basin development and syntectonic sedimentation associated with kinematically coupled strike slip and detachment faulting southern nevada a previously unrecognized 12 10 ma clastic sequence is distributed widely south of the las vegas valley shear zone and north of lake mead in southern nevada structural and stratigraphic evidence from these sedimentary and associated volcanic rocks suggests that basin development was kinematically related to movement along the las vegas valley shear zone major movement along the nearby saddle island detachment also occurred within this time interval we suggest that spatially and temporally related basin development sedimentation and stratal tilting reflect kinematically coupled movement along a combined las vegas valley shear zone saddle island detachment fault system

an early history of pure shear in the upper plate of the raft river metamorphic core complex black pine mountains southern idaho abstract although commonly obscured by simple shear pure shear fabrics occur locally within many metamorphic core complexes the cover rocks to the raft river metamorphic core complex exposed within the black pine mountains display an early coaxial strain history which developed prior to the formation of low angle fault bounded allochthons at higher structural levels this is documented by pressure shadows with straight sutures and oppositely rotated antitaxial calcite veins these rocks were sub horizontally extended parallel to bedding by up to 160 in an e w direction the marbles of the lowest exposed levels exhibit well developed crystallographic preferred orientation fabrics calcite c and a axis pole figures are symmetric to slightly asymmetric with obliquities ranging from 0 to 8 which indicate that the percent pure shear ranges between 100 and 76 initial constraints on the timing of this layer parallel extension suggest that extension was occurring locally within the sevier belt hinterland during the late cretaceous these data coupled with previous studies suggest that pure shear might be a much more common component of strain in rocks deformed in extensional environments than is generally acknowledged

the location and source parameters of the lompoc california earthquake of 4 november 1927 in this paper we address the relocation magnitudes and the style of faulting of the lompoc earthquake from a sparse assortment of teleseismic and regional seismograms the highest quality teleseismic waveform data come from a station at de bilt netherlands that remains in operation thus recordings of numerous modern events in central coastal california i e the 1969 santa lucia banks 1983 coalinga 1978 santa barbara and 1989 loma prieta earthquakes have been used for comparison with the 1927 records location constraints for the lompoc event were established from the de bilt recording by comparing s p and sss s waveform matches against the above master events to avoid the effect of unknown clock errors on locations that use absolute times these same seismograms were modeled to estimate the depth faulting parameters and source strength a similar approach using observational comparisons and numerical modeling was applied to the regional waveform data obtained from the stations at berkeley tucson and pasadena our results indicate a north northwesterly striking reverse event located about 40 km west of point conception which is in excellent agreement with the recent tsunami modeling results by satake and somerville 1992 this location is 25 km south of that proposed by hanks 1979 and well within his error bars we obtain a body wave moment of 1 10 26 dyne cm a trapezoidal time history of 2 2 2 sec and a source depth of 10 km the weak beginning of the pnl wavetrain at berkeley indicates some source complexity which is characteristic of many large events the fault parameters are strike n20 w dip 66 ne and rake 95 most seismicity catalogs report a m s 7 3 for this event after gutenberg and richter 1956 but this was a long period body wave magnitude and not a surface wave result their original worksheets indicate a smaller ms 7 0 the body waves of the loma prieta event m s 7 1 appear distinctly larger than those of the lompoc event at de bilt in agreement with our lower estimate of source strength

estimating locations and magnitudes of earthquakes in southern california from modified mercalli intensities modified mercalli intensity mmi assignments instrumental moment magnitudes m and epicenter locations of thirteen 5 6 m 7 1 training set events in southern california were used to obtain the attenuation relation mmi 1 64 1 41 m 0 00526 h 2 63 log h where h is the hypocentral distance in kilometers and m is moment magnitude intensity magnitudes m i and locations for five 5 9 m 7 3 independent test events were consistent with the instrumental source parameters fourteen historical earthquakes between 1890 and 1927 were then analyzed of particular interest are the m i 7 2 9 february 1890 and m i 6 6 28 may 1892 earthquakes which were previously assumed to have occurred near the southern san jacinto fault a more likely location is in the eastern california shear zone ecsz these events and the 1992 m 7 3 landers and 1999 m 7 1 hector mine events suggest that the ecsz has been seismically active since at least the end of the nineteenth century the earthquake catalog completeness level in the ecsz is m 6 5 at least until the early twentieth century online material figures and table of empirical mmi site corrections

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

pure shear and simple shear calcite textures comparison of experimental theoretical and natural data abstract the pattern of lattice preferred orientation texture in deformed rocks is an expression of the strain path and the acting deformation mechanisms a first indication about the strain path is given by the symmetry of pole figures coaxial deformation produces orthorhombic pole figures while non coaxial deformation yields monoclinic or triclinic pole figures more quantitative information about the strain history can be obtained by comparing natural textures with experimental ones and with theoretical models for this comparison a representation in the sensitive three dimensional orientation distribution space is extremely important and efforts are made to explain this concept we have been investigating differences between pure shear and simple shear deformation incarbonate rocks and have found considerable agreement between textures produced in plane strain experiments and predictions based on the taylor model we were able to simulate the observed changes with strain history coaxial vs non coaxial and the profound texture transition which occurs with increasing temperature two natural calcite textures were then selected which we interpreted by comparing them with the experimental and theoretical results a marble from the santa rosa mylonite zone in southern california displays orthorhombic pole figures with patterns consistent with low temperature deformation in pure shear a limestone from the tanque verde detachment fault in arizona has a monoclinic fabric from which we can interpret that 60 of the deformation occurred by simple shear

cretaceous to early tertiary paleogeography in the hinterland of the sevier thrust belt east central nevada the cretaceous to early tertiary hinterland of the sevier thrust belt in east central nevada underwent two periods of sedimentary basin development the older early cretaceous episode is characterized by basins that formed coeval with deformation within a belt of contractional structures basins were characterized by periods of through flowing fluvial drainage the younger late cretaceous to eocene episode of basin development was characterized by internally drained alluvial basins that subsequently expanded into more regional lacustrine basins basal alluvial detritus indicates that basin inception occurred as a result of rapid development of topographic relief syn sedimentary tectonism was characterized by horizontal extension due to attainment of equilibrium topography in response to crustal thickening

strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

the mesozoic cenozoic tectonothermal evolution of the ruby mountains east humboldt range nevada a cordilleran metamorphic core complex a combined k ar and 40ar 39ar geochronological study of the ruby mountains east humboldt range nevada a cordilleran metamorphic core complex has revealed a complex tectonothermal history the three structural subdivisions of the core complex migmatitic core mylonitic zone and cover share magmatic and deformational histories in part but also display important contrasts in structural style metamorphic paragenesis and thermal history during the late jurassic ca 160 ma miogeoclinal rocks were polyphase deformed metamorphosed under amphibolite facies conditions and pervasively intruded by peraluminous granitic magmas thereby forming an igneous and metamorphic complex late jurassic muscovite granite porphyry plugs also intruded unmetamorphosed cover rocks late cretaceous cooling of the igneous and metamorphic complex is locally suggested by poorly resolved cretaceous incremental release 40ar 39ar hornblende spectra and by u pb monazite ages from pelitic schist exposed in the northern ruby mountains between ca 45 ma and 20 ma the igneous and metamorphic rocks experienced an episode of complex extensional tectonism which involved the development of low angle normal sense simple shear zones as a result rocks metamorphosed at depth in the mesozoic were translated to higher crustal levels which resulted in cooling through temperatures required for the intracrystalline retention of argon within minerals near concordance of hornblende and biotite plateau ages from the migmatitic core of the complex suggest relatively rapid exhumation however it is not certain if this cooling followed a prolonged maintenance at elevated temperatures after the ca 160 ma metamorphism or if distinct locally superposed late mesozoic and or middle tertiary thermal overprints had affected the cooling history a major extensional simple shear zone is marked by an approximately 1 kilometer thick mylonite zone that experienced late kinematic superposed brittle deformation a geometric consequence of the development of low angle extensional fault zones is the eventual translation of colder rocks over hotter rocks such a history is supported by the 40ar 39ar data for example mylonitic mafic orthogneisses from a major allochthon record 40ar 39ar hornblende plateau ages between 44 and 48 ma and biotite plateau ages between 32 and 33 ma and are structurally positioned above mylonitic rocks of the northern ruby mountains which record biotite cooling ages of ca 22 24 ma such relations suggest either multiple episodes of tertiary mylonitization or a protracted multiphase history of mylonitization however after ca 20 ma all of the presently exposed mylonitic rocks were maintained below 100 c and have only experienced upper crustal brittle deformation this article contains supplementary material

the middle mountain shear zone southern idaho kinematic analysis of an early tertiary high temperature detachment the middle mountain shear zone is exposed in the idaho portion of the albion raft river grouse creek metamorphic core complex the structure strikes north northeast south southwest and dips shallowly to the west it is defined on the basis of well developed mylonitic fabrics and had an initial thickness of at least 4 km at least some movement on the shear zone occurred at depths of 22 26 km during eocene amphibolite facies metamorphism but shear zone fabrics may be found in some granite bodies of late oligocene age the zone may thus have been active in the ductile regime over a period of as much as 10 15 m y mylonitic rocks within the middle mountain shear zone display both type i and type ii s c fabrics augen structures pressure shadows and quartz c axis orientations are strongly asymmetric these fabrics and textures indicate that deformation within the zone was characterized by noncoaxial laminar flow during which the hanging wall of the structure moved down and to the west northwest relative to the footwall we interpret the middle mountain shear zone as a major detachment that played an important role in middle tertiary extension of this part of the eastern great basin the top to the west displacement sense contrasts with late cenozoic predominantly top to the east extension in this area implying a major change in extension geometry in early to middle miocene time

tracing the subducted oceanic crust beneath the central california continental margin results from ocean bottom seismometers deployed during the 1986 pacific gas and electric edge experiment large aperture seismic data were collected on several ocean bottom seismometers obs deployed along a deep crustal seismic profile that was shot across the central california continental margin the line of shots extends from the oceanic crust seaward of the santa lucia escarpment to the california coast near morro bay and crosses the santa lucia basin santa lucia bank santa maria basin and hosgri fault zone the obs data permit us to trace the subducted oceanic crust from seaward of the santa lucia escarpment to beneath the central part of the santa maria basin a complementary study using onshore recordings of the same shots extends the crustal model onshore just seaward of the santa lucia escarpment the oceanic crust is subhorizontal and covered by a thin layer of low velocity sediment the velocity 4 5 km s and gradient 1 20 1 25 km s km of the upper oceanic crust in this region are well determined and agree with earlier determinations of the crustal structure of the eastern pacific beneath the santa lucia escarpment and santa lucia basin the oceanic crust dips approximately 16 to the east it is overlain by material with a velocity that increases from 4 8 to 6 4 km s at a depth of 1 7 5 5 km below the seafloor beneath the santa lucia basin a low velocity zone may be sandwiched between the subducted crust and this shallow high velocity material which we interpret to represent obducted oceanic crustal material beneath the eastern edge of santa lucia basin the dip of the subducted oceanic crust decreases to less than 2 the configuration of the subducted crust in this region is consistent with imbrication of the subducted crust as suggested by others based on common midpoint reflection data beneath the central santa maria basin the top of the subducted oceanic crust is at a depth of about 14 16 km and the moho is at 19 21 km

plasma drifts associated with a system of sun aligned arcs in the polar cap a series of four sun aligned arcs passed over sondre stromfjord greenland on the night of the 17th and 18th of february 1985 observations of these arcs were made using the sondrestrom incoherent scatter radar and an intensified all sky imaging tv system that was operated at the radar site the first of the four arcs crossed the sondre stromfjord meridian just before local midnight moving westward and the other three arcs followed at approximately half hour intervals when we account for the earth s rotation the arc drift in an inertial frame was eastward or dusk to dawn the half hour interval between meridian crossings of the arcs implies that the mean spacing between the arcs was 180 km a defense meteorological satellite program dmsp f6 satellite pass at 0110 ut revealed the presence of highly structured electron and ion precipitation throughout the polar cap the dmsp visible imager detected a single sun aligned arc associated with the largest peak in precipitating electron flux this arc was also observed at thule greenland with an intensified film camera these observations suggest that at least one of the arcs that were observed at sondre stromfjord extended across a large part of the polar cap the radar at sondre stromfjord measured electron density and ion drift velocities associated with the four arcs the radar drift measurements were superimposed on the all sky video images to determine the location of the measurements relative to the arcs plasma drifts outside the arcs were found to be both sunward and antisunward while within the arcs the drifts were predominantly antisunward the variability of the drifts in the direction parallel to the arcs indicates that the electric fields were highly structured even though the configuration and motion of the arcs were well behaved the motion of the auroral forms perpendicular to the arcs was generally in the same direction as the perpendicular component of the plasma velocity

ionospheric currents associated with a sun aligned arc connected to the auroral oval on the evening of 14 november 1985 the sondrestrom incoherent scatter radar made measurements of ionization and drifts associated with a sun aligned polar cap arc near local midnight all sky photographs at 630 0 nm showed that the arc which appeared to move westward across the sondrestrom meridian was connected to the auroral oval the radar measured height integrated hall and pedersen conductances in the arc of 2 to 3 mhos the plasma drift within the arc was anti sunward with velocities greater than 1 km sec indicating a westward electric field across the arc of about 60 mv m the radar data imply a model current system that includes a northward hall current within the sun aligned arc and a westward electrojet in the auroral oval to the south the model currents produce ground magnetic perturbations that are consistent with those measured by stations in the greenland magnetometer chain in particular the sun aligned arc is detectable by a negative deflection in the d component of the magnetometers divergence of the currents at the edges of the auroral forms yield field aligned currents that are upward on the west edge of the arc and downward on the east edge an upward current is required at the point where the polar cap arc joins the auroral oval because of the gradient in the conductivity and electric field at that location this upward current implies the presence of enhanced electron precipitation which can account for the observed brightening of polar cap arcs at the point where they intersect the auroral oval

dayside and nightside segments of a polar arc the particle characteristics 1 using multiple satellites and a ground based radar network the physical characteristics of a polar arc and its evolution are investigated under quiet solar wind conditions the average electron energy is higher at the near midnight segment than at the sunward extension of the polar arc both polar arc segments are positioned primarily on closed magnetic field lines the convection reversal boundary in the fitted convection pattern was located close to the transition region in the average electron energy seen by the satellites the topside plasma density at an altitude of 680 km is clearly enhanced around the polar arc although the enhancement boundary does not coincide precisely with that of the precipitation the temporal sequence of auroral images suggests that the polar arcs emerged from a double oval structure this suggestion is supported by the observed physical characteristics of the polar arc which are quite similar to those of double oval structures that have been reported previously

the 1978 earthquake sequence near thessaloniki northern greece summary this paper presents a detailed study of teleseismically and locally recorded foreshocks and aftershocks of the 1978 thessaloniki earthquake sequence and discusses the relations between the locations and mechanisms of these shocks and the three dimensional geometry of faulting in the source region sixteen teleseismically recorded events were relocated using a relative relocation method and positioned geographically using the accurate locally determined location of the largest aftershock all these shocks had depths shallower than 15 km the spatial distribution of the foreshocks showed that the activity was initiated at the central part of the epicentral region progressed westwards and for about 20 days clustered in the region of the imminent mainshock no teleseismically recorded foreshock was located west of the mainshock location in less than 1 hr after the mainshock large aftershocks occurred west of it and in later stages spanned the entire western part of the epicentral region no teleseismically recorded aftershocks were located east of the mainshock locally recorded aftershocks were accurately located and provided a more detailed picture of the seismic deformation they occurred at depths between 3 and 12 km and in three distinct clusters separated by well defined gaps the largest foreshock on may 23 had a moment of 5 6 1024dyne cm and a mean displacement of 31cm the mainshock on june 20 had a moment of 5 2 1025dyne cm a strain drop of 4 0 10 5 static stress drop of 12 bar and a mean displacement of 64 cm these values are for one of the two possible interpretations of the aftershock distribution which suggests that the largest earthquakes of the sequence occurred on two different but adjacent fault segments the first event 1 on may 23 occurred along the central fault segment and was followed 28 days later by the second event 2 on june 20 along a western fault segment finally 61 days later another smaller earthquake event 3 occurred along another fault east of event 1

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

precise determinations of the equilibria kyanite sillimanite and kyanite andalusite and a revised triple point for al 2 sio 5 polymorphs

comments on the lompoc california earthquake november 4 1927 m 7 3 and its aftershocks by thomas c hanks

thermomechanics of an extensional shear zone raft river metamorphic core complex nw utah abstract a detailed structural and microstructural analysis of the miocene raft river detachment shear zone nw utah provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes combined microstructural electron backscattered diffraction strain and vorticity analysis of the very well exposed quartzite mylonite show an increase in intensity of the rock fabrics from west to east along the transport direction compatible with observed finite strain markers and a model of necking of the shear zone microstructural evidence quartz microstructures and deformation lamellae suggests that the detachment shear zone evolved at its peak strength close to the dislocation creep exponential creep transition where meteoric fluids played an important role on strain hardening embrittlement and eventually seismic failure empirically calibrated paleopiezometers based on quartz recrystallized grain size and deformation lamellae spacing show very similar results indicate that the shear zone developed under stress ranging from 40 mpa to 60 mpa using a quartzite dislocation creep flow law we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10 12 and 10 14 s 1 we suggest that a compressed geothermal gradient across this detachment which was produced by a combination of ductile shearing heat advection and cooling by meteoric fluids may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone

sense of nappe emplacement from quartz c axis fabrics an example from the betic cordilleras spain preferred crystallographic orientation patterns fabrics in zones of ductile deformation beneath nappes are a potentially useful indicator of the sense of nappe translation quartz c axis fabrics from a shear zone beneath the pile of higher betic nappes in the betic cordilleras spain have distinctly asymmetric topologies revealed by constructing a fabric skeleton these skeletons closely resemble those produced by computer simulations of quartz fabric development basedon taylor bishop hill theory the fabric asymmetry indicates a northerly sense of transport for the nappes consistent with field evidence from the same area analyses of both c and a axis fabrics based on asymmetric intensity distributions within the fabric give equivocal results and may reflect the presence of earlier preferred orientation patterns predating nappe emplacement

evidence for the tongue of ionization under northward interplanetary magnetic field conditions 1 the activities of the international ionospheric tomography community open up new possibilities of simultaneously imaging the large scale spatial structure of the ionosphere in different longitude sectors in the study tomography receiver chains in scandinavia and greenland were used to provide a wide view of the plasma density structure in the winter magnetic postnoon sector under conditions of stable positive interplanetary magnetic field bz component the spatial distributions of the plasma are discussed in light of a high latitude plasma convection pattern pertinent to the conditions which is supported by dmsp flow measurements the observations are consistent with a tongue of dayside photoionization being drawn antisunward by the convection pattern to form an arc of enhanced plasma density around the periphery of the polar cap

postmylonitic deformation in the raft river metamorphic core complex northwestern utah evidence of a rolling hinge much of the recent debate over low angle normal faults exposed in metamorphic core complexes has centered on the rolling hinge model the model predicts tilting of seismogenic high angle normal faults to lower dips by footwall deformation in response to isostatic forces caused by footwall exhumation this shallow brittle deformation should visibly overprint the mylonitic fabric in the footwall of a metamorphic core complex the predicted style and magnitude of rolling hinge strain depends upon the macroscopic mechanism by which the footwall deforms two end members have been proposed subvertical simple shear and flexural failure each mechanism should generate a distinctive pattern of structures that strike perpendicular to the regional extension direction subvertical simple shear svss should generate subvertical faults and kink bands with a shear sense antithetic to the detachment for an svss hinge the hinge related strain magnitude should depend only on initial fault dip rolling hinge structures should shorten the mylonitic foliation by 13 for an initial fault dip of 30 in flexural failure the footwall behaves as a flexed elastic beam that partially fails in response to bending stresses resulting structures include conjugate faults and kink bands that both extend and contract the mylonitic foliation extensional sets could predominate as a result of superposition of far field and flexural stresses strain magnitudes do not depend on fault dip but depend on the thickness and radius of curvature of the flexed footwall beam and vary with location within that beam postmylonitic structures were examined in the footwall of the raft river metamorphic core complex in northwestern utah to test these predictions observed structures strike perpendicular to the regional extension direction and include joints normal faults tension gash arrays and both extensional and contractional kink bands aside from the subvertical joints the extensional structures dip moderately to steeply and are mainly either synthetic to the detachment or form conjugate sets range wide the extensional structures accomplish about 4 elongation of the mylonitic foliation contractional structures dip steeply mainly record shear antithetic to the detachment and accomplish 1 contraction of the foliation these observations are consistent with the presence of a rolling hinge in the raft river mountains but a rolling hinge that reoriented a high angle normal fault by svss is excluded the pattern and magnitudes of strain favor hinge related deformation mainly by flexural failure with a subordinate component of svss

coherent mesoscale convection patterns during northward interplanetary magnetic field all sky imaging photometer asip and coincident de 2 satellite plasma drift and particle data have been combined to study polar ionospheric convection in the presence of subvisual intensity soft particle excited f region 6300 a sun aligned polar cap arcs coincident de b drift meter data identify these arcs electrodynamically as lines of negative electric field divergence based on conductivities derived from the de b low altitude plasma instrument measured particle fluxes and the measured electric field gradients the divergence of horizontal current across these particle impact excited arcs is in good quantitative agreement with upward birkeland currents carried by the measured particle fluxes although velocity structure can be found without arcs given the asip identified condition of stable weak hundreds of rayleighs 6300 a sun aligned arcs in the polar cap electric field negative divergence is consistently found these arcs of the order of 100 km in width are found by asips in the polar cap about half the time under bz 0 interplanetary magnetic field conditions they are regularly seen by asip data to extend 1000 to over 2000 km in the sunward direction and to persist in time often for over an hour we are thus led to conclude that velocity gradients of this noon midnight elongated scale are typical of bz 0 conditions we further conclude that combined polar asip images and electrostatic potentials calculated along transpolar satellite tracks offer a valuable diagnostic for polar ionospheric convection studies under bz 0 conditions the asip time continuous two dimensionality and the satellite equipotential scaling allow individual snapshots of these polar convection boundaries application here demonstrates highly anisotropic temporally stable convection with greater order than has previously been suspected

fold fault relationships in low angle detachment systems a close kinematic and timing relationship between low angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well exposed neogene detachment systems in the simplon region of the alps and in the death valley region of california fold amplitude is largest in the oldest mylonitic foliation of the footwall these folds are transected by younger mylonitic foliation s and cataclastic detachment faults which are themselves folded about similar axes in general the amplitude decreases and the wavelength increases for progressively younger fold structures consistent with a history of progressive folding during faulting the interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes this strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

relations between hinterland and foreland shortening sevier orogeny central north american cordillera the tectonic relations between foreland and hinterland deformation in noncollisional orogens are critical to understanding the overall development of orogens the classic central cordilleran foreland fold and thrust belt in the united states late jurassic to early tertiary sevier belt and the more internal zones to the west central nevada thrust belt provide data critical to understanding the developmere of internal and external parts of orogens the garden valley thrust system part of the central nevada thnkst belt crops out in south central nevada within a region generally considered to be the hinterland of the jurassic to eocene sevier thrust belt the thrust system consists of at least four principal thrust plates composed of strata as young as pennsylvanian in age that are unconformably overlain by rocks as old as oligocene suggesting that contraction occurred between those times new u pb dates on intrusions that postdate contraction combined with new paleomagnetic data showing significant tilting of one area prior to intrusion suggest that regionally these thrusts were active before 85 100 ma the thrust faults are characterized by long relatively steeply dipping ramps and associated folds that are broad and open to close upright and overturned although now fragmented by cenozoic crustal extension individual thrusts can be correlated from range to range for tens to hundreds of kilometers along strike we correlate the structurally lowest thrust of the garden valley thrust system the golden gate mount irish thrust southward with the gass peak thrust of southern nevada this correlation carries the following regional implications at least some of the slip across jurassic to mid cretaceous foreland thrusts in southern nevada continues northward along the central nevada thrust belt rather than noaheastward into utah this continuation is consistent with age relations which indicate that thrusts in the type sevier belt in central utah are synchronous with or younger than the youngest thrusts in southern nevada this in turn implies that geometrically similar sevier belt thrusts in utah must die out southward before they reach nevada that slip along the southem nevada thrusts is partitioned

geochronology of intrusive and metamorphic rocks in the pilot range utah and nevada and comparison with regional patterns k ar and u pb isotopic studies demonstrate middle mesozoic metamorphism and plutonism followed by eocene plutonism in the pilot range northeastern great basin combined with field relations the age constraints indicate that local amphibolite facies and widespread greenschist facies metamorphism peaked between 165 and 150 ma and that plutons were emplaced early in the deformational and metamorphic history between 165 and 155 ma cenozoic plutons and widespread dikes that were emplaced at about 40 ma cut folds and faults including a major detachment fault that placed unmetamorphosed on metamorphosed rocks the mesozoic and early cenozoic history of metamorphism and plutonism in the pilot range is consistent with age data and field relations throughout the northeastern great basin that indicate late jurassic plutonism and metamorphism followed by eocene and oligocene plutonism and local metamorphism in many areas rocks that were metamorphosed during late jurassic time were cooled through ar blocking temperatures for micas between 85 and 60 ma the widespread occurrence of these mica ages suggests a regional mechanism for cooling

observations of nightside auroral cavities radar observations of field aligned auroral f region density depletions cavities have been identified in a portion of the sondre stromfjord incoherent scatter radar isr data base covering the period february 1986 to january 1988 these auroral cavities are nightside phenomena with localized field aligned f region density depletions of 20 to 70 percent below surrounding values they occur during moderate to quiet geomagnetic conditions when the pillared edge of the auroral oval is within view of sondre stromijord seasonally they are a wintertime phenomena occurring just pillared of the statistical auroral oval unlike the previously reported polar hole the average width of the cavities is less than 100 km case studies show that the cavities closely track the pillared edge of the most pillared auroral arc sequential radar scans show that cavities appear on time scales as short as several minutes suggestive of local electrodynamic formation or rapid transport data from january 24 1987 collected during coordinated optical radar and satellite observations spanning an hour of local time were examined for possible cavity formation mechanisms the cavity formation processes examined herein include locally enhanced chemical loss vertical diffusion drifting horizontal gradients in the background plasma and evacuation as a result ofmore field aligned currents the formation time scales calculated evacuation fluxes close proximity to e region aurora and field aligned current signatures seen in magnetometer and radar observations suggest a strong association of the cavities with upward flowing electrons carrying region 1 downward field aligned currents 46 refs 9 figs 1 tab less

case of the phantom foredeep early cretaceous in west central utah a reconstruction of the structure in west central utah shows that a major foredeep with sedimentary deposits possibly as thick as 6 km now eroded probably existed at the present site of the sevier desert during early cretaceous time and conceivably during late jurassic time concurrent emplacement and erosion of the canyon range thrust sheet provided the load and sediment much like the willard paris thrust sheet did for the better documented early cretaceous foredeep in northern utah and eastern idaho the onset of major thrusting in western utah may have been as early as callovian or as late as albian a 50 m y time span

late cretaceous extension in the hinterland of the sevier thrust belt northwestern utah and southern idaho cover rocks of the raft river metamorphic core complex located in the sevier belt hinterland preserve a structural and metamorphic history that predates the middle tertiary extension of the region in the eastern raft river mountains cambrian permian rocks form two allochthons that occur in the hanging wall of the mid miocene raft river detachment fault dramatically attenuated metamorphosed cambrian pennsylvanian strata of the lower allochthon yield late cretaceous muscovite sup 40 ar sup 39 ar plateau ages ca 82 90 ma that are interpreted to date cooling during final stages of layer parallel extension devonian permian rocks exposed in the black pine mountains underwent east west extension approximately 160 associated with syntectonic growth of white mica along cleavage sup 40 ar sup 39 ar whole rock spectra of associated slates also indicate a late cretaceous metamorphism together field and sup 40 ar sup 39 ar results suggest that late cretaceous extension occurred in the sevier belt hinterland at the same time as shortening in the eastern foreland and at depth in the hinterland sufficient topography must have been present to drive upper crustal extension in the eastern hinterland

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

plasma characteristics of polar cap f layer arcs first results are reported of a comprehensive campaign to measure ionospheric structure and dynamics of nighttime polar cap f layer or f region arcs airborne optical and digital ionosonde data were collected simultaneously with ground based incoherent scatter radar data continuously over many hours in time and 500 to 1000 km in space polar cap f region sub visual auroral arcs which are commonly observed during quiet magnetic conditions are found to represent boundaries or shears in the polar cap plasma circulation pattern f region electron concentration enhancements are found in these features and can be of significance to polar thermospheric circulation composition and thermal structure the quiet polar ionosphere is so structured that high time resolution intensified optical images of thermospheric emmissions can make a major contribution to interpretation of incoherent scatter radar data

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

mesozoic structure of the newfoundland mountains utah horizontal shortening and subsequent extension in the hinterland of the sevier belt the newfoundland mountains of northwestern utah preserve an unusually complete record of mesozoic deformation in the hinterland of the sevier orogenic belt structural relations of paleozoic miogeoclinal strata with the newfoundland stock which has biotite and hornblende k ar ages of 153 2 4 6 and 147 7 4 4 m y respectively and dated quartz monzonite dikes with k ar biotite ages of 150 7 4 5 and 143 7 4 3 m y and a muscovite age of 149 1 4 5 m y provide the basis for dating mesozoic deformation in the range geologic mapping at 1 24 000 has identified four geometric classes of faults with known or probable mesozoic ages 1 older over younger thrust faults including the desert peak thrust named herein 2 inward facing folds in the contact aureole of the stock 3 low angle normal faults at a high angle to bedding intruded by undeformed mesozoic dikes and 4 younger over older faults oriented at a low angle to bedding microfabric analysis of recrystallized rocks in the contact aureole and unrecrystallized country rocks shows that most of the intracrystalline strain is pre latest jurassic in age and is characterized by layer parallel shortening post latest jurassic intracrystalline strain is minor and records shortening originally oriented nearly vertical the map geometries geochronology and microfabric analysis suggest the following structural history from oldest to youngest 1 pre latest jurassic regional shortening and thrusting and 2 syn or post latest jurassic horizontal extension during the mesozoic in the cretaceous and early tertiary whereas strata in the idaho wyoming northern utah thrust belt to the east were shortened by 140 150 km the rocks in the newfoundland mountains experienced little internal strain or were being extended even though they were also probably being translated eastward above the deep seated westward extension of the thrust belt decollement

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

prograde metamorphism strain evolution and collapse of footwalls of thick thrust sheets a case study from the mesozoic sevier hinterland u s a abstract analysis of prograde metamorphic fabrics in the exhumed footwall of the mesozoic windermere thrust northeast nevada reveals fabrics and style of ductile flow developed during tectonic burial metamorphism of a sedimentary footwall of a thick thrust sheet in response to structural burial footwall strata underwent barrovian metamorphism synchronous with development of bedding parallel or near parallel s and s l tectonites footwall rocks range from unmetamorphosed at high structural levels and progressively increase in metamorphic grade to upper amphibolite facies at deep levels attenuation of footwall stratigraphic units accompanied tectonite development whereby the amount of attenuation varies with metamorphic grade from 0 in lower greenschist facies to 30 50 in upper amphibolite facies rocks microstructures in metacarbonate metapsammite and metapelite and analysis of crystallographic preferred orientations of quartz c axes in quartzite suggest a dominantly coaxial strain path with flattening strain predominating at lower metamorphic grade and plane strain more dominant at higher metamorphic grade the microstructural data indicate that dominantly coaxial deformation during metamorphism accommodated extension of the footwall ductile extension most likely records footwall collapse induced by a loss of strength due to relative upward migration of isotherms during and or following burial extension may act as a mechanism that in part facilitates isostatic accommodation or sinking of the overlying load resulting in a reduction of topography attenuation of footwalls of thick thrust sheets and production of predominantly coaxial layer parallel or near parallel fabrics may be an intrinsic process once sufficient structural overburden is developed and thermal relaxation and prograde metamorphism ensues

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

the tarskavaig nappe of skye northwest scotland a re examination of the fabrics and their kinematic significance early petrofabric studies of quartz c axis preferred orientation within the tarskavaig nappe located at the southern end of the moine thrust zone have been interpreted as indicating that structures within the nappe have been produced by tectonic movements acting at right angles to those responsible for formation of structures within the overlying but immediately adjacent moine nappe such a dramatic contrast in inferred transport direction has not been recognized within the rest of the thrust zone re examination of the microstructures and c axis fabrics within mylonitic metasediments from the western part of the tarskavaig nappe has revealed that these tectonites are characterized by maximum principal extension axes which trend sub parallel to the wnw trending moine thrust zone transport direction later folding has reorientated and modified these structures within the eastern part of the nappe producing fabrics which have erroneously been taken to indicate a transport direction orientated sub perpendicular to that of the rest of the moine thrust zone wnw trending maximum principal extension axes and wnw directed overthrust senses are indicated by microstructures within schists from the adjacent moine nappe suggesting that structures within both the tarskavaig and moine nappes may be associated with a common west northwest transport direction essentially coaxial pure shear strain paths indicated by symmetrical c axis fabrics dominate the internal parts of the tarskavaig nappe close to the base of the nappe non coaxial strain paths originally associated with wnw directed overthrusting are indicated by asymmetrical c axis fabrics and oblique grain shape alignments strain compatibility considerations indicate that the lower kinematic domain which must either be contemporaneous with or later than formation of the upper domain must be characterized by a combination of pure and simple shear deformation

development of preferred orientation in plane strain deformed limestone experiment and theory carbonate rocks deform preferentially by twin gliding on e 01 18 and slip on r 10 14 and f 02 21 in polycrystalline aggregates strong textures develop we report on experimentally produced textures in triaxial plane strain geometry with orthorhombic symmetry at 200 c and 400 c pole figure of the experimentally deformed specimens are compared quantitatively with theoretical simulations based on the taylor theory using both slip and mechanical twinning as mechanisms agreement at low and high temperature is satisfactory and documents that models developed for f c c metals can be applied to low symmetry minerals provided that deformation mechanisms are known and that mechanical twinning is properly accounted for comparison with experimental results indicates that strain was nearly homogeneous at the conditions considered and the same may apply to many geological textures three texture types are described which are differentiated mainly by the relative importance of e twinning

reply to w gawthrop s comments on the lompoc california earthquake november 4 1927 m 7 3 and its aftershocks

construction of a composite pressure temperature path revealing the synorogenic burial and exhumation history of the sevier hinterland usa metamorphic pressure temperature p t paths derived from 16 growth zoned garnets nine from this study and seven from a previous study have been combined to construct a detailed composite path for an area in the hinterland of the cretaceous to early tertiary sevier orogenic belt in southern idaho and north west utah samples are from two proterozoic units in the footwall of the basin elba thrust the schist of mahogany peaks in the central albion mountains idaho and the schist of stevens spring in the basin creek area of the grouse creek mountains utah 40 km to the south the simulated portions of the garnets analysed in this study grew from reactions involving the breakdown of chlorite in the upper greenschist to lower amphibolite facies multiple garnets were analysed from three samples overlapping segments of p t paths from different garnets in the same sample correlate with respect to slope and garnet mn concentration the composite p t path documents three episodes of sharply increasing pressures separated by two episodes of pressure decrease all during progressively increasing temperatures the path is interpreted to represent alternating episodes of synconvergent thrusting and extensional exhumation in the hinterland of the sevier orogen burial was probably caused by the basin elba fault the only major thrust exposed in the region extensional exhumation may have occurred along the mahogany peaks or emigrant spring faults or by extensional reactivation of the basin elba fault this method of correlating partial p t paths to reveal a more complete composite path provides a powerful tool in unveiling orogenic histories in metamorphic terranes where evidence of major structures responsible for burial and exhumation is commonly obscured by later events

neogene lateral extension during convergence in the central alps evidence from interrelated faulting and backfolding around the simplonpass switzerland abstract the simplon fault zone occurs at the western end of the lepontine metamorphic dome in the central alps and represents a major low angle 25 dip normal fault associated with wsw ene directed lateral extension movement began in the early miocene and continued until at least 3 ma accumulating a total vertical throw of around 10 km corresponding to 24 km displacement parallel to the fault this displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic detachment fault the simplon line in the central region simplon village to zwischbergen some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone due to the higher exposed structural level the importance of the cataclastic component increases towards the northwest in the northern steep belt around the simplonpass retrograde greenschist facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with sse vergent large scale backfolding representing nnw sse shortening and crustal thickening the combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen allowing lateral escape of continental material normal faulting and the resultant lateral escape was not just superficial it extended to depths of at least 15 km and probably to depths exceeding 25 km the overall geometry and kinematics of the simplon fault zone and in particular the interplay between folding and faulting is very similar to that described from the metamorphic core complexes and detachment faults of the cordillera of the southwestern usa within the central alps extension was associated with rapid exhumation of the lepontine metamorphic dome as evidenced by very rapid cooling rates determined from mineral ages in the footwall similar to the cenozoic tectonics of the himalayas and tibet orogen parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction more limited extension of the same age and orientation is known from a large area of the european foreland greater extension in the mountain belt compared with the foreland was accommodated by major strike slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern alps of haute provence france coeval folding and orogen parallel extension such as at the simplonpass were developed in a dextral transpressive regime due to oblique wnw ese convergence across the alps during the neogene

formation of recumbent folds during synorogenic crustal extension austroalpine nappes switzerland in sedimentary rocks of the austroalpine nappes in eastern switzerland several generations of alpine cretaceous tertiary folds can be distinguished by overprinting relations the second generation d 2 probably late cretaceous in age includes spectacular kilometre scale folds refolding the earlier formed nappe pile d 2 folds are interpreted here as resulting from east southeast directed crustal extension rather than as previously assumed from north south crustal shortening for the following reasons 1 d 2 folds are restricted to a predominantly ductile layer underlying the brittlely extended orogenic lid and 2 d 2 folds are associated with structures indicating extension in a direction parallel to the extension direction of the brittle layer before the east southeast extension started the layering of the rocks was already steeply inclined owing to earlier d 1 folding the deformation producing the second phase folds probably had a noncoaxial component with a top to the east southeast shear sense and a component of coaxial deformation leading to shortening in the vertical direction and stretching in an east southeast horizontal direction the folds developed with axes oblique to parallel to the direction of bulk stretching i suggest that folding of initially nonhorizontal layers by ductile crustal extension is an important deformation mechanism in the austroalpine and possibly also in other parts of the alps initially steep orientations of layers resulted from transpressive deformation in the obliquely convergent alpine belt and crustal extension parallel to the chain resulted from gravitational spreading i propose that this process can lead to the often encountered phenomenon of stretching parallel to the axes of folds

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

allochthonous quartzite sequence in the albion mountains idaho and proposed proterozoic z and cambrian correlatives in the pilot range utah and nevada

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

oxygen and carbon isotope profiles in metasediments from lizzies basin east humboldt range nevada constraints on mid crustal metamorphic and magmatic volatile fluxes 18o 16o ratios vary systematically in the 1 000 m section of interlayered metasediments and granitoids at lizzies basin the deepest structural level exposed in the east humboldt range metamorphic core complex in the lower 300 m of the section the lower zone 18o is homogeneous and low 6 6 to 8 8 in silicate rocks 8 7 to 12 1 in marbles a detailed oxygen isotope profile across an individual marble layer within the lower zone has a similar range in 18o 9 5 to 11 9 with the highest values preserved in the middle of the layer in the upper 700 m of the section the upper zone metasediments have been less strongly 18o depleted the 18o values are higher and more heterogeneous and in a profile across a marble layer of similar thickness rise steeply from marginal values of 12 to core values of 23 quartz from silicate metasediments throughout the upper zone ranges from 11 to 13 and is thus fairly homogeneous particularly where detailed profiles have been measured adjacent to the 18o rich marble layers covariation of 18o 16o and 13c 12c ratios in marbles suggests that the isotopic composition of these elements has been altered by exchange with infiltrating water rich co2 h2o fluids with mantle like isotopic composition in most cases marble cores preserve protolith 13c values and these vary systematically throughout the section according to structural level including some exceptionally 13c rich values up to 12 this range may reflect stratigraphic variation in 13c of the proterozoic sedimentary protoliths of these rocks the upper lower zone boundary and the contrast in isotope systematics to either side has been previously explained either as an impermeable barrier to fluid flow or as an infiltration front wickham and peters 1990 the second alternative has been tested using a numerical model in which low 18o aqueous fluid flows vertically upward through an alternating sequence of monomineralic quartz 18o 12 and calcite 18o 22 layers in a regime in which the oxygen isotopic composition is controlled by advection diffusion in the fluid and fluid solid exchange solutions to the relevant transport equations indicate that the abrupt change in the average 18o value of the layers at the upper lower zone boundary can be reproduced by the model with a uniform porosity throughout the system but the observed contrast in the shapes of the marble layer profiles in the upper and lower zones cannot be reproduced under these conditions however if the calcite layers have two orders of magnitude lower porosity in comparison with the quartz layers and exchange within them is diffusion dominated as opposed to advection dominated in the quartz the contrasting shapes of the marble profiles in the two zones are reproduced as well as the upper lower zone discontinuity the range of conditions that generates both an infiltration front at the appropriate scale and contrasting marble profiles at the appropriate scale is quite narrow but requires a volatile flux that could be generated by plausible volumes of mantle derived magma crystallizing at depth beneath the area providing support for this mechanism as a viable agent of 18o depletion in the deep level rocks of this terrane

space time patterns and tectonic controls of tertiary extension and magmatism in the great basin of the western united states structural and stratigraphic relations in the great basin indicate widespread pre middle miocene crustal extension that appears to define two north trending belts most extension in these belts was oligocene age but locally it began earlier or lasted into early miocene time the eastern belt straddles the nevada utah border and includes the snake range nevada area with its southern end near 37 5 n and its western edge at the seaman butte mountains breakaway the southern boundary of the eastern belt is occupied by the 26 15 ma caliente caldera complex and approximately coincides with the present east west trending margin of the great basin north of saint george utah crust north of this boundary extended approximately east west before volcanism began at 30 32 ma but to the south extension began after about 15 ma this boundary may have been a rooted zone of left slip faults that allowed the footwall of the stampede detachment to move west relative to unextended terrain to the south the eastern margin of the eastern belt is probably located near the present eastern edge of the great basin but its northern end is poorly defined the western belt runs from the funeral and grapevine mountains california to the ruby mountains nevada and north northeast to the albion range idaho tens of kilometers of crustal extension occurred at least locally in both belts but magnitude of extension is poorly known for large areas of each tertiary volcanism in the great basin began in the north in eocene time with predominantly effusive volcanism and swept southward ending with voluminous oligocene miocene ignimbrite eruptions from calderas in an irregular discontinuous belt between marysvale utah and reno nevada a result of the south ward migration of volcanism is that the onset of extension in both belts was syn or post volcanic in the north but was pre volcanic in the south late paleogene extension and crustal magmatism coincided in both time and space only locally where south migrating magmatism overlapped active north south trending extensional belts most calderas in the southern great basin formed in previously extended belts or on their margins southward migration of ignimbrite sources was apparently blocked by unextended crust to the south in contrast volcanism north of the ignimbrite province was dominated by nonexplosive effusion of lava prior to or during crustal extension this is consistent with observations in the southern basin and range where volcanism and crustal extension were generally synchronous and volcanism was dominantly effusive thus caldera formation may be controlled by the distribution of upper crustal extension although the physical mechanism for this control remains speculative the space time patterns of late paleogene extension in the great basin are consistent with extension being triggered by thermal weakening of subducted oceanic lithosphere rather than by effects transmitted from the plate margin but being driven by gravitational collapse of thick crust space time patterns of tertiary volcanism in the basin and range also appear to conform to patterns of thermal weakening or destruction of the subducted slab both active and passive rifting mechanisms are inapplicable on the scale of the extensional belts because both predict close spatial and temporal association of extension and magmatism which is not generally observed

time of initial thrusting in the sevier orogenic belt idaho wyoming and utah reexamination of the distribution of fossils found in the earliest preserved synorogenic conglomerates within the sevier thrust belt suggests that initial thrust movement may be no older than aptian age this interpretation is corroborated by subsidence analyses of sedimentary sequences lying within and east of the idaho wyoming and utah thrust belts a major episode of middle cretaceous aptian cenomanian subsidence is interpreted as recording the initiation of thrust loading deformation in the adjacent sevier orogenic belt an earlier subsidence event took place during middle jurassic time more than 30 m y prior to the cretaceous event and may be the result of tectonic events to the west that are unrelated to thrust deformation in the idaho wyoming and utah thrust belts we find no verifiable evidence to support previous interpretations that sevier belt deformation and uplift began in late jurassic time

transition from contraction to extension in the northeastern basin and range new evidence from the copper mountains nevada abstract new mapping structural analysis and 40ar 39ar dating reveal an unusually well constrained history of late eocene extension in the copper mountains of the northern basin and range province in this area the northeast trending copper creek normal fault juxtaposes a distinctive sequence of metacarbonate and granitoid rocks against a footwall of upper precambrian to lower cambrian quartzite and phyllite correlation of the hanging wall with footwall rocks to the northwest provides an approximate piercing point that requires 8 12 km displacement in an ese direction this displaced fault slice is itself bounded above by another normal fault the meadow fork fault which brings down a hanging wall of dacitic to rhyolitic tuff that grades conformably upward into conglomerate these relationships record the formation of a fault bounded basin between 41 3 and 37 4 ma the results are consistent with a regional pattern in which volcanism and extension swept southward from british columbia to southern ne

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

calcite fabrics in a natural shear environment the helvetic nappes of western switzerland the aim of this contribution is to describe and interpret the crystallographic orientations and microstructures of calcite in the upper jurassic limestones of the helvetic nappes between the mont blanc and aar massifs over a distance along strike of about 180 km the c axis preferred orientation pattern which could be inferred is regionally very constant the c axis point maxima are everywhere subperpendicular to the regional strike of the chain there is an obliquity between the orientation of the macroscopically dominant cleavage and the microscopically visible long axes of individual grains the c axis maxima in most specimens are parallel to the axis of shortening of the microscopic grain shape fabric this fabric geometry indicates either continuous shear deformation or alternatively a two stage deformation history the obliquity of the grain shape fabric may be due to the fact that the flattening plane of the individual grains reflects only the last increment of a continuous shear process or it may represent a later finite deformation which has led to the formation of a regionally developed second cleavage there is a systematic relationship between the direction of these late shear movements and the main overthrust direction as given by the finite strain pattern this suggests that the microfabric of the rocks was acquired during a progressive rotation with time of the main overthrust direction the observed pattern of preferred orientation has been correlated by several authors with intracrystalline deformation mechanisms a combination of slip on r and twinning on e however the e twinning observed in the specimens investigated is younger than the development of the crystallographic fabric a new method for stress determination from e twinning is proposed

location and size of the 1927 lompoc california earthquake from tsunami data abstract the lompoc earthquake of 4 november 1927 m s 7 0 was one of the largest offshore events in california and the associated tsunami was recorded in hawaii as well as on the california coast the epicenter of this event has long been debated one location is offshore where the ocean is about 1000 m deep whereas another location is very near shore beneath a water depth of less than 100 m we analyzed the tsunami records from this event from tsunami travel times the source location was estimated to be well offshore at about 34 2 n 120 75 w consistent with recent seismic waveform analysis waveform modeling of the tsunami also supports an outer continental shelf location the seismic moment is estimated to be 3 10 26 dyne cm corresponding to a moment magnitude of 7 0 the tsunami computation reproduced the large tsunami in hawaii indicating that the observed large tsunami was due to bathymetry effects particularly near hilo bay the tsunami magnitude of 7 6 obtained from the hawaiian recordings by abe thus appears to be an overestimate

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

constrictional strain in a non coaxial shear zone implications for fold and rock fabric development central mojave metamorphic core complex california finite strain and fold analyses of footwall mylonites in the central mojave metamorphic core complex cmmcc reveal two distinct deformation paths that generated non coaxial constrictional strain the first path recorded in the waterman hills accomplishes constrictional strain through the formation of l tectonites at the grain scale the second path recorded in the mitchel range involves a combination of plane strain at the grain scale and y axis shortening through syn mylonitic folding the ductile shear zone in the waterman hills is approximately 500 m thick and is developed entirely within a miocene syn kinematic granodiorite strain magnitude decreases structurally downward the mylonites contain abundant ne directed non coaxial microstructures yet are l tectonites rf o analysis of quartz ribbon grain shapes indicate prolate strains k 6 and bulk chemical data suggest that mylonitization was isochemical or involved volume loss therefore ductile shearing in the waterman hills involved true constrictional strain at the grain scale immediately to the southeast in the mitchel range the lateral continuation of the shear zone is more than 1000 m thick and primarily involves lithologically heterogeneous pre tertiary basement the mylonitic fabric comprises a well developed foliation and ne trending stretching lineation rf o strain analysis was applied to grain shapes of cataclasized and strongly altered garnet porphyroclasts in a peraluminous granite in contrast to the waterman hills finite strains in these l s tectonites approximate plane strain k 1 1 however the mylonitic fabric is strongly folded about axes uniformly oriented subparallel to the stretching lineation the coaxial folds range from open to isoclinal we interpret the folds to have nucleated with axes parallel to the stretching lineation during mylonitization and to have accomplished y axis shortening in the shear zone macroscopic folds of the brittle detachment spatially correspond to open folds in the ductile shear zone suggesting that y axis shortening continued after the fault zone had reached the brittle regime corrugations and folds oriented subparallel to the transport direction are characteristic features of fault zones in cordilleran metamorphic core complexes therefore constrictional strain may be common in core complexes strain compatibility with upper plate rocks in core complexes may be maintained by y axis shortening along conjugate strike slip faults a nearly uniaxial stress field 3 2 1 is consistent with most of the structures formed in basin and range extension and could generate the observed constrictional strain

linking thermodynamic modelling lu hf geochronology and trace elements in garnet new p t t paths from the sevier hinterland major element trace element and lu hf geochronological data from amphibolite facies pelitic schist in the raft river and albion mountains of northwest utah and southern idaho indicate that garnet grew during increasing pressure interpreted to be the result of tectonic burial and crustal thickening during sevier orogenesis garnet growth was interrupted by hiatuses interpreted from discontinuities in major element zonation pressure temperature paths were determined from the pre hiatus portions of the garnet chemical zoning profiles and indicate an increase of 2 kbar and 50 c in the western raft river mountains garnet lu hf dates of 150 1 ma in the western raft river mountains and 138 7 0 7 ma and 132 5 ma in the southern albion mountains indicate the timing of garnet growth lutetium garnet zoning profiles indicate that the lu hf ages are biased towards the post hiatus or outer pre hiatus segments indicating that the determined ages likely post date the recorded p t path history or date the tail end of the paths crustal thickening associated with sevier orogene sis in the western raft river mountains thus began slightly before 150 1 ma in the late jurassic this study shows that integrating p t paths determined from garnet growth zoning with lu hf gar net geochronology and in situ garnet trace element analyses is an effective approach for interpreting and dating deformation events in orogenic belts

calcite texture development in experimentally induced ductile shear zones specimens of fine grained micritic limestone were deformed in plane strain geometry in pure shear a combination of simple and pure shear and in simple shear temperatures were 400 c and 500 c confining pressure was 100 mpa in the experiments with a simple shear component strain is concentrated and approximately homogeneous in a 2 3 mm wide shear zone shear displacement is documented by marker lines and circles shear strain varies between 0 84 and 1 56 strain is recorded by flattening of individual grains defining a foliation normal to the axis of principal finite shortening e1 no twinning is observed on a macroscopic scale

the lower cretaceous king lear formation northwest nevada implications for mesozoic orogenesis in the western u s cordillera cretaceous paleogeography of the u s cordillera west of the sevier fold and thrust belt is poorly known due to the scarcity of cretaceous supracrustal rocks in the arc and backarc regions the lower cretaceous king lear formation exposed in the jackson mountains and the krum hills of northwest nevada provides a rare and important record of early cretaceous tectonism and paleogeography our work shows that the king lear formation everywhere overlies deformed and metamorphosed triassic jurassic rocks across a major unconformity the king lear formation is dominated by conglomerates and sandstones deposited in subaerial alluvial fan and gravelly braided river systems and it can be divided into three new members based on differences in clast provenance a lower member derived from local arc and backarc rocks a middle member dominated by externally derived quartzite and chert clasts and an upper member derived largely from an intrabasinal volcanic complex sedimentary features and provenance analysis indicate that clasts in the middle member were derived from nearby exposures of paleozoic rocks such as the roberts mountains and golconda allochthons which must have formed a topographically elevated area in central nevada during the early cretaceous the stratigraphic record provides evidence of deposition in a tectonically active basin in contrast with some prior studies inferring that deposition was synchronous with contractional deformation our new field observations structural data and a shallow seismic reflection profile confirm that the king lear formation was deposited in an active half graben in the jackson mountains these results provide new upper age constraints on the timing of shortening deformation in the arc and backarc regions shortening must have been complete not only before deposition of the king lear formation in the middle early cretaceous but also prior to several kilometers of exhumation of its depositional basement possible driving forces for extension leading to development of the king lear basin include relaxation of thick crust in the hinterland of thrust belts to the east stresses associated with regional tilting and development of dynamic topography and stresses related to dextral strike slip deformation to the west

cretaceous crustal structure and metamorphism in the hinterland of the sevier thrust belt western u s cordillera combined structural and geochronologic studies to the west of the sevier belt at lat 39 degree 40 degree n indicate that peak metamorphism of the upper 15 km of crust occurred during the late cretaceous 70 90 ma overlapping in time with the intrusion of muscovite bearing granites and pegmatites metamorphism was accompanied by penetrative deformation top to the east layer parallel shear both were localized at the currently exposed structural levels resulting in impressive vertical and lateral strain gradients and t sub max gradients of 50 c km inception of thrust faulting in the sevier belt may have predated peak metamorphism and intrusion of granites in the hinterland by 10 15 m y but it continued during and ended simultaneously with the end of metamorphism at about 70 ma the rise of muscovite bearing granites and hot fluids generated by metamorphism and crustal anatexis at greater depth provides and explanation for the observed localized metamorphism and deformation at relatively high structural levels

thermal evolution of a portion of the sevier hinterland the northern ruby mountains east humboldt range and wood hills northeastern nevada conventional thermobarometry and gibbs method modeling have been used to obtain new information pertaining to the mesozoic tertiary thermal evolution of the ruby mountains east humboldt range and wood hills metamorphic complexes northeastern nevada kyanite grade assemblages from clover hill in the extreme northeastern part of the east humboldt range were produced at temperatures of 780 810 k and depths in excess of 35 km previously published geochronologic data suggest an early cretaceous or late jurassic age for this event the clover hill area subsequently experienced a major unroofing interval prior to late cretaceous time during which it was brought roughly 20 km closer to the surface the lack of stratigraphic evidence for substantial cretaceous erosion in the region and the topology of the pressure temperature paths for the clover hill samples suggest that the unroofing mechanism may have been tectonic denudation related to the relaxation of large topographic gradients produced by crustal thickening in the hinterland of the sevier orogen metamorphic rocks at clover hill and within the wood hills core complex equilibrated at 820 900 k and 500 640 mpa 18 24 km in cretaceous time prior to 115 ma other parts of the ruby mountains east humboldt range metamorphic complex presently structurally separated from the clover hill sequence were intruded extensively by late cretaceous granitic magmas and attained substantially higher temperatures than the clover hill wood hills block sillimanite grade assemblages from these areas apparently underwent roughly 10 km of unroofing and 130 k of cooling prior to final equilibration at 820 920 k and 360 430 mpa 13 16 km during oligocene extension

history of cenozoic extension in central sevier desert west central utah from cocorp seismic reflection data cocorp seismic reflection data show that cenozoic extension beneath the sevier desert has had movement along a major west dipping surface reflections were obtained from several stratigraphic zones of known age within the sevier desert basin the geometric relationships of these zones indicate that extension has occurred in at least two stages a large first stage of movement 20 30 km or 12 18 mi began after the middle oligocene 26 28 ma and continued into the miocene this activity was followed by a probably short period of quiescence and erosion a later episode of movement of smaller proportions 8 km or 5 mi began during or after the eruption of a middle pliocene 4 2 ma basalt this later stage is still active as indicated by fault scarps which offs t recent sediments in the basin the record of extension within the sevier desert reflects the history of movement along the underlying sevier desert detachment a major low angle normal fault that has been traced over 70 km 43 mi perpendicular to strike and to depths of 12 15 km 7 5 9 3 mi the record of displacement along the sevier desert detachment may have implications for the general history of extension in the basin and range province

grain boundary migration microstructures in a naturally deformed quartzite abstract in this study several grain scale microstructures are presented that are thought to demonstrate the migration direction of once mobile grain boundaries in a naturally deformed quartzite an analysis is presented of the sense of migration of the boundaries and the characteristics of the patterns of relative grain growth and shrinkage grain boundary migration seems to be correlated with the relative crystallographic orientations of neighbouring grains for the quartz quartz grain boundaries and the pattern of preferred grain growth is roughly symmetrical about the mica foliation plane

late cretaceous paleocene synorogenic sedimentation and kinematic history of the sevier thrust belt northeast utah and southwest wyoming integration of cross cutting structural relationships overlapping sedimentary units new conglomerate provenance data and radiometric and palynological dates provides a basis for reinterpretation of the distribution and timing of late cretaceous through paleocene thrust faulting in the northeast utah southwest wyoming part of the sevier thrust belt these data indicate a general eastward progression of deformation that was punctuated by local out of sequence and hinterlandward verging events provenance data delimit a sequential restoration of a regional cross section the principal thrust systems in northeast utah and southwest wyoming are the willard ogden crawford absaroka and hogsback thrusts the willard is the westernmost structurally highest and oldest of the thrusts it carries a unique section of thick proterozoic sedimentary rocks as well as paleozoic rocks and was folded during displacement on younger thrust systems the next youngest thrust system is the ogden which comprises several basement rooted imbricate thrusts that together form a large antiformal stack with a structural culmination in the wasatch range growth of the wasatch culmination took place during coniacian through paleocene time contemporaneous with sequential displacement on the frontal crawford absaroka and hogsback thrusts total shortening in this part of the thrust belt was 100 km and structural relief of 25 km developed in the area of the wasatch culmination during coniacian through paleocene time thrusting followed an overall eastward progression that was interrupted by local out of sequence and hinterlandward verging events several episodes of synchronous displacement on two or more thrusts can be demonstrated shortening occurred in three main episodes the first episode 89 84 ma involved 33 km of shortening on the crawford thrust and its footwall imbricates approximately 19 km of structural relief on the basement cover contact developed in the area of the wasatch culmination the second episode 84 62 ma with a break between 75 69 ma involved 30 km of shortening mainly on the absaroka thrust and development of an additional 6 km of structural relief in the culmination the third episode of shortening 56 50 ma took place on the hogsback thrust involved 21 km of horizontal shortening and produced no significant increase in structural relief in the culmination long term rates of shortening ranged between 3 0 mm yr and 6 6 mm yr these three episodes of shortening produced three large accumulations of synorogenic conglomerate totaling 3 km in thickness the henefer formation and echo canyon and weber canyon conglomerates were deposited during crawford thrusting the evanston formation was deposited during and after absaroka thrusting and the lower conglomeratic part of the wasatch formation was deposited during and after hogsback thrusting most of the sediment in these synorogenic units however was derived from repeated uplift of the willard thrust sheet and from the eastern flank of the wasatch culmination in the rear part of the thrust belt only local minor accumulations were derived from the frontal ramp anticlines sediment accumulation and structural deformation were generally out of phase periods of regional shortening and uplift were marked by development of unconformities and sediment bypassing to distal parts of the foreland basin periods of structural inactivity were marked by accumulation of aerially widespread braided river conglomerate on top of the thrust belt one exception to this pattern is the henefer echo canyon weber canyon conglomerate deposit which contains evidence of progressive deformation in close proximity to the tip of the crawford thrust comparison of the sequential restoration with the late cretaceous subsidence history and isopach patterns in the distal foreland basin of western wyoming demonstrates that the principal controls on regional subsidence and sediment supply were the growth and erosion of the wasatch culmination growth of the duplex beneath the culmination may have been a means of maintaining critical taper in the thrust wedge

seismicity and rates of relative motion on the plate boundaries of western north america summary the consistency of earthquake data and plate tectonic models and other estimates of fault slip may be tested by estimating rates of motion from the earthquakes using the concept of seismic moment the contribution of individual events may simply be summed but a generally better estimate of long term average slip rate is obtained by integration over magnitudefrequency of occurrence relations estimates of fault motion rates associated with earthquakes are possible within about a factor of 2 using this approach if the major sources of uncertainty are given careful consideration e g incompleteness and inaccuracies in the earthquake data empirical moment magnitude relations and the effect of their stochasticity fault widths or depth extents the recurrence relations maximum magnitudes and the form of truncation at the maximum magnitude a formulation for the recommended magnitude density truncation is developed application of the latter method to the earthquake data of the offshore transform faults of the juan de fuca ridge system the queen charlotte fault zone and the northern vancouver island area in each case gives good agreement with rates from plate tectonic models for the southern san andreas fault and gulf of california area there is also good agreement with previous estimates obtained by summing the contributions of individual events however the displacement rate in the margin convergence zone of southern british columbia washington and oregon computed from the seismicity is at least a factor of 10 lower than from plate models and from other convergence estimates and primarily aseismic slip is suggested the fault motions as a function of time computed from the seismicity records have also been plotted and compared to the longterm average rates the plots permit estimates of the minimum present accumulated elastic strain and show if there are any temporal relations among earthquake displacements on different fault zones

m 6 earthquakes in southern california during the twentieth century no evidence for a seismicity or moment deficit abstract a broadly based report on seismic hazards in southern california wgcep 1995 concluded that the predicted seismicity exceeds that observed since 1850 a subsequent independent analysis argued that infrequent huge m 8 earthquakes are needed to explain the low rate of large earthquakes jackson 1996 frequency magnitude relationships and earthquake reporting suggest that the 1903 to 1997 catalog we present here with a b value of 1 0 and a rate of m 6 shocks of 0 42 to 0 49 yr 1 is nearly complete in contrast the 1850 to 1994 catalog used by wgcep is incomplete before the turn of the century and thus its reported seismicity rate of 0 32 m 6 shocks yr 1 is too low principally because the wgcep 1995 model results in b values of up to 4 0 for regions of lesser and blind faults the rate of m 6 shocks off the san andreas system predicted by the wgcep 1995 model is three times greater than that observed in this century because they obtained b 0 4 for m b 2 2 for m 7 3 on major faults their expected rate of m 7 san andreas shocks is twice as high as observed thus part of the seismicity and moment discrepancy identified by wgcep was caused by use of an incomplete catalog and part was caused by inappropriate b values we obtain a southern california moment release rate of 8 to 12 10 18 n m yr 1 which cannot be distinguished from the moment release estimated by fault slip or the moment accumulation inferred from plate motions or geodetically measured shear strain we thus find no evidence for a moment deficit significant aseismic moment release or for rare m 8 earthquakes off the san andreas fault system finally the number of m 6 earthquakes per decade does not depart significantly from a poisson process during this century and thus we find no evidence that the rate of seismicity is increasing now or at any other time since 1900

response of intracontinental deformation in the central andes to late cenozoic reorganization of south american plate motions new stratigraphic chronologic and fault kinematic data from the quebrada del toro in northwestern argentina provide a detailed view of late cenozoic kinematic reorganization in the southern central andes the quebrada del toro is a thrust bounded basin filled with 2 km of synorogenic clastic deposits ranging in age from early miocene to holocene as observed elsewhere in northwestern argentina faults in the quebrada del toro record an early phase of horizontal nw se contraction beginning between early and late miocene and ending after 0 98 ma and a later phase of horizontal ne sw contraction beginning between late miocene and 4 17 ma and still active these results require both kinematic regimes to have been active between 4 17 and 0 98 ma compatible with timing and kinematic data from the adjacent central andes and best explained by a temporal pattern of serial recurrence of the kinematic regimes a hypothesis that intraplate deformation in the central andes is driven by the absolute motion of the south american plate with respect to the hotspot reference frame is tested by comparing quebrada del toro kinematics to plate reconstructions and space based geodetic results nazca south america relative motion and absolute nazca plate motion show consistent azimuths throughout the past 20 5 myr in contrast absolute south american plate motion azimuths mimic the changing contraction directions determined from intraplate andean faults although the timing of the south american plate motion change is poorly resolved by current data it is compatible with the quebrada del toro data absolute upper plate motion might control intraplate kinematics along many of the earth s convergent margins

alternating contraction and extension in the hinterlands of orogenic belts an example from the raft river mountains utah combined macroscopic to microscopic structural analyses detailed geologic mapping and thermochronology were used to study the structural evolution of midcrustal rocks of the sevier orogenic belt hinterland in northwestern utah these results when combined with previous studies provide new insight into the structural history of this region and suggest alternating tectonic contraction and extension during mesozoic to early cenozoic time two allochthons form the upper plate of the miocene raft river detachment fault in the eastern raft river mountains the lower allochthon comprises neoproterozoic ordovician and pennsylvanian strata and is bounded below by the raft river detachment fault and above by the middle detachment fault strata within the lower allochthon were dramatically attenuated by two episodes of ductile deformation the first deformation d1 took place at metamorphic temperatures of 500 c and resulted in penetrative fabrics throughout these rocks that record combined flattening and top to northeast shearing strains the second deformation d2 resulted in significant stratigraphic attenuation along discrete top to the west shear zones that are generally parallel to lithologic contacts separates of synkinematic muscovite from the penetrative fabric yield 40ar 39ar cooling ages that indicate that d1 deformation occurred prior to cooling ca 90 ma both fabrics were subsequently folded about d3 kilometer scale recumbent folds the areally extensive middle allochthon composed chiefly of pennsylvanian and permian rocks metamorphosed in the greenschist facies was emplaced d4 along the low angle middle detachment fault this fault cuts across various structural levels of the recumbently folded lower allochthon in its footwall and juxtaposes greenschist facies over amphibolite facies metamorphic rocks the lower and middle allochthon were subsequently deformed d5 into open folds with north trending axes neogene extension d6 produced an 200 m thick top to east ductile shear zone in precambrian rocks and formed the younger raft river detachment fault which forms the present upper contact of the ductile shear zone northeast vergent d1 fabrics probably record shortening deformation on the basis of fabric correlations with the grouse creek and albion mountains deformation kinematics and synkinematic prograde metamorphism d2 attenuation faults have been interpreted to record crustal extension of late cretaceous age if d3 recumbent folds developed during extension then the deformation sequence records early shortening followed by protracted extensional unroofing with variable structural styles the favored alternative is that d3 recumbent folds developed during shortening in this case d1 through d4 record episodic alternations of contraction and extension such alternations are consistent with observations from analog and theoretical models of contractional mountain belts and suggest that the hinterland of the sevier orogenic belt underwent dynamic adjustments in crustal thickness and deformation kinematics in response to changes in the boundary conditions of the orogenic belt

pressure temperature paths from garnet zoning evidence for multiple episodes of thrust burial in the hinterland of the sevier orogenic belt garnets are present in two horizons of the schist of stevens spring from the basin creek area of the grouse creek mountains in northwest utah the two horizons possess different bulk compositions which resulted in garnet growth by different reactions along the same pressure temperature p t path garnet in the upper horizon grew from the breakdown of chlorite at upper greenschist facies conditions and garnet in the lower horizon grew from the breakdown of staurolite at upper amphibolite facies conditions from the upper horizon five garnets from three samples were analyzed all display growth zoning ragged morphologies associated with secondary rim consumption and yield garnet biotite geothermometry temperatures of 460 490 c from the lower horizon one garnet from each of two samples was analyzed these also display growth zoning but differ from garnet in the upper horizon in that they are dominantly idioblastic and yield garnet biotite geothermometry temperatures of 635 c garnet biotite geothermometry calculated for every point along detailed compositional traverses across the garnets revealed localized reequilibration along rims cracks and inclusions in both generations of garnet garnets from the upper horizon display prograde reequilibration and the garnets from the lower horizon display retrograde reequilibration numerical simulations of garnet growth using the gibbs method with duhem s theorem were carried out to determine p t paths the p t path defined by the modeling of five garnets from the upper horizon is an isothermal pressure increase of 1 7 kbar the p t path defined by the modeling of two garnets from the lower horizon has a steep p t trajectory d p d t 32 bars c and a total pressure change of 0 9 kbar both paths are indicative of thrust burial however the two paths cannot be reconciled as products of a single thrust episode these data are interpreted to indicate two different episodes of thrust burial during the sevier orogeny separated by 150 c of heating and partial exhumation