sequence age and source of silicic fallout tuffs in middle to late miocene basins of the northern basin and range province the latest cenozoic 6 ma ash beds in the western united states have been intensively studied for several decades the more widespread of these ash beds are well documented event horizons that are of great value in studies of the timing and pace of geological climatological and biological events throughout the region because explosive volcanism was not restricted to latest neogene time in this region many older ash beds are likely to prove as useful as younger beds as event horizons once they are located characterized and dated as a first step in developing a useful chronology of older cenozoic ash beds in the western united states we have sampled and analyzed silicic fallout tuffs in middle to late miocene sedimentary basins across the northern basin and range province the northern basin and range basins ideally situated in the vicinity of major coeval silicic volcanic centers contain numerous relatively unaltered silicic fallout tuffs we have correlated tuffs between all sampled sections on the basis of glass shard composition the composite stratigraphic sequence established by the correlations contains more than 200 individual tuffs including 59 widely distributed tuffs termed correlative tuffs the tuffs vary widely in composition but most are in one of two compositional groups gray metaluminous vitric tuffs gm tuffs or white metaluminous vitric tuffs wm tuffs distribution patterns compositional characteristics and correlation with ash flow tuffs show that the source for most gm tuffs was the snake river plain volcanic province along the northern edge of the northern basin and range and the source for most wm tuffs was the southwestern nevada volcanic field in the southern part of the northern basin and range the northern basin and range tuffs range in age from ca 16 6 ma the ages of individual tuffs are determined variously by direct isotopic dating by correlation to previously dated fallout and ash flow tuffs or by interpolation age estimation ages for most tuffs are known to within 0 25 m y 1 or less and for many tuffs to within 0 1 m y or less the sequence and ages of tuffs established in this study provide insights into the evolution of the northern basin and range basins and patterns of explosive volcanism in coeval volcanic centers and contribute to the development of a high resolution stratigraphy and chronology of coeval sedimentary deposits throughout the western united states

temporal changes in fault strike to 90 and extension directions during multiple episodes of extension an example from eastern nevada in areas with prolonged extensional histories or multiple episodes of extension temporally distinct extensional fault systems including dip slip normal oblique slip and or strike slip faults may form these fault systems may be spatially distinct overlapped or completely superimposed and different fault systems may have different fault strikes and extension directions although fault strike is not a very sensitive indicator of principal stress directions changes through time in fault strike of as much as 90 exceed the uncertainty overprinted extensional fault systems of different ages with significantly different to 90 fault strikes and extension directions provide strong evidence of changes in the regional stress field with time tens of millions of years of extension occurred in the basin and range province of the united states and many temporally distinct fault sets formed through that time we document at least four extensional episodes and the fault kinematics of each where possible in the central hiko range eastern nevada the four cenozoic extensional events are prevolcanic synvolcanic postvolcanic miocene pliocene and postvolcanic pliocene quaternary the prevolcanic faults strike north south and are interpreted as footwall faults to the regional eocene oligocene snake stampede extensional system which caused approximately east west extension the synvolcanic faults are late oligocene miocene age and strike east west although they are rare and have minor offset they accommodated north south directed extension postvolcanic faults part of the timpahute lineament are approximately east west striking normal and northeast striking oblique slip faults they also accommodated approximately north south extension during miocene time the northeast striking left normal oblique slip fault set formed as new faults and records a change between north south and east west extension in miocene time later east west extension occurred along the pliocene to quaternary hiko fault zone these four overprinted extensional systems show two separate changes in normal fault strike of 90 from north south to east west and from east west back to north south consequently the extension direction similarly changed through time we suggest that causes for the changes in direction of upper crustal extension include a change from stresses created by 1 plate interactions which generated approximately east west extension to 2 mantle motions which generated approximately north south extension and 3 back again the extension related to mantle motions is associated with a band of volcanism that passed across the northern basin and range province moving generally from north to south during the protracted period of extension therefore a change from plate boundary or interaction derived stresses to mantle or volcanism related stresses and back again is possible

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

geometry mechanisms and significance of extensional folds from examples in the rocky mountain basin and range province u s a abstract geologic mapping and structural analysis in paleogene half graben of idaho and montana have revealed over 70 extensional folds that form orthogonal sets with mostly ne and se trending axes on a regional scale they parallel or lie perpendicular to the strikes of the largest normal faults transverse folds at a high angle to the fault and folds oblique to the fault comprise more than half of the folds and reflect the highly three dimensional nature of the bulk strain detailed geometric analysis of 15 folds in the salmon horse prairie and medicine lodge basins shows that they typically have an upright to steeply inclined bisecting surface an interlimb angle of 141 and a plunge of 19 based on our synthesis of the eight common mechanisms of extensional folding and their distinguishing characteristics we were able to determine how some of the investigated folds formed fault bend folding above both fault parallel and fault perpendicular bends in the underlying normal fault produced most of the folds in the study area but displacement gradients along normal faults fault drag isostatic adjustments and transtension were also factors in the deformation most of the compound folds which result from more than one mechanism of folding are oblique to all adjacent normal faults recognition of such extensional folds is critical because they might be misinterpreted as contractional structures they influence sediment thickness patterns and dispersal in rift basins and may control the migration and trapping of petroleum and groundwater resources

evolution of a metamorphic fluid and its role in lode gold mineralisation in the central lberian zone the central iberian zone of spain and portugal contains numerous gold deposits of varying size age and host lithology previous work concentrated on single deposits leading to a need to apply a range of techniques to the region as a whole to form a model for metallogenesis throughout the region with this aim a number of occurrences of gold mineralisation in the south western central iberian zone were studied the central iberian zone consists essentially of greenschist facies metasedimentary units intruded by hercynian granitoids hercynian deformation occurred in three main stages the earliest d1 being the most intense quartz veining is common throughout the region and a vein chronology was established relative to these deformation events v1 veins are pre hercynian and may predate the precambrian sardic deformation v2 veins are syndeformational d1 and associated with peak greenschist metamorphism v3 veins are late kinematic syn to post d2 v4 veins are syn to post d3 occurring in the final stages of the hercynian and often associated with or post dating granites reactivation of vein sets by later fluids was common fluid chemistry studies of samples of known chronology indicate three main sources of fluid the dominant fluid throughout the region from v2 to v4 is a dilute aqueo carbonic fluid of varying composition the combination of fluid inclusion analyses and geochemical studies of wallrock alteration indicates that the fluid was the result of devolatilisation and dehydration during prograde metamorphism at varying depths the resulting aqueo carbonic fluids flowed upwards mixing with each other and with meteoric waters contact metamorphism associated with granite intrusions during the d3 deformation released further aqueo carbonic fluids which in some cases unmixed to form separate carbonic and aqueous inclusions circulation of meteoric waters was important during later stages d3 prolonged low temperature fluid rock interaction produced highly saline aqueous fluids which post date all major quartz veining no single set of characteristics exists for gold mineralisation in the central iberian zone deposits cover a wide spectrum but three main types have been identified 1 early d1 v2 syn peak metamorphic gold quartz veins associated with an aqueo carbonic fluid with high n2 and ch4 2 late kinematic d2 v3 veins often associated with late brittle faults again the fluid present at the time of formation was aqueo carbonic but with lower n2 and ch4 3 late v4 low temperature veins formed by a mainly meteoric fluid which was also responsible for remobilisation and concentration of gold in earlier deposits the gold mineralisation in the central iberian zone supports the continuum model for lode gold deposits gold was leached from metasediments during prograde metamorphism transported to shallower crustal levels and deposited wherever suitable traps occurred channelling of fluid from deeper levels resulted in the localisation of deposits close to major shear zones depositional sites were usually dilational zones in faults and more rarely folds but locally lithological controls were important h2o co2 unmixing was common but was not necessarily related to gold deposition

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

strain partitioning in the northern walker lane western nevada and northeastern california abstract paleomagnetic studies constrain both the kinematics and the timing of neogene intracontinental strain in the walker lane of northwestern nevada northeastern california the northwest trending walker lane is a zone of complex faulting and dextral offset along the boundary between the sierra nevada and the basin and range here an extensive tertiary volcanic section provides a paleomagnetic test for vertical axis rotation the 25 1 ma nine hill tuff covered the two structural domains of northern walker lane and parts of the adjacent provinces in the northernmost domain the pyramid lake domain the nine hill tuff is unrotated or rotated slightly counter clockwise in contrast nine hill tuff is rotated 35 44 clockwise in the northeastern part of the adjacent carson domain usable exposures of the tuff are lacking elsewhere in the carson domain but 9 13 ma basalts record 37 51 clockwise vertical axis rotation these basalts were sampled at three sites encompassing the n s length of the carson domain younger basalts from the carson domain show smaller clockwise rotations the paleomagnetic studies show that the deformation in the northern walker lane is partitioned into domains dominated by translation rotation and extension dextral slip along the walker lane is accommodated primarily by northwestward translation in the pyramid lake domain and by clockwise vertical axis rotation in the carson domain some neogene extension is also accommodated in the walker lane but most of it is occurring to the west of the walker lane along the edge of the sierra nevada comparison of paleomagnetic results from rocks of different ages constrains the timing of the present deformation regime in the walker lane the similar rotations recorded in 25 ma and 9 13 ma rocks indicates that virtually all of the rotation in the carson domain occurred after the eruption of the 9 13 ma basalts smaller rotations recorded in younger rocks constrain rotation rate

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

origin ore forming fluid evolution and timing of the logrosan sn w ore deposits central iberian zone spain the logrosan sn w ore deposits in the metallogenic sn w province of the european variscan belt consist of endo and exogranitic greisen type and quartz cassiterite veins associated with a s type granite mineral characterization fluid inclusion study isotope geochemistry and ar ar geochronology have been combined in order to reconstruct the conditions for sn w mineralization the endo and exogranitic mineralization must have been developed in a relatively long lived system 308 303 ma during or soon after the emplacement of the logrosan related granite at ca 308 ma the mineralizing fluids are characterized by complex aqueous and volatile h2o n2 co2 ch4 nacl fluid inclusions microthermometry and raman analyses indicate that fluid composition evolved from n2 ch4 to n2 rich followed by co2 rich fluids with varying amounts of h2o the presence of n2 and ch4 suggests the interaction with fluids derived from the nearby metasedimentary host rocks a model of host rock interaction assimilation and mixing of metamorphic and magmatic fluids resulting in change of the redox conditions is proposed for tin deposition later sulfide minerals were precipitated as a result of pressure and temperature release

raw material quality and oldowan hominin toolstone preferences evidence from kanjera south kenya the role of raw material quality in oldowan technology has not been fully explored there are numerous studies suggesting oldowan hominins preferred certain types of stone for artifact manufacture previous studies of the artifact assemblage from the early pliocene oldowan locality of kanjera south south rachuonyo district kenya show that raw material selection and transport was an important aspect of late pliocene hominin adaptations yet the exact properties of stones that hominins were selecting remain enigmatic two potentially important features of artifact raw material are durability and fracture predictability we investigate fracture predictability through mechanical tests of stone and investigations of the affect of stone properties on fracture patterns in archaeological collections we investigate stone durability with actualistic studies of edge attrition combined with further mechanical tests of various lithologies oldowan hominins at kanjera appear to have selected raw materials based on their durability the ability for a stone to fracture consistently does not appear to be as important in hominin toolstone preference as previously assumed hominins that produced the assemblages at kanjera south appear to have incorporated an extensive understanding of various attributes of raw material in the transport and production of stone artifacts when combined with previous research on the transport patterns at kanjera the results of this study provide evidence for a more complex raw material acquisition strategy than has previously been suggested for late pliocene oldowan hominins

cretaceous to early tertiary paleogeography in the hinterland of the sevier thrust belt east central nevada the cretaceous to early tertiary hinterland of the sevier thrust belt in east central nevada underwent two periods of sedimentary basin development the older early cretaceous episode is characterized by basins that formed coeval with deformation within a belt of contractional structures basins were characterized by periods of through flowing fluvial drainage the younger late cretaceous to eocene episode of basin development was characterized by internally drained alluvial basins that subsequently expanded into more regional lacustrine basins basal alluvial detritus indicates that basin inception occurred as a result of rapid development of topographic relief syn sedimentary tectonism was characterized by horizontal extension due to attainment of equilibrium topography in response to crustal thickening

formation evolution and inversion of the middle tertiary diligencia basin orocopia mountains southern california the diligencia basin located in the eastern orocopia mountains of southern california contains 1500 2000 m of oligocene miocene continental siliciclastic sedimentary rocks and subordinate limestone and evaporite deposits intercalated with basaltic lavas these rocks are locally intensely folded and faulted defining in present day coordinates an elongate east west trending basin and are unconformably overlain by flat lying late pliocene and pleistocene alluvial deposits the sedimentological stratigraphic and structural history of the basin is compatible with late oligocene early miocene formation as a half graben basin produced by orthogonal basin and range type extension and latest miocene to pliocene basin inversion in either a localized transpressional transverse range regime or a more diffuse compressional regime associated with north south shortening of the entire mojave block facies associations in the lower part of the diligencia basin display a distinctly asymmetric distribution across the basin indicating deposition in a half graben with a steep possibly fault controlled south facing northern escarpment and a more gentle north facing southern slope paleocurrent data particularly from high energy deposits on the northern basin margin indicate stream flow toward the southeast to south southeast oblique to the deformed basin margins and suggest an approximate northeast to east northeast strike for the basin before deformation previously published paleomagnetic data however indicate that the elongate currently east west trending fault block containing the diligencia basin has rotated clockwise by as much as 110 about a vertical axis angle depends on data and model used since deposition ceased if correct this would suggest that the basin may have originally opened to between the northeast and east northeast subparallel to well documented extension directions in miocene age basins in the mojave desert to the east clockwise rotation on east west fault blocks exposed in this region has previously been bracketed as between ca 10 and 4 5 ma structural and paleomagnetic data indicate that inversion of the diligencia basin occurred after block rotation implying a latest miocene to pliocene age for inversion we speculate that basin inversion within a north south and still active compressive stress field resulted from the locking and subsequent internal deformation of this previously passively rotating elongate crustal block

late cenozoic extensional transfer in the walker lane strike slip belt nevada mid miocene to pliocene 11 to 6 ma displacement on the shallowly northwest dipping silver peak lone mountain detachment system transferred 30 to 40 km of right slip from the furnace creek transcurrent fault 85 km east northeast to the central walker lane strike slip faults top to the northwest slip on the detachment was accompanied by upper plate extension on steeply dipping faults and large scale exhumation of midcrustal rocks of the lower plate complex doubly plunging northwest trending folds turtleback structures control the outcrop distribution of the extensional complex and formed in response to simple shear of the footwall complex activity on the detachment ceased as miocene pliocene 6 4 8 ma volcanic rocks were erupted and deformed during the final stage of northwest trending folding late pliocene to holocene displacement transfer shifted north to a curved array of steeply dipping faults that may be underlain by a shallowly northwest dipping detachment

sierra nevada basin and range transition near reno nevada two stage development at 12 and 3 ma relative and absolute elevations of the sierra nevada and adjacent basin and range province timing of their differentiation and location amount and timing of strike slip movement between them are controversial the provincial boundary near reno developed in two stages 1 at ca 12 ma the 700 km 2 verdi boca sedimentary basin formed across what was to become the boundary probably as a result of a small magnitude but regional extensional episode that affected much of the western basin and range 2 at 3 ma the basin was complexly faulted and folded during a larger magnitude extensional episode that established the modern sierran structural and topographic boundary in this area the boundary is really a transition zone with a western edge along the donner pass california fault zone which is farther west than previously placed both episodes appear to have resulted from east west extension only which suggests that northwest motion of the sierra nevada relative to the basin and range shown by geodetic data began after 3 ma or was taken up farther east

spatial and temporal distribution of the orogenic gold deposits in the late palaeozoic variscides and southern tianshan how orogenic are they abstract a principal uncertainty in models of orogenic ore deposits concerns their ages relative to orogenic processes the yardstick of the relation has resided loosely in the peak of metamorphism age estimates in the variscides and tianshan indicate that most orogenic ore deposits were formed in the course of the late carboniferous to middle triassic with a peak between 305 and 280 ma their locations settings and ages suggest an association with coeval lithosphere scale strike slip deformation which played a role in the destruction of the orogenic system following crustal shortening this destruction tends to invalidate the orogenic association of the orogenic ore deposits observation of a mantle signature in some of these ore deposits and in coeval magmatic rocks suggests that the strike slip belts tapped into sub crustal levels here lithological composition had been modified by subduction during the stage of construction consequently any orogenic element in the ore deposit models is apart from location only an element of inheritance of subduction modified lithosphere features acquired during construction of the orogen these features may however well have been crucial for the formation of the ore deposits in view of the translithospheric extent of the strike slip belts fluids which contributed to the ore deposits may have come from at least the entire in part metasomatized lithosphere column the ore deposits could probably not have been formed without these deep reaching strike slip systems a model is proposed for the geological setting of the orogenic gold deposits in the variscides and the southern tianshan it consists of plutono metamorphic elements in the mantle in the lower and in the middle crust within a transpressional to transtensional tectonic framework of translithospheric faults and associated domes and pull apart basins with shallow marine to terrestrial sediments and interlayered felsic and mafic volcanics the thermal engines of these systems were probably in localized upwellings of the asthenosphere prompted and controlled by the lithosphere scale deformation at the time at an even larger scale oroclinal bending of the old orogenic backbone may have played yet another role in the localization of the ore deposits the thesis of specific localized tectono thermal engines is at odds with the original proposals of a relation between orogenic ore deposits and regional metamorphism it suggests that the late palaeozoic gold antimony mercury ore deposits in the variscides and the southern tianshan are more akin to the intrusion related ore systems

fluid rock interaction processes related to hydrothermal vein type mineralization in the siegerland district germany implications from inorganic and organic alteration patterns altered wallrocks of vein type pb zn sb mineralization siegerland district rheinisches schiefergebirge have been investigated by a combination of inorganic and organic geochemical methods including major and trace element analysis vitrinite reflectance measurements c isotope and elemental analysis of kerogen alteration features of the siliciclastic pelitic psammitic lower devonian wallrocks are increased k na ratios significant desilicification and relative immobility of a number of elements notably al ti zr cr v wallrock kerogens display elevated vitrinite reflectance values decrease in h c atomic ratios coupled with increase in s c atomic ratios and heavier c isotope compositions compared to the unaltered precursor sedimentary rocks interaction processes between the hydrothermal fluids and the respective wallrocks related to injection of high temperature silica undersaturated solutions are dominated by quartz dissolution coupled with sericitization reactions heat transfer due to fluid infiltration convection and wallrock reactions caused fluid cooling which promoted the sequential deposition of quartz and stibnite sulphosalts within the vein systems hydrocarbons detected in ore assemblages of pb zn and sb mineralization were most probably derived from the lower devonian very low grade meta sedimentary rocks high maturity levels and pronounced typical organic alteration patterns indicate that thermochemical so2 4 reduction tsr played an important role in precipitation of metal sulphides the present study demonstrates that a combination of inorganic and organic investigations on fluid rock interaction processes is particularly useful for deciphering precipitation mechanisms of base metal sulphides

quantitative edxrf studies of obsidian sources in northern hokkaido quantitative trace element data from six obsidian sources on the japanese island of hokkaido is presented previous work by japanese scholars has utilized neutron activation analysis naa focusing on the rare earth elements or qualitative energy dispersive x ray fluorescence edxrf in this study non destructive edxrf is used to generate trace element data for 9 elements bivariate plots of the incompatible trace elements ba rb sr y and zr can be used to separate the obsidian sources in this study source separation was confirmed by using linear discriminant analysis stepwise discriminant analysis indicates that ba sr rb ti y and zr are the most discriminating elements

geochemical and physical characterization of lithic raw materials in the olduvai basin tanzania abstract the invention and proliferation of stone tool technology in the early stone age esa marks a watershed in human evolution patterns of lithic procurement manufacture use and discard have much to tell us about esa hominin cognition and land use however these issues cannot be fully explored outside the context of the physical attributes and spatio temporal availability of the lithic raw materials themselves the olduvai basin of northern tanzania which is home to both a wide variety of potential toolstones and a rich collection of esa archaeological sites provides an excellent opportunity to investigate the relationship between lithic technology and raw material characteristics here we examine two attributes of the basin s igneous and metamorphic rocks spatial location and fracture predictability a total of 244 geological specimens were analyzed with non destructive portable xrf pxrf to determine the geochemical distinctiveness of five primary and secondary sources while 110 geological specimens were subjected to schmidt rebound hardness tests to measure fracture predictability element concentrations derived via pxrf show significant differences between sources and multivariate predictive models classify geological specimens with 75 80 accuracy the predictive models identify naibor soit as the most likely source for a small sample of three lithic artifacts from bed ii which supports the idea that this inselberg served as a source of toolstone during the early pleistocene clear patterns in fracture predictability exist within and between both sources and rock types fine grained volcanics show high rebound values associated with high fracture predictability while finer grained metamorphics and coarse grained gneisses show intermediate and low rebound values respectively artifact data from bed i and ii suggest that fracture predictability played a role in raw material selection at some sites but other attributes like durability expediency and nodule size and shape were more significant

provenancing of hominid and mammalian fossils from kanjera kenya using edxrf abstract we describe a rapid non destructive energy dispersive x ray fluorescence edxrf technique for provenancing fossil bone and apply it to surface collected fossils from kanjera kenya the locality is best known for its controversial anatomically modern human fossils potentially among the earliest from africa excavations of the 1987 1988 smithsonian expedition provided the in situ fossil sample necessary to provenance the previous finds relative sr y zr u rb rn and th concentrations were used to characterize 301 fossils from six stratigraphic units the sources of the surface fossils important fossils from past kanjera expeditions including the hominid and theropithecus oswaldi samples were analysed and their relative elemental values used to assign them to a reference sample the hominids post date kanjera formation deposition the t oswaldi fossils are largely attributable to the early pleistocene kanjera bed kn 2a secondary deposition of kanjera formation fossils into later strata was noted

mineralogical petrographic and geochemical study to identify the provenance of limestone from two archaeological sites in the sulmona area l aquila italy abstract mineralogical petrographic paleontological and geochemical studies were carried out in order to identify the limestone used to build the ercole curino sanctuary iv iii centuries b c and the foundation walls of a roman domus i century b c ii century a d which was brought to light during restoration works in 1991 in the s s annunziata complex xiv xv centuries a d at sulmona l aquila italy results identified the lithofacies of the stone used for these monuments most of the ercole curino sanctuary samples are calcareous dolomitic lithotypes from the dolomie di castelmanfrino formation lias dogger samples from the artistically more remarkable stone show features similar to those of the limestone of the calcari cristallini formation upper cretaceous paleocene samples from the s s annunziata complex show petrographic characteristics and fossil contents similar to those of the limestone of the terratta formation dogger lower cretaceous the provenance of the monuments stone from a quite small area is supported by the homogeneity of trace element contents being substantially close to those of samples collected from outcrops near the monument for the ercole curino sanctuary and at cave di introdacqua located at about 3 km sw sulmona for the s s annunziata complex carbon and oxygen isotopic compositions supply further support to the identity of provenance of the stone of the sanctuary on the contrary isotopic data of samples from the domus at the s s annunziata complex do not reveal any consistent or clear correspondence with the sampled geological formations the stone used in the artistically more valuable features of the sanctuary shows chemical characteristics similar to those of samples collected at the colle mitra outcrop about 5 km from the archaeological site

oldowan behavior and raw material transport perspectives from the kanjera formation the archaeological record of oldowan hominins represents a diverse behavioral system it has been suggested that exploitation of lithic resources by oldowan hominins was simplistic and represented mostly use of local sources of stone here we investigate the raw material selection and transport behaviors of oldowan hominins reflected in the stone artifact assemblages from the kanjera south formation south rachuonyo district kenya using geochemical methods ed xrf artifacts are linked to primary and secondary source outcrops throughout southwestern kenya these data show that hominins selected raw materials for transport at frequencies that are significantly different from their availability on ancient landscapes furthermore a substantial proportion of the assemblage represents transport over relatively long distances 10 km our study further suggests that in the early stages of stone tool use hominins used a wide variety of raw materials and selected these materials at some distance from their eventual discard locations early hominin behavior may have incorporated an understanding of raw material source distributions across a more extensive landscape than has been previously documented this supports the growing perspective that oldowan technology represents a more complex behavioral pattern than is usually associated with the beginnings of hominin tool use

fault tectonics and deformation a method of quantification using field data abstract a new method for analysing fault populations in terms of infinitesimal strain is discussed a fractured rock volume taken as a whole can be considered to be affected by continuous deformation provided fault offsets are small compared to its size each fault then acts as a penetrative simple shear we combinen simple shears to define an average deformation ellipsoid for the given faulted volume the method has been tested using accurate fault slip data collected in two superposed levels of coal mines of the lorraine basin the studied rock volume has been affected by extensional phenomena during permo triassic results show that within a major tilted block the average deformation corresponds to e w extension with a total factor of extension of 1 03 this result is in good agreement with independent estimates

block rotation by strike slip faulting structural and paleomagnetic evidence geometric analysis shows that motion of sets of strike slip faults should cause block rotation otherwise the distorted fault domain would not fit with its surrounding the sense of the block rotation depends on the sense of fault slip and the spacing and orientation of the faults all of which can be obtained from structural data the predictions of the geometric model can then be independently tested by paleomagnetic measurements the model was tested in northern israel structural data reveal several domains of contemporaneous right and left lateral strike slip faults paleomagnetic measurements show that in a domain of nnw trending left lateral faults blocks rotated 23 3 8 2 clockwise and in a domain of right lateral faults blocks rotated 22 4 9 0 anticlockwise these results are in agreement with the prediction of the geometric analysis of the structure the combined results of the heterogenous deformation of the area resembles a pure shear which allows n s extension by about 1 3 in other domains rotations of 34 6 9 1 and 53 1 11 0 were found in their original orientation the left and right lateral faults intersected at angles of 60 70 enclosing the principal axis of shortening in accordance with theories of brittle failure as a result of block rotations this angle is now larger and may reach 110 the results demonstrate large block rotations in different senses in domains of strike slip faults the independent structural and paleomagnetic data can be interpreted within the framework of a simple geometric model northern israel is considered to be a good model for this efficient mechanism of intraplate deformation which is expected to occur in similar tectonic settings elsewhere

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

patterns and timing of late quaternary faulting in the great basin province and relation to some regional tectonic features large scale surface faulting events each tens of kilometers long and involving more than a meter of displacement of late quaternary age have not been uniformly distributed but have been concentrated in subprovincial and smaller belts and areas within the great basin province furthermore faulting apparently has not been uniformly distributed over time the average recurrence interval of large scale faulting events on individual faults in the province is measured in thousands or tens of thousands of years in contrast to such long recurrence intervals in the central nevada and eastern california seismic belts large scale faulting events have occurred during the last 111 years in an apparently coherent sequential belt filling pulse of activity in which events were separated by only a few years or a few decades the questions are where are other pulses of large scale faulting likely to occur and what controls the localization of such belts and areas characterized by higher rates of faulting two examples of tectonic subprovinces that may play a role in delimiting smaller zones of faulting are the black rock carson sink zone of extension and the central nevada downwarp the central nevada seismic belt lies along the dividing line between these two subprovinces and the northwest margin of the black rock carson sink zone of extension coincides with the margin of a fingerlike belt of dense faulting in northwestern nevada

accommodation of missing shear strain in the central walker lane western north america constraints from dense gps measurements abstract we present 264 new interseismic gps velocities from the mobile array of gps for nevada transtension magnet and continuous gps networks that measure pacific north american plate boundary deformation in the central walker lane relative to a north america fixed reference frame northwestward velocities increase smoothly from 4 mm yr in the basin and range province to 12 2 mm yr in the central sierra nevada resulting in a central walker lane deformation budget of 8 mm yr we use an elastic block model to estimate fault slip and block rotation rates and patterns of deformation from the gps velocities right lateral shear is distributed throughout the central walker lane with strike slip rates generally

physical models of transtensional folding physical models of transtensional deformation indicate the formation of folds for a range of plate divergence angles from 0 to 30 folds are upright and fold axial planes are vertical for all angles of oblique divergence the transtensional fold hinges are initiated parallel to the direction of the maximum horizontal infinitesimal extension and rotate with the maximum horizontal finite extension toward parallelism with the oblique movement direction not the shear zone boundary large amounts of hinge parallel extension occur during fold formation and material lines are observed to rotate through the fold hinges during progressive deformation the transtensional folding experiments reinforce the pitfalls of assuming a simple coincidence between geological structures and strain or stress for the 22 and 30 obliquely divergent experiments the observed fold axial planes are not perpendicular to the maximum contraction direction s 3 or the inferred maximum compressive stress direction 1 consequently one must be cautious in making interpretations of principal strain directions and especially principal stress directions based on the orientations of folds formed in transtension

geologic versus geodetic deformation adjacent to the san andreas fault central california we combine geologic and global positioning system gps data to characterize the style and magnitude of off fault deformation across the san andreas fault system in central california geologic structures record 12 km of both fault parallel and fault perpendicular displacements across creeping and locked portions of the san andreas fault analysis of 150 gps site velocities suggests that the borderlands record 4 6 mm yr of fault parallel and 3 5 mm yr of fault perpendicular motion alongside the creeping segment where elastic strain is minimized the distribution of both long term geologic and short term geodetic deformation is affected by basement type where more deformation is concentrated northeast of the san andreas fault on franciscan basement we suggest that at least half the fault parallel gps deformation measured by gps bordering the creeping segment must be accommodated by geologic structures this permanent deformation needs to be incorporated into dynamic models of the fault system elastic modeling of the san andreas fault in central california which incorporates its well known transition from locked to creeping behavior near parkfield predicts first order variations in the gps velocity field along the fault and corresponding variations in dilatational strain rates the strain rate pattern is dominated by a large contractional region northeast of the transition from locked to creeping behavior and a large extensional region southwest of the transition the former coincides with the coalinga and kettleman hills anticlines the growth and development of which seem to have occurred under at least two kinematic conditions we suggest that the onset of fault creep in central california promoted the growth of these folds by implication fault creep has been active over geologic time scales

late pleistocene and holocene paleoearthquake activity of the olinghouse fault zone nevada paleoseismic trenching and fault surface trace mapping indicate that the olinghouse fault zone a northeast trending left lateral strike slip fault located in the northern walker lane nevada has been the source of multiple latest pleistocene and holocene surface rupturing earthquakes a trench exposure near the eastern end of the fault records two and possibly three earthquakes after 3360 190 cal yr b p and two trenches at the western end of the fault contain evidence for two earthquakes after 19 800 630 cal yr b p with the most recent earthquake occurring after 1935 70 cal yr b p the apparent higher frequency of recent earthquakes at the eastern end of the fault may reflect triggered slip on the olinghouse fault zone due to earthquakes on the conjugate northwest trending right lateral pyramid lake fault repeated late pleistocene and holocene earthquakes on the olinghouse fault zone demonstrate that northwest directed right lateral shear of the northern walker lane is accommodated in part by northeast trending left lateral faults

gold deposit formation during collapse tectonics structural mineralogical geochronological and fluid inclusion constraints in the ouro preto gold mines quadril atero ferrifero brazil the gold deposits of the ouro preto area minas gerais brazil occur along the main tectonic contact responsible for the thrusting of the paleoproterozoic units minas supergroup onto the gneissic and metasedimentary archean units rio das velhas supergroup during the brasiliano ca 600 ma tectonic event these deposits are claimed to be the most complex of the quadrilatero ferrifero with orebodies lying parallel to the thrust related stretching lineation based on structural mineralogical geochronological and fluid inclusion constraints our study demonstrates that the gold bearing quartz veins were formed in response to late orogenic collapse tectonics that led in this area to gravity backsliding of the nappes during these movements sigmoidal voids were created and filled by elongate quartz grains associated with sulfides tourmaline and carbonates inferred hydrothermal effects were responsible for the formation of illite sulfides carbonate garnet and green biotite within the matrix large arsenopyrites within the veins were affected by three stages of fracturing during which the metallic phases and associated gold were deposited gold wherever it is in contact with bi related minerals occurs in the form of electrum maldonite or aurostibite fluid inclusion studies show that this event was associated with the entrapment of both co2 rich and h2o salt rich fluids the process was initiated after the brasiliano thrusting event ca 500 c 8 kbars and in p t conditions close to 300 to 400 c and 2 kbars an age of ca 485 to 490 ma has been obtained by 40ar 39ar dating of hydrothermal biotite and muscovite this study demonstrates that the ouro preto gold deposits were formed in a context of late orogenic collapse tectonics drastically different from the conventional auriferous shear zone model this could account for their relative paucity compared to other mesothermal gold deposits

apatite u th he thermochronometry as an innovative geothermal exploration tool a case study from the southern wassuk range nevada abstract extensional type geothermal systems are complicated by the interrelation between footwall advection during exhumation and the subsequent redistribution of heat by migrating hydrothermal fluids in the hanging wall the southern wassuk range wr hanging wall hosts a moderate temperature extensional type geothermal system and is ideal for studying this duality by analyzing a suite of apatite ahe and zircon zhe helium samples from the footwall and hanging wall of the southern wr young 3 4 ma ahe ages along the wr front are concentrated along the se corners of segmented footwall blocks marking the location of focused mio pliocene transtension fracture dilation and advection hydrothermally reset ahe ages along the footwall range front suggest that the hawthorne geothermal system 85 135 c is long lived and has resided at a prominent structural boundary in the wr footwall marked by localized advection and range front deformation in contrast the presence of both hydrothermally reset and non reset ahe ages from a 1 4 km deep borehole in the hanging wall basin probably indicate that the geothermal plumbing system and current manifestation as a thermal anomaly 113 c are juvenile and are controlled by the generation of newly formed faults in the hanging wall ahe ages have been shown here to greatly enhance the focusing of regional scale geothermal exploration efforts and for the first time have been used to identify and estimate the temperature of unseen hydrothermal fluids

tectonic setting and fluid evolution of auriferous quartz veins from the la codosera area western spain the boundary between the central iberian and the ossa morena zones of the iberian peninsula is marked by the badajoz shear zone along which movements were focused during the hercynian orogeny to the north of the shear zone in the la codosera area auriferous veins are located in a series of late hercynian north south to northeast southwest trending faults which are antithetic to the main shear zone the passage of dilute co 2 rich fluids at 350 degrees to 400 degrees c and 2 800 to 3 300 bars along these faults resulted in mineralization where dilatant structures for example offsets intersections or fault terminations developed within black slate lithologies by combining geological and geochemical systematics including vein orientation lithology and fluid chemistry with remotely sensed fracture data a testable exploration model for the region has been developed

relation between regional lineament systems and structural zones in nevada analysis of diverse geologic geophysical and geochemical data shows that six of eight extensive lineament systems delineated in landsat images of nevada are morphologic and tonal expressions of three broad structural zones the lineament systems defined in this report are the walker lane rye patch midas pancake range warm springs timpahute pahranagat and northern nevada rift the three structural zones are the walker lane structural zone the humboldt structural zone and the southern nevada structural zone the northwest trending walker lane structural zone which includes the walker lane lineament system is 150 to 200 km wide and is marked by northwest oriented faults a broad belt of high frequency aeromagnetic anomalies and concentrations of volcanic rocks 34 to 6 m y old the northwest trending rye patch lineament system may mark the northeast boundary of this structural zone and is characterized by stratigraphic discontinuities alignment of aeromagnetic anomalies and contrasting aeromagnetic anomaly patterns on opposing sides northern nevada is dominated by the northeast trending 150 to 200 km wide humboldt structural zone which is marked by the midas lineament system northeast oriented faults broad gravity anomalies and the battle mountain heat flow high this zone appears to extend into central montana the humboldt structural zone is transected by the northern nevada rift lineament system which is partly defined by a relatively narrow belt of north northwest trending basaltic dikes that produce a series of pronounced aeromagnetic highs the east trending 150 to 175 km wide southern nevada structural zone includes the pancake range warm springs and timpahute lineament systems this zone is marked by disruption and termination of ranges stratigraphic discontinuities east to east northeast trending faults and an easterly grain in the aeromagnetic and gravity patterns the other lineament system in southern nevada the northeast trending pahranagat may lie within a poorly defined southward extension of a broad structural zone defined in utah by a belt of volcanic rocks 34 to 0 m y old stratigraphic discontinuities coincidence with three extensive gravity lineaments and a belt of aeromagnetic anomalies this paper proposes that the lineament systems except for the northern nevada rift lineament system are conjugate shears formed during and after middle miocene extension of the great basin these lineament systems probably reflect only the most recent movement along broad crustal zones which may be of pre miocene age the three documented structural zones and possibly one including the pahranagat lineament system may reflect transition zones between the central great basin where 30 extension has been documented and the adjacent regions that responded differently to spreading since middle miocene time the humboldt structural zone appears to be especially important for geothermal and mineral exploration in nevada because of the presence of the battle mountain heat flow high and a high concentration of metal districts within this zone in nevada

fold fault relationships in low angle detachment systems a close kinematic and timing relationship between low angle extensional faulting and upright to moderately inclined folding with fold axes parallel to the extension direction is established for two well exposed neogene detachment systems in the simplon region of the alps and in the death valley region of california fold amplitude is largest in the oldest mylonitic foliation of the footwall these folds are transected by younger mylonitic foliation s and cataclastic detachment faults which are themselves folded about similar axes in general the amplitude decreases and the wavelength increases for progressively younger fold structures consistent with a history of progressive folding during faulting the interplay between folding and faulting in the footwall reflects a component of shortening perpendicular to fold axial surfaces during extension parallel to fold axes this strain pattern can develop in regions with a transcurrent component or during lateral extrusion and does not necessarily require overall crustal thinning

provenancing hornfels in the dingcun industry the exploitation of the vicinity source in this paper we attempt to determine the source of the hornfels used to make the famous late middle pleistocene lithic industry from dingcun in north china by combining geological mapping and geochemical fingerprinting xrf we demonstrate that the source of the hornfels was around the dagudui mountain 7 km east of dingcun taking into consideration the gujiao site which is also located along the fenhe river the palaeolithic inhabitants of the fenhe river drainage basin had a good knowledge of the local rock sources and used them over a very long period from the middle pleistocene to the neolithic our geochemical data not only helps us to establish the source of the dingcun industry but also provides a reliable basis for the sub classification of the hornfels that is more accurate than that resulting from visual inspection in addition our research initiates a new method for studying raw material sources of chinese paleolithic sites

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

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

archaeological carved slabs of the langobard art in churches of peligna valley and spoleto italy provenance of the stones abstract in this study we investigate the provenance of the limestone and marble used to made carved slabs today placed in three churches located in raiano and vittorito peligna valley abruzzo italy and in spoleto umbria italy the slabs representative examples of the langobard art middle ages are dated approximately from 8th to 9th centuries a d and have the same pattern in terms of decoration suggesting a progressive expansion of langobards in central and southern italy after the foundation of the pavia kingdom a multi analytical approach including petrographic characterization chemical carbon and oxygen isotope composition was used for the complete characterization of these stones the comparison of the results of the limestone slabs with those of the geological units outcropping in the area proves a local quarrying from the calcari cristallini formation conversely the carved marble slab has petrographic and isotopic signatures comparable to those of carrara marble and presumable was imported from this famous locality

seismic evidence for conjugate slip and block rotation within the san andreas fault system southern california the pattern of seismicity in southern california indicates that much of the activity is presently occurring on secondary structures several of which are oriented nearly orthogonal to the strikes of the major through going faults slip along these secondary transverse features is predominantly left lateral and is consistent with the reactivation of conjugate faults by the current regional stress field near the intersection of the san jacinto and san andreas faults however these active left lateral faults appear to define a set of small crustal blocks which in conjunction with both normal and reverse faulting earthquakes suggests contemporary clockwise rotation as a result of regional right lateral shear other left lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data many of these structures predate the modern san andreas system and may control the pattern of strain accumulation in southern california geodetic and paleomagnetic evidence confirm that block rotation by strike slip faulting is nearly ubiquitous particularly in areas where shear is distributed and that it accommodates both short term elastic and long term nonelastic strain a rotating block model accounts for a number of structural styles characteristic of strike slip deformation in california including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults domains of evenly spaced antithetic faults that terminate against major fault boundaries continued development of bends in faults with large lateral displacements anomalous focal mechanisms and differential uplift in areas otherwise expected to experience extension and subsidence since block rotation requires a detachment surface at depth to permit rotational movement low angle structures like detachments of either local or regional extent may be involved in the contemporary strike slip deformation of southern california a block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge bounding faults but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction this behavior coupled with the presence of possible regional detachments accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges although fault displacements along secondary structures associated with block rotations remain small they may still influence the nucleation and the characteristic rupture length of large earthquakes a more complete description of what these structures are and how they interact may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

microplate capture rotation of the western transverse ranges and initiation of the san andreas transform as a low angle fault system tectonic rotation of the western transverse ranges block is explained by capture of the partially subducted monterey microplate by the pacific plate at about anomaly 6 time ca 20 ma as pacific monterey spreading slowed and eventually ceased the slip vector along the gently northeast dipping subduction interface beneath the california margin changed from slightly oblique subduction to transtensional dextral transform motion this change in slip vector and a shift of pacific plate motion eastward along the already subducted monterey plate interface imply that the san andreas transform began as a system of low angle faults that locally subjected the overriding continental margin to distributed basal shear and crustal extension this basal shear produced the rotated western transverse ranges this model helps explain the timing of initial rotation and basin formation the sudden appearance of widely distributed transform motion well inland of the margin in early miocene time why the western transverse ranges uniquely rotated as a large coherent crustal block and several fundamental structural characteristics of central and southern california the model also provides major constraints on the amount of pacific north america strike slip motion the position through time of offshore oceanic plates relative to onshore geology and a general explanation for what may happen as a spreading ridge approaches a trench and the subduction zone evolves into a transform system

rapid localization of pacific north america plate motion in the gulf of california correlation of late miocene volcaniclastic strata across the northern gulf of california shows that the pacific north america plate boundary localized east of the baja california peninsula ca 6 ma dextral offset of the 12 6 ma tuff of san felipe and a pair of overlying ca 6 3 ma pyroclastic flows indicate at least 255 10 km of displacement along an azimuth of 310 isopach and facies trends of the tuff of san felipe support no more than a few tens of kilometers of additional dextral displacement between 12 6 and 6 3 ma these constraints indicate that nearly all of the dextral displacement between the pacific and north american plates prior to 6 3 ma was accommodated outside of the gulf region and by 4 7 ma the plate boundary motion was localized in the gulf of california although continental extension has accounted for a component of plate boundary motion in northwestern mexico since cessation of subduction offshore of southern baja california at 12 5 ma transfer of baja california to the pacific plate was delayed by at least 6 7 m y

neogene paleostress changes in the basin and range a case study at hoover dam nevada arizona at hoover dam 40 km southeast of las vegas nevada well exposed highly faulted miocene rocks provide an excellent opportunity to study the paleostress history of a very small area within a region where previous geologic studies indicate clockwise rotation of paleostress and a nearness to a major strike slip fault zone boundary within 0 5 km3 of rock the sense of slip was determined on almost 500 separate faults the fault slip data show internal consistency with respect to lithology size of faults and location within the small area with respect to fault slip however the data represent an in homogeneous mixture of primarily strike slip and dip slip motions from this mixture it is possible to resolve two distinct stress fields with directions of extension that differ by 60 each stress field corresponds to a mixture of strike slip and dip slip faults and thus the orientations of 1 and 2 are not tightly constrained if however the data are first separated into strike slip and dip slip faulting modes and subsequently searched for sub populations that correspond to contrasting paleostress orientations the computations yield tightly constrained tensors that illustrate two distinct stress fields with subhorizontal 3 axes that trend n50 e and n75 w the 1 and 2 axes permutate in two vertical planes that strike n40 w and n15 e because 1 and 2 are close in value relative to 3 these relationships suggest that strike slip and dip slip faulting belong to the same tectonic regime qualitative observations of polyphase slip fault fault offsets and fault bedding geometric relationships when evaluated in the context of changes in 3 orientation and permutations of 1 and 2 provide a basis for a two stage late cenozoic structural evolution at hoover dam these stages are 1 strike slip faulting partly pre tilt and dip slip faulting and associated stratal tilting and 2 mostly post tilt complexly interrelated strike slip oblique slip and dip slip faulting the qualitative evaluations indicate that strike slip and dip slip faulting alternated in time during the second stage of deformation and may have done so during the first these mixed mode movements probably represent stress oscillations in time and space rather than discrete stress reorganizations in contrast the two different orientations of 3 either represent a major clockwise rotation of the stress field or a major counterclockwise rotation of the rocks during the faulting history each alternative is consistent with regional geologic relationships and the choice of which is correct cannot be made within the small area that was studied

melrose green rhyolite its geologic setting and petrographic and geochemical characteristics abstract fine grained lithic material called melrose green is common in middle archaic to late woodland period archaeological sites in southeastern new england a quarried source of this material crops out on a hillside in melrose massachusetts at this site a crystal rich volcanic tuff is intruded by three fine grained felsic dikes composed of rock similar to archaeological materials identified as melrose green locally parts of the dikes are pockmarked and saddle shaped in cross section due to removal of quarried material the aphanitic dike rock is characterized by sparse feldspar microphenocrysts in a matrix of granular quartz and feldspar epidote titanite opaque minerals and calcite are accessory minerals flow bands and relict shard texture are also present the rock is of rhyolitic dacitic composition some previous workers interpreted melrose green artifacts as chert or argillite field relationships petrography and geochemistry demonstrate that melrose green is of igneous parentage while geochemistry shows that melrose green is similar to felsic volcanic rocks of the nearby lynn mattapan volcanics the trace element composition of the dikes distinguish them from most other known prehistoric quarry sites within the lynn mattapan complex chipping debris from the dikes is widespread at the melrose quarry site demonstrating that the dikes were extensively worked for sources of raw material three artifacts macroscopically similar to melrose green rhyolite were collected from three coastal sites within boston harbor analyses showed that two of these are of igneous origin and generally similar to rocks of the lynn mattapan complex only one of these yielded data consistent with a source from the dike outcrops in melrose the third artifact is most likely a non igneous argillite these examples show the importance of using petrographic and geochemical analysis to correctly characterize macroscopically similar lithic material

the mari rosa late hercynian sb au deposit western spain geology and geochemistry of the mineralizing processes the central iberian zone of the hesperian massif hosts a series of late hercynian vein type sb deposits one of them is the mari rosa mineralization hosted by metagreywackes and slates of the schist greywacke complex upper precambrian the mineralization is characterized by a complex paragenesis comprising three hydrothermal stages stage h1 arsenopyrite pyrite stage h2 stibnite gold and stage h3 pyrite pyrrhotite galena sphalerite chalcopyrite tetrahedrite boul angerite stibnite of these only the second episode was of importance and gave rise to the main mineralized bodies of the deposit hydrothermal alteration consists of a mild sericitization chloritization and carbonatization of the metasedimentary rocks around the veins chemical changes in the hydrothermal halos include a remarkable increase in the ratio k2o na2o and a decrease in the ratio sio2 volatiles together with a sharp increase in sb mo au and n fluids associated with ore deposition lie in the h2o nacl co2 ch4 n2 compositional system these fluids evolved progressively cooling from initial circulaion temperatures close to 400 c in the early stage h1 to temperatures of approximately 150 c in the late one h3 fluid composition evolution was characterized by a progressive increase in the bulk water content of the fluids and with an increase in the relative proportion of n2 with respect to ch4 and co2 in the volatile fraction massive stibnite deposition resulted from a boiling process developed at 300 c and 0 9 1 kb at a depth of 4 5 km geological geochemical and fluid inclusion evidence suggest that the intrusion of the alburquerque batholith late hercynian s type granitoids triggered hydrothermal activity leading to the transport and deposition of sb and au in mari rosa

geochronology and physical context of oldowan site formation at kanjera south kenya oldowan sites in primary geologic context are rare in the archaeological record here we describe the depositional environment of oldowan occurrences at kanjera south kenya based on field descriptions and granulometric analysis excavations there have recovered a large oldowan artefact sample as well as the oldest substantial sample of archaeological fauna the deposits at kanjera south consist of 30 m of fluvial colluvial and lacustrine sediments magneto and bio stratigraphy indicate the kanjera south member of the kanjera formation was deposited between 2 3 and 1 92 million years ago ma with 2 0 ma being a likely age for the archaeological occurrences oldowan artefacts and associated fauna were deposited in the colluvial and alluvial silts and sands of beds ks 1 to ks 3 in the margins of a lake basin field descriptions and granulometric analysis of the sediment fine fraction indicates sediments from within the main archaeological horizon were emplaced as a combination of tractional and hyperconcentrated flows with limited evidence of debris flow deposition this style of deposition is unlikely to significantly erode or disturb the underlying surface and therefore promotes preservation of surface archaeological accumulations hominins were repeatedly attracted to the site locale and rapid sedimentation minimal bone weathering and an absence of bone or artefact rounding further indicate that fossils and artefacts were quickly buried

strain accumulation and rotation in western nevada 1993 2000 1 the positions of 44 gps monuments in an array extending from the sierra nevada at the latitude of reno to near austin nevada have been measured several times in the 1993 2000 interval the western half of the array spans the walker lane belt whereas the eastern half spans the central nevada seismic zone cnsz the principal strain rates in the walker lane belt are 29 6 5 3 nstrain yr 1 n88 4 e 5 4 and 12 8 6 0 nanostrain yr 1 n01 6 w 5 4 extension reckoned positive and the clockwise as seen from above the earth rotation rate about a vertical axis is 13 6 4 0 nrad yr 1 the quoted uncertainties are standard deviations the motion in the walker lane belt can then be represented by a zone striking n35 w subject to 16 8 4 9 nstrain yr 1 extension perpendicular to it and 19 5 4 0 nstrain yr 1 right lateral simple shear across it the n35 w strike of the zone is the same as the direction of the local tangent to the small circle drawn about the pacific north america pole of rotation the principal strain rates for the cnsz are 46 2 11 0 nstrain yr 1 n49 9 w 6 0 and 13 6 6 1 nstrain yr 1 n40 1 e 6 0 and the clockwise rotation rate about a vertical axis is 20 3 6 3 nrad yr 1 the motion across the cnsz can then be represented by a zone striking n12 e subject to 32 6 11 0 nstrain yr 1 extension perpendicular to it and 25 1 6 3 nstrain yr 1 right lateral simple shear across it the n12 e strike of the zone is similar to the strikes of the faults rainbow mountain fairview peak and dixie valley within it

magmatic and hydrothermal ammonium in granites of the tuscan magmatic province italy abstract the granites of the tuscan magmatic province have total ammonium contents that vary from 0 to 636 ppm in the three largest intrusions of elba giglio and montecristo the granites have not been affected by hydrothermal alteration and have low ammonium contents ranging up to 50 ppm there are detectable differences between the separate intrusions and also within the biggest intrusion granitic samples from the smaller outcrops and boreholes on the tuscan mainland are nearly all hydrothermally altered and these rocks show much higher ammonium contents ranging up to 636 ppm in the hydrothermally altered rocks the ammonium contents vary considerably over short distances and where they have been most closely sampled in the s2 borehole of castel di pietra there is a clear correlation between the ammonium content and the intensity of sulphide mineralization the major conclusion of the study is that the amount of ammonium present in the granites is determined much more by post magmatic hydrothermal alteration than by magmatic variation the mean ammonium contents of the fresh and hydrothermally altered granites are 17 and 95 ppm respectively

an experimental approach to distinguishing different stone artefact transport patterns from debitage assemblages this paper experimentally demonstrates the ability of a set of indices to distinguish between different stone artefact transport patterns represented in debitage assemblages stone artefacts were transported extensively in the past and this is an important component of technological organisation however most stone artefacts occur as part of debitage assemblages from these assemblages where mostly non transported artefacts remain it can be challenging to identify what artefacts if any were transported in anticipation of future use a series of indices the cortex ratio volume ratio flake to core ratio non cortical to cortical flake ratio and flake core diminution tests are presented to meet this challenge these are tested on an experimental assemblage where three different transport scenarios are simulated results suggest that the indices are sensitive to artefact transport and are capable of empirically distinguishing between the three transport scenarios even when raw material form varies the results also indicate that artefact transport is capable of exerting a significant influence on stone artefact assemblage formation

tectonic fabric of sierra leone west africa implications for mesozoic continental breakup a lineament map interpreted from landsat images has been integrated with available geological and geophysical data from both offshore and onshore regions of sierra leone the lineament patterns are related to intraplate and plate marginal reaction of pre existing structures during mesozoic rifting events that resulted in strong tectonic controls on magmatism of four main lineament trends the nnw sse to n s nne ssw and ene wsw trends are directly relatable to archaean fabric in the leo uplift a nw se trend reflects coast parallel late mesozoic dykes that follow the rokelide pan african fabric reactivated archaean nnw and n s trending structures nw se trending faults defining offshore basins are segmented and offset by ene wsw trending continental extensions of ocean fracture zones both the guinea and sierra leone fracture zones have nucleated on ene wsw trending sinistral archaean shear zones in the leo uplift the four lineament trends focused mesozoic magmatic events through protracted reactivation spatial and geometric relationships between the magmatic provinces and tectonic fabric indicate that intraplate deformation occurs far inland during rifting events the angular relationships between the permo triassic ne trending guinea belt the jurassic triassic nw se trending coast parallel dykes and the ene wsw trending ocean fracture zones centred on the eariy jurassic freetown basic igneous complex suggest that the sierra leone liberia continental margin evolved as an obliquely sheared rift rift transform passive margin during mesozoic continental breakup

timing and extent of neogene tectonic rotation in the western transverse ranges california paleomagnetic data collected from lower miocene rocks throughout the western transverse ranges province have consistently clockwise deflected magnetic declinations which suggests that the entire province has experienced clockwise tectonic rotation during neogene time paleomagnetic declinations in lower miocene rocks increase westward from 33 5 c 11 in the san gabriel block to 91 7 7 in the western santa ynez range suggesting a westward increase in the net amount of clockwise tectonic rotation paleomagnetic data that constrain the tuning of rotation suggest that the entire western transverse ranges rotated uniformly 50 60 clockwise during the middle miocene this implies a minimum of 200 km of distributed dextral shear along the california coast and offshore during middle miocene time little tectonic rotation appears to have occurred in the western transverse ranges during late miocene time since miocene time continued clockwise rotation in the western santa ynez range 31 3 combined with counterclockwise rotation in the san gabriel block 16 has oroclinally bent the transverse ranges to the west of the san andreas fault the pliocene pleistocene clockwise rotation implies an additional 30 60 km of distributed dextral shear across the southern california coast ranges during post miocene time

chapter 3 structural development of a major extensional accommodation zone in the basin and range province northwestern arizona and southern nevada implications for kinematic models of continental extension

structures that accommodated differential vertical axis rotation of the western transverse ranges california 1 new paleomagnetic and structural data from the northwestern transverse ranges of california define the location geometry and kinematics of structures that facilitated differential vertical axis rotation paleomagnetic declinations in the southern coast ranges indicate negligible amounts of vertical axis rotation which contrasts sharply with data from rocks of the same age in the western transverse ranges that record approximately 90 of clockwise rotation since early miocene time this change in paleomagnetic declinations occurs across an east west trending zone of reverse faults and folds that includes the western big pine pine mountain fault the santa ynez fault and the structures in between the structures in this zone exhibit increased amounts of shortening to the west such that the zone closed in a fan like fashion resulting in rotation of the southern edge with respect to the northern edge the new data and interpretations refine previous models of vertical axis rotation in southern california by describing the deformation present at rotation boundaries that allowed distinct areas to rotate relative to the surrounding crust

variscan sb au mineralization in central brittany france a new metallogenic model derived from the le semnon district numerous sb au deposits are known in the european variscan belt and are often associated with the latevariscan hydrothermal events linked to postorogenic extension and strike slip faults through the whole belt thecentral part of the armorican massif central brittany hosts several sb au deposits such as the le semnondeposit however little is known about its formation its structural control or hydrothermal history in order toprovide new constraints on the sb au mineralization framework of the variscan armorican massif structural geochronological and chemical analyses have been performed on the le semnon sb au deposit first wehighlight that sb au mineralization consists of fault fill and flat extensional vein systems resulting from smallscalefault valve mechanism controlled by fluid overpressure at shallow depth indeed this deposit is characterizedby the presence of dolerite dykes which acted as the plumbing system for fluids within impermeableslates the low permeability of the slates permitted the establishment of local supralithostatic fluid pressure in apredominantly hydrostatic regime coupled with new micro textural evidences a reappraisal of the parageneticevolution and new physico chemical data 40ar 39ar and u pb analyses show that the le semnon sb aumineralization was emplaced around 360 ma and therefore indicates an early hydrothermal event in thevariscan history of the armorican massif moreover absolute dating of mafic magmatism coupled with hydrothermalalteration associated with the deposition of sb au mineralization strongly suggest that the emplacementof the sb au mineralization was coeval with the emplacement of the host rock doleritic dyke furthermore the high temperature recorded in fluid inclusions and the paragenetic evolution suggests an elevatedthermal anomaly in central brittany thus we propose that the emplacement of the widespread maficmagmatism at ca 360 ma played a major role in the mobilization of fluid flows and the redistribution of metalsin the armorican crust

the influence of raw material size on stone artefact assemblage formation an example from bone cave south western tasmania this paper specifically examines the influence of raw material size on stone artefact assemblage formation in conjunction with two behavioural assemblage formation processes reduction intensity and artefact transport the aim is to establish whether raw material size can influence the way in which reduction intensity and artefact transport form a stone artefact assemblage and should a significant influence be established whether this has any bearing on human behavioural reconstructions this is first investigated in an experimental setting followed by an archaeological setting using a quartzite and chert assemblage from bone cave in south western tasmania reduction intensity and artefact transport are respectively measured using the scar density index flake to core ratio non cortical to cortical flake ratio and cortex ratio while tool blank sizes are also reconstructed the combined experimental and archaeological analyses find that raw material size variation is capable of exerting a substantial influence on stone artefact assemblage formation this particularly with reference to the nature of change in stone artefact assemblages both reduction intensity and artefact transport will alter assemblage composition at faster rates when nodule sizes are smaller this can have significant implications for intra and inter site reconstructions of past human behaviour

antimony quartz and antimony gold quartz veins from northern portugal abstract antimony and pb sb quartz veins from the braganca district portugal are mainly hosted by silurian phyllites antimony au quartz veins from the durico beira region are mainly hosted by a cambrian schist metagraywacke complex as well as ordovician phyllites and quartzites the deposits were mostly exploited in the late 19th century mineralogical characteristics and chemical compositions of individual ore minerals are similar in the two areas first and second generations of arsenopyrite precipitated at 390 and 300 c respectively berthierite and stibnite are the most abundant sb bearing minerals and precipitated between 225 and 128 c native antimony at

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

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

geothermal exploration with hymap hyperspectral data at brady desert peak nevada abstract we explore the use of hymap hyperspectral mapper to remotely map unique geothermal indicator minerals over the brady desert peak geothermal fields geothermal related minerals and rocks such as sinter tufa and sulfates display diagnostic characteristics in the visible and shortwave infrared their presence and distribution can be used to guide more detailed field work for geothermal exploration the brady desert peak geothermal fields are located about 80 km east of reno nevada in the hot springs mountains north northeast striking en echelon faults offset tertiary volcanic and lacustrine rocks two geothermal power plants produce electricity from two separate geothermal systems one with numerous fumaroles and mudpots the other showing no active surface expression of geothermal activity surface occurrences of gypsum calcium carbonate hematite and opaline silica were identified at both sites with the hyperspectral data these minerals when considered together are indicative of geothermal activity at both sites mapping results were synthesized with other spatial data in a geographic information systems gis database that was used to help draw structural interpretations of faulting and fault controls at the brady desert peak area the same processing methods can be applied to new hyperspectral data sets for future exploration in the great basin especially in areas that lack obvious thermal expressions

the biards sb au bearing shear zone massif central france an indicator of crustal scale transcurrent tectonics guiding late variscan collapse structural textural mineralogic microthermometric and isotopic data obtained on the biards sb au district located in the south limousin area massif central france provide new evidence for the tectonic setting of stibnite and gold deposition during the late orogenic variscan collapse in western europe a variety of geologic and geophysical data define a major crustal scale left lateral strike slip shear zone that acted as a trap for sb au mineralization during these movements fault fill veins and then extensional veins of limited extent were created along rheologic heterogeneities that focused brittle deformation geometry and orientation of both vein sets indicate two main episodes of emplacement of ore related to a progressive wrench movement in response to an counterclockwise rotation of the regional stress axes both veins sets are filled by quartz and sulfides in three stages an early arsenopyrite pyrite stage with fine grained quartz a sphalerite pb sb sulfosalt native gold stage with comb quartz and a late stibnite berthierite stage with comb quartz combined fluid inclusion and stable isotope studies on quartz muscovite and sulfides reveal that the three stages result from the continuous evolution of a deep seated hydrothermal system in which the latest stages mixed with meteoric water these data argue for cooling and a pressure drop during ore deposition and indicate a shallow level of emplacement for sb au veins a comparison of the biards deposits with other upper paleozoic gold and antimony deposits in the massif central the western variscides and elsewhere in the world illustrates their common association with crustal scale strike slip shear zones the upper carboniferous age 305 300 ma of the sb au deposits indicates that they formed during the early stages of the postthickening extension of the variscan lithosphere sb au bearing strike slip shear zones resulted from the late variscan crustal collapse guided by the very large scale dextral shearing between central europe northern africa and the northern appalachians and associated with the opening of the paleo tethys and the closing of the theic ocean during the late paleozoic gondwana laurasia translation

the world s oldest stone artefacts from gona ethiopia their implications for understanding stone technology and patterns of human evolution between 2 6 1 5 million years ago the systematic archaeological and geological survey and excavations at gona between 1992 1994 led to the discovery of well flaked stone artefacts which are currently the oldest known from anywhere in the world more than 3000 surface and excavated artefacts were recovered at 15 localities documented east and west of the kada gona river based on radioisotopic dating 40 ar 39 ar and magnetostratigraphy the artefacts are dated between 2 6 2 5 million years ago ma eg10 and eg12 from east gona are the most informative with the highest density providing the best opportunity for characterizing the oldest assemblages and for understanding the stone working capability of the earliest tool makers slightly younger artefact occurrences dated to 2 4 2 3 ma are known from hadar and omo in ethiopia and from lokalalei in kenya cut marked bones dated to 2 5 ma from bouri in ethiopia are now providing important clues on the function of these artefacts in addition australopithecus garhi known from contemporary deposits at bouri may be the best candidate responsible for the oldest artefacts surprisingly the makers of the gona artefacts had a sophisticated understanding of stone fracture mechanics and control similar to what is observed for oldowan assemblages dated between 2 0 1 5 ma this observation was corroborated by the recent archaeological discoveries made at lokalalei because of the similarities seen in the techniques of artefact manufacture during the late pliocene early pleistocene it is argued here that the stone assemblages dated between 2 6 1 5 ma group into the oldowan industry the similarity and simplicity of the artefacts from this time interval suggests a technological stasis in the oldowan 2000 academic press

kinematics of the northern walker lane an incipient transform fault along the pacific north american plate boundary in the western great basin of north america a system of dextral faults accommodates 15 25 of the pacific north american plate motion the northern walker lane in northwest nevada and northeast california occupies the northern terminus of this system this young evolving part of the plate boundary offers insight into how strike slip fault systems develop and may reflect the birth of a transform fault a belt of overlapping left stepping dextral faults dominates the northern walker lane offset segments of a w trending oligocene paleovalley suggest 20 30 km of cumulative dextral slip beginning ca 9 3 ma the inferred long term slip rate of 2 10 mm yr is compatible with global positioning system observations of the current strain field we interpret the left stepping faults as macroscopic riedel shears developing above a nascent lithospheric scale transform fault the strike slip faults end in arrays of n striking normal faults suggesting that dextral shear diffuses into extension in the great basin coeval extension and dextral shear have induced slight counterclockwise fault block rotations which may ultimately rotate riedel shears toward the main shear zone at depth thus facilitating development of a throughgoing strike slip fault

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

volatile production during contact metamorphism the role of organic matter in pelites elemental analyses of palaeozoic mudrocks from south cornwall indicate the presence of 0 04 0 74 wt carbon primarily in an elemental and or organic form in addition to 0 04 0 10 wt nitrogen as nh 4 in micas and in organic material both elements are depleted in samples from the lithologically homogeneous mylor slate formation next to the tregonning granite an offshoot of the cornubian batholith the depletions are due to an h 2 o co 2 n 2 ch 4 fluid formed during contact metamorphic devolatilization of the pelitic country rocks fluids of this type have been reported from a number of hercynian sn w provinces and may play an important role in mobilizing metals in pelitic rocks during granite related contact metamorphism and hence in the metallogenic evolution of such regions

miocene extension and fault related folding in the highland range southern nevada a three dimensional perspective abstract the highland range of southern nevada contains a major anticline and syncline that constitute the westernmost segments of the black mountains accommodation zone in the highly extended colorado river extensional corridor the folds are defined by thick tilted sections of miocene volcanic and sedimentary strata that accumulated immediately prior to and during regional extension the folds are generally symmetrical with interlimb angles that exceed 90 subhorizontal hingelines and steeply inclined axial surfaces east and west dipping normal faults dominate the west and east tilted limbs of the folds respectively the limbs of the folds are parts of major half grabens tilt fanning within these half grabens and 15 new 40ar 39ar dates bracket major extension between about 16 5 and 11 ma tilting of the opposing fold limbs occurred simultaneously and was contemporaneous with extension the anticline and syncline are therefore interpreted as fault related extensional folds produced by the partial along strike overlap of oppositely dipping normal fault systems and attendant tilt block domains the anticline developed between overlapping listric normal faults that dip toward one another including the east dipping mccullough range and west dipping keyhole canyon faults each limb of the anticline is a rollover fold developed in the hanging wall of the inwardly dipping listric normal faults the syncline formed between overlapping outwardly dipping listric faults as adjacent fault blocks were tilted toward one another the dominant folding style was fault bend folding with drag folding and displacement gradient folding playing subsidiary roles the anticline and syncline significantly affected depositional patterns with synextensional units including two major ash flow tuffs thinning toward the anticlinal hinge and thickening toward the synclinal hinge the black mountains accommodation zone is largely composed of intersecting northwest trending anticlines and northeast trending synclines both occurring at a variety of scales depending on the amount of overlap between the opposing normal fault systems this three dimensional geometry contrasts with the typical style of folding in contractional settings and may characterize extensional accommodation zones that include the partial along strike overlap of multiple oppositely dipping normal faults the folds in the highland range serve as well exposed analogues of similar features in hydrocarbon rich areas of submerged continental margins

normal vs strike slip faulting during rift development in east africa the malawi rift kinematic analysis of neogene and quaternary faults demonstrates that the direction of extension in the malawi rift rotated from east northeast to southeast rift development commenced with the formation of half grabens bounded by northwest north and northeast striking normal faults owing to slightly oblique rifting the northwest striking faults in the northernmost rift segment show a small dextral oblique slip component whereas north and northeast oriented faults in the central part of the rift display a sinistral oblique slip component this first event resulted in block faulting and basin subsidence which is largely responsible for the present day basin morphology of lake malawi a major change in fault kinematics occurred because of rotation of the extension direction and permutation of the principal compressive and intermediate axes the structural pattern inherited from the first rifting phase was no longer suitably oriented to accommodate extensional deformation and strike slip faulting assumed a major role the strike slip regime amplified uplift of basement ridges within the rift in regions of local transpression but it also created alluvial basins because of local transtension this new kinematic style is compatible with the recent seismicity older faults that show mainly the first deformational increment are restricted to the outermost parts of the rift toward the center the faults depict an increase in strike slip components of movement suggesting deformation propagation toward the rift center which results in a narrowing of the active rift environments with time

are obsidian subsources meaningful units of analysis temporal and spatial patterning of subsources in the coso volcanic field southeastern california archaeologists frequently assign artifacts to chemically discrete subsignatures of major obsidian sources while the technical ability to do so has been demonstrated it remains to be shown that such information is behaviorally meaningful indeed some analysts choose not to make such determinations under the presumption that the data are not anthropologically relevant using a case study from the coso volcanic field which has at least four distinct subsignatures we examine this problem and conclude that subsource identification can be useful and quite interesting this is particularly so when large datasets encompassing spatially expansive areas can be assembled and statistically analyzed

the p v t x f o2 evolution of h2o co2 ch4 bearing fluid in a wolframite vein reconstruction from fluid inclusion studies aqueous carbonaceous and later pure aqueous fluid inclusions in quartz from a ferberite fe 95mn 05 wo4 vein within the low grade metamorphic aureole of the borne granite french massif central have been studied by microthermometry and raman spectrometry the bulk v x properties of the aqueous carbonaceous inclusions have been derived using the equation of state of heyenet al 1982 for the low temperature co2 ch4 system a p t path has been proposed for their trapping using the equations of state of jacobs and kerrick 1981a for the h2o co2 ch4 system two main episodes were reconstructed for the history of the aqueous carbonaceous fluid 1 primary h2o co2 ch4 vapourrich inclusions in quartz indicated the early circulation of a low density fluid 65 mole h2o 34 mole co2 1 mole ch4 and traces of n2 d 0 35 gcm 3 at around 550 50 c and 700 100 bar fluid cooled approximately isobarically to 450 400 c and was progressively diluted by h2o with a concomitant increase in density the fo2 of the h2oco2 ch4 fluid estimated from the equilibrium co2 2h2o ch4 2o2 first ranged from 10 22 to 10 27 bar close to the q f m buffer within analytical errors these values were consistent with the presence of graphite in equilibrium with the fluid 2 a drop in pco2 and therefore a drop in fo2 was recorded by the secondary liquid rich inclusions in quartz the inclusions formed at and below 400 c were composed of h2o and ch4 only and fo2 at that stage was below that fixed by the graphite fluid equilibrium this second episode in the fluid rock system could be explained by the drop of temperature below the blocking temperature of the graphite fluid equilibrium according to this interpretation the blocking of the graphite fluid equilibrium occurred at t 370 c and probably at 400 c on account of the pressure correction mass spectrometric data show that ferberite contains h2o co2 and ch4 in fluid inclusions which lie in the gap of the v x properties of the aqueouscarbonaceous fluid in quartz deposition of ferberite probably occurred at around 400 c the previously inferred blocking temperature resulting from either the drop in pco2 the drop fo2 and or the related ph increase it is concluded that the existence of a blocking temperature for the graphite fluid chemical equilibrium may be a critical factor for maintaining a stable fluid pressure gradient in geothermal systems occurring under greenschist facies conditions in graphite bearing rocks

large magnitude late holocene earthquakes on the genoa fault west central nevada and eastern california the genoa fault a principal normal fault of the transition zone between the basin and range province and the northern sierra nevada displays a large and conspicuous prehistoric scarp three trenches excavated across this scarp exposed two large displacement late holocene events two of the trenches contained multiple layers of stratified charcoal yielding radiocarbon ages suggesting the most recent and penultimate events on the main part of the fault occurred 500 600 cal b p and 2000 2200 cal b p respectively normal slip offsets of 3 5 5 m per event along much of the rupture length are comparable to the largest historical basin and range province earthquakes suggesting these paleoearthquakes were on the order of magnitude 7 2 7 5 the apparent late holocene slip rate 2 3 mm yr is one of the highest in the basin and range province based on structural and behavioral differences the genoa fault is here divided into four principal sections the sierra diamond valley carson valley and jacks valley sections and is distinguished from three northeast striking faults in the carson city area the kings canyon carson city and indian hill faults the conspicuous scarp extends for nearly 25 km the combined length of the carson valley and jacks valley sections the diamond valley section lacks the conspicuous scarp and older alluvial fans and bedrock outcrops on the downthrown side of the fault indicate a lower activity rate activity further decreases to the south along the sierra section which consists of numerous distributed faults all three northeast striking faults in the carson city area ruptured within the past few thousand years and one or more may have ruptured during recent events on the genoa fault

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

intracrustal detachment within zones of continental deformation geologic mapping of active to recently active geologic structures in panamint valley western united states and in the haiyuan region and northern tibetan plateau china suggests detachment in the middle and lower crust on a scale of tens of kilometres to at least several hundred kilometres detachment occurs similarly in predominantly extensional panamint valley and in predominantly compressional china environments it involves structures with displacements of more than 10 km and displacement rates of more than 3 mm yr perhaps more than 10 mm yr the steeply dipping strike slip faults present in all three areas hunter mountain fault haiyuan fault and altyn tagh fault zone terminate in zones of extension or compression and geometric relations indicate that all structures including strike slip faults are thin skinned and restricted to the upper crust in panamint valley and in the haiyuan region deformation within these systems can be reconstructed in three dimensions displacement on the strike slip faults is absorbed by extension or compression occurring at the termination of the faults so strike slip displacement is roughly equal to and in the same direction as shortening or extension we propose that left slip on the altyn tagh fault zone in northern tibet is similarly absorbed by shortening southeast of the fault zone within the qaidam basin and the nan shan

lead isotope systematics of vein type antimony mineralization rheinisches schiefergebirge germany a case history of complex reaction and remobilization processes mesothermal vein type sb mineralization in the rheinisches schiefergebirge germany is characterized by two different mineralization styles which are 1 extensional quartz stibnite vein systems and 2 cu pb sb sulphosalt assemblages in overprinted pre existing pb zn veins a detailed pb isotope study of 52 representative samples from both mineralization types indicates distinct compositional patterns cu pb sb sulphosalts meneghinite boulangerite bournonite formed by reaction remobilization are characterized by pb isotope compositions 206pb 204pb 18 179 18 223 which are identical to the precursor galena 206pb 204pb 18 168 18 223 the pb isotope composition of sulphosalt minerals in these vein systems was controlled by lead inherited from pre existing galena stibnite and pb sulphosalts zinkenite semseyite plagionite formed in quartz stibnite vein systems display pb isotope ratios 206pb 204pb 18 250 18 354 which are more radiogenic than galenas from variscan pb zn ore veins 206pb 204pb 18 162 18 303 detailed small scale investigation of thrust zones hosting pb zn ores and crosscutting quartz ankerite fissure veins ramsbeck deposit indicates that the pb isotope compositions of recrystallized galena and remobilized phases boulangerite semseyite bournonite are arranged along a linear trend this is interpreted as mixing between primary galena with 206pb 204pb 18 206 and overprinting hydrothermal fluids with a more radiogenic composition 206pb 204pb 18 354 expressed by intermediate compositions 206pb 204pb 18 256 18 334 of newly formed sulphosalts the pb isotope systematics of the vein type sb mineralization is in accordance with a model of pb extraction from similar crustal sources palaeozoic sedimentary sequences at different times

deformation along the yammuneh the restraining bend of the dead sea transform paleomagnetic data and kinematic implications a simple geometrical model shows that the plate margin along the restraining bend segment of the transform plate boundary be deformed in order to accommodate overlap of the crust this model predicts both the maximum width of the deformed zone and the magnitude of deformation when the general geometry of the plate margin is known as well as the plate slip vector and the amount of cumulative displacement this idea has been tested along the plate margin of the yammuneh restraining bend of the dead sea transform the plate margin has been deformed by nne folding parallel to the restraining bend and by east west trending right lateral strike slip faults paleomagnetic measurements yield counterclockwise rotation of r r 61 0 9 6 and f f 23 4 17 2 the paleomagnetic rotational data and fault kinematic suggest that the mechanism which accommodates regional left lateral shear is simultaneous right lateral strike slip faulting on secondary faults and block rotation the magnitude of deformation is the same as that predicted by the model which is 100 shortening in a direction parallel to the plate slip vector and negligible deformation in a direction parallel to the transform the large amount of rotation implies that probably more than one fault set is involved and present day seismic activity is the current manifestation of this crustal deformation process if this prediction is correct then the current deformation of the plate margin is accommodated by more favorable newly formed nnw right lateral strike slip faults

multiple strike slip fault sets a case study from the dead sea transform in many strike slip tectonic settings large rotations of crustal blocks about vertical axes have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which presumably accommodate the relative motion between the blocks as regional deformation progresses a mechanical model suggests that rotations greater than c 25 to 45 must be accommodated by more than one set of faults with the angle c between their directions consequently the sum of the angles between sets must be roughly equal to the total tectonic material rotation to test this model we investigated the fault geometry and field relationship of fault sets in the mt hermon area in northern israel where paleomagnetic declination data imply 69 13 counterclockwise ccw block rotation the statistical and field relationship analysis of over 315 faults shows that the faulting is predominantly right lateral strike slip consisting of three distinct sets the oldest set strikes 254 the second oldest set strikes 295 and the youngest strikes 331 this last direction is consistent also with the current north south direction of the maximum principal stress axis the angle c between the first and second sets is 41 and between the second and third sets 36 in good agreement with the c angle predicted from mechanical considerations the sum of the two angles is 77 ccw in good agreement with the 69 13 ccw paleomagnetically derived rotation the results suggest specifically that the sequential development of multiple intersecting fault sets is responsible for the faulting in the mt hermon area and generally that the model of block rotation with multiple faults provides very good simple rules for analyzing very complex fault patterns