late cenozoic structure and tectonics of the northern mojave desert in the fort irwin region of the northern mojave desert late cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral thrust slip on northwest striking moderately dipping segments at the east ends of the blocks crustal blocks 7 10 km wide by 50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block faulting initiated after 11 ma and quaternary deposits are faulted and folded we document a minimum of 13 km cumulative sinistral offset in a north south transect from south of the bicycle lake fault to north of the drinkwater lake fault paleomagnetic results from 50 sites reveal two direction groups in early and middle miocene rocks the north to northwest declinations of the first group are close to the middle miocene reference pole however rock magnetic studies suggest that both primary and remagnetized directions are present in this group the northeast declinations of the second group are interpreted as primary and 63 5 7 6 clockwise from the reference pole and suggest net post middle miocene clockwise rotation of several of the east trending blocks in the northeast mojave domain the jurassic independence dike swarm in fort irwin may be rotated 25 80 clockwise relative to the swarm north of the garlock fault thus supporting the inference of clockwise rotation using a simple shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks we predict 23 clockwise rotation using the observed fault slip or one third that inferred from the paleomagnetic results the discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks where most of our paleomagnetic sites are located however at least 25 40 of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible rigid body rotation of the region evidenced by clockwise bending of northwest striking domain bounding faults our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the eastern california shear zone over the last 10 m y occurred within the northeast mojave domain the remainder must be accommodated in adjacent structural domains e g east of the avawatz mountains and west of the goldstone lake fault supporting appendices 1 and 2 are available on diskette or via anonymous ftp from kosmos agu org directory apend username anonymous password guest diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 or by phone at 800 966 2481 15 00 payment must accompany order

the geomorphic dynamics and environmental history of an upper deltaic floodplain tract in the sacramento san joaquin delta california usa a multi proxy approach was used to examine the geomorphic dynamics and environmental history of an upper deltaic oodplain tract in the sacramento san joaquin delta california three long cores were collected from the mccormack williamson tract mwt and these cores were analyzed for bulk density loss on ignition ne clay and silt content al concentration magnetic susceptibility pollen and charcoal radiocarbon dates obtained for the cores were converted into calendar years and an age depth model was constructed long term vertical accretion and sedimentation rates were estimated from the age depth model cross core relations show that coarse sediment generally accumulates more rapidly and has greater magnetic susceptibility compared to ne sediment percentage ne and loi data show a strong linear relationship that indicates ooding is the primary mechanism for the deposition of particular organic matter on the oodplain and that landscape wash load has contributed a highly consistent fraction of persistent organic matter averaging 5 5 per cent to the site down core grain size pro les show two hydrological domains in the cores namely millennial ne coarse uctuations superimposed on general up core ning coarse sediment is viewed as channel or near channel overbank deposits whereas ne deposits are considered to be distal overbank ood deposits the coarse ne uctuations are indicative of changing depositional settings as channels migrated laterally across mwt whereas the upward ning trend re ects a combination of self limiting overbank deposition as oodplain elevation increased and decreasing competence as sea level rise reduced ood pulse energy slopes mwt has been cross cut and incised numerous times in the past only to have the channels abandoned and subsequently lled by ne sediment the channels around mwt attained their modern con guration about 4000 years ago mwt likely came under tidal in uence at about 2500 cal bp wetlands have recently developed on mwt but they are inorganic sediment dominated copyright 2004 john wiley sons ltd

palinspastic reconstruction of southeastern california and southwestern arizona for the middle miocene a paleogeographic reconstruction of southeastern california and southwestern arizona at 10 ma has been made based on available geologic and geophysical data clockwise rotation of 39 has been reconstructed in the eastern transverse ranges consistent with paleomagnetic data from late miocene volcanic rocks and with slip estimates for left lateral faults within the eastern transverse ranges and nw trending right lateral faults in the mojave desert this domain of rotated rocks is bounded by the pinto mountain fault on the north the model requires that the western part of the late miocene pinto mountain fault was a thrust fault which gained displacement to the west because of the absence of evidence for rotation of the san bernardino mountains or for significant right slip faults within the san bernardino mountains the squaw peak thrust system of meisling and weldon 1989 may be a western continuation of this fault system the sheep hole fault bounds the rotating domain on the east east of this fault an array of nw trending right slip faults and south trending extensional transfer zones has produced a basin and range physiography while accumulating up to 16 km of right slip this maximum is significantly less than the 37 5 km of right slip required in this region by a recent reconstruction of the central mojave desert dokka and travis 1990a geologic relations along the southern boundary of the rotating domain are poorly known but this boundary is interpreted to involve a series of curved strike slip faults and linking normal faults that accommodated noncoaxial extension quaternary movement on the pinto mountain and nearby faults is unrelated to the rotation of the eastern transverse ranges and a hiatus during part of pliocene time followed the deformation which produced the rotation the reconstructed clemens well fault in the orocopia mountains proposed as a major early miocene strand of the san andreas fault projects eastward towards arizona where early miocene rocks and structures are continuous across its trace making large displacements on this structure unlikely the model predicts a 14 clockwise rotation and 32 extension during late miocene and early pliocene time along a nw trending line parallel to the present trace of the san andreas fault palinspastic reconstructions of the san andreas system based on this proposed reconstruction may be significantly modified from current models

paleomagnetism of middle miocene volcanic rocks in the mojave sonora desert region of western arizona and southeastern california paleomagnetic directions have been obtained from 190 early to middle miocene 12 20 ma mafic volcanic flows in 16 mountain ranges in the mojave sonora desert region of western arizona and southeastern california these flows generally postdate early miocene tectonic deformation accommodated by low angle normal faults but predate high angle normal faulting in the region after detailed demagnetization experiments 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations trm because of the episodic nature of basaltic volcanism in this region the 179 flows yielded only 65 time distinct virtual geomagnetic poles vgps the angular dispersion of the 65 vgps is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation the paleomagnetic pole calculated from the 65 cooling unit vgps is located at 85 5 n 108 9 within a 4 4 circle of 95 confidence this pole is statistically indistinguishable at 95 confidence from reference poles calculated from rocks of similar age in stable north america and from a paleomagnetic pole calculated from rocks of similar age in baja california the coincidence of paleomagnetic poles from the mojave sonora desert region with reference poles from the stable continental interior indicates that 1 significant vertical axis net tectonic rotations have not accompanied post middle miocene high angle normal faulting in this region 2 there has been no detectable post middle miocene latitudinal transport of the region and 3 long term nondipole components of the middle miocene geomagnetic field probably were no larger than those of the recent 0 5 ma geomagnetic field in contrast paleomagnetic data indicate vertical axis rotations of similar age rocks in the transverse ranges the eastern transverse ranges and the mojave block we speculate that a major structural discontinuity in the vicinity of the southeastward projection of the death valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not experienced such rotations

deformation processes adjacent to active faults examples from eastern california 1 major seismogenic faults are embedded within narrow zones of inelastic off fault deformation ofd where both distributed displacement and modification of rock properties occur active distributed displacement may affect slip rate estimates seismic energy radiation and geodynamic models this study addresses the role of ofd in the displacement history and mechanical behavior of seismogenic faults by multisite study of deformed geologic features adjacent to 30 60 km long active strike slip faults of 10 km of dextral displacement in the mojave desert eastern california we find that distributed displacement accommodates 0 to 25 of the total displacement over zones of one to two kilometers width displacement occurs mostly within 100 200 m of faults and decreases nonlinearly away from the main fault we show evidence for distributed displacement through simple shear in the form of parallel secondary faults and progressive deformation of linear markers adjacent to the calico fault we also find evidence for shear via rotation and progressive fragmentation adjacent to the harper lake fault analysis of block dimensions show that blocks tend to decrease in size toward faults and that cumulative length of secondary faults is longer than the main fault by at least a factor of 10 based on crosscutting relationships and the relationship of ofd to geophysically imaged compliant zones around active faults we argue that distributed displacement is an active process and suggest that zones of diminished rigidity near faults may be at least in part driven by secondary faulting during the rupture propagation along the main fault

fluvial development and the sediment regime of the lower calder ribble catchment northwest england in the lower reaches of the calder lancashire tributary of the ribble catchment northwest england five river terraces set into the surrounding glacial terrain have been mapped and their underlying sediments radiocarbon dated the earliest terrace t1 reflects sandur style deposition during deglaciation and it aggraded in a reach cut into glacial diamict and glaciolacustrine muds incision below t1 spanned the start of the holocene driven by a combination of reduced sediment supply under a stabilising landscape and lower base levels t2 had aggraded and was being abandoned by 4000 bc its formation latterly involving a period of comparative stability in terms of channel migration perhaps a function of extensive forest cover and sea levels reaching above present day levels the last 3000 years were marked by substantial changes in the sedimentary regime with increased and varying sediment supply driving cycles of cut and fill and greater lateral channel migration these changes coincided with the first widespread and substantial reductions in forest cover and a progression towards an agricultural landscape between the aggradation of terraces t3 and t4 there was a substantial increase in sediment supply which is broadly in keeping with the erosion history in the surrounding uplands this sediment slug appears to have induced a switch from highly sinuous meandering to lower sinuosity channels characterised by lateral scroll style migration late holocene vegetation changes particularly woodland removal driven by anthropogenic land pressure appear to have rendered the floodplain prone to channel migration and the landscape more susceptible to erosion

global climate change and active tectonics effective tools for teaching and research abstract multidisciplinary student teams foster innovative research projects that can discern the locations times and styles of late quaternary faulting field studies in the area of the 1992 landers magnitude 7 3 earthquake in southeastern california provide excellent opportunities for teaching and research in climatic and tectonic geomorphology graduate student work begins with descriptions of type sections for mappable units of quaternary alluvium climate change induced aggradation events these are stair stepped flights of alluvial fan and terrace surfaces topographic profiles along faulted stream channels and fans provide estimates of vertical tectonic deformation and insight into the responses of fluvial systems to wrench fault tectonics time lines in this tectonically deforming landscape are the result of global climate changes that caused synchronous pulses of alluviation in the mojave desert at about 125 55 and 10 12 and 8 ka these dated alluvial surfaces record tectonic deformation resulting from cumulative late quaternary surface ruptures tectonic geomorphologic analyses use topographic pedogenic stratigraphic and structural data but do not require expensive and time consuming trenching projects ages of faulted and unfaulted alluvium constrain the dates of two pre 1992 earthquakes the 1992 landers event is not characteristic of earlier faulting because it ruptured through adjacent segments of the emerson fault that previously ruptured in separate events one at 3 1 2 ka and the other before 8 ka styles of tectonic deformation and locations of active faults however may not have changed during the past 125 ka

sks splitting beneath southern california measurements of sks phase splitting were obtained from nineteen seismic stations in southern california the fast polarization directions are 53 at the southern end of the great valley 82 8 in the western transverse ranges and northern peninsular ranges 95 4 in mojave desert and 70 on san clemente island the splitting time ranges from 0 8 to 1 8 seconds which is consistent with an anisotropic layer of 100 to 200 km thick for 4 anisotropy

the deep crustal structure of the mojave desert california from cocorp seismic reflection data cocorp seismic reflection profiling in the western and northern mojave desert of southern california has revealed the presence of numerous major low angle reflecting horizons within the crust these complex though laterally continuous horizons are interpreted to represent major southwesterly dipping crustal fault zones and as such they place important constraints on the tectonic evolution of the region the upper most horizon is interpreted to be the rand thrust which where exposed places precambrian and late cretaceous crystalline rocks over possibly younger pelona rand orocopia schist this reflecting horizon extends for 25 km southwest of the rand mountains where it appears to be truncated at a depth of about 7 4 km by another horizon which may be a later low angle normal fault the other reflecting horizons are not traceable to the surface and so greater ambiguity remains in their interpretation the most prominent of these horizons occurs at midcrustal depths 15 6 km exhibits a ramp and flat geometry and extends over the northern area of the mojave survey into the basin and range province a lower horizon at depths of 20 30 km in the northern part of the survey area is antiformal and appears to terminate above a flat and relatively continuous moho depth horizon the crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s 33 km in the north of the survey and at about 8 9 s 26 29 km in the south these reflections are offset in the vicinity of the town of mojave the deep intracrustal fault zones inferred from the cocorp survey may represent 1 the deep crustal continuation of the system of mesozoic thrusts which crops out in southern nevada and southeastern california 2 late cretaceous to early cenozoic northeast vergent thrusts related to the uplift of the pelona orocopia rand schist or 3 low angle normal faults related to early miocene northeast southwest directed crustal extension the cocorp survey also traversed the major strike slip faults that bound the mojave block the san andreas fault zone appears to truncate reflectors at depths of 6 8 and 20 km within the mojave basement suggesting that it is a major vertical feature which extends to at least 20 km depth conversely the garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature

large magnitude miocene extension in the central mojave desert implications for paleozoic to tertiary paleogeography and tectonics this is the published version copyright 1990 american geophysical union all rights reserved

the nature of the landers mojave earthquake line the landers california earthquake of 28 june 1992 magnitude 7 3 is the latest of six significant earthquakes in the past 60 years whose epicenters and slip directions define a 100 kilometer alignment running approximately n15 w across the central mojave region this pattern may indicate a geologically young throughgoing fault that replaces numerous older strike slip faults by obliquely cutting across them these older faults and perhaps also the bend in the san andreas fault may be losing their ability to accommodate upper crustal deformation because they have become unfavorably oriented with respect to the regional stress field

slip rate of the calico fault implications for geologic versus geodetic rate discrepancy in the eastern california shear zone 1 long term 10 5 years fault slip rates test the scale of discrepancy between infrequent paleoseismicity and relatively rapid geodetic rates of dextral shear in the eastern california shear zone ecsz the calico fault is one of a family of dextral faults that traverse the mojave desert portion of the ecsz its slip rate is determined from matching and dating incised pleistocene alluvial fan deposits and surfaces displaced by fault slip a high resolution topographic base acquired via airborne laser swath mapping aids in identification and mapping of deformed geomorphic features the oldest geomorphically preserved alluvial fan unit b is displaced 900 200 m from its source at sheep springs wash in the northern rodman mountains this fan deposit contains the first preserved occurrence of basalt clasts derived from the pipkin lava field and overlies quaternary conglomerate deposits lacking these clasts the 40 ar 39 ar dating of two flows from this field yields consistent ages of 770 40 ka and 735 9 ka an age of 650 100 ka is assigned to this fan deposit based on these ages and on the oldest cosmogenic 3 he exposure date of 653 20 ka on a basalt boulder from the surface of unit b this assigned age and offset together yield a mid pleistocene to present average slip rate of 1 4 0 4 mm yr ayounger fan surface unit k records 100 10 m of dextral displacement and preserves original depositional morphology of its surface granitic boulders and pavement samples from this surface yield an average age of 56 4 7 7 ka after taking into account minimal cosmogenic inheritance of granitic clasts the displaced and dated k fans yield a slip rate of 1 8 0 3 mm yr distributed deformation of the region surrounding the fault trace if active could increase the overall displacement rate to 2 1 0 5 mm yr acceleration of slip rate from an average of 1 4 mm yr prior to 50 ka to 1 8 mm yr since 50 ka is possible though a single time averaged slip rate of 1 6 0 2 mm yr satisfies the data these rates are faster than any other paleoseismic or long term slip rate yet determined for other dextral faults in the mojave desert and imply that fault slip rates and earthquake productivity are heterogeneous across this portion of the ecsz total displacement across the calico fault diminishes northward as shear is distributed into folding and sinistral faults in the calico mountains this pattern is consistent with an approximately threefold drop in geologic slip rate as the calico fault steps over onto the blackwater fault and demonstrates the significance of fault interaction for understanding the pattern of present day strain accumulation in the ecsz

nature of the southwestern boundary of the central mojave tertiary province rodman mountains california tertiary volcanic and sedimentary rocks in the central mojave desert are largely confined to the barstow bristol trough an elongate tectonic depression filled with lower miocene strata the southwestern boundary of the trough is sharply defined and separates thick 2 km tertiary sequences from an area to the southwest that is virtually devoid of tertiary rocks segments of this boundary have been interpreted as a buttress unconformity as a right slip sidewall fault that formed the lateral boundary of an early miocene extensional terrane and as a dip slip fault caused by flexure of the north american plate above the subducted mendocino fracture zone the silver bell fault in the rodman mountains central mojave desert is the best exposed segment of the boundary and relations exposed along it can be used to test these hypotheses the tertiary sequence north of the silver bell fault is at least 2 3 km thick and comprises at least 1 9 km of mafic lava flows overlain by and intercalated with coarse clastic rocks derived from the south side of the fault stratigraphic relations require faulting during volcanism and indicate that the minimum throw across the fault is at least equal to the thickness of the exposed tertiary section tuff layers intercalated in the lower part of the clastic deposits are 23 24 m y old indicating that faulting and volcanism were contemporaneous with extension and plutonism in the nearby central mojave metamorphic core complex structural and stratigraphic data indicate that the silver bell fault is primarily a dip slip fault the tertiary section is folded into a west trending upright to overturned syncline located near the southern edge of exposed tertiary rocks the syncline records late cenozoic north south shortening along the northwest striking calico fault which in this area accommodated about 10 km of right slip and perhaps 1 km of reverse slip southwestward dipping strata in the areally extensive north limb of the syncline resemble tilted strata in nearby highly extended areas and have been interpreted to reflect crustal extension however bedding dip does not decrease upward in the tertiary section paleocurrent directions trend obliquely updip and northeast dipping normal faults are rare stratal tilting postdated the accumulation of the exposed section which therefore was not deposited during rotational normal faulting the new data contradict interpretation of the silver bell fault as a major right slip fault bounding the central mojave extensional terrane although they permit small magnitude extension across the fault the amount of early miocene crustal extension in this area therefore is apparently small in spite of stratal tilting and the lateral limit of large magnitude crustal extension in the central mojave block is farther northwest the data are consistent with the fault having formed by failure during plate flexure above the subducted mendocino fracture zone

distributed extensional deformation in a zone of right lateral shear implications for geodetic versus geologic rates of deformation in the eastern california shear zone walker lane 1 the eastern california shear zone ecsz walker lane belt represents an important evolving component of the pacific north america plate boundary geodetic data suggest the northern ecsz is accumulating dextral shear at a rate of 9 3 mm a more than double the total measured late pleistocene rate at 37 5 n at this latitude the silver peak lone mountain splm extensional complex plays an important role in accommodating and transferring slip among the strike slip and normal faults of the ecsz and walker lane to better understand the recent geodynamic evolution of this region we determined late pleistocene extension rates for the clayton valley fault zone one of a series of down to the northwest normal faults comprising the splm using geologic mapping differential gps fault scarp surveys and cosmogenic nuclide geochronology extension rates along the clayton valley fault zone are time invariant at 0 1 0 1 to 0 3 0 1 mm a depending on fault dip since 137 ka when combined with other published fault slip rates at this latitude the cumulative late pleistocene geologic slip rate is 3 3 to 5 2 mm a this rate is lower than both the geodetic rate of dextral shear and other long term slip rate budgets in the northern ecsz our results suggest that deformation in clayton valley is spread across a diffuse set of normal faults and that not all of the deformation is recorded in the surficial geology we suggest that the low cumulative geologic slip rate in the northern ecsz may be a result of this distributed extension which can cause long term rates of deformation to be significantly underestimated

braiding and meandering parameters modifications to standard definttrons of brarding and meandertng are proposed to indicate the morphology of every rrver channel reach quantrtatrvely whether tt has srngle channel or multiple chinnels srnuosrty p rs defined as p l llr where l ts the length of the midline of the channel rn single channel rivers or the wldest channel inlnulti channel rivers and lp is the overall length of the reach braiding rs a measure of channel multiplicity and a new term brard channel ratlo b has been defined as b l rorll o where l o is the total of the mrd channel lengths of a ll the channels tn a reactt anoiiiir e p ssion for the brard channel ratto which gives the same numerical rcsult is b p rorl p where the total sinuosity p o l o1 lp and p has been defined above scatter pio on braid channel ratlo slnuoslty axes show a negatlve correlatton between these parameters as would be expected from the above relatlonshrps stnglechannel rivers b l have relatrvely hrgher srnuosrtres and the upper hmrt of these rs the point at which cut off becomes hrghly probable for multi channel rivers a i sinuoslty iemains low reflecting the limrtrng effect of braid bars on the development of fully dcveloped spiral seconciary flow data analyses showing that increasesin channel slope and bankfull diiharge are associated with changes from meandering to braided morpiology are diverting attenti on from the importance of the rncreased avarlability of bed load grade sediment as a control a variety of river systems supplying water and sediment to the gangetic al luvial plarns of indta have been investigated these alluvral plains are some of the most extensive in the world so the examination has dcpcndcd hcirvi ly t l thc tr c l topographical maps and satel l l te imagcs i he degree of braiding and meanderrng displayed by these rivers varies greatly from rlver to rtver and along the course of part lcular r lve rs ollc of thc first steps has therefore been to devise methods of measuring the degree of brardrng and meandering which can be applied to al l the rivers using maps and satel l i te tmagcs wrth exist ing schemes a r ivcr channel rcach cun either be expressed ln terms of brardrng or meandering sinuosrty although arguments have been developed against the thresholds between brarding and meandering that were suggested by leopold wolman 1957 there have been very few attempts at devlsing common parameters to quantl tat ively express the morphology of channels that vary from one type to the other the f irst part of this paper discusses definit ions of braiding and nrcanderrng rnd thc modif icat ions of these definrtrons that havc been adopted variat ions of thcsc nume rtcal measurements are then r l lustrated hrghhghting some of thc factors that may bc tnvolvcd tn causlng the variatton the meandering parameter sinuosity two minor modrf icat ions have been adopted to the procedures previously used in defining the slnuosity of channcls e g l copold wolman 1957 first ly the channel length has been measured along a l ine that runs mid way between the channel banks this has al lowed mcasuremcnt readl ly from topographical maps aenal photographs and satel lr te images and also has the advantage that the channel length is unlrkely to change ln a major way with changes of r rver water leve l secondly the r ivers rn india show so many transit ions between srngle channel and brarded morphologies that i t was found necessary to abandon the hard and fast dist inct ion be ween braided and meandering classes of r ivers and employ definit ions of brarding and meandering that can each be applied to every rrver no matter whether i t has single or multrple channels or both therefore the meandering sinuostty parameter has been extended to mult i channel situations where i t is based on the mid channel length o f the channe l tha t rs wtdes t tn each reach of the channel belt the modif ied sinuosttv parameter p rs

paleogeographic and tectonic implications of the geology of the tiefort mountains northern mojave desert california geologic mapping petrologic and structural analyses and u pb dating in the tiefort mountains northern mojave desert california provide data on paleozoic paleogeography and mesozoic intra arc tectonics metasedimentary rocks consist primarily of quartzose schists and gneisses that are provisionally correlated with late precambrian cordilleran passive margin facies recognition of precambrian augen gneiss and metasedimentary rocks with craton margin protoliths suggests that the craton miogeocline boundary is in or near the tiefort mountains the presence of eugeoclinal rocks in ranges to the west implies truncation or telescoping of the miogeocline in this area the metasedimentary units are intruded by middle jurassic granite and diorite orthogneisses dated by u pb as ca 164 and ca 160 ma deformed rocks record amphibolite facies metamorphism and mylonitization during middle to late jurassic southeast vergent ductile shear pervasively annealed microstructures indicate that recrystallization outlasted deformation at south tiefort mountain the mylonitic fabric is deformed by northwest trending folds and crosscut by a swarm of mafic and felsic dikes dated as 148 14 ma at tiefort mountain synkinematic biotite granite gneiss is dated as ca 105 ma and mylonitic fabrics are folded about north trending axes the presence of the cretaceous orthogneiss suggests that two deformational events jurassic and cretaceous in age may have affected part of the study area dikes dated as ca 82 ma that crosscut all fabrics are interpreted to be part of a regional intrusive event the timing and vergence of shear zones at south tiefort mountain are similar to contractional shear zones in adjacent areas forming a belt of contractional deformation from the central mojave desert to the east sierran thrust system this deformation may be related to a period of late middle to late jurassic sinistral oblique subduction

cenozoic crustal rotations in the mojave desert from paleomagnetic studies around barstow california paleomagnetic data from the mojave desert around barstow california suggest 55 to 75 of clockwise rotation of this area in earliest miocene time followed by 23 of counterclockwise rotation in the early miocene and no rotation after 18 ma earliest miocene clockwise rotation of the barstow area is suggested by data from nine sites in the oligocene miocene lane mountain quartz latite and jackhammer formation and may be related to oroclinal bending of the southern sierra nevada batholith data from 13 sites in the early miocene pickhandle formation indicate counterclockwise rotation which coincides with early miocene e ne w sw extension and detachment faulting in the mojave desert other parts of the mojave desert experienced variable senses and amounts of rotation at this time which may be related to drag along transfer zones within the upper plate while the lower plate was not rotated these results suggest that miocene extension in the mojave desert is related to similarly oriented extension in the colorado river extensional corridor early miocene to pliocene rocks of the andesite and dacite of the calico mountains barstow formation andesite of murphy s well and black mountain basalt show no evidence for significant rotation of the barstow area after 18 ma most other paleomagnetic data from elsewhere in the mojave are consistent with a lack of significant post 18 ma rotations suggesting that post 10 ma right lateral faulting there has produced relatively little crustal rotation the exception to this generalization is the northeast corner of the mojave desert which underwent substantial post 18 ma clockwise rotation paleomagnetic flattening data also indicate that post oligocene n s translation of crustal blocks in the mojave desert has been insignificant

fault slip rates and earthquake histories for active faults in southern california abstract within the context of the historical record of seismicity we review the geological data bearing on the quaternary slip rates and paleoearthquake histories of active faults in southern california

reassessment of the basal sauk supersequence boundary across the laurentian craton margin hinge zone southeastern california abstractin the death valley and mojave desert regions of southeastern california the contact separating the lower and middle members of the wood canyon formation wcf is currently interpreted as a regional scale unconformity coincident with the base of the sauk sequence regional mapping of this surface however reveals a nonconformable contact with underlying crystalline basement in cratonic settings and a relatively conformable contact atop a northwest thickening wedge of off craton strata that is capped by the lower member of the wcf consistent with an unconformity the progressive loss of three carbonate units within the lower member of the wcf has been attributed to incision by the base of the middle member wcf however fossil evidence and correlation based on carbon isotope compositions of each lower member wcf dolostone unit reject top down erosion to describe their loss and overall cratonward thinning results from multiple detailed measured stratigraphic sections of a conglomerate at the b

geodetic measurement of deformation in the central mojave desert california data from triangulation and trilateration surveys made during 1934 1982 are used to calculate shear strain rates in the central mojave desert of california for the region between the helendale and camp rock faults the shear strain rate was determined to be 0 16 0 03 strain yr with maximum right lateral shear strain occurring on a plane oriented n41 w 5 if we assume that this deformation is due to right lateral motion across the northwest trending local faults the average shear straining corresponds to a relative displacement of 6 7 1 3 mm yr across this portion of the network accounting for 12 of the predicted 56 mm yr of relative motion between the north american and pacific plates from the camp rock fault eastward across the network there is a transition from significant to very low strain rates examination of nine focal mechanisms and their relation to the local geology and the strain data suggests that most of the long term displacement occurs on the major northwest trending faults oriented nearly along the direction of relative motion between the north american and pacific plates secondary faulting controlled by a coulomb anderson failure mechanism or by slip on preexisting faults can account for the occurrence of earthquakes on faults of other orientations

river response to rapid holocene environmental change evidence and explanation in british catchments macklin m g jones a f lewin j 2010 river response to rapid holocene environmental change evidence and explanation in british catchments quaternary science reviews 29 13 14 1555 1576 sponsorship welsh assembly government and the higher education funding council for wales for funding this study through its support of the centre for catchment and coastal research at aberystwyth university

active faulting and block rotations in the western transverse ranges california the geometry of faulting slip vectors of major earthquakes and results of very long baseline interferometry vbli are consistent with active clockwise rotation of crustal blocks in the western transverse ranges about vertical axes oblique left lateral thrust faults trending roughly east west across the western transverse ranges apparently separate long east west blocks of upper crust and the large components of lea lateral shear on east striking planes suggest that the blocks rotate with respect to the san andreas fault in the mojave region between the western transverse ranges and the mojave desert paleomagnetic declinations indicate some 35 and possibly 60 of clockwise rotation since about 10 ma and more importantly the relative velocity between the only two frequently measured vbli sites in the western transverse ranges is consistent with a rotation rate of 6 ma 3 ma during a 5 year period in the 1980s azimuths of slip vectors of major earthquakes are essentially orthogonal to the segment of the san andreas fault across the western mojave desert indicating that crustal shortening across the region is absorbed by crustal thickening and not by lateral translation extension of material parallel to that segment of the fault plate motions late quaternary offsets on the san andreas and san jacinto faults and geodetic measurements in the mojave region yield a calculated direction of relative motion between the san gabriel mountains and the pacific plate across the western transverse ranges that also is perpendicular to the san andreas fault in the mojave region rotation rates associated with simple mechanisms for applying essentially normal tractions to the lateral margins of blocks are not consistent with the observed geometry of faulting and with average rotation rates the rotation rate of 2 8 5 3 ma calculated assuming that the long east west blocks rotate in response to a uniform compression of a viscous substratum to the western transverse ranges however overlaps the range of observed average rotation rates thus the hypothesis that blocks of continental crust move as if carried passively by a substratum undergoing continuous deformation passes one test that can be made of it

block rotation and deformation by strike slip faults 2 the properties of a type of macroscopic discontinuous deformation study of the general features of deformation by systems of subparallel strike slip faults shows that the governing constraints are kinematic the fault blocks must remain in contact with each other and the deformed area must fit with its surroundings as a result fault blocks that move laterally without significant internal deformation also rotate about vertical axes relative to boundaries of the fault domain by an amount that is quantitatively related to fault slip spacing and orientation areas where left and right lateral faults move simultaneously are usually divided into domains that contain faults of one kind only deformation of such multidomain areas is nonhomogeneous but the differences between domains can compensate for each other so that the regional deformation is quite simple strike slip faulting combined with block rotation is an efficient mechanism of deformation because modest offsets and rotations change the linear dimensions of faulted areas by several tens of percents which may often greatly exceed the results of other types of coeval deformation paleomagnetic data can provide an independent measure of block rotations and should be used to supplement structural data block rotation changes the initial angles between right and left lateral faults and also their orientation relative to the stress field the faults probably tend to rotate away from the axis of maximum compression rotated faults continue to move because they are weak surfaces since the deformation depends on geometric factors such as fault orientation and domain geometry it is not simply related to the stress field these properties including rotation of material units characterize discontinuous deformation in general

displacements on late cenozoic strike slip faults of the central mojave desert california field studies demonstrate that displacements on northwest striking wrench faults of the central mojave desert are too small to support hypotheses suggesting large interior translations and associated rotation of the province during late cenozoic time the margin of an early miocene structural belt provides the marker with which to establish lateral displacement on individual faults displacement values for faults are as follows lenwood fault 1 5 3 km camp rock fault 1 6 4 0 km calico fault 8 2 km and rodman pisgah faults 6 4 14 4 km cumulative displacement on all the major northwest striking faults of the mojave desert is about 26 7 38 4 km most if not all regionally distributed right shear presumably related to pacific north american plate interaction developed in the central mojave desert after 20 m y b p right shear was preceded by significant amounts of kinematically unrelated northeast southwest crustal extension

the san andreas and walker lane fault systems western north america transpression transtension cumulative slip and the structural evolution of a major transform plate boundary abstract relative right lateral pacific north american transform motion across the western us is largely taken up by the san andreas and walker lane fault systems cumulative lateral displacement on active strands of the san andreas system is 3 4 or more times greater than across faults of the walker lane the san andreas system is transpressional while the walker lane system is transtensional the relatively more complex discontinuous and broader system of faults composing the walker lane system is attributed to lower cumulative slip and an attendant extensional component of motion greater slip accompanied by a small component of contraction has yielded the simpler more continuous and generally throughgoing set of faults that comprise the san andreas system with respect to the contrasting amounts of cumulative slip the san andreas is at a more mature stage of structural development than the walker lane despite differences in gross patterns of faulting the systems also share similarities in deformation style slip along each is locally accommodated by clockwise rotation of crustal blocks that has produced structural basins and oblique components of slip are commonly partitioned into subparallel strike slip and dip slip faults

enriched sr nd isotopic characteristics of amphibolites in the oki dogo island japan evidence for crust mantle interaction by deeply subducted continental crust

episodic growth of the chocolate mountains anticlinorium recorded by the neogene bear canyon conglomerate southeastern california u s a abstract between indian pass and picacho state recreation area southeastern california the neogene bear canyon conglomerate crops out in and around the chocolate mountains anticlinorium and can be subdivided into three unconformity bounded sequences that are designated from oldest to youngest i ii and iii each of these sequences is composed of interstratified debris flow channelized stream flow and sheet flood deposits debris flow deposits are poorly sorted inversely graded pebble to boulder paraconglomerates with subangular clast populations large clasts extruding into overlying layers are a characteristic of such beds in contrast sheet flood and channelized stream flow deposits are well sorted orthoconglomerates with subrounded to rounded clasts the bear canyon conglomerate unconformably overlies a lower miocene volcanic and epiclastic unit metasedimentary and metavolcanic rocks of the jurassic winterhaven formation a probable jurassic orthogneiss and the upper cretaceous to paleogene orocopia schist geochemical paleocurrent and clast count data suggest that alluvial material in the bear canyon conglomerate was derived from local exposures underlain by one or more of these units and then was funneled into an approximately north south trending structural depression in the axial trace of the chocolate mountains anticlinorium though the exact timing of deposition of the three sequences in the bear canyon conglomerate is difficult to constrain stratigraphic and published geochronological data when viewed in the context of regional tectonic patterns suggest that sequence i probably was deposited after 17 ma and before 9 45 ma in contrast sequence ii was deposited after sequence i and then was folded either sometime prior to or after 9 45 ma sequence iii is interstratified with the 9 45 ma basalts of black mountain and was folded prior to deposition of flat lying quaternary alluvial sediments the east west trending copper basin reverse fault cuts sequence iii in addition key components of a conjugate set of northwest and northeast trending dextral and sinistral slip faults respectively transect the copper basin fault the axis of the chocolate mountains anticlinorium and the 9 45 ma basalts of black mountain however neither the copper basin nor the conjugate faults disrupt flat lying quaternary alluvial sediments episodic growth of the east west trending chocolate mountains anticlinorium during deposition of the neogene bear canyon conglomerate and subsequent formation of the east west trending copper basin fault and the conjugate set of dextral and sinistral faults all record transient north south shortening within the margin of the north american plate these transient periods of north south shortening followed extensional deformation of lower miocene continental arc volcanic rocks and are strain compatible with the 12 10 ma to 6 ma development of the eastern california shear zone numerous other examples of late cenozoic east west trending folds and reverse faults are reported in the literature to have formed during development of this 100 110 km wide zone of dextral shear we therefore conclude that the geology and development of the chocolate mountains anticlinorium between indian pass and picacho state recreation area reflect changing continental margin strain patterns as the boundary between the farallon and north american plates was transformed from a paleogene convergent margin to a wholly neogene transform setting

state of stress from focal mechanisms before and after the 1992 landers earthquake sequence the state of stress in the eastern california shear zone ecsz changed significantly because of the occurrence of the 1992 m w 6 1 joshua tree and the m w 7 3 landers earthquakes to quantify this change focal mechanisms from the 1975 galway lake sequence the 1979 homestead valley sequence background seismicity from 1981 to 1991 and the 1992 landers sequence are inverted for the state of stress in all cases the intermediate principal stress axis s2 remained vertical and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes s 1 and s 3 and small variations in the value of the relative stress magnitudes in general the stress state in the ecsz has s 1 trending east of north and 0 43 to 0 65 suggesting that the ecsz is a moderate stress refractor and the style of faulting is transtensional south of the pinto mountain fault in the region of the 1992 joshua tree earthquake the stress state determined from the 1981 to 1991 background seismicity changed on 23 april and again on 28 june 1992 in the central zone s 1 rotated from n14 5 e to n28 5 e on 23 april and back again to n16 5 e on 28 june thus the landers mainshock in effect recharged some of the shear stress in the region of the m w 6 1 joshua tree earthquake comparison of the state of stress before and after 28 june 1992 along the landers mainshock rupture zone showed that the mainshock changed the stress orientation the s1 trend rotated 7 to 20 clockwise and became progressively more fault normal from south to north along the emerson camp rock faults the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor the complex distribution of p and t axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock the san bernardino mountains region of the m w 6 2 big bear earthquake has a distinctively different state of stress as compared to the landers region with s 1 trending n3 5 w this region did not show any significant change in the state of stress following the 1992 m w 6 2 big bear sequence

regional coseismic deformation from the june 28 1992 landers california earthquake results from the mojave gps network the 1992 landers california earthquake sequence and its aftershocks delineate an active part of the eastern california shear zone the surface rupture lies within the mojave desert providing a unique opportunity to characterize far field deformation with a regional global positioning system gps network that was installed and occupied in may 1991 with uncertainties of less than 1 cm on interstation baseline vectors changes in absolute displacement vectors on the decimetre and centimetre level were determined for individual sites providing more samples of the sparsely sampled landers displacement field measured displacements result from secular strain across the mojave desert coseismic elastic recovery and postseismic deformation during the six weeks between the earthquake and the gps experiment secular strain and postseismic displacement are relatively small thus first order modeling of these fields allows calculation and subtraction of their signals leaving a coseismic residual field the gps determined far field coseismic displacements differ significantly from elastic half space models offering new insight on the role of regional scale heterogeneity in crustal structure

earthquakes quaternary faults and seismic hazard in california data describing the locations slip rates and lengths of quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in california the expected seismic moment m0e and the strength of ground shaking resulting from the entire rupture of each mapped fault or fault segment are estimated using empirical relations between seismic moment m0 rupture length source to site distance and strong ground motions assuming a fault model whereby the repeat time t of earthquakes on each fault equals m0e m0g where the moment rate m0g is proportional to fault slip rate it is observed that the moment frequency distribution of earthquakes predicted from the geologic data agrees well with the distribution determined from a 150 year historical record the agreement is consistent with the argument that the geologic record of quaternary fault offsets contains information sufficient to predict the average spatial and size distribution of earthquakes through time in california the estimates of t for each fault are the foundation for constructing maps that depict the average return period of 0 1g peak horizontal ground accelerations and the horizontal components of peak acceleration peak velocity and the pseudovelocity response at 1 period and 5 damping expected to occur at the level of 0 1 probability during a 50 year period of time a map is also formulated to show the probability that 0 1g horizontal ground accelerations will occur during the next 50 years the maps serve to illustrate the potential value of quaternary fault studies for assessing seismic hazard interpretation of available slip rates indicates that the largest and most frequent occurrence of potentially destructive strong ground motions are associated principally with the san andreas san jacinto calaveras hayward and ventura basin fault zones other regions of similarly high hazard may yet remain unrecognized this inadequacy results primarily from an incomplete data set numerous faults for example are mapped along the coastal region of northern california and within the modoc plateau but relatively few studies relating to fault slip rate are reported a similar problem exists for other stretches of coastal california where marine reflection studies provide evidence of active faulting offshore yet yield little or no information of fault slip rate geological and geophysical field studies can work to remove these deficiencies a concerted effort to locate and define rates of activity on all faults in california is the most promising means to further quantify present levels of seismic hazard in california

quantifying dextral shear on the bristol granite mountains fault zone successful geologic prediction from kinematic compatibility of the eastern california shear zone abstractfor regional kinematic compatibility to be a valid boundary condition for continental tectonic reconstructions there must be tests that validate or invalidate kinematic model predictions in several reconstructions of western north america the displacement history of the mojave block continues to be unresolved the magnitude of displacement along the bristol granite mountains fault zone bgmfz which is the eastern margin of the eastern california shear zone ecsz in the mojave block is a key example of a long standing kinematic prediction that has defied a positive field test until now the ecsz is a network of late neogene and quaternary right lateral strike slip faults that extend from the gulf of california north through the mojave desert linking pacific north america plate motion with basin and range extension this network of faults accounts for 15 of post 16 ma plate transform motion geologic estimates of net dextral offset along the mojave portion of the ecsz 53 6 km are appro

horizontal deformation in the mojave desert near barstow california 1979 1983 the barstow trilateration network spans late cenozoic right lateral strike slip faults in the mojave desert block in southern california the network has been surveyed four times between 1979 and 1984 a marginally significant increase of 0 47 0 18 rad in fault parallel right lateral shear strain occurred in 1980 this increase may reflect random error in measurement since more than half of it is due to changes in only three lines while other lines favorably oriented to detect shear show no change the overall rate of fault parallel shear is 0 06 0 05 rad yr a rate which does not differ significantly from zero the maximum shear strain rate is 0 08 0 05 rad yr oriented n21 w 17

magnetic fabric flow directions and source area of the lower miocene peach springs tuff in arizona california and nevada we have used anisotropy of magnetic susceptibility ams to define the flow fabric and possible source area of the peach springs tuff a widespread rhyolitic ash flow tuff in the mojave desert and great basin of california arizona and nevada the tuff is an important stratigraphic marker from the colorado plateau to barstow california a distance of 350 km however the location of its source caldera is unknown dated at 18 5 ma by 40ar 39 ar the tuff erupted during the early stages of miocene extension along the lower colorado river the thicker accumulations 100 m occur at kingman arizona and in the piute mountains california on opposite sides of the colorado river extensional corridor our ams studies produced well defined magnetic lineations in 30 of 42 sites distributed throughout the tuff typical ratios of the principal ams axes are 1 01 for the magnetic lineation kmax kint and 1 02 for the foliation kint kmin the bulk magnetic susceptibility of the peach springs tuff averages 2 0 10 3 in the si unit system the subhorizontal lineations which presumably parallel the flow directions form a pattern radiating outward from the approximate center of the outcrop area magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff the lineation and imbrication indicate a source region near the southern tip of nevada defining the best intersection of the ams lineations required restoration of major extension strike slip faulting and associated tectonic rotation in the disrupted tuff the optimum intersection of magnetic lineations lies in the southern black mountains of arizona on the eastern side of the colorado river extensional corridor no caldera structures are known from that area but the area contains thick sections of the peach springs tuff above a silicic volcanic center the caldera may be buried under younger deposits in the mohave valley of arizona tertiary granite in the newberry mountains may represent a deeper level of the peach springs tuff vent that has been exhumed by detachment faulting

the co seismic slip distribution of the landers earthquake we derived a model for the co seismic slip distribution on the faults which ruptured during the landers earthquake sequence of 28 june 1992 the model is based on the inversion of surface geodetic measurements primarily vector displacements measured using the global positioning system gps the inversion procedure assumes that the slip distribution is to some extent smooth and purely right lateral strike slip for a given fault geometry a family of so lutions of varying smoothness can be generated we choose the optimal model from this family based on cross validation which measures the predictive power of the data and the trade off of misfit and roughness solutions which give roughly equal weight to misfit and smoothness are preferred and have certain features in common 1 there are two main patches of slip on the johnson valley fault and on the homestead valley emerson and camp rock faults 2 virtually all slip is in the upper 10 to 12 km and 3 the model reproduces the general features of the geologically measured sur face displacements without prior constraints on the surface slip in all models regardless of smoothing very little slip is required on the fault that represents the big bear event and the total moment of the landers event is 9 1019 n m the nearly simultaneous rupture of multiple distinct faults suggests that much of the crust in this region must have been close to failure prior to the earthquake

the mojave extensional belt of southern california it is proposed that the regional pattern of early miocene low and high angle normal faulting and dike swarm emplacement in the western and central mojave desert is the result of regional extension developed within a roughly east trending zone named here the mojave extensional belt this zone of extension although now disrupted by late cenozoic right slip faulting can be traced from the intersection of the san andreas and garlock faults to the eastern mojave desert near bristol lake and the granite mountains fault restoration of post oligocene movement along the san andreas fault system places the now buried western part of the mojave extensional belt opposite a similar age extended area of west central california surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the mojave extensional belt was low angle normal sense simple shear detachment faulting strain within the mojave extensional belt is partitioned between four domains edwards waterman daggett and bullion terranes that each consist of one or more half grabens each half graben is composed of tilted normal fault bounded blocks that lie above a rooted low angle brittle ductile normal sense shear zone tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping 80 planar to slightly curviplanar normal faults at least one generation of high angle faults postdates the formation of the detachments differential extension between the domains was accommodated by strike slip faults transfer zones similarly the lack of early miocene extensional elements north of the western segment of the garlock fault suggests that the garlock may have been active in the early miocene and served as the northern transfer boundary of the mojave extensional belt the mojave extensional belt initially began to open about 22 m y ago with the major phase of extension occurring between 22 and 20 ma local high angle normal faults cut older detachments and were active between 20 and 17 ma kinematic indicators suggest that the western part of the mojave extensional belt opened in a ne sw direction whereas the eastern portion extended in an ene wsw direction present day reference frame rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the mojave extensional belt was originally oriented n s and was rotated as much as 50 clockwise between 20 and 18 ma early miocene

river entrenchment and terrace formation in the uk holocene abstract a meta analysis of a large database of 14c dated fluvial units is used to assess the chronology and controls of episodic holocene river entrenchment and terrace formation in the uk most holocene terraces are of a fill cut type developed in pleistocene sediments in places now reaching down to pre holocene bedrock holocene terraces are widespread in higher relief areas of the uk and peripheral higher energy rivers and include up to 7 levels in some valley floor reaches using 14c constrained data sub sets for incision episodes the onset vertical ranges formation times and rates of entrenchment are examined together with geographical distributions the height range of terrace separation is relatively small 0 5 3 5 m with a long term averaged incision rate over the late and mid holocene of 0 43 0 5 0 67 0 7 and 0 81 m ka in the tweed rheidol severn ouse and ribble catchments and a regional similarity in the scale of incision events the periods 4200 3700 3100 2900 2100 1900 1800 1500 cal bp and most notably the last 1000 years with prominent peaks at 900 800 and 700 600 cal bp were times of accelerated incision it appears likely that extreme flood events triggered the formation of incision slots rather than entrenchment being a direct response to glacio tectonic uplift or the result of incremental valley floor lowering by combined incision and lateral reworking river entrenchment has also been rapid in recent centuries reflecting the coupling of extreme events with anthropogenic effects on catchment hydrology incision results in changes of river channel flood power and overbank flood extent and improved data on the long term and large scale vertical tendency of uk rivers are needed for flood risk management purposes

fault trace complexity cumulative slip and the shape of the magnitude frequency distribution for strike slip faults a global survey summary we examine whether the shape of the magnitude frequency distribution for strike slip faults is described by the gutenburg richter relationship log n a bm or by the characteristic earthquake model by analysing a data set of faults from california mexico japan new zealand china and turkey for faults within regional seismic networks curves of the form log n yrpl a bm where n yr is the number of events per year equal to magnitude m are fit to the instrumental record of seismicity and geological data are used to estimate independently the size and recurrence rate of the largest expected earthquakes that would rupture the total length of the fault extrapolation of instrumentally derived curves to larger magnitudes agrees with geological estimates of the recurrence rate of the largest earthquakes for only four of the 22 faults if uncertainties in curve slope are considered and significantly underestimates the geological recurrence rates in the remaining cases also if we predict the seismicity of the faults as a function of fault length and slip rate and the predicted seismicity is distributed in accord with the gutenburg richter relationship we find the predicted recurrence rate to be greater than the observed recurrence rates of smaller earthquakes along most faults if individual fault zones satisfy the gutenburg richter relationship over the long term our observations imply that during the recurrence interval of the largest expected earthquakes the recurrence of lesser sized events is not steady but rather strongly clustered in time however if the instrumental records provide an estimate of the long term rate of small to moderate earthquakes along the faults our observations imply that the faults generally exhibit a magnitude frequency distribution consistent with the characteristic earthquake model also we observe that the geometrical complexity of strike slip faults is a decreasing function of cumulative strike slip offset the four faults we observe to be consistent with the gutenburgrichter relationship are among those characterized by the least amount of cumulative slip and greatest fault trace complexity we therefore suggest that the ratio of the recurrence rate of small to large earthquakes along a fault zone may decrease as slip accumulates and the fault becomes smoother

north south contraction of the mojave block and strike slip tectonics in southern california the mojave block of southern california has undergone significant late cenozoic north south contraction this previously unappreciated deformation may account for part of the discrepancy between neotectonic and plate tectonic estimates of pacific north american plate motion and for part of the big bend in the san andreas fault in the eastern mojave block contraction is superimposed on early miocene crustal extension in the western mojave block contractional folds and reverse faults have been mistaken for extensional structures the three dimensional complexity of the contractional structures may mean that rigid block tectonic models of the region based primarily on paleomagnetic data are unreliable

coupling relationships hillslope fluvial linkages in the hodder catchment nw england abstract variations in the coupling of sediment transfer between different parts of a fluvial catchment e g hillslope to axial stream can hamper understanding but are an integral part of the geomorphological record depositional environments respond to a combination of land use climate storms floods and autogenic conditioning the distribution of sediment in the upland landscapes of nw england is out of equilibrium with contemporary climate and geomorphological processes more a function of peri and paraglacial mobilisation of glacigenic deposits soil and vegetation development after deglaciation have interrupted any progression toward sediment exhaustion with sediment release controlled largely by extrinsic perturbation with late holocene anthropogenic activity climate and extreme hydrological events the likely candidates this paper presents a new radiocarbon dated holocene geomorphological succession for the river hodder nw england alongside evaluating new palaeoecological and geoarchaeological data to discern the impacts of human activity these data show a late holocene expansion in human occupation and use of the landscape since the iron age 700 0 cal b c with more substantial changes in the character and intensity of upland land use in the last 1300 years the geomorphological responses in the uplands were the onset of considerable and widespread hillslope erosion gullying and associated alluvial fan development interpretation of the regional radiocarbon chronology limits gullying to four more extensive and aggressive phases after 500 cal b c the downstream alluvial system has responded with considerable valley floor deposition and lateral channel migration that augmented sediment supply by remobilising the existing floodplain terraces and led to the aggradation of a series of inset alluvial terraces the timing of these changes between states of aggradation and incision in alluvial reaches reflects the increased connectivity between the hillslope and alluvial systems aspects of both the regional climate and land use histories are conducive to increasing discharge and sediment flux but the region wide lowering of erosion thresholds appears a key driver conditioning these sediment rich conditions and producing a landscape that was more susceptible to erosion under lower magnitude flows

modeling of neogene crustal block rotation case study of southeastern california numerical modeling using a two dimensional distinct element method dem was performed to give an additional understanding of the mechanism of crustal block rotation in a highly active portion of the pacific north america transform plate boundary paleomagnetic and structural investigations have shown that large 30 50 horizontal rotations of crustal blocks occurred in the eastern transverse zone and the northeast mojave block southeastern california since the end of early miocene the main aspect of the modeling was to investigate whether an anisotropic horizontal stress state can be responsible for the rotation of crustal blocks with a length of 100 km or not the simulations were performed using different stress boundary conditions and different parameters of the fault strength applying an assumed ratio of 2 for the horizontal stress components as boundary condition block rotations in the order of 40 only occur if 1 the major horizontal stress sh is oriented n20 e with a maximal deviation of 15 to the east or west and 2 the fault strength described by the friction coefficient is smaller than 0 12 or 14 only of the value 0 85 for intact rocks byerlee s law for a weakening model of the fault strength an initial friction coefficient of 0 85 allows also rotations of about 40 if the friction coefficient decreases rapidly due to an increasing shear displacement to a basic friction coefficient of 0 12 or smaller for the same amount of rotation 40 a decreasing stress ratio requires a lowering of the friction coefficient used whereas an increase of the ratio allows a higher friction coefficient the simulation also show that the modeling with elastic blocks can adequately describe the observed main features of the crustal deformation especially the cumulative slip values discrepancies in the observations and modeling results for the western parts of the mojave desert are the result of a neglect of the three dimensional deformation in our two dimensional modeling

paleoseismology of the johnson valley kickapoo and homestead valley faults clustering of earthquakes in the eastern california shear zone paleoseismic data from 11 trenches at seven sites excavated across the southern johnson valley kickapoo and homestead valley faults that ruptured in the 1992 landers earthquake as well as the northern johnson valley fault which did not fail in 1992 indicate that the return period for large surface rupturing events in this part of the eastern california shear zone is in the range of 5 15 ka the inferred slip rates based on their respective recurrence intervals are in the range of 0 2 0 6 mm yr for each of the faults studied a previous large earthquake ruptured the southern johnson valley and kickapoo faults about 5 ka b p the northern johnson valley fault also failed at about this time at 5 8 ka b p and may have been part of the same rupture in contrast the penultimate large earthquake that we identify on the homestead valley fault occurred about 15 ka b p much earlier than other faults involved in the 1992 rupture from these observations combined with paleoseismic work by others after the 1992 earth quake it appears that previous events along the southern johnson valley and kick apoo faults were different than those of 1992 and may have involved other fault segments it has been over 5 ka since the most recent rupture on the northern johnson valley fault therefore it is surprising that it did not fail in the 1992 rupture from our observations dextral shear appears to be distributed across the entire eastern california shear zone with individual faults taking only a small proportion of the overall slip release of this regional strain appears to occur in temporal clusters of large earthquakes with the 1992 event apparently the most recent of a sequence of late holocene 0 1 ka earthquakes that have ruptured the nine faults we have trenched in the southwestern mojave desert previous clusters of earthquake activity occurred in the early 8 9 ka and middle 5 6 ka holocene and possibly the latest pleistocene 15 ka

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

the importance of plant macrofossils in late glacial climatic reconstructions an example from western norway a contribution to the north atlantic seaboard programme of igcp 253 termination of the pleistocene abstract late glacial pollen diagrams from two sites on the outermost coast of western norway are discussed on the islands of utsira paus 1990 and blomoy mangerud 1970 during the allerod chronozone 12 000 11 000 bp their climatic reconstructions depend on the assumed local occurrence of tree betula b pubescens at blomoy and on the presence of pollen of thermophilous taxa at utsira the plant macrofossil analyses at these same sites do not indicate the presence of betula at either during the allerod reconstruction of the local vegetation is of willow shrubs and empetrum dwarf shrub heath at utsira and of salix herbacea dominated vegetation at blomoy these reconstructions accord well with modern analogues above the tree line in the west norwegian mountains the inferred summer temperatures during the allerod are thus 2 6 c lower than had been proposed from pollen evidence alone late glacial plant macrofossil assemblages can generally be easily matched with modern vegetation analogues on the west norwegian coast it would appear that the pollen of betula and thermophilous indicators arrived by long distance dispersal facilitated by the strong winds during the late glacial the frequently found no analogue pollen assemblages of the late glacial period may therefore be due to their previously unsuspected content of extra regional pollen dispersed by strong winds this emphasises that climatic reconstructions should only be made using taxa that were certainly growing locally it demonstrates the importance of using locally derived ecologically sound vegetational reconstructions for the reconstruction of late glacial climates and for this plant macrofossil analysis is invaluable

faults in the mojave desert california as revealed on enhanced landsat images previously unknown strike slip and normal faults in the central and eastern mojave desert have been revealed on landsat thematic mapper images enhanced by four component processing this method provides color images on which lithologies are discriminated by their contrasting absorption and reflection primarily at infrared wavelengths and particularly with regard to their ferric iron ferrous iron and hydroxyl contents while retaining landform depiction these discriminants represent a new type of geophysical display for geologic mapping in regions of well exposed bedrock faults are revealed on the images by abrupt spectral and textural contrasts that coincide with aligned topographic features the newly discovered faults form part of an extensive regional network of right shear that connects faults in the death valley region with the san andreas fault system they support a heterogeneous strain model for late cenozoic tectonic evolution of the region regional structural relations indicate a westward migration of the locus of strain through time some of the newly identified faults bound blocks that have experienced contrasting rotational histories since early miocene time

cenozoic evolution of the mojave block of southern california the recorded cenozoic history of the mojave desert region of southern california began in the latest oligocene when intense volcanism and tectonism interrupted a long early tertiary silence volcanism commenced across the region in an east west band ca 24 22 ma northwest of barstow volcanism was accompanied by intense crustal extension and development of a metamorphic core complex outside of this relatively restricted area extension was minor or absent after extension ceased ca 18 ma volcanism shifted to small volume eruptions of basalt post extensional deformation has largely been by strike slip faulting along northwest striking dextral faults and west striking sinistral faults and total dextral slip across the mojave desert region since the early miocene is 45 60 km strike slip deformation has been overprinted locally by intense north south contraction that is the dominant style of deformation in the western mojave block paleomagnetic data indicate that parts of the mojave block were rotated clockwise although the magnitude and timing of this rotation are poorly determined the best evidence for large 45 rotation comes from the area east of barstow where large clockwise declination anomalies and mesozoic and cenozoic dikes with anomalous strikes may reflect early miocene clockwise deflection along the mojave river fault volcanism and tectonism in the mojave block resulted from interactions among the north american pacific and various oceanic plates patterns of volcanism and tectonism do not correlate with growth of slab windows beneath the continent but do correlate with the position of the subducted mendocino fracture zone plate circuit reconstructions suggest that the driving force for extension was divergence between the pacific and north american plates along the transform margin that separated the two this hypothesis accounts for the direction magnitude and rate of extension in the mojave block

hillslope gullying in the solway firth morecambe bay region great britain responses to human impact and or climatic deterioration abstract in the solway firth morecambe bay region of great britain there is evidence for heightened hillslope instability during the late holocene after 3000 cal bp little or no hillslope geomorphic activity has been identified occurring during the early holocene but there is abundant evidence for late holocene hillslope erosion gullying and associated alluvial fan and valley floor deposition interpretation of the regional radiocarbon chronology available from organic matter buried beneath alluvial fan units suggests much of this geomorphic activity can be attributed to four phases of more extensive gullying identified after 2500 2200 1300 1000 1000 800 and 500 cal bp both climate and human impact models can be evoked to explain the crossing of geomorphic thresholds and palaeoecological data on climatic change bog surface wetness and human impact pollen together with archaeological and documentary evidence of landscape history provide a context for addressing the causes of late holocene geomorphic instability high magnitude storm events are the primary agent responsible for gully incision but neither such events nor cooler wetter climatic episodes appear to have produced gully systems in the region before 3000 cal bp increased gullying after 2500 2200 cal bp coincides with population expansion during iron age and romano british times the widespread and extensive gullying after 1300 1000 cal bp and after 1000 800 cal bp coincides with periods of population expansion and a growing rural economy identified during norse times 9 10th centuries ad and during the medieval period 12 13th centuries ad these periods were separated by a downturn associated with the harrying of the north ad 1069 to 1070 the gullying episode after 500 cal bp also coincides with increased anthropogenic pressure on the uplands with population growth and agricultural expansion after ad 1500 following 150 years of malaise caused by livestock and human the black death plagues poor harvests and conflicts on the scottish english border the increased susceptibility to erosion of gullies is a response to increased anthropogenic pressure on upland hillslopes during the late holocene and the role of this pressure appears crucial in priming hillslopes before subsequent major storm events in particular the cycles of expansion and contraction in both population and agriculture appear to have affected the susceptibility of the upland landscape to erosion and the hillslope gullying record in the region therefore contributes to understanding of the timing and spatial pattern of human exploitation of the upland landscape

receiver structure beneath the southern mojave block california an inversion of a receiver function recorded at lawrence livermore national laboratory nevada test site network station lac reveals a range of 1 d velocity structures that are consistent the observed waveform the shear velocity structure southeast of lac is relatively simple consisting of an approximately 30 to 34 km thick crust all models contain relatively sharp crust mantle boundaries with a change in velocity from about 3 5 km sec to approximately 4 6 km sec over a depth range of 4 to 6 kilometers the range of models can be roughly divided into two groups that differ mostly in the mid to lower crust the first model of the earth structure contains a large low velocity zone in the middle and lower crust shear wave velocities as low as 2 8 km sec the second model contains nearly constant velocities approximately 3 5 km sec through the middle to lower crust in contrast with the simplicity of the southeast receiver function the receiver functions corresponding to the northwest and southwest show the influence of lateral velocity heterogeneity and can at best be discussed qualitatively

the chronology of late quaternary fluvial activity in part of the milfield basin northeast england the sediment stratigraphy of a 4 m thick intercalated holocene alluvial fill and valley floor peat at a site in the milfield basin northumberland has been dated by a series of eight 14c assays and related to a previously analysed pollen record the sequence extends from the earliest holocene until c 2800 cal bp prior to the onset of peat inception substantial amounts of channel trenching can be demonstrated to have occurred in the milfield basin during the loch lomond stadial there is no measurable early holocene accelerated fluvial activity but a major flooding event occurred at c 7500 cal bp much earlier than recorded elsewhere in the region the explanation for this is not clear however the cessation of mid holocene overbank sedimentation at c 4000 3500 cal bp is tentatively correlated with slope stability associated with woodland regeneration 1998 john wiley sons ltd

a kinematic model for the evolution of the eastern california shear zone and garlock fault mojave desert california abstract the eastern california shear zone in the mojave desert california accommodates nearly a quarter of pacific north america plate motion in south central mojave the shear zone consists of six active faults with the central calico fault having the fastest slip rate however faults to the east of the calico fault have larger total offsets we explain this pattern of slip rate and total offset with a model involving a crustal block the mojave block that migrates eastward relative to a shear zone at depth whose position and orientation is fixed by the coachella segment of the san andreas fault saf southwest of the transpressive big bend in the saf both the shear zone and the garlock fault are assumed to be a direct result of this restraining bend and consequent strain redistribution the model explains several aspects of local and regional tectonics may apply to other transpressive continental plate boundary zones and may improve seismic hazard estimates in these zones

a kinematic model of southern california we propose a kinematic model for southern california based on late quaternary slip rates and orientations of major faults in the region internally consistent motions are determined assuming that these faults bound rigid blocks relative to north america most of california west of the san andreas fault is moving parallel to the san andreas fault through the transverse ranges and not parallel to the motion of the pacific plate this is accomplished by counterclockwise rotation of california south of the san andreas fault and by the westward movement of central california north of the gar lock fault the velocities of the blocks are calculated along several paths in southern california that begin in the mojave desert and end off the california coast a path that crosses the western transverse ranges accumulates the accepted relative north america pacific plate velocity whereas paths to the north and south result in a significant missing component of motion this implies the existence of a zone of active deformation in southern california that is interpreted to include the western transverse ranges and northwest trending predominately strike slip faults close to the coast both north and south of the transverse ranges strain on this system accounts for about a third of the total north america pacific plate motion

middle precambrian detrital monazite and zircon from the hida gneiss on oki dogo island japan their origin and implications for the correlation of basement gneiss of southwest japan and korea abstract monazite and zircon from seven paragneiss samples of the upper amphibolite facies grade from oki dogo island in the hida terrane western japan were analyzed for th u and pb on an electron microprobe most monazite grains are chronologically unzoned and give unequivocal chime chemical th u total pb isochron ages of 250 ma but some grains show a distinct core overgrowth relationship the cores retain evidence of pre 250 ma ages and are interpreted to be of detrital origin suggesting that monazite can be more immune to diagenetic and low grade metamorphic decompositions than had been thought previously zircon grains rounded to nearly euhedral give chime ages ranging from 3000 to 350 ma of particular interest are several rounded 3000 ma recycled zircons which perhaps originated from an archean terrain in the sino korean craton the youngest age of detrital zircons constrains that the gneiss protolith deposited after the 350 ma event the chime monazite and zircon ages for the paragneisses from oki dogo island and from the main part of the hida terrane reveal that the gneisses formed from permo carboniferous clastic sediments during a single metamorphism at ca 250 ma the results are in marked contrast to the current view that 1 the hida gneiss is a polymetamorphosed precambrian complex 2 the central part hida gneiss region and the eastern marginal part unazuki schist region of the hida terrane are correlative with the precambrian kyonggi massif and the paleozoic okchon zone in the korean peninsula respectively instead we consider that the okchon zone alone can be correlatable with the paleozoic hida terrane

permian and triassic rocks of the mojave desert and their implications for timing and mechanisms of continental truncation upper paleozoic and lower mesozoic rocks in the mojave desert and adjacent areas record the change from a passive continental margin to a subduction margin in the southwestern united states this change was apparently accompanied by truncation and reorientation of the continental margin in the mojave desert and western basin and range province from late proterozoic to mississippian time the western margin of north america had a northeast southwest trend through nevada and eastern california truncation of the margin began in pennsylvanian time when a northwest trending left lateral strike slip fault zone formed across the antler orogenic belt the cordilleran miogeocline and the more cratonal character mojave desert rocks of the antler belt were apparently displaced southward from east central california to their present position in the northwestern mojave desert sedimentary rocks deposited during this interval record an active continental borderland characterized by alternately subsided and uplifted marine basins this transcurrent faulting continued to late permian time magmatic activity and deformation started in the mojave desert in late permian time and affected both miogeoclinal cratonal and displaced antler rocks magmatic activity ceased and marine sedimentation resumed in early triassic time lower triassic rocks constitute an overlap sequence on both deformed and undeformed miogeoclinal cratonal rocks and possibly on displaced antler rocks facies trends in these rocks are northwest southeast indicating the new margin trend truncation and reorientation of the margin were completed therefore by early triassic time or possibly as early as late permian time upper permian magmatism and deformation in the mojave desert are older than and distinct from the early triassic sonoma orogeny of nevada lower triassic rocks in the mojave desert are post tectonic with respect to deformation in that region whereas correlative lower triassic rocks in nevada are pretectonic to syntectonic with respect to the sonoma orogeny this difference reflects the variation in deformation and tectonic setting along a newly developed cordilleran margin the southern part of the cordillera was apparently a subduction margin as early as late permian or middle triassic time while the central nevada segment was the site of collision tectonics

basal layered deposits of the peach springs tuff northwestern arizona usa basal layered deposits of the large volume peach springs tuff occur beneath the main pyroclastic flow deposit over a minimum lateral distance of 70 km in northwestern arizona usa the basal deposits are interpreted to record initial blasting and pyroclastic surge events at the beginning of the eruption the pyroclastic surges traveled a minimum of 100 km from the as yet unknown source changes in bedding structures with increasing flow distance are related to the decreasing sediment load of the surges some bed forms in the most proximal part of the study area kingman arizona can be interpreted as being shock induced reflecting a blast origin for the surges component analyses support a hydrovolcanic origin for some of the blasting and subsequent pyroclastic surges the eruption apparently began with magmatic blasts which were replaced by hydrovolcanic blasts hydrovolcanic activity may be partially related to failure of the conduit walls that temporarily plugged the vent a single large volume pyroclastic flow immediately followed the blast phase and no evidence has been observed for a plinian eruption column the stratigraphic sequence indicates that powerful hydrovolcanic blasting rapidly widened the vent thus bypassing a plinian fallout phase and causing rapid evolution to a collapsing eruption column similar processes may occur in other large volume ignimbrite eruptions which commonly lack significant plinian fallout deposits

kinematic evolution of the olinghouse fault and the role of a major sinistral fault in the walker lane dextral shear zone nevada usa abstract the olinghouse fault is a highly oblique sinistral fault in the northern walker lane dextral shear zone in this study we use geologic mapping and structural analysis of 56 fault surfaces and several folds to constrain the evolution and kinematic role of the olinghouse fault within the walker lane we document a two stage development of the olinghouse fault with an early phase of extension between 12 and 10 ma followed by left lateral shear with a component of extension beginning at 8 to 5 ma the sinistral olinghouse fault has played a relatively complex kinematic role in the evolution of the walker lane including acting as a boundary for clockwise vertical axis block rotation a strain transfer zone between loci of extension and a structural domain boundary redistributing strain between the carson and pyramid lake domains the fault has accumulated 2 5 3 km of left lateral offset at an average rate of 0 3 0 6 mm yr the complexity of this sinistral normal fault zone within a broad zone of dextral shear was likely influenced by the propagation of walker lane dextral shear into a zone of pre existing crustal structure

deformation across the western united states a local estimate of pacific north america transform deformation we obtain a locally based estimate of pacific north america relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern north america from east of the rio grande rift to near the continental escarpment data are primarily quaternary geologic slip rate estimates and resulting deformation determinations therefore are instantaneous in a geologic sense but long term with respect to earthquake cycles we deduce a rate of motion of the pacific plate relative to north america that is 48 2 mm yr a rate indistinguishable from that predicted by the global kinematic models rm2 and nuvel 1 however we obtain an orientation that is 5 9 counterclockwise of these models a more westerly motion of the pacific plate with respect to north america is calculated from all three paths the relatively westerly motion of the pacific plate is accommodated by deformation in the north american continent that includes slip on relatively counterclockwise oriented strike slip faults including the san andreas fault which is especially relevant in and south of the transverse ranges and a margin normal component of net extension across the continent which is especially relevant north of the transverse ranges deformation of the sw united states occurs in regionally coherent domains within which the style of deformation is approximately uniform in the vicinity of the transverse ranges two important shear systems splay from the san andreas fault the eastern california shear zone trending nnw from the eastern transverse ranges and the trans peninsular faults trending sse from the western and central transverse ranges within the transverse ranges the right lateral san andreas fault steps left seemingly requiring large amounts of convergence there however most of this convergence is avoided through a funneling flow of the crust toward the western transverse ranges and into the relatively narrow central california coast ranges and the northern motion of the mojave the former process involves an alternation of rotation direction from counterclockwise in and south of the central transverse ranges to clockwise in the western transverse ranges

active faulting in the walker lane 1 deformation across the san andreas and walker lane fault systems accounts for most relative pacific north american transform plate motion the walker lane is composed of discontinuous sets of right slip faults that are located to the east and strike approximately parallel to the san andreas fault system mapping of active faults in the central walker lane shows that right lateral shear is locally accommodated by rotation of crustal blocks bounded by steep dipping east striking left slip faults the left slip and clockwise rotation of crustal blocks bounded by the east striking faults has produced major basins in the area including rattlesnake and garfield flats teels columbus and rhodes salt marshes and queen valley the benton springs and petrified springs faults are the major northwest striking structures currently accommodating transform motion in the central walker lane right lateral offsets of late pleistocene surfaces along the two faults point to slip rates of at least 1 mm yr the northern limit of northwest trending strike slip faults in the central walker lane is abrupt and reflects transfer of strike slip to dip slip deformation in the western basin and range and transformation of right slip into rotation of crustal blocks to the north the transfer of strike slip in the central walker lane to dip slip in the western basin and range correlates to a northward broadening of the modern strain field suggested by geodesy and appears to be a long lived feature of the deformation field the complexity of faulting and apparent rotation of crustal blocks within the walker lane is consistent with the concept of a partially detached and elastic brittle crust that is being transported on a continuously deforming layer below the regional pattern of faulting within the walker lane is more complex than observed along the san andreas fault system to the west the difference is attributed to the relatively less cumulative slip that has occurred across the walker lane and that oblique components of displacement are of opposite sense along the walker lane extension and san andreas contraction respectively despite the gross differences in fault pattern the walker lane and san andreas also share similarities in deformation style including clockwise rotations of crustal blocks leading to development of structural basins and the partitioning of oblique components of slip onto subparallel strike slip and dip slip faults

long dormancy low slip rate and similar slip per event for the emerson fault eastern california shear zone excavations in a playa along the 1992 rupture of the emerson fault reveal evidence of two paleoseismic events with only one large prehistoric rupture in the past 15 millennia accelerator mass spectrometer radiocarbon ages of charcoal from playa sediments and from fault scarp colluvium directly beneath the playa beds indicate that the last large prehistoric slip event occurred about 9000 ka trench wall exposures revealed clear evidence of at least one pre 9 ka rupture at the playa site the event horizon of this earthquake is between two pedogenic carbonate layers that have radiocarbon ages of 14 8 ka and 24 1 ka implying that the earthquake occurred about 20 000 years b p the actual bracketing ages for this rupture are likely a few thousand years older because of the mean residence time for the pedogenic carbonate and calibration of the 14c ages by 230th dating despite the large uncertainties a dormant period of at least 6 kyr to as much as 13 kyr separates the older event from the 9 ka event because the scarp formed by the penultimate event is similar in height to the scarp formed by the 1992 mw 7 3 landers earthquake the penultimate rupture was at least locally similar in size to the most recent rupture this similarity supports the concept of characteristic slip for the emerson fault preliminary results from paleoseismic studies at other sites on the 1992 rupture suggest that large ruptures occurred on other nearby faults within a few hundred years of this penultimate event on the emerson fault the interseismic period that preceded the 1992 earthquake on the emerson fault was about 40 times longer than the average interval between large events on the nearby san andreas fault therefore in comparison to events on the san andreas fault the 1992 landers earthquake was an exceedingly rare event

changes in frequency of moderate size earthquakes and coulomb failure stress before and after the landers california earthquake of 1992 changes in the frequency of moderate size events before and after the 28 june 1992 landers earthquake are investigated and their implications are discussed in the context of coulomb failure stress cfs evolution since 1812 in southern california we systematically considered circular regions and equal area annuli centered on the epicenter of the landers earthquake frequency magnitude relationships for two 10 year periods before and two 5 year periods around the landers event are compared only events with magnitude m 4 0 are included aftershocks are removed for the larger circular regions with radii of 140 to 160 km the rate and slope of the frequency magnitude distribution for moderate size events just before the mainshock appear to be anomalous compared to those for either the preceding or subsequent periods for areas closer to the 1992 epicenter however the number of events is few and the differences in the distributions are less obvious when we examined the seismic activity in annuli of equal area however the largest changes occurred about 150 km from the epicenter of the mainshock not closer as would be expected for a precursor to the landers event we also derive an index value to better quantify differences in the frequency of occurrence of moderate size events as a function of time the index value and the frequency magnitude distribution show similar spatial dependence since 1812 a large region near landers has moved closer to failure in terms of changes in cfs for faults of san andreas type these changes however are dominated by coseismic changes associated with the 1812 and 1857 earthquakes and by tectonic stress buildup related to the san andreas fault not by stress buildup associated with the landers faults themselves which are characterized by very slow long term displacements hence the most pronounced changes in the frequency of moderate size earthquakes before 1992 do not appear to be related to stress buildup to the landers sequence itself they along with the landers sequence may be indicative of a broad region that is approaching a high stress state prior to an eventual future great earthquake the failure to find a pronounced increase in moderate size shocks close in to landers is in accord with the idea that such increases on a timescale of years to decades are associated with the regional buildup of stress to large earthquake along faults of high not low long term slip rates

is extension in death valley accommodated by thinning of the mantle lithosphere beneath the sierra nevada california while relatively rapid and large scale extension occurred in the death valley region of california during neogene time little or no shallow extension occurred in the adjacent sierra nevada this contrast in tectonic history has often been extrapolated to include the entire lithosphere but geophysical and geologic observations indicate that more extension of the mantle lithosphere has occurred under the sierra than under the death valley region upper mantle seismic velocities observed beneath the high sierra are lower than those observed in other regions with comparable surface heat flow this discrepancy could be resolved if the mantle lithosphere beneath the high sierra had become warmer presumably by tectonic thinning in the last 10 m y upper mantle seismic velocities averaged topography and bouguer gravity anomalies all are consistent with the presence of thinner mantle lithosphere beneath the high sierra than beneath the california portion of the basin and range province to the east this suggests that extension of the crust near death valley might be accommodated at a deeper level by thinning of the mantle lithosphere beneath the sierra nevada the extension in the crust of the california basin and range province and the thinning of the mantle lithosphere under the high sierra appear to share the same bounds in time and space the uplift of the high sierra occurred over the past 9 m y which coincides with most of the extension that occurred in the california basin and range province because the orientation of extension in the california basin and range province is inferred to be approximately n60 w from geologic geodetic and in situ stress measurements the northern and southern edges of the death valley extensional subprovince may extend n60 w from the inferred northern and southern limits of west dipping low angle normal faults of the death valley region pronounced changes in the averaged topography and bouguer gravity anomaly across these two bounds both in the basin and range province and in the sierra nevada support a connection between the tectonics of both regions the geomorphic history of the southern sierra suggests an up to the north warp of the sierra across this southern bound during latest cenozoic time hence extension near death valley may be localized in the crust and may be laterally connected to thinning of the mantle lithosphere beneath the sierra nevada this geometry requires

topographic effects of the eastern california shear zone in the mojave desert digital elevation data were used to evaluate the topographic implications of the late neogene eastern california shear zone ecsz analysis shows that the ecsz has directly affected 18 of the surface of the mojave desert block mdb uncertainties regarding the distribution of strain in basins suggest that the area of effect may be larger by perhaps a factor of 2 remaining areas are likely due to quaternary erosion or are inherited from pre ecsz times major drainages and basins of the mdb are developed along transtensional depressions within the ecsz these depressions established the regional and local base levels that have governed erosion by running water local topography is closely tied to the style of strain highlands of the mdb are spatially associated with 1 areas that have or are currently undergoing contractional strain associated with transpression within the ecsz and 2 areas that lie adjacent to uplifted transpressional belts that bound the mojave desert san bernardino and san gabriel mountains in contrast lowland areas are spatially associated only with transtensional areas within the ecsz examination of the topography in the transtensional areas also reveals two distinct populations of elevation spatial analysis indicates that areas that have undergone the greatest local transtension associated with dextral shear are also the lowest topographically these areas formed during the earliest phase of movement of the ecsz late miocene to early pleistocene the second population is coextensive with recently formed transtensional basins that continue to be active

comparison of earthquake scaling relations derived from data of the instrumental and preinstrumental era estimates of surface rupture displacement and magnitude for crustal earthquakes from the preinstrumental era pre 1900 tend to be greater than the corresponding estimates derived from modern scaling relations we investigate this tendency using an expanded and updated version of the earthquake dataset of wells and coppersmith 1994 to fit regression relations of moment magnitude on surface rupture length and rupture area and average surface displacement on surface rupture length separate relations are fitted to preinstrumental and instrumental data and the results compared to the equivalent relations of wells and coppersmith we find that our relations for instrumental data remove some but not all of the differences between the preinstrumental data and the relations of wells and coppersmith we attribute the remaining differences largely to natural censoring of surface displacements less than about 1 m and surface rupture lengths less than about 5 km from the dataset for the preinstrumental era because regressions constructed from similarly censored instrumental data are indistinguishable from the preinstrumental regressions since the regressions for our censored instrumental data i e restricted to moderate to large earthquakes are different from regressions for our complete dataset of instrumental earthquakes and from the regressions of wells and coppersmith both with a larger proportion of small to moderate earthquakes the results may indicate that large earthquakes have different scaling relationships from those of smaller earthquakes

comparing strain events a case study for the homestead valley earthquakes we present a procedure for comparing strain events at different sites and apply it to observations for the homestead valley earthquake sequence of 15 march 1979 coseismic strain steps occurred on two laser strainmeters at pinon flat observatory pfo and on a creepmeter spanning the san andreas fault at wiebe ranch xwr the creepmeter records were studied by leary and malin 1984 the pfo coseismic strains are consistent with a source model derived from near field geodetic measurements while the xwr coseismic extension is 1200 times too large and of the wrong sign the xwr instrument showed an anomalous extension about 20 hours before the earthquake leary and malin 1984 but no such signals were detected on the pfo strainmeters at this time to compare these preseismic observations we must make some assumption about the preseismic source and we choose to assume that any precursory slip occurred on the fault plane that eventually ruptured under this assumption the preseismic xwr and pfo records cannot be reconciled unless the preseismic dislocation is left lateral or mostly thrust and the fault zone at xwr magnifies strain by a factor of 2900 or more this strain magnification implies that the fault zone shear modulus at xwr is 107 pa three orders of magnitude smaller than the shear modulus of typical crustal rocks however earth tide observations at xwr constrain the strain magnification at this site to be less than about 55 thus the preseismic extension at xwr is probably not a precursor to the homestead valley earthquake

earthquakes of the holocene geologic studies of earthquakes involve mapping of coseismic surface faulting and secondary deformation from historical events trenching and geomorphic analysis to define the timing and size of past earthquakes and investigations of fault zone structure and geometry in both unconsolidated sediments and bedrock this research is now being referred to as paleoseismology seismic geology and earthquake geology since the mid 1970s it has led to some of the most exciting and important contributions to the understanding of earthquake behavior in space and time hanks 1985 allen 1986 the present report is the first of what is hoped will become a regularly contributed summary of the geologic aspects of the study of earthquakes

near field investigations of the landers earthquake sequence april to july 1992 the landers earthquake which had a moment magnitude m w of 7 3 was the largest earthquake to strike the contiguous united states in 40 years this earthquake resulted from the rupture of five major and many minor right lateral faults near the southern end of the eastern california shear zone just north of the san andreas fault its m w 6 1 preshock and m w 6 2 aftershock had their own aftershocks and foreshocks surficial geological observations are consistent with local and far field seismologic observations of the earthquake large surficial offsets as great as 6 meters and a relatively short rupture length 85 kilometers are consistent with seismological calculations of a high stress drop 200 bars which is in turn consistent with an apparently long recurrence interval for these faults