paleomagnetic study of some cretaceous and tertiary sedimentary rocks of the klamath mountains province california paleomagnetic investigation of cretaceous outliers and tertiary sedimentary strata of the klamath mountains province and of onlapping cretaceous strata has shown the rocks to be largely remagnetized samples studied are from the upper jurassic to upper cretaceous great valley sequence upper cretaceous hornbrook formation eocene montgomery creek formation and oligocene weaverville formation cretaceous samples that survived the remagnetization have a mean remanence direction that is very close to the expected direction of the cretaceous magnetic field at the locality of the klamath mountains data from both primary and remagnetized samples suggest the possibility of 11 5 15 8 of post cretaceous clockwise rotation of the klamath mountains province none of the data from either the primary or remagnetized samples shows evidence of the large amounts 70 of clockwise rotation that other workers have measured for the lower tertiary of the oregon coast range our data indicate that the oregon coast range and klamath mountains province did not behave as a single rigid block during the early tertiary they also suggest that any post oligocene rotation of the klamath mountains province is less than the approximately 30 post oligocene rotation recently proposed for a combined oregon coast range klamath mountains cascade range block

combined visual and quantitative techniques in the study of natural turbulent flows an important difficulty in understanding turbulence in natural flows is that one does not see what is being measured by devices recording velocity fluctuations this study shows how visualization of turbulent structures can be effectively implemented in natural river flows in order to match turbulent flow structures with their velocity signatures flow visualization is combined with high frequency 20 hz velocity measurements from electromagnetic current meters two fluvial environments are examined in a river confluence where the difference in turbidity provides a natural tracer very strong negative velocity fluctuations for the streamwise component are concomitant with an upwelling motion a video recording made from a bridge of the passage of turbulent structures delineated by turbid and clear water allowed us to relate the marked fluctuations in velocity to eddies of a duration of 1 2 s in the mixing layer in a gravel bed river we have examined patterns of eddy shedding in the lee of a pebble cluster these patterns were highlighted by dye milk injected in the separation zone downstream black and white images were obtained from an underwater lens linked to a video camera recorder we observed a complex dynamic characterized by two dominant patterns of eddy shedding one with a clear motion of eddies toward the water surface and another with a downward expansion toward the bed of the structures this pattern is related to the passage of intermittent wedges of low speed fluid separated by wedges of fast moving fluid

the effect of sparse vegetation on the transport of dune sand by wind the precise effect of a sparse plant cover on the transport of dune sand by wind is a critical factor in understanding the morphology evolution and global distribution of aeolian dunes1 3 it is also of considerable practical importance in sand stabilization rehabilitation and restoration ecology here i report the first detailed experimental quantification of this effect using living plants in patterns approximating natural vegetation for wind velocities v at 0 5 m elevation up to 15 m s 1 and plant covercup to 17 the rate of aeolian sand transport q g cm 1 s 1 is closely approximated by q b v 1 kc vt 3 where b is bagnold s constant4 vt is a threshold velocity and k is a constant dependent on plant geometry for typical small erect or spreading herbaceous dune plants k 0 018 for small rounded stemless plants k 0 046

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

magnitude of crustal extension across the northern basin and range province constraints from paleomagnetism abstract the magnitude of crustal extension across the northern basin and range province is a matter of longstanding controversy estimates range from 10 to 300 recently published estimates of extension across the southern basin and range province 36 n are in the range of 80 100 thus the larger values suggested for the northern part of the province 40 n seem to require substantial counterclockwise rotation of the sierra nevada during tertiary extension paleomagnetic data from the range however limit rotation to 4 10 at the 95 confidence level these limits combined with estimates of extension near the garlock fault allow severe constraints to be placed on the magnitude of extension across more northerly parts of the province we conclude that the maximum extension at 40 n is about 50 and that values of 39 12 188 43 km are likely

model for late mesozoic early tertiary tectonics of coastal california and western mexico and speculations on the origin of the san andreas fault cretaceous and early tertiary arc and forearc rocks found along the coast in southern california and baja california have been shown by paleomagnetic measurements to have originated many hundreds or even several thousands of kilometers south of their current locations northward transport also is found in cretaceous batholithic rocks near the edge of the continent in washington and british columbia the consistency of this pattern suggests that slices of arc and forearc rock originating on the western edge of north america have been translated along the coast by strike slip faulting strike slip faults are shown to be an expected response to north oblique subduction of the farallon and or kula plates faulting and subduction probably were concurrent activities i e the arc and forearc grew and were displaced simultaneously because displacement was dominantly parallel to subduction related lithic belts arc and forearc rocks may have traveled large distances without producing either significant hiatuses in the rock record or conspicuous disruptions in the distribution of lithic belts subduction related strike slip faulting is shown to be favored by high angles of oblique convergence shallow dip of the subducting slab relatively easy slip on potential faults within the arc or forearc and the existence of a place for the moving sliver to go it is suggested that these conditions are not difficult to satisfy and that subduction related strike slip faulting may thus be a common feature of convergent orogens the location of the san andreas fault inboard from the continental margin may be attributable to localization of the pacific north american transform by a system of precollision subduction related faults

paleomagnetic constraints on rotation within mount abbot quadrangle central sierra nevada california the regional photolineament pattern in mount abbot quadrangle in the central sierra nevada is sigmoidal in plan view the fractures in the southwest section of the quadrangle near florence lake strike n10 20 e while those in the central section along bear creek strike n50 70 e and those in the northeast strike n25 40 e a paleomagnetic investigation was undertaken to determine if this sigmoidal pattern was formed by kinking of originally straight photolineaments after the rock mass cooled below the blocking temperature if the inner limb of the kink rotated 40 60 clockwise with respect to the outer limbs then there should be a similar difference in the paleomagnetic declinations between the florence lake and bear creek areas the paleomagnetic results indicate a clockwise difference in declination of 13 4 7 of the bear creek area with respect to florence lake thus the sigmoidal pattern could not have formed entirely by rotation after cooling below the blocking temperature it is also possible that the observed declination discrepancy is due to tilting apparent polar wander or paleomagnetic dispersion instead of rotation sigmoidal patterns formed by kink folding occur on the outcrop scale with inner limb rotations of 15 30 three of these kinks were sampled to test the paleomagnetic method in an area of demonstrable rotation the data from these sites indicate that although no difference in declination is evident a reheating event may have occurred which could have removed any rotation from the paleomagnetic record reheating possibly due to the circulation of hydrothermal fluids may have been causally related to the outcrop scale kinking since reheating is not observed away from the outcrop scale kinks the data further indicate that there may have been continuous deformation of the rock mass during the formation of the kinks

geochemical contrasts between late triassic ore bearing and barren intrusions in the weibao cu pb zn deposit east kunlun mountains nw china constraints from accessory minerals zircon and apatite the weibao copper lead zinc skarn deposit is located in the northern east kunlun terrane nw china igneous intrusions in this deposit consist of barren diorite porphyry u pb zircon age of 232 0 2 0 ma and ore bearing quartz diorite and pyroxene diorite u pb zircon ages of 223 3 1 5 and 224 6 2 9 ma respectively whole rock major and trace element and accessory mineral zircon and apatite composition from these intrusions are studied to examine the different geochemical characteristics of ore bearing and barren intrusions compared to the barren diorite porphyry the ore bearing intrusions have higher ce4 ce3 ratios of zircon and lower mn contents of apatite indicating higher oxidation state besides apatite from the ore bearing intrusions shows higher cl contents and lower f cl ratios these characteristics collectively suggest the higher productivity of ore bearing quartz diorite and pyroxene diorite when compared with ore bearing intrusions from global porphyry cu deposits those from cu pb zn skarn deposits display lower ce4 ce3 and eun eun ratios of zircon and lower cl and higher f cl ratios of apatite we conclude that these differences reflect a general geochemical feature and that zircon and apatite composition is a sensitive tool to infer economic potential of magmas and the resulting mineralization types in intrusion related exploration targets

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

post 12 m y rotation of southwest washington paleomagnetic field directions from the basalt of pack sack lookout are compared to those from the pomona member of the saddle mountains basalt of the columbia river basalt group the pomona crops out over a wide region on the columbia plateau east of the cascade range and the basalt of pack sack lookout crops out well to the west of the cascades about 30 to 60 km east of the washington coast our paleomagnetic results support existing geologic and geochemical evidence that both these units are part of a single 12 m y old flow that erupted in western idaho and flowed to the washington coast the magnetic data further indicate that southwest washington has undergone clockwise rotation with respect to the columbia plateau and stable north america the data suggest that both a large scale regional rotation of 16 and locally complex small scale rotations exist the two being present to different degrees in the eastern and western regions of southwest washington the cascade range appears to coincide with the tectonic boundary separating rotated and unrotated regions of washington state rotation of southwest washington appears to have been associated with the rotation of large pieces of western oregon and southern washington possibly as a result of either a ridge push force from the basin and range province or shear along the pacific north america plate boundary

paleomagnetism of the middle proterozoic electra lake gabbro needle mountains southwestern colorado the electra lake gabbro is a small 1 435 ga pluton that intrudes 1 7 to 1 6 ga gneisses and schists of the needle mountains in southwestern colorado paleomagnetic samples were collected from the main phases of the gabbro diabase dikes granite and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton gabbro diabase and some melt zone samples have a single polarity characteristic magnetization of northeast declination d and moderate negative inclination i demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite after correction for the minor west dip of overlying paleozoic strata we obtain a mean direction of d 32 1 i 41 9 k 94 95 3 3 n 21 sites and a paleomagnetic pole at 21 1 s 221 1 e k 89 a95 3 4 this pole is similar to poles from the middle proterozoic belt supergroup but is located at a higher southerly latitude than poles from other 1 47 1 44 ga plutons from north america most of which plot at equatorial latitudes the reason for this discrepancy is not clear but may result from a combination of factors including unrecognized tilting of the gabbro the failure of this relatively small pluton to fully average paleosecular variation and uncertainties in the overall reliability of other 1 5 1 4 ga poles of the north american apparent polar wander path

application of paleomagnetism to accretionary tectonics and structural geology the use of paleomagnetism to determine translations and rotations of geologic terranes has contributed to a better understanding of deformation within accretionary complexes shear zones and rifts in this report to the i u g g we review the progress resulting from paleomagnetic studies during the last four years in the cordillera of north america the appalachian orogen and the circum pacific region

transpressional shearing and strike slip partitioning in the late cretaceous sierra nevada magmatic arc california the rosy finch shear zone rfsz represents an example of synmagmatic strike slip tectonics in the east central sierra nevada and provides information on transpressional tectonics within magmatic arcs the rfsz 1 4 km wide and 80 km long is contained in late cretaceous granitoids 92 83 ma of the mono pass intrusive suite dextral strike slip deformation is indicated by a continuous band of orthogneiss mylonitic and cataclastic deformation characterized by subvertical foliations with subhorizontal lineations the style of dextral shearing in the rosy finch shear varies along strike from wide zones of ductile deformation in the youngest plutons to narrow zones of cataclastic deformation in the older plutons deformation of the youngest granitoids is synmagmatic as constrained by both field observations and isotopic studies dextral shearing occurs concurrently in contemporaneous intrusive suites to the north and south tuolumne and mount whitney suggesting strike slip movement along the axis of active plutonism field studies also suggest that contraction across the arc acted concurrently with strike slip movement consistent with a transpressional setting for the late cretaceous sierra nevada magmatic arc comparison of the field data with strain models of oblique plate convergence suggests that the rfsz is a preserved ductile signature of transcurrent motion of strike slip partitioning within a transpressional orogenic system recent plate reconstructions indicate a switch from sinistral to dextral oblique convergence at 95 ma which is consistent with the timing constraints on the dextral movement on the rfsz

stability of the earth with respect to the spin axis for the last 130 million years abstract recently continental paleomagnetic data rotated into a hotspot reference frame have been used to suggest the earth rotated 18 with respect to the spin axis at 110 ma we test this true polar wander tpw hypothesis using paleomagnetic data from widespread granitic rocks of cratonal north america these data conflict with tpw predictions leading to rejection of the hypothesis the calculated polar displacement is instead consistent with motion of the atlantic hotspots during the mid cretaceous this analysis suggests that the time averaged position of the spin axis has deviated by no more than 5 over the last 130 million yr indicating that mantle mass heterogeneities have not changed rapidly enough to drive tpw

tillamook volcanic series further evidence for tectonic rotation of the oregon coast range paleomagnetic directions from the eocene tillamook volcanic series of the oregon coast range point 46 o clockwise from the expected eocene field direction potassium argon dating of six dikes and flows from this formation yields a mean age of 44 3 0 6 m y these results establish that the oregon coastal block of simpson and cox 1977 extends north to the oregon washington border and that this block has rotated clockwise 46 o 13 o during the past 44 m y the block has undergone no detectable north south translation a two phase model is developed to explain the tectonic history and anomalous magnetic field directions of the pacific northwest region phase i of the rotation active between 50 and 42 m y b p results from fragmentation of the farallon plate with rotation of a fragment during its accretion to north america phase ii of the rotation active between 20 m y b p and the present occurs in association with extension in the basin and range province

paleolatitude of the butano sandstone california and its implications for the kinematic histories of the salinian terrane and the san andreas fault the lower to middle eocene butano sandstone of the santa cruz mountains california is part of the central portion of the salinian terrane a slivered granitic fragment west of the san andreas fault the butano sandstone has been correlated with the point of rocks sandstone across the san andreas fault approximately 315 km to the southeast the magnitude of this offset is the same as the amount of neogene san andreas fault offset documented in other studies thus the correlation of these two eocene units implies that no pre san andreas system motion of the butano sandstone has occurred paleomagnetic studies indicating large northward translations of cretaceous and paleocene rocks of coastal california however raise the possibility of continued northward motion of the salinian terrane during or after deposition of the butano sandstone to better our understanding of the eocene position of the salinian terrane the paleolatitude of the butano sandstone was determined by paleomagnetic analysis of azimuthally unoriented cores from exploratory oil wells specimens from 30 core intervals from the top to the middle or lower parts of the formation were subjected to alternating field and thermal demagnetization the resulting paleolatitudes group better when bedding is horizontal than in their drill hole attitudes providing a positive fold test several magnetic polarity zones are present the mean paleolatitude 34 6 2 5 is significantly lower than that expected if the rocks were deposited in their present day location on an underformed north american craton when neogene slip on the san andreas and on smaller inboard faults is restored however the measured paleolatitude does not differ significantly from that expected this result supports the correlation of the butano and point of rocks sandstones and implies that the salinian terrane experienced no significant motion between deposition of the butano sandstone and establishment of the neogene san andreas dextral fault system

continental tectonics selected topics cause and effect relations between plate boundary processes and orogeny were investigated using new and refined plate models for the mesozoic and cenozoic terrane accretion and large components of horizontal shear were shown to be fundamental aspects of orogen evolution deep crustal thrust systems with large lateral transport and vertical stacking of slices of crystalline basement were documented in the cordillera and appalachians new geomechanical models of crystalline thrust sheets and foreland thrust belts emphasized the dominance of tectonic rather than gravitational stresses lithospheric thickening was shown to cause buoyancy forces that control the lateral spread of deformation outward from the interior of an orogen and may modify regional stresses in the vicinity of topographic fronts resulting in extension within the upper part of the lithosphere coeval with shortening at depth

very large scale motions in the atmospheric surface layer a field investigation a field observation array for the atmospheric surface layer asl was built on a dry flat bed of qingtu lake in minqin china as the qingtu lake observation array qloa site which is similar to the surface layer turbulence and environmental science test sltest site in the utah usa western desert the present observation array can synchronously perform multi point measurements of wind velocity and temperature at different vertical and streamwise positions in other words three dimensional turbulent asl flows can be measured at the qloa station and reynolds numbers as high as can be achieved with steady wind conditions by careful selection and pretreatment for measured data of more than 1200 h the qloa data have been validated to be reliable for high reynolds number turbulent boundary layer research results from correlation and spectral analysis confirm that very large scale motions vlsms exist in the asl at a reynolds number up to through premultiplied spectral analysis it is revealed that the spectral energy in the high wavenumber region decreases with height similar to turbulent boundary layers at low or moderate reynolds numbers while it increases with height in the low wavenumber region resulting in a log linear increase of vlsms energy with height which is different from turbulent boundary layers at low or moderate reynolds numbers the present analyses support the view that the evolution of the vlsms cannot be fully attributed to a bottom up mechanism alone and probably other mechanisms including a top down mechanism also play a role

paleomagnetism of the middle proterozoic laramie anorthosite complex and sherman granite southern laramie range wyoming and colorado we present the results of a combined paleomagnetic and 40ar 39ar geochronologic investigation of the middle proterozoic laramie anorthosite complex and sherman granite in the southern laramie range of wyoming and colorado anorthosites and monzosyenites of the laramie anorthosite complex yield a well defined characteristic magnetization of northeast declination d and moderate negative inclination i although antipodal normal and reverse polarity magnetizations are present at three sites a grand mean direction from 29 of 35 sites in the complex is d 44 6 i 48 7 k 77 4 95 3 1 alternating field af and thermal demagnetization behavior and rock magnetic experiments indicate that magnetization is carried by low ti titanomagnetite of single or pseudo single domain character that occurs as elongate to rod shaped inclusions in plagioclase and potassium feldspar the sherman granite contains a dual polarity magnetization that is less well defined than that of the laramie anorthosite complex but similar in declination and inclination d 53 1 i 48 1 k 46 5 95 7 6 n 8 14 sites rock magnetic data indicate the primary carrier of remanence in sherman granite is magnetite the 40ar 39ar geochronologic data from sherman granite hornblende biotite and microcline indicate that subsolidus cooling was moderate to relatively rapid through the range of temperatures over which magnetization was blocked and that the age of remanence is about 1415 ma microcline data indicate that the laramie anorthosite complex and sherman granite have probably not been thermally remagnetized paleomagnetic poles from the laramie anorthosite complex and sherman granite are indistinguishable at the 95 confidence level and individual virtual geomagnetic poles vgps from both units are combined to provide a mean pole at 215 0 e 6 7 s k 46 9 a95 3 5 n 37 vgps the location of this pole is similar to paleomagnetic poles derived from 1480 ma to 1450 ma intrusions elsewhere in north america but it plots significantly north of those from middle proterozoic sedimentary strata of the belt supergroup and sibley group in addition to the characteristic magnetization samples from some sites in sherman granite contain a remanence of southeast declination and shallow negative inclination d 154 9 i 16 0 k 90 6 95 9 7 n 4 sites this secondary magnetization was probably acquired during late paleozoic time

paleomagnetism and tectonic significance of albian and cenomanian turbidites ochoco basin mitchell inlier central oregon 1 understanding continental growth and convergent margin dynamics associated with terrane accretion and modification of the cordilleran margin of north america is prevented by conflicts in paleogeographic models for major terranes oceanic plates and the north american margin we present new paleomagnetic data suggesting that the blue mountains superterrane located at an inboard portion of the cordilleran margin has undergone substantial northward translation and clockwise rotation relative to north america since mid cretaceous time positive baked contact conglomerate and fold tests provide evidence that the magnetization of these rocks dates from their deposition these results yield a mean direction of d 10 8 i 58 5 k 127 95 4 1 and n 11 comparison of magnetic inclinations and magnetic fabrics shows no evidence for inclination error a paleolatitude of 39 2 4 8 4 3 n is obtained indicating a paleolatitude anomaly of 15 9 4 1 northward translation of 1760 460 km and cw rotation of 37 7 3 with respect to north america since circa 93 ma application of an ad hoc 5 inclination error yields translation of 1200 460 km through comparison with other units in the western cordillera we infer that the mitchell inlier was located adjacent to the sierra nevada arc during mid cretaceous time and that the blue mountains province may correlate with the intermontane superterrane in british columbia other interpretations such as large inclination errors or an independent displacement history for the mitchell inlier are possible but unlikely more work is needed to fully test these hypotheses and resolve major questions of cordilleran tectonics

mid cretaceous paleomagnetic reference field for north america restudy of 100 ma intrusive rocks from arkansas we have obtained new paleomagnetic data from three middle cretaceous alkalic intrusive complexes in central arkansas magnet cove and potash sulfur springs intrusions about 100 ma 20 sites and granite mountain stock about 88 ma 6 sites out of 254 specimens measured 147 from 20 sites were accepted the remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region the geologic evidence indicates that the intrusions have not been tilted since emplacement the directions of magnetization calculated with respect to present day horizontal yield a mean paleopole at 192 5 e 74 1 n n 20 k 34 a95 5 7 this does not differ at 95 confidence from that determined previously by scharon and hsu but the error is diminished by half the angular dispersion sd of site virtual geomagnetic poles vgps is 13 6 similar to the value of sd about 15 predicted by current paleosecular variation models the revised arkansas paleopole is combined with four other cratonic north american paleopoles of mid cretaceous age to yield a revised middle cretaceous reference pole at 196 e 71 n a95 4 9 the mean of nine paleopoles derived from overprints in rock units from the craton and craton margin which range in age from proterozoic to jura cretaceous is 202 e 68 n a95 8 2 this is not different from the reference paleopole indicating that overprinting was very widespread across north america in mid cretaceous time

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

paleomagnetic evidence that the central block of salinia california is not a far traveled terrane new paleomagnetic results from late cretaceous 75 85 m y red beds on the central block of salinia indicate that salinia was located within 6 in latitude of its current cratonal north american position during the late cretaceous after correction for neogene san andreas fault transport the red beds formed as alluvial fan overbank deposits with hematite cement deposited directly on salinian granites in the la panza range paleomagnetic analysis shows two components of magnetization in the red beds a low blocking temperature present day overprint residing in goethite and a high blocking temperature 600 component residing in hematite the hematite magnetization is a chemical remanent magnetization which formed soon after deposition during pedogenesis the bedding corrected hematite remanence contains a magnetic polarity stratigraphy with antipodal normal and reversed directions twenty three class i sites 95 20 have an average hematite direction with inclination 54 4 and declination 18 2 95 6 1 after structural correction these paleomagnetic data suggest that salinia resided at about 35 n latitude during the late cretaceous within 6 of its current location adjacent to cratonal north america by contrast a summary of paleomagnetic data from the peninsular ranges terrane and the sur obispo terrane which are currently outboard of salinia shows northward transport of these terranes of 12 to 22 relative to their current locations in north america since the cretaceous the offsets increase systematically away from the craton with the most outboard sur obispo terrane which is composed of accretionary prism and distal forearc material showing the largest degree of northward translation

on the shape of paleomagnetic data sets a quick and easy method to analyze the shape of paleomagnetic data sets is described and illustrated the elongation e of a set of directions of virtual geomagnetic poles vgp is defined as e k2 k3 where k2 and k3 are the second and third eigenvalues of the bingham distribution it is shown that e 3 or so indicates that the distribution is significantly elongate although the size of the sample and the paleolatitude both affect this conclusion because the azimuth of k2 gives the direction of elongation it can be used to help determine why a particular distribution is elongate it is shown that for most units from the craton both directions and vgp are nearly circular except for units magnetized at very low latitudes in general poles are more circular than directions some units formed in plate margin locations also are circular but many are highly elongate many units from the western cordillera of north america have highly elongate distributions that are compatible with origin within a shear zone others are obviously underformed internally units formed during extension are less commonly elongate but some show the probably effects of normal faulting it is hoped that shape analysis will be part of any complete paleomagnetic study in the future

the effect of turbulent flow structures on saltation sand transport in the atmospheric boundary layer the effect of turbulent flow structures on saltation sand transport was studied during two convective storms in niger west africa continuous synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 hz the shear stress production was determined from the vertical and streamwise velocity fluctuations the greatest stress bearing events were classified as turbulent structures with sweep ejection inward interaction and outward interaction described according to the quadrant technique the classified turbulent structures accounted for 63 5 per cent of the average shear stress during the first storm and 56 0 per cent during the second storm the percentage of active time was only 20 6 per cent and 15 8 per cent respectively high saltation fluxes were associated with sweeps and outward interactions these two structures contribute positively sweeps and negatively outward interactions to the shear stress but have in common that the streamwise velocity component is higher than average therefore the horizontal drag force seems primarily responsible for saltation sand transport and not the shear stress this was also reflected by the low correlation coefficients r between shear stress and saltation flux 0 12 and 0 14 respectively while the correlation coefficients between the streamwise velocity component and saltation flux were much higher 0 65 and 0 57 respectively

seismic refraction study of upper crustal structure in the lassen peak area northern california in september 1982 the u s geological survey recorded a 125 km long refraction profile through the lassen peak area of the cascade range this profile extended northwestward to the klamath mountains and southeastward to the sierra nevada foothills interpretation of the refraction data determined the upper crustal structure in this volcanic region of northern california near surface velocities 1 5 5 55 km s are consistent with a thickness of 2 4 km for the cascade volcanic rocks between 0 5 km above and 1 8 km below sea level velocities are about 4 7 5 55 km s the highest velocities are modeled below the devils kitchen terminal geyser area possibly indicating hydrothermal alteration of the volcanic rocks the absence of low velocities less than about 3 4 5 km s deeper than 1 km above sea level suggests that the cretaceous sedimentary rocks exposed west of lassen peak do not underlie the volcano southeast of lassen peak at 1 8 km below sea level a 0 3 km thick layer with a velocity of 6 1 km s overlies a lower velocity 5 7 5 8 km s 2 9 km thick layer because these modeled velocities do not change across the boundary between the sierra nevada foothills and the cascade range sierran basement could continue under the cascade volcanic rocks as far northwest as lassen peak the velocity model does not indicate the presence of a magma chamber larger than 3 km in diameter within the top 8 km of the crust

a plate tectonic model for late jurassic ophiolite genesis nevadan orogeny and forearc initiation northern california recently published age and structural data allow the reconciliation of previously conflicting models for late jurassic genesis of the josephine smartville and coast range ophiolites and the nevadan orogeny in the klamath mountains and sierra nevada the resulting model is consistent with the mode of initiation location and geometry of the great valley forearc basin and with the lack of a significant forearc basin west of the klamath mountains the coast range ophiolite formed by backarc spreading west of an east facing intraoceanic arc soon thereafter a remnant arc was calved off the west side of this arc and the smartville ophiolite formed by backarc interarc spreading during this time the sierran phase of the nevadan orogeny began as the intraoceanic arc encountered the west facing continental margin arc of north america an east west trending calcalkaline dike swarm in the sierra nevada foothills may mark the trajectory of the colliding arcs at the initiation of the collision simultaneously a new subduction zone was initiated west of the collision suture zone and this new trench propagated southward thus trapping the coast range ophiolite in the new forearc area south of the klamath area intense deformation in the sierran region resulted from this collision and both magmatic arcs became inactive as the last remnant of intervening oceanic crust was subducted continued westward relative movement of the north american arc was permitted north of the sierra nevada owing to the lack of a colliding intraoceanic arc the result was the westward rifting of the continental margin arc by intraarc spreading which formed the josephine ophiolite in the klamath area the klamath phase of the nevadan orogeny resulted from contraction of the west facing intraoceanic arc and josephine backarc basin beneath the continental margin basal sediments of the great valley forearc basin were derived primarily from the sutured arc ophiolite terranes and were deposited on top of the coast range ophiolite the southern edge of the klamaths and the western side of the sierra nevada a new late mesozoic magmatic arc was superposed across the previously accreted terranes and formed the primary sediment source for the cretaceous forearc basin

paleomagnetism of the tertiary clarno formation of central oregon and its significance for the tectonic history of the pacific northwest the clarno formation a mostly eocene and partly early oligocene sequence of andesitic lavas and volcaniclastic rocks is the oldest tertiary formation exposed in north central oregon remanent magnetization directions at 46 sites in the lavas provide a paleomagnetic pole at 84 n 278 e with a 95 confidence cone of 7 comparison of this pole with the north american reference pole for eocene time indicates that the clarno formation has rotated 16 clockwise with an uncertainty of 10 but has undergone no significant latitudinal displacement further comparison with paleomagnetic data from the miocene columbia river basalt group and steens basalt shows that large scale block rotations in the eastern columbia embayment ceased sometime between 38 m y and 15 m y a paleogeographic reconstruction for 38 m y is offered which is consistent with the observed rotation of the clarno and with the other paleomagnetic data from the pacific northwest which are briefly reviewed this reconstruction shows that it is possible to account for virtually all of the paleomagnetically indicated rotations in pre miocene tertiary rocks of the pacific northwest by an extensional tectonic model as a consequence only part of the rotations in the pre tertiary rocks of the eastern columbia embayment need to be the result of accretionary tectonics the initiation of magmatism in the eastern columbia embayment that is represented by the clarno formation itself may have been contemporaneous with the beginning of crustal extension in the pacific northwest

the basin and range province origin and tectonic significance the basin and range province is a vast arid tract of regionally corrugated angular topography of high relief in the western cordillera it is characterized by evenly spaced parallel mountain ranges and intervening desert basins fig ure la the range flanks are marked by poorly sorted gravel aprons that slope smoothly basinward interrupted here and there by low fault scarps that paral lel the range front faults and by alluvial fans at the mouths of canyons draining the ranges thermal springs located at or near range bounding faults attest to vigorous hydrothermal circulation within zones of fracture porosity created and maintained by faulting in the southern part of the province especially in southeastern california and southwestern arizona range fronts have been worn back by erosion leaving a thin veneer of gravel on an erosion cut bedrock surface that slopes gently outward the range bounding faults of these mountain blocks are buried at the outer edge of such pediments often at considerble distances from the erosional remnants of the ranges themselves the american physiographer n m fenneman 1928 1931 named the basin and range province and defined its general boundaries as thus circum scribed the province includes some 800 000 km2 of area in eight western states later students pardee 1950 lawrence 1976 reynolds 1979 eaton 1979b have observed that many of the fundamental geological and geo physical characteristics of the province are found well beyond the boundaries drawn by fenneman which were based on physiography alone as a tec tonophysical entity its areal extent is greater than 1 million km2 more than 10 of the area of the united states figure 1 fenneman 1931 subdivided the province into five physiographic sections the largest of which is the great basin see figure 1b it is not as its name implies a single regional depression with a common topographic center but is characterized instead by isolated networks of interior drainage divisible into

northward translation of baja british columbia along the late cretaceous to paleocene margin of western north america paleomagnetic data suggest that a large part of british columbia and southern alaska was at the latitude of baja california mexico in early late cretaceous time this allochthonous block baja bc includes the insular talkeetna in alaska and intermontane composite terranes the miniterranes between them and possibly part of the omineca metamorphic belt assuming that baja bc was adjacent to north america by middle cretaceous an analysis using plate kinematics of the pacific basin produces a model of northward movement of baja bc which is compatible with the known geology of the reconstructed margin of western north america baja bc moved north 2400 km as part of the kuia plate along a transform margin with north america from 85 to 66 ma this time interval is marked by the cessation of igneous activity in the sierra nevada development of wrench faults and basins along coastal california and initiation of laramide style tectonics to the east within baja bc diminished igneous activity to the west was coeval with possible wrench tectonic basins along its eastern margin by the end of the cretaceous baja bc was positioned from oregon to northern british columbia between 66 and 56 ma baja bc became attached to north america in a dextral transpressive stage during which the kula north america plate boundary expanded to encompass the whole block with a series of northwest trending strike slip faults a fold and thrust belt in the east and large scale uplifts and the formation of an incipient volcanic arc on the west southern alaska was the site of convergence during the whole of the period of northward translation of baja bc a late cretaceous subduction zone in alaska evolved into a major compressional belt as the basin and miniterranes on the northern margin of baja bc collided with interior alaska the collision culminated in the paleocene with formation of the mckinley granites and coeval dextral slip on the denali fault system

paleomagnetic results from the central sierra nevada constraints on reconstructions of the western united states paleomagnetic results from three plutons of the central sierra nevada limit the motion of the sierra nevada relative to the north american craton to 5 8 of poleward motion and 7 11 of clockwise rotation since about 90 ma the indicated rotation and translation are not significant at the 95 confidence level a new tectonic model is presented that is consistent with all available paleomagnetic data from the sierra nevada great valley klamaths oregon coast range cascades and western nevada the paleogeographic reconstructions of this model associate the displacement of major crustal blocks with basin and range extension starting at about 30 ma this model accounts for about 15 of clockwise rotation of the klamaths and southern cascades about 2 of clockwise rotation of the sierra nevada and great valley and 26 of clockwise rotation of the oregon coast range since 30 ma the model is consistent with 40 of clockwise rotation of the oregon coast range before 30 ma as proposed by magill and cox 1980 the amount of basin and range extension in this model at latitude 41 n is 225 km directed e w and at latitude 35 n is 245 km directed wsw ene

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

paleomagnetism of jurassic rocks in the western sierra nevada metamoprhic belt and its bearing on the structural evolution of the sierra nevada block the western metamorphic belt of the sierra nevada consists of two eugeosynclinal terranes separated by the melones and sonora faults subvertical bedded mesozoic volcanic rocks metamorphosed to low greenschist facies predominate to the west whereas paleozoic metamorphic rocks of higher grade and greater structural complexity predominate to the east in order to study the structural development of the faults 121 samples of basalt and diabase were collected for paleomagnetic analysis from three jurassic formations the logtown ridge and penon blanco formations west of the melones fault and the sonora dike swarm to the east of the sonora fault a northwesterly downward directed magnetization occurs in each unit three fold tests and a conglomerate test on the two formations west of the faults show that the magnetization is secondary postdating nevadan late jurassic folding and is probably coeval with peak metamorphism an average of five paleomagnetic poles from the sierra nevada three derived from the secondary magnetizations given herein and two previously published all of probable kimmeridgian age yields 67 2 n 161 2 e and 95 6 5 southeasterly magnetizations also occur in the logtown ridge formation and sonora dike swarm directions from the sonora dikes are approximately antipodal to the secondary directions and are reversed magnetizations from the logtown ridge formation yield similar results only if corrected for the tilt of bedding the logtown ridge magnetizations tilt corrected yield a pole position near to that expected for north america the data from the sonora dikes require a tilt correction of 25 30 toward the south southwest about a horizontal axis parallel to the regional structure in order to yield a north american pole position we conclude that the eastern wall rocks of the melones and sonora faults have been rotated 25 30 in response to nevadan deformation in contrast to the western wall rocks which have been rotated about 90

cainozoic evolution of the state of stress and style of tectonism of the basin and range province of the western united states cainozoic evolution of the modern plate boundary along the western united states from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western united states extensions tectonism responsible for the modern basin and range province appears to represent a unique late stage episode of a much longer period of extension initiated in an in tra arc setting contemporaneously with calc alkaline magmatism basin range extension is distinguished from early extension on the basis of angular unconformities differences in fault trends and spacing and associated magmatism basaltic prebasin range extension i e extension preceding the break up of the region into ranges resembling the modern ones was under way locally by at least 30 ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks by large regions of the crust underlain by passively emplaced subvolcanic batholiths and by the thickness and distribution of stratigraphic units locally high strain rates that accompanied early extensions of as much as 50 100 are implied data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform wsw ene least principal stress orientation in the period ca 20 10 ma during this early extension the change from early extension to basin range style faulting of the upper 15 km of crust which resulted in broadly spaced ranges 25 35 km crest crest spacing was time transgressive and probably not abrupt locally both types occurred concurrently southern basin and range block faulting occurred largely in the period 13 10 ma in response to a stress field orientated similarly to that responsible for the early extension in contrast northern basin and range block faulting developed after 10 ma and continues to the present in response to a stress field orientated approximately 45 clockwise to the earlier stress field this modern stress field with a wnw ese to e w directed least principal stress characterizes the entire modern basin and range province and rio grande rift region the 45 change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the san andreas transform fault inclusion of pre basin range extension may help resolve the discrepancy between estimates of 15 30 for basin range block faulting and total extension estimates of 100 300 for the basin and range province

shear stress partitioning in sparsely vegetated desert canopies shear velocity ratios were derived for six neighbouring localities representing rangeland abandoned farmland and a playa in the semi arid region of the sonoran desert near surface and above canopy wind speeds mostly below saltation threshold were used to evaluate the partitioning of shear velocities and shear stresses between the plant canopy and the surface the technique utilized represents an alternative to the threshold shear velocity ratio rt approach of musick and gillette land degradation and rehabilitation 1990 2 87 94 and results demonstrate the need for partitioning over a range of wind speeds shear velocity ratios were variable over the playa and abandoned farmland while variance in near surface wind speeds was highest over shrub dominated sites the relationship between lateral cover lc and shear velocity r is comparable to previous findings utilizing rt although methodological differences result in mean shear velocity ratios less than shear velocity ratios derived for threshold conditions with respect to the model of raupach et al journal of geophysical research 1993 98 d2 3023 3029 this may be attributed to the difference between measured shear stresses and the average shear stress on the exposed surface at high lateral covers shear velocity ratios are lower than threshold shear velocity ratios due to increased wake interactions or decreased drag on the shrub elements at higher wind speeds

dispersed remnants of a northeast pacific fringing arc upper paleozoic terranes of permian mccloud faunal affinity western u s two fragmentary subparallel belts of terranes within the western north american cordillera contain upper paleozoic rocks and are characterized by contrasting lithotectonic assemblages and contrasting permian faunal affinity these two belts are a volcanic arc related successions of permian mccloud faunal affinity mccloud belt and b subduction related accretionary complexes of permian tethy an faunal affinity cache creek belt this paper supports the hypothesis that the fragmentary terranes of the mccloud belt once constituted parts of a northeast pacific fringing arc system and in constrast to some earlier interpretations concludes that the volcanic arc evolved above an eastward dipping subduction zone the absolute distance between this arc and western north america during the late paleozoic cannot be constrained however there is little evidence to suggest closure of a major 10 km ocean basin or protracted periods of westward dipping subduction parts of devonian to permian volcanic island arc sequences of the western u s cordillera are represented in the northern sierra eastern klamath bilk creek grindstone and chilliwack terranes these scattered volcanic arc remnants share several fundamental characteristics 1 the sequences were constructed across continental affinity basement assemblages 2 they underwent similar tectonic evolution during late paleozoic time such as coeval pulses in volcanism and related depositional histories 3 they contain early permian mccloud type fauna of distinctive biogeographic affinity 4 mccloud belt terranes are spatially and possibly genetically related to westward lying accretionary complexes of the cache creek belt which contain fragments of upper triassic blueschist and permian limestone blocks bearing tethyan permian fusulinids and corals based on the presence of distinctive early permian mccloud fauna the island arc remnants discussed in this paper are referred to as the mccloud belt the similarities between mccloud belt terranes suggest that they were once parts of a long lived late paleozoic volcanic arc complex the stratigraphy of these terranes records the tectonic evolution of the volcanic arc volcanism within the paleozoic arc began as early as the early devonian and was succeeded or accompanied by carbonate and epiclastic sedimentation during the middle and late devonian an influx of detritus derived from sedimentary and metasedimentary rocks during the late devonian and mississippian records the uplift and erosion of arc basement sequences an event that was at least locally accompanied by ongoing volcanism a pulse in volcanism during the pennsylvanian and coeval uplift led to the establishment of shallow marine and deltaic volcanogenic and carbonate sedimentation and locally subaerial exposure which persisted into the early early permian elsewhere during the early permian shallow water carbonate deposition was established with only minor influx of volcanogenic debris during the late permian and locally as early as late early permian a major pulse in volcanism occurred and is now represented by voluminous sequences of volcanogenic debris and lavas most of these sequences are overlain by upper triassic carbonate platformal sequences arc volcanism was recorded at least locally until the middle jurassic paleomagnetic data are used to constrain a schematic pre cretaceous reconstruction of these volcanic arc sequences and their associated subduction assemblages the inferred paleogeography of island arc sequences and the adjacent accretionary terranes suggests that volcanism occurred above an east dipping subduction zone with a long lived paleogeographic relation to north america the current structural position of these terranes is largely the result of mesozoic and locally younger deformation

using leaf margin analysis to estimate the mid cretaceous albian paleolatitude of the baja bc block abstract the baja bc hypothesis which postulates that western washington state british columbia and southern alaska originated at the latitude of mexico has pitted paleomagnetic results against long held interpretations about the tectonic evolution of western north america in this paper we develop a new paleobotanical method for estimating paleolatitude and apply it to this problem we start by showing that the modern mat field for north america is well correlated with latitude demonstrating the feasibility of using mat to estimate paleolatitude a compilation of mat and floral data from 84 modern sites in central and north america is used to establish a new prediction relationship mat 1 32 28 99p where p is the proportion of smooth margined species within a floral sample at a site our analysis also includes a more complete estimate of the uncertainties associated with estimating mat from a measurement of p using modern data we show that mat and p can be used to estimate latitude as well we then apply this approach to resolve the paleolatitude of baja bc eleven floral sites from stable north america are used to establish the latitudinal mat profile for north america during the albian and cenomanian a floral site from the winthrop formation a mid cretaceous 110 100 ma fluvial unit in the methow basin of northern washington state is linked to the baja bc block and predates its proposed northward offset forty three morphospecies of dicotyledonous angiosperm leaves from the winthrop formation give p 0 76 which is equivalent to a mat of 23 4 c indicating a subtropical to tropical climate we use the north american mat profile to estimate a paleolatitude of 38 4 n for the winthrop flora indicating 2200 km of northward offset relative to stable north america

quantifying sand storage capacity of large woody debris on beaches using lidar abstract the sedimentological role of large woody debris lwd on beaches is understudied and is relevant for the morphodynamics of sandy high energy beach dune systems of the northeast pacific ocean on the west coast of canada this debris consists largely of historical escape logs from the coastal logging industry in areas with competent wind regimes lwd can trap appreciable amounts of windblown sand in the backshore which can alter beach foredune sediment budgets and initiate incipient dune formation as this additional store of sediment must be reworked first during high water events it provides an important buffer that reduces erosion of established foredunes this study examines the morphology and sand storage capacity of three backshore lwd deposits of varying morphologies on northeastern graham island haida gwaii queen charlotte islands british columbia canada a new method was developed using coincident high spatial resolution lidar data and digital orthophotographs to derive dems for distinct ground cover classes sand lwd these dems were then used to quantify relative storage capacities of lwd and sand in the backshore significant amounts of sand are stored within and around lwd on beaches in the study region existing storage quantities above hhwmt range from 9 19 10 4 to 1 39 10 5 kg m 1 beach width or 1 14 to 1 60 m storage depth the same lwd deposits have a further potential storage capacity ranging from 1 04 to 1 70 10 4 kg m 1 beach width or 0 21 to 0 28 m depth the relative storage capacity of these features is reflected in the backshore morphology of each site with sediment transport further into the backshore dependent upon the morphology and relative in filling of the log debris jam with this additional sediment storage log debris could enhance development of large incipient dunes in the backshore thereby buffering against increasing storminess and gradual sea level rise in the region as the use of precise lidar data is increasing in geomorphic research the development of a verifiable methodology for estimating beach dune storage volumes is timely

paleomagnetism and tectonics of the crescent formation northern olympic mountains washington a stable prefolding magnetization has been discovered in pillow basalts of the eocene lower crescent formation of the northern olympic mountains the curved outcrop pattern of the crescent formation has been the target of several unsuccessful studies to test for oroclinal bending the success of this study is due in part to the development of a small diameter electric core drill for sampling the fractured rims of basalt pillows thermal demagnetization produced stable endpoints by 580 c in 12 of the 34 sites sampled large within site scatter was common in the remaining sites among the accepted sites within site scatter was small and correction for bedding tilt significantly reduced the scatter between sites the mean paleomagnetic pole for newly sampled pillow basalts 86 4 north latitude 170 0 east longitude a95 16 5 is indistinguishable from the early to middle eocene pole expected for north america including previous results from sites in subaerial basalt of the upper crescent formation in and near the easterm olympic mountains results in a more regional paleomagnetic pole 80 7 north latitude 192 0 east longitude a95 8 0 n 46 that shows no significant rotation 0 8 14 4 or poleward displacement 3 6 8 5 analysis of the magnetic mineralogy suggests that the remanence is early probably primary the pole therefore should be valid for tectonic interpretation of the region a circular distribution of virtual geomagnetic poles after correction for bedding tilt supports the hypothesis that the northern crescent formation experienced deformation due to the rise in a domelike fashion of the sediments of the olympic core terrane erosion of a partial dome open to the west could have produced the curvature seen in the outcrop pattern of the crescent formation the lack of significant rotation of the northernmost coast ranges contrasts with the net clockwise rotation seen to the south the difference could be that irrotational northward translation paleomagnetically insignificant yet geologically important driven by oblique convergence was accommodated by the north south trending strike slip faults to the east of the olympic mountains

high frequency sediment transport responses on a vegetated foredune wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and wenglor laser particle counters the foredune had a compound seaward slope with a wave cut scarp about 0 5 m high separating the upper vegetated portion from the lower dune ramp which was bare of vegetation wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction but overall magnitudes were similar over the whole length ten minute average transport intensity correlates strongly with mean wind speed measured at the dune crest and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique but for wind angles 45 from shore perpendicular some sand is transported onto the lower slope copyright 2012 john wiley sons ltd

boundary layer turbulence characteristics during aeolian saltation 1 a great deal of effort has been expended in measuring turbulence phenomena in clean air flows however no previous measurements have been successfully made of the vertical distributions of turbulence intensity and reynolds stress in a fully adjusted boundary layer flow saturated with saltating particles the present wind tunnel study addresses this knowledge gap using a custom designed laser doppler anemometer lda the amount of turbulence is found to increase with the introduction of saltating particles to the airflow over the lowest 15 of boundary layer vertical profiles of the streamwise wind speed provide friction velocities that lie well within the narrow range of those derived from direct measurement of the reynolds stress relative to clean air aeolian saltation is demonstrated to increase the magnitude but not the frequency of burst sweep events that primarily contribute to the total fluid stress within several millimeters above the bed surface all vertical profiles of wind speed converge upon a focal point as the local fluid stress declines toward the mobile bed

magnitude of crustal extension in the southern great basin strike slip faults in the southern great basin separate areas of cenozoic upper crustal extension from relatively stable tectonic blocks linear geologic features offset along the garlock fault las vegas valley shear zone and lake mead fault system allow reconstruction of the southern great basin to a pre extension configuration the sierra nevada mojave desert spring mountains and colorado plateau are treated as stable unextended blocks that have moved relative to each other in response to crustal extension with the spring mountains held fixed to the mojave block our reconstruction indicates a minimum of 65 extension 140 km between the southern sierra nevada and colorado plateau

geomorphology of desert sand dunes a review of recent progress through the 1980s and 1990s studies of the geomorphology of desert sand dunes were dominated by field studies of wind flow and sand flow over individual dunes alongside these there were some attempts numerically to model dune development as well as some wind tunnel studies that investigated wind flow over dunes as developments with equipment allowed field measurements became more sophisticated however by the mid 1990s it was clear that even these more complex measurements were still unable to explain the mechanisms by which sand is entrained and transported most importantly the attempt to measure the stresses imposed by the wind on the sand surface proved impossible and the use of shear or friction velocity as a surrogate for shear stress also failed to deliver at the same time it has become apparent that turbulent structures in the flow may be as or more important in explaining sand flux in a development paralleled in fluvial geomorphology aeolian geomorphologists have attempted to measure and model turbulent structures over dunes progress has recently been made through the use of more complex numerical models based on computational fluid dynamics cfd some of the modelling work has also suggested that notions of dune equilibrium form may not be particularly helpful this range of recent developments has not meant that field studies are now redundant for linear dunes careful observations of individual dunes have provided important data about how the dunes develop but in this particular field some progress has been made through ground penetrating radar images of the internal structure of the dunes the paradigm for studies of desert dune geomorphology for several decades has been that good quality empirical data about wind flow and sand flux will enable us to understand how dunes are created and maintain their form at least some of the difficulty in the past arose from the plethora of undirected data generated by largely inductive field studies more recently attention has shifted although not completely to modelling approaches and very considerable progress has been made in developing models of dune development it is clear however that the models will continue to require accurate field observations in order for us to be able to develop a clear understanding of desert sand dune geomorphology

crustal genesis on the oregon plateau the areal and temporal distribution and the chemical and isotopic characteristics of cenozoic volcanic rocks from the oregon plateau reflect the changing tectonic conditions affecting this area over the last 20 m y following subduction zone related calc alkaline volcanism flood basalt eruptions occurred in the time interval 18 14 ma this primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the juan de fuca ridge that resulted in changes in the geometry of subduction against the pacific northwest the compositions of the mid miocene flood basalts show signs of extensive crystal fractionation in some cases accompanied by crustal contamination roughly 11 m y ago basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions this implies that as extension in the oregon plateau proceeded the crust became less of an impediment to the passage of relatively unfractionated mantle melts initial sr nd and pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area these observations suggest that an area of some 130 000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern north america over the last 20 m y the intensity of basalt volcanism as manifest in the columbia river group and in the oregon plateau does not appear to reflect the presence of a hot spot beneath the area both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by back arc spreading in a manner analogous to the formation of the marginal basins of the western pacific this spreading initiated along the western border of the archean wyoming craton with the location of mantle ascent melt production and extraction controlled by the changing geometry of subduction the existence of a zone of lithospheric weakness and the presence of a thick low density subcontinental mantle keel existing beneath the archean craton in this area

native terranes of the central klamath mountains california the klamath mountains of northern california and southern oregon contain several good examples of terranes which have developed in situ the term native is proposed for five of these terranes and all terranes whose development can be tied to an adjacent cratonal area the klamath terranes discussed herein include disrupted stratigraphic and metamorphic types three disrupted terranes the rattlesnake creek eastern hayfork and north fork contain fossiliferous blocks derived from both north american and exotic sources the unique mixed faunal assemblage stratigraphic ties to north america in the source terranes of the blocks and paleomagnetic evidence indicate that the tectonic and sedimentary processes responsible for mixing these blocks occurred in proximity to north america not distant from the terranes present positions coeval blueschist metamorphism in a fourth inboard terrane the stuart fork suggests that all four terranes developed during a late triassic to early jurassic subduction event a fifth stratigraphic terrane the western hayfork was constructed upon the assembled disrupted and metamorphic terranes in the middle jurassic disrupted terranes with similar mixtures of north american and exotic faunas occur throughout the cordillera from central california possibly as far north as british columbia late triassic deformation has been documented in several of these terranes suggesting that 1 subduction operated along at least this portion of north america during the late triassic early jurassic and 2 many additional cordilleran terranes should also be considered native

metallogenic setting and mineralization alteration characteristics of major skarn fe polymetallic deposits in qimantag area western qinghai province fe polymetallic skarn deposits are widely distributed in the qimantag area of western qinghai province recently this area has become a major metallogenic belt with most promising ore potential geotectonically it belongs to eastern kulun orogenic belt mineralization is related to indosinian 204 237 ma diorite granodiorite and mozonitic granite ore controlling strata are composed of marble and silicalite of middle proterozoic langyashan formation marble clasolite silicalite and andesite basaltic volcanic rocks of ordovician silurian tanjianshan group and crystalline limestone and clasolite of carboniferous period in the ore deposits three types of skarns are developed i e calcic magnesian and manganoan types with the first type being dominant calcic skarn is commonly associated with fe cu mo pb zn mineralization magnesian skarn is mainly associated with fe mineralization and manganoan skarn is always associated with pb zn ag mineralization mineral assemblage of the ores is extremely varied and has certain zonation potassic metasomatic phenomena are very entensive in the endo contact zones of the deposits in addition to different types of skarns potassic metasomatism also constitutes one of the important ore searching indicators

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

the role of streamline curvature in sand dune dynamics evidence from field and wind tunnel measurements abstract field measurements on an unvegetated 10 m high barchan dune in oman are compared with measurements over a 1 200 scale fixed model in a wind tunnel both the field and wind tunnel data demonstrate similar patterns of wind and shear velocity over the dune confirming significant flow deceleration upwind of and at the toe of the dune acceleration of flow up the windward slope and deceleration between the crest and brink this pattern including the widely reported upwind reduction in shear velocity reflects observations of previous studies such a reduction in shear velocity upwind of the dune should result in a reduction in sand transport and subsequent sand deposition this is not observed in the field wind tunnel modelling using a near surface pulse wire probe suggests that the field method of shear velocity derivation is inadequate the wind tunnel results exhibit no reduction in shear velocity upwind of or at the toe of the dune evidence provided by reynolds stress profiles and turbulence intensities measured in the wind tunnel suggest that this maintenance of upwind shear stress may be a result of concave unstable streamline curvature these additional surface stresses are not recorded by the techniques used in the field measurements using the occurrence of streamline curvature as a starting point a new 2 d model of dune dynamics is deduced this model relies on the establishment of an equilibrium between windward slope morphology surface stresses induced by streamline curvature and streamwise acceleration adopting the criteria that concave streamline curvature and streamwise acceleration both increase surface shear stress whereas convex streamline curvature and deceleration have the opposite effect the relationships between form and process are investigated in each of three morphologically distinct zones the upwind interdune and concave toe region of the dune the convex portion of the windward slope and the crest brink region the applicability of the model is supported by measurements of the rate of sand transport and the change of the dune surface in the field

development of cretaceous transpressional cleavage synchronous with batholith emplacement central sierra nevada california metamorphosed strata of the ritter range pendant record structures formed during the emplacement of granitic plutons at upper crustal levels 8 km in the late cretaceous sierra nevada magmatic arc paleozoic to middle jurassic strata of the pendant have been penetratively shortened 30 50 forming slates phyllites and schists with bedding inclined cleavage and associated linear structures the cleavage secondary cleavages and folds associated with these cleavages have orientations and geometries consistent with dextral transpression between the farallon and north american plates microstructural relations show that cleavage development accompanied greenschist to amphibolite facies metamorphism metamorphic hornblende muscovite and biotite in the wall rocks show weak to strong preferred orientation and have 40ar 39ar plateau ages ranging from about 85 to 80 ma hornblende and possibly muscovite are expected to retain 40ar at or near estimated peak metamorphic temperatures of 500 c thus their 40ar 39ar ages closely correspond to the time of cleavage formation granitic plutons adjacent to the ritter range pendant have yielded u pb and other ages that cluster from 91 to 82 ma we conclude that bedding inclined cleavage and metamorphism in the ritter range wall rocks developed in a dextral transpressive strain regime penecontemporaneously with emplacement of adjacent granitic plutons

ore genesis of the weibao lead zinc district eastern kunlun orogen china constrains from ore geology fluid inclusion and isotope geochemistry the weibao lead zinc district composed by two deposits named the east weibao deposit and the west weibao deposit respectively is located in the qimantagh area eastern kunlun orogen although both controlled by skarn alternation and experienced similar mineralization processes the east weibao deposit is dominated by massive ores while the west weibao deposit is characterized by banded ores the ore forming processes of both the deposits can be divided into four stages including prograde skarn stage retrograde skarn stage sulfide stage and carbonate stage three types of fluid inclusions i e pure co2 fluid inclusion pc type co2 bearing fluid inclusion c type and aqueous types w type have been identified in quartz calcite and sphalerite from different stages noteworthily only sulfide stage of the east weibao deposit contains carbonic fluid inclusions whereas only aqueous fluid inclusions were observed in sulfide stage of the west weibao deposit the co2 rich and aqueous type fluid inclusions in sulfide stage of the east weibao deposit were homogenized at temperatures between 263 and 424 c concentrating at 360 400 c with salinities ranging between 3 2 and 13 6 wt nacl eqv the w type fluid inclusions in sulfide stage of the west weibao deposit were homogenized at temperatures between 228 and 381 c concentrating at 280 340 c with salinities between 1 6 and 9 0 wt nacl eqv and the aqueous type fluid inclusions in carbonate stage in both deposits were homogenized at temperatures between 115 and 336 c concentrating at 200 240 c with salinities between 0 2 and 5 9 wt nacl eqv the ore forming fluid in sulfide stage at the east weibao deposit yielded calculated updelta 18 text o text h 2 text o and 18d values of 4 2 to 5 6 and 88 to 83 respectively the results are 8 8 to 10 8 99 to 92 for the sulfide stage at the west weibao deposit and 1 0 to 0 2 50 to 45 for carbonate stages in both deposits the lead isotopic ratios of sulfide samples from the weibao lead zinc district range between 18 259 and 18 336 for 206pb 204pb 15 580 and 15 659 for 207pb 204pb and 38 007 and 38 253 for 208pb 204pb respectively and the corresponding sulfur isotopic values range between 2 2 and 3 6 the fluid inclusion and isotope studies on the two deposits in the weibao lead zinc district suggest that they both have magmatic origin but mineralized by different mechanisms mineralization at the east weibao deposit may be caused by fluid immiscibility and mixing while at the west weibao deposit mineralization may result from water rock reactions between magmatic fluid and impure carbonate wall rocks combined with geological observation we conclude that the weibao lead zinc district as a united magmatic hydrothermal fluid system associated with the triassic magmatism that widely distributes in the qimantagh area

wavelet power spectra of aeolian sand transport by boundary layer turbulence 1 this letter reports on wavelet spectral analysis of collocated time series of streamwise wind speed and aeolian sand transport collected at 0 04 m above the bed in a field experiment on top of a large sand mound in a desert environment results suggest three distinct spatio temporal scales of interaction between boundary layer turbulence and sediment transport a long term scale associated with the regional mixing layer an integral time scale associated with the internal boundary layer and a streamer scale associated with the inertial subrange eddy cascade and saltation inertia

fast strain rates during pluton emplacement magmatically folded leucocratic dikes in aureoles of the mount stuart batholith washington and the tuolumne intrusive suite california finite strain analysis and thermal modeling of magmatically folded leucocratic dikes in the mount stuart batholith washington and the tuolumne intrusive suite california yield strain rates in the range of 10 2 to 10 1 3 s 1 compared to published regional strain rates 10 1 3 to 10 1 5 s 1 wall rock strain rates associated with dike fed expansion 10 7 s 1 and rates resulting from numerical modeling of crystal plastic creep in aureoles 10 1 1 s 1 our fast rates are several orders of magnitude higher field and microstructural observations suggest that multiple material transfer processes including rigid rotation ductile flow cracking and potentially melt assisted granular flow operated in the aureoles to accommodate emplacement of these two plutons calculated durations of pluton construction and bulk shortening in the aureoles indicate that aureole deformation requires only slow long term strain rates of 10 1 4 s 1 thus our local fast strain rates indicate that aureoles may be characterized by pulsating high strain rate surges we suggest that magmatically folded dikes in these and other pluton aureoles around the world may be used as evidence for fast host rock strain rates during pluton emplacement

neogene tectonics and kinematics of western north america recent quantitative studies of deformation in western north america including pacific and north american relative plate motion estimates of neogene displacement in the southern basin and range and anomalous paleomagnetic declinations of the pacific northwest are self consistent and together determine a simple kinematic model of the deformation that is an extension of the 1970 plate tectonic model of atwater a large continental lithospheric block called here the sierra nevada klamath block was displaced about 300 km to the w nw relative to the north american craton during the neogene the southern end of the overriding plate in the subduction zone along the pacific northwest has been shifted the same amount resulting in the subduction zone and overlying accretionary complex rotating clockwise about 30 the pivot is located near seattle and the several tens of kilometers of n s shortening required to accommodate the rotation is taken up in the major thrusts of the juan de fuca strait and olympic peninsula

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

crescentic dunes at schiermonnikoog the netherlands this paper describes the appearance and maintenance of crescentic dunes in high wind speed conditions on a frozen beach at schiermonnikoog the netherlands the dunes were cresentic forms with horns they were barchanoidal in plan view but had reverse morphologies to typical barchans the highest and steepest slopes were upwind and led to long low slopes downwind slipfaces were absent it is hypothesized that such crescentic dunes may be a stable aerodynamic form under high to very high c 15 20 m s 1 flow conditions 1997 by john wiley sons ltd

wind flow and sedimentation in artificial vegetation field and wind tunnel experiments abstract the flow dynamics within and immediately above an artificial plastic vegetation canopy are measured in the field and wind tunnel artificial plants created from wooden dowels and cable zip ties were arranged in a variety of arrays on pegboards where plant cover or lateral cover could be accurately controlled plant covers examined ranged from 12 to 60 wind velocity profiles and the bedforms incipient dunes that developed within and beyond the vegetation in a few hours were measured the results indicate that flow deceleration and drag increases within the canopy as plant density increases in general there is a marked reduction and inflection in velocity at the canopy top and a second major inflection in the velocity profiles occurs immediately below the base of the four arrays equal or 23 density tall vegetation allows flow to sweep under the canopy and local high speed maxima flows may develop leading to minimal sedimentation in those canopies but deposition downwind beyond the canopies short vegetation where the leaves fill the canopy close to the ground display significantly greater drag and rapid rates of sand deposition flow velocities above the canopy increase as plant cover or density increases due to flow compression and acceleration bedform development is strongly correlated with plant cover density or lateral cover and short plants such that shorter length higher bedforms develop in higher density and or shorter plants the relationship between plant cover density or lateral cover and within canopy velocity is non linear and a new tentative flow regime is proposed canopy flow which may occur between wake interference flow and skimming flow this study further confirms that plant density plays a defining role in determining bedform and dune morphology the results also apply to flow in semi arid grasslands