effects of fault slip distribution on the geometry and kinematics of the southern junggar fold and thrust belt northern tian shan abstract at the foreland edge of fold and thrust belts ftbs tectonic shortening could either be accommodated by thick skinned faulting and or structural wedges developed near range fronts or be transferred along detachment layers further into basins forming thin skinned structures although the geometry and kinematics of each structure have been well investigated studies that characterize slip distribution along a specific fault as well as the effects of variations in magnitude of fault displacement on the development of fault related folds are still lacking in this study we present a case study of the southern junggar thrust sjt a seismically active fault developed in the southern junggar ftb northern tian shan with an overall vergence to the north the results of structural interpretations of seismic reflection profiles combined with the age constraints provided by previous magnetostratigraphic studies demonstrate that 1 the kinematics of the sjt vary along strike from purely structural wedging to classical fault bend folding besides these end member models intermediate cases exist between them in which fault slip is partitioned by two stage structural deformation i e fault bend folding subsequent to structural wedging at the range front as a result part of the displacement is transferred northward forming the tugulu anticline 2 along strike variations in displacement along the tugulu fault frontal fault of the sjt also affect the geometry and kinematics of the tugulu anticline varying from detachment folding trishear fault propagation folding with a pure shear fault bend folding back limb and classical trishear fault propagation folding to fault bend folding with increasing displacement 3 the sjt has an average quaternary slip rate of 3 9 0 4 mm yr serving as a principal structure accommodating tectonic shortening in the southern junggar ftb the results of this study are helpful for seismic hazard assessments of the study area which may also shed light on structural evolution of fault related folds in other ftbs

stress strain paradox plate coupling and forearc seismicity at the cascadia and nankai subduction zones abstract in the continental forearcs of the cascadia and nankai subduction zones geodetic strain measurements indicate crustal contraction in the direction of plate convergence nearly margin normal but earthquake focal mechanisms and other stress indicators show maximum compression along strike margin parallel it is evident that the geodetic strain signals reflect the temporal fluctuations of elastic stress associated with subduction earthquake cycles not the absolute stresses the absolute margin normal stress is not only much less than the margin parallel stress but also no greater than the lithostatic value indicating a very weak subduction thrust fault weak subduction faults are also consistent with geothermal data which require very low frictional heating along the faults great subduction earthquakes occur at very low shear stresses and cause small perturbations to the forearc stress regime because these small elastic stress perturbations are relatively fast they give large strain rates that are detected by geodetic measurements the present nearly margin normal contraction in both places is due to the locking of the subduction fault and the consequent increase in elastic stress in the direction of plate convergence with the margin parallel compression being dominant the small increase in margin normal stress may change the forearc seismicity in cascadia from a mixture of thrust and strike slip types into mainly thrust before the next great subduction earthquake in nankai the change should be from strike slip to quiescence as observed in the nankai forearc prior to the 1944 1946 great subduction earthquakes

the makran earthquake of 1983 april 18 a possible analogue to the puget sound earthquake of 1965 summary on 1983 april 18 a large earthquake mb 6 5 m0 1 2x1019 nm occurred in the makran region near the pakistan iran border this earthquake is the only event of mb 6 2 in the entire makran region since the occurrence of the great makran earthquake m 8 0 8 3 in 1945 we determined the source parameters of the 1983 event by formally inverting its p and sh waves recorded by the world wide standard seismograph network wwssn and the canadian seismograph network csn at teleseismic distances the best fitting centroidal solution shows predominantly normal faulting at a depth of about 65 5 km these results provide an additional constraint on the configuration of the down going slab previously inferred from the occurrence of three small to moderate sized events located 100 300 km to the west of the event reported here we also discuss the possibility of drawing an analogy between the occurrence of the 1983 makran event and that of the puget sound earthquake of 1965 m0 1 4x1019nm mb 6 5 and between the seismotectonic settings of the two regions if the very large m 8 0 8 3 earthquake off the makran coast in 1945 is a low angle thrust due to slip along the plate interface its occurrence calls for the possibility of similar seismic risk in the puget sound region however in contrast to inferences made from geological observations in the field about a dozen p wave first motions read from historical seismograms and those reported by the international seismological summary suggest that the 1945 makran earthquake has a large component of normal faulting which can be attributed to the relatively old age of the subducted lithosphere

a tectonic boundary for the southern colorado plateau abstract a tectonic boundary should be defined by changes in tectonic elements tectonic elements would include such parameters as structural style stress orientations volcanism heat flow seismicity and changes in crustal thickness examination of these tectonic elements for the southern colorado plateau suggests that the southwestern part of the physiographic plateau appears to be tectonically part of the basin and range province i therefore propose a boundary at 112 w longitude the locus of seismicity and recent volcanism and also the site of crustal thickening and a change of structural style the tectonic boundary of the southern colorado plateau is suggested to form part of the eastern boundary of the southern basin and range seismotectonic region this seismotectonic region is bounded on the north by the las vegas shear zone nevada seismic zone and has a different stress orientation than the northern basin and range from which it began to evolve separately 13 m y ago the present principal stress orientation of the southern basin and range of arizona appears to be the result of compound movements of the san andreas transform and garlock las vegas shear zone resulting in a shear couple affecting the crust in arizona and an associated ne sw least principal stress orientation

analysis of the 4 february 1976 chino valley arizona earthquake abstract this m b 4 9 earthquake was located about 20 km north of prescott in the transition zone between the basin and range and colorado plateau provinces several lines of approach were used to determine the focal mechanism p wave first motions surface wave amplitude and phase spectra and synthetic seismograms the preferred focal mechanism displays northwest trending nodal planes with predominantly normal movement the inferred fault plane strikes 120 dips 40 sw with a slip of 100 and a depth of 10 to 15 km the seismic moment is 1 0 10 23 dyne cm the azimuth of least horizontal stress is inferred to be 37 lithostatic stress generated by variations in elevation and density between the crusts of the southern basin and range and colorado plateau may contribute to the inferred stress orientations uplift and resultant extension in this zone of thin lithosphere and relatively high heat flow may also be important microearthquake studies in the chino valley during the summers of 1978 and 1979 indicate a microseismicity rate of 0 3 events per day this is an order of magnitude less than that observed in the intermountain seismic belt

ductile and brittle deformations in the northern snake range nevada abstract the northern snake range east central nevada is one of the metamorphic core complexes of the sevier hinterland within the range a major decollement separates an upper plate composed of brittlely deformed paleozoic sedimentary rocks mostly carbonates from a lower plate composed of metamorphic upper precambrian lower cambrian rocks intruded by gneissic granites a study of the geometry and kinematics of structures and fabrics at outcrop scale and in thin sections indicates that the northern snake range decollement has been a zone of intense non coaxial e vergent shear and transport in a n115 e direction outstanding asymmetric boudinage within the marble sheet capping the lower plate testifies for late ductile shear strains of at least 20 the interface between brittlely and ductilely deformed rocks seems too sharp to represent a regional rheological transition but might result from two distinct phases of deformation ductile deformation in and below the decollement could have occurred before brittle deformation in the upper plate brittle faulting in the upper plate related to basin and range extension reactivated the upper surface of the ductile shear zone high topographic relief on the normal faults bounding the range triggered the slide of olistolites from the upper plate into the adjacent oligo miocene basins

contemporary tectonics of the wasatch front region utah from earthquake focal mechanisms we have completed a comprehensive study of focal mechanisms of digitally recorded earthquakes ml 4 4 that occurred in the wasatch front region in utah during 1980 to 1986 single event solutions for 24 events were determined using recently revised crustal models and a computerized grid search technique overall the mechanisms show predominantly normal faulting on n s striking nodal planes of moderate to steep dip 30 tension axis azimuths average 96 12 thus in general the mechanisms indicate e w to ese wnw crustal extension and vertical crustal shortening oblique slip when observed is characterized by left lateral motion on planes striking n to ne or right lateral motion on planes striking n to nw most of the mechanisms with significant amounts of oblique slip motion occur in the southern part of the study area where compression axis orientations range from near vertical to near horizontal thus the mechanisms suggest a possible change in stress regime from north to south along the wasatch front despite geologic evidence for low angle faults in the study area shallowly dipping nodal planes are relatively uncommon

thrust front zone of the precordillera argentina a thick skinned triangle zone the frontal thrust zone of the precordillera thrust belt of western argentina only superficially resembles a classical alberta style triangle zone in this thick skinned triangle zone the thrust front is the result of interaction between the eastward verging thin skinned central precordillera and the westward verging thick skinned eastern precordillera system the western boundary formed by the niquivil thrust plate has been refolded and faulted by thrust faults from the eastern precordillera at several localities the deformation is still active we divide the thick skinned triangle zone into three segments based on the system s activity independent late interfering and collision zones balanced cross sections across a thick skinned triangle zone show a uniform s ortening for the eastern precordillera 17 km the structural geometry suggests multiple reactivation of the main faults the thick skinned triangle zone began to grow after 2 6 ma and is still active episodic and alternating opposing movement is controlled by buttressing of the thin skinned thrust system against the basement structures of the eastern precordillera

late quaternary slip rates across the central tien shan kyrgyzstan central asia 1 slip rates across active faults and folds show that late quaternary faulting is distributed across the central tien shan not concentrated at its margins nearly every intermontane basin contains neogene and quaternary syntectonic strata deformed by holocene north south shortening on thrust or reverse faults in a region that spans two thirds of the north south width of the central tien shan slip rates on eight faults in five basins range from 0 1 to 3 mm yr fault slip rates are derived from faulted and folded river terraces and from trenches radiocarbon optically stimulated luminescence and thermoluminescence ages limit ages of terraces and aid in their regional correlation monte carlo simulations that sample from normally distributed and discrete probability distributions for each variable in the slip rate calculations generate most likely slip rate values and 95 confidence limits faults in basins appear to merge at relatively shallow depths with crustal scale ramps that underlie mountain ranges composed of pre cenozoic rocks the sum and overall pattern of late quaternary rates of shortening are similar to current rates of north south shortening measured using global positioning system geodesy this similarity suggests that deformation is concentrated along major fault zones near range basin margins such faults separated by rigid blocks accommodate most of the shortening in the upper crust index terms 8107 tectonophysics continental neotectonics 9320 information related to geographic region asia 8010 structural geology fractures and faults

miocene rise of the shillong plateau and the beginning of the end for the eastern himalaya abstract a common feature of convergent plate boundaries is the self organization of strain exhumation and topography along discrete arcuate boundaries deviations from this geometry can represent first order changes in stress applied at a plate boundary that must affect how strain is partitioned within the interior of an orogen the simplicity of the himalayan fold and thrust belt seen along its central portion breaks down along the eastern extremity of the arc where the 400 km long shillong plateau has developed this change in strain partitioning affects nearly 25 of the arc and has not previously been considered to be important to the orogen s development new low temperature thermochronometry data suggest this structure initiated in mid to late miocene time significantly earlier than was previously estimated from the sedimentary record alone development of the shillong plateau may be linked to a number of kinematic changes within the himalayan and burman collision zones that occur at the same time these events include the onset of e w extension in central tibet eastward expansion of high topography of the tibetan plateau onset of rotation of crustal fragments in southeastern tibet and re establishment of eastward subduction beneath the indo burman ranges we suggest that the coincidence of these tectonic events is related to the dismemberment of the eastern himalayan arc signifying a change in regional stress applied along the india eurasia burma plate boundaries discrepancies between vertical long term faulting rates and geodetically derived far field convergence rates suggest that the collisional boundary in the eastern himalayan system may be poorly coupled due to introduction of oceanic and transitional crust into the eastern plate boundary the introduction of dense material into the plate boundary late in the orogen s history may explain regional changes in the strain field that affect not only the himalaya but also the deformation field more than 1000 km into the tibetan plateau

distinct erosional progressions in the medusae fossae formation mars indicate contrasting environmental conditions abstract the form of erosional remnants of the medusae fossae formation mff on mars provide evidence of their development progression and implicate two spatially distinct environments in the equatorial regions of mars ubiquitous yardangs are clearly the product of strong unidirectional winds acting over time on variably indurated deposits yardang orientation is used as a proxy to map regional and local wind direction at meso scale resolution in other more limited areas not subjected to strong unidirectional winds randomly oriented kilometer scale mesas and buttes are found to be remnants of progressive cliff recession through mass wasting as support is lost from within the mff lithology at its margins the differing processes that dominate the formation of the distinctive landforms have implications for meso scale variations in climate that remain unresolved by current modeling efforts additionally the variability of erosional forms within the deposit emphasizes the overall complexity of this extensive formation

mechanical analysis of the geometry of forced folds a mechanical model of forced folding comprised of an anisotropic cover overlying displaced rigid basement blocks is used to investigate the influence of various parameters on theoretical fold form shape and dip of basement fault strength of basement cover contact and degree of anisotropy of the cover we show that the degree of anisotropy in the cover largely influences the geometry of the forelimb of the forced fold folds produced in isotropic cover display forelimbs that taper from large dip angles near the basement cover contact to low dip angles at the ground surface in contrast dips in the forelimbs of folds in anisotropic cover are nearly uniform with depth we show that the basement cover contact and the shape of the basement fault largely influence the geometry of the backlimb backlimb rotation occurs in cover welded to the basement and in cover underlying curved basement faults in addition the kinematic features of the theoretical folds are compared with the fold geometry generated by parallel kink and trishear models folds in isotropic cover overlying straight basement faults closely resemble the fold forms produced by the trishear kinematic model while fold forms in anisotropic cover more closely resemble folds produced by parallel kink geometric constructions

mining induced microseismicity monitoring and applications of imaging and source mechanism techniques the study of microseismicity in mines provides an ideal method for remote volumetric sampling of rock masses the nature and uniqueness of microseismic monitoring is outlined in the context of acquisition hardware and software requirements several topics are used to highlight the potential for novel applications of microseismicity and to outline areas where further study is required these topics reflect some of the current interest areas in seismology namelyb values and source parameters fault plane solutions modes of failure and moment tensor inversion imaging and seismicityvelocity correlations these studies suggest potential correlations between zones of high seismic velocity high microseismic activity and maximal stress drops which can be interpreted spatially to be the locations of highly stressed ground with a potential for rock bursting fault plane solutions are shown to be useful in determining the slip potential of various joint sets in a rock mass source parameter studies and moment tensor analysis clearly show the importance of non shear components of failure andb values for microseismicity appear to be magnitude limited and related to spatial rather than temporal variations in effective stress levels

microseismicity of the kaibab plateau northern arizona and its tectonic implications abstract a microearthquake investigation of the kaibab plateau in northern arizona revealed swarms of small to very small magnitude earthquakes a 12 station network of portable seismographs recorded 296 events with magnitudes from 0 5 m l t axes from these fault plane solutions yields an average least principal horizontal stress direction of ene wsw the inferred orientation of principal stresses found in this study indicates the kaibab plateau lies within a region of predominantly normal faulting which extends from central arizona northward into southwestern utah although data are sparse in the region surrounding the kaibab plateau evidence for the location of a basin range colorado plateau stress boundary through the kaibab plateau is not supported by the results of this study

thermal control on post orogenic extension in collision belts abstract intracontinental extension develops in two different ways in the rhine graben the baikal or the east african rifts it is localized in a long and narrow zone along a few normal faults which often delineate a single graben in contrast in the basin and range province or the tibetan plateau it is distributed over a much broader area up to more than 1000 km across this difference arises from differences in rheology and applied forces widespread extension probably reflects the thinning of a previously thickened crust geological observations indicate that only wide orogenic belts undergo late or post orogenic thinning a related fact is that anatectic granites seem to be most abundant in wide orogenic belts the general correlation between belt width and occurrence of both extension and anatectic granites suggest that it reflects in part the characteristics of horizontal heat transfer between thickened crust and surroundings in wide orogenic plateaux the large amount of heat produced in the crust by radioactive decay and lack of erosion leads to high temperatures in narrow mountain belts horizontal heat transfer and high erosion rates concur to prevent temperatures from rising to values at which the crust may yield in extension we have tested this hypothesis using transient two dimensional numerical models of orogenic belts focussing on the effects of the shortening rate the final width of the belt and the rate of crustal radiogenic heat production in the range of geological values the shortening rate has little influence on the results crustal temperatures increase as a function of belt width we find that the sensitivity of crustal temperatures to belt width is most pronounced in the 100 300 km width range and for large values of crustal heat generation the pressure temperature paths followed by geological formations depend on the belt width which is consistent with available data the integrated shear strength of the lithosphere decreases with time as the belt is being built and then eroded this evolution is sensitive to belt width if extension occurs when the integrated shear strength drops below a critical value our results yield a framework for comparing different orogenic belts we predict that extension starts late in the history of wide belts and never occurs in narrow belts a final implication of this study is that the average rate of crustal heat production is probably close to 1 1 10 6 w m 3 which is in the upper range of possible values we present a simple and convenient diagram summarizing the thermal characteristics of orogenic belts as a function of width and shortening rate

map view interference of monoclinal folds abstract outcrop observations and laboratory experiments show that many small chevron folds form by interference of monoclinal kink bands in multilayer buckling kink band interference has also been proposed for some map scale folds furthermore fault related folding provides additional mechanisms of monoclinal fold generation other than buckling and thus makes kink band interference all the more conceptually plausible as a significant large scale process in this paper we document several relatively simple examples of map scale monoclinal fold interference including three interfering monoclines of the colorado plateau a seismically imaged example in the perdido foldbelt of the gulf of mexico and a more complex example from seismic mapping in the santa barbara channel california kink band interference has normally been analyzed in cross section here we emphasize the map view phenomena and present a simple balanced three dimensional model of the interference geometry treating the monoclines as two independent kink bands which does not depend of the kink band folding mechanism this model predicts the shape of the jog produced by crossing monoclines and is used to help evaluate the role of interference in the map view geometry of our examples the documentation of these simple examples supports the concept that more complex monoclinal fold interference could be a significant phenomenon in the upper crust

geodetic measurement of deformation in the central mojave desert california data from triangulation and trilateration surveys made during 1934 1982 are used to calculate shear strain rates in the central mojave desert of california for the region between the helendale and camp rock faults the shear strain rate was determined to be 0 16 0 03 strain yr with maximum right lateral shear strain occurring on a plane oriented n41 w 5 if we assume that this deformation is due to right lateral motion across the northwest trending local faults the average shear straining corresponds to a relative displacement of 6 7 1 3 mm yr across this portion of the network accounting for 12 of the predicted 56 mm yr of relative motion between the north american and pacific plates from the camp rock fault eastward across the network there is a transition from significant to very low strain rates examination of nine focal mechanisms and their relation to the local geology and the strain data suggests that most of the long term displacement occurs on the major northwest trending faults oriented nearly along the direction of relative motion between the north american and pacific plates secondary faulting controlled by a coulomb anderson failure mechanism or by slip on preexisting faults can account for the occurrence of earthquakes on faults of other orientations

sequential limb rotation and kink band migration recorded by growth strata almazan basin north spain abstract the geometry of growth strata in the almazan monocline demonstrates that limb rotation and kink band migration can coexist as folding mechanisms the almazan basin showing an overall synclinal geometry was one of the most actively subsiding areas during the tertiary in the inner part of the iberian plate with more than 3500 m of preserved tertiary non marine sediments they are arranged in five tectonosedimentary units tsu a1 to a5 bounded by unconformities at the basin borders the southern basin border is defined by the wnw ese trending almazan monocline that can be followed for 25 km along strike with a maximum structural relief of about 2500 m the maximum changes in thickness on both limbs of the monocline occur in units a2 and a3 which constrains the maximum deformation period to between the middle upper eocene headonian top of a1 and upper oligocene lower miocene agenian base of a4 the detailed geometry of the mesozoic pre growth and the tertiary growth strata of the almazan monocline was determined from the analysis of six seismic reflection sections at high angles or perpendicular with the fold axis the average dip of the monocline changes from 15 in the east to more than 45 in the western sector being replaced westwards by a north verging steeply dipping thrust in some sections growth strata in the dipping limb show rotational offlap onlap syntectonic unconformities with progressively decreasing dips from bottom to top indicating that limb rotation took place during the first stages of folding at the limits of the dipping limb anticline and syncline two kink bands appear in some of the sections the one located on the syncline being more developed within each kink band beds show a constant dip of 30 40 the axial surfaces of each kink band converge upwards in the third and fourth tecto sedimentary units being parallel in the older units the fixed axial surface occurs in a down dip position with respect to the active axial surface the geometry of growth strata indicates that the mechanism of folding followed a sequence of i a first stage of large scale limb rotation wavelength larger than 5 km with minor development of kink bands in some sections and ii a second stage of more localised kink band migration probably favoured by fault propagation in the basement

mechanical models of trishear like folds previous workers have formulated velocity descriptions of the trishear kinematic model of fault propagation folds which are inherently non unique we present two complete mechanical models of fault related folding and assess the validity of the assumptions used in the assignment of velocities in the trishear description and to eliminate the untenable situation of an infinite number of possible solutions for velocity fields the mechanical model of forced folding forced fold based on viscous folding theory is used to derive the velocity fields in an anisotropic sedimentary cover overlying faulted and displaced rigid basement blocks the solution of displacements around a stress free fault in an elastic body is used to model fault arrest folds that form around a fault imbedded in a deformed medium the results indicate that the velocity fields assumed in the trishear model more closely resemble the velocities derived in the mechanical forced fold model than the mechanical model of fault arrest folding the forced fold model produces a triangular region of concentrated deformation similar to the trishear region assumed in the kinematic models while the deformation produced by the fault arrest model is not concentrated within a triangular zone

exhumation history of the tatry mountains western carpathians constrained by low temperature thermochronology this study tests alternative models for the growth of the tatry mountains central western carpathians by the application of low temperature thermochronology zircon u th he ages from the north of the range are mostly between 48 to 37 ma and indicate cooling prior to the onset of forearc sedimentation in the region 42 39 ma in contrast zircon u th he ages in the south of the range are around 22 ma apatite fission track ages across the sampled sites range from 20 to 15 ma apatite u th he ages range from 18 to 14 ma with little variation with elevation or horizontal location based on thermal modelling and tectonic reconstructions these miocene ages are interpreted as cooling in the hanging wall of a northward dipping thrust ramp in the current location of the sub tatric fault with cooling rates of 20 c myr at 22 14 ma modeled cooling histories require an abrupt deceleration in cooling after 14 ma to 5 c myr this is associated with termination of deformation in the outer carpathians and is synchronous with the transition of the pannonian basin from a syn rift to a post rift stage and with termination of n s compression in the northern part of the central western carpathians overall the timing of shortening and exhumation is synchronous with the formation of the outer carpathian orogen and so the miocene exhumation of the tatry record retro vergent thrusting at the northern margin of the alcapa microplate

implications of principal stress axes and eigenvalue ratios on critical orientation of fractures application to two tectonic regions in alaska the principal stress axes and eigenvalue ratios of the stress tensors from two active seismic regions in alaska eastern aleutian arc and eastern gulf of alaska are computed these results are obtained using focal mechanisms of earthquakes with magnitudes ranging from 5 5 to 8 2 and show for both regions slightly different compressional states with an orientation of the pressure axis of n26 w for the eastern gulf of alaska and n27 w for the east aleutian arc the eigenvalue ratios for the eastern gulf of alaska and east aleutian arc are 0 84 and 0 99 respectively the algorithm used in computations allows us to detect some focal mechanisms which are incoherent with respect to the obtained stress tensors these incoherences are explained in terms of some tectonic features of the region the orientations of possible pre existing fractures which would need an unrealistically high maximum shear stress to start a slip on their fault planes are also investigated these critical orientations depend on a constitutive frictional law as well as on eigenvalue ratios and eigenvectors of the stress tensors the orientation which is given in our case by the angle between the normal vector to a fracture and the regional tensional axis is approximately 59 for the eastern aleutian arc this angle ranges from 49 to 59 for the eastern gulf of alaska it must be pointed out that fractures which need unrealistically high shear stress to start slip on their fault planes are defined by a very narrow band of possible angles between tensional direction and the normal vector to fault plane

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

the deep structure of south central taiwan illuminated by seismic tomography and earthquake hypocenter data abstract the taiwan mountain belt is generally thought to develop above a through going basal thrust confined to within the sedimentary cover of the eurasian continental margin surface geology magnetotelluric earthquake hypocenter and seismic tomography data suggest however that crustal levels below this basal thrust are also currently being involved in the deformation here we combine seismic tomography and earthquake hypocenter data to investigate the deformation that is taking place at depth beneath south central taiwan in this paper we define the basement as any pre eocene rifting rocks and use a p wave velocity of 5 2 km s as a reference for the interface between these rocks and their sedimentary cover we found that beneath the coastal plain and the western foothills clustering of hypocenters near the basement cover interface suggests that this interface is acting as a detachment this detachment is located below the basal thrust proposed from surface geology for this part of the mountain belt inherited basement faults appear to determine the geometry of this detachment and their inversion in the alishan area result in the development of a basement uplift and a lateral structure in the thrust system above them however across the shuilikeng and the chaochou faults earthquake hypocenters define steeply dipping clusters that extend to greater than 20 km depth above which higher velocity basement rocks are uplifted beneath the hsuehshan and central ranges we interpret these clusters to form a deeply penetrating east dipping ramp that joins westward with the detachment at the basement cover interface it is not possible to define a basal thrust to the east beneath the central range

fracture porosity creation and persistence in a basement involved laramide fold upper cretaceous frontier formation green river basin usa fracture hosted porosity and quartz distribution along with crack seal texture and fluid inclusion assemblage sequences in isolated bridging quartz deposits show that open fractures can persist through protracted burial and uplift in foreland basins fractures oriented at a high angle to current maximum compressive stress remain open and were weak mechanical discontinuities for millions of years even at great depth upper cretaceous frontier formation sandstones in the basement involved laramide table rock anticline eastern greater green river basin wyoming sampled by two horizontal wells cut parallel or nearly parallel to bedding and at a high angle to steeply dipping fractures have 41 5 m of rock in four cores at depths of 4538 4547 m cores intersect older e striking set 1 fractures are abutted by or locally cross cut by n striking set 2 fractures both sets contain quartz and porosity sequenced using quartz crack seal cement texture maps set 1 fluid inclusion assemblage fia trapping temperatures increase progressively from 140 to 165 c then decrease to c 150 c compatible with fracture opening over c 15 ma during rapid burial followed by uplift in eocene oligocene time set 2 opened at c 160 c probably near maximum burial after a period of quiescence set 2 reopened at c 5 ma at c 140 c on a cooling trajectory intermittent set 2 movement could reflect local basement involved fault movement followed after a pause by further set 2 reactivation in the modern stress field during uplift interpretations are sensitive to available burial thermal histories which have considerable uncertainty

fault related fold styles and progressions in fold thrust belts insights from sandbox modeling fault related folds of variable structural styles and assemblages commonly coexist in orogenic belts with competent incompetent interlayered sequences despite their commonality the kinematic evolution of these structural styles and assemblages are often loosely constrained because multiple solutions exist in their structural progression during tectonic restoration we use a sandbox modeling instrument with a particle image velocimetry monitor to test four designed sandbox models with multilayer competent incompetent materials test results reveal that decollement folds initiate along selected incompetent layers with decreasing velocity difference and constant vorticity difference between the hanging wall and footwall of the initial fault tips the decollement folds are progressively converted to fault propagation folds and fault bend folds through development of fault ramps breaking across competent layers and are followed by propagation into fault flats within an upper incompetent layer thick skinned thrust is produced by initiating a decollement fault within the metamorphic basement progressive thrusting and uplifting of the thick skinned thrust trigger initiation of the uppermost incompetent decollement with formation of a decollement fold and subsequent converting to fault propagation and fault bend folds which combine together to form imbricate thrust breakouts at the base of the early formed fault ramps along the lowest incompetent layers which may correspond to basement cover contacts domes the upmost decollement and imbricate thrusts to form passive roof duplexes and constitute the thin skinned thrust belt structural styles and assemblages in each of tectonic stages are similar to that in the representative orogenic belts in the south china southern appalachians and alpine orogenic belts

case for very low coupling stress on the cascadia ssubduction fault a fundamental problem in plate tectonics is the shear strength of major plate boundary faults this translates to the question whether the generally observed small earthquake stress drops of 3 10 mpa on major faults release most of the accumulated stress or only a small fraction of it there is strong evidence that the san andreas fault a major transform plate boundary is weak 20 mpa shear resistance it is not yet clear whether subduction thrust faults are also weak we present two types of evidence from the northern cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault comparable to that estimated for the san andreas fault first the well defined surface heat flow and heat generation allow negligible frictional heating on the plate interface the average shear stress on the fault must thus be very low over a time scale of a few million years second focal mechanism solutions for small crustal earthquakes in the southern vancouver island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress this inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low one explanation for the weakness of the fault is the presence of near lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence the conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked accumulating strain energy toward a future great earthquake the surface deformation responds to the small 20 mpa temporal changes of the stress field associated with the subduction earthquake cycle this transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin the weakness of the cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards for example a subduction earthquake may induce large strike slip earthquakes in the forearc that affect a large area

structural constraints in neotectonic studies of thrust faults from the magnetotelluric method kochkor basin kyrgyz republic 1 a continuous high resolution magnetotelluric mt profile 5 4 km long reveals a 1 to 2 km thick section of neogene strata thrust beneath a crystalline range front along a gently dipping decollement in the kyrgyz tien shan central asia the imaged decollement links the intrabasin akchop hills fault which currently accommodates 15 of the total shortening across the central tien shan to a crustal ramp that dips 45 beneath the range bounds from the mt data on the base of the sediments and on the dip of the crustal ramp suggest 2 5 7 5 km of total shortening across the basin margin of which at least 2 5 4 8 km clearly involves the decollement and the recently developed akchop hills fault thus a significant fraction of the total shortening across the margin has been accommodated by this young fault system combined with preliminary studies that suggest a mid miocene age for the base of the stratigraphic section results drawn from the mt section indicate that shortening rates have increased since the onset of deformation perhaps within the past 1 2 million years

fault displacement distance relationships as indicators of contractional fault related folding style we present a method of using fault displacement distance profiles to distinguish fault bend shear fault bend and fault propagation folds and use these insights to guide balanced and retrodeformable interpretations of these structures we first describe the displacement profiles associated with different end member fault related folding models then provide examples of structures that are consistent with these model based predictions natural examples are imaged in high resolution two and three dimensional seismic reflection data sets from the niger delta sichuan basin sierras pampeanas and cascadia to record variations in displacement with distance updip along faults termed displacement distance profiles fault bend folds exhibit constant displacement along fault segments and changes in displacement associated with bends in faults shear fault bend folds demonstrate an increase in displacement through the shearing interval and fault propagation folds exhibit decreasing displacement toward the fault tip more complex structures are then investigated using this method demonstrating that displacement distance profiles can be used to provide insight into structures that involve multiple fault related folding processes or have changed kinematic behavior over time these interpretations are supported by comparison with the kinematics inferred from the geometry of growth strata overlying these structures collectively these analyses illustrate that the displacement distance approach can provide valuable insights into the styles of fault related folding

regional tectonic and earthquake hazard implications of a crustal fault zone in southwestern washington the st helens seismic zone shz is a crustal earthquake zone in southwestern washington that is defined by small to moderate magnitude 2 5 to 5 5 earthquakes and focal mechanisms this zone seismically active along a distance of 90 km is interpreted as a fault zone capable of generating a moderate to large magnitude shallow earthquake at the northern end of this seismic zone about 15 km north of the cowlitz river a large area of seismic quiescence for earthquakes above magnitude 3 0 has predominated since 1977 this quiescence leaves open the question whether the shz continues northward into southern puget sound earthquake focal mechanisms along the shz indicating nearly vertical strike slip faulting with the preferred fault planes striking north south from these focal mechanisms we infer that the direction of maximum compression is northeast approximately parallel with the direction of plate convergence between the north american and juan de fuca plates we interpret the focal mechanism data as evidence for locked subduction this tectonic model raises the possibility of a large magnitude subduction earthquake neither the possibility of a moderate to large shallow crustal earthquake nor a major subduction event is part of the current earthquake hazard assessment for washington

delineation and interpretation of seismotectonic domains in western north america indicators of crustal stress and regional seismicity are used to delineate six seismotectonic domains in western north america the san andreas east central california ecc mendocino triple junction mtj and pacific northwest pnw domains are found to have similar stress fields although the style and rate of deformation in each is different we propose that all of these domains are generated by shear between the pacific and north american plates in the mtj and pnw domains this shear may be evidence for the initiation of a transform fault connection between the san andreas and the queen charlotte island fault systems the ecc a region including much of the sierra nevada appears to result from the relatively stiff nature of the san andreas fault system the big bend and region of relatively new faulting in northern california act to make the san andreas fault system appear stiff on a regional scale two other domains the northern basin and range nbr and the intermountain seismic belt isb show evidence of being controlled by more local forces the nbr has been heated and uplifted in the past and is presently extending to the northwest because of a release of tectonic compression at the juan de fuca north america plate boundary the isb can be explained by readjustments between the colorado plateau and the basin and range province which may be caused by the lowering of the basin and range due to extension

depth dependent earthquake focal mechanism orientation evidence for a weak zone in the lower crust the traction free boundary condition across the earth s surface provides an opportunity for studying the relationship between stress orientation and earthquake focal mechanisms because it requires alignment of principal stress axes with vertical and horizontal orientations a survey of earthquake focal mechanisms in northern california shows that their principal axes are also closely aligned with the vertical and the horizontal in the upper few kilometers of earth s crust thus the signature of the free surface boundary condition on stress appears in focal mechanism orientations as well the focal mechanism alignment can also be characterized by the relative magnitude of the off diagonal elements mxz and myz of the seismic moment tensor we find significant and systematic depth variations in the horizontal moment tensor element ms which relates to the shear traction acting on a horizontal plane for the special case of perfect alignment between principal stress and focal mechanism axes values of ms near earth s surface are small but increase with depth to a maximum between 5 and 8 km at greater depths there is a gradual decrease which suggests decreasing horizontal shear traction toward the base of the seismogenic zone we interpret this tendency of axes to become oriented near the base of the seismogenic zone and its expression in ms as the signature of a weak zone in the lower crust if correct this observation would have important implications for the mechanics of lithospheric deformation

using petrophysics and cross section balancing to interpret complex structure in a limited quality 3 d seismic image a study was done to characterize deep prolific ellenburger gas reservoirs at lockridge waha west waha and worsham bayer fields in pecos ward and reeves counties in west texas a major effort of the study was to interpret a 176 mi2 3 d seismic data volume that spanned these fields well control defined the depth of the ellenburger the principal interpretation target to be 17 000 21 000 ft 5200 6400 m over the image area ellenburger reflection signals were weak because of these great target depths additionally the top of the ellenburger had a gentle ramp like increase in acoustic impedance that did not produce a robust reflection event a further negative influence on seismic data quality was the fact that a large portion of the 3 d seismic area was covered by a variable surface layer of low velocity tertiary fill that was in turn underlain by a varying thickness of high velocity salt anhydrite these complicated near surface conditions attenuated seismic reflection signals and made static c

a microearthquake study of southwest utah northwest arizona transition between the basin and range province and intermountain seismic belt abstract we conducted a microearthquake study of southwest utah northwest arizona during june and july of 1982 in order to better determine the variation of the stress field in what appears to be a transition between stress domains eleven single event and composite fault plane solutions were determined including two from a microearthquake swarm which occurred near enterprise utah the region can be divided into two domains defined by fault plane solutions the western and southern parts contain no intermountain seismic belt type fault plane solutions while the remaining area contains both the intermountain seismic belt and northern basin and range type fault plane solutions the mixed fault plane solutions may be caused by a low horizontal deviatoric stress which corresponds to the stress transition between the northern basin and range colorado plateau and structural complexities which may locally affect the stress field the average t axis direction determined from the fault plane solutions for the entire study area is approximately n65 w as compared to the minimum horizontal principal stress direction of n65 w inferred from holocene geologic indicators and n85 e calculated from seismic moment tensors found by other workers in addition we observed as have other workers in the intermountain seismic belt no relationship between seismicity and quaternary faulting finally the microearthquake swarm which is characteristic of southwest utah northwest arizona displayed two distinctly different fault plane solutions from events that migrated in time and space the two fault plane solutions are inferred to represent stress release by the earlier microearthquakes which thereby altered the local stress field

spatial temporal variations in seismicity preceding the may 1980 mammoth lakes california earthquakes abstract anomalous temporal variations in the southern part of the sierra nevada great basin boundary zone sngbz from 1977 to 1980 followed a pattern that several authors have identified as precursory to strong earthquakes from january 1977 to september 1978 a general decrease in seismicity was observed in the southern part of the zone followed by a burst of moderate earthquakes from doyle in the northern part of the sngbz to bishop in the south the 1977 to 1979 variations are particularly noteworthy because they occurred over the entire sngbz indicating a regional rather than local cause in the southern part of the zone a magnitude 5 7 shock 25 km northwest of bishop on 4 october 1978 was followed by an earthquake swarm in the mammoth lakes area with peaks in activity during late 1979 and the spring of 1980 the mammoth lakes activity spread gradually to the west during the first year of this sequence apparently involving several northerly trending fracture zones in an area of about 900 km 2 bursts of activity from december 1979 to april 1980 had increasingly dense spatial clustering temporal clustering showed monthly peaks in the spring of 1980 and intense swarming occurred in mid may about 10 days before the largest events in the sequence a search for changes in p wave velocity changes within the sngbz based on recordings of nevada test site explosions at pairs of stations has so far produced negative results

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

structural and microstructural evolution of the rattlesnake mountain anticline wyoming usa new insights into the sevier and laramide orogenic stress build up in the bighorn basin the rocky mountains in western us provide among the best examples of thick skinned tectonics following a period of thin skinned tectonics related to the sevier orogeny the compressional reactivation of basement faults gave birth to the so called laramide uplifts arches the bighorn basin located in wyoming is therefore a key place to study the transition from thin to thick skinned tectonics in orogenic forelands especially in terms of microstructural and stress strain evolution our study focuses on a classic laramide structure the rattlesnake mountain anticline rma wyoming usa a basement cored anticline located in the western part of the bighorn basin stress and strain evolution analysis in folded sedimentary layers and underlying faulted basement rocks were performed on the basis of combined analyses of fractures fault slip data and calcite twinning paleopiezometry most of the fractures are related to three main tectonic events the sevier thin skinned contraction the laramide thick skinned contraction and the basin and range extension serial balanced cross sections of rma and displacement profiles suggest that all thrust faults were coeval evidencing strain distribution in the basement during faulting the comparison of rma with another structure located in the eastern edge of the bighorn basin i e the sheep mountain anticline sma allows to propose a conceptual model for the geometric and kinematic evolution of laramide related basement cored anticlines finally the stress evolution is reconstructed at both the fold scale and the basin scale we show that the evolution of stress trends and magnitudes was quite similar in both structures rma and sma during laramide times thick skinned tectonics in spite of different stress regimes during sevier thin skinned tectonics and post laramide times stress trends and fracture patterns were different in these two structures these results suggest that the distance to the orogenic front influenced the fracture patterns but not the foreland stress magnitudes which were likely controlled by the structural style

kinematics of compressional and extensional ductile shearing deformation in a metamorphic core complex of the northeastern basin and range abstract analysis of shear criteria enables the kinematics of two main ductile shearing events d 1 and d 2 to be established in the raft river grouse creek and albion metamorphic core complex the first event d 1 is a nne thrusting and corresponds to mesozoic shortening a well developed non coaxial ductile deformation d 2 of cenozoic age is marked by the occurrence of opposing eastward in raft river and westward shear criteria in albion grouse creek these characterize an arch structure where the shear strain increases outwards in the axial zone of the complex d 2 seems coaxial cenozoic extension is considered to be related to gravitational instability induced by mesozoic overthickening of the crust involving uplift erosion and abnormal heating brittle extension occurs in the upper part of the uplifted domain it is transformed laterally above undeformed basement towards stretched domains of the middle and lower crust through the ductile shear zones localized at the precambrian paleozoic interface this extension of the middle and lower crust occurred in the vicinity of the root zones of the former mesozoic thrusts which may thus have been reactivated as ductile normal faults during cenozoic extension

an explanation for contradictory geodetic strain and fault plane solution data in western north america in a few areas of western north america the principal strain azimuths from geodetic strain measurements differ dramatically from the principal stress directions inferred from fault plane solutions of earthquakes the reason for this difference may be related to the amount of strain measured by each technique the strain relieved in an earthquake is about two orders of magnitude greater than that observed by geodetic measurements taken over approximately a seven year time span strain measurements may thus sense short term fluctuations in strain while earthquakes may average these short term fluctuations responding to the total strain accumulated to produce a more reliable measure of the longer term tectonic strain

kinematics of postseismic relaxation from aftershock focal mechanisms of the 1994 northridge california earthquake geodetic observations of surface deformation associated with the 1994 northridge southern california earthquake generally are reproduced by simple models of a large scale elastic dislocation on a blind or buried thrust fault the smaller scale aftershocks of the northridge earthquake are distributed throughout much of the volume of crust that appears to have deformed elastically during the mainshock these aftershocks averaged over volumes that are large relative to their rupture radii reflect a distributed permanent deformation that is accommodated by local brittle fracture we use a micropolar continuum model to invert the aftershocks in such volumes for the average incremental strain and we compare that deformation both with the elastic strain from the dislocation model of the mainshock and with geodetically measured strain aftershock deformation that occurred at depths below about 6 km and which is associated with the primary rupture zone is consistent with slow continuation of the southwest side up reverse slip on the blind northridge thrust fault in contrast aftershock deformation from the upper 5 7 km of the hanging wall block directly above the thrust fault can be characterized by horizontal ne sw shortening and horizontal nw se i e fault parallel extension this pattern of deformation is similar to that associated with the mainshock as observed geodetically and as calculated from the elastic dislocation model we interpret that the aftershock activity in the hanging wall represents the quasi ductile accommodation by brittle deformation mechanisms of a permanent strain distributed through the hanging wall block the aftershocks along the mainshock rupture zone are interpreted as resulting from either 1 the time dependent release along a weakened fault zone of part of the remaining accumulated elastic strain in the upper crust or 2 the continued slip in the weakened fault zone driven by the deformation of a ductile elastic lower crustal layer that relaxes under the stress transferred by the coseismic loss of cohesion in the upper crust in either case the aftershock activity suggests that the crust undergoes quasi ductile flow as a brittle elastic material and is not a strictly elastic material

variable fault tip propagation rates affected by near surface lithology and implications for fault displacement hazard assessment abstract the fabric of reverse fault zones close to the surface is usually partitioned in between a narrow discrete rupture zone and a more distributed one where folding is predominant this makes quite challenging the adoption of proper setbacks in surface rupture hazard studies for critical facilities or microzoning some of the parameters controlling fault zone fabric are related to mechanics of near surface geology lithology overburden thickness cohesion and water content whose interaction is complex and only partially understood nevertheless these can be seldom measured or derived kinematic models conversely express such an interaction of complex variables as simple synthetic parameters such as the amount of upward propagation of the fault tip for unit of slip usually referred to as the p s ratio propagation on slip here we discuss results on the trishear kinematic inverse modeling of a contractional fault propagation fold at monte netto hill capriano del colle n italy observing a two stage fault and fold growth evolution marked by a significant shift in the p s parameter at this site exceptional sequence of exposures due to ca 10 years of quarry excavations allowed to obtain a series of cross sections across the fault zone we use this detailed high resolution example as a natural analogue for more general large scale surface ruptures involving a thick alluvial cover a very common setting for the siting of critical facilities during the early stage of displacement the fault cut through clast supported fluvial gravels with a high propagation rate p s 7 and a discrete rupture width then during the latest movements of the thrust fault tip propagation slowed down to p s 2 9 as the fault started cutting through several stacked bodies of clays and silty clays pedogenized aeolian silts and overbank deposits causing a pronounced folding of the layers over a wider deformation zone these results strongly suggest that lithological changes in the underlying shallow stratigraphy common in an alluvial plain depositional setting would significantly affect the potential for surface faulting across the same tectonic structure with relevant implications in the fault displacement hazard assessment

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

near surface in situ stress 1 strain relaxation measurements along the san andreas fault in southern california strain relaxation stress measurements were made in the mojave desert southeast of palmdale california at two sites during the summers of 1979 and 1980 using the u s bureau of mines technique to depths of about 30 m the field data and finite element modeling studies demonstrate that thermally induced stress dominates the results obtained in the upper 6 m at depths greater than 6 m the average orientation for the horizontal maximum compressive stress at these sites is n22 w 9 at 2 km south of the san andreas fault and n13 w 2 at 20 km north of the fault these azimuths compare favorably with the average of n21 w determined with nearby hydrofracture stress measurements zoback et al 1980 savage et al 1981 also found a nnw orientation for the maximum shortening from a geodetic network with a 15 km aperature in the palmdale area the fact that essentially the same orientation is recovered by three different techniques which sample to different depths and over different areal extents argues strongly for a contemporary tectonic origin for the stress finite element models of the san andreas fault in southern california develop a stress field similar to that observed regionally 15 e for the maximum compressive stress away from the fault when displacements corresponding to relative motion between lithospheric plates are applied on the boundaries of the models near the fault however the model principal stresses are rotated counterclockwise similar to those measured near palmdale demonstrating the influence of the faults on the principal stress orientations

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

extensional shearing deformation and kilometer scale a type folds in a cordilleran metamorphic core complex raft river mountains northwestern utah cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations in the raft river mountains a mylonitic zone and a late superimposed detachment fault separate precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous paleozoic rocks which suffered a brittle extensive deformation we suggest that the mylonitic ductile deformation is the result of tertiary extensional tectonics strain analysis shows that 1 strain results from large scale progressive shearing deformation in a non coaxial regime 2 e w regional stretching lineations represent the direction of shearing and transport 3 shear criteria indicate an east directed movement 4 strain magnitude is increasing eastward 5 kilometer scale folds parallel to the lineations are interpreted as a type folds formed by eastward shearing of a previously northward dipping layering measurement of striated faults in the upper plate gives an e w direction of extension parallel to the lineations thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics a model is proposed to explain the observed geometry and kinematics of the structures the mesozoic to paleocene cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes important uplift abnormal heating and granite emplacement occurred later during the collapse of the former belt the combined effects of thermal and gravitational instabilities during tertiary extensional tectonics enabled the development of a ductile shear zone localized at the precambrian basement paleozoic cover interface the brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust

inferring fault characteristics using fold geometry constrained by airborne laser swath mapping at raplee ridge utah 1 we used airborne laser swath mapping alsm data to define the geometry of strata in raplee ridge in southeastern utah that were likely deformed by slip on an underlying fault the fold geometry was used to infer the fault geometry and loading responsible for the folding using the boundary element model poly3d monte carlo markov chain mcmc methods created probability density functions pdfs that reveal the most likely fault geometry and loading conditions and their uncertainties despite the restrictive assumptions of poly3d in which body deformations occur from slip on displacement discontinuities in a homogeneous linear elastic half space the strata deflections are matched remarkably well and the resulting inferences of fault geometry and stress field appear reasonable we propose that similar methods may be used with models that include more complicated rheologies and heterogeneous and anisotropic material properties to estimate fault geometries and loading conditions where only deflection of strata record these parameters

thermochronologic insight into late cenozoic deformation in the basement cored terskey range kyrgyz tien shan 1 basement cored ranges formed by reverse faulting within intracontinental mountain belts are often composed of poly deformed lithologies geological data capable of constraining the timing magnitude and distribution of the most recent deformational phase are usually missing in such ranges in this paper we present new low temperature thermochronological and geological data from a transect through the basement cored terskey range located in the kyrgyz tien shan using these data we are able to investigate the range s late cenozoic deformation for the first time displacements on reactivated faults are constrained and deformation of thermochronologically derived structural markers is assessed these structural markers postdate the earlier deformational phases providing the only record of cenozoic deformation and of the reactivation of structures within the terskey range overall these structural markers have a southern inclination interpreted to reflect the decreasing inclination of the reverse fault bounding the terskey range our thermochronological data are also used to investigate spatial and temporal variations in the exhumation of the terskey range identifying a three stage cenozoic exhumation history 1 virtually no exhumation in the paleogene 2 increase to slightly higher exhumation rates at 26 20 ma and 3 significant increase in exhumation starting at 10 ma

earthquake hazards of active blind thrust faults under the central los angeles basin california we document several blind thrust faults under the los angeles basin that if active and seismogenic are capable of generating large earthquakes m 6 3 to 7 3 pliocene to quaternary growth folds imaged in seismic reflection profiles record the existence size and slip rates of these blind faults the growth structures have shapes characteristic of fault bend folds above blind thrusts as demonstrated by balanced kinematic models geologic cross sections and axial surface maps we interpret the compton los alamitos trend as a growth fold above the compton ramp which extends along strike from west los angeles to at least the santa ana river the compton thrust is part of a larger fault system including a decollement and ramps beneath the elysian park and palos verdes trends the cienegas and coyote hills growth folds overlie additional blind thrusts in the elysian park trend that are not closely linked to the compton ramp analysis of folded pliocene to quaternary strata yields slip rates of 1 4 0 4 mm yr on the compton thrust and 1 7 0 4 mm yr on a ramp beneath the elysian park trend assuming that slip is released in large earthquakes we estimate magnitudes of 6 3 to 6 8 for earthquakes on individual ramp segments based on geometric segment sizes derived from axial surface maps multiple segment ruptures could yield larger earthquakes m 6 9 to 7 3 relations among magnitude coseismic displacement and slip rate yield an average recurrence interval of 380 years for single segment earthquakes and a range of 400 to 1300 years for multiple segment events if these newly documented blind thrust faults are active they will contribute substantially to the seismic hazards in los angeles because of their locations directly beneath the metropolitan area

structural geometry of raplee ridge monocline and thrust fault imaged using inverse boundary element modeling and alsm data abstract we model the raplee ridge monocline in southwest utah where airborne laser swath mapping alsm topographic data define the geometry of exposed marker layers within this fold the spatial extent of five surfaces were mapped using the alsm data elevations were extracted from the topography and points on these surfaces were used to infer the underlying fault geometry and remote strain conditions first we compare elevations extracted from the alsm data to the publicly available national elevation dataset 10 m dem digital elevation model ned 10 and 30 m dem ned 30 while the spatial resolution of the ned datasets was too coarse to locate the surfaces accurately the elevations extracted at points spaced 50 m apart from each mapped surface yield similar values to the alsm data next we used a boundary element model bem to infer the geometry of the underlying fault and the remote strain tensor that is most consistent with the deformation recorded by strata exposed within the fold using a bayesian sampling method we assess the uncertainties within and covariation between the fault geometric parameters and remote strain tensor inferred using the model we apply these methods to the raplee ridge monocline and find that the resolution and precision of the alsm data are unnecessary for inferring the fault geometry and remote strain tensor using our approach however the alsm data were necessary for the mapping of the spatial distribution of surface outcrops our models considered two scenarios one in which fault geometry and remote strains were inferred using a single deformed stratum and another in which all mapped strata were used in the inversion modeled elevations match those observed to within a root mean squared error of 16 18 m and show little bias with position along the fold both single and multilayer inversions image a fault that is broadly constrained to be 4 5 14 km in down dip height 13 30 km in along strike width with a tip line 2 0 9 5 km below the surface at the time of deformation poisson s ratio was not well resolved by the inversion the idealized elastic model is oversimplified when considering the complicated layered nature of this fold however it provides a good fit to the observations thus comparable surface displacements may be produced with a variety of rheological models so independent constraints on factors such as the fault geometry may be required to ascertain the appropriate rheology of the fold

p wave fault plane solutions and the generation of surface waves by earthquakes in the western united states surface waves recorded at regional distances are used to study the source mechanisms of seven earthquakes in the western united states with magnitudes between 4 3 and 5 5 the source mechanisms of events in or on the margins of the basin and range show t axis with an azimuth of n85 w 16 and a plunge of 12 16 of the seven events four have p wave solutions that are inconsistent with surface wave observations azimuths of the t axis obtained from the surface wave mechanisms and from the p wave solutions differ by up to 45 these events have dip slip or oblique slip mechanisms and the source depths for three of the events are 5 km or less their source mechanisms and small magnitudes make identification of the p wave first motion difficult due to poor signal to noise ratio of the initial p wave and close arrivals of pp or sp with significant amplitude we suggest that mis identification of the p wave first motion and distortion of the body wave ray paths due to non planar structure were sources of error in determining the nodal planes for these events

direction of maximum horizontal compression in western oregon determined by borehole breakouts elliptical borehole enlargements or breakouts caused by systematic spalling of a borehole wall were identified in 18 wells drilled in the coast range and willamette valley of western oregon the breakouts generally indicate a nnw to nne orientation of maximum horizontal compression hmax that agrees with the predominant direction of hmax determined from earthquake focal mechanisms from post middle miocene structural features and from alignments of holocene volcanic centers in the pacific northwest however this orientation is inconsistent with the n50 e convergence between the juan de fuca and north american plates determined by riddihough 1984 from juan de fuca plate magnetic lineations as young as 730 ka the brunhes matuyama boundary the predominant nnw to nne orientation of hmax may be due to the complex interaction of a northwestward moving pacific plate driving into the gorda and juan de fuca plates and indirectly transmitting n s compression across the strongly coupled cascadia subduction zone into the overriding north american plate spence 1989 alternatively the predominant nnw to nne orientation of hmax may be due to a landward counterclockwise rotation of the direction of hmax from n50 e compression offshore to n s compression in the coast range

transform push oblique subduction resistance and intraplate stress of the juan de fuca plate the juan de fuca plate is a small oceanic plate between the pacific and north america plates in the southernmost region referred to as the gorda deformation zone the maximum compressive stress 1 constrained by earthquake focal mechanisms is n s off oregon and possibly off washington nw trending left lateral faults cutting the juan de fuca plate indicate a 1 in a ne sw to e w direction the magnitude of differential stress increases from north to south this is inferred from the plastic yielding and distribution of earthquakes throughout the gorda deformation zone to understand how tectonic forces determine the stress field of the juan de fuca plate we have modeled the intraplate stress using both elastic and elastic perfectly plastic plane stress finite element models we conclude that the right lateral shear motion of the pacific and north america plates is primarily responsible for the stress pattern of the juan de fuca plate the most important roles are played by a compressional force normal to the mendocino transform fault a result of the northward push by the pacific plate and a horizontal resistance operating against the northward or margin parallel component of oblique subduction margin parallel subduction resistance results in large n s compression in the gorda deformation zone because the force is integrated over the full length of the cascadia subduction zone the mendocino transform fault serves as a strong buttress that is very weak in shear but capable of transmitting large strike normal compressive stresses internal failure of the gorda deformation zone potentially places limits on the magnitude of the fault normal stresses being transmitted and correspondingly on the magnitude of strike parallel subduction resistance transform faults and oblique subduction zones in other parts of the world can be expected to transmit and create stresses in the same manner

composition and morphology of early devonian microbial and metazoan patch reefs qasr member of the jauf formation northwestern saudi arabia abstract late pragian early emsian 405 ma microbial and metazoan reefs in the al jawf area of northwestern saudi arabia occur in the 20 m thick qasr limestone member of the jauf formation within a dominantly fine grained marine siliciclastic sequence deposited on a broad shelf adjacent to the arabian nubian shield microbial reefs composed of stromatolites thrombolites and to a lesser extent leiolites are circular to elliptical in plan and have mounded domical cross sections individual microbial reefs up to 6 m in height and 15 m across are commonly laterally and vertically juxtaposed in composite buildups up to 10 m high and 60 m across metazoan reefs dominated by poorly preserved bryozoans range from small subcircular mounds 10 60 m across through ridges up to 300 m in length to more complex structures and reach 4 m in height the qasr member provides a rare example of well exposed early devonian reefs in plan view satellite images show the spatial arrangement of qasr mounds as hundreds of small patch reefs over a wide area regardless of type the reefs and composite bioherms are generally nne ssw oriented and many are grouped into parallel rows this pattern as in some modern patch reef complexes may reflect direct influence of paleotopography and water circulation patterns the reefs are enclosed and separated by bedded deposits marls siltstones and limestones intraclastic floatstones bioclastic floatstones rudstones and peloidal grainstones the qasr reef complex probably developed in generally low energy environments between normal and storm wave base in turbid to clear water early devonian increase in microbial reefs such as qasr challenges the view that post cambrian stromatolites were common only in normal marine subtidal settings in the wake of metazoan mass extinctions

the shuilikeng fault in the central taiwan mountain belt for over 200 km along strike the shuilikeng fault of taiwan separates miocene rocks of the western foothills from the largely eocene and oligocene rocks of the hsuehshan range to the east despite its importance in the taiwan mountain belt the structure and kinematics of the shuilikeng fault are not well known here we present results from new geological mapping along 100 km of its strike length at the surface the shuilikeng fault is a steeply east dipping brittle fault with a series of splays and bifurcations along its southern part it cuts an earlier fold and fault system outcrop kinematic data vary widely from thrusting to strike slip the surface data are integrated with a relocated and collapsed seismicity database to interpret the fault location at depth these data indicate that the shuilikeng fault can be traced to greater than 20 km depth some 260 focal mechanisms from this dataset indicate that its kinematics is overall transpressive from a regional perspective we interpret the shuilikeng fault to reactivate a pre existing rift related basin bounding fault to the east of which rocks in the hsuehshan range are being exhumed

state of stress in the lithosphere recent estimates of the magnitude and orientation of lithospheric stresses are reviewed data come from a variety of sources including earthquake focal mechanisms fault slip data young volcanic dikes and feeders in situ stress measurements at depth lithospheric flexure models microstructure paleopiezometry and consideration of constraints on maximum stress differences from laboratory determined friction and flow laws

multibend fault bend folding this paper shows how slightly complex but angular fault shapes with only a few bends lead to broadly curved and or highly complex folds a common perception exists that simple angular fold geometries are a necessary consequence of the normal assumptions of fault bend fold theory namely angular fault bends straight faults and flexural slip to the contrary it is shown here that exact application of these low level assumptions produces great complexity from simple fault shapes this complexity is caused by the combination of 1 generation of new axial surfaces by displacement of hangingwall cut offs past successive fault bends and 2 fragmentation of axial surfaces by mutual interference the result is entwined arrays of fold axial surfaces that produce quasi curved and or complex folds from discrete angular fault bends fold complexity grows in a highly non linear way in the most extreme case the number of fold axial surface segments grows as the fourth power of the number of fault bends this paper presents examples of the immense variety of complex fold geometries that are thus generated 0 1997 elsevier science ltd all rights reserved

structure of the mountain front flexure along the anaran anticline in the pusht e kuh arc nw zagros iran insights from sand box models this is a contribution of the group of dynamics of the lithosphere gdl financed by a collaborative project between the institute of earth sciences jaume almera csic of barcelona spain and statoilhydro tehran and statoilhydro research centre with the partial support of project team consolider ingenio 2010 no csd 2006 00041

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