taking mylonites temperatures we measured titanium in quartz titaniq temperatures by ion microprobe for three deformed rocks having different quartz microstructure to assess the possibility of constrain ing the temperature of mylonitization directly from dynamically recrystallized quartz grains calibration via ion microprobe indicates analytical precisions and accuracies of 2 c and 10 c respectively 2 high versus low temperature mylonites yield high versus low ti concentrations and temperatures that are consistent with other fi eld studies of mylonites that have similar microstructures these observations imply that titaniq accurately measures dynamic recrystallization temperatures variations in temperature in a single domain exceed analytical errors implying that one microstructure in a rock can refl ect different tempera tures and presumably strain rates combined with paleopiezometric and phase equilibrium estimates of differential stress and water fugacity strain rates may be estimated

new insights into the tectonic evolution of the andaman basin northeast indian ocean multibeam swath bathymetry data acquired over an area of about 30 000 km2 together with magnetic and single channel seismic data have been analyzed to understand the tectonic evolution of the andaman basin northeast indian ocean swath bathymetry data revealed several morphotectonic features that divide the basin into a complex western part comprising arc parallel seamount chains and n s trending fault systems and a relatively smooth eastern part a sw ne trending spreading ridge bisects the basin an offset of about 11 8 km at 94 21 e longitude divides the spreading ridge into sw and ne segments high resolution bathymetry and magnetic data enabled us to identify two finer scale segments over the sw part of the ridge that display distinct topographic fabric compared to the ne segment thin sediment cover and presence of axial seamounts characterize sw segments of the ridge whereas the ne segment has thick sediment cover and well maintained rift valley kinematics of basin evolution derived from the identification of the seafloor spreading magnetic anomalies suggest very slow initial opening rates of about 1 6 cm yr and an increase in the spreading rate to about 3 8 cm yr from anomaly 2 to present although this interpretation relies only on magnetic anomalies in the two westernmost segments it is suggested that the true seafloor spreading started about 4 ma in the andaman backarc basin rather than the 11 ma postulated previously we infer based on the integrated analysis of swath bathymetry magnetic and seismological data that the basin has evolved as a consequence of extrusive tectonics that prompted extension and rifting along the plane joining the sagaing and semengko fault systems extension by seafloor spreading during the past 4 myr resulted in the formation of the deep andaman backarc basin this phase has also experienced westward propagation of the spreading center

alpine and pacific styles of phanerozoic mountain building subduction zone petrogenesis of continental crust a broad continuum exists between two distinct end member types of mountain building alpine type orogenic belts develop during subduction of an ocean basin between two continental blocks resulting in collision they are characterized by an imbricate sequence of oceanward verging nappes some alpine belts exhibit superimposed late stage backthrusting sediments are chiefly platform carbonates and siliciclastics in some cases associated with minor amounts of bimodal volcanics pre existing granitic gneisses and related continental rocks constitute an autochthonous parautochthonous basement metamorphism of deeply subducted portions of the orogen ranges from relatively high pressure hp to ultrahigh pressure uhp calcalkaline volcanic plutonic rocks are rare and have peraluminous s type bulk compositions in contrast pacific type orogens develop within and landward from long sustained oceanic subduction zones they consist of an outboard oceanic trench accretionary prism and an inboard continental margin island arc the oceanic assemblage consists of first cycle in part melanged volcaniclastics and minor but widespread cherts deep water carbonates intimately mixed with disaggregated ophiolites the section recrystallized under hp conditions recumbent fold vergence is oceanward a massive slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable non subducted plate it consists of abundant dominantly intermediate metaluminous i type volcanics resting on old crust both assemblages are thrown into open folds intruded by comagmatic i type granitoids and metamorphosed locally to regionally under high t low p conditions in the subduction channel of collisional and outboard circumpacific terranes combined extension above and subduction below allows buoyancy driven ascent of ductile thin aspect ratio slices of hp uhp complexes to midcrustal levels where most closely approached neutral buoyancy exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume at massive sialic buildups associated with convergent plate cusps syntaxes tectonic aneurysms may help transport hp uhp complexes from mid to upper crustal levels the closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts so volcanic plutonic belts are poorly developed in alpine orogens compared with circumpacific convergent plate junctions generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation phase equilibrium studies show that under typical subduction zone p t trajectories clinoamphibole ca al hydrous silicates constitute the major hydroxyl bearing phases in deep seated metamorphic rocks of morb composition other hydrous minerals are of minor abundance ca and na clinoamphiboles dehydrate at pressures of above approximately 2 gpa but low temperature devolatilization may be delayed by pressure overstepping thus metabasaltic blueschists and amphibolites expel h2o at melt generation depths and commonly achieve stable eclogitic assemblages partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions for reasonable subduction zone geothermal gradients however white micas biotites remain stable to pressures 3 gpa accordingly attending descent to depths of 100 km mica rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of h2o and transform incompletely to stable eclogite facies assemblages underflow of amphibolitized oceanic lithosphere thus generates most of the deep seated volatile flux and the consequent partial melting to produce the calcalkaline suite along and above a subduction zone where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths volatile flux severely diminishes thus continental collision in general does not produce a volcanic plutonic arc whereas in contrast the long continued contemporaneous underflow of oceanic lithosphere does

structural decoupling in a convergent forearc setting southern crete eastern mediterranean a multidisciplinary approach is used to investigate the structure of the southern cretan margin which is located in one of the most seismically active forearc regions in europe bathymetric seismic reflection and fault plane solution data were used to identify the main tectonic features on the margin correlating their evolution with the main sedimentary sequences recognized on crete in contrast to the majority of forearc settings in the pacific and indian oceans southern crete is a region of predominantly oblique movement above well defined detachment zones north dipping thrust faults identified on seismic reflection profiles reveal significant crustal shortening during the miocene due to the westward propagation of the hellenic fold and thrust system in addition east dipping thrust faults rooted on top of pre neogene strata were also identified but only a few of these thrusts affect neogene to holocene strata small scale domes derived from evaporitic messinian intrusions deform pliocene quaternary strata west and east dipping normal faults were also recognized within the mesozoic and cenozoic successions and these are related to regional extension during forearc convergence in such a setting the fault bounded continental slope of crete effectively separates a region of uplift crete from subsiding troughs to the south our work shows that structural segmentation at depth is complex with multiple crustal levels recording contrasting styles of deformation and distinct moment tensor solutions this complexity derives from the oblique style of convergence recorded south of crete which reactivates distinct crustal levels depending on their rheology and relative degree of metamorphism inherited during alpine compression as a result a strong correlation between seafloor morphology and transtensional movements is recorded in the upper 10 15 km of the crust while transpression prevailed after the serravallian below these depths

collapse of the luzon volcanic arc during onset of arc continent collision evidence from a miocene pliocene unconformity eastern taiwan a regional miocene pliocene unconformity in the coastal range of eastern taiwan provides information about vertical motions that affected the colliding volcanic arc during initiation of arc continent collision the unconformity separates precollisional volcanic and volcaniclastic rocks of the miocene luzon arc and forearc basin from an overlying thick sequence of syncollisional pliocene pleistocene sedimentary rocks derived from the metasedimentary accretionary wedge of taiwan the duration of time represented by the unconformity averages 1 2 m y and locally ranges up to 4 m y the basal unit of the younger syncollisional sequence consists of bathyal mega slump folded mudstone pebbly mudstone and thin bedded turbidites sharp stratigraphic juxtaposition of these deep marine deposits directly above shallow marine limestone and epiclastic rocks upper part of the older arc sequence requires extremely rapid subsidence during initiation of the collisional basin the coastal range unconformity is interpreted to represent a period of latest miocene nondeposition and or erosion in the luzon arc that was related to initial subduction of the eurasian continental margin along the manila trench this period was followed in pliocene time by very rapid subsidence or collapse of the arc platform to 1 2 km water depth there are two dissimilar mechanisms of basin subsidence that may have controlled initiation of the collisional basin 1 extension rifting and crustal thinning in the luzon arc massif to form a series of small rift or pull apart subbasins or 2 flexural subsidence of oceanic lithosphere in a foreland style basin due to crustal loading in the expanding accretionary wedge of proto taiwan although data are insufficient to provide a unique solution the flexural subsidence model is supported by systematic younging of the initial subsidence event from west to east which may record eastward migration of an outer flexural bulge notable differences between the unconformity and paleogeography of the early collisional basin and analogous present day basins in the offshore region to the south suggest that arc continent collision and basin development in eastern taiwan have experienced secular change since late pliocene time

5 9 mw 18th june 2010 earthquake and fault segment linkage at andaman a study based on macroseismic survey gps geodesy and coulomb stress changes abstract 18th june 2010 5 9 mw earthquake at north andaman triggered along nw se pre existing fault with reverse fault mechanism macroseismic survey and gps geodesy reveal maximum damages following ne sw trend due to normal fault mechanism coulomb stress modeling for post and inter seismic earthquakes after the 2004 mega earthquake show different stages of fault segment linkage at north andaman the present earthquake has been explained as co shock due to asiesmic soft linkage of fault propagation

transcurrent movements in the burma andaman sea region the drag produced by the northward movement of the indian plate has caused the indoburman ranges an accretionary prism to decouple from the burma plate along the kabaw fault and the burma plate itself has decoupled from the eastern highlands of burma along the sagaing fault the indoburman ranges have probably been dragged north slightly ahead of the burma plate although at times the two plates may have locked together to move as one unit it is suggested that the double arc formed by the indoburman ranges and the andaman nicobar sumatra islands was formerly one large single arc northward translation of this larger single arcuate structure has caused the tail end of the arc i e the andaman nicobar sumatra section to rotate into the burma plate causing a bending of the arc to form the present double arc and compression within the burma plate to form the pegu yoma folding northward movement of the burma plate has also resulted in offset by over 400 km of the irrawaddy river from its former connection through the channel of what is now the chindwin river

implications of the 26 december 2004 sumatra andaman earthquake on tsunami forecast and assessment models for great subduction zone earthquakes results from different tsunami forecasting and hazard assessment mod els are compared with observed tsunami wave heights from the 26 december 2004 indian ocean tsunami forecast models are based on initial earthquake information and are used to estimate tsunami wave heights during propagation an empirical forecast relationship based only on seismic moment provides a close estimate to the observed mean regional and maximum local tsunami runup heights for the 2004 indian ocean tsunami but underestimates mean regional tsunami heights at azimuths in line with the tsunami beaming pattern e g sri lanka thailand standard forecast models developed from subfault discretization of earthquake rupture in which deep ocean sea level observations are used to constrain slip are also tested forecast models of this type use tsunami time series measurements at points in the deep ocean as a proxy for the 2004 indian ocean tsunami a transect of deep ocean tsunami amplitudes recorded by satellite altimetry is used to constrain slip along four subfaults of the m 9 sumatra andaman earthquake this proxy model performs well in comparison to observed tsunami wave heights travel times and inundation patterns at banda aceh hypothetical tsunami hazard assessments models based on end member estimates for average slip and rupture length m w 9 0 9 3 are compared with tsunami observations using average slip low end member and rupture length high end member m w 9 14 consistent with many seismic geodetic and tsunami inversions adequately estimates tsunami runup in most regions except the extreme runup in the western aceh province the high slip that occurred in the southern part of the rupture zone linked to runup in this location is a larger fluctuation than expected from standard stochastic slip models in addition excess moment release 9 deduced from geodetic studies in comparison to seismic moment estimates may gen erate additional tsunami energy if the exponential time constant of slip is less than approximately 1 hr overall there is significant variation in assessed runup heights caused by quantifiable uncertainty in both first order source parameters e g rupture length slip length scaling and spatiotemporal complexity of earthquake rupture

ti diffusion in quartz abstract we have measured ti diffusion in quartz under dry 1 atm conditions experiments were performed using synthetic and natural quartz and a tio2 powder source with ti profiles obtained by rutherford backscattering spectrometry rbs over the temperature range 700 1150 c the following arrhenius relation was obtained for diffusion parallel to 001 d ti 7 10 8 exp 273 12 kj mol 1 r t m 2 sec 1 similar diffusivities were obtained for both synthetic and natural quartz and for a range of ti source materials including natural titanite powder and rutile quartz single crystal diffusion couples although ti diffusion appears slightly slower in the direction normal to c this difference is not great using these diffusion parameters calculations indicate that distances over which diffusional alteration of ti concentrations in quartz could occur in a million years would be on order of 500 m at 800 c and 15 m at 600 c the arrhenius relation above should be broadly applicable and may find application in constraining metamorphic histories under conditions where the ti in quartz geothermometer can be reasonably applied this relation can also be used to constrain peak temperatures and cooling histories from observations of exsolved rutile needles in quartz and the width of the ti depleted zone that develops around the growing needles finally these findings may find utility in evaluating crystal residence times in magmatic systems when coupled with observations of the sharpness of zoning patterns in quartz

deep structure and tectonics of the burmese arc constraints from earthquake and gravity data abstract active subduction of the indian plate is currently occurring beneath the burmese arc along an east dipping benioff zone which extends to a depth of about 180 km the overriding burma plate has an appearance of an inland seismic slab that is deflected downwards in the vicinity of the benioff zone a crustal seismic zone some 60 80 km east of the benioff zone correlates to backarc activity a triangular aseismic wedge in the top part of the crust outlines the central belt molasse basin east of the burmese foldbdt fault plane solutions show that the burmese benioff zone is characterized by shallow angle thrusting at its upper edge whereas down tip tensional events dominate its lower edge most of the backarc seismicity is accounted for by the sagaing transform or by the activity of the shan scarp normal fault zone at the margin of the asian plate a gravity anomaly pair with amplitude of 175 mgal coincides with the 1100 km long burmese arc lying in a north south direction the gravity anomalies along a profile in central burma and in adjacent areas of the bengal basin are interpreted in terms of plate subduction as well as near surface mass anomalies this suggests that sediments below the central belt may have an average thickness of the order of 10 km but may be as thick as 15 km at the subduction zone the oceanic crust underlying deeper parts of the bengal basin experiences phase transition at about 30 km depth in a benioff zone environment east of the burmese foldbelt several thrust planes are present within the folded and deformed cretaceous tertiary sediments of the fold belt these are often associated with ophiolites and basic to ultrabasic rocks a low density zone at least 60 km wide underlies the andesitic volcanic axis in the overriding plate

the effect of deformation on the titaniq geothermobarometer an experimental study we performed high strain up to 47 axial compression experiments on natural quartz single crystals with added rutile powder tio2 and 0 2 wt h2o to investigate the effects of deformation on the titanium in quartz titaniq geothermobarometer one of the objectives was to study the relationships between different deformation mechanisms and incorporation of ti into recrystallized quartz grains experiments were performed in a griggs type solid medium deformation apparatus at confining pressures of 1 0 1 5 gpa and temperatures of 800 1 000 c at constant strain rates of 1 10 6 or 1 10 7 s 1 mobility of ti in the fluid phase and saturation of rutile at grain boundaries during the deformation experiments are indicated by precipitation of secondary rutile in cracks and along the grain boundaries of newly recrystallized quartz grains microstructural analysis by light and scanning electron microscopy the latter including electron backscatter diffraction mapping of grain misorientations shows that the strongly deformed quartz single crystals contain a wide variety of deformation microstructures and shows evidence for subgrain rotation sgr and grain boundary migration recrystallization gbmr in addition substantial grain growth occurred in annealing experiments after deformation the gbmr and grain growth are evidence of moving grain boundaries a microstructure favored by high temperatures electron microprobe analysis shows no significant increase in ti content in recrystallized quartz grains formed by sgr or by gbmr nor in grains grown by annealing this result indicates that neither sgr nor moving grain boundaries during gbmr and grain growth are adequate processes to facilitate re equilibration of the ti content in experimentally deformed quartz crystals at the investigated conditions more generally our results suggest that exchange of ti in quartz at low h2o contents which may be realistic for natural deformation conditions is still not fully understood thus the application of the titaniq geothermobarometer to deformed metamorphic rocks at low fluid contents may not be as straightforward as previously thought and requires further research

evolution of the eastern indian ocean since the late cretaceous constraints from geosat altimetry we propose a new model for the tectonic evolution of the eastern indian ocean from the late cretaceous to the present two types of data are used to improve previously published reconstructions first recent reinterpretations of seafloor magnetic anomalies between australia and antarctica and in the wharton basin provide new constraints on spreading rates and the timing of major reorganizations second vertical deflection profiles i e horizontal gravity anomaly derived from 22 repeat cycles of geosat altimeter data reveal the tectonic fabric associated with fracture zones these new geosat data provide tight constraints on paleospreading directions for example three prominent fracture zones can be traced from south of tasmania to the george v basin antarctica providing an important constraint on the relative motions of australia and antarctica through the late eocene in addition the geosat profiles are used to locate the conjugate continental margins and continent ocean boundaries of australia and antarctica as well as the conjugate rifted margins of kerguelen plateau and broken ridge based on a compilation of magnetic anomaly data from the crozet basin the central indian basin the wharton basin and the australian antarctic basin ten plate tectonic reconstructions are proposed reconstructions at chrons 5 11 ma 6 21 ma 13 36 ma and 18 43 ma confirm that the southeast indian ridge behaved as a single plate boundary since chron 18 the constraints from the geosat data provide an improvement in the fit of the kerguelen plateau and broken ridge at chron 20 46 ma to avoid overlaps between broken ridge and the kerguelen plateau prior to their breakup our model includes relative motions between the northern and southern provinces of the kerguelen plateau finally we examine the implications of our model for the relative motions of india australia and antarctica on the tectonic evolution of the kerguelen plateau and broken ridge and the adjacent labuan basin and diamantina zone as well as the emplacement of the ninetyeast ridge and the kerguelen plateau over a fixed hot spot

numerical study of winter water formation in the chukchi sea roles and impacts of coastal polynyas winter water formation is examined in the chukchi sea for the winters of 1992 2006 using a primitive equation ocean model forced by ncep wind and surface salinity flux derived from ssm i thin ice thickness estimates the model is also forced by an external inflow of 0 8 sv through the bering strait the model successfully reproduces the oceanic circulation on the chukchi shelf thus providing numerous insights into behaviors of salt enriched water produced on the shelf the experiments show that under northeasterly winds northward throughflow across barrow canyon is reduced this results in salinity buildup under freezing conditions and ultimately in greater enhancement of salinity of the waters carried into the arctic basin the flow and salinity enhancement of the flow through herald canyon is less extreme but more steady than through barrow canyon together with moored salinity in the bering strait the model results estimate the actual salinity to be 32 9 0 8 psu and 32 7 0 3 psu respectively for waters moving through the barrow and herald canyons both estimates are less than 33 1 psu that is typically observed for the cold halostad layer in the canada basin suggesting the importance of diapycnal mixing with saltier atlantic origin water

gravity field and deep structure of the bengal fan and its surrounding continental margins northeast indian ocean abstract a revised gravity anomaly map for the northeast indian ocean shows that the shelf edge underlying the eastern continental margin of india is a rather narrow but extensively linear gravity low minimum free air 149 mgal the bengal fan seaward of the shelf has a depressed gravity field average free air 20 to 30 mgal in spite of the enormous thickness of sediments of as much as 10 15 km the two buried ridges below the bengal fan the 85 east and 90 east ridges have a large negative 75 mgal and a substantial positive 40 mgal free air anomaly respectively the andaman and burmese arcs lying along the east margin of the bengal fan are active subduction areas which have typical bipolar gravity signatures with a maximum amplitude of 300 mgal gravity interpretation for three regional traverses across the central and northern parts of the bengal fan and their surrounding continental margins suggests that a thickened oceanic crustal wedge juxtaposes the transitional crust under the eastern continental slope of india the 85 east ridge that was created when the indian ocean lithosphere was very juvenile appears to underlie a nearly 10 km thick and 120 km wide oceanic crustal block consisting of the ridge material embedded in the upper lithosphere while the 90 east ridge submarine topography buried load below the bengal fan is probably isostatically compensated by a low density mass acting as a cushion at the base of the crust the bengal fan crust with its thick sediment layer is carried down the andaman subduction zone to a depth of about 27 km where possibly phase transition takes place under higher pressure the maximum sediment thickness at the andaman burmese subduction zone is of the order of 10 12 km the gravity model predicts a low density zone about 60 km wide below the andaman burmese volcanic arc penetrating from crustal to subcrustal depths in the overriding burma plate a more complex density distribution is however envisaged for the andaman volcanic arc that is split by the neogene back arc spreading ridge the ocean continent crustal transition possibly occurs farther east of the volcanic arc below the shan plateau margin in burma or below the mergui terrace at the malayan continental margin east of the andaman sea

transtension to transpression a study of strain evolution in andaman islands based on gps measurements following great sumatra andaman earthquake 2004 we use campaign mode and continuous gps data from 2005 to 2008 to examine the evolving strain pattern in andaman islands after the great sumatra andaman earthquake two and half years after the earthquake the post seismic period changes to inter seismic period at port blair post seismic displacement rate reduced to about 95 with clockwise rotation of azimuths and towards north andaman velocity vector gradually rotates towards north with drop in magnitude the inter seismic displacement was dominated by trench parallel movement strain calculations on inclined transpession model shows deformations at andaman were highly heterogeneous both in post and inter seismic periods with dominant strike slip and dip slip shearing along with layer parallel shortening and extension post seismic and inter seismic periods had suffered simultaneous subduction perpendicular extension and contraction leading to transtension and transpression deformational environments respectively occurrence of earthquakes during post and inter seismic periods were results of interplay between simple shear and pure shear components of the strain matrix

plastic anisotropy and fabric evolution in sheared and recrystallized quartz single crystals 1 the effect of a lattice preferred orientation on the flow strength of quartz aggregates dynamically recrystallized from single crystals of synthetic quartz was investigated using general shear experiments in a griggs apparatus experiments were conducted at shear strains up to 5 at a temperature of 900 c confining pressure of 1 5 gpa and shear strain rate of 10 5 s 1 three starting orientations of crystal were used to activate three slip systems basal a prism c and prism a although slip induced rotation of the crystal axes in the first two orientations led to the activation of additional slip systems for crystals with higher water contents basal a and prism a orientations are relatively weak and prism c orientations are stronger all three initial crystal orientations undergo dynamic recrystallization with increasing shear strain although the strain required for 100 recrystallization varies 2 for prism c slip 3 8 for basal a slip and 5 for prism a slip for all three starting orientations distinct domains of recrystallized grains develop with c axes parallel to y of the strain ellipsoid ymax replacing recrystallized grains of other orientations the ymax domains increase in size with increasing strain in addition strain markers show that strain is highly localized within the ymax domains indicating geometrical softening of up to an order of magnitude in effective viscosity

seismicity gravity anomalies and lithospheric structure of the andaman arc ne indian ocean abstract the andaman arc in the northeastern indian ocean defines nearly 1100 km long active plate margin between the india and burma plates where an oblique benioff zone develops down to 200 km depth several east trending seismologic sections taken across the andaman benioff zone abz are presented here to detail the subduction zone geometry in a 3 d perspective the slab gravity anomaly computed from the 3 d abz configuration is a smooth long wavelength and symmetric gravity high of 85 mgal amplitude centering to the immediate east of the nicobar island where a prominent gravity high follows the nicobar deep the slab residual gravity anomaly srga and mantle bouguer anomaly mba maps prepared for the andaman plate margin bring out a double peaked srga low in the range of 150 to 240 mgal and a wider cum larger mba low having the amplitude of 280 to 315 mgal demarcating the andaman arc trench system the gravity models provide evidences for structural control in propagating the rupture within the lithosphere the plate margin configuration below the andaman arc is sliced by the west andaman fault waf as well as by a set of sympathetic faults of various proportions often cutting across the fore arc sediment package some of these fore arc thrust faults clearly give rise to considerably high post seismic activity but the seismic incidence along the waf further east is comparatively much less particularly in the north although the lack of depth resolution for many of the events prohibits tracing the downward continuity of these faults tectonic correlation of the gravity derived models presented here tends to favour the presence of oceanic crust below the andaman nicobar outer arc ridge

downdip tensional earthquakes beneath the tonga arc a double seismic zone characteristics of seismicity at intermediate depth beneath the tonga arc are studied special attention is paid to newly identified downdip tensional events because the dominance of downdip compression has been known in this area we reexamine the relatively shallow 70 km downdip tensional events reported in the harvard centroid moment tensor solutions by modeling bodywave waveforms of world wide standard seismograph network seismograms and conclude that they are in fact intraplate downdip tensional events furthermore these downdip tensional events are consistently located below and seaward of downdip compressional events and appear to constitute a double seismic zone the result of a relative relocation technique applied to these earthquakes also supports this observation which was originally based on routinely determined epicenters the subducting slab beneath the tonga arc has been considered as a prototype of a compressionally loaded slab the existence of a double seismic zone in tonga therefore would suggest that double seismic zones located at intermediate depth in subducting slabs are more common than previously thought

the rate of water penetration in experimentally deformed quartzite implications for hydrolytic weakening abstract in order to determine the rate at which water penetrates the grains of a quartzite we performed several types of experiments including water added deformation and hydrostatic experiments on vacuum dried samples and hydrostatic annealing experiments on previously deformed quartzites samples of heavitree quartzite with an average grain size of 200 m and intragranular water concentration of 1750 420 h 10 6 si as measured by ftir were vacuum dried at 800 c 6 pa for 12 h after vacuum drying the grains showed thermal cracks with an average spacing of 50 m and an intragranular water concentration of 240 60 h 10 6 si one constraint on the rate of water penetration into quartz was obtained by deforming vacuum dried samples with 0 3 wt 20 000 h 10 6 si water added at 800 c 1500 mpa and a strain rate of 10 6 s conditions at which as is quartzite undergoes climb accommodated dislocation creep the time between reaching experimental conditions and yielding of the sample was between 2 and 9 h the samples had yield stresses of 100 250 mpa values well below the yield stress of 1500 mpa for a vacuum dried sample deformation of the grains was homogeneous on both optical and tem scales indicating that the grains had fully equilibrated with the water related defect that is responsible for water weakening ftir measurements are consistent with this interpretation one of the deformed samples has an intragranular water concentration of 820 160 h 10 6 si and vacuum dried samples hydrostatically annealed for 3 or 10 h at the same conditions as the deformed samples have an average intragranular water concentration of 730 180 h 10 6 si to obtain a second constraint on the rate of water penetration hydrothermal annealing experiments were performed on portions of a sample of heavitree quartzite which were previously deformed without water added at 700 c 1200 mpa and 10 5 s and have a high 10 16 m 2 and homogeneous dislocation density two samples were annealed at 700 c 1500 mpa for 24 h one with 0 2 wt water and one with no water added the grains in the water added sample have a rim approximately 15 m wide with dislocation recovery microstructures indicating equilibration with the water related defect the sample with no water added has a homogeneous dislocation density throughout the grains which is similar to that of the original deformed sample the 15 m wide recovered rim suggests a penetration rate approximately an order of magnitude faster than the rate of oxygen diffusion under hydrothermal conditions the observed rate of penetration of the water related species is consistent with theoretical considerations and calculations which predict a diffusion rate for h 2 o that is an order of magnitude faster than the rate of hydrothermal oxygen diffusion we infer from our experiments that the rate of water penetration into quartz is consistent with diffusion of h 2 o

cathodoluminescence imaging and titanium thermometry in metamorphic quartz cathodoluminescence cl of quartz from metamorphic rocks representing a range of conditions from the garnet grade to the migmatite grade reveals a variety of textures that is a function of metamorphic grade and deformation history ti concentrations determined by electron microprobe and ion microprobe generally correlate with cl intensity blue wavelengths and application of the ti in quartz thermometer titaniq reflects the temperature of quartz growth or recrystallization and in some settings modification by diffusion quartz from garnet grade samples is not visibly zoned records temperatures of 425 475 c and is interpreted to have recrystallized during fabric formation quartz grains from staurolite grade samples are zoned in cl with markedly darker cores and brighter rims some of which are interpreted to have been produced by the dominant staurolite producing reaction whereas others are interpreted as having formed by diffusion of ti into quartz rims quartz from the matrix of kyanite and sillimanite grade samples are generally unzoned although locally displays slightly brighter rims higher ti quartz inclusions within garnet and staurolite have distinctly brighter rims which are interpreted as having been produced by diffusive exchange with the host mineral quartz from migmatite grade samples displays highly variable cl intensity which is dependent on the location of the grain matrix grains in melanosomes are largely unzoned or rarely zoned with darker cores leucosome quartz is strongly zoned with bright cores and dark rims and is interpreted as having formed during crystallization of the melt locally within the leucosome is observed oscillatory zoned quartz which is interpreted as a subsolidus recrystallization to achieve strain relaxation quartz inclusions within garnet or plagioclase crystals often show bright domains separated by zones of dark cl these enigmatic textures possibly reflect local melting fluxed by fluid inclusions temperatures calculated from the ti in quartz thermometer are a function of the metamorphic grade of the sample the textural setting of the quartz the reaction history and the deformation history of the rock the titaniq temperatures can be used to constrain the conditions at which various metamorphic processes have occurred

high density crustal intrusive bodies beneath shillong plateau and indo burmese range of northeast india revealed by gravity modeling and earthquake data abstract modeling and analysis of gravity and earthquake data of a critically dynamic northeastern region of india assumes a special significance for understanding the seismicity and geodynamics in this paper the relation between lateral heterogeneity of gravity interfaces with subsurface structures and seismic characteristics of northeast india are studied in detail to understand the seismicity and dynamics of the region using bouguer gravity earthquake and magnetotelluric data the modern scaled power spectral analysis of gravity data has brought out crustal lateral heterogeneity at a depth of 8 33 km 19 73 km and 37 14 km that correlate well with 10 km upper crust 20 km lower crust and a moho at 28 42 km revealed by 1d seismic crustal velocity and tomography studies the inferred crustal depths support thinner crust over uplifted shillong plateau and gradual increase of depths over subduction interface of indo burmese range 2d gravity modeling constrained by seismic velocity model and 3d euler deconvolution technique along two specific e w and three n s regional gravity transects delineated three significant high density anomalous heterogeneous intrusive bodies presumably due to underplating beneath shillong plateau mikir massif and indo burmese range based on the seismicity and stress pattern analysis through seismic b value variation we infer that the study region exhibits compressive stress concentrations in the form of brittle deformation at shallow depths and weakening of lower crust by high density intrusive mafic ultramafic bodies of ductile nature which may cause intense seismicity besides the ongoing collision of north drifting indian plate with eurasia to the north and the eastern subduction interface of indo burmese range we also infer that the ne to nw trending v shaped areas outlined north of shillong plateau south of mikir massif and ne sw trending thrust belt of schuppen exhibit aseismic nature and may be the future impounding seismic sources in this region

development of crystallographic preferred orientations by nucleation and growth of new grains in experimentally deformed quartz single crystals abstract deformation experiments have been carried out to investigate the effect of dynamic recrystallisation on crystallographic preferred orientation cpo development cylindrical samples of natural single crystals of quartz were axially deformed together with 1 vol of added water and 20 mg of mn2o3 powder in a griggs solid medium deformation apparatus in different crystallographic orientations with compression direction i parallel to ii at 45 to and 45 to and iii parallel to the experiments were performed at a temperature of 800 c a confining pressure of 1 2 gpa a strain rate of 10 6 s 1 to bulk finite strains of 14 36 the deformed samples were analysed in detail using optical microscopy electron backscatter diffraction ebsd and transmission electron microscopy tem two different microstructural domains were distinguished in the deformed samples i domains with undulatory extinction and deformation lamellae and ii domains with new recrystallised grains within the domains of undulatory extinction crystal plastic deformation caused gradual rotations of the crystal lattice up to 30 away from the host orientation new recrystallised grains show a strong cpo with c axis maxima at 45 to the compression direction this is the case in all experiments irrespective of the initial crystallographic orientation the results show that c axes are not continuously rotated towards the new maxima the new grains thus developed through a mechanism different from subgrain rotation recrystallisation new grains have a subeuhedral shape and numerous microcavities voids fluid channels and fluid inclusions at their grain boundaries no host control is recorded in misorientation axes across their large angle grain boundaries new grains might have been created by nucleation from solution in the m scale voids and microfractures the cpo most likely developed due to preferred growth of the freshly precipitated grains with orientations suitable for intracrystalline deformation at the imposed experimental conditions

stress state seismicity and subduction geometries of the descending lithosphere below the hindukush and pamir abstract the paper presents an analysis of spatial distribution of 6600 earthquake events which occurred during the period 1964 to 1999 between latitude 34 to 40 n and longitude 68 to 76 e this large volume event is reported in the international seismological centre isc catalog in addition to this a total of 248 focal mechanism solutions are considered to derive a generalised predominant stress prevailing in the descending lithosphere below the hindukush and pamir regions the analysis of spatial distribution shows that the epicentres of the events at shallow level depth 170 km the epicentres below the hindukush are mainly concentrated in a triangular shaped zone and the mean points of concentration of the epicentres appear to be shifted towards southwest at increasing depth the distribution of epicentres at the intermediate and deeper layers of the pamir is observed to be diffused except a cluster of few events in each layer appears to be shifted towards south southeast at increasing depth the distribution of hypocentres changes its concentration from lesser to considerably higher at about 70 km depth and further takes a minimum at about 170 km depth below the hindukush and pamir the present study further involves in analysing the composite group effect of stresses associated with the descending lithosphere below the hindukush and pamir after deriving the best fit generalized predominant directions of stresses it shows that the intermediate depth seismic zone below the hindukush is acted upon by maximum compressive stresses p axes from two directions while the deeper depth zone from three directions and may convincingly be correlated with the changing shape of the respective seismic zones another interesting phenomenon observed here is the change in direction of maximum compressive stresses in clockwise fashion from intermediate to deep seismic zones below the hindukush at shallow depths below the pamir the maximum and minimum t axes compressive stresses are acting almost along nnw sse and ene wsw and are oriented horizontally t axes for few events at these depths show almost vertical orientation the observed down dip extension is predominantly parallel with the descending lithosphere below the hindukush the entire analysis along with the observed scattering of p and t axes of some events at intermediate depths might be indicating a slight contortion of the middle layer below the hindukush the spatial distribution of seismicity and the generalised stress pattern of both the regions infer the existence of two isolated subducting lithosphere it perhaps has created the eastward expulsion or lateral extrusion of tibet along the major strike slip faults like karakorum altyn tagh kunlun and red river finally the whole analysis confirms the existence of shield like continental rigid slab at depths greater than 170 km below the hindukush

seismicity and plate deformation below the andaman arc northeastern indian ocean abstract the seismic activity originating below the andaman arc sea region is generally discernible into fore and back arc seismic zones which are traceable for nearly 1500 km in a n s direction at the junctures between the indian burma and se asia plates the fore arc seismicity displays an east dipping 40 55 benioff zone upto about 200 km focal depths details of the benioff zone in correspondence to the observed gravity field are discussed in four n s sectors which suggest some significant variations in the configuration of the benioff zone the back arc seismicity affects only the top 40 45 km of the lithosphere below the andaman sea where the back arc spreading ridge splits the volcanic arc stress distribution and faulting due to earthquakes below the andaman west sunda arc are studied here using 68 focal mechanism solutions their most significant results are low angle thrust events occur along the upper edge of the descending indian plate downdip tensional events have steeply dipping 60 nodal planes and normal faulting takes place in most parts of the benioff zone along moderately dipping 30 45 planes downdip compressional events high angle reverse fault nodal plane dip 60 or reverse faulting along moderately dipping 30 45 nodal planes also occur below the andaman arc the compressive earthquakes dominate the shallower level of the subducting slab and the tensional stress observed locally in north part of the andaman sea may be an outcome of the weak coupling between the descending and overriding plates generally a more or less complete sequence of faulting i e thrusting below the trench normal faulting below the fore arc and strike slip motion along the inner edges of the fore arc characterize the andaman west sunda arc in the southern andaman region a rather oblique convergence between the indian ocean and the se asia plates is needed to explain the existence of a somewhat contorted benioff zone in which compressional stress dominates in deeper lithosphere oceanward the ninetyeast ridge also impinges on the subduction zone in this region left lateral shear motion along the east margin of the ninetyeast ridge is further inferred by the results of focal mechanism solutions

titaniq a titanium in quartz geothermometer titanium is one of many trace elements to substitute for silicon in the mineral quartz here we describe the temperature dependence of that substitution in the form of a new geothermometer to calibrate the titaniq thermometer we synthesized quartz in the presence of rutile and either aqueous fluid or hydrous silicate melt at temperatures ranging from 600 to 1 000 c at 1 0 gpa the ti contents of quartz in ppm by weight from 13 experiments increase exponentially with reciprocal t as described by text log left x text qtz text ti right 5 69 pm 0 02 frac 3765 pm 24 t k application of this thermometer is straightforward typically requiring analysis of only one phase quartz this can be accomplished either by epma for crystallization temperatures above 600 c or by sims for temperatures down to at least 400 resulting temperature estimates are very precise usually better than 5 c potentially allowing detailed characterization of thermal histories within individual quartz grains although calibrated for quartz crystallized in the presence of rutile the thermometer can also be applied to rutile absent systems if tio2 activity is constrained

deformation thermometry based on quartz c axis fabrics and recrystallization microstructures a review abstract for quartz rich tectonites two types of deformation thermometer are currently commonly employed 1 the quartz c axis fabric opening angle thermometer that provides an estimate of deformation temperatures when fabrics were locked in during dislocation creep and dynamic recrystallization 2 the quartz recrystallization thermometer that indicates a range of likely deformation temperatures based on observed microstructures and inferred mechanisms of dynamic recrystallization a critically important caveat in applying both thermometers is the assumption that deformation temperature is the primary controlling factor in recrystallization mechanisms and fabric development however fabric opening angles and recrystallization mechanisms are also sensitive to other variables such as strain rate and water weakening in this paper the development of these thermometers is reviewed and their potential sensitivities to competing factors such as temperature strain rate water weakening and in the case of opening angles 3d strain type are discussed examples of the application of these potential thermometers to naturally deformed quartz rich rocks are given and case studies of correlations between deformation temperatures estimated by these thermometers and temperatures of synkinematic metamorphism determined by petrology based thermobarometers are highlighted in the review attention is focused on problems associated with applying these thermometers to natural deformation and examples of such problems are discussed

titaniq recrystallized experimental confirmation of the original ti in quartz calibrations several studies have reported the p t dependencies of ti in quartz solubility and there is close agreement among three of the four experimental calibrations new experiments were conducted in the present study to identify potential experimental disequilibrium and to determine which ti in quartz solubility calibration is most accurate crystals of quartz rutile and zircon were grown from sio2 tio2 and zrsio4 saturated aqueous fluids in an initial synthesis experiment at 925 c and 10 kbar in a piston cylinder apparatus a range of quartz crystal sizes was produced in this experiment both large and small examples were analyzed by electron microprobe to determine whether ti concentrations are correlated with crystal size cathodoluminescence images and epma measurements show that intercrystalline and intracrystalline variations in ti concentrations are remarkably small regardless of crystal size the average ti in quartz concentration from the synthesis experiment is 392 1 ppmw ti which is within 95 confidence interval of data from the 10 kbar isobar of wark and watson contrib mineral petrol 152 743 754 2006 and thomas et al contrib mineral petrol 160 743 759 2010 as a cross check on the ti in quartz calibration we also measured the concentration of zr in rutile from the synthesis experiment the average zr in rutile concentration is 4337 32 ppmw zr which is also within the 95 confidence interval of the zr in rutile solubility calibration of ferry and watson contrib mineral petrol 154 429 437 2007 the p t dependencies of ti solubility in quartz and zr solubility in rutile were applied as a thermobarometer to the experimental sample the average ti in quartz isopleth calculated from the calibration of thomas et al contrib mineral petrol 160 743 759 2010 and the average zr in rutile isopleth calculated from the calibration of tomkins et al j metamorph geol 25 703 713 2007 cross at 9 5 kbar and 920 c which is in excellent agreement with the p t conditions of the synthesis experiment separates of the high ti quartz from the initial synthesis experiment described above were used as starting material in subsequent experiments at 20 kbar at which pressure the solubility of ti in quartz is expected to be significantly lower in the recrystallized quartz these recrystallization experiments were conducted under wet and dry conditions at 925 c and under wet conditions at 850 c both wet and dry recrystallization experiments produced polycrystalline quartzites rutile occurs as inclusions in quartz and as individual crystals dispersed along quartz grain boundaries quartz that grew during the recrystallization experiments has dark cathodoluminescence indicating substantially lower ti concentrations the average ti concentrations in quartz from the recrystallization experiments are within the 95 confidence interval of a linear fit to the 20 kbar data of thomas et al contrib mineral petrol 160 743 759 2010 collectively the results from the synthesis and recrystallization experiments confirm that the ti in quartz concentrations used to calibrate the p t dependencies of ti in quartz solubility in thomas et al s contrib mineral petrol 160 743 759 2010 calibration represent the equilibrium concentrations of ti in quartz

comprehensive probabilistic seismic hazard analysis of the andaman nicobar regions an updated catalog of earthquakes has been prepared for the andaman nicobar and adjoining regions the catalog was homogenized to a unified magnitude scale and declustering of the catalog was performed to remove aftershocks and foreshocks eleven regional source zones were identified in the study area to account for local variability in seismicity characteristics the seismicity parameters were estimated for each of these source zones and the seismic hazard evaluation of the andaman nicobar region has been performed using different source models and attenuation relations probabilistic seismic hazard analysis has been performed with currently available data and their best possible scientific interpretation using an appropriate instrument such as the logic tree to explicitly account for epistemic uncertainty by considering alternative models source models maximum magnitude and attenuation relationships the hazard maps for different periods have been produced for horizontal ground motion on the bedrock level

aftershocks and seismotectonic implications of the 13 september 2002 earthquake mw 6 5 in the andaman sea basin a medium size earthquake mainshock m w 6 5 occurred on 13 september 2002 22 h 28 m 31 s utc in the andaman sea about 20 km offshore diglipur north andaman island the epicenter at 13 087 n and 93 112 e and centroid depth at 31 km were estimated by the u s geological survey usgs the main shock was a reverse faulting event four digital seismograph stations were established in the andaman islands to monitor aftershocks about 200 aftershocks m d 1 0 were recorded during a period from 19 september to 7 october 2002 the epicenter map shows a northwest southeast trending aftershock cluster in an area of about 140 70 km2 which reflects the rupture area of the mainshock the aftershocks occurred mostly at a depth range of 5 20 km except one at 38 km the moment tensor solutions usgs of the mainshock and the largest aftershock m w 5 8 which occurred 22 hr after the mainshock revealed consistent reverse faulting the northeast dipping northwest southeast trending nodal plane which is comparable with the aftershock trend is inferred to be the fault plane the northeast southwest compressional stress p axis obtained by the fault plane solutions is compatible with the north northeast movement of the indian plate the mainshock and the two well located largest aftershocks located by the usgs occurred within the subducted plate one composite fault plane solution is obtained for the shallower aftershocks recorded by the temporary network the solution shows normal faulting with a northeast southwest tensional t axis these aftershocks occurred off the subducted plate at a much shallower depth 10 km and are due to local tension in the overriding plate

earthquake source mechanisms from body waveform inversion and intraplate tectonics in the northern indian ocean double couple point source parameters for 11 of the largest intraplate earthquakes in the northern indian ocean during the last 20 y were determined from a formal inversion of long period p and sh waveforms nine of the events have centroid depths at least 17 km below the seafloor well into the upper mantle two have centroid depths as great as 39 km using the source mechanisms of these earthquakes we distinguish two major intraplate tectonic provinces in the northern indian ocean to the west of the ninetyeast ridge in the southern bay of bengal intraplate earthquakes have thrust faulting mechanisms with p axes oriented n s the centroid depths of these earthquakes range from 27 to 39 km below the seafloor lithospheric shortening in this region is thus accomplished by thrust faulting in the strong core of the oceanic upper mantle while other geophysical evidence suggests that shallow sedimentary and crustal layers apparently deform predominantly by folding in the immediate vicinity of the ninetyeast ridge earthquakes display strike slip mechanisms with left lateral motion on planes parallel to the ridge this type of faulting occurs from at least 10 s to the northern end of the ninetyeast ridge near 10 n where the ridge meets the sunda arc seismic activity diminishes to the east of the ninetyeast ridge but is also characterized by strike slip faulting despite these variations in deformational style the inferred orientation of greatest compressive stress in the northern indian ocean displays a consistent long wavelength pattern over a large portion of the indian plate varying smoothly from nearly n s in the bay of bengal to nw se in the northeastern indian ocean this plate wide stress pattern and the high level of intraplate seismicity in the northern indian ocean are likely the results of substantial resistance along the himalayan continental collision zone to the continued northward motion of the western portion of the indian plate oceanic intraplate earthquakes in other regions where the level of deviatoric stress associated with the long wavelength part of the stress field is likely to be smaller need not be comparably reliable indicators of the plate wide stress field

tectonic collision processes after plate rupture abstract rupture of a continental plate subducted below a forearc produces a fold and thrust mountain belt with fast overthrusting of nappes post rupture plate unflexing leading to reflexure provides a mechanism for foreland basin formation application to the australia banda arc collision accounts for the origin of the timor trough its imbrication and contemporaneous extension in outer arc as well as reversal of subduction direction after the emplacement of nappes

large scale thin skin tectonics the lithosphere is to a first approximation a layered system generally pressure temperature and composition are strongly dependent on depth and vary relatively little laterally these parameters can affect greatly the response to stress of rocks in the crust and upper mantle and are responsible for the stratification of many characteristics such as seismic response rheology and kinematics in the lithosphere asthemosphere system e g chen and molnar 1983 on a smaller scale lithology which often is also horizontally stratified plays a major role in determining strength e g engelder and geiser 1980 pore pressure can also greatly affect strength hubbert and rubey 1959 suppe 1981

very low solubility of rutile in h2o at high pressure and temperature and its implications for ti mobility in subduction zones the solubility of rutile in h2o has been measured at 1000 1100 c 1 2 gpa the data indicate that solubility is very low over the investigated range with a maximum of 4 7 millimol kg h2o at 1100 c 2 gpa the data were fit with the equation log m ti 4 892 10470 t 0 1923 p where m ti is ti molality t is in kelvins and p in gpa when compared to previous results the new data indicate substantially lower solubility opposite pressure dependence and thermodynamic properties of the reaction rutile tio2 aq that are now consistent with other oxide hydrolysis reactions calculations of ti transport during mantle metasomatism by h2o in subduction zone environments predict much lower ti mobility at all conditions these results offer strong support for models of ti retention in eclogites during slab devolatilization and require that examples of significant ti mass transfer be explained by complexing agents in solution most likely aluminosilicate complexes

seismicity and tectonics of the bay of bengal evidence for intraplate deformation of the northern indian plate abstract seismicity and fault plane solutions of several earthquakes in the bay of bengal have been studied to focus attention on the nature of intraplate deformation in the northern indian plate a widely dispersed diffuse seismicity provides evidence that the intraplate region of the entire bay of bengal and peninsular india of the indian plate are seismically active of which the gangetic delta and the eastern flank of the ninety east ridge show somewhat higher seismicity in comparison to the thickly populated epicenters at the eastern plate boundary predominance of thrust faulting earthquake mechanisms with consistent north dipping fault planes and more or less n s pressure axes imply that n s compression is more prevalent in the intraplate region of the bay of bengal which is analogous to the himalayan arc this intraplate seismicity and n s compressive deformation is presumed to be the result of collisional resistance to the north moving indian plate with the eurasian plate at the himalayas where subduction of the indian lithosphere in mantle has stopped recently as a result to compensate the horizontal compressive force by ridge push at the carlsberg ridge the intraplate region of the northern indian plate in continuation to the central indian ocean basin no longer remains passive due to a strong repulsive force at the collisonal plate boundary himalaya that causes stress transmission towards south and makes the intraplate region seismically active

the effects of quartz recrystallization and reaction on weak phase interconnection strain localization and evolution of microstructure abstract we conducted axial compression and general shear experiments at t 900 c and p 1 5 gpa on samples of banded iron formation bif and synthetic aggregates of quartz hematite and magnetite to investigate how dynamic recrystallization of quartz promotes strain localization and the role of weak second phases oxides on the rheology and microstructural evolution of the aggregates experiments showed strain localization into oxide rich layers and that the oxide content and oxide distribution are key factors for the strength of the aggregate only 2 10 wt hematite leads to pronounced weakening and increasing hematite content above 10 has only a minor additional effect where oxide grains are dispersed the initial strength contrast with quartz induces stress concentrations at their tips promoting high stress recrystallization accommodated dislocation creep of quartz fine recrystallized quartz reacts with oxide forming trails of fine reaction product ferrosilite fayalite leading to the interconnection percolation of a weaker matrix the strength contrast between the quartz framework and these fine grained trails promotes strain localization into micro shear zones inducing drastic strain weakening thus dynamic recrystallization of quartz promotes syn deformational reactions leading to a microstructurally controlled evolution of phase strength contrast it results in a rheologic transition from load bearing framework to a matrix controlled rheology with transition from s c to s c fabric with increasing strain

application of resistivity and gpr techniques for the characterization of the coastal litho stratigraphy and aquifer vulnerability due to seawater intrusion abstract the objective of this study is to identify geological properties and structural settings allowing salt water intrusion to the aquifers in oceanic originated rock formations of the south andaman island to evaluate the impact of seawater in groundwater aquifer several geophysical surveys using ground penetrating radar gpr and resistivity sounding were performed through the most significant sites located near coastal areas tidal channels and agriculture land high resolution gpr images were obtained and combined with resistivity sounding which allows to identify the mixing transition zone of fresh and saline groundwater and also successfully delineated the structural geometry and litho stratigraphic details of aquifers we show that the sedimentary formations are highly susceptible to seawater intrusion than the mantle originated ophiolites further potential impacts from seawater intrusion are higher along the west coast than the east coast because of macro pore tendency of soils allows seawater filtration

accurate differential stress measurement using the molten salt cell and solid salt assemblies in the griggs apparatus with applications to strength piezometers and rheology abstract calibrations have been developed for the molten salt cell and solid salt assemblies used in tri axial griggs type piston cylinder deformation apparatus griggs apparatus that enable accurate differential stress determinations in both assemblies at high pressures and high resolution differential stress measurements in the molten salt cell elastic response of the apparatus was characterized by loading alumina and wc rods and flow strengths were measured for high purity nickel and molybdenum and a ti zr mo alloy in replicate experiments in a griggs apparatus and a heard tri axial gas apparatus gas apparatus at identical pressures temperatures and strain rates the elastic response of the griggs apparatus is nonlinear at low total loads 1 760 mpa on a 5 mm diameter sample but becomes linear at loads greater than 15 kn plastic deformation experiments on water insensitive metals indicate that differential stresses measured in the molten salt cell msc and solid salt ssa assemblies in the griggs apparatus are greater than those measured in the gas apparatus at all conditions but vary systematically and reproducibly a simple linear calibration gas apparatus 0 73 griggsmsc 10 mpa where differential stress 1 3 can be applied to the differential stresses obtained using the msc in the griggs apparatus to obtain sample flow strength a similar linear calibration gas apparatus 0 73 griggsssa 48 mpa 30 mpa can be applied to the differential stresses obtained using the griggs apparatus with a solid salt assembly to obtain accurate sample flow strength these calibrations have been tested by applying them to mechanical data for carbonates deformed in the griggs apparatus and gas apparatus the calibrations are also applied to data from previous studies performed in the griggs apparatus on clinopyroxenites and quartzites and compared to results from similar experiments using the gas apparatus when the experimental starting materials and conditions are the same the recalculated flow strengths measured in the griggs apparatus experiments agree with those measured in the gas apparatus however discrepancies in differential stress are still observed in cases where starting materials differ or water available to silicate samples varies presumably through hydrogen diffusion through sample jackets and hydrogen fugacities of the surrounding confining media

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

syndeformational antigorite dehydration produces stable fault slip we conducted deformation experiments in which temperature was increased above the thermal stability of antigorite to directly test the dehydration embrittlement hypothesis and its potential link to intermediate depth earthquakes the experiments were designed to maximize the possibility for instability pore pressure was undrained pre dehydration stress was high and deformation was localized prior to initiation of reaction however stick slip instabilities or unstable fault behavior did not develop rather we find that weakening occurs stably at a rate that depends on the ratio of the temperature ramp rate to the strain rate resulting in laboratory slow slip or stress relaxation events

plate rupture by hydraulic fracture resulting in overthrusting lithospheric plates bending into subduction zones may become ruptured by hydraulic fracturing initiated by the injection of mobilized sediments thick poorly indurated water laden sediments of the proximal continental rise are the most obvious potential source for the large volume of material required for this process rupture of the descending plate may result in the subducted oceanic part of the lithosphere becoming detached and sinking independently while the more buoyant continental part rises by isostatic rebound we show here that applying this model to eastern indonesia provides an explanation for the banda arc geometry the apparent absence of a normal forearc and for the position of the thrust sheets relative to the benioff zone1 we also report that it can account for the short phase of deformation of the australian continental margin preceding its uplift of 3 km ref 1

tio2 activity in metamorphosed pelitic and basic rocks principles and applications to metamorphism in southeastern canadian cordillera the activity of tio2 can be precisely defined as a function of pressure temperature and activities of other components for common mineral assemblages in metapelites ilmenite quartz garnet plagioclase al2sio5 and in metabasites plagioclase sphene ilmenite quartzgarnet these mineral assemblages can be modelled by the equilibria 1 3ilmenite al2sio5 2quartz almandine 3tio2 2 anorthite 2sphene grossular 2tio2 quartz 3 3anorthite 3quartz 6ilmenite grossular 6tio2 2almandine these mineral assemblages can be used at a text tio text 2 1 rutile saturation and a given t to get maximum pressure limits of some metapelites and metabasites when electron microprobe analyses of mineral grains adjacent to ti bearing phases are made these data give maximum pressure estimates in reasonable agreement with other geobarometers the activity of tio2 in many metapelites is very near rutile saturation but for metabasites the activity of tio2 in some sillimanite zone rocks is as low as 0 6 the solubility of tio2 in biotite hornblende and garnet is a complex function of t p the activities of components in coexisting minerals and crystal chemical constraints in these minerals at a given p and t the solubility of tio2 in biotite and hornblende does not appear to be strongly dependent upon a text tio text 2 1 for sphene and ilmenite versus rutile bearing assemblages

seismotectonics of subduction and back arc rifting under the andaman sea abstract active underthrusting of the indian plate below the burma plate produces seismic activity under the andaman and burmese arcs it is suggested that significant differences exist in the configuration of the benioff zone underlying the two arcs several parallel and linearly extensive fault zones are outlined by epicentral distribution in the andaman arc the andaman back arc spreading ridge is defined by a relatively narrow shallow seismic zone that orients nne to sse in north and south parts of the andaman sea respectively the spreading ridge is offset by active transform faults on the far north the spreading ridge appears to connect to a major transform fault in burma across the burmese coast focal mechanism study provides evidence for tensional stress near the inner wall of the andaman trench dominant compressive deviatoric stress within the overriding burma plate and tensional stress under the back arc spreading ridge

rates of neogene and quaternary tectonic movements in the southern banda arc based on micropalaeontology it is generally considered that the outer banda arc islands are composed in part of strongly deformed sediments that accumulated at the north australian continental margin the onset and completion of folding nappe emplacement imbrication and uplift that produced the islands of the outer banda arc have been dated by means of planktonic foraminifera the dating of the onset of uplift of timor from a deep submarine position at the end of the neogene nappe emplacement to mountains now reaching nearly 3 km high indicates the rate of post collision uplift to have been initially 3 mm yr then it slowed to about 1 5mm yr where the australian continental margin meets the eastern end of the present java trench there are indications that the australian margin has overridden the trench in the timor region by 240 km micropalaeontological dating of the neogene collision between the australian continental slope proximal rise and the trench provides constraints for checking the overriding hypothesis by calculating rates of convergence derived from plate motions 7 5cm yr against the rates based on the tectonic model 7 4cm yr after nappe emplacement possibly faster than 6 25 cm yr shortening of the continental crust appears to have migrated towards the australian continent so that the shelf itself has become involved in the imbrication with shortening of the cover rocks between the nappes and the present shelf edge amounting to about 40 km during the last 2 ma

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

double seismic zones kinematics the kinematics of a double seismic zone of subducting lithosphere at an intermediate depth is studied a simple geometrical argument shows that the strain rate due to unbending of subducting lithosphere can be as large as 10 15 s 1 and large enough to account for the seismic strain rate in this depth range the geometrical unbending of subducting lithosphere in the mantle and the presence of a double seismic zone there can be therefore considered essentially equivalent phenomenon we model a double seismic zone required from this unbending in order to investigate what constraint the presence of a double seismic zone can place on the properties of the descending lithosphere the thickness of a double seismic zone is controlled by the thermal structure and the rheological properties of the subducting slab a preliminary result for the double seismic zone beneath tohoku japan indicates a possibility that the currently available flow law of dry olivine does not properly describe the double seismic zone or at least presents some inconsistency in our knowledge of the mechanical properties e g thermal structure rheology of oceanic lithospheres

subslab anisotropy in the andaman subduction zone controlled by slab dip abstract the mechanisms that govern anisotropy in subduction zones are debated in this study we present evidence of subslab mantle crustal and mantle wedge anisotropy in the andaman nicobar subduction zone using 52 teleseismic and 103 local shear wave splitting sws measurements the local sws measurements reveal a dominant trench parallel anisotropy in the crust that can be attributed to spo associated with arc parallel fault planes of normal earthquakes and strike slip faults parallel to the trench 3d flow from the subslab mantle possibly caused by a tear in the subducting slab is a plausible mechanism for anisotropy in the mantle wedge synthesis of the present results with the previous ones from the andaman sumatra java arc reveals a clear relation between the fast polarization azimuths in a ray coordinate system and slab dip the sub slab anisotropy is trench parallel in steeply subducting 40 in java to trench normal in gently subducting segments in the andaman and sumatra regions

back arc thrusting in the eastern sunda arc indonesia a consequence of arc continent collision the structure of the eastern sunda back arc region is dominated by two large north directed thrusts the wetar and flores thrusts and one or more minor thrusts which may represent early stages of subduction polarity reversal of the arc the relationships between thrusting and magmatic activity surface slopes cross arc faulting uplift forearc structure and collision by the australian continental margin provide constraints on the likely driving mechanisms of thrust formation driving mechanisms are considered in two broad categories one set of possible mechanisms originates within the upper plate and includes gravity sliding gravity spreading and magmatic intrusion the other set focuses on stress propagation between the upper and lower plates and for this arc system the major mechanism is that of continental collision while no one mechanism explains the range of observations we see in the back arc region all of these aspects play some role in the development of the thrusts we can eliminate upper plate mechanisms as primary driving forces because slope effects are similar in areas of both thrusting and nonthrusting in addition the wetar thrust shows a negative relation between thrusting and volcanism and side looking airborn radar images show no evidence of postvolcanic rifting that could accompany intrusion without volcanism collision between the arc and the australian continent provides the clearest driving mechanism but it does not explain the discontinuous nature or the particular location of the thrusts we suggest a sequence that appears to account for the data and to provide insight into the controversies often associated with other such thrust belts thrusting was initiated in those areas where the crust of the forearc region was thick a factor that facilitated stress propagation across the arc and where the crust of the back arc was thin which facilitated back arc thrusting thrusting concentrated at the base of the slope where the slope stress is maximum the early thrusting may have been helped also by thermal weakening of the crust due to volcanism as convergence proceeded volcanism waned then ceased in the eastern part of the arc the crust then strengthened and thrusting required higher stress to maintain and propagate this later stage should show higher seismic energy release which appears to be the case for the wetar thrust region displacement on the flores thrust appears to be matched by deflection of the volcanic arc we have estimated shortening in the back arc from interpretation of reflection profiles and for the flores thrust the value of approximately 30 km is consistent with that estimated for deflection of the arc compressional deformation continues west of the collision zone across the bali basin and into java it is reported in ne sumatra as well this deformation involves much less convergence than the eastern sunda thrusts and it may be related to magmatic processes to subduction of oceanic plateaus such as the roo rise or in the case of the bali basin to efficient lateral propagation of the flores thrust

grain growth kinetics of quartz and calcite aggregates grain growth rates for novaculite flint and jasper have been determined between 1000 and 800 c and 15 to 2 kb water pressure a pre treatment established an equilibrium microstructure and a nearly uniform mean grain diameter bar d 0 at 1000 c and 15 kb the grain growth data are linear on a log bar d bar d 0 vs log time plot and there is no significant difference between the three samples although the jasper contained 5 10 vol iron oxide at lower pressures and temperatures the growth curves are approximately parallel to the 1000 c and 15 kb data but growth is slower and that for jasper is slower than for the other two the growth rate is strongly dependent on the solubility of quartz in the intergranular fluid and the activation energy is about 11 kcal mole the data are used to estimate the growth rate of quartz at metamorphic conditions and to help evaluate the conditions where growth of dynamically recrystallized grains may be significant the growth rate of calcite grains s

preparation and deformation of synthetic aggregates of quartz synthetic quartz aggregates of low porosity have been fabricated from natural quartz powder impure silica gel and high purity silicic acid by hydrothermal isostatic pressing at 300 mpa the resulting specimens have water contents comparable to those of natural quartzites and have been used for deformation tests the flow stress at 1200 1300 k was found to vary in an inverse relationship to the water pressure estimated under the assumption that the measured water content homogeneously filled the measured porosity it is questionable that the natural quartz origin specimens were fully equilibrated in respect to the activity of the water but equilibration has probably been achieved in the amorphous silica origin specimens which crystallized under the experimental conditions in the deformation of the latter materials the experimental activation energy at water pressures approaching the confining pressure was found to be about 150 kj mol 1 for both however the impure gel origin material gave a stress exponent of about 2 3 whereas the high purity silicic acid origin material gave a stress exponent of about 4 in spite of the grain sizes being about 90 m and 20 m respectively it is concluded that while the impure specimens have higher intragranular strengths there is also a significant contribution of grain boundary processes to the strain in them which is absent in the high purity specimens

geoelectrical method in the investigation of groundwater resource and related issues in ophiolite and flysch formations of port blair andaman island india geoelectrical resistivity investigation using vertical electrical sounding ves technique was conducted at port blair south andaman island to locate the fractures in different formations and to decipher its groundwater potential a total of 40 ves were carried out covering the entire study area using schlumberger electrode configuration out of which 34 ves fall in andaman flysch formations and the remaining ves in ophiolite formations the interpreted resistivity results were integrated with nine borehole lithologs for the subsurface analysis the combination of ves with borehole litholog data has provided a close correspondence on subsurface hydrogeological conditions the interpreted ves data of various formations showed drastic variations in the resistivity ranging from higher in ophiolite moderate in andaman flysch and very low in valleys of andaman flysch formations the study further revealed that the weathered and fractured volcanics of ophiolite groups of rocks and sandstone that occur in the andaman flysch formations constitute the productive water bearing zones categorized as good groundwater potential zone based on the geoelectrical parameters viz thickness of layers and the layer resistivity values a groundwater potential map was prepared in which good moderate and poor groundwater zones were demarcated further numerical spatial and litho geoelectric models of resistivity were analyzed in terms of groundwater potential and these models have thus enabled to prepare a comprehensive groundwater development and management plans proving its efficacy in this art of exploratory investigations

ground motion relations for active regions in india in this article a study on development of ground motion prediction equations gmpes is undertaken for seismically active regions in india to derive the equations the seismically active regions are divided into four units based on seismotectonic setting and geology due to lack of strong motion data a stochastic finite fault simulation method is used for generating a complete synthetic database with respect to magnitude and distance the input parameters in the stochastic seismological model such as site amplification and stress drop are first derived from the past strong motion data a total of 236 three component records from 62 earthquakes with magnitudes ranging from m w 3 4 to 7 8 are used to calibrate the seismological model the obtained stress drops of these 62 events lie in between 60 and 165 bars with the help of a large synthetic database generated from the calibrated seismological model ground motion relations for 5 damped spectral acceleration are obtained by regression analysis the developed ground motion relations are compared with the existing gmpes of the other active regions in the world although the proposed equations have trends similar to those of the existing relations there are some differences attributed to stress drop and the quality factor of active regions in india these relations will be useful to prepare spectral acceleration hazard maps of india for a given annual probability of exceedance

earthquakes plate subduction and stress reversals in the eastern aleutian arc plate subduction beneath the 1500 km long segment of the eastern aleutian arc between kodiak and atka islands 154 w and 176 w longitude is studied with observations from teleseismic data the primary data base consists of hypocenters of earthquakes for the period 1965 1975 carefully selected from the bulletins of the international seismological centre and of 44 new focal mechanism solutions the principal results of this study are that hypocenters of intermediate depth earthquakes in the eastern aleutians appear to define a weakly developed double seismic zone at depths between 70 and 170 km additional evidence for a double seismic zone comes from focal mechanisms which generally show downdip directed p axes for earthquakes in the upper zone and downdip directed t axes in the lower zone major features of the double zone can be explained by thermoelastic stresses in the downgoing plate the observed predominant downdip stress polarity at intermediate depths in the descending plate reverses along strike of the arc this stress reversal coincides in map view with a change from a continental to an oceanic arc the coincidence may result from spatial differences either in the coupling between the plates at shallow depths or in the rheology of the surrounding oceanic versus continental mantle alternatively the stress reversal may be related to the time since the last great earthquake if the observed stress polarity relates to the elapsed time since the last great earthquake it may be a significant observation for long to intermediate term prediction of great thrust earthquakes in subduction zones portions of the eastern aleutian arc where downdip tension predominates contain one or more seismic gaps that appear to have a high probability for great earthquakes in the next few decades a moderate sized bathymetric feature in the pacific plate the amlia fracture zone modifies subduction and produces a bend or offset in the downgoing plate as well as an offset in the line of active volcanoes in the upper plate subduction of the fracture zone appears also to cause formation of summit basins on the arc platform and some aseismic behavior of a 100 km long segment of the main thrust zone

evolution of c axis pole figures and grain size during dynamic recrystallization results from experimentally sheared quartzite 1 quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation cpo with increasing shear strain up to 8 and degree of recrystallization up to 100 we used optical methods to determine the c axis cpo for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the cpos in five different orientation domains with increasing the c axis pole figure evolves from a broad peripheral maximum indicative of basal a slip to an inclined single girdle with two maxima indicative of rhomb a slip and finally an elongate single maximum at the girdle center indicative of prism a slip throughout this sequence the fabric skeleton rotates with the sense of shear the bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains grain boundary density analysis indicates that the size of prism a recrystallized grains is 1 5 times that of average and 2 times that of rhomb grains indicating that prism a slip is significantly easier at our experimental conditions this inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain similar progressive changes in c axis cpo are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates these results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

the recrystallized grain size piezometer for quartz d 1 0 3 56 0 27 s 1 26 0 13 with no change in slope at the regime 2 3 transition and no effect of temperature or a b stability field another experimental piezometer relation for regime 1 of hirth and tullis 1992 differs in slope suggesting that different recrystallization mechanisms require different piezometer calibrations index terms 3902 mineral physics creep and deformation 5120 physical properties of rocks plasticity diffusion and creep 8030 structural geology microstructures 8159 tectonophysics rheology crust and lithosphere 8164 tectonophysics stresses crust and lithosphere citation stipp m and j tullis the recrystallized grain size piezometer for quartz geophys res lett 30 21 2088 doi 10 1029 2003gl018444 2003

the sumba enigma is sumba a diapiric fore arc nappe in process of formation abstract the anomalous updomed morphological expression of sumba island its enigmatic lack of strong neogene deformation and the northward morphological indentation of southern sumbawa and flores require explanation the stratigraphy of sumba may be correlated with the cretaceous to miocene part of the timor allochthon the sedimentary and eruptive rock succession in sumba shows remarkable similarities with the allochthonous palelo wiluba and cablac deposits of timor in both islands the cretaceous parts of these sequences are regarded as characteristic of fore arc deposits built on thin continental crust the timor nappe is interpreted as a 5 km thick tectonic flake of the banda fore arc thrust onto the australian continental margin in the mid pliocene collision the postulated sumba nappe has not yet been thrust onto the australian margin which in the sumba region has not yet converged as close to the arc as in the timor area the postulated sumba nappe is interpreted as a diapiric elongated dome of the sunda fore arc that is being squeezed by the converging margin of australia against the volcanic islands of sumbawa and flores the absence of indications on the seismic reflection profiles for the presence of the thrust fault of the sumba nappe may perhaps be explained by the thrusts being nearly horizontal within flat lying strata the savu thrust is correlated with the probably older pre late pliocene wetar suture as a major southward dipping lithospheric rupture east of 124 e this suture does not seem to have moved much since the mid pliocene collision that emplaced the nappes on timor however microearthquake data suggest some activity is continuing

compositional changes of minerals associated with dynamic recrystallizatin the rate of compositional and isotopic exchange between minerals may be enhanced significantly if the rock is deformed simultaneously the enhanced exchange rate may result from a reduction in grain size shorter distance for volume diffusion dissolution and growth of grains by diffusion creep pressure solution or the movement of high angle grain boundaries through strained grains during recrystallization in the dislocation creep regime the migration of high angle grain boundaries provides high diffusivity paths for the rapid exchange of components during recrystallization the operation of the latter process has been demonstrated by deforming aggregates consisting of two plagioclases an1 and an79 at 900 c 1 gpa confining pressure and a strain rate of 2x10 6s 1 the polygonal recrystallized grains were analyzed using an analytical transmission electron microscope and have a variable but often intermediate composition at the conditions of these experiments the volume interdiffusion rate of nasi caal is too slow to produce any observable chemical change and microstructural chemical relations indicate that the contribution from diffusion creep was insignificant except for initially fine grained 2 10 m aggregates these results indicate that strain induced recrystallization can be an effective mechanism for enhancing the kinetics of metamorphic reactions and for resetting the isotope systematics of minerals such as feldspars pyroxenes and amphiboles

seismic history and seismotectonics of the sunda arc historic records of the last 300 years reveal two great interplate earthquakes 1833 mw 8 1861 mw 8 8 which ruptured major segments of the sumatra fore arc in western indonesia a significant percentage of interplate slip along this portion of the plate boundary appears to occur seismically the ends of these rupture zones are coincident with clusters of large ms 7 and moderate 6 ms 7 shocks and with heterogeneities in the plate interface as inferred from geologic and geophysical data the northern extent of the 1833 rupture zone is coincident with a group of moderate and large earthquakes a structural arch in the fore arc and a fracture zone on the subducted plate the southern portion is coincident with a very prominent cluster and the along strike extension of a sediment filled trough on the subducted plate prior to this study sumatra was characterized as relatively aseismic as inferred from the lack of great earthquakes in the instrumental record of this century java and the lesser sunda islands had major ms 6 earthquakes in the historic record but none are of the same magnitude as the great events near sumatra there is no clear evidence that shocks in the java fore arc were interplate events yet geophysical data indicate recent uplift of the trench slope break and a change in strike of the trench axis where the northernmost flank of the roo rise a prominent bathymetrie high appears to be interacting with the fore arc this feature was formed at a ridge crest and is more buoyant and thus more difficult to subduct than the surrounding seafloor there is no increase in seismicity associated with this tectonism however that may indicate a lack of contact between the crystalline portions of the plates at this time we infer that a majority of slip on the plate interface near java occurs aseismically the variation in occurrence of great interplate earthquakes along the sunda arc may be interpreted in terms of a model that attributes variation in interplate coupling to the age of the subducted lithosphere great interplate earthquakes occur near sumatra where the age of the youngest crust is 46 ma the characteristics of the sunda arc and analogies with pacific arcs imply that the entire length of sumatra has the potential to produce great thrust earthquakes its seismic potential should be considered high as there has been no recurrence of the great events of 1833 and 1861 in contrast no such events have been reported off java and the lesser sunda islands where the oldest crust is 152 ma the plate interface near java and the lesser sunda islands should be considered to have low seismic potential

geodynamic evolution of the outer arc forearc belt in the andaman islands the central part of the burma java subduction complex the andaman islands the central part of burma java subduction complex expose tectonostratigraphic units of an accretionary prism in an outer arc setting and turbidites of a forearc setting a number of n s trending dismembered ophiolite slices of cretaceous age occurring at different structural levels with eocene trench slope sediments were uplifted and emplaced by a series of e dipping thrusts subsequently n s normal and e w strike slip faults resulted in the development of a forearc basin with deposition of oligocene and mio pliocene sediments metapelites and metabasics of greenschist to amphibolite grade occur in a melange zone of ophiolites the eocene mithakhari group represents pelagic trench sediments and coarser clastics derived from ophiolites evidence of frequent facies changes predominance of mass flow deposits syn sedimentary basinal disturbance and wide palaeogeographic variation indicate deposition of eocene sediments in isolated basins of an immature trench slope setting deposition of the oligocene andaman flysch group in a forearc setting is indicated by the large scale persistence of beds lack of small scale lithological variation bimodal provenance less deformation a well defined submarine fan sequence and development predominantly on the eastern part of the outer arc the mio pliocene archipelago group includes alternations of siliciclastic turbidites and subaqueous pyroclastic flow deposits in the lower part and carbonate turbidites in the upper part suggesting its deposition in the shallower forearc compared to the siliciclastic oligocene sediments

aftershock investigation in the andaman nicobar islands of india and its seismotectonic implications six three component short period digital seismograph four reftek and two kinemetrics stations were established in different parts of the andaman nicobar islands following the 26 december 2004 devastating sumatra andaman mainshock m w 9 3 here we analyze about 18 000 aftershocks m 3 0 recorded from 6 january to 16 march 2005 to better understand the seismotectonics of the region a sudden burst of aftershock activity with irregular trend in the month of january 2005 was observed at almost all seismograph stations the estimate of p value 0 9532 from aftershocks m 4 5 is near to normal value of 1 0 which suggests a slow decay sequence of aftershock with complex and nonuniform stress change in a fault system creep effects and history dependent stress changes the frequency magnitude relation of the aftershocks followed the power law with average b value of 0 7723 and it varies from 0 49 to 1 03 indicating the compressive stress state of the region and its heterogeneous structure with possible variations in frictional conditions along the fault the distribution of the located aftershocks by a multistation method shows a north south trending aftershock cluster in an area of about 800 300 km 2 which reflects an approximate rupture dimension of the mainshock beneath the andaman nicobar islands most of aftershocks occurred in a depth range of 5 65 km the determination of composite fault plane solutions of the best located aftershock clusters at three different depth ranges 0 15 16 30 and 31 km in the ten blocks of the region suggests that the mainshock rupture propagated through normal reverse and strike slip earthquakes the areas in the vicinity of barren and narcondum volcanic zones show a dominantly normal fault mechanism due to predominant tensional forces suggesting the facilitation of brittle failure in the weakened crust by the process of underheating the zone near baratang mud volcano is associated predominantly with thrust fault at depth of 0 30 km suggesting a high compressive force beneath the baratang that ejected a huge amount of mud and slurry materials to the surface after the 26 december 2004 sumatra andaman earthquake m w 9 3 here we propose that the relocation of our large aftershock dataset using converted sp depth phase technique for precise depth control is needed and its sharing with global earthquake data can render more information on the subduction dynamics and geodynamical processes of the region

what drives centuries long polygenetic scoria cone activity at barren island volcano abstract barren island in the andaman sea is an active mafic stratovolcano which had explosive and effusive eruptions followed by caldera formation in prehistoric time poorly dated a scoria cone within the caldera marking volcanic resurgence was active periodically from 1787 to 1832 the historic eruptions since 1991 the same scoria cone has produced six eruptions commonly including lava flows links between barren island s eruptions and giant earthquakes such as the 26 december 2004 great sumatra megathrust earthquake have been suggested though there is no general correlation between them the 227 year long activity of the scoria cone named here shanku cone is normally driven by purely magmatic processes i present a source to surface model for barren island and shanku including the source region deeper and shallow magma chambers volcanotectonics dyking from magma chambers and eruptions and eruptive style as controlled by crustal stresses composition and volatile content calculations show that dykes 0 5 m thick and a few hundred meters long originating from shallow level magma chambers 5 km deep are suitable feeders of the shanku eruptions shanku a polygenetic scoria cone at least 13 eruptions since 1787 has three excellent analogues namely anak krakatau 40 eruptions since 1927 cerro negro 23 eruptions since 1850 and yasur persistent activity for the past hundreds of years this is an important category of volcanoes gradational between small monogenetic scoria cones and larger polygenetic volcanoes

source area and rupture parameters of the 31 december 1881 mw 7 9 car nicobar earthquake estimated from tsunamis recorded in the bay of bengal 1 on the morning of 31 december 1881 a submarine earthquake beneath the andaman islands generated a tsunami with a maximum crest height of 0 8 m that was recorded by eight tide gauges surrounding the bay of bengal since the earthquake occurred 8 years before the construction of the world s first teleseismic recording seismometer little has been known about its rupture parameters or location waveform and amplitude modeling of the tsunami indicate that it was generated by a mw 7 9 0 1 rupture on the india andaman plate boundary resulting in 10 60 cm of uplift of the island of car nicobar the rupture consisted of two segments the northern 40 km long segment is separated from the southern 150 km long segment by a 100 km region corresponding to the westward projection of the west andaman spreading center the main rupture occurred between 8 5 n and 10 n with a total area of 150 km 60 km dipping 20 e with a mean slip of 2 7 m the recurrence time for 1881 type events is estimated to be 114 200 years on the basis of inferred gps convergence rates and inferred plate closure vectors although slip partitioning in the region may extend this estimate by as much as 30

regional conductance map of andaman and nicobar region abstract to image the electrical conductivity distribution fluxgate magnetometers are operated at five sites in andaman and nicobar region transfer functions are estimated for the period range 8 128 min from nighttime transient geomagnetic variations using robust regression analysis the observed induction arrows in andaman islands are found to point towards east despite deep sea located towards its west this indicates that fore arc basin andaman nicobar deep is more conducting than the region of outer non volcanic island arc thin sheet model requires the conductance of 10 000 35 000 s with increase conductivity towards the south for explaining the observed induction pattern the observed induction pattern at andaman nicobar stations can be explained in terms of high conducting cretaceous tertiary sediments filling the andaman nicobar deep high conductivity over invisible bank has been attributed to the partial melts volatile fluids derived from the subducting indian plate that are intruding into the eastern margin of fore arc basin through the west andaman fault waf the induction pattern at great nicobar station campbell bay may be related to the highly conducting sediments filling the mergui basin along with mafic intrusions also crustal transition occurs below the mergui terrace at the malayan coast contributing to the enhanced conductivity anomaly

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

experimental constraints on the role of dynamic recrystallization on resetting the ti in quartz thermobarometer we conducted deformation experiments on ti doped quartz aggregates to investigate the effect of crystal plastic deformation and dynamic recrystallization on ti substitution in quartz shear experiments were conducted at 1 0 gpa and 900 c at a constant shear strain rate 5 10 6 s 1 for progressively longer intervals of time 24 48 and 72 h equivalent experiments were conducted under hydrostatic stress to compare the effect of deformation relative to static crystallization a novel quartz doping technique is used to synthesize a quartz aggregate consisting of two layers with ti concentrations above and below the predicted solubility level for the experimental conditions samples deformed to progressively higher shear strain by dislocation creep accommodated by a combination of subgrain rotation and grain boundary migration recrystallization show a strengthening of the crystallographic preferred orientation that correlates with a progressive evolution of ti concentrations quartz grains in the highest strain most strongly recrystallized samples have ti concentrations that are most similar to undeformed quartz grown in hydrostatic annealing experiments cathodoluminescence cl analysis of intragrain variations in ti content reveals core and rim zoning present in the starting material that becomes progressively homogenized with increasing strain recrystallized quartz in the highest strain samples exhibits a uniform cl signal we conclude that dynamic recrystallization enhances the kinetics of trace element equilibration in quartz illustrating that ti in quartz thermobarometry is capable of recording the conditions of ductile shearing