high grade magnetite mineralization at 1 86 ga in neoarchean banded iron formations gongchangling china in situ u pb geochronology of metamorphic hydrothermal zircon and monazite

the cryogenian record in the southern kingston range california the thickest death valley succession in the hunt for a gssp abstract the kingston peak formation of the death valley area california allows valuable insight into both regional cordilleran stratigraphy and the number of glacial cycles preserved in the cryogenian record in the kingston range the eponymous strata have been previously interpreted to record both sturtian and marinoan pan glacial events in the context of a search for a global boundary stratotype section and point gssp for the cryogenian we provide the first detailed description of the thickest diamictite bearing interval in the western usa two clast poor muddy diamictite intervals within the succession one at the base and one near the top have been used to support sturtian and marinoan events previously however new data from the southern part of the kingston range suggest that the upper diamictite interval is genetically related to underlying strata the deposits are interpreted as glaciogenic debris flow deposits which probably represent the proximal tract of a subaqueous fan medial to distal portions of this fan are dominated by turbidites which were transported down a consistent se oriented palaeoslope lowermost beds of the upper diamictite interval are intercalated with graded sandstones and sandy matrix supported conglomerates the graded beds turbidites and matrix supported conglomerates debrites testify to a subaqueous setting with the compositionally and texturally distinct diamictites indicating a glacial origin

cap carbonate platform facies model noonday formation se california the neoproterozoic outcrop belt of the death valley region california preserves an oblique cross section of the noonday formation a mixed carbonate siliciclastic platform that hosts distinctive basal ediacaran cap carbonate affiliated sedimentary structures stromatolite textures and 13 c carb values the noonday platform encompasses two depositional sequences that reveal two cycles of relative sea level change within strata conventionally considered to record a single rapid postglacial sea level rise in updip localities facies of the first depositional sequence record the transition from a carbonate ramp to a stromatolite bearing tubestone textured reef rimmed platform downdip localities seaward of the reefal escarpment variably preserve a thin and condensed onlapping foreslope wedge base level fall exposed the reef crest to karstic dissolution and propagated submarine incised valleys into the seaward margin of the reef overlying strata record the backfilling of a submarine incised valley and reestablishment of a back stepping carbonate dominated ramp prior to a second subaerial exposure event that defines the contact between the noonday and johnnie formations we address the relative contributions of syndepositional tectonism and recovery from low latitude deglaciation in dictating noonday platform architecture and the intra noonday formation sequence boundary noonday formation deposition coincided with extension of the laurentian margin during disaggregation of the rodinian supercontinent within this framework previous work has suggested that the intra noonday formation sequence boundary records growth faulting that reinforced differential topography uplifting reef rimmed horsts exposing the reef crest to karstic dissolution and downdropping grabens however we trace the intra noonday formation sequence boundary seaward of the reef crest and demonstrate that for a time wave base was situated downdip of the reef escarpment on putatively downdropped fault blocks thus if the noonday margin were undergoing extension then the creation of the intra noonday formation sequence boundary required a concomitant decrease in accommodation due perhaps to postglacial isostatic uplift attendant with low latitude deglaciation we speculate that noonday formation sequence architecture records 1 immediate deglacial flooding 2 shoaling and exposure due to isostatic rebound induced by either a hiatus in meltwater flux or rapid ice sheet collapse against a background of global deglaciation and 3 resumed flooding following complete deglaciation as rift related tectonism could amplify or counter glacial isostasy inferences of the amplitude of local postglacial sea level change will require robust estimates of syndepositional extension across the noonday margin

tectono stratigraphic evolution of the comondu group from bahia de la paz to loreto baja california sur mexico abstract the late oligocene to mid miocene volcanic and volcaniclastic rocks of the comondu group are well exposed along the main rift escarpment of baja california sur from the bahia de la paz region to bahia concepcion new mapping and stratigraphic analysis of the comondu group from bahia de la paz to loreto reveal facies trends and correlations that form the foundation for a continuous stratigraphic framework for the comondu group along a 300 km long transect on the eastern coast of the baja california peninsula broad but distinct lithostratigraphic trends alluvial fan facies and volcanic and volcaniclastic facies record an overall coarsening upwards package that includes ignimbrite deposits within increasingly proximal alluvial fan deposits both derived from the east geochronology of the unit including 32 isotope ages and 12 previously unpublished 40ar 39ar ages provide the timing of four main increasingly proximal depositional events non marine sandstone defining the base of the comondu group was first deposited between 26 ma and 24 ma emplacement of rhyolitic ignimbrites initiated between 24 ma and 23 ma and marked a westward expansion of volcanic activity affiliated with the sierra madre occidental ignimbrite sequences in southern sinaloa western durango and northern nayarit a change in volcanism occurred at 19 ma to 18 ma with more ignimbrites increased intermediate compositions and the appearance of local vents and proximal volcanic facies a final localized change of volcanism occurred from 14 to 12 ma in the loreto area with an increase of proximal alluvial fan deposits and local volcanoes in the upper comondu group the bulk of the upper comondu group is absent south of the loreto area and has either been removed by erosion as a source for the magdalena fan in the pacific ocean or was focused primarily in the loreto area and northward we use a pre rift tectonic reconstruction of the gulf of california to align broad stratigraphic trends along the peninsula the gulf conjugate margins and within the gulf the cascadia arc of oregon and northern california may be a modern analog for the comondu group with a linear volcanic arc formed above shrinking subducting microplates and a broad backarc region of moderate extension and scattered volcanism

observation of shear wave splitting from natural events south moat of long valley caldera california june 29 to august 12 1982 three component seismograms from local earthquakes recorded by three university of wisconsin stations in the south moat of long valley caldera in 1982 have been analyzed for shear wave splitting the average polarization direction of the first arriving shear waves observed at the two stations south of the resurgent dome is nnw sse time delays between the fast and slow shear waves are proportional to hypocentral distances and the maximum velocity anisotropy inferred is 6 4 the average shear wave polarization observed at the station above the resurgent dome is north south the time delays there are not a function of hypocentral distance and the normalized time delays ms km from events at the southwest boundary of the caldera are larger than those at other azimuths indicating maximum 9 6 velocity anisotropy the east west to ene wsw variation of the tensional stress fields implied by the shear wave polarizations indicates the interaction between the inflating caldera and the sierra nevada range front faults under the generally wnw ese tension expected from the regional tectonics

shear wave splitting in the crust in north china stress faults and tectonic implications summary this paper uses data from the capital area seismograph network in north china nc currently the largest regional seismograph network in china to investigate the compressional stress orientations from the polarizations of faster shear waves pfs a parameter of shear wave velocity anisotropy data are restricted to earthquakes deeper than 5 km to reduce the influence of the heterogeneous uppermost few kilometres of the crust the results show that the regional compressional stress direction is ene wsw to nearly ew in nc which agrees with seismic mechanisms and drilling and gps data predominantly pfs are also nearly ew in nc and are influenced by faults geology and tectonics pfs orientations at stations on seismically active faults are consistent with the strike of strike slip faults and indicate large seismically active faults breaking or nearly breaking the free surface pfs orientations are influenced by both regional compressional stress and by nearby faults it is an effective way to study detailed spatial distribution of crustal compressional stress by shear wave anisotropy with dense seismograph network shear wave splitting may also be used to indicate hitherto unknown faults

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

slickenside kinematic indicators abstract a new classification of slickenside kinematic indicators is presented based on 61 criteria these slickensides have been subdivided into eleven major groups v or crescentic markings steps fractures trains of inclined planar structures trailed material asymmetric elevations deformed elements mineralogical crystallographic orientations asymmetric plan view features asymmetric cavities and asymmetric folds this classification constitutes a useful tool for geologists interested in the determination of the shear sense in fault surfaces bearing slickensides examples of application of this classification to natural fault surfaces at different scales are presented

dynamics and implications of slab detachment due to ridge trench collision 1 the approach of a buoyant spreading ridge to a subduction zone may lead to detachment of a subducted slab previous work has called upon the detachment process as an explanation for observed ridge abandonment and slab window related magmatism e g in baja california but such a scenario has not been tested using fully dynamic numerical models we use dynamic two dimensional models including a non newtonian rheology to study the approach of a trench parallel spreading ridge to a subduction zone we find that before the ridge approaches within 100 km of the trench detachment of the subducted slab occurs due to the combined effects of increased buoyancy and reduced strength of the lithosphere with ridge proximity in models exploring effects of subducted slab length distance of the ridge from the trench shear zone strength and lithospheric yield strength we find the following 1 a decrease in subduction velocity as the ridge approaches the trench 2 a shrinking surface plate that maintains a uniform plate like subduction velocity 3 ridge abandonment distances 100 275 km from the trench and 4 slab gap distances 195 285 km from the trench these results are consistent with observations in baja california where detachment of the cocos slab may explain abandonment of observed segments of the east pacific rise 50 200 km outboard of the trench and the presence of nonarc related magmatism located 100 250 km inboard of the trench ridge subduction does not occur within any of these two dimensional cases of ridge trench collision indicating that 3 d geometric effects may be responsible for ridge subduction

observation and modelling of fault zone fracture seismic anisotropy i p sv and sh travel times summary three component vsp borehole seismograms taken in the vicinity of an active normal fault in california show strong systematic shear wave splitting that increases with proximity to the fault using cerven s method of characteristics for ray tracing in anisotropic heterogeneous media and hudson s formulation of elastic constants for media bearing aligned fractures we have fitted a suite of p sv and sh hanging wall and foot wall travel times with a simple model of aligned fractures flanking the fault zone the dominant fracture set is best modelled as parallel to the fault plane and increasing in density with approach to the fault the increase in fracture density is non uniform power law or gaussian with respect to distance to the fault although the hanging wall and the foot wall rock are petrologically the same unit the fracture halo is more intense and extensive in the hanging wall than in the foot wall upon approach to the fault plane the fracture density or fracture density gradient becomes too great for the seismic response to be computed by hudson cerven procedures the maximum fracture density that can be modelled is about 0 08 within this 25 m fracture domain it appears more useful to model the fault and near field fractures as a low velocity waveguide we observe production of trapped waves within the confines of the intense fracture interval

early miocene shortening in the lower comondu group in baja california sur mexico abstract the late oligocene early miocene volcaniclastic deposits of baja california sur form most of the exposed western margin of the gulf of california rift in some places these deposits collectively referred to as comondu group show complex deformation patterns given by the coexistence of tectonic and gravitational features the area north of la paz is characterized by the occurrence of several slump bodies which are displaced by normal faults connected with the rift opening in some places we have identified 100 s m scale thrust related folds and reverse faults that we have interpreted as shortening features the latter displace the slump layers and are offset by the normal faults if confirmed this would represent the first report of a shortening event in the early miocene volcaniclastic deposits of baja california sur the observed shortening has modest magnitude ca 3 5 bulk shortening and has been detected in a sector extending over 100 km north from la paz new 40 ar 39 ar ages integrated with existing radiometric age datasets constrain the timing of this shortening episode the rocks affected by shortening have ages between 24 and 21 ma and are capped by undeformed volcanic rocks with ages spanning between 19 4 and 17 2 ma these relationships define an intra early miocene unconformity which we interpret to be related to the shortening deformation the available timing constraints allow us to infer that a main ene to ese trending shortening was short lived possibly ca 19 4 21 ma the account of this shortening event may shed some light on the complex subduction and microplate processes that preceded the continental rifting of the gulf of california

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

pulling apart the mid to late cenozoic magmatic record of the gulf of california is there a comondu arc the composition of the lithosphere can be fundamentally altered by long lived subduction processes such that subduction modified lithosphere can survive for 100 s myrs incorrect petrotectonic interpretations result when spatial temporal compositional trends of and source contributions to magmatism are not properly considered western mexico has had protracted cenozoic magmatism developed mostly in board of active oceanic plate subduction beneath western north america a broad range of igneous compositions from basalt to high silica rhyolite were erupted with intermediate to silicic compositions in particular showing calc alkaline and other typical subduction related geochemical signatures a major oligocene rhyolitic ignimbrite flare up 300 000 km3 switched to a bimodal volcanic phase in the early miocene 100 000 km3 associated with distributed extension and opening of numerous grabens extension became more focussed 18 ma resulting in localised volcanic activity along the future site of the gulf of california this localised volcanism known as the comondu arc was dominantly effusive and andesite dacite in composition past tectonic interpretations of comondu age volcanism may have been incorrect as these regional temporal compositional changes are alternatively interpreted as a result of increased mixing of mantle derived basaltic and crust derived rhyolitic magmas in an active rift environment rather than fluid flux melting of the mantle wedge above the subducting guadalupe plate

shear wave anisotropy of active tectonic regions via automated s wave polarization analysis abstract using an automated aspect ratio method to evaluate shear wave anisotropy from earthquakes recorded on arrays of three component digital seismographs over the bucaramanga nest of colombia the east rift zone of hawaii and the phlegraean fields uplift area of italy a shear wave velocity anisotropy of 4 7 is observed the anisotropy is attributed to tectonic processes acting either to align anisotropic minerals or to align stress related cracks at most fixed seismographs the polarization of the leading s phase is consistent from event to event independent of earthquake location on a regional basis the polarization may vary from station to station but a regional trend is apparent for events from the bucaramanga nest colombia a small seismogenic volume at 160 km depth in a subducted plate shear wave splitting averages 0 5 0 1 s for more than 100 events the polarization direction of the fast shear wave is 355 n 15 parallel to the strike of the benioff zone at the phlegraean fields in italy which represent an active quaternary caldera the average shear wave splitting observed from 51 events is 0 2 s corresponding to a minimum velocity anisotropy of 7 the strike of the inferred maximum compressive stress is consistent with values obtained from fault plane solutions in hawaii the polarization of the leading shear wave tends to parallel the strike of the east rift zone average velocity anisotropy is about 5 if one assumes that the entire travel path contributes to the observed splitting however station variations in fast shear wave polarization direction imply that some of the splitting is due to near station effects

shear wave splitting and anisotropy in the charlevoix seismic zone quebec during the month of august 1984 an experiment was undertaken in the charlevoix seismic zone to test the hypothesis that shear wave splitting could be observed in a seismically active region seismograms recorded by digital 3 component seismographs located very close to the epicentres of three earthquakes showed shear wave splitting the amount of splitting was estimated to be 1 6 to 2 of the shear wave velocity from particle motion diagrams the average azimuth of the direction of polarization was 30 these observations can be explained by the presence of vertical cracks striking 30 east of north with a minimum crack density of e 0 03

estimating the internal structure of reservoirs with shear wave vsps in the past 10 years vertical seismic profiles vsps have played an increasingly important role in identifying and delineating in situ layers such vsps have usually recorded p wave data three component shear wave trains carry three or four times more information about the source and structure of the rocks along the raypath than the equivalent p wave train so in principle shear wave vsps should be a particularly valuable technique for examining in situ layers

modelling the compliance of crustal rock ii response to temporal changes before earthquakes summary there have been several claims that seismic shear waves respond to changes in stress before earthquakes the companion paper develops a stress sensitive model ape for the behaviour of low porosity low permeability crystalline rocks containing pervasive distributions of fluid filled intergranular microcracks and this paper uses ape to model the behaviour before earthquakes modelling with ape shows that the microgeometry and statistics of distributions of such fluid filled microcracks respond almost immediately to changes in stress and that the behaviour can be monitored by analysing seismic shear wave splitting the physical reasons for the coupling between shear wave splitting and differential stress are discussed in this paper we extend the model by using percolation theory to show that large crack densities are limited at the grain scale level by the percolation threshold at which interacting crack clusters lead to pronounced increases in rock matrix permeability in the simplest formulation the modelling is dimensionless and almost entirely constrained without free parameters nevertheless ape modelling of the evolution of fluid saturated rocks under stress reproduces the observed fracture criticality and the narrow range of shear wave azimuthal anisotropy in crustal rocks it also reproduces the behaviour of temporal variations in shear wave splitting observed before and after the 1986 m 6 north palm springs earthquake southern california and several other smaller earthquakes the agreement of ape modelling with a wide range of observations confirms that fluid saturated crystalline rocks are stress sensitive and respond to changes in stress by critical fluid rock interactions at the microscale level this means that the effects of changes in stress and other parameters can be numerically modelled and monitored by appropriate observations of seismic shear waves

indication of high pore fluid pressures in a seismically active fault zone summary polarisations of seismic shear wave splitting observed above small earthquakes in iceland are typically approximately ne to sw parallel to the direction of maximum horizontal stress in contrast the polarisations of shear waves at three new stations sited over the husav ik flatey fault a major seismically active transform fault in northern iceland are approximately nw to se orthogonal to the stress aligned polarisations elsewhere modelling suggests that these 90 flips in polarisations are caused by propagation through cracks containing fluids at high pore fluid pressures within one or two mpa of the critical stress these observations suggest that high pore fluid pressures which play a key role in earthquake source mechanisms can be monitored by analysing shear wave splitting above seismically active fault planes

local shear wave observations in joao câmara northeast brazil 1 we present shear wave splitting data from a 10 km aperture eight station digital three component seismograph network which operated from december 1992 to august 1994 near joao c mara 5 33 s 35 51 w to record shear waves from continuing microearthquake activity persisting 6 years after a main shock in november 1986 previous aftershock recording had shown remarkably impulsive simple seismograms and hypocentral errors of typically 200 m in a structure of near uniform wave speed but only one station was three component the pattern of polarization directions shown is consistent with the trend of precambrian foliation observed in the field it is therefore concluded that these shear wave observations are controlled by the seismic anisotropy associated with the precambrian foliation and hence bear the signature of the paleo stress field i e the precambrian deformation regime this conclusion has important implications for the interpretation of shear wave splitting observations made in crustal crystalline rock because other authors have asserted that shear wave splitting normally results from the effect of stress aligned fluid filled cracks extensive dilatancy anisotropy or eda eda is by contrast a signature of the present day stress field in this region there is firm evidence that the present day maximum compressive stress is e w this evidence comes from earthquake focal mechanisms and borehole breakout data and is summarized in the world stress map we therefore conclude that our present data are in maximum conflict with an interpretation in terms of eda our interpretation is supported by the observation that delay times are consistent with those expected from modeling of elastic waves in the crust

upper proterozoic rift related rocks in the pensacola mountains antarctica precursors to supercontinent breakup sedimentological and structural studies in the pensacola mountains antarctica suggest that upper precambrian clastic sedimentary rocks of the patuxent formation and associated bimodal volcanic rocks formed in an intracontinental rift setting the turbidites of the patuxent formation are part of a large depositional system derived from a continental source interbedded pillow basalts and basaltic sills have trace and rare earth element signatures enriched relative to mid ocean ridge basalt and similar to some rift related tholeiitic suites nd and sr isotopic values are compatible with derivation from a lithospheric mantle source in a continental setting associated felsic volcanic rocks have crustal trace element and isotopic characteristics the rifting may have been a prelude to the fragmentation of a supercontinent and according to recent hypotheses the separation of laurentia from antarctica comparisons between the late precambrian and cambrian records of western north america and antarctica suggest that if these were conjugate margins separation must have been neoproterozoic rather than cambrian in age

the digital array at anza california processing and initial interpretation of source parameters seismicity along the san jacinto fault suggests that a 40 km long section near the town of anza may constitute a gap in the occurrence of ml 6 7 earthquakes the potential for a gap filling shock and the high rate of seismicity at the southern end of the gap five events of 4 0 ml 5 5 since 1970 provided the impetus for deploying a digital seismic array to collect high quality ground motion recordings of all events 2 ml 4 5 on scale recording for shocks with magnitudes above ml 4 can be obtained from an existing array of eight strong motion accelerographs the anza site also had the advantage of being in the southern california batholith which appears to be relatively homogeneous compared to the franciscan gablian contrast of the central san andreas we expected that the granitic rocks of the batholith would yield relatively accurate earthquake locations and efficiently propagate high frequencies the field instrumentation is specifically designed for broadband recording up to 70 hz and high dynamic range 96 db in the on site digitizer alone since both are necessary for determining the rupture history of earthquakes both local vhf and microwave digital telemetry transmit the data from the anza region to san diego for computer data logging in the first 30 months of the array s operation approximately 292 events have been recorded located and processed for source parameters most events occur in one of five clusters or in a diffuse zone near the buck ridge fault two of these clusters are located at right stepping en echelon offsets coyote creek san jacinto and san jacinto hot springs two others are directly below and about 8 km west of anza respectively the fifth cluster is just to the northeast of the hot springs fault although event depths are generally between 11 and 14 km at the southern end of the hot springs fault depths extend to 18 km these are some of the deepest strike slip earthquakes on the san andreas system we calculate source parameters such as the scalar moment and stress drop for the analysis of high frequency waves scaling relations and earthquake interaction the largest event recorded thus far had a moment of 4 4 1021 dyn cm m 3 8 and a stress drop of 55 bars both arms and brune stress drops increase with moment source radius increases only slowly with moment the maximum values of both the brune and arms stress drops increase with depth down to 10 km remain approximately constant to 14 km and may decrease below 14 km the data suggest that stress drops of a group of earthquakes can be related to the strength of the upper crust calculated from frictional and quasi plastic flow laws although individual events may not be related stress drops on the san jacinto fault are high compared to those on the central creeping section of the san andreas where stress drops are about 10 bars or less this observation is consistent with the relative rupture area of events of equal ml 5 5 and may be related to the lithologie differences between the two fault segments and the amount of normal stress compared to shear stress

quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction we analyze shear wave polarizations from local earthquakes recorded by the anza network in southern california using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival the linearity interval initial shear wave particle motions are strongly aligned at four stations a feature that is not predicted by focal mechanisms the particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations a result supported by the clear shear wave splitting seen in a borehole recording near one of the anza stations these results are consistent with an earlier analysis of these data by peacock et al 1988 however our analysis does not support claims by crampin et al 1990 that shear wave splitting delay times at station knw exhibit temporal variations which can be correlated with the occurrence of the north palm springs earthquake ml 5 6 of july 8 1986 automatically determined linearity intervals scatter widely from 0 02 to 0 15 s and exhibit no clear temporal trends we find a correlation between earthquake moment and the linearity interval possibly a result of longer effective source time functions for the larger events the inability to identify a distinct slow quasi shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion even in the very early shear wave coda analysis of earthquake clusters with similar waveforms recorded at knw shows that seismic green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events if shear wave splitting is causing the observed delay times between horizontal components the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5 10

late precambrian tectonism in the kingston range southern california speculation on the early history of the cordilleran geosyncline of north america has relied in part on interpretation of the tectonic setting of the precambrian pahrump group recent mapping in the kingston range resulted in new data to aid our speculation specifically precambrian tectonic events involving folding and later low angle normal faulting affected the crystal spring formation beck spring dolomite and lower part of the kingston peak formation unconformably overlying these units is a relatively undeformed conglomerate diamictite occurs both above and below the unconformity hence cordilleran wide correlations of the kingston peak formation based on diamictite require reassessment the nature of boundaries localized deformation and outcrop distribution of the pahrump group suggest that these rocks may be preserved in transtensional basins associated with strike slip faulting possibly occurring during development of the late precambrian continental margin

stratigraphy and geochronology of the comondu group near loreto baja california sur mexico abstract upper oligocene to middle miocene volcanic and sedimentary rocks in the loreto region baja california sur are widely exposed and make up the comondu group as redefined here following mcfall the comondu group is part of a volcanic arc and forearc basin that formed along the northwestern margin of mexico regional to detailed scale mapping stratigraphic analysis and geochronology in a 10 20 km wide and 70 km long belt from the gulf escarpment to the coast near loreto reveal three main units in the comondu group and a composite thickness of 1 5 2 km 1 the lower clastic unit 30 19 ma contains 200 300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt flows that accumulated in a forearc basin 2 the middle breccia and lava flow unit 19 15 ma is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris flows interbedded with minor andesite lava flows 3 the upper lava flow and breccia unit 15 12 ma consists almost entirely of up to 600 meters of andesite lava flows a few kilometers west of loreto that are part of a composite volcano north and south of loreto for up to 25 km the unit is composed of andesite lava flows and massive andesite breccia that formed from thick debris flows near that and other volcanic centers the three units of the comondu group in the loreto area correlated with the salto pelones and ricason formations of the bahia concepcion area the lower clastic unit of this study is time equivalent to the marine san gregorio el cien and isidro formations on the western and southern baja california peninsula the middle and upper units of the loreto area are the proximal equivalent to the type section of the comondu group near the village of comondu during deposition of the lower part of the comondu group the arc lay to the east on mainland mexico in the sierra madre occidental and then migrated to the position of the gulf of california at about 25 ma at 19 ma the proximal part of the arc migrated abruptly 50 km westward to approximately the position of carmen island and at 15 ma the arc once again stepped west forming eruptive centers along the coast near loreto until 12 ma also at 15 ma the proximal forearc region changed from aggradational to neutral or erosional such that no rocks of the upper unit are preserved in the gulf escarpment

oblique rifting ruptures continents example from the gulf of california shear zone we show that a belt of clockwise vertical axis block rotation associated with dextral oblique rifting in the basin and range province in mexico hosted the localization of plate boundary strain that led to formation of the gulf of california ocean basin paleomagnetism of miocene ignimbrites distributed widely across the rift reveals the magnitude distribution and timing of rotation using new high precision paleomagnetic vectors 95 1 from tectonically stable exposures of these ignimbrites in baja california we determine clockwise rotations up to 76 for intrarift sites low reference site error permits isolation of intrarift block rotation during proto gulf time prior to rift localization ca 6 ma we estimate that 48 locally 0 75 of the net rotation occurred between 12 5 ma and 6 4 ma sites of large 20 block rotation defi ne an 100 km wide belt associated with strike slip faulting herein named the gulf of california shear zone which was embedded within the wide rift basin and range province and kinematically linked to the san andreas fault after a protracted history of diffuse extension and transtension rift localization was accomplished by focusing of pacifi c north america dextral shear into the gulf of california which increased strain rates and connected nascent pull apart basins along the western margin of the province oblique rifting thus helped to localize and increase the rate of continental break up and strongly controlled the three dimensional architecture of the resultant passive margins

ductile precambrian fabric control of seismic anisotropy in the acu dam area northeastern brazil 1 in the joao c mara area ne brazil seismic anisotropy has been interpreted as the result of either precambrian ductile fabric or stress aligned fluid filled grain boundary caused by high pore fluid pressures the first interpretation is based on the fact that the direction of the fastest s wave splitting agrees with the north south trending precambrian fabric the second interpretation related to the extensive dilatancy anisotropy eda is based on the stress field orientation and on the assumption that the pore fluid pressure in the region is high enough to modify microcrack distribution and cause s wave polarization to become parallel to the north south oriented minimum horizontal stress shmin here we address this question by investigating this ambiguity we have analyzed the seismicity in the acu dam area 150 km west of joao c mara over a 3 year period in the acu area the northeast southwest trending ductile precambrian fabric is not parallel or orthogonal to the east west trending maximum horizontal stress shmax we combined the interpretation of fastest s wave polarizations their time delays and fabric orientation measured in the field with side looking airborne radar imagery taken before the lake impoundment thus we discriminate between 1 stress aligned fluid filled cracks 2 high pore pressure fluids modifying the microcrack distribution and therefore causing 90 flips and 3 fabric or paleostrain as causes for the observed s wave anisotropy our main result indicates that the direction of the fastest s wave polarization in the acu dam area agrees with the northeast southwest trending fabric n40 e eda or 90 flips control of seismic anisotropy if occurring in acu have minor effects

comment on quantitative measurements of shear wave polarizations at the anza seismic network southern california implications for shear wave splitting and earthquake prediction by richard c aster peter m shearer and jon berger

evolution of the margin of the gulf of california near loreto baja california peninsula mexico the gulf of california is a prime example of a young oblique divergent plate boundary this type of plate boundary is much less well understood than classic rifts and passive margins in divergent plate settings a complexity in the gulf of california is that the modern oblique rifting was preceded by a stage of orthogonal rifting we have used extensive mapping as well as stratigraphic geomorphic and structural analysis to determine the history of deformation near loreto on the southern baja california peninsula during formation of the gulf of california our data support the suggestion that middle to late miocene protogulf stage 12 to ca 6 ma orthogonal rifting was overprinted by transtensional structures during pliocene to quaternary time though we cannot closely define the time of change we further demonstrate that the plate margin was structurally segmented in the protogulf stage and then complexly overprinted by transtensional structures that suggest one type of process for initiating transform faults the loreto segment is 85 km long along the rift and is bounded on the west by a discontinuous series of aligned down tothe east extensional monoclines and normal faults these structures lie along or as far as 4 km in front east of a steep 1000 1600 m high main gulf escarpment that defines the western topographic margin of the rift zone the loreto segment is further defined by a rise in elevation of the main gulf escarpment of up to 1 km and an 500 800 m increase in structural relief of prerift strata in the escarpment from the segment boundaries to the center of the segment structural analysis of secondary faults of the loreto segment shows that fault populations that are known to be pliocene to quaternary in age are mixed normal and dextral normal faults with a bulk extension direction of west northwest eastsoutheast 2808 1008 in contrast fault populations that cut only prerift oligocene middle miocene volcanic and sedimentary rocks are mainly normal faults with northeast southwest to east northeast west southwest bulk extension directions that average 2458 658 these faults are interpreted to be late miocene in age and formed during protogulf orthogonal rifting thus the pliocene faults record a major change of extension direction of 358 from latest miocene to pliocene time compatible with the onset of oblique rifting in the gulf of california oblique divergent overprinting is mainly expressed as the loreto fault and loreto basin in the northern part of the segment the loreto fault and basin evolved with major fault reorganizations and partial basin inversion many observations suggest that the pliocene loreto fault was linked to two nascent transform faults in the gulf and that carmen island rotated 358 408 clockwise within the complex fault system

evidence from shear wave splitting for the restriction of seismic anisotropy to the upper crust the existence of anistropy in the propagation speed of seismic waves through the earth s crust has been attributed to the non random alignment of microcracks in the crust1 for sparsely distributed parallel vertical cracks perturbation theory predicts the splitting of a shear wave into two orthogonally polarized waves2 4 here we present extremely clear evidence for shear wave splitting in three component seismograms from crustal and sub crustal microearthquakes recorded in the shikoku district of japan quantitative analysis of travel time differences between split shear waves leads to the conclusion that the observed anisotropy is limited to the upper crust shallower than 10 15 km this model is consistent with recent results from reflection seismology5 and implies that microcrack density or orientation changes from the upper to the lower crust

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

active continental rifting in southern baja california mexico implications for plate motion partitioning and the transition to seafloor spreading in the gulf of california three major earthquake series have occurred during the past 30 years on fault systems that are spatially and kinematically distinct from oceanic spreading centers and transform faults in the southern gulf of california focal mechanism solutions for earthquakes of the april 1969 and june 1995 series indicate nearly pure normal displacement across nnw striking planes this seismicity falls along different strands of a major system of normal faults that extends at least 300 km along strike and defines the western limit of the gulf extensional province the dominant normal faults control distribution of neogene basins and have been active since middle to late miocene time epicenters of the august 1969 earthquake series lie south of the baja california peninsula and focal mechanism solutions indicate strike slip displacement this seismic activity is likely related to active wnw striking faults in the continental borderland which isolate baja california as a microplate within the pacific north american plate boundary therefore modern pacific north america plate motion is partitioned into at least three distinct fault systems oceanic spreading centers and transforms in the gulf of california normal faults along the western margin of the gulf of california and faults along the pacific margin of the baja california peninsula kinematic partitioning of plate motion is required to accommodate the strongly three dimensional strain of transtensional rifting continental rifting and seafloor spreading have overlapped in time for the past 3 6 myr which also is likely attributed to the highly oblique kinematics of rifting

farallon slab detachment and deformation of the magdalena shelf southern baja california 1 subduction of the farallon plate beneath northwestern mexico stalled by 12 ma when the pacific farallon spreading ridge approached the subduction zone coupling between remnant slab and the overriding north american plate played an important role in the capture of the baja california bc microplate by the pacific plate active source seismic reflection and wide angle seismic refraction profiles across southwestern bc 24 5 n are used to image the extent of remnant slab and study its impact on the overriding plate we infer that the hot buoyant slab detached 40 km landward of the fossil trench isostatic rebound following slab detachment uplifted the margin and exposed the magdalena shelf to wave base erosion subsequent cooling subsidence and transtensional opening along the shelf starting 8 ma starved the fossil trench of terrigenous sediment input slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes

millisecond accurate monitoring of seismic travel times over 13 and 18 kilometer baselines using a repeatable airgun source we have monitored the travel times of several seismic phases along 13 and 18 km baselines from bouquet reservoir to boreholes on the san andreas fault near palmdale california the first arriving phases refract at a depth of 2 3 km while one later arrival is possibly a reflection from 10 km the majority of some 200 travel time determinations for the most prominent phases were made between august 1979 and march 1980 isolated measurements were made in november 1978 and march 1981 there exists ample geodetic and other evidence for significant strain changes during this period allowing for source variation due to fluctuating reservoir water levels we observed no travel time changes to the limit of our precision of 1 2 ms for the first arrivals and 3 5 ms for secondary arrivals or about 0 1 of the total travel time an earlier study which monitored rays penetrating only 500 m into the crust found that travel times varied by up to 0 3 over a period of several days the variations showed some correlation with strain changes due to local seismic activity and were not permanent to understand this different behavior of crustal rocks between 500 and 2000 m we investigate the depth dependence of their velocity strain sensitivity using measured p and s velocity profiles to gauge the rate at which confining pressure increases velocity we find that 1 bar of overpressure at 500 m can vary the p velocity by 0 1 while 1 bar at 2000 m will vary the velocity by only 0 01 applying the o connell budiansky physical model for cracked rock to the velocity depth profiles we can further characterize the local crust as having a high crack density a mobile fluid component and an average crack aspect ratio of 2 5 10 3 the combination of a mobile fluid content and high crack density suggests that fluids play a role in the transient velocity anomalies observed at a depth of 500 m and argues against significant undersaturation phenomena at seismogenic depths on the san andreas fault

microstructures in metasedimentary rocks from the neoproterozoic bonahaven formation scotland microconcretions impact spherules or microfossils abstract microscopic spherules in relatively undeformed mudstones of the neoproterozoic bonahaven formation islay scotland are differentiated from their matrix by a sharp micron scale smoothly rounded boundary these elongate spherules were earlier interpreted as hollow bodies filled penecontemporaneously by glauconite and subsequently metamorphosed to phengite but their origin remains a matter of debate spherules observed in thin section are predominantly rounded 74 but can exhibit a flat edge or protrusion at one end in 11 of a sample population two or more spherules are conjoined x ray diffraction indicates that spherule bearing mudstones consist mainly of muscovite with variable amounts of kaolin group minerals and minor iron chlorites a range of physical origins for the spherules including microconcretions or metamorphic microstructures deposition from the sky as micrometeorites microtektites microkrystites or accretionary volcanic ash particles and detrital grains is considered but rejected on distributional morphological and mineralogical evidence biological origins are considered most likely especially protistan tests similar to the vase shaped microfossils found in somewhat older neoproterozoic rocks if correct this provides the first report of eukaryotic life in the dalradian succession that passes critical tests for biogenicity and new evidence for testate microfossils in post sturtian but pre marinoan aged rocks

oceanic ridge subduction vs slab break off plate tectonic evolution along the baja california sur continental margin since 15 ma the interaction of the pacific farallon spreading centers with the north american convergent margin off baja california mexico supposedly ceased at 12 ma when plate convergence and seafloor spreading stopped we propose a new geodynamic evolution based on full bathymetry coverage and magnetic profiles from 23 n to 27 n famex cruise of the r v l9atalante april 2002 the data unveil a major clockwise rotation of the pacific farallon spreading direction starting ca 14 ma that formed a series of short spreading centers that became extinct ca 8 7 ma we suggest that the transcurrent motion between the pacific and north america along baja california was accommodated by seafloor spreading and oblique convergence along the trench this change in spreading direction was followed by a concomitant progressive demise of both pacific farallon seafloor spreading and farallon north america subduction that are attributed to the break off of the farallon slab this also resulted in the opening of a trench parallel slab window beneath baja california

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

shear wave splitting in the anza seismic gap southern california temporal variations as possible precursors shear wave splitting above small earthquakes in the anza seismic gap on the san jacinto fault southern california displays the effective anisotropy of vertical fluid filled microcracks the delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence these changes can be modeled by increasing the aspect ratio bowing of stressoriented microcracks which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid filled microcracks this is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake

the scatter of time delays in shear wave splitting above small earthquakes summary measurements of time delays in seismic shear wave splitting above small earthquakes typically display a scatter of often as much as 80 per cent about the mean changes in the average timedelay appear to be related to changes of stress but applications of this potentially powerful tool have been handicapped by the previously inexplicable scatter in time delays above earthquakes in contrast measurements of shear wave time delays in controlled source exploration seismics are typically well controlled and display little scatter previous estimates of possible causes of scatter cannot produce sufficient variation specifically above earthquakes here we show that 90 flips in shear wave polarizations due to fluctuating high pore fluid pressures on seismicallyactive fault planes are the most likely cause of the scatter

shear wave vibrator signals in transversely isotropic shale the experiments of robertson and corrigan 1983 on shale are among the first three component field observations of shear waves in transversely isotropic media to be published their data are reprocessed to highlight the effects of the shale s anisotropy on shear waves two results emerge first shear wave splitting in a transversely isotropic substrate is most easily observed when the vibrator baseplate is oriented so that both sh and sv waves reach the geophone second the sv wave polarization deviates significantly from perpendicular to the raypath both results may significantly affect the interpretation both are found to agree with theoretical results and are modeled successfully by synthetic seismograms

ignimbrite flare up and deformation in the southern sierra madre occidental western mexico implications for the late subduction history of the farallon plate geologic mapping and structural data for the southern part of the smo demonstrating that most of this volcanic province was built in two episodes of ignimbrite flare up in oligocene 31 5 28 ma and early miocene 23 5 20 ma time and that extensional deformation occurred mostly before the transfer of baja california to the pacific plate extensive ignimbrite successions with 40 ar 39 ar ages clustering at 23 and 21 ma cover most of the southern smo thus correlating in age with ignimbrites exposed in southern baja california and central mexico grabens with a 020 to n s orientation developed in the east almost concurrently with this volcanic episode half grabens and nnw striking listric normal fault systems formed at the end of middle miocene as far as 150 km from the present coast a belt of left lateral transpressional structures formed along the southern boundary of the smo during the same period we link these magmatic and tectonic events to the evolution and dynamics of the farallon and north america plates during the miocene particularly we propose that a first detachment of the lower part of the farallon plate in early miocene time produced a transient thermal event and partial melting of the crust via mafic underplating middle miocene extension would be related to a second detachment event resulting from the slowing subduction that preceded the final capture of the magdalena microplate by the pacific plate at 12 5 ma transpression at the southernmost end of the smo occurred along the inland projection of the magdalenacocos plate boundary and may be explained by a difference in subduction rate and by a temporal convergence between the two plates in the eve of the end of subduction of the magdalena plate index terms 5480 planetology solid surface planets volcanism 8450 8150 tectonophysics evolution of the earth plate boundary general 3040 8109 tectonophysics continental tectonics extensional 0905 keywords ignimbrite flare up sierra madre occidental western mexico extensional tectonics slab detachment

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

high pore fluid pressures at bhuj inferred from 90 flips in shear wave polarizations summary shear wave splitting above local earthquakes in the aftershock zone of the 2001 m w7 6 bhuj earthquake display a spatial variation of anisotropy related to crack orientation at most seismic stations the fast polarization directions are approximately nne ssw parallel to the inferred directions of maximum horizontal stress suggesting the dilatancy induced anisotropy resulting from approximately stress aligned parallel vertical microcracks in contrast the fast polarization directions at station samkhyali skl very close to the epicentre of the main shock are fault parallel approximately ew and almost orthogonal to the stress aligned polarizations elsewhere ape modelling and observations elsewhere suggest that these are 90 flips in shearwave polarizations caused by propagation through microcracks containing fluids at critically high pore fluid pressures surrounding the seismically active fault plane the presence of high pore fluid pressures on all seismogenic faults also explains the high scatter in shear wave time delays the average path normalized delay time varies from 2 to 4 ms km 1 except at station skl where higher delays of up to 13 ms km 1 are found the large scatter in measured time delays is caused by cracks saturated with high pressure pore fluids these observations can be explained by the presence of both normal stress aligned and 90 flipped shear wave polarizations at bhuj this suggests that the kutch mainland fault is a major tectonic feature

timing of rifting in the southern gulf of california and its conjugate margins insights from the plutonic record the gulf of california is a young example of crustal stretching and transtensional shearing leading to the birth of a new oceanic basin at a formerly convergent margin previous studies focused along the southwestern rifted margin in baja california indicated rifting was initiated after subduction and related magmatism ceased at ca 14 12 5 ma however the geologic record on the mexico mainland sinaloa and nayarit states indicates crustal stretching in the region began as early as late oligocene the timing of cooling and exhumation of pre and synrift plutonic rocks can provide constraints on the timing and rate of rifting here we present results of a regional study on intrusive rocks in the southern gulf of california sampled along the conjugate baja california and nayarit sinaloa rift margins as well as plutonic rocks now exposed on submerged rifted blocks inside the gulf forty one samples were dated via u pb zircon and 40ar 39ar mineral ages providing emplacement age and thermochronological constraints on timing and rate of cooling we found an extensive suite of early and middle miocene plutons emplaced at shallow depths within the basement cretaceous paleocene peninsular range and sinaloa jalisco batholiths early miocene granitoids occur in an elongated wnw ese belt crossing the entire southern gulf from southern baja california to nayarit and sinaloa most have an intermediate composition 75 sio2 wt was emplaced 20 1 18 3 ma near the end of the early miocene age span and chemical composition of the early miocene silicic plutons essentially overlap ignimbrites and domes exposed in the southern sierra madre occidental and in southern baja california suggesting that eruptive sources for the early miocene ignimbrite flare up may also have been located within the southern gulf of california early miocene plutons cooled below the 40ar 39ar biotite closure temperature 350 400 c in less than 2 5 m y which we interpret as evidence of a regional extensional event leading to the opening of the gulf of california a less widely distributed suite of intermediate composition middle miocene granitoids 15 13 ma was sampled from the central western part of the gulf west of the pescadero basin and these correspond to an episode of scarce volcanism recorded by the middle and upper members of the onshore comondu group in baja california our widely spaced sampling of the generally sediment covered igneous crust suggests that middle miocene primary volcanic rocks are much less abundant than implied by previous models in which the gulf was the site of a robust comondu arc thermobarometry data also indicate a very shallow depth 5 km of emplacement for the middle miocene plutonic rocks some of these rocks also show a distinctive inequigranular texture indicative of at least two crystallization stages at different pressure early and middle miocene granitoids away from the gulf axis yielded 40ar 39ar cooling ages very close to u pb zircon ages demonstrating rapid cooling to 350 c which we attribute to their shallow emplacement and possibly to exhumation soon after intrusion since comondu age and middle miocene magmatism in the gulf region coincided with rapid cooling of young plutons that predate the end of subduction we suggest that intense crustal stretching controlled the pattern and timing of comondu age magmatism rather than the middle miocene magmatism controlling the locus of 12 ma extension

sedimentology and sequence stratigraphy of neoproterozoic and cambrian units across a craton margin hinge zone southeastern california and implications for the early evolution of the cordilleran margin abstract neoproterozoic basal cambrian strata exposed in eastern california represent the deposits of a craton margin hinge zone that formed in response to the fragmentation of the neoproterozoic supercontinent rodinia one unresolved question regarding the late neoproterozoic stratigraphy in the southwestern united states has been the interpretation of paleotectonic affinity for rocks deposited above the glaciogenic kingston peak formation of central concern has been attempts to identify the position in this stratigraphy where rift related sediments give way to passive margin deposits subsidence analyses suggest that rifting occurred near the start of the cambrian perhaps coincident with the feldspathic middle member wood canyon formation it rests on a disconformity that represents the base of the sauk supersequence and would likely represent the break up unconformity in subsidence analysis models middle member wood canyon and overlying lower middle cambrian strata extend far into the craton detailed sedimentary facies and sequence stratigraphic analysis of post kingston peak pre middle wood canyon rocks indicate that depositional facies in this succession distal alluvial through shallow subtidal are similar to units deposited on the stable craton further these pre sauk units which can be divided into multiple principally eustatically driven depositional sequences are correlated across long distances and even lap onto the edge of the stable craton we suggest the kingston peak formation is the principal rift generated deposit and the overlying thick predominantly siliciclastic section represents the basal deposits of the cordilleran passive margin kingston peak strata consist of thick coarse diamictites interbedded with turbidites such facies are unique in the overall stratigraphy but are similar to other known rift deposits although the age of the kingston peak formation is not well constrained and may be as young as 590 ma it may correlate with the rapitan sturtian glaciation 720 ma which would place middle wood canyon strata some 150 ma younger than the start of rifting

temporal changes in shear wave splitting during an earthquake swarm in arkansas shear wave splitting has been observed in seismograms of two aftershock and one foreshock sequence recorded over 12 days by a local network of nine three component digital seismometers during the 1982 enola arkansas earthquake swarm polarizations of the faster split shear wave show alignments within the shear wave window which correlate with the regional stress field the time delay between split shear waves appears to increase before and to decrease at or after the time of each main shock this behavior is similar to observations before and after a m 6 0 event near the anza seismic gap southern california and may correspond to variations in crack geometry caused by changes in the stress field before and after the earthquakes there are indications that the decrease in delay may occur a few hours before the main shock and could be a possible precursor for earthquake prediction

miocene detachment faulting predating epr propagation southern baja california at the southern tip of the baja california peninsula we characterize the onshore structures and kinematics associated with crustal necking leading up to the pliocene breakup and early east pacific rise seafloor spreading from a combination of tectonic field investigations k ar and cosmogenic isotope dating and geomorphology we propose that the los cabos block represents the exhumed footwall of a major detachment fault this north trending detachment fault is marked by a conspicuous low dipping brittle ductile shear zone showing a finite displacement with top to the se ending to the ese this major feature is associated with fluid circulations which led to rejuvenation of the deformed cretaceous magmatic rocks at a maximum of 17 5 ma the detachment footwall displays kilometer scale corrugations controlling the present day drainage pattern this major detachment is synchronous with the development of the san jose del cabo basin where syntectonic sedimentation took place from the middle miocene to probably the early pliocene we propose that this seaward dipping detachment fault accommodates the proximal crustal necking of the baja california passive margin which predates the onset of formation of the east pacific rise spreading axis in the cabo puerto vallarta segment our data illustrate an apparent anticlockwise rotation of the stretching direction in baja california sur from 17 ma to the pliocene

a complex microbiota from snowball earth times microfossils from the neoproterozoic kingston peak formation death valley usa a thin carbonate unit associated with a sturtian age 750 700 million years ago glaciogenic diamictite of the neoproterozoic kingston peak formation eastern california contains microfossil evidence of a once thriving prokaryotic and eukaryotic microbial community preserved in chert and carbonate stratiform stromatolites oncoids and rare columnar stromatolites also occur the microbial fossils which include putative autotrophic and heterotrophic eukaryotes are similar to those found in chert in the underlying preglacial units they indicate that microbial life adapted to shallow water carbonate environments did not suffer the significant extinction postulated for this phase of low latitude glaciation and that trophic complexity survived through snowball earth times

a diamictite dichotomy glacial conveyor belts and olistostromes in the neoproterozoic of death valley california usa multiple intercalations of glacially derived and slope derived diamictites testify to the drawbacks of correlating neoproterozoic diamictites more widely but shed new light on the close interrelationship of these processes in the cryogenian world in the neoproterozoic of death valley california usa rifting of rodinia occurred concomitantly with a major glacial event that deposited the kingston peak formation a new sedimentologic investigation of this formation in the silurian hills demonstrates for the first time that some diamictites are ultimately of glacial origin abundant dropstone textures occur in interstratified heterolithic deposits with clasts of identical composition gneiss schist granite metabasite quartzite to those of boulder bearing diamictites suggesting a common source the glacial conveyor belt in stark contrast megaclast bearing diamictites yielding clasts of carbonate and siliciclastic preglacial strata as much as 100 m across are interpreted as olistostromes the occurrence of syn sedimentary faults within the succession allows glacial versus slope derived material to be distinguished for the first time

extensional segmentation of the main gulf escarpment mexico and united states periodic reversal of upper plate transport direction on miocene and younger extensional fault systems occurs in the northwestern gulf extensional province baja california and adjacent united states with segments in which the peninsular ranges are in either footwall or hanging wall positions accommodation zones separate these segments and are defined by changes of structural style and topography in the peninsular ranges and or the gulf extensional province these and other characteristics allow speculative definition of segments 50 150 km long along the baja california peninsula

paleontology of proterozoic shales and mudstones examples from the belt supergroup chuar group and pahrump group western usa abstract proterozoic black shales and other fine grained siliciclastic rocks contain a variety of fossils these fossils are illustrated by examples from three proterozoic units in the western usa the belt supergroup middle proterozoic the chuar group late proterozoic and the upper pahrump group late proterozoic a variety of microfossils and megafossils occur in the belt supergroup western montana acritarchs and filamentous microfossils are moderately well preserved in sub greenschist facies black shale in the little belt mountains the biologic affinity of the acritarchs is uncertain they could represent the vegetative or encystment stage of eukaryotic algae or the outer envelope of colonial coccoid cyanobacteria most of the filaments in these shales are of a size and morphology that is consistent with prokaryotic affinities however rare 20 and 40 m wide filaments could be either prokaryotic or eukaryotic higher grade sub greenschist facies dark gray to black mudstones in glacier national park contain poorly preserved acritarchs and spheroidal and filamentous pseudomicrofossils they are of interest because they provide information useful for distinguishing authentic microfossils from pseudofossils in mudstones and shales that have undergone substantial burial metamorphism several authentic megafossils occur in lower belt shales in the little belt and big belt mountains montana one of these is the carbonaceous compression beltina danai which could represent either fragments of a microbial mat or fragments of a sheet like megascopic alga the megafossil grypania spiralis has the form of a coiled ribbon and tentatively is interpreted as a megascopic alga green to purple mudstones and muddy sandstones in the lower belt supergroup in glacier national park contain markings that resemble a string of beads they are interpreted as megafossils that may have eukaryotic algal affinities black shales in the late proterozoic kwagunt formation chuar group grand canyon arizona contain a variety of acritarchs and filamentous microfossils of special interest is the vase shaped microfossil melanocyrillium which is interpreted as the sporangium of a eukaryotic and probably megascopic alga melanocyrillium also occurs in black mudstone in the late proterozoic pahrump group southeastern california

stratigraphic investigations of carbon isotope anomalies and neoproterozoic ice ages in death valley california an unusual richness of biogeochemical events is recorded in neoproterozoic cambrian strata of the death valley region california united states eight negative carbon isotope d13c excursions are found in carbonate units between 1 08 ga and the precambrian cambrian boundary four of these excursions occur in carbonates that contain textural features similar to those found globally in postglacial cap carbonates including one or more of the following laminite with rollup structures apparent tube rocks seafloor precipitates and sheet crack cements however only two of these units the sourdough limestone member of the kingston peak formation and the noonday dolomite rest directly upon glacial strata the basal beck spring dolomite and the rainstorm member of the johnnie formation each contain negative excursions and cap carbonate like lithofacies but do not rest on known glacial deposits if the negative d13c excursions are assumed to record depositional processes two equally interesting hypotheses are possible 1 the death valley succession records four glacial pulses in neoproterozoic time but glacial units are not preserved at two stratigraphic levels 2 alternatively other global oceanographic processes can cause negative excursions and cap carbonate like facies in addition to or independent of glaciation

shear wave anisotropy spatial and temporal variations in time delays at parkfield central california summary shear wave splitting is analysed on data recorded by the high resolution seismic network hrsn at parkfield on the san andreas fault central california during the three year period 1988 1990 shear wave polarizations either side of the fault are generally aligned in directions consistent with the regional horizontal maximum compressive stress at some 70 to the fault strike whereas at station mm in the immediate fault zone shear wave polarizations are aligned approximately parallel to the fault normalized time delays at this station are found to be about twice as large as those in the rock mass either side this suggests that fluid filled cracks and fractures within the fault zone are elastically or seismically different from those in the surrounding rocks and that the alignment of fault parallel shear wave polarizations are associated with some fault specific phenomenon temporal variations in time delays between the two split shear waves before and after a ml 4 earthquake can be identified at two stations with sufficient data mm within the fault zone and vc outside the immediate fault zone time delays between faster and slower split shear waves increase before the ml 4 earthquake and decrease near the time of the event the temporal variations are statistically significant at 68 per cent confidence levels earthquake doublets and multiplets also show similar temporal variations consistent with those predicted by anisotropic poroelasticity theory for stress modifications to the microcrack geometry pervading the rock mass this study is broadly consistent with the behaviour observed before three other earthquakes suggesting that the build up of stress before earthquakes may be monitored and interpreted by the analysis of shear wave splitting

precambrian olistoliths masquerading as sills from death valley california olistolith production and magmatism are processes commonly associated with extensional tectonic settings such as rift basins we present a cautionary exemplar from one such precambrian basin in which we reinterpret metabasite bodies previously documented as sills to be olistoliths we nevertheless demonstrate that on the basis of field observation alone the previous but erroneous sill interpretation is parsimonious indeed it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed we present new data from metabasites and metasedimentary strata of the kingston peak formation cryogenian and crystal spring formation mesoproterozoic of death valley usa these include field observations u pb apatite ages u pb zircon ages detrital and igneous and whole rock geochemistry these data also provide a new maximum age for the base of the pahrump group and suggest that the crystal spring diabase was more tholeiitic than previously thought similar sill olistolith misinterpretations may have occurred elsewhere potentially producing erroneous age and tectonic setting interpretations of surrounding strata this is particularly relevant in precambrian rocks where fossil age constraints are rare this is illustrated herein using a potential example from the neoproterozoic literature of the lufilian belt africa we caution others against precambrian olistoliths masquerading as sills supplementary material details of a meta igneous boulder from p12 of the silurian hills la icp ms and whole rock geochemistry methods and standards and u pb apatite and zircon isotopic data including standards and selected cathodoluminescence images are available at https doi org 10 6084 m9 figshare c 3990639

processing modelling and predicting time lapse effects of overpressured fluid injection in a fractured reservoir summary time lapse seismology is important for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs we analyse two 4 d 3c onshore surveys from vacuum field new mexico usa where the reservoir of interest is a fractured dolomite in phase vi a time lapse survey was acquired before and after a pilot tertiary recovery programme of overpressured co2 injection which altered the fluid composition and the pore fluid pressure phase vii was a similar time lapse survey in the same location but with a different lowerpressure injection regime applying a processing sequence to the phase vi data preserving normal incidence shearwave anisotropy time delays and polarization and maximizing repeatability interval time analysis of the reservoir interval shows a significant 10 per cent change in shear wave velocity anisotropy and 3 per cent decrease in the p wave interval velocities a 1 d model incorporating both saturation and pressure changes is matched to the data the saturation changes have little effect on the seismic velocities there are two main causes of the time lapse changes any change in pore fluid pressures modifies crack aspect ratios additionally when there are overpressures as there are in phase vi there is a 90 change in maximum impedance directions and the leading faster split shear wave instead of being parallel to the crack face as it is for low pore fluid pressures becomes orthogonal to the crack face the anisotropic poro elasticity ape model of the evolution of microcracked rock calculates the evolution of cracked rock to changing conditions ape modelling shows that at high overburden pressures only nearly vertical cracks to which normal incidence p waves are less sensitive than s waves remain open as the pore fluid pressure increases ape modelling matches the observed time lapse effects almost exactly demonstrating that shear wave anisotropy is a highly sensitive diagnostic of pore fluid pressure changes in fractured reservoirs in this comparatively limited analysis ape modelling of fluid injection at known pressure correctly predicted the changes in seismic response particularly the shear wave splitting induced by the high pressure co2 injection in the phase vii survey ape modelling also successfully predicted the response to the lower pressure injection using the same phase vi model of the cracked reservoir the underlying reason for this remarkable predictability of fluid saturated reservoir rocks is the critical nature and high crack density of the fluid saturated cracks and microcracks in the reservoir rock which makes cracked reservoirs critical systems

agglutinated tests in post sturtian cap carbonates of namibia and mongolia abstract paleomagnetic data suggest that the early cryogenian sturtian glaciation extended to sea level at low latitude the impact of this dramatic environmental change on biota and the composition of ecosystems in the immediate aftermath of the sturtian glaciation remain virtually unknown here we report the discovery of abundant agglutinated tests in organic rich carbonates directly overlying sturtian glacial deposits from two different paleocontinents the rasthof formation of the congo craton in northern namibia and the tsagaan oloom formation of the dzabkhan terrane in mongolia the most abundant tests preserve morphological and compositional characters consistent with those found in at least two different families of modern lobose testate amoebae amoebozoa a group of heterotrophic microbial eukaryotes the presence of spatially and compositionally variable clay minerals quartz and microcline on the test walls is a signature of widespread biological agglutination the post glacial fossil assemblages differ from the most common pre sturtian vase shaped fossil testate amoebae perhaps as a result of different preservational mechanisms or of the appearance of new forms after the glaciation the apparent local abundance of eukaryotic body fossils in the post sturtian carbonates suggests that the cryogenian limestones and dolostones may host a currently unexplored fossil record of modern eukaryotes

seismic anisotropy of the shallow crust in the umbria marche italy region abstract small magnitude earthquake data recorded during the 1997 umbria marche italy seismic crisis have been used to study the shear wave splitting in the central apennine using the method of diagonalization of the covariance matrix a set of 1234 s waves impinging the earth at an angle less than the critical angle has been selected the cross correlation method was used to estimate the splitting time between the two horizontal s components and the direction of the symmetry axis of the anisotropic structure a further analysis based on visual inspection of corrected traces allowed us to estimate the anisotropic parameters for 884 s waves an average splitting time 32 3 4 ms has been estimated splitting times do not generally show a clear pattern with distance which is an indication of possible spatial variations of seismic anisotropy a minimum 2 km depth of the anisotropic layer is estimated the trend of normalized splitting time vs depth roughly indicates a maximum 4 5 km depth of the anisotropic layer the distribution of the fast symmetry axis indicates a main symmetry axis oriented along the direction of the maximum compressive stress in the area and a secondary symmetry axis almost orthogonal to the first one the stereographic plots of fast axis direction seem exclude a depth dependence of anisotropic parameters a clear spatial correlation between the directions of the fast symmetry axes at the considered stations and the orientations of the nearest fault systems and thrust fronts is found which confirms the spatial variability in the anisotropic properties of the area as inferred by the analysis of splitting times based also on previous studies results are interpreted in terms of the extensive dilatancy anisotropy eda model the estimated crack density is always smaller than the critical coalescence value enforcing the hypothesis that fluid filled separated crack systems control the polarization of s waves

shear wave splitting above small earthquakes in the kinki district of japan abstract we have identified shear wave splitting diagnostic of the effective anisotropy induced by aligned microcracks in the wavetrains of micro earthquakes at four stations of the abuyama network in the kinki district of japan we find that the directions of polarization of the faster split shear waves are nearly parallel for all azimuths of arrival and for all angles of incidence less than the critical angle at three of the four stations these directions of polarization are consistent with the axis of maximum compression obtained from earthquake fault plane mechanisms and also agree with the directions of the general trends of geological structures which represent the orientations of the cleavage or lamination these results suggest that crack induced anisotropy is present in the brittle upper crust beneath japan as has been found elsewhere but we could not distinguish whether this reflects the distributions of cracks induced by the present stress field or results from the general trends of surface geology although the delay times between faster and slower shear waves are difficult to estimate reliably because of their high sensitivity to internal interfaces the delay times can be interpreted as the result of a distribution of parallel vertical cracks with a crack density of about 0 04 the consistency or lack of consistency of the directions of the shear wave polarizations at the four stations demonstrates the effects of surface topography and near surface layering on the shear wave polarizations

the effect of crustal scattering on observed high frequency earthquake seismograms summary a comparison of high frequency 1 100 hz local out to 40 km earthquake seismograms at stations of the southern black forest digital seismic network is made in an attempt to isolate the effects of near receiver structure and of crustal scattering on the waveforms of the direct body waves and on the coda waves angle of incidence and azimuth of approach analyses from lower crustal earthquakes show that near surface weathering layers with low body wave velocities and with laterally varying thicknesses exist under all of the stations even those at hard rock sites of the black forest the similarity of direct p wave pulse shapes at each site for events of different sizes the time domain form of the so called fmax phenomenon is attributed to the response from this weathering zone the amplitude of the p wave coda of an event is found to be independent of the earthquake radiation pattern and to depend only on the absolute size of the event and on epicentral distance to the receiver furthermore the high frequency p wave coda is found to be generated at least in part from random crustal scattering including that within 100 m of the earthquake focus direct s waveforms are shown to be more complicated than their p wave counterparts probably due to the same near receiver and crustal scattering phenomena however in the case of s waves scattered energy can arrive before as well as at and after the direct s arrival making the identification of the direct s difficult especially for nodal arrivals

life on land in the precambrian microfossils have been discovered in cavity fill and replacement silica that occurs between chert breccia clasts in 1200 million year old paleokarst at the top of the mescal limestone central arizona and in 800 million year old paleokarst at the top of the beck spring dolomite southeastern california microbial communities on precambrian g550 million years ago land may have been extensive enough to affect weathering erosion sedimentation and geochemical processes

changes in shear wave splitting at anza near the time of the north palm springs earthquake changes in shear wave splitting are observed at knw before and after the m 6 north palm springs earthquake of july 8 1986 knw is a station of the anza seismic network monitoring the anza seismic gap on the san jacinto fault southern california the gradual increase in the delays between the split shear waves over 3 years at knw reported by peacock et al 1988 ended in june 1986 the further 2 years of observations analyzed here show that the behavior of the delays changed abruptly near the time of the north palm springs earthquake 33 km north of knw peacock et al demonstrated that the increase in delays could be simulated by increasing the aspect ratio of stress aligned fluid filled inclusions and speculated that this increase might be the result of a buildup of stress before an impending earthquake the new data appear to confirm this speculation but the temporal variations require a more complex interpretation although they still suggest that the changes in shear wave splitting are caused by earthquake induced stress changes to the fluid filled inclusions throughout the rockmass central to interpretation of temporal changes in shear wave splitting is the well established existence throughout at leastmore the uppermost 10 to 20 km of the crust of small fluid filled cracks microcracks and pores the existence of such inclusions introduces a compliant quality to otherwise stiff crustal rock the authors term these distributions of inclusions extensive dilatancy anisotropy or eda and the individual inclusions eda cracks because although they may include a wide range of shapes many of the seismic properties can be simulated by distributions of thin parallel cracks less

newly discovered uranium mineralization at 2 0 ma in the menggongjie granite hosted uranium deposit south china abstract the southeastern part of the nanling metallogenic province south china contains numerous economically important granite hosted hydrothermal vein type uranium deposits the miao ershan mes uranium ore field is one of the most important uranium sources in china hosts the largest chanziping carbonaceous siliceous pelitic rock type uranium deposit and several representative granite hosted uranium deposits the geology and geochemistry of these deposits have been extensively studied however accurate and precise ages for the uranium mineralization are scarce because uranium minerals in these deposits are usually fine grained and may have formed in several stages thus hindering the understanding of the uranium metallogenesis of this province the menggongjie mgj uranium deposit is one of the largest granite hosted uranium deposits in the mes ore field uranium mineralization in this deposit occurs at the central part of the mes granitic complex accompanied with silicification fluorination k metasomatism and hematitization the ore minerals are dominated by uraninite occurring in quartz or fluorite veinlets along fractures in altered granite in situ sims u pb dating on the uraninite yields the u pb isotopic age of 1 9 0 7 ma which is comparable to the chemical u th pbtol uraninite age of 2 3 0 1 ma such ages agree well with the eruption ages of the extension related quaternary volcanics 2 1 1 2 ma in south china suggesting that the uranium mineralization have formed at an extensional setting possibly related to the quaternary volcanic activities therefore our robust new dating results of the mgj uranium deposit make it the youngest granite hosted uranium deposit reported so far in south china and the mineralization event represents a newly identified mineralization epoch