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

fault interaction may generate multiple slip vectors on a single fault surface slip vectors of multiple orientations on a single fault surface may be explained by the interaction of crustal scale faults in response to nearby earthquakes field observations of at least five sets of striae on a single planar fault surface can be reproduced in numerical experiments these experimental results suggest that slip vectors having variable orientations may reflect a highly transient local stress state in an otherwise constant regional stress field our results suggest that stress inversion techniques should be applied with caution to areas with complex regional scale fault patterns

transtensional model for the sierra nevada frontal fault system eastern california active strike slip and normal faults along the eastern margin of the sierra nevada primarily accommodate northwestward translation of the sierra nevada central valley i e sierran microplate with respect to stable north america strike slip faults bordering the eastern sierran microplate are subparallel to small circles about the sierra nevada north american euler pole normal faults of the sierra nevada frontal fault system strike 45 clockwise of the small circle trajectories and exhibit well defined left stepping en echelon patterns consistent with formation in dextral transcurrent regime major graben bordering the northeastern sierran microplate are located in regions where the locus of range front deformation steps abruptly eastward in a releasing geometry relative to sierra nevada north american motion crustal shortening occurs at the northern end of the sierran microplate where a component of northwest dextral shear steps westward in a left restraining geometry across the sierran crest to the sacramento valley kinematic inversions of earthquake focal mechanisms from the walker lane belt bordering the eastern sierra nevada indicate that seismogenic deformation primarily is characterized by horizontal shearing and oblique crustal thinning directions of macroscopic dextral shear inferred from the inversions are subparallel to the trajectories of small circles about the sierra nevada north american euler pole normal faulting along most of the eastern sierran range front thus appears to primarily accommodate microplate translation rather than sierran uplift or regional basin and range extension

inversion of field data in fault tectonics to obtain the regional stress iii a new rapid direct inversion method by analytical means summary a new method for determining the reduced stress tensor with four degrees of freedom the orientations of the three principal stress axes as well as the ratio of principal stress differences using fault slip data or focal mechanisms of earthquakes is presented from a computational point of view the inversion of fault slip data is made in a direct way by purely analytical means as a result the determination process is extremely fast and adaptable on small microcomputers from a physical point of view the method aims at simultaneously i minimizing the angles between theoretical shear stress and actual slip vector and ii having relative magnitudes of shear stress large enough to induce slip despite rock cohesion and friction examples of application to actual fault slip data sets with good or poor variety of fault slip orientations are shown the double significance of the basic criterion adopted results in a more realistic solution of the inverse problem than the single minimization of the shear stria angle

crustal structure and seismicity distribution adjacent to the pacific and north america plate boundary in southern california new three dimensional 3 d v p and v p v s models are determined for southern california using p and s p travel times from local earthquakes and controlled sources these models confirm existing tectonic interpretations and provide new insights into the configuration of geological structures at the pacific north america plate boundary the models extend from the u s mexico border in the south to the southernmost coast ranges and sierra nevada in the north and have a 15 km horizontal grid spacing and an average vertical grid spacing of 4 km down to 22 km depth the heterogeneity of the crustal structure as imaged by v p and v p v s models is larger within the pacific plate than the north american plate similarly the relocated seismicity deepens and shows more complex 3 d distribution in areas of the pacific plate exhibiting compressional tectonics the models reflect mapped changes in the lithology across major geological terranes such as the mojave desert the peninsular ranges and the transverse ranges the interface between the shallow mono of the continental borderland and the deep moho of onshore california forms a broad zone to the north beneath the western transverse ranges ventura basin and the los angeles basin and a narrow zone to the south along the peninsular ranges the near surface increase in velocity from the surface to up to 8 km depth is rapid and has a logarithmic shape for stable blocks and mountain ranges but is slow with a linear shape for sedimentary basins at midcrustal depths a rapid increase in v p is imaged beneath the sediments of the large sedimentary basins while beneath the adjacent mountain ranges the increase is small or absent

analysis of fault slip inversions do they constrain stress or strain rate fault slip data commonly are used to infer the orientations and relative magnitudes of either the principal stresses or the principal strain rates which are not necessarily parallel or equal at the local scale of an individual fault the shear plane and slip direction define the orientations of the local principal strain rate axes but not in general the local principal stress axes at a large scale the orientations of p and t axes maxima for sets of fault slip data do not provide accurate inversion solutions for either strain rate or stress the quantitative inversion of such fault slip data however provides direct constraints on the orientations and relative magnitudes of the global principal strain rates to interpret the inversion solution as constraining the global principal stresses requires that 1 the fault slip pattern must have a characteristic symmetry no lower than orthorhombic 2 the material must be mechanically isotropic and 3 there must be a linear constitutive relationship between the global stress and the global strain rate isotropic linear elastic constitutive equations are appropriate to describe the local deformation surrounding an individual slip discontinuity fault slip inversions however constrain the characteristics of a large scale cataclastic flow which is described by constitutive equations that are probably but to an unknown degree anisotropic and nonlinear such material behavior would not strictly satisfy the requirements for the stress interpretation thus at the present state of knowledge fault slip inversion solutions are most reliably interpreted as constraining the principal strain rates

vertical axis rotations in the las vegas valley shear zone southern nevada paleomagnetic constraints on kinematics and dynamics of block rotations paleomagnetic study of vertical axis rotations at 23 localities along the right lateral las vegas valley shear zone lvvsz in southern nevada indicates that clockwise rotations generally increase with proximity to the lvvsz reaching 100 at the closest localities rates of rotation determined at four localities range from l m y to 12 m y the maximum characteristic size of rotating blocks is 2 4 km considerably smaller than the dimensions of the zone of deformation associated with the shear zone 100 km in length 15 km in half width thus the deformation does not accord with kinematic models of block rotations that assume uniform rotations within domains having dimensions of the order of the width of the deforming zone instead the deformation appears to be quasi continuous when considered on a scale of 10 km we suggest that it is consistent with the deformation of a highly plastic layer of upper crustal material mechanically decoupled from deeper parts of the crust and that the across strike distribution of rotations is controlled by the rheological properties of the upper crust the length of the shear zone and the total amount of offset across the shear zone

coseismic growth of basement involved anticlines the northridge laramide connection seismogenic thrust faults in the transverse ranges of southern california may provide analogues for interpreting kinematic data from some laramide anticlines in the southern rocky mountains the 1994 northridge earthquake m w 6 7 occurred on a southwest dipping blind thrust fault beneath the transverse ranges and resulted in incremental growth of a basement involved fault propagation fold analyses of geodetic data and aftershock focal mechanisms show that deformation of the hanging wall block consisted of two major components 1 a large scale elastic strain characterized by horizontal shortening normal to the fault and horizontal lengthening parallel to the fault that was imposed by reverse slip on the underlying blind thrust during the main shock and 2 postseismic cataclastic flow with a geometry comparable to the coseismic strain including horizontal fault parallel lengthening of the hanging wall block which was accommodated at least in part by distributed aftershock activity fault and fold parallel lengthening strains similar to those observed during the northridge earthquake have been obtained by inversion of fault slip data from some laramide basement involved anticlines the northridge analogue suggests that distributed brittle faulting of the crystalline basement may reflect aftershock deformation during incremental growth of the laramide folds if so then the laramide structures provide examples now exposed by erosion of the mature internal structure of the active basement involved folds in the transverse ranges

kinematics of transpressional deformation in the eastern san francisco bay region california inversions of earthquake focal mechanisms for brittle strain show that the dextral hayward fault forms the western boundary of a distinct seismotectonic domain in the eastern san francisco bay region east of the hayward fault the seismogenic strain field is characterized by subhorizontal extension oriented northwest southeast and subhorizontal shortening oriented northeast southwest major folds and thrust faults east of the hayward fault are oriented normal to the direction of maximum shortening but have formed in a predominantly strike slip tectonic setting and exhibit a right stepping en echelon geometry typical of dextral wrench folds the direction of maximum right lateral shear strain in this domain is n20 w which generally is parallel to the strike of the dextral calaveras greenville and concord faults but oriented 15 25 more northerly than the strike of the san andreas and hayward faults to the west the clockwise rotation of maximum right lateral shear strain from west to east across the hayward fault is confirmed by geodetic measurements the kinematic consequences of this variation in regional strain for deformation east of the hayward fault include the following 1 dextral faults must form left restraining contractional stepovers in order to maintain continuity and conserve slip along strike thus creating localized fold and thrust belts 2 dextral faulting accommodates a component of shortening normal to the boundary between the pacific plate and the sierra nevada central valley microplate thus obviating the need for a laterally continuous boundary parallel zone of thrust faulting along the eastern margin of the coast range coast ranges sierran block boundary zone to accommodate plate normal motion at the latitude of the east bay domain the transpressional kinematics of the east bay domain may be present in other obliquely convergent orogenic belts and they have implications for seismic hazard assessment in the highly urbanized san francisco bay area

strain partitioning between structural domains in the forearc of the hikurangi subduction zone new zealand the pacific plate obliquely converges with the australian plate at latitude 39 50 s along the hikurangi margin off the east coast of the north island of new zealand an extensive and youthful subaerially exposed forearc on the east coast of the north island in the hawke s bay area provides the opportunity to document contemporaneous forearc deformation in this obliquely convergent margin setting geologic mapping and analysis of strain at both mesoscale and megascale indicates that strain is partitioning into domains of extension contraction and strike slip the domains are elongate and trend parallel to the margin measurements of net shortening and transcurrent slip in the forearc show that the obliquely convergent motion is transferred across the plate interface deformation rates calculated for the past 1 2 m y for structures in all six forearc domains account for 50 70 of the margin parallel motion required by pacific australian plate convergence and about 6 of the plate motion perpendicular to the plate boundary at the surface in the forearc this obliquely convergent motion is manifest not by transpressional faults but rather by paired structural domains that consist of a strike slip fault zone and an accompanying contractional fault and fold zone on the trenchward side there are two such transcurrent faulting and contraction couplets one where the backstop daylights at the arcward edge of the forearc and another couplet trenchward of a relatively undisturbed forearc basin the small amount of shortening relative to strike slip in the onshore part of the forearc suggests that shortening perpendicular to the plate boundary may be concentrated offshore and that most of the component of plate motion perpendicular to the plate boundary may be accommodated by slip along the subduction zone megathrust

stress and the direction of slip on fault planes the shear stress direction on a fault plane depends only on four of the six components of the stress tensor assuming only that the slip direction marks the shear stress direction on any fault plane and that stress is homogeneous it is possible to estimate these four stress parameters from populations of fault planes with known slip directions as several workers have observed different formulations of the problem may yield varying best fitting stresses and estimates of uncertainty in the simplest case no assumptions are made regarding the orientations of fault planes relative to the stress tensor thus the technique allows for the possibility that the fault planes may be very weak here we present simple analytical and graphical descriptions of the field of admissible fault geometries relative to any four parameter stress model which can be used to illustrate the significance of various inverse strategies in particular this paper explores the effects of using two alternative measures of misfit between an observed fault datum and stress model 1 the pole rotation the angle between the observed and predicted slip direction on the observed fault plane and 2 the minimum rotation the smallest angle between the observed fault geometry and any fault geometry which is consistent with the model by allowing for variation of the fault plane as well as the slip vector the minimum rotation procedure generally achieves a more stable and presumably realistic estimate of the actual discrepancy between a fault observation and stress model than the pole rotation procedure in a test case using 17 earthquake focal mechanisms from the yuii region of eastern taiwan separate inversions based on the two misfit criteria yield different optimum stress models and uncertainty estimates additional constraints on the stress tensor such as the effect of friction can be superimposed on the ones used here

present day kinematics of the eastern california shear zone from a geodetically constrained block model we use global positioning system gps data from 1993 2000 to determine horizontal velocities of 65 stations in eastern california and western nevada between 35 and 37 n we relate the geodetic velocities to fault slip rates using a block model that enforces path integral constraints over geologic and geodetic time scales and that includes the effects of elastic strain accumulation on faults locked to a depth of 15 km the velocity of the sierra nevada block with respect to nevada is 11 1 0 3 mm yr with slip partitioned across the death valley 2 8 0 5 mm yr panamint valley 2 5 0 8 mm yr and airport lake owens valley 5 3 0 7 4 6 0 5 mm yr faults the western mojave block rotates at 2 1 0 8 my clockwise with 3 7 0 7 mm yr of left lateral motion across the western garlock fault we infer 11 2 mm yr of right lateral motion across the mojave region of the eastern california shear zone

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

curved slickenfibers a new brittle shear sense indicator with application to a sheared serpentinite abstract brittle fault zones are commonly characterized by penetrative fracturing of the rock to form an aggregate of blocks so that the rock becomes in essence like a granular material composed of rigid grains the surfaces of the blocks or grains have a wide distribution of orientations and shearing of the blocks past one another on these planes accommodates the large scale motion i e the macromotion in the fault zone during a macromotion that is a non coaxial deformation the rigid blocks and their surfaces may undergo a progressive rigid rotation that is distinct from the macromotion and is described by the microspin if a macroscopic non coaxial deformation has monoclinic symmetry this symmetry should be reflected in the sense of rigid rotation of the blocks and their surfaces on the surfaces of the blocks which are local shear planes the history of displacement may be recorded by mineral fibers that grow progessively with displacement because of the rigid rotation of the local shear planes slickenfiber lineations commonly are curved the sense of curvature from the youngest to the oldest part of the fiber looking down the normal to the local shear plane is different for those planes whose normals are on opposite sides of the unique monoclinic symmetry plane for the macroscopic shearing the intersection of the symmetry plane with the plane of the fault zone defines the macroscopic slip direction and the sense of rigid rotation of the local shear planes determined from the lineations defines the shear sense application of this technique to the disrupted and sheared margins of the feather river peridotite in the northern sierra nevada of california indicates that late deformation involved dextral normal oblique slip along the melones fault zone

fine scale partitioning of contemporary strain in the southern walker lane implications for accommodating divergent strike slip motion treating the crust as a micropolar continuum and inverse modeling a high quality catalog of earthquake focal mechanisms reveals that km scale heterogeneous strain is partitioned at km scales into pairs of plane strains with sub perpendicular principal axes these earthquakes occurred in a shallow crustal volume centered on the western indian wells valley of eastern california the site of divergent right lateral strike slip motion along the boundary between the sierran microplate and the southern basin and range province the seismic events defining the plane strains occur in a diffuse seismic source zone and cannot be additionally subdivided on the basis of the type of strain they accommodate the results indicate that seismic moment release for background earthquakes is partitioned into events associated with either fault zone perpendicular extension by normal faults or horizontal shearing by conjugate strike slip faults the spatial scales over which this partitioning occurs suggest that outcrop scale brittle faults may in some cases record nearly perpendicular plane strains related to a single continuous deformation

regional state of stress fault kinematics and adjustments of blocks in a fractured body of rock application to the microseismicity of the rhine graben a simple mechanical model of a homogeneous state of stress and independent fault motions is generally used to interpret fault kinematics in terms of stress however analysis of focal mechanisms of microearthquakes shows that as a result of incompatibilities of block motions in an assemblage of rigid or elastic blocks deformation of a fractured body of rocks is in general heterogeneous analyses of focal mechanism sets show that they are in general composed of a main group whose fault motions result from a mean state of stress tm in good agreement with the regional state of stress the remaining complex fault motions are explained by a compressional tc and or a tensional deviator te an important observation is that within the computed uncertainties these tc and te deviators are co axial with the mean state of stress tm these deviators statistically model the local fault slips probably due to the incompatibilities of block motions and cannot be considered as other mean states of stress thus if the regional state of stress is known for a fault plane whose strike and dip are given its kinematics cannot be surely predicted two types tc and te of motions different from that tm predicted by the regional state of stress may exist they are the kinematic instabilities as a consequence all the motions shown by focal mechanisms or by striated faults are not significant of the regional tectonic regime this is well illustrated by the microseismicity of the rhine graben where it is shown that the compressional fault motions are kinematic instabilities and the extensional fault motions are representative of a regional state of stress 1 vertical with a n45 trending 3 axis hence the simple mechanical model of homogeneous state of stress and independant fault motions is valid only if there are no incompatibilities of block motions possibly these incompatibilities may be accommodated by a pressure solution crystallization process for creep faulting or by internal cataclastic deformation of large blocks separated by major stick slip faults

fault kinematics and past plate motions at a convergent plate boundary tertiary shimanto belt southwest japan early and late stage outcrop scale faults have been documented in paleogene strata of the muroto peninsula of southwest japan the former were active early in the consolidation history of the sediment and are crosscut by the latter which were active after regional scale imbrication folding and penetrative deformation i e regional spaced pressure solution cleavage orientation distributions of striae and kinematic axes indicate that the two generations of faults reflect distinct kinematic regimes the orientations and crosscutting relations indicate a counterclockwise viewed downward rotation of the principal shortening direction between early and late stage faulting existing fossil and radiometric age data suggest this transition occurred during eocene oligocene time the fault kinematic data are interpreted to reflect a change in relative plate motions between eurasia and an oceanic plate subducting beneath eurasia in paleogene time these results coupled with existing paleothermal and paleopressure data are consistent with the presence of a spreading ridge in the western pacific basin during this time

paleomagnetism and geology of eocene volcanic rocks of southwest washington implications for mechanisms of tectonic rotation paleomagnetic and geologic investigations in eocene volcanic rocks of the southwest washington coast range demonstrate a close relationship between tectonic rotations and the local structural geology the allochthonous middle eocene submarine basalt basement of the crescent formation consists of several fault bounded structural domains up to 30 km across that are characterized by different amounts of clockwise rotation 20 to perhaps as much as 65 when compared to the eocene reference pole for north america structural analysis shows the differential rotations postdate middle eocene folding of the crescent formation against the continental margin and predate the unconformably overlying upper eocene goble volcanics which are rotated about 23 and do not show the same domains of rotation as the underlying crescent formation post goble rotations may be accommodated by a fault pattern very similar to that expected for areas caught in a simple dextral shear couple along transcurrent faults major north northwest trending faults with several kilometers of dextral displacement form the boundaries of cross faulted shear domains in which the clockwise rotation of elongate crustal slices is accommodated by west northwest trending sinistral r riedel shears thirty five to 100 of the observed post late eocene rotations could have occurred by this shear rotation mechanism other paleomagnetic study areas in the coast range and western cascades have a similar fault geometry and may also have undergone significant shear rotations long term northward oblique subduction of the farallon plate beneath the coast range throughout most of the tertiary could have been the driving force for the shear rotations and could explain the rapid eastward decrease in rotation away from the continental margin shear rotations could eliminate many of the structural and stratigraphic difficulties associated with models involving rotation of large rigid plates

evidence for the importance of small faults on block rotation field and earthquake data are presented which show that small faults can be important during block rotation small here means those faults which are at least an order of magnitude smaller than the largest faults at a particular scale of observation the nw se trending dextral faults of sw england have millimetres to kilometres of displacement the largest of these faults such as the sticklepath lustleigh fault do not appear to cause more than a few degrees of rotation from the general e w strike of beds tens of degrees of rotation are often visible however on faults with millimetres of displacement similarly the apparent rotation of beds in the badajoz cordoba shear zone spain increases as the resolution of the faults is increased the power law scaling relationship of earthquake magnitudes in the san andreas fault zone illustrates that small faults can also be important in regions of active block rotation small faults can allow deformation within rotating blocks and can allow high displacement gradients to occur on the block bounding faults this would reduce the need for void creation which is a requirement of rigid block rotation models a tentative model is presented which incorporates the concept that fault behaviour is fractal emphasising the contribution of small faults it is suggested that future studies of block rotation should rigorously test the contribution of small faults and that particular care is needed when comparing palaeomagnetic data with regional scale structures

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

strain partitioning and crack seal growth of chlorite muscovite aggregates during progressive noncoaxial strain an example from the slate belt of taiwan abstract chlorite muscovite aggregates from the slate belt of southeastern taiwan have an ellipsoidal geometry k 1 on a flinn diagram with an aspect ratio a c ranging from about 7 1 to 12 1 and a crystallographic preferred orientation with the basal planes of the chlorite and muscovite at a high angle to the slaty fabric in the rock the principal axes of the aggregates are approximately coaxial with the principal finite strain directions determined from fibrous pressure shadows in cleavage parallel sections straight fibers parallel to the downdip mineral lineation record plane strain in cleavage perpendicular sections however pressure shadows have curved fibers that are consistent with w directed thrusting we propose a model in which the chlorite mica aggregates grow syntectonically by cracking of detrital chlorite grains parallel to the basal planes and precipitation of secondary chlorite and mica in the open cracks this model involves 1 rotation of preexiting detrital chlorite grains during progressive noncoaxial deformation 2 cracking of these grains parallel to the basal plane when the 001 planes rotate into the incremental shortening field with nucleation and growth of secondary chlorite and muscovite in the open microcracks as the aspect ratio increases the grains stabilize in orientation with the long axes normal to the basal planes at a small angle to the shear direction extension normal to 001 is also accompanied by some shortening parallel to 001 but with an overall increase in size of the aggregates the final grain size is controlled by the cumulative displacement associated with crack seal events microprobe analyses of both detrital and secondary chlorite grains show distinct compositional differences within individual grains perpendicular to 001 particularly in the detrital grains which we interpret as primary and secondary chlorite consistent with a primary and secondary origin for the aggregates

lava fingerprinting using paleomagnetism and innovative x ray fluorescence spectroscopy a case study from the coso volcanic field california 1 we demonstrate an efficient method of rigorously separating difficult to distinguish lavas into eruptive units based on paleomagnetic remanence direction and rapid x ray fluorescence spectroscopy xrf for rb sr y zr and nb of polished paleomagnetic core samples called pc xrf combined use of paleomagnetic remanence and pc xrf for lava fingerprinting allows correlation of individual eruptive units from one locality to another permitting compilation of composite stratigraphy and paleomagentic measurement of relative vertical axis rotation of fault bounded blocks we test this lava fingerprinting method on rocks from the coso volcanic field california against similar fingerprinting using xrf analysis by established methods resulting unit definitions and correlations are the same by both xrf techniques when coupled with paleomagnetic data but at great time and cost savings with pc xrf pc xrf analysis is possible because 1 matrix and grain size effects are minimal for the element set analyzed 2 moderately phyric to aphyric polished paleomagnetic cores are already homogenous on spatial scales of 2 cm and 3 use of element ratios cancels out some analytical uncertainties as well as minimizes effects of concentration variations due to fractional crystallization paleomagnetic remanence direction is an indispensable part of fingerprinting because it distinguishes flows of similar chemistry and can also place constraints on the duration of emplacement of each eruptive unit

fault block kinematics at a releasing stepover of the eastern california shear zone partitioning of rotation style in and around the coso geothermal area and nascent metamorphic core complex abstract pliocene lavas and sediments of wild horse mesa in the coso range ca exhibit clockwise vertical axis rotation of fault bounded blocks this indicates localization of one strand of the eastern california shear zone walker lane belt within a large scale transtensional dextral releasing stepover we measured rotations paleomagnetically relative to two different reference frames at two localities we averaged secular variation through sedimentary sections to reveal rotation or its absence relative to paleogeographic north where sediments are lacking we used areally extensive lava flows from individual cooling units or short eruptive episodes to measure the relative rotation of localities by comparing their paleomagnetic remanence directions to one another at the western edge of wild horse mesa the fanglomerate member of the coso formation c a 3 ma exhibits between 8 4 7 8 and 26 2 9 0 two endmember models of a continuum absolute clockwise rotation within wild horse mesa 3 3 5 ma lavas at 5 different localities exhibit about 12 0 4 6 weighted mean clockwise rotation relative to the margins of the area a result statistically indistinguishable from the absolute rotation hence the segment of the eastern california shear zone passing through wild horse mesa has caused vertical axis rotation of fault bounded blocks as part of the overall dextral shear strain the magnitude of block rotation at wild horse mesa suggests that rotation has accommodated 1 1 5 km of dextral shear along an azimuth of about north 30 west since ca 3 ma between the area s bounding faults and 2 2 km of extension perpendicular to the coso wash normal fault during this same period this corresponds to 13 25 extension across the mesa in contrast to wild horse mesa the opposite western side of the trace of the coso wash normal fault hosts the coso geothermal area and what monastero et al f c monastero a m katzenstein j s miller j r unruh m c adams k richards dinger the coso geothermal field a nascent metamorphic core complex geol soc amer bull 117 2005 1534 1553 characterize as a nascent metamorphic core complex consistent with upper plate disruption above a detachment surface rocks i e the upper plate of the detachment system at the coso geothermal area are tilted westward however they appear to exhibit no detectable rotation thus the style of block rotation may be partitioned with clockwise vertical axis rotation dominating in the wild horse mesa and horizontal axis rotation tilting in the geothermal area

modeling the shortening history of a fault tip fold using structural and geomorphic records of deformation 1 we present a methodology to derive the growth history of a fault tip fold above a basal detachment our approach is based on modeling the stratigraphic and geomorphic records of deformation as well as the finite structure of the fold constrained from seismic profiles we parameterize the spatial deformation pattern using a simple formulation of the displacement field derived from sandbox experiments assuming a stationary spatial pattern of deformation we simulate the gradual warping and uplift of stratigraphic and geomorphic markers which provides an estimate of the cumulative amounts of shortening they have recorded this approach allows modeling of isolated terraces or growth strata we apply this method to the study of two fault tip folds in the tien shan the yakeng and anjihai anticlines documenting their deformation history over the past 6 7 myr we show that the modern shortening rates can be estimated from the width of the fold topography provided that the sedimentation rate is known yielding respective rates of 2 15 and 1 12 mm yr across yakeng and anjihai consistent with the deformation recorded by fluvial and alluvial terraces this study demonstrates that the shortening rates across both folds accelerated significantly since the onset of folding it also illustrates the usefulness of a simple geometric folding model and highlights the importance of considering local interactions between tectonic deformation sedimentation and erosion

composition of modern stream sands derived from sedimentary source rocks in a temperate climate northern apennines italy abstract the northern apennine is a moderate relief thrust fold belt maximum elevation of less than 2000 m above sea level lying in an area of temperate climate drained by short mean 50 km transversal streams flowing northwards into the po river main channel the mountain range is mostly made up of sedimentary rocks with minor ophiolite slices the source rock types of nine contiguous watersheds draining an overall area of more than 2800 km 2 were quantitatively defined through a geographical information system gis the bedrock lithology at the source area was compared with the composition of the medium sized fluvial sand to evaluate how the various types of sedimentary source rocks are represented in the stream sand population the sand provenance approach has two approximation levels the first level distinguishes the source rock types and constrains some assumptions about the degree of recycling the second level calculates the contribution of different source rock types according to the previous results provenance modelling obtained through this method fits well with the real geology of the eroded landscape this provides an actualistic example that can be useful for a quantitative provenance approach to the study of ancient sediments fed by recycled sedimentary thrust fold belts

theory of slickenline patterns based on the velocity gradient tensor and microrotation abstract brittle fault zones commonly are characterized by a multitude of lineated shear planes having a wide distribution of orientations in our model we assume the faulted rock comprises an aggregate of rigid blocks whose surfaces are the shear planes we define two independent scales of motion to distinguish the local rigid rotation of the blocks with their shear planes the micromotion from the average deformation of the material on a macroscopic scale the macromotion the macromotion is defined by the macrovelocity gradient tensor which is separated into symmetric and antisymmetric parts the deformation rate and the macrospin tensors respectively the micromotion is defined by a microspin tensor on any shear plane slickenlines form parallel to the direction of the maximum rate of shear which we assume is determined by the direction of the maximum component of the macrovelocity gradient tensor tangent to the shear plane and by a component of the microspin in the shear plane for a restricted kinematic model patterns of slickenline orientations are defined on a uniform distribution of shear plane orientations by the values of d and w d is a ratio of differences in the principal values of the deformation rate tensor and w is a ratio of the net spin to the maximum rate of macroshear we distinguish between instantaneous slickenline patterns and finite deformation patterns the two are different if the shear planes rotate relative to the principal deformation rate axes in which case finite deformation slickenlines form curved or crossing lineations if w 0 the instantaneous slickenline patterns have orthorhombic symmetry 0 d d 1 or 0 if w 0 the instantaneous slickenline patterns have monoclinic symmetry for pure shear in an isotropic body instantaneous slickenline patterns have orthorhombic symmetry w 0 d 0 5 whereas for pure shear accommodated by shear on a planar anisotropy w 1 d 0 5 the pattern is monoclinic for simple shear with the rigid rotation of local shear planes defined by the shear induced spin w 0 d 0 5 the instantaneous slickenline pattern is orthorhombic but the finite deformation pattern is monoclinic for simple shear with no shear plane rotation w 1 d 0 5 the instantaneous pattern is monoclinic special cases of our hypothesis are equivalent to the hypothesis that slickenlines are parallel to the direction of maximum resolved shear stress e g angelier 1979 1984 the slickenline patterns predicted for this case are always of orthorhombic or higher symmetry and are equivalent to our instantaneous patterns for which w 0 and to our finite deformation patterns for which in addition there is no microspin