geology of panamint valley saline valley pull apart system california palinspastic evidence for low angle geometry of a neogene range bounding fault geological studies support the interpretation that northern panamint valley and saline valley southeast california form paired pull apart basins on opposite sides of the right slip hunter mountain fault zone eight to ten km of late cenozoic net slip can be established on the hunter mountain fault zone palinspastic reconstruction of northern panamint valley indicates that the valley was formed by movement on a shallow crustal low angle normal fault of 0 15 degree west dip during the last 3 0 ma this interpretation appears to contradict the notions that little extension is accomodated in the uppermost crust by low angle faulting and that the most recent extension in the basin and range province is accomodated exclusively by high angle faulting saline valley however is interpreted to have formed by extension on closely spaced rotated planar normal faults thus within one geometric system of paired pull apart basins extension appears to have been accommodated in the shallow crust in two different ways

topography and tectonics of the central new madrid seismic zone results of numerical experiments using a three dimensional boundary element program we present results of a series of numerical experiments designed to test hypothetical mechanisms that drive deformation in the new madrid seismic zone experiments are constrained by subtle topography and the distribution of seismicity in the region we use a new boundary element algorithm that permits calculation of the three dimensional deformation field surface displacement fields are calculated for the new madrid zone under both far field plate tectonic scale and locally derived driving strains results demonstrate that surface displacement fields cannot distinguish between either a far field simple or pure shear strain field or one that involves a deep shear zone beneath the upper crustal faults thus neither geomorphic nor geodetic studies alone are expected to reveal the ultimate driving mechanism behind the present day deformation we have also tested hypotheses about strain accommodation within the new madrid contractional step over by including linking faults two southwest dipping and one vertical recently inferred from microearthquake data only those models with step over faults are able to predict the observed topography surface displacement fields for long term relaxed deformation predict the distribution of uplift and subsidence in the contractional step over remarkably well generation of these displacement fields appear to require slip on both the two northeast trending vertical faults and the two dipping faults in the step over region with very minor displacements occurring during the interseismic period when the northeast trending vertical faults are locked these models suggest that the gently dipping central step over fault is a reverse fault and that the steeper fault extending to the southeast of the step over acts as a normal fault over the long term

vector constraints on western u s deformation from space geodesy neotectonics and plate motions the rate of slip vector on the san andreas fault in central california estimated from geodetic and holocene geological data 34 3 mm yr n41 w 2 is inconsistent with the prediction of rigid plate models such as rm2 56 3 mm yr n35 w 2 this well known san andreas discrepancy is diagnostic of plate deformation distributed both east of the fault in the basin and range and west of the fault along the california continental margin we construct constraints on the integrated deformation rates across these two regions consistent with 1 the kinematical boundary conditions imposed by the rigid plate model 2 neotectonic and paleoseismic estimates of deformation rates 3 ground based geodetic measurements and 4 rates of change observed by very long baseline interferometry along seven baselines to western u s sites the space geodetic data on basin and range extension taken over a 4 year interval are compatible with geological observations averaged over the holocene the best estimate of its integrated deformation rate provided by the joint inversion of both data types is 9 7 2 1 mm yr n56 w 10 too small and in the wrong direction to account entirely for the san andreas discrepancy we therefore attribute much of it to deformation west of the sierra nevada great valley block accommodated for example on faults such as the san gregorio hosgri system of coastal california the integral of this deformation estimated by subtracting the basin and range contribution from the discrepancy vector requires significant right lateral shear parallel to the san andreas 13 5 mm yr and some compression perpendicular to it 9 3 mm yr

seismicity and detection location threshold in the southern great basin seismic network a spatially varying model of the detection location capabilities of the southern great basin seismic network sgbsn has been derived that is based on simple empirical relations and statistics this permits use of almost all the catalog data gathered instead of ignoring data that are below the threshold of completeness a spatially varying threshold model is developed so that subregions having lower completeness levels than the network as a whole can be outlined and the completeness level of each sub region determined such a model is required to unambiguously identify regions that are aseismic due to natural processes rather than to limited detection and or location capabilities accounting for spatial variations in detection location threshold is also important for studies in which magnitude frequency distributions are interpreted in terms of source scaling properties the characteristics of the spatial distribution of earthquakes where earthquake clusters and aseismic regions locate appear to be stable at all magnitude levels so that inferences about where strain is being accommodated will be the same whether numbers of earthquakes or strain estimated from seismic moments are examined for the southern great basin region these principal characteristics include clusters at the northern end of the furnace creek fault and in the pahranagat shear zone and a relatively large number of earthquakes in the northern and southeastern portions of the nevada test site these clusters cover regions much larger than the surface projections of any of the mapped faults the extent to which seismicity is induced by nuclear testing is unclear the predominantly aseismic regions include the area west of the death valley furnace creek fault system and an almost complete absence of events at yucca mountain finally a considerable number of isolated events in the sgbsn catalog cannot be correlated with mapped faults

strike slip faulting and block rotation in the lake mead fault system strike slip faults in the basin and range province have often been considered passive boundaries between differentially extended domains of tilted normal faults and are thus considered secondary in accommodating regional horizontal deformation paleomagnotic investigation of late miocene age volcanic rocks displaced by the left lateral fault system of lake mead nevada shows 1 that these rocks have not been affected by significant structural tilt the difference between observed and expected inclinations being only 0 6 14 9 and 2 a significant horizontal counterclockwise rotation of 29 4 8 5 about a vertical axis this rotation was accommodated by slip on northwest trending right lateral strike slip faults this implies significant west northwest elongation results of the investigation indicate that strike slip faulting is the primary process accommodating crustal deformation along the lake mead fault system and that tilting in response to normal faulting is secondary

a 22 000 year record of vegetation change in the north central sonoran desert abstract plant macrofossils from eleven packrat neotoma sp middens provide a history of vegetation change over the last 22 140 years at eagle eye mountain near the northern border of the sonoran desert in arizona this site is located in the arizona upland subdivision departures and arrivals of various perennial plant species during the holocene were similar to other records from more southerly sites and support earlier generalizations of van devender van devender t r 1990 late quaternary vegetation and climate of the sonoran desert united states and mexico in betancourt j l van devender t r martin p s eds packrat middens the last 40 000 years of biotic change university of arizona press tucson pp 134 165 regarding the tempo and mode of late quaternary changes in the arizona upland evergreen woodland containing pinus monophylla singleleaf pinyon juniperus osteosperma utah juniper and quercus turbinella shrub live oak occupied the site during full glacial times the latter two species but not p monophylla persisted into the early holocene and were joined by a limited number of desertscrub species including acacia greggii catclaw carnegiea gigantea saguaro and several other succulents with the arrival of many other species during the middle holocene a desertscrub community that differed considerably from the modern vegetation developed acacia greggii and cercidium floridum blue paloverde were the principal large woody species on exposed hillslopes assemblages similar in composition to the modern desertscrub did not appear until approximately 4000 years ago when the more xerophytic cercidium microphyllum foothills paloverde arrived and c floridum disappeared from hillslopes the delayed arrival of c microphyllum in comparison to other sonoran desertscrub species with similar climatic tolerances is attributed to the limited dispersal distance of its seeds by seed eating heteromyid rodents at 4540 and 6425 yr b p the cold intolerant shrub encelia farinosa was present on northeasterly aspect exposures where it is absent today suggesting slightly warmer winter conditions during the middle holocene the presence of the tree cercidium floridum and the perennial c 4 grass setaria leucopila on exposed slopes during the middle holocene indicates substantially moister conditions during that time than in the late holocene or today at sites on the xeric limits of the modern distributions of both species average annual precipitation is from 50 to 100 greater than that presently received at eagle eye mountain in particular the responses of s leucopila and cacti during the middle holocene indicate substantial increases in warm season precipitation

paleoclimatic implications of holocene plant remains from the sierra bacha sonora mexico abstract a total of 93 plant taxa were identified from 11 packrat neotoma sp midden samples from the sierra bacha on the coast of the gulf of california near puerto libertad sonora mexico nine indurated samples have radiocarbon dates ranging from 9970 to 320 yr b p sonoran desertscrub was present on rocky slopes throughout the holocene early holocene assemblages dominated by fouquieria columnaris boojum tree reflect vegetation and climate more like modern baja california with greater winter rainfall and cooler summers middle holocene vegetation was essentially modern with modest indications of greater monsoonal rainfall even though cold water upwelling locally inhibits summer precipitation the results are similar to all previous midden reconstructions of early and middle holocene climates in the sonoran desert but contradict general atmospheric circulation model simulations

small scale stress heterogeneity in the anza seismic gap southern california focal mechanism inversions reveal significant lateral variations in stress orientations along the anza segment of the san jacinto fault zone the most notable stress anomaly is within the 20 km aseismic seismic gap portion of the fault zone where 1 the maximum compressive stress is nearly horizontal and is oriented at 74 13 relative to the fault strike this contrasts with orientations ranging from 62 11 to 49 7 along the more seismically active portions of the fault zone immediately to the northwest and southeast of the seismic gap regional stress results found by inverting all focal mechanisms simultaneously indicate that 1 is horizontal and trends north south while 3 is horizontal and trends east west approximately 15 km west of the seismic gap in the off fault cahuilla swarm area 1 and 3 solutions are rotated clockwise by about 25 relative to the regional model roughly 10 km southeast of the seismic gap near the buck ridge fault 1 and 3 are rotated counterclockwise by about 10 relative to the regional solution northwest of the seismic gap along the fault zone 3 plunges about 30 from the horizontal correlating with a local increase in reverse faulting between the hot springs and san jacinto faults southeast of the seismic gap 1 plunges about 45 from the horizontal correlating with a local increase in normal faulting in the trifurcation region of the buck ridge clark and coyote creek faults we propose a simple mechanical model in which a block rotation superimposed on the dominant right lateral strikeslip motion of the fault zone satisfies the first order observations of stress orientation faulting and horizontal surface strain under this model the anza seismic gap is the region of zero convergence between the northeast and southwest sides of the fault and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap

wet conditions during the last glaciation in the chihuahuan desert alta babicora basin mexico abstract a 467 cm core b94 3 from the alta babicora basin in the chihuahuan desert northern mexico 29 n 108 w documents lake level and climate changes over the past ca 65 000 yr chronological control is provided by four accelerator mass spectrometry 14 c dates and five u series dates on diatom silica the core has been analyzed for magnetic susceptibility loss on ignition carbonate content sediment chemistry and mineralogy and pollen and diatom composition the basin was occupied by a deep freshwater lake throughout the late pleistocene which based on shoreline evidence was at least 19 m deeper than today the lake shallowed after ca 57 000 yr b p high variability typified the period between 54 600 and 38 000 yr b p probably with periodic desiccation and deflation a deep water lake was reestablished after 38 000 yr b p and persisted until ca 29 000 yr b p shallowing occurred through the last glacial maximum although the lake was still deeper than at present the pleistocene holocene transition was marked by a distinctive change in the diatom flora there is no diatom record for much of the holocene but other proxies indicate generally dry stable catchment conditions the babicora record is more akin to those from the southwest united states than it is to central america and northern south america wet conditions in the late pleistocene are attributed to winter rainfall from midlatitudes in contrast to the modern tropical summer rainfall regime

pacific north america plate motions new results from very long baseline interferometry expansion and densification of the very long baseline interferometry vlbi network by the nasa crustal dynamics project continues to clarify details of pacific north america motions in this paper i construct for the first time rigid plate tectonic models based purely on space geodetic data construction of vlbi plate models has the advantages of averaging the bias and errors of individual site velocities making failures in the rigid plate assumption easy to document and allowing the consistency of specified plate models to be quantified against the vlbi data set as a whole vlbi based rigid plate models vm2 and vm3 obtained by fitting vector velocities of 10 sites in the stable interiors of the north america and pacific plates reveal a misfit of vlbi observations and rm2 minster and jordan 1978 predictions significant at the 99 confidence level nuvel l demets et al 1990 reproduces the velocities of these 10 sites far better than rm2 and offers a fit to the data which is statistically indistinguishable from vm2 and vm3 by including velocities of 16 additional stations in the deforming plate margin vm2 and vms can resolve all four terms vpn total pacific north america motion vsa san andreas slip ve w path integrated deformation east and west of the san andreas in the vector equation characterizing pacific north america plate boundary deformation vpn vsa ve vw for several points along the san andreas without reference to a global plate model geological or local geodetic measurements or restrictions on distributed shear in most cases instantaneous vlbi determined rates agree with geologically based estimates mapped to point c 36 0 n 120 6 w in central california vm2 gives vpn c 46 8 1 4 mm yr n36 2 w vsa c 38 6 1 3 mm yr n36 2 w and ve c 8 2 1 3 mm yr n34 7 w vlbi substantially eliminates vw c the component of the san andreas discrepancy west of c although a few millimeters per year of right lateral strike slip on offshore faults can not be rejected at point d 34 1 n 117 0 w in southern california vm2 returns vsa d 25 0 1 1 mm yr n52 3 w ve d 8 8 1 1 mm yr n25 8 w and vw d 13 3 1 6 mm yr n21 5 w vw d is accommodated west of the san andreas to the channel islands the azimuth of san andreas slip suggests that the transverse ranges north of los angeles are absorbing 8 1 mm yr of compression perpendicular to the trend of the fault

fault kinematics in the 1989 loma prieta rupture area during 20 years before that event the loma prieta lp compressional bend of the san andreas fault zone saf appears to be multi stranded and vertically segmented thus faults in this segment other than the one that ruptured in 1989 may 1 be the source of prior large events associated with this portion of the saf and 2 be highly stressed after the 1989 lp event the fault that ruptured in 1989 was remarkably aseismic during the preceding 20 years seismicity occurred on some of the neighboring faults of the system but others remained aseismic the lake elsman burst of seismicity within 1 6 years of the main shock is on a set of secondary faults within the foot wall of the rupture the shallow 5 km portion of the saf morphologically and structurally characterized as steep and predominantly strike slip and the imbricate thrust zone to the northeast were also quiescent these upper crustal faults may partition the oblique motion accomplished by lp like ruptures in the lower crust into fault parallel transcurrence and fault normal motion convergence respectively while the overall level of seismicity in the region around lp increased over the 20 years before 1989 the level of seismicity number of events on the creeping portion of the saf next to the lp segment decreased moreover the level of background seismicity on this and other fault segments varied dramatically both up and down in conjunction with major regional earthquakes on other segments thus improved models for precursory patterns of seismicity should account for mechanistically more realistic changes than simple seismicity increase or decrease everywhere around an impending rupture

seismic slip segmentation and the loma prieta earthquake we have plotted the cumulative seismic slip projected onto a vertical plane for earthquakes occurring during the last 20 years along 210 km of the san andreas fault that includes the section that moved in the loma prieta earthquake these plots illustrate the differences in depth and character of the seismicity between the locked and creeping portions of the fault or fault zone and define the segment upon which the loma prieta earthquake occurred working by analogy from the relation between pre main shock microseismicity and presumed main shock slip regions at parkfield and loma prieta we identify a segment on the san francisco peninsula where we believe the m 7 1838 earthquake occurred and which we believe may have accumulated sufficient strain that rupture should be expected in the coming decades

paleoclimatic changes during the late pleistocene holocene in laguna babicora near the chihuahuan desert mexico the paleoclimatic variability of northern mexico since the late wisconsinan is determined from sedimentological geochemical and stratigraphic studies of the lacustrine sequence of laguna babicora 29 4 n 107 7 w elevation 2100 m asl chronological control is based on 13 radiocarbon dates 6 from this study and 7 previously reported from four stratigraphic profiles that cover a range from 16 342 to 2800 bp two major periods of increased humidity are recognized by lake level variation during the late wisconsinan and early holocene the late wisconsinan humid period may be related to jet stream migration to southerly latitudes and the influence of the mobile polar high this period has been identified in the sedimentary record of el diablo profile with a radiocarbon date of 16 342 201 bp the early holocene humid period is recorded in el diablo 10 976 115 bp and cano profiles 9614 130 bp it is interpreted as a result of the younger dryas climatic anomaly characterized by a temperature decrease and cold paleoclimatic conditions similar to those for the late wisconsinan the early holocene was followed by an overall gradual drying that reached a maximum around 6000 bp during the middle holocene for the late holocene evidence of widespread erosion sedimentation changes flooded surfaces and paleosoils indicates that laguna babicora was subjected to short term climatic changes

a middle holocene vegetation record from the mojave desert of north america and its paleoclimatic significance abstract packrat middens from the mccullough range in the southeastern mojave desert of nevada record vegetation conditions during the periods 6800 to 5060 yr b p and 1250 yr b p to present their age and location are ideal to test for middle holocene aridity the period between 6800 and 5060 yr b p does appear to have been more arid than during the late holocene period of record this agrees with other evidence for middle holocene aridity from the southwest arguments for strengthened middle holocene monsoons in the region are based chiefly on packrat midden samples older than 7500 yr b p or on samples that are younger than 5500 yr b p simulations of 6000 yr b p climate that fail to show strong subtropical flow into the southwest are therefore supported by evidence for enhanced aridity in that region

the origin of large local uplift in extensional regions large localized uplift is commonly observed in continental regions undergoing extension these observations can be modelled by planar high angle normal faulting of an elastic upper crust overlying an inviscid lower crust isostasy provides the necessary driving force the model quantifies the role of flexural rigidity density variations in the crust and erosion and deposition of sediment

zircon u pb chronology of the nyingtri group southern lhasa terrane tibetan plateau implications for grenvillian and pan african provenance and mesozoic cenozoic metamorphism abstract the nyingtri group of the lhasa terrane in southern tibet consists dominantly of metasedimentary rocks and orthogneiss these rocks have a similar mineral paragenesis of plagioclase k feldspar biotite quartz sillimanite garnet staurolite muscovite amphibole indicating amphibolite facies metamorphic conditions inherited detrital zircons from the metasedimentary rocks show magmatic features and yield widely variable 206pb 238u ages ranging from 3300 to 50 ma the data define two prominent age populations 1200 1000 and 600 500 ma indicating that the source of the nyingtri group preserves the records of both grenville and pan african magmatic thermal events inherited magmatic zircon cores from the orthogneiss yield a crystallization age of 496 ma limiting the depositional age of the metasedimentary sequence to cambrian or older overgrowth rims on the detrital zircons from one metasedimentary rock yield a metamorphic age of 32 ma on the basis of these results together with the

geochemistry and sm nd and rb sr isotopic composition of eclogite in the lhasa terrane tibet and its geological significance abstract a new occurrence of eclogite has been discovered in the lhasa terrane northeast of lhasa tibet along the northern margin of the gangdese magmatic arc the eclogite occurs as a 500 1000 m wide belt that extends over 60 km in an e w direction and consists of coarse grained massive rutile eclogite quartz eclogite and phengite eclogite whole rock geochemistry and relatively high end t for the eclogites suggest that their protoliths were low k oceanic tholeiites similar to typical n morb derived from a depleted mantle source however all analyzed samples have negative nb anomalies in morb normalized trace element patterns suggesting a suprasubduction zone influence zircon shrimp u pb ages of the eclogite ranging from 242 15 ma to 291 13 ma are interpreted as the age of peak metamorphism a sm nd isochron age of 306 50 ma 2 likely reflects the protolith age these data suggest that the paleo tethyan ocean existed along the northern margin of the gangdese island arc until the early permian

evidence for an active shear zone in southern nevada linking the wasatch fault to the eastern california shear zone previous studies have shown that 5 of the pacifi c north america relative plate motion is accommodated in the eastern part of the great basin western united states near the wasatch fault zone and other nearby faults deformation is currently concentrated within a narrow zone of extension coincident with the eastern margin of the northern basin and range farther south the pattern of active deformation implied by faulting and seismicity is more enigmatic to assess how present day strain is accommodated farther south and how this relates to the regional kinematics we analyze data from continuous global positioning system gps stations and model the strain rate tensor fi eld using the horizontal gps velocities and earthquake focal mechanisms the results indicate an 100 km wide zone of 3 3 mm yr extension at 40 5 n that broadens southward from the wasatch fault zone to a width of 400 km at 36 n this broadening involves at least one zone of localized extension in northwestern arizona that encroaches into the southwestern plateau and an eastnortheast trending sinistral shear zone the pahranagat shear zone through southern nevada this shear zone may accommodate as much as 1 8 mm yr and is a key feature that enables westward transfer of extension thereby providing a kinematic connection between the wasatch fault zone and the eastern california shear zone

contemporary block tectonics california and nevada well determined fault plane solution and the gross pattern of late cenozoic faulting in california and nevada show a systematic relation between the orientation of fault planes and slip directions in general normal faults have northerly strikes reverse faults have easterly strikes and dextral and sinstral strike slip faults have northwesterly and northeasterly strikes respectively kinematically this relation is consistent with the response of clusters of fault bounded crustal blocks to a regional stress field generated by the relative motion between the pacific and north american plates in this stress field the greatest and least principal compressive stresses are restricted to northerly and easterly striking vertical planes respectively clusters of crustal blocks bound by faults having the above attitudes form three basic regimes 1 a spreading regime with a combination of normal and conjugate strike slip faults 2 a converging regime with a combination of reverse and conjugate strike slip faults and 3 a wrench regime with sets of subparallel throughgoing northwest striking dextral faults or northeast striking sinstral faults these three regimes are typified by faulting patterns in the basin and range province spreading the transverse ranges converging and the san andreas garlock fault systems wrench respectively the gross deformation of each regime resulting from relative displacements between individual blocks is characterized by north south shortening and east west extension with the ratio of extensional to shortening strains and the areal dilitation decreasing systematically from spreading to wrench to compressional regimes the wrench regime involves a component of net rotational deformation clockwise for dextral slip and counter clockwise for sinstral slip while deformation of the spreading and converging regimes is irrotational local deviations from regional kinematic directions are concentrated along the boundaries between regimes reflecting the mismatch in gross deformation fields between regimes maximum principal and shear stress magnitudes will increase systematically from spreading to wrench to converging regimes provided that fault slip is controlled by frictional strength byerlee s law along preexisting fractures and that pore pressure in the brittle crust is laterally uniform a minimum strength difference between active block bounding faults and block interiors is 15 30 simple arrangements of such block clusters mimic the gross kinematic pattern of quaternary faulting in california and nevada some implications for contemporary tectonics emphasized by this model involve the westward displacement of the sierra nevada block with respect to the stable interior of the north american plates oblique thrusting of the salinian block over the pacific plate and a progressive increase in the offset of the san andreas fault represented by the big bend through the transverse ranges

paleomagnetism and crustal rotations along a shear zone las vegas range southern nevada in southern nevada paleozoic isopachs mesozoic structures and mountain ranges trend north south but as they approach the right lateral west northwest trending las vegas valley shear zone lvvsz they are bent clockwise up to 90 in oroflexes 30 km long in the las vegas range the declination of the characteristic magnetization of the cambrian bonanza king formation has likewise been rotated clockwise more than 50 for samples from eight sites 13 23 samples per site in the las vegas range and in its extension south of the lvvsz stepwise thermal demagnetization to 600 c removed recent and tertiary overprints revealing a nearly horizontal late paleozoic viscous partial thermoremanent magnetization vptrm which predates both the tertiary oroflexural bending and the sevier thrusting in the area the declination of the characteristic magnetization after correcting for the dip of bedding varies from 133 2 at a site 24 km north of the shear zone to 196 4 13 km north of it and 204 4 11 km south of the shear zone this parallels a similar change in the structural trend the paleomagnetic data indicate that the las vegas range is broken into rather small blocks 5 km which rotate in response to more continuous deformation at depth this deformation of the upper crust can be modelled using a thin non newtonian viscous plate model which when applied to the las vegas valley shear zone predicts rotations similar to those seen in the las vegas range

seismicity and shear strain in the southern great basin of nevada and california this study examines the relationship between the distribution of small earthquakes ml 4 3 and mechanisms of strain accumulation and relaxation in an area with long repeat times between large events the southern great basin seismic network sgbsn region the great basin is a unique continental extensional province characterized by normal and strike slip faulting high heat flow crust of thin to normal thickness and high elevations the sgbsn is operated to provide data to address suitability issues pertaining to yucca mountain nevada which is being evaluated as a potential site for a national mined geologic nuclear waste repository suitability issues include estimation of the probability of occurrence of future damaging earthquakes the characterization of the mechanisms that drive hydrologic flow and the identification of fractures faults that might act as flow conduits or barriers this study attempts to explain the distribution of small earthquakes in terms of spatial variations in the shear strain field where strain concentrates there should be a greater number of small earthquakes strain field models are constructed under the assumption that long term fault behavior perturbs an otherwise uniform strain field these strain field models are then interpreted with regard to the regional tectonics and site suitability issues modeling results provide one possible explanation of why earthquake clusters cover regions much larger than the surface projections of any of mapped major faults clusters in a wide band along and extending northeast of the northern half of the furnace creek fault may correspond to elevated shear strains along the fault and a broad cluster in the pahranagat shear zone may be associated with shear strain arising from a distribution of smaller localized faults the relatively large number of small earthquakes in the southern and eastern portions of the nevada test site is consistent with the strain field models a minimum in shear strain at yucca mountain is predicted by all models consistent with an almost total lack of earthquakes observed there the region to the west of the death valley furnace creek fault system the portion of the study area with the most active deformation but few small earthquakes is an area of low shear strain a possible reason for this is that the fault configuration in the area is optimal for accommodating regional deformation via large earthquakes or creep while there is also a relative lack of earthquakes at yucca mountain this may be indicative of a lack of accumulating strain energy and thus a lower potential for a large earthquake

inferences about the local stress field from focal mechanisms applications to earthquakes in the southern great basin of nevada focal mechanisms determined from regional network earthquake data or aftershock field investigation often contain members ranging from strike slip to normal slip in extensional tectonic environments or from strike slip to thrust slip in compressional environments although the coexistence of normal and strike slip faulting has suggested to some investigators that the maximum and intermediate principal stresses are of approximately equal magnitude several have asserted that the directions of principle stresses can or must interchange to accommodate both types of mechanisms zoback and zoback 1980b vetter and ryall 1983 a coulomb navier criterion of slip is invoked to demonstrate that both types of mechanisms as well as oblique members having preferred nodal plane dips intermediate between those of the strike slip and normal mechanisms may be observed in a region where the stress field resolved into principal components is axially symmetric the proximate coexistence of earthquakes having diverse focal mechanisms could be interpreted as evidence for an approximately axially symmetric stress field in a region where optimally oriented planes of weakness are known to exist in the host rock 10 refs 6 figs

state of stress and modern deformation of the northern basin and range province constraints on the current stress regime of the actively extending northern basin and range province are provided by deformation data focal mechanisms and fault slip studies hydraulic fracturing in situ stress measurements borehole elongation breakouts analyses and alignment of young volcanic vents the integrated data indicate significant variations both in principal stress orientations and magnitudes an approximately e w least principal stress direction appears to characterize both the eastern and western margins of the basin and range province whereas in the active interior parts of the province extension occurs in response to a least principal stress oriented nw to n60 w the contrast in stress orientations between the province boundaries and in the interior suggests that along the margins the least principal stress direction may be locally controlled by the generally northerly trending profound lithospheric discontinuities associated with these margins active deformation along the southeastern and western province margins is characterized by a combination of strike slip and normal faulting focal mechanisms along northeastern province margin wasatch front and in central nevada indicate a combination of normal and oblique normal faulting temporal regional and depth dependent variations in the relative magnitudes of the vertical and maximum horizontal stresses can explain much of the observed variations in deformation styles however some depth variation in faulting style inferred from focal mechanisms may be apparent and simply a function of the attitude of fault planes being reactivated evidence for significant temporal variation or multiple cycles of variation in relative stress magnitude comes from the sierran front basin and range boundary region where recent earthquakes are predominantly strike slip whereas the profound relative vertical relief across the sierra frontal fault zone in the last 9 10 m y implies a normal faulting stress regime using the best data on stress orientation relative stress magnitudes are constrained from slip vectors of major earthquakes and young fault displacements analysis of well constrained slip vectors in the owens valley california area indicate that large temporal variations in the magnitude of the approximately n s oriented maximum horizontal stress are required to explain dominantly dip slip and strike slip offsets on subparallel faults similar faulting relations are observed throughout much of the boundary zone between the basin and range sierra nevada including the walker lane belt along the eastern province margin in the wasatch front area in utah available data suggest that the maximum and minimum horizontal stresses may be approximately equal at depths of 4 5 km earthquake focal mechanisms in this area suggest more variability in relative magnitude of the two horizontal stresses with depth furthermore superimposed sets of young fault striae along a segment of the wasatch fault also indicate temporal variations of relative stress magnitudes sources of regional and temporal variations in the stress field may be linked to variable shear tractions applied to the base of the brittle crust related to intrusion thermally induced flow and the influence of the san andreas plate boundary although difficult to date accurately the fault slip data suggest that the temporal variations in relative magnitudes stress may occur on the time scale of both a single major earthquake cycle 1000 5000 years and multiple earthquake cycles 10 000 years

fault interaction near hollister california a numerical model is used to study fault stress slip near hollister california the geometrically complex system of interacting faults including the san andreas calaveras sargent and busch faults is approximated with a two dimensional distribution of short planar fault segments in an elastic medium the steady stress and slip rate are simulated by specifying frictional strength and stepping the remote stress ahead in time the resulting computed fault stress is roughly proportional to the observed spatial density of small earthquakes suggesting that the distinction between segments characterized by earthquakes and those with aseismic creep results in part from geometry a nonsteady simulation is made by introducing in addition stress drops for individual moderate earthquakes a close fit of observed creep with calculated slip on the calaveras and san andreas faults suggests that many changes in creep rate averaged over several months are caused by local moderate earthquakes in particular a 3 year creep lag preceding the august 6 1979 coyote lake earthquake on the calaveras fault seems to have been a direct result of the november 28 1974 thanksgiving day earthquake on the busch fault computed lags in slip rate preceding some other moderate earthquakes in the area are also duemore to earlier earthquakes although the response of the upper 1 km of the fault zone may cause some individual creep events and introduce delays in others the long term rate appears to reflect deep slip less

high resolution paleoceanography of the guaymas basin gulf of california during the past 15 000 years abstract deep sea drilling project site 480 27 54 10 n 111 39 34 w 655 m water depth contains a high resolution record of paleoceanographic change of the past 15 000 years for the guaymas basin a region of very high diatom productivity within the central gulf of california analyses of diatoms and silicoflagellates were completed on samples spaced every 40 50 yr whereas icp aes geochemical analyses were completed on alternate samples sample spacing 80 100 yr the bolling allerod interval 14 6 12 9 ka note ka refers to 1000 calendar years bp throughout this report is characterized by an increase in biogenic silica and a decline in calcium carbonate relative to surrounding intervals suggesting conditions somewhat similar to those of today the younger dryas event 12 9 11 6 ka is marked by a major drop in biogenic silica and an increase in calcium carbonate increasing relative percentage contributions of azpeitia nodulifera and dictyocha perlaevis a tropical diatom and silicoflagellate respectively and reduced numbers of the silicoflagellate octactis pulchra are supportive of reduced upwelling of nutrient rich waters between 10 6 and 10 0 ka calcium carbonate and a nodulifera abruptly decline at dsdp 480 while roperia tesselata a diatom indicative of winter upwelling in the modern day gulf increases sharply in numbers a nearly coincident increase in the silicoflagellate dictyocha stapedia suggests that waters above dsdp 480 were more similar to the cooler and slightly more saline waters of the northern gulf during much of the early and middle parts of the holocene 10 to 3 2 ka at about 6 2 ka a stepwise increase in biogenic silica and the reappearance of the tropical diatom a nodulifera marks a major change in oceanographic conditions in the gulf a winter shift to more northwesterly winds may have occurred at this time along with the onset of periodic northward excursions el nino driven of the north equatorial countercurrent during the summer beginning between 2 8 and 2 4 ka the amplitude of biogenic silica and wt fe al and ti proxies of terrigenous input increase possibly reflecting intensification of enso cycles and the establishment of modern oceanographic conditions in the gulf increased numbers of o pulchra after 2 8 ka suggest enhanced spring upwelling

paleomagnetic constraints on the geometry and timing of deformation at yucca mountain nevada paleomagnetic data from miocene ash flow sheets indicate that southern yucca mountain has undergone about 30 of clockwise vertical axis rotation since emplacement of the tiva canyon member of the paintbrush tuff at about 13 ma declinations of remanent magnetization from 32 sites in the reversely magnetized tiva canyon member display a systematic southward increase over the 25 km north south extent of yucca mountain probable errors in estimating structural attitudes are a major source of uncertainty in determining the amount of rotation between any two sites analysis of tilt corrected directions of remanent magnetization from the tiva canyon sites indicates that structural attitudes determined from outcrop patterns on geologic maps are generally more accurate than field measurements of attitudes of compaction foliation or contacts attitudes determined from map patterns apparently minimize effects of 1 initial dips 2 poorly defined compaction foliation and 3 tilting that occurred after compaction but before blocking of remanence however such attitudes cannot accurately represent tilting related to unmapped local structures rotations implied by data from three older ash flow sheets the topopah spring member of the paintbrush tuff seven sites and the prow pass four sites and bullfrog three sites members of the crater flat tuff are generally consistent with the amount and sense of rotation documented by data from the tiva canyon member uncertainties in the declinations of tilt corrected site mean directions from these older units preclude evaluating whether older units have undergone more rotation than the tiva canyon member

the mechanics of the south iceland seismic zone the south iceland seismic zone appears to act as a transform fault between offset sections of the mid atlantic ridge however there is no morphological expression of a throughgoing transform between the ridges the anticipated throughgoing east west transform is represented by a series of north south strike slip faults we study the mechanics of the south iceland seismic zone using boundary element methods first we examine the implications of the differences between the geometry of an orthogonal ridge transform ridge system and the geometry of the structures observed in iceland then we consider the role of the north south faults in accommodating transform motion in an east west direction we conclude that the north south faults can accommodate deformation if they are longer or more spatially frequent than mapped the south iceland seismic zone is subject to sequences of large earthquakes every 45 112 years in comparing the seismic moment release derived from earthquake magnitudes with that predicted by our models we confirm that the system of north south faults can act as a transform fault the seismic moment release derived from the earthquake magnitudes is 1 4 times greater than that predicted by our models if our application of the moment magnitude relation is correct the above could imply that the brittle layer extends to 14 km rather than the 10 km assumed in our models other possibilities are that the magnitudes of the earthquakes have been slightly overestimated or that the historical seismic record documents a period of seismicity greater than that expected from rates of plate motion inferred from geological and very long baseline interferometry vlbi

evidence of holocene climatic changes from aeolian deposits in baja california sur mexico abstract well defined late pleistocene and holocene dune fields are present on the surface of the purisima iray magdalena sedimentary basin and on top of barrier islands of the magdalena lagoonal complex in baja california sur mexico thermoluminescence amino acid epimerization and radiocarbon techniques were used for dating dune sands and shell samples respectively during late pleistocene to early holocene 14 to 8 ka bp mega barchans and linear dunes were formed during early holocene through middle holocene warm dry climate dune fields were still active at about 5 to 6 ka bp the stabilization of sea level developed the present lagoonal system in the area sandy barrier islands capped by foredunes were developed at this time coastal dunes were eroded and in part inundated forming large flooding flats and sabkha deposits in the old inter dune areas during late holocene after the lagoonal system was fully developed dunes were emplaced along the lagoonal coasts the presence of some younger dates in older deposits seems to represent dune reactivation

atlantic ocean modulated hydroclimate of the subtropical northeastern mexico since the last glacial maximum and comparison with the southern us abstract proxy records generated from chemical composition and radiocarbon dating of a sediment sequence collected from the el potosi basin provide information about variable hydroclimate of the subtropical northeastern mexico since the last glacial maximum lgm minimal amounts of runoff and more carbonate deposition during 19 ka represent the lgm the deglaciation was wetter compared to the lgm and enhanced runoff during 19 12 ka transported more ti bearing clastics from a relatively humid watershed over the deglaciation the basin received more runoff during the bolling allerod compared to the heinrich stadial 1 and younger dryas reduction in the amount of runoff during 10 1 ka and deposition of unweathered si bearing clastics during 4 1 ka suggest that the holocene was drier compared to the deglaciation hydroclimates of the northeastern mexico and southern us were synchronous during the lgm as well as deglaciation change from the drier lgm to wetter bolling allerod occurred as the gulf of mexico became warmer and the region received more summer precipitation however the dissimilarities observed in regional hydroclimates of the holocene need further evaluation

sedimentary record of late holocene relative sea level change and tectonic deformation from the guerrero seismic gap mexican pacific coast studies of the coastal sedimentary record have allowed both the reconstruction of relative sea level changes and the determination of local rates and magnitudes of tectonic deformation particularly in tectonically active areas despite their successful use elsewhere studies of this type are much less common for the mexican pacific coast which parallels the cocos north america subduction plate boundary stratigraphic geochemical and microfossil data from sediments in laguna mitla the pacific coast of guerrero mexico document late holocene sea level changes induced by tectonic activity in the mexican subduction zone three major events are identified first the formation of the lagoon by c 4630 yr bp as indicated by a freshwater to brackish peat second a relative sea level rise or land subsidence as indicated by a shift from a freshwater marginal lagoon environment to a marine setting preceded by a marine inundation represented by a sand unit possibly a tsunami deposit by c 3400 yr bp and finally a return to lagoonal conditions indicating a drop in relative sea level or coastal uplift by c 2300 yr bp the laguna mitla stratigraphy indicates general coastal subsidence or relative sea level rise of c 1 mm yr we argue that these relative sea level land level changes have been induced by tectonic activity associated with the mexican megathrust a plausible explanation for the 3400 yr bp marine inundation is probably a tsunami produced by a large seismic event accompanied by coastal subsidence discrete fining upward fine to coarse sand units with an erosional basal contact medium to poor sorting and clay mud rip up clasts an increase in na and sr elemental concentrations indicative of a marine origin and the landward extent of the sands support a tsunamigenic source for these deposits however these apparent tsunami deposits require further study to determine their lateral extent and to assess whether they can be correlated from one site to another this study demonstrates the applicability of a multiproxy sedimentary approach in interpreting relative sea level land level changes and to derive data on related earthquake and tsunami events in tropical coastal lagoons

right lateral displacements and the holocene slip rate associated with prehistoric earthquakes along the southern panamint valley fault zone implications for southern basin and range tectonics and coastal california deformation the n20 w trending panamint valley fault zone is linked to the n60 w trending hunter mountain strike slip fault and the saline valley fault system which represents one of the three major fault systems accommodating active crustal extension in the southern great basin a 25 km long zone of fault scarps along the southern panamint valley fault zone is recognized as the surface rupture zone associated with the most recent prehistoric earthquake the displacement associated with the most recent event determined through six detailed topographic maps of offset features is 3 2 0 5 m and a number of larger offsets in range of 6 7 m and 12 m are also observed if the larger displacements represent respectively two and three events each of 3 m then the fault zone appears to be associated with a characteristic earthquake which we estimate from the length of the rupture zone and the displacement to be between ms 6 5 and 7 2 the holocene slip rate is 2 36 0 79 mm yr is determined from the displacement of two alluvial features whose maximum age is estimated from pluvial shorelines assuming a characteristic earthquake model the recurrence interval is between 860 and 2360 years the holocene slip rate appears to be similar to the 4 million year slip rate of 2 2 7 mm yr determined from the hunter mountain fault which we suggest reflects the relatively constant tectonics in this region over the last 4 million years we further speculate that this supports the san andreas discrepancy in that the holocene slip rate of the san andreas fault probably represents its very long term several ma slip rate the total slip vector of the southern panamint valley fault system is oriented toward n35 w making this a predominantly strike slip fault in conjunction with the n60 w orientation of the hunter mountain strike slip fault we suggest that the displacement vector for the southern great basin is toward the nw consistent with results from vlbi data rather than wnw as determined by combining vlbi and geological data this in turn suggests that the coastal california deformation component involves respectively less shortening and more strike slip displacement perpendicular and parallel to the san andreas fault than is currently proposed

a holocene record from a former coastal lagoon in bahia kino gulf of california mexico a 10 m long core was recovered from the coastal flood plain on the eastern margin of the gulf of california bahia kino sonora sediments were analysed for loss on ignition magnetic susceptibility pollen diatoms ostracodes and foraminifera three levels were dated by 1 4 c the combined data suggest that the site was flooded during the middle holocene as sea level increased at about 6600 yr bp the area was a freshwater to brackish marsh dominated by typha domingensis and cyperaceae freshwater to brackish conditions suggest the presence of a local source of freshwater during this time and therefore a climate wetter than present continued rise in sea level was associated with an increase in depth and salinity by c 6350 6250 yr bp the coring site was an inner lagoon by c 6125 yr bp a central lagoon and by c 5900 yr bp the site was close to the mouth of the lagoon lower lagoon lower lagoon conditions persisted until the top of the record showing sea level stabilization which was followed by the formation of the dune ridge that presently separates the coring site from the sea

tectonic deformation in the new madrid seismic zone inferences from map view and cross sectional boundary element models the lack of instrumental recordings and obvious fault scarps associated with the 1811 1812 new madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting associated with these events morphologic and geologic features and the distribution of modern seismicity are used to infer the slip distribution and type of faulting strike or dip slip number strike length and width of the major faults in the new madrid seismic zone nmsz this is accomplished through two dimensional boundary element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains tectonic deformation is reflected in 1 the seismicity 2 the lake county uplift 3 reelfoot lake 4 the fractured rocks of the blytheville arch and 5 the st francis sunk lands many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left stepping right lateral strike slip faults that are coincident with the northeast trending zones of seismicity and the blytheville arch the morphology appears to be at least in part a consequence of major earthquakes that rupture these faults the locations of the 1811 1812 and largest post 1812 earthquakes and the models are consistent with a process in which the 1811 1812 earthquakes relieved accumulated regional shear strain causing the greatest post 1812 shear strains to exist at the ends of the fault zone modeling results also suggest that the numerous small earthquakes in the nmsz are not aftershocks of the 1811 1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field the bootheel lineament does not appear to be significant in shaping the morphology geologic structure and pattern of seismicity in the nmsz the absence of fault scarps and coherence of shallow reflectors overlying the blytheville arch suggests that the major nmsz faults may not have ruptured to the surface during the 1811 1812 earthquakes the inferred lengths of the 1811 1812 earthquake ruptures also suggest that their sizes may have been overestimated model predicted subsidence within the st francis sunk lands indicates that tectonic deformation also may have influenced alluvial processes in the nmsz

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

neogene paleostress changes in the basin and range a case study at hoover dam nevada arizona at hoover dam 40 km southeast of las vegas nevada well exposed highly faulted miocene rocks provide an excellent opportunity to study the paleostress history of a very small area within a region where previous geologic studies indicate clockwise rotation of paleostress and a nearness to a major strike slip fault zone boundary within 0 5 km3 of rock the sense of slip was determined on almost 500 separate faults the fault slip data show internal consistency with respect to lithology size of faults and location within the small area with respect to fault slip however the data represent an in homogeneous mixture of primarily strike slip and dip slip motions from this mixture it is possible to resolve two distinct stress fields with directions of extension that differ by 60 each stress field corresponds to a mixture of strike slip and dip slip faults and thus the orientations of 1 and 2 are not tightly constrained if however the data are first separated into strike slip and dip slip faulting modes and subsequently searched for sub populations that correspond to contrasting paleostress orientations the computations yield tightly constrained tensors that illustrate two distinct stress fields with subhorizontal 3 axes that trend n50 e and n75 w the 1 and 2 axes permutate in two vertical planes that strike n40 w and n15 e because 1 and 2 are close in value relative to 3 these relationships suggest that strike slip and dip slip faulting belong to the same tectonic regime qualitative observations of polyphase slip fault fault offsets and fault bedding geometric relationships when evaluated in the context of changes in 3 orientation and permutations of 1 and 2 provide a basis for a two stage late cenozoic structural evolution at hoover dam these stages are 1 strike slip faulting partly pre tilt and dip slip faulting and associated stratal tilting and 2 mostly post tilt complexly interrelated strike slip oblique slip and dip slip faulting the qualitative evaluations indicate that strike slip and dip slip faulting alternated in time during the second stage of deformation and may have done so during the first these mixed mode movements probably represent stress oscillations in time and space rather than discrete stress reorganizations in contrast the two different orientations of 3 either represent a major clockwise rotation of the stress field or a major counterclockwise rotation of the rocks during the faulting history each alternative is consistent with regional geologic relationships and the choice of which is correct cannot be made within the small area that was studied

on eustatic sea level history last glacial maximum to holocene abstract this paper addresses the question of the magnitude and time dependence of the globally averaged eustatic rise of sea level that occurred subsequent to the time of last glacial maximum lgm at approximately 21 000 calendar years before present through the analysis of relative sea level rsl histories predicted by a realistic mass conserving and gravitationally self consistent theory of postglacial sea level change it is demonstrated that there are preferred oceanic locations at which this eustatic function is well approximated by local sea level history one such location is the island of barbados in the caribbean sea a site from which a coral based record exists that extends from mid holocene to lgm because of the sea level ambiguity that is inherent to coral based records however it is important that the global ice equivalent eustatic sea level curve inferred on the basis of the barbados data be tested against observations at other locations from which similarly extensive records are also available but which are derived on the basis of sea level indicators which are not subject to the ambiguities inherent to corals it is shown that when the eustatic function employed in the global theoretical model is tuned so as to enable the model to fit the barbados observations where the maximum relative sea level rsl depression is assumed to be near 120 m then the theory misfits the record from the sunda shelf in the indonesian archipelago as well as the record from j bonaparte gulf in northern australia both of these recently published records appear to constrain the lgm low stand of rsl to a value above 120 m the implications of these results for interpretation of the long coral derived records from the huon peninsula of papua new guinea and the island of tahiti are also discussed

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

coseismic stress changes induced by the 1989 loma prieta california earthquake earthquake focal mechanisms from before and after the 1989 loma prieta california earthquake are used to infer the coseismic stress change before the main shock most earthquakes correspond to right lateral slip on planes sub parallel to the san andreas fault and imply a generally n s most compressional stress axis and a vertical intermediate stress axis aftershocks within the main shock rupture zone however display almost every style and orientation of faulting implying an extremely heterogeneous stress field this suggests that the main shock relieved most if not all of the shear stress acting on its fault plane aftershocks that lie on the perimeter of the rupture agree with spatially uniform stress states but only when considered in three groups north south and above the main shock rupture area in each of these areas the stress state may reflect stress transfer by the main shock

stress orientation determined from fault slip data in hampel wash area nevada and its relation to contemporary regional stress field fault slip data were collected from an area of relatively young faulting in a seismically active part of the nevada test site 12 km nw of mercury nevada the data come primarily from intensely faulted miocene tuffaceous sedimentary rocks in hampel wash which is bounded on the north by the quaternary ene trending rock valley fault and on the south by a parallel unnamed fault data from faults with known sense of displacement exhibit a bimodal distribution of slip angles rakes faults exhibiting steep rakes typically 75 to 90 cluster about a n30 35 e strike most dip 65 to 80 faults having shallow rakes generally less than 20 exhibit a wide range of strikes from n6 w to n80 e and mostly dip between 80 and 90 the predominant n30 35 e strike of the steep rake faults and the quasi conjugate nature of a consistent subset of the shallowrake faults suggest a maximum horizontal stress orientation of about n30 35 e and a least horizontal principal stress direction of n55 60 w analysis of the data using a least squares iterative inversion to determine a mean deviatoric principal stress tensor indicates a normal faulting stress regime s1 vertical with principal stress axes in approximately horizontal and vertical directions s1 trend n 19 e and plunge 82 n s2 n30 e and 8 s and s3 n60 w and 2 e the maximum horizontal stress s2 was found to be nearly intermediate in magnitude between s1 and s3 the n60 w least horizontal principal stress orientation obtained from the fault slip inversion agrees with our geometric analysis of the data and is consistent with a modern least horizontal principal stress orientation of n50 70 w inferred from earthquake focal mechanisms well bore breakouts and hydraulic fracturing measurements in the vicinity of the nevada test site this solution fits all the data well except for a subset of strike slip faults that strike n30 45 e subparallel to the normal faults of the data set nearly pure dip slip and pure strike slip movement on similarly oriented faults however cannot be accommodated in a single stress regime superposed sets of striae observed on some faults suggest temporal rotations of the regional stress field or local rotations within the region of the fault zone

the growth of geological structures by repeated earthquakes 1 conceptual framework in many places earthquakes with similar characteristics have been shown to recur if this is common then relatively small deformations associated with individual earthquake cycles should accumulate over time to create geological structures following this paradigm we show that existing models developed to describe leveling line changes associated with the seismic cycle can be adapted to explain geological features associated with a fault in these models an elastic layer containing the fault overlies a viscous half space with a different density fault motion associated with an earthquake results in immediate deformation followed by a long period of readjustment as stresses relax in the viscous layer and isostatic equilibrium is restored deformation is also caused as a result of the loading and unloading due to sediment deposition and erosion in this paper the parameters that control the growth of dip slip structures are identified we find that the flexural rigidity of the crust or the apparent elastic thickness provides the main control of the width of a structure the loading due to erosion and deposition of sediment determines the ratio of uplift to subsidence between the two sides of the fault the flexure due to sediment load is much more important in this respect than whether the fault is normal or reverse in character we find that in general real structures are associated with apparent elastic thicknesses of 4 km or less and thus with very low flexural rigidities

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

discovery of an eclogite belt in the lhasa block tibet a new border for paleo tethys abstract a newly discovered eclogite belt in the eastern part of the lhasa block tibet is about 500 1000 m wide and at least 60 km long in an e w direction the eclogites occur as tectonic slices in garnet bearing mica quartz schist they are generally fresh and form thick massive layers that consist chiefly of garnet grt omphacite omp phengite phe rutile rut quartz qtz p t calculations based on the grt omp phe mineral assemblage yielded peak metamorphic conditions of 2 7 gpa and 730 c close to the phase boundary between coesite and quartz and thus the eclogites can be regarded as part of a very high pressure metamorphic belt petrochemical data suggest that the eclogite protoliths were typical morb basalts derived from depleted mantle shrimp u pb dating of zircons from the eclogite yielded metamorphic ages ranging from 242 15 to 292 13 ma with an average value of 262 5 ma the morb eclogites are interpreted to be remnants of paleo tethyan oceanic lithosphere the eclogites along with permian island arc volcanic rocks to the north are believed to mark a carboniferous permian suture zone dividing the lhasa block into a northern and southern segment this newly identified suture zone suggests that the border of the paleo tethyan ocean jumped southward from north of the bangong nujiang suture to within what is now the lhasa block

upb geochronology of gangdese transhimalaya plutonism in the lhasa xigaze region tibet abstract a series of different plutons from the lhasa xigaze segment of the gangdese transhimalaya belt has been studied by high resolution u pb analyses of zircon using zircon fractions of 5 100 grains each selected upon specific grain characteristics for two diorites located east of xigaze dazhuka the zircons yield concordant ages of 93 4 1 0 and 94 2 1 0 m y respectively also concordant ages of 41 1 0 4 and 41 7 0 4 m y have been obtained for two granodiorites collected southwest of lhasa qushui the precision on the ages of two granites from the xigaze and lhasa area is limited by two factors the presence of inherited radiogenic lead and the occurrence of subsequent lead loss however some concordant zircons detected in both granites define approximate ages of about 67 and 53 m y respectively the inherited lead components show that melting of precambrian material was involved in magma genesis the u pb ages substantiate a magmatic activity lasting from mid cretceous cenomanian to eocene lutetian time such a period of plutonism at the southern margin of eurasia as well as the occurrence of magma generation from continental crust suggest that the gangdese range results from the subduction of tethys oceanic lithosphere indian plate underneath eurasia eurasian plate if this model is true the collision of india with eurasia along the lhasa xigaze sector postdates the emplacement of the 41 m y old gangdese granodiorites i e the collision occurred after lutetian time

refining the eustatic sea level curve since the last glacial maximum using far and intermediate field sites the eustatic component of relative sea level change provides a measure of the amount of ice transferred between the continents and oceans during glacial cycles this has been quantified for the period since the last glacial maximum by correcting observed sea level change for the glacio hydro isostatic contributions using realistic ice distribution and earth models during the last glacial maximum lgm the eustatic sea level was 125 5 m lower than the present day equivalent to a land based ice volume of 4 6 4 9 107 km3 evidence for a non uniform rise in eustatic sea level from the lgm to the end of the deglaciation is examined the initial rate of rise from ca 21 to 17 ka was relatively slow with an average rate of ca 6 m ka 1 followed by an average rate of ca 10 m ka 1 for the next 10 ka significant departures from these average rates may have occurred at the time of the younger dryas and possibly also around 14 ka most of the decay of the large ice sheets was completed by 7 ka but 3 5 m of water has been added to the oceans since that time

temporary seismic quiescence se turkey summary the border zone that is the contact of the arabian plate with the three plates of turkey eurasia and iran to the north has been remarkably aseismic during this century by extending the period of observation backwards in time by a few centuries it is shown that this seismicity is atypical of the long term behaviour of the zone and due to a quiescent period in the activity of the area during the 20th century in turn this implies that short term data alone do not provide a reliable assessment of earthquake hazard

identification of early carboniferous granitoids from southern tibet and implications for terrane assembly related to the paleo tethyan evolution abstractthis article presents zircon u pb and hf isotope data together with the whole rock major and trace element composition of early carboniferous granitoids newly identified from the jiacha and langxian areas in the southern lhasa terrane southern tibet the jiacha rocks are monzogranites that yield zircon u pb ages of 347 345 ma and ehf t values from 5 4 to 4 9 the langxian rocks are granodiorites with slightly older zircon u pb ages of 355 352 ma and lower ehf t values from 6 8 to 6 5 our data suggest that these granitoids were generated largely by reworking of paleoproterozoic ga basement materials in conjunction with literature data it is further argued that the southern and central parts of the lhasa terrane separated by the sumdo eclogite belt should have been an integrated block before the late paleozoic our study supports the notion that the lhasa terrane was derived from the northern margin of gondwanaland in association with formation of at least two stages of tethyan oc

strike slip fault system in amargosa valley and yucca mountain nevada abstract the amargosa desert fault system part of a 250 km long system of dextral strike slip faults east of the death valley region is a throughgoing long lived nw striking dextral strike slip shear system that lies beneath crater flat and yucca mountain evidence for the fault system includes gravity seismic structural stratigraphic and paleomagnetic data the distribution of springs and basaltic volcanic centers and patterns of late quaternary surface faulting where the amargosa desert fault system passes beneath miocene ash flow tuffs of the southwestern nevada volcanic field we infer that the fault system changes into a series of detachments near the basement cover contact and is expressed as a system of rotational normal faults in the ash flow tuffs displaced geologic features indicate that total dextral displacement on the amargosa desert fault system exceeds 30 km vertical axis rotations in ash flow tuffs suggest that about 25 km of dextral displacement may have occurred since about 12 75 ma this fault system has important implications for the proposed high level nuclear waste repository at yucca mountain since it has experienced quaternary and possibly holocene displacements both normal faults and basaltic volcanic cones near the proposed repository may be controlled by the evolution of this regional strike slip fault system

p wave velocity structure in the yucca mountain nevada region 1 we have performed a crustal tomographic inversion using over 250 000 p arrival times from local earthquake sources and surface explosions in the yucca mountain nevada region within the shallowest 2 3 km topographic features tend to dominate the structure with high velocities imaged under bare mountain the funeral mountains and higher terrain to the east of yucca mountain and low velocities imaged under crater flat jackass flat the amargosa desert and the caldera complexes imaged shallow velocities also show correlation with several known gravity and aeromagnetic anomalies below the basins 2 3 km depth velocities vary between 5 5 and 6 5 km s and lose many of the correlations seen in the shallowest layers however a few major structures such as the bare mountain block can be traced to at least 10 km depth additionally we image structures that may be associated with the wahmonie intrusion and pre tertiary structural trends yucca mountain itself is underlain by a high velocity upper crustal scale structure similar to other structures in the region such as bare mountain and may represent a basin and range style back tilted block which may provide a structural explanation for yucca mountain s topographic expression additionally the imaged relatively low velocity basement under crater flat may provide a preferred conduit for magma intrusion into crater flat compared to yucca mountain accounting for the lack of post miocene volcanism observed at the mountain proper we explore our tomographic results in the context of four major tectonic models that have been proposed for the yucca mountain region

mid to late wisconsin pollen record of san felipe basin baja california abstract a lacustrine sequence from laguna seca san felipe baja california in the southwestern sonoran desert provides the first record of mid to late wisconsin vegetation in one of the driest regions of north america pollen analysis was performed in the intermediate part of the sequence from ca 44 000 to ca 13 000 14 c yr b p according to six 14 c radiocarbon dates sedimentation of eolian sands prior to 44 000 14 c yr b p indicates dry conditions high pollen concentration of montane and chaparral species in the pollen assemblages contrast with the modern desert vegetation sixty four pollen types were identified the source of mid to late wisconsin sedimentation must have been the plant communities in the surrounding highlands of the basin mid wisconsin pollen assemblages dominated by pines junipers and artemisia reflect humid conditions in the area by late wisconsin time a significant increment in junipers indicates a lowering in the altitudinal ranges of woodlands chaparral pollen from wind pollinated species is abundant while desert plants specifically animal or insect pollinated species are poorly represented the san felipe record does not identify the last glacial maximum as the time of greatest effective moisture as suggested by pluvial lake levels and other paleoclimatic evidence in the soutwestern united states

evolution of extensional basins and basin and range topography west of death valley california neogene extension in the death valley region se california has produced a variety of sedimentary basins diachronous movements on an array of strike slip and normal fault systems have resulted in the uplift and preservation of older basins in modern ranges one of the best exposed of these is the nova basin on the western flank of the panamint mountains the nova basin includes over 2000 m of sedimentary and volcanic rocks deposited during denudation of the panamint mountains metamorphic core complex in late miocene early pliocene time the principal growth structure for the basin was the emigrant detachment which initiated and moved at a low angle modern panamint valley west of the range developed as a consequence of late pliocene recent kinematically linked movement on the right slip high angle hunter mountain fault zone and the low angle panamint valley detachment detailed mapping of the intersection between the emigrant and panamint valley detachments demonstrates that segments of the earlier system remained active during development of panamint valley and thus during development of modern basin and range topography as well these results indicate that large scale extension in the death valley region accommodated by movement on low to moderate angle normal fault systems and high angle strike slip fault systems is a continuing process basin and range topography in the panamint valley death valley area was generated at least in part by displacements on low angle detachments rather than high angle normal faults

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

the morphology of strike slip faults examples from the san andreas fault california fault zones are seldom straight nor are fractures within them continuous permanent strain fields are imprinted near a fault of complex geometry as a result of continued slip these strain fields may be manifest as vertical deformation near the fault strain fields that increase area result in subsidence and strain fields that reduce area result in uplift although these effects are modified by erosion and deposition the general features of fault zone morphology are a consequence of the geometry and slip distribution along strike graphical views of strain fields associated with simple vertical strike slip faults are derived using boundary element methods if the fault is straight and parallel to the applied shear strain the hills and valleys resulting from finite fault rupture are separated by the fault trace this is rarely observed in nature instead strike slip faults cut through hills and valleys apparently of their own making we show this to be a consequence of segments of the fault being nonparallel to each other and to the overall applied shear strain coseismic strains are large but transient since they are substantially annulled by rupture on contiguous fault segments we test our theory with examples from parts of the san andreas fault system in california the morphologies of the parkfield region and the southern san andreas fault in the coachella valley are shown to be consistent with plausible distributions of slip on mapped surface faults the location of the salton sea appears to be a consequence of a reduction of slip on the san andreas fault toward the brawley seismic zone

diatoms in the gulf of california seasonal flux patterns and the sediment record for the last 15 000 years flux and taxonomic composition of the diatom assemblage was determined for biweekly sediment traps in the central gulf of california over a 2 year period fluxes and relative abundance of taxa are not significantly different in traps from the eastern side of the guaymas basin and the western side of the carmen basin indicating that patterns of diatom production are uniform across the central gulf at all seasons of the year the annual maximum in diatom flux occurred during the winter season of both years either as a sustained event over many weeks 1990 or as a series of short blooms 1991 1992 of the most abundant taxa only hyalochaete chaetoceros species showed a clear seasonal cycle with a flux maximum in the spring of both years factor analysis defines five assemblages representing 1 summer and fall conditions of surface stratification and nutrient limitation 2 winter bloom triggered by onset of strong northwesterly winds 3 late spring to summer production probably related to sporadic events during periods of weak southeasterly winds 4 spring 1991 inferred to reflect the distal edge of a coastal upwelling phase 5 lateral advection of resuspended sediments from the continental shelf possibly resulting from trapped coastal waves during summer hurricanes and from storms during the 1991 1992 el nino southern oscillation enso the diatom data support the inference that subtropical pacific water penetrates deeper into the gulf during enso events in order to compare the sediment trap record to a sediment core a second factor analysis was done using only the taxa which are preserved in the sediment and omitting the most abundant taxa which showed little downcore variation this produces five factors representing 1 summer fall low production 2 early winter bloom 3 late winter spring upwelling 4 enso conditions and 5 a late spring summer assemblage of uncertain significance variation of species characterisic of each factor was examined in piston core atlantis ii 125 8 56jpc taken from the oxygen minimum zone in guaymas basin correlation to nearby deep sea drilling project hole 480 indicates that the core penetrates to about 15 ka with laminated sediments in the holocene 0 10 ka and late deglacial 11 13 ka and massive nonlaminated sediments around younger dryas time 10 11 ka and before 13 ka downcore variations in the diatom assemblage do not correspond directly to presence of laminations implying that the processes controlling bottom water oxygen content are not determined by conditions of surface water production presence of calcareous nannofossils downcore correlates inversely with two diatom assemblages enso and winter bloom indicators and not with occurrence of laminations suggesting that occurrence of calcareous nannofossils in the sediment is at least partly a function of surface water production prior to 12 ka production was lower than at present during all seasons implying weaker winds throughout the year subtropical waters extended to the midgulf as they do today during enso events between 11 and 12 ka conditions were similar to those of the present with relatively high winter and spring production resulting from northwesterly winds enso events were still common although less so than during the preceding period younger dryas time was one of transition with intermediate conditions a slackening of winter spring northwesterly winds and reduction of total annual production and increased occurrence of enso conditions in the early to middle holocene winter northwesterlies increased to a maximum resulting in very high production but apparently did not persist into spring subtropical waters mostly disappeared from the gulf in the late holocene northwesterly winds have supported high production during both winter and spring seasons

aftershock patterns and main shock faulting we have compared aftershock patterns following several moderate to large earthquakes with the corresponding distributions of coseismic slip obtained from previous analyses of the recorded strong ground motion and teleseismic waveforms well located aftershock hypocenters are projected onto the main shock fault plane and their positions are examined relative to the zones of coseismic displacement indicated by the estimated distributions of main shock slip we also examine the aftershock focal mechanisms when these data are available in an attempt to identify possible patterns of secondary faulting within the aftershock zone our results are consistent with a hypothesis of aftershock occurrence that requires a secondary redistribution of stress following primary failure on the earthquake fault aftershocks following the earthquakes examined in this study occur mostly outside of or near the edges of the source areas indicated by the patterns of main shock slip the spatial distribution of aftershocks reflects either a continuation of slip in the outer regions of the areas of maximum coseismic displacement or the activation of subsidiary faults within the volume surrounding the boundaries of main shock rupture

why is it downhill from tonopah to las vegas a case for mantle plume support of the high northern basin and range a major abrupt southward decline in elevation 800 m with an accompanying increase in bouguer gravity anomaly 80 mgal crosses a seismically active region of southern nevada at about 37 n the steepness of the gravity gradient requires significant crustal density contrast some of which is probably caused by the plutonic roots of voluminous tertiary volcanism to the north but the amplitude of the anomaly requires additional contrast within the mantle the topographic step probably developed in mid miocene time coincident with the arrival of the starting head of the yellowstone thermal plume at the base of the lithosphere a plausible combination of crustal and deep buoyancy sources related to the heat and melt input of an anomalously hot asthenospheric source is consistent with gravity seismic heat flow and isotopic observations and explains the origin of the topographic step

back arc basin basalt systematics the mariana east scotia lau and manus back arc basins babs have spreading rates that vary from slow 100 mm yr and extension axes located from 10 to 400 km behind their island arcs axial lava compositions from these babs indicate melting of mid ocean ridge basalt morb like sources in proportion to the amount added of previously depleted water rich arc like components the arc like end members are characterized by low na ti and fe and by high h2o and ba la the morb like end members have the opposite traits comparisons between basins show that the least hydrous compositions follow global morb systematics and an inverse correlation between na8 and fe8 this is interpreted as a positive correlation between the average degree and pressure of mantle melting that reflects regional variations in mantle potential temperatures lau manus hotter than mariana scotia this interpretation accords with numerical model predictions that faster subduction induced advection will maintain a hotter mantle wedge the primary compositional trends within each bab a positive correlation between fe8 na8 and ti8 and their inverse correlation with h2o 8 and ba la are controlled by variations in water content melt extraction and enrichments imposed by slab and mantle wedge processes systematic axial depth as a proxy for crustal production variations with distance from the island arc indicate that compositional controls on melting dominate over spreading rate hydrous fluxing enhances decompression melting allowing depleted mantle sources just behind the island arc to melt extensively producing shallow spreading axes flow of enriched mantle components around the ends of slabs may augment this process in transform bounded back arcs such as the east scotia basin the re circulation by mantle wedge corner flow to the spreading axes of mantle previously depleted by both arc and spreading melt extraction can explain the greater depths and thinner crust of the east lau spreading center manus southern rifts and mariana trough and the very depleted lavas of east scotia segments e8 e9 the crust becomes mid ocean ridge mor like where the spreading axes further away from the island arc and subducted slab entrain dominantly fertile mantle

nitrogen isotopic variations in the gulf of california since the last deglaciation response to global climate change high resolution records of the nitrogen isotopic composition of organic matter 15norg opal content and opal accumulation rates from the central gulf of california reveal large and abrupt variations during deglaciation and gradual holocene changes coincident with climatic changes recorded in the north atlantic homogenous sediments with relatively low 15norg values and low opal content were deposited at the end of the last glacial period during the younger dryas event and during the middle to late holocene in contrast laminated sediments deposited in the two deglacial stages are characterized by very high 15norg values 14 and opal accumulation rates 29 41 mg cm 2 yr 1 abrupt shifts in 15norg were driven by widespread changes in the extent of suboxic subsurface waters supporting denitrification and were amplified in the central gulf record due to variations in upwelling vertical mixing and or the latitudinal position of the intertropical convergence zone

paleomagnetic evidence for the age and extent of middle tertiary counterclockwise rotation dixie valley region west central nevada paleomagnetic data obtained from oligocene to lower miocene igneous rocks and middle miocene basaltic rocks at fifteen localities from a region surrounding dixie valley in west central nevada indicate that parts of the area experienced counterclockwise vertical axis rotation and these data provide constraints on the extent and timing of rotation counterclockwise vertical axis rotation probably exceeding 30 is indicated for oligocene to lower miocene rocks in the central part of the study area calculated rotations increase with increasing age of the oligocene to lower miocene rocks indicating that vertical axis rotation was concurrent with ash flow deposition about 23 33 ma paleomagnetic data indicate that middle miocene basaltic rocks about 10 17 ma postdate significant counterclockwise vertical axis rotation although this interpretation is complicated because the rocks were erupted episodically in pulses that apparently were short lived with respect to geomagnetic secular variation counterclockwise rotation was related to a deformation event that predated development of the present basin and range physiography of the area the total amount of crustal rotation in tertiary time in the area is poorly known rotation estimates calculated from time averaged mean directions that incorporate data from a broad range of the oligocene to lower miocene units probably underestimate total tertiary rotation because the mean directions include data from units that postdate much of the rotation for example a rotation estimate for a composite sequence from the central part of the study area is 23 15 whereas estimates from older and younger halves of the sequence are 37 21 and 11 16 respectively paleomagnetic data indicate that oligocene to lower miocene rocks at some localities in the northern and southern parts of the study area e g the golconda canyon locality probably did not experience significant tertiary counterclockwise rotation although the paleomagnetic data alone are insufficient to fully define the geographic area subjected to tertiary counterclockwise rotation the data suggest that a discrete subregion was affected rather than the entire basin and range province