structural geometry of meade thrust plate in northern blackfoot mountains southeastern idaho one of the principal generalizations concerning structural geology in foreland thrust belts is that thrust faults climb to the surface by running parallel with incompetent units and cutting upsection ramping across competent members thus forming folds in the hanging wall but not in the footwall detailed geologic mapping at the front of the meade plate in the northern blackfoot mountains southeastern idaho shows that this rule can be applied only to the initial phase of deformation the initial forms of folds created over ramps in the basal decollement have strongly influenced the geometry of subsequent imbricate thrusts within the meade plate so that later faults were locally required to cut downsection in the direction of translation the allochthon has been divided into three subsidiary thrust plates by two major imbricate thrusts folds within thick bedded upper paleozoic strata of the meade plate have sharp hinges and planar limbs with curved axial surfaces that progressively steepen to the west these folds are of a modified box or kink form folds in less competent mesozoic rocks below the meade thrust in the parautochthon are more open and concentric fold geometry was apparently determined by thickness of overburden during folding proximity to ramps in the thrust surface and mechanical properties of the strata involved major thrust imbrications dip gently westward locally cutting downsection toward the east direction of transport where they cross more steeply west dipping upright fold limbs this geometry implies that folding preceded imbricate faulting and that anisotropy of the sequence no longer controlled stratigraphic levels of thrusting after bedding became moderately inclined to the west generally greater than 35 depending on competence the inferred ramp associated genesis of folds within the meade allochthon in the blackfoot mountains implies that these folds are rootless although rootless hanging wall folds are productive farther east in the idaho wyoming thrust belt e g above the absaroka thrust in southwestern wyoming those described here in the blackfoot mountains as a result of erosional level and shallow dip of the meade thrust do not provide significant structural traps folds of the underlying parautochthon upper plate absaroka st johns thrust are more promising

thrust sheet rotation and out of plane strains associated with oblique ramps an example from the wyoming salient u s a abstract movement over oblique thrust ramps results in local thrust sheet rotation and principal strains which deviate from the transport plane resulting folds faults and deformation fabrics are superposed on those associated with displacement on frontal ramps the south fork thrust a hangingwall imbricate of the absaroka thrust sheet sw wyoming has a steeply dipping 70 small displacement 300 m oblique ramp in the hangingwall and footwall which makes an angle of 25 to the regional transport direction two penetrative deformation events are recorded by folding stylolitic cleavage and calcite twinning strains in the deformed hangingwall and footwall the first is e to se directed layer parallel shortening prior to or during motion of the absaroka thrust sheet the second event indicates ne directed shortening unique to the oblique ramp at a high angle to ramp strike at the trailing frontal ramp oblique ramp intersection folds in the footwall and normal faults in the hangingwall reflect local strike parallel shortening and extension due to out of plane deflection of the wallrock during displacement the out of plane strains arc consistent with those predicted by kinematic and mechanical models in the hangingwall of the oblique ramp cross strike fractures and bedding are rotated 30 counterclockwise about a vertical axis with respect to structural domains to the north and south due to differential displacement on the south fork thrust stylolitic cleavage and caleite strains unique to the oblique ramp show a similar change in attitude indicating the rotation was the latest recorded stage of deformation in general if an oblique ramp is shallowly dipping the hangingwall may move from the lower flat up the ramp and onto the upper hat if an oblique ramp has a steep dip as in the case of the south fork thrust relatively greater sliding and bending resistance may impede displacement up the ramp and upper flat further transport of the thrust sheet is then accommodated by footwall deformation and hangingwall rotation

analysis of a complex high strain zone at cap de creus spain abstract the structural analysis of a high strain zone developed in medium to high grade metamorphic micaschists from the cap de creus area spain provides an example of the complex relationships between geometry strain and kinematics to be found in deep crustal shear zones this high strain zone is composed of e w trending structural domains characterized by different strain intensities and associated with syntectonic emplacement of pegmatite dykes the main phase of deformation discussed here d2 affects steeply dipping bedding boudinaged quartz veins and s1 developed parallel to bedding d2 deformation of these features and of syn d2 pegmatite dykes gives rise to fold boudin structures in map view a d2 high strain zone coincides with a km scale dextral flexure of s0 s1 and s2 although all small scale structures on outcrop surfaces close to horizontal indicate a prevalent sinistral shear sense in addition a subvertical stretching lineation is present approximately parallel to the axis of the foliation deflection and thus normal to the apparent displacement direction in the high strain zone it is proposed that this high strain zone did not form by horizontal dextral simple shear with a flow plane parallel to the boundaries of high and low strain zones but acted as a zone of highly vortical sinistral non coaxial flow with a strong vertical extension component the dextral flexure of s0 s1 and s2 can be explained by considering that sinistral non coaxial flow in the low strain zones was either slower or earlier that in the high strain core this example stresses the necessity of being careful when interpreting displacement directions from the deflection of older fabric elements in deep seated high strain zones

microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels the cataclasite pseudotachylyte connection in pelitic schists of the nason terrane of washington state pseudotachylyte can be demonstrated to have developed subsequent to the formation of foliation parallel cataclasite zones numerous microstructural lines of evidence point to a genetic link between pseudotachylyte and cataclasite and chemical evidence indicates that cataclasis played an important role in controlling pseudotachylyte chemistry besides optical and textural differences between the cataclasite and pseudotachylyte which may occur together there is unambiguous evidence of a melt origin for the pseudotachylyte while such evidence within the cataclasite is absent tem evidence indicates that glass is present in at least some of the veins as demonstrated by a lack of electron diffraction microlites present in the glass lack dislocations high temperature microlites of a mineralogy and composition not present in the host schist occur in various habits which depend on the distance from the vein margin also present are assimilation textures apparently quenched sulfide droplets and vesicles and amygdules in a pseudotachylyte vein studied in detail five microstructural domains occur these include vein margin cataclasite a transitional dark pseudotachylyte vein interior glass opaque and sulfide rich blebs and translucent haloes surrounding some of the blebs the cryptocrystalline cataclasite matrix grades into the dark pseudotachylyte which optically grades into the glass after adjustment for silica the host schist and cataclasite matrix are chemically very similar the dark pseudotachylyte and glass with the exception of na2o and cao are chemically quite similar to the cataclasite matrix the main difference may be the result of the segregation of the opaque plagioclase blebs thus the pseudotachylyte chemistry is strongly dependent upon the chemistry of the material comprising the cataclasite matrix which in turn is dependent on mineral susceptibility to incorporation into the cataclasite matrix fluids infiltrated along the cataclasite zones prior to pseudotachylyte generation as recorded by hydration reactions sulfide veining na2o loss and fluid inclusions this demonstrates that pseudotachylytes can form under fluid rich conditions as opposed to the commonly assumed dry conditions

mechanical stratigraphy and fault fold interaction absaroka thrust sheet salt river range wyoming abstract a distinct contrast in deformation style from tightly folded and faulted cambrian through ordovician strata to broadly folded and faulted mississippian and younger strata is displayed in the hanging wall of the absaroka thrust salt river range wyoming based on style the paleozoic section can be divided into two main structural lithic units each unit is characterized by a relatively weak ductile anisotropic lower section and a relatively strong brittle more isotropic upper section where the two units were stacked within the thrust sheet inverted fault propagation folds formed in the center of each unit and the overall transition upward from close to wide spaced folds and imbricate faults developed in the multilayer where the upper unit was isolated deformation was dominated by imbricate faulting with little associated folding and inverted fault propagation folds did not form observations illustrate that the mechanical interaction between the two units and boundary conditions imposed on them were significant to defining the deformation response it is note worthy that the boundary between the two structural lithic unts in part defined by the roof thrust of the stewart passive roof duplex occurs within a dolostone rather than in overlying shale of the darby formation i propose that the roof thrust and other detachments in dolostone reflect the relatively low frictional strength of dolostone under shallow crustal conditions cross cutting relations suggest that the absaroka sheet was cut by large imbricate faults during late stage movement over a major dip ramp a high deviatoric and low mean compressive stress state just beyond the upper ramp flat corner promoted break back imbrication and allowed tectonic thickening of the thrust wedge necessary to reestablish critical taper and for forward translation

brittle ductile brittle deformation during cooling of tonalite adamello southern italian alps abstract late to post magmatic deformation in slightly diachronous contiguous intrusions of the north western adamello batholith southern alps italy is recorded as from oldest to youngest i joints ii solid state ductile shear zones iii faults associated with epidote k feldspar veins and iv zeolite veins and faults structures ii to iv are localized on the pervasive precursory network of joints i which developed during the earliest stages of pluton cooling high temperature 500 c ductile overprinting of joints produced lineations defined by aligned biotite and hornblende on the joint surfaces and highly localized mylonites the main phase of faulting producing cataclasites and pseudotachylytes occurred at 250 c and was associated with extensive fluid infiltration cataclasites and pseudotachylytes are clustered along different e w striking dextral strike slip fault zones correlated with the activity of the tonale fault a major tectonic structure that bounds the adamello batholith to the north ductile deformation and cataclastic veining episodes occurred at p 0 25 0 3 gpa during rapid cooling of the batholith to the ambient temperatures 250 c that preceded the exhumation of the batholith timing of the sequence of deformation can be constrained by 39 ar 40 ar ages of 30 ma on pseudotachylytes and various existing mineral ages in the whole composite adamello batholith multiple magma pulses were intruded over the time span 42 30 ma and each intrusive body shows the same ductile to brittle structural sequence localized on the early joint sets this deformation sequence of the adamello might be typical of intrusions undergoing cooling at depths close to the brittle ductile transition

eocene seismogenic reactivation of a jurassic ductile shear zone at cap de creus pyrenees ne spain abstract the cap de creus peninsula in ne spain exposes variscan amphibolite facies rocks transected by greenschist facies mylonitic shear zones accommodating displacements of up to 1 km one of these shear zones contains pseudotachylytes with 1 4 mm thick fault veins and cm scale high angle injection veins and breccias the pseudotachylyte matrix encloses mm to m scale wallrock fragments and consists of ultrafine biotite and feldspar often with a microporphyritic structure suggesting crystallization from a melt recent dating of the host shear zone yielded a jurassic 170 160 ma age here we present 40ar 39ar dating of the pseudotachylytes yielding early eocene ages between 52 76 1 64 ma and 44 55 0 77 ma indicating alpine brittle faulting at times that jurassic low angle ductile shears had been tilted to their present vertical position following alpine thrusting in the pyrenees the kinematics of the pseudotachylytes suggest strike slip reactivation along the host shear zone consistent with plate kinematic analyses indicating coeval iberia europe motion dominated by strike slip the dimensions of the pseudotachylyte bearing zone and estimated brittle displacements suggest a moment magnitude of 4 7 4 9 the average thicknesses of the fault veins indicate deformation at 4 km depth an energy density of 9 106 j m2 and a frictional resistance of 23 mpa

deformation along a salient transverse zone junction an example from the leamington transverse zone utah sevier fold thrust belt usa abstract within fold thrust belts the junctions between salients and recesses may hold critical clues to the overall kinematic history of fold thrust belts the deformation history within these junctions is best preserved in areas where thrust sheets extend from a salient through an adjacent recess we examine one such junction within the sevier fold thrust belt western united states along the leamington transverse zone northern utah the canyon range thrust sheet can be traced continuously from the leamington transverse zone to its adjacent salient to the south the central utah segment deformation within the canyon range thrust sheet took place by faulting and cataclastic flow analyses of these fault networks preserved throughout the canyon range thrust sheet are used to develop a kinematic history of the leamington transverse zone field data is supplemented by analog sandbox experiments this study suggests that in detail deformation within the overlying thrust sheet may not directly reflect the underlying basement structure moreover these junctions may contain several types of accommodating structures that helped to maintain critical taper and that serve as potential targets for natural resource exploration

pseudotachylites in the eastern peninsular ranges of california abstract a continuous zone containing pseudotachylites in the eastern peninsular ranges of california extends over more than 15 km from deep canyon in the north to at least toro canyon in the south pseudotachylites are found in rocks of tonalitic to dioritic composition while the overall compositions of host rock and pseudotachylite veins are similar veins are characteristically enriched in fe and ti and depleted in si many veins are cataclastic and all contain fragments but a large number have a groundmass with skeletal and spherulitic microlites of calcic plagioclase an 40 50 biotite and ilmenite indicative of partial melting the pseudotachylites formed mainly during a late brittle event 56 62 ma and postdate the high strain ductile mylonitic deformation in the santa rosa mylonite zone 65 87 ma as ascertained by 40ar 39ar ages the size of veins ranges from less than 1 mm to more than 10 cm in thickness and aspect ratios display a broad log normal distribution three quarters of the veins are more or less parallel to the foliation the orientation of the veins is consistent with a top to the west displacement similar to that for the ductile mylonites from the sizes energies required for melting are estimated to range between 105 j for smaller veins and 1010 j for larger veins interestingly the energy distribution and geometry of these pseudotachylites correspond closely with energy distributions of current microseismic events along the san andreas fault at parkfield suggesting that basic mechanisms may be similar and due to intrinsic mechanical properties of rocks in particular these indicate that local stress concentrations rather than average crustal stresses are relevant for earthquakes

timing of deformation in overthrust belt and foreland of idaho wyoming and utah a review of the timing and displacement evidence of the major structures of the western wyoming overthrust belt and foreland shows there is a progression in thrust displacement apparent duration of motion and palinspastic position of thrust traces from west to east those toward the west moved farther for an apparently longer period of time and are more widely spaced in their restored positions than those toward the east however average thrust velocities are all on the order of 0 5 0 5 cm yr 0 2 in yr foreland events are in part synchronous with thrust belt events and had an effect on them although dating precision varies widely on major normal faults present evidence does not contradict the generally held view that all normal faults postdate the youngest thrusting

dating fault reactivation by ar ar laserprobe an alternative view of apparently cogenetic mylonite pseudotachylite assemblages apparently cogenetic mylonite pseudotachylite assemblages are commonly reported in major fault zones they represent two very incompatible modes of deformation and complex formation mechanisms have been proposed to explain this paradox we report here one such assemblage from central norway in which apparently synchronous mylonite and pseudotachylite formation is separated by 100 ma mean muscovite ar ar laserprobe spot ages from mylonite and pseudotachylite matrix are 406 11 ma and 290 10 ma respectively in preference to a complex cogenetic model we invoke fault reactivation as a viable alternative and suggest that assumed contemporaneity in such assemblages may be invalid in many cases

fracture geometry of pseudotachylyte generation zones a study of shear fractures formed during seismic events abstract the orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the ikertoq shear belt west greenland are described and the fracturing sequence is inferred the generation zones occur in strongly foliated gneisses and consist of concordant usually paired principal displacement shear fractures paired shears on which most melt was generated and a system of minor shear fractures minor shear fracturing was most intense between principal displacement shear fractures and most melt intruded from these paired shears inwards with respect to the zone intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures is believed to be contemporaneous with minor shear fracturing in the generation zones generation zones affect intact rocks and individual principal displacement shear fractures have a maximum displacement of 4 6 m paired principal displacement shear fractures are up to 3 m apart and have a maximum observed length of 1 km in all the zones described the fracturing appears to be the result of a single episode of slip both dextral and sinistral generation zones occur within the study area and have different geometries in each case the displacement is strike slip

the terminology of structures in thrust belts abstract a review of structures and geometric relationships recognized in thrust belts is presented a thrust is defined as any contractional fault a corollary being that thrusts must cut up section in their transport direction flats are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ramps are those portions of thrusts which cut across datum surfaces ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike slip movement an array of thrusts which diverge in their transport direction may form by either of two propagation models these are termed piggy back propagation which is foreland directed and overstep propagation which is opposed to the thrust transport direction an array of thrust surfaces is termed an imbricate stack and should these surfaces anastamose upwards a duplex will result the fault bounded blocks are termed horses a duplex is bounded by a higher roof thrust and a lower floor thrust the intersection of any two thrust planes is termed a branch line thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut a further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one the movement of thrust sheets over corrugated surfaces or the local development of thrust structures beneath will fold higher thrust sheets these folds are termed culminations and their limbs are termed culmination walls accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust these accommodation surfaces are termed hangingwall detachments and they need not root down into the active thrust this category of detachment includes dip slip hangingwall drop faults which are developed by differential uplift of duplex roofs and out of the syncline thrusts which develop from overtightened fold hinges back thrusts as well as forming as hangingwall detachments may also form due to layer parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

significance of the deformation history within the hinge zone of the pennsylvania salient appalachian mountains abstracttwo competing models exist for the formation of the pennsylvania salient a widely studied area of pronounced curvature in the appalachian mountain belt the viability of these models can be tested by compiling and analyzing the patterns of structures within the general hinge zone of the pennsylvania salient one end member model suggests a nw directed maximum shortening direction and no rotation through time in the culmination an alternative model requires a two phase development of the culmination involving nnw directed maximum shortening overprinted by wnw directed maximum shortening structural analysis at 22 locations throughout the valley and ridge and southern appalachian plateau provinces of pennsylvania are used to constrain orientations of the maximum shortening direction and establish whether these orientations have rotated during progressive deformation in the pennsylvania salient s hinge outcrops of paleozoic sedimentary rocks contain several orders of folds conjugate faults steep

the origin of the basse normandie duplex boulonnais france abstract two hercynian duplexes are developed in visean limestones in the basse normandie quarry the lower duplex is completely exposed in a subvertical quarry wall the partially exposed upper duplex lies immediately above the lower duplex the duplexes are both located in the footwall of the hydrequent thrust which emplaced devonian clastic sediments above the visean limestones the lower duplex exposes all the internal thrusts a reference bed of chalky limestone the roof and floor thrusts and the duplex tip the duplex has been graphically restored to its pre deformation geometry by line length and area balancing and its resultant geometry is close to the model of boyer elliott the lower duplex shortened by two different mechanisms an initial phase of layer parallel shortening which produced no cleavage followed by thrust imbrication the average contraction of the front portion of the duplex was 49 natural strain of which 27 is layer parallel shortening and 22 is thrust imbrication however locally the bulk shortening increases from zero at the duplex tip to over 120 in a down dip direction the area balancing provides the most accurate estimates of bulk shortening line length balance calculations give minimum estimates only an area balance on the whole of the lower duplex gives a bulk shortening of 84 an area balance of the upper duplex yields an average contraction of 75 and the total contraction produced by both duplexes is 92

an apparent mechanism dependence of radiated seismic energy we develop an extension to the method of boatwright and choy 1986 for determining the radiated seismic energy es that accounts for factors that bias the estimate we apply our technique to 204 events worldwide during the period 1992 1999 and find that the apparent stress is on average largest for strike slip events 0 70 mpa while for reverse and normal events it is significantly smaller 0 15 and 0 25 mpa respectively these results support the mechanism dependence of es reported by choy and boatwright 1995 although we find that once likely sources of bias are accounted for the mechanism dependence is not as strong as found previously the source of the mechanism dependence is unclear but one possibility is that it reflects a mechanism dependent difference in the stress drop this hypothesis is suggested by the scaling of slip with width in large strike slip earthquakes and makes two predictions which could be used to test it the first is that the discrepancy should disappear for the very largest dip slip earthquakes as the length of the fault greatly exceeds the downdip extent the second is that the discrepancy ought to disappear for smaller earthquakes the first can not yet be tested due to a lack of recent very large dip slip earthquakes the second is supported by the lack of mechanism dependence to es for smaller earthquakes an alternative hypothesis is that the apparent mechanism dependence could result if faults are opaque during rupture blocking seismic radiation across them brune 1996 this could cause radiated seismic energy to be trapped preferentially in the crust near the source volume for dipping faults there remains however a large discrepancy between estimates of es obtained from teleseismic versus regional data this discrepancy indicates a problem with teleseismic and or regional estimates of the seismic energy and must be resolved before a definite conclusion can be drawn

basement control on oblique thrust sheet evolution seismic imaging of the active deformation front of the central andes in bolivia abstract in the area of the bolivian orocline we examine the deformation pattern associated with the active development of a new thrust sheet a dense grid of reprocessed 2 d seismic lines from hydrocarbon exploration industry is interpreted and a 3 d simplified structural and kinematic model is deduced in the boomerang hills onlapping paleozoic and foredeep sediments are detached from the underlying s dipping basement they are thrust northeastwards by less than 2 km two zones can be differentiated along the andean deformation front 1 a w e to nw se striking frontal segment of predominantly orthogonal shortening comprising a thrust and anticline system 2 a wsw ene striking lateral zone of oblique shortening within a complex system of thin skinned strike slip faults and minor folds the deformation front always follows a pronounced edge in the topography of the top basement surface close to the boundary of the paleozoic basin the observed deformation pattern indicates intensified strain partitioning caused by the interaction of contraction direction and basement topography which provides a near oblique ramp for the onlapping wedge of sediments the sw ne thrusting direction is divided into orthogonal and tangential components these are accommodated by convergent and strike slip structures respectively which sole into a common detachment horizon the structural evolution of the new thrust sheet in the bolivian orocline is primarily controlled by the paleorelief of the brazilian shield because 1 the shape of the basement affects the taper of the thrust wedge and localizes the deformation front and 2 small asperities in close to the top of the basement promote fault localization the coincidence of a relatively high basement position and a structural high of the eastern cordillera leads to the conclusion that the shape of the brazilian shield also controls the structural evolution of the pronounced eastern border of the bolivian orocline

pseudotachylyte bearing strike slip duplex structures in the fort foster brittle zone s maine abstract detailed 1 60 scale mapping of the fort foster brittle zone in the mylonitic rye formation of southernmost maine has revealed the intricate internal duplex structure of a system of probable paleozoic age dextral strike slip faults that have produced abundant pseudotachylyte and minor breccia the internal configuration of this brittle zone consists of a mosaic of individual pseudotachylyte generation zones as slab duplex structures individual duplex zones are up to 100 m in length and 1 m or less in width and are defined by pairs of layer parallel slip surfaces along which frictional melts were produced these slab duplex structures are interpreted as zones of displacement transfer between long overlapping layer parallel en echelon strike slip fault surfaces contractional duplexes develop layer parallel compressional structures that tend to shorten and thicken the fault bounded slabs by the formation of lateral ramps and conjugate faults kinks and asymmetric folds extensional duplexes develop layer parallel stretching and thinning by the formation of oblique dextral shears high angle conjugate pairs and localized fault breccias the production of pseudotachylyte by friction melting along layer parallel fault surfaces in these exposures is attributed to rapid slip during paleoseismic events the rupture structures developed during these events may be characteristic of fault structure and mechanics at near focal depths in a strike slip seismogenic zone

al rich calcic pyroxene in pseudotachylite an indicator of high pressure and high temperature abstract an aluminous pyroxene with an approximate formula ca 0 60 na 0 05 0 20 mg 0 58 fe 0 11 2 al 0 60 vi si 1 68 a1 0 32 o 6 or jd 0 05 ts 0 32 es 0 20 wo 0 10 en 0 27 fs 0 06 has been found in a pseudotachylite vein from gabbros in the musgrave ranges central australia textural evidence indicates that it formed by breakdown of calcic plagioclase augite and olivine which constitute the isochemical country rock there is a large amount of m 2 vacancy compared to most pyroxenes the vein mineral is most similar in composition to natural pyroxenes in kimberlites and eclogites and especially to synthetic pyroxenes which formed at pressures in excess of 3 gpa 30 kb we suggest that these high pressures were not lithostatic but represent dynamic stresses released during catastrophic brittle rupture either at tips of propagating fractures or spallation of unsupported crackwalls such transient shear stresses may reach the shock range we propose such a fracture mechanism as an alternative to frictional melting for the productoin of pseudotachylite veins

hydrochemical evolution and reaction simulation of travertine deposition of the lianchangping hot springs in yunnan china located in western yunnan of china the lianchangping hot springs are of so 4 hco 3 ca na type with total dissolved solids tds ranging from 3560 to 4240 mg l large amounts of travertine are deposited near the hot springs with a variety of travertine morphology including travertine terrace slope and ridge travertine is precipitating along the slope near one of the spring s thresholds chemical compositions of the hot water samples collected along travertine slope show that p co 2 p co 2 tds and contents of hco 3 and ca decrease downstream along the travertine slope due to travertine deposition the initial milliequivalent ratio of ca 2 hco 3 is greater than 1 5 before travertine precipitation and there is an increasing trend in the ratio downstream along the travertine slope high levels of sulphate are probably the result of gypsum dissolution in the sediments widespread in the lianchangping area the spring waters issuing from the land surface are slightly supersaturated with respect to calcite dolomite fluorite kaolinite halite plagioclase and carbon dioxide hydrochemical evolution of the lianchangping spring water along the slope is calculated with the phreeqc interactive 2 12 5 sets of minerals and gases reacted in appropriate amounts are recognized through reaction simulation quantitatively accounting for the variations in chemical compositions especially depositing minerals along the flow path on the travertine slope

structural geology of stewart peak culmination idaho wyoming thrust belt a structural culmination is the highest point along a structural axis or fold system away from which the folds plunge structural culminations are an important and predictable component of foreland fold thrust belts and are prime targets for oil and gas accumulations the stewart peak culmination is a doubly plunging anticlinorium in the absaroka thrust sheet of western wyoming located in the northern salt river range major thrust faults in the stewart peak culmination comprise a large scale duplex fault zone the roof fault of the duplex is the absaroka thrust the floor fault is the basal or regional decollement and the intervening murphy and firetrail thrusts are trailing edge imbricate faults in the darby plate that have locally uplifted and deformed the absaroka into an asymmetric breached anticlinorium or culmination in addition these major thrust faults subdivide the culmination into fault bound structural horses each with a characteristic style of deformation as a function of its stratigraphic and structural level in the duplex a recent exploratory test of the culmination provided data which suggest that the absaroka thrust fault locally cuts downsection in the transport direction this may be explained by initial folding and breaching of the firetrail anticline followed by main motion on the absaroka thrust which overrode the fold the available data suggest that the stewart peak culmination is the result of 1 polyphase uplift or arching of the absaroka thrust sheet by the murphy and firetrail thrust sheets 2 lateral imbrication and ramping and 3 arching of the precambrian basement complex beneath the northern salt river range in addition a basic kinematic model of deformation applicable to structural culminations includes emplacement of an allochthonous mass over a parautochthonous substratum closely followed by or contemporaneous with deformation and translation of the footwall section to produce a duplex fault zone that folds the overlying allochthon the stewart peak culmination bears many similarities in structural style and origin to polyphase folded fault domes in the canadian rocky mountains specifically the stewart peak area is compared to the savanna creek and moose mountain culminations

decollement folding as a mechanism for thrust ramp spacing the hypothesis that early formed decollement folds localize thrust ramps explaining the regular spacing of major thrust ramps observed in many mountain belts is examined the model for folding instability consists of a rigid basement a weak decollement zone and a stiff thrust sheet the model incorporates the effects of topographic stresses and syntectonic erosion and redeposition and uses a linearized power law rheology for the thrust sheet this approximation is valid for small angles of fold limb dip 15 and is appropriate for analysis of the wave length selection of folds at the onset of deformation the spacing of major thrust ramps and the stratigraphic thicknesses of the decollement zone and thrust sheet from the wyoming portion of the sevier fold and thrust belt are used to test the model the ratio of the fold dominant wavelength to thrust sheet thickness ld h2 increases with the ratio r of the viscosity of the stiff layer to that of the decollement zone and decreases with the power law stress exponent n2 the fold amplification factor increases with both n2 and r as the topographic decay ratio d increases ld h2 increases at fixed n2 and r acceptable models for the absaroka and darby thrust sheets have n2 3 only when r 1000 and significant erosion and redeposition of sediments accompany the folding of the thrust sheet d 103 does the required r fall below a reasonable limit thus a model in which decollement folding is accompanied by syntectonic erosion and redeposition precedes thrusting is consistent with the available observational constraints

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

fault structure wear mechanisms and rupture processes in pseudotachylyte generation abstract fault generated pseudotachylyte is found within both cataclastic and mylonitic host rocks suggesting that rapid catastrophic displacements have occurred at a variety of depths within paleoseismogenic zones pseudotachylyte bearing fault zones represent a composite of structural features associated with the process of earthquake rupture propagation and coseismic slip the development of multiple pseudotachylyte veins in fault linkages duplexes sidewall ripouts en echelon arrays and brittle zones suggests repeated rupturing during a series of characteristic earthquakes each earthquake as a coseismic slip event can be subdivided into initial rupture acceleration stable sliding and final deceleration stages these evolve through distinctive sequences of wear and deformation mechanisms that vary with sliding velocity duration of slip total displacement and the hydrodynamics of the developing fault zone slip is thought to proceed toward surface refinement and possible frictional melting following the propagation of leading shear fracture process zones the passage of the initial process zone of oblique fracturing would be followed by linkage to a throughgoing structure with asperity reduction through brecciation comminution and refined cataclastic flow for frictional melting in an abrasive wear dominant model at greater depths in the presence of mylonitic anisotropy slip would proceed through initial layer parallel surfaces and duplex linkages with rapid surface refinement through plastic smearing and laminar flow for frictional melting in an adhesive wear dominant model

zooming on northern cap de creus shear zones abstract the northern cap de creus shear belt provides fine examples of shear zones developed at different scales in rocks bearing pre existing penetrative foliations the shear zones developed under retrograde metamorphic conditions and they are preferentially located in crystalline schists lower grade metasedimentary rocks accommodated deformation by folding the shear zones are linked in an anastomosing framework with self similar properties they are interpreted to have formed as the result of a progressive wrench dominated deformation affecting anisotropic rocks a progressive non coaxial deformation regime is considered responsible for the development of the complex kinematic pattern wherein shear zones with opposite senses of shear may come to lie in close parallelism in such a framework individual shear zones are characterised by marked strain gradients not only across the shear zone but also along it these features together with the geometrical analysis of shear zone marginal domains indicate that these structures developed with a component of shortening across the shear zones the shear zones are interpreted to have nucleated as buckling instabilities that gradually evolved to become shear zones approaching the simple shear model microstructural and quartz c axis fabric analyses indicate that shear zone formation is associated with strong strain partitioning on all scales

asymmetric growth of the pyrenees revealed through measurement and modeling of orogenic fluxes orogenic asymmetry is ubiquitous in terms of topography structure and metamorphism but the relative influences of climate and tectonics in determining such asymmetry remain contentious this study demonstrates that for the case of the european pyrenees orogenic asymmetry is not defined by inherited structural asymmetry nor by orographic asymmetry of precipitation but is the simple consequence of the inherent tectonic growth of a doubly vergent orogen the pyrenees record approximately 60 myr of upper crustal accretion erosion and sedimentation that developed from late cretaceous to early miocene times approximately 165 km of plate convergence was accommodated by four phases of orogenic growth that can be summarized in terms of their generic orogenic development as follows 1 early inversion of passive margin extensional faults initiating the pro wedge 2 growth by a combination of frontal accretion and underplating to the pro wedge symmetrical and steady record of erosion and foreland basin sedimentation 3 cessation of pro wedge frontal accretion with propagation of the retro wedge deformation front 4 accelerated growth of pro wedge antiformal stack induced by underplating accelerated erosion over antiformal stack and increase in sediment discharge to pro foreland basin stabilization of outer pro wedge by sediment blanketing discrete element model experiments are used to explore the natural tendency of the system towards asymmetry model output demonstrates that during active frontal accretion to the pro wedge of an orogen the maxima in erosional denudation is offset towards the pro wedge relative to the maximum depths of exhumation this asymmetry is fundamentally driven by the progressive reduction in advection velocities of material within the orogen from the pro wedge into the retro wedge hence for an efficient erosional regime a dynamic coupling between erosion and structural uplift of the pro wedge will reduce activity of the retro wedge and hence drive asymmetry in terms of low temperature thermochronometry erosion and sedimentation the models also demonstrate the tendency for pro wedge growth through frontal accretion to slow however the cessation of frontal accretion as recorded by the south pyrenean thrust belt phases 3 and 4 requires additional forcing it is proposed that the thick wedge top sedimentation recorded at this time increased taper angles and hence stabilized the wedge accretionary influx into the orogen at this time was accomplished through underplating these processes governing asymmetry do not require a spatially variable climate or a laterally heterogeneous underthrust plate

pseudotachylyte and the development of ultramylonite bands in the saint barthelemy massif french pyrenees abstract in mylonitic gneisses of the saint barthelemy massif in the french pyrenees evidence was found that ultramylonite bands developed by ductile deformation of pseudotachylyte in the gneiss a mylonitic shape fabric was produced by a continuous structural event during retrogression from amphibolite to upper greenschist facies decrease in temperature caused gradual hardening in the gneiss which led to the development of pseudotachylyte bands by intermittent seismic slip these acted as weak zones in which ductile deformation was concentrated a microstructure typical of ultramylonite was produced in these deformed pseudotachylyte bands