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

analysis of landslide frequencies and characteristics in a natural system coastal british columbia two hundred and one debris slides and debris ows were analyzed in a 286 km2 study area on the west coast of vancouver island british columbia canada the study area remains essentially untouched by humans and therefore affords a natural setting in which to examine slope processes landslides were identi ed and characterized on aerial photographs from 1 15 000 to 1 31 680 and were then mapped and transferred to a gis for analysis based on detailed landslide surveys we propose a new method to accurately determine volume of landslides of this type by measured total area results indicate average denudation rates of 56 m3 y 1 km 2 and higher natural rates of failure than analogous regions in coastal british columbia in contrast the landslide rates are substantially less than those from forested watersheds landslide distribution is spatially clustered in air photograph epochs and we propose intense storm cells within regional events as the causal mechanism further failures occurred preferentially over the west coast crystalline complex by 1 4 times a metamorphic assemblage of gabbros schists and amphibolites but 1 5 times less often over the island plutonic suite a granitic intrusive formation the former result represents a new nding while the latter corroborates ndings of previous authors we examined magnitude frequency relationships of the data set and present for the rst time a strong argument that the rollover effect is not merely an artefact but is instead a consequence of the physical characteristics of the landslides themselves we subsequently analyzed magnitude frequency relationships from two other complete data sets from coastal british columbia and produced a family of curves corroborating this result copyright 2004 john wiley sons ltd

thermal stresses differential subsidence and flexure at oceanic fracture zones geosat geoid undulations over four pacific fracture zones have been analyzed after correcting for the isostatic thermal edge effect the amplitudes of the residuals are shown to be proportional to the age offset the shape of the residuals seems to broaden with increasing age both geoid anomalies and available ship bathymetry data suggest that slip must sometimes occur on the main fracture zone or secondary faults existing models for flexure at fracture zones cannot explain the observed anomalies a combination model accounting for slip and including flexure from thermal stresses and differential subsidence is presented our model accounts for lateral variations in flexural rigidity from brittle and ductile yielding due to both thermal and flexural stresses and explains both the amplitudes and the shape of the anomalies along each fracture zone the best fitting models have mechanical plate thicknesses that are described by the depth to the 600 700 c isotherms

landsliding and the evolution of normal fault bounded mountains much of the tectonic and climatic history in high relief regions such as the mountains of the western u s basin and range province is contained in the morphology of hillslopes drainage networks and other landforms that range in scale from 10 1 to 101km to understand how these landforms evolve we have developed a numerical landscape evolution model that combines a detailed tectonic displacement field with a set of physically based geomorphic rules bedrock landsliding long recognized as a significant geomorphic process in mountainous topography is for the first time explicitly included in the rule set in a series of numerical experiments we generate synthetic landscapes that closely resemble mountainous topography observed in the basin and range the production of realistic landscapes depends critically on the presence of bedrock landslides and landsliding yields rates of long term erosion that are comparable in magnitude to those of fluvial erosion the erosive efficiency of bedrock landsliding implies that hillslopes may respond very quickly to changes in local base level and that fluvial erosion is the rate limiting process in steady state experimental landscapes our experiments generate power law distributions of landslide sizes somewhat similar to both field and laboratory observations thus even a simple model of bedrock landsliding is capable of quantitatively reproducing mountainous topography and landslide distributions and represents a significant step forward in our understanding of the evolution of normal fault bounded ranges

the sandstone derived provenance record of the gualala basin northern california u s a abstract detrital zircon u pb ages and pb isotope compositions of detrital feldspars from the late cretaceous eocene gualala basin in northern california indicate that the basin received sediments from the southern sierra nevada the mojave block and the salinian block mesozoic detrital zircon age components and pb isotope compositions of detrital feldspars from the basin are similar to u pb zircon ages and pb isotope compositions from previously studied coeval conglomerate clasts the detrital zircons and feldspars however also include additional age and pb isotope components that are unobserved or poorly represented in the conglomerate based provenance record these new data suggest a more complex provenance history than that inferred from previously studied conglomerate clasts and strongly imply a connection to the mojave desert provenance evolution of the sediments in the basin indicates that significant contribution of sediment from the mojave desert began during the paleocene when it is inferred that the drainage network expanded eastward from its previous extent the presence of mojave derived late permian zircons as well as feldspars that have high 208pb 204pb ratios in the eocene age german rancho formation indicates that the basin remained in the vicinity of the mojave desert until the eocene this result demonstrates that the gualala basin had undergone very little northward motion at that time and consequently requires that no significant northward translation took place prior to 50 ma the results of this study highlight the value of additional provenance constraints combined with age distributions when identifying sediment source terranes

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

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

structure and evolution of the east sierran thrust system east central california 1 a belt of arc parallel northeast vergent contractional deformation the east sierran thrust system ests crops out for 150 km along the east side of the sierran continental margin arc the ests is nowhere wider than 20 km and it accommodated an estimated minimum of 9 3 km of horizontal shortening remarkably it experienced repeated episodes of broadly coaxial and coaxial planar contractional deformation beginning prior to 188 ma and continuing past 140 ma we postulate that the ests resulted primarily from episodic underthrusting of the back arc lithosphere beneath the east edge of the sierran arc facilitated by a buttressing effect of the arc as a result of this process rocks along the east flank of the batholith including the ests were episodically shortened against the arc buttress the ests experienced significant deformation during the nevadan orogeny indicating that contractional to transpressive deformation affiliated with this event affected the eastern wall rocks of the arc as well as its western wall rocks index terms 8102 tectonophysics continental contractional orogenic belts 9609 information related to geologic time mesozoic 8015 structural geology local crustal structure 9350 information related to geographic region north america keywords eastern california mesozoic structure cordilleran thrust system citation dunne g c and j d walker 2004 structure and evolution of the east sierran thrust system east central california tectonics 23 tc4012 doi 10 1029 2002tc001478

strength of fracture zones from their bathymetric and gravitational evolution fracture zone evolution is investigated using dynamic models that allow the fault zones to freely slip this is an improvement over past formulations where bathymetric offsets were imposed kinematically the models use a viscoelastoplastic rheology that incorporates the influence of fault friction on fracture zone slip history using viscoelastic plates we assess the role of small scale convection on removal of the lowermost thermal lithosphere beneath fracture zones through a comparison of synthetic gravity to free air gravity across fracture zones we find that the amplitude of the gravity jump across fracture zones is best fit by models with weak faults that have depth averaged yield strengths 10 mpa fracture zones with such low strengths can convert to subduction zones with 100 km of convergence many fracture zones do not fit plate subsidence models with locked or slipping faults but are better fit by systems that are tectonically deformed by modest amounts of extension

supply and removal of sediment in a landslide dominated mountain belt central range taiwan abstract a strong coupling between hillslope and valley systems is often inferred for mountain landscapes dominated by bedrock landsliding we reveal the nature of this link using data sets on landsliding and sediment transport from two montane catchments draining the eastern central range of taiwan here the magnitude frequency distribution of landslides can be modeled by a robust power law but this scale invariance is not mirrored in the sediment discharge at the mountain front instead downstream sediment loads reflect a complex response to both sediment supply and ambient hydraulic conditions the rivers do not transport significant amounts of sediment unless it is provided by hillslope mass wasting in the catchment removal of landslide debris is a function of the transport capacity of the stream at the site of entry thus there is a dual supply and transport control on sediment loads in bedrock floored streams over a monitoring period of 25 yr the bulk of the sediment leaving the mountain bel

sediment transfer by shallow landsliding in the queen charlotte islands british columbia despite the importance of landsliding in routing sediment through mountainous drainage basins few studies have documented landsliding rates over extended time and space scales we have investigated landsliding in surficial material in the queen charlotte islands using a large inventory of events derived from aerial photography covering an area of 166 7 km 2 the mean erosion rate for shallow landsliding is 0 10 mm a 1 which is at the upper end of shallow landsliding rates observed in the pacific northwest and coastal british columbia but several orders of magnitude lower than rock based landsliding rates reported in the literature probability distributions for landslide area and volume are somewhat convex in form flattening of the curve found at low magnitudes may be due to sampling bias or physical mechanisms inhibiting failure and the steepening for high values may exist because the sampling period is not long enough to adequately represent large events landslides generally initiate on hillslope gradients greater than 0 50 0 60 the largest numbers of landslides occur on south to southwest facing slopes and east to northeast facing slopes most events occur on concave and straight hillslopes in upper slope positions landsliding rates were found not to be af fected by rock type hillslopes in the queen charlotte islands are often mantled by weathered quaternary deposits and hence landsliding events are not directly controlled by weathering of bedrock about 31 of landslides identified in this study deposited material in stream reaches with about 83 of these landslides deposited in reaches with gradients between 3 and 10 resume malgre la grande contribution des glissements de terrain a acheminer des sediments a travers des bassins de drainage montagneux peu d etudes ont documente les taux de glissements de terrain sur une longue periode de temps et sur de grandes superficies nous avons fait des recherches sur des glissements de materiaux de surface dans les iles de la reine charlotte en utilisant un grand inventaire d evenements tires de photographies aeriennes couvrant une re

lithospheric flexure at fracture zones bathymetric profiles across six major fracture zones fz s in the north pacific are used to demonstrate the absence of vertical slip on the fossil fault planes the scarp heights on these fz s are constant with age and equal to the initial vertical offsets at the ridge transform fault fz intersections because of the frozen in scarp and the differential subsidence of lithosphere far from the fz the lithosphere bends in the vicinity of the fz this flexure results in a characteristic ridge trough topographic fz signature the flexural amplitude which is the difference between the scarp height and the overall change in depth across the fz increases with age good fits to the bathymetric profiles across the mendocino and pioneer fz s are obtained by modelling the topography as the flexure of a thin elastic plate with an age dependent effective elastic thickness results of the modelling indicate that the base of the elastic lithosphere is approximately defined by the 450 c isotherm maximum bending stresses at fz s are on the order of 100 mpa substantially less than the stresses encountered at subduction zones because the mendocino and pioneer fz s are separated by less than a flexural wavelength they are elastically coupled

shear strength of the great pacific fracture zones shipboard bathymetry and gravity data from 30 crossings of 6 great pacific fracture zones fzs the mendocino murray molokai clarion clipperton and udintsev are compared with the predictions of a model in which fzs are locked beyond the ridge transform intersection such that no vertical motion occurs on the fault in response to differential thermal subsidence at least some sections of all of these fzs except the molokai are consistent with this model and sustain shear stresses as high as 20 mpa however none of the fzs is locked along its entire length as inferred from observed shear stresses dropping below 75 of the value necessary to maintain a locked fault there is some suggestion that the unlocking may be related to excess volcanism

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

biomarker reconstruction of the early eocene paleotopography and paleoclimate of the northern sierra nevada we reconstruct ancient temperature and elevation gradients across the early eocene 52 49 ma northern sierra nevada california united states using organic molecular proxies that record atmospheric and ground level effects of topography paleoelevation was determined by reconstructing the change in the hydrogen isotopic composition of precipitation d precip and mean annual temperature t gdgt glycerol dialkyl glycerol tetraethers from the isotopic composition of fossil angiosperm leaf n alkanes and the distribution of microbially produced soil tetraethers preserved in leaf bearing sediments organic molecular data produce equivalent range scale d n alkane and channel t gdgt paleoelevation estimates that show the northern sierra nevada was a warm 6 8 c warmer than modern high elevation 2 km and moderate to low relief landscape at the eocene climatic optimum modern northern sierra nevada topography likely refl ects post paleocene reduction of mean surface elevation and late cenozoic increases in relief

are segmented fracture zones weak analytical and numerical models constrain anomalous bathymetry at the clarion and murray fracture zones abstract between 150 w and 135 w the clarion fracture zone fz coalesces from six discrete fz traces to a single fz over a period of 30 million years coinciding with a change in plate motion that placed the associated transform fault tf in compression between 160 and 157 the murray fz is also comprised of several closely spaced

instability in eight sub basins of the chilliwack river valley british columbia canada a comparison of natural and logging related landslides logging causes increased landslide frequency in british columbia in this study the slope type initiation location aspect rate bedrock geology and size of mass movements located in eight logged tributary valleys of the chilliwack river valley are investigated a landslide inventory was created by digitizing landslides identified on aerial photographs dating from 1941 to 2002 this database was analysed using qualitative observations and simple statistical tests and a comparison between natural and logging related landslides was made slope tests displayed a significant difference between natural and logging related events indicating that on average logging related landslides require lower slope gradients to initiate than natural events although aspect initiation location and landslide type did not show a significant relationship between natural and logging related landslides they do suggest subtle differences landslide rate was calculated and compared for natural and logging related landslides it was significantly higher for logging related events attaining a maximum of 31 times the natural rate in 1978 and an overall average increase of nine times the natural rate logging related landslides were on average smaller than natural landslides but this nevertheless resulted in a 3 1 times increase in area affected by logging related landslides compared to natural landslides geology may influence landslide frequency as well it appears to be higher over the less resistive sedimentary rocks of the cultus formation and chilliwack group and lower in granodioritic areas finally roads affected slope stability more than clearcuts in the early decades of the study but this effect decreased over time suggesting a correlation with improved road construction

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

paleobotanical evidence for high altitudes in nevada during the miocene leaf physiognomy provides estimates of environmental parameters including mean annual enthalpy which is a thermodynamic parameter of the atmosphere that varies with altitude analyses of 12 mid miocene floras from western nevada indicate that this part of the basin and range province stood 3 kilometers above sea level at 15 to 16 million years ago which is 1 to 1 5 kilometers higher than its present altitude much if not all of the collapse to present day altitudes seems to have been achieved by 13 million years ago the crust in much of this area has been extended and thinned throughout the past 40 to 50 million years and the isostatic balance of a thinning crust requires subsidence not uplift as suggested by previous paleobotanical work

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

are oceanic fracture zones locked and strong or weak new evidence for volcanic activity and weakness traditionally oceanic fracture zones and other lithospheric and crustal faults have been viewed as zones of weakness that are vulnerable to seismic and volcanic activity this notion has recently been questioned on the basis of evidence showing that initial depth offsets scarp height across some pacific fracture zones are preserved even after 100 m y of differential subsidence this evidence suggests that some parts of oceanic fracture zones are locked shortly after they drift away from ridge transform intersections because the fracture zones seem to be strong welds differential cooling stresses cause flexure in the lithosphere on both sides of the fracture in this paper we briefly review the seemingly contradictory evidence for the thermomechanical characteristics of oceanic fracture zones locked and strong vs slipped and weak and present new evidence indicating that parts of some fracture zones are volcanically active volcanism is an indication of relative weakness because strain and failure are required to open volcanic conduits this and other evidence for the inherent weakness of fracture zones can be reconciled with evidence for strength if different segments of fracture zones exhibit contrasting thermomechanical behavior more data are needed to determine whether this is so and whether there are systematic patterns of distribution for weak and strong parts of individual fracture zones

topographic lithologic and glaciation style influences on paraglacial processes in the upper sil and luna catchments cantabrian mountains nw spain abstract deglaciated mountainous areas are usually affected by a great range of paraglacial processes that involve progressively denudation of glacial imprints this study focuses on paraglacial processes and glacial landform preservation in six zones located in two catchments of the cantabrian mountains nw spain these zones are adjacent but display important topographic lithologic and glaciation style differences an analysis of how these variables conditioned the intensity and diversity of paraglacial processes in these zones was investigated through a detailed study of their relief lithology and paleo glacier surface using gis a comprehensive cartography of paraglacial landforms was also accomplished considering landslides alluvial fans and rock glaciers that show signs of having been conditioned by glaciation moraines and debris mantled slopes were also included in the analysis to examine their grade of preservation depending on the studied variables data comparison show that differences in glaciation style influenced paraglacial processes in the ancient icefield area relief modification by glacial action was limited and scarce paraglacial processes occur by contrast in the upper areas located in the southern ranges where an alpine style of glaciation was installed paraglacial processes were very active generating many rock glaciers lithology explains some paraglacial landforms distribution 1 landslides are associated with incohesive sandy and shale rocks 2 rock glaciers mainly occur in quartzite areas 3 in limestone areas subterranean drainage contributes to glacial landscape preservation but suffosion dolines affect some glacial deposits topography is also a key factor in paraglacial processes 1 moraines are well preserved in gently sloping valleys but rare in the steeper where they are usually transformed into moraine or debris mantled slopes 2 alluvial fans are more frequent in the steeper valleys but more dissected by postglacial river action 3 elevation and orientation show little influence on paraglacial landforms except in the generation of some periglacial features such as rock glaciers

u th he dating of apatite a potential thermochronometer abstract we found a gem quality crystal of durango fluorapatite to have a 4he content consistent with complete retention of radiogenic helium since its formation at about 31 ma isothermal heating and step heating analysis reveal 4he loss to occur systematically by volume diffusion at low temperatures the linear low temperature portion of the diffusion data yields an activation energy of 38 5 8 1 kcal mol and a frequency factor of in d 0 a 2 16 4 2 8 sec 1 corresponding to a closure temperature of 105 c 30 c cooling rate 10 c m y it appears that u th he dating of apatite might represent a useful new thermochronometer with a range similar to that of fission track dating of apatite from these results we infer that a number of the too young u th he dates reported in the literature on minerals such as zircon and magnetite may in fact represent valuable records of the low temperature thermal history of their host rocks

deglaciation of chilliwack river valley british columbia quaternary sediments and landforms in chilliwack river valley southwestern british columbia provide a detailed record of deglaciation of this area between 12 000 and 11 000 years bp stratigraphic sedimentological and radiocarbon data show that a large glacier in eastern fraser lowland part of the cordilleran ice sheet blocked the mouth of chilliwack valley at a time when the middle reaches of the valley were ice free a lake existed between the ice dam in the lower part of the valley and a delta sandur complex west of chilliwack lake in the upper part of the valley two relatively minor advances of the fraser lowland ice lobe into lower chilliwack valley occurred about 11 500 and 11 200 years bp these were separated by a brief period of recession during which tephra was deposited and a coniferous forest and soil developed on freshly deglaciated terrain shortly after 11 200 years bp a glacial lake formed in cultus lake basin two sets of terraces in lower chilliwack valley are graded to diffe

evolution and strength of pacific fracture zones previous studies have shown that pacific fracture zones are strong in some locations sustaining the stresses associated with differential subsidence across a locked fault while at other locations they show signs of weakness acting as preferential conduits for volcanism or supporting anomalously low shear stresses we find that half of all geosat crossings of central pacific fracture zones are inconsistent with a flexurally maintained scarp we test two hypotheses for the origin of such anomalous crossings 1 anomalous structures formed at the transform fault during times of changes in plate motions and preserved at strong fracture zones and 2 anomalous structures representing a posttransform response of the fracture zone to subsequent tectonic activity we find that three quarters of the anomalous crossings occur over the parts of fracture zones that formed during or immediately subsequent to times of changes in spreading directions with the exception of several locations overprinted by hot spot volcanism these same crossings show no obvious correlation with regional tectonic processes these observations suggest that most anomalous fracture zone topography is inherited from the transform fault and is not a product of subsequent activity consistent with the hypothesis that fracture zones generally remain strong throughout their lifetimes

assessment of shallow landslide susceptibility by means of multivariate statistical techniques several multivariate statistical analyses have been performed to identify the most influential geological and geomorphological parameters on shallow landsliding and to quantify their relative contribution a data set was first prepared including more than 30 attributes of 230 failed and unfailed slopes the performance of principal component analysis t test and one way test allowed a preliminary selection of the most significant variables which were used as input variables for the discriminant analysis the function obtained has classified successfully 88 5 per cent of the overall slope population and 95 6 per cent of the failed slopes slope gradient watershed area and land use appeared as the most powerful discriminant factors a landslide susceptibility map based on the scores of the discriminant function has been prepared for ensija range in the eastern pyrenees an index of relative landslide density shows that the results of the map are consistent copyright 2001 john wiley sons ltd language en

dating topography of the sierra nevada california using apatite u th he ages the upward motion of rock masses relative to the earth s surface has been documented for most of the main mountain belts using thermochronological and petrological techniques more fundamental to the physical processes of mountain building however is the motion of the earth s surface itself which remains elusive here we describe a technique for estimating the age of topographic relief by mapping the low temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium thorium and helium contents the technique exploits horizontal variations in temperature in the shallow crust that result from range normal river drainages because cooling beneath ancient river valleys occurs earlier than beneath intervening ridges our results from the sierra nevada california indicate that two of the modern transverse drainages the kings and the san joaquin had developed deep canyons by the late cretaceous period suggesting that the high topography of the range is 50 60 million years older than generally thought

paleoenvironmental and paleoclimatic interpretations of the late paleocene goler formation southern california u s a based on paleosol geochemistry the paleocene was characterized by high global temperatures and elevated concentrations of atmospheric co2 paleoclimate records of this epoch are of particular interest to understanding the effects of anthropogenic climate forcing as well as linkages between the paleocene climate and major tectonic and evolutionary events however terrestrial paleoclimatic records of the paleocene are limited in their spatial extent and few such records are known from western north america in this study paleosols from two members in the late paleocene goler formation of southern california were used to investigate local paleoclimatic conditions the paleosols were described in detail and analyzed for morphology clay mineralogy and bulk geochemistry carbonate nodules present in paleosols from the upper member member 4a were characterized using petrographic and cathodoluminescence microscopy and were analyzed for carbon and oxygen stable isotope composition enrichment and retention of soluble elements ca mg and na and the mobilization of fe and mn in the paleosol profiles suggest a climate with strongly seasonal precipitation this interpretation is further supported by the presence of pedogenic slickensides and the clay minerals smectite and illite the appearance of pedogenic carbonate nodules in paleosols from the upper member is suggestive of a shift to more arid or more seasonal conditions that is coincident with a prominent change in sediment provenance and an inferred increase in sedimentation rate uplift of the ancestral sierra nevada during the late paleocene was likely an important control on regional paleoclimate due to the effects of elevated topography on mean annual precipitation and seasonal distribution of precipitation the correlation of a shift in pedogenic processes with changes in provenance and sedimentation rate suggests that the evolution of regional tectonic activity was an important control on these processes during the deposition of the goler formation

geophysical constraints on the shear stress along the marquesas fracture zone we examine bathymetry and gravity observations acquired by surface ships and deflection of the vertical data derived from the geosat radar altimeter for the purpose of determining if the bathymetric relief along the marquesas fracture zone in the central pacific ocean is caused by differential thermal subsidence across a locked fault if the fracture zone can maintain between 20 and 40 mpa of shear stress we would predict that a ridge and a parallel trough would develop to the south and north respectively of the fracture zone on account of flexural modification of the step in bathymetry locked into the plate along the fracture zone at the ridge transform intersection we find that only 2 out of 32 deflection of the vertical profiles from the geosat altimetric mission are consistent with this model while none of the 27 bathymetric profiles or 9 shipboard gravity profiles fits the theoretical predictions in cases where bathymetry and gravity can be modeled simultaneously it appears that the gravity anomaly is simply not large enough in magnitude to be consistent with the moho flexing conformably with the observed bathymetry as required if the relief is produced by deeper thermal variations across the locked fracture zone rather the potential field data allow shear stresses only 25 of those required to lock the fault and are more consistent with a model in which the relief on the moho is opposite in sign to that of the surface topography as would be produced by local or regional compensation of surface loads arising from volcanic activity along the fracture zone thus we conclude that the marquesas fracture zone in contrast to those other pacific fracture zones which fit the predictions of the locked fault high stress model may be unusually weak along almost its entire length

oceanic intraplate seismicity the occurrence of seismicity at the surface of the globe largely along preferential lines now recognized as mid oceanic ridges and subduction zones was key evidence for the development of the theory of plate tectonics in the 1960s indeed many plate boundaries especially some of the southern less accessible ridges were drawn at least initially on the basis of this evidence alone the mere concept of intraplate seismicity may then appear as somewhat of a paradox if not as a failure of the whole theory however and as shown later in this paper the relatively low level of this seismicity warrants its consideration as a perturbation in the general framework of nearly rigid plates additionally intraplate earthquakes are of great value since in the oceanic environment they are our only clue to the state of stress of the lithosphere thereby providing us with critical insight into the forces responsible for its motion understanding para meters that control these earthquakes is made easier with respect to a continental situation by the generally younger age of the ocean floor and the much simplified tectonic history of any given oceanic province on the other hand seismic detection capabilities are greatly reduced at sea because of both logistics and generally higher seismic attenuation thus our knowledge of many aspects of oceanic intraplate seismicity is still rudimentary it is in this general framework that the present paper reviews the following points

insights from low temperature thermochronometry into transpressional deformation and crustal exhumation along the san andreas fault in the western transverse ranges california 1 the san emigdio mountains are an example of an archetypical transpressional structural system bounded to the south by the san andreas strike slip fault and to the north by the active wheeler ridge thrust apatite u th he and apatite and zircon fission track ages were obtained along transects across the range and from wells in and to the north of the range apatite u th he ages are 4 6 ma adjacent to the san andreas fault and both u th he and fission track ages grow older with distance to the north from the san andreas the young ages north of the san andreas fault contrast with early miocene u th he ages from mount pinos on the south side of the fault restoration of sample paleodepths in the san emigdio mountains using a regional unconformity at the base of the eocene tejon formation indicates that the san emigdio mountains represent a crustal fragment that has been exhumed more than 5 km along the san andreas fault since late miocene time marked differences in the timing and rate of exhumation between the northern and southern sides of the san andreas fault are difficult to reconcile with existing structural models of the western transverse ranges as a thin skinned thrust system instead these results suggest that rheologic heterogeneities may play a role in localizing deformation along the big bend of the san andreas fault as the san emigdio mountains are compressed between the crystalline basement of mount pinos and oceanic crust that underlies the southern san joaquin valley

a reexamination of the flexural deformation beneath the hawaiian islands seismically derived depth estimates to the top of the oceanic crust beneath the hawaiian islands indicate that the curvature of the deflected lithosphere is much larger than commonly believed the conservative and model independent curvature estimates exceed 10 7 m 1 and are comparable in magnitude to curvatures at trenches and outer rise systems the depth estimates are used to constrain both two dimensional 2 d and three dimensional 3 d flexural models the curvature constraints require a 2 d variable elastic thickness that decreases from 35 km in areas away from the volcanic load to 25 km directly beneath the load in an attempt to understand the nature of the yielding beneath the hawaiian islands we introduce two new 3 d models the first model combines a realistic yield strength based rheology with a new technique for 3 d flexure calculations in which the elastic plate thickness is curvature dependent the new variable rigidity model predicts an undeformed mechanical plate thickness of 44 km decreasing to 33 km beneath the big island of hawaii the best fitting mechanical thickness corresponds approximately to the depth to the 600 c isotherm in 90 m y old lithosphere the second model uses a broken plate but here the crack is oriented along the weak molokai fracture zone rather than along the island chain trend this unconventional flexure model can explain the observed asymmetry in the depth data across the fracture zone without requiring the excessively large elastic thickness of more conventional broken plate models both the proposed models imply that modeling with constant thickness plates may underestimate the true mechanical plate thickness by being unduly influenced by the weak zone beneath the seamounts

geochemistry of lavas from mohns ridge norwegian greenland sea implications for melting conditions and magma sources near jan mayen mohns ridge lavas between 71 and 72 30 n 360 km have heterogeneous compositions varying between alkali basalts and incompatible element depleted tholeiites on a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts from the island jan mayen and mohns ridge tholeiites the variation in isotopes suggests a heterogeneous mantle which appears to be tapped preferentially by low degree melts 5 close to jan mayen but also shows its signature much further north on mohns ridge three lava types with different incompatible element compositions e g chondrite normalized la sm n 2 occur in the area at 72 n and were generated from this heterogeneous mantle the relatively depleted tholeiitic melts were mixed with a small degree melt from an enriched source the elements ba rb and k of the enriched melt were probably buffered in the mantle by residual amphibole or phlogopite that such a residual phase is stable in this region of oceanic mantle suggests both high water contents and low mantle temperatures at odds with a hotspot origin for jan mayen instead we suggest that the melting may be induced by the lowered solidus temperature of a wet mantle mohns morb mid ocean ridge basalt and jan mayen area alkali basalts have high contents of ba and rb compared to other incompatible elements e g ba la 10 these ratios reflect the signature of the mantle source ratios of ce pb and rb cs are normal morb mantle ratios of 25 and 80 respectively thus the enrichments of ba and rb are not indicative of a sedimentary component added to the mantle source but were probably generated by the influence of a metasomatizing fluid as supported by the presence of hydrous phases during the petrogenesis of the alkali basalts geophysical and petrological models suggest that jan mayen is not the product of hotspot activity above a mantle plume and suggest instead that it owes its existence to the unique juxtaposition of a continental fragment a fracture zone and a spreading axis in this part of the north atlantic

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

refining paleoaltimetry reconstructions of the sierra nevada california using air parcel trajectories stable isotope paleoaltimetry studies often implicitly assume that atmospheric flow interactions with topography can be simply modeled as a rayleigh distillation process in which air parcels consistently ascend topographic barriers we present a modern a d 1979 2010 air parcel trajectory analysis using the hybrid single particle lagrangian integrated trajectory hysplit model that shows that this fundamental assumption is often violated in the sierra nevada region of the western united states observed trajectory patterns and scaling calculations indicate that windward orographic blocking frequently occurs when trajectories encounter high elevations 2 5 km in the central and southern sierra nevada as a result trajectories reaching the sierran lee commonly travel around rather than over the highest range elevations redirection effects are particularly pronounced at leeward sites distal 150 km to the sierran crest but are also evident in trajectory patterns in the northern sierra nevada this trajectory analysis improves the interpretability of regional meteoric water and proxy isotopic records and has particular relevance to stable isotope based reconstructions of sierran paleoelevations specifically stable isotope methods alone provide only limited insight into the elevation history of the sierra nevada and are likely insufficient to resolve proposed late cenozoic elevation gains on the order of 1 2 km that may have raised the sierra to its modern elevations

modeling of the temperature sensitivity of the apatite u th he thermochronometer apatite u th he apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion to characterize the response of apatite he ages to thermal histories involving partial he retention we explored solutions to the he production diffusion equation under thermally static conditions the analytical solution to this equation coupled with published diffusivity data demonstrates that the zone of partial he retention extends from about 40 c to 85 c this zone lies at temperatures 35 c cooler than the analogous fission track partial annealing zone he ages within the partial retention zone ultimately achieve a balance between he production and loss yielding a steady state age both the ultimate age and the time it takes to achieve this age are temperature dependent for example an apatite held at 75 c equilibrates to an age of 2 ma after 17 myr regardless of whether equilibrium is approached from a higher or a lower initial he age for representative dynamic thermal histories we evaluated apatite he ages using a numerical solution to the ingrowth diffusion equation the results illustrate the sensitivity of he ages to various geologic histories and are useful for understanding he age elevation relationships and for testing time temperature paths derived from apatite fission track length distributions in addition although he diffuses rapidly from apatite at shallow crustal temperatures modeling of ambient temperature fluctuations indicates that he ages are nearly unaffected by surficial processes

evolution of the southern san joaquin basin and mid tertiary transitional tectonics central california a cenozoic tectonic and sedimentary history is proposed for the southern san joaquin basin ssjb and tehachapi mountains that evolved adjacent to the plate margin off central california seismic reflection borehole field biostratigraphic and paleomagnetic data are integrated into geologic and fault structure maps cross sections and geohistory plots and are analyzed with previous work in the region to develop a model relating the sequence timing and distribution of complex tectonically linked events the largely buried structures and strata in the ssjb preserve an unusually complete record of the mid tertiary transition from convergent to transform plate boundary as well as the regional transition to contraction during the pliocene significant structural relief existing across both extensional and contractile features is preserved in the subsurface and an active fold thrust belt propagates basinward along the margin of the u shaped tejon embayment the cenozoic evolution of the ssjb reflects the regional deformation of central california as different tectonic events followed each other along the adjacent north american plate margin five oligocene miocene basin phases are identified in the ssjb 1 late oligocene early miocene extensional subsidence with high and low angle normal faulting accompanied by volcanism and deposition of coarse syntectonic conglomerates 2 middle miocene uplift 3 later mid miocene transtensional subsidence to lower bathyal depths 4 alternating subsidence and uplift until the late miocene and 5 flexural subsidence due to pliocene to recent contraction reconstructions of mid tertiary california place the southern san joaquin tehachapi extensional terrane as a paleotectonic block located between the western mojave terrane then to the east and the western california terrane then to the south and west regional extension occurred during a long transition period between convergent and transform boundaries along the north american plate margin significant slip along the san andreas transform postdates this extensional event its origin apparently coincided with a regional middle miocene uplift event indicating that the san andreas fault is younger than previously supposed the tertiary ssjb has subsided due to extension transtension crustal tilting and thrust related loading the maricopa subbasin floor is likely composed of ensimatic and mafic rocks like those along the west side of the sierra nevada and locally has subsided beyond 12 km in contrast the relatively stable tejon embayment underlain by sierran crystalline crust achieved its maximum subsidence in the miocene since pliocene time the ssjb has filled and continued to deepen after the basin was tectonically shut off from the pacific ocean

provenance and stratigraphy of the eocene tejon formation western tehachapi mountains san emigdio mountains and southern san joaquin basin california abstract the eocene tejon formation of the western tehachapi and san emigdio mountains of california provides a unique east west trending outcrop belt of otherwise deeply buried strata that underlie the san joaquin basin to the north its stratigraphy and framework mineralogy have been used to define offsets along the san andreas fault a major neogene plate boundary transform fault that has translated coastal california northward more than 300 km by right slip the tejon formation was deposited directly on crystalline basement rocks as a result of an eastward transgression of at least 45 km from the early to the late middle eocene followed by a major regression in the late middle eocene the basal uvas conglomerate member consists entirely of coarse grained transgressive system tract tst deposits characterized by abundant current formed sedimentary structures it is overlain by offshore marine shales of the liveoak shale member which consist of lower tst deposits overlain by highstand system tract hst deposits the overlying regressive metralla sandstone member consists mostly of highly bioturbated fine grained shallow marine hst sandstones with local thin bedded turbidites present in some western exposures the quartzofeldspathic sandstones grand mean qm51 12f42 12lt7 4 of the tejon formation suggest a provenance consisting of plutonic metamorphic and volcanic rocks such as those present in the basement of the san emigdio and techachapi mountains northwestern mojave desert and southern sierra nevada petrologic parameters however suggest variable contributions from these source areas through time three distinctive petrofacies coincide with the three principal lithostratigraphic subdivisions the shallow marine tst sandstones from the uvas conglomerate member are more quartzose than those of the hst metralla sandstone member possibly because higher energy deposition preferentially preserved quartz thin bedded turbidites in the western exposures of the metralla sandstone in contrast have more abundant feldspar and volcanic lithic fragments than the uvas and liveoak members the composition of the tejon formation suggests derivation from a basement uplift provenance with progressive upsection changes from qm60 f33 lt8 uvas to qm46 f44 lt10 liveoak and qm 43 f52 lt5 metralla the composition and provenance of the tejon formation is different from that of the coeval butano sandstone and point of rocks sandstone submarine fan deposits to the north and west

the impacts of logging on landslide activity at clayoquot sound british columbia abstract the objective of this study was to investigate the impacts of logging on landslide activity in clayoquot sound on the west coast of vancouver island british columbia a total of 1004 landslides were documented in order to test the hypothesis that areas affected by logging activities show different density frequency and magnitude characteristics of landsliding than areas unaffected by logging the frequency of landslides in logged terrain was found to be nine times higher than in undisturbed forest an exponential increase in landslide frequency within the area logged was observed on a large watershed scale failures in logged terrain occur on gentler slopes than in natural terrain partly because fewer slopes steeper than 40 have been logged debris slides and debris flows are the most frequently occurring mass movements initiating mostly from road fill failures and from within cutblocks most landslides initiated on se facing slopes with the highest landslide frequency near the coast and diminishing exponentially further inland this pattern of activity most likely reflects the greater impact of severe storms arriving from the pacific with winds dominating from the southeast concave and straight slopes are most susceptible to landslide initiation a preliminary examination of geologic control on the frequency of landslides suggests that the quatsino formation the island plutonic suite and the sicker group are significantly above the average rate further work is needed to identify and quantify factors confounding the trends and correlations identified in this overview study results obtained in this study will be used to implement changes in forest management

the effects of changes in plate motions on the shape of the marquesas fracture zone the marquesas fracture zone mfz one of the great fracture zones that formed on the pacific farallon ridge system shows dramatic variations in morphology along its strike that cannot be explained by models for lithospheric flexure in response to thermal stresses and differential subsidence marine geophysical data collected by the r v maurice ewing in fall 1991 show that major structures in the mfz can be correlated with changes in plate motions as the fracture zone formed the western mfz shows two significant changes in trend at 209 e and at 224 e the change in strike at 209 e records a reorientation that put the transform fault under compression and created a 100 km long sinuous ridge within the fracture zone the change in plate motions at 224 e induced extension in the transform fault creating three subparallel fracture zones these results are in accord with observations of morphological changes observed at active transform faults and other fracture zones and suggest that even relatively small changes 6 in spreading direction can produce significant structures that are preserved in the fracture zone

magnetotelluric evidence of lithospheric mantle thinning beneath the southern sierra nevada a wideband 0 01 1000 s magnetotelluric survey across the southern sierra nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10 20 km the eastern zone underlies the highest topography of the range and extends eastward this eastern conductive zone extends to depths in excess of 100 km based on model sensitivities and lies well below the moho depth of 32 38 km from seismological studies therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt such an interpretation is consistent with gravitational seismological and geologic evidence estimates of the partial melt fraction range from 2 to 5 at depths of 40 70 km which are consistent with fractions of melt inclusions observed in xenoliths from the eastern sierra nevada this partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations the other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes the western conductive zone also straddles the moho and extends into the mantle beneath western foothills of the sierra nevada and the great valley sediments to the west however xenoliths from this region indicate high velocity crustal rocks to depths in excess of 60 km and we therefore attribute the enhanced conductivity to graphitic metasediments and or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the sierran plutons

cenozoic thermal evolution of the central sierra nevada california from uth he thermochronometry apatite u th he cooling ages are reported for igneous apatite samples from the central sierra nevada and compared to published apatite fission track ages and track length data from the same mineral separates helium ages are youngest at low elevations and increase systematically toward higher elevations ranging from 43 to 84 ma at yosemite valley 32 to 74 ma at kings river canyon and 23 to 75 ma at mt whitney helium ages from high elevation samples are generally concordant with corresponding fission track ages while lower elevation helium ages are substantially younger cooling histories inferred from present laboratory derived fission track annealing and helium diffusion models do not match well suggesting that either helium diffusion rates or fission track annealing rates are miscalibrated at temperatures below about 60 c for geologic exposure periods unlike the fission track results the helium data do not indicate a very low geothermal gradient in the sierra nevada during early to middle tertiary time

detrital k feldspar pb isotopic evaluation of extraregional sediment transported through an eocene tectonic breach of southern california s cretaceous batholith abstract sedimentary provenance studies have come to be overwhelmingly based upon u pb geochronologic measurements performed with detrital zircon while alternative and potentially complementary approaches such as conglomerate clast studies and heavy mineral analysis have faded in importance measurement of pb isotopic compositions in detrital k feldspar is among the under utilized approaches available to ascertain sedimentary source regions while it has been long recognized that common pb isotope compositions recorded by k feldspar vary widely and reflect the crustal provinces from which the host basement rocks crystallized use of the approach has suffered due to a lack of appropriate statistical models and ground truth compositional data from source regions in this paper we 1 present high throughput la icpms analysis protocols needed to generate statistically meaningful detrital k feldspar pb isotope data sets 2 develop an interpretative approach based upon 208pb 206pb vs 207pb 206pb that incorporate information from the u and th decay systems into one two dimensional plot that is amenable to analysis using two dimensional kolmogorov smirnoff statistical tests 3 generate new pb isotopic data from basement rocks from southwestern north america to improve knowledge of the pb isotopic properties of potential source regions and 4 generate new pb isotopic data from lower eocene to lower miocene sedimentary rocks to evaluate changes in drainage patterns that occurred in response to deformation that affected the southern california margin through this case study we demonstrate how our new analytical and interpretative methods could be profitably applied to future geochemical and provenance studies and tectonically driven re organization of drainage patterns

18o 16o and d h in goethite from a north american oxisol of the early eocene climatic optimum abstract an early eocene oxisol in the ione formation of california formed in a coastal continental weathering environment at a paleolatitude of 38 n the dominant minerals in the oxisol are goethite quartz and kaolinite material balance calculations were applied to new measurements of chemical composition d h and 18 o 16 o ratios of oxisol samples to determine the d 150 3 and 18 o 2 4 0 3 values of the goethite feooh these data in combination with the global meteoric water line mwl yielded an isotopic temperature of 21 4 c the nominal value of 21 c contrasts with the modern mean annual temperature mat of 16 c in that area the warmer temperature is consistent with formation of the goethite during the early eocene climatic optimum the isotopic composition of the goethite and a temperature of 21 c imply ancient water with a d value of 61 4 and a 18 o value of 8 9 0 5 this early eocene 18 o or d value is more negative than values in the range of isotopic scatter observed for modern global precipitation at sites with a mat of 21 c at times of warm global climates the location of a near surface atmospheric isotherm would generally shift relative to its location under modern climatic conditions a simple rayleigh type condensation model indicates that if one follows the isotherm the associated scatter in d and 18 o of precipitation in very warm global climates should shift for a given isotherm to more negative values that may be detectable in proxy records the isotopic results from the goethite of the early eocene oxisol appear to add to evidence in support of this idea

thermomechanical evolution of oceanic fracture zones a fracture zone fz model is constructed from existing models of the thermal and mechanical evolution of the oceanic lithosphere as the lithosphere cools by conduction thermal and mechanical boundary layers develop and increase in thickness as age 1 2 surface expressions of this development such as topography and deflection of the vertical i e gravity field are most apparent along major fz s because of the sharp age contrast a simple model including the effects of lateral heat transport but with no elastic layer predicts that variations in seafloor depth and deflection of the vertical will become increasingly smooth and ultimately disappear as the fz ages observations however show that both the fz topography and the deflection of the vertical remain sharp as the fz evolves these two observations as well as the observed asymmetry in deflection of the vertical profiles across the udintsev romanche and mendocino fz s are explained by including a continuous elastic layer in the model the asymmetry in deflection of the vertical is a consequence of elastic thickness variations across the fz modeling also shows that the evolution of the fz topography is extremely sensitive to the initial thermal structure near the ridge transform intersection model geoid steps and their development with age are used to access techniques for measuring geoid offsets across fz s reasonable step estimation techniques will underestimate the overall step amplitude by up to 50 this implies that abnormally thin thermal boundary layers derived from studies of geoid height versus age are not required by the data

changes in plate motions and the shape of pacific fracture zones geosat passes the new 2 min global gravity grid smith and sandwell 1995 and shipboard bathymetry across central pacific fracture zones were used to identify features common to fracture zone segments that formed during times of changes in plate motions these features are not predicted by current locked fault fracture zone models during a change in spreading direction that induces tension across the transform fault large offset greater than 500 km transforms develop multiple parallel faults spaced 50 to 100 km apart the gravity signature of small offset transform faults under tension includes a broader and more symmetric trough than observed on segments that formed during periods of steady spreading parts of fracture zones that form subsequent to a spreading reorientation that causes compression across the transform fault generally exhibit a single fault scarp that fits the locked fault model seafloor formed during a period of change usually marks a transition between structural styles for example between multiple fracture zone strands and a narrower single fault fracture zone widening of the transform fault zone under tension and narrowing under compression are consistent with the assumption that during a change in spreading direction the new spreading ridges propagate to but not across the old transform fault

evaluating debris slide occurrence using digital data paraglacial activity in chilliwack valley british columbia debris sliding is one of the most important processes acting to transport sediment within mountainous regions detailed study of debris slide activity at the basin scale typically involves landslide inventories generated from aerial photographs however it has been shown that some types of rapid mass movement can be accurately identified using a combination of high resolution satellite imagery and digital elevation data this approach is beneficial as the digital products allow for a more accurate and efficient data throughput into various types of geomorphic analysis here we demonstrate the use of an automated inventory in the geomorphometric evaluation of debris slide initiation for the chilliwack basin british columbia canada our results indicate that the occurrence of debris sliding is primarily determined by topographical controls for basins that are in equilibrium with the existing climate the frequency of debris sliding should demonstrate a strong relationship to bedrock geology as the production of unconsolidated materials available for failure is a function of weathering rates under these conditions the lack of bedrock control within the chilliwack basin suggests a state of paraglacial relaxation wherein glacial deposits dominate the sediment cascade within the area therefore topographic parameters can be used to discriminate the location of metastable slopes where debris slide erosion will be active the use of digital data in the characterization of debris slide occurrence would seem to be a viable alternative to the more traditional methods

sediment flux from a mountain belt derived by landslide mapping in humid uplands landsliding is the dominant mass wasting process in the western southern alps of new zealand landslides are scale invariant and have a power law magnitude frequency distribution independent studies from other regions suggest that this is a general property of landsliding this observation is of critical importance to the evaluation of the impact of events of different length scales over different time intervals on landscape evolution it is particularly useful when estimating regional geomorphic rates because it constrains the frequency and overall significance of extreme events which cannot otherwise be evaluated by integrating the complete response of the system we estimate the regional denudation rate due to landsliding to be 9 4 mm yr 1 sediment discharge from the western southern alps is dominated by landslide derived material

a reappraisal of the early slip history of the san andreas fault central california usa the modern san andreas fault system california united states is widely considered to have formed in response to the initiation of pacific north american plate interaction ca 27 ma although there is general consensus on the magnitude and timing of neogene displacement along the san andreas system its paleogene history remains unresolved in particular 100 km of right lateral offset between mid cretaceous plutonic rocks of the northern salinian block and the western edge of sierra nevada basement remains unaccounted for after restoration of neogene displacement along strike slip faults of the san andreas system our detrital zircon data invalidate a key paleogene piercing point by demonstrating that displaced portions of the hypothesized middle eocene butano point of rocks submarine fan were never contiguous across the san andreas fault we instead show that the eocene provenance characteristics exhibited by northern salinian strata closely match those of the southern sierra nevada and northwestern mojave desert this implies that the northern salinian block was located at least 75 50 km farther south in eocene time than previously recognized our data require 1 pre 23 ma dextral slip along the san andreas fault in central california and or 2 slip along a predecessor fault that formed prior to pacific north american plate interaction this previously undocumented slip may indicate that significant pacific north american plate interaction propagated from the plate margin into the continental interior much earlier than conventionally believed alternatively late paleogene slip could predate the development of the modern plate boundary and represent inboard dextral strike slip displacement along the eastern margin of the salinian block similar to the deformation that occurs today along the strike slip sumatra fault system

geology of the inyo mountains volcanic complex implications for jurassic paleogeography of the sierran magmatic arc in eastern california an 3 1 km thick volcanic complex exposed in the southern inyo mountains east central california records jurassic subaerial depositional environments along the east flank of the sierran arc this complex which we name the inyo mountains volcanic complex is subdivided into lower middle and upper stratigraphic intervals the 200 580 m thick lower interval comprises predominantly epiclastic strata deposited on alluvial fans and adjacent river flood plains that were inclined northeast mafic lava flows and rare reworked tuff in this interval record the onset of jurassic volcanism in this part of the arc the 300 700 m thick middle interval is composed predominantly of intermediate to silicic lava flows and tuffs representing a major episode of volcanism ending at ca 169 ma that is contemporaneous with emplacement of numerous plutons in the region the 2260 m thick upper interval is composed of epiclastic strata with minor intercalations of volcanic rock most of this interval accumulated on low gradient flood plains that hosted evaporative lakes and that were episodically invaded by alluvial fan complexes three new u pb age determinations constrain the lower half of the upper interval to have been deposited during the interval from ca 169 ma to 150 ma the uppermost part of the complex remains undated but probably accumulated prior to 140 ma the inyo mountains volcanic complex is part of a belt of volcanic complexes that are the easternmost preserved jurassic complexes of the sierran arc these complexes share sufficient similarities to suggest that they represent a distinctive arc flank depositional province significantly different from that represented by coeval volcanic complexes preserved in roof pendants farther west closer to the magmatic axis of the arc similarities among arc flank complexes include predominantly to exclusively subaerial settings substantial 30 portions of epiclastic strata and existence at times of north to northeast inclined paleoslopes we infer on the basis of the varying types and amounts of volcanic rocks that whereas most complexes in the arc flank province were rarely if ever proximal to major eruptive centers complexes in two areas white mountains and eastern mojave desert were at times located in or adjacent to such centers these differences lead us to speculate that the east flank of the jurassic arc consisted of eastward projecting volcanic salients separated by arc recesses typified by the inyo mountains area in which epiclastic deposition was dominant

thermal stresses in the oceanic lithosphere evidence from geoid anomalies at fracture zones models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate however there has so far been little direct evidence for the magnitude and distribution of thermal stresses we present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones specifically our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones including the effect of varying thermal bending moments thin plate theory predicts vertical nonisostatic displacements of the lithosphere by plate flexure the predicted amplitude of the resulting geoid anomaly is large enough to be observed in seasat altimeter profiles furthermore the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable the anomaly due to thermal stresses has been clearly identified in geoid profiles across the clarion and the udintsev fracture zones the amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 c consistent with the maximum depth of seismicity in the oceanic lithosphere the distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes

magnitude and frequency of rock falls and rock slides along the main transportation corridors of southwestern british columbia the two main transportation corridors of southwestern british columbia are subject to a range of rock slope movements rock falls rock slides and rock avalanches that pose significant risks to r

magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of california effects of assimilation crustal thickness and depth of crystallization in order to better understand geologic fac tors controlling pronounced regional variations in whole rock chemistry mineralogy and mineral chemistry in the batholiths of california we calculate the magmatic intensive variables f hf f h 2 o f hf f hcl and f o 2 regional scale west to east increases in f oh in mafic silicates corresponding to the systematic i wc to i mc and i sc progression reflect orders of magnitude increase in f hf f h 2 o attending pluton crystallization low f hf f h 2 o of formation of western i wc types is consistent with their derivation from low fluorine source rocks in subducted oceanic slabs or the upper mantle in contrast higher f hf f h 2 o of crystallization of i mc and i sc types to the east implies the involvement of 1 progressively greater amounts of continental crustal source material such as biotite bearing metamorphic rocks their unweathered sedimentary derivatives retaining f rich mafic minerals or their fusion products and or 2 source materials which become more f rich toward the continental interior the regional distribution of i mc and i sc types suggests that the precambrian craton of western north america or derivative sediments may extend farther north in california and be morphologically more complex than previously thought from seemingly out of place occurrences of i wc plutons on the eastern slopes of the sierra nevada batholith we infer the existence of regions where precambrian basement was thin or absent in mesozoic time which prevented extensive cratonal contamination of subducted slab or upper mantle derived magmas new methods for estimating t f o 2 relations in the magmas demonstrate that i scr granites crystallize at oxygen fugacities as much as five orders of magnitude lower than those of i wc i mc and i sc types under conditions at or below the maximum stability limit of graphite in equilibrium with a c o h s gas phase the local scale formation of i scr granites in plutonic belts within specific wall rock terranes containing highly reducing sediments or metasediments may occur by contamination of i types with graphitic pelite or in some cases by the direct fusion of this reducing pelitic wall rock the spatial distribution of i scr granite provinces therefore is controlled simply by wall rock lithology amphibole geobarometry demonstrates a general west to east decrease in crystallization pressure across the sierra nevada batholith in contrast the peninsular ranges batholith displays a west to east crystallization pressure increase the bulk of the california batholiths crystallized at pressures less than 4 5 kb and depths less than about 15 19 km in the southern sierra nevada batholith the san gabriels and the eastern peninsular ranges however plutons crystallized at pressures exceeding 6 kb at deep crustal levels 23 km reconstruction of the pre erosion top of the batholith shows that in an east west cross section the central sierra nevada batholith was a horizontal tabular body with an aspect ratio of at least five to one

a general model of paraglacial landscape response a unifying attribute of all forms of paraglacial response is that deglaciation results in the exposure of unstable or metastable sediment sources that are subsequently tapped by a wide variety of processes over a wide range of timescales the paraglacial period in different geomorphological contexts is therefore con ditioned by two variables glacially conditioned sediment release and rate of sediment reworking under steady state conditions the latter may be approximated by an exhaustion model in which sediment yield is related to the amount of remaining available sediment by a negative exponential function this model offers an approximation for the behaviour of primary paraglacial systems in which glacigenic sediment sources are not replenished the long term behaviour of secondary primarily uvial paraglacial systems in which sediment inputs include both in situ and reworked glacigenic sediment appears intrinsically more complex but consistent with an exhaustion model if initial sediment avai

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

identification of topographic settings conducive to landsliding from dem in nelson county virginia u s a the steep terrains in the blue ridge mountains of virginia are potentially vulnerable to landsliding hundreds of slope failures were initiated by a severe storm in 1969 in nelson county virginia the outlines of 87 landslide scars were delineated on aerial photographs within a 28 8 km2 area a digital elevation model dem of the same area was constructed from which three topographic variables slope orientation gradient and form were derived overlay analyses of the landslide data and the topographic data followed by chi square tests revealed that the topographic variables studied are statistically significant to the spatial distribution of the sites disturbed by landslide paths while the landsliding potential is low at very low and high elevations it is found to be high in the median range of elevation west northwest north and northeast facing slopes are consistently more vulnerable to landsliding than their east southeast south and southwest facing counterparts terrains with a gentle gradient are stable but the potential for landslides to develop increases greatly as the terrains become steeper the slopes with a concave cross section are more susceptible to landsliding than those having other shapes furthermore the landsliding potential does not vary significantly with slope forms in the profile direction it is concluded that the introduction of a dem to this study has provided renewed insight into topographic settings conducive to landsliding which could not have been achieved otherwise

structure and morphology of submarine volcanism in the hotspot region around reunion island western indian ocean the form of the deccan maldives mascarene reunion hotspot trace suggests that it has at least in part been strongly controlled by crustal structures especially fracture zones this makes it difficult to assess the present day or past location of the hotspot and thus complicates the interpretation of african plate motion reconstruction we present here results of a cruise to the reunion area of which the aims were a to determine the extent of present day volcanism associated with the reunion hotspot in the region and b to examine the role of pre existing oceanic crustal structures in controlling the location of present day volcanism additionally we examined the morphology and geology of the important extinct spreading centre southwest of reunion abandoned when spreading jumped to separate seychelles from india during the deccan flood basalt episode some 60 65 ma ago the extensive bathymetrie seismic and geological investigation shows that significant present day hotspot volcanism is confined to the piton de la fournaise edifice on reunion island itself apparently the location of recent reunion volcanism has not been controlled by a crustal fracture and the major fracture zones on both sides of the island are not acting as magma conduits for plate motion reconstruction and plume flux calculation purposes piton de la fournaise must be taken as the present location of the reunion hotspot accretion at the extinct spreading centre progressively ceased at the time of anomaly a27 63 ma and was associated with marked propagation of the rift tips

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

paleocene to holocene paleodeltas of ancestral colorado river offset by the san andreas fault system southern california upper paleocene to middle miocene fluvial deltaic rocks in the los angeles and ventura basins were deposited by a colorado paleoriver prior to 300 km of dextral displacement on the san andreas fault during the late miocene movement on the fault and associated rifting in the salton trough rerouted the paleoriver into the proto gulf of california since the pliocene translation of a moving colorado deltaic depocenter along the fault has filled the salton trough progressively southeastward the postulated colorado paleoriver extended north approximately along the lower course of the present river where it joined with gila and amargosa paleotributaries late cenozoic tectonism disrupted the amargosa paleoriver and the modern course of the colorado river through the lower grand canyon was developed

sedimentologic and isotopic constraints on the paleogene paleogeography and paleotopography of the southern sierra nevada california detrital zircon u pb provenance and stable isotopic studies of three paleogene southern sierra nevada california basins place new constraints on the paleoelevation history of the region age spectra from the paleocene witnet formation within the southernmost sierra nevada link these sediments to source terranes that were at or near sea level in the early cenozoic while age spectra from the paleocene goler formation east of the sierra nevada demonstrate isolation of southern sierra nevada basins from the continental interior and tapping of jurassic and triassic arc flank sources during the paleocene west of the sierra nevada strata of the eocene tejon formation are dominated by cretaceous zircons sourced from the sierran batholith goler formation carbonate 18 o suggests paleocene paleoelevations of 1 2 km for the central and southern sierra nevada taken together these data indicate a paleogene southern sierra nevada with modest elevations locally dissected to sea level by rift basins formed by late cretaceous lithospheric collapse these results place new limits on the amount of regional middle to late cenozoic elevation gain that may have resulted from the loss of dense mantle lithosphere from below the central and southern sierra nevada and point to possible north south variations in the topographic evolution of the sierra nevada

representing the landslide magnitude frequency relation capilano river basin british columbia we use magnitude frequency analysis as a statistical tool to quantify the erosion caused by landslides and debris ows using air photo and ground derived data we show that the departure from power law distribution customarily observed for small magnitude is an artefact of sampling de ciencies nonetheless the total distribution is not sensitive to the frequency of small slides and total erosion remains adequately represented in the air photo derived data our data also demonstrate a real departure from simple scaling at much larger magnitudes the cause of which is not de nitively established copyright 2004 john wiley sons ltd

source mechanisms of earthquakes near mid ocean ridges from body waveform inversion implications for the early evolution of oceanic lithosphere to investigate the early tectonic evolution of oceanic lithosphere we present a synthesis of the characteristics of near ridge earthquakes i e earthquakes which occur off the axis of major mid ocean spreading centers but in lithosphere less than 35 m y old near ridge seismicity for the past two decades has been rather evenly distributed along the major ridge systems with the exception of the central indian ocean and the eastern part of the cocos plate which have experienced a high rate of earthquake activity during that time for 32 near ridge earthquakes we determined the double couple orientation seismic moment centroid depth and source time function using a formal inversion technique based on matching synthetic and observed p and sh waveforms all types of faulting styles are observed seismogenic deformation in young oceanic lithosphere is concentrated in the first 15 m y and occurs almost entirely in the uppermost mantle from just below the moho to depths at which the temperature estimated from standard thermal models exceeds 800 c thrust faulting is not observed at depths greater than 10 km below the seafloor the deepest near ridge earthquakes are generally characterized by normal faulting focal mechanisms with the t axis oblique or perpendicular to the local spreading direction most of these earthquakes occur in the central indian ocean the transition from mechanisms characteristic of ridge axes to those characteristic of the near ridge environment occurs within the first few million years for lithosphere less than 35 m y old horizontal compressive stress associated with the cooling and subsidence of the lithosphere ridge push does not appear to be the dominant source of stress released in earthquakes this result may be used to place a lower bound of several hundred bars on the typical stress difference in young oceanic lithosphere this bound together with the observed depth of faulting implies that the strain rate associated with seismic deformation is locally at least 10 15 s 1 we propose that thermoelastic stresses associated with plate cooling play an important role in near ridge earthquakes this source of stress can account for the mechanisms and depths of many near ridge earthquakes and for the concentration of activity in very young lithosphere the comparative aseismicity of the oceanic crust may reflect the concentration of thermoelastic stress below the moho due to different material properties for the crust and upper mantle or the early release of thermoelastic stresses through hydrothermal circulation and cracking throughout the crust at or near the ridge axis the higher level of near ridge seismicity in the indian ocean may be the result of modification or enhancement of the more typical near ridge stress state by such local processes as the continental collision between india and asia or active secondary convection beneath the young lithosphere of the indian plate

thrusting in oceanic crust during continental drift offshore niger delta equatorial africa 1 two and three dimensional 3 d seismic reflection data acquired over oceanic crust in the deepwater west niger delta reveal convincing evidence for compressional tectonics during oceanic crustal spreading using the 3 d seismic data set we describe numerous inclined seismic reflections that dissect the entire oceanic crust from the top of the crust to the level of the moho that are interpreted as thrusts thrust propagation results in the development of associated hanging wall anticlines and footwall synclines these structures are orthogonal to and clearly postdate normal faults that formed during the accretion of oceanic crust during continental drift and strike at right angles to them the charcot ridge is located 140 km south of these thrusts and is a significantly larger structure it is a triangular shaped uplifted region of oceanic crust measuring 80 by 150 km and is located along the ne sw oriented charcot fracture zone two interpretations are possible for the role of the fracture zone in the development of the charcot ridge 1 a thin skinned model whereby the oceanic crust west of the fracture zone has been thrust southeastward with detachment occurring close to the level of the moho the ridge forms as a result of translation and folding above a crustal scale ramp flat thrust geometry 2 a thick skinned model where there is no detachment close to the moho with the thrust fault being much steeper penetrating the crust and probably the mantle lithosphere in this interpretation the structure formed owing to the compressional reactivation of the fracture zone approximate dating of onlapping reflections on either side of the ridge constrains the timing of its formation as between 25 and 120 ma ago the charcot ridge represents one of the largest thrust structures to be identified in a passive margin setting many other compressional folds with the same orientation formed to the northeast in the benue trough probably during the santonian as a result of a change in the spreading direction during south atlantic rifting we speculate that the same causal mechanism applies for the formation of the charcot ridge

why there are no earthquakes on the marquesas fracture zone although the plate tectonic paradigm does not predict relative horizontal motion between lithosphere on opposing sides of a fracture zone the fact that younger more rapidly subsiding seafloor lies adjacent to older seafloor implies relative vertical motion the observation that fracture zones are notably aseismic has led to the proposition of high strength along fracture zones such that the differential subsidence is accommodated by flexure across a locked fault this model predicts that a ridge develops on the young side of the fracture zone flanked by a foredeep trough on the old side with parallel warping of the moho and large associated gravity anomalies previous analyses of satellite altimetric passes over pacific fracture zones have shown that the amplitude and shape of the gravity anomalies frequently do not conform to the predictions of this simple model one of the most notable departures has been the marquesas fracture zone mfz where only one limited section was determined to be high strength curiously the only earthquake to rupture the mfz in the last 35 years is located in this locked region elsewhere where we would predict vertical slip must be occurring the fault is aseismic to better understand the history of vertical motion on this fault we have analyzed geophysical data obtained during the recent survey ew9106 aboard the r v maurice ewing our detailed hydrosweep gravity and seismic data resolve this paradox by showing no evidence for vertical slip along the fault shear stresses caused by differential thermal subsidence do not exceed the strength of this fault rather the failure of the altimetry signal to conform to the predictions of the high strength model along much of the mfz is caused by changes in the pacific farallon pole of rotation reorientation of the plate boundary was accommodated by propagating rifts intratransform tension or compression and changes in transform offset that complicated the signal from differential subsidence across a locked fault for example a counterclockwise rotation of the transform in the cretaceous caused overthrusting in the transform and thus compensation of the depth differential by flexural loading of a very young plate a clockwise rotation of this right stepping transform fault at about 35 ma led temporarily to a crenulated plate boundary and later to the development of intra transform spreading centers after accounting for these complications the fracture zone appears capable of sustaining at least 20 mpa of shear stress and remains locked along the entire length of the fault except perhaps locally where it passed over the tuamotu and society hot spots

quantitative analysis of land surface topography land surface topography significantly affects the processes of runoff and erosion a system which determines slope aspect and curvature in both the down slope and across slope directions is developed for an altitude matrix also the upslope drainage area and maximum drainage distance are determined for every point within the altitude matrix a fortran 66 program performs the analysis

disequilibrium of holocene sediment yield in glaciated british columbia it is generally supposed that specific sediment yield the quantity of sediment passing a monitored river cross section per unit area drained upstream of that section declines as the area drained increases1 3 part of the sediment mobilized from the land surface is supposed to go back into storage at field edges and on footslopes and floodplains in contrast we show here that data from british columbian rivers reveal a pattern of increasing specific sediment yield at all spatial scales up to 3 104 km2 this results from the dominance of secondary remobilization of quaternary sediments along river valleys over primary denudation of the land surface the result controverts the conventional model which has been derived from studies of small highly disturbed agricultural catch ments the rivers are still responding to the last glaciation giving a landscape relaxation time greater than 10 kyr this holds pro found implications for geomorphological theory and for studies of erosion

a hot and high eocene sierra nevada despite broad interest in determining the topographic and climatic histories of mountain ranges the evolution of california s sierra nevada remains actively debated prior stable isotope based studies of the sierra nevada have relied primarily on hydrogen isotopes in kaolinite hydrated volcanic glass and leaf n alkanes here we reconstruct the temperature and elevation of the early eocene sierra nevada using the oxygen isotope composition of kaolinitized granite clasts from the ancestral yuba and american rivers that drained the windward pacific flank of the sierra nevada first we evaluated the possible contributions of hydrogen isotope exchange in kaolinite by direct comparison with oxygen isotope measurements next we utilized differences in the hydrogen and oxygen isotope fractionation in kaolinite to constrain early eocene midlatitude weathering temperatures oxygen isotope geochemistry of in situ kaolinites indicates upstream eastward depletion of 18o in the northern sierra nevada the 18o values ranging from 11 4 to 14 4 at the easternmost localities correspond to paleoelevations as high as 2400 m when simulating the orographic precipitation of moisture from a pacific source using eocene boundary conditions this result is consistent with prior hydrogen isotope studies of the northern sierra but oxygen isotope based paleoelevation estimates are systematically 500 1000 m higher than those from hydrogen based estimates from the same samples kaolinite geothermometry from 16 samples produced early eocene weathering temperatures of 13 0 36 7 c with an average of 23 2 6 4 c 1 these kaolinite temperature reconstructions are in general agreement with paleofloral and geochemical constraints from eocene california localities and climate model simulations our results confirm prior hydrogen isotope based paleoelevation estimates and further substantiate the existence of a hot and high eocene sierra nevada