central baffin electromagnetic experiment cbex mapping the north american central plains nacp conductivity anomaly in the canadian arctic abstract over the summers of 2001 and 2002 a 45 station 500 km long regional magnetotelluric mt profile was acquired on central baffin island in the eastern canadian arctic this central baffin electromagnetic experiment cbex profile traverses the northern margin of the paleoproterozoic trans hudson orogen tho in its southern segment within the juvenile rocks of the orogen the profile lies on paleoproterozoic meta sedimentary strata known as the piling group and the profile extends northwards onto the archean rae craton the primary goal of the experiment was to determine the subsurface geometry of major geological boundaries and to define regional electrical structures field observations and laboratory analyses show that one particular horizon within the piling group the sulphidic graphitic astarte river formation is highly conductive and can be mapped and used as structural proxy for the base of the piling group the laboratory results imply that the source of the enhanced conductivity in the astarte river formation is the high content of interconnected graphite and that the host rocks are highly anisotropic due to bedding mapping this formation in depth images the base of the piling group basin well there is high contrast in electrical conductivity between the piling group meta sedimentary rocks and the archean granites and gneissic complexes of the rae craton to the north the lower crust of the rae craton in this area is moderately conductive some 100 s m in contrast to rae lower crust observed elsewhere in canada and this observation is not readily explained the lithospheric mantle beneath the profile exhibits a strong north to south gradient in decreasing resistivity suggesting resistive archean mantle 3000 m subducted beneath moderately resistive proterozoic mantle 300 m

u pb ages of neoarchean granitoids from the black hills south dakota usa implications for crustal evolution in the archean wyoming province abstract zircons in basement rocks from the eastern wyoming province black hills south dakota usa have been analyzed by ion microprobe shrimp in order to determine precise ages of archean tectonomagmatic events in the northern black hills nbh near nemo phanerozoic and proterozoic meta sedimentary rocks are nonconformably underlain by archean biotite feldspar gneiss bfg and little elk gneissic granite leg both of which intrude older schists the archean granitoid gneisses exhibit a pervasive nw se trending fabric whereas an earlier ne sw trending fabric occurs sporadically only in the bfg which is intruded by the somewhat younger leg zircon crystals obtained from the leg and bfg exhibit double terminations oscillatory zoning and th u ratios of 0 6 0 3 thereby confirming a magmatic origin for both lithologies in situ analysis of the most u pb concordant domains yields equivalent 207 pb 206 pb ages upper intercept u pb concordia of 2559 6 and 2563 6 ma both 2 for the leg and bfg respectively which constrains a late neoarchean age for sequential pulses of magmatism in the nbh unzoned in bse patches of 2560 ma zircon commonly truncate coeval zonation in the same crystals with no change in th u ratio suggesting that deuteric fluid assisted recrystallization accompanied post magmatic cooling a xenocrystic core of magmatic zircon observed in one leg zircon yields a concordant age of 2894 6 ma 2 this xenocryst represents the oldest crustal material reported thus far in the black hills whether this older zircon originated as unmelted residue of 2900 ma crust that potentially underlies the black hills or as detritus derived from 2900 ma crustal sources in the wyoming province cannot be discerned in the southern black hills sbh the peraluminous granite at bear mountain bmg of previously unknown age intrudes biotite plagioclase schist zircon crystals from the bmg are highly metamict and altered but locally preserve small domains suitable for in situ analysis a u pb concordia upper intercept age of 2596 11 ma 2 obtained for zircon confirms both the late neoarchean magmatic age of the bmg and a minimum age for the schist it intrudes taken together these data indicate that the neoarchean basement granitoids were emplaced at 2590 2600 ma sbh and 2560 ma nbh most likely in response to subduction associated with plate convergence final assembly of supercontinent kenorland in contrast thin rims present on some leg bfg zircons exhibit strong u pb discordance high common pb and low th u ratios suggesting growth or modification under hydrothermal conditions as previously suggested for similar zircons from se wyoming the leg bfg zircon rims yield a nominal upper intercept date of 1940 2180 ma which may represent a composite of multiple rifting events known to have affected the nemo area between 2480 and 1960 ma together these observations confirm the existence of a paleoproterozoic rift margin along the easternmost wyoming craton moreover the 2480 1960 ma time frame inferred for rifting in the black hills nemo area corresponds closely to a 2450 2100 ma time frame previously inferred for the fragmentation of supercontinent kenorland

fluid inclusions and hydrothermal alteration on the dixie valley fault nevada footwall rocks of the 1954 rupture segment of the dixie valley fault show extensive hydrothermal alteration related to fluids that were present on the fault during tectonic events hydrothermal alteration of granitic host rocks consists of temporally and spatially overlapping mineral assemblages an early biotite feldspar assemblage is followed by later fe chlorite and epidote both chlorite and epidote are replaced by hydrothermal sericite and cross cut by calcite hematite and quartz calcite veins biotite is partially replaced by prehnite the latest hydrothermal minerals are stilbite laumontite kaolinite alunite smectite illite and pervasive replacement of rock units with fine grained quartz chalcedony and opal secondary fluid inclusions trapped in healed microfractures in igneous quartz include type i inclusions that contain a moderate salinity aqueous liquid and vapor type ii inclusions that contain a moderate salinity aqueous liquid and co2 type iii inclusions that show eutectic melting temperatures below the nacl h2o eutectic and contain substantial cacl2 and type iv inclusions containing halite and other daughter minerals microthermometric measurements on these inclusions yield variable compositions and homogenization temperatures salinities of type i inclusions vary from 0 1 to 12 9 wt nacl with the mode in the interval 0 to 1 salinities of type ii co2 bearing inclusions range from 0 62 to 6 81 wt nacl relative to h2o and salinities of type iii inclusions with low eutectic melting temperatures are 12 9 to 25 3 nacl equivalent wt salinities of halite bearing inclusions are 30 1 to 39 2 wt nacl homogenization temperatures span the range 120 to 400 c the processes of isochemical cooling with upward displacement of the footwall mixing of cool low salinity water with hotter components and mixing of cool evaporite brine with hotter components could be responsible for variable fluid inclusion compositions homogenization temperatures and densities the p t path of the fault fluids is established by mineral equilibria and fluid inclusion characteristics the path includes a lithostatic fluid pressure at 305 c and 1570 bars along with cooling and escape of co2 from fluids the fluid p t path probably approaches hydrostatic pressure conditions at lower temperatures hydrothermal alteration product minerals fluid temperatures pressures and compositions in the footwall of the dixie valley fault constrain minimum fault age to 20 to 25 ma displacement to 6 km with about 3 km of pre 10 to 13 ma and 3 km of post 10 ma uplift fluid compositions and p t data suggest the following mechanism for rupture initiation and arrest ruptures may be initiated as a result of high fluid pressures then opening of dilatant fractures causes drastic decrease in fluid pressure separation of steam and co2 the drastic reduction in fluid bulk modulus that accompanies volatile phase separation permits propagation of the ruptures even though fluid pressure is reduced in areas where fluid pressure reduction is not accompanied by phase separation fractures are arrested by dilatant hardening

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

paleoclimatic implications of late pleistocene sediment yield rates for the bonneville basin northern utah abstract climate factors such as precipitation and temperature as well as climate driven geomorphic processes such as glaciation affect rates of erosion and the amount of sediment supplied from a drainage basin sediment yield thus sediment yield rates provide paleoclimatic implications although very little work has been published on ancient sediment yield rates late pleistocene lake bonneville utah idaho and nevada with its well documented lake level history and well preserved deltaic sequences provides a unique opportunity to estimate ancient sediment yield rates two end member delta systems coarse grained gravel classic gilbert type deltas and fine grained sand silt and clay deltas are exposed along the eastern margin of paleolake bonneville one example of each end member delta is examined in this study american fork and weber river deltas the established bonneville hydrograph can be used to determine time of deposition for a given package of sediments because these shoreline deltaic sediments were deposited in shallow water the geomorphic expressions of these deltas are well preserved and their sediment volumes can be determined river drainage basin area can be combined with time constraints and delta volumes to estimate sediment yield rates sediment yield rate estimates for the coarse grained american fork and fine grained weber river deltas are 478 m 3 km 2 yr 1 34 and 705 m 3 km 2 yr 1 15 respectively the similarity of these values is somewhat surprising due to the large difference in drainage basin sizes american fork 160 km 2 and weber river 3328 km 2 however this similarity suggests that precipitation may be the dominant control on sediment yield using sediment yield precipitation regression equations developed for similar modern basins in the southern alps of new zealand a first order approximation of paleoprecipitation is 1 6 m yr 14 for american fork delta and 1 9 m yr 7 for weber river delta these paleoprecipitation value are comparable to modern precipitation values of 0 4 1 5 m yr range of mean values over both drainage basins a maximum late pleistocene precipitation increase of 33 over the modern day value is suggested this is determined by the difference between the highest paleoprecipitation rate 1 9 m yr 7 of the weber river delta and the modern value 1 5 m yr previous climate models and this 33 paleoprecipitation increase suggest a maximum decrease in basin floor paleotemperature of approximately 13 c this paleotemperature decrease is consistent with previous estimates of late pleistocene temperatures using amino acid epimerization rates and periglacial wedges in the bonneville basin and adjacent areas these estimates suggest that although late pleistocene precipitation rates may have been slightly higher than present up to 33 higher late pleistocene temperature and evaporation rates were more important in the development of lake bonneville and associated glaciers

facies architecture and sequence stratigraphy of fine grained lacustrine deltas along the eastern margin of late pleistocene lake bonneville northern utah and southern idaho this study documents the effects of changing lake level limnostasy tectonics sediment yield and basin physiography on the facies architecture sequence stratigraphy and reservoir quality of three fine grained deltas deposited along the eastern margin of late pleistocene lake bonneville analysis of facies architecture indicates that the weber and spanish fork deltas were strongly wave modified because they were situated along openly exposed portions of the shoreline the bear river delta nestled in a relatively isolated northeast arm of the lake records both fluvial and wave processes these fine grained deltas were fed by low gradient rivers that drained large regions that were sparsely glaciated whereas other contemporaneous coarse grained gilbert deltas were fed by steep gradient rivers that drained local source areas that were strongly influenced by glaciers limnostasy tectonics and sediment yield were similar for all three fine grained deltas implying that the most influential forcing parameter on sequence stratigraphy is basin physiography basin physiography specifically ramp length and accommodation most strongly controlled the external and internal geometry of the lowstand systems tract of each delta potential hydrocarbon reservoir quality e g grain size and sorting is largely a product of drainage basin size and stream gradient relative differences in ramp lengths seemed to determine thickness and lateral continuity of delta front deposits relative differences in accommodation appeared to determine the internal geometry of the delta front deposits especially in the lowstand systems tract these deltas can serve as analogs for lacustrine exploration and production where many forcing parameters typically are unknown

horizontal strain across the wasatch front near salt lake city utah two adjacent geodetic networks span a 105 km extent of the wasatch fault zone in north central utah one of the networks located in the vicinity of salt lake city was observed by the national ocean service national geodetic survey nos ngs in 1962 1963 triangulation mode and again in 1973 1974 combined triangulation trilateration mode to the north in the vicinity of ogden the u s geological survey usgs performed five separate trilateration surveys of the second network during the 1972 1981 interval when fit to a model assuming both temporal and spatial homogeneity for the strain rate the two independent data sets yield mutually consistent strain rate tensors subsets of the nos ngs data however suggest spatial variation in the strain field and a subset of the usgs data indicates a possible temporal variation in the strain rate the quality of the data is such that we are uncertain whether or not these apparent variations represent actual tectonic signals the strain rate tensors for the two complete data sets suggest an underlying deformational pattern characterized by east west e w maximum principal strain extension positive this pattern may also be inferred from regional seismicity and quaternary geology the suspected variations would imply that the area to the north of salt lake city experienced significant e w compression preceding 1978 in contradiction to the underlying strain pattern this compression may be caused by the mountainous block eastward adjacent to the fault zone displacing westward along a shallow angle west dipping detachment surface we speculate on the existence of an e w trending zone of weakness for example secondary faults located north of salt lake city to explain how the stress field to the north may be decoupled from that to the south where e w compression has not been detected while seismic and geologic information contribute some plausibility to our speculations the evidence for the existence of the suspected variations is inconclusive

feldspar influenced rock rheologies progressive fracture subsequent cataclasis and syntectonic alteration of feldspar to phyllosilicates transformed a massive granite into a quartz mica phyllonite during early tertiary deformation in a 20 30 m wide shear zone in southeastern arizona the ductile deformation proceeded at temperatures and pressures approximating middle to upper crustal levels most of the deformation was taken up by the feldspar and its alteration products quartz deformed by both brittle fracture and crystal plastic mechanisms these results show that under certain temperature pressure strain rate conditions feldspars are weaker than quartz the observed deformation mechanisms indicate that the brittle to ductile transition in fault zones is not simply a function of the onset of temperature activated plastic deformation in quartz nor of the frictional behavior of rocks as inferred in the familiar strength vs depth curves it is more likely that the transition is complex and depends on a variety of parameters one such factor delineated in this study is the fracture and fracture facilitated syntectonic alteration of feldspar under hydrated conditions

extensional reactivation of an abandoned thrust a bound on shallowing in the brittle regime abstract shallow dip angles 45 suggested by field observations of continental extensional faults are not predicted by classical isotropic mohr coulomb anderson theory earthquake data indicate that normal faults exist in the upper crust with dip angles commonly as shallow as 30 some structural evidence suggests brittle normal faulting with dip angles as shallow as 10 one explanation of the apparent conflict between theory and structural seismic observations is that intrinsically weak shallow dipping pre existing faults are preferentially reactivated any reduction in frictional strength of the pre existing structure below that of surrounding rock increases the likelihood that such structures will be first breaking when extension is initiated enhanced fluid pore pressure on the pre existing fault reduces the effective strength and can further enhance shallow fault reactivation an analytical treatment clarifies the roles of geometry intact pre existing fault strengths and fluid pore pressures frictional strength ratios of 3 or greater could account for extremely shallow normal faults dips 10 20 without consideration of pore pressures in excess of the least principal stress or of principal stress systems rotated away from the gravity vector moderate reduction in friction 3 4 with respect to wall rock can reduce the dip to 30 and can account for shallow normal slip earthquakes

on the spatial and temporal distribution of near inertial energy in the southern ocean 1 we use an eddying realistic primitive equation model of the southern ocean to examine the spatial and temporal distribution of near inertial wind power input wpi and near inertial energy nie in the southern ocean we find that the modeled near inertial wpi is almost proportional to inertial wind stress variance iwsv while the modeled nie is modulated by the inverse of the mixed layer depth we go on to assess recent decadal trends of near inertial wpi from trends of iwsv based on reanalysis wind stress averaged over the southern ocean annual mean iwsv is found to have increased by 16 over the years 1979 2011 part of the increase of iwsv is found to be related to the positive trend of the southern annular mode over the same period finally we show that there are horizontal local maxima of nie at depth that are almost exclusively associated with anticyclonic eddies

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

the energy flux from the wind to near inertial motions in the surface mixed layer abstract time series of wind stress computed from long term meteorological buoy data off north america are used to examine the forcing of surface inertial currents by the wind a simple damped slab model of the mixed layer is used to compute h the average flux of energy from the wind to mixed layer inertial currents in a mixed layer of fixed depth h the forcing of mixed layer inertial motions is highly intermittent most of the forcing occurs during the winter months with a few dozen events accounting for typically over half the total energy flux major forcing events are usually associated with translating cold fronts or small lows with scales of about 100 km the larger synoptic scale features have little energy at the inertial frequency and thus result in only weak forcing of inertial currents a strong seasonal signal exists in the inertial forcing at ows p 50 n 145 w 50 m is largest from october to february and is a factor of 12 above the june and july values if seasonally varying

hydrothermal fluid venting along a seismogenic detachment fault in the moresby rift woodlark basin papua new guinea we investigate the conditions of hydrothermal alteration occurring along an active seismogenic normal fault dipping 30 in the moresby rift woodlark basin papua new guinea petrological and fluid inclusion studies were conducted in the samples recovered from the ocean drilling program leg 180 that cored the footwall scarp and hanging wall of the fault chemical analyses of the thermodependent si al substitutions in chlorites show that synrift alteration is coeval with a temperature rise from 200 c to 300 c in the footwall of the detachment whereas no thermal event is recorded in the hanging wall in the same samples fluid inclusions indicate that footwall rocks followed an isothermal exhumation path from 5 7 km to the subsurface taken together the results strongly suggest that the 100 c temperature rise results from the upward migration of h2o nacl fluids expelled by dehydrating metamorphic rocks from the lower crust fluid release began with the onset of detachment faulting and is still actively occurring nowadays the fact that low angle earthquakes also occur now at 5 7 km along the fault suggests that the dehydration of the metamorphic rocks and the subsequent circulation of hot hydrothermal fluids may be responsible for the weakening of the middle crust and the formation of a detachment in the moresby rift

synthetic fluid inclusions in natural quartz i compositional types synthesized and applications to experimental geochemistry synthetic fluid inclusions having a wide range of geologically applicable compositions in the c o h s na k ca cl cu fe system have been formed by healing fractures in inclusion free natural quartz and by precipitating new quartz overgrowths on an original seed crystal inclusion types synthesized include 1 1 liquid rich pure h2o inclusions of primary and secondary origin 2 2 pure h2o inclusions with the critical density 3 3 liquid rich inclusions containing undersaturated aqueous solutions of nacl or kcl or cacl2 or mixtures of the three salts 4 4 h2o nacl inclusions containing halite daughter minerals 5 5 h2o nacl kcl inclusions containing halite and sylvite daughter minerals 6 6 h2o co2 inclusions of various compositions containing liquid h2o and either co2 vapor or co2 liquid or both at 25 c 7 7 h2o co2 nacl inclusions containing an aqueous phase liquid and vapor co2 and halite at 25 c 8 8 c o h inclusions containing liquid h2o a co2 ch4 liquid phase and graphite daughter minerals at 25 c 9 9 h2o nacl inclusions that contain a chalcopyrite daughter mineral and 10 10 inclusions representing trapping of the coexisting immiscible fluids in the h2o nacl h2o co2 nacl and na c o h systems the inclusions exhibit uniform phase ratios at room temperature and the temperatures of various phase changes within individual inclusions agree with those predicted from experimental and theoretical data indicating that the inclusion fluid has the same composition and density as the parent solution these miniature autoclaves thus may be used to study various problems related to fluid inclusion research to calibrate analytical equipment used to study natural inclusions and to study phase equilibria solubility and pvt relations of a variety of chemical systems

a high resolution seismic reflection and gravity survey of quaternary deformation across the wasatch fault utah high resolution reflection seismic and detailed gravity data were acquired across an exposure of the wasatch fault utah near a trench excavated for dating of quaternary fault displacement the seismic data across the quaternary wasatch fault are interpreted to show three subparallel surfaces dipping from 70 w at the surface to an estimated 45 w at 40 m displaced unconsolidated sediments and colluvial material abutting the main fault detailed gravity data were of assistance in mapping the sediment bedrock interface from the rangefront across the wasatch fault zone a combined high resolution seismic and gravity interpretation reveals that bedrock is 80 m deeper in the hanging wall than in the footwall suggesting that displacement has occurred on several en echelon blocks in the near surface of the wasatch fault movement on the most recent wasatch fault trace began prior to deposition of pleistocene lacustrine sediments the twelvefold high resolution reflection seismic data were recorded and processed to achieve vertical resolution of approximately 1 m and horizontal resolution fresnel zone radii of as good as 5 m on the deepest reflecting interfaces at 40 m reasonably high frequencies and good bandwidth about 80 300 hz in the stacked seismic data permitted a direct comparison to the trenched cross section good correlation between the trench stratigraphy and structure and the seismic data suggests high resolution seismic data can be a valuable tool for analyzing near surface faulting in unconsolidated sediment and for locating potential trench sites

ion probe and fluid inclusion evidence for co seismic fluid infiltration in a crustal detachment we have investigated the geochemical pattern of fluid infiltration in the extensional detachment of tinos island cyclades greece ion microprobe o isotope analyses and fluid inclusion studies have been conducted in strain fringes developing around pyrite blasts in the mylonite of the shear zone micro scale traverses in quartz and calcite fibres show that 18o increases from 17 18 to 20 21 in 1 mm towards the blast drops of 3 in 200 m then rises again in the direction of growth 18o variations are interpreted as transient influxes of exotic fluids into the shear zone between periods of closed system buffering by the host rock fluid inclusions trapped in the fibres show fluctuating salinities 0 4 wt nacl eq and densities that reflect drops of the pore pressure from lithostatic 1 to hydrostatic 0 4 values during fringe growth isotopic and microthermometric data are consistent with models of seismic pumping developed for compressive shear zones we therefore suggest that co seismic pore pressure variations developed suction forces sufficient to drive large scale fluid migration in the tinos detachment as in convergent tectonic settings

incipient metamorphism and deformation in the variscides of sw dyfed wales first steps towards isotopic equilibrium incipient metamorphism accompanying thrusting folding and cleavage development has been investigated in a varied sequence of palaeozoic sediments near the variscan front in sw dyfed wales the aim was to evaluate a critical stage in the progression from heterogeneous sediment whose detrital phases are neither in equilibrium with one another nor with pore fluids through indurated sedimentary rock to metamorphic rock comprising newly formed crystals that equilibrated with one another as they grew quartz veins are widely developed in the area especially in the more psammitic lithologies while finer grained rocks became cleaved during tectonic deformation mineralogical constraints and fluid inclusion measurements suggest maximum temperatures around 200 310d c slightly higher in the marloes musselwick thrust sheet than in other parts of the structural succession at depths of the order of 6 13 km quartz veins yield distinctly heavier oxygen isotopic compositions than detrital quartz grains in the adjacent wall rocks although care must be taken in interpreting the data because slivers of detrital grains may become incorporated into veins while matrix detrital grains may incorporate veinlets or rims of newly formed quartz it is concluded that vein quartz grew in isotopic equilibrium with a fluid phase whose isotopic composition was primarily controlled by exchange with phyllosilicates not detrital quartz grains vein and matrix quartzes from the marloes musselwick thrust sheet are distinctly lighter 18oveins 14 to 18 and 18omatrix 11 to 14 than those from other thrust sheets 18o 17 to 20 and 14 to 17 respectively we conclude that vein quartz and phyllosilicate grains in cleavage domains probably attained equilibrium with a locally buffered pore fluid at the peak of metamorphism but many relict grains of different chemical and isotopic composition remained elsewhere in the rock local fluid migration along veins and through cleavage lamellae facilitated the attainment of equilibrium but there is little evidence for large scale infiltration of externally derived fluids with further metamorphism the quartz in these rocks would attain an isotopic composition intermediate between that of the heavy vein material and light detritus which coexist here

wind driven motions in the northeast pacific as measured by lagrangian drifters abstract analysis is presented of the time dependent motion of 47 surface drifters in the northeast pacific during fall 1987 and 16 drifters in fall and winter 1989 90 the drifters were designed at 15 m depth and were designed to have wind produced slips less than 2 cm s 1 for wind speeds up to 20 m s 1 the coherence of velocity and local wind is presented for motions with periods between 1 day and 40 days for periods between 5 and 20 days drogue motion at 15 m depth is found to be highly coherent with local wind with an average phase of 70 to the right of the rotating wind vector these results differ from analyses of fgge type drifters as reported by mcnally et al and niiler in the same area a model of wind produced slip as a function of drifter design is used to provide a possible explanation of the differences a linear regression which accounts for 20 40 of the current variance gives water motion al 0 5 of wind speed and 68 to the right of the wind vector assuming an ekman type balance

latitudinal dependence of wind induced near inertial energy midlatitudestorms accountingforthemajorityof windenergyinputtonear inertialmotionsintheocean are known to shift their track significantly from one year to another the consequence of such storm track shifts on wind inducednear inertialenergy nie isyetunknown here thelatitudinaldependenceofwind inducednie isfirstanalyzedintheframeworkoftheslabmodelandthentestedusingtwonumericaloceanmodels itisfound that the nie input by pure inertial wind stress forcing which dominates the wind energy input to near inertial motions is independent of latitude as a consequence the nie generated by white noise wind stress forcing is also latitudinally independent in contrast the nie generated by red noise wind stress forcing shows strong dependence on latitude owing to longer inertial periods at lower latitudes capable of sampling greater inertial wind stress forcing given that the observed surface wind stress spectra are red results from this study suggest that an equatorward shift of the storm track is likely to result in an increase in wind induced nie in the ocean while the opposite is true for a poleward shift

variability of the observed midlatitude storm tracks in relation to low frequency changes in the circulation pattern abstract the principal modes of month to month variability of the wintertime storm tracks over the north pacific and north atlantic are identified by empirical orthogonal function analysis of the root mean square statistics of bandpass 2 5 6 day filtered geopotential height data for 19 yr one of the two leading modes depicts fluctuations in the level of synoptic scale activity without any noticeable spatial displacement of the storm track axes whereas the other mode is associated with meridional shifts of the storm tracks from their time averaged positions higher order modes are indicative of diversion or truncation of cyclone tracks in particular geographical regions it is demonstrated that the leading storm track modes are linked to some of the best known monthly averaged teleconnection patterns the dipolar western pacific and western atlantic patterns for the monthly mean flow are seen to be accompanied by marked changes in the intensity of the storm tracks over the western oceans whereas the m

north american central plains conductivity anomaly goes east the north american central plains electrical conductivity nacp anomaly has been known for almost two decades and its influence on palaeotectonic models of the north american continent has been significant magnetometer array studies have located the structure in a gross sense but there has not been until now a comprehensive magnetotelluric mt survey of it in this letter we report on the preliminary interpretation of data from thirty five mt sites recorded along a 400 km east west profile just north of the us canadian border we show that the anomaly at this latitude is 75 km farther east of the location previously mapped by the array studies modelling indicates that the conductive body has its top surface at about 10 km depth and appears to be anticlinal in shape the profile also extends sufficiently east that it traverses another previously known electrical structure referred to herein as the tobe anomaly we consider briefly other geophysical and geological data for the region and tentatively infer that the nacp structure is terminated to the east by the interpreted southern extension of the tabbernor fault fold system and that the tobe anomaly is an expression of the known southern extension of the thompson nickel belt both beneath phanerozoic sediments of the williston basin

the north american central plains conductivity anomaly and its correlation with gravity magnetic seismic and heat flow data in saskatchewan canada abstract the north american central plains conductivity anomaly nacp lies virtually in its entirety within the trans hudson orogen accordingly should these two features prove to be contemporaneous then the geometrical relationship between these two is of foremost importance to any evolutionary tectonic model proposed to explain the collision of the superior and churchill provinces in the hudsonian and a model that does not include a mechanism for the generation of this anomaly is obviously untenable in order to map better the trend of the nacp in the province of saskatchewan canada two magnetotelluric mt profiles were conducted over the nacp previously defined by magnetometer arrays and profiles data from these two profiles along with an earlier mt profile just north of the u s canadian border suggest that the nacp is not a continuous feature but rather that it exhibits a definite break at latitude 51 n other geophysical evidence examined herein is concordant with this characteristic if a second mt anomaly mapped to the northwest of this break is in fact a manifestation of the same geological structure then one possible interpretation is of a major nw se trending sinistral fault in the deep crust previously undetected with a movement of some 100 150 km along strike the mt data from the southernmost profile after correction for static shift are modelled in a 2d manner and it is shown that the nacp is consistent with an arcuate structure in vertical section of high conductivity 2 s m 1 beginning at a depth of 10 km centred on 103 w dipping down to the west reaching possibly the base of the crust it is also shown that such an arcuate shape in section can explain an observed gravity high of 40 mgal such high conductivities cannot be explained in terms of connected fluids as it would require implausibly high porosities 12 20 a comprehensive interpretation of all the geophysical data would have to account for the observed characteristics of the anomalous region which are high electrical conductivity positive density contrast no magnetization and high heat flow in the basement in terms of a single causative body one possible explanation is presented in terms of a zone of lithospheric weakness along which was emplaced low density differentia from a mantle derived body an alternative explanation could be the phase transformation of obducted material into eclogite or serpentinite however difficulties exist with both of these explanations and further data are required

structural analysis of the brigham city weber segment boundary zone wasatch normal fault utah implications for fault growth and structure we investigate the history kinematics principal stress orientations and geometry of deformation at the end of a bent normal fault segment of the wasatch fault zone utah three fault types developed in archean crystalline rocks reflect progressive uplift of fault related footwall rocks chlorite breccias and phyllonites reflect deep level reaction assisted plastic deformation along the north striking part of the segment planar fretted faults which formed by cataclasis cut the phyllonites and breccias and are developed throughout the footwall of the segment youngest faults are hematitecoated extremely narrow polished surfaces slip vectors and kinematic analyses of small faults developed in the footwall indicate oblique normal slip along the north striking portion of the segment slip vectors and fault orientation along the northwest striking portion of the segment reflect complexly oriented slip on faults which strike subparallel and at high angles to the main fault trace yet slip is confined to a broad fault parallel zone small faults at the southernmost tip of the segment indicate a strong influence of the north striking weber segment to the south inversion of fault data for principal stress orientations document complexly oriented principal stresses through the segment boundary zone and suggest that 3 may have reoriented approximately 60 over the life of the segment subsurface structure combined with small fault data indicate the segment boundary is comprised of a southwest plunging bedrock high which is reflected by a sharp bend in the brigham city segment the southern end of the brigham city segment may have started as a straight north striking fault which has bent due to changes in stress orientations and or interaction with the adjacent weber segment

slip rate variations on normal faults during glacial interglacial changes in surface loads geologic and palaeoseismological data document a marked increase in the slip rates of the wasatch fault and three adjacent normal faults in the basin and range province during the late pleistocene early holocene epochs the cause of this synchronous acceleration of fault slip and the subsequent clustering of earthquakes during the holocene has remained enigmatic although it has been suggested that the coincidence between the acceleration of slip and the shrinkage of lake bonneville after the last glacial maximum may indicate a causal relationship here we use finite element models of a discrete normal fault within a rheologically layered lithosphere to evaluate the relative importance of two competing processes that affect fault slip postglacial unloading the removal of mass which decreases the slip rate and lithospheric rebound which promotes faster slip we show that lithospheric rebound caused by regression of lake bonneville and deglaciation of adjacent mountain ranges provides a feasible mechanism for the high holocene rates of faulting in the wasatch region our analysis implies that climate controlled changes in loads applied to earth s surface may exert a fundamental control on the slip history of individual normal faults

isotopic composition of uranium and thorium in crystalline rocks the 238u 234u 230th 232th system has been investigated in 84 silicic crystalline rocks obtained from drill cores surface and near surface samples in california wyoming colorado and illinois results of these analyses displayed on ternary diagrams with apexes for 238u 234u and 230th indicate five predominant geochemical processes that affected uranium in the rock 1 bulk uranium leaching where 238u and 234u were removed with little or no fractionation 2 preferential 234u leaching by alpha recoil displacement 234u recoil loss with lesser 238u loss 3 234u recoil loss with little or no 238u loss 4 uranium assimilation where both 238u and 234u were added with present day 234u 238u activity ratios varying from 0 8 to 1 2 and 5 addition of 234u and 230th by daughter emplacement processes 234u 230th recoil gain evidence for the existence of 234u and 230th recoil gain in rocks is the most important finding of this investigation radioactive disequilibrium occurs in the majority of rocks analyzed where 234u recoil loss is the predominant process associated with incipient weathering u assimilation and 234u 230th recoil gain occur under conditions of substantial water penetration along fractures and into weathered zones in the rocks relatively unfractured and petrographically fresh rocks from the uph 3 drill hole in northern illinois are closest to being in radioactive equilibrium for any suite of rocks included in this study and they demonstrate that equilibrium during the last 0 5 m y can be maintained over a substantial vertical distance where there has been little or no movement of water in the basement rock the 238u 234u 230th system can be a sensitive indicator of geologically recent u mobility and rock water interaction both in petrographically fresh core samples and in sealed fracture zones

the impact of oceanic near inertial waves on climate abstractthe community climate system model version 4 ccsm4 is used to assess the climate impact of wind generated near inertial waves niws even with high frequency coupling ccsm4 underestimates the strength of niws so that a parameterization for niws is developed and included into ccsm4 numerous assumptions enter this parameterization the core of which is that the niw velocity signal is detected during the model integration and amplified in the shear computation of the ocean surface boundary layer module it is found that niws deepen the ocean mixed layer by up to 30 but they contribute little to the ventilation and mixing of the ocean below the thermocline however the deepening of the tropical mixed layer by niws leads to a change in tropical sea surface temperature and precipitation atmospheric teleconnections then change the global sea level pressure fields so that the midlatitude westerlies become weaker unfortunately the magnitude of the real air sea flux of niw energy is poorly con

compressive and tensile failure at high fluid pressure where preexisting fractures have cohesive strength with application to the san andreas fault in thrusting and strike slip situations when the maximum principal horizontal stress s1 acts nearly normal to a fault a misoriented fault such as the san andreas pore fluid pressure the lithostatic load pf sv is required to reactivate movement on that fault pf sv may be achieved without causing hydraulic tensile fracturing if 1 previously existing cracks have regained cohesive strength by chemical processes 2 subcritical crack growth has been blunted and 3 the least principal horizontal stress s3 nearly equals sv where pf sv has been attained within a misaligned fault increasing the stress difference s1 s3 at constant pf sv will not lead to shear failure while a decrease in s1 s3 can lead to shear failure of that fault however where the cohesive strength of material in a broad misaligned fault zone is less than that of the surrounding intact rock increasing s1 s3 while pf sv can result in shear failure of fractures at near optimum angles to s1 but confined within this weak fault zone if this faulting results in the local short lived attainment of pf sv cataclastic deformation and frictional heating overcoming dilation and a simultaneous decrease in s1 s3 this combination of effects can trigger movement along the main trace of the misaligned fault when increasing pf results in hydraulic failure anisotropy in tensile strength or fracture toughness resulting from foliation within faults allows fractures to propagate along the planes of weakness rather than across the foliation perpendicular to s3

strike slip fault bridge fluid pumping mechanism insights from field based palaeostress analysis and numerical modelling abstract we present a finite element study of stress perturbation in evolving compressive and extensional strike slip fault bridges the results are compared with a fracture study of a compressive bridge at st donats uk horizontally interbedded calcareous mudstone and bioclastic calcilutite at st donats have a distinct vertical permeability anisotropy this sedimentary sequence behaves as a set of horizontal aquifers the fluid flow in these aquifers is sensitive to mean stress gradients paleostress analysis of field fracture data verified by finite element modelling indicates a rotation of 1 towards parallelism with boundary faults inside the growing compressive bridge boundary faults and bridge faults recorded numerous fluid flow events the modelled mean stress pattern shows a regional maximum within the bridge and local maxima minima pairs at boundary fault tips finite element modelling of an extensional bridge indicates that 3 rotates towards parallelism with boundary faults the mean stress pattern is similar to the pattern in compressive bridge but with maxima and minima locations interchanged the stress patterns are reestablished by each stress build up preceding the rupturation of the boundary faults throughout the development stages of strike slip fault bridges mean stress gradients developed pre failure control the fluid flow in fractures of the strike slip fault system at and after the end of each stress build up and the fluid flow in boundary faults post failure fracture reactivation and new fracture generation within an evolving bridge is a process consisting of multiple successive events that retain the storage capacity of the bridge rupture and sealing of the main bounding faults is a step wise process that opens and closes fluid conduits between areas with different pressures

structural and fluid chemical properties of seismogenic normal faults abstract structures in extensional fault zones can be classified using the configuration of fault branch lines and tip lines important classes include the segment bend segment termination segment branch cross fault intersection and segment offset the effect of these structures on rupture history is not necessarily consistent neither between individual earthquakes nor between different fault zones rupture behavior is dependent on several other factors including loading conditions regional and localized rupture tip stress fields fluid mechanical processes and chemical processes fracture toughness is partly controlled by the angular discordance between slip directions on adjacent fault segments greater discordance between slip directions and intersection lines of fault segments results in greater strain incompatibility an internal fracture network generally evolves within a segment boundary to maintain compatibility and transfer slip between the segments the dimensions and structure of this fracture network may also partly control rupture propagation presumably activation of a fault network with large angular discordances between slip directions and intersection lines will generate numerous asperities as the subsidiary faults mutually interact and offset each other the geometry of a segment boundary may change with depth and the three dimensional nature of the structure may be important in controlling rupture history fluids influence rupturing via fluid pressure effects and time dependent chemical processes fracture propagation by stress corrosion may favor instability and chemical alteration may produce minerals of lower strengths allowing time dependent creep sealing and healing of fractures however may remove damage and increase strength elementary computations indicate that representative times for sealing and chemical alteration are between 1 and 1000 years for reasonable physical conditions well within the recurrence intervals of most large earthquakes time to failure for stress corrosion cracking is more highly variable and strongly sensitive to applied stress and fluid pressure

pore fluid and seismogenic characteristics of fault rock at depth on the wasatch fault utah fluid inclusions and structural fabric in fault rock in the exhumed footwall of the wasatch fault allow measurement of fluid pressure temperature and composition and estimation of rheological characteristics of fault behavior hydrothermally altered and deformed oligocene quartz monzonite of the little cottonwood stock forms a partially preserved footwall carapace at the southern end of the salt lake segment of the wasatch normal fault zone cataclasite and phyllonite contain two syndeformational alteration mineral assemblages that formed during progressive faulting and displacement of the footwall an early epidote chlorite sericite magnetite alteration assemblage developed in phyllonite and cataclasite and a later laumontite prehnite hematite alteration assemblage developed during cataclasis fluid inclusions associated with the formation of alteration minerals are preserved in healed fractures in quartz grains fluid inclusions that record the transition temperature from quasi plastic flow to frictional deformation are associated with hydrothermal alteration to chlorite epidote and sericite and were trapped at minimum temperatures of 223 c 353 c and minimum fluid pressures of 1150 2800 bars the fluids average 13 mol co2 and 8 2 wt nacl a radiometric age of hydrothermal sericite suggests that the age of the alteration is 17 6 0 7 m y if maximum fluid pressure is near lithostatic the minimum depth of formation of the alteration was 11 km at 337 c consistent with a thermal gradient of 30 c km the ratio of pore fluid pressure to lithostatic pressure varied from 0 52 to nearly 1 0 and indicates that effective normal stress on the fault at these depths may have approached zero structural fabrics and fluid inclusion characteristics are used to estimate seismogenic characteristics of the fault cleavage lineation and s c bands in the phyllonite indicate that deformation at temperatures above 350 c and depths greater than 11 km was dominated by aseismic creep unstable fracture propagation in cataclasite developed during seismic slip at shallower levels the downdip seismogenic length of the salt lake segment is estimated as 16 km using a fault zone dip of 45 and a minimum depth of 11 km for the quasi plastic frictional transition the segment strike length is 35 km and the seismogenic area is about 600 km2 compatible with generation of m 7 0 earthquakes

crustal structure and evolution of the trans hudson orogen results from seismic reflection profiling a 400 km long deep seismic reflection transect across northeastern montana and northern north dakota reveals the crustal scale structural fabric of the early proterozoic trans hudson orogen beneath the williston basin comparison with deep seismic reflection data across the canadian portion of the same orogen 700 km to the north reveals first order similarities in crustal architecture but documents significant along strike variation in orogenic evolution both transects display a broad crustal scale antiform axial to the orogen in the north geologic data suggest that this antiform is cored by an archean microcontinent in the south west dipping reflections on the western flank of the antiform extend from the upper crust to the uppermost mantle and truncate prominent subhorizontal lower crustal reflections of the archean wyoming craton within the wyoming craton the eastern limit of east dipping midcrustal reflections coincides with the subsurface age boundary between the craton and the early proterozoic trans hudson orogen as interpreted from potential field and drill core data on the basis of subsurface geochronologic data from the crystalline basement and by analogy with the glennie domain within the exposed trans hudson orogen in canada we suggest that the southern antiform is cored by an archean crustal fragment that was caught up in the terminal collision of the wyoming and superior cratons during hudsonian orogeny the eastern side of the trans hudson orogen is characterized on both seismic transects by predominantly east dipping crustal penetrating reflections we interpret the easterly dip of these reflections as evidence that the superior province was thrust westward over the interludes of the orogen during terminal collision although juvenile early proterozoic terranes characterize the exposed segment of the trans hudson orogen in canada limited drill core information within the dakota segment of the orogen shows a predominance of granulitic archean age crust this difference in basement lithologies along strike within the orogen may indicate the following either juvenile crust comparable to that exposed in the northern trans hudson was never present in the south or it was removed by progressive over thickening erosion and or faulting postorogenic extensional collapse may be responsible for preservation of juvenile terranes in the north

reduced seismic velocities in the source zone of new madrid earthquakes a three dimensional inversion of p wave travel time residuals from local earthquakes reveals a remarkable pattern of low seismic velocities in crustal rocks immediately adjacent to the active portions of the new madrid fault system seismic velocities are lowest in regions of greatest concentration of earthquake activity near two intersections of linear trends in seismicity the maximum reduction in compressional wave velocity is at least 7 per cent in the upper 5 km of the crust and at least 4 per cent in the depth range of 5 to 14 km the reductions are consistent with a velocity decrease which would be expected if crustal rocks in the source zone contain fluid filled cracks in which pore pressure is a substantial fraction of external pressure the presence or absence of such fluids may explain why some portions of the faults in and surrounding the upper mississippi embayment are active while others are not

earthquake supercycles as part of a spectrum of normal fault slip styles like thrust boundaries in subduction zones some normal faults show evidence of temporally transient slip patterns such as temporally clustered earthquakes or periods of elevated creep rate we create rate and state friction numerical models to assess whether typical distributions of frictional parameters can explain transient slip on normal faults we constrain our models with geometry from the wasatch fault zone where 5 10 kyr timescale earthquake clusters are recorded on fault scarps we use numerical models to test whether these earthquake clusters could be the geologic representation of supercycles which we define as long period stress cycles containing multiple seismic cycles field observations laboratory data and analogue experiments suggest that the ratio of velocity weakening to velocity strengthening material varies with the ratio of brittle to ductile material in the shear zone we therefore employ a linear transition of frictional parameters with depth to simulate bulk frictional properties of the shear zone we find that for generalized normal faults rate and state friction predicts a spectrum of slip behaviors including steady aseismic creep aseismic transients clustered earthquakes and regular seismic cycle earthquakes the dominant control on slip behavior is the degree of weakening material in the potentially seismogenic fault patch earthquake clusters occur with higher proportions of velocity weakening material which correspond to more competent less ductile shear zone assemblages we find that rate and state friction predicts a wider spectrum of slip behavior on active normal faults than has been previously recognized potentially due to geologic preservation and identification issues

on the loss of wind induced near inertial energy to turbulent mixing in the upper ocean wind induced near inertial energy has been believed to be an important source for generating the ocean mixing required to maintain the global meridional overturning circulation in the present study the near inertial energy budget in a realistic 1 12 degrees model of the north atlantic ocean driven by synoptically varying wind forcing is examined the authors find that nearly 70 of the wind induced near inertial energy at the sea surface is lost to turbulent mixing within the top 200 m and hence is not available to generate diapycnal mixing at greater depth assuming this result can be extended to the global ocean it is estimated that the wind induced near inertial energy available for ocean mixing at depth is at most 0 1 tw this confirms a recent suggestion that the role of wind induced near inertial energy in sustaining the global overturning circulation might have been overemphasized

electromagnetic image of the trans hudson orogen tho94 transect the north american central plains nacp anomaly in enhanced electric conductivity and its relationship with the paleoproterozoic trans hudson orogen tho has been studied under the auspices of lithoprobe for over a decade the nacp anomaly was the first geophysical evidence of the existence of the tho beneath the phanerozoic sediments of the central plains this anomaly detected geomagnetically in the late 1960s has been the subject of a number magnetotelluric studies from the early 1980s the pancanadian and geological survey of canada experiments in the 1980s and the lithoprobe experiments in the 1990s together comprise four east west and one north south re gional scale profiles in saskatchewan perpendicular to the strike of the orogen in this paper data from the northern most line coincident with seismic line s2b are analysed and interpreted and are shown to be key in determining the northern extension of the nacp anomaly dimensionality analysis confirms the rotation of deep crustal structures east ward to hudson bay as earlier proposed on the basis of broad scale geomagnetic studies on this profile as with the profile at the edge of the paleozoic sediments the nacp anomaly is imaged as lying within the la ronge domain in contact with the rottenstone domain and structurally above the guncoat thrust a late compressional feature the location of the anomaly together with the surface geology suggests that the anomaly is caused either by sulphide mineralization concentrated in the hinges of folds by graphite or by a combination of both our interpretation of the data is consistent with that from other profiles and suggests that the nacp anomaly was formed as a consequence of subduction and collisional processes involving northward subduction of the internides of the tho beneath the hearne craton on the southern part of this profile a resistive structure is identified as the sask craton suggesting that proterozoic rocks are above archean rocks in the tho 493

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

fluid pressure transients on seismogenic normal faults abstract fluid inclusions in hydrothermally altered footwall rocks of the dixie valley fault nevada and the wasatch fault utah indicate that pore fluid pressure fluctuated minimum entrapment pressures for fluid inclusions consisting of h2o co2 nacl ranged from 295 mpa to 60 mpa in the temperature range 350 to 170 c on the wasatch fault and from 158 to 35 mpa in the temperature range 350 to 200 c on the dixie valley fault scatter in the pressure estimates at constant temperature is interpreted as paleo fluid pressure transients at depths of up to 11 km on the wasatch fault and 3 to 5 km on the dixie valley fault observed pressure transients range from 5 mpa within the limits of error in pressure determination to 120 mpa on the wasatch fault and 7 to 126 mpa on the dixie valley fault the pressure transients are greatest on both faults in the temperature range 270 to 310 c the fluids represented by fluid inclusions play a key role in nucleation and propagation of earthquake ruptures high fluid pressures may initiate rupture then dilatancy pore pressure reduction and dilatant hardening may arrest the rupture however decompression of the fluids and phase separation produces a decrease in fluid bulk modulus of 41 to 90 which reduces the dilatant hardening effect and may permit ruptures to propagate

dating of archean basement in northeastern wyoming and southern montana rb sr whole rock and u pb zircon ages of granite and gneiss cores from three deep drill holes extend known occurrences of archean rocks in the subsurface of northeastern wyoming and southern montana rb sr and k ar mineral ages are discordant and reflect early or middle proterozoic disturbance highly altered rocks occur in a thin zone immediately below the sub cambrian unconformity samples from a few metres deeper in the basement are much fresher but show the effects of this alteration in filled fractures and thin adjacent alteration haloes whole rock rb sr systems have retained a fair degree of integrity in spite of increased susceptibility to modification because of the disturbed mineral systems interaction of the rocks with water a few metres below the sub cambrian unconformity probably occurred for only a relatively short time fractures filled rapidly with secondary minerals such as chlorite anhydrite and carbonate to maintain a relatively impermeable crystalline basement in which the silicates and their contained isotopic systems were preserved ages of cores from two wells extend the archean basement to the nacp anomaly a possible suture related to early proterozoic tectonism these and published ages limit a possible ensimatic mobile belt to a zone 300 km wide beneath the williston basin however the possibility remains that archean crust was continuous between the wyoming age province and the superior province

electromagnetic images of the trans hudson orogen the north american central plains anomaly revealed magnetotelluric studies of the trans hudson orogen over the last two decades prompted by the discovery of a significant conductivity anomaly beneath the north american central plains nacp from over 300 sites yield an extensive database for interrogation and enable three dimensional information to be obtained about the geometry of the orogen from southern north dakota to northern saskatchewan the nacp anomaly is remarkable in its continuity along strike testimony to along strike similarity of orogenic processes where bedrock is exposed the anomaly can be associated with sulphides that were metamorphosed during subduction and compression and penetratively emplaced deep within the crust of the internides of the orogen to the boundary of the hearne margin a new result from this compilation is the discovery of an anomaly within the upper mantle beginning at depths of 80 100 km this lithospheric mantle conductor has electrical properties similar to those for the central slave craton mantle conductor

low angle normal fault lanf zone architecture and permeability features in bedded carbonate from inner northern apennines rapolano terme central italy abstract fault zones have the capacity to be hydraulic conduits within upper crustal levels allowing migration of large volume of fluids through shallow and deeper geological environments low angle normal faults lanfs crosscutting carbonate rocks produce damaged volumes that may have a relevant role in channelling or hosting geothermal fluids therefore deserving of investigation to better predict mining targets deformation along lanfs zones dissecting carbonate successions produces permeable volumes presently exploited in the larderello and monte amiata geothermal areas italy in this paper the architectural and permeability features of an exhumed lanf zone exposed in the northern apennines rapolano terme central italy affecting cretaceous bedded limestone are presented such a fault was not affected by circulation of geothermal fluids but its features could reveal much on the potential impact on fluids migration in the active geothermal areas therefore resulting an intriguing analogue the study lanf zone consists of faults which enucleated at depth 4 km during its earlier stage of evolution dissolution seams often arranged in s c fabric characterised the whole damage zone dissolution seams developed under very low grade metamorphism t 100 150 c as indicated by illite crystallinty analyses fault zone architecture and permeability features changed during the fault growth and exhumation permeability heterogeneity and anisotropy characterised the lanf zone during its development if geofluids circulated within the fault zone it could be an effective barrier during its earlier evolution being accompanied by dissolution seams on the contrary it could play as combined barriers conduits during its later evolution progressively at shallower levels being characterised by intersecting fault planes which define pipe like conduits parallel to the direction of the tectonic transport such a configuration could have the capacity to impact on fluids migration for long paths across the upper crustal levels this adds significant information for predicting deeper geothermal circuits and the location of potential exploitable reservoirs in active tectonic environments

wyoming on the run toward final paleoproterozoic assembly of laurentia paleoproterozoic suture zones mark the formation of supercontinent nuna and provide a record of north america s assembly conspicuously young ages ca 1 715 ga associated with deformation in southeast wyoming craton argue for a more protracted consolidation of laurentia long after peak metamorphism in the trans hudson orogen using paleomagnetic data from the newly dated 1899 5 ma sourdough mafic dike swarm wyoming craton we compare the relative positions of wyoming superior and slave cratons before during and after peak metamorphism in the trans hudson orogen with these constraints we refine a collisional model for laurentia that incorporates wyoming craton after superior and slave cratons united redefining the paleoproterozoic sutures that bind southern laurentia

structure of the mount raymond transverse zone at the southern end of the wyoming salient sevier fold thrust belt utah abstract the mount raymond transverse zone mrtz forms the east west trending boundary between the wyoming salient of the sevier fold thrust belt and the uinta cottonwood arch in north central utah major faults in the zone dip 40 to 45 north our structural analysis indicates that the mrtz contains both contractional and extensional structures the contractional structures thrusts and related folds initially formed as part of a southeast verging northeast trending thrust system this system gradually curved and merged to the north with the east verging absaroka thrust system in the apex of the wyoming salient the contrast in trend between structures in the mrtz and those in the apex of the wyoming salient reflects the initial curvature of the salient this curve formed because the stratigraphic sequence involved in thrusting thinned gradually southward toward the axis of a proto uinta arch the present east west trend of the mrtz developed during laramide uplift of the unita cottonwood arch an event which tilted the faults into their present position cenozoic crustal extension subsequently reactivated segments of the mrtz in sum the evolution of the mrtz illustrates how post thrusting processes can affect the map view geometry of thrust belts to create transverse zones

high and low density fluids in a quartz vein from the irish variscides abstract combined microstructural and fluid inclusion studies on a multiple quartz vein from the irish variscides reveal a kinematic history consisting of three separate dynamically distinct extensional opening events the first of these is characterized by trapped fluid at densities of 0 87 0 98 g cm 3 indicative of pressures approximating the estimated lithostatic load while fluid densities associated with the following two events 0 76 0 91 g cm 3 clearly indicate sub lithostatic fluid pressures the combined evidence suggests that these later two opening events were extensional fractures occurring well below the critical depth for tensional 3 least effective principal stress it is argued here that the preservation of such low density fluids in an environment that conventionally requires supra lithostatic fluid pressures for hydraulic failure is due to i the influence of a preexisting material anisotropy in the rock that would result in hybrid extensional failure ii the effect of stress heterogeneities associated with the development of crude boudins in the vein

seismic moment rates in the utah region an estimate of the long term seismicity of utah is obtained through the calculation of seismic moment rates from available geologic data on quaternary faulting the recurrence rates calculated from these geologic seismic moment rates agree well with recurrence rates estimated from historical earthquake data a return period of 80 to 190 yr for a magnitude 7 0 to 7 5 earthquake in the utah region is predicted by historical seismic and geologic data the estimation of recurrence rates from geologic data requires the use of a moment versus magnitude relationship for earthquakes characteristic of the utah region spectral studies of 19 utah earthquakes define this relationship in addition moment tensors and other source parameters are estimated for these earthquakes a resultant strain tensor for utah indicates the amount of horizontal east west extensional strain is nearly equal to the amount of vertical compressional strain in the region

ductile creep and compaction a mechanism for transiently increasing fluid pressure in mostly sealed fault zones a simple cyclic process is proposed to explain why major strike slip fault zones including the san andreas are weak field and laboratory studies suggest that the fluid within fault zones is often mostly sealed from that in the surrounding country rock ductile creep driven by the difference between fluid pressure and lithostatic pressure within a fault zone leads to compaction that increases fluid pressure the increased fluid pressure allows frictional failure in earthquakes at shear tractions far below those required when fluid pressure is hydrostatic the frictional slip associated with earthquakes creates porosity in the fault zone the cycle adjusts so that no net porosity is created if the fault zone remains constant width the fluid pressure within the fault zone reaches long term dynamic equilibrium with the hydrostatic pressure in the country rock one dimensional models of this process lead to repeatable and predictable earthquake cycles however even modest complexity such as two parallel fault splays with different pressure histories will lead to complicated earthquake cycles two dimensional calculations allowed computation of stress and fluid pressure as a function of depth but had complicated behavior with the unacceptable feature that numerical nodes failed one at a time rather than in large earthquakes a possible way to remove this unphysical feature from the models would be to include a failure law in which the coefficient of friction increases at first with frictional slip stabilizing the fault and then decreases with further slip destabilizing it

response of the internal wave field to remote wind forcing by tropical cyclones abstractin the tropical north atlantic mean winds introduce relatively little energy into the internal wave field but hurricanes act as very energetic sources for near inertial waves in addition to the eventlike passage of such tropical cyclones changes in the wind speed north of the trade wind system induce a seasonal cycle in surface swell with potential implications for the high frequency part of the internal wave field using a 5 yr mooring time series in the interior of the tropical north atlantic at 16 n the temporal variability of internal wave energy south of the main hurricane track in different frequency bands is studied and the magnitude of its variability along with possible energy transfer mechanisms is analyzed the results show that changes in near inertial energy are dominated by the passage of internal waves generated by hurricanes centered several hundred kilometers north of the mooring the major role of hurricanes in the generation of near inertial waves is also seen in an ext

chemistry source and behavior of fluids involved in alpine thrusting of the central pyrenees quartz veins occupying extensional fractures that formed relatively late in the alpine thrusting history of the pyrenees contain abundant inclusions of hypersaline brines small amounts of bitumen also occur a range of brine compositions is apparent from the freezing behavior of fluid inclusions from different samples with large variations in na ca ratio but many individual samples contain only one brine composition all brine types display a bimodal distribution of homogenization temperature which is interpreted as the result of fluctuation in fluid pressure from a maximum value equal to lithostatic at 5 7 km depth and around 270 c to a value up to 700 bars lower this is ascribed to dilatancy pumping chemical analyses of brines leached from two samples yields balanced analyses for 14 elements both analyses appear to represent sedimentary formation waters and are closely analogous to oil field brines a basement origin is precluded by the presence of bitumens and the low k ca ratios differences between the analyses probably reflect different source bed mineralogy including the presence or absence of dolomite but triassic red beds are likely to be a major source the occurrence of these brines in the fault planes provides constraints on both the sources and movement of the fluids involved in thrusting significant infiltration of either surface waters or metamorphic water of dehydration can be ruled out because of the high salinities typically 20 25 wt dissolved salts hence although permeability was locally high due to fracturing around the faults fluids were pumped into these fractures from sedimentary formations adjacent to the fault individual fluid pathways were short lived relative to the duration of the deformation event so that different fluids were pumped into the faults at different times in response to local fault propagation

archean geochronological framework of the bighorn mountains wyoming the bighorn mountains of the central wyoming province expose a large tract of archean crust that has been tectonically inactive and at relatively high crustal levels since 2 7 ga seven sensitive high resolution ion microprobe shrimp u pb zircon and titanite age determinations on samples of the main lithologic units provide a geochronological framework for the evolution of this area the oldest precisely dated magmatic event occurred at 2950 5 ma when diorite to granite dykes and sills intruded an older gneiss complex exposed in the central and southern bighorn mountains rocks as old as 3 25 ga may be present in this gneissic basement as indicated by the oldest dates obtained on areas of zircon grains that are interpreted as inherited cores a tonalitic gneiss was intruded into the gneiss complex at 2886 5 ma deformation of the central and southern gneisses preceded the intrusion of the bighorn batholith a tonalitic to granitic intrusion that occupies the northern portion of the uplift this