wrinkle ridge assemblages on the terrestrial planets the wrinkle ridge assemblage is a group of associated features that have been observed on the moon mars and mercury members of the wrinkle ridge assemblage are classified as either arches or ridges based on morphology and ridges are subdivided into first second or third order ridges on the basis of dimensions the ridge assemblages described in this study are known to occur in mare basalts on the moon and smooth plains units that may be volcanic in origin on mars and mercury the structures that occur in the continental flood basalts of the columbia plateau in the northwestern united states are morphologically and dimensionally similar to features in the ridge assemblages on the terrestrial planets the anticlinal ridges of the columbia plateau and many first order ridges on the moon mars and mercury are interpreted to be folds with reverse or thrust faulting developed as a result of buckling flexure fracture or buckling as a consequence of reverse or thrust faulting fracture flexure estimates of the unit shortening across first order ridges using simple geometric forms to approximate the ridge morphology are lower than those determined for the anticlines of the columbia plateau where displacements are the results of folding and reverse to thrust faulting evidence of layer parallel extension that often accompanies folds developed by fracture flexure mechanisms i e drape or fault bend folds is not found in the anticlinal ridges of the columbia plateau and has only been observed in one ridge on the moon a kinematic model involving buckling followed by reverse to thrust faulting flexure fracture is favored for the origin of the anticlinal ridges of the columbia plateau and many of the first order ridges on the moon mars and mercury

structural analysis and interpretation of the surface deformations of the el asnam earthquake of october 10 1980 the el asnam earthquake of october 10 1980 represents the major seismotectonic event of these last decades in the west mediterranean area it has induced a large amount of surface breaks this earthquake reveals the importance of compressional phenomena that characterize nowadays the tectonics of north africa the tectonic analysis of surface rupture shows the complexity of the deformation mechanism the principal mechanism consists of the activation of a nw se trending thrust fault accompanied with a left lateral motion and with an intense deformation of the northwestern overthrusting block an accurate description of these deformations shows that for a large part the breaks geometry depends closely on the substratum structure finally the analysis of the dislocations on irrigation ducts just above the fault traces allowed a quantitative study of the deformation

patterns of imbricate thrusting through the use of seismic reflection profiles satellite images and balanced kinematic models we describe patterns that help to identify imbricate structures and define the sequence of thrusting in fold and thrust belts measures of folding shear strains are employed to define break forward systems in which younger deeper faults refold overlying thrust sheets in contrast break backward sequences form a wide range of viable geometries that are not expected to conserve these folding strains thus measures of folding shear strains provide a means to define thrusting sequence we also explore map view patterns of imbricate thrusting by using three dimensional models and stereoscopic satellite images we present an example from the peruvian andes in which these map patterns are combined with seismic reflection profiles to define a complex imbricate system with components of both break forward and break backward thrusting these integrated methods provide new tools for understanding the deformational histories of fold and thrust belts including the definition of complex structural closures with opportunities for hydrocarbon reservoir duplication

earthquakes and folding coalinga california the magnitude 6 6 coalinga earthquake of may 2 1983 and most of the other earthquakes of magnitude 4 and greater near that town are tentatively attributed here to flexural slip on the flanks of growing folds this explanation apparently conflicts with the 100 yr old theory that all tectonic earthquakes are caused by slip on faults perhaps the coalinga events in an area where no active faults had been mapped will serve to suggest another possible mechanism for the generation of some earthquakes here the strongly folded and long coalinga anticline and syncline in a thick stratigraphic section seem to require a mechanism of bedding plane slip the magnitude 2 5 lompoc earthquake of april 7 1981 can also be attributed to flexural slip on the limb of a fold flexural slip bedding plane slip usually unseen and unmappable is clearly revealed in several areas in southern california many small earthquakes may be produced in these and other areas where direct evidence of active flexural slip by folding is unavailable if the may 2 coalinga earthquake was caused by folding unusual geologic conditions may be responsible for its high intensity

microbial alteration of subsurface natural gas accumulations bacterial alteration of subsurface reservoired hydrocarbon accumulations extends into the range of the c1 c4 gaseous hydrocarbons preferentially attacking the wet gas components in most instances during the initial stages of bacterial gas alteration propane seems to be selectively attacked resulting in anomalous propane carbon isotopic compositions as well as unusual carbon isotopic distributions among the wet gas components extensive bacterial alteration appears to be capable of removing all but traces of the wet gas components resulting in a dry gas that could be confused with overmature dry gas whose wet components have undergone extensive thermal cracking recognition of microbial alteration in subsurface reservoired hydrocarbon gas accumul tions is important for both correlation purposes and maturity estimates

radar and photoclinometric studies of wrinkle ridges on mars earth based radar altimetry and image derived photoclinometric profiles were analyzed to examine both the long and short wavelength topography associated with wrinkle ridges on mars photoclinometrically derived elevation data across wrinkle ridges were evaluated to determine the sensitivity of profiles to two empirical photoclinometric parameters the horizontal digital number hdn and the scattered light value slv the photoclinometric profiles are extremely sensitive to small variations in hdn the sense of slope of a profile can be completely reversed over a range in hdn of as little as 1 comparably small variations in the slv have relatively minor effects on the photoclinometrically derived elevations the existence of elevation offsets from one side of the ridge to the other reported in previous photoclinometric studies of martian wrinkles were not confirmed through photoclinometry in addition no evidence of elevation offsets were found in earth based radar altimetry profiles across wrinkle ridges in order to more accurately model wrinkle ridge topography we controlled photoclinometrically derived elevations with long wavelength topography obtained from the radar altimetry the results of this study do not support kinematic models for the origin of planetary wrinkle ridges that involve deeply rooted thrust faults which separate crustal blocks at different elevations a kinematic model involving buckling of shallow crustal layers into concentric folds that close leading to the development of thrust faults is consistent with wrinkle ridge morphology and terrestrial analogs recent geophysical studies of terrestrial analogs and the influence of shallow subsurface structures particularly buried craters on the localization of many wrinkle ridges on mars suggest that thrust faults associated with the ridges are confined to the ridged plains material and do not extend into the lithosphere

stress transfer by the 1988 1989 m 5 3 and 5 4 lake elsman foreshocks to the loma prieta fault unclamping at the site of peak mainshock slip we study the stress transferred by the june 27 1988 m 5 3 and august 8 1989 m 5 4 lake elsman earthquakes the largest events to strike within 15 km of the future loma prieta rupture zone during 74 years before the 1989 m 6 9 loma prieta earthquake we find that the first lake elsman event brought the rupture plane of the second event 0 3 1 6 bars 0 03 0 16 mpa closer to coulomb failure but that the lake elsman events did not bring the future loma prieta hypocentral zone closer to failure instead the lake elsman earthquakes are calculated to have reduced the normal stress on or undamped the loma prieta rupture surface by 0 5 1 0 bar 0 05 0 10 mpa at the site where the greatest slip subsequently occurred in the loma prieta earthquake this association between the sites of peak unclamping and slip suggests that the lake elsman events did indeed influence the loma prieta rupture process unclamping the fault would have locally lowered the resistance to sliding such an effect could have been enhanced if the lowered normal stress permitted fluid infusion into the undamped part of the fault although less well recorded the ml 5 0 1964 and ml 5 3 1967 corralitos events struck within 10 km of the southwest end of the future loma prieta rupture no similar relationship between the normal stress change and subsequent loma prieta slip is observed although the high slip patch southwest of the loma prieta epicenter corresponds roughly to the site of calculated coulomb stress increase for a low coefficient of friction the lake elsman loma prieta result is similar to that for the 1987 m 6 2 elmore ranch and m 6 7 superstition hills earthquakes suggesting that foreshocks might influence the distribution of mainshock slip rather than the site of mainshock nucleation

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

wrinkle ridges stress domains and kinematics of venusian plains wrinkle ridges are nearly ubiquitous landforms on the plains of venus by analogy with similar structures on other planets venusian wrinkle ridges are inferred to trend normal to the direction of maximum principal compression in the crust an inference that is verified by geometrical relationships with positive and negative relief features on venus because plains are the dominant terrain on venus wrinkle ridges provide an excellent opportunity to determine the orientations of shallow crustal principal stress trajectories over most of the planet in most places there are two or more sets of wrinkle ridges and commonly one of these persists over a very large area defining a regional stress domain intersection relationships indicate that these domains differ in age

viscoelastic gravitational deformation by a rectangular thrust fault in a layered earth previous papers in this series have been concerned with developing the numerical techniques required for the evaluation of vertical displacements which are the result of thrust faulting in a layered elastic gravitational earth model this paper extends these methods to the calculation of fully time dependent vertical surface deformation from a rectangular dipping thrust fault in an elastic gravitational layer over a viscoelastic gravitational half space the elastic gravitational solutions are used together with the correspondence principle of linear viscoelasticity to give the solution in the laplace transform domain the technique used here to invert the displacements into the time domain is the prony series technique wherein the transformed solution is fit to the transformed representation of a truncated series of decaying exponentials purely viscoelastic results obtained are checked against results found previously using a different inverse transform method and agreement is excellent the major advantage in using the prony series technique is that deformations can be computed for arbitrary time intervals a series of results are obtained for a rectangular 30 dipping thrust fault in an elastic gravitational layer over viscoelastic gravitational half space time dependent displacements are calculated out to 50 half space relaxation times a or 100 maxwell times 2 m a significant effects due to gravity are shown to exist in the solutions as early as several a the difference between the purely viscoelastic solution and the viscoelastic gravitational solutions grows as time progresses typically the solutions with gravity reach an equilibrium value after 10 20 relaxation times when the purely viscoelastic solutions are still changing significantly additionally the length scaling which was apparent in the purely viscoelastic problem breaks down in the viscoelastic gravitational problem two independent length scales one of which changes with time are now seen to characterize the problem

influence of layering and boundary conditions on fault bend and fault propagation folding the influence of heterogeneous layering and boundary conditions on the structural development of fault bend and fault propagation folds has been investigated through petrographic study of nonscaled rock models the models are deformed in a triaxial rock deformation apparatus at room temperature and a 50 mpa confining pressure the models consist of a single layer of sandstone containing a saw cut ramp that is inclined 20 to the layering and an overlying intact thinly layered unit that is composed of limestone interlayered with lead or mica analysis of the fold thrust structures generated in sequentially shortened models with different loading conditions and layer types suggests that the mode of fold thrust interaction activated upon shortening will depend on fault zone drag bending and shearing resistance of the hanging wall shear strength of layer interfaces and loading conditions for the models these parameters may be expressed as a strength ratio describing the resistance to foreland translation relative to the resistance to internal deformation of the thrust system low strength ratios favor fault bend folding high strength ratios favor internal shortening of the sheet isotropic and thick relative to ramp height units above a propagating thrust tip will shorten primarily by faulting whereas thinly layered anisotropic units will shorten by fault propagation folding during both modes of fold thrust interaction the dips of the fold limbs increase interlimb angles decrease and imbricate faults form in the hanging wall or footwall with shortening in one model suite the imbrication is associated with a transition from fault bend folding to fault propagation folding and produces a highly asymmetric ramp anticline similar to a second mode fault bend fold or to a transported fault propagation fold the model data suggest that fault propagation folding in heterogeneously layered rock occurs by the discontinuous formation growth and linkage of faults below the growing fold amplification of the fault propagation fold is affected by the amount of slip transferred out of the deforming region imbrication and buckling the changes in the mode of fold thrust interaction and modifications in the local geometry and strain distribution that occur during shortening result from slip hardening on faults or from rotation or strain induced variations in the strength of the layers

changes in static stress on southern california faults after the 1992 landers earthquake the magnitude 7 5 landers earthquake of 28 june 1992 was the largest earthquake to strike california in 40 years the slip that occurs in such an earthquake would be expected to induce large changes in the static stress on neighbouring faults these changes in stress should in turn affect the likelihood of future earthquakes stress changes that load faults towards failure have been cited as the cause of small1 5 moderate6 and large7 earthquakes conversely those that relax neighbouring faults have been related to a decrease in seismicity5 here we use an elastic half space model8 to estimate the stress changes produced by the landers earthquake on selected southern california faults including the san andreas we find that the estimated stress changes are consistent with the triggering of four out of the five aftershocks with magnitude greater than 4 5 and that the largest changes 1 10 bar occurring on part of the san bernardino segment of the san andreas fault may have decreased the time to the next magnitude 8 earthquake by about 14 years

bedding plane slip in initial stages of fault related folding abstract frictional slip along bedding planes contributes to fault related folding of layered rocks we use numerical experiments to investigate the deformation of frictional bedding planes near dipping faults under layerparallel contraction and extension within the numerical experiments contraction boundary conditions produce asymmetric anticlines and extension produces asymmetric synclines the fold shape may be used to infer dip of the underlying fault in situations where the fault may not be observable additionally sense of slip along bedding planes may indicate proximity to the fault tip under uniform remote tectonic strain fault slip induces deformation in both the hangingwall and the footwall at depths as shallow as 1 km there is no significant difference between fold amplitudes in the hangingwall and the footwall this result is contrary to many kinematic models currently in use kinematic models of fault related folds commonly include the development of flat ramp and flat ramp flat fault geometries which may be attributed to initial ramp thrusting and later flat development our mechanical models show that fault flats may be produced from fault ramps due to slip along frictional bedding planes near the thrust fault tips our numerical experiments also evaluate joint initiation joints perpendicular to bedding are promoted in extensional environments we compare the results of the mechanical model to kinematic models of fault propagation folds and conclude that mechanical models offer important insights to better understand the folding process

the shallow structure of the martian lithosphere in the vicinity of the ridged plains wrinkle ridges in the smooth plains of the coprates and lunae planum regions of mars form linear or concentric patterns with a regular spacing of 25 50 km the authors test the hypothesis that the periodic development of deformation was a consequence of unstable horizontal compression that occurred prior to probable ridge related faulting of a strength stratified lithosphere which consists of a mechanically strong surface plains unit that successively overlies a weak megaregolith and a strong lithospheric basement results show that a range of models with both rigid and deformable megaregolith basement interface conditions yield solutions which can explain the ridge spacing within the constraint provided by the estimated thickness of the smooth plains materials in models that incorporate viscous and plastic rheologies uniform and exponentially varying vertical strength distributions and the presence or absence of interfacial slip at the base of the surface plains unit the ridge spacing is primarily controlled by the power law exponent of the lithosphere the megaregolith plains unit thickness ratio and the plains unit megaregolith strength contrast deformable basement models with a viscous rheology 1 le n approx le 3 can explain the ridge spacing if the megaregolith was thicker than and approximately 1 3 orders of magnitude weaker than themore plains unit similar models with a perfectly plastic n yields infinity rheology require plains unit thicknesses greater than 5 km which exceeds most observational estimates ridge spacing in the combined viscous and plastic models can be consistent with either a dry water rich or ice rich megaregolith at the time of ridge formation less

wrinkle ridges of arcadia planitia mars wrinkle ridges of arcadia planitia were examined to determine their morphology spatial distribution and the amount of crustal shortening and strain they accommodate ridges trend generally northward but their orientation and distribution are strongly controlled by the relief of the underlying knobby material ridges begin or end at inselbergs of older terrain and are associated with buried craters arcadia planitia ridges have an average width of 3425 m and accommodate an average folding shortening of 3 m and a faulting shortening of 55 m mean total shortening is 57 m three east west transects were constructed at 20 25 and 28 n to estimate regional shortening and strain average total shortening across the transects is about 900 m corresponding to a regional compressive strain of 0 06 the total shortening and compression across arcadia planitia are less than in lunae planum faults associated with the arcadia ridges are inferred to have a westward dip compared with an eastward dip for lunae planum ridges the generally levels of compression and symmetric orientation of the ridges suggest a regionally organized stress system differences in the shortening and strain between lunae planum and arcadia planitia may reflect the effects of local lithology or azimuthal variations in the stress field

the global distribution of wrinkle ridges on venus abstract new digital mapping of over 65 000 wrinkle ridges of the plains of venus shows a strong correlation between the location and orientation of these long low amplitude compressive anticlines and major features of the geoid and long wavelength topography regions with wrinkle ridges occupy 43 of the plains and are strongly skewed toward low elevations and negative geoid anomalies about 93 of the wrinkle ridge plains lie below mean planetary radius and 72 have negative geoid anomalies in contrast the extensional rift zones of venus are strongly skewed toward high elevations and positive geoid anomalies thus compressive deformation dominates the topographic and geoid lows whereas extensional rifting is generally restricted to the highs these observations are consistent with geoid based stress models that predict compression in geoid lows and extension in geoid highs the orientations of wrinkle ridges are generally consistent over regions extending for 1000 10 000 km many regions have multiple sets of wrinkle ridges of different orientations reflecting multiple episodes and directions of compression however about 80 of the regions display a single dominant wrinkle ridge orientation with secondary orientations subordinate the dominant wrinkle ridge orientations in many regions follow the contours of long wavelength topography and geoid or lie along the axes of troughs in the geoid thus the maximum horizontal compression recorded by the folds is commonly parallel to the present day gradient in geoid and topography although some regions are strong exceptions perhaps reflecting changes in topography and geoid with time the dominant wrinkle ridge trends ring several major geoid and topographic swells especially western aphrodite terra and lada terra with ring diameters of 75 120 8000 13 000 km in addition there are smaller rings of wrinkle ridges around the swells in themis eistla and bell regiones especially gula mons with ring diameters of 25 45 2500 5000 km in contrast there are no rings of compressive deformation surrounding some other geoid topographic highs including beta and atla regiones which have been proposed to be the youngest swells and display the largest peak geoid topography ratios and largest geoid gradients these observations are consistent with the idea that not all the major topographic geoid swells existed on venus at the time of wrinkle ridge formation

geometry and slip distribution in normal fault systems implications for mechanics and fault related hazards normal faults are important sources of large destructive earthquakes in continental rift provinces we investigate the mechanical characteristics of multiple linked master and subsidiary faults using two dimensional boundary element models the master fault system is a coulomb frictional fault that dips 50 from the surface to a depth of 15 km where it is linked to a subjacent shear zone that creeps under low resolved shear stress we consider two basic configurations one in which the shear zone and master fault are coplanar and the other in which the shear zone dips 20 forming a bent or crudely listric fault system antithetic and splay faults are added to the master fault system in each case to model failure of multiple fault systems that are typical of half grabens we focus on the mechanical interactions between the various faults in order to identify points of initial failure spatial variations in the amount of fault slip and the patterns of surface deformation that may precede catastrophic failure of the master fault system in all cases failure on the master frictional fault initiates at the base where it joins the creeping shear zone and at the earth s surface during the same stress increment the listric fault system fails at lower regional differential stress than the planar fault system failure of a large antithetic fault that dips at 70 from the surface to the base of the frictional master fault initiates at lower regional differential stress for the listric than for the planar case several repeated failure episodes may be required to cause surface offset on the antithetic fault if the master fault system is planar but antithetic surface faulting may occur during each master fault failure episode in the listric case modeled splay faults dip from the surface at either greater 70 or lower 50 angles than the frictional master fault to a depth of 5 km where they intersect the master fault in both cases surface slip is greater on the master fault and lesser on the splay fault surface deformation prior to catastrophic failure of the frictional master fault is generated by creep on the deep seated shear zone and during the initial stages of master fault rupture footwall uplift is predicted above a steeply dipping locked coulomb fault and creeping shear zone but the footwall uplift is absent in the listric fault shear zone case a localized zone of increased extensional strain is centered on the surface trace of the master fault as the result of near surface extensional and shear fracturing during the initial stages of fault slip these geodetic anomalies are of interest because they are a near surface phenomenon which may also alter fluid flow and gas flux and be detectable by hydrologic and geochemical instrumentation as well as geodetic measurements

wrinkle ridges reverse faulting and the depth penetration of lithospheric strain in lunae planum mars abstract wrinkle ridges in the lunae planum region of mars formed in response to compressional stresses associated with the development of the tharsis rise in this study we investigate the accomodation of lithospheric strain associated with wrinkle ridge development in order to constrain the depth penetration of deformation associated with the formation of tharsis using observations that indicate that the shallow martian lithosphere in lunae planum consists of a surface plains unit underlain successively by an unconsolidated megaregolith and competent basement we construct models of an elastic lithosphere in which a ridge nucleates along a single fault in either the surface plains unit or basement we then calculate strain and displacement fields for a range of plausible lithosphere structures assuming uniform horizontal shortening of the lithosphere transmission of strain between the surface layer and basement is most effective for a thin megaregolith and small young s modulus contrast between the surface layer megaregolith and basement however results under a broad range of conditions for both models show straining of both the surface layer and basement if lithosphere structure information and strains estimated by kinematic studies of the wrinkle ridges are applicable then thatsis related compressional deformation must have involved the lithosphere rather than being restricted to the near surface

coseismic folding earthquake recurrence and the 1987 source mechanism at whittier narrows los angeles basin california static deformation associated with the october 1 1987 whittier narrows california m 6 0 earthquake was detected by geodetic elevation changes the earthquake uplifted a 1 5 km high quaternary fold the santa monica mountains anticlinorium by 50 mm but caused no fault rupture at the ground surface this suggests that folding and faulting of the los angeles basin sediments are coincident and continuing after correction for surveying errors and nontectonic subsidence we model the 214 geodetic observations with a simple dislocation in an elastic half space a thrust fault with reverse slip of 1 1 0 3 m and dipping 30 4 n with an upper edge at a depth of 12 1 km and a lower edge at 17 1 km fits the geodetic data best and is consistent with the main shock hypocenter fault plane solution and initial aftershock distribution the upper limit on the static stress drop is 17 5 5 0 mpa 175 50 bar the lower limit on the geodetic moment is 1 0 0 2 1025 dyne cm in accord with seismic estimates indicating that most of the slip took place during seismic rupture if earthquakes of m 6 characterize the blind thrust fault on which the earthquake occurred we estimate a 200 year repeat time at whittier narrows and a 5 18 year rate of earthquake occurrence within a 150 km long band in the northern los angeles basin and santa monica bay to the west the 25 year rate of historical occurrence since 1860 is less than this prediction the deficit in moment release implies that either vigorous aseismic slip or infrequent larger earthquakes occur here

origin of periodically spaced wrinkle ridges on the tharsis plateau of mars ridged plains are a major geologic unit on mars and are a predominant unit on the tharsis plateau a region that has undergone uplift extension and extensive shield and flood volcanism these units probably flood volcanic in origin are characterized by landforms classed as wrinkle ridges wrinkle ridges are interpreted to be folds resulting from buckling followed by reverse to thrust faulting flexure fracture a prominent characteristic of many of the wrinkle ridges on mars particularly those in the tharsis ridge system is the periodic nature of their spacing the periodic spacing has been evaluated in six major provinces on the tharsis plateau with regions further divided into domains based largely on variation in ridge orientation the average spacing of the tharsis ridges based on 2934 measurements is 30 km in an effort to account for the periodic nature of the wrinkle ridges the ridged plains material is modeled as both a single member and a multilayer with frictionless contacts that has buckled at a critical wavelength of folding free slip between layers is assumed based on the possible existence of mechanically weak interbeds in the ridged plains sequence separating groups of flows the near surface mechanical structure in the area of ridged plains is approximated by a strong layer or layers overlying a weak megaregolith of finite thickness overlying a strong basement buckling of the ridged plains is assumed to be decoupled from the basement by the weak megaregolith decoupling is enhanced if the megaregolith were water or ice rich at the time of deformation the rheologic behavior of the ridged plains and megaregolith is approximated by a linear elastic and linear viscous material the models are examined for a range in 1 the strength contrast between the ridged plains material and the underlying megaregolith 2 thickness of the ridged plains material 3 thickness of the megaregolith and 4 number of layers the elastic model can explain the observed ridge spacing only if the ridged plains are relatively thick 2400 m with multiple layers n 1 and relatively high contrasts in young s modulus e e0 100 the high e e0 required is possible only if the megaregolith was water rich at the time of deformation over the same range in values of the parameters viscous buckling is much less restricted the viscous model can explain the observed ridge spacing over a wide range in ridged plains thickness 250 m to several kilometers megaregolith thickness number of layers n 1 to 8 and contrasts in viscosity 0 10 in addition viscous buckling is viable if the megaregolith were dry water rich or ice rich at the time of deformation the absence of folds with a cross sectional geometry in the shape of a sinusoid anticline syncline pairs may be the result of initial deformation of the ridged plains material into low amplitude folds in infinitesimal strain followed by plastic yielding in the cores of the anticlines in finite strain initial elastic or viscous buckling coupled with plastic yielding confined to the hinge area followed by the development of reverse to thrust faulting could account for the asymmetric fold geometry of many of the wrinkle ridges

the role of bed parallel slip in the formation of blind thrust faults abstract a finite element code is used to model the mechanical behaviour of elastoplastic sedimentary layers that are deformed by the movement of a faulted deformable basement the propagation of the blind thrust fault is analysed in terms of the evolution of strain localization in the elastoplastic layers of strain softening and strain hardening materials the role of layer thickness bedding parallel slip and fault dip are also studied we found that if no interlayer slip is allowed blind thrusts tend to propagate with constant dip regardless of the rheological properties of the sediment layers however if bedding parallel slip is present a mechanical decoupling appears between layers allowing stress transfers along large distances and favouring the formation of backthrusts fault dip also has significant influence on the qualitative behaviour when interlayer slip is possible a steep fault will tend to split

relative scale and the strength and deformability of rock masses abstract the strength and deformation of rocks depend strongly on the degree of fracturing which can be assessed in the field and related systematically to these properties appropriate mohr envelopes obtained from the rock mass rating rmr classification system and the hoek brown criterion for outcrops and other large scale exposures of fractured rocks show that rock mass cohesive strength tensile strength and unconfined compressive strength can be reduced by as much as a factor often relative to values for the unfractured material the rock mass deformation modulus is also reduced relative to young s modulus a cook book example illustrates the use of rmr in field applications the smaller values of rock mass strength and deformability imply that there is a particular scale of observation whose identification is critical to applying laboratory measurements and associated failure criteria to geologic structures

time dependent changes in failure stress following thrust earthquakes two dimensional 2 d viscoelastic finite element models were used to calculate the time dependent changes in coulomb failure stresses following thrust earthquakes due to respective effects of relaxation of viscous lower crust or upper mantle and postseismic creep on the main fault or its downdip extension results suggest that thrust earthquakes cause a coseismic increase in coulomb stress along antithetic lobes normal to the slip plane following a quake creep processes that reduce stresses in a ductile lower crust or upper mantle are calculated to cause a transfer of stress to the upper crust under certain conditions transfer of stress may lead to a further buildup of high coulomb stress along the base of the upper crust potentially shortening the time to failure of other faults in the region the conditions under which an antithetic lobe of high coulomb stress are favored to expand at the base of the upper crust postseismically within a few decades include the following the lower crust or upper mantle has an effective viscosity not greater than 1019 pa s the thrust fault has a moderate dip angle 40 50 the brittle ductile transition is deep enough to provide a corridor at the base of the upper crust for expansion and the crust has a low apparent coefficient of friction 0 2 postseismic increases in coulomb stress within the upper crust may also be caused by aseismic creep on the fault stress changes due to this mechanism are maximized with a high apparent coefficient of friction analysis of experimentally determined non newtonian flow laws suggests that wet granitic quartz and feldspar aggregates may have a viscosity of the order of 1019 pa s the calculated rate of stress transfer from a viscous lower crust or upper mantle to the upper crust becomes faster with increasing values of the power law exponent and the presence of a regional compressive strain rate results of this 2 d analysis suggest a potentially important role of viscous flow in controlling time dependent postseismic stress changes that warrant further investigation using 3 d viscoelastic analysis

why blind thrust faults do not propagate to the earth s surface numerical modeling of coseismic deformation associated with thrust related anticlines high fault tip stress concentrations are associated with coseismic slip on blind thrust faults and suggest that these structures should readily propagate to the earth s surface seismic profiles of blind thrust related earthquakes reveal diffuse zones of aftershocks surrounding the fault tip which are attributed to inelastic deformation such as flexural slip or extensional fracturing the complex interaction between blind thrust faults and secondary structures may control the evolution of blind thrust systems the influence of bedding plane slip on fault propagation is simulated with numerical models using the boundary element method we use two parameters to estimate the tendency for thrust fault propagation 1 the mode ii stress intensity factor and 2 the maximum coulomb stress near the fault tip calculations from both analyses suggest that shallow thrust faults may exhibit an increased tendency to propagate as a result of interaction with the earth s surface and slip along bedding planes above the fault tip and a decreased tendency to propagate due to slip along bedding planes at or below the fault tip our results demonstrate that the magnitude and style of inelastic deformation in active fault systems control fault propagation

origin of planetary wrinkle ridges based on the study of terrestrial analogs wrinkle ridges linear asymmetric topographic highs are common physiographic features on the moon mars and mercury we describe here some terrestrial features of similar morphology and scale that have formed under compressional stress systems similar to planetary wrinkle ridges all of the terrestrial analogs are produced by the anticlinal folding of rocks above reverse faults that shallow and typically break the surface characteristics common to both terrestrial and planetary ridges include 1 a linear asymmetric antiformal shape 2 tension cracks and grabens along the hinges of the antiform 3 overlapping en echelon lobes 4 reversals of the sense of asymmetry along strike 5 occurrence in compressional environments and 6 similarity to features produced during compression of various materials in laboratory scale models these similarities strongly suggest that planetary wrinkle ridges result from deformation associated with shallow thrust faults that either break or come close to the surface kinematic models capable of explaining the development of wrinkle ridges include 1 anticlinal folding of rocks over a buried thrust fault that may subsequently propagate to the surface and 2 fault bend folding of upper plate rocks over the surface flattening bend of a thrust fault

displacement length scaling for terrestrial and martian faults implications for valles marineris and shallow planetary grabens the displacement length ratio for the central composite graben in valles marineris inferred from topography and likely stratigraphic offsets to be d max l 4 6 x 10 3 is consistent with the scaling relations derived from terrestrial faults and grabens that also demonstrate a systematic variation in displacement with position along the structure the large 10 km depth in central valles marineris is therefore not unusual but rather should be expected for a fault system 2240 km long in contrast existing measurements of throw along the numerous shallow radial grabens in tharsis while apparently inconsistent with the scaling relations demonstrate the importance of recognizing mechanical segment length along linked or echelon graben sets faults on other planetary surfaces should scale with their terrestrial counterparts as long as they extend substantially deeper than the gravity dependent near surface rock mass zone 1 km for earth 2 5 km for mars

critical taper wedge mechanics of fold and thrust belts on venus initial results from magellan fold and thrust belts exist on venus at the margins of crystal blocks such as plateaus tesserae and coronae and as ridge belts within the plains these fold belts display a number of key features that are consistent with their formation by critical taper wedge mechanics a mechanics that is well known for fold and thrust belts and accretionary wedges on earth for example an analysis of fold geometry at the toe of the artemis chasma fold belt indicates fault bend folding above a regionally extensive decollement horizon at a depth of about 1 5 km near surface deformation on venus is interpreted to be brittle and is anticipated to be dominated by cohesive strength in the upper 1 2 km critical taper wedge mechanics under anticipated venus conditions suggests that brittle wedges should have maximum surface slopes in the range 10 20 which is similar to some estimated slopes in the steep parts of the fold belts the low taper toes of fold belts may be cohesion dominated toes on either brittle or plastic decollement horizons once the base of the wedge undergoes the brittle plastic transition the surface slope is expected to flatten to near horizontal in qualitative agreement with many topographic profiles of fold and thrust belts on venus the estimated depth of the brittle plastic transition is uncertain based on rock mechanics data but is expected to be close enough to the surface to be affected by the atmospheric temperature gradient the relief of fold belts measured between the toe of the wedge and the flat crest displays a remarkable linear dependence on absolute elevation figure 15 ranging from 6 km for maxwell montes at an elevation of 10 km to a few hundred meters at the lowest planetary elevations 0 to 2 km this remarkable phenomenon appears to reflect an absolute elevation dependence of the depth of the brittle plastic transition possibly controlled by an isostatic coupling of elevation lithospheric thickness and geothermal gradient

wrinkle ridges in lunae planum mars implications for shortening and strain photoclinometric data for lunae planum wrinkle ridges indicate average relief of approximately 130 m and distinct elevation discontinuities across the ridge of 55 m modeling ridges as the result of thrust faulting and associated upper plate folding indicates shortening across individual ridges of approximately 131 m 90 faulting total shortening at 20 degree n across lunae planue is approximately 1 840 m corresponding to a regional compressive strain of 0 29 strain appears uniform across lunae planum although it is accommodated by larger and few structures in the west than in the east

localization of bedding plane slip and backthrust faults above blind thrust faults keys to wrinkle ridge structure a mechanically based model for wrinkle ridge development is developed that combines wrinkle ridge morphologies regional topographic offsets suggestive of subsurface thrust faults and folding of near surface layers this model provides explicit relationships between observed morphologic elements characteristic of wrinkle ridges and plausible mechanisms in the subsurface a key component that is absent in previous qualitative fault based scenarios for these structures as developed in this paper wrinkle ridges are the surface expression of anticlines that grow above a blind thrust fault as a result of both flexural slip folding of near surface strata and the nucleation and growth of echelon arrays of backthrust faults calculations of displacements both horizontal shortening and vertical uplift and coulomb stress change related to slip along blind thrust faults in the model demonstrate physically important spatial inhomogeneities in these quantities with revealing and useful implications 1 the ratio of shortening due to folding at the surface to the shortening due to faulting at depth is characteristically small for coupled wrinkle ridge blind thrust fault systems and decreases with increasing fault depth the depth of the blind thrust fault s upper tip thus profoundly influences the surface strains 2 folding and uplifted topography forming the topographic ridge are produced above the area of the slipping blind thrust fault plane horizontal and vertical deformation at the surface extend over several ridge widths or a total of at least 50 km for a 10 km wide ridge implying that topographic profiles and geologic studies must extend sufficiently far from the wrinkle ridge to fully characterize the surface deformation 3 calculations of coulomb stress changes suggest that fault slip can localize both bedding plane slip in overlying strata and new backthrust faults that propagate upward to become wrinkles on the trailing side of the ridge initiation of bedding plane slip in association with slip along the blind thrust fault likely determines whether the resulting surface structure becomes a wrinkle ridge or a lobate scarp wrinkle ridge spacing may also be related to stress changes associated with slip along the underlying blind thrust fault

stress transferred by the 1995 mw 6 9 kobe japan shock effect on aftershocks and future earthquake probabilities the kobe earthquake struck at the edge of the densely populated osaka kyoto corridor in southwest japan we investigate how the earthquake transferred stress to nearby faults altering their proximity to failure and thus changing earthquake probabilities we find that relative to the pre kobe seismicity kobe aftershocks were concentrated in regions of calculated coulomb stress increase and less common in regions of stress decrease we quantify this relationship by forming the spatial correlation between the seismicity rate change and the coulomb stress change the correlation is significant for stress changes greater than 0 2 1 0 bars 0 02 0 1 mpa and the nonlinear dependence of seismicity rate change on stress change is compatible with a state and rate dependent formulation for earthquake occurrence we extend this analysis to future mainshocks by resolving the stress changes on major faults within 100 km of kobe and calculating the change in probability caused by these stress changes transient effects of the stress changes are incorporated by the state dependent constitutive relation which amplifies the permanent stress changes during the aftershock period earthquake probability framed in this manner is highly time dependent much more so than is assumed in current practice because the probabilities depend on several poorly known parameters of the major faults we estimate uncertainties of the probabilities by monte carlo simulation this enables us to include uncertainties on the elapsed time since the last earthquake the repeat time and its variability and the period of aftershock decay we estimate that a calculated 3 bar 0 3 mpa stress increase on the eastern section of the arima takatsuki tectonic line attl near kyoto causes fivefold increase in the 30 year probability of a subsequent large earthquake near kyoto a 2 bar 0 2 mpa stress decrease on the western section of the attl results in a reduction in probability by a factor of 140 to 2000 the probability of a mw 6 9 earthquake within 50 km of osaka during 1997 2007 is estimated to have risen from 5 6 before the kobe earthquake to 7 11 afterward during 1997 2027 it is estimated to have risen from 14 16 before kobe to 16 22