hummocky cross stratification and megaripples a geological double standard abstract hummocky cross stratification in ancient sedimentary deposits is considered to be diagnostic of inner shelf storm deposits the geometry of the cross strata sets implies a low relief oval bedform of a few meters spacing orientation is poorly defined or random hummocky cross strata sets have been attributed to the action of strong storm wave surges megaripples of 2 5 m spacing are typical responses of the inner atlantic shelf of north america to storm flows side scan sonar records have shown that megaripples on the inner shelf are commonly hummocky oval to circular in plan view and lack asymmetry box cores and vibracores reveal gently inclined to horizontal lamination as the characteristic primary structure the megaripples am produced by combined flow storm currents these are alongcoast geostrophic flows with a mean flow component of 20 60 cm sec 1 a high frequency wave orbital current of similar or weaker intensity is present but its role in megaripple formation is subordinate to that of the mean flow the high velocity coastal margin of the flow is often jetlike and downwelling in nature and is characterize by a down current decrease in sand flux and rapid deposition of sand the bottom boundary layer of such flows is characterized by dense sediment suspensions 40 60 mgl 1 and is highly turbulent due to the interaction of the mean flow and wave orbital current components eddy diffusivity up to 200 cm2 sect at 40 cm above the bed sand transport in this coastal boundary zone is obliquely offshore but neither the density of the sediment suspension nor storm surge ebb appear to be important driving forces it is difficult to account for either modern hummocky megaripples or ancient hummocky cross stratification as responses to purely oscillatory flows combined flow currents are the typical response of mostmodern shelves and epicontinental seas to storms and presumably were so in the past hummocky cross stratified beds are generally too thick 20 80 cm to be accounted for by in situ resuspension they are more likely to have been despoited by combined flow currents in areas where these currents were experiencing a downstream velocity decrease and sediment deposition throughout much of the storm s duration evidence for strong scour in swales between hummocks and the smoothly rounded surface of the hummocks suggests a transitionlike bed regime measurements on the atlantic shelf likewi e suggest a transitionlike regime whose boundary layer is characterized by lower velocity than the transitional regime of unidirectional flow in flumes but more intense turbulence and a higher suspended sand concentration the lack of normal cross bedding in bothmodern and ancient hummocky bedforms indicates that these features did not migrate this characteristic may be a consequence of the complex trajectories followed by near bottom water particles in response to combined flow currents

fluid flow and volume loss during mylonitization an origin for phyllonite in an overthrust setting north carolina u s a abstract in the western blue ridge province of the southern appalachians north carolina granite gneisses are converted to phyllitic mylonites phyllonites along the base of the rector branch thrust sheet the mylonites show evidence for fluid infiltration and large volume losses with decreasing distance from the thrust and local shear zones feldspars undergo grain size reduction by cracking followed by chemical breakdown to muscovite sericite and zoisite the following reaction was important during mylonitization 3 k feldspar 2 h muscovite 2 k 6 sio 2 quartz displays evidence for dynamic recrystallization whereas feldspar deforms brittely which together with the secondary mineral assemblages suggest that deformation occurred at temperatures of between 300 c and 500 c in the presence of a fluid the mylonites are enriched by a factor of at least 2 5 in elements such as tio 2 p 2 o 5 zr y v la ce and depleted in sio 2 in comparison to their protolith gneisses the immobile element enrichments are attributed to enrichment in residual phases such as ilmenite zircon apatite and epidote in the mylonites and are interpreted as due to minimum volume losses of 60 mass balance calculations based on major element chemistry indicate the mylonites lost large amounts of sio 2 and smaller amounts of alkalies and were infiltrated by fluid rock weight ratios of between 10 2 and 10 3 phyllonitization as a process involves several deformation mechanisms including cataclasis and solution of feldspar as well as dislocation creep of quartz and micas reaction enhanced softening was apparently important in strain localization

scaling of the critical slip distance for seismic faulting with shear strain in fault zones theoretical and experimentally based laws for seismic faulting contain a critical slip distance1 5 dc which is the slip over which strength breaks down during earthquake nucleation on an earthquake generating fault this distance plays a key role in determining the rupture nucleation dimension6 the amount of premonitory and post seismic slip7 10 and the maximum seismic ground acceleration1 11 in laboratory friction experiments d c has been related to the size of surface contact junctions2 5 12 thus the discrepancy between laboratory measurements of dc 10 5m and values obtained from modelling earthquakes 10 2m has been attributed to differences in roughness between laboratory surfaces and natural faults5 this interpretation predicts a dependence of dc on the particle size of fault gouge2 breccia and wear material but not on shear strain here we present experimental results showing that dc scales with shear strain in simulated fault gouge our data suggest a new physical interpretation for the critical slip distance in which dc is controlled by the thickness of the zone of localized shear strain as gouge zones of mature faults are commonly 102 103 m thick12 17 whereas laboratory gouge layers are 1 10 mm thick our data offer an alternative interpretation of the discrepancy between laboratory and field based estimates of dc

seismic guided waves trapped in the fault zone of the landers california earthquake of 1992 a mobile seismic array of seven stations was deployed at 11 sites along the fault trace of the m7 4 landers earthquake of june 28 1992 with a maximum offset of 1 km from the trace we found a distinct wave train with a relatively long period following the s waves that shows up only when both the stations and the events are close to the fault trace this wave train is interpreted as a seismic guided wave trapped in a low velocity fault zone to study the distribution of amplitude of the guided waves with distance from the fault trace and also their attenuation with travel distance along the fault zone we eliminated source and recording site effects by the coda normalization method the normalized amplitudes of guided waves show a spectral peak at 3 4 hz which decays sharply with distance from the fault trace spectral amplitudes at high frequencies 8 15 hz show an opposite trend increasing with distance from the fault trace the normalized amplitudes of guided waves at 3 4 hz also show a systematic decrease with hypocentral distance along the fault zone from which we infer an apparent q of 50 in order to confirm the existence of the guided waves a dense array of 31 stations was deployed at one of the 11 sites the resultant records revealed unequivocal evidence for the existence of guided waves associated with the fault zone by modeling the waveforms as s waves trapped in a low velocity waveguide sandwiched between two homogeneous half spaces with velocity vs 3 0 km s we infer a waveguide width of about 180 m a shear velocity of 2 0 2 2 km s and a q of 50 hypocenters of aftershocks with clear guided waves show a systematic distribution both laterally and with depth delineating the extent of the low velocity fault zone in three dimensions we find that the zone extends to a depth of at least 10 km this zone apparently continues to the south across the pinto mountain fault because guided waves are observed at stations north of the pinto mountain fault for earthquakes with epicenters south of it on the other hand the zone appears to be discontinuous at the fault bend located about 20 km north of the mainshock epicenter guided waves were observed for stations and epicenters which are located on the same sides of the fault bend but not for those on the opposite sides

volume loss fluid flow and state of strain in extensional mylonites from the central mojave desert california abstract compositional changes in mylonitic rocks from the central mojave metamorphic core complex california indicate that large volume loss 20 70 attended formation of mylonite along the waterman hills detachment fault strong silica depletion and significant mobility of normally immobile elements imply large fluid rock ratios during mylonitization possibly as a result of multiple pass hydrothermal convection connecting near surface and mid crustal alteration regimes dissolution of quartz during mylonitization indicates that the fluid was undersaturated with respect to silica and therefore was probably not magmatic or connate large volume loss in a shear zone should result in apparent flattening strains yet mylonites throughout the complex including the mylonites that record large volume loss exhibit constrictional strains this inconsistency raises questions about the applicability of the shear zone model to cordilleran metamorphic core complexes

the frictional behavior of lizardite and antigorite serpentinites experiments constitutive models and implications for natural faults laboratory studies of the frictional behavior of rocks can provide important information about the strength and sliding stability of natural faults we have conducted friction experiments on antigorite and lizardite serpentinites rocks common to both continental and oceanic crustal faults we conducted both velocity step tests and timed hold tests on bare surfaces and gouge layers of serpentinite at room temperature we find that the coefficient of friction of lizardite serpentinite is quite low 0 15 0 35 and could explain the apparent low stresses observed on crustal transform faults while that of antigorite serpentinite is comparable to other crustal rocks 0 50 0 85 the frictional behavior of both types of serpentinite is well described by a two mechanism model combining state variable dominated behavior at high slip velocities and flow dominated behavior at low velocities the two mechanism model is supported by data from velocity step tests and timed hold tests the low velocity behavior of serpentinite is strongly rate strengthening and should result in stable fault creep on natural faults containing either antigorite or lizardite serpentinite

sequence stratigraphy of the upper oligocene lower miocene of eastern jylland denmark role of structural relief and variable sediment supply in controlling sequence development the upper oligocene lower miocene succession in eastern jylland can be subdivided into three sequences a c from older to younger deposited on and around the ringkobing fyn high the development of the sequences reflects a complex interaction between eustatic sea level changes physiography and variable sediment supply superimposed on this frequent storms promoted longshore sediment transport and the development of spit systems adjacent to structural highs as a consequence sequence boundaries and flooding surfaces are not always expressed as portrayed in conventional sequence models sequence boundaries or flooding surfaces may only be marked by subtle changes in depositional environment that can only be revealed by careful integration of sedimentological observations with palynological data the influence of the topography resulted in the development of brackish water basins that were sufficiently large to permit the deposition of hummocky cross stratified sands with muds these deposits are overlain by clean hummocky and swaley cross stratified sands that were deposited in a fully marine high energy environment this evolution from mud rich storm influenced sediments to sand dominated shoreface sediments resulted from a rise in sea level and was not the result of shoreface progradation and downstepping during a sea level fall in addition to the topographic control on sequence development sediment supply to the study area changed significantly during the deposition of the three sequences initially the basin was sediment starved favouring the formation of glaucony rich sediments the sediment input gradually increased and the influence of structural highs and lows became less significant with time consequently both sequence boundaries and flooding surfaces are characterized by more conventional features in the younger part of the succession where a basinward displacement of the shoreline resulted in thick lowstand delta deposits

fluid rock reaction weakening of fault zones the presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well developed fabrics the growth of phyllosilicates is favored in meteoric water dominated granitic fault systems where mineral aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions in deeper more alkaline rock dominated regimes the reactions reverse and feldspars tend to replace phyllosilicates in mg rich mafic rocks however phyllosilicates chlorite biotite can replace stronger framework and chain silicates in both shallower approximately 10 km meteoric h sub 2 o dominated and in deeper alkaline rock dominated regimes where these phyllosilicates precipitate in active fault zones they contribute directly to reaction softening because low temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure independent dislocation glide the strength of phyllosilicate rich fault rocks can be low at all depths low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites where preferred orientations are strong and contiguity of phyllosilicates is large strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well defined folia can exhibit aseismic slip especially where mafic mg rich rocksmore occur along the fault like parts of the san andreas fault 104 refs 6 figs 1 tab less

microfracturing paleostress and the growth of faults abstract we have analyzed the microfractures in samples taken near one thrust fault and five normal faults of known displacement to test the dependence of microfracture density and distribution on fault slip we have also recorded the orientation of microfractures as a function of distance from the associated fault surface in order to control as many parameters as possible shallow faults with similar lithologies were chosen they showed the same kind of deformation namely microfracturing without block rotation recrystallization extensive granulation or crystal plasticity the results of over 8000 microfracture measurements suggest that maximum microfracture density is independent of net slip we interpret this relationship as indicating that the preponderance of microfracturing occurs in proximity to the propagating fault tip the angular relationship between the shear plane and microfractures was commonly found to be between 5 and 20 significantly less than the 30 expected between the far field maximum principal stress and the shear surface this low angle between microfracture orientation and shear plane orientation is predicted for a local stress field associated with a propagating mode ii fault tip other possible explanations for the small angle for tensional microfractures and the shear surface are 1 the effective confining pressure was very low or 2 shear strength at failure was much larger than expected

3 d stochastic modeling and simulation of fault zones in the albala granitic pluton sw iberian variscan massif a distribution of fracture index fi is obtained from stochastic modeling and simulation to characterize quantitatively the fault system of the mina ratones area in three dimensions 3 d fi is a quantitative estimate of the fracture intensity in discrete domains of the granitic rock massif the resulting 3 d grids expressed as block cells models or contoured isosurfaces of fi show high and low fi zones the correlation of these zones with mapped faults allows two structural domains to be distinguished 1 variably irregular surfaces of high fi and 2 rhomboidal blocks of low fi located inside them high fi domains fi 4 2 m 1 are interpreted as fault zones since there is a good correlation at the surface between the domains and traces of the major fault zones low fi blocks fi 2 5 m 1 correspond to less fractured granite the contact between high and low fi domains is gradual the high and low fi structural domains may correspond with the damage zone fault core and the protolith in the model for fault zone architecture of caine et al 1996 therefore 3 d grids of the fi in granitic areas affected by strike slip brittle tectonics such as mina ratones constitute an image of fault zone architecture

two dimensional geostatistical modeling and prediction of the fracture system in the albala granitic pluton sw iberian massif spain abstract we use a fracture index distribution method of geostatistical modelling and prediction to characterize quantitatively the fracture system in two dimensions 2d in the mina ratones area located in the albala granitic pluton sw iberian massif the fracture index fi is a quantitative estimate of the fracture density in discrete domains to validate the results of geostatistical modeling a detailed structural map of the area was also made on a scale of 1 1000 the resulting grids expressed as pixel maps describe the continuous value of the fi in 2d for the whole mina ratones area based on the modelled distribution of the fi and their correlation with mapped faults we distinguish two structural domains in the studied area elongated bands of fracture zones with high fi values and romboidal blocks located between them with low fi values the separation between both domains is gradual though a threshold value of the fi that separate both structural domains is not clearly defined the fracture zones generally present fi 1 and the individualized blocks fi

swaley cross stratification in medium to coarse sandstone produced by oscillatory and combined flows examples from the proterozoic kansapathar formation chhattisgarh basin m p india abstract the proterozoic kansapathar formation is the topmost unit of the siliciclastic chandarpur group of rocks which records a fluvial to marine transition and comprises a fining upwards transgressive succession overlain by a coarsening upwards prograding succession the kansapathar formation around panduka m p is composed of medium to coarse grained well sorted quartzarenite and is replete with swaley cross strata scs trough cross strata and other structures of intermediate morphology in laterally continuous exposures scs are more common in the northeastern part of the study area whereas trough cross strata are more dominant in the southwestern part we have recognised the scs on the basis of the stratal geometry originally described by leckie and walker 1982 scs occur as nested sets of elliptical swales which are commonly symmetrically filled the trough cross strata are variable in scales and dominantly sw directed the major axes of swales show a strong nnw sse preferred orientation orthogonal to the orientation of trough cross strata and at a low angle to the shoreline orientation determined from the megaripple orientation in the immediately underlying unit scs described herein in contrast to many examples reported from elsewhere formed in medium to coarse sand and the maximum inclinations of their lower bounding surfaces and of the infilling laminae are greater than those of fine grained hummocky cross strata hcs and scs this is explicable since medium to coarse grained sediments did not experience significant suspension transport and thus the development of low angle lamination was suppressed the symmetrically filled swales do not show any evidence of lateral migration and might have been formed by bidirectional oscillatory flow the trough cross strata could have been formed by translatory current dominated combined flow the spectrum of structures may owe their origin to symmetrical oscillatory flow transforming into asymmetric flow during onshore propagation of waves this is expected to be common in shoreface environment the spatial variation in relative frequency of occurrence of various types of cross strata is also consistent with the palaeogeographic reconstruction of the study area the trough cross strata seem to be more common towards the constantly shifting sands of nearshore thus the relative frequency of trough cross strata and scs has been used as a clue to determine relative proximity to shoreline

precambrian storm wave base deposits of early proterozoic age 1 9 ga preserved in andalusite cordierite rich granofels and quartzite ramsberg area varmland sweden abstract well preserved laminites wave ripples and thin hummocky cross stratified beds arranged in several thickening and thinning upward sequences have been interpreted as lobe like storm wave base deposits delicate features in thin sections confirm the importance of wave activity over current activity these features include thinning and fining upwards sequences on a centimetre scale with a great variety of laminae graded inversely graded non graded with diffuse or sharp tops due to winnowing micro flaser bedding on millimetre scale with 2 cm wavelength lateral sorting and grain size differences and silty streaks of single grain thickness of approximately 0 5 1 cm in length these structures are exceptionally well preserved in the medium grade metamorphics due to the lack of burrowing in early proterozoic sediments and to the non deformational character of the thermal metamorphism the proterozoic sequence is compared with existing depositional models of storm influenced shelves from the recent and fossil record

the role of fluids in faulting deformation a case study from the dead sea transform jordan the geochemistry of carbonate fault rocks has been examined in two areas of the arava fault segment which forms the major branch of the dead sea transform between the dead sea and the gulf of aquaba the role of fluids in faulting deformation in the selected fault segment is remarkably different from observations at other major fault zones our data suggest reduced fluid rock interactions in both areas and limited fluid flow the fault did not act as an important fluid conduit there are no indications that hydrothermal reactions cementation dissolution did change the strength and behavior of the fault zone although the two areas show considerable differences with respect to fluid sources and fluid flow in one area the investigated calcite mineralization reveals an open fluid system with fluids originating from a variety of sources stable isotopes 13c 18o strontium isotopes and trace elements indicate both infiltration of descending meteoric and or sea water and ascending hydrothermal fluids in the other area all geochemical data indicate only local small scale fluid redistribution these fluids were derived from the adjacent limestones under nearly closed system conditions

cataclastic rocks of the san gabriel fault an expression of deformation at deeper crustal levels in the san andreas fault zone the san gabriel fault a deeply eroded late oligocene to middle pliocene precursor to the san andreas was chosen for petrologic study to provide information regarding intrafault material representative of deeper crustal levels cataclastic rocks exposed along the present trace of the san andreas in this area are exclusively a variety of fault gouge that is essentially a rock flour with a quartz feldspar biotite chlorite amphibole epidote and fe ti oxide mineralogy representing the milled down equivalent of the original rock anderson and osborne 1979 anderson et al 1980 likewise fault gouge and associated breccia are common along the san gabriel fault but only where the zone of cataclasis is several tens of meters wide at several localities the zone is extremely narrow several centimeters and the cataclastic rock type is cataclasite a dark aphanitic and highly comminuted and indurated rock the cataclastic rocks along the san gabriel fault exhibit more comminution than that observed for gouge along the san andreas the average grain diameter for the san andreas gouge ranges from 0 01 to 0 06 mm for the san gabriel cataclastic rocks it ranges from 0 0001 to 0 007 mm whereas the san andreas gouge remains particulate to the smallest grain size the ultra fine grain matrix of the san gabriel cataclasite is composed of a mosaic of equidimensional interlocking grains the cataclastic rocks along the san gabriel fault also show more mineralogiec changes compared to gouge from the san andreas fault at the expense of biotite amphibole and feldspar there is some growth of new albite chlorite sericite laumontite analcime mordenite and calcite the highest grade of metamorphism is laumontite chlorite zone zeolite facies mineral assemblages and constrained uplift rates allow temperature and depth estimates of 200 30 c and 2 5 km thus suggesting an approximate geothermal gradient of 50 c km such elevated temperatures imply a moderate to high stress regime for the san andreas which is consistent with experimental rock failure studies moreover these results suggest that the previously observed lack of heat flow coaxial with the fault zone may be the result of dissipation rather than low stress much of the mineralogy of the cataclastic rocks is still relict from the earlier igneous or metamorphic history of the protolith porphyroclasts even in the most deformed rocks consist of relict plagioclase oligoclase to andesine alkali feldspar quartz biotite amphibole epidote allanite and fe ti oxides ilmenite and magnetite we have found no significant development of any clay minerals illite kaolinite or montmorillonite for many sites the compositions of these minerals directly correspond to the mineral compositions in rock types on one or both sides of the fault whole rock major and trace element chemistry coupled with mineral compositions show that mixing within the zone of cataclasis is not uniform and that originally micaceous foliated or physically more heterogeneous rock units may contribute a disproportionally large amount to the resultant intrafault material as previously found for the gouge along the san andreas chemical mobility is not a major factor in the formation of cataclastic rocks of the san gabriel fault we see only minor changes for si and alkalies however there is a marked mobility of li which is a probable result of the alteration and formation of new mica minerals the gouge of the san andreas and san gabriel faults probably formed by cataclastic flow there is some indication presently not well constrained that the fine grained matrix of the cataclasite of from the san gabriel fault formed in response to superplastic flow

swaley cross stratification produced by unidirectional flows bencliff grit upper jurassic dorset uk unusual bedforms occur in the bencliff grit at osmington mills dorset the geometry of cross strata in stacked predominantly fine grained sandstone units is dominated by low angle troughs with locally perfectly preserved antiforms which wedge out onto climbing low angle erosion surfaces many of the antiforms hummocks and troughs swales appear to have grown at similar rates of sediment fall out others show draped antiforms and thickening into swales in both cases laminae are concordant with the lower bounding surfaces of the cross sets demonstrating that vertical accretion was the dominant process this geometry together with the conspicuous absence of angle of repose cross strata give the bedforms a resemblance to amalgamated hummocky cross stratification or swaley cross stratification many workers consider amalgamated hummocky cross stratification and swaley cross stratification to be identified with storm sedimentation on the shelf to shoreface thereby bracketting the structures both in terms of process and environment however the structures in the bencliff grit suggest that such a precise interpretation of amalgamated hummocky cross stratification or swaley cross stratification is inadvisable the geometry of the cross stratification suggests deposition under a highly sediment charged but essentially unidirectional flow this study therefore supports recent views that hummocky and swaley styles of cross stratification are in reality a form of low angle trough cross stratification produced primarily by unidirectional flows clearly since undulatory forms of stratification are substrate and process dependent and not environmentally specific great care must be taken in the interpretation of palaeoenvironmental settings especially where exposures are limited as for instance in borehole cores it is unlikely that core data can be used to identify bedforms analogous to hummocky cross stratification or swaley cross stratification unless boreholes are highly deviated

marine and nonmarine deposition in a long term low accommodation setting an example from the middle pleistocene qm2 unit eastern central italy abstract facies anatomy and large scale stratigraphic organization of mid pleistocene strata qm2 unit exposed near the town of ortona eastern central italy have been defined through an integrated outcrop and subsurface dataset and record sedimentation within a long term low accommodation setting the studied interval can be subdivided into nine facies associations reflecting a range of continental marginal marine nearshore and innermost shelf environments and contains a well preserved record of high frequency relative sea level changes in the form of a stack of three unconformity bounded sequences each of the three sequences designated qm23 qm22 and qm21 from oldest to youngest consists of a specific subset of the nine identified facies associations as a function of preserved depositional systems and systems tracts typically the following architectural elements occur in ascending stratigraphic order i a basal unconformity molded by fluvial incision and subaerial exposure during sea level fall and shoreface erosion during subsequent rise ii a maximum regressive surface which is coincident with the sequence boundary iii a transgressive systems tract including braided river conglomerates and floodplain fines within the confines of an incised valley palustrine carbonates and back barrier lagoon mudstones and a deepening and fining upward shoreface to offshore transition facies succession bounded at the base by a wave ravinement surface iv a sharp regressive surface of marine erosion produced by shoreface incision during relative sea level fall and v a prograding downstepping and offlapping sharp based lithofacies assemblage of either beach conglomerates or sandstones ascribed to the falling stage systems tract overlain by a rooted paleosol marking subaerial exposure sequences were deposited on the uplifting palaeo adriatic shelf in response to repeated high frequency and high amplitude eustatic changes in sea level that would promote the creation of accommodation space by subtracting space to deposition the contemporaneous regional uplift of the basin margin may be accounted for the shingled downstepping configuration of individual sequences to form a falling stage sequence set of a larger composite sequence in dip view the erosional contact at the base of the qm2 falling stage sequence set is a composite regressive surface resulting from the downdip convergence and amalgamation of multiple erosional surfaces produced by the progressive basinward translation of successive sequences results from this study have direct implications for the oil and gas field development and exploration when making predictions of reservoir and seal geometries within shallow marine marginal marine and nonmarine sediments emplaced in similar long term limited accommodation settings

fault zone trapped seismic waves two instances of fault zone trapped seismic waves have been observed at an active normal fault in crystalline rock near oroville in northern california trapped waves were excited with a surface source and recorded at five near fault borehole depths with an oriented three component borehole seismic sonde at parkfield california a borehole seismometer at middle mountain recorded at least two instances of the fundamental and first higher mode seismic waves of the san andreas fault zone at oroville recorded particle motions indicate the presence of both love and rayleigh normal modes the love wave dispersion relation derived for an idealized wave guide with velocity structure determined by body wave travel time inversion yields seismograms of the fundamental mode that are consistent with the observed love wave amplitude and frequency applying a similar velocity model to the parkfield observations we obtain a good fit to the trapped wavefield amplitude frequency dispersion and mode time separation for an asymmetric san andreas fault zone structure a high velocity half space to the southwest a low velocity fault zone a transition zone containing the borehole seismometer and an intermediate velocity half space to the northeast in the parkfield borehole seismic data set the locations depth and offset normal to fault plane of natural seismic events which do or do not excite trapped waves are roughly consistent with our model of the low velocity zone we conclude that it is feasible to obtain near surface borehole records of fault zone trapped waves because trapped modes are excited only by events close to the fault zone proper thereby fixing these events in space relative to the fault a wider investigation of possible trapped mode waveforms recorded by a borehole seismic network could lead to a much refined body wave tomographic velocity model of the fault and to a weighting of events as a function of offset from the fault plane it is an open question whether near surface sensors exist in a stable enough seismic environment to use trapped modes as an earth monitoring device

a hypothesis for delayed dynamic earthquake triggering 1 it s uncertain whether more near field earthquakes are triggered by static or dynamic stress changes this ratio matters because static earthquake interactions are increasingly incorporated into probabilistic forecasts recent studies were unable to demonstrate all predictions from the static stress change hypothesis particularly seismicity rate reductions however current dynamic stress change hypotheses do not explain delayed earthquake triggering and omori s law here i show numerically that if seismic waves can alter some frictional contacts in neighboring fault zones then dynamic triggering might cause delayed triggering and an omori law response the hypothesis depends on faults following a rate state friction law and on seismic waves changing the mean critical slip distance dc at nucleation zones

grain fabric of hummocky and swaley cross stratification abstract bed form and grain fabric of two types of hummocky and swaley cross stratification from the pleistocene formations in the kanto plain cental japan display clearly distinctive flow conditions of storm typhoon generated combined flow the grain alignment in the vertical profile of a domal structure of hummocky cross set together with variable strike directions of a lamina suggest that the cross stratification was produced under the waning stage of oscillatory dominant combined flow elsewhere the preferred grain orientation along the maximum dip direction of a large swaley structure shows a small angle of upcurrent imbrication both on the stoss and lee sides of the laminae the stratification within the swale is asymmetric but the strike directions of a lamina are persistently oriented in a preferred direction the fabric and the structure of the swale indicate that the cross stratification was formed as a very low angle mega ripple under unidirectional dominant combined flow the grain fabric of both cross sets clearly shows that in spite of a superficial resemblance to antidunes neither of the two structures was formed in flow conditions for antidunes

structures in natural serpentinite gouges abstract serpentinite gouges from cataclastic fault zones in the voltri massif nw italy are dominated by few discrete boundary parallel y shears and oblique synthetic riedel r 1 shears separating domains of massive gouge unlike natural clay rich fault gouges the cohesive microbreccia domains only rarely show foliations these structures dominated by discrete shears indicate a significant degree of localization of the deformation at the scale of the gouge the dominant gouge forming mechanisms were fracturing of olivine and pyroxene grains along serpentinized grain boundaries and initiation of transgranular fractures in pyroxene grains in domains of lattice bending and grain indentation syn tectonic serpentinization added to the weakening of fragments textural similarities with experimentally formed gouges suggest that the voltri fault zones were dominated by stick slip sliding hence that they may represent fossil seismogenic faults a notable aspect of the gouge texture in the voltri fault zones is a sigmoidal shape of the riedel shears which bend into parallelism with the y shears it is argued that this geometry was largely controlled by local perturbations of the stress field near heterogeneities on y or p shears the average spacing between the r 1 shears increases linearly with 0 47 times the distance between bounding y shears this proportionality may reflect a balance between the cohesive strength of the gouge material and the shear stresses acting on the bounding slip planes

hummocky cross stratification from the lower jurassic hasle formation of bornholm denmark abstract the exposed part of the lower pliensbachian hasle formation is composed of coarse siltstone and fine grained sandstone and is hummocky cross stratified throughout the stratification can be described as amalgamated h and or hf divisions the dott bourgeois ideal sequence for hummocky cross stratification is slightly modified so that the basal pebbly lag is extended to include the graded and laminated pebbly portion at the base of the h division the suggestion to include a parallel laminated p division below the h division is not followed the hummocks of the hasle formation are mainly of erosional nature but hummocks representing growing bed forms have been observed the formation was deposited in an open marine shelf 1 2 km from the fault controlled coast lines the hummocky cross stratification was probably formed in depths of 10 40 m by combined flow storm currents representing along coast geostrophic flows storms were frequent and the coastline faced a wide epeiric sea with a fetch towards the west of possibly 1000 km the hettangian of bornholm represents an overall transgressive or deepening sequence passing from lacustrine over fluvial to tidal flat deposits the recognition of hummocky cross stratification in the hasle formation thus indicates that the deepening continued in the pliensbachian this conforms with published curves on eustatic sea level changes

upper triassic lower cretaceous seismic sequence stratigraphy and basin tectonics at bornholm denmark tornquist zone nw europe abstract an integrated analysis of seismic sequence stratigraphy and structural evolution of a dynamic intraplate fault basin was conducted along a segment of the tornquist zone at the boundary between the northwest european craton and the baltic shield east european platform high quality regional seismic reflection lines document the late triassic through early late cretaceous tectonic history and depositional sequence architecture the stratigraphic package was divided into three major transgressive regressive cycles 1st order sub cycles the upper upper triassic through lower cretaceous siliciclastic cycles i and ii are divided into 2nd order transgressive regressive facies cycles composed of higher frequency 3rd order sequences the lowstand basin fill comprises stacked successions of lowstand systems tracts produced by relative sea level rises and falls during the 3rd order regressive intervals highstand dominated depositional conditions prevailed to the northwest while lowstand dominated conditions occurred to the southeast cycle iii records an upper cretaceous major siliciclastic chalk cycle the magnitude of the relative sea level highs and lows increased up through the studied interval a two dimensional structural evolution analysis is based on the variation of the sediment thickness across the fault blocks up through the sedimentary column basin subsidence can be divided into successive structural tectonic regimes fault block rotation and structural inversion characterized the early subsidence of the basin floor during the second interval inversion ceased while fault block rotation continued in the third interval the rotation also stopped and no deformation of the basin is recognized the three major transgressive regressive facies cycles and associated relative sea level cycles are linked with the structural regimes contemporaneous deposition of 3rd order highstand and lowstand dominated complexes related to differential subsidence and uplift is also evidence of basin tectonics as the controlling factor on relative sea level changes comparison of the relative sea level curve developed for the study area with eustatic sea level curves shows that they follow different trends the deviations between these curves and the correlation of sequence boundaries with changes of the tectonic regime support the conclusion that tectonics was a major control on relative sea level fluctuations in the study area the 2nd order cycles linked with deformation episodes along the tornqist zone correlate with lower middle jurassic 2nd order cycles in england deformation along the tornquist zone may thus have influenced the sequence stratigraphy across the entire northwest european craton

micro scale damage characterized within part of a dismembered positive flower structure san jacinto fault southern california usa abstract to better understand the processes that control sub grain fracturing in fault damage zones we studied micro scale damage in sandstones adjacent to the san jacinto fault sjf where it is exhumed from a total depth of 220 m beneath a northeast verging thrust that comprises part of a relic and dismembered flower structure the thrust places high grade gneiss of the pre middle cretaceous burnt valley complex over sedimentary rocks of the pleistocene bautista formation an 10 12 cm thick zone of cataclasite is present along the northeast side of the fault adjacent to a narrow black ultracataclasite core non pervasive microscopic damage characterized by pulverized sand grains extends outward from the zone of cataclasites tens of meters such textures are better developed in sandstones that contain

mesoscopic structure of the punchbowl fault southern california and the geologic and geophysical structure of active strike slip faults we examine the distribution density and orientation of outcrop scale structures related to the punchbowl fault an exhumed ancient trace of the san andreas fault southern california in order to determine the structure of the fault zone the punchbowl fault has 44 km of right lateral slip and cuts the cretaceous pelona schist in the study area the mesoscopic structures examined include fractures small faults and veins they were inventoried using scan lines at closely spaced stations along three strike perpendicular traverses 200 250 m long across the fault the fault zone thickness is a function of the type of structure measured slip along narrow 2 m wide ultracataclasite cores of the faults results in foliation reorientation over a distance of 50 m from the cores fracture and fault densities appear to increase 50 80 m from the fault cores and vein densities are highly variable across the fault zone fractures and faults in the damaged zone have a variety of orientations but most are at high angles to the main fault zone when coupled with previous geochemical and microstructural data these data show that large displacement faults of the san andreas system are up to 200 250 m thick and enclose zones of mineralogic and geochemical alteration that are 20 30 m thick extreme slip localization occurs over zones 1 5 m thick when reconciled with geophysical imaging our data suggest that trapped headwaves travel in the damaged zone and that some aftershock events produce slip on faults and fractures which often have orientations very diaerent from the principal slip surfaces 7 2000 elsevier science ltd all rights reserved

an introduction to the mesozoic biotas of scandinavia and its arctic territories the mesozoic biotas of scandinavia have been studied for nearly two centuries however the last 15 years have witnessed an explosive advance in research most notably on the richly fossiliferous t

growth linkage and termination processes of a 10 km long strike slip fault in jointed granite the gemini fault zone sierra nevada california abstract field based structural analysis of an exhumed 10 km long strike slip fault zone elucidates processes of growth linkage and termination along moderately sized strike slip fault zones in granitic rocks the gemini fault zone is a 9 3 km long left lateral fault system that was active at depths of 8 11 km within the transpressive late cretaceous sierran magmatic arc the fault zone cuts four granitic plutons and is composed of three steeply dipping northeast and southwest striking noncoplanar segments that nucleated and grew along preexisting cooling joints the fault core is bounded by subparallel fault planes that separate highly fractured epidote chlorite and quartz breccias from undeformed protolith the slip profile along the gemini fault zone shows that the fault zone consists of three 2 3 km long segments separated by two zones of local slip minima slip is highest 131 m on the western third of the fault zone and tapers to zero at the eastern termination slip vectors plunge shallowly west southwest and show significant variability along strike and across segment boundaries four types of microstructures reflect compositional changes in protolith along strike and show that deformation was concentrated on narrow slip surfaces at or below greenschist facies conditions taken together we interpret the fault zone to be a segmented linked fault zone in which geometrical complexities of the faults and compositional variations of protolith and fault rock resulted in nonuniform slip orientations complex fault segment interactions and asymmetric slip distance profiles

internal waves vs surface storm waves a review on the origin of hummocky cross stratification hummocky cross stratification hcs is considered a diagnos tic structure of surface storm activity at the shoreface offshore transition however the origin of hcs is still debated laboratory experiments have not yet reproduced it and direct observations on the continental shelves do not exist most hydrodynamic interpretations invoke pure oscilla tory flows unidirectional dominated combined flows and oscillatory dominated flows but they all share the assump tion of hcs to reflect the combined action of surface storm waves and related currents within this context of uncertain ties internal waves gravity waves propagating along the pycnocline provide an alternative mechanism to explain the origin of hcs internal waves breaking on the shelf create episodic high turbulence events and induce upslope and downslope currents as well as oscillatory flows at the depth where the pycnocline intersects the sea floor in this scenario both the oscillatory and the unidirectional components needed for hcs to form are not necessarily linked to surface storm waves but can occur at various depth as far and near there is a pycnocline where internal waves can form

clustering and periodic recurrence of microearthquakes on the san andreas fault at parkfield california the san andreas fault at parkfield california apparently late in an interval between repeating magnitude 6 earthquakes is yielding to tectonic loading partly by seismic slip concentrated in a relatively sparse distribution of small clusters 20 meter radius of microearthquakes within these clusters which account for 63 of the earthquakes in a 1987 92 study interval virtually identical small earthquakes occurred with a regularity that can be described by the statistical model used previously in forecasting large characteristic earthquakes sympathetic occurrence of microearthquakes in nearby clusters was observed within a range of about 200 meters at communication speeds of 10 to 100 centimeters per second the rate of earthquake occurrence particularly at depth increased significantly during the study period but the fraction of earthquakes that were cluster members decreased

effects of simulated clay gouges on the sliding behavior of tennessee sandston abstract the effects of simulated fault gouge on the sliding behavior of tennessee sandstone are studied experimentally with special reference to the stabilizing effect of clay minerals mixed into the gouge about 30 specimens with gouge composed of pure clays of homogeneously mixed clay and anhydrite or of layered clay and anhydrite along a 35 precut are deformed dry in a triaxial apparatus at a confining pressure of 100 mpa with a shortening rate of about 5 10 4 sec and at room temperature pure clay gouges exhibit only stable sliding and the ultimate frictional strength is very low for bentonite mont morillonite intermediate for chlorite and illite and considerably higher for kaolinite anhydrite gouge shows violent stick slip at 100 mpa confining pressure when this mineral is mixed homogeneously with clays the frictional coefficient of the mixed gouge determined at its ultimate frictional strength decreases monotonically with an increase in the clay content the sliding mode changes from stick slip to stable sliding when the frictional coefficient of the mixed clay anhydrite gouge is lowered down below 90 95 of the coefficient of anhydrite gouge the stabilizing effect of clay in mixed gouge is closely related to the ultimate frictional strength of pure clays that is the effect is conspicuous only for a mineral with low frictional strength only 15 20 of bentonite suppresses the violent stick slip of anhydrite gouge in contrast violent stick slip occurs even if the gouge contains as much as 75 of kaolinite the behavior of illite and chlorite is intermediate between that of kaolinite and bentonite bentonite anhydrite two layer gouge exhibits stable sliding even when the bentonite content is only 5 thus the presence of a thin clay rich layer in a fault zone stabilizes the behavior much more effectively than do the clay minerals mixed homogeneously with the gouge this result brings out the mechanical significance of internal structures of a fault zone in understanding the effects of intrafault materials on the fault motion based on the present experimental results incorporated with some other experimental data it is argued that although the stabilizing effect of montmorillonite and vermiculite is indeed remarkable at room temperature the effect should be much less pronounced at elevated temperatures due perhaps to the dewatering of the clays in most geological environments where shallow earthquakes occur the stabilizing effect of clays is probably not so conspicuous as to completely suppress the unstable motion of a fault

fault plane processes and mesoscopic structure of a strong type seismogenic fault in tonalites adamello batholith southern alps abstract the gole larghe fault is an exhumed paleoseismic fault crosscutting the adamello tonalites italian southern alps ambient conditions of faulting were 9 11 km in depth and 250 300 c in the study area the fault accommodates 1100 m of dextral strike slip over a fault thickness of 550 m displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons faults1 2 3 fault displacement is in the range of few centimeters faults3 to a maximum of a few tens of meters in major faults1 faults1 2 nucleated on pre existing joints whereas faults3 are newly generated fractures produced during slip along faults1 2 each fault within the gole larghe fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production pseudotachylytes are usually generated at the host rock cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins pseudotachylytes and cataclasites have a similar chemical composition which is enriched in loss on ignition k rb ba u and fe3 compared to host rock we envision two models for the evolution of the gole larghe fault in both models synkinematic fluid rock interaction along a fault causes fault hardening by precipitation of abundant k feldspar epidote and minor chlorite in the cataclasite matrix conducive to final production of pseudotachylyte in the first model induration occurs progressively by differential precipitation related to fabric evolution in cataclasites in the second model induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir whatever the model the gole larghe fault represents a strong fault where hardening processes resulted in a low displacement fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement fault thickness ratios

structural geology and kinematic history of rocks formed along low angle normal faults death valley california several late cenozoic low angle normal or detachment faults on the western flank of the black mountains are each characterized by the following in descending structural order 1 a hanging wall of upper tertiary to quaternary sedimentary and volcanic strata that displays little evidence for fault related damage other than widely spaced planar or listric normal faults 2 a sharp planar and locally striated principal slip plane forming the lower boundary of the hanging wall 3 an upper zone zone i of very fine grained fault generated rocks composed dominantly of gouge 4 a lower zone zone ii of coarser grained fault rocks consisting chiefly of foliated breccia and 5 a variably damaged footwall consisting of partly mylonitic precambrian units or tertiary plutonic rocks that has been exhumed from depths of 10 12 km since late miocene time the fault rocks which were mostly derived from the footwalls preserve evidence for cataclastic and particulate flow fault rocks contain authigenic minerals but lack the cyclically deformed mineral filled syntectonic veins that are abundant in some other late cenozoic high angle and strike slip faults meso and microscale fabrics in zones i and ii indicate that finite shear strains increase progressively upward toward the principal slip plane in a conceptual kinematic model of a shear box displacements of the hanging wall produced a shearing flow in the fault rocks below some of the slip on the principal slip plane was also partitioned into localized slip on discrete sliding surfaces in zones i and ii since 770 ka during the latest stages of incremental deformation in the brittle shear zones distributed flow in the fault rocks alternated with slip that was chiefly localized on the principal slip planes in this respect the detachment faults differ from inactive segments of the san andreas system along which displacements were progressively and irreversibly localized onto a single principal slip surface the strain gradients and corresponding changes in grain size in the shear zones resemble those in mylonitic shear zones except in their symmetry strain gradients in the black mountains are notably asymmetric they are present only below the principal slip planes not above

evidence for and implications of self healing pulses of slip in earthquake rupture dislocation time histories of models derived from waveforms of seven earthquakes are discussed in each model dislocation rise times the duration of slip for a given point on the fault are found to be short compared to the overall duration of the earthquake 10 however in many crack like numerical models of dynamic rupture the slip duration at a given point is comparable to the overall duration of the rupture i e slip at a given point continues until information is received that the rupture has stopped propagating alternative explanations for the discrepancy between the short slip durations used to model waveforms and the long slip durations inferred from dynamic crack models are 1 the dislocation models are unable to resolve the relatively slow parts of earthquake slip and have seriously underestimated the dislocations for these earthquakes 2 earthquakes are composed of a sequence of small dimension short duration events that are separated by locked regions barriers 3 rupture occurs in a narrow self healing pulse of slip that travels along the fault surface evidence is discussed that suggests that slip durations are indeed short and that the self healing slip pulse model is the most appropriate explanation a qualitative model is presented that produces self healing slip pulses the key feature of the model is the assumption that friction on the fault surface is inversely related to the local slip velocity the model has the following features high static strength of materials kilobar range low static stress drops in the range of tens of bars and relatively low frictional stress during slip less than several hundreds of bars it is suggested that the reason that the average dislocation scales with fault length is because large amplitude slip pulses are difficult to stop and hence tend to propagate large distances this model may explain why seismicity and ambient stress are low along fault segments that have experienced large earthquakes it also qualitatively explains why the recurrence time for large earthquakes may be irregular

frictional slip of granite at hydrothermal conditions sliding on faults in much of the continental crust likely occurs at hydrothermal conditions i e at elevated temperature and elevated pressure of aqueous pore fluids yet there have been few relevant laboratory studies to measure the strength sliding behavior and friction constitutive properties of faults at hydrothermal conditions we slid laboratory granite faults containing a layer of granite powder simulated gouge velocity stepping experiments were performed at temperatures of 23 to 600 c pore fluid pressures ph2o of 0 dry and 100 mpa wet effective normal stress of 400 mpa and sliding velocities v of 0 01 to 1 m s 0 32 to 32 m yr conditions were similar to those in earlier tests on dry granite to 845 c by lockner et al 1986 the mechanical results define two regimes the first regime includes dry granite up to at least 845 and wet granite below 250 c in this regime the coefficient of friction is high 0 7 to 0 8 and depends only modestly on temperature slip rate and ph2o the second regime includes wet granite above 350 c in this regime friction decreases considerably with increasing temperature temperature weakening and with decreasing slip rate velocity strengthening these regimes correspond well to those identified in sliding tests on ultrafine quartz we infer that one or more fluid assisted deformation mechanisms are activated in the second hydrothermal regime and operate concurrently with cataclastic flow slip in the first cool and or dry regime is characterized by pervasive shearing and particle size reduction slip in the second hot and wet regime is localized primarily onto narrow shear bands adjacent to the gouge rock interfaces weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide or from a dependence of strength on gouge particle size major features of the granite data set can be fit reasonably well by a rate and temperature dependent three regime friction constitutive model chester this issue we extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural slipping faults for sliding rates as low as 31 mm yr we do this for two end member cases in the first case pore pressure is assumed hydrostatic at all depths shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data while at depths below about 9 13 km depending on slip rate strength becomes less sensitive to depth but sensitive to slip rate in the second case pore pressure is assumed to be near lithostatic at depths below 5 km strength is low at all depths in this case 20 mpa in agreement with observations of weak faults such as the san andreas the predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm yr in rough agreement with the seismic aseismic transition depth observed on mature continental faults these results highlight the importance of fluid assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry large displacements higher concentrations of phyllosilicates and time dependent fault healing

a revised sea level curve for the early jurassic a comparison is undertaken of three different methods of eustatic sea level analysis 1 shallowing and deepening transgressive events inferred from the facies sequence 2 changes through time in the areal spread of epicontinental seas 3 seismic stratigraphy the results of these methods are compared mainly with reference to the lower jurassic new data are brought forward from several continents in a stage by stage study and it is concluded that thelias was characterized by interrupted sea level rise with no major regressions both the transgressions and regressions were relatively rapid with the major transgressions associated with widespread anoxic bottom waters estimates of the net amount of sea level rise through early jurassic time range from 75 m to 110 m

the stress heat flow paradox and thermal results from cajon pass conventional friction models predict a substantial thermal anomaly associated with active traces of strike slip faults but no such anomaly is observed from over 100 heat flow determinations along 1 000 km of the san andreas fault the cajon pass well is being drilled to bring deep heat flow and stress data to bear on this paradox preliminary heat flows to 1 7 km in the neighboring arkoma well indicated a positive anomaly of about 30 as would be expected if frictional resistance of the fault were 50 mpa or so but the pliocene and pleistocene history of the transverse ranges particularly high rates of uplift and erosion could also explain the anomaly preliminary stress results from cajon pass and a new interpretation of regional data by mark d zoback and colleagues suggests that the maximum compressive stress near the fault is almost normal to the trace and hence the resolved shear stress is low and the fault weak although this is consistent with long standing evidence from low heat flow vexing questions remain regarding the physics of strike slip faulting the heat flow data show large variability with depth probably from three dimensional structure and an overall decrease from over 90 mw m in the upper kilometer to less than 80 mw m in the lower 300 m with no evidence of advective heat transfer whether this decreasing trend is significant and whether it continues downward to the background value 70 mw m less the radiogenic heat from overlying rocks as expected for a weak fault must be verified with data from greater depth

storm influenced siliciclastic and carbonate ramp deposits the lower ordovician dumugol formation south korea the dumugol formation lower ordovician in the southern part of the baegunsan syncline south korea contains mixed siliciclastic and carbonate ramp deposits the ramp sediments were frequently influenced by storm events resulting in tempestites of sandstone mudstone couplets bioclastic grainstones to packstones flat pebble conglomerates a skeletal lag layer and laminated calcisiltites all tempestites are characterized by an erosive to sharp base poor grading and a transitional upper boundary the difference in lithology of tempestites appears to have been controlled by the nature of substrates and by proximality for example laminated calcisiltites have developed on the shallow carbonate ramp flat pebble conglomerates are closely associated with nodular limestones on shallow and deep ramps and thin skeletal lag layers from fossiliferous argillaceous sediments formed in a basinal setting the stratigraphic succession of the dumugol formation represents an initial transgression followed by a regression the vertical facies change records the transition from a shallow siliciclastic ramp to a deep carbonate ramp to a basin shallowing to a deep carbonate ramp and to a shallow carbonate ramp storm effects are mostly well preserved in shallow to deep ramp deposits

geological and geomorphic evidences of neotectonic activity along the himalayan frontal thrust nahan salient nw himalaya india abstract in this paper we report geomorphic signatures of neotectonic activity identified from the area around nahan salient along himalayan frontal thrust hft using remote sensing and field investigations we document two newly discovered exposures of hft fault zone and active fault traces along the mountain front at khetpurali nahan salient nw himalaya india we demonstrate that hft fault zone at khetpurali is not blind instead it is exposed at the surface along with highly deformed folded strata in the hanging wall and its geometry is quite variable with the possibility of along strike segmentation the geometry of fold and various deformational features preserved in the hanging wall indicate a process of fault propagation fold this is in contrast to the previous observation of fault bent folding within the nahan salient two three levels of river terraces are preserved and truncated in the hanging wall along with the active fault scarps which are linked to uplift along hft fault zone the khetpurali anticline ka is a fault propagating fold formed due to displacement and shortening along hft which dips 30 towards north and strike wnw ese the uplift u along the ka is calculated to be 73 m and accommodated 7 3 shortening of the original length before the fault tip truncated the fold various deformational structures have been identified within the hanging wall around tip damage zone of 80 m perpendicular to hft fault zone and in the core of ka northwest of khetpurali a south facing hogback structure 100 m high is formed due to erosion of forelimb of the anticline at khetpurali tectonic control due to folding of strata and uplift of river terraces is evident as the dudhgarh river forms v shaped profile near the thrust compared to the wide floodplain before and after crossing the hft fault zone along strike segmentation of hft and associated anticlines is observed as the surface trace of hft is offset at khetpurali trilokpur and sabilpur due to the strike slip faults the study reveals that hft has been truncated by tear faults and has different subsurface geometries in study area which may have implication on seismic hazard in the nahan salient where three thrusts himalayan frrontal thrust hft nahan thrust nt and main boundary fault mbf are located at relatively shorter distance of 6 km as compared to the surrounding recesses

mineral precipitation as a mechanism of fault core growth abstract faults vary in structural style from simple planes to complex systems composed of fault cores and damage zones increased fault complexity results from the interaction of mechanical and chemical processes including fracture growth shear and linkage and mineral dissolution and precipitation although water rock interaction is traditionally associated with fault rock weakening and shear localization we investigate processes of fault core widening by water rock interactions that resulted in quartz precipitation we combine field and petrographic observations with prior mechanical characterization to assess the impact of alteration and cementation on fault architecture at the dixie comstock epithermal gold deposit nevada usa mineralized portions of the fault contain strong thick silicified fault cores and wide weak damage zones with evidence for widening of the core through entrainment of damage zone material and repeated cycles of embrittlement dilation and cementation we present a model of fault zone evolution in which the hydrothermal regimes favoring either alteration weakening or precipitation strengthening result in distinct fault zone architecture and mechanical and flow properties of fault systems alteration weakening favors localization of the fault into thinner clay rich low permeability fault cores precipitation strengthening promotes thick strong and low permeability fault cores with mineralization embrittlement enhancing transient permeability following coseismic failure

role of storm in ramgarh and west bokaro coalfields and its implication in adjacent peninsular gondwana coalfields india abstract the permian gondwana sediments in the damodar koel valley rajmahal and deoghar basins were deposited on a northwest palaeoslope in broad valleys within uplands e g aravalli vindhyan chattisgarh chotanagpur and santhal parganas upland the main winds during permian period were mostly the westerly winds coming from the northwest low pressure belt in the tethys which found no barrier between the vindhyan and santhal parganas uplands and then changing its direction moving towards east frequent storm activity also took place from the northwest tethyan low pressure belt and followed the same path of wind possibly there was a barrier in the form of a dense rain forest or elevated land at the place of east bokaro coalfield and its adjoining southern part which acted as a deterrent and minimized the effect of storm towards further east of the damodar valley intense tropical hurricanes for the generation of storm generated bedforms cannot be ruled out which probably in some cases causes marine inundation flooding deep into the landmass and also mass uprooting of trees

spatial variability in microscopic deformation and composition of the punchbowl fault southern california implications for mechanisms fluid rock interaction and fault morphology abstract we examine the distribution and nature of microstructures geochemistry and mineralogy along two traverses across the punchbowl fault southern california to determine the morphology and deformation mechanisms of the fault zone in schistose rocks the punchbowl fault is an exhumed fault that has two main strands of slip localization and has a total of 44 km of right slip protolith in the study area consists of the pelona schist which is primarily a quartz albite muscovite actinolite schist with thin to medium banding and rare metabasalts the traverses are 1 3 km apart and were conducted at a site with a single fault strand and a site where both principal strands of the fault are exposed the fault zone thickness is a function of the type of measurement that is used to define it for the single strand site analysis of the distribution of microfractures shows that the fault zone consists of a roughly 40 m thick damaged zone adjacent to the fault core the damaged zone is marked by an increase in veins thin cataclasite bands inter and intragranular fracturing and alteration relative to the country rock brittle grain size reduction occurs in a zone 10 m thick as measured from the fault core which consists of a continuous 10 cm thick very fine grained cataclasite that experienced repeated alteration vein injection and grain size reduction whole rock geochemical analyses of the fault related rocks suggest that the geochemically defined fault zone is less than 10 m thick volume loss at the site with one fault strand appears to have been small the dominant alteration reactions associated with the fault core are the hydration of hornblende and actinolite accompanied by the alteration of muscovite to produce a quartz chlorite albite epidote ultracataclasite the composition of the fault core is variable and locally influenced by one of the adjacent protoliths the examination of the two fault strand sites shows that two damaged zone fault core structures are present the region between the two strands experienced a greater degree of deformation than the protolith but the total deformation is much less than immediately adjacent to the fault cores the total thickness of the damaged zone around the two strands is less than 200 m the fault core enveloped by a damaged zone morphology as well as the textures of the fault core rocks are similar to rocks associated with the north branch san gabriel fault which formed in crystalline rocks with a total displacement of 22 km fault thickness is less in the pelona schist than in the crystalline rocks perhaps owing to more efficient strain localization in the schists thus the faults of the san andreas system may be thinner in regions where schists or franciscan rocks are the protolith but the main fault core may be a constant feature of the fault zone transformation induced weakening is less important in this part of the punchbowl fault since the protolith has a large amount of mica the structure of the fault zone with two principal slip surfaces is marked by both chemical changes and microstructures and indicates that some parts of the san andreas fault system may consist of multiple slip surfaces each with a damaged zone that together may create a fault zone 100 m thick and in which slip is localized to zones meters to decimeters thick

architecture of fault zones determined from outcrop cores 3 d seismic tomography and geostatistical modeling example from the albala granitic pluton sw iberian variscan massif abstract the 3 d seismic tomographic data are used together with field core and well log structural information to determine the detailed 3 d architecture of fault zones in a granitic massif of volume 500 575 168 m at mina ratones area in the albala granitic pluton to facilitate the integration of the different data geostatistical simulation algorithms are applied to interpolate the relatively sparse structural hard control data conditioned to abundant but indirect 3 d soft seismic tomographic data to effectively integrate geologic and tomographic data 3 d migration of the velocity model from the time domain into the depth domain was essential the resulting 3 d model constitutes an image of the fault zone architecture within the granitic massif that honours hard and soft data and provides an evaluation of the spatial variability of structural heterogeneities based on the computation of 3 d experimental variograms of fracture index fault intensity data this probabilistic quantitative 3 d model of spatially heterogeneous fault zones is suitable for subsequent fluid flow simulations the modeled image of the 3 d fault distribution is consistent with the fault architecture in the mina ratones area which basically consists of two families of subvertical structures with nne ssw and ene wsw trends that displaces the surfaces of low angle faults north fault and follows their seismically detected staircase geometry these brittle structures cut two subvertical dykes 27 and 27 dykes with a nne ssw to n s trend the faults present high fi fi 12 adjacent bands of irregular geometry in detail that intersect in space delimiting rhombohedral blocks of relatively less fractured granite fi

lake mire deposition earthquakes and wildfires along a basin margin fault ronne graben middle jurassic denmark abstract regional uplift of structural highs took place in the greater north sea region in early middle jurassic times resulting in a change from marine shelf deposition in the early jurassic to widespread emergence erosion and localized deposition in grabens the middle jurassic baga formation exposed on the island of bornholm in the baltic sea was deposited in the ronne graben which forms a dog leg pull apart basin linking the northern and southern branches of the tornquist zone the studied succession is situated on the hanging wall immediately adjacent to the main eastern graben margin fault and the footwall consists of deeply weathered granite it is the only locality along the northern branch of the tornquist zone where active tectonic influence on middle jurassic deposition can be demonstrated in outcrop small alluvial fans composed of poorly sorted washout sands and muddy debris flows with weathered granite boulders fringed the footwall scarp syn depositional soft sediment deformation and chaotic bedding structures interpreted as seismites caused by earthquake shocks are characteristic at several levels on the hanging wall further away from the active fault the depositional environment was dominated by shallow freshwater lakes with deposition of clay lake deposition outpaced subsidence and open oxygen deficient freshwater mires were formed during periods of no or only incremental fault movement the mires were only occasionally isolated from clastic input due to doming and dense vegetation resulting in accumulation of pure peat beds the content of spores and pollen shows that the mires were covered by a vegetation of arborescent and herbaceous filicopsida ferns lycopsida club mosses cycadales bennettitales ginkgoales caytoniales taxodiaceae or cupressaceae and araucariaceae wildfires were common in the mires and also on the footwall probably followed by episodes of increased run off and erosion of the upland the presence of common washout sands debrites seismites and abundant lacustrine flooding surfaces indicating sudden drowning of the mires was caused by abrupt movements on the fault with associated earthquakes the succession thus provides an example of temporal changes in deposition in response to changes in tectonic conditions the study illustrates how regional uplift of structural highs caused a restriction of deposition to narrow grabens like the ronne graben facies patterns changed dramatically from laterally extensive uniform packages controlled by sea level changes in the early jurassic to strongly localized deposition with marked facies changes controlled by faulting and earthquakes in the middle jurassic the succession thus serves as an excellent example of tectonically controlled deposition under humid conditions in a continental graben

geochemistry mineralization structure and permeability of a normal fault zone casino mine alligator ridge district north central nevada abstract we examine the geochemical signature and structure of the keno fault zone to test its impact on the flow of ore mineralizing fluids and use the mined exposures to evaluate structures and processes associated with normal fault development the fault is a moderately dipping normal fault zone in siltstone and silty limestone with 55 100 m of dip slip displacement in north central nevada across strike exposures up to 180 m long 65 m of down dip exposure and 350 m of along strike exposure allow us to determine how faults fractures and fluids interact within mixed lithology carbonate dominated sedimentary rocks the fault changes character along strike from a single clay rich slip plane 10 20 mm thick at the northern exposure to numerous hydrocarbon bearing calcite filled nearly vertical slip planes in a zone 15 m wide at the southern exposure the hanging wall and footwall are intensely fractured but fracture densities do not vary markedly with distance from the fault fault slip varies from pure dip slip to nearly pure strike slip which suggests that either slip orientations may vary on faults in single slip events or stress variations over the history of the fault caused slip vector variations whole rock major minor and trace element analyses indicate that au sb and as are in general associated with the fault zone suggesting that au and silica bearing fluids migrated along the fault to replace carbonate in the footwall and adjacent hanging wall rocks subsequent fault slip was associated with barite and calcite and hydrocarbon bearing fluids deposited at the southern end of the fault no correlation exists at the meter or tens of meter scale between mineralization patterns and fracture density we suggest that the fault was a combined conduit barrier system in which the fault provides a critical connection between the fluid sources and fractures that formed before and during faulting during the waning stages of deposit formation the fault behaved as a localized conduit to hydrocarbon bearing calcite veins the results of this study show that fault zone character may change dramatically over short deposit or reservoir scale distances the presence of damage zones may not be well correlated at the fine scale with geochemically defined regions of the fault even though a gross spatial correlation may exist

differences in magnetic properties of fragments and matrix of breccias from the rupture of the 2008 wenchuan earthquake china relationship to faulting abstract rock magnetic and mineralogical analyses were performed on fragments and matrix of six fault breccias from the zhaojiagou outcrop of yingxiu beichuan fault which was the main fault ruptured during the 2008 wenchuan mw 7 9 earthquake at leigu town beichuan county sichuan province china the matrix which is generally enriched in dolomite feldspar and clay minerals but depleted in calcite has much higher low field magnetic susceptibility and magnetization and lower coercivity in comparison with fragments and bulk samples magnetic behavior of the bulk samples and fragments is dominated by dia paramagnetic components in contrast small amounts of partially oxidized magnetite and lepidocrocite are present in the matrix a simple conceptual model associating faulting related effects was proposed to explain these significant differences in magnetic properties of fragments and matrix during coseismic slip intense shear may crush the pre existing magnetic grains in fault rocks into finer ones while stress and frictional heating to a little extent induce magnetic changes and thermochemical alterations of magnetic mineralogy in fault breccias respectively during interseismic periods meteoric fluids would infiltrate and percolate into fault zone and cause dissolution precipitation and recrystallization of fe bearing minerals as the diverse permeability structure and grain size of fragments and matrix these effects would modify their magnetic mineralogy at various levels consequently faulting related effects especially the fluid movements taking place over many previous earthquake cycles would be the most likely reasons for the observed different magnetic properties in fragments and matrix it further proposes that magnetic studies of fault breccias would provide clues to help understand seismic faulting and history of fault activity

petrography diagenesis and pore water evolution of a shallow marine sandstone hasle formation lower jurassic bornholm denmark abstract lower jurassic marine sandstones in the hasle formation on bornholm island can easily be separated in two different petrographic types in the lower part of the type locality type 1 sandstones are friable with layers and lenses of concretionary siderite they are separated by a major erosional surface from the overlying type 2 well cemented sandstones all parts of the sandstones were subjected to chloritization of original grain coatings neomorphism of chlorite and local precipitation of authigenic pyrite during early diagenesis diagenetic pathways differed between the two sandstone types type 1 sandstones were subjected to compaction before kaolinitization and some quartz authigenesis occurred type 2 sandstones immediately above the major erosional surface were cemented by concretionary siderite at a very early stage and thus further diagenetic modifications were prevented above this zone sandstones are tightly cemented by various carbonate minerals diagenetic modifications mainly involved alternating events of carbonate precipitation and dissolution the sequence of carbonate precipitation was 1 zoned stable metastable siderite 2 siderite gradually with significant ca and mg substitution 3 ankerite and 4 calcite reflecting a sequence of precipitating carbonates containing progressively lower amounts of iron a period of kaolinitization predated calcite cementation six diagenetic stages were recognized 1 early burial reducing conditions resulting in precipitation of pyrite 2 precipitation of chlorite and siderite caused from compactional flow from underlying brackish and fresh water sediments 3 dissolution of carbonate and partly oxidation of siderite caused by fresh water flushing related to uplift 4 precipitation of siderite with gradually increasing substitution by caco3 and mgco3 and eventually precipitation of ankerite caused by renewed burial 5 fresh water diagenesis with precipitation of kaolinite and quartz 6 precipitation of calcite in remnant porosity

san andreas fault zone head waves near parkfield california microearthquake seismograms from the borehole seismic network on the san andreas fault near parkfield california provide three lines of evidence that first p arrivals are head waves refracted along the cross fault material contrast first the travel time difference between these arrivals and secondary phases identified as direct p waves scales linearly with the source receiver distance second these arrivals have the emergent wave character associated in theory and practice with refracted head waves instead of the sharp first breaks associated with direct p arrivals third the first motion polarities of the emergent arrivals are reversed from those of the direct p waves as predicted by the theory of fault zone head waves for slip on the san andreas fault the presence of fault zone head waves in local seismic network data may help account for scatter in earthquake locations and source mechanisms the fault zone head waves indicate that the velocity contrast across the san andreas fault near parkfield is approximately 4 percent further studies of these waves may provide a way of assessing changes in the physical state of the fault system

fabric induced weakness of tectonic faults 1 mature fault zones appear to be weaker than predicted by both theory and experiment one explanation involves the presence of weak minerals such as talc however talc is only a minor constituent of most fault zones and thus the question arises what proportion of a weak mineral is needed to satisfy weak fault models existing studies of fault gouges indicate that 30 of the weak phase is necessary to weaken faults a proportion not supported by observations here we demonstrate that weakening of fault gouges can be accomplished by as little as 4 wt talc provided the talc forms a critically aligned through going layer observations of foliated fault rocks in mature large offset faults suggest they are produced as a consequence of ongoing fault displacement and thus our observations may provide a common explanation for weakness of mature faults

washover fan and brackish bay sedimentation in the berriasian valanginian of bornholm denmark abstract the robbedale and jydegard formations berriasian valanginian of the danish island of bornholm represent a 100 m thick vertical sequence from shoreface foreshore and beach sands of a high energy coast through backbarrier flat bay margin pond and distal washover fan sand and clay brackish bay clay to fluvial sands the longevity of the backbarrier bay system c 10 myr thickness 100 m of the bay deposits and apparently stacked nature of the facies belts suggest a relatively stationary position of the individual subenvironments with only minor progradation this reflects strong tectonic control of the depositional system during an important phase of synsedimentary block faulting and wrenching along the tornquist zone the importance of washover fan sands in the backbarrier deposits and the lack of tidal indications in the whole sequence suggest a microtidal regime a system of migrating mud banks formed in shallow water on the landward side of the barrier the bay waters varied from almost fresh to brackish and anoxic conditions commonly occurred at the bottom adverse living conditions for most organisms in the bay caused seasonal possibly toxic dinoflagellate blooms resulting in mass mortality of infaunal bivalves bay margin ponds underwent periodic desiccation leading to mass mortality of freshwater gastropods as a general scenario it is envisaged that longshore currents redistributed bedload from a major delta and formed an extensive nw se barrier spit which partly enclosed a major bay to the ne the barrier was breached during heavy storms and the sand transported along the resulting washover channels was deposited on the backbarrier flat made up of the subaerial parts of coalescent washover fans enormous amounts of suspension load from the delta travelled further along the barrier to be deposited in the lee side bay

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

rock anelasticity measurements at high pressure low strain amplitude and seismic frequency a new apparatus has been constructed which will ultimately facilitate the study of rock anelasticity under conditions which closely approach those of teleseismic wave propagation simultaneous high pressure and temperature low frequency and strain amplitude and controlled pore pressure of volatiles its performance has been tested in a series of preliminary experiments conducted at room temperature measurements on a steel standard demonstrate the sensitivity of the apparatus to very small departures from ideal elasticity q 1000 experimental data for a fine grained granitic rock show that both the shear modulus g and quality factor q increase sharply with increasing pressure below 100 mpa beyond which pressure both parameters become markedly less pressure sensitive these observations are in accord with those of previous studies at higher frequencies and larger strains and are consistent with the view that the anelasticity of rocks at ambient pressure is dominated by mechanisms operative at open cracks and grain boundaries

controls on peat accumulation and depositional environments of a coal bearing coastal plain succession of a pull apart basin a petrographic geochemical and sedimentological study lower jurassic denmark abstract the paralic lower middle jurassic baga formation of the island of bornholm denmark was deposited in a fault bounded subsiding pull apart basin the formation is up to 400 m thick and contains more than 50 coal seams twelve of these have been investigated petrographically and geochemically to provide basic information on the composition of the relatively unknown jurassic coals the peat forming environments represented by the seams and the associated siliciclastic sediments are interpreted the seams represent three types of environments with organic matter deposition peat accumulation occurred in low lying areas situated between river channels in a coastal plain environment undergoing overall transgression the coals have a relatively uniform huminite rich petrographic composition indicating that the precursor mires were dominated by persistent water saturated and anoxic conditions the swamps were probably occupied by a small statured flora with cellulose rich tissues significant bacterial activity in the peat swamps is suggested by an abundance of hopanes influence from marine water was not common but occurred occasionally during peat accumulation the depositional conditions were stable and quiet the small thicknesses of the seams 8 57 cm thick indicate relatively short periods of peat formation average c 2300 yr due to continued base level rise controlled by subsidence and an overall eustatic rise causing repeated changes in the sedimentary regimes the coal seams are of low rank and were buried to a depth of 1100 1200 m before uplift due to late cretaceous early tertiary basin inversion and neogene uplift

high resolution sequence stratigraphy of a hettangian sinemurian paralic succession bornholm denmark sequence stratigraphic interpretation of paralic successions is complicated by the complex interfingering of marine and continental strata the successions may also include terrestrial extensions of marine parasequences and completely independent lacustrine parasequence analogues failure in recognizing the possible interbeddding of these two independent parasequence types may lead to construction of sequence stratigraphic schemes based on incompatible data sets we have studied a lower jurassic paralic section from the baltic island of bornholm situated in the tornquist zone which demarcates the transition from the stable precambrian baltic shield to the subsiding danish basin and danish polish trough the hettangian sinemurian sose bugt member ronne formation of bornholm includes lacustrine fluvial and restricted marine estuarine deposits reflecting the basin margin position biostatigraphic resolution is poor and a sequence stratigraphic interpretation of the paralic succession is far from straightforward a multidisciplinary approach including facies analysis recognition and lateral trading of key surfaces palynostratigraphy palynofacies coal petrography palaeopedology clay mineralogy and source rock geochemistry is applied in order to obtain a high degree of precision in the interpretation of the paralic facies in this way four sequences are recognized in the overall backstepping lacustrine to estuarine succession marine and marginal marine parasequences are distinguished from their purely lacustrine analogues and an internally consistent sequence stratigraphic scheme is proposed this is compared and tentatively correlated with fossiliferous marine sediments in the danish basin and with published eustatic cycle charts

dinosaur tracks and possible lungfish aestivation burrows in a shallow coastal lake lowermost cretaceous bornholm denmark abstract the mesozoic succession of the island of bornholm in the baltic sea has hitherto yielded only very few remains after vertebrate life we describe a new find of a well exposed vertebrate trample ground and large densely spaced vertical to inclined burrows from shallow lake deposits belonging to the berriasian skyttegard member of the rabekke formation the succession is terrestrial but a coastal setting is indicated by the great abundance of pyrite concretions and framboidal pyrite organic geochemical and palynological data show that the lake was surrounded by a low diversity gymnospermous vegetation the trampling ground is exposed in a vertical section at the boundary between light grey structureless or trough cross bedded fluvial sand and dark brown organic rich lacustrine mud with densely spaced vertical rootlets wood fragments and large pyrite concretions the tracks are bowl shaped have an irregular flat or slightly conical base are on average 22 cm deep and on average 43 cm wide in the upper part and 23 cm in the lower part but some reach widths up to about 1 m and have a highly irregular shape the underlying sand in some cases shows a faint stratification that is bent downwards adjacent to the mud filled tracks which have near vertical and commonly overhanging margins the morphology of the bowl shaped depressions suggests that they were made by dinosaurs most likely sauropods they were clearly emplaced after deposition of the dark brown mud long vertical to inclined commonly slightly curved j shaped or spiraling cylindrical burrows with a diameter averaging 4 4 cm and lengths up to 45 cm occur densely spaced in the mud they are unbranched have no mud or lag linings the top of the burrows is indistinct or not preserved whereas the base is rounded concave up commonly with a slightly enlarged chamber the length is difficult to estimate precisely because of the missing top and because the burrows cross cut and go in and out of the plane of exposure the burrows have a passive infill of dark mud and at some levels of irregularly alternating laminae of white sand and mud the sand may be of aeolian origin or may represent distal crevasse splays or washovers from adjacent rivers or the nearby sea skeletal remains have not been found in or associated with the burrows which are similar to burrows referred to lungfish or crayfish and they are with hesitation interpreted as representing lungfish aestivation burrows based on the morphological characteristics this is only the second report of dinosaur tracks in denmark and the first in situ find where environmental conditions can be interpreted with some confidence possible lungfish burrows are here reported for the first time from the danish mesozoic the overall rarity of vertebrate remains in the mesozoic succession of bornholm may reflect that the area was a horst island or submerged shoal during much of the jurassic and cretaceous and may not always have had direct land connection with surrounding land areas of the baltic shield or mainland europe

fault zone waves observed at the southern joshua tree earthquake rupture zone waveform and spectral characteristics of several aftershocks of the m 6 1 22 april 1992 joshua tree earthquake recorded at stations just north of the indio hills in the coachella valley can be interpreted in terms of waves propagating within narrow low velocity high attenuation vertical zones evidence for our interpretation consists of 1 emergent p arrivals prior to and opposite in polarity to the impulsive direct phase these arrivals can be modeled as headwaves indicative of a transfault velocity contrast 2 spectral peaks in the s wave train that can be interpreted as internally reflected low velocity fault zone wave energy and 3 spatial selectivity of event station pairs at which these data are observed suggesting a long narrow geologic structure the observed waveforms are modeled using the analytical solution of ben zion and aki 1990 for a plane parallel layered fault zone structure synthetic waveform fits to the observed data indicate the presence of ns trending vertical fault zone layers characterized by a thickness of 50 to 100 m a velocity decrease of 10 to 15 relative to the surrounding rock and a p wave quality factor in the range 25 to 50

strong seismic shaking of randomly prestressed brittle rocks rock damage and nonlinear attenuation strong seismic waves produce frequent episodes of transient oscillating dynamic stress in the shallow subsurface within seismically active areas dynamic stress drives slip on a network of preexisting small fracture planes and leaves residual stresses once shaking ceases the residual stresses act as prestresses during the next strong earthquake local failure occurs on fractures at low dynamic stresses where the prestresses are favorably aligned while most of the bulk rock remains intact hence nonlinear attenuation of strong seismic waves commences at dynamic stresses well below those for pervasive coulomb failure consideration of the fractal distribution of prestresses indicates that nonlinear attenuation increases rapidly with dynamic stress vertically propagating shear waves provide simple scaling relationships for quick application 1 nonlinear attenuation is concentrated near the quarter wavelength depth 2 the coulomb stress ratio dynamic lithostatic stress above the quarter wavelength depth scales with the sustained dynamic acceleration in g s rate and state friction thus provides estimates of the maximum sustained acceleration from coulomb stress ratio at failure 1 for regolith that has repeatedly failed 1 5 for shallow intact sandstone and 2 for shallow intact granite a potentially observable effect occurs with shallow brittle regolith on hillslopes where gravity causes net downslope creep during strong shaking analogous crack network behavior with prestress occurs in the damage zone surrounding the rupture zone of major faults pulverized rock forms at shallow depths when rock is repeatedly deformed in strong shaking to just beyond the elastic limit but strain does not localize into a crack network

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

a structural interpretation of the aftershock cloud of the 1992 mw 7 3 landers earthquake we analyze the spatial relationship of relocated aftershocks to the principal rupture planes of the m w 7 3 1992 landers mainshock from a structural point of view we find that the aftershocks constitute primarily a several kilometer wide damage zone centered on the mainshock rupture planes the intensity of damage decreases away from the principal faults less than half of the aftershocks occurred within 1 km of the mainshock planes and perhaps only 5 of the aftershocks are candidates for rerupture of the mainshock faults moreover it seems that aftershocks along the landers rupture have b values that correlate well with the complexity of the mainshock rupture low b values occur along segments that are simple whereas higher b values correlate with sections that are more complex thus structural complexity appears to correlate with a greater relative abundance of small earthquakes these observations imply that aftershock populations reflect fault populations in the medium surrounding the principal faults rather than the behavior of the mainshock planes themselves online material arcview information about the surface rupture of the landers mainshock

giant quartz vein formation and high elevation meteoric fluid infiltration into the south armorican shear zone geological fluid inclusion and stable isotope evidence giant quartz veins associated with the south armorican shear zone record important fluid circulation during the hercynian period regional scale mapping of veins allows two groups of veins to be identified on the basis of their geometric relationship with the south armorican shear zone veins in the first group are parallel to the shear zone whereas those in the second group developed in a direction oblique to it the former probably record infiltration of fluids along permeable pathways in highly deformed zones the latter may represent crustal scale tension gashes in the regional context most quartz veins have 18 o values between 10 and 16 indicating a mid crustal origin for the fluids microthermometry on fluid inclusions from euhedral quartz indicates that late fluids were mostly aqueous with very low salinity 0 1 7 wt eq and with homogenization temperatures ranging between 150 and 270 c together with very low 18 o values of some euhedral quartz down to 2 these features argue for a surface origin corresponding 18 o fluid values estimated near 11 are probably related to the high palaeo elevation of meteoric precipitation scarce but significant h 2 o co 2 fluid inclusions in euhedral quartz indicate a metamorphic contribution these were probably sourced from the exhumed metamorphic basement in the southern part of the region

sedimentary structures sorby and the last decade henry clifton sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures together with their use as palaeocurrent indicators research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant particularly as regards the physical explanation of structures and to present significant challenges and opportunities perhaps the most pressing needs are for a better understanding of 1 bedforms in gravels silts and carbonate sediments 2 tidal and especially sand wave bedding 3 hummocky and swaley cross stratification and 4 soft sediment and dewatering structures in turbidites many sedimentary structures present a little exploited opportunity to quantify process rates and define short time periods from the rock record

scaling and geometric properties of extensional fracture systems in the proterozoic basement of yemen tectonic interpretation and fluid flow implications abstract multi scale mappings of fracture systems in the crystalline basement of yemen are presented fracture datasets are described through statistical analyses of direction length spacing density and spatial distribution results are combined with field observations and can be directly used to model the geometry of the fracture networks in analog basement rocks from multi kilometric to decametric scales the fractured reservoir analog is defined with a dual porosity model in which tectonic and joint systems correspond to the basement reservoir backbone and matrix respectively these two end members reveal contrasting geometrical reservoir and scaling properties in tectonic systems multi scale geometries are self similar the fracture network shows fractal behavior power law length distribution and clustered spacing and fault zones show hierarchical organization of geometrical parameters such as length thickness and spacing in joint systems the fracture network is scale dependent with exponential length distribution and shows anti clustered spacing however these two end members have both well connected properties with fault zones acting as main drain and joint systems acting as the fluid supply

implications for mechanical properties of brittle faults from observations of the punchbowl fault zone california field observations of the punchbowl fault zone an inactive trace of the san andreas are integrated with results from experimental deformation of naturally deformed punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting the punchbowl fault zone consists of a single continuous gouge layer bounded by zones of extensively damaged host rock fault displacements were not only localized to the gouge layer but also to discrete shear surfaces within the gouge deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms textural data also suggest that significant amounts of pore fluids were present during faulting and that fluid assisted mechanisms such as dissolution diffusion and precipitation were operative the experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone the gouge layer has fairly uniform mechanical properites and it has significantly lower strength elastic modulus and permeability than both the damaged and the undeformed host rock for the punchbowl fault and possibly other brittle faults the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock in addition the damaged zone acts as the permeable unit of the fault zone and surrounding rock it appears that the gouge primarily governs whether displacements are localized and it therefore may have a significant influence on the mode of slip

determination of volume loss and element mobility patterns associated with the development of the copper basin fault picacho state recreation area se california usa abstract the fault zone architectural elements of the copper basin fault include a fault core inner damage zone and outer damage zone utilizing yb as a reference frame element significant elemental mass changes were documented across all three architectural zones with the greatest change in the fault core porosity and volumetric strain both increase toward the principal slip surface reaching a maximum in the inner damage zone and then dropping significantly in the fault core in the damage zone and unaffected wall rocks smectite is the most prominent clay species whereas illite dominates the fault core suggesting that temperatures may have reached 100 150 c or more during the production of the fault core a number of studies have interpreted similar mass changes and clay mineral reactions to indicate high water rock ratios such interpretations imply that during rupture permeability increased within the fault core allowing it to act as a pathway for hot chemically reactive fluids then during and after rupture co 2 and sio 2 rich fluids moved into the inner and outer damage zones where calcite and quartz precipitation healed the fractured and permeable rock

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

fault deformation mechanisms and fault rocks in micritic limestones examples from corinth rift normal faults abstract a multidisciplinary study investigates the influence of different parameters on fault rock architecture development along normal faults affecting non porous carbonates of the corinth rift southern margin here some fault systems cut the same carbonate unit pindus and the gradual and fast uplift since the initiation of the rift led to the exhumation of deep parts of the older faults this exceptional context allows superficial active fault zones and old exhumed fault zones to be compared our approach includes field studies micro structural optical microscope and cathodoluminescence geochemical analyses 13 c 18 o trace elements and fluid inclusions microthermometry of calcite sin kinematic cements our main results in a depth window ranging from 0 m to about 2500 m are i all cements precipitated from meteoric fluids in a close or open circulation system depending on depth ii depth in terms of p t condition determines the development of some structures and their sealing iii lithology marly levels influences the type of structures and its cohesive non cohesive nature iv early distributed rather than final total displacement along the main fault plane is the responsible for the fault zone architecture v petrophysical properties of each fault zone depend on the variable combination of these factors

ductile creep compaction and rate and state dependent friction within major fault zones the shear traction on major strike slip faults during earthquakes is much lower than that expected on a frictionally sliding surface in equilibrium with hydrostatic pressure the low shear traction is explained if the fluid pressure at the time of the earthquake is much greater than hydrostatic pressure ductile creep within mostly sealed fault zones compacts the matrix and thus increases fluid pressure between earthquakes frictional dilatancy during earthquakes decreases fluid pressure below hydrostatic and over the earthquake cycle the fault zone is in long term equilibrium with the country rock this ductile mechanism is formally unified with rate and state theory for time dependent friction when the difference between a critical porosity where the rock loses all strength and the actual porosity of cracks is used as a state variable this choice is justified by percolation theory of mostly broken lattices time dependent behavior associated with changes in normal traction in the laboratory is explained by the formalism instability earthquakes sometimes occurs in the numerical experiments however fairly small amounts of frictional dilatancy during initial frictional creep decrease fluid pressure and preclude unstable sliding two coupled mechanisms for producing dilatancy on faults once an instability is well underway are evident 1 expansion of pore fluids associated with frictional heating increases fluid pressure offsetting the effects of increased pore volume during earthquakes there is some tendency for pore volume increase to balance fluid expansion so that fluid pressure stays relatively constant 2 production of isolated voids that do not immediately decrease fluid pressure throughout the fault zone during earthquakes can occur to the extent that the fault zone is not significantly strengthened although the extent of both processes is constrained by energy considerations the variation of fluid pressure during earthquakes is not yet well enough understood to predict stress drop from observable material properties

a statistical framework for calculating and assessing compositional linear trends within fault zones a case study of the ne block of the clark segment san jacinto fault california usa utilizing chemical data derived from the various fault zone architectural components of the clark strand of the san jacinto fault southern california usa we apply for the first time non central principal component analysis to calculate a compositional linear trend within molar a cn k space in this procedure a cn k are calculated as the molar proportions of al2o3 a cao na2o cn and k2o k in the sum of molar al2o3 na2o cao and k2o cao is the molar cao after correction for apatite we then derive translational invariant chemical alteration intensity factors t for each architectural component through orthogonal projection of analyzed samples onto the compositional linear trend the chemical alteration intensity factor t determines the relative change in composition compared to the original state i e the composition of the altered wall rocks it is dependent on the degree of intensity to which the process or processes responsible for the change in composition of each architectural component has been active these processes include shearing fragmentation fluid flow and generation of frictional heat non central principal component analysis indicates that principal component 1 explains 99 7 of the spread of a cn k data about the calculated compositional linear trend i e the variance the significance level for the overall one way analysis of variance anova is 0 0001 such a result indicates that at least one significant difference across the group of means of t values is different at the 95 confidence level following completion of the overall one way anova the difference in means t test indicated that the mean of the t values for the fault core are different than the means obtained from the transition and damage zones in contrast at the 95 confidence level the means of the t values for the transition and damage zones are not statistically distinguishable the results of xrd work completed during this study revealed that the 2 m fraction is composed primarily of illite smectite with 15 illite in the damage zone of illite smectite with 30 illite in the transition zone and of discreet illite with very minor smectite in the fault core these changes parallel the increasing values of the chemical alteration intensity factors i e t based on the above results it is speculated that when fault zones are derived from tonalitic wall rocks at depths of 400 100 m the onset of the illite smectite to illite conversion will occur when t values exceed 0 20 0 12 the average chemical alteration intensity factor calculated for the transition zone under such conditions during repeated rupturing events frictional heat is produced and acidic fluids with elevated temperatures 125 c are flushed through the fault core over time the combination of shearing fragmentation and frictionally elevated temperatures eventually overcomes the kinetic barrier for the illite smectite to illite transition such settings and processes are unique to fault zones and as a result they represent an underappreciated setting for the development of illite from illite smectite the success of non central principal component analysis in this environment offers the first statistically rigorous methodology for establishing the existence of compositional linear trends in fault zones this method also derives quantifiable alteration intensity factors that could potentially be used to compare the intensity of alteration at different segments of a fault as well as offer a foundation to interpret the potential driving forces for said alteration and differences therein

facies architecture and depositional processes of the holocene recent accretionary forced regressive skagen spit system denmark spit systems are seldom recognized in the pre quaternary sedimentary record compared to their common occurrence along present day coasts and in quaternary successions this lack of recognition may partly be due to the lack of widely accepted depositional models describing the facies characteristics of spit systems and their subaqueous platforms in particular the skagen spit system is a large active system that began to form 7150 yr bp and from 5500 bp to recent times it has prograded 4 m year 1 and accumulated 3 5 109 m3 of sand the spit system provides a unique opportunity for establishing a well constrained depositional model because uplift and erosion have made large windows into the preserved facies while active spit forming processes can be examined at the young prograding end of the same system the depositional model presented here thus builds on excellent outcrops surface morphology a well defined palaeogeography and detailed c14 age control supplemented with observations from continuous well cores and profiles obtained by ground penetrating radar and transient electromagnetic surveys the factors that have governed the development of the spit system such as relative sea level change wave and current climate tidal range sediment transport and depositional rates are also well understood the sedimentary facies of the spit system are grouped into four principal units consisting from below of thick storm sand beds dune and bar trough deposits beach deposits and peat beds these four units form a coarsening and shallowing upward sand dominated succession up to 32 m thick which overlies offshore silt with a transition zone and is topped by a diastem overlain by young aeolian dune sand the sedimentary structures and depositional processes are described in detail and integrated into a depositional model which is compared to other spit systems and linear shoreface models

a rheologic model for wet crust applied to strike slip faults the strength and stability of crustal faults are not adequately addressed by the widely used two mechanism rheologic models of the crust based on byerlee s law and power law creep the models neglect the fluid assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments state variable constitutive relations are used in a multiple mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures this multimechanism description of friction is substituted for byerlee s law in the two mechanism models to generate a multimechanism rheologic model for the crust the multimechanism model is used to predict the rate and temperature dependence of crustal strength and the slip rate and depth temperature conditions over which each mechanism dominates application of the model to strike slip faults in the crust illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability the model predicts that frictional strength at midcrustal depths is significantly reduced relative to byerlee s law only for thick fault zones the reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer the rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust