscopic and mesoscopic kinematic indicators exhibit a downward transition from a subequal mix of top north and top south shear in garnet zone rocks to dominantly top north shear in staurolite kyanite zone and deeper rocks kinematic vorticity values indicate an increase in pure shear component with depth from 48 pure shear in chloritoid zone rocks through 62 in garnet zone to staurolite kyanite zone rocks to 68 pure shear in an orthogneiss the deepest exposed rocks deformation temperatures inferred from grain scale microstructures and quartz lattice preferred orientations increase from 300 c 400 c in chloritoid zone rocks to 600 c in the deepest exposed rocks these temperatures are equivalent to temperatures derived from garnet biotite thermobarometry indicating that wm was recorded during peak metamorphism this ductile deformation zone was cut by the brittle southern tibetan detachment system stds that juxtaposed metasedimentary rocks upon the orthogneiss on the basis of these relations midcrustal rocks in the core of kangmar dome record 1 general shear vertical thinning and n s horizontal extension with a component of top north shear during peak metamorphism within a ductile shear zone corresponding to the northern and deeper portion of the stds 2 an increase in pure shear with structural depth a consequence of an increase in lithostatic load and 3 displacement of the high temperature shear zone by the brittle stds our data are compatible with the deformation patterns predicted for the top part of a southward flowing midcrustal channel

structural insights into the early stages of exhumation along an orogen scale detachment the south tibetan detachment system dzakaa chu section eastern himalaya abstract structural transects through the south tibetan detachment system stds in the dzakaa chu valley tibet reveal a 1000 m thick low angle

late miocene early pliocene onset of n s extension along the southern margin of the central andean puna plateau evidence from magmatic geochronological and structural observations abstract extensional faulting typifies many of the earth s cenozoic orogenic plateaus the altiplano puna plateau in the southern central andes is the world s second largest orogenic plateau it contains a rich array of extensional fractures and normal faults with closely associated mafic volcanic flows dykes and cones this is in stark contrast to the foreland areas which record protracted shortening and continuous eastward growth of the mountain belt within the puna area a kinematic change over from shortening to extension has previously been inferred to have occurred during the quaternary however our research in the southern puna together with earlier petrologic investigations in the south central part of the plateau indicates an earlier onset for the extensional tectonism our analysis has focused on the eastern portion of the cordillera de san buenaventura in the southernmost part of the plateau where we mapped two upper miocene pliocene volcanic units associated with extensional faulting the aguada alumbrera ignimbrite and the la hoyada volcanic complex the structural inventory of these units records n s extensional deformation in the puna associated with the development of e w oriented fractures dykes normal and strike slip faults and folds new 40ar 39ar dating of magmatic rocks together with their relationships with the mapped structures reveal that widespread extension in the southern puna had already commenced by 5 ma and possibly as early as 7 ma although the extensional structures on the puna are typically associated with mafic volcanism the upper miocene pliocene la hoyada volcanic complex hosts dykes of intermediate compositions mafic to intermediate magmatism and extensional tectonism in this region is thus contemporaneous with and structurally akin to that in the central and northern parts of the plateau the ubiquitous cogenetic relationship between mafic volcanism and extensional tectonism both within the puna plateau and along its eastern flanks clearly indicates that the normal faulting is a regional phenomenon that is broadly coeval with an inferred rapid and large scale uplift of the altiplano puna plateau and a decrease in plate convergence rates these observations are compatible with a mantle delamination scenario in which the delamination of lithospheric mantle may have produced an ensuing weakening of the crust and regional uplift which may in turn have generated the conditions necessary to trigger gravitational extensional collapse beginning in the late miocene to early pliocene

structural evidence for the relationship between thrusts extensional faults and granite intrusions in the variscan belt of galicia spain abstract new structural and petrofabric data are presented from the hombreiro pluton a synkinematically emplaced granite located in the northern part of the variscan belt of spain this pluton shows the imprint of two plastic deformations the first is related to the motion of the mondonedo nappe with a top to the east shear sense and the later one to the vivero fault an extensional shear zone with a top to the west motion these deformations initiated at high temperatures close to the granite solidus as shown by the preservation of c slip fabrics in quartz this fact indicates that extensional faulting was activated soon after the thrusting of the mondonedo nappe during which the hombreiro granite was emplaced we propose that the position of the vivero fault was controlled by a crustal instability induced by the intrusion of a number of plutons that delineate the footwall block of this fault

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

channel flow and the himalayan tibetan orogen a critical review the movement of a low viscosity crustal layer in response to topographic loading provides a potential mechanism for 1 eastward flow of the asian lower crust causing the peripheral growth of the tibetan plateau and 2 southward flow of the indian middle crust to be extruded along the himalayan topographic front thermomechanical models for channel flow link such extrusion to focused orographic precipitation at the surface isotopic constraints on the timing of fault movement anatexis and thermobarometric evolution of the exhumed garnet to sillimanite grade metasedimentary rocks support mid crustal channel flow during the early to mid miocene exhumed metamorphic assemblages suggest that the dominant mechanism of the viscosity reduction that is a requirement for channel flow was melt weakening along the upper surface defined by the south tibetan detachment system and strain softening along the base bounded by the main central thrust neotectonic extrusion bounded by brittle quaternary faults south of the main central thrust is positively correlated with the spatial distribution of precipitation across a north south transect suggesting climate tectonic linkage over a million year time scale a proposed orogen wide eastward increase in extrusion rate over 20 ma reflects current precipitation patterns but climate tectonic linkage over this time scale remains equivocal

crustal scale thrusting and extension in the hercynian schwarzwald and vosges central europe the hercynian outcrop areas in the schwarzwald of sw germany and vosges of easternmost france consist from north to south of three major lithotectonic complexes very low to medium grade metamorphics of the saxothuringian belt the polymetamorphic central gneiss complex and the tectonic melange of the southern complex juxtaposition of these three complexes occurred by oblique convergent motions in early carboniferous time the saxothuringian belt is part of a major se dipping accretionary wedge that consists of lower paleozoic meta sedimentary and metaigneous rocks and superimposed upper devonian to lower carboniferous synorogenic volcanics and elastics the central gneiss complex consists of polymetamorphic paragneisses orthogneisses metabasites and meta ultramafics dipping to the nw the southern complex consists of an imbricated melange of north to nw dipping synorogenic elastics volcanics and blocks of ultramafics and gneissic basement the central gneiss complex cgc experienced pervasive hercynian low pressure high temperature metamorphisrn before it was emplaced against and thrust over the sedimentary and volcanic fill of the adjacent synorogenic basins this convergence 340 330 ma produced mylonitic cataclastic shear zones in the cgc the most significant shear zones are the se verging todtnau fault zone between the cgc and the southern complex and the nw verging lubine fault zone between the central gneiss complex and the rocks of the saxothuringian belt these two fault zones enclose a bivergent pop up of the polymetamorphic basement the todtnau fault zone probably extends for more than 100 km along strike and can be traced to a depth of 10 km by deep seismic reflection studies detachment probably occurred along a mid crustal zone of anatexis expressed by a domain of low seismic reflectivity between about 10 and 15 km crustal thickening during early carboniferous convergence led to an extensional phase of crustal reordering between about 330 and 280 ma that was accompanied by the massive rise of posttectonic granitic plutons from middle to high crustal levels simultaneous creation of a structural lamination in the refractory lower crust neglected the main convergent tectonic boundaries it produced a basin and range type lower crust and mantle crust transition in early permian time and involved a major advective input of heat

geodynamic models of mature continental collision evolution of an orogen from lithospheric subduction to continental retreat delamination 1 the behavior of deep lithospheric processes in continental collision are frequently speculated on but poorly understood in this study we build on previous work to explore the styles of continental lithosphere deformation during mature collision where 1800 km of convergence has been accommodated by horizontal shortening we conducted a suite of numerical geodynamic experiments that test the sensitivity of mature continent collision to varying mantle lithosphere density mantle lithosphere yield stress lower crustal strength and to the presence of phase change related density changes in the lower crust i e eclogitization of mafic lower crust the models suggest that the early stages of collision are accommodated by subduction of lower crust and mantle lithosphere along a discrete shear zone beneath the overriding plate following this initial stage of subduction the subducting lower crust and mantle lithosphere can retreat from the collision zone permitting the sub lithospheric mantle to upwell and intrude the overriding plate as a result the lower crust and mantle lithosphere of the overriding plate delaminate from the overlying crust with local isostatic adjustment subduction and delamination driven crustal processes plateau like uplift occurs the sub crustal evolution also causes bands of syn convergent crustal extension to develop in models with a rheologically weaker lower crust surface crustal response to the deep lithosphere dynamics becomes more diffuse as an example the numerical experiments satisfy a number of surface observables of the himalayan tibetan orogen namely 1 the current general mantle lithosphere architecture as defined by seismic analyses 2 the long wavelength plateau uplift of the western tibetan plateau 3 the surface heat flow measurements in india the tethyan himalaya qiangtang and in qaidam basin and 4 the anomalous syn convergent extension in the southern portion of the orogen

structure of the chamba nappe and position of the main central thrust in kashmir himalaya abstract the chamba nappe composed of an approximately 8 km thick sequence of late precambrian to jurassic age rocks is located between the higher himalaya crystallines hhc and the lesser himalayan lh formations of panjal imbricate zone piz in the kashmir himalaya to the south the panjal thrust demarcating the base brings the chamba nappe rocks over the panjal imbricate zone to the north the chamba nappe rocks lie over the metamorphic hhc along the south dipping chenab normal fault cnb a pervasive stretching lineation defined by a mineral lineation stretched pebbles and felspar phenocrysts plunges ne nne and occurs on the foliation cleavage surface this lineation is related to southward displacement of the chamba nappe the chamba nappe is folded by regional scale fold viz the chamba tandi and bharmor synclines and the tisa anticline these nw se trending folds structures were developed synchronously with southward thrusting of the chamba nappe the chamba nappe results from southwestward sliding of cover rocks from their basement hhc due to topographic uplift the main central thrust mct in kashmir himalaya is different from that of the kumaun and nepal himalaya the mct vaikrita thrust does not extend west of the beas river but it is exposed in the rampur window and the kishtwar window separating the hhc from the underlying lh rocks southward propagation of the mct from the window zone up cutting the overlying hhc is transferred to the panjal thrust which transports the chamba nappe to the south over the lesser himalayan formations

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

crustal accretion of thick mafic crust in iceland implications for volcanic rifted margins rifting near hotspots results in mantle melting to create thick mafic igneous crust at volcanic rifted margins vrms this mafic crust is transitional between rifted continental crust with mafic intrusions landward and oceanic crust into which it grades seaward seismic velocities crustal drilling and exhumed margins show that the upper crust in these areas is composed of basaltic lava erupted in subaerial to submarine conditions intruded by downward increasing proportions of dikes and sparse gabbroic intrusions the lower crust of these regions is not exposed but is inferred from seismic velocities vp 6 5 km sec and petrological constraints to be gabbroic to ultramafic in composition limited access to crustal sections generated along vrms have raised questions regarding the composition and structure of this transitional crust and how it evolves during the early stages of rifting and subsequent seafloor spreading active processes in iceland provide a glimpse of subaerial spreading with the creati

experimental constraints on hercynian anatexis in the iberian massif spain decisive in determining at which stage of the tectonic according to their chemical and mineralogical com cycle anatexis took place with these arguments in mind positions the hercynian granitoids both older andwe have studied experimentally the melting relationships younger can be grouped into two families peraluminous of the ollo de sapo gneiss the geochemical relationships two mica granites and leucogranites and biotite bearingand the compositions of melts derived from this mica crd hb granodiorites the widespread ocurrencerich gneissic rock were established by castro

time dependent triggered afterslip following the 1989 loma prieta earthquake we employ a modified version of the network inversion filter to investigate time dependent slip following the 1989 loma prieta earthquake previous analysis of global positioning system gps and leveling data suggests afterslip on the loma prieta rupture as well as aseismic slip on a thrust fault northeast of the san andreas fault which we identify with the foothills thrust belt we analyzed 173 daily gps solution files at 62 stations collected from 1989 8 to 1998 3 a total of 1 134 three dimensional relative baseline determinations the observed position changes are assumed to result from secular deformation random benchmark motions and temporally varying fault slip the data reveal temporal variations in slip rate but poorly resolve spatial variations in fault slip the amount of temporal smoothing is estimated by maximum likelihood conditional on this estimate reverse slip on the foothills thrust decays from 45 12 mm yr immediately after the earthquake to zero by 1992 reverse slip on the loma prieta rupture surface decays from 57 11 mm yr to zero by 1994 right lateral slip on the loma prieta rupture surface decays monotonically from 30 10 mm yr to zero by 1994 these results suggest that 1 triggered afterslip can occur off the main rupture zone on adjacent faults 2 shallow afterslip dominated the postseismic deformation for the 8 years following the earthquake and 3 postseismic slip on the foothills thrust may account for a significant portion of its total slip budget

how perfect is the himalayan arc the himalayan plate boundary because it is entirely subaerial is both the most dramatic and the most accessible to direct observation of all active convergent boundaries on earth the shape of this boundary can be described as a small circle of radius 1696 55 km centered at long 91 6 1 6 e and lat 42 4 2 1 n for the extent of the arc between long 77 2 and 92 1 e the pole of this small circle is consistent whether seismicity topography or stress state is used to define the position of the tectonic boundary the defined small circle also coincides with a peak in microseismicity the maximum horizontal strain rate and a peak in the vertical velocity field this quantitative definition of a stable curved tectonic boundary is a prerequisite to modeling the dynamics of curvature in convergent arcs and applying appropriate boundary conditions to other regional models

evidence for extension in the western alpine orogen the contact between the oceanic piemonte and overlying continental sesia units abstract in the alps and other orogens major tectonic contacts must have been established during convergence and overthrusting what is less clear is how strongly these contacts were modified as the orogen evolved during alpine collision continental crust of the overriding plate the sesia unit is inferred to have been thrust northwest over oceanic material of the piemonte unit both units preserve in parts eclogite facies metamorphism however all structures along this contact in our study area indicate that final movement was southeast directed with no clear evidence of the kinematics of earlier movement these structures include se verging folds se directed shear bands and larger normal faults downthrowing to the southeast it is inferred that there was a se directed shear regime below the contact which although locally buckling and shortening earlier lithological contacts was dominantly extensional with respect to pre existing tectonic layering the lower limit of this shear regime has not been identified although se directed shear is common throughout the upper greenschist facies part of the piemonte unit we show that this shear passes beneath the sesia zone and does not re emerge it is therefore a shear which was net extensional relative both to the modern surface and to the palaeosurface it may have contributed to the unroofing of eclogite facies rocks in the lower part of the piemonte unit although additional timing data are required to clarify this nearby se directed shears buckle and imbricate earlier layering within the piemonte unit these are normally identified as backthrusts yet they do not appear to breach the base of the sesia unit and might merge with the extensional shear even though these structures shorten layering they too could have been extensional relative to the earth s surface the significance of alpine backthrusts should be reappraised in this context this study indicates that hinterland directed extension could have been an important phenomenon during alpine evolution

time dependent inversion study of the slow thrust event in the nankai trough subduction zone southwestern japan we estimate time variation of interplate coupling between the subducting philippine sea and the overriding continental plates along the western part of the nankai trough japan during the period between 1996 and 1999 after subtracting annual components and average annual velocities deviation from steady crustal deformation in the shikoku and kyushu islands is examined around 15 gps stations facing the bungo channel detected the following three stage ground displacements of up to 3 cm in 1997 with a duration of 1 year 1 increased displacement rate associated with the 1996 hyuga nada earthquakes 2 an increasingly rapid anomalous period of deformation from june 1997 to the end of 1997 and 3 a slowdown period and return to normal rates of deformation after the end of 1997 linearized least squares locate the slip area of this slow event near the plate boundary beneath the bungo channel the estimated slip is opposite the motion of the philippine sea plate these results support the hypothesis that the observed anomalous caistal deformation is due to a rebound process of the philippine sea plate time dependent inversion analysis shows the following slip history during the initial period of increased displacement rate a visible slip area is resolved beneath the coastal area of southwestern shikoku in the next stage of accelerated displacements the slip area expanded southwestward to the bungo channel beginning around june 1997 when postseismic slip of the 1996 hyuga nada earthquakes almost ended after this accelerated period the estimated thrust slip ends with time

mesozoic evolution hinterland of the sevier orogenic belt west of the mesozoic foreland sevier orogenic belt in northern utah and southern idaho a regional detachment of inferred mid jurassic age occurs in upper mississippian shale and can be traced westward to the margin of the coeval magmatic arc represented by spaced calc alkalic intrusions the detachment may be the oldest and structurally highest intrusions the detachment may be the oldest and structurally highest thrust of the idaho wyoming thrust belt if so it bridges the apparent spatial and temporal gap between magmatism and thrusting thus suggesting a direct genetic link thin skinned deformation is interpreted here to be driven from the heated magmatic arc toward the colder craton by lateral tectonic compression created by lateral volume increase due to intrusion elevated crustal geotherms and possible thickening in the arc the metamorphic complexes occur at the crustal interface between these two thermal tectonic regimes and may themselves be allochthonous

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

southern california permanent gps geodetic array error analysis of daily position estimates and site velocities we analyze time series of daily positions estimated from data collected by 10 continuously monitoring global positioning system gps sites in southern california during the 19 month period between the june 28 1992 mw 7 3 landers and january 17 1994 mw 6 7 northridge earthquakes each time series exhibits a linear tectonic signal and significant colored noise spectral power at frequencies in the range 5 yr 1 to 0 5 d 1 is dominated by white noise or possibly fractal white noise and is several orders of magnitude higher than what would be expected from random walk noise in this short period range attributed by others to geodetic monument motions estimating a single slope for the time series power spectra suggests fractal white noise processes with spectral indices of about 0 4 site velocity uncertainties assuming this fractal white noise model are 2 4 times larger than uncertainties obtained assuming a purely white noise model a combination white noise plus flicker noise spectral index of 1 model also fits the data and suggests that the velocity uncertainties should be 3 6 times larger than for the white noise model we cannot adequately distinguish between these two noise models nor can we rule out the possibility of a random walk signal at the lowest frequencies these questions await the analysis of longer time series in any case reducing the magnitude of low frequency colored noise is critical and appears to be best accomplished by building sites with deeply anchored and braced monuments otherwise rate uncertainties estimated from continuous gps measurements may not be improved significantly compared to those estimated from infrequent campaign mode measurements

geometry and kinematics of continental basement deformation during the alpine orogeny mt olympos region greece abstract in the hellenic alps continental crust of the leading edge of the apulian plate was subducted beneath the european plate during late cretaceous early tertiary closure of tethyan ocean basin s carbonate rocks of the external hellenides are exposed in the mt olympos tectonic window overthrust by several thin 0 2 4 km thick thrust sheets of granitic basement derived from the internal hellenides field mapping and structural data from the mt olympos region reported here establish the geometry kinematics and strain history of the continental basement before during and after continental collision and subduction seven deformational events document the change from ductile conditions obtained at depth in the early stages of deformation to lower temperature ductile and semi ductile shortening related fabrics to brittle extensional features related to exhumation of the metamorphic rocks d 1 d 4 occurred during subduction of basement nappes and are marked by thrust faulting and folding under blueschist facies conditions d 5 d 7 are related to unroofing of blueschist facies metamorphic rocks and are associated with extension each of the major shortening phases d 1 d 3 verges toward the foreland southwest oppositely directed d 5 d 7 extension occurred behind the active thrust belt during continued regional shortening normal faults have strongly modified the original thrust related geometry in the mt olympos region and elsewhere in the internal hellenides

the role of pre existing thrust faults and topography on the styles of extension in the gran sasso range central italy abstract structural analysis and field mapping together with simple geometrical and flexural elastic models document that two styles of quaternary extensional tectonics characterized the gran sasso range central apennines italy in the western part of the range extension took place on 10 15 km long range front normal faults with associated 600 1000 m high escarpments showing evidence of late glacial holocene activity this topography has been reproduced with a thin elastic plate subjected to the isostatic forces induced by the movement along high angle 55 65 planar normal faults in the eastern part of the belt extension occurred on shallow dipping normal faults 30 35 which reactivated progressively deeper pre existing thrusts in this area antithetic domino faults formed to accommodate the mechanical adjustment of the hanging wall over a variably dipping major fault surface the eastward increase in shortening due to the earlier compressional phase documented in the gran sasso belt by previous authors accounts for the more developed zones of weakness and high topographic relief in the eastern sector this setting could explain the different styles of extension and the more advanced northeastern limit of normal faulting in the eastern sector this work suggests that normal faults can originate either with low or high angle geometry in the upper crust according to the pre existing tectonic setting and that topography could be important in controlling the geometry and pattern of migrating normal faulting

simultaneous development of noncylindrical folds frontal ramps and transfer faults in a compressional regime experimental investigations of himalayan examples experiments were performed with modeling clay models to understand the simultaneous development of folds frontal ramps and transfer faults oblique ramps in layered sequences the models were deformed by layer parallel contraction at a late stage of deformation the models were subjected to a maximum contraction parallel to the layering and perpendicular to the early axis of maximum contraction to study the effect of the superposed deformation the experimental results reveal that a transfer fault joining two frontal thrust ramps acts as a shear zone and controls the orientation of fold hinge lines in the adjacent region hence two orientations of fold hinge lines may be observed 1 oblique to the axis of maximum compression in the vicinity of the transfer fault and 2 normal to the axis and parallel to the frontal ramps away from the transfer fault during the longitudinal fold propagation the two hinge lines may coalesce to form a fold curvature opposite to the tectonic transport direction the curvature varies along the transfer fault depending on the shear zone geometry additional fold hinge line curvatures may result during the superposed deformation both in the early and the superposed fold hinge lines all these curvatures produce a systematic pattern which can be analyzed with the help of the experimental results the fold hinge line orientation may also be used to establish an oblique ramp in field as demonstrated by examples from the lesser himalayas india

formation of an active thrust triangle zone associated with structural inversion in a subduction setting eastern new zealand 1 we analyze a thrust triangle zone which underlies the continental shelf of hawke bay eastern new zealand within the hikurangi subduction margin this triangle zone differs from many other examples in that it is active 90 km from the leading edge of the overriding plate and formed due to polyphase deformation involving opposed dipping thrust duplex and backthrust with the later structure forming in response to inversion of an extensional graben the component structures of the zone mainly developed sequentially rather than synchronously high quality marine seismic reflection lines tied to well and seabed samples reveal the three dimensional structure of the zone together with its 25 myr evolution and late quaternary activity the triangle zone occurs in the lateral overlap between a stack of nw dipping blind thrusts and a principal backthrust the kidnappers fault the nw dipping thrusts initiated in the early middle miocene during the early stages of subduction with subsequent thrust duplex formation producing major uplift and erosion in the late miocene early pliocene the active backthrust formed during the late miocene to early pliocene as a thin skinned listric extensional fault confined to the cover sequence structural inversion of the extensional fault commenced in the early middle pliocene produced the backthrust and marks the formation of the thrust triangle zone the thrust duplex and backthrust accrued strain following inversion however the later structure accommodated most of the surface deformation in the quaternary section balancing of the triangle zone together with a detailed analysis of reverse displacements along the backthrust reveal spatial and temporal variations of strain accumulation on the two principal components of the zone although the formation of the triangle zone is strongly influenced by regional tectonics of the subduction system these variations may also in part reflect local fault interaction for example high quaternary displacement rates on the backthrust accounts for 70 of the displacement loss that occurs on the southern segments of the overlapping lachlan fault understanding the tectonic evolution of such complex polyphase thrust triangle zones requires the preservation of growth strata that record sequential deformation history in the absence of such data synchroneity of opposed dipping thrusts in triangle zones cannot be assumed

a silent slip event on the deeper cascadia subduction interface continuous global positioning system sites in southwestern british columbia canada and northwestern washington state usa have been moving landward as a result of the locked state of the cascadia subduction fault offshore in the summer of 1999 a cluster of seven sites briefly reversed their direction of motion no seismicity was associated with this event the sudden displacements are best explained by 2 centimeters of aseismic slip over a 50 kilometer by 300 kilometer area on the subduction interface downdip from the seismogenic zone a rupture equivalent to an earthquake of moment magnitude 6 7 this provides evidence that slip of the hotter plastic part of the subduction interface and hence stress loading of the megathrust earthquake zone can occur in discrete pulses