paleomagnetism of coastal california and baja california alternatives to large scale northward transport paleomagnetic data from the santa lucia orocopia sloa and baja borderland bba allochthons of coastal california and baja california have been interpreted to indicate up to 2500 km of post mid cretaceous northward transport of these regions with respect to interior north america however with neogene strike slip offsets taken into account geological interpretations correlate basement rocks of the coastal allochthons with continental basement rocks directly across the san andreas and related fault systems we have examined paleomagnetic data from sloa and bba and conclude that apparent discordances can be explained without large scale pre neogene tectonic transport three major observations are fundamental to this analysis 1 paleolatitudes derived from volcanic rocks of the jurassic eugenia formation of bba and coast range ophiolite of sloa are concordant when compared to revised jurassic reference paleomagnetic poles from interior north america 2 isotopic and paleobarometric data from the peninsula ranges batholith in southern california indicate that the batholith has been tilted northeast side up by an amount that can account for discordant paleomagnetic directions observed in plutonic rocks of the batholith without large scale northward transport 3 literal interpretation of the paleolatitudes determined from paleomagnetic directions in upper cretaceous and paleogene marine sedimentary rocks of sloa and bba requires north then south then north transport and a complex motion history between the two allochthons however concordant paleolatitudes are indicated by some sedimentary rocks while coeval or younger sedimentary rocks of the same allochthon have discordant paleolatitudes coupled with recent documentations of compaction shallowing of paleomagnetic inclination in other marine sedimentary rocks these inconsistencies suggest that paleolatitudes derived from most of the marine sedimentary rocks of sloa and bba are biased towards low paleolatitudes by compaction shallowing

tomographic images of the upper crust from the los angeles basin to the mojave desert california results from the los angeles region seismic experiment we apply inversion methods to first arriving p waves from explosive source seismic data collected along line 1 of the los angeles region seismic experiment larse extending northeastward from seal beach california to the mojave desert in order to determine a seismic model of the upper crust along the profile we use resolution information to quantify the extent of blurring in the larse images and to smooth a damped least squares dls image by postinversion filtering pif most of the original data fit is preserved while minimizing model artifacts we compare dls pif and smoothing constraint inversion images using both real and synthetic data a preferred pif image includes larger scale features in the smoothing constraint inversion image and finer scale features in the dls inversion image that are consistent with geologic information we interpret principal model features in terms of geology including faulting the maximum depth of low velocity sedimentary and volcanic rocks in the los angeles basin is 8 9 km and in the san gabriel valley is 4 5 5 km a horst like uplift of basement rocks occurs between these basins the northeastern boundary of the san gabriel valley is imaged as a tabular moderately north dipping low velocity zone that projects to the surface at the southernmost trace of the sierra madre fault system in the central and southern san gabriel mountains velocity depth profiles are consistent with intermediate velocity mylonites overlying lower velocity pelona schist along a shallowly southwest dipping vincent thrust fault tomography does not provide a definitive dip for the san andreas fault but combined with other larse results is consistent with a vertical to steep northeast dip

melt generation during rifting in the north sea in a forthcoming paper mckenzie and o nions1 invert rare earth element concentrations in mid ocean ridge basalts to calculate the variation of melt fraction with depth in the asthenospheric mantle beneath ocean ridges they find that melting persists to greater depths about 80 km than would be predicted by parameterizations of melting experiments2 about 50 km this result has important implications for predicting the volume and composition of melt produced from asthenosphere of normal potential temperature during continental rifting the north sea rift is a good location to test models for melt generation because the physical conditions of rifting are well constrained3 8 using the distribution of melt fraction with depth derived in ref 1 we calculate the volume and composition of melt produced from the asthenosphere in the most stretched part of the north sea the forties region when this melt is mixed in approximately equal proportions with an alkaline melt similar to that produced from metasomatized lithosphere by very small amounts of stretching on the rift flanks it accounts for the volume and the elemental and isotopic composition of the basalts in the most extended region

seismic mountain building landslides associated with the 2008 wenchuan earthquake in the context of a generalized model for earthquake volume balance here we assess earthquake volume balance and the growth of mountains in the context of a new landslide inventory for the mw 7 9 wenchuan earthquake in central china coseismic landslides were mapped from high resolution remote imagery using an automated algorithm and manual delineation which allow us to distinguish clustered landslides that can bias landslide volume calculations employing a power law landslide area volume relation we find that the volume of landslide associated mass wasting 2 8 0 9 0 7 km3 is lower than previously estimated 5 7 15 2 km3 and comparable to the volume of rock uplift 2 6 1 2 km3 during the wenchuan earthquake if fluvial evacuation removes landslide debris within the earthquake cycle then the volume addition from coseismic uplift will be effectively offset by landslide erosion if all earthquakes in the region followed this volume budget pattern the efficient counteraction of coseismic rock uplift raises a fundamental question about how earthquakes build mountainous topography to provide a framework for addressing this question we explore a group of scaling relations to assess earthquake volume balance we predict coseismic uplift volumes for thrust fault earthquakes based on geophysical models for coseismic surface deformation and relations between fault rupture parameters and moment magnitude mw by coupling this scaling relation with landslide volume mw scaling we obtain an earthquake volume balance relation in terms of moment magnitude mw which is consistent with the revised wenchuan landslide volumes and observations from the 1999 chi chi earthquake in taiwan incorporating the gutenburg richter frequency mw relation we use this volume balance to derive an analytical expression for crustal thickening from coseismic deformation based on an index of seismic intensity over a defined area this model yields reasonable rates of crustal thickening from coseismic deformation e g 0 1 0 5 km ma 1 in tectonically active convergent settings and implies that moderate magnitude earthquakes mw 6 7 are likely responsible for most of the coseismic contribution to rock uplift because of their smaller landslide associated volume reduction our first order model does not consider a range of factors e g lithology climate conditions epicentral depth and tectonic setting nor does it account for viscoelastic effects or isostatic responses to erosion and there are important large uncertainties on the scaling relationships used to quantify coseismic deformation nevertheless our study provides a conceptual framework and invites more rigorous modeling of seismic mountain building

gravity anomalies flexure and the elastic thickness structure of the india eurasia collisional system wehaveusedbouguergravityanomalyandtopographydatatodeterminetheequivalentelasticthicknessofthelithosphere te in the region of the india eurasia collisional system comparison of observed and modelled gravity anomalies along 1dimensional profiles suggest there are significant variations in te along strike of the himalaya foreland estimates decrease from a high of 70 km in the central region to 30 50 km in the east and west we have verified these inferences of spatial variations using a 2 dimensional non spectral interative flexure and gravity anomaly modelling technique the himalaya foreland forms a high te 40bteb100 km rigid block with a well defined edge as shown by the localisation of faulting and deformation along its northern margin other highteblocks occur to the north beneath the qaidam and sichuan basins the tibetan plateau forms a low te 0bteb20 km weak region that extends from the central part of the plateau into south western china tectonic styles in the india eurasia collisional system therefore involve both drigidt and dnon rigidt blocks where high te rigid blocks are present the stylesdominatedbyunderthrustingofthemorerigidblock wherethecollisionalzoneisnotconstrainedbyrigidblocks however the style appears to be dominated by lower crustal flow and a more continuous style of deformation d 2005 elsevier b v all rights reserved

small scale heterogeneity of phanerozoic lower crust evidence from isotopic and geochemical systematics of mid cretaceous granulite gneisses san gabriel mountains southern california an elongate belt of mid cretaceous compositionally banded gneisses and granulites is exposed in cucamonga terrane in the southeastern foothills of the san gabriel mountains of southern california banded gneisses include mafic granulites of two geochemical types type 1 rocks are similar to high al arc basalts and andesites but have higher hfse high field strength element abundances and extremely variable lile largeion lithophile element abundances while type 2 rocks are relatively low in al and similar to alkali rich mor midocean ridge or intraplate basalts intercalated with mafic granulites are paragneisses which include felsic granulites aluminous gneisses marble and calc silicate gneisses type 1 mafic granulites and calcic trondhjemitic pegmatites also oceur as cross cutting synmetamorphic dikes or small plutons small scale heterogeneity of deep continental crust is indicated by the lithologic and isotopic diversity of intercalated ortho and paragneisses exposed in cucamonga terrane geochemical and isotopic data indicate that k rb and u depletion and sm nd fractionation were associated with biotite muscovite dehydration reactions in type 1 mafic granulites and aluminous gneisses during high grade metamorphism field relations and model initial isotopic ratios imply a wide range of protolith ages ranging from early proterozoic to phanerozoic

multi scale depositional successions in tectonic settings observations in sedimentary basins affected by deformation show that the fault induced depositional accommodation at various spatial and temporal scales is closely linked to basin kinematics the tectonically driven sediment infill displays the history of deepening and shoaling facies that are controlled by the activation of faults and changes in their offset rates simply stated this results in shifting sedimentary facies towards the source area or towards the basin centre in response to increasing or decreasing depositional space we propose a first principle conceptual model for tectonic successions controlled by the balance between the rates of creation of depositional space and sediment supply these sediment bodies are bounded by succession boundaries and comprise sourceward or basinward shifting facies tracts that are separated at a point of reversal due to the relatively steep slopes associated with the evolution of faults changes in sediment supply rates and mass wasting are common in these systems and may complicate the normal rhythm of the shifting facies tracts once tectonic quiescence is achieved and if the basin is connected to the open ocean eurybatic or eustatic base level changes may take over and play a greater role in sedimentary rhythm and cyclicity we illustrate the efficacy of the new concept with a review of examples from extensional contractional and strike slip basins we show that the basic tectonic succession model is applicable at all temporal and spatial scales and whether the tectonics cause subsidence or uplift and in all types of tectonic settings that determine the evolution of sedimentary basins

identification of high frequency pulses from earthquake asperities along chilean subduction zone using strong motion the chilean subduction zone is one of the most active of the world with m 8 or larger interplate thrust earthquakes occurring every 10 years or so on the average the identification and characterization of pulses propagated from dominant asperities that control the rupture of these earthquakes is an important problem for seismology and especially for seismic hazard assessment since it can reduce the earthquake destructiveness potential a number of studies of large chilean earthquakes have revealed that the source time functions of these events are composed of a number of distinct energy arrivals in this paper we identify and characterize the high frequency pulses of dominant asperities using near source strong motion records two very well recorded interplate earthquakes the 1985 central chile ms 7 8 and the 2007 tocopilla mw 7 7 are considered in particular the 2007 tocopilla earthquake was recorded by a network with absolute time and continuos recording from the study of these strong motion data it is possible to identify the arrival of large pulses coming from different dominant asperities the recognition of the key role of dominant asperities in seismic hazard assessment can reduce overestimations due to scattering of attenuation formulas that consider epicentral distance or shortest distance to the fault rather than the asperity distance the location and number of dominant asperities their shape the amplitude and arrival time of pulses can be one of the principal factors influencing chilean seismic hazard assessment and seismic design the high frequency pulses identified in this paper have permitted us to extend the range of frequency in which the 1985 central chile and 2007 tocopilla earthquakes were studied this should allow in the future the introduction of this seismological result in the seismic design of earthquake engineering

paleogene strata of the eastern los angeles basin california paleogeography and constraints on neogene structural evolution post paleogene dextral slip of 8 9 km is demonstrated for the southeastern part of the whittier fault zone in the eastern los angeles basin area of southern california a linear axis of greatest thickness for the combined upper paleocene and lower to lower middle eocene clastic formations intersects the fault zone and is offset by it to give the new measure fragmentary evidence hints that the whittier structural zone may have exerted control on bathymetric topographic relief and sedimentation even in latest paleocene ca 54 ma a clear topographic influence was exerted by 20 17 ma strike slip and present deformational style is younger than ca 8 ma our paleogene isopach map extends as far west as long 117 58 w and is a foundation for companion zonal maps of predominant lithology and depositional environments integration of new palynological data with published biostratigraphic results and both new and published lithologic and sedimentological interpretations support the zonal maps reconstruction of marine nonmarine facies and fragmented basin margins yields a model for the northeastern corner of a paleogene coastal basin palinspastic adjustment for the neogene quaternary whittier fault offset and a reasoned westerly extension of the northern edge of the basin model yield a reconstruction of paleogene paleogeography paleoceanography our reconstruction is based partly on the absence of both paleocene and eocene deposits beneath the unconformable base of the middle miocene topanga group in a region nowhere less than 15 km wide between the raymond sierra madre cucamonga fault zone and the northern edge of the paleocene basin thus paleogene strata of the santa monica mountains could not have been offset from the northern extension of the santa ana mountains by sinistral slip on those boundary faults structural rearrangements needed to accommodate the clockwise rotation of the western transverse ranges from the early miocene starting position are thereby fixed

the importance of earthquake induced landslides to long term slope erosion and slope failure hazards in seismically active regions abstract this paper describes a general method for determining the amount of earthquake induced landsliding that occurs in a seismically active region over time this determination can be used as a quantitative measure of the long term hazard from seismically triggered landslides as well as a measure of the importance of this process to regional slope erosion rates and landscape evolution the method uses data from historical earthquakes to relate total volume of landslide material dislodged by an earthquake to the magnitude m and seismic moment m0 of the earthquake from worldwide data a linear regression relation between landslide volume v and m0 is determined as v m0 1018 9 0 13 where v is measured in m3 and m0 is in dyn cm to determine the amount of earthquake generated landsliding over time this relation is combined with data on seismic moment release for a particular region which may be derived from either earthquake history or fault slip data the form of the m0 v relation allows the rate of production of earthquake induced landslides over time to be determined from total rate of seismic moment release without regard to the distribution of individual events thus simplifying and generalizing the determination application of the method to twelve seismically active regions with areas ranging from 13 275 to 2 308 000 km2 shows that erosion rates from earthquake induced landslides vary significantly from region to region of the regions studied the highest rates were determined for the island of hawaii new zealand western new guinea and the san francisco bay region of california significantly lower rates were determined for iran tibet the sierra nevada great basin region of california and central japan for the time period from 715 ad to the present intermediate rates were determined for peru southern california onshore california turkey and central japan for the time period from 1586 ad to the present to determine the relative long term importance of seismically triggered landslides these erosion rates are compared to erosion rates calculated for other slope processes and to rates calculated from fluvial sediment discharge comparisons with other slope processes indicate that earthquake induced landslides are the predominant agents of slope erosion on the island of hawaii in the san francisco bay region and in western new guinea for hawaii the san francisco bay region and sierra nevada great basin region of california the erosion rates calculated for earthquake induced landslides also exceed the regional erosion rates calculated from fluvial sediment discharge

the tectonic history of the southwestern united states and sonora mexico during the past 100 m y the area between the colorado plateau on the north new mexico and chihuahua to the east and southern and baja california to the southwest has undergone a complicated thermal and tectonic history climaxing in medial cretaceous time and manifested in a sequence of events which continue to the present time independent of plate boundary transform faulting this sustained sequence of events was initiated by subduction related magmatic emplacement largely between 120 and 50 ma beginning in the west and sweeping eastward across the region the emplacement of these rocks generated a large isostatic welt which began to elevate in late medial cretaceous time in the west central portion of the area and spread outward until in oligocene time the west coast of southern california was emergent almost to the continental escarpment both erosional and tectonic unloading accompanied elevation with the axis of the welt achieving its maximum elevation in earliest cenozoic time when north central sonora may have had elevations of the order of 5000 m body forces caused the welt to relax laterally producing thrust and nappe structures which were westward vergent in the west and eastward vergent in the east to the east these structures are included in what is called the larimide orogeny to the west they include such features as the santa rosa mylonite belt and related structures as with the magmatic emplacement the gravitational spreading began in the west during the eocene additional rebound in response to deep erosion and tectonic unroofing began to expose midcrustal rocks within the core area of the welt and subhorizontal detachment began along the brittle ductile boundary during oligocene and early miocene time renewed isostatic uplift resulted from rising magma and continued erosional and tectonic denudation spawned both brittle and ductile extension hydrothermal alteration and mineral deposition from mid miocene to the present time the central portion of the area has subsided while the marginal mountain ranges have elevated

predicting orogenic wedge styles as a function of analogue erosion law and material softening 1 the evolution of a compressive frictional wedge on a weak frictional and planar decollement subjected to frontal accretion is predicted with a two step method called sequential limit analysis the first step consists in finding with the kinematic approach of limit analysis the length of the active decollement and the dips of the emerging ramp and of the conjugate shear plane composing the emerging thrust fold the second step leads to a modification of the geometry first because of the thrust fold development due to compression and second because of erosion erosion consists in removing periodically any material above a fictitious line at a selected slope as done in analogue experiments this application of sequential limit analysis generalizes the critical coulomb wedge theory since it follows the internal deformation development with constant frictional properties the deformation is mostly diffuse a succession of thrust folds being activated so that the topographic slope reaches exactly the theoretical critical value frictional weakening on the ramps results in a deformation style composed of thrust sheets and horses applying an erosion slope at the critical topographic value leads to exhumation in the frontal central or rear region of the wedge depending on the erosion period and the weakening erosion at slopes slightly above or below the critical value results in exhumation toward the foreland or the hinterland respectively regardless of the erosion period exhumation is associated with duplexes imbricate fans antiformal stacks and major backthrusting comparisons with sandbox experiments confirm that the thickness dips vergence and exhumation of thrust sheets can be reproduced with friction and erosion parameters within realistic ranges of values

tectonics of the san gabriel basin and surroundings southern california the sangabriel basin is a pliocene pleistocene transverse ranges structure bounded by the san gabriel mountains on the north the san jose and puente hills on the east and the repetto and montebello hills on the west a basement boundary between gneissoid rocks of san gabriel mountains aspect and peninsular ranges rocks strikes east across the basin and connects with the san jose fault this boundary may represent the eastward continuation of the canton fault a middle miocene precursor of the san gabriel fault system with up to 30 km of right slip canton fault displacement is largely canceled by 22 5 km of left slip on a precursor to the raymond fault such that an oligocene dacite in the san jose hills and clasts derived from it show little evidence of large horizontal displacement between the san gabriel mountains and puente hills clockwise rotation in the middle miocene probably influenced the distribution of lateral slip the fill in the northeast trending pliocene pleistocene san gabriel basin is a basal shallow marine sequence overlain by the nonmarine duarte conglomerate in contrast to the deep water fernando formation of the los angeles basin the paleoslope trends south in the direction of the repetto and puente hills evidence that uplift of these hills postdated deposition the triangular san gabriel basin is currently a stable block bounded by the right slip northwest striking east montebello fault on the west and the left slip northeast striking walnut creek fault on the east farther east the san jose and puente hills are underlain by an east trending active fold belt including the san jose reverse fault and anticline the walnut anticline and the puente hills anticline adjacent to the right lateral whittier fault the east montebello fault is bounded on the west by the elysian park anticlinorium a fold belt in downtown los angeles the left slip holocene raymond fault joins the sierra madre reverse fault at its segment boundary marked by the clamshell sawpit fault the triangular stable block between east trending fold belts is bounded on the south by the puente hills blind thrust and on the north by the sierra madre fault documenting north south quaternary convergence east west extension is also indicated although not confirmed by geodetic measurements

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

a seismologically consistent expression for the total area and volume of earthquake triggered landsliding we present a new seismologically consistent expression for the total area and volume of populations of earthquake triggered landslides this model builds on a set of scaling relationships between key parameters such as landslide spatial density seismic ground acceleration fault length earthquake source depth and seismic moment to assess the model we have assembled and normalized a catalog of landslide inventories for 40 shallow continental earthquakes low landscape steepness causes systematic overprediction of the total area and volume of landslides when this effect is accounted for the model predicts the total landslide volume of 63 of 40 cases to within a factor 2 of the volume estimated from observations r2 0 76 the prediction of total landslide area is also sensitive to the landscape steepness but less so than the total volume and it appears to be sensitive to controls on the landslide size frequency distribution and possibly the shaking duration some outliers are likely associated with exceptionally strong rock mass in the epicentral area while others may be related to seismic source complexities ignored by the model however the close match between prediction and estimate for about two thirds of cases in our database suggests that rock mass strength is similar in many cases and that our simple seismic model is often adequate despite the variety of lithologies and tectonic settings covered this makes our expression suitable for integration into landscape evolution models and application to the anticipation or rapid assessment of secondary hazards associated with earthquakes

mineralogy and pressure temperature time path of cretaceous granulite gneisses south eastern san gabriel mountains southern california mid cretaceous granulite gneisses crop out in a narrow belt in the cucamonga region of the south eastern foothills of the san gabriel mountains southern california interlayered mafic granulites and pelitic carbonate calc silicate and quartzofeldspathic metasediments record hornblende granulite subfacies metamorphism at approximately 8 kbar and 700 800 c regional deformation and formation of banded gneisses ceased by c 108 ma although mafic intermediate magmatism and high grade metamorphism continued locally as late as c 88 ma garnet zoning in metapelitic gneisses suggests that peak metamorphism was followed locally by a period of near isobaric cooling but this interpretation requires diachronous cooling of the granulite belt which cannot be demonstrated without detailed thermo chronological data it is more likely that the entire terrane remained at granulite facies p t conditions until 88 ma followed by rapid uplift associated with juxtaposition against adjacent middle and upper crustal arc terranes uplift occurred between c 88 and 78 ma at rates of approximately 1 2 km ma 1 the geotectonic evolution of the cucamonga granulites is similar to mid cretaceous high p granulites in the sierra nevada and salinian block of central california late cretaceous uplift common to these granulites may provide an important tectonic link between dismembered mesozoic batholithic terranes in the california cordillera

evaluation of factors controlling earthquake induced landslides caused by chi chi earthquake and comparison with the northridge and loma prieta events over 10 000 landslides were triggered by the september 21 1999 chi chi earthquake a large number of these landslides have been mapped from spot images and a smaller number were described in a detailed field investigation geographic information systems gis was used to conduct a spatial characterization of the slope failures including distribution of type size slope angle bedrock geology ground motion and distance from earthquake source the most abundant landslides were shallow disaggregated rock and soil slides landslides occurred primarily in tertiary sedimentary rocks which are well known for their susceptibility to landsliding in many parts of the world landslide concentration values diminish beyond epicentral distances of 40 and 70 km from the epicenter and the surface projection of the fault plane respectively ground motion was found to be the most significant factor in triggering the shallow landslides in the chi chi earthquake overall 74 of all slope failures occurred in regions with vertical ground motions greater than 0 2g and 81 of all slope failures occurred in the region with mean horizontal peak ground accelerations pga greater than 0 15g these factors were used to compare landslides generated by the chi chi earthquake to the landslides triggered by the 1989 loma prieta earthquake and the 1994 northridge earthquake the major difference in distribution of landslides between the chi chi earthquake and the two california earthquakes was the distribution of slope angles in the chi chi earthquake 90 of the failures occurred on slopes steeper than 45 while more than 80 of failures occurred on slopes less than 50 in california apparently reflecting much steeper and tectonically more active geologic setting in taiwan

structural evidence for northeastward movement on the chocolate mountains thrust southeasternmost california the late cretaceous chocolate mountains thrust of southeastern california and southwestern arizona places a block of proterozoic and mesozoic continental crust over the late mesozoic continental margin oceanic sedimentary and volcanic rocks of the regionally distinctive orocopia schist the chocolate mountains thrust is interpreted as a thrust burial subduction fault rather than a low angle normal exhumation unroofing uplift fault an important parameter required to understand the tectonic significance of the chocolate mountains and related thrusts is their sense of movement the chocolate mountains thrust zone contains sparse to locally abundant mesoscopic asymmetric folds fabric relations supported by regional geologic evidence indicate that these folds are an integral part of and coeval with the thrust zone on a lower hemisphere equal area plot representing the orientation and sense of asymmetry of 80 thrust zone folds from 36 localities spread over an area 60 by 10 km z folds plot northwest of and s folds plot southeast of a northeast southwest striking vertical plane of overall monoclinic symmetry the only sense of movement consistent with the collective asymmetry of the thrust zone folds is top to the northeast asymmetric microstructures studied at several localities also indicate top to the northeast movement paleomagnetic data suggest that the original sense of thrusting prior to neogene vertical axis tectonic rotation related to the san andreas fault system was northward the essential point is that movement of the upper plate of the chocolate mountains thrust evidently was continentward continentward thrusting suggests a tectonic scenario in which an insular or peninsular microcontinental fragment collided with mainland southern california the suture predicted by this model is elusive but the chocolate mountains thrust and underlying orocopia schist themselves may represent the suture at the present level of exposure alternative tectonic models involving subduction of the orocopia schist eastward beneath continental southern california circumvent the suture problem but are presently not supported by any direct structural evidence

seismically induced mass movements and volumetric fluxes resulting from the 2010 mw 7 2 earthquake in the sierra cucapah mexico the observation that sediment flux from mountain ranges struck by high magnitude earthquakes can be strongly influenced by coseismic mass movements brings into question the nature of coseismic deformation as a net contributor to mountain building to better constrain the role of high magnitude earthquakes in orogenesis high resolution data of earthquake induced mass wasting is required for areas of differing tectonic morphological and climatic settings here we compare the erosional flux to the tectonic flux associated with the 2010 mw 7 2 sierra el mayor earthquake in mexico and examine the landslide patterning of coseismic mass movements associated with this event the ruptured fault system has a significant strike slip component with subsidence along the eastern flank and uplift on the western flank of the range peak ground acceleration was highest along the steepest sections of the range such that the frequency of landslide occurrence was strongly correlated to slope gradient both vertical and horizontal coseismic displacement demonstrated a strong control over landslide initiation this result suggests that strike slip systems experience very different landslide patterning to thrust faults during earthquakes based on interferometric analysis of synthetic aperture radar images the earthquake resulted in a total uplifted volume of 41 6 106 m3 and a loss of 95 2 106 m3 due to subsidence this suggests a net tectonic volumetric flux of 53 6 106 m3 sediment mobilisation by coseismic landslides is estimated at 2 7 106 m3 derived from a manually mapped inventory using spot 5 multispectral imagery thus the net volume loss through coseismic subsidence of the mountain range generated a strongly negative mass flux which was only marginally enhanced by mass wasting

intrusion of horizontal dikes tectonic significance of middle proterozoic diabase sheets widespread in the upper crust of the southwestern united states initially horizontal sheet intrusions of middle proterozoic diabase are abundant in a region 650 by 300 km across in arizona and california the diabase forms discordant sheets in basement granite and gneiss and sills in overlying shelf sedimentary sequences massive granite is the most common basement host for the sheets probably because it fractured more easily than foliated hosts during sheet emplacement steep feeder dikes are rare compared to the sheets the diabase in many places is exposed in fault blocks that were tilted during tertiary tectonic extension structure sections restored from the map patterns of upended blocks show that the sheets were intruded at levels throughout the upper crust to depths of at least 13 km sheet intrusion implies a vertical orientation of the least compressive stress so i conclude that the crust was under tectonic compression or in an isotropic state of stress at the time of diabase intrusion about 1 1 ga magma overpressures water encountered by rising magma vertical changes in the crustal stress regime and flotation of low density granite all may be important factors for sheet intrusion the stress conditions suggested by the presence of the sheets argue against an extensional tectonic regime earlier proposed for the diabase event the diabase province contrasts structurally with similar age provinces of basaltic magmatism elsewhere in north america that show evidence of tectonic extension such as the midcontinent rift the consistent orientations of the sheets commonly allow them to be used as structural markers for postdiabase deformation of basement blocks sheet intrusions in the geologic record may be seriously underreported their recognition is important for the interpretation of seismic reflection profiles of continental crust sheets in arizona may be responsible for the bagdad reflection sequence which extends to depths of at least 15 km

a deterministic and stochastic velocity model for the salton trough basin and range transition zone and constraints on magmatism during rifting as a high resolution addition to the 1992 pacific to arizona crustal experiment pace a 45 km long deep crustal seismic reflection profile was acquired across the chocolate mountains in southeastern california to illuminate crustal structure in the transition between the salton trough and the basin and range province the complex seismic data are analyzed for both large scale deterministic and fine scale stochastic crustal features a low fold near offset common midpoint cmp stacked section shows the northeastward lateral extent of a high velocity lower crustal body which is centered beneath the salton trough off end shots record a high amplitude diffraction from the point where the high velocity lower crust pinches out at the moho above the high velocity lower crust moderate amplitude reflections occur at midcrustal levels these reflections display the coherency and frequency characteristics of reflections backscattered from a heterogeneous velocity field which we model as horizontal intrusions with a von karman fractal distribution the effects of upper crustal scattering are included by combining the mapped surface geology and laboratory measurements of exposed rocks within the chocolate mountains to reproduce the upper crustal velocity heterogeneity in our crustal velocity model viscoelastic finite difference simulations indicate that the volume of mafic material within the reflective zone necessary to produce the observed backscatter is about 5 the presence of wavelength scale heterogeneity within the near surface upper and middle crust also produces a 0 5 s thick zone of discontinuous reflections from a crust mantle interface which is actually a first order discontinuity

empirical tests of carbon isotope thermometry in granulites from southern california abstract peak metamorphic carbon isotope fractionations between calcite and graphite cal gr in marbles and calc silicates from the cucamonga granulite terrane san gabriel mountains california range from 3 48 to 2 90 the data are used to test three previously published calibrations of the calcite graphite carbon isotope thermometer an empirical calibration of the calcite graphite carbon isotope thermometer gives temperatures of 700 750 c a theoretical experimental calibration of the system gives temperatures of 760 870 c an experimental calibration gives temperatures of 870 1300 c temperatures calculated using the empirical calibration are in agreement with those calculated from garnet based cation exchange thermometry when uncertainty is considered temperatures calculated using the theoretical experimental calibration overlap the upper range of cation exchange thermometry temperatures and range to 50 c higher the experimental calibration yields temperatures from 50 to 480 c higher than those from cation exchange thermometry moreover temperatures from the experimental calibration are also inconsistent with mineral and melt equilibria in the granulite phase assemblage despite the better agreement between cation exchange thermometry and the empirical calibration of the calcite graphite system temperatures calculated using the theoretical experimental calibration may be real peak metamorphic temperatures if retrograde diffusion partially reset garnet based cation exchange thermometers by c 50 c then the cation exchange temperatures are consistent with those from the theoretical empirical calibration thermometric evidence from biotite dehydration melting equilibria is consistent with either the empirical calibration if melting was fluid present or the theoretical experimental calibration if melting was fluid absent

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

seismic mapping of shallow fault zones in the san gabriel mountains from the los angeles region seismic experiment southern california during the los angeles region seismic experiment larse a reflection refraction survey was conducted along a profile line 1 extending from seal beach california northeastward to the mojave desert and crossing the los angeles and san gabriel valley basins and san gabriel mountains in most shot gathers from the southern and central san gabriel mountains clear secondary arrivals are seen that merge or appear to merge with first arrivals at three locations including the location of the vincent thrust fault an exposed late mesozoic early cenozoic megathrust these secondary arrivals are interpretable as reflections in the shallow crust 5 km depth from a concave upward interface that projects to the surface in the north near the vincent thrust fault is offset in its central part at the san gabriel fault an old branch of the san andreas fault and terminates in the south at 1 to 2 km depth at the southern mountain front the velocity structure above and below this interface strongly suggests it is the vincent thrust fault intermediate velocities 6 2 km s consistent with mylonites overlying the vincent thrust fault are observed above it lower velocities 5 8 km s consistent with the pelona schist underlying the vincent thrust fault are observed below it problems arise however in attempting to match this reflector to the exposed vincent thrust fault which is seen in outcrops east of line 1 the vincent thrust fault is shallower than the reflector in most places an unmapped structure steep fault monocline or thrust fault is required between line 1 and the outcrops that either drops the vincent thrust fault down to the depths of the reflector or repeats the vincent thrust fault beneath line 1 in the footwall of another thrust fault an alternative interpretation of the reflector is a deep greenstone horizon within the pelona schist although this alternative is not favored by the velocity structure

prolonged seismically induced erosion and the mass balance of a large earthquake large earthquakes deform the earth s surface and drive topographic growth in the frontal zones of mountain belts they also induce widespread mass wasting reducing relief the sum of these two opposing effects is unknown using a time series of landslide maps and suspended sediment transport data we show that the mw7 6 chi chi earthquake in taiwan was followed by a period of enhanced mass wasting and fluvial sediment evacuation peaking at more than five times the background rate and returning progressively to pre earthquake levels in about six years therefore it is now possible to calculate the mass balance and topographic effect of the earthquake the choshui river has removed sediment representing more than 30 of the added rock mass from the epicentral area this has resulted in a reduction of surface uplift by up to 0 25 m or 35 of local elevation change and a reduction of the area where the chi chi earthquake has built topography for other large earthquakes erosion may evolve in similar predictable ways reducing the efficiency of mountain building in fold and thrust belts and the topographic expression of seismogenic faults prolonging the risk of triggered processes and impeding economic regeneration of epicentral areas

the mass balance of earthquakes and earthquake sequences large compressional earthquakes cause surface uplift as well as widespread mass wasting knowledge of their trade off is fragmentary combining a seismologically consistent model of earthquake triggered landsliding and an analytical solution of coseismic surface displacement we assess how the mass balance of single earthquakes and earthquake sequences depends on fault size and other geophysical parameters we find that intermediate size earthquakes mw 6 7 3 may cause more erosion than uplift controlled primarily by seismic source depth and landscape steepness and less so by fault dip and rake such earthquakes can limit topographic growth but our model indicates that both smaller and larger earthquakes mw 7 3 systematically cause mountain building earthquake sequences with a gutenberg richter distribution have a greater tendency to lead to predominant erosion than repeating earthquakes of the same magnitude unless a fault can produce earthquakes with mw 8 or more

regional patterns of earthquake triggered landslides and their relation to ground motion 1 we have documented patterns of landsliding associated with large earthquakes on three thrust faults the northridge earthquake in california chi chi earthquake in taiwan and two earthquakes on the ramu markham fault bounding the finisterre mountains of papua new guinea in each case landslide densities are shown to be greatest in the area of strongest ground acceleration and to decay with distance from the epicenter in california and taiwan the density of co seismic landslides is linearly and highly correlated with both the vertical and horizontal components of measured peak ground acceleration based on this observation we derive an expression for the spatial variation of landslide density analogous with regional seismic attenuation laws in its general form this expression applies to our three examples and we determine best fit values for individual cases our findings open a window on the construction of shake maps from geomorphic observations for earthquakes in non instrumented regions

thermobarometric constraints on the depth of exposure and conditions of plutonism and metamorphism at deep levels of the sierra nevada batholith tehachapi mountains california we present thermodynamic estimates of pressures temperatures and volatile activities in variably deformed gabbroic to granitic cretaceous 115 100 ma batholithic and framework rocks of the tehachapi mountains southernmost sierra nevada california al contents of hornblende in granitoids imply igneous emplacement at 8 kbar in the southernmost tehachapi mountains with lower pressures 3 7 kbar to the north metamorphic pressures and temperatures for garnet bearing paragneisses and metaigneous rocks were estimated on the basis of garnet hornblende plagioclase quartz and garnet biotite plagioclase quartz thermobarometers disparate results for the metaigneous rocks from the latter system point to the difficulty of applying pelite based thermobarometers to rocks of contrasting composition and mineralogy preferred pressures cluster at 7 1 9 4 and 3 6 4 3 kbar incomplete knowledge of reaction histories however limits our interpretation of the lower pressures because they are minimum estimates the 4 kbar samples are all from a small area and if our interpretation is correct they imply a local more shallow event superimposed on crust once residing at deeper structural levels garnet hornblende and garnet biotite temperatures are less coherent likely owing to retrograde fe mg exchange and range from 570 to 790 c the majority of the rocks are igneous and affected by recrystallization and metamorphism during subsolidus cooling they are not granulites country rock paragneisses are typically migmatized at peak metamorphic conditions near that of the wet granite solidus 690 c veinlike paragenesis of garnet in the metaigneous rocks suggests formation related to the presence of a fluid phase thermodynamic estimates of volatile activities in these garnet bearing assemblages suggest variable mostly co 2 rich fluid compositions in the absence of any pervasive fluid flux the igneous rocks of the tehachapi mountains were thus intruded at depths of 30 km making them the deepest known exposed components of the cretaceous sierra nevada batholith metamorphism occurred at these great depths and perhaps locally after 15 km of uplift before 87 ma implying an uplift rate of 1 2 mm yr a minimum uplift rate is 0 6 mm yr this original uplift and possible subsequent uplift events may have been related to underthrusting of a block of rand schist from what is now the southeast with concomitant widespread ductile deformation the deduced pressure temperature and uplift history is similar to those of high pressure high temperature cretaceous batholithic rocks in salinia and the san gabriel mountains but direct correlation is not wan anted when compared with higher level intrusive rocks from analogous portions of the sierra nevada batholith to the north the tehachapi rocks reveal a deep batholith that is more heterogeneous and somewhat more mafic on average but displaying a similar level of isotopic hybridization involving mantle and crustal sources the batholith is quartz rich at these levels suggestive of a weak ductile middle crust susceptible to prolonged deformation and possible delamination

late cretaceous intra arc thrusting in southern california thick zones of mylonitic rock are exposed in or between a number of the suspect crystalline terranes in the transverse ranges and eastern peninsular ranges of southern california after palinspastic reconstruction of strike slip displacements along neogene faults in the region the mylonite zones appear in a semicontinuous belt several hundred kilometers long mylonitic deformation in these areas is of a similar style and age lineation trends in the mylonitic rocks are closely aligned when palinspastic reconstruction takes into account the block rotations in the region and the sense of shear is consistently west directed the mylonite zones appear to have formed concurrently in a major midcrustal thrust system that juxtaposed the basement terranes during the late cretaceous a kinematic model is proposed in which a west directed ductile thrust zone along the eastern side of the peninsular ranges is offset by a sinistral tear fault or lateral thrust ramp along the southern margin of the san gabriel mountains from a ductile thrust underlying crystalline basement terranes to the north this model requires the suspect basement rocks of the peninsular and transverse ranges to be linked tectonically by the latest cretaceous this late cretaceous thrust system also appears to continue directly into autochthonous basement rocks in the san bernardino mountains the late cretaceous westward displacement of basement rocks in the transverse ranges relative to the northern end of the peninsular ranges batholith is mirrored by similar tectonic movement around the southern end of the sierra nevada batholith it is argued here that a formerly continuous batholithic belt was breached by ductile west directed thrusting during the late cretaceous between the two still recognizable batholiths the salinia terrane represents the westernmost portion of this westward escaping arc segment and would have been established as a crustal salient by the latest cretaceous this model implies that the basement terranes involved in this thrusting episode are parautochthonous fragments of a cretaceous continental arc and refutes suggestions that they have been accreted in the cenozoic

from geodetic imaging of seismic and aseismic fault slip to dynamic modeling of the seismic cycle understanding the partitioning of seismic and aseismic fault slip is central to seismotectonics as it ultimately determines the seismic potential of faults thanks to advances in tectonic geodesy it is now possible to develop kinematic models of the spatiotemporal evolution of slip over the seismic cycle and to determine the budget of seismic and aseismic slip studies of subduction zones and continental faults have shown that aseismic creep is common and sometimes prevalent within the seismogenic depth range interseismic coupling is generally observed to be spatially heterogeneous defining locked patches of stress accumulation to be released in future earthquakes or aseismic transients surrounded by creeping areas clay rich tectonites high temperature and elevated pore fluid pressure seem to be key factors promoting aseismic creep the generally logarithmic time evolution of afterslip is a distinctive feature of creeping faults that suggests a logarithmic dependency of fault friction on slip rate

effects of progressive mylonitization on ar retention in biotites from the santa rosa mylonite zone california and thermochronologic implications the santa rosa mylonite zone developed predominantly from a granodiorite protolith in the eastern margin of the peninsular ranges batholith a wide variation in k ar biotite dates within the zone is shown to reflect the times of cooling through closure temperatures whose variability is chiefly a result of deformation induced reduction in grain size we suggest that such variation generally may be exploited to place constraints on the timing of deformation episodes previous workers have shown that deformation in the santa rosa mylonite zone involved the formation of mylonites and an imbricate series of low angle faults which divide the area into structural units field work petrographic studies and tem analysis of deformation mechanisms in biotite show that the granodiorite mylonite the lowermost structural unit formed below the granodiorite solidus the granodiorite mylonite varies from protomylonite to ultramylonite with regions of high strain distributed heterogeneously within the zone previously reported biotite and hornblende k ar dates from the granodiorite protolith below 82 89 ma and the asbestos mountain granodiorite above 61 68 ma the mylonite zone indicate dramatically dissimilar thermal histories for the lowermost and uppermost structural units other workers fission track dates on sphene zircon and apatite from the granodiorite mylonite and the asbestos mountain granodiorite suggest thermal homogenization and rapid cooling to 100 c by ca 60 ma within and adjacent to the mylonite zone k ar dates on 5 samples of biotite from variably deformed granodiorite range from 62 76 ma dates are not correlated with structural depth but clearly decrease with degree of deformation and concomitant grain size reduction 40ar 39ar incremental heating analyses of biotite from the granodiorite protolith reveals no evidence of excess argon and produces a relatively flat age spectrum 40ar 39ar incremental heating analysis of biotite from the granodiorite mylonite discloses discordance consistent with 39ar recoil loss k analysis of samples allowing k ar dates to be calculated is therefore recommended as an adjunct to 40ar 39ar step heating analysis in rocks that have experienced similar deformation during mylonitization biotite grain size reduction through intracrystalline cataclasis results in estimated grain dimensions as small as 0 05 m locally within porphyroclasts as large as 1 mm because biotite compositions are relatively uniform fe fe mg mn ti alvi 0 47 0 52 and show no systematic variation with grain size compositional dependence of activation energy and diffusivity can be eliminated as sources of variation in ar retention ar closure temperatures calculated with appropriate diffusion parameters for the observed grain sizes are in the range 220 280 c and define a cooling curve consistent with a thermal history intermediate between those of the granodiorite protolith below and the asbestos mountain granodiorite above the mylonite zone changes in the slope of the cooling curve indicate that the main deformation episode initiated at or above ca 330 c 80 ma above the closure temperature for thermally activated diffusion of argon in biotite and continued to a minimum of ca 220 260 c 62 ma

mass wasting triggered by the 2008 wenchuan earthquake is greater than orogenic growth shallow earthquakes lead to the uplift of mountain ranges but also trigger landslides that remove mass an analysis of the 2008 wenshuan earthquake suggests that more material was removed from the orogen by widespread landslides than was added by coseismic uplift

estimating the return times of great himalayan earthquakes in eastern nepal evidence from the patu and bardibas strands of the main frontal thrust the return times of large himalayan earthquakes are poorly constrained despite historical devastation of cities along the mountain range definitive links between events and specific segments of the main frontal thrust mft are not established and paleoseismological records have not documented the occurrence of several similar events at the same location in east central nepal however recently discovered primary surface ruptures of that megathrust in the a d 1255 and 1934 earthquakes are associated with flights of tectonically uplifted terraces we present here a refined longer slip history of the mft s two overlapping strands patu and bardibas thrusts in that region based on updated geomorphic neotectonic mapping of active faulting two 1 3 km long shallow seismic profiles and logging of two river cut cliffs three paleoseismological trenches and several pits with constraints from 74 detrital charcoals and 14 cosmogenic nuclide ages the amount of hanging wall uplift on the patu thrust since 3650 450 years requires three more events than the two aforementioned the uplift rate 8 5 1 5mm yr thrust dip 25 5 n and apparent characteristic behavior imply 12 17 5m of slip per event on the bardibas thrust discrete pulses of colluvial deposition resulting from the coseismic growth of a flexural fold scarp suggest the occurrence of six or seven paleo earthquakes in the last 4500 50 years the coeval rupture of both strands during great himalayan earthquakes implies that in eastern nepal the late holocene return times of such earthquakes probably ranged between 750 140 and 870 350 years

origin of gneisses in the aureole of the san gabriel anorthosite complex and implications for the proterozoic crustal evolution of southern california granulite facies gneisses along the southwestern margin of the san gabriel anorthosite complex include compositionally banded locally migmatitic felsic gneisses and mafic granulites with rare interlayered augen gneiss and garnet hypersthene spinel aluminous gneiss previous workers accepted a 1 4 ga age for these granulites and a 1 2 ga age for the adjacent anorthosite complex and hence that this proterozoic basement is allochthonous with respect to the southwestern united states part of the north american craton solvus geothermometry in felsic and mafic granulites yields minimum metamorphic temperatures of 900 950 c and new u pb zircon data yield a minimum metamorphic age of 1186 ma coincident with that of the adjacent anorthosite we reinterpret these rocks as a 1190 ma anorthosite complex and its granulite grade thermal aureole geochemical data suggest that the anorthosite complex intruded early proterozoic rocks correlative with the mojave crustal province representing the north american craton exposed to the northeast we conclude that these proterozoic basement rocks represent the southwestern margin of the north american craton and not an exotic terrane accreted in mesozoic or tertiary time

landslide patterns reveal the sources of large earthquakes the distribution of fault slip in an earthquake controls seismic surface deformation and ground shaking and is the most direct record of fault response to tectonic stress detailed information about slip patterns in large earthquakes is only available where a dense local network of seismometers is in place elsewhere the evaluation of earthquake source and mechanism relies on observation of proxies such as surface change or damage intensity in upland areas the density of seismically induced landslides is strongly influenced by the intensity and duration of strong ground motion and therefore contains information about the earthquake source mechanism here we show that density patterns of co seismic landslides closely track the distribution of fault slip in two large thrust earthquakes in japan when adjusted for site effects landslide distributions readily obtained from remote sensed images can supplement or replace instrumental records of earthquakes opening historical and remote events for close geophysical investigation

lower edge of locked main himalayan thrust unzipped by the 2015 gorkha earthquake faults are unlocked by earthquakes analysis of seismic data from the 2015 nepal earthquake shows that only part of the main himalayan thrust fault was unzipped by the quake leaving much of the fault locked and ready to slip in a future event

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

density distribution of landslides triggered by the 2008 wenchuan earthquake and their relationships to peak ground acceleration the distribution of landslides induced by the 2008 m w 7 9 wenchuan earthquake was investigated following the method of meunier et al 2007 to examine the relationships to peak ground acceleration pga results suggest that landslide densities are quantifiably related to pga thresholds of pgacr for landslide failure are graphic 1 for average horizontal pga and graphic 2 for average vertical pga as with other earthquakes the distribution of the wenchuan earthquake triggered landslides can be described by two equations based on area and source densities respectively the similar damping factors of 20 in these two equations suggest that both density distribution patterns of landslides reflect the dissipation of seismic energy landslide derived pga equations were then produced for the hanging wall region of the earthquake affected area the results indicate that it should be possible to produce an earthquake shake map from the distribution of earthquake triggered landslides in an area where there are few or no seismic instruments but many landslides 1 embed inline graphic 1 gif 2 embed inline graphic 2 gif

late cretaceous juxtaposition of metamorphic terranes in the southeastern san gabriel mountains california new structural petrological and geochronological data from the southeastern san gabriel mountains define the amalgamation history of suspect terranes in this region four metamorphic terranes constitute the area the cucamonga san antonio san gabriel and baldy terranes and were juxtaposed in two discrete episodes during the late cretaceous the cucamonga san antonio and san gabriel terranes were juxtaposed along subparallel east west trending left lateral mylonitic fault zones during the first episode the cucamonga terrane consists of lower cretaceous granulite facies gneissic rocks these rocks were intruded in the late cretaceous by granitic rocks as the cucamonga terrane was faulted ductilely against the southern margin of the san antonio terrane mylonitized units were subjected concurrently to retrograde amphibolite facies metamorphism metasedimentary pendants in the san antonio terrane were metamorphosed under upper amphibolite facies conditions as plutonic rocks were emplaced syntectonically along its northern boundary the san antonio terrane was juxtaposed concurrently against the san gabriel terrane a terrane consisting of precambrian gneiss and precambrian and mesozoic intrusive rocks the boundary is represented by middle fork complex a ductile zone intermingling rocks of the two terranes after palinspastic restoration of local cenozoic fault displacements the left lateral mylonitic fault zones separating the terranes appear as parts of a major sinistral transform system along the southern margin of the san gabriel mountains displacement for each of the mylonitic fault zones was approximately several tens of kilometers and was part of a broad left lateral shear system this system is modeled as a lateral ramp or tear fault to a west directed synplutonic ductile thrust system underlying the san gabriel terrane crosscutting syn and post tectonic intrusives indicate that most of the displacement occurred between 88 and 78 m y ago no lithologic isotopic or structural data support previous assertions that these terranes were accreted to north america during the tertiary the late cretaceous synplutonic juxtaposition of the cucamonga san antonio and san gabriel terranes was followed by underthrusting of oceanic rocks of the baldy terrane that is pelona schist along the vincent thrust precise timing of this episode is as yet uncertain

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

deciphering cumulative fault slip vectors from fold scarps relationships between long term and coseismic deformations in central western taiwan we document the 30 ka cumulative slip history and long term slip vector azimuth on the northern chelungpu fault based on a series of fault bend folded alluvial terraces and draw quantitative relationships between geological structure deformation observed from the geomorphology and coseismic displacements during the 1999 mw 7 6 chi chi earthquake in our study area three main terrace levels show progressive folding by kink band migration in relation to the underlying fault geometry forming a main n s fold scarp up to 193 m high and secondary e w scarps detailed analysis using 5 m resolution digital elevation model allows us to characterize the scarp morphology and quantify the deformation parameters namely terrace heights fold scarp relief and fold limb width and slope angle the 3 d deformation of the highest terrace dated by optically stimulated luminescence at 30 2 4 0 ka enables to simultaneously determine amplitude and azimuth of the long term slip vector based on scarp relief the long term slip vector oriented n338 6 is found to parallel the chi chi coseismic displacements in this area cumulative slip and dating results yield a constant slip rate of 17 7 2 2 mm a in the direction n338 6 late quaternary shortening rates observed at four sites vary along strike in a similar way to chi chi coseismic displacements together with the collinearity of long term and coseismic slip vectors at our study site this suggests that chi chi earthquake is a characteristic earthquake for the chelungpu thrust with recurrence interval 470 years we also discuss implications for the regional and long term distribution of shortening in the central western foothills