observations from the basin and range province western united states pertinent to the interpretation of regional detachment faults this paper summarizes the results of completed and ongoing research in three areas of the basin and range province of the western united states that casts doubt on the interpretation of specific regional detachment faults and the large extensional strains with which such faults are commonly associated given that these examples were influential in the development of ideas about low angle normal faults and particularly in making the case for frictional slip at dips of appreciably less than the 308 lock up angle for m 0 6 where m is the coefficient of friction we advocate a critical re examination of interpreted detachments elsewhere in the basin and range province and in other extensional and passive margin settings the sevier desert detachment of west central utah is reinterpreted as a palaeogene unconformity that has been traced to depth west of the northern sevier desert basin along an unrelated seismic reflection most probably a splay of the cretaceous age pavant thrust the absence of evidence in well cuttings and cores for either brittle deformation above or ductile deformation below is inconsistent with the existence of a fault with as much as 40 km of displacement the pavant thrust and the structurally higher canyon range thrust are erosionally truncated at the western margin of the southern sevier desert basin and are not offset by the detachment in the manner assumed by those inferring large extension across the basin the mormon peak detachment of se nevada is reinterpreted as a series of slide blocks on the basis of detachment characteristics and spatially variable kinematic indicators that are more closely aligned with the modern dip direction than the inferred regional extension direction a particularly distinctive feature of the detachment is a basal layer of up to several tens of centimetres of polymictic conglomerate that was demonstrably involved in the deformation with clastic dykes of the same material extending for several metres into overlying rocks in a manner remarkably similar to that observed at rapidly emplaced slide blocks the castle cliff detachment in the nearby beaver dam mountains of sw utah is similarly regarded as a surficial feature as originally interpreted and consistent with its conspicuous absence in seismic reflection profiles from the adjacent sedimentary basin the middle miocene eagle mountain formation of eastern california interpreted on the basis of facies evidence and distinctive clast provenance to have been moved tectonically more than 80 km ese from a location close to the jurassic age hunter mountain batholith of the cottonwood mountains is reinterpreted as having accumulated in a fluvial lacustrine rather than alluvial fan lacustrine setting with no bearing on either the amount or direction of tectonic transport the conglomeratic rocks upon which the provenance argument was based are pervasively channelized with erosional relief of less than 1 m to as much as 15 m fining upwards successions at the same scale and abundant trough cross stratification all characteristic features of fluvial sedimentation and not of alluvial fans the interpretation of the eagle mountain formation as having been deposited within a few kilometres of the hunter mountain batholith which depends strongly on assumptions about the dimensions of alluvial fans is therefore not required the result is important because the eagle mountain offset has been viewed as representing the strongest evidence for extreme extension in this part of california and for the existence of detachment faults of regional

cap carbonate platform facies model noonday formation se california the neoproterozoic outcrop belt of the death valley region california preserves an oblique cross section of the noonday formation a mixed carbonate siliciclastic platform that hosts distinctive basal ediacaran cap carbonate affiliated sedimentary structures stromatolite textures and 13 c carb values the noonday platform encompasses two depositional sequences that reveal two cycles of relative sea level change within strata conventionally considered to record a single rapid postglacial sea level rise in updip localities facies of the first depositional sequence record the transition from a carbonate ramp to a stromatolite bearing tubestone textured reef rimmed platform downdip localities seaward of the reefal escarpment variably preserve a thin and condensed onlapping foreslope wedge base level fall exposed the reef crest to karstic dissolution and propagated submarine incised valleys into the seaward margin of the reef overlying strata record the backfilling of a submarine incised valley and reestablishment of a back stepping carbonate dominated ramp prior to a second subaerial exposure event that defines the contact between the noonday and johnnie formations we address the relative contributions of syndepositional tectonism and recovery from low latitude deglaciation in dictating noonday platform architecture and the intra noonday formation sequence boundary noonday formation deposition coincided with extension of the laurentian margin during disaggregation of the rodinian supercontinent within this framework previous work has suggested that the intra noonday formation sequence boundary records growth faulting that reinforced differential topography uplifting reef rimmed horsts exposing the reef crest to karstic dissolution and downdropping grabens however we trace the intra noonday formation sequence boundary seaward of the reef crest and demonstrate that for a time wave base was situated downdip of the reef escarpment on putatively downdropped fault blocks thus if the noonday margin were undergoing extension then the creation of the intra noonday formation sequence boundary required a concomitant decrease in accommodation due perhaps to postglacial isostatic uplift attendant with low latitude deglaciation we speculate that noonday formation sequence architecture records 1 immediate deglacial flooding 2 shoaling and exposure due to isostatic rebound induced by either a hiatus in meltwater flux or rapid ice sheet collapse against a background of global deglaciation and 3 resumed flooding following complete deglaciation as rift related tectonism could amplify or counter glacial isostasy inferences of the amplitude of local postglacial sea level change will require robust estimates of syndepositional extension across the noonday margin

re evaluation of the middle miocene eagle mountain formation and its significance as a piercing point for the interpretation of extreme extension across the death valley region california u s a abstract the death valley area of eastern california and southern nevada has been highly influential in the development of ideas about extreme crustal extension one of the tightest constraints on death valley extension is apparently provided by clasts in inferred alluvial fan deposits of the eagle mountain formation 15 11 ma and their source in the hunter mountain batholith now located 100 km from some of the deposits because alluvial fans are usually less than 10 20 km in radius the remaining separation has been interpreted as tectonic new research reported here suggests that the eagle mountain formation at its type location was deposited in a fluvial lacustrine setting and provides no constraint on either the magnitude or the direction of tectonic transport and or crustal extension confidence in palinspastic reconstruction thus depends on resolving ambiguities in the correlation of pre extensional markers or on the recognition of demonstrably proximal facies tectonically distributed across the region the succession at eagle mountain comprises 1 diffusely stratified monolithologic carbonate breccia and sandstone 140 m onlapping cambrian carbonate rocks at an unconformity with 110 140 m of relief fluvial or fluvially influenced 2 10 m of tabular bedded siltstone diamictite and sandstone lacustrine 3 cross stratified and channelized sandstone and polymict conglomerate bearing hunter mountain clasts with minor siltstone and carbonate 110 m mostly fluvial and 4 tabular bedded sandstone siltstone and minor carbonate 140 m mostly lacustrine eight prominent stratigraphic discontinuities mapped within the third interval are characterized by up to 15 m of local erosional relief and by abrupt upward coarsening from siltstone or carbonate to conglomerate or sandstone a fluvial interpretation for the same critical part of the succession is based upon the existence of the mapped surfaces the ubiquitous development of channels trough cross stratification and upward fining trends particularly between the mapped surfaces and the abundance of well rounded clasts in conglomerate paleocurrents are generally directed between southward and eastward although with considerable dispersion and they shift from approximately southward or southeastward in the mostly fluvial deposits to approximately eastward in the upper lacustrine interval an unusual feature of the eagle mountain formation at eagle mountain is the presence of five crosscutting conglomerate bodies interpreted as vertically infilled fissures of tectonic origin numerous normal reversed normal and oblique slip faults with up to 34 m of stratigraphic separation are thought to postdate sedimentation and tilting of the eagle mountain formation after 11 ma

pore pressure stress increase and fault weakening in low angle normal faulting low angle dip 30 normal faults accommodate much extension of the continental crust they apparently move under low resolved shear stress and are anomalously weak characteristics that they share with the san andreas fault structural textural and geochemical arguments suggest that low angle normal faults are weak in both the ductile and brittle regimes partly or totally due to elevated pore fluid pressure high pore pressure in detachment zones may be contained by upper plate strata mineral precipitation in their hanging walls formation of low permeability microbreccia layers threshold pressure gradients and low permeability mylonites below chlorite breccia mechanical analysis shows that fault weakening may preclude equality of the regional and fault zone stress tensors and predicts reorientation and increase of principal stresses in weak fault zones these changes suppress hydraulic fracturing in the brittle detachment zone and allow slip under frictional sliding conditions typical of upper crustal rocks fault weakening focuses extension in the upper crust onto low angle normal ductile brittle shear zones in the midcrust promoting propagation of low angle brittle normal faults into the upper crust

role of crustal flexure in initiation of low angle normal faults and implications for structural evolution of the basin and range province the mechanics of low angle normal faulting has proved difficult to establish despite clear geological evidence for the existence of such faults gently dipping shear stress trajectories are necessary for nucleation and propagation of low angle faults in a structurally isotropic medium such trajectories are not produced by simple horizontal extension but do arise in the presence of flexural stresses we model stress conditions in a cross section through the strong upper crust using airy stress functions for a two dimensional elastic medium numerical modeling indicates that high ratios of flexural stress to extensional stress produce conditions favorable for low angle normal fault initiation at intermediate depths in an elastic medium we propose that the gentle dip of cenozoic low angle normal faults in the basin and range province resulted from flexural stresses that were produced by isostatically uncompensated surface and especially moho relief resurgent mid cenozoic magmatism and the consequent reduction of the thermally dependent flexural strength of the mantle lithosphere transferred flexural stresses to the upper crust causing or promoting initiation of low angle normal faults temporal changes in structural style from dominantly low angle to dominantly high angle normal faulting may reflect reduction of the magnitude of flexural stresses in the upper crust due to flattening of the moho and cooling of the mantle lithosphere

the death valley turtlebacks reinterpreted as miocene pliocene folds of a major detachment surface determining the origin of extension parallel folds in metamorphic core complexes is fundamental to understanding the development of detachment faults an excellent example of such a feature occurs in the death valley region of california where a major undulatory detachment fault is exposed along the well known turtleback antiformal surfaces of the black mountains in the hanging wall of this detachment fault are deformed strata of the copper canyon formation new age constraints indicate that the copper canyon formation was deposited from 6 to 3 ma the formation was folded during deposition into a se plunging syncline with an axial surface coplanar with that of a synform in the underlying detachment this relation suggests the turtlebacks are a folded detachment surface formed during large scale extension in an overall constrictional strain field the present more planar black mountains frontal fault system may be the result of out stepping of a normal fault system away from an older detachment fa

low angle range flank faults in the panamint inyo and slate ranges california implications for recent tectonics of the death valley region four of the mountain ranges near death valley california display exhumed low angle normal faults on their flanks features originally referred to as turtleback structures these fault surfaces are smooth planar to curviplanar and defined by faceted spurs and coincident interfluve crests the lowest parts of these faults are overlain in tectonic contact by poorly indurated fanglomerate and locally by quaternary volcanic rocks steep fault scarps in alluvium are present at the foot of each turtleback type flank although these may be examples of active low angle normal faults it is difficult to establish the dip histories of these faults the problem with constraining the dip histories is one of missing and concealed information because the exposed hanging wall rocks are some of the youngest material cut and displaced by the currently shallow dipping faults another important relationship along these range flanks is the intersection between the low angle faults and steep scarp forming faults that cut alluvium at the range fronts although this relationship remains unconstrained in most of the ranges a shallow dipping fault is cut by one of the steep faults in panamint valley suggesting that in one area at least the low angle faults are not active the low angle faults and not the steep neotectonic faults are probably responsible for most of the opening of the present valleys because each low angle fault intersects the valley floor at the range front none of the low angle faults are perched high in the ranges as would be the case if the steep faults had uplifted them significantly therefore even if there has been a geologically recent transition from low angle normal faulting to steep normal with or without strike slip faulting the modern topography still reflects the former regime strictly speaking the death valley region may not be an example of active low angle normal faults and supradetachment basins

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

the butte valley and layton well thrusts of eastern california distribution and regional significance the butte valley and layton well thrusts are major structural features in two adjacent mountain ranges west of southern death valley the butte valley thrust in the southern panamint range underlies most of the range and emplaces proterozoic rocks over strata as young as jurassic in age the layton well thrust to the southwest in the slate range has been interpreted to have proterozoic rocks juxtaposed on rocks as young as jurassic suggesting that the butte valley thrust and the layton well thrust might be correlative new information indicates that the allochthonous rocks of the layton well thrust are mesozoic in age and are not likely part of the same allochthon as that above the butte valley thrust in addition the butte valley thrust cuts sharply downward to the north and west across lower plate paleozoic strata suggesting that the fault roots beneath the layton well thrust the layton well thrust probably belongs to the east sierran thrust system and thus would be in the upper plate of the butte valley thrust

rolling hinge or fixed basin a test of continental extensional models in death valley california united states two models are typically offered to explain severe continental extensional tectonism low angle detachment faults with alloch thonous sedimentary basins above a crustal rolling hinge or differential displacement on systems of distinct deeply rooted normal faults with in place sedimentary basins structural and stratigraphic relationships between the badwater turtleback and the adjacent furnace creek basin in death valley provide a test of these contrasting models there ductile extensional deformation of basement rock at the badwater turtleback coincided with brittle extensional deformation and local uplift in the adjacent furnace creek basin between 13 and 18 ma in the furnace creek basin crustal extension of this age is evident as 1 greater fault density and offset in pre 13 ma rocks relative to post 13 ma rocks and 2 the presence of laterally variable sedimentary breccias and conglomerates which indicate a syndepositional paleotopographic high this high is approximately on line with the likely projection of an early badwater turtleback fault the surface expression of the turtleback mylonites these relationships indicate that the badwater turtleback and furnace creek basin were contiguous prior to 13 ma and that the basin formed in place relative to the black mountains basement these findings support arguments for extension in the death valley area by distinct deeply rooted fault zones rather than by a rolling hinge detachment fault

east trending dextral faults in the western great basin an explanation for anomalous trends of pre cenozoic strata and cenozoic faults two postulated major east trending strike slip fault zones here named the coaldale and excelsior fault zones are delineated mainly on the basis of faults in cenozoic rocks in eastern california and western nevada and coincide with apparent major right lateral offsets in the distribution and facies of pre cenozoic rocks apparent right lateral offset of pre cenozoic rocks on the coaldale fault zone is 60 to 80 km and on the excelsior fault zone is 45 to 55 km this offset accounts for major disruption in trends of pre cenozoic rocks in eastern california and western nevada a disruption previously interpreted as the result of either an originally curving continental margin or tectonic distortion of originally linear trends by large scale bending oroflexural folding or by crustal scale folding related to north east southwest compression main offset on the coaldale and excelsior fault zones appears to be late mesozoic in age a pre mid cretaceous age is indicated because mid cretaceous and younger plutonic rocks are not cut by major strike slip faults on line with the fault zones major movement is probably younger than the dunlap formation which is dated paleontologically as early jurassic but which may also contain rocks as young as cretaceous local reactivation of the faults in the cenozoic perhaps under a different stress regime accounts for offset of cenozoic rocks the coaldale and excelsior fault zones terminate major northwest trending late cenozoic and possibly older faults the owens valley white mountain fault system in eastern california and the furnace creek fault zone in southeastern california and westernmost nevada terminate northward at the coaldale fault zone a system of south east trending right lateral and high angle faults in western nevada including the bettles well fault terminate southward at the excelsior fault zone major movement on northwest trending faults such as the furnace creek and bettles well fault may be mostly late cenozoic in age but present information does not preclude the possibility that these fault zones initiated prior to and are offset by the coaldale and excelsior fault system the unusual trends of pre tertiary rocks and of the coaldale and excelsior fault zones in eastern california and western nevada appear to be restricted to the walker lane belt a broad northwest trending structural zone characterized by right lateral shear in western nevada and eastern california possibly initial shear produced the northwest trending faults and a later kink in this system produced oroflexural folding and the east trending right lateral coaldale and excelsior fault zones this kink may have resulted from a change in the stress regime and structural complexities where the shear zone obliquely crossed the paleozoic continental margin

distribution and provenance of the middle miocene eagle mountain formation and implications for regional kinematic analysis of the basin and range province conglomeratic strata from middle miocene sections in the central resting spring range and nearby eagle mountain california contain a clast assemblage including marble orthoquartzite fusulinid grainstone and coarse 1 m monzogabbro interstratified with tephras yielding laser fusion 40 ar 39 ar ages of 11 6 13 4 and 15 0 ma petrographic and geochronologic evidence ties the clast assemblage to a source area in the southern cottonwood mountains california 100 km west northwest of their present location in the upper 100 m of the resting spring range section conglomerates are derived almost exclusively from the southern cottonwoods source and sandstone modes are as much as 50 angular plagioclase derived from the monzogabbro the lack of dilution of this detritus by other sources and sedimentary features in both sections indicate 1 that deposition occurred on an alluvial fan with a north northeast paleoslope and 2 that transport of the gravels by sedimentary processes was probably 20 km north northeast in a direction normal to the present azimuth to their source therefore we interpret most or all of the net east southeast transport as a result of extensional and strike slip faulting between the cottonwood mountains and the resting spring range since 11 12 ma restoration of these deposits to a position 10 20 km north northeast of the eastern margin of the monzogabbro source east margin of the hunter mountain batholith yields a net tectonic displacement of the cottonwood mountains relative to the resting spring range of 104 km n67 w this result confirms previous reconstructions based on the restoration of isopachs in the cordilleran miogeocline pre cenozoic structural features and other proximal tertiary deposits in the region

three dimensional model of the late cenozoic history of the death valley region southeastern california the accumulation of a large database on the timing and kinematics of late cenozoic deformation in the death valley region of southeastern california indicates a complex three dimensional history on the basis of paleogeographic reconstructions we suggest the system was initiated as a localized pull apart between two conjugate strike slip faults the garlock and furnace creek faults and evolved into a system characterized by distributed transtension related to the eastern california shear zone our reconstructions differ from previous models in the incorporation of significant vertical axis rotations of a number of crustal blocks to explain paleomagnetic data from the region the model may resolve 1 a long standing problem of the eastern termination of the garlock fault which is explained here as a complex system of splays that initially terminated in the pull apart between the furnace creek and garlock systems and 2 the complex architecture of the black mountains which is explained here in terms of initial extreme attenuation between the garlock and furnace creek systems with overprinting by a fold and normal fault system that operated simultaneously as a result of distributed transtension this model suggests much of the displacement field is taken up in rotations and translations and the actual crustal thinning in our model is relatively small 50 66 of original thickness

slip rate of the western garlock fault at clark wash near lone tree canyon mojave desert california the precise tectonic role of the left lateral garlock fault in southern california has been controversial three proposed tectonic models yield significantly different predictions for the slip rate history orientation and total bedrock offset as a function of distance along strike in an effort to test these models we present the first slip rate estimate for the western garlock fault that is constrained by radiocarbon dating a channel referred to here as clark wash incised into a latest pleistocene alluvial fan has been left laterally offset at least 66 6 m and no more than 100 m across the western garlock fault indicating a left lateral slip rate of 7 6 mm yr 95 confidence interval of 5 3 10 7 mm yr using dendrochronologically calibrated radiocarbon dates the timing of aggradational events on the clark wash fan corresponds closely to what has been documented elsewhere in the mojave desert suggesting that much of this activity has been climatically controlled the range front fault located a few hundred meters northwest of the gar lock fault has probably acted primarily as a normal fault with a holocene rate of dip slip of 0 4 0 7 mm yr the record of prehistoric earthquakes on the garlock fault at this site though quite possibly incomplete suggests a longer interseismic interval 1200 2700 yr for the western garlock fault than for the central garlock fault the relatively high slip rate determined here indicates that the western and central segments of the garlock fault show similar rates of movement that are somewhat faster than rates inferred from geodetic data the high rate of motion on the western garlock fault is most consistent with a model in which the western garlock fault acts as a conjugate shear to the san andreas fault other mechanisms involving extension north of the garlock fault and block rotation at the eastern end of the fault may be relevant to the central and eastern sections of the fault but they cannot explain a high rate of slip on the western garlock fault

right lateral displacements and the holocene slip rate associated with prehistoric earthquakes along the southern panamint valley fault zone implications for southern basin and range tectonics and coastal california deformation the n20 w trending panamint valley fault zone is linked to the n60 w trending hunter mountain strike slip fault and the saline valley fault system which represents one of the three major fault systems accommodating active crustal extension in the southern great basin a 25 km long zone of fault scarps along the southern panamint valley fault zone is recognized as the surface rupture zone associated with the most recent prehistoric earthquake the displacement associated with the most recent event determined through six detailed topographic maps of offset features is 3 2 0 5 m and a number of larger offsets in range of 6 7 m and 12 m are also observed if the larger displacements represent respectively two and three events each of 3 m then the fault zone appears to be associated with a characteristic earthquake which we estimate from the length of the rupture zone and the displacement to be between ms 6 5 and 7 2 the holocene slip rate is 2 36 0 79 mm yr is determined from the displacement of two alluvial features whose maximum age is estimated from pluvial shorelines assuming a characteristic earthquake model the recurrence interval is between 860 and 2360 years the holocene slip rate appears to be similar to the 4 million year slip rate of 2 2 7 mm yr determined from the hunter mountain fault which we suggest reflects the relatively constant tectonics in this region over the last 4 million years we further speculate that this supports the san andreas discrepancy in that the holocene slip rate of the san andreas fault probably represents its very long term several ma slip rate the total slip vector of the southern panamint valley fault system is oriented toward n35 w making this a predominantly strike slip fault in conjunction with the n60 w orientation of the hunter mountain strike slip fault we suggest that the displacement vector for the southern great basin is toward the nw consistent with results from vlbi data rather than wnw as determined by combining vlbi and geological data this in turn suggests that the coastal california deformation component involves respectively less shortening and more strike slip displacement perpendicular and parallel to the san andreas fault than is currently proposed

initiation of deformation of the eastern california shear zone constraints from garlock fault geometry and gps observations 1 we suggest a 2 stage deformation model for the eastern california shear zone ecsz to explain the geometry of the garlock fault trace we assume the garlock fault was originally straight and then was gradually curved by right lateral shear deformation across the ecsz in our 2 stage deformation model the first stage involves uniform shear deformation across the eastern part of the shear zone and the second stage involves uniform shear deformation across the entire shear zone in addition to the current shape of the garlock fault our model incorporates constraints on contemporary deformation rates provided by gps observations we find that the best fitting age for initiation of shear in eastern part of the ecsz is about 5 0 0 4 ma and that deformation of the western part started about 1 6 myr later

the yaqui ridge antiform and detachment fault mid cenozoic extensional terrane west of the san andreas fault the narrow zone of mid cenozoic detachment terrane in the southwestern united states can be widened and traced across the southern strands of the san andreas fault zone into the eastern margin of the peninsular ranges batholith the yaqui ridge core complex and detachment fault in southern borrego valley california exemplifies the nature of detached terranes in southcentral california the detachment fault dips 10 40 to the northeast and separates a lower core of gneissic late cretaceous granodiorite from an unconsolidated unmetamorphosed megabreccia of eocene to miocene age foliations in the lower plate generally conform to the strike and dip of the overlying detachment fault and become less distinct away from the fault the megabreccia which forms the upper plate is composed of unsorted fairly well rounded clasts characteristic of batholithic and metasedimentary rocks of the region plio pleistocene lacustrine sediments unconformably overlie the megabreccia in some areas the detachment fault and upper plate are expressed as a series of klippen which parallel yaqui ridge along the northeast and southeast flanks the fault is marked by a typically narrow 6 10 cm band of intensely sheared cataclasite a chlorite breccia zone occurs below the cataclasite and gradually grades into the less deformed regionally foliated gneissic granodiorite on the southeast flank what appears to be an extension of the cataclasite and detached upper plate is exposed only locally near the southeast nose of the antiform here the upper plate consists of blocks of pale gray to tan aphanitic fault gouge superimposed upon and cutting through the detachment related features is a broad zone of left oblique slip faulting termed the yaqui ridge shear zone this fault zone trends wnw ese parallel to yaqui ridge and separates the detachment fault from the ridge merging with the detachment fault near the eastern margin further east the shear zone widens into a massive zone of gouge 400 m wide and disappears under the sediments of borrego valley and the salton trough to the west the shear zone slices into the eastern margin of the peninsular ranges batholith and extends the marginal plutons in an e w direction four dominant tectonic episodes can be recognized from exposures at and adjacent to yaqui ridge 1 a late cretaceous synkinematic cataclasis and metamorphism accompanying and following emplacement of the yaqui ridge and related plutons and forming a pervasive regional nw trending foliation and ne trending lineation 2 a subsequent mid cenozoic shallow low temperature event of detachment faulting and folding which produces localized brittle cataclasis which overprints the earlier nw trending fabrics 3 the late miocene early pliocene development of yaqui ridge shear zone along the northeast margin of yaqui ridge shearing rotating and overprinting the fabrics formed during the late cretaceous plutonism and regional deformation 4 a pleistocene episode of high angle faulting and folding which produce a series of e w trending anticlines and synclines in upper pliocene to pleistocene sediments linear elements which have formed throughout the several late cretaceous to cenezoic deformations are concordant

the stratigraphic expression of a large negative carbon isotope excursion from the ediacaran johnnie formation death valley the rainstorm member of the ediacaran johnnie formation of the southern basin and range us records a large negative excursion in carbon isotope ratios of carbonate strata 13 c vpdb 6 the character of the excursion raises fundamental questions about whether this isotopic pattern is accurately capturing the time series behavior of marine dissolved inorganic carbon dic or is a product of diagenesis to explore this issue we examined the expression of this isotopic excursion within the johnnie oolite a 2 m thick marker bed which records the highest rate of change in 13 c sedimentologically the oolite unit is thought to be time transgressive if the isotopic excursion reflects time series behavior of the carbon cycle its expression in the oolite across the basin should systematically align the sections according to their slight diachroneity detailed carbon and oxygen isotopic stratigraphy of the oolite at seven different locations indicates the magnitude of the excursion at the base of the oolite is spatially variable such that after a palinspastic reconstruction the sections align along a systematic north to south gradient in the isotopic data the oolite preserved in the old dad mountains the most southerly section measured is an outlier to this trend and the most difficult to ordinate accurately in the reconstruction showing the largest isotopic range between the onset of carbonate deposition and top of the oolite several hypotheses are congruent with these data but the sum of observations is best explained by a scenario wherein the oolite is time transgressive and deposited in a north to south manner throughout the onset of the isotopic excursion if correct this implies that the stratigraphic 13 c pattern reflects time series behavior of marine dic

field tests of rolling hinges existence mechanical types and implications for extensional tectonics in rolling hinge models a locus of isostatic footwall uplift migrates together with the withdrawing hanging wall of a large displacement tens of kilometers normal fault a fault bend fold that migrates through the footwall results causing progressive deactivation of the updip part of the fault as it is tilted to a lower dip rolling hinge migration should leave a discernible geologic record but compelling tests are difficult to design because predictions of rolling hinge models are nonunique and alternative models commonly can explain individual data sets field tests based on thermochronology paleomagnetism and structural analysis are typically applied to footwall rocks whereas sedimentology geochronology of volcanic rocks and structural analysis are typically applied to upper plate rocks we review existing theoretical models and case studies in 14 detachment terranes some negative results are reasonably compelling but positive tests as yet tend to permit rather than demand a rolling hinge locally both positive and negative conclusions have been reached about the passage of a hinge within the same terrane such differences in interpretation may depend more on the scale of observation or on the specific model considered than on the geological development of the area subvertical simple shear and flexural failure mechanisms with or without significant induced flow of lower crustal material are thought to dominate hinge related footwall deformation in individual sites although rolling hinge models offer an alternative to low initial fault dips hinge related deformation does not demand a steep initial fault dip few areas in which rolling hinge tectonics are permissible had initially steep faults 35 rather where the evidence most strongly favors a rolling hinge detachment faults had initial dips 35 in the future conclusive results are most likely to be obtained if diverse tests are applied to both the upper and lower plates of detachment terranes where other geological sources of ambiguity are minimal at best the rolling hinge mechanism does not universally explain the observation that earthquakes are rare on low angle normal faults nor solve the mechanical problems of frictional slip on such faults

the black mountains turtlebacks rosetta stones of death valley tectonics the black mountains turtlebacks expose mid crustal rock along the western front of the black mountains as such they provide keys to understanding the tertiary structural evolution of death valley and because of the outstanding rock exposure they also provide valuable natural laboratories for observing structural processes there are three turtlebacks the badwater turtleback in the north the copper canyon turtleback and the mormon point turtleback in the south although important differences exist among them each turtleback displays a doubly plunging antiformal core of metamorphic and igneous rock and a brittle fault contact to the northwest that is structurally overlain by miocene pleistocene volcanic and or sedimentary rock the turtleback cores contain mylonitic rocks that record an early period of top southeastward directed shear followed by top northwestward directed shear the earlier formed mylonites are cut by and locally appear concurrent with 55 61 ma pegmatite we interpret these fabrics as related to large scale basement involved thrust faults at the turtlebacks now preserved as areally extensive metamorphosed basement over younger cover contacts the younger and far more pervasive mylonites record late tertiary extensional unroofing of the turtleback footwalls from mid crustal depths available geochronology suggests that they cooled through 300 8c at different times 13 ma at badwater 6 ma at copper canyon 8 ma at mormon point at mormon point and copper canyon turtlebacks these dates record cooling of the metamorphic assemblages from beneath the floor of an 11 ma tertiary plutonic complex collectively these relationships suggest that the turtlebacks record initiation of ductile extension before 14 ma followed by injection of a large plutonic complex along the ductile shear zone ductile deformation continued during extensional uplift until the rocks cooled below temperatures for crystal plastic deformation by 6 8 ma subsequent low angle brittle fault slip led to final exposure of the igneous and metamorphic complex the turtleback shear zones can constrain models for crustal extension from map view as well as cross sectional perspectives in map view the presence of basement involved thrust faults in the turtlebacks suggest the black mountains were a basement

assessing vertical axis rotations in large magnitude extensional settings a transect across the death valley extended terrane california models for neogene crustal deformation in the central death valley extended terrane southeastern california differ markedly in their estimates of upper crustal extension versus shear translations documentation of vertical axis rotations of range scale crustal blocks or parts thereof is critical when attempting to reconstruct this highly extended region to better define the magnitude aerial extent and timing of vertical axis rotation that could mark shear translation of the crust in this area paleomagnetic data were obtained from tertiary igneous and remagnetized paleozoic carbonate rocks along a roughly east west traverse parallel to about 36 n latitude sites were established in 7 to 5 ma volcanic sequences greenwater canyon and brown s peak and the 10 ma chocolate sundae mountain granite in the greenwater range 8 5 to 7 5 ma and 5 to 4 ma basalts on the east flank of the black mountains the 10 6 ma little chief stock and upper miocene basalts in the eastern panamint mountains and paleozoic pogonip group carbonate strata in the north central panamint mountains at the site level most materials yield readily interpretable paleomagnetic data group mean directions after appropriate structural corrections suggest no major vertical axis rotation of the greenwater range e g d 359 i 46 95 8 0 n 12 7 normal n 5 reversed r polarity sites little post 5 ma rotation of the eastern black mountains e g d 006 i 61 95 4 0 n 9 n 6 r sites and no significant post 10 ma rotation of the panamint range e g d 181 i 51 95 6 5 n 9 r sites in situ data from the greenwater canyon volcanic rocks chocolate sundae mountain granite funeral peak basalt rocks the little chief stock and paleozoic carbonate rocks remagnetized are consistent with moderate south east side down tilting of the separate range blocks during northwest directed extension the paleomagnetic data reported here suggest that the panamints shared none of the 7 ma to recent clockwise rotation of the black mountains crystalline core as proposed in recent models for transtensional development of the central death valley extended terrane

curie point depth estimates from aeromagnetic data from death valley and surrounding regions california aeromagnetic data were analyzed to determine the curie point depth cpd by power density spectral and three dimensional inversion methods within and surrounding death valley in southern california we calculated the cpd for 0 5 regions using 2d power density spectral methods and found that the cpds varied between 8 and 17 km however the 0 5 region may average areas that include shallow and deep cpds and because of this limitation we used the 3d inversion method to determine if this method may provide better resolution of the cpds the final 3d model indicates that the depth to the bottom of the magnetic susceptible bodies varies between 5 and 23 km even though both methods produced roughly similar results the 3d inversion method produced a higher lateral resolution of the cpds the shallowest cpds occur within the central and southern death valley panamint valley coso geothermal field and the tecopa hot springs region deeper 15 km cpds occur over outcropping granitic and precambrian lithologies in the panamint range grapevine mountains black mountains and the argus range the shallowest cpd occurs within the central death valley over a possible seismically imaged magma body and slightly deeper values occur within the panamint valley southern death valley and tecopa hot springs the shallow cpd values suggest that partially molten material may also be found in these latter regions the cpd computed heat flow values for the region suggest that the entire area has high heat flow values 100 mw m 2 on the other hand locally extremely high values 200 mw m 2 occur within the panamint valley the southern and central death valley and tecopa hot springs region these locally high heat flow values may be related to midcrustal magma bodies but additional geophysical experiments are needed to determine if the magma bodies exist

surficial offsets on the central and eastern garlock fault associated with prehistoric earthquakes geomorphic features offset along the central and eastern garlock fault record the amount of surface slip associated with prehistoric earthquakes along the easternmost 90 km of the fault the smallest offsets cluster around 2 3 m of left lateral slip apparently associated with the most recent earthquake on this portion of the fault larger offsets along this part of the fault especially in pilot knob valley cluster around values consistent with 2 4 m of slip in each of the past several events farther west south of el paso mountains offset geomorphic features suggest that each of the past two earthquakes on this stretch of the garlock fault was produced by about 7 m of slip whereas the third event back was produced by about 4 m of slip vertical displacements of geomorphic features range from 0 to 30 of the left lateral offsets within pilot knob valley along the southern side of the slate range vertical displacements are consistently up on the northern side whereas within the avawatz mountains both north and south side up vertical displacements are present on the basis of the geomorphic offsets the geometry of the garlock fault and the precedents set by historical strike slip earthquakes elsewhere a number of different rupture patterns are plausible these range from rupture of the entire garlock fault in a single event with a maximum magnitude of about mw 7 8 to separate rupture of the western segment and of the central and eastern segments combined with approximate magnitudes mw 1 1 and mw 7 5 respectively to separate rupture of even shorter segments producing earthquakes of magnitudes mw 6 6 to mw 7 5 in conjunction with available slip rates for the garlock fault the geomorphic offsets suggest that average recurrence intervals are probably within the range of 600 1200 years south of el paso mountains about 200 750 years in searles valley about 200 1300 years in pilot knob valley and about 200 3000 years near leach lake and in the avawatz mountains

extensional tectonics in the death valley area california transport of the panamint range structural block 80 km northwestward in the hypothesis presented here the panamint range is a structural block that was detached during late cenozoic time from underlying rocks that now form the black mountains and was transported tectonically an estimated 80 km to the northwest transport occurred along a single low angle westward dipping detachment fault or perhaps along a system of such faults the estimate of 80 km of transport distance is based on the apparent right lateral offset of late precambrian and paleozoic facies and thickness trends along the furnace creek fault zone that bounds the detached block on the northeast the death valley turtleback surfaces are considered to be gigantic mullions related to the detachment surface

the neoproterozoic noonday formation death valley region california the neoproterozoic cambrian succession in the death valley region of the southwestern united states is among the best exposed and easily accessible in the world the largest single exposure of these strata occurs in the panamint range on the west side of death valley and it although variably metamorphosed contains the most complete sections of the noonday formation cap carbonate sequence new geological mapping measured sections and high resolution carbon isotope data for the noonday formation in this range enable establishment of a unified stratigraphy across the death valley region that consists of three units from the base upward they include the sentinel peak member of light gray massive to laminated fine dolostone locally containing vugs and tubes variably filled with micrite spar or quartz and rarely galena and varying from 2 to 200 m in thickness the radcliff member composed of feldspathic shale and sandstone and thin bedded limestone 0 200 m thick and the newly defined mahogany flats member of gray commonly stromatolitic thin to medium bedded fine dolostone that is 200 m thick at its type locality carbon isotopic trends in the panamint range match to within 1 2 reproducibility those known for the equivalent nonmetamorphosed strata in the eastern death valley sections a composite section of the noonday formation displays a chemostratigraphic profile with values near 3 through the cap dolostone of the sentinel peak member a decline to 6 in the lower part of the radcliff member followed by a recovery to near 0 and subsequent decline toward 2 in the remainder of the radcliff units and lower mahogany flats member and then a return to positive values 4 through the remainder of the mahogany flats member this pattern matches the ediacaran cap carbonate in namibia remarkably well and assuming our carbon isotopic correlations are correct it indicates that noonday deposition occurred at the beginning of the ediacaran period and that the immediately underlying wildrose diamictite of the kingston peak formation probably represents the younger cryogenian marinoan glacial episode

how canyons evolve by incision into bedrock rainbow canyon death valley national park united states incising rivers may be confined by low slope erodible hillslopes or steep resistant sidewalls in the latter case the system forms a canyon we present a morphodynamic model that includes the essential elements of a canyon incising into a plateau including 1 abrasion driven channel incision 2 migration of a canyon head knickpoint 3 sediment feed from an alluvial channel upstream of the knickpoint and 4 production of sediment by sidewall collapse we calculate incision in terms of collision of clasts with the bed we calculate knickpoint migration using a moving boundary formulation that allows a slope discontinuity where the channel head meets an alluvial plateau feeder channel rather than modeling sidewall collapse events we model long term behavior using a constant sidewall slope as the channel incises our morphodynamic model specifically applies to canyon rather than river hillslope evolution we implement it for rainbow canyon ca salient results are as follows 1 sediment supply from collapsing canyon sidewalls can be substantially larger than that supplied from the feeder channel on the plateau 2 for any given quasi equilibrium canyon bedrock slope two conjugate slopes are possible for the alluvial channel upstream with the lower of the two corresponding to a substantially lower knickpoint migration rate and higher preservation potential 3 knickpoint migration occurs at a substantially faster time scale than regrading of the bedrock channel itself underlying the significance of disequilibrium processes although implemented for constant climactic conditions the model warrants extension to long term climate variation

late palaeogene neogene evolution of the triple junction region near maras south central turkey the marao region of turkey includes the anatolian arabian african triple junction the african arabian margin is marked by left lateral transtension along the dead sea kara su rift motion on the anatolian arabian boundary is partitioned between the left lateral east anatolian fault and a fold thrust belt termed the engizek zone equivalent to the bitlis suture further east the african anatolian boundary lies along the cyprus misis andirin trend and is expressed near the triple junction as the aslantao iskenderun zone now undergoing slow left lateral transtension the neogene evolution of these boundaries is largely recorded in the stratigraphy and structure of the basins that lie along the anatolian arabian lice basin and anatolian african aslantao iskenderun basin boundaries these basins appear to have developed in the early miocene along major strike slip zones large half graben in the pre neogene 9basement9 of the western misis andirin block developed contemporaneously with motion on the anatolian african boundary these extensional features evolved subsequent to the intense shattering of the mesozoic carbonate sequence in the misis andirin block and its mixing with panels of deep water palaeogene strata to form an extensionally disrupted assemblage transpression and conversion of both anatolian african and anatolian arabian boundaries to fold thrust belts occurred in the late miocene transpression continues along the anatolian arabian boundary but transtension has recurred along the anatolian african boundary since the pliocene

velocity models for the highly extended crust of death valley california we employ nonlinear optimization to obtain shallow velocity structure from cocorp seismic reflection profiles in the highly extended death valley region california we also use group velocity analysis of regional earthquake surface waves that have traversed in and near the extended domain when compared with regional models for surrounding provinces our results show no evidence for significant uplift of the middle crust or removal of the upper crust we do not find fundamental upper crustal variations between regions proposed to have widely varying extensional histories these observations constrain previously proposed extensional mechanisms toward less than extreme amounts of extension

physical analog modeling of pull apart basin evolution abstract pull apart basins are structural depressions formed by localized extension along strike slip fault systems typically at releasing bends or steps in the fundamental strike slip system analog modeling is used to evaluate the sequence of structural evolution of pull apart basins and factors that control their degree of structural asymmetry and geometry basin evolution internal structure and overall symmetry are investigated for oblique releasing step angles and with varying rates of displacement between brittle and ductile crust on opposing sides of the strike slip system while maintaining relative rates between brittle crust on opposing sides of the basin pull apart basin evolution is separated into three stages incipient early and mature incipient pull apart basins are characterized by formation of a normal fault bounded graben or half graben parallel to the oblique step between main strike slip zones in the early stage of formation additional normal faults form toward the basin interior from the original bounding faults and cross basin strike slip faults cut diagonally across the basin interior basin bounding normal fault systems are characterized by lateral variations of fault throw and localized relay ramps in the mature stage of evolution strike slip and normal faults join to completely bound the pull apart basin analog model results indicate that displacement associated with cross basin faults causes development of a through going strike slip fault that links the two main strike slip displacement zones ultimately resulting in a decline in normal fault activity asymmetric symmetric and hybrid pull apart basins all follow the same overall deformation sequence just described the asymmetry of a pull apart basin is controlled by the degree of decoupling between brittle and ductile crust beneath the two crustal blocks in relative motion this is modeled by maintaining a constant relative rate of motion between opposing fault blocks in all models but varying the rate between the blocks and the fixed model basement absolute rate models in which one side of the detachment is fixed with respect to the basement form asymmetric pull apart basins defined by a half graben with the master fault on the mobile side in models where opposing sides of the fault system are equally decoupled from the basement symmetrical pull apart basins form defined by horst and graben structures and master fault dominance switching sides along the length of the basin fault segmentation associated with relatively immature pull apart basins may be capable of arresting earthquake rupture late linkage of main strike slip zones by a cross basin fault could extend potential rupture area dramatically increasing the possibility for large magnitude earthquakes

origin and trajectory of the frenchman mountain block an extensional allochthon in the basin and range province southern nevada abstract the central basin and range province has provided a natural laboratory for the study of continental extension since the pioneering studies of chester r longwell it is here that large magnitude crustal extension along systems of normal faults was first documented by r ernest anderson estimates of crustal extension in the lake mead extensional domain eastern central basin and range province have been based largely on restoration of displaced blocks most notably the frenchman mountain block the original position of frenchman mountain an allochthonous block that has been translated more than 60 km westward during regional extension has been controversial for more than 2 decades we use correlation of mass wasting deposits with source areas paleozoic stratigraphic characteristics and subunconformity pinchout relations to evaluate previously published models for the original position of the frenchman mountain block and use new evidence to propose a more tightly constrained preextension posi

a new hypothesis for the amount and distribution of dextral displacement along the fish lake valley northern death valley furnace creek fault zone california nevada 1 the fish lake valley northern death valley furnace creek fault zone a 250 km long predominantly right lateral structure in california and nevada is a key element in tectonic reconstructions of the death valley area eastern california shear zone and walker lane and central basin and range province total displacement on the fault zone is contested however with estimates ranging from 30 to 63 km or more here we present a new synthesis of available constraints preextensional thrust faults folds and igneous rocks indicate that offset reaches a maximum of 50 km neogene rocks constrain its partitioning over time most offset is interpreted as 13 10 ma accruing at 3 5 mm yr in the middle of the fault zone and more slowly toward the tips the offset markers imply 68 14 km of translation between the cottonwood mountains and resting spring nopah range 60 14 km since 15 ma through a combination of strike slip and crustal extension this suggests that a previous interpretation of 104 7 km based on the middle miocene eagle mountain formation is an overestimate by 50 our results also help to mitigate a discrepancy in the 12 0 ma strain budget for the eastern california shear zone displacement has previously been estimated at 100 10 km and 67 6 km for the basin and range and mojave portions of the shear zone respectively our new estimate of 74 17 km for the basin and range is within the uncertainty of the mojave estimate

on the importance of higher order ice dynamics for glacial landscape evolution abstract alpine glacial landforms such as u shaped troughs hanging valleys and cirques have been linked to the meltwater modulated sliding motion of glaciers several of these alpine landforms are associated with characteristic length scales for width and spacing indicating that the viscosity of ice exerts a first order control on the processes responsible for their formation here we present the results of three dimensional computational experiments focusing on the influence of higher order ice dynamical effects such as horizontal stress gradients on long term glacial landscape evolution some of the experiments presented have highly simplified bed conditions other experiments use topography from an existing mountain range as basis for simulating glacial erosion the experiments demonstrate how gradients in horizontal stress play a primary role in scaling the bed shear stress of glaciers we also demonstrate how higher order ice dynamics may influence the feedback between glacial sliding and erosion hereby providing important stabilization mechanisms that prevent runaway effects associated with the steepening of longitudinal profiles and the formation of overdeepenings

the northern cascadia subduction zone at vancouver island seismic structure and tectonic history the structure and tertiary tectonic history of the northern cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the

the origin and evolution of the southern snake range decollement east central nevada regional and local stratigraphic metamorphic and structural constraints permit reconstruction of the southern snake range extensional deformational system in east central nevada the dominant structure of the range the southern snake range decollement ssrd operated during oligocene and miocene extensional deformation to exhume a footwall of multiply deformed metasedimentary and plutonic rocks intrusion of three plutons 160 ma 79 1 0 5 ma and 36 1 ma respectively and development of two cleavages preceded the onset of extensional deformation plastic deformation of lower plate metasedimentary rocks accompanied the early phases of regional extension and produced bedding parallel grain shape foliations and wnw trending stretching lineations these fabrics parallel the ssrd even in low strain domains suggesting that a significant component of pure shear strain probably accompanied noncoaxial deformation associated with motion on the ssrd consistent with other lines of evidence meanwhile hanging wall rocks were greatly extended by at least two generations of tilt block style normal faults soling into the ssrd with the earlier faults antithetic to the ssrd and the later faults dipping in the same direction as the ssrd a retrodeformed regional cross section sequence illustrates plausible alternative schemes for reconstructing the southern snake range extensional system in one scheme the ssrd forms as a crustal scale stretching shear zone separating an upper plate that extends on steeply inclined normal faults from a lower plate that stretches by penetrative flow in the other lower plate deformation incorporates a component of coaxial stretching but the ssrd also functions as a conventional shear zone accommodating through going displacement between opposing plates in either case as tectonic unroofing proceeds differential isostatic unloading induces the ssrd to rotate to steeper dips as it migrates into the frictional sliding regime thus enabling it to remain active as a brittle normal fault until it finally rotates to its present shallow inclination in either scenario cross section constraints suggest that total extension accommodated by the ssrd was probably between 8 km and 24 km

coupling the flow of ice water and sediment in a glacial landscape evolution model abstract the processes of subglacial erosion depend not only on the sliding motion of glaciers over bedrock but also on the presence of meltwater and sediment in the subglacial environment in particular theoretical models for subglacial quarrying and abrasion as well as hypothesized erosion thresholds associated with subglacial sediment transport include both positive and negative effects of subglacial water and sediment on the rate of erosion in order to incorporate the existing theoretical models for subglacial erosion by quarrying and abrasion in a long term glacial landscape evolution model we here present a coupled computational framework for simulating the simultaneous flow processes of ice water and sediment we supplement a higher order ice sheet model with simple long term models for glacial hydrology and subglacial sediment transport as well as fluvial and hillslope related erosion processes an important strength of the modeling framework presented relates to the morphological detail of the landscapes produced which facilitates direct comparison with existing landforms this improves the connection between glacial landscape evolution models geomorphological observations and the existing knowledge of the physical processes that operate under glaciers we demonstrate with model examples how increased basal meltwater pressure and transport limited subglacial erosion lead to both positive and negative feedbacks related to glacial erosion and the formation of overdeepenings

interpretation and tectonic implications of cooling histories an example from the black mountains death valley extended terrane california abstract in the death valley extended terrane of california the black mountains have long been considered unique because they largely lack the miogeoclinal cover rocks characteristic of the surrounding ranges fission track ages presented here are combined with published40ar 39ar ages and used to construct cooling path envelopes for samples of precambrian crystalline basement and miocene plutonic rocks collected across the entire range the cooling history reconstructions are used to differentiate between contrasting miocene unroofing histories proposed for this range apatite and zircon fission track ages from the southeastern portion of the range suggest unroofing occurred there at 13 8 5 ma from temperatures well below 300 c cooling age data from the central black mountains indicate major unroofing at 8 5 6 0 ma from temperatures greater than 300 c old cooling ages from directly beneath the highly extended amargosa chaos rocks are consistent with the chaos rocks being part of an allochthonous slice that was tectonically transported from high crustal levels onto deeper crustal levels scenarios for the miocene unroofing history of this range rely heavily on interpretations of the depth of emplacement of miocene plutons in the core of the range thermochronologic and geobarometric data and thermal modeling of intrusion cooling suggest emplacement of an 11 6 ma pluton into the crystalline core at a depth of 10 15 km both the cooling age data and considerations of the local geology seem to preclude an unroofing history dominated by erosion of the overlying miogeoclinal section the morphology of the cooling path envelopes constructed here are similar to those constructed for detachment fault terranes the data are most consistent with unroofing involving tectonic denudation 10 15 km along a single westerly dipping detachment zone diachronous rapid cooling from southeast to northwest within the range is interpreted as a result of the lower plate undergoing flexural deformation as it pulls out from underneath a relatively rigid scoop shaped hanging wall block the rolling hinge model migration of unroofing and tilting within the range mimics the overall east to west sequential tilting and unroofing of range blocks in the death valley extended terrane similar thermochronologic evidence for sequential range scale tilting has been obtained from the lake mead extended region suggesting that this style of extension might provide an explanation for strongly extended domains in the basin and range province

thermal and barometric constraints on the intrusive and unroofing history of the black mountains implications for timing initial dip and kinematics of detachment faulting in the death valley region california unroofing of the black mountains death valley california has resulted in the exposure of 1 7 ga crystalline basement late precambrian amphibolite facies metasedimentary rocks and a tertiary magmatic complex the 40ar 39ar cooling ages obtained from samples collected across the entire length of the range 55 km combined with geobarometric results from synextensional intrusions provide time depth constraints on the miocene intrusive history and extensional unroofing of the black mountains data from the southeastern black mountains and adjacent greenwater range suggest unroofing from shallow depths between 9 and 10 ma to the northwest in the crystalline core of the range biotite plateau ages from 13 to 6 8 ma from rocks making up the death valley turtlebacks indicate a midcrustal residence with temperatures 300 c prior to extensional unroofing biotite 40ar 39ar ages from both precambrian basement and tertiary plutons reveal a diachronous cooling pattern of decreasing ages toward the northwest subparallel to the regional extension direction diachronous cooling was accompanied by dike intrusion which also decreases in age toward the northwest the cooling age pattern and geobarometric constraints in crystalline rocks of the black mountains suggest denudation of 10 15 km along a northwest directed detachment system consistent with regional reconstructions of tertiary extension and with unroofing of a northwest deepening crustal section mica cooling ages that deviate from the northwest younging trend are consistent with northwestward transport of rocks initially at shallower crustal levels onto deeper levels along splays of the detachment the well known amargosa chaos and perhaps the badwater turtleback are examples of this splaying process considering the current distance of the structurally deepest samples away from moderately to steeply east tilted tertiary strata in the southeastern black mountains these data indicate an average initial dip of the detachment system of the order of 20 similar to that determined for detachment faults in west central arizona and southeastern california beginning with an initially listric geometry a pattern of footwall unroofing accompanied by dike intrusion progresses northwestward this pattern may be explained by a model where migration of footwall flexures occur below a scoop shaped hanging wall block one consequence of this model is that gently dipping ductile fabrics developed in the middle crust steepen in the upper crust during unloading this process resolves the low initial dips obtained here with mapping which suggests transport of the upper plate on moderately to steeply dipping surfaces in the middle and upper crust

dating the detachment fault system of the ruby mountains nevada significance for the kinematics of low angle normal faults 1 the mechanics of low angle normal faulting and metamorphic core complexes continue to be a subject of debate we investigate the conditions timing and kinematics of slip in the late upper crustal stages of core complex evolution of the ruby mountains detachment fault at the well exposed secret pass locality with an x ray diffraction xrd and ar ar study of clay gouge samples from three separate faults two from the low angle detachment system and one from a high angle normal fault that soles into the main detachment fault system xrd analysis and modeling of xrd analysis show that authigenic illite rich illite smectite i s in gouge at secret pass is distinguishable from clay phases in hanging wall rocks because the i s in the gouges contains only one water layer as opposed to the more common two water i s phases found in both the hanging wall and footwall ar ar ages for the monomineralic one water i s found in the hanging wall high angle fault the main detachment and a low angle normal fault structurally above the main detachment are 11 6 0 1 ma 12 3 0 1 ma and 13 8 0 2 ma respectively the not quite flat ar ar spectra indicate the gouge illites grew over some interval of time and not in discrete events the nearly overlapping ages indicate that gouge formation and thus the last major period of activity on the detachment were at 11 13 ma and were active coevally as part of a kinematically linked fault system with the main detachment active at dips 45 and possibly as low as 22

interseismic deformation and geologic evolution of the death valley fault zone 1 the death valley fault zone dvfz located in southeastern california is an active fault system with an evolved pull apart basin that has been deforming over the past 6 myr we present a study of the interseismic motion and long term stress accumulation rates to better understand the nature of both past and present day loading conditions of the dvfz using a 3 d semi analytic viscoelastic deformation model combined with geodetic velocities derived from the mobile array of gps for nevada transtension magnet network and the southern california earthquake center scec crustal motion map version 4 cmmv4 gps data we establish parameters for interseismic slip rate and apparent locking depth for four dvfz fault segments our preferred model provides good fit to the data 1 0 mm yr and 1 5 mm yr rms misfit in the fault perpendicular and fault parallel directions respectively and yields apparent locking depths between 9 8 17 1 km and strike slip rates of 3 7 mm yr for the segments we also determine subsidence 0 5 0 8 mm yr and extension 1 0 1 2 mm yr rates in the pull apart basin region with these parameters we construct a dvfz evolution model for the last 6 myr that recreates the motion of the fault blocks involved in the formation of the present day geological structures in death valley finally using coulomb stress accumulation rates derived from our model 0 25 0 49 mpa 100 yr combined with earthquake recurrence interval estimates of 500 to 2600 years we assess present day seismic hazards with calculated moment magnitudes ranging from 6 7 7 7

strike slip fault system in amargosa valley and yucca mountain nevada abstract the amargosa desert fault system part of a 250 km long system of dextral strike slip faults east of the death valley region is a throughgoing long lived nw striking dextral strike slip shear system that lies beneath crater flat and yucca mountain evidence for the fault system includes gravity seismic structural stratigraphic and paleomagnetic data the distribution of springs and basaltic volcanic centers and patterns of late quaternary surface faulting where the amargosa desert fault system passes beneath miocene ash flow tuffs of the southwestern nevada volcanic field we infer that the fault system changes into a series of detachments near the basement cover contact and is expressed as a system of rotational normal faults in the ash flow tuffs displaced geologic features indicate that total dextral displacement on the amargosa desert fault system exceeds 30 km vertical axis rotations in ash flow tuffs suggest that about 25 km of dextral displacement may have occurred since about 12 75 ma this fault system has important implications for the proposed high level nuclear waste repository at yucca mountain since it has experienced quaternary and possibly holocene displacements both normal faults and basaltic volcanic cones near the proposed repository may be controlled by the evolution of this regional strike slip fault system

geochemical consequences of flow differentiation in a multiple injection dike trinity ophiolite n california abstract a clinopyroxene rich dike of the trinity ophiolite sheeted dike complex shows three different magmatic pulses probably injected in a short period of time no well developed chilled margin and important variations of the clinopyroxene and plagioclase percentages between its core highly porphyritic and margins aphyric this variation interpreted as related to a flow differentiation phenomenon mechanical phenocryst redistribution has important geochemical consequences it produces increases in the feo mgo cao cr and ni contents from the margin to the core together with increases in the clinopyroxene percentage and decreases in the sio 2 zr y nb and ree contents together with a decrease in the percentage of the fine grained groundmass toward the core of the dike this mineralogical redistribution which also affects the incompatible trace element ratios because of the difference in plagioclase and clinopyroxene mineral liquid partition coefficients illustrate the importance of fractionation processes outside of a magma chamber

petrologic constraints on the unroofing history of the funeral mountain metamorphic core complex california this is the published version copyright 1990 american geophysical union all rights reserved

timing and tectonic setting of tertiary low angle normal faulting and associated magmatism in the southwestern united states major episodes of tertiary low angle normal faulting in the southwestern united states south of latitude 37 n moved northward with time occurring about 25 million years ago in southern arizona 20 million years ago in the mojave desert region and 10 to 15 million years ago in the las vegas area in most areas faulting and associated volcanism occurred when the mendocino triple junction lay to the west this correspondence probably resulted from the unstable configuration of the triple junction east west to northeast southwest horizontal stretching as plates pulled apart at the triple junction prevented development of a hole in the lithosphere at the triple junction the regional strain history predicted by this model is consistent with observed tertiary low angle faults strike slip faults and folds

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

paleoelevation estimates for the northern and central proto basin and range from carbonate clumped isotope thermometry 1 quantitative paleoelevation histories can help explain both why and how widespread cenozoic extension occurred in the basin and range province of western north america we present new estimates of preextensional paleoelevations for the northern and central basin and range using clumped isotope 47 thermometry of lacustrine carbonates collected from each region comparison of carbonate 47 derived mean annual air temperature maat estimates 16 c 20 c for the late cretaceous eocene sheep pass basin of east central nevada with published maat estimates for the eocene coastal northern sierra nevada 20 c 25 c suggests that the early paleogene sheep pass basin had a paleoelevation of 2 km such a modest paleoelevation suggests that either 1 the proto northern basin and range did not attain maximum paleoelevations of 3 4 km until the late eocene early oligocene or 2 the sheep pass basin was a local high relief 1 km setting contained within a 3 km orogenic highland nevadaplano similarity of 47 derived maat estimates 17 c 24 c for carbonates from the central basin and range and the near sea level southern sierra nevada bena basin indicate that middle miocene paleoelevations in the death valley region were 1 5 km these fairly low paleoelevations are incompatible with preextensional crustal thicknesses 52 km and indicate that mean elevation change was minor 500 m and lithospheric mass was not conserved during 100 neogene extension of the central basin and range but was instead likely compensated by synextensional magmatic additions to the crust

palaeomagnetic dating of fault rocks evidence for permian and mesozoic movements and brittle deformation along the extensional dalsfjord fault western norway summary palaeomagnetic and structural studies of the dalsfjord nappe western norway show that the basal ow angle detachment dalsfjord fault is a long lived fault zone and that the most important phase of faulting was of devonian extension probably nucleated on an earlier silurian scandian thrust fault rocks produced during subsequent movements indicate that the dalsfjord fault underwent periods of brittle low angle extensional reactivation during the permian 250 260 ma and upper jurassic lower cretaceous c 150 ma corresponding with stages of major extensional movements on the continental shelf palaeomagnetic studies may be of great importance for dating faults and major movement stages in long lived fault systems the particular importance of the results is that they show that low angle normal faults can operate in a brittle upper crustal regime

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