history and isostatic effects of the last ice sheet in southern british columbia abstract the late wisconsinan cordilleran ice sheet covered british columbia southern yukon territory and parts of alaska washington idaho and montana its major source areas were the high mountain ranges of the canadian cordillera and flow was strongly controlled by topography the last ice sheet began to develop about 30 000 25 000 14c yr ago but it did not achieve its maximum extent until 15 000 14 000 14c yr bp ice sheet growth was interrupted locally by stillstands and retreat ice sheet decay was rapid and was characterized by complex frontal retreat at the periphery accompanied locally by brief readvances and by downwasting and stagnation by 10 000 14c yr bp

late pleistocene holocene eolian deposits of the southern buenos aires province argentina a preliminary model abstract the aim is to reconstruct the sedimentological history of the late pleistocene holocene eolian deposits of southern buenos aires province hence a preliminary model is proposed in order to analyze the different stages of the sedimentological cycle the deposits include sandy loess fine sand sheets and dunefields the southern buenos aires province represents the depositional area whereas the source and transport environments are located in the northern patagonian andes and northern extra andean patagonia according to the mineralogical characteristics of the material explosive volcanism is considered the predominant process of particle formation particularly as generator of pyroclastic deposits which were reworked by different erosive processes glacial fluvial eolian during the late glacial maximum and the holocene alluvial deposits which were mainly generated by fluvial erosion of volcaniclastic sedimentary units were deposited along the floodplains of the colorado and negro rivers including those distal segments located in the emerged offshore platform the fluvial regime depended upon the regime of the glaciers hence it reflected the major glacial fluctuations which took place in the andes floodplains were deflated by southwest winds transporting the material towards the southern buenos aires province sandy loess and loessial sands and to la pampa province dunefields

late quaternary slip rates across the central tien shan kyrgyzstan central asia 1 slip rates across active faults and folds show that late quaternary faulting is distributed across the central tien shan not concentrated at its margins nearly every intermontane basin contains neogene and quaternary syntectonic strata deformed by holocene north south shortening on thrust or reverse faults in a region that spans two thirds of the north south width of the central tien shan slip rates on eight faults in five basins range from 0 1 to 3 mm yr fault slip rates are derived from faulted and folded river terraces and from trenches radiocarbon optically stimulated luminescence and thermoluminescence ages limit ages of terraces and aid in their regional correlation monte carlo simulations that sample from normally distributed and discrete probability distributions for each variable in the slip rate calculations generate most likely slip rate values and 95 confidence limits faults in basins appear to merge at relatively shallow depths with crustal scale ramps that underlie mountain ranges composed of pre cenozoic rocks the sum and overall pattern of late quaternary rates of shortening are similar to current rates of north south shortening measured using global positioning system geodesy this similarity suggests that deformation is concentrated along major fault zones near range basin margins such faults separated by rigid blocks accommodate most of the shortening in the upper crust index terms 8107 tectonophysics continental neotectonics 9320 information related to geographic region asia 8010 structural geology fractures and faults

mapping crustal stress and strain in southwest british columbia 1 this paper investigates the orientation and sources of stress in the forearc of the cascadia subduction zone in southwest british columbia using bayesian inversion results from focal mechanism data and comparing results with gps derived short term strain rates the subduction margin in this region includes a change in orientation from n s in washington state to nw se in british columbia over 1000 focal mechanisms from north american crustal earthquakes have been calculated to identify the dominant style of faulting and 600 were inverted to estimate the principal stress orientations and the stress ratio our results indicate the maximum horizontal compressive stress orientation changes with distance to the trench from approximately margin normal along the coast to approximately margin parallel 100 150 km inland from the coast comparing stress orientations with gps data we relate the margin normal stress direction to subduction related strain rates due to the locked interface between the north american and juan de fuca plates just west of vancouver island further from the margin the plates are coupled less strongly and the margin parallel maximum horizontal compressive stress in the north american plate relates to the northward push of the oregon block which is also observed in the horizontal shortening direction of the residual strain rates after the subduction component is removed

holocene surface rupture history of an active forearc fault redefines seismic hazard in southwestern british columbia canada author s morell kd regalla c amos c bennett s leonard l graham a reedy t levson v telka a

two regions of seafloor deformation generated the tsunami for the 13 november 2016 kaikoura new zealand earthquake the 13 november 2016 kaikoura new zealand m w 7 8 earthquake ruptured multiple crustal faults in the transpressional marlborough and north canterbury tectonic domains of northeastern south island the hikurangi trench and underthrust pacific slab terminate in the region south of kaikoura as the subdution zone transitions to the alpine fault strike slip regime it is difficult to establish whether any coseismic slip occurred on the megathrust from on land observations the rupture generated a tsunami well recorded at tide gauges along the eastern coasts and in chatham islands including a 4 m crest to trough signal at kaikoura where coastal uplift was about 1 m and at multiple gauges in wellington harbor iterative modeling of teleseismic body waves and the regional water level recordings establishes that two regions of seafloor motion produced the tsunami including an m w 7 6 rupture on the megathrust below kaikoura and comparable size transpressional crustal faulting extending offshore near cook strait

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

earthquake rupture through a step over fault system an exploratory numerical study of the leech river fault southern vancouver island the leech river fault lrf zone located on southern vancouver island is a major regional seismic source we investigate potential interactions between earthquake ruptures on the lrf and the neighb

textural characterization and stratigraphic aspects of the late pleistocene holocene eolian sediments in the sierra de la ventana area buenos aires province argentina abstract textural characteristics of late pleistocene holocene eolian sediments in the sierra de la ventana area buenos aires province argentina are defined and compared with loessic sediments from other areas of the province and with the typical loess of the northern hemisphere these deposits correlate with the stratigraphic units previously established in the study area the study was carried out on samples collected with pedological criterion in the less altered c horizon of two profiles the textural homogeneity which characterizes the sediments of one of the profiles is due to the fact that they were originated on the same parent material on the other hand the differences found within the other profile were inherited from the parent materials the textural features of the sediments analyzed in the present study show the similarity with the loessic sediments described in buenos aires province and show a greater fine sand content than typical loess of the northern hemisphere based on these analyses without taking into account the calcium carbonate content the late pleistocene holocene eolian sediments from sierra de la ventana area are classified as loess sandy loess and eolian silt clayey sand

complex pedogenesis related to differential aeolian sedimentation in microenvironments of the southern part of the semiarid region of argentina abstract the aim of this paper is to explain soil genesis and spatial variability in the soil cover of a flat landscape in the southern part of argentina s semiarid region soil survey data indicate random differences in the properties of soils lying within a few hundred meters of one another as reflected in an intricate distribution pattern which cannot be explained by the climatic theory of soil genesis in a single pedogenetic cycle this pattern is unrelated to the actual vegetation cover the soil parent materials consist of a

dust transport and the question of desert loess formation although contemporary dust storms are frequent in arid and semi arid areas desert loess deposits are poorly developed much of the world s loess occurs in mid latitude areas which experienced glaciation during the pleistocene ocean core evidence indicates that dust transport from sub tropical deserts increased during cold stages of the pleistocene but loess formed only on certain desert margins for reasons which have not been fully explained this paper re examines the mechanisms of dust transport and deposition and the circumstances leading to the accumulation of thick loess typical loess is composed mainly of medium silt grains which are transported in short term suspension a few metres above the ground significant thicknesses of loess form only when dust is trapped within a limited area often relatively close to the source dust particles finer than 20 m are transported mainly in long term suspension over a greater height range and may be widely dispersed the availability of silt and the frequency magnitude and direction of dust transporting winds are important factors governing the potential for loess formation but the existence of a suitable dust trap is particularly important traps may be formed by topographic obstacles areas of moist ground or vegetated surfaces vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid latitudes during the pleistocene dust blown during glacial periods from certain deserts notably in sinai soviet central asia and china accumulated as loess in neighbouring semi arid regions on the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps during most of the holocene net dust deposition rates in all desert marginal areas have been too low for significant loess accumulation this is mainly due to a reduction in silt availability and a tendency towards landscape stability reported dust storm frequencies during the past 50 years over estimate the longer term holocene dust flux due to the effects of human activities much modern dust owes its origin to erosion of cultivated soils in semi arid areas and is finer than typical loess

simultaneous reactivation of two subparallel inland normal faults during the mw 6 6 11 april 2011 iwaki earthquake triggered by the mw 9 0 tohoku oki japan earthquake the gigantic m w 9 0 11 march 2011 tohoku oki earthquake suddenly changed the overriding inland area to an extensional stress regime and triggered massive seismic swarms in the coastal region the largest earthquake of m w 6 6 struck southern fukushima on 11 april 2011 and ruptured two previously mapped faults the northwest nw trending yunodake fault and the north northwest nnw trending itozawa fault clear 15 km long and 14 km long surface ruptures appeared along both faults respectively exhibiting a predominantly normal sense of slip down to the west the maximum vertical offset on the yunodake fault is 0 8 m whereas that on the itozawa fault is 2 1 m the itozawa fault rupture is in part marked by uphill facing scarps which is discordant with the large scale topography but consistent with saddled ridges and ponded alluvium the yunodake fault which bounds a neogene half graben structure juxtaposing mesozoic metamorphic rocks against neogene sedimentary rocks shows left lateral deflections of streams seismological data and interviews of local residents revealed that the two subparallel faults ruptured simultaneously based on the location of its hypocenter we however interpret that the itozawa rupture was primary and then triggered normal faulting on the yunodake fault under the heightened coulomb stress caused by the preceding tohoku oki earthquake our paleoseismic trench across the itozawa fault exposed evidence for a penultimate earthquake that occurred sometime between 12 620 and 17 410 cal yr b p there is no evidence that the itozawa fault ruptured during or immediately after the a d 869 jogan earthquake which is believed to be the penultimate giant megathrust earthquake along the japan trench

crustal extensional faulting triggered by the 2010 chilean earthquake the pichilemu seismic sequence 1 we report a sequence of crustal quakes that began after the mw 8 8 thrust subduction maule earthquake that affected the central chile margin on 27 february 2010 this activity lasted by several months having the most important events on 11 march 2010 mw 6 9 and mw 7 0 with normal focal mechanisms seismicity shows a rupture oriented along a nw striking and sw dipping normal fault from the surface down to the interplate contact seismicity can be correlated with neotectonics extensional structures similarly oriented in the region which have coexisted with nne ssw reverse faults since the late pliocene even though both have older periods of activity since the paleozoic this crustal rupture would have been triggered by the high coulomb stress change produced by the maule earthquake enhanced by likely fluid presence along weakened zones of the forearc crust as evidenced by high vp vs ratio the occurrence of relevant neotectonic activity in coincidence with short term deformation suggests a relationship with long term tectonic features of this region which would have been acting as a barrier during the interseismic period increasing the strain accumulation and triggering contractional faulting in the crust as well as producing high slip patches during great subduction ruptures favoring triggering of crustal extensional faulting crustal faulting in pichilemu suggests that this kind of events should be considered in seismic hazard analysis despite the absence of historical crustal seismic activity before the maule earthquake

sedimentological study of aeolian soil parent materials in the rio sauce grande basin buenos aires province argentina abstract sedimentological characteristics of aeolian soil parent materials in the rio sauce grande basin southern buenos aires province were analysed using grain size distributions transport mechanisms dynamics of the sedimentary process and the nature of calcareous layers samples were collected from c horizon of 13 profiles distributed over all the piedmont areas and plains calcium carbonate in these materials is either inherited from the parent material or was redistributed by pedogenic processes their amounts in ck horizons increase from the upper to the lower basin and are inversely related to the thickness of soil parent material to mean annual rainfall and to the calcrete depth the cumulative grain size plots show that the sediments were transported by both aeolian saltation and aeolian suspension most of them have coarser and more heterogeneous grain size distributions than typical loess of the northern hemisphere indicating a larger and more fluctuating wind energy than is required for the formation of typical loess deposits in the upper and middle basins were deposited in two main aeolian episodes which can be related to the dry periods of the late pleistocene and the late holocene recorded in southern buenos aires province in some thin profiles of the middle and lower basins the large clay content of the calcic c horizon was probably inherited from one or more previous pedogenic events there is a systematic increase of particle size towards the lower basin which may be related to an increase of energy towards the lower basin or to the influence of littoral dune fields along much of the atlantic coastal zone

slip rate variations on normal faults during glacial interglacial changes in surface loads geologic and palaeoseismological data document a marked increase in the slip rates of the wasatch fault and three adjacent normal faults in the basin and range province during the late pleistocene early holocene epochs the cause of this synchronous acceleration of fault slip and the subsequent clustering of earthquakes during the holocene has remained enigmatic although it has been suggested that the coincidence between the acceleration of slip and the shrinkage of lake bonneville after the last glacial maximum may indicate a causal relationship here we use finite element models of a discrete normal fault within a rheologically layered lithosphere to evaluate the relative importance of two competing processes that affect fault slip postglacial unloading the removal of mass which decreases the slip rate and lithospheric rebound which promotes faster slip we show that lithospheric rebound caused by regression of lake bonneville and deglaciation of adjacent mountain ranges provides a feasible mechanism for the high holocene rates of faulting in the wasatch region our analysis implies that climate controlled changes in loads applied to earth s surface may exert a fundamental control on the slip history of individual normal faults

the open system geochemical evolution of alkalic cap lavas from haleakala crater hawaii usa lavas ranging from alkalic basalt to mugearite are exposed in a continuous stratigraphic section in haleakala crater on the island of maui these lavas define a series of discrete magma batches separated by sharp compositional breaks characterized by upsection increases in incompatible element e g k2o nb ree th ba rb contents al2o3 cao ratios and la sm ratios and complementary decreases in compatible element e g mgo ni cr v sc contents within magma batches there are systematic upsection decreases in incompatible element contents al2o3 cao ratios and la sm ratios and systematic increases in compatible element contents the observed compositional cyclicity can be explained as the result of alternating periods of low and high magma chamber recharge rates the upsection interbatch compositional breaks represent periods of low or negligible magma chamber recharge during which eruption is suppressed and the geochemical effects of crystal fractionation dominate those imposed by magma mixing at the initiation of recharge the influx of mafic magma into the magma chamber produces an eruption resulting in partial evacuation of the magma chamber following eruption recharge magma mixes with magma remaining in the magma chamber continued recharge and mixing during these eruptive periods cause the compositions of lavas to become more mafic with time thus during a recharge period the bulk composition of the magma chamber approaches that of the recharge magmas the most mafic lavas i e the youngest lavas within each magma batch typically are relatively evolved mg 48 mgo 7 wt and therefore cannot represent primary magmas evidence for polybaric fractionation suggests that these lavas represent recharge magmas that have undergone crystal fractionation in a deeper level reservoir prior to injection into the shallow level haleakala magma chamber the observed fine scale geochemical cyclicity is superimposed over the larger scale systematic changes in isotope ratios and highly incompatible trace element ratios that are attributed to time dependent changes in the proportions of distinct mantle components in the source of these lavas these contrasting scales of temporal geochemical variability illustrate how shallow level magma chamber processes have imposed short time period 2000 years compositional controls on magmas whose compositions were also changing over relatively long time periods 800 000 years due to source related effects the decrease in magma supply rates during the alkalic cap phase of hawaiian volcanism probably results in increased magma chamber residence times thus creating the greater magmatic diversity observed the specifics of the petrologic and geochemical evolution of individual hawaiian volcanoes during this period differ at least in part because of differences in magma supply rates

implications of fault slip rates and earthquake recurrence models to probabilistic seismic hazard estimates abstract increasingly fault slip rates are being used to constrain earthquake recurrence relationships for site specific probabilistic seismic hazard ground motion assessments this paper shows the sensitivity of seismic hazard assessments to variations in recurrence models and parameters that incorporate fault slip rates two models are considered to describe the partitioning of the slip rate or seismic moment rate into various magnitude earthquakes an exponential magnitude distribution and a characteristic earthquake distribution assuming an exponential distribution the activity rate n m 0 is constrained by the upper bound magnitude m u the b value for the region and the fault slip rate s for a given s variations in m u and b value have significant effects on recurrence and computed hazard depending on whether the assumption is made that the seismicity rate is constant or the moment rate is constant there is increasing evidence that the characteristic earthquake model is more appropriate for individual faults than the exponential magnitude distribution based on seismicity data from areas having repeated large earthquakes a generalized recurrence density function is developed and the resulting recurrence relationship requires only m u b value and s a comparison of the recurrence relationships from this model with the historical seismicity and paleoseismicity data on the wasatch and san andreas faults shows a good match the computed hazard based on the characteristic earthquake model differs from that obtained for the exponential model as a function of the fault to site distance and the acceleration level one check on the recurrence models using slip rate is to compare over large regions activity rates based on seismicity data and slip rate data such a comparison in the western transverse ranges shows a reasonable match for both the exponential and characteristic earthquake models in the moderate to large magnitude range

loess of southern south america abstract loess and loessoid loess like reworked loess sediments extend across the chaco pampean plains and the nw mountain environments of argentina and in neighbouring countries paraguay brazil uruguay bolivia loessoid sediments are much more abundant than primary loess the beginning of the loessoid sedimentation cycle was related to a phase of late miocene orogeny in the andes the plio pleistocene record is mostly composed of loessoid sediments modified by pedogenesis which produced welded palaeosols the late pleistocene holocene loess record reveals a heterogeneous composition across the region coarse textures and an andean derived volcanic composition prevail in the southern pampas finer textures and material coming from the andes and the other two sources sierras pampeanas and the parana basin characterize the loess deposits of the northern pampas and the eastern chaco a southern patagonian source is also suggested for the mountain valley loess of tucuman although a western andean provenance has been proposed it is believed that the material in the western chaco was derived from the bolivian andes loess deposition was related to a multistage transport mechanism involving fluvial and aeolian processes inferred westerly and southwesterly wind directions as dominant carriers of the aeolian deposits are in agreement with westerly palaeowind simulations using climate models however the role played by westerly tropospheric winds and northerly winds remains to be established

paleoseismic results from the alpine site wasatch fault zone timing and displacement data for six holocene earthquakes at the salt lake city provo segment boundary

probabilistic seismic hazard analysis of victoria british columbia considering an active fault zone in the nearby leech river valleyprobabilistic seismic hazard analysis of victoria british columbia

temporal behavior of deep seated gravitational slope deformations a review abstract deep seated gravitational slope deformations dsgsds are slow moving hillslope scale mass movements featuring characteristic landforms such as double crested ridges and upslope facing scarps that occur in diverse landscapes throughout the world although these deformations have been studied since the 1960s significant insights to the rates of dsgsds have only been gained within the last two decades owing to progress in geochronology remote sensing and instrumental monitoring absolute age and monitoring data indicate that dsgsd movements are observable over long term 102 years and short term future research directions include constraining the lifespan of dsgsds with absolute age dating and precise high resolution monitoring of spatio temporal displacement across dsgsd bodies it is of great importance to include absolute time scales in the numerical models of dsgsds furthermore combining remote sensing techniques such as synthetic aperture radar interferometry with high resolution lidar scanning and near surface geophysics offers a promising toolkit for collecting very detailed and accurate data on the short term deformation rates of dsgsds

the devils mountain fault zone an active cascadia upper plate zone of deformation pacific northwest of north america abstract the devils mountain fault zone dmfz extends east to west from washington state to just south of victoria british columbia in the northern strait of juan de fuca of canada and the usa recently collected geophysical data were used to map this fault zone in detail which show the main fault trace and associated primary and secondary conjugate strands and extensive northeast southwest oriented folding that occurs within a 6 km wide deformation zone the fault zone has been active in the holocene as seen in the offset and disrupted upper quaternary strata seafloor displacement and deformation within sediment cores taken close to the seafloor expression of the faults data suggest that the present dmfz and the re activated leech river fault may be part of the same fault system based on the length and previously estimated slip rates of the fault zone in washington state the dmfz appears to have the potential of producing a strong earthquake perhaps as large as magnitude 7 5 or greater within 2 km of the city of victoria

late holocene earthquakes on the toe jam hill fault seattle fault zone bainbridge island washington five trenches across a holocene fault scarp yield the first radiocarbon measured earthquake recurrence intervals for a crustal fault in western washington the scarp the first to be revealed by laser imagery marks the toe jam hill fault a north dipping backthrust to the seattle fault folded and faulted strata liquefaction features and forest soil a horizons buried by hanging wall collapse colluvium record three or possibly four earthquakes between 2500 and 1000 yr ago the most recent earthquake is probably the 1050 1020 cal calibrated yr b p a d 900 930 earthquake that raised marine terraces and triggered a tsunami in puget sound vertical deformation estimated from stratigraphic and surface offsets at trench sites suggests late holocene earthquake magnitudes near m7 corresponding to surface ruptures 36 km long deformation features recording poorly understood latest pleistocene earthquakes suggest that they were smaller than late holocene earthquakes postglacial earthquake recurrence intervals based on 97 radiocarbon ages most on detrital charcoal range from 12 000 yr to as little as a century or less corresponding fault slip rates are 0 2 mm yr for the past 16 000 yr and 2 mm yr for the past 2500 yr because the toe jam hill fault is a backthrust to the seattle fault it may not have ruptured during every earthquake on the seattle fault but the earthquake history of the toe jam hill fault is at least a partial proxy for the history of the rest of the seattle fault zone

recent state of stress change in the walker lane zone western basin and range province united states the nw to north trending walker lane zone wlz is located along the western boundary of the northern basin and range province with the sierra nevada this zone is distinguished from the surrounding basin and range province on the basis of irregular topography and evidence for both normal and strike slip holocene faulting inversion of slip vectors from active faults historic fault offsets and earthquake focal mechanisms indicate two distinct quaternary stress regimes within the wlz both of which are characterized by a consistent wnw 3 axis these are a normal faulting regime with a mean 3 axis of n85 9 w and a mean stress ratio r value r 2 1 3 1 of 0 63 0 74 and a younger strike slip faulting regime with a similar mean 3 axis n65 70 w and r values ranging between 0 1 and 0 2 this younger regime is compatible with historic fault offsets and earthquake focal mechanisms both the extensional and strike slip stress regimes reactivated inherited mesozoic and cenozoic structures and also produced new faults the present day strike slip stress regime has produced strike slip normal oblique slip and normal dip slip historic faulting previous workers have explained the complex interaction of active strike slip oblique and normal faulting in the wlz as a simple consequence of a single stress state with a consistent wnw 3 axis and transitional between strike slip and normal faulting maximum horizontal stress approximately equal to vertical stress or r 0 in both regimes with minor local fluctuations the slip data reported here support previous results from owens valley that suggest deformation within temporally distinct normal and strike slip faulting stress regimes with a roughly constant wnw trending 3 axis zoback 1989 a recent change from a normal faulting to a strike slip faulting stress regime is indicated by the crosscutting striae on faults in basalts 300 000 years old and is consistent with the dominantly strike slip earthquake focal mechanisms and the youngest striae observed on faults in plio quaternary deposits geologic control on the timing of the change is poor it is impossible to determine if there has been a single recent absolute change or if there is rather an alternating or cyclical variation in stress magnitudes our slip data in particular the cross cutting normal and strike slip striae on the same fault plane are inconsistent with postulated simple strain partitioning of deformation within a single regional stress field suggested for the wlz by wesnousky and jones 1994 the location of the wlz between the deep seated regional extension of the basin and range and the right lateral strike slip regional tectonics of the san andreas fault zone is probably responsible for the complex interaction of tectonic regimes in this transition zone in early to mid tertiary time the wlz appears to have had a similarly complex deformational history in this case as a back arc or intra arc region accommodating at least part of the right lateral component of oblique convergence as well as a component of extension