distinctive triassic megaporphyritic monzogranite evidence for only 160 km offset along the san andreas fault southern california distinctive megaporphyritic bodies of monzogranite to quartz monzonite that occur in the mill creek region of the san bernardino mountains and across the san andreas fault on liebre mountain share identical modal and chemical compositions intrusive ages and petrogenesis and similar thermal histories both bodies are strontium rich and contain large potassium feldspar phenocrysts and hornblende u pb determinations on zircon from both bodies indicate triassic intrusive ages 215 ma and derivation in part from homogeneous precambrian continental crust u pb analyses on apatite and sphene and k ar analyses on hornblende and biotite show that the bodies suffered a late cretaceous thermal event 70 75 ma the strong similarities between the two bodies suggest that they constitute segments of a formerly continuous pluton that has been offset about 160 km by movement on the san andreas fault about 80 km less than the generally accepted distance plutons having monzonitic compositions reassembled with the megaporphyritic bodies are used as a piercing point form a relatively coherent province within the varied suite of mesozoic batholithic and prebatholithic rocks in southern california

kinematics of sand avalanches using particle image velocimetry abstract the avalanching of granular material at slope angles between the angle of repose and the angle of maximum stability such as on a sandpile or on the lee slip face of a sand dune typically produces droplet shaped flows that consist of a well defined head at the front we have performed the first experiments on granular materials using the particle image velocimetry piv technique in which we have studied avalanching flows of sand for different surface properties our results suggest that the presence of a deformable bed layer of loose or erodible particles on the surface is a necessary condition for the occurrence of these flows we measured for the first time the surface velocity field in avalanches propagating at angles between the angle of repose and the maximum angle of stability and observed a transition in the flow behavior with downstream distance from a surface like flow to a compressing flow that has the characteristics of a shock wave propagating through the deformable bed measurements for sand avalanches seem to indicate that the features of this transition depend on the inclination angle of the surface in the small range between these two critical angles the results suggest that grainflow laminae and scoria cone flanks for example are formed by flows that in their upper part propagate as a surface flow and in their lower part propagate as a shock wave near the surface of the debris pile

holocene rate of slip and tentative recurrence interval for large earthquakes on the san andreas fault cajon pass southern california abstract detailed mapping of the san andreas fault zone where it crosses cajon creek southern california has revealed a number of late quaternary deposits and geomorphological features offset by the fault radiocarbon dates from alluvial and swamp deposits provide a detailed chronology with which to characterize the activity of the san andreas fault four independent determinations of the slip rate on the san andreas fault yield an average rate of 24 5 3 5 mm yr for the past 14 400 years the similarity of the four values which span different intervals of time from 5900 to 14 400 years ago suggest that the slip rate has been constant during this period the slip rate confirms that the san andreas fault is accumulating slip faster than any other fault in the plate boundary throughout california also the sum of the slip rates on the san andreas 24 5 mm yr and the san jacinto 10 mm yr faults south of their junction is the same as the san andreas north of their junction 35 mm yr an excavation provided evidence for at least two and up to four earthquakes that caused rupture on the fault between 1290 and 1805 a d and tentative evidence for six earthquakes in about the last 1000 years both lines of evidence imply an average recurrence interval for large earthquakes of about 1 to 2 centuries combined with the historic record this investigation indicates that the last major earthquake at cajon creek was probably at the beginning or middle of the 18th century while the record at cajon creek may suggest that the next major earthquake on the san andreas fault is overdue the data can be interpreted to indicate that the recurrence interval for earthquakes on the san andreas south of the 1857 rupture is 3 to 4 centuries and that the anticipated event should not be expected until the next century

rapid deposition of sand and gravel mixtures downstream of a negative step the role of matrix infilling and particle overpassing in the process of bar front accretion an experimental flow channel was used to investigate the rapid deposition of sand and gravel mixtures downstream of a negative step hydraulic processes give rise to the quasi simultaneous accumulation of openwork gravels matrix filled contact framework and dilated framework gravels the resultant structure is analogous to a bar front in gravel bed streams flow conditions were non uniform but steady in time whereas open and closed gravel fabrics are usually interpreted as representing deposition from unsteady flows for example a varying discharge regime consideration of the percentage of sand in the deposits indicated that in terms of packing the sediments may be considered as a simple binary mixture with partial framework dilation occurring with sand percentages in excess of c 32 despite dilation for sand percentages less than 50 structural stability of the foreset gravels was maintained so that the bar front exhibited a distinct brinkpoint and a planar slip face at the angle of initial yield which made an angular contact with the base of the flume for sand percentages in excess of 80 individual gravel clasts were completely supported by sand the original structural stability was lost so that the bar front eroded by reverse currents assumed a tangential contact with the flume base below the angle of initial yield the brinkpoint was rounded and indistinct in addition to altering the bar front slope increasing the quantity of sand in the bed sediments had a major influence on the dip of gravel particles on the foresets in all experimental runs a surface sand seal with openwork gravels beneath developed upstream of the brinkpoint the mechanics of sand seal formation require further investigation however an overpassing mechanism was noted to be very important in sorting gravel particles rolling over the brinkpoint a published mathematical model describing the overpassing mechanism was found to be appropriate to the present experimental observations

analogue models of folds above a wrench fault abstract this paper describes the experimental deformation of models made with sheets of paraffin wax simulating a bedded cover resting on a basement wrench fault during the experiments en echelon folds appear in the cover as a result of early fault motion folds first appear at heterogeneities in the bedding and with axes at about 45 to the trace of the wrench fault further fault displacement causes a bulk rotation of fold axes towards parallelism with the basement wrench fault and a resulting curvature of fold axes at larger fault displacement folding affects an area which tends to quickly stabilize in width since folding weakens the sheared cover and subsequent deformation is concentrated in it axial surfaces of folds are initially upright then tend to become inclined with an external vergence forming a fan centered on the basement wrench fault deeper layer deformation close to the basement involves fold reorientations that are greater than in the upper layers therefore down a given vertical line there is no continuity between surface and deep structures the geometry and orientation of folds appearing at later stages of wrenching is controlled by the geometry and orientation of already extant folds

helium chronometry of apatite and titanite using nd yag laser heating we developed a technique for extracting 4he from apatite and titanite using laser heating that permits measurement of u and th and hence u th he ages on a single sample aliquot unlike direct laser heating quantitative he extraction and u th retention can be achieved when samples are indirectly laser heated to temperatures below their melting point resulting he ages are consistent with those obtained from conventional furnace heating of larger samples the major advantages of this technique include faster analysis and far lower more reproducible he blanks that permit single crystal analyses single crystal dating opens a variety of new avenues for u th he chronometry

sorting in grain flows at the lee side of dunes abstract sediment sorting at the lee side of ripples dunes and bars has already been recognized long ago a predictive model of the sorting is necessary but unavailable for implementation in sediment transport models for sediment mixtures relevant processes in sedimentological and physical literature are reviewed and compared to the sparse data of sediment sorting a synthesis is given of the most important variables governing the sorting processes for the benefit of future experimentation and modelling these variables are the sorting standard deviation skewness and bimodality of the sediment mixture delivered to the brink point the height of the dune or bar relative to the average grain size of the mixture the velocity of the flow above the brink point relative to the settling velocity for all grain size fractions and the frequency of the grain flows in addition the initiation mechanism and frequency of the grain flows affect the pattern and effectiveness of sorting

shear instability in a highly stratified estuary abstract shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary a field program involving high resolution velocity and density measurements as well as high frequency acoustic imagery allowed direct comparison of instantaneous richardson number distributions to the occurrence of shear instability the theoretical stability threshold of 0 25 is consistent with the measurements based on estimates of gradients that contain the mean as well as fluctuations due to internal waves an effective stability threshold based on mean gradients is found to be approximately one third reflecting a significant contribution of internal wave shear the integral effect of the mixing process is to homogenize the gradients of velocity and density producing linear profiles of these quantities across the pycnocline a turbulent prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin

hematite u th he thermochronometry constrains intraplate strike slip faulting on the kuh e faghan fault central iran abstract the kuh e faghan strike slip fault system kff located to the northern edge of the lut block in central iran developed through a neogene quaternary pulsed history of eastward fault propagation and fault related exhumation this system is a consequence of the residual stresses transmitted from the arabia eurasia convergent plate boundary here we integrate structural and textural analysis with new and previously published apatite fission track aft and apatite u th he apatite he results chlorite thermomentry and hematite u th he data from hematite coated brittle fault surfaces to constrain the timing of tectonic activity and refine patterns of late miocene pliocene burial and exhumation associated with the propagation of the kff twenty nine hematite u th he hematite he dates from three striated hematite coated slip surfaces from the kff fault core and damage zone yield individual dates from 12 2 ma petrographic analysis and chlorite thermometry of a polyphase fossil fluid system in the kff fault core document that fluid circulation and mineralization transitioned from a closed system characterized by pressure solution and calcite growth to an open system characterized by hot hydrothermal t 239 10 c fluids and hematite formation hematite microtextures and grain size analysis reveal primary and secondary syntectonic hematite fabrics no evidence of hematite comminution and similar hematite he closure temperatures 60 85 c in each sample integration of these results with thermal history modeling of aft and apatite he data shows that kff activity in the late miocene is characterized by an early stage of fault nucleation fluid circulation hematite mineralization and eastward propagation not associated with vertical movement that lasted from 12 to 7 ma hematite he aft and apatite he data track a second phase of fault system activity involving fault related exhumation initiating at 7 ma and continuing until present time our new data constrain the onset of the recognized late miocene pliocene tectonic reorganization in north central iran

two phase exhumation of the santa rosa mountains low and high angle normal faulting during initiation and evolution of the southern san andreas fault system low angle detachment fault systems are important elements of oblique divergent plate boundaries yet the role detachment faulting plays in development of such boundaries is poorly understood the west salton detachment fault wsdf is a major low angle normal fault that formed coeval with localization of the pacific north america plate boundary in the northern salton trough ca apatite u th he thermochronometry ahe n 29 samples and thermal history modeling of samples from the santa rosa mountains srm reveal that initial exhumation along the wsdf began at ca 8 ma exhuming footwall material from depths of 2 to 3 km an uplifted fossil miocene helium partial retention zone is present in the eastern srm while a deeper crustal section has been exhumed along the pleistocene high angle santa rosa fault sfr to much higher elevations in the southwest srm detachment related vertical exhumation rates in the srm were 0 15 0 36 km myr with maximum fault slip rates of 1 2 3 0 km myr miocene ahe isochrons across the srm are consistent with northeast crustal tilting of the srm block and suggest the post wsdf vertical exhumation rate along the srf was 1 3 km myr the timing of extension initiation in the salton trough suggests that clockwise rotation of relative plate motions that began at 8 ma is associated with initiation of the southern san andreas system pleistocene regional tectonic reorganization was contemporaneous with an abrupt transition from low to high angle faulting and indicates local fault geometry may at times exert a fundamental control on rock uplift rates along strike slip fault systems

fission track analysis of the late cenozoic vertical kinematics of continental pacific crust south island new zealand during the late cenozoic the pacific plate has been converging obliquely with the australia plate in south island new zealand a result of this convergence has been the growth of a major mountain range the southern alps at the leading edge of the pacific plate the results of fission track analysis of 140 samples from 13 transects across the alps reported here establish the late cenozoic vertical kinematics amount age and rate of rock uplift of the pacific crust underlying the alps the late cenozoic rock uplift of the pacific crust is asymmetrical with respect to the alpine fault being a maximum 19 km immediately east of the central part of the fault with lesser values at the eastern 3 km northern 10 km and southern 8 km extremities of the alps the age of the start of rock uplift varies spatially across the southern alps the earliest indications from fission track analysis being at 8 ma at the southern end of the alps decreasing to 5 ma at the northern end and 3 ma along the southeastern margin this age variation reflects the longer time over which the southern parts of the alps have been in collision the rate of propagation of rock uplift southeastward into the pacific plate has been 30 mm yr nearly 4 times the late cenozoic average rate of convergence normal to the plate boundary late cenozoic mean rock uplift rates range from a maximum of 2 8 mm yr at the alpine fault to a minimum of 1 0 mm yr in the east and have been sustained for periods of 3 8 m y accompanying denudation has exhumed amphibolite grade rocks immediately east of the alpine fault the rock uplift has been controlled by oblique slip displacement on the alpine fault a continental crustal section at least 19 km thick has been uplifted on the alpine fault comparison of the late cenozoic mean rock uplift rates with uplift rates derived from reset zircon data 2 10 mm yr near the alpine fault shows that uplift has accelerated over time but only significantly since 1 3 0 3 ma the amount of mesozoic uplift ranged from minimal amounts north of arthur s pass to 3 km near mount cook to 10 km in the south at lake wanaka

tidal computations for morecambe bay summary a numerical finite difference technique for computing tides in areas of shoaling water has been established and used to calculate the m2 tide in morecambe bay an essential feature of the method is the representation of drying banks which appear as the water level falls on the ebb and which are re submerged on the flood as the level rises again since the non linear advective acceleration should be important in shallow water schemes for incorporating these terms are examined and the influence of advection on the tide assessed in a series of tests the open sea tides applied to the outer boundary of the numerical bay model were taken first from existing tidal charts then from calculations with a larger scale encompassing model and finally from observations of current in each case the computed tidal elevations within the bay turned out to be in satisfactory agreement with those obtained from observations however it is significant that in the first case inaccuracy of the input tidal distribution deduced from charts gives rise to erroneous currents near the boundary

geochemistry of river borne clays entering the east china sea indicates two contrasting types of weathering and sediment transport processes the east china sea is characterized by wide continental shelf receiving a huge input of terrigenous matter from both large rivers and mountainous rivers which makes it an ideal natural laboratory for studying sediment source to sink transport processes this paper presents mineralogical and geochemical data of the clays and bulk sediments from the rivers entering the east china sea aiming to investigate the general driving mechanism of silicate weathering and sediment transport processes in east asian continental margin two types of river systems tectonically stable continental rivers and tectonically active mountainous rivers coexist in east asia as the direct weathering products clays can better reflect the silicate weathering regimes within the two river systems provenance rock types are not the dominant factor causing silicate weathering intensity difference existed in the east asian rivers the silicate weathering intensity of tectonically stable river basins is primarily driven by monsoon climate and the sediment transfer is relatively slow because of natural trapping process and increasing damming effect the geochemistry of these river borne sediments can thus indicate paleo weathering intensities in east asian continent in contrast silicate weathering intensity in tectonically active mountainous rivers is greatly limited by strong physical erosion despite the high temperature and highest monsoon rainfall the factors controlling silicate weathering in tectonically active catchments are complex and thus it should be prudent to use river sediment records to decipher paleoclimate change these two different silicate weathering regimes and sediment transport processes are manifestations of the landscape evolution and overall dominate the sedimentation in asian continental margin

formation and entrainment of fluid mud layers in troughs of subtidal dunes in an estuarine turbidity zone 1 the formation and entrainment of fluid mud layers in troughs of subtidal dunes were investigated in the weser estuary north sea germany based on hydroacoustic measurements near bed suspension layers were found to consist of a suspension of large mud flocs of variable concentration ranging from 25 g l below the lutocline to 70 g l at the river bed whereas the gelling concentration was below 70 g l sites of fluid mud formation coincided with the location of the estuarine turbidity zone during slack water on average near bed density gradients were initially observed in dune troughs 1 2 h before slack water and all fluid mud layers were entrained 2 3 h after slack water no shear instabilities occurred until 1 8 h after slack water while the flow was oriented in the dune direction rapid entrainment was related to the development of the turbulent flow field behind dunes and is explained to be induced by advection of strong turbulence during accelerating currents fluid mud layers in dune troughs were entrained at an earlier point in time after slack water compared to adjacent layers formed on a comparatively flat bed where dune crests did not protrude from the lutocline

fluidization of mud in estuaries the south alligator river located in the northern territory australia is a macrotidal estuary with suspended sediment concentration values reaching 10 g 1 1 in september 1986 in the dry season the estuary was well mixed in temperature and salinity while the vertical gradients in suspended sediment concentration were small at flood tides for most of the ebb tide duration a lutocline separated a clear upper layer from an extremely turbid bottom layer both layers being of comparable thickness the tidal evolution of the suspended sediment concentration is consistent with that computed by a numerical model based on the equation of conservation of mass of suspended sediment in this model sediment is entrained from the bottom and mixed vertically upward by eddy diffusion but through a richardson number dependence sediment induced buoyancy effects inhibit vertical mixing the final depth of the turbid layer can be readily estimated analytically as a result of a balance between the rate of kinetic energy input and the buoyancy flux determined by the particle fall velocity the presence of a lutocline helps form mud banks on the inner side of a meander

near field response to transpression along the southern san andreas fault based on exhumation of the northern san gabriel mountains southern california 1 near field mountain building is common along transpressive faults but how vertical deformation relates to oblique plate motion versus local structural complexity has not been universally established we have investigated the mechanisms of exhumation along the southern san andreas fault in the northern san gabriel mountains to examine the effects of pure shear dominated transpression obliquity 20 although rugged topography suggests near field uplift low temperature thermochronometry indicates minimal exhumation apatite u th he ages are 10 ma along narrow ridges within the fault zone implying near field rock uplift is not pervasive along this oblique plate boundary ages farther from the fault are younger 5 ma and suggest a deformation event occurred during the transition of activity from the san gabriel fault to the modern san andreas strand these results reveal a heterogeneous crustal response to transpression and imply that secondary structures or geometrical complexities play a primary role in near field bedrock uplift along transpressive faults

late cenozoic geomorphic and tectonic evolution of the patagonian andes between latitudes 42 s and 46 s an appraisal based on fission track results from the transpressional intra arc liquine ofqui fault zone fission track ft thermochronology has been applied to investigate the low temperature cooling and denudation history of the patagonian andes along the southern part of the intra arc transpressional liquine ofqui fault zone between 42 and 46 s the liquine ofqui fault is shown to have been the focus of enhanced cooling and denudation initiated between ca 16 and 10 ma several fault blocks with different cooling histories are identified these are separated by major oblique or reverse slip faults proposed to form the eastern part of a major crustal scale dextral transpression zone local very fast rates of cooling and denudation between ca 7 and 2 ma were coeval with collision of the chile rise an active mid oceanic ridge with the peru chile trench between 47 and 48 s this location is close to the southern termination of the liquine ofqui fault implying that the collision of the ridge was a major force driving late cenozoic transpression the lack of significant cooling and denudation before ca 16 ma is indicative of pure strike slip or transtensional movement along the liquine ofqui fault before the collision of the ridge digital landscape analysis supports glacial and periglacial erosion as the main contributor to denudation since ca 7 ma leading to restriction of topographic development the combination of transpression induced rock uplift and glacial erosion is shown to be very effective at causing localized denudation anomalously young ft ages along the liquine ofqui fault are attributed to the existence of a late cenozoic localized heat flow anomaly along the fault

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

a magnetostratigraphic study of the sediments of the ridge basin southern california and its tectonic and sedimentologic implications a magnetic polarity zonation has been established for the upper castaic formation and lower ridge route and peace valley formations of the ridge basin of the central transverse ranges of southern california the zonation is based on the analysis of over 1900 samples from 669 sites in the 5800 m thick sedimentary sequence rock magnetic studies provide evidence that the carrier of the primary remanence is magnetite and that the magnetization was acquired at or shortly after deposition of the sediment although many samples contain a secondary normal overprint systematic analysis of the changes in direction upon demagnetization leads to an accurate determination of the original polarity thirteen polarity intervals are recognized on the basis of biostratigraphic constraints imposed by a marine invertebrate faunaat the base of the sequence and a terrestrial vertebrate fauna at the top the magnetic polarity zonation can be correlated to the magnetic polarity time scale the studied interval extends from the epoch 5 epoch 6 boundary to the middle of epoch 8 polarity changes within the studied interval represent chronostratigraphic horizons which can be used to analyze tectonic and sedimentologic processes which are recorded in the sediments of the ridge basin the locus of the tectonic activity within the basin has been dated as occurring on the clearwater fault from 8 1 to 7 8 m y ago on the liebre faultfrom 7 3 to 6 1 m y ago and on the north liebre fault from 6 0 to 5 0 m y ago during this entire period of time the san gabriel fault on the western margin of the basin was continuously active and a period of particularly vigorous activity has been dated at 7 3 to 7 0 m y the mean declinations of samples from the normal magnetozones provide no evidence for the existence of a major rotation of the ridge basin subsequent to about 8 5 m y ago sedimentation rates for interbedded sandstone and shale units within the basin have been separately determined as 0 2 m 1000 years and 3 0 m 1000 years respectively the water content of certain deformed structures within the sequence has been determined to have been 35 and 40 at the time of deformation

the motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike slip faults geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere if the dominant forces acting on crustal blocks are tractions at their bases then the long term motion of each block will be given by the average velocity of the underlying lithosphere slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the south island of new zealand and southern california are in good agreement with slip rates estimated geologically

neogene tectonic rotation of the san gabriel region california suggested by paleomagnetic vectors previous paleomagnetic work in the western transverse ranges and the islands offshore of california has suggested that these regions have experienced large clockwise rotations since middle miocene time this study is aimed at testing how far east these rotations extend into the continent neogene volcanic rocks from the region bounded by the san gabriel and san andreas faults were selected for paleomagnetic study the sampled units are lavas from the late oligocene to early miocene continental vasquez formation and water laid tuff beds from the middle to late miocene mint canyon formation these rocks possess a stable remanent magnetization which was acquired soon after deposition and is believed to be primary seventy four lava flows from eleven locations in the vasquez formation were studied the average direction for seven locations near the center of the study area suggests that this area has rotated a net clockwise amount of 37 1 12 2 since the early miocene there is no suggestion of north or south translation a location west of the central region is rotated almost 90 clockwise although this is believed to be a local effect detailed magnetic stratigraphy at the western and central location can be correlated over 6 km locations in the east near the san andreas fault yielded highly dispersed directions which cannot now be interpreted in terms of rotation the mean paleomagnetic direction from six sites four different beds in the mint canyon formation west of the vasquez study area is deflected in a counterclockwise sense by 16 30 these data are consistent with a proposed model in which the san gabriel region first was rotated clockwise 53 37 16 in the early miocene and later rotated 16 counterclockwise as it slid northward into the leftward bend region of the san andreas fault the clockwise rotation is believed to be due to dextral shear within the pacific north american plate shear zone counterclockwise rotation in the mojave region is believed to be responsible for the bend in the san andreas fault

architecture of transpressional thrust faulting in the san bernardino mountains southern california from deformation of a deeply weathered surface to investigate the architecture of transpressional deformation and its long term relationship to plate motion in southern california we have studied the deformation pattern and structural geometry of orogeny within the san andreas fault system the san bernardino mountains have formed recently at the hub of several active structures that intersect the san andreas fault east of los angeles this mountain range consists of a group of crystalline blocks that have risen in association with transpressive plate motion along both high and low angle faults of a complex structural array we have used a deeply weathered erosion surface as a structural datum to constrain the pattern of vertical deformation across fault blocks in and adjacent to this mountain range by subtracting the hanging wall and footwall positions of this preuplift horizon we have determined vertical displacement along two major thrust faults we conclude that one fault the north frontal thrust has played a more significant role in raising the large fault blocks and can explain the uplift of all but a few crustal slivers on the basis of the pattern of displacement associated with this thrust fault we have also inferred fault zone geometry beneath the range rather than simply steepening into a high angle fault zone or flattening into a decollement the thrust fault may have a complex curviplanar geometry the pattern of rock uplift also enables us to calculate the total motion accommodated by this orogeny we estimate that 6 km of convergence 5 of the total plate motion in the last 2 myr has occurred this horizontal shortening is associated spatially with the 15 km wide restraining bend in the san andreas fault zone near san gorgonio pass the entire range may thus have risen because of a small geometric complexity in the san andreas fault rather than the obliquity of far field plate motion

insights from low temperature thermochronometry into transpressional deformation and crustal exhumation along the san andreas fault in the western transverse ranges california 1 the san emigdio mountains are an example of an archetypical transpressional structural system bounded to the south by the san andreas strike slip fault and to the north by the active wheeler ridge thrust apatite u th he and apatite and zircon fission track ages were obtained along transects across the range and from wells in and to the north of the range apatite u th he ages are 4 6 ma adjacent to the san andreas fault and both u th he and fission track ages grow older with distance to the north from the san andreas the young ages north of the san andreas fault contrast with early miocene u th he ages from mount pinos on the south side of the fault restoration of sample paleodepths in the san emigdio mountains using a regional unconformity at the base of the eocene tejon formation indicates that the san emigdio mountains represent a crustal fragment that has been exhumed more than 5 km along the san andreas fault since late miocene time marked differences in the timing and rate of exhumation between the northern and southern sides of the san andreas fault are difficult to reconcile with existing structural models of the western transverse ranges as a thin skinned thrust system instead these results suggest that rheologic heterogeneities may play a role in localizing deformation along the big bend of the san andreas fault as the san emigdio mountains are compressed between the crystalline basement of mount pinos and oceanic crust that underlies the southern san joaquin valley

evolution of the northern santa cruz mountains by advection of crust past a san andreas fault bend the late quaternary marine terraces near santa cruz california reflect uplift associated with the nearby restraining bend on the san andreas fault excellent correspondence of the coseismic vertical displacement field caused by the 17 october 1989 magnitude 7 1 loma prieta earthquake and the present elevations of these terraces allows calculation of maximum long term uplift rates 1 to 2 kilometers west of the san andreas fault of 0 8 millimeters per year over several million years this uplift in concert with the right lateral translation of the resulting topography and with continual attack by geomorphic processes can account for the general topography of the northern santa cruz mountains

observation and modeling of the storm induced fluid mud dynamics in a muddy estuarine navigational channel abstract observations of storm induced fluid mud dynamics have been conducted at the north passage deepwater navigational channel dnc of the yangtze estuary in october to december 2010 during the occurrence of a cold air front the measurement data reveal that just after the critical wind wave event a large amount of fine sediment was trapped in a state of fluid mud along the channel the observed thickness of the fluid mud was up to about 1 5 m which caused some significant economic and safety problems for shipping traffic in the yangtze delta area the mechanisms and transport processes of the storm induced fluid mud are analyzed and presented from the angles of both process oriented and engineering oriented methods with the help of hydrodynamics and wave modeling it could be inferred that the behavior of the storm induced fluid mud event mainly depends on the overall hydrodynamic regimes and the exchanges of sediment which is released by storm wave agitation from adjacent tidal flats these sediments are accumulated as fluid mud and subsequently oscillate and persist at those locations with weaker longitudinal residuals in the river and tide dominated estuary in addition the downslope transport of fluid mud is also thought to have stimulated and worsened the fluid mud event observed in this study our modeling results and observations demonstrate that 1 the transport of fluid mud is an advective phenomenon determining the central position of fluid mud layer along the channel and it s also a tidal energy influenced phenomenon controlling the erosion and accumulation of fluid mud and 2 both suspended particulate matter availability and local residual flow regime are of critical importance in determining the trapping probability of sediment and the occurrence of fluid mud

transpression along the southern san andreas fault durmid hill california transpression is expressed at durmid hill in the development and subsequent rotation of en echelon folds and ongoing vertical uplift the transpression related strain field can be quantified by an analysis of 1 orientation of rotated fold axes which display an angle of about 20 between the san andreas fault and the fold axial traces 2 shortening perpendicular to the fold axes 40 60 3 extension parallel to the fold axes 58 68 measured using a boudinaged ash bed and 4 uplift determined from an antecedent stream 1 1 1 7 mm yr the topographic expression of durmid hill and the exposed stratigraphy the observed deformation may be approximated by a homogeneous transpressional deformation this model suggests that an estimated 9 14 of the total displacement predicted by plate motion models across the san andreas fault has been accommodated by the durmid hill deformation zone adjacent to the fault this off fault deformation is not directly coupled to coseismic or interseismic movement on the fault itself

one dimensional ocean surface layer modeling a problem and a solution abstract the first part of this paper is generic it demonstrates a problem associated with one dimensional ocean surface layer model comparisons with ocean observations unlike three dimensional simulations or the real ocean kinetic energy can inexorably build up in one dimensional simulations which artificially enhances mixing adding a sink term to the momentum equations counteracts this behavior the sink term is a surrogate for energy divergence available to three dimensional models but not to one dimensional models the remainder of the paper deals with the mellor yamada boundary layer model there exists prior evidence that the model s summertime surface temperatures are too warm due to overly shallow mixed layer depths if one adds a sink term to approximate three dimensional model behavior the warming problem is exacerbated creating added incentive to seek an appropriate model change guided by laboratory data a richardson number dependent dissipation is introduced and this simple modificat

interseismic strain accumulation and anthropogenic motion in metropolitan los angeles 1 we use global positioning system gps geodesy and synthetic aperture radar sar interferometry to distinguish between interseismic strain accumulation and anthropogenic motion in metropolitan los angeles we establish a relationship between horizontal and vertical seasonal oscillations of the santa ana aquifer use this relationship to infer cumulative horizontal anthropogenic motions from cumulative vertical motions caused by water and oil resource management and estimate horizontal interseismic velocities corrected for anthropogenic effects vertical anthropogenic rates from 1992 to 1999 are slower than 3 mm yr 1 in the santa ana and san gabriel aquifers and faster than 5 mm yr 1 in the chino aquifer and in many oil fields inferred horizontal anthropogenic velocities are faster than 1 mm yr 1 at 18 of 46 gps sites northern metropolitan los angeles is contracting with the 25 km south of the san gabriel mountains shortening at 4 5 1 mm yr 1 95 confidence limits the thrust fault in an elastic edge dislocation model of the observed strain is creeping at 9 2 mm yr 1 beneath and north of a position 6 2 km deep and 8 8 km north of downtown los angeles the model fault is near the los angeles segment of the puente hills thrust but south of the sante fe springs segment of the thrust disagreement between the 6 km locking depth in the model and the 15 km seismogenic depth inferred from earthquakes suggests that the elastic continuum model may be unsatisfactory models with different stiffnesses of sedimentary basin and crystalline basement must be investigated

fault systems of the 1971 san fernando and 1994 northridge earthquakes southern california relocated aftershocks and seismic images from larse ii we have constructed a composite image of the fault systems of the m 6 7 san fernando 1971 and northridge 1994 california earthquakes using industry reflection and oil test well data in the upper few kilometers of the crust relocated aftershocks in the seismogenic crust and larse ii los angeles region seismic experiment phase ii reflection data in the middle and lower crust in this image the san fernando fault system appears to consist of a decollement that extends 50 km northward at a dip of 25 from near the surface at the northridge hills fault in the northern san fernando valley to the san andreas fault in the middle to lower crust it follows a prominent aseismic reflective zone below and northward of the main shock hypocenter interpreted upward splays off this decollement include the mission hills and san gabriel faults and the two main rupture planes of the san fernando earthquake which appear to divide the hanging wall into shingle or wedge like blocks in contrast the fault system for the northridge earthquake appears simple at least east of the larse ii transect consisting of a fault that extends 20 km southward at a dip of 33 from 7 km depth beneath the santa susana mountains where it abuts the interpreted san fernando decollement to 20 km depth beneath the santa monica mountains it follows a weak aseismic reflective zone below and southward of the main shock hypocenter the middle crustal reflective zone along the interpreted san fernando decollement appears similar to a reflective zone imaged beneath the san gabriel mountains along the larse i transect to the east in that it appears to connect major reverse or thrust faults in the los angeles region to the san andreas fault however it differs in having a moderate versus a gentle dip and in containing no mid crustal bright reflections

transpression and the build up of the cordillera the example of the bucaramanga fault eastern cordillera colombia widespread wrench tectonics have been described along the northern andes the bucaramanga fault described as sinistral strike slip bounds the high santander massif we combine structural and thermochronological data from the central southern portion of the fault to investigate the vertical displacement the structural survey data show old phases of activity preserved in the host rocks along the fault trace with the superimposition of different generations of slickenlines and both strike slip and dip slip kinematics indicators new and previous thermochronological data show that differential exhumation of the fault walls has been ongoing for the last 50 ma the hanging wall the santander massif records in the central portion decreasing exhumation rates from the early miocene to the mid to late miocene and in the southern portion constant rates from the late oligocene to the pliocene combining such observations the thermochronological offset resulting from the relative motion of the two fault walls is comparable with the observed drop in elevation across the fault suggesting that the present topography of the santander massif is related to vertical movement along the bucaramanga fault we infer that the fault has a significant neogene reverse component consistent with the present day horizontal global positioning system vector data long term exhumation rates and the structural assemblage supplementary material the supplementary materials contain methodological information on the thermochronological procedures a compilation of the previously published data used in this study and the structural geology dataset https doi org 10 6084 m9 figshare c 4620140

modified dynamic cptu penetrometer for fluid mud detection one particularly complex phenomenon is the episodic tidally driven variation of navigable depth level as a result of fluid mud settlement this paper presents results from dynamic cone penetration testing with pore pressure measurement cptu as a nonacoustical direct device to support surveying and management of these areas the new technique is modular and uses a disk configuration for fluid mud detection both disk resistance and pore pressure measurements accurately identify suspended matter concentrations of 90 g l or more and the transition from fluid mud to consolidating mud once concentrations exceed 150 g l hence the procedure attests the potential for rapid reliable assessment of a fluid mud layer and concurrent characterization of the underlying consolidated sediment by monitoring the pore pressure and strength changes during penetration

role of lithospheric strength heterogeneities in the tectonics of tibet and neighbouring regions unlike rigid oceanic plates continents show diffuse deformation for example the tibetan plateau that has accommodated much of the convergence of india with asia we interpret the tertiary deformation of asia as the three dimensional strain of a non linear viscous continental lithosphere lateral strength contrasts modify this deformation stress concentration around the relatively strong tarim basin gives rise to local crustal thickening in the tien shan to the north and to strike slip in the altyn tagh along its southern margin

near field transpressive deformation along the san andreas fault zone in southern california based on exhumation constrained by u th he dating low temperature thermochronometry reveals that a narrow crustal sliver trapped within strands of the san andreas fault zone in southern california has experienced recent rapid exhumation eight apatite u th he ages from a 1 km relief section along yucaipa ridge in the san bernardino mountains range from 1 4 to 1 7 ma the minimal change in age with elevation implies exhumation of 5 7 mm yr 1 sustained for at least several hundred thousand years three titanite helium ages from the ridge are much older ranging from 57 to 82 ma these show a steep gradient with elevation representing either an exhumed partial retention zone or slow cooling through much of the tertiary these data imply that a total exhumation of 3 to 6 km has occurred since 1 8 ma it is uncertain whether this exhumation terminated as early as 1 ma or has continued up to the present at a decelerated rate we surmise that this exhumation represents rock uplift in the absence of major surface uplift in that it kept pace with tectonic uplift as the narrow fault block maintained steady state relief the record of sedimentation in adjacent basins is consistent with the implied magnitude of erosion such rapid large magnitude exhumation within the strands of the san andreas fault zone is important for models of transpressional tectonics it is consistent with a strain partitioning model which predicts that pure shear dominated fault zones experience significant vertical strain however it is inconsistent with a stress partitioning model which predicts that fault zone weakness limits pure shear deformation to the borderlands of the master strike slip fault in addition a concentration of secondary contraction within the fault zone may require modification of coupling models between strong upper mantle and brittle upper crust via the weak lower crust these models predict that transpressional deformation will either be uniformly distributed across the plate boundary or be limited to the far field borderlands rather than concentrated in the near field alternatively the exhumation of yucaipa ridge may have been driven by the nearby restraining bend in the san andreas fault at san gorgonio pass in which case it represents local fault geometry rather than accommodation of far field plate motion

four dimensional transform fault processes progressive evolution of step overs and bends many bends or step overs along strike slip faults may evolve by propagation of the strike slip fault on one side of the structure and progressive shut off of the strike slip fault on the other side in such a process new transverse structures form and the bend or step over region migrates with respect to materials that were once affected by it this process is the progressive asymmetric development of a strike slip duplex consequences of this type of step over evolution include 1 the amount of structural relief in the restraining step over or bend region is less than expected 2 pull apart basin deposits are left outside of the active basin and 3 local tectonic inversion occurs that is not linked to regional plate boundary kinematic changes this type of evolution of step overs and bends may be common along the dextral san andreas fault system of california we present evidence at different scales for the evolution of bends and step overs along this fault system examples of pull apart basin deposits related to migrating releasing right bends or step overs are the pliopleistocene merced formation tens of km along strike the pleistocene olema creek formation several km along strike along the san andreas fault in the san francisco bay area and an inverted colluvial graben exposed in a paleoseismic trench across the miller creek fault meters to tens of meters along strike in the eastern san francisco bay area examples of migrating restraining bends or step overs include the transfer of slip from the calaveras to hayward fault and the greenville to the concord fault ten km or more along strike the offshore san gregorio fold and thrust belt 40 km along strike and the progressive transfer of slip from the eastern faults of the san andreas system to the migrating mendocino triple junction over 150 km along strike similar 4d evolution may characterize the evolution of other regions in the world including the dead sea pull apart the gulf of paria pull apart basin of northern venezuela and the hanmer and dagg basins of new zealand d 2004 elsevier b v all rights reserved

paleomagnetic measurement of nonbrittle coseismic deformation across the san andreas fault at pallett creek paleomagnetic data have been obtained to address a problem at the pallett creek paleoseismological site the 9 mm yr slip rate determined from three dimensional mapping of late holocene offsets across discrete faults is only a quarter of the expected value we suspected that nonbrittle deformation adjacent to the faults might account for the 26 mm yr discrepancy in our search for the missing slip we collected and analyzed 264 paleomagnetic samples from a 53 m wide transect across the fault zone half the samples came from a unit deposited immediately after a large earthquake of about a d 1480 these samples were affected by two large earthquakes that involved rupture at the site in 1812 and 1857 we collected the other half of the samples from a slightly older bed one that was deposited before the earthquake of about a d 1480 relative to control groups composed of 10 samples and collected 50 m from the fault samples closer to the fault display clockwise rotations of 30 or less if interpreted as block rotations the data from the older unit imply that it has sustained a total of 14 0 2 9 m of dextral warp during the past three major earthquakes and that the younger unit has experienced a total of 8 5 1 0 m of warp during the most recent two combining these values with the amounts of dextral slip across the mapped fault planes yields dextral offsets of 5 5 6 25 and 6 25 m for the events of a d 1480 1812 and 1857 and a slip rate of 35 6 6 7 mm yr this slip rate averaged over the past three complete seismic cycles is consistent with published rates from other sites offsets associated with the past three events are remarkably similar these amounts however appear independent of the length of interseismic cycles these observations suggest 1 that this part of the san andreas fault has a characteristic strength and 2 that conventional concepts of strain accumulation and relief for example time and slip predictable models of earthquake occurrence are unrealistic

the key role of fluvial dunes in transport and deposition of sand gravel mixtures a preliminary note abstract sediment transport and deposition in sand gravel bed rivers is shown to depend on dunes the sediment mixture is vertically sorted in avalanches at the lee side of the dunes part of the resulting upward fining sets of the largest dunes that occur during a discharge wave is preserved in the bed furthermore the sediment is entrained and deposited size selectively in the dune troughs which results in an additional upward fining accumulation of lag deposits this deposit is the source for sediment entrained during the next discharge wave which will depend on the relict vertical sorting and on the depth from which it is entrained the entrainment and deposition depth of the sediment depends on the dune trough level below the average bed level and therefore on the dune height thus subsequent discharge waves of decreasing magnitude will leave the upward fining cross bedded sets at depths below the bed surface related to the concurrent dune height this is demonstrated with flume experiments and with new vibrocores collected from the river waal the netherlands

mountain building strike parallel motion and mantle anisotropy abstract geophysical and geological evidence indicates that tectonic movements parallel to the strike of orogenic belts and suture zones are the most important mode of displacement during continental collision and oblique subduction several hundred to a few thousand kilometers of displacement may have occurred this way resulting in a pervasive ductile deformation of the lithosphere consequently both crustal and mantle fabrics may reflect these movements and their study using both geological and geophysical methods could supply important clues to lithosphere behavior during mountain building on the basis of kinematic analysis of ductilely deformed crustal rocks in zones of movement a variety of tectonic models have been proposed which differ in 1 1 the timing of strike parallel motions early before or during continental accretion or late after accretion was completed 2 2 the geometric pattern of strike parallel displacement zones low angle or high angle and 3 3 the location of orogen parallel movement zones in outboard terranes on the subducting plate along the suture zone or within the continental hangingwall large scale displacements parallel to mountain belts also suggest that they occurred along lithospheric faults rooted in the upper mantle although few studies have been carried out from this view point thermal structural and petrological data support the conclusion that lithospheric faults involving the upper mantle may be discriminated from crustal faults rooted in some crustal decoupling zone where available data on mantle flow beneath orogenic belts are in agreement with mountain parallel displacements inferred from crustal studies this has an important bearing on the seismic anisotropy pattern which can be predicted in the lithospheric mantle below mountain belts the fast velocity direction which is parallel to the flow direction should therefore be recorded parallel to the strike of the belt

settling behaviour of concentrated dispersions and some volcanological applications abstract experiments have been carried out on the settling of non aggregating poorly sorted aqueous dispersions laced with lowdensity particles in order to gain insight into the sedimentation behaviour of volcaniclastic flows which contain pumice two poorly sorted dispersions of silicon carbide particles with continuous size distributions were studied plastic beads were added as a light component experiments were carried out at solids concentrations between 10 and 60 vol dispersions of low concentration generated normally graded deposits in which particles of different densities were in approximate hydraulic equivalence above a critical concentration 45 to 60 depending on the grain size distribution particle segregation was suppressed and the dispersion settled to form a virtually ungraded bed hindered settling at intermediate concentrations generated normally graded deposits with elutriation pipes floating rafts of plastic and capping layers of fines elutriated by escaping pore fluid buoyancy forces resulted in strong hydraulic inequivalence between light and dense components in the deposits from intermediate concentrations two mechanisms of elutriation pipe growth were observed at intermediate concentrations pipes grew progressively upwards in the deposit as the latter aggraded whereas at high concentrations they nucleated and grew uniformly throughout the whole dispersion elutriation pipes form readily during sedimentation of some poorly sorted aqueous dispersions the presence of elutriation pipes cannot be used alone to distinguish between deposits of epiclastic and pyroclastic flows reverse grading of pumice pumice rafts elutriation pipes and layers of elutriated fines should characterise deposits from pumice rich submarine mass flows despite differences in the fluid phase the experiments also have implications for fluidization mechanisms in pyroclastic flows hindered settling of a concentrated poorly sorted dispersion is a type of self fluidization in which the sedimentation of large particles causes a sufficiently strong vertical fluid flux to fluidize or elutriate smaller ones hindered settling is intrinsic to all pyroclastic flows as they spread out and deflate across the landscape and may reinforce other gas sources as an effective fluidization mechanism during emplacement of type 2 and type 3 ignimbrites and of coignimbrite lag breccias

uplift and erosion of the san bernardino mountains associated with transpression along the san andreas fault california as constrained by radiogenic helium thermochronometry apatite helium thermochronometry provides new constraints on the tectonic history of a recently uplifted crystalline mass adjacent to the san andreas fault by documenting aspects of the low temperature 40 100 c thermal history of the tectonic blocks of the san bernardino mountains in southern california we have placed new constraints on the magnitude and timing of uplift old helium ages 64 21 ma from the large big bear plateau predate the recent uplift of the range and show that only several kilometers of exhumation has taken place since the late cretaceous period these ages imply that the surface of the plateau may have been exposed in the late miocene and was uplifted only 1 km above the mojave desert in the last few myr by thrusting on the north and south a similar range in helium ages 56 14 ma from the higher san gorgonio block to the south suggests that its crest was once contiguous with that of the big bear block and that its greater elevation represents a localized uplift that the big bear plateau did not experience the structure of the san gorgonio block appears to be a gentle antiform based on the geometry of helium isochrons and geologic constraints young ages 0 7 1 6 ma from crustal slices within the san andreas fault zone indicate uplift of a greater magnitude than blocks to the north these smaller blocks probably experienced 3 4 km of uplift at rates 1 5 mm yr in the past few myr and would stand 2 5 km higher than the big bear plateau if erosion had not occurred the greater uplift of tectonic blocks adjacent to and within the san andreas fault zone is more likely the result of oblique displacement along high angle faults than motion along the thrust fault that bounds the north side of the range we speculate that this uplift is the result of convergence and slip partitioning associated with local geometric complexities along this strike slip system transpression thus appears to have been accommodated by both vertical displacement within the san andreas fault zone and thrusting on adjacent structures

critical shear stress of natural sediments the critical shear stress of individual fractions ci in unimodal and weakly bimodal sediments shows little variation with grain size and depends only on the mean grain size bf the mixture for strongly bimodal sediments ci increases with grain size an apparent result of a lateral segregation of the finer and coarser fractions on the bed surface that causes ci to deviate from size independence in the direction of unisize shields values a quantitative definition of mixture bimodality may be used to estimate the degree of mixture bimodality beyond which a substantial size dependence of ci is observed and to predict the variation of ci with grain size in bimodal sediments because properties of sediment grain size distributions other than bimodality appear to have little influence on ci the trends presented here for 14 unimodal and bimodal sediments may be quite general

late cenozoic uplift of denali and its relation to relative plate motion and fault morphology apatite fission track analysis of samples that cover a 4 kilometer vertical section from the western flank of denali mount mckinley north america9s highest mountain suggests that the mountain massif was formed by rapid uplift 1 kilometer per million years beginning 6 million years ago ma uplift was a result of the morphology of the denali fault and a change in motion of the pacific plate with respect to north america at 5 ma which created opposing tangential vectors of relative movement along the fault and forced the intervening crustal blocks upward

clockwise rotation of the western mojave desert paleomagnetic results from miocene volcanic rocks in the western mojave desert suggest about 25 of clockwise rotation for this region twelve sites 110 oriented cores were collected from the 20 ma saddleback basalt and seven sites 56 oriented cores from the red buttes quartz basalt around boron california structural corrections were made from the attitude of the basalt or the tropico group sedimentary rocks in which the sampled basalts are interbedded paleomagnetic directions are coincident for the two basalts and show both normal and reversed polarity after demagnetization to 40 or 60 mt application of structural corrections and inversion of reversed sites the data yield an average direction of i 51 6 and d 15 6 k 19 95 8 2 comparison with the expected direction for miocene rocks for stable north america shows a clockwise rotation of 23 8 11 3 and a flattening of 2 1 7 4 clockwise rotation of the western mojave agrees in direction with oroclinal bending of the southern sierra nevada due to right lateral shear along the western margin of north america most of this rotation is constrained by other paleomagnetic and structural information to have occurred soon after the sampled basalts were deposited about 20 ma and before about 16 ma these clockwise declination anomalies also indicate that any subsequent counterclockwise rotation of the mojave is small and or compensated by previous clockwise rotation

rapid exhumation of cretaceous arc rocks along the blue mountains restraining bend of the enriquillo plantain garden fault jamaica using thermochronometry from multiple closure systems abstract the effect of rapid erosion on kinematic partitioning along transpressional plate margins is not well understood particularly in highly erosive climates the blue mountains restraining bend bmrb of eastern jamaica bound to the south by the left lateral enriquillo plantain garden fault epgf offers an opportunity to test the effects of highly erosive climatic conditions on a 30 km wide restraining bend system no previous thermochronometric data exists in jamaica to describe the spatial or temporal pattern of rock uplift and how oblique 20 plate motion is partitioned into vertical strain to define the exhumation history we measured apatite n 10 and zircon n 6 u th he ages 40ar 39ar n 2 amphibole and k spar ages and u pb zircon n 2 crystallization ages late cretaceous u pb and 40ar 39ar ages 74 68 ma indicate rapid cooling following shallow emplacement of plutons during north south subduction along the great caribbean arc early to middle miocene zircon helium ages 19 14 ma along a vertical transect suggest exhumation and island emergence at 0 2 mm yr older zircon ages 10 15 km to the north 44 35 ma imply less rock uplift apatite helium ages are young 6 1 ma across the entire orogen suggesting rapid exhumation of the bmrb since the late miocene these constraints are consistent with previous reports of restraining bend formation and early emergence of eastern jamaica an age elevation relationship from a vertical transect implies an exhumation rate of 0 8 mm yr while calculated closure depths and thermal modeling suggests exhumation as rapid as 2 mm yr the rapid rock uplift rates in jamaica are comparable to the most intense transpressive zones worldwide despite the relatively slow 5 7 mm yr strike slip rate we hypothesize highly erosive conditions in jamaica enable a higher fraction of plate motion to be accommodated by vertical deformation thus strike slip restraining bends may evolve differently depending on erosivity and local climate

crustal structure and tectonics from the los angeles basin to the mojave desert southern california a seismic refraction and low fold reflection survey known as the los angeles region seismic experiment larse was conducted along a transect line 1 extending from seal beach california to the mojave desert crossing the los angeles and san gabriel valley basins and san gabriel mountains the chief result of this survey is an interpreted cross section that addresses a number of questions regarding the crustal structure and tectonics of southern california that have been debated for decades and have important implications for earthquake hazard assessment the results or constraints are as follows 1 the maximum depth of the los angeles basin along line 1 is 8 9 km 2 the deep structure of the sierra madre fault zone in the northern san gabriel valley is as follows the duarte branch of the sierra madre fault zone forms a buried 2 5 km high moderately north dipping buttress between the sedimentary and volcanic rocks of the san gabriel valley and the igneous and metamorphic rocks of the san gabriel mountains for deeper structure see following 3 there are active crustal decollements in southern california at middle crustal depths the sierra madre fault zone appears to sole into a master decollement that terminates northward at the san andreas fault and projects southward beneath the san gabriel valley to the puente hills blind thrust fault 4 the dip and depth extent of the san andreas fault along line 1 dips steeply 83 northward and extends to at least the moho 5 the subsurface lateral extent of the pelona schist in southern california is as follows along line 1 the pelona schist underlies much if not all of the san gabriel mountains south of the san andreas fault to middle crustal depths north of the san andreas fault it is apparently not present along the transect

an experimental study of subaquaeous slipface deposition abstract a flume study indicates that grainflow on slipfaces accounts for most cross strata formed in unidirectional shallow water flows the slipfaces studied were on small megaripples and delta like steps 0 06 0 28 m high during intermittent avalanching at relatively low flow velocities periods between avalanches were marked by grainfall onto the slipface the intensity of which was greatest near the brink of the slipface and increased with current velocity nearly all grainfall deposits however were incorporated into subsequent grainflows grain flow cross strata were made up of relatively distinct layers at least near the base of the slipface continuous avalanching at high flow velocity was marked by a steady stream of grains forming more poorly defined cross strata although the fundamental cause of grain flow is the gradual buildup of sediment on the upper slipface to the angle of initial yield four other processes were recognized as promoting avalanching 1 migration of superimposed bedforms to the brink 2 generation of turbulent pulses upstream of the brink 3 lee eddy impingement on the lower slipface and 4 extension of the lee eddy above the brink the lee eddy proved very significant in slipface processes by redistributing grainfall sediments and both promoting and impeding grainflow regression analyses showed that the slipface advance per avalanche sa is strongly correlated with the slipface height h expressed approximately by sa 0 060h in addition sa is a direct function of the rate of slipface advance vb the relationship among sa h and vb can be expressed as sa h 0 0385 1 0 134 min cm vb 1 cross strata dip angles between 28 and 34 show no systematic relation to h and vb but dip angles greater than 34 occurred only when both h and vb were small and dip angles less than 28 occurred only when both h and vb were large

oblique plate motion and continental tectonics three dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins kinematic models of strike slip partitioning for transpression and transtension allow the exact relation among three critical parameters plate motion instantaneous strain axes and degree of strike slip partitioning to be calculated application to two end member tectonic environments characterized by a low south island new zealand and high central california degree of strike slip partitioning demonstrates a remarkable consistency among the three parameters suggesting that strike slip partitioned transpression is a valid model for deformation in these regions the extreme degree of strike slip partitioning in wrench dominated systems such as central california is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes

new views of granular mass flows concentrated grain fluid mixtures in rock avalanches debris flows and pyroclastic flows do not behave as simple materials with fixed rheologies instead rheology evolves as mixture agitation grain concentration and fluid pressure change during flow initiation transit and deposition throughout a flow however normal forces on planes parallel to the free upper surface approximately balance the weight of the superincumbent mixture and the coulomb friction rule describes bulk intergranular shear stresses on such planes pore fluid pressure can temporarily or locally enhance mixture mobility by reducing coulomb friction and transferring shear stress to the fluid phase initial conditions boundary conditions and grain comminution and sorting can influence pore fluid pressures and cause variations in flow dynamics and deposits

late cenozoic tectonics of the northwestern san bernardino mountains southern california the late cenozoic structural and stratigraphic history of the northwestern san bernardino mountains supports two distinct episodes of uplift in late miocene to earliest pliocene and quaternary time that we hypothesize are related to movements on low angle structures beneath the range in this paper we document the nature distribution and timing of late cenozoic deformation and deposition in the northwestern san bernardino mountains and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams in the first episode of deformation late miocene to earliest pliocene motion on the south southwest directed squaw peak thrust system disrupted drainage in pre existing miocene nonmarine basins and uplifted the western third of the present range to form the ancestral san bernardino mountains crystalline rocks of the san bernardino mountains were thrust southward across the present site of the san andreas fault between 9 5 and 4 1 ma at a time when the san gabriel fault was the active strand of the san andreas transform system we speculate that the liebre mountain crystalline block at the northern margin of the ridge basin may be the missing upper plate of the squaw peak thrust now offset along the san andreas fault the second episode of deformation began with uplift of the northern plateau of the modern san bernardino mountains on north directed range front thrusts in early pleistocene time between 2 0 and 1 5 ma synchronous uplift of the northern plateau recorded in early pleistocene fanglomerates on the northwestern margin of the range is interpreted to be the result of movement of a relatively coherent crustal block northward up a south dipping detachment ramp beneath the central range in middle pleistocene time activity on the northern range front began to wane and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the san andreas fault which subsequently migrated rapidly northwestward along the san andreas fault from the western san bernardino mountains into the northeastern san gabriel mountains we attribute this pattern of deformation to the passage of a bulge or strike slip ramp attached to the southwest side of the san andreas fault at depth

denudation processes and rates in the transverse ranges southern california erosional response of a transitional landscape to external and anthropogenic forcing 1 quantification of denudation in the transverse ranges of california permits reconstruction of spatial and temporal variations in erosion that represent both the response and evolution of interacting hillslopes and channels on the southern front of the san gabriel mountains observational records of the infilling of debris basins and dams define twentieth century landscape erosion rates averaging 1 6 and 0 9 mm yr 1 respectively although all major sediment transport occurs during intense winter storms debris production on hillslopes is greatly enhanced by recurrent fires consequently in this populated region anthropogenic fires have augmented the natural erosion rates we perform a global inversion to estimate the role of precipitation intensity burned areas and local slope on catchment denudation rates after subtracting the effects of anthropogenic fires we estimate landscape denudation rates given natural fire ignition rates increased fire during the past century has augmented sediment production in debris basins by an average of 60 and individual basins show increases up to 400 to identify the dominant hillslope erosion processes the volumetric contribution of landslides was estimated using repeat aerial photographs for the same time interval over which the debris basins have been operative between 1928 and 1973 landsliding produced only 10 of the sediment in debris basins even in the long term when infrequent but volumetrically important landslides occur bedrock landslides appear to contribute a maximum of 50 to the long term landscape denudation previous mapping of soil slippage in the san dimas experimental forest within the san gabriel mountains rice et al 1969 rice and foggin 1971 indicates that shallow landsliding is likely to be the dominant modern hillslope erosion process when compared to incision rates derived from a fluvial shear stress model and to exhumation rates based on low temperature thermochronological data blythe et al 2000 modern natural erosion rates are comparable to denudation rates since the pliocene comparisons of modern erosion rates suggest that debris production on hillslopes and first order channels is directly dependent on vegetation cover and precipitation intensity for higher order channels drainage areas 2 km2 only major storms convey the sediments down valley during the past century some temporal decorrelation occurs between small scale and large scale catchments because hillslope produced sediment is stored in second order and higher channels until major storms mobilize it thus the larger fluvial network damps the episodic fire induced hillslope sediment pulses that occur within small watersheds at a longer temporal scale however uplift and denudation may have been sustained sufficiently long in much of the san gabriel mountains for the topography to reach a macroscale steady state in contrast to rapidly eroding 2 mm yr 1 ranges for which quantification of bedrock landsliding will approximate the sediment flux the san gabriel mountains occupy a niche of intermediate rates 0 1 1 0 mm yr 1 in which a broad suite of hillslope processes including shallow seated and deep seated landslides debris flows and wet and dry ravel contribute to the sediment flux

rapid pliocene exhumation in the karakoram pakistan revealed by fission track thermochronology of the k2 gneiss rapid pliocene denudation in the karakoram pakistan as revealed by fission track dating of rocks from k2 godwin austen was probably caused by tectonic uplift apatite separated from samples collected from elevations of 5300 to 8611 yielded fission track ages of 2 1 0 6 to 4 3 1 4 ma and suggest an initial apparent denudation rate of 3 6 mm yr commencing after 5 ma one zircon separate from 6600 m gave a mean fission track age of 32 6 ma the mid tertiary zircon age delimits the maximum amount of pliocene denudation to 7000 m the total amount of denudation at the present mean surface elevation of 6000 m was estimated to be 6000 m

3 d fluid mud dynamics in the jiaojiang estuary china a 3 d model has been developed for the muddy jiaojiang estuary and adjoining coastal waters and verified against field observations to simulate fluid mud formation the model uses a fine resolution grid near the bottom and involves coupling processes between hydrodynamics and fluid mud such as the sediment induced buoyancy increasing turbulent kinetic energy sink and kinematic viscosity mixing by internal waves riding on the lutocline and non newtonian properties of fluid mud the effective hydrodynamic drag was reduced in the presence of fluid mud it is shown that the estuary is infilled by tidal pumping and that longitudinal and transversal gradients of suspended sediment concentration salinity and currents control the formation of mud banks thus a 3 d model is necessary to estimate the fate of mud although the model results are very sensitive to details of the parameterization of the hydrodynamics mud feedback processes

seismic refraction evidence for steep faults cutting highly attenuated continental basement in the central transverse ranges california a 2 d upper crustal structural profile with seven steeply dipping faults was constructed from the los angeles region seismic experiment phase ii larse ii seismic refraction data recorded over the north central transverse ranges and adjacent mojave desert in southern california the profile extends to a depth of approximately 4 km the faults were identified from distinctive features in the seismic first arrival data while the velocity structure was determined from travel times and refined by forward modelling the resulting seismic velocity structure is correlative to the geological structure along the profile and is used to help constrain a geological cross section six of the seven faults detected from the seismic data correspond directly to geologically mapped faults in the central transverse ranges and adjacent mojave desert from south to north they are the pelona fault the san francisquito fault the clearwater fault the san andreas fault and two faults that cut the portal ridge the seventh fault is buried beneath quaternary deposits of the western mojave desert and appears to correlate with an additional fault that cuts portal ridge out of the section line the southernmost fault the pelona fault separates the younger vasquez formation in the south from the pelona schist in the north and is determined to be an intermediate 40 60 south dipping normal fault this coupled with field observations indicate that the pelona fault was instrumental in the late oligocene early miocene formation of the soledad basin and the structural ascent and exhumation of the pelona schist along the proto sierra pelona intrabasin ridge analogous neogene extensional deformation is found to have affected the western mojave desert region where faults 6 and 7 appear to have had north side down normal displacement and to have been instrumental in forming the western antelope basin a 6 0 km s 1 basal layer to our shallow seismic structure is correlated with the pelona schist south of the san andreas fault and with the tectonically related rand schist beneath the western mojave desert granitic and gneissic crystalline rocks which form regional upper plate complexes along the vincent thrust above the pelona schist and along the rand thrust above the rand schist yield consistently low seismic velocities as compared with the expected velocities for the constituent rock types these anomalous velocities are reconciled by the degraded structural and textural state of the upper plate crystalline rocks that resulted from extreme shearing brittle fracturing and related retrogressive hydration reactions starting during or shortly after the latest cretaceous early palaeocene underthrusting of the schists extension translation along the san andreas transform system and compressional faulting with the uplift of the transverse ranges further accentuated the basement deformations the integrated result is that the survival of the precambrian through mesozoic granitic crust of the region became a rootless and highly attenuated upper crustal layer

shortening and thickening of metropolitan los angeles measured and inferred by using geodesy geodetic observations using the global positioning system gps and other techniques record a high rate of north south shortening in an east southeast trending 5 40 km wide belt in northern metropolitan los angeles california downtown los angeles is observed to be converging upon the southern san gabriel mountains at 6 mm yr aside from the elastic strain that will be released during earthquakes rupturing the san andreas and san jacinto faults east west lengthening across northern metropolitan los angeles is minor 2 5 mm yr therefore north south shortening is accommodated mainly by vertical crustal thickening

a precise 40ar 39ar reference age for the durango apatite u th he and fission track dating standard the distinctive durango apatite is widely distributed as a reference material for mineralogy geochemistry and thermochronology the apatite occurs within the cerro de mercado open pit iron mine located in the northern portion of durango city mexico it is largely confined to vugs and open fractures within the lower portion of the ore deposit apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the chupaderos caldera complex this activity occurred between eruptions of two major ignimbrites from the caldera complex four single crystal sanidine anorthoclase 40 ar 39 ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31 44 0 18 ma 2 for the apatite itself four additional multi crystal measurements are in general agreement with this age also in agreement are a set of 24 u th sm he ages recently determined directly on the apatite which have a mean of 31 02 1 01 ma 1 with a standard error s e of 0 22 ma

dynamic response of the fluid mud to a tropical storm fluidmud fm isauniquesedimentaryfeatureinhigh turbidityestuaries whereitcanmake a rapid contribution to morphodynamics insuf cient eld measurements and xed point monitoring lead to de cient understandings of the formation transport and breakdown of the fm under extreme weather conditions a eld survey was conducted in the changjiang estuary during the period of turbidity maximum just after typhoon haikui the measurements captured the formation of the fm beneath the suspended layers particularly around the lower reach of the north passage the thickness of the observed fm gradually decreased landward along the channel with the maximum value reaching 0 9 m the major features of the observed storm induced fm were simulated using the finite volume community ocean model the results indicated that the initial appearance of the fm was the result of a typhoon intensi ed salinity induced strati cation in the outlet region the subsequent landward propagation of the fm was driven by the combined effects of the fm induced mud surface pressure gradient force and saltwater intrusion near the bottom weak mixing during the subsequent neap tidal period sustained the fm as it rapidly extended into the middle region of the north passage this produced a large velocity shear at the interface of the fm and upper suspension layer increasing the entrainment from the fm to the upper suspensionlayer asaresultoftheincreasedtidalmixing thefmweakenedandthen nallybrokedownin the subsequent spring tidal period

stochastic prediction of sediment transport in sand gravel bed rivers classical deterministic bedload transport predictors are applied to sand gravel bed rivers the turbulent bed shear stress is modeled according to a probability distribution to obtain realistic bedload transport rates at incipient motion in extending the predictors to stochastic predictors for nonuniform sediment many parameters that represent near bed turbulence and the particle size distribution must be chosen the parameters that give realistic results are chosen by analyzing the results of a new experimental flume dataset with relatively large water depths choosing other combinations of parameters may give equal total bedload transport rates but at the cost of large errors in fractional transport rates attention is given to the hiding exposure phenomenon and a hindrance effect related to nonuni form sediment validation based on two independent field datasets shows that successful predictions of particle sizes near the threshold for motion are feasible using the stochastic approach while the deterministic approach gives successful predictions well above incipient motion

cohesive sediment transport in the jiaojiang river estuary china extensive data were obtained of the water and fine sediment dynamics during spring tides in the extremely turbid jiaojiang river estuary at spring tides these data enabled the calibration of a two dimensional 2 d width integrated model four processes dominated the mud dynamics these were firstly the formation at slack tide of soft mud deposits with an erosion constant much smaller than that of the underlying compacted sediment secondly the sediment induced buoyancy effects thirdly the collapse of the turbulence by the sediment suspension in the fluid mud range and fourthly the inflow of sediment at the mouth of the estuary these findings demonstrate the necessity to have detailed field data to enable quantitative as opposed to qualitative modelling of mud dynamics in turbid estuaries the estuary is infilling with sediment from the east china sea and not from riverine inflow this sediment presumably originates from the yangtze river located 200 km further north

morphology sedimentology and palaeohydraulic significance of large gravel dunes altai mountains siberia coarse gravel bedforms which resulted from pleistocene glacial outburst floods are identified as subaqueous dunes comparison of the morphology of these fossil structures with modern dunes shows that the form of two dimensional 2 d transverse dunes and 3 d cuspate and lunate dunes developed in coarse gravels is comparable with sand dune morphology within lesser scale geophysical flows the similarity of the steepest gravel dunes with equilibrium dunes in sand indicates that grain size is not a major factor in constraining primary duneform internal structure indicates that flow over 2 d dunes was relatively uniform but over 3 d bedforms flow was locally variable flow separation and complex streaming of flow occurred over the steepest 3 d dunes cross beds are thin and few approach the angle of repose consequently most dunes did not migrate primarily by avalanching but by stoss entrained gravel transported over the crests rolling down and depositing on the lee slopes the lee formed when flood power was waning some dunes were siightly planed down during falling stage because lee side cross beds tend to be steeper than the angle of the preserved lee slope however silt rich caps indicate that any height reduction was contemporary with the final deposition of foresets post flood modification has been negligible although the modern topography is subdued by loess deposits within the dune troughs lee side sediments are often finer than the stoss slope sediments which indicates

continental tectonics in the aftermath of plate tectonics the success of plate tectonics required an acceptance of continental drift and thus a reinterpretation of the large scale geological history of most of the earth but the basic tenet of plate tectonics rigid body movements of large plates of lithosphere fails to apply to continental interiors where buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity

change in motion of pacific plate at 5 myr bp alignments of volcanoes such as the hawaiian seamount chain record the passage of oceanic plates over mantle plumes or hot spots and changes in the orientation of a seamount chain may indicate changes in the direction of motion of the plate within the hotspot frame of reference1 this hypothesis can be tested for the pacific plate by comparing data from the hawaiian volcanoes with tectonic movements inferred from the geology of adjacent plates we have now found that the motion of the pacific plate changed to a more northerly direction 5 myr ago probably as a result of the detachment of a piece of pacific sea floor which had been subducted beneath the fiji plateau during the tertiary

three dimensional numerical experiments of strain partitioning at oblique plate boundaries implications for contrasting tectonic styles in the southern coast ranges california and central south island new zealand a three dimensional numerical model is used to investigate the nature and distribution of crustal scale deformation and strain partitioning across obliquely convergent transpressive plate boundaries a uniform elastic plastic crustal layer obeying murrell s extension to three dimensions of the griffith failure criterion was deformed under basal kinematic boundary conditions that correspond to the oblique convergence between two mantle lithospheres one of which detaches and subducts beneath the other the results show deformation concentrated in relatively narrow shear zones the character planar versus curved and dip of these shear zones or crustal scale faults depend on the relative amount of convergence versus transcurrent motion imposed across the plate boundary in models dominated by transcurrent boundary conditions two separate sets of coexisting structures develop 1 a pair of planar relatively shallow dipping thrust zones arranged in a v structure that accommodate convergent motion and 2 a set of curved near vertical strike slip zones arranged in a flower structure that accommodates the transcurrent motion in models dominated by convergence both contractional and transcurrent deformations are accommodated by oblique slip across a single pair of structures that have the same geometry as the thrust shear zones there is a gradational behavior between these end member partitioned and nonpartitioned modes for intermediate boundary conditions in all cases the dip of the shear zones predicted by the numerical model is in good agreement with the dip of ideal faults predicted by mohr s incipient faulting hypothesis the two basic modes are interpreted to underlie the contrasting tectonic styles of the southern coast ranges california and central south island new zealand factors other than obliquity of convergence which play a role in strain partitioning are discussed