seismic and aseismic weakening effects in transtension field and microstructural observations on the mechanics and architecture of a large fault zone in se tibet

damped regional scale stress inversions methodology and examples for southern california and the coalinga aftershock sequence 1 we present a new focal mechanism stress inversion technique to produce regional scale models of stress orientation containing the minimum complexity necessary to fit the data current practice is to divide a region into small subareas and to independently fit a stress tensor to the focal mechanisms of each subarea this procedure may lead to apparent spatial variability that is actually an artifact of overfitting noisy data or nonuniquely fitting data that does not completely constrain the stress tensor to remove these artifacts while retaining any stress variations that are strongly required by the data we devise a damped inversion method to simultaneously invert for stress in all subareas while minimizing the difference in stress between adjacent subareas this method is conceptually similar to other geophysical inverse techniques that incorporate damping such as seismic tomography in checkerboard tests the damped inversion removes the stress rotation artifacts exhibited by an undamped inversion while resolving sharper true stress rotations than a simple smoothed model or a moving window inversion we show an example of a spatially damped stress field for southern california the methodology can also be used to study temporal stress changes and an example for the coalinga california aftershock sequence is shown we recommend use of the damped inversion technique for any study examining spatial or temporal variations in the stress field

comments on mafic ultramafic sills new insights from m and s shaped mineral and whole rock compositional profiles by v egorova and r latypov j petrology 54 2155 2191 2013

the typical large scale superposed folds in the central south china implications for mesozoic intracontinental deformation of the south china block abstract the south china block has experienced polyphase intensive intracontinental deformation in the mesozoic time large scale superposed folds in central south china document the mesozoic tectonic events within the south china block here we present new structural data related to synfolding deformation coupling with chronological data to reconstruct the deformation sequence and tectonic regime during crustal shortening these data indicate that the superposed folds dominated by a two stage tectonic compressive regime experienced two phases of superposed buckle folding leading to the orthogonal superposition of ne sw trending folds onto wnw ese trending folds between the late middle jurassic and the earliest early cretaceous our structural analysis together with geochronological data for this area suggests that the south china block predominately underwent two phases of intracontinental deformation during the mesozoic the early phase of tectonism d1 is characterized by a late middle triassic to earliest early jurassic ne sw compression causing the occurrence of an orogeny perpendicular shortening accompanying with evident magmatism this tectonic event was most likely associated with progressive clockwise collision of the south china block toward the north in the indosinian event subsequent tectonic activity d2 between the late middle jurassic and the earliest early cretaceous contributed to a phase of nw se contraction that overprinted the early ne sw shortening in the interior of the south china block generating a large scale nw convex fold belt and the typical large scale superposed folds within the central south china block the later tectonism was probably driven by the nw directed subduction of the paleo pacific plate beneath the eastern part of the asian continent

inversion of field data in fault tectonics to obtain the regional stress i single phase fault populations a new method of computing the stress tensor summary we attempt a general definition of the inverse problem of computing the components of the regional stress tensor from a set of field data including the measurements of the strike and dip of several faults and the directions and senses of relative motion along these faults as indicated by slickenslides in previous treatments of this problem no experimental errors could be taken into account except those in measuring the pitch of slickenslides thus errors in the orientation of the fault strike and dip which have considerable practical importance were neglected in our work all experimental errors in the field measurements are taken into account so that the agreement between the computed stress tensor and the set of field measurements can be rigorously checked

the estimation of fault slip from map data the separation pitch diagram although an essential feature of faults is that they produce a relative movement of two blocks the total or net slip is known only for a very small percentage of faults this note proposes a diagram for the graphical determination of net slip from the strike separation of markers recorded on geological maps with or without knowledge of the net slip direction the strike separation of one marker allows the drawing of a solutions line on the diagram depicting the range of slip vectors compatible with the measured separation the net slip vector can be obtained by either the intersection of two or more solutions lines or from a single solutions line if the direction of slip is independently known the diagram gives a visual appreciation of the effects of data uncertainty on the estimate of net slip and is a useful device for the representation of the kinematic characteristics of a collection of faults the use of the diagram is illustrated with examples from the south wales coalfield and the moine thrust zone

tectonic evolution of the northern oman mountains part of the strait of hormuz syntaxis new structural and paleothermal analyses and u pb dating of synkinematic calcite

arc deformation and marginal basin opening japan sea as a case study we discuss the opening mechanism of the japan sea in miocene time using 1 tectonic and published paleomagnetic data along the eastern margin from the north of hokkaido island to sado island 2 a mechanical model which is tested by small scale physical modeling and 3 crustal structure and bathymetric features in the japan sea which constrain our kinematic model and preopening reconstructions our main conclusions are the following the eastern margin of the japan sea was as a whole a dextral shear zone about 100 km wide this conclusion is supported by the existence of a ductile dextral shear zone in central hokkaido hidaka mountains and associated brittle deformation in western hokkaido and northeastern honshu the stress field during the opening which ended about 12 ma ago at the end of the middle miocene changes from right lateral transpression in the north to right lateral transtension in the south the western margin along the korean peninsula during the same period also was an active dextral shear zone paleomagnetic results indicate that clockwise rotations occurred in the south during the opening and counterclockwise rotations in the north we propose a model of right lateral pull apart deformation with clockwise rotations of rigid blocks in the southern transtensional domain and counterclockwise rotations in the transpressional one small scale physical models show that the clockwise rotation in transtension is possible provided that the eastern boundary pacific side is free of stress the opening stopped and compression subsequently began about 12 ma ago finally we show that the dextral shear which is distributed over the whole japan sea area is accommodated by n s trending right lateral faults and rotation of blocks located between these right lateral faults

role of faults and layer interfaces on the spatial variation of stress regimes in basins inferences from numerical modelling abstract the spatial variation of the computed state of stress is studied in a series of numerically solved boundary value problems of basin scale deformation these numerical experiments illustrate simple situations of decollement tectonism under compression the study focuses on the local variation of the state of stress in a sedimentary overburden where major discontinuities such as large scale faults and layer interfaces are present the states of stress are computed using either the distinct element method or the finite element method classic elasto plasticity laws are used for the behaviour of the continuum and the discontinuities are modelled as coulomb contact surfaces a geologic interpretation of the numerical experiments is proposed to highlight the role played by major structural discontinuities in the local variations of the stress regime the models are two dimensional in space and assume plane strain but the full three dimensional stress strain constitutive relation is used therefore the stress regime variations can be represented by constructing the contour map of the wallace bott stress ratio this simple technique together with the construction of diagrams of principal stress magnitude across the model enables us to rapidly examine the stress field pattern and further discuss the deformation modes of faulting and fracturing which may take place at a small scale we have found that the local variations of the stress tensor are in good agreement with common geologic interpretation of brittle deformations in foreland and thrust systems we demonstrate how the presence of layer interface and the difference in mechanical properties of formation lithology may strongly affect the variation of stress in space this modelling approach has important issues when interpreting well tests to gain access to in situ stresses

an inverse problem in microtectonics for the determination of stress tensors from fault striation analysis abstract a method is proposed to compute some characteristics of a possible stress tensor related to striations measured in a given faulted area if several tectonic phases are responsible for the striations the method separates and determines the successive stress tensors the iterative algorithm consists of alternatively a sorting the data attributed to a tectonic phase and b computing the characteristics of the stress tensor of this phase the method is tested using an application to synthetic and actual data

a transtensional basin model for the organya basin central southern pyrenees based on magnetic fabric and brittle structures summary the purpose of this paper is to explain the kinematics of the inverted extensional organy basin centralsouthernpyrenees duringitsextensionalevolution thestudyoftheanisotropy of magnetic susceptibility ams and the study of brittle mesostructures are the instruments used to decipher the extensional kinematics separation of magnetic subfabrics analyses of ams at low temperature and anisotropy of the anhysteretic remanence aarm has been also done to properly interpret the obtained magnetic fabric data the combination of all magnetic fabric and brittle mesostructures analyses has provided new information to clarify thestructuralevolutionofthebasin whichsupportanewtectonicmodelfortheaptian albian basinal stage 20 sites for the magnetic fabric analysis were sampled in aptian lower albian marls the ams ellipsoids obtained at room temperature show their kmin axes perpendicular to bedding and kmax axes oriented nw se in 70 per cent of the samples n s in 15 per cent and ne sw in 15 per cent analyses of ams at low temperature show a better clustering of the axes of the magnetic ellipsoid pointing out that phyllosilicates constitute the carriers of the ams whereas the analyses of the aarm confirm the scattering of the ferromagnetic grains these results suggest that the obtained ams in the organy a basin was acquired during or shortly after deposition of the aptian lower albian marls without any subsequent overprint the direction of the magnetic lineation obtained from the ams at room and low temperatures agrees with the orientation of the stress axes obtained from the analysis of brittle mesostructures mainly faults and calcite filled tension gashes indicating its tectonic origin we interpret the orientation of the magnetic ellipsoids related to the transtensional movements linked to the formation of the organy a basin during the aptian albian interval which are consistent with the eastward displacement and rotation of iberia with respect to europe analysis oftensiongashesalsoindicatesamainnw seextensiondirection whereaspalaeostressanalysis of striated faults shows a more n s trending 3 the transtensional model proposed from magnetic fabric and brittle mesostructures for the organy a basin during the aptian albian is consistent with the creation of a strongly subsiding area south of the axial zone during the cretaceous and implies that rotation and separation of iberia respect to europe took place simultaneously

changes in paleostress state along a subduction zone preserved in an on land accretionary complex the yokonami melange in the cretaceous shimanto belt kochi southwest japan a change in paleostress along a subduction zone plate interface in the shallow portion of a seismogenic zone was detected in an on land accretionary complex the yokonami melange in southwest japan using the microfault inversion method microfaults were classified into two groups based on location those occurring throughout the yokonami melange and those occurring in the goshikinohama fault zone which is considered to be a fossil seismogenic fault and is located at the northern end of the yokonami melange stresses obtained for these classified microfaults indicate that two different stress states exist for each deformation feature one is subhorizontal 1 and subvertical 3 with a smaller stress ratio and the other is subvertical 1 and subhorizontal 3 with a larger stress ratio the difference between these stress states could be related to the horizontal stress change by stress drop after large earthquakes that were recently observed after the tohoku oki great earthquake

tertiary tectonic evolution of the external east carpathians romania abstract paleostress calculation and analysis of mesoscopic structures are integrated with depth interpreted geological profiles based on seismic studies and well correlation to derive a tertiary tectonic model for the east carpathians following early miocene and older orogenic phases the first tectonic event that affected the studied area is characterised by a wsw ene oriented shortening of middle miocene late burdigalian in age resulting deformations induced ene ward thrusting of tarcau and marginal units as well as the internal part of the subcarpathian nappe a second shortening event with an e w to wsw ene contraction direction took place in the late miocene sarmatian characterised by further foreland thrusting of the subcarpathian nappe and out of sequence deformation in the tarcau and marginal folds nappes along strike differences in deformation mechanisms are controlled by the friction coefficients along the main detachment layers by the lateral variations in the wedge thickness and by the involvement in the northern part of the thrusting system of the thick competent east european platform tear faulting occurred in both tectonic events the main resulting structure being the triangle zone developed south of the trotus valley the latest miocene latest sarmatian early pliocene is characterised by a strike slip stress field with nne ssw compression and wnw ese tension axis left lateral faults being dominant the last deformation which affected the studied area is characterised by nnw sse shortening during the pliocene major deformations taking place mainly in the sw most bending zone

forty years of paleostress analysis has it attained maturity abstract powerful methods of paleostress analysis from fault slip data have been used since the 1970 s and have demonstrated their capacity for reconstructing stress fields with tectonic significance however in some cases these methods have been applied to samples that do not fulfill basic requirements or developed on the basis of unsound conceptual models most methods of fault slip are based on the wallace bott principle dynamic paradigm while others invoke the necessity of remaining within the alternative kinematic paradigm therefore inferring only finite strain or strain rate axes the scientific community shares neither standard procedures for applying these methods nor standard parameters for assessing the quality of results this indicates that paleostress analysis has not attained maturity it is necessary to see back to the conceptual and methological bases following accurate procedures of data collection using additional field observations and critically assessing the results within the tectonic setting spatial and temporal heterogeneity of stress fields over all scales makes the mean stress tensor obtained from site analysis just an abstract notion though indispensable for understanding lithosphere dynamics

segmentation and along strike asymmetry of the passive margin in socotra eastern gulf of aden are they controlled by detachment faults on the island of socotra the southern passive margin of the gulf of aden displays along its strike two different types of asymmetric structures western socotra is made up of a series of southward tilted blocks bounded by consistently northward dipping normal faults eastern socotra consists of a broad asymmetric anticline with a steep northern limb and a gently dipping southern limb a zone of ne sw striking strike slip and normal faults separates the two areas the overall structure is interpreted as representing two rift segments separated by a transfer zone the along strike juxtaposition of crustal scale asymmetric structures on the southern margin of the gulf of aden is complemented by the asymmetry of the conjugate margins on either side of the gulf whereas the western socotra margin is narrow and characterized by oceanward dipping normal faults the conjugate oman margin is broader and dominated by horsts and graben considering that asymmetric structures in the upper crust are often associated with synthetic shear zones at deeper ductile levels we propose that the western and eastern socotra margin segments were controlled at depth by two detachment faults with opposite dips and senses of shear the normal faults of western socotra would sole out into a top to the north ductile shear zone whereas the asymmetric anticline of eastern socotra would be associated with a top to the south detachment fault

the tectonic development of the hellenic arc and the sea of crete a synthesis abstract we use field analyses together with interpretation of aerial photographs orbital imagery and sea beam data to define more accurately the geometry of fault systems in the hellenic arc the tectonics of the area are dominated by extensional processes related to normal faulting we then use sea beam data previous oceanographic studies and extrapolation of field analyses on surrounding islands to describe the fault pattern of the cretan sea basin which is found consistent with the land fault pattern the mechanisms of the late neogene and quaternary deformation of the central and southern aegean region are analysed in terms of stress trajectories as revealed by field studies of fault populations we then examine the subsidence of the sea of crete in terms of a model of uniform stretching of the lithosphere the opening of the sea of crete was cylindrical and did not result in bending of crete this leads us to modify the southern portion of an earlier kinematic reconstruction of aegea in agreement with recent paleomagnetic results the implications of the opening of the sea of crete on the tectonic evolution of the straits to the west and east of crete are discussed finally the uplift of the outer arc is explained by underplating due to subduction

complex fault patterns transtension and structural segmentation of the lofoten ridge norwegian margin using digital mapping to link onshore and offshore geology an integrated onshore offshore study involving regional to outcrop scale fault analysis is used to develop a self consistent structural model for transtension along the lofoten ridge the lofoten vesteralen archipelago lva is a segmented basement high showing distinct lateral variations in trend deformational style and structural complexity this study investigates whether segmentation can be linked to differences in the obliquity of preexisting structures relative to plate movement vectors regional analysis of fault lineament patterns using geographic information systems gis reveals that the lva can be subdivided into a series of distinct lineament domains these domains are closely coincident with changes in ridge trend and variations in structure within offshore models derived from seismic reflection studies digital field mapping and spatial analysis of faulting in the north lofoten reveal that multimodal faulting is dominated by transtensional dip slip and oblique slip movements which are comparable to analogue models where the ridge axis is 30 oblique to regional extension the overall change in fault orientation fault geometry and deformation style are consistent with models for transtension where the ridge bounding structure becomes increasingly oblique to regional extension previously identified transfer zones simply reflect segment domain boundaries and are not reactivating basement structures this model is a possible analogue for other orthogonal and oblique rift structures on the norwegian and other margins

progressive strain localization in a major strike slip fault exhumed from midseismogenic depths structural observations from the salzach ennstal mariazell puchberg fault system austria 1 analysis of a strike slip fault exhumed from midseismogenic depths reveals that the fault experienced progressive strain localization toward a high strain fault core we focus on the ennstal segment of the 400 km long salzach ennstal mariazell puchberg semp strike slip fault system in the eastern alps which accommodated 60 km of left lateral displacement during oligo miocene time macroscopic and microscopic observations reveal a zoned fault featuring a high strain core at least 10 m wide within a fault zone that is at least 150 m wide grain size distribution analysis shows how the ennstal segment of the semp evolved our data reveal a 10 m wide high strain fault core characterized by a power law relationship of grain sizes d2 2 0 bordered by a 54 m wide transition zone where the largest and smallest grains are characterized by two power law relationships d2 2 0 and 1 6 respectively this zone is in turn bordered by a region with grain sizes that show a single low strain power law relationship of d2 1 6 we interpret these relationships to be the result of concentrated shear overprinting an initial low strain power law grain size distribution before strain localized to the core this is consistent with the theory that faults mature by smoothing geometrical complexities forming a highly localized high strain fault core the data do not support the idea that damage forms primarily in response to dynamic stresses during seismic rupture although they do suggest that this mechanism may operate within tens of meters of the fault once it has developed its zoned structure

an example of stress tensor distribution deduced from the aftershocks of the november 23 1980 southern italy earthquake abstract the total inversion algorithm tarantola and valette 1982 is applied to a set of focal mechanisms corresponding to the aftershocks of the southern italy november 23 1980 earthquake m s 6 9 and the elements of several stress tensors are computed the obtained complex distribution of stress for this region is analyzed and two zones around the main event corresponding to areas of tension and compression distributed at different depths are delimited a stress tensor was sought for a third region corresponding to events which occurred along the strike of the main event and with magnitudes greater than 2 2 but with ambiguous results

polyphased block tectonics along the north anatolian fault in the tosya basin area turkey abstract the tosya basin is located in the bending segment of the north anatolian fault naf in turkey we have obtained original observations on the neotectonics from sar ers images digital elevation model dem and field structural analysis regional neogene deformation is characterised by the occurrence of several basins that are superimposed in time and space they result from differently oriented movements since 12 ma including southwestward motion along a fault subparallel to the naf we propose a model of polyphased tectonics related to the displacement of several individualised blocks in the first stage tortonian the north tosya block has moved toward the n250 azimuth parallel to the dextral n70 striking segment of the naf as a consequence a triple junction related compatibility basin was opened at the intersection with a n60 to n30 striking fault this pattern is similar to the karliova corner where the naf and the east anatolian fault meet in the second stage early pliocene middle pleistocene a segment of the former n70 naf was abandoned and the naf propagated eastward to form a n90 striking segment n90 naf cutting the former tosya block and basin into two parts the north tosya block has moved again and this new geometry has permitted a south tosya block to move parallel to the naf but with a higher rate which has induced compression in the tosya basin in the third stage holocene the south tosya block moved toward n240 obliquely to any of the naf segments this has resulted in the formation of two holocene pull apart type basins along the previous n60 to n30 striking fault while extensional faults were formed in the south tosya block estimated dextral displacement along the naf is 5 9 to 8 5 km at this stage this model of blocks moving in different directions including holocene local movements toward n240 means that the naf can be considered not to be a simple transform fault our model implies that the n90 naf was non existent before the early pliocene

neogene strike slip faulting in sakhalin and the japan sea opening we describe structural data from a 2000 km n s dextral strike slip zone extending from northern sakhalin to the southeast corner of the japan sea satellite images field data and focal mechanisms of earthquakes in sakhalin are included in the interpretation since miocene time the deformation in sakhalin has been taken up by n s dextral strike slip faults with a reverse component and associated en echelon folds narrow en echelon neogene basins were formed along strike sup faults and were later folded in a second stage of deformation we propose a model of basin formation along extension al faults delimitating dominos between two major strike slip faults and subsequent counterclockwise rotation of the dominos in a dextral transpressional regime basins becoming progressively oblique to the direction of maximum horizontal compression and undergoing shortening the association of both dextral and compressional focal mechanisms of earthquakes indicates that the same transpressional regime still prevails today in sakhalin we present fault set measurements undertaken in noto peninsula and yatsuo basin at the southern end of the sakhalin east japan sea strike slip zone early and middle miocene formations recorded the same transtensional regime as observed along the west coast of ne honshu during the early and middle miocene the strike slip regime was transpressional to the north in sakhalin and hokkaido and transtensional to the south along the west coast of ne honshu as far as noto peninsula and yatsuo basin dextral motion accommodated the opening of the japan sea as a pull apart basin with the tsushima fault to the west the opening of the japan sea ceased at the end of the middle miocene when transtension started to change to e w compression in the japan arc subduction of the japan sea lithosphere under the japan arc started 1 8 ma ago the evolution of the stress regime from transtensional to compressional in the southern part of the strike slip zone is related to the inception of the subduction of the young philippine sea plate lithosphere under the japan arc during the late miocene subduction related extension is a necessary condition for the opening of the japan sea two possible mechanisms can account for dextral shear in this area 1 counterclockwise rotation of crustal blocks due to the collision of india with asia 2 extrusion of the okhotsk sea block squeezed between the north america and eurasia plates

slip vectors and fault mechanics in the makran accretionary wedge southwest pakistan the makran is a broadly east west trending arcuate accretionary wedge currently forming where the oceanic floor of the gulf of oman is being subducted north northeastward beneath the south asian margin two traverses across the makran coast ranges extending about 80 km inland from the coastline which lies about 100 km north of the wedge front show that the structure of this region is dominated by large back rotated south directed thrust faults and associated folds related structures include duplexes fault gouge containing reidel shears and melange along fault zones these structures are cut by later north directed backthrusts out of sequence south directed thrusts and apparently conjugate sets of nne trending sinistral and nw trending dextral strike slip faults slip vectors were determined from slickenlines fiber lineations gouge fabrics and riedel shears vectors from early thrusts and bedding parallel slip surfaces corrected for tectonic rotation form a broad south directed concentration lying between the normal to the regional strike varying from 160 to 180 and the present plate convergence vector 200 locally the concentration is bimodal this pattern may represent the interaction between plate forces and body forces in the accretionary wedge the late thrusts have widely variable slip vectors and stair step off the early thrusts they overlap in orientation and slip vector with gently dipping sinistral strike slip faults suggesting that they also stair step in plan off the latter the steep strike slip faults represent the final tectonic stage involving minor n s shortening and longitudinal extension these three stages probably occurred in sequence in any one area but migrated diachronously southward as the wedge grew

calcite twinning and determination of paleostress orientations three methods compared the classic turner 1953 method of determining 1 and 3 from an analysis of calcite twins is compared with a graphical method modified after dietrich and song 1984 and a newly devised method which treats calcite twin glide directions and twin planes in an analogous way to slickensides on fault planes all three methods provide similar positions for 1 but none gives a reliable position for 3 if a c axes preferred orientation exists 3 can most successfully be identified as the axis of a girdle of turner compression axes computer assisted adaptations of the turner method are recommended as the most rapid and internally consistent means of determining 1 and 3 the manual graphical method provides a useful check on turner determinations of 1 and a means of cleaning data

estimating geomechanical parameters from microseismic plane focal mechanisms recorded during multistage hydraulic fracturing abstractwe extend a full waveform modeling method to invert source focal plane mechanisms for microseismic data recorded with dual borehole seismic arrays combining inverted focal plane mechanisms with geomechanics knowledge we map the pore pressure distribution in the reservoir determining focal mechanisms for microseismic events is challenging due to poor geometry coverage we use the p wave polarities the p and s wave similarities the sv p amplitude ratio and the sh p amplitude ratio to invert the focal plane mechanisms a synthetic study proves that this method can effectively resolve focal mechanisms with dual array geometry we apply this method to 47 relatively large events recorded during a hydraulic fracturing operation in the barnett shale the focal mechanisms are used to invert for the orientation and relative magnitudes of the principal stress axes the orientation of the planes slipping in shear and the approximate pore pressure perturbation that caused the slip the analysis of the

analysis of normal fault populations in the kumano forearc basin nankai trough japan 2 principal axes of stress and strain from inversion of fault orientations 1 we use a high resolution 3 d seismic survey to map a population of recent normal faults within the kumano basin of the nankai subduction zone in order to quantify patterns of strain and stress state over the last 0 44 myr we identify distinct fault populations that define three phases of extension phases 1 and 2 comprise nw se striking faults located along the western basin edge and in the northwestern portion of the study area respectively the ne sw striking faults of phase 3 comprise the largest population and extend 20 km landward from the basin s seaward edge phase 2 faults typically terminate within a few reflectors of the seafloor whereas most phase 3 faults form seafloor scarps inversion of the fault populations documents ne sw extension during phases 1 and 2 and nw se extension during phase 3 consistent with both core scale structures and horizontal stress orientations observed at integrated ocean drilling program iodp boreholes slip on phase 3 faults accommodates strain of up to 1 2 concentrated near the basin s seaward edge inversion for a best fit stress tensor yields a subvertical 1 and subhorizontal 2 and 3 for all faulting phases we find that during phase 3 in most portions of the basin 2 3 shmax shmin reflecting widely varying fault strikes this contrasts with distinct shmax and shmin magnitudes inferred from iodp borehole data these observations may be reconciled if the orientation of maximum horizontal stress fluctuates due to variation of subduction parallel compression through the seismic cycle

tectonic paleostress fields and structural evolution of the nw caucasus fold and thrust belt from late cretaceous to quaternary abstract the nw caucasus fold and thrust belt essentially corresponds to the inverted western flysch zone of the great caucasus mountains a deep basin that developed from late jurassic to eocene times between the scythian plate to the north and the transcaucasian terranes to the south the shatsky ridge sw of the nw caucasus zone the flysch basin was strongly affected by compression in late eocene times when the characteristic wnw trending folds and thrusts of the nw caucasus belt developed some authors regard the main compressive deformation as miocene in age by means of remote sensing analysis we elucidate the geometry of major structures in the belt wnw trending south vergent thrusts and folds and major vertical and transverse nne ssw to ne sw deep fault zones the later structures are interpreted as ancient faults that were active during the development of the flysch basin paleostress investigations reveal seven main tectonic episodes in the evolution of the nw caucasus since late cretaceous combining structural interpretation remote sensing analysis and paleostress field reconstruction we propose a model for the structural evolution of the belt during the late cretaceous paleocene the western caucasus zone was under transtensional regime with an e w to ne sw trending 3 that generated oblique normal movements along nne ssw transverse faults and wnw ese margins of the flysch basin this tectonism could correspond to rifting related to the formation of the eastern black sea basin at the paleocene eocene boundary a transpressional event with an e w to nw se trending 1 developed and the nne ssw to ne sw trending faults could have been inverted this event could correspond to an attenuation in the eastern black sea basin formation or to the incipient accretion of the transcaucasian terranes during the eocene another e w to nw se oblique extension transtensional event affected the flysch basin that could be related to a known rifting phase in the eastern black sea basin strong nne ssw to ne sw compression characterises late eocene tectonism the fold and thrust belt developed at this time as a result of the direct collision of the shatsky ridge with the scythian plate a ne sw extension followed the late eocene event related to basin development around the newly formed fold belt a wnw ese oblique contraction affected the belt during the early miocene as the result of arabian plate convergence with the caucasian system the latest inferred event is a compressional regime with nnw sse trending 1 that is affecting the nw caucasus belt from sarmatian times until the present under this oblique compression the belt has deformed as in a dextral shear zone and the thrust surfaces have acquired lenticular shapes this study highlights the occurrence of oblique movements in the nw caucasus area prior to and after the dominant late eocene compression from the late cretaceous until the eocene the structural development of the nw caucasus was closely related to the evolution of the eastern black sea basin from the late eocene until quaternary times it was rather related to the arabia eurasia plate convergence

tectonics of the jurassic early cretaceous magmatic arc of the north chilean coastal cordillera 22 26 s a story of crustal deformation along a convergent plate boundary the tectonic evolution of a continental magmatic arc that was active in the north chilean coastal cordillera in jurassic early cretaceous times is described in order to show the relationship between arc deformation and plate convergence during stage i circa 195 155 ma a variety of structures formed at deep to shallow crustal levels indicating sinistral arc parallel strike slip movements from deep crustal levels a sequence of structures is described starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions bolfin complex the high grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike slip and thrust kinematics with a nw se directed maximum horizontal shortening i e parallel to the probable late jurassic vector of plate convergence a kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds thrusts and high angle normal faults which formed during the same time interval as the discrete shear zones during stage ii 160 150 ma strong arc normal extension is revealed by brittle low angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels during stage iii 155 147 ma two reversals in the stress regime took place indicated by two generations of dikes an older one trending ne sw and a younger one trending nw se sinistral strike slip movements also prevailed during stage iv until 125 ma when the atacama fault zone originated as a sinistral trench linked strike slip fault the tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates the structures of stages i and iv suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations however decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages ii and iii

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

seismic evidence for a slab tear at the puerto rico trench 1 the fore arc region of the northeast caribbean plate north of puerto rico and the virgin islands has been the site of numerous seismic swarms since at least 1976 a 6 month deployment of five ocean bottom seismographs recorded two such tightly clustered swarms along with additional events joint analyses of the ocean bottom seismographs and land based seismic data reveal that the swarms are located at depths of 50 150 km focal mechanism solutions found by jointly fitting p wave first motion polarities and s p amplitude ratios indicate that the broadly distributed events outside the swarm generally have strike and dip slip mechanisms at depths of 50 100 km while events at depths of 100 150 km have oblique mechanisms a stress inversion reveals two distinct stress regimes the slab segment east of 65 w longitude is dominated by trench normal tensile stresses at shallower depths 50 100 km and by trench parallel tensile stresses at deeper depths 100 150 km whereas the slab segment west of 65 w longitude has tensile stresses that are consistently trench normal throughout the depth range at which events were observed 50 100 km the simple stress pattern in the western segment implies relatively straightforward subduction of an unimpeded slab while the stress pattern observed in the eastern segment shallow trench normal tension and deeper trench normal compression is consistent with flexure of the slab due to rollback these results support the hypothesis that the subducting north american plate is tearing at or near these swarms the 35 year record of seismic swarms at this location and the recent increase in seismicity suggest that the tear is still propagating

fault slip analysis and late exhumation of the tauern window eastern alps exhumation of the eastern alps from the early tertiary to the late miocene was localized mainly in the tauern window a thermal and structural dome located in front of the dolomites indenter stress inversions based on new structural investigations over the entire tauern window indicate a regional zoning of the paleostress field with a predominance of strike slip states of stress in the core of the investigated area and dominant extensional regimes in the eastern and western borders of the dome few inverse fault structures have been highlighted we propose a two stage deformation history in order to explain the different types of structures that characterise the ductile and the brittle domains during the first stage of exhumation of the tauern window corresponding to the folding event the brittle crust was probably dominated by n s shortening and compression the second stage of exhumation was marked by normal faulting at the borders of the dome and strike slip faulting in the core during the second stage the brittle part of the crust that was previously affected by compressive structures belonging to the first stage was eroded normal faulting associated to e w extension along the eastern and western borders of the tauern window was accommodated by strike slip faulting located in the core of the tauern window yielding e w extension and n s shortening the orientation of the extensional axes and the nature of the stress tensors are similar to the ones inferred for the late stage brittle overprint of the internal basement massifs of the central alps pointing to a possible common large scale state of stress acting well beyond the area of the eastern alps

analysis of submarine slumping in the gabon continental slope the gabon continental slope is selected as a case study for slope stability analysis because of evidence of previous slide activities different types of data were collected from the continental slope in the gulf of guinea off west africa during guiness and zaiango surveys the offshore investigation was carried out using swath bathymetry and associated imagery deep towed high resolution subbottom profiles side scan sonar images observation from remotely operated vehicle victor and kullenberg cores these data reveal different examples of seafloor instabilities commonly related to fluid escape features these slides occur on the continental slope at low declivities showing that slope gradient has a secondary role on the marine slope instability with respect to external triggering mechanisms such as fluid flow earthquake shallow gas and gas hydrates one case of mass slide with small downslope displacement was studied on the gabon slope in this work a pseudothree dimensional slope stability analysis sultan et al 2001 was undertaken three scenarios of instability were tested to identify the possible trigger mechanism of the observed slide instability 1 under static gravity loading 2 under earthquakes and 3 under upward fluid flow simulation results show that static stability of the area is satisfactory however the stability is very sensitive to fluid escape these results agree with sonar images showing seepage features aligned along the upslope limit of the observed slide

determination of stresses from heterogeneous focal mechanism data an adaptation of the multiple inverse method abstract a new method is proposed to separate stresses from earthquake focal mechanism data double couple component of focal mechanisms from spatially or temporarily varying state of stress the underlying multiple inverse method was originally devised for geological fault slip data but we have adapted its resampling scheme to focal mechanism data to cope with the nodal planes ambiguity problem the stresses compatible with a dataset are recognized as the clusters of reduced stress tensors the clusters are identified with the k means clustering the uncertainty of the stresses are evaluated from the spread of the clusters the technique is shown to successfully determine valid stresses from simulated data we have also applied the technique to natural data from southwestern japan

multiple strike slip fault sets a case study from the dead sea transform in many strike slip tectonic settings large rotations of crustal blocks about vertical axes have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which presumably accommodate the relative motion between the blocks as regional deformation progresses a mechanical model suggests that rotations greater than c 25 to 45 must be accommodated by more than one set of faults with the angle c between their directions consequently the sum of the angles between sets must be roughly equal to the total tectonic material rotation to test this model we investigated the fault geometry and field relationship of fault sets in the mt hermon area in northern israel where paleomagnetic declination data imply 69 13 counterclockwise ccw block rotation the statistical and field relationship analysis of over 315 faults shows that the faulting is predominantly right lateral strike slip consisting of three distinct sets the oldest set strikes 254 the second oldest set strikes 295 and the youngest strikes 331 this last direction is consistent also with the current north south direction of the maximum principal stress axis the angle c between the first and second sets is 41 and between the second and third sets 36 in good agreement with the c angle predicted from mechanical considerations the sum of the two angles is 77 ccw in good agreement with the 69 13 ccw paleomagnetically derived rotation the results suggest specifically that the sequential development of multiple intersecting fault sets is responsible for the faulting in the mt hermon area and generally that the model of block rotation with multiple faults provides very good simple rules for analyzing very complex fault patterns