composition of modern sand and cretaceous sandstone derived from the sierra nevada california usa with implications for cenozoic and mesozoic uplift and dissection abstract the california sierra nevada represents the roots of a primarily cretaceous magmatic arc that is presently being dissected extensively in the south and to a lesser degree in the north where it is covered by cenozoic volcanic rocks the cenozoic magmatic arc built on top of the cretaceous arc has been dissected concurrently with northward migration of the mendocino triple junction south of which the magmatic arc is inactive a north to south transect from the modern cascade arc along the sierra nevada therefore acts as a proxy for evolution from an active undissected magmatic arc to a transitional arc to a dissected arc to intensely uplifted basement discriminant analysis of cascade and sierra nevada modern alluvial sand defines four compositional groups and clearly distinguishes volcaniclastic undissected arc and plutoniclastic uplifted basement end members two intermediate compositional groups are primarily volcaniplutoniclastic and metamorphiplutoniclastic the former group represents dickinson s 1985 transitional arc the latter represents dissected arc sand composition in the southern sierra nevada reflects significant uplift resulting from a combination of latest cretaceous paleogene uplift and late cenozoic basin and range extension footwall uplift of the western rift shoulder possibly enhanced by mantle lithosphere delamination high concentrations of quartz and feldspar and near absence of aphanitic lithic fragments are typical of the southern sierra nevada and are characteristic of dickinson s basement uplift cretaceous sandstone petrofacies of the great valley group gvg of western california document dissection of the cretaceous magmatic arc as well as erosion of residual orogenic highlands analogous to modern taiwan formed during latest jurassic arc continent collision nevadan orogeny when cretaceous petrofacies data are entered into the modern sand discriminant analysis as unknowns they are classified into the modern cascade sierra nevada groups and a group from taiwan the results confirm that the lowest gvg petrofacies represent erosion of the nevadan suture belt and the undissected post nevadan arc increasing dissection of the arc occurred during the cretaceous but most samples represent transitional arc indicating that even by the latest cretaceous laramide cessation of arc activity in california dissection of the magmatic arc was less than what we see today in the central and southern sierra nevada no cretaceous samples have compositions equivalent to the basement uplift of the southern sierra nevada the application of actualistic sand stone petrofacies in this study provides important constraints on models for the cretaceous to modern history of the sierra nevada as well as implications for arc evolution in general

origin of high mountains in the continents the southern sierra nevada active and passive seismic experiments show that the southern sierra despite standing 1 8 to 2 8 kilometers above its surroundings is underlain by crust of similar seismic thickness about 30 to 40 kilometers thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range but little subcrustal lithosphere is present beneath the eastern high sierra and adjacent basin and range these and other data imply the crust of both the high sierra and basin and range thinned by a factor of 2 since 20 million years ago at odds with purported late cenozoic regional uplift of some 2 kilometers

nd sr isotopic geochemical and petrographic stratigraphy and paleotectonic analysis mesozoic great valley forearc sedimentary rocks of california measurements of nd sr isotopes major and trace elements and model mineralogy were made on upper jurassic and cretaceous great valley forearc sedimentary rocks to test models for the temporal and spatial evolution of sierra nevada arc sources isotopes and major and trace elements are sensitive provenance indicators because of the large west east isotopic geochemical and age gradients in the plutonic rocks of the sierra nevada batholith and because petrographic models indicate that source areas moved east during the cretaceous isotopic and chemical variations are correlated in the forearc sandstone as nd decreases th u la nb zr hf pb rb sio2 and k2o concentrations increase and feo mgo tio2 ni and cr concentrations decrease this relation is the same as that observed in the plutonic rocks and indicates that the arc was the primary source of sediment and that the sandstone chemistry was not disturbed by sedimentary processes the nd sr relation of san joaquin valley sandstone is the same as the plutonic rocks but sacramento valley sandstone is elevated in sr because of seawater exchange weathering and diagenesis whole rock sandstone decreases in nd from 7 to 5 and increases in 87sr 86sr from 0 7045 to 0 7073 with decreasing stratigraphic age the nd sr isotopic composition is correlative with the plagioclase to feldspar ratio and indicates that source areas moved inland during the cretaceous upper cretaceous san joaquin valley shale is similar in nd to the sandstone indicating that sandstone and shale were derived from the same source and that the nd isotopic composition is independent of grain size the shale is higher in 87sr 86sr than the sandstone possibly due to concentration of biotite in the fine fraction during transport and subsequent rb loss during diagenesis nd sr isotopes were used to construct models to locate source areas parameters include lithology drainage basin geometry and erosion rate the age and isotopic compositions of the calculated igneous component of the sandstone correspond to the age and isotopic compositions of the plutonic rocks of the batholith this correspondence indicates that 1 the isotopic composition of the plutonic rocks and the coeval volcanic cover were similar 2 the volcanic front was denuded within a few million years and 3 the sediment was derived from the head of the drainage basin located at the migrating volcanic front

isotopic systematics of pb u zircon and 40ar 39ar biotite hornblende from rocks of the central foothills terrane sierra nevada california pb u zircon and 40 ar 39 ar biotite horn blende studies in conjunction with detailed structural and textural studies clarify the age and structural relations of a unique cluster of plutons in the central sierra nevada western metamorphic belt the plutons range from gabbro to tonalite trondhjemite and include the guadalupe igneous complex magmatic emplacement ages for the plutons are for the most part between 150 and 145 m y as recorded in both the zircon and hornblende isotopic systems additional ages of 138 and 123 m y were determined on small plutons having key structural settings in general the pb u and 40 ar 39 ar isotopic systems are well behaved second order complications are locally present in the pb u systems owing to the entrainment of older zircon from wall rocks and in 40 ar 39 ar systems owing to minor compositional differences in mica and amphibole phases the age data and structural relations of the plutons in conjunction with 40 ar 39 ar metamorphic wall rock biotite and hornblende age data constrain regional ductile deformation in the study area to have occurred between 150 and 125 ma such deformation was accompanied by upper greenschist to lower amphibolite facies metamorphism and is at least in part closely related to pluton emplacement such deformation and metamorphism has in the past been attributed to the late jurassic nevadan orogeny this study shows clearly however that ductile deformation and metamorphism in the study area are post nevadan if modern concepts of the nevadan orogeny are strictly adopted

refrigeration of the western cordilleran lithosphere during laramide shallow angle subduction the laramide orogeny has generally been attributed to a shift from normal angle to shallow angle subduction beneath the western margin of the north american plate in addition to important mechanical effects this shift may have had important thermal effects on the lithosphere in the western cordillera before the laramide geothermal gradients in the western cordillera were probably normal to high because of the presence below of a hot asthenospheric wedge with the shift to shallow subduction the wedge was probably expelled and replaced by a cold subducting slab that extended just below the cordilleran lithosphere this would shift the western cordillera into a cold forearc like thermal setting dominated by the refrigeration effects of the subducting slab thermochronologic data from the sierra nevada great basin mojave desert western arizona and perhaps the colorado plateau record latest cretaceous early tertiary cooling that may be evidence of such refrigeration if regional refrigeration occurred it would have several important implications 1 metamorphism in the western cordillera would have waned with the start of the laramide orogeny 2 the crust in the western cordillera would have strengthened and become much more resistant to deformation e g gravitational collapse and extension of thickened crust in the sevier hinterland would have been impeded and 3 latest cretaceous early tertiary isotopic cooling ages may not be valid indicators of a period of major regional uplift and unroofng

pace of landscape evolution in the sierra nevada california revealed by cosmogenic dating of cave sediments we report 26 al 10 be based ages of sierra nevada caves that constrain detailed late pliocene and quaternary river incision his tories for five river canyons rapid incision of 0 2 mm yr from 2 7 to ca 1 5 ma slowed markedly to 0 03 mm yr thereafter likely reflecting the combined effects of a transient erosional re sponse to pliocene rock uplift and periodic mantling of riverbeds with glacially derived sediment in the late quaternary while 400 m of incision has occurred in the past 2 7 m y outpacing interfluve erosion and thereby increasing the local relief canyons as deep as 1 6 km existed prior to that time these new erosion rates strength en the case for tectonically driven late cenozoic uplift

uplift of the sierra nevada of california gravity and most seismic interpretations agree that the sierra nevada is at present isostatically compensated by a crustal root the most reasonable timing for emplacement of the root was during mesozoic batholithic intrusions this is difficult to reconcile with the evidence that major uplift of the mountains occurred in the past 10 m y a simple quantitative model for flexural isostasy of an elastic plate before and after breaking resolves this problem and explains the tilt of the sierra nevada block and variations in topography along the range we postulate that cooling and elastic strengthening of the mesozoic magmatic arc prevented its reaching local isostatic equilibrium during erosion in the cenozoic thus an overcompensated residual mountain range was held down elastically until basin and range extension broke the elastic plate along the owens valley and allowed rapid uplift the moho density contrast that best models the amplitude of both late cenozoic uplift and prior topography is slightly larger than seismic and gravity models suggest this implies that up to 20 of the uplift may be driven by density contrasts in the upper mantle the idea of overcompensated erosional remnants and their release upon lithospheric faulting has a more general application to anorogenic uplift of mountain blocks

geomorphically driven late cenozoic rock uplift in the sierra nevada california geologists have long accepted that the sierra nevada california experienced significant late cenozoic tectonically induced uplift a flexural isostatic model presented here shows however that a large fraction of the primary evidence for uplift could be generated by the lithospheric response to coupled erosion of the sierra nevada and deposition in the adjacent central valley and therefore requires less tectonic forcing than previously believed the sum of range wide erosion and the resultant isostatic rock uplift would have lowered sierra mean elevation by 200 to 1000 meters since 10 million years ago and could also have increased summit elevations during the current period of relief production

the stratigraphic evolution of the el paso basin southern california implications for the miocene development of the garlock fault and uplift of the sierra nevada the ricardo group is a 1 700 m thick sequence of miocene volcanic rocks and continental sedimentary rocks deposited between 19 and 7 ma in the el paso basin near the junction of the garlock fault and the sierra nevada frontal fault in southern california the combination of stratigraphic and structural data from the ricardo group with chronologic and rotational histories derived from magnetostratigraphic and radiometric studies provides some new constraints for the morphotectonic emergence of the sierra nevada the initiation of strike slip movement on the garlock fault and related east west extension in the southern basin and range region the miocene sequence documents 1 volcanism and north south tension without net extension at 18 15 ma 2 relative uplift about 15 13 5 ma 3 the beginning of sinistral slip on the garlock fault and east west extension in the southwestern basin and range region about 10 9 ma and 4 emergence of the sierra nevada as a source area and topographic high by 8 ma stratigraphic evidence from the ricardo group does not indicate significant miocene dip slip movement along the large scale down to the north fault that has been previously hypothesized to have preceded the present garlock fault in the same location during the early tertiary the evolution of this sequence however can be explained by tectonic models which propose a time space linkage between basin evolution east west extension in southern california and the mendocino triple junction system

thin crust and active upper mantle beneath the southern sierra nevada in the western united states abstract analysis of seismic refraction wide angle reflection travel time and seismic amplitude data collected during the 1993 southern sierra nevada continental dynamics sscd project resolves a small crustal root 40 42 km thick centered 80 km west of the sierran topographic crest the sscd experiment consisted of a west east profile across the sierra nevada at approximately 36 5 n and a north south profile extending the length of owens valley located eastward of the sierran topographic crest two dimensional finite difference travel time inversion of p g and p n arrivals resolves upper crustal velocities of 6 0 to 6 4 km s 1 within the sierran batholith and basin and range an upper crustal west dipping wedge of higher velocities 6 8 7 2 km s 1 consistent with ophiolitic material underlying the great valley sedimentary sequence and higher velocities in the lower crust beneath the basin and range 6 8 7 0 km s 1 than those beneath the sierran batholith 6 6 km s 1 low average p n velocities 7 6 7 9 km s 1 and a laminated transitional moho imaged beneath the sierran batholith also differ from the higher p n velocities 7 9 8 0 km s 1 and sharp first order moho observed beneath the basin and range the crust decreases in thickness both westward of the root to 28 34 km beneath the great valley and eastward to 35 km beneath the highest sierran topography and decreases further to 27 30 km beneath the basin and range crustal thickness also appears to increase southeast to northwest from 29 30 km beneath the garlock fault in the south to 38 40 km beneath the north end of chalfant valley juxtaposition of the crustal model with previous p wave tomography models of the south ernmost sierra nevada upper mantle reveal that the thickest sierran crust on the west east profile overlies a pronounced upper mantle high velocity anomaly 5 whereas the region of laminated moho overlies a flanking upper mantle low velocity region 3 the upper mantle velocity anomalies relatively low p n and relatively flat moho observed beneath the sierran crest suggest that the recent uplift of the sierra nevada is due to asthenospheric flow and or lithospheric thinning beneath the southeastern sierra nevada and basin and range

collision orogen provenance western alps detrital signatures and unroofing trends abstract the alps are perhaps the best studied thrust belt formed during continent continent convergence collision orogen shallow intermediate and deep structural levels can be distinguished within the alpine thrust stack each shedding sediments with distinct petrographic and mineralogical signatures in modern first cycle sands carried by alpine rivers bulk composition rank of metamorphic lithic grains metamorphic index mi and dense mineral assemblages all faithfully mirror the tectono metamorphic history of exposed basement and cover units the shallow structural level widely preserved in the ligurian alps consists of remnant ocean turbidites shedding sedimentary to very low rank metasedimentary detritus documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcomman

petrology and provenance of cenozoic sand from the indus cone and the arabian basin dsdp sites 221 222 and 224 abstract the closing of the remnant ocean basin between india and asia has produced the uplifted gneissic sedimentary and metasedimentary terrane of the western himalayas sediment derived from this source is presently being deposited in the submarmefan and basin plain complex of the indus cone and arabian abyssal plain the location of dsdp sites 221 222 and 224 point counts of 15 cenozoic sand samples from these sites have mean qfl percentages of 43 30 27 the plagioclase total feldspar mean is 0 66 lithic types are dominated by quartz mica tectonite argillite shale polycrystalline carbonate and other sedimentary and metasedimentary varieties volcanic grains present are mainly felsic these sands resemble those of the bengal and nicobar fans which were also derived from the himalayas these plus other sands derived from collision orogenic belts have a distinctive signature p f close to 0 66 lv l low 0 10 or less and q dominant with f and l subequal

hydrogen isotopes in eocene river gravels and paleoelevation of the sierra nevada we determine paleoelevation of the sierra nevada california by tracking the effect of topography on precipitation as recorded in hydrogen isotopes of kaolinite exposed in gold bearing river deposits from the eocene yuba river the data compared with the modern isotopic composition of precipitation show that about 40 to 50 million years ago the sierra nevada stood tall 2200 meters a result in conflict with proposed young surface uplift by tectonic and climatic forcing but consistent with the sierra nevada representing the edge of a pre eocene continental plateau

cenozoic migration of topography in the north american cordillera continental topography is the result of complex interactions among mantle convection continental dynamics and climatic and erosional processes therefore topographic evolution of mountain belts and continental interiors reflects directly upon the coupling between mantle and surface processes it has recently been proposed that the modern topography of western north america is partly controlled by the removal of the subducting farallon plate and replacement of lithospheric mantle by hot asthenosphere creating surface uplift of the colorado plateau the southwestern united states and northern mexico while concomitant subsidence characterizes the central united states how the topography of the cenozoic north american cordillera evolved in the past is largely unknown yet currently debated tectonic models each have a predictable topographic response here we examine cenozoic surface uplift patterns of western north america based on a record of 3000 stable isotope proxy data this data set is consistent with eocene north to south surface uplift in the cordillera culminating in the assembly of an eocene oligocene highland 3 4 km in elevation the diachronous record of surface uplift and associated magmatism further supports tectonic models calling for the convective removal of mantle lithosphere or removal of the farallon slab by buckling along an east west axis the eocene oligocene development of rainout patterns similar to present day patterns along the flanks of the cordilleran orogen is therefore unlikely to be the result of late mesozoic crustal thickening and associated development of an andean style altiplano

the uplift of the sierra nevada and implications for late cenozoic epeirogeny in the western cordillera tilted cenozoic strata in the eastern great valley california record progressive late cenozoic uplift of the sierra nevada the magnitude of tilting is combined with other geologic data using a technique pioneered by grant and others to infer the rate and timing of post late miocene uplift between the kings river and the feather river angular unconformities of late neogene age have been reported throughout the east central great valley typically stratigraphic units of late oligocene through miocene pliocene age beneath the unconformities dip approximately 1 2 1 6 southwest middle to late pliocene units above the unconformities dip approximately 0 6 0 9 southwest and progressively overlap younger units from east to west these angular unconformities are interpreted to represent the onset of late cenozoic uplift of the sierra nevada due to westward tilting dated volcaniclastic units in the east central sacramento valley bracket the onset of tilting there between 3 4 and 8 4 ma stratigraphic and geologic relations from the stanislaus river area eastern san joa quin valley suggest that the late cenozoic tilting probably began approximately 5 ma previously published tilt data from the san joaquin valley combined with new work in the eastern sacramento valley indicate that the rate of post late miocene tilting has been approximately uniform in the east central great valley early to middle pleistocene units typically dip 0 5 0 7 southwest middle pleistocene units dip 0 2 0 4 middle to late pleistocene units dip 0 1 0 15 late pleistocene units dip 0 05 0 1 if it is assumed that major westward tilting began 5 ma these data indicate an approximately uniform tilting rate of 0 28 per million years these data have important implications for models of the uplift simultaneous inception of tilting in the east central great valley at approximately 5 ma does not support models that link uplift of the sierra nevada to the migration of the mendocino triple junction late cenozoic tilting of the sierra nevada block does not appear to coincide temporally with the onset of major basin and range extension the uplift may be due to thinning of the mantle lithosphere beneath the sierra nevada but the tilt data do not afford a direct test of this hypothesis post late miocene tilting of the sierra nevada is temporally associated with late cenozoic uplift of the northern basin and range uplift of the cascades uplift of the colorado plateau and uplift of the southern rocky mountains supporting the contention that tilting of the sierra nevada may have occurred as part of a cordillera wide uplift event

tropical glaciers potential for ice core paleoclimatic reconstructions the objective of this work is to evaluate the potential of tropical glaciers and ice caps for the reconstruction of past climatic conditions by the analysis of firn and ice cores samples were collected in pits and from cores retrieved on three snow fields in the south american andes quelccaya ice cap and huascaran in peru and chimborazo in ecuador these are compared with results from west africa and indonesia measurements include the microparticle concentration oxygen isotopic abundance ratios and total beta radioactivity which in polar regions often exhibit seasonal depositional cycles annual variations in these three parameters are fairly well defined on quelccaya and huascaran and are apparently absent on chimborazo the quelccaya ice cap contains the best preserved annual signal for all three parameters and although in some instances the annual cycle is difficult to distinguish the measurement of more than one stratigraphic parameter aids in the interpretation of the firn core records the relatively flat bottom topography under quelccaya suggests a simple ice flow regime and depth age calculations indicate that an ice core to bedrock should contain a record of 600 to 1300 years this inference was confirmed by drilling to bedrock in 1983 this investigation of tropical glaciers indicates that the quelccaya ice cap on the eastern edge of the peruvian andes has excellent potential to contain a lengthy and interpretable record of climatic conditions within the tropics of south america

the effects of source lithology transport deposition and sampling scale on the composition of southern california sand the transverse ranges of southern california represent an uplifted and variably dissected mesozoic magmatic arc and mesozoic to holocene sedimentary and volcanic strata deposited in convergent and transform tectonic settings modern sand within part of the western transverse ranges represents first order sampling scale of the santa monica and the san gabriel mountains second order sampling scale of the santa clara river draining both mountain ranges and third order sampling scale of the beach system between the mouth of the santa clara river and the eastern santa monica mountains and turbidite sand of the hueneme mugu submarine fan source lithology includes plutonic and metamorphic rocks of the san gabriel mountains and sedimentary and volcanic rocks of the santa monica mountains first order sands have large compositional variability sand from local coastal drainage of the santa monica mountains ranges from basaltic feldspatholithic to quartzofeldspathic sand of the san gabriel mountains local drainages has three distinct petrofacies ranging from metamorphiclastic feldspatholithic to mixed metamorphi plutoniclastic and plutoniclastic quartzofeldspathic second order sand is represented by the main channel of the santa clara river the sand has an abrupt downstream compositional change from feldspathic to quartzofeldspathic third order sand beaches and deep sea turbidite samples of the santa monica basin is quartzofeldspathic beach sand is more quartz rich than is santa clara river sand whereas turbidite sand is more feldspar rich than is beach sand deep sea sand has intermediate composition with respect to second order samples of the santa clara river and third order samples of the beach system suggesting that 1 the santa clara river is the main source of sediments to the marine environment and 2 local entry points from canyons located near local drainages may generate turbidity currents during exceptional flood conditions petrologic data of modern sand of the study area are highly variable at first and second order scale whereas third order sand is homogenized the homogenized composition of deep marine sand is similar to the composition of most ancient sandstone derived primarily from the mesozoic dissected magmatic arc of southern california this study of the western transverse ranges illustrates the effects of source lithology transport depositional environment and sampling scale on sand composition of a complex system which provides insights regarding actualistic petrofacies models

quantitative analysis of detrital modes statistically rigorous confidence regions in ternary diagrams and their use in sedimentary petrology abstract detrital modes are the basic type of quantitative data used in sedimentary petrology they represent estimates of the petrographic framework composition of sand or sandstone obtained by point counting of thin sections at present geological inferences from analysis of these numerical data are semi quantitative only because many of the data processing methods employed in sedimentary petrology lack a firm theoretical basis a review of existing methods reveals a number of fundamental statistical problems associated with the use of univariate statistics and the construction of so called hexagonal fields of compositional variation it is shown that these problems can be overcome by using multivariate methods that honour the non negativity and unit sum constraints on compositional data and incorporate the covariance structure spread in compositional data can be modelled on two levels 1 by regarding the grain as unit of observation and the total point count as a sample from a multinomial distribution 2 by regarding the total count as unit of observation and a set of counts as a sample from an additive logistic normal distribution numerical methods are presented for constructing statistically rigorous confidence regions in ternary diagrams by inversion of significance tests for these two distributions statistical analyses of the framework composition and heavy mineral assemblages of modern beach and river sands illustrate their use the case histories are followed by a brief overview of popular multivariate methods for reconstructing sediment provenance

petrofacies and provenance of late mesozoic forearc basin northern and central california data from the great valley group sequence represent the most complete information regarding sandstone petrology of sediment derived from a magmatic arc this information is useful in documenting tectonic and magmatic events within the arc and related terranes and forms the basis for the establishment of petrostratigraphic units for mapping and correlation sandstone and conglomerate compositions are controlled by changes in provenance many of which were basinwide and synchronous clay mineral composition is controlled primarily by burial metamorphism careful attention to sample collection sample preparation and petrographic techniques is essential for uniform results seven petrographic parameters p f lv l m qp q q f and l listed in decreasing importance to petrofacies discrimination define eight petrofacies stony creek platina lodoga grabast boxer cortina los gatos and rumsey listed in approximate order of decreasing age the upper jurassic lower cretaceous petrofacies stony creek platina and lodoga contain higher lithic contents supracrustal sources whereas the upper cretaceous petrofacies especially the rumsey contain higher proportions of plutoniclastic components quartz feldspar and micas the proportion of potassium feldspar increases from near zero in the upper jurassic to nearly 50 of all feldspars in the uppermost cretaceous the lower part of the great valley group upper jurassic and lower cretaceous contains significant quantities of sedimentaclastic and metamorphiclastic material eroded from accreted and deformed terranes tectonic highlands formed by the arc arc collision nevadan orogeny that occurred prior to initiation of the franciscan great valley sierra nevada arc trench system the klamath mountains area provided a major proportion of this detritus ophiolite and serpentinite detritus was deposited locally near the base of the great valley group as a result of deformation along the east side of the growing franciscan subduction complex volcaniclastic detritus was fed into the entire forearc basin as magmatism increased in the sierra nevada area during the cretaceous as the volcanic cover was stripped off plutoniclastic and metamorphiclastic detritus from the underlying batholithic terranes was provided in abundance to the forearc basin crustal components were more continental in the southern sierra nevada and more oceanic in the northern sierra nevada as demonstrated by the higher proportions of metamorphiclastic detritus and by the more felsic nature of volcaniclastic detritus to the south by the middle of the late cretaceous extensive batholithic terranes provided potassium feldspar rich arkosic detritus to the entire forearc basin by the paleogene arc magmatism had migrated eastward sufficiently that deeper levels of the california part of the arc were exposed by erosion tectonic activity decreased in the forearc basin and the basin was filled to sea leve in most parts

ophiolitic basement to a forearc basin and implications for continental growth the coast range great valley ophiolite california we present a compilation of 18 published models from the length of the great valley forearc basin california based on seismic reflection borehole seismic refraction gravity and aeromagnetic data to address long standing questions about the nature of the basement to the great valley its origin its tectonic history and it s mechanism of incorporation into the north american continental margin the geophysical models permit a 700 km long 70 km wide complete ophiolite sequence beneath the entire great valley in the northern great valley the ophiolite is overlain by ophiolitic breccia the ophiolite crust is 7 8 km thick and the mantle section is mostly unserpentinized beneath the southern great valley there is no ophiolitic breccia the crust may be up to 10 12 km thick and the mantle section if present at all is serpentinized to such a degree that it cannot be distinguished from sierran basement or mafic ophiolite members on the basis of velocity or density data geochemical petrological and paleomagnetic data support suprasubduction zone ophiolite formation at north american paleolatitudes and geological data and geophysical models are consistent with ophiolite formation by back arc spreading behind an east facing arc in the north this was apparently followed by obduction of back arc crust onto older continental basement during the late jurassic nevadan orogeny in the south the newly formed intraoceanic arc and back arc apparently collided with the continental margin during the nevadan orogeny but were not obducted onto it instead the arc and back arc docked with the continental margin leaving the arc itself to become the basement to the great valley basin cretaceous sierran magmatism then intruded plutons beneath the docked ophiolite and mafic arc irrespective of the detailed accretionary history our cross sections show a rapid pulse of continental growth by ophiolite accretion of more than 500 km km 1 in less than 10 myr

petrofacies and provenance of the great valley group southern klamath mountains and northern sacramento valley lower cretaceous sandstone and conglomerate of the great valley group rest depositionally on the southern klamath mountains at the north end of the sacramento valley exposures include nonmarine shallow marine and deep marine sediments which are part of the platina formation petrofacies approximately 150 sandstone point counts 500 points per section using gazzi dickinson method and 32 conglomerate clast counts indicate a klamath provenance of mixed magmatic arc and recycled orogenic character standard triangular plots and multivariate analyses combined with paleocurrent and paleogeographic data suggest discrete source areas within the klamath terrane however the general homogeneity of both sandstone and conglomerate petrology is consistent with all of the units being included within the platina petrofacies

state of stress and modern deformation of the northern basin and range province constraints on the current stress regime of the actively extending northern basin and range province are provided by deformation data focal mechanisms and fault slip studies hydraulic fracturing in situ stress measurements borehole elongation breakouts analyses and alignment of young volcanic vents the integrated data indicate significant variations both in principal stress orientations and magnitudes an approximately e w least principal stress direction appears to characterize both the eastern and western margins of the basin and range province whereas in the active interior parts of the province extension occurs in response to a least principal stress oriented nw to n60 w the contrast in stress orientations between the province boundaries and in the interior suggests that along the margins the least principal stress direction may be locally controlled by the generally northerly trending profound lithospheric discontinuities associated with these margins active deformation along the southeastern and western province margins is characterized by a combination of strike slip and normal faulting focal mechanisms along northeastern province margin wasatch front and in central nevada indicate a combination of normal and oblique normal faulting temporal regional and depth dependent variations in the relative magnitudes of the vertical and maximum horizontal stresses can explain much of the observed variations in deformation styles however some depth variation in faulting style inferred from focal mechanisms may be apparent and simply a function of the attitude of fault planes being reactivated evidence for significant temporal variation or multiple cycles of variation in relative stress magnitude comes from the sierran front basin and range boundary region where recent earthquakes are predominantly strike slip whereas the profound relative vertical relief across the sierra frontal fault zone in the last 9 10 m y implies a normal faulting stress regime using the best data on stress orientation relative stress magnitudes are constrained from slip vectors of major earthquakes and young fault displacements analysis of well constrained slip vectors in the owens valley california area indicate that large temporal variations in the magnitude of the approximately n s oriented maximum horizontal stress are required to explain dominantly dip slip and strike slip offsets on subparallel faults similar faulting relations are observed throughout much of the boundary zone between the basin and range sierra nevada including the walker lane belt along the eastern province margin in the wasatch front area in utah available data suggest that the maximum and minimum horizontal stresses may be approximately equal at depths of 4 5 km earthquake focal mechanisms in this area suggest more variability in relative magnitude of the two horizontal stresses with depth furthermore superimposed sets of young fault striae along a segment of the wasatch fault also indicate temporal variations of relative stress magnitudes sources of regional and temporal variations in the stress field may be linked to variable shear tractions applied to the base of the brittle crust related to intrusion thermally induced flow and the influence of the san andreas plate boundary although difficult to date accurately the fault slip data suggest that the temporal variations in relative magnitudes stress may occur on the time scale of both a single major earthquake cycle 1000 5000 years and multiple earthquake cycles 10 000 years

origin of conglomerate stratigraphy in the franciscan assemblage and great valley sequence northern california in the northern california coast ranges the forearc basin great valley sequence of late jurassic and cretaceous age contains three distinctive types of conglomerate 1 chert rich conglomerate of tithonian to valanginian age 2 volcanic rich conglomerate of mainly valanginian age and 3 mixed clast conglomerate of valanginian and younger age all three types are found in stratigraphic sequence along the west side of the sacramento valley but in three belts of outliers that overlie the franciscan assemblage to the west the oldest conglomerate present in each belt is progressively younger southwestward this may reflect the widening of the forearc basin over time as progressively younger conglomerate types lapped onto the elevated western margin of the basin the same three conglomerate types occur in coeval rocks of the accretionarywedge franciscan assemblage and show a similar southwestward progression in composition possibly due to trench deposition of conglomerate with the same sources as the great valley sequence each conglomerate type being accreted successively to the west side of the accretionary wedge the franciscan must have underthrust the forearc basin become deeply buried been metamorphosed and returned to the surface without major disruption of the distribution of conglomerate types

uranium lead isotopic ages from the sierra nevada batholith california this study provides new information on the timing and distribution of mesozoic magmatic events in the sierra nevada batholithic complex chiefly between 36 and 37 n latitude u pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks granitoid rocks in this area range in age from 217 to 80 m y triassic intrusions are restricted to the east side of the batholith jurassic plutons occur south of the triassic plutons east of the sierra nevada as isolated masses within the cretaceous batholith and in the western foothills of the range cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the sierra nevada no granitic intrusions were emplaced for 37 m y east of the sierra nevada following the end of jurassic plutonism however following emplacement of the eastern jurassic granitoids regional extension produced a fracture system at least 350 km long into which the dominantly mafic calc alkalic independence dike swarm was intruded 148 m y ago the dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the nevadan orogeny intrusion of cretaceous granitic plutons began in large volume about 120 m y ago in the western sierra nevada and migrated steadily eastward for 40 m y at a rate of 2 7 mm y this slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone such steady conditions could be necessary for the production of large batholithic complexes such as the sierra nevada the abrupt termination of plutonism 80 m y ago may have resulted from an increased rate of convergence of the american and eastern pacific plates and dramatic flattening of the subduction zone u pb ages of the giant forest alaskite sequence in sequoia national park are all in the range 99 3 m y indicating a relatively short period of emplacement and cooling for this nested group of plutons u pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma comparison of isotopic ages determined by different methods such as zircon u pb sphene u pb hornblende k ar and biotite k ar suggests that zircon u pb ages generally approximate the emplacement age of a pluton however some plutons probably contain inherited or entrained old zircons and the zircons of some samples are disturbed by younger thermal and metamorphic events the ages reported here are consistent with u pb age determinations previously made on granitic rocks to the north stern et al 1981 the age distribution of granitic belts determined here is in general agreement with those established by k ar dating evernden and kistler 1970 but does not differentiate the five epochs of plutonism determined in their study

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

sourceland controls on the composition of beach and fluvial sand of the northern tyrrhenian coast of calabria italy implications for actualistic petrofacies abstract the northern tyrrhenian margin of calabria in southern italy provides a natural laboratory for sampling sand at different scales small drainages first order rivers draining mountain ranges second order and marine environments beach to deep marine third order calabrian mountain ranges represent an uplifted and variable dissected thrust belt constituted by palaeozoic through pleistocene plutonic metamorphic ophiolitic carbonate and siliciclastic rocks the modern setting includes a mountainous coast having high rates of fluvial discharge and the deep marine paola basin the composition of modern fluvial and beach sands is useful for the interpretation of sediment transported into deeper water environments modern beach and fluvial sands of the northern tyrrhenian margin of calabria define three distinct petrologic provinces from north to the south 1 the lao littoral province has calclithite sand derived from erosion of dominantly carbonates of the southern apennines 2 the coastal range littoral province has quartzolithic sand derived from dominantly metamorphic schist and phyllite coastal range and 3 the santa eufemia littoral province has quarzofeldspathic sand derived from dominantly metamorphic coastal range and sila massif and plutono metamorphic mount poro provenances deep marine turbidites of the paola basin have basinwide quartzolithic turbidite sands having close compositional relations with coastal range littoral petrofacies only at the northern boundary of the paola basin calclithite turbidite sand record deep water dispersal of the lao littoral sands comparison of detrital modes from mainland to deep marine environments contribute to the models of dispersal pathways and geographical extension of actualistic sand petrofacies

triple junction instability as cause for late cenozoic extension and fragmentation of the western united states regional extension in the western united states began soon after 30 m y b p the time of first interaction between the pacific and north american plates previous explanations for extension include lithospheric active processes and asthenospheric active processes the former are favored because of coincidence of timing with lithospheric interactions relative timing of volcanism and deformation and coincidence with pre existing structures although extension has occurred in part in a back arc setting it cannot be attributed to age of subducted crust or azimuth of subduction because both attributes favor back arc contraction characteristic behavior before 40 m y b p the mendocino triple junction has been unstable since its inception the continental margin was relatively straight before 30 m y b p and it has become more convex westward as the triple junction has migrated northward the continental margin and arc have been anchored to the subducting slab farallon juan de fuca whereas the triple junction must move parallel to the san andreas transform the combined result has been the northwestward and clockwise movement of coastal blocks relative to the continental interior and the eastward stepping of the san andreas transform relative to the coast these effects result from the unstable geometry of the mendocino triple junction lack of a subducted slab slab window beneath the extended lithosphere enables asthenospheric rise but does not cause extension

tectonic control on southern sierra nevada topography california in this study we integrate the apatite u th he thermochronometric technique with geomorphic structural and stratigraphic studies to pursue the origin and evolution of topographic relief related to extensive late cenozoic faulting in the southern sierra nevada the geomorphology of this region reflects a transition from a vast region to the north characterized by nonequilibrium fluvial modification of a relict low relief landscape little affected by internal deformation to a more complex landscape affected by numerous faults regionally the relict landscape surface is readily resolved by age elevation relationships of apatite he ages coupled to geomorphology these relationships can be extended into the study area and used as a structural datum for the resolution of fault offsets and related tilting on the basis of 63 new apatite he ages and stratigraphic data from proximal parts of the san joaquin basin we resolve two sets of normal faults oriented approximately n s and approximately nw quaternary west side up normal faulting along the n s breckenridge kern canyon zone has resulted in a southwest step over from the owens valley system in the controlling structure on the regional west tilt of sierran basement this zone has also served as a transfer structure partitioning neogene quaternary extension resulting from normal displacements on the nw fault set this fault system for the most part nucleated along late cretaceous structures with late cenozoic remobilization representing passive extension by oblate flattening as the region rose and stretched in response to the passage of a slab window and the ensuing delamination of the mantle lithosphere from beneath the region

the continental crust as a source of sand southern alps cross section northern italy abstract all tectonostratigraphic levels of the continental crust from carbonate platforms in the dolomites to granulites in the ivrea zone are exposed along strike in the world famous southern alps cross section in this extraordinary natural laboratory we studied the composition of modern stream sands shed by each distinct type of source rock in order to improve on existing models of sediment provenance and specifically to reconstruct the step by step changes of detrital signatures ideally produced during unroofing of a continental block detritus from progressively deeper crustal levels of the southern alps changes systematically from lithic sedimentaclastic unmetamorphosed cover sequences to lithoquartzose quartzolithic and quartzofeldspathic metamorphiclastic greenschist facies to amphibolite facies basement units and finally to feldspathoquartzose and feldspathic composition granulite facies basement units six progressively richer types of heavy mineral assemblages ztr brookite garnet

was the laramide orogeny related to subduction of an oceanic plateau numerous models have been presented to explain the late cretaceous early cenozoic laramide orogeny which affected the foreland region of the western cordillera within the us1 the most attractive models invoke low angle subduction2 4 which can develop for various reasons5 6 evidence from south america indicates that one such reason may be the subduction of buoyant ocean floor we here extend the low angle subduction models by attributing the shallowing of the subduction angle during the laramide orogeny to subduction of a large oceanic plateau of anomalously thick and buoyant oceanic crust pacific plate history indicates that a twin to the hess rise may represent the oceanic plateau that triggered laramide events

current sierra nevada north america motion from very long baseline interferometry implications for the kinematics of the western united states we use geodetic measurements from very long baseline interferometry to estimate the motion of the sierra nevadan microplate which is composed of the sierra nevada and the great valley the motion of the sierra nevadan microplate relative to the north american plate is described by a right handed rotation of 0 61 m y about lat 32 n long 128 w this euler pole lies only 10 southwest of the sierra nevadan microplate and predicts a significant counterclockwise rotation about a local vertical axis it further predicts a velocity of the eastern edge of the sierra nevada at 38 0 n 119 3 w relative to stable north america of 11 1 mm yr toward n36 3 w quoted uncertainties are plus or minus one standard error which accounts for about one fourth of the velocity between the pacific and north american plates and is 25 clockwise of many prior estimates the velocity nearly parallels the boundary between the sierra nevada and the great basin which implies that current motion within the great basin results in a rotational noncoaxial deformation we use this velocity to estimate how motion is distributed across the broad deforming zone taking up pacific north america plate motion we find that the vector sum of strike slip along the san andreas fault and motion of the sierra nevada relative to stable north america taken up by deformation within the great basin differs little from the pacific north america plate velocity the difference can be described at 36 n along the san andreas fault by a vector of 6 mm yr directed toward n20 w this vector resolves into components of 5 mm yr parallel to the fault and 2 mm yr perpendicular to the fault with 95 confidence intervals of 0 to 10 mm yr and 1 to 5 mm yr respectively the component perpendicular to the fault is several times smaller than found in prior studies and places a small upper bound on fault perpendicular shortening we conclude that motion previously inferred to be taken up by deformation other than strike slip along the san andreas fault or deformation within the great basin is much smaller than previously thought

guadalupe pluton mariposa formation age relationships in the southern sierran foothills onset of mesozoic subduction in northern california we report a new 153 2 ma sims u pb date for zircons from the hypabyssal guadalupe pluton which crosscuts and contact metamorphoses upper crustal mariposa slates in the southern sierra a 950 m thick section of dark metashales lies below sandstones from which clastic zircons were analyzed at 152 2 ma assuming a compacted depositional rate of 120 m myr accumulation of mariposa volcanogenic sediments which overlie previously stranded middle jurassic and older ophiolite chert argillite belts in the sierran foothills began no later than 160 ma correlative oxfordian kimmeridgian strata of the galice formation occupy a similar position in the klamath mountains we speculate that the late jurassic was a time of transition from 1 a mid paleozoic middle jurassic interval of mainly but not exclusively strike slip and episodic docking of oceanic terranes 2 to transpressive plate underflow producing calcalkaline igneous arc rocks outboard blueschists at 170 150 ma whose erosion promoted accumulation of the mariposa galice overlap strata 3 continued transpressive underflow attending 200 km left lateral displacement of the klamath salient relative to the sierran arc at 150 140 ma and development of the apparent polar wander path cusps for north and south america and 4 then nearly orthogonal mid and late cretaceous convergence commencing at 125 120 ma during reversal in tangential motion of the pacific plate after 120 ma nearly head on subduction involving minor dextral transpression gave rise to voluminous continent building juvenile and recycled magmas of the sierran arc providing the erosional debris to the great valley fore arc and franciscan trench

composition of modern stream sand derived from a mixture of sedimentary and metamorphic source rocks henares river central spain the henares river central spain flows westward from the iberian range mesozoic sedimentary rocks under semiarid climatic conditions in the middle and lower reaches the henares river receives sediment from three tributaries canamares bornova and sorbe rivers that drain the central system paleozoic crystalline rocks low grade metamorphic rocks and minor amounts of mesozoic sedimentary rocks modern sands from the henares river and its tributaries offer an excellent opportunity to evaluate the importance of lithology and physiography in determining detrital modes from mixed metamorphic and sedimentary source terrain sand modes from the henares river and its tributaries are quartzolithic they plot in a restricted area on a qmflt diagram with low contents of feldspar and variable amounts of quartz and carbonate metamorphic and minor siliciclastic lithic fragments higher feldspar abundances in the canamares and bornova stream sands are related to the erosion of gneissic rocks by contrast sands from sorbe river are low in feldspar reflecting the absence of coarse metamorphic sources thus proportions of bedrock lithologies in the drainage sub basins are the main control on detrital modes of the tributaries in addition slope acts on the sand productivity of source rocks sedimentary source rocks in the upper reaches of the three tributaries have a poor productivity in the carbonate lithic fragments lsc compared with the lsc productivity of sedimentary sources located downstream with higher relief the percentage of bedrock types in the source area versus petrographic indices lm l lss l and lsc l provide a useful contrast between source and sand composition in the tributaries thus the high content of metamorphic lithic grains in the lithic grain population over represents this lithology slate plus schist at the source terrain lithic grains from sedimentary clastic sources generate few recognizable grains lss in the lithic grain population local supplies from carbonate sources may produce important increases of lsc in short reaches of the channels sand compositions in the henares river seem to be very homogeneous in a qmflt diagram with only minor differences caused by the supplies from the tributaries these differences can be modeled in terms of end member mixing processes between henares detritus and detritus from the relevant tributary these mixing processes are more evident when lithic grain contents lmlvls and lmlsslsc diagrams are compared changes produced by inputs from the canamares bornova and sorbe rivers are not permanent along the henares course an important homogenization of henares sand composition takes place by the mixing of tributary deposits with previously deposited henares river terraces abrasion during transport does not appear to cause significant changes in the sand composition along the henares river

detrital modes in sedimenticlastic sands from low order streams in the iberian range spain the potential for sand generation by different sedimentary rocks the composition of modern stream sands derived from sedimentary source rocks in the iberian range has been analyzed in order to evaluate the contributions of the different bedrock types mainly sandstones limestones and dolostones temperate to subhumid climate and short transport conditions promote a weathering limited denudation regime as expected sand composition proved to be essentially quartzolithic with variable amounts of penecontemporaneous carbonates sand compositional data were compared with the exposure areas of the different bedrocks in the drainage sub basins considered for semi quantitative assessment of the sand generation potential of each bedrock type siliciclastic formations sandstones appear to be by far the most significant sand producers with sand generation indices sgis in the medium sand fraction ranging from 4 to 20 i e 4 to 20 times greater than the sgi of carbonate rocks composition and texture are the main factors controlling carbonate sand generation sparitic limestones yield higher sgis 2 8 to 20 when source terrains are constituted exclusively by carbonate rocks high sparite grain content in the sands is enhanced by supplies from additional sources such as calcitized dolostones dolomicrite sources are strongly under represented in the sands analyzed very low sgi whereas the proportion of micritic limestone grains tends to be an accurate reflection of that bedrock at the source even though the results presented here refer to the first stage of sand generation with negligible transport effects we think they may be helpful in the analysis and reconstruction of source terrains in ancient sedimenticlastic deposits

stream incision tectonics uplift and evolution of topography of the sierra nevada california abstract stream incision faulting thermochronologic and geobarometric data suggest that sierra nevada topography is a consequence of two periods of uplift stream incision of up to 1 km has occurred since 5 ma maximum eocene miocene incision was 150 m uplift of the sierra nevada westward tilting stream incision and east down normal and dextral faulting along the present eastern escarpment of the range began at 5 ma estimates of late cenozoic crestal rock uplift for different areas in the sierra nevada range from 1440 to 2150 m low summit erosion rates suggest that the rock uplift approximates the surface uplift of crestal summits tertiary stream gradients were lower than modern ones suggesting that the bottoms of the canyons have been uplifted in the late cenozoic and that the mean elevation of the sierra nevada has increased the elevation of pre cenozoic basement rocks above the base of tertiary paleochannels ranges from 1000 m in the south and

paleogeographic and paleotectonic evolution of the himalayan range as reflected by detrital modes of tertiary sandstones and modern sands indus transect india and pakistan detrital modes of sandstones derived from the himalayan suture belt record the history of the mountain range since initial collision between india and asia which began in latest paleocene time tertiary clastic wedges deposited in fore arc foreland and remnant ocean basins and exposed along the indus transect from northernmost india to the arabian sea represent the best opportunity to study sedimentary responses to successive tectonic events during continental collision quartzose continental block and feldspatholithic magmatic arc sandstones were deposited respectively on the passive indian tethys himalayan succession and active asian indus group continental margins during late cretaceous paleocene time closure of the neotethys was marked by sudden arrival of volcanic and ophiolitic detritus on the passive continental margin of the indian plate during deposition of sediments dated at foraminiferal zones p6 pakistan to p8 india starting in early eocene time deposition of chulung la formation and murree supergroup volcanic and ophiolitic to metasedimentary detritus was accumulated in rapidly subsiding piggy back and foreland basins homogeneous petrographic composition within the eocene lower miocene murree supergroup with only slight progressive increase of detrital feldspars suggests erosion of largely supracrustal rocks involved in thrusting in the north in middle miocene time marked enrichment in medium to high grade metamorphic detritus in foreland sandstones siwalik group reflects rapid uplift of a warm wedge of indian crust which was carried southward along the main central thrust this major paleogeographic change was recorded also by quartzolithic remnant ocean turbidites which were fed great distances along transverse fracture zones and later accreted in the coastal makran subduction complex panjgur association and makran group recycled orogen detritus derived from the elevated himalayan chain is still accumulating today in the indus fan enrichment in feldspars with respect to ancient sandstones reflects deep erosion levels into mid crustal rocks along the core of the growing orogen

cenozoic tectonic and topographic evolution of the northern sierra nevada california through stable isotope paleoaltimetry in volcanic glass we determine the paleoelevation of the northern sierra nevada california in the oligocene based on hydrogen stable isotope compositions of meteoric water preserved within volcanic glass from ignimbrites sampled across the range a 48 decrease in the isotopic composition of hydrated glass from ignimbrites located near paleosea level to ignimbrites 100 km to the east refl ects the effect of ancient high topography on precipitation these data show that 31 28 ma ago the northern sierra nevada had a steep western gradient and elevations similar to the present this study placed in the context of other paleoaltimetry studies suggests that the range was a high topographic feature throughout the cenozoic and that the majority of uplift occurred in the late cretaceous to early cenozoic much earlier than some studies have proposed

paleoconvergence rates determined from k2o sio2 ratios in magmatic rocks and their application to cretaceous and tertiary tectonic patterns in southwestern north america spatial temporal and chemical data may be used to calculate paleodip angles for paleosubduction zones assuming the position of the paleotrench is reasonably well known in turn paleoconvergence rates may be calculated according to the equation developed by luyendyk 1970 that relates dip angle to convergence rate data from cretaceous tertiary magmatic rocks indicate paleoconvergence rates of 7 to 8 cm yr for the period 130 to 85 m y ago convergence rates markedly increased to an average of about 14 cm yr for the period 85 to 45 m y decreased convergence rates are indicated from 45 to 20 m y paleoconvergence rates determined from magmatic rocks are generally in agreement with those determined independently by other workers from paleomagnetic data on ocean floor rocks the anomalously high rates of convergence determined from the igneous rock data may be due to subduction of younger more buoyant oceanc crust after 65 m y b p although recent observations from independent data suggest that eocene convergence may have been a high 20 to 26 cm yr paleomagnetically the period of constant convergence constant dip subduction from 130 to 85 m y ago correlates almost exactly with the long cretaceous normal polarity interval as suggested on the new lowrie alvarez 1981 geomagnetic time scale in contrast the post 85 m y period of variable convergence and variable dip subduction correlates with polarity events of short duration and frequent reversal tectonically the period of constant convergence rates from 130 to 85 m y ago correlates with sevier orogeny in nevada and utah the period of increased convergence from 85 to 45 m y ago correlates with classic laramide orogeny in wyoming and colorado the period of decreased convergence from 45 to about 20 m y ago correlates with a poorly understood and incompletely defined mid tertiary orogenic event

petrology of holocene sand peninsular ranges california and baja norte mexico implications for provenance discrimination models abstract the peninsular ranges of southern california and baja california norte mexico represent an uplifted and variably dissected mesozoic magmatic arc they are composed mostly of cretaceous plutonic rocks and mesozoic greenschist to amphibolite facies metasedimentary and metavolcanic rocks in order to evaluate the reliability of the diamond shaped qnuquqp provenance discrimination diagram developed by basu et al 1975 and the qfl provenance discrimination diagram developed by dickinson et al 1983 we collected 62 samples of holocene sand from four drainage basins along the west side of the peninsular ranges the qfl composition of each specimen was determined by the gazzi dickinson point counting method the relative percentages of med um grained fragments of qnu qu and qp were determined by counting 100 grains in each specimen the results of our study indicate that holocene sands derived from the peninsular ranges have the following average petrological characteristics 1 sands derived mostly or exclusively from plutonic rocks are composed of 16 46 percent q 54 83 percent f and 0 2 0 5 percent l the medium grained quartz population produced by erosion of mostly or exclusively plutonic rocks consists of 61 77 percent qnu 4 17 percent qu and 13 27 percent qp 2 sands originating from the erosion of metasedimentary rocks are composed of 63 percent q 13 percent f and 24 percent l the medium grained quartz population derived from a metasedimentary source is made up of 25 percent qnu 9 percent qu and 66 percent qp 3 sands produced by the erosion of mostly or exclusively metavolcanic rocks are composed of 19 percent q 39 44 percent f and 38 43 percent l the medium grained quartz population originating from a mostly or exclusively metavolcanic source consists of 48 74 percent qnu 6 percent qu and 20 46 percent qp petrological data obtained during this study indicate that the qfl provenance discrimination model generally yields a correct interpretation of tectonic setting when applied to holocene sand from the peninsular ranges data however also show that local source areas produce sands that when interpreted on the basis of their position on the qfl provenance discrimination diagram yield an erroneous interpretation of tectonic setting data obtained from counting varieties of medium grained quartz show that detrital quartz populations produced by plutonic rocks are discriminated from those derived from metasedimentary rocks by the qnuquqp provenance discrimination diagram in contrast quartz populations originating from a mostly or exclusively metavolcanic source are difficult to interpret on the qnuquqp diagram therefore we recommend that the qfl provenance discrimination model be used with caution when sand or sandstone samples are from a limited geographic area and that the qnuquqp provenance discrimination diagram be used with prudence when metavolcanics are suspected in the provenance

carson pass kirkwood paleocanyon system paleogeography of the ancestral cascades arc and implications for landscape evolution of the sierra nevada california tertiary strata of the central sierra nevada are dominated by widespread voluminous volcanic breccias that are largely undivided and undated the origin of which is poorly understood these dominantly andesitic strata are interpreted to be eruptive products of the ancestral cascades arc deposited and preserved within paleocanyons that crossed the present day sierra nevada before basin and range faulting began there these strata are thus important not only for understanding the paleogeography of the ancestral cascades arc but also for reconstructing the evolution of the sierra nevada landscape our regional scale mapping shows that paleocanyon fills of the central sierra nevada are dominated by intrusions and vent proximal facies along the present day sierran crest with more distal facies extending westward down the paleocanyons vent proximal facies consist of lava domes that collapsed to generate block and ash flow tuffs which in turn were remobilized down canyon to produce coarse grained volcanic mudflow and dilute flow fluvial deposits lava flows are rare instead magmas invaded wet volcaniclastic sediment to form in situ peperite piles that were partly remobilized to form debris flows or block and ash flows with peperite domains detailed mapping and dating of previously undifferentiated tertiary strata in the carson pass kirkwood area of the central sierra nevada has allowed us to identify 6 unconformity bounded sequences preserved within a paleocanyon as deep as 650 m cut into mesozoic granitic basement the carson pass kirkwood paleocanyon trends ne sw with a paleo transport direction roughly parallel to the modern mokelumne river drainage toward the sw sequence 1 consists of oligocene silicic ignimbrites sourced from nevada sequences 2 through 6 consist of dominantly andesitic rocks of the miocene ancestral cascades arc with 40 ar 39 ar ages ranging from 14 69 0 06 ma to 6 05 0 12 ma these new dates provide constraints on the ages of unconformities vertical relief on the unconformities within the paleocanyon ranges from 12 to 303 meters with paleoslope gradients range from 3 to 48 no evidence exists for widening or deepening of the paleocanyon into mesozoic basement during tertiary time so we infer that unconformity 1 the paleocanyon floor and walls was inherited from cretaceous time the deepest unconformities within the paleocanyon fill reincised into granitic basement in the early miocene unconformity 2 between 14 and 10 ma unconformity 5 and between 10 and 6 ma unconformity 6 the paleocanyon was also beheaded cut off from sources to the east by ca 10 ma we suggest that the early miocene reincision records tectonism related to the onset of arc magmatism in the sierra nevada the middle miocene reincision may record the onset of basin and range faulting in the central sierra the late miocene reincision may correspond to uplift attendant with the northward sweep of the triple junction through the latitude of the central sierra paleocanyons of the central sierra differ from those of the northern and southern sierra by showing steeper local paleorelief and the bouldery stream deposits attest to higher axial gradients than envisioned for other parts of the range this may indicate that the uplift history of the range is not uniform from segment to segment

actualistic ophiolite provenance the cyprus case abstract the island of cyprus represents an excellent site to assess quantitatively petrologic clastic response to actively obducting oceanic sources in order to define an actualistic reference for ophiolite provenance in terms of framework composition and heavy mineral suites an improved methodology an extension of the classic ternary qfl logic to include a wider spectrum of key indexes and ratios provides an accurate synthesis of modal data and allows differentiation of three main petrographic provinces and at least seven subprovinces diagnostic signatures of detritus from various levels of an oceanic lithospheric source and criteria for distinguishing provenance from suprasubduction versus mid oceanic ophiolites are also outlined modern sands derived from the troodos ophiolite contain variable proportions of largely pelagic carbonate to chert boninite to basalt diabase to metabasite plagiogranite to gabbroic and cumulate grains supplied from progressively deeper seated levels of the multilay

speculative tectonic model for the early mesozoic arc of the southwest cordilleran united states the early mesozoic continental arc of arizona california and western nevada is here interpreted to have occupied a continuous graben depression more than 1000 km long similar to the modern extensional are of central america high calculated rates of subsidence the presence of syndepositional normal faults and fault talus breccias the abundance of calderas and high level intrusions and interstratification of cale alkalic and alkalic volcanic rocks provide support for this model the arc graben depression acted as a long lived more than 40 m y trap for craton derived quartz sands which were funneled along the length and across the width of the depression

deuterium and oxygen isotopes paleoelevations of the sierra nevada and cenozoic climate although geomorphic observations suggest that the sierra nevada has tilted so that the crest has risen 1 2 km since late miocene time deuterium and oxygen 18 isotope concentrations in cenozoic geologic materials decrease eastward across california and nevada similarly to those in modern orographically induced precipitation as if little change in sierra nevadan elevations has occurred since eocene time orographic precipitation however depends on the amount of moisture in the atmosphere which in turn can be much larger in warm air as in eocene or oligocene time and in summer than in the cooler air characteristic of present day dominantly winter precipitation moreover the integrated rainout of vapor and hence presumably in stable isotope concentrations in the remaining vapor depends largely on the difference in heights traversed by air masses not slopes of mountain ranges thus if due simply to orographically induced rainout both eocene and oligocene variations in deuterium isotopes across the sierra nevada and miocene quaternary differences in deuterium and oxygen isotopes between the great valley of california and the basin and range place only weak constraints on the slope or past elevations of the sierra nevada they do not necessarily contradict the inference that the crest of the sierra nevada has risen 1000 m or more since late miocene time

nd sr isotopic provenance analysis of upper cretaceous great valley fore arc sandstones the provenance of fore arc sandstones of the upper cretaceous great valley group was investigated by using measurements of sm nd and rb sr isotope ratios and major and trace element concentrations the objective was to elucidate the erosional history of the adjacent sierra nevada arc and its relation to magmatism and fore arc sedimentation relatively detailed provenance information is obtainable because the currently exposed plutonic rocks of the arc and by inference the cretaceous volcanic rocks have large geographic gradients in isotopic composition mainly from west to east across the arc the relation of nd to sr and to major and trace elements in great valley sandstones is the same as in the plutonic rocks of the sierra nevada this observation confirms that the arc was the primary source of sediment and that the chemistry of the detritus was not significantly disturbed by sedimentary processes with decreasing nd there is an increase in k 2 o rb pb ba la and u and mgo ni and cr decrease the nd of sandstones decreases with decreasing depositional age from 0 7 to 5 0 in 97 to 73 ma strata the nd and age of the sandstones closely match those of the plutonic rocks of the sierra nevada this observation indicates that the location of the mean sandstone source and the active volcanic front were nearly coincident in time and space thus the isotopic compositions of the sandstones directly record the temporal and geographic variations in arc magmatism the volcanic roof of the sierra nevada arc was evidently chemically and isotopically equivalent to its plutonic roots prearc metasedimentary rocks did not significantly contribute to the isotopic or geochemical composition of the fore arc sandstones

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

sierra nevada uplift a ductile link to mantle upwelling under the basin and range province recent geophysical studies in the southern sierra nevada found no significant airy type crustal root the mountain range seems largely supported by the buoyant asthenosphere beneath an abnormally thin mantle lithosphere we suggest that the late cenozoic uplift of the sierra nevada may have resulted from mantle upwelling under the basin and range province which tends to push ductile material within the surrounding lithosphere causing it to flow away and downward numerical modeling indicates that such ductile flow could lead to pronounced lithospheric thinning under the high sierra and lithospheric thickening under the western sierra nevada comparable to the observed structure

composition of modern sand from the sierra nevada california u s a implications for actualistic petrofacies of continental margin magmatic arcs abstract the sierra nevada of california represents the roots of a long lived magmatic arc primarily cretaceous that is presently being dissected as the range is uplifted beginning in the south and progressing northward this dissection is occurring concurrently with northward migration of the mendocino triple junction south of which magmatic arc activity is absent and north of which magmatic arc activity continues a north to south transect along the sierra nevada represents transitions of active magmatic arc to transitional arc to dissected arc and in the southern sierra nevada to uplifted basement thus analysis of composition of modern sand derived from these separate parts of the sierra nevada provides a test for actualistic sand stone petrofacies of magmatic arcs at all stages of dissection rigorous statistical analysis of the composition of modern sand from the sierra nevada characterizes stages of magmatic arc dissection discriminant analysis of point count data from sierra nevada modern sand defines four compositional groups and distinguishes clearly between undissected arc and uplifted basement end members two intermediate compositional groups reflect the complex geologic history of the sierra nevada volcanic and plutonic rocks dominate one intermediate group metamorphic and plutonic rocks dominate the other the former group exhibits characteristics of dickinson s 1985 transitional arc whereas the latter is equivalent to dissected arc no single compositional parameter is diagnostic of the degree of magmatic arc dissection although volcanic lithic content clearly differentiates between undissected and basement groups regional compositional trends are evident when recalculated point count data are plotted against latitude sand composition in the southern sierra nevada reflects significant cenozoic uplift of the batholith resulting from basin and range extension high concentrations of quartz and feldspar and extreme lithic depletion are typical at the southernmost sierra nevada this extreme depletion characterizes basement uplift the primary drainage divide of the sierra nevada forms a rough compositional boundary between eastern and western groups potassium feldspar content best distinguishes these groups with higher values in the east rigorous confidence regions in ternary diagrams yield results that are similar but not identical to hexagonal fields of compositional variation arithmetic means associated with hexagonal fields are not identical to geometric means associated with logratio confidence regions definition of sand stone provenance groups in general depends on both data acquisition methods e g gazzi dickinson method and statistical methods e g weltje method so that direct comparison of data analyzed using different methods is unwarranted

the role of oceanic plateau subduction in the laramide orogeny the cause of the laramide phase of mountain building remains uncertain conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant hess or shatsky oceanic plateaux independent verification of these models has remained elusive because the putative ridges or plateaux are no longer at the earth s surface inverse convection models have identified two prominent seismic anomalies on the recovered farallon plate here we combine inverse convection models with reconstructions of plate motions to show that these seismic anomalies coincide palaeogeographically with the restored positions of the shatsky and hess conjugate plateaux as they subducted beneath north america specifically the distribution of laramide crustal shortening events tracked the passage of the shatsky conjugate beneath north america whereas the effects of the hess conjugate subduction were restricted to the northern mexico foreland belt we propose that continued subduction caused the oceanic crust to undergo the basalt eclogite phase transformation during which the shatsky conjugate lost its extra buoyancy and was effectively removed increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward followed by regional scale surface rebound we conclude that laramide uplift resulted from the removal rather than emplacement of the shatsky conjugate

bedding types in holocene tidal channel sequences knik arm upper cook inlet alaska abstract uplifted convoluted and horizontal to subhorizontal beds of varying thickness in intertidal silt as old as 3 280 90 yr b p are exposed in the banks of tidal channels of unknown depth in the intertidal zone in knik arm of upper cook inlet internal discordances may occur both within convoluted beds and between convoluted and horizontal to subhorizontal beds at the base of many convoluted beds there is a rapid gradation upward into laminae which are severely deformed that is in some places the contortions appear to have originated along a single bedding plane where the convoluted sequences are truncated by nearly horizontal sequences the distortion must have resulted from syndepositional or postdepositional events prior to their burial by the overlying beds various forms of gravitational and tidal processes caused the deformation of the knik arm deposits events triggering the movement of the sediment include any or all of the following 1 seismic activity 2 sediment or ice loading 3 wave induced liquefaction 4 pore pressure changes brought on by changing levels of tidewater 5 undercutting of sediments by channel migration and 6 freezing and thawing