paleomagnetic and 40ar 39ar geochronologic data bearing on the structural evolution of the silver peak extensional complex west central nevada the silver peak extensional complex located in the silver peak range of west central nevada is a displacement transfer system linking the furnace creek fish lake valley fault system and transcurrent faults of the central walker lane late neogene northwest directed motion of an upper plate composed of lower paleozoic sedimentary rocks and late tertiary volcanic and volcaniclastic strata exhumed a lower plate assemblage of metamorphic tectonites with proterozoic and mesozoic protoliths paleomagnetic investigation of miocene pliocene pyroclastic and sedimentary rocks of the upper plate and miocene mafic dikes in the lower plate reveals modest horizontal axis tilting northwest side up and vertical axis rotation clockwise within the extensional complex eight to ten samples from each of 123 sites were demagnetized 95 sites yielded interpretable results dual polarity results from one population of mafic dikes in the lower plate assemblage indicate moderate northwest side up tilting declination d 329 inclination i 37 95 4 3 number n 30 sites in situ 95 the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction some dikes yield exclusively normal polarity results that are interpreted to indicate modest clockwise vertical axis rotation d 021 i 57 95 4 3 n 19 sites in situ concurrent with uplift of the lower plate rocks and nine sites yield magnetization directions that are north directed with positive inclinations of moderate steepness similar to an expected miocene field late miocene pyroclastic rocks in the upper plate yield normal polarity magnetizations suggestive of moderate clockwise vertical axis rotation d 032 i 53 95 8 8 n 10 sites the apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field because the overall dispersion of the vgp virtual geomagnetic pole positions is high for the latitude of the site location middle miocene sedimentary rocks probably were remagnetized shortly after deposition of eight 40ar 39ar determinations from mafic dikes in the lower plate five groundmass concentrates yield saddle shaped age spectra and one separate provided a plateau date of low confidence isochron analysis reveals that all six groundmass concentrates contain excess ar if rapid cooling and ar retention below 250 c are assumed the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10 5 ma the 40ar 39ar age spectrum data are consistent with existing fission track cooling and k ar isotopic age information from lower plate granitic rocks and indicate rapid cooling of the lower plate assemblage from well above 300 c to 100 c between 13 and 5 ma rapid cooling may explain the overall distribution of paleomagnetic results from lower plate intrusions such that the earliest acquired magnetizations reflect both northwest side up tilt and clockwise rotation and the younger magnetizations reflect northwest side up tilt overall the paleomagnetic data from the silver peak extensional complex are interpreted to suggest that vertical axis rotation of crustal scale blocks associated with displacement transfer in the central walker lane may play an integral part in accommodating strain within a continental displacement transfer system

tectonic implications of a late mesozoic fold and thrust belt in northwestern nevada the luning fencemaker fold and thrust belt of northwestern nevada formed east of the mesozoic sierran arc in a back arc setting where noncontinental crust localized marine deposition and subsequent deformation the fold thrust belt was active in middle or late jurassic and early cretaceous time and evolved during major northwest southeast contraction comparable shortening was not developed in the sierran arc and for much of the mesozoic the arc and the back arc regions were at least partially decoupled and separated by the regionally extensive pine nut fault system which underwent substantial left lateral displacement development of the fold thrust belt and associated strike slip fault is attributed to oblique subduction the shortening axis of the fold thrust belt and inferred displacement of the strike slip fault are incompatible with right oblique subduction and call for an interval of left oblique convergence during part of jurassic cretaceous time

error analysis for the determination of tectonic rotation from paleomagnetic data appendix b is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b83 001 2 50 payment must accompany order recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25 this has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence 95 an analytical calculation which is valid when error bars are small shows that the correct error bars are 75 to 80 of the values derived in previous studies i have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used the analytical approximation may be used whenever 95 is less than 10 and the mean inclination is not too close to vertical confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20 25 narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data

magnetic structural and geochronologic evidence bearing on volcanic sources and oligocene deformation of ash flow tuffs northeast nevada magnetic properties of mid tertiary volcanic rocks west of jiggs in northeast nevada were investigated for the purposes of interpreting igneous structural and tectonic processes in this part of the basin and range province anisotropy of magnetic susceptibility ams patterns reflect flow fabrics and suggest previously unknown sources for these ash flow tuffs paleomagnetic and structural evidence suggest counterclockwise relative rotation of the southern part of the area with respect to the north new stratigraphic chemical and k ar isotopic data support these interpretations dacite to rhyolite ash flow tuffs of the indian well formation were subdivided into two main units the lower and predominant tuff of jiggs 35 8 37 0 ma and the unconformably overlying but areally restricted tuff of hackwood 30 8 ma the jiggs unit has two polarities whereas the hackwood has only a reversed polarity remanence together these units with tilt correction define a pole 92 e 87 n dp 6 dm 8 for approximately 30 37 ma this pole is concordant with coeval north american reference poles indicating that this zone of approximately 30 km ns extent has not undergone significant vertical axis rotation relative to the north american reference andesite lavas of normal magnetic polarity and of 32 5 ma age characterize the diamond hills immediately to the south we interpret this region from both structural evidence and discordant paleomagnetic direction to have rotated approximately 25 counterclockwise relative to the indian well volcanic units to the north the apparent rotation of the diamond hills is possibly the result of drag on the left lateral garcia fault which limits the diamond hills on the southwest analysis of ams data suggests by patterns of the k1 axes two distinct sources for the jiggs unit a northern buried source and a central partially buried source lithologic evidence consistent with proximal vent facies is found near the latter source an unusual pattern in the ams k3 axes suggested as resulting from the tilt correction applied to initial dip is consistent with the interpreted flow directions tight age constraints can be placed on the deformation of the main ash flows of the jiggs unit in the 35 8 to 30 8 ma interval because the diamond hills unit is slightly younger and appears to have been affected by the same deformative episode as the jiggs the regional deformation affecting both units can be more narrowly confined to the 32 5 30 8 ma interval this deformation is inferred to coincide with the major early tertiary episode of crustal extension in this region of the basin and range

ophiolitic thrust nappes in western nevada implications for the cordilleran orogen new field and geophysical evidence from jurassic mafic and ultramafic rocks in western nevada indicate the presence of two major ophiolite complexes tectonically overlying the rocks of the north american continental margin coupled with the existence of jurassic and older island arc marginal basin complexes and overthrust relations in the adjacent sierran klamath region these ophiolitic thrust nappes in nevada support collision models for the middle jurassic deformation the geometry of the observed structures and their relative age relationships suggest that this deformation might have resulted from the collision of the north american continent with a west dipping subduction zone which resulted in ophiolite emplacement and continental underthrusting continued westward motion of north america beneath the allochthonous oceanic terranes during the late mesozoic developed extensive thrust and possible strike slip faulting in the western us cordillera and caused migration of deformation towards the east this eastward propagation of deformation is interpreted to have resulted in the development of the sevier foreland thrust belt of latest jurassic to latest cretaceous age these collisional processes and the geometry of the proposed collision orogen are reminiscent of the alpine type collision belts

paleomagnetism and crustal rotations along a shear zone las vegas range southern nevada in southern nevada paleozoic isopachs mesozoic structures and mountain ranges trend north south but as they approach the right lateral west northwest trending las vegas valley shear zone lvvsz they are bent clockwise up to 90 in oroflexes 30 km long in the las vegas range the declination of the characteristic magnetization of the cambrian bonanza king formation has likewise been rotated clockwise more than 50 for samples from eight sites 13 23 samples per site in the las vegas range and in its extension south of the lvvsz stepwise thermal demagnetization to 600 c removed recent and tertiary overprints revealing a nearly horizontal late paleozoic viscous partial thermoremanent magnetization vptrm which predates both the tertiary oroflexural bending and the sevier thrusting in the area the declination of the characteristic magnetization after correcting for the dip of bedding varies from 133 2 at a site 24 km north of the shear zone to 196 4 13 km north of it and 204 4 11 km south of the shear zone this parallels a similar change in the structural trend the paleomagnetic data indicate that the las vegas range is broken into rather small blocks 5 km which rotate in response to more continuous deformation at depth this deformation of the upper crust can be modelled using a thin non newtonian viscous plate model which when applied to the las vegas valley shear zone predicts rotations similar to those seen in the las vegas range

cretaceous magnetizations in northwestern nevada and tectonic implications paleozoic and mesozoic arc related rocks of the black rock desert region of northwestern nevada have acquired a very stable postfolding secondary magnetization probably in late cretaceous time the remagnetization which appears to be of regional extent probably accompanied emplacement of granitic plutons in a northeast trending belt across northwestern nevada near the end of mesozoic time the region has experienced no appreciable tectonic rotation since that time

kinematics of late mesozoic thrusting pilot mountains west central nevada u s a abstract the northern pilot mountains of west central nevada consist of a complexly deformed terrane of imbricate thrust nappes composed of rocks of permian triassic through jurassic and possible cretaceous ages three episodes of fold and thrust generation are recognized on the basis of folded thrusts and thrusted folds and deformation and emplacement of the nappes is constrained as having occurred during the late mesozoic folds are apparently coeval with thrust faults and fold geometry is used in determining approximate directions of thrust displacement the history of thrust displacement is complex and involves three directions of motion on a regionally extensive detachment surface the luning thrust the first motion from nw to se results in displacements of the order of several tens of kilometres and is the probable result of nw se regional compression the final two episodes of motion are ne sw followed by e w they result in small displacements and are possibly the product of gravity sliding of the thrust sheet into depressions in the autochthon sites of downwarp in the autochthon may have been formed either by load induced subsidence or regional compression

remagnetization and tectonic rotation of upper precambrian and lower paleozoic strata from the desert range southern nevada in the desert range of southern nevada miogeoclinal sedimentary rocks mostly shallow water limestones of latest precambrian through early ordovician age yield three components of magnetization having different blocking temperature spectra 1 a low blocking temperature component near the direction of the present axial dipole field 2 an intermediate blocking temperature component with northerly declination and inclination 60 and 3 a characteristic component with southeasterly declination and inclination 20 combinations of alternating field and thermal demagnetization indicate that the intermediate and characteristic components reside in magnetite the intermediate magnetization probably reflects a viscous partial thermoremanent magnetization vptrm imposed between the late cretaceous and mid cenozoic it was probably acquired when the strata were dipping slightly eastward the characteristic magnetization is probably a vptrm imposed during regional uplift in the late permian the degree of heating required to have destroyed any primary magnetization is consistent with the conodont color alteration index observed in the ordovician rocks additionally the characteristic magnetization in those rocks is younger than bedding disruption caused by major late stylolitization red purple mudstones from the middle member of the wood canyon formation in which a directionally similar characteristic magnetization resides in fine grained hematite also appear to have been remagnetized in these rocks the remagnetization probably reflects partial recrystallization as the blocking temperatures are too high to have been reset by burial heating the sampled sections have undergone relative tectonic rotation about a vertical axis consistent with late tertiary oroflexural bending that had been proposed on independent geologic evidence the characteristic magnetization probably provides a reliable estimate of the magnitudes of the vertical axis rotations as the regional geology suggests the desert range strata were essentially horizontal throughout the paleozoic the total observed relative rotation is 44 5 representing absolute counterclockwise rotation 27 4 of the northern part of the desert range and absolute clockwise rotation 17 5 of the southern part the only unit in the desert range sequence that may retain a primary magnetization is the late precambrian rainstorm member of the johnnie formation the characteristic magnetization of this unit exhibits two polarities and probably resides in specular hematite after correction for 18 of counterclockwise rotation the resulting pole 5 n 151 e is near other late hadrynian poles from north america

the 1999 chi chi taiwan earthquake earthquake fault and strong motions we report the results of a field survey on the chelungpu surface rupture zone and building damage as well as the analysis of strong motions of the chi chi taiwan earthquake damage due to this earthquake is classified into the three main types 1 damage due to large displacements along the surface rupture zone 2 widespread spots or patches of demolished areas that might be attributed to localized strong ground motions and 3 damage in remote regions as in taipei caused by anomalous amplifications of strong motions damage tends to be localized in areas of several tens to hundreds of meters long and is characterized by intermittent arrivals of intense shaking the surface rupture occurred mainly along the active chelungpu fault we also found some evidence that large buildings might have affected the rupture process on the surface manuscript received 31 july 2000

regional crustal structures and their relationship to the distribution of ore deposits in the western united states based on magnetic and gravity data upgraded gravity and magnetic databases and associated filtered anomaly maps of western united states define regional crustal fractures or faults that may have guided the emplacement of plutonic rocks and large metallic ore deposits fractures igneous intrusions and hydrothermal circulation tend to be localized along boundaries of crustal blocks with geophysical expressions that are enhanced here by wavelength filtering in particular we explore the utility of regional gravity and magnetic data to aid in understanding the distribution of large mesozoic and cenozoic ore deposits primarily epithermal and porphyry precious and base metal deposits and sediment hosted gold deposits in the western united states cordillera on the broadest scale most ore deposits lie within areas characterized by low magnetic properties the mesozoic mother lode gold belt displays characteristic geophysical signatures regional gravity high regional low to moderate background magnetic field anomaly and long curvilinear magnetic highs that might serve as an exploration guide geophysical lineaments characterize the idaho montana porphyry belt and the la caridad mineral park belt from northern mexico to western arizona and thus indicate a deep seated control for these mineral belts large metal accumulations represented by the giant bingham porphyry copper and the butte polymetallic vein and porphyry copper systems lie at intersections of several geophysical lineaments at a more local scale geophysical data define deep rooted faults and magmatic zones that correspond to patterns of epithermal precious metal deposits in western and northern nevada of particular interest is an interpreted dense crustal block with a shape that resembles the elliptical deposit pattern partly formed by the carlin trend and the battle mountain eureka mineral belt we support previous studies which on a local scale conclude that structural elements work together to localize mineral deposits within regional zones or belts this study of mineral deposits of the western united states demonstrates the ability of magnetic and gravity data to elucidate the regional geologic framework or structural setting and to contribute in locating favorable environments for hydrothermal mineralization

evolution of a late mesozoic back arc fold and thrust belt northwestern great basin u s a abstract the luning fencemaker fold and thrust belt was active from the middle or late jurassic through the early cretaceous and involves rocks of the mesozoic marine province of the northwestern great basin rocks of the marine province were deposited in a back arc basin bound on the west by the sierran arc they are underlain by simatic crust formed either in a paleozoic marginal basin formed on the west coast of north america or in an exotic oceanic arc accreted to north america in the permo triassic deposition in the marine province was localized by the underlying simatic crust which later controlled the locus of back arc thrusting to the south in areas of the back arc region overlying continental crust mesozoic rocks are terrestrial volcanic and volcanogenic deposits rocks of this area are not disrupted by thrusts of significant magnitude until the southwestern end of the sevier thrust belt is encountered during contraction the luning fencemaker thrust belt underwent several hundred kilometers of nw se shortening which was not developed in the sierra nevada to the west the arc and back arc regions were separated by a regionally extensive left lateral fault system that was active during back arc thrusting the marine province was partially coupled with the sierra nevada during transpressional motion on the left lateral fault resulting in imbrication and emplacement of the luning fencemaker thrust belt in the continental region to the south decoupling was more complete and only minor thrusting was associated with the southerly displacement of the sierran arc farther south where the paleozoic miogeoclinal hingeline was encountered conditions apparently changed and the sevier thrust belt developed timing relations suggests that the andean type margin of the western united states had paired back arc thrust belts luning fencemaker and sevier for much of the late mesozoic the belts were separated by a region that underwent relatively minor internal shortening the intervening region probably was decoupled at depth and the thrust belts may have shared a decollement development of the luning fencemaker thrust belt and the associated left lateral fault system extended over a period of 40 50 m y and is best explained as a product of plate convergence the nw se shortening axis for the thrust belt and the inferred left slip on the strike slip fault are not compatible with the generally accepted right oblique subduction proposed for the sierran arc since the early triassic they suggest an interval of left oblique convergence for the middle to late jurassic and early cretaceous regionally developed structures indicate a return to right oblique convergence in the late early cretaceous

anisotropy of magnetic susceptibility data bearing on the transport direction of mid tertiary regional ignimbrites candelaria hills area west central nevada in west central nevada the oligocene candelaria pyroclastic sequence reaches a local thickness of up to 1 3 km in what has been referred to as the candelaria trough but more generally the accumulation of ash flow tuffs and related volcanic rocks is less than 300 m thick complete to near complete outcrops are scattered over about 3200 km2 in the candelaria hills and surrounding ranges of the southern walker lane structural zone three regionally extensive compound cooling units within the overall sequence 25 8 ma metallic city 24 1 ma belleville and 23 7 ma candelaria junction tuffs have distinguishing characteristics and are the focus of study at 106 sites anisotropy of magnetic susceptibility ams data provide an estimate of transport direction of each tuff inferred transport directions based on the ams data are corrected for a modest clockwise yet variable magnitude vertical axis rotation that affected these rocks in late miocene to pliocene time as revealed by paleomagnetic studies the ams data show a somewhat orderly pattern of magnetic fabrics that we interpret to define unique transport directions for the metallic city and candelaria junction tuffs the low susceptibility and degree of anisotropy of the belleville tuff limits our interpretation from this pyroclastic deposit the metallic city and candelaria junction tuffs typically show gentle south southeast and southeast dipping magnetic fabric imbrication respectively and very gently plunging magnetic lineations these ams fabric elements indicate the tuffs were transported to the north northwest and northwest respectively the ams fabric data from the metallic city and candelaria junction tuffs suggest relatively unrestricted flow during emplacement evidence across the 3 200 km2 area to support more regionally controlled channelized flow into and or flow along the east northeast west southwest axis of the candelaria trough is lacking the ignimbrites clearly filled a topographic depression inferred to have formed concurrent with early localized basin and range extension during pyroclastic emplacement but based on the uniformity of ams fabric data we infer that the depression quickly filled and did not hinder flow across the region unrecognized eruptive centers for the three ignimbrites may lie buried beneath neogene basin fill sediments south southeast of the candelaria hills or concealed below younger deposits farther southeast into the palmetto mountains

paleomagnetic evidence for motions of parts of the canadian cordillera abstract paleomagnetic evidence from major terranes of the western cordillera in canada referred to collectively as baja british columbia indicates that they have moved as a whole about 2300 km from the south and rotated 60 clockwise relative to north america in the late cretaceous and earliest tertiary 85 50 ma sometime between the early jurassic and the mid cretaceous 190 100 ma these same terranes moved south over 1000 km and underwent irregular but where well constrained counter clockwise rotations the terranes amongst themselves seem to have shared much the same paleolatitude during and since the late triassic although they may have undergone relative rotations in the later jurassic and early cretaceous these motions are now known to be broadly consistent with the motions of oceanic plates to the west as calculated by independent procedures and indicate that the terranes of the western cordillera were for part of their existence coupled to oceanic plates first to the farallon plate during the late jurassic and early cretaceous and then to the kula plate during the late cretaceous and early tertiary it was this change of allegiance from farallon to kula that caused first the southerly and then the northerly motion the paleomagnetic evidence despite claims to the contrary is consistent with geologically inferred displacements along major faults in the canadian cordillera when due account is taken of the age constraints on those motions

petrogenesis of garnet two mica granites in the ruby mountains nevada garnet two mica granites of jurassic 160 3 m y and cretaceous 83 1 3 m y ages intrude amphibolite facies metasedimentary rocks of precambrian age and lower paleozoic sedimentary rocks in the northern part of the ruby mountains nevada high initial 87sr 86sr and high values of 18o for minerals suggest a dominantly sedimentary source for the magmas total rare earth contents are low and less fractionated cen ybn 10 than granitic rocks of similar composition in the sierra nevada and show a negative eu anomaly eu eu 0 57 these data are consistent with melting in the stability field of plagioclase with little contribution to the melt from refractory minerals such as zircon sphene and apatite minor euhedral garnets are manganese rich and magnesium poor al60sp34py3 6gr2 8 compared to garnet al79sp6py13gr2 in the intruded metamorphic rocks and show an increase in manganese and a decrease in calcium in the outer few microns fractionation of magnesium iron between garnet and biotite and of sodium potassium between k feldspar and plagioclase in the granites suggest submagmatic equilibration temperatures in the range of 365 to 505 c whereas oxygen isotope equilibration temperatures for two granites are higher at 480 and 570 c the compositions of the granites plot near minimum melting compositions in the water saturated quartz albite orthoclase system comparison of muscovite plus quartz stability the water saturated granite solidus and experimental garnet melt equilibria suggests pressures of crystallization no lower than about 3 5 kbar this pressure is compatible with that estimated from garnet plagioclase sillimanite quartz equilibria for the intruded metasedimentary rocks

spatial variability in the structure of the roberts mountains allochthon western nevada structural variability in rocks of the upper plate of the roberts mountains thrust indicates a complex history of thrust emplacement during the mid paleozoic antler orogeny the roberts mountains allochthon consists predominantly of a highly deformed assemblage of structurally imbricated lower continental slope and continental rise sedimentary and volcanic rocks of early paleozoic age it has been interpreted variously as a back arc thrust and as a grounded accretionary prism that overrode the upper precambrian and lower paleozoic passive margin of western north america current understanding of structural and stratigraphic relations supports the second interpretation the trace of the roberts mountains thrust trends south southwesterly across nevada from near the idaho border to the vicinity of tonopah 38 n lat where it is interpreted to swing westerly and extend to the sierra nevada subparallel to the thrust trace is the trend of the western boundary of the late precambrian passive continental margin as deduced from lower paleozoic facies patterns and the 87sr 86sr 0 706 line the westerly deflection of these and younger trends has been attributed by some workers to crustal deformation of late mesozoic and or cenozoic age and by others to an original bend in the continental margin results presented here support the latter interpretation structures in the northern and central segments of the roberts mountains thrust characterized by south southwest trace consist of a single phase of folds and indicate easterly transport the inferred southern segment of the thrust with a westerly trend exhibits two and locally three phases of folds interpreted as being genetically related to emplacement of the thrust first phase folds in the south are correlated with folds in the central and northern segments of the thrust and after the effects of superimposed structures are geometrically removed also yield an easterly transport direction cross folds in the southern part of the allochthon and their absence to the north indicate different structural histories for the regions structures in the allochthon are interpreted as having developed during the formation of an accretionary prism differences in structural history are related to differential rates of slip on the allochthon s basal thrust during transport and emplacement development of cross folds in the south is thought to be due to a local reduction in the rate of displacement and corresponding internal shortening of the upper plate rocks caused by resistance to subduction of a west facing promontory of continental crust

transitional field clusters from uppermost oligocene volcanic rocks in the central walker lane western nevada abstract three sections of the candelaria hills volcanic sequence west central nevada appear to have recorded parts of two transitional field records or reversal excursions paleomagnetic data and 40 ar 39 ar laser fusion sanidine age estimates for pyroclastic rocks and associated flows show that these rocks recorded the unusual field behavior at about 25 7 ma and about 23 8 ma fifteen sites yield northeast declination moderate to shallow negative inclination mean directions and 16 sites yield west to southwest declination moderate negative inclination directions both populations of site mean directions representing a total of 12 independent eruptive units are highly discordant to a time averaged late tertiary field direction and neither can be explained by a geologically reasonable magnitude of vertical axis rotation virtual paleomagnetic poles vgps estimated from the directional data lie at low to intermediate latitudes 29 of the 31 flows at intermediate latitudes

understanding kinematics of intra arc transcurrent deformation paleomagnetic evidence from the liquine ofqui fault zone chile 38 41 s the liquine ofqui fault zone lofz is a major 1000 km long dextral shear zone of southern chile likely related to strain partitioning of nazca plate oblique convergence with south america to understand block rotation pattern along the lofz we paleomagnetically sampled 55 sites 553 samples between 38 s and 41 s we gathered oligocene to pleistocene volcanics and miocene granites at a maximum distance of 20 km from the lofz and at both sides of it rotations with respect to south america evaluated for 36 successful sites show that crust around the lofz is fragmented in small blocks 1 to 10 km in size while some blocks at both fault edges undergo very large 150 170 rotations others do not rotate even adjacent to fault walls we infer that rotations affected equidimensional blocks while elongated crust slivers were translated subparallel to the lofz without rotating rotation pattern across the lofz is markedly asymmetric east of the fault and adjacent to it rotations are up to 150 170 clockwise and fade out 10 km east of fault these data support a quasi continuous crust kinematics characterized by small rigid blocks drag by the underlying ductile crust flow and imply 120 km of total fault offset conversely crust west of the lofz is cut by seismically active nw se sinistral antithetic faults and yields counterclockwise rotations up to 170 at 8 10 km from lofz besides the unrotated blocks further data from the chile fore arc are needed to understand block rotation kinematics and plate dynamics west of the lofz

a paleomagnetic study of the structural deformation in the yerington district nevada 2 mesozoic basement units and their total and pre oligocene tectonism

preliminary paleomagnetic data from the jurassic humboldt lopolith west central nevada evidence for thrust belt rotation in the fencemaker allochthon layered and massive gabbros of the humboldt lopolith and extrusive equivalents in west central nevada yield well grouped paleomagnetic data on a site basis with corrections for attitudes of compositional layering in gabbros and flow contacts in lavas kcorr kin situ 5 5 the mean of 15 site means is d 307 i 21 95 9 k 17 while k ar data imply a late jurassic age we prefer an older probably early jurassic age for the lopolith and its magnetization based upon magnetization data and relations of these rocks with host strata compared with early jurassic expected directions the data reveal a discordancy in declination of 37 14 22 which implies a significant counterclockwise rotation of the lopolith local rotation of fence maker allochthon units most likely occurred when southeasterly directed thrust sheets obliquely contacted a generally north south trending shelf margin mesozoic deformation will be more accurately estimated with better age control and measurement of possible cenozoic crustal rotation

kinematics of plate convergence deduced from mesozoic structures in the western cordillera mesozoic structures in the western cordillera of the united states yield important constraints for kinematic reconstruction of plate motions relative to north america structures in the mesozoic arc s of the northern sierra nevada and of northeastern oregon are remarkably uniform in timing style and with late cretaceous and or cenozoic rotations removed orientation rocks of both regions experienced three major episodes of contraction occurring in the triassic early jurassic the late jurassic and the early cretaceous cross folds were developed after the first two deformations but were not associated with significant shortening the three major deformations are nearly homoaxial and have shortening axes oriented subperpendicular ne sw to the regional trend of the arc system in the back arc region of the northwestern great basin deposition was continuous during the triassic and early jurassic and the region was shielded from tectonism in the arc to the west development of a regional fold thrust belt associated with major nw se shortening was initiated in the back arc region during the middle or late jurassic and continued through the early cretaceous corresponding deformation did not occur in the arc and the arc and back arc were decoupled by a major left lateral fault system that subparallelled the arc axis in the mid cretaceous approximately 100 m y b p regional compression in the back arc abruptly changed to ne sw and both the arc and back arc experienced major shortening development of homoaxial structures in the sierran arc is probably analogous to shortening in present day arc systems undergoing oblique subduction in which the shortening axis is parallel to the normal component of convergence thus the dominant structures in the sierran arc simply may be a reflection of the orientation of the trench and may not constrain the sense of obliquity of convergence strike slip faults and related compressional structures such as the back arc left lateral fault and associated fold thrust belt are probably the response to the strike slip component of convergence and indicate a period of left oblique subduction relative plate motions constrained by paleomagnetic data are in agreement with regional structures and indicate strong right lateral components of motion for the last 100 m y before 100 m y b p relative plate motions are inadequately constrained by paleomagnetic data and regional structures indicate a period of left oblique convergence from the middle or late jurassic to approximately 100 m y b p during the triassic and early jurassic the relative sense of convergence is equivocal and the possibility exists that continuous left oblique convergence occurred throughout the late paleozoic and early mesozoic until 100 m y b p based on the available data we cannot discount the possibility that a complex convergence history existed however with changes from right to left convergence in the middle or late jurassic and from left to right convergence in the mid cretaceous

structure and stratigraphy of the luning allochthon and the kinematics of allochthon emplacement pilot mountains west central nevada the northern pilot mountains of west central nevada consist of a complexly deformed terrane of imbricate thrust sheets composed of rocks of mesozoic and possible late paleozoic ages thrust nappes within the allochthonous terrane named the luning allochthon are principally constituted by the upper triassic luning formation locally the nappes contain numerous imbricate thrust slices some of which are composed of undifferentiated sunrise gabbs formation triassic and jurassic and unnamed volcanic carbonate assemblages of probable permian or triassic age three episodes of folding and thrusting are recognized on the basis of folded thrusts and thrusted folds and deformation and emplacement of the allochthon is constrained as late mesozoic folds apparently formed coevally with thrust faults and fold geometry is used to determine approximate directions of thrust transport thrust displacements are responsible for 35 to 40 km of an estimated 70 km of intra allochthon contraction which is inferred from lithofacies analysis of rocks juxtaposed by thrust nappes and from the structural overlap of imbricate thrusts the history of thrust displacement is complex and involves three directions of motion on a regionally extensive detachment surface the luning thrust first motion from northwest to southeast resulted in the major component of stratal contraction and is the probable result of northwest southeast regional compression the final two episodes of motion are northeast to southwest followed by east to west they resulted in small displacements and are possibly the product of gravity sliding of the thrust sheets into a depression formed beneath part of the allochthon within the autochthon the site of downwarp in the autochthon may have formed by either load induced subsidence or regional compression

speculative reconstruction of the mojave snow lake fault implications for paleozoic and mesozoic orogenesis in the western united states available evidence suggests that the mojave snow lake fault a major early cretaceous dextral fault with up to 400 km displacement formerly existed in what is now the axial part of the sierra nevada batholith regional relations suggest that the fault formerly continued in western nevada the mojave desert and in rocks now exposed in the salinian block restoration of 400 km of early cretaceous dextral displacement on the mojave snow lake fault leads to speculative but testable reconstructions of mesozoic and paleozoic tectonic features of california and nevada insights derived from these reconstructions include the following 1 two regionally extensive jurassic thrust belts of opposed vergence appear to have intersected in the northern sierra nevada the eastern east vergent thrust belt which includes the fencemaker and liming thrusts in western nevada and several east vergent thrusts in the northern sierra may be a contractional belt along which an early jurassic interarc or marginal basin was closed in middle to late jurassic time the western west vergent thrust belt is part of the classic nevadan orogen of the western klamath mountains and the western sierra metamorphic belt in which collisions and or thrusting of various island arc terranes occurred during middle and late jurassic time 2 the early mesozoic continental arc included paired belts of triassic jurassic blueschist and melange and igneous rocks including late triassic and younger plutons and volcanic rocks this conclusion is not new but the reconstruction provides a slightly different geometry than previously thought 3 sonomia the late paleozoic island arc terrane of the western u s may have formed a relatively narrow belt extending from northwestern nevada to the latitude of the western mojave desert the geologic development of the paleozoic basement complex of sonomia was very different from that of the cordilleran miogeocline in death valley and the mojave desert requiring that a continuation of the sonoma suture belt or golconda thrust must have extended southward through the sierra nevada batholith to the mojave region 4 a heterogeneous belt of permian to triassic or triassic deformation in the mojave and eastern sierra nevada together with the golconda allochthon in nevada may represent the protracted response to the collision of sonomia with the western margin of north america the contractional deformation continued during the evolution of the early mesozoic continental arc 5 several predictions can be made about the nature of concealed basement within the sierra nevada batholith and within western nevada the reconstruction suggests that elements of the antler and sonoma orogenic belts should underlie areas of the batholith and western nevada as a result of northward displacement along the mojave snow lake fault from the western mojave and southern sierra nevada

paleomagnetic assessment of oroflexural deformation in west central nevada and significance for emplacement of allochthonous assemblages abrupt westerly deflections of north south trending facies boundaries the isopleth of 87sr 86sr 0 076 and arcuate structural trends of paleozoic mesozoic and cenozoic age in the west central basin and range province have been explained by 1 post early jurassic oroflexural folding and shear 2 the existence of a pre late jurassic resistant block along the western continental margin possibly representing late precambrian breakup and 3 regional flexure mina deflection prior to intrusion of late cretaceous plutons which crosscut flexure related structural trends paleomagnetic data gathered from late cretaceous plutons and remagnetized metasedimentary rocks along and north of the northern margin of the zone of east west structural trends are not in support of significant oroflexural folding and attending major clockwise rotation since late cretaceous time the data water canyon pluton d 1 i 57 95 4 k 9 n 132 gunmetal pluton d 354 i 60 95 6 k 21 n 29 east garfield hills pluton d 349 i 74 95 11 k 14 n 12 la panta pluton d 22 i 66 95 10 k 16 n 13 gillis canyon pluton d 354 i 50 95 4 k 20 n 50 and folded units of the excelsior mountains d 309 i 79 95 7 k 12 n 37 are dispersed from expected cretaceous directions for the mina deflection region but they do not conform to a model of major systematic regional clockwise rotation the generally small deflections of unit means from expected directions could be explained by irregular components of late cenozoic tilting and or rotation the paleomagnetic data do not deny the possibility of pre late cretaceous oroclinal bending nevertheless regional structural analysis complimented by the paleomagnetic results indicate that the mina deflection is in all likelihood a primary feature whose geometry reflects the late precambrian fragmentation of western north america structures related to the accretion of allochthonous assemblages between middle devonian and cretaceous time in this part of the cordillera also reflect the primary nature of this feature

rotation of the southernmost sierra nevada california structural paleogeographic and magmatic trends of the sierra nevada batholith swing from a characteristic nnw trend in the northern and central parts of the batholith to a nearly e w trend in the tehachapi and san emigdio mountains paleomagnetic results from eight sites in the upper cretaceous 80 86 m y bear valley springs pluton in the tehachapi mountains yield a mean direction d 022 i 56 k 97 95 5 6 this mean direction is significantly different from the expected late cretaceous field direction for this locality and suggests that the pluton has undergone a clockwise rotation of 45 14 in accord with the change in structural trend in that area lower to middle miocene volcanic and sedimentary rocks of the kinnick formation overlie the plutonic rocks and yield a mean direction d 354 i 64 a paleomagnetic pole derived from this mean direction is not significantly different from the expected miocene pole for north america this constrains the time of rotation of the pluton to be between about 80 and 20 m y probably predating establishment of the present san andreas transform in this area we suggest that the observed rotation is part of a large oroclinal bend affecting to different degrees the entire southern sierra nevada the driving mechanism for such a bending may be right lateral shear along a proto san andreas transform for which there is now a significant body of evidence

paleomagnetic evidence for the age and extent of middle tertiary counterclockwise rotation dixie valley region west central nevada paleomagnetic data obtained from oligocene to lower miocene igneous rocks and middle miocene basaltic rocks at fifteen localities from a region surrounding dixie valley in west central nevada indicate that parts of the area experienced counterclockwise vertical axis rotation and these data provide constraints on the extent and timing of rotation counterclockwise vertical axis rotation probably exceeding 30 is indicated for oligocene to lower miocene rocks in the central part of the study area calculated rotations increase with increasing age of the oligocene to lower miocene rocks indicating that vertical axis rotation was concurrent with ash flow deposition about 23 33 ma paleomagnetic data indicate that middle miocene basaltic rocks about 10 17 ma postdate significant counterclockwise vertical axis rotation although this interpretation is complicated because the rocks were erupted episodically in pulses that apparently were short lived with respect to geomagnetic secular variation counterclockwise rotation was related to a deformation event that predated development of the present basin and range physiography of the area the total amount of crustal rotation in tertiary time in the area is poorly known rotation estimates calculated from time averaged mean directions that incorporate data from a broad range of the oligocene to lower miocene units probably underestimate total tertiary rotation because the mean directions include data from units that postdate much of the rotation for example a rotation estimate for a composite sequence from the central part of the study area is 23 15 whereas estimates from older and younger halves of the sequence are 37 21 and 11 16 respectively paleomagnetic data indicate that oligocene to lower miocene rocks at some localities in the northern and southern parts of the study area e g the golconda canyon locality probably did not experience significant tertiary counterclockwise rotation although the paleomagnetic data alone are insufficient to fully define the geographic area subjected to tertiary counterclockwise rotation the data suggest that a discrete subregion was affected rather than the entire basin and range province