shallow landsliding root reinforcement and the spatial distribution of trees in the oregon coast range the influence of root reinforcement on shallow landsliding has been well established through mechanistic and empirical studies yet few studies have examined how local vegetative patterns influence slope stability because root networks spread outward from trees the species size and spacing of trees should influence the spatial distribution of root strength we documented the distribution and characteristics of trees adjacent to 32 shallow landslides that oc curred during 1996 in the oregon coast range although broadly classified as a conifer dominated forest we observed sparse coniferous and abundant hardwood trees near landslide scars in an industrial forest mapleton that experienced widespread burning in the 19th century in industrial forests that were burned selectively harvested and not replanted elliott state forest swordfern was ubiquitous near landslides and we observed similar numbers of live conifer and hardwood trees proximal to landslide scarps we demonstrate that root strength quantified in landslide scarps and soil pits correlates with a geometry based index of root network contribution derived from mapping the size species condi tion and spacing of local trees indicating that root strength can be predicted by mapping the distribution and charac teristics of trees on potentially unstable slopes in our study sites landslides tend to occur in areas of reduced root strength suggesting that to make site specific predictions of landslide occurrence slope stability analyses must account for the diversity and distribution of vegetation in potentially unstable terrain

correction of deposit ages for inherited ages of charcoal implications for sediment dynamics inferred from random sampling of deposits on headwater valley floors abstract inherited age is defined herein as the difference between times of carbon fixation in a material and deposition of that material within sediments from which it is eventually sampled in order to estimate deposit age via radiocarbon dating inheritance generally leads to over estimation of the age by an unknown amount and therefore represents unquantified bias and uncertainty that could potentially lead to erroneous inferences inherited ages in charcoal are likely to be larger and therefore detectable relative to analytic error where forests are dominated by longer lived trees material is stored for longer periods upslope and downstream post fire delivery of that material is dominated by mass movements such as in the near coastal mountains of northwestern north america inherited age distribution functions were estimated from radiocarbon dating of 126 charcoal pieces from 14 stream bank exposures of debris flow deposits fluvial fines and fluvial gravels along a headwater stream in the southern oregon coast range usa in the region these 3 facies are representative of the nearly continuous coalescing fan fill complexes blanketing valley floors of headwater streams where the dominant transport mechanism shifts from debris flow to fluvial within each depositional unit and for each charcoal piece within that unit convolution of the calibrated age distribution with that of the youngest piece yielded an inherited age distribution for the unit fits to the normalized sums of inherited age distributions for units of like facies provided estimates of facies specific inherited age distribution functions finally convolution of these distribution functions with calibrated deposit age distributions yielded corrections to published valley floor deposit ages and residence time distributions from nearby similar sites residence time distributions were inferred from the normalized sums of distributions of 30 deposit ages at each of 4 sites 2 adjacent valley reaches 103 m long and within 102 m of 2 tributary confluences mean inherited ages from the observed distributions are 666 688 and 1506 yr for debris flow deposits fluvial fines and fluvial gravels respectively on average correction reduced estimates of individual deposit age means by a factor of 0 71 0 56 0 94 and increased standard deviations by a factor of 6 1 0 97 43 across sites mean residence times decreased by 24 0 and standard deviations by 12 5 on average corrected residence time distributions have thicker tails as indicated by gamma distribution fits with smaller shape factors and these changes are significant relative to the bootstrapped 95 confidence limits representing potential error in the sampling for inherited ages the ratio of the means of sediment age and residence time ranged from 1 03 to 1 80 across sites before correction and 1 21 to 2 18 after correction where a value of one implies that probability of evacuation from the reservoir comprising valley floor deposits is independent of time since deposition corrected values of this ratio therefore indicate that evacuation favors younger deposits at all sites whereas uncorrected results implied age independent evacuation from the more downstream valley reach

drainage basin sensitivity to tectonic and climatic forcing implications of a stochastic model for the role of entrainment and erosion thresholds long term average rates of channel erosion and sediment transport depend on the frequency magnitude characteristics of ood ows that exceed an erosion threshold using a poisson model for rainfall and runoff analytical solutions are developed for average rates of stream incision and sediment transport in the presence of such a threshold solutions are derived and numerically tested for three erosion transport formulas the howard kerby shear stress incision model the bridge dominic sediment transport model and a generic shear stress sediment transport model results imply that non linearity resulting from threshold effects can have a rst order impact on topography and patterns of dynamic response to tectonic and climate forcing this non linearity becomes signi cant when fewer than about half of ood events are capable of detaching rock or sediment predicted morphology and uplift gradient scaling is more closely consistent with observations and laboratory experiments than conventional slope linear or shear linear erosion laws these results imply that particle detachment thresholds are not details that can be conveniently ignored in long term landscape evolution models copyright 2004 john wiley sons ltd

tethyan and indian subduction viewed from the himalayan high to ultrahigh pressure metamorphic rocks abstract the himalayan range is one of the best documented continent collisional belts and provides a natural laboratory for studying subduction processes high pressure and ultrahigh pressure rocks with origins in a variety of protoliths occur in various settings accretionary wedge oceanic subduction zone subducted continental margin and continental collisional zone ages and locations of these high pressure and ultrahigh pressure rocks along the himalayan belt allow us to evaluate the evolution of this major convergent zone 1 cretaceous 80 100 ma blueschists and possibly amphibolites in the indus tsangpo suture zone represent an accretionary wedge developed during the northward subduction of the tethys ocean beneath the asian margin their exhumation occurred during the subduction of the tethys prior to the collision between the indian and asian continents 2 eclogitic rocks with unknown age are reported at one location in the indus tsangpo suture zone east of the nanga parbat syntaxis they may represent subducted tethyan oceanic lithosphere 3 ultrahigh pressure rocks on both sides of the western syntaxis kaghan and tso morari massifs formed during the early stage of subduction exhumation of the indian northern margin at the time of the paleocene eocene boundary 4 granulitized eclogites in the lesser himalaya sequence in southern tibet formed during the paleogene underthrusting of the indian margin beneath southern tibet and were exhumed in the miocene these metamorphic rocks provide important constraints on the geometry and evolution of the india asia convergent zone during the closure of the tethys ocean the timing of the ultrahigh pressure metamorphism in the tso morari massif indicates that the initial contact between the indian and asian continents likely occurred in the western syntaxis at 57 1 ma west of the western syntaxis the higher himalayan crystallines were thinned rocks equivalent to the lesser himalayan sequence are present north of the main central thrust moreover the pressure metamorphism in the kaghan massif in the western part of the syntaxis took place later 7 m y after the metamorphism in the eastern part suggesting that the geometry of the initial contact between the indian and asian continents was not linear the northern edge of the indian continent in the western part was 300 to 350 km farther south than the area east of the nanga parbat syntaxis such en baionnette geometry is probably produced by north trending transform faults that initially formed during the late paleozoic to cretaceous gondwana rifting farther east in the southern tibet the collision occurred before 50 6 0 2 ma finally high pressure to ultrahigh pressure rocks in the western himalaya formed and exhumed in steep subduction compared to what is now shown in tomographic images and seismologic data

fire and the evolution of steep soil mantled landscapes recent burns in the western united states attest to the signif icant geomorphic impact of fire in mountainous landscapes yet we lack the ability to predict and interpret fire related erosion over millennial time scales a diverse set of geomorphic processes is often invoked following fire the magnitude of postfire erosional processes coupled with temporal variations in fire frequency dic tate the extent to which fires affect sediment production and land scape evolution in the oregon coast range several models for long term rates of soil production and transport have been tested and calibrated although treatment of fire related processes has been limited following recent fires in the oregon coast range we observed extensive colluvial transport via dry ravel localized bed rock emergence due to excess transport and talus like accumula tion in adjacent low order valleys soils exhibited extreme but dis continuous hydrophobicity and no evidence for rilling or gullying was observed using a field based data set for fire induced dry rav el transport we calibrated a physically based transport model that indicates that soil flux varies nonlinearly with gradient the post fire critical gradient 1 03 which governs the slope at which flux increases rapidly is lower than the previously estimated long term value 1 27 reflecting the reduction of slope roughness from in cineration of vegetation by using a high resolution topographic data set generated via airborne laser swath mapping we modeled the spatial pattern of postfire and long term erosion rates postfire erosion rates exceed long term rates which average 0 1 mm yr 21 by a factor of six and subtle topographic variations generated local patches of rapid postfire erosion commonly 1 mm yr 21 our sim ulations indicate that fire related processes may account for 50 of temporally averaged sediment production on steep hillslopes our analysis provides a mechanistic explanation for the coincident early holocene timing of increased fire frequency and regional ag gradation in oregon coast range drainage basins given the sen sitivity of steep hillslopes to fire driven transport changes in cli mate and fire frequency may affect soil resources by perturbing the balance between soil transport and production

processes and rates of sediment and wood accumulation in headwater streams of the oregon coast range usa channels that have been scoured to bedrock by debris flows provide unique opportunities to calculate the rate of sediment and wood accumulation in low order streams to understand the temporal succession of channel morphology following disturbance and to make inferences about processes associated with input and transport of sediment dendrochronology was used to estimate the time since the previous debris flow and the time since the last stand replacement fire in unlogged basins in the central coast range of oregon debris flow activity increased 42 per cent above the background rate in the decades immediately following the last wildfire changes in wood and sediment storage were quantified for 13 streams that ranged from 4 to 144 years since the previous debris flow the volume of wood and sediment in the channel and the length of channel with exposed bedrock were strongly correlated with the time since the previous debris flow wood increased the storage capacity of the channel and trapped the majority of the sediment in these steep headwater streams in the absence of wood channels that have been scoured to bedrock by a debris flow may lack the capacity to store sediment and could persist in a bedrock state for an extended period of time with an adequate supply of wood low order channels have the potential of storing large volumes of sediment in the interval between debris flows and can function as one of the dominant storage reservoirs for sediment in mountainous terrain copyright 2003 john wiley sons ltd

large himalayan frontal thrust paleoearthquake at khayarmara in eastern nepal abstract an exposure created by excavation of a trench across the himalayan frontal thrust provides the basis to interpret that a single earthquake produced vertical separation of 7 m at khayarmara a small community 80 km southwest of kathmandu the fault trace at khayarmara is expresssed by a topographic ridge resulting from folding up dip and toward the surface at the expense of greater fault slip taking place at depth structure stratigraphy and radiocarbon data are interpreted to indicate displacement occurred after about 1050 ce to 1200 ce the timing and displacement at khayarmara are compared to that reported previously at six contiguous sites that extend from 200 km to the west and 250 km to the east of khayarmara respectively the comparison leads us to conclude that the surface rupture at khayarmara was part of a 250 km long synchronous surface rupture earthquake of magnitude approaching if not surpassing mw 9 we observe in the exposure no record of surface rupture associated with the great 1934 bihar nepal earthquake

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

effect of eolian fe supply change on the phytoplankton productivity and community in central equatorial pacific ocean during the pleistocene a lipid biomarker approach abstract the effect of iron fe fertilization on biological productivity and community structure in the equatorial pacific a region with high nutrient and low chlorophyll concentrations has been debated for many years here we present lipid biomarker and geochemical proxy records in a sediment core from the central equatorial pacific spanning from 1100 to 600 ka to investigate the response of marine phytoplankton communities to eolian fe supply and to assess links to environmental fluctuations during the pleistocene we found a strong link between change in eolian fe input and phytoplankton community structure our data demonstrate that an increase in eolian fe supply across the 800 ka boundary resulted in a shift in the major primary producer from coccolithophores alkenones to diatoms epi brassicasterol and dinoflagellates dinosterol at the study site this strong link between changes in eolian fe supply and the community structure of plankton during 800 600 ka may have been associated with a change in atmospheric and ocean conditions due to southward migration of the intertropical convergence zone itcz the lipid biomarker results show that such a high supply of eolian fe might have led to a change in the dominant phytoplankton group however the opal record which is often used as a primary productivity indicator was not consistent with the biomarker results therefore the results indicate that multi proxy approaches should be used in paleoenvironmental studies because different proxies can reveal different aspects of environmental conditions thus contributing to a better understanding of paleoceanographic conditions

debris dams and the relief of headwater streams in forested mountain landscapes where debris flows are common their deposition commonly forms valley spanning dams of wood boulders or complex mixtures of both in headwater valleys sediment impoundment behind these dams causes alluviation in what would otherwise be bedrock channels in this paper the effects of debris dams on the evolution of headwater valley profiles over geologic time are examined in the oregon coast range usa longitudinal profiles of three headwater channels approximately 2 km 2 maximum contributing area two of which are nominally bedrock and all debris dams were surveyed channel and valley widths were measured and surface bed material measurements pebble counts were taken at several locations along one stream cumulative relief in debris dams is highly variable within and among streams reaching a maximum of 58 of profile relief at one location the surveyed dams comprise 9 8 19 and 6 4 of total basin relief in the three corresponding drainage basins respectively a model of bedrock erosion and channel profile evolution is derived that accounts for a fractional coverage by the channel of total valley width at any given time and b shielding by sediment impounded behind debris dams the equation for incision rate assumed constant is solved for valley gradient as a function of contributing area and parameter values are estimated from or supplied by field data results are compared to stream gradient contributing area relationships obtained for the field sites these comparisons suggest a strong effect of network structure which varies significantly among the sites on profile shape and relief because of the different susceptibilities to debris flow deposition and therefore debris dam formation longitudinal profiles with and without the effects of debris dams are modeled for one forced alluvial i e nominally bedrock channel for these model profiles which extend only as far as the surveyed profile at this site 55 of the profile relief is due to debris dams and the concomitant shielding of the bed by impounded sediment the results suggest that a significant fraction of the relief of such forested mountain landscapes is due to the effects of relatively immobile wood and boulders deposited in the valleys by debris flows

ablative subduction a two sided alternative to the conventional subduction model subduction is conventionally modelled as a convergence and overlapping of two semirigid plates such a convergence is one sided in that one lithospheric plate descends while the other remains on the surface we suggest an alternative model one based on the dynamics of fluids in this model the viscous lower lithosphere flows downward and the brittle upper lithosphere deforms in passive response this process is potentially double sided for we find that even a buoyant plate can be dragged downward by a dense descending neighbor thus an apparent overriding plate may be worn away by a process of viscous ablation with the rate of ablation a function of plate buoyancy we call this process ablative subduction ablative subduction allows us to simply interpret observations concerning slab profiles interplate seismicity back arc tectonics and complex processes such as double subduction and subduction polarity reversal in performing experiments modelling the evolution of simple fluid slabs we find that slab profile is strongly influenced by ablation in the overriding plate when ablation is weak as when a buoyant continent borders the trench deformable slabs adopt shallow andean style profiles these profiles develop over time from an initially steep shape more vigorous ablation rates as might occur in an ocean ocean convergence yield steeper marianas style profiles thus differences between marianas style and andean style slabs may result from differences in ablation rate and subduction duration the occurrence of ablation might not be easily detected at depth because material from subducting and ablating plates adhere closely as a single slab these slabs show downdip deformations that are consistent with seismic focal mechanisms plate deformations associated with ablative subduction are also consistent with observed patterns of seismicity in particular the relative aseismicity of marianas style plate boundaries is consistent with the ablative model we may thus explain how plates can converge aseismically without requiring that these plates be decoupled ablation should be weakest in andean style subduction although pulses of rapid ablation might lead to episodes of crustal shortening in oceanic regions the manifestation of ablation may depend upon tectonic regime in an extensional area like the marianas ablation may be temporally self limiting and ablative cycles may explain conflicting observations of tectonic erosion and accretion in a compressional regime vigorous ablation might lead to observations of double subduction and subduction reversal in each case the behavior of the lower lithosphere is relatively simple what varies is the rate of ablation and the response of the upper lithosphere

evidence for regional stream aggradation in the central oregon coast range during the pleistocene holocene transition abstract low nearly continuous terraces of similar age are present along streams in drainage basins that range in size from drift creek 190 km 2 to the umpqua river 11 800 km 2 in the oregon coast range radiocarbon ages from near the bose of fluvial sediments underlying these terraces are clustered at about 9000 11 000 14 c yr b p beveled bedrock surfaces straths that underlie the fluvial sediments are 1 8 m above summer stream levels and are present along most of the nontidal reaches of the rivers that we studied where exposed the bedrock straths are overlain by 2 11 m of fluvial sediment that consists of a bottom stratum channel facies of sandy pebble cobble gravel and a top stratum overbank facies of sandy silt or silt eight radiocarbon ages from the fluvial sediments allow correlation of the lowest continuous terrace over a wide area and thus indicate that a regional aggradation episode occurred in coast range drainage basins during the pleistocene holocene transition the cause of such widespread aggradation is unknown but may be related to climate induced changes in the frequency of evacuation of colluvium from hollows which are common in all drainage basins in the region

formation and exhumation of ultra high pressure rocks during continental collision role of detachment in the subduction channel 1 uhp rocks commonly form and exhume during the transition from oceanic subduction to continental collision their exhumation in subduction channels depends on the balance between down channel shear traction and up channel buoyancy thermal mechanical upper mantle scale numerical models are used to investigate how variations in material properties of the subducting continental margin affect this balance changes in shear traction leading to crustal decoupling detachment are investigated by varying the onset of strain weakening thermal parameters and convergence velocity variations in buoyancy force are investigated by modifying subducted material density and volume the model results are interpreted in terms of the exhumation number e which expresses the role of the pressure gradient channel thickness effective viscosity and subduction velocity peak metamorphic conditions exhumation velocity and timing of exhumation are temporally and spatially variable and are sensitive to the evolution of e the models reproduce natural ptt constraints and indicate that neither slab breakoff nor surface erosion is required for uhp exhumation

erosion of steepland valleys by debris flows episodic debris flows scour the rock beds of many steepland valleys along recent debris flow runout paths in the western united states we have observed evidence for bedrock lowering primarily by the impact of large particles entrained in debris flows this evidence may persist to the point at which debris flow deposition occurs commonly at slopes of less than 0 03 0 10 we find that debris flow scoured valleys have a topographic signature that is fundamentally different from that predicted by bedrock river incision models much of this difference results from the fact that local valley slope shows a tendency to decrease abruptly downstream of tributaries that contribute throughgoing debris flows the degree of weathering of valley floor bedrock may also decrease abruptly downstream of such junctions on the basis of these observations we hypothesize that valley slope is adjusted to the long term frequency of debris flows and that valleys scoured by debris flows should not be modeled using conventional bedrock river incision laws we use field observations to justify one possible debris flow incision model whose lowering rate is proportional to the integral of solid inertial normal stresses from particle impacts along the flow and the number of upvalley debris flow sources the model predicts that increases in incision rate caused by increases in flow event frequency and length as flows gain material downvalley are balanced by rate reductions from reduced inertial normal stress at lower slopes and stronger less weathered bedrock these adjustments lead to a spatially uniform lowering rate although the proposed expression leads to equilibrium long profiles with the correct topographic signature the crudeness with which the debris flow dynamics are parameterized reveals that we are far from a validated debris flow incision law however the vast extent of steepland valley networks above slopes of 0 03 0 10 illustrates the need to understand debris flow incision if we hope to understand the evolution of steep topography around the world

experiments in waterlogging and sedimentology of charcoal results and implications abstract fossil charcoal has a sporadic occurrence in sedimentary rocks since devonian time it is moderately common as a component of the organic material found in a wide variety of facies but there are also some notable concentrations which occur locally these occurrences have considerable palaeobotanical and palaeoecological value because the process of charring may result in excellent preservation of plant tissue if assemblages of charred material are to be used to interpret palaeoenvironments it is important to understand the behaviour of charcoal during transport from the site of the fire charcoal is an unusual sedimentary material because most fresh material floats but with prolonged immersion becomes waterlogged and sinks we carried out a series of waterlogging experiments on uncharred material and charcoal from a range of different plant types and tissues these show considerable variations in the rates of waterlogging of different charred and uncharred plant tissues and it is suggested that buoyancy contrasts are likely to result in separation during transport and deposition furthermore experiments in a flume tank have shown that the process of deposition of charcoal in sand is controlled by the rate of migration of bedforms which is in turn determined by depth flow velocity and sediment supply allochthonous assemblages of charred and uncharred plant debris will most probably be biased and not fully representative of the contemporaneous plants or vegetation the presence of charcoal concentrations may be used as an aid in interpreting palaeohydraulic conditions

indian plate motion and shape constraints on the geometry of the himalayan orogen abstract sea floor palaeomagnetic data that reflect variations in rate and vector of indian plate movement and rotation suggest that initial collision between india and asia occurred at about 50 55 ma ago as the pre collisional indian plate was diamond shaped with the northern margin comprised of two oblique boundaries collision was earliest where these boundaries meet or in what is now the northwest himalaya oblique convergence along each of these two boundaries would generate rotation of thrust sheets as they climb on to the indian plate the oroclinal shape of the main himalayan chain to the east of the northwest himalayan syntaxes reflects a combination of the effects of oblique convergence post collisional anticlockwise rotation of the indian plate and the pinning of the main thrusts at their northwestern terminations by crust thickened during the earliest collisional stage the indian plate rotation enhances a strike slip component of movement along the western oblique margin with the transpressively sinistral chaman fault zone now acting as a continental escape structure

charcoal recognition taphonomy and uses in palaeoenvironmental analysis abstract charcoal predominantly the product of wildfires is abundant in many sedimentary rocks deposited in a wide range of environments from terrestrial to marine it also occurs in some volcanic rocks this paper outlines aspects of charcoal formation both natural and experimental and briefly considers the taphonomic processes leading to a final assemblage this is done using examples from recent fires and through experimentation the ways in which charcoal assemblages are recognized in the field and extraction in the laboratory are also considered methods of charcoal identification are presented the range of charcoalified plant organs that can be found is discussed and a wide range of study methods outlined including light microscopy dark field light microscopy reflectance microscopy scanning electron microscopy and synchrotron radiation x ray tomographic microscopy emphasis in this paper is on the study of macro and meso charcoal above180 m finally there is a consideration of the broad use of charcoal from plant evolution studies fire history studies vegetation studies anatomical studies climate and atmospheric studies and the wider importance of charcoal for the earth and biological sciences charcoal is information rich but yet is an under utilized resource

vegetative and climatic controls on holocene wildfire and erosion recorded in alluvial fans of the middle fork salmon river idaho the middle fork salmon river watershed spans high elevation mixed conifer forests to lower elevation shrub steppe in recent decades runoff from severely burned hillslopes has generated large debris flows in steep tributary drainages these flows incised alluvial fans along the mainstem river where charcoal rich debris flow and sheetflood deposits preserve a record of latest pleistocene to holocene fires and geomorphic response through deposit sedimentology and 14c dating of charcoal we evaluate the processes and timing of fire related sedimentation and the role of climate and vegetation change fire related deposits compose 66 of the total measured fan deposit thickness in more densely forested upper basins versus 33 in shrub steppe dominated lower basins fires during the middle holocene 8000 5000 cal yr bp mostly resulted in sheetflood deposition similar to modern events in lower basins decreased vegetation density during this generally warmer and drier period likely resulted in lower sev

effects of wood on debris flow runout in small mountain watersheds 1 debris flows have typically been viewed as two phase mixtures of sediment and water but in forested mountain landscapes wood can represent a sizable fraction of total flow volume the effects of this third phase on flow behavior are poorly understood to evaluate whether wood can have a significant effect on debris flow runout in small mountainous watersheds we used a landscape scale model combining empirical stochastic and physical submodels of storms fires forest growth tree fall wood decay soil production and diffusion landslide initiation debris flow runout and fluvial sediment transport we examined changes in the cumulative distribution function of debris flow runout lengths in a small 2 km2 watershed in the oregon coast range due to presence or absence of two hypothesized effects of wood 1 velocity reduction due to entrainment of wood in the runout path and 2 velocity reduction due to changes in flow direction angle the model was calibrated such that the distribution for simulations including both effects was similar to that measured in the study basin and amounts of wood in the simulation and the field both fallen in small valleys and incorporated by debris flows were comparable removal of either effect or both significantly shifted runout length distributions to longer lengths simulations and field observations indicate that with wood fluvial transport is a significant source of sediment output few debris flows reach the outlet and debris flow deposits are widely distributed throughout the network simulations indicate that without wood basin sediment yield greatly increases that yield is dominated by longer runout debris flows and that debris flow deposits are concentrated in the low gradient reach near the outlet

modeling the evolution of continental subduction processes in the pamir hindu kush region several geological and geophysical studies suggest the presence of two converging subduction zones in the western syntaxis of the india eurasia collision zone with steep northward subduction of indian lithosphere beneath the hindu kush and southward subduction of asian lithosphere under the pamir we investigate the geometry and the timing of these subduction processes seismic tomography images are used to constrain the geometry of the two slabs under the pamir and the hindu kush by measuring the tomographically inferred length of the indian slab under the hindu kush region and by comparison with paleomagnetic reconstructions we estimate that the process of indian slab break off most likely occurred at the early stage of the collision at ca 44 48 ma we infer that after slab break off western india continued its northward motion until its northern boundary reached the present hindu kush region where it began to subduct we estimate an age for the initiation of subduction in this region of about 8 ma we apply 2d thermo kinematic and rheological numerical modeling to compute the temperature distribution and brittle field along two vertical sections across the hindu kush and pamir regions for the hindu kush we introduce parameters reproducing fast and near vertical subduction whereas in the pamir region we use the results from published studies to reproduce slower subduction modeling results indicate that faster subduction under the hindu kush results in a deeper brittle region which is consistent with observed deeper seismicity than in the pamir region

sediment reservoirs at mountain stream confluences dynamics and effects of tributaries dominated by debris flow and fluvial processes radiocarbon age estimates n 68 from bank terrace riser and in channel materials sampled from random locations near two channel confluences a debris flow dominated tributary to cedar creek and a fluvially dominated tributary to golden ridge creek in the oregon coast range are proxies for sediment transit times through tributary and mainstem sediment reservoirs separated from one another by incised bedrock risers geomorphic volumetric stratigraphic and sedimentologic data aided reservoir characterizations inferred transit time distributions for tributary deposits are right skewed and heavy tailed indicating preferential evacuation of younger deposits the debris flow fan is much larger than fluvial terraces on the other tributary but mean transit times in both reservoirs are similar 1370 2240 yr and 1660 2130 yr for fan and terrace deposits respectively whereas tributary deposits are much larger than mainstem deposits at both sites mainstem deposits adjacent to the fan have a relatively short mean transit time of 442 491 yr but mean transit time in mainstem deposits adjacent to the fluvial terrace is much greater 3870 6720 yr reservoir flux estimates indicate that most 60 of the debris flow fan tributary9s sediment yield enters fan storage but only a small part 3 of the fluvial tributary9s yield enters storage at the confluence debris flows from the debris flow fan tributary apparently promote both greater storage of mainstem sediments and more rapid unbiased evacuation of mainstem deposits whereas old mainstem deposits adjacent to the fluvial tributary have a much greater probability of preservation

transient hot channels perpetrating and regurgitating ultrahigh pressure high temperature crust mantle associations in collision belts abstract two dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite and diamond bearing rocks metamorphosed at 700 to 900 c in the presence of dense supercritical silicate fluids and melts may be explained by a transient hot channel effect anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper crustal rocks especially sediments of passive margin origin and mantle rocks heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration deserpentinization of the overriding mantle lithosphere that has been hydrated during earlier subduction stages the channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate 150 200 km and is characterized by metamorphic temperatures reaching 700 to 900 c low effective viscosity of rocks subsequent to increased temperature partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks the hot channel exists only during early collision but rapidly produces large amounts of ultrahigh pressure high temperature rocks further collision closes the channel by squeezing rheologically weak partially molten buoyant rocks between the strong lithospheric mantles of the two colliding plates assemblages of complicated p t paths with repetitive loops characterize exhumation of ultrahigh pressure rocks in the convoluted flow pattern of the hot channel

flat versus steep subduction contrasting modes for the formation and exhumation of high to ultrahigh pressure rocks in continental collision zones abstract flat and steep subduction are end member modes of oceanic subduction zones with flat subduction occurring at about 10 of the modern convergent margins and mainly around the pacific continental margin subduction normally follows oceanic subduction with the remarkable event of formation and exhumation of high to ultrahigh pressure hp uhp metamorphic rocks in the continental subduction collision zones we used 2d thermo mechanical numerical models to study the contrasting subduction collision styles as well as the formation and exhumation of hp uhp rocks in both flat and steep subduction modes in the reference flat subduction model the two plates are highly coupled and only hp metamorphic rocks are formed and exhumed in contrast the two plates are less coupled and uhp rocks are formed and exhumed in the reference steep subduction model in addition faster convergence of the reference flat subduction model produces extrusion of uhp rocks slower convergence of the reference flat subduction model results in two sided subduction collision the higher lower convergence velocities of the reference steep subduction model can both produce exhumation of uhp rocks a comparison of our numerical results with the himalayan collisional belt suggests two possible scenarios 1 a spatially differential subduction collision model which indicates that steep subduction dominates in the western himalaya while flat subduction dominates in the extensional central himalaya and 2 a temporally differential subduction collision model which favors earlier continental plate flat subduction with high convergence velocity in the western himalaya and later flat subduction with relatively low convergence velocity in the central himalaya

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

the intercept is a poor estimate of a calibrated radiocarbon age intercept based methods of generating a point estimate of a calibrated radiocarbon date are very popular but exhibit undesirable behaviour they are highly sensitive to the mean of the radiocarbon date and to adjustments of the calibration curve other methods give more stable results the weighted average of the probability distribution function is recommended as the best central point estimate but more consideration should be given to using the full probability distribution rather than a point estimate in developing age depth models

continental subduction and exhumation of high pressure rocks insights from thermo mechanical laboratory modelling abstract thermo mechanical physical modelling of continental subduction is performed to investigate the exhumation of deeply subducted continental crust the model consists of two lithospheric plates made of new temperature sensitive analogue materials the lithosphere is underlain by liquid asthenosphere the continental lithosphere contains three layers the weak sedimentary layer the crust made of a stronger material and of a still stronger lithospheric mantle the whole model is subjected to a constant vertical thermal gradient causing the strength reduction with depth in each lithospheric layer subduction is driven by both push force and pull force during subduction the subducting lithosphere is heating and the strength of its layers reduces the weakening continental crust reaches maximal depth of about 120 km and cannot subduct deeper because its frontal part starts to flow up the subducted crust undergoes complex deformation including indicated upward ductile flow of the most deeply subducted portions and localised failure of the subducted upper crust at about 50 km depth this failure results in the formation of the first crustal slice which rises up between the plates under the buoyancy force this process is accompanied by the delamination of the crustal and mantle layers of the subducting lithosphere the delamination front propagates upwards into the interplate zone resulting in the formation of two other crustal slices that also rise up between the plates average equivalent exhumation rate of the crustal material during delamination is about 1 cm year the crust asthenosphere boundary near the interplate zone is uplifted the subducted mantle layer then breaks off removing the pull force and thereby stopping the delamination and increasing horizontal compression of the lithosphere the latter produces shortening of the formed orogen and the growth of relief the modelling reveals an interesting burial exhumation evolution of the sedimentary cover during initial stages of continental subduction the sediments of the continental margin are dragged to the overriding plate base and are partially accreted at the deep part of the interplate zone at 60 70 km depth these sediments remain there until the beginning of delamination during which the pressure between the subducted crust and the overriding plate increases this results in squeezing the underplated sediments out part of them is extruded upwards along the interplate zone to about 30 km depth at an equivalent rate of 5 10 cm year

flow laws of multiphase rocks calculated from experimental data on the constituent phases abstract an attempt has been made to develop expressions for the flow laws of polyphase rocks in terms of the volume fractions and flow laws of their constituent phases the flow strengths predicted by the proposed model for granite diorite diabase and lherzolite are in good agreement with previously determined experimental values the study allows one to calculate to the first approximation the flow laws of a large number of rocks at geologic conditions based on the experimentally determined flow laws of a relatively small number of monomineralic aggregates

fire and alluvial chronology in yellowstone national park climatic and intrinsic controls on holocene geomorphic processes we employed a systemwide approach a large and robust set of radiocarbon ages and modern process analogs to interpret the holocene history of forest fire related sedimentation and overall alluvial activity in northeastern yellowstone national park debris flow and flood events following the 1988 fires provided facies models for interpreting the stratigraphic record of fire related sedimentation within valley side alluvial fans of soda butte creek fire related deposits make up approximately 30 of the late holocene fan alluvium fifty 14 c ages on fire related events cluster within the intervals of 3300 2900 2600 2400 2200 1800 and 1400 800 yr b p and suggest earlier episodes of large fires and fan aggradation around 7500 5500 and 4600 4000 yr b p a major pulse of fire related debris flow activity between 950 and 800 yr b p coincided with the height of the widely recognized medieval warm period ca a d 1050 1200 instrumental climate records over the last 100 yr in yellowstone imply that the intensity and interannual variability of summer precipitation are greater during warmer periods enhancing the potential for severe short term drought major forest fires and storm generated fan deposition along lower soda butte creek fill cut terrace treads were created by lateral migration of channels and accumulation of overbank sediments ca 8000 yr b p terrace level t1a 7000 5600 t1b 3100 2600 t2 2000 1300 t3 and post 800 yr b p t4 these periods coincide with overbank sedimentation on slough creek and the lamar river but alternate with intervals of fire related fan deposition implying a strong climatic control local paleoclimatic data suggest cooler effectively wetter conditions during terrace tread formation in warmer drier intervals reduced average runoff in axial streams results in meander belt narrowing concurrent channel incision may be caused by infrequent large floods greater resistance to downcutting however allowed fewer terraces to be formed along slough creek and the lamar river alluvial systems in northeastern yellowstone show a clear response to millennial scale climatic cycles wherein alluvial fans aggrade and prograde over flood plains in drier periods axial streams widen their flood plains and trim back the fans during wetter periods small scale climatic fluctuations of the holocene thus had substantial impact on postglacial landscapes in northeastern yellowstone

erosion rates in the southern oregon coast range evidence for an equilibrium between hillslope erosion and sediment yield the relationship of hillslope erosion rates and sediment yield is often poorly defined because of short periods of measurement and inherent spatial and temporal variability in erosion processes in landscapes containing hillslopes crenulated by alternating topographic noses and hollows estimates of local hillslope erosion rates averaged over long time periods can be obtained by analysing colluvial deposits in the hollows hollows act as local traps for a portion of the colluvium transported down hillslopes and erosion rates can be calculated using the age and size of the deposits and the size of the contributing source area analysis of colluvial deposits in nine oregon coast range hollows has yielded average colluvial transport rates into the hollows of about 35 cm3cm 1yr 1 and average bedrock lowering rates of about 007 mm yr for the last 4000 to 15000 yr these rates are consistent with maximum bedrock exfoliation rates of about 0 09 mm yr calculated from six of the hollows supporting the interpretation that exfoliation rates limit erosion rates on these slopes sediment yield measurements from nine coast range streams provide similar basin wide denudation rates of between 0 05 and 0 08 mm yr 1 suggesting an approximate steady state between sediment production on hillslopes and sediment yield in addition modern sediment yields are similar in basins varying in size from 1 to 1500 km2 suggesting that erosion rates are spatially uniform and providing additional evidence for an approximate equilibrium in the landscape

stochastic forcing of sediment supply to channel networks from landsliding and debris flow sediment influx to channel networks is stochastically driven by rainstorms and other perturbations which are discrete in time and space and which occur on a landscape with its own spatial variability in topography colluvium properties and state of recovery from previous disturbances the resulting stochastic field of sediment supply interacts with the topology of the channel network and with transport processes to generate spatial and temporal patterns of flux and storage that characterize the sedimentation regime of a drainage basin the regime varies systematically with basin area we describe how the stochastic sediment supply is generated by climatic topographic geotechnical and biotic controls that vary between regions the general principle is illustrated through application to a landscape where sediment is supplied by mass wasting and the forcing variables are deterministic thickening of colluvium random sequences of root destroying wildfires and random sequences of rainstorms that trigger failure in a population of landslide source areas with spatial variance in topography and colluvium strength landslides stop in channels or convert to scouring debris flows depending on the nature of the low order channel network sediment accumulates within these channels for centuries before being transferred downstream by debris flows time series of sediment supply transport and storage vary with basin scale for any combination of climatic topographic and geotechnical controls in a companion paper benda and dunne this issue we use simulations of timing volumes and locations of mass wasting to study the interaction between a stochastically forced sediment supply and systematic changes of storage and flux through channel networks

revised dates of large earthquakes along the carrizo section of the san andreas fault california since a d 1310 30 1 precise knowledge of the age and magnitude of past earthquakes is essential for characterizing models of earthquake recurrence and key to forecasting future earthquakes we present 28 new radiocarbon analyses that refine the chronology of the last five earthquakes at the bidart fan site along the carrizo section of the south central san andreas fault which last ruptured during the fort tejon earthquake in a d 1857 the new data show that the penultimate earthquake in the carrizo plain occurred not earlier than a d 1640 and the modeled 95th percentile ranges of the three earlier earthquakes and their mean are a d 1540 1630 1585 a d 1360 1425 1393 and a d 1280 1340 1310 indicating an average time interval of 137 44 years between large earthquakes since a d 1310 30 a robust earthquake recurrence model of the carrizo section will require even more well dated earthquakes for thorough characterization however these new data imply that since a d 1310 30 the carrizo section has failed more regularly and more often than previously thought

fire related sedimentation events on alluvial fans yellowstone national park u s a abstract we document initiation and flow processes deposit facies and geomorphic effects of forest fire related sedimentation on small alluvial fans in yellowstone national park brief intense convective storm precipitation on steep basins burned in the 1988 fires produced sedimentation events involving a variety of depositional processes on fans over the course of all documented events flows on fans progressed from higher to lower sediment concentration events were often dominated by either debris flows or relatively sediment poor streamflow processes in some events however flows ranging a over wide spectrum of sediment concentration produced significant fan deposits debris flows were generated by progressive sediment bulking involving pervasive surface runoff and rill erosion on st ep upper basin slopes followed by deep incision as flows progressed down channels debris flow deposits show a marked decline in thickness and coarse gravel content downfan often with extensive distal gravel poor facies we recognized a relatively minor percentage of noncohesive debris flow and hyperconcentrated flow facies with sorting and stratification intermediate between muddy debris flow and streamflow facies these were deposited where dilute flows bulked with coarse sediment by eroding channel alluvium or earlier deposits of the event below incised fan channels streamflows expanded as sheetfloods which prograded lower fans with distally fining deposits basins 3 km2 typically produced streamflow events on fans but sediment texture and availability on slopes and in channels are primary factors determining flow processes on fans of smaller basins burned soil surfaces provided abundant silt and clay for debris flow generation but because soil surface sediment was stripped and or compacted over time the lack of available fines resulted in dominance of streamflow processes in later events

sediment storage and evacuation in headwater valleys at the transition between debris flow and fluvial processes sediment from landscape disturbance often enters temporary storage in valleys and evacuates over longer times which in steeplands are poorly delimited we hypothesize that across process transitions e g debris fl ow versus fl uvial transport distributions of sediment transit times also change we use fi eld surveys and extensive radiocarbon dating to assess the distribution of transit residence times through the proxy measurement of ages of bank deposits in two mainstem reaches of a 2 23 km 2 watershed in the oregon coast range in the downstream reach debris fans impound fl uvial deposits debris fl ow fi ne fl uvial and coarse fl uvial deposits compose nearly equal parts of the valley fi ll and fl uvial erosion evacuates deposits transit times have a sample mean of 1 22 10 3 14 c yr and an exponential distribution indicating uniform probability of evacuation from storage in the upstream reach valleyspanning debris jams impound debris fl ow deposits composing 95 of the valley fi ll which is routinely scoured by debris fl ows transit times have a sample mean of 4 43 10 2 14 c yr and if 100 14 c yr a power law distribution indicating preferential evacuation of younger deposits and retention of older deposits in both reaches most sediment has short transit times 600 14 c yr but signifi cant volumes remain for millennia less than 20 of basin wide denudation passes through these reservoirs but the latter are still signifi cant buffers between hillslope disturbance and downstream aquatic habitat especially for coarse sediment

thermal anomalies and magmatism due to lithospheric doubling and shifting abstract we present some thermal and magmatic consequences of the processes of lithospheric doubling and lithospheric shifting lithospheric doubling concerns the obduction of a cold continental or old oceanic lithospheric plate over a young and hot oceanic lithosphere upper mantle system including an oceanic ridge lithospheric shifting concerns the translation and rotation of a lithospheric plate relative to the upper mantle in both cases the resulting thermal state of the upper mantle below the obducting or shifting lithosphere may be perturbed relative to a normal continental or oceanic geothermal situation the perturbed geothermal state gives rise to a density inversion at depth and thus induces a vertical gravitational instability which favours magmatism we speculate about the magmatic consequences of this situation and infer that in the case of lithospheric doubling our model may account for the petrology and geochemistry of the resulting magma the original layering and composition of the overridden young oceanic lithosphere may strongly influence magmatic processes we dwell shortly on the genesis of kimberlites within the framework of our lithospheric doubling model and on magmatism in general lithospheric recycling is inherent to the mechanism of lithospheric doubling

post fire geomorphic response in steep forested landscapes oregon coast range usa abstract the role of fire in shaping steep forested landscapes depends on a suite of hydrologic biologic and geological characteristics including the propensity for hydrophobic soil layers to promote runoff erosion during subsequent rainfall events in the oregon coast range several studies postulate that fire primarily modulates sediment production via root reinforcement and shallow landslide susceptibility although few studies have documented post fire geomorphic response here we describe field observations and topographic analyses for three sites in the central oregon coast range that burned in 1999 2002 and 2003 the fires generated strongly hydrophobic soil layers that did not promote runoff erosion because the continuity of the layers was interrupted by pervasive discontinuities that facilitated rapid infiltration at each of our sites fire generated significant colluvial transport via dry ravel consistent with other field based studies in the western united states fire driven dry ravel accumulation in low order valleys of our sulphur creek site equated to a slope averaged landscape lowering of 2 5 mm given holocene estimates of fire frequency these results suggest that fire may contribute 10 20 of total denudation across steep dissected portions of the oregon coast range in addition we documented more rapid decline of root strength at our sites than has been observed after timber harvest suggesting that root strength was compromised prior to fire or that intense heat damaged roots in the shallow subsurface given that fire frequencies in the pacific northwest are predicted to increase with continued climate change our findings highlight the importance of fire induced dry ravel and post fire debris flow activity in controlling sediment delivery to channels

sediment entrainment by debris flows in situ measurements from the headwaters of a steep catchment 1 debris flows can dramatically increase their volume and hence their destructive potential by entraining sediment yet quantitative constraints on rates and mechanics of sediment entrainment by debris flows are limited using an in situ sensor network in the headwaters of a natural catchment we measured flow and bed properties during six erosive debris flow events despite similar flow properties and thicknesses of bed sediment entrained across all events time averaged entrainment rates were significantly faster for bed sediment that was saturated prior to flow arrival compared with rates for sediment that was dry bed sediment was entrained from the sediment surface downward in a progressive fashion and occurred during passage of dense granular fronts as well as water rich inter surge flow en massefailure of bed sediment along the sediment bedrock interface was never observed large magnitude high frequency fluctuations in total normal basal stress were dissipated within the upper 5 cm of bed sediment within this near surface layer concomitant fluctuations in coulomb frictional resistance are expected irrespective of the influence of pore fluid pressure or fluctuations in shear stress if the near surface sediment was wet as it was overridden by a flow additional large magnitude high frequency pore pressure fluctuations were measured in the near surface bed sediment these pore pressure fluctuations propagated to depth at subsonic rates and in a diffusive manner the depth to which large excess pore pressures propagated was typically less than 10 cm but scaled as d fi 0 5 in which d is the hydraulic diffusivity and fiis the frequency of a particular pore pressure fluctuation shallow penetration depths of granular normal stress fluctuations and excess pore pressures demonstrate that only near surface bed sediment experiences the full dynamic range of effective stress fluctuations and as a result can be more easily entrained than deeper sediment these data provide robust tests for mechanical models of entrainment and demonstrate that a debris flow over wet bed sediment will be larger than the same flow over dry bed sediment

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

forest clearing and regional landsliding the influence of forest clearing on landsliding is central to longstanding concern over the effects of timber harvesting on slope stability here we document a strong topographic control on shallow landsliding by combining unique ground based landslide surveys in an intensively monitored study area with digital terrain modeling using high resolution laser altimetry and a coarser resolution regional study of 3224 landslides as predicted by our digital terrain based model landslides occur disproportionately in steep convergent topography in terrain predicted to be at low risk of slope failure a random model performs equally well to our mechanism based model our monitoring shows that storms with 24 hr rainfall recurrence intervals of less than 4 yr triggered landslides in the decade after forest clearing and that conventional monitoring programs can substantially underestimate the effects of forest clearing our regional analysis further substantiates that forest clearing dramatically accelerates shallow landsliding in steep terrain typical of the pacific northwest

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

evolution of a natural debris flow in situ measurements of flow dynamics video imagery and terrestrial laser scanning many theoretical and laboratory studies have been undertaken to understand debris flow processes and their associated hazards however complete and quantitative data sets from natural debris flows needed for confirmation of these results are limited we used a novel combination of in situ measurements of debris flow dynamics video imagery and pre and postflow 2 cm resolution digital terrain models to study a natural debris flow event our field data constrain the initial and final reach morphology and key flow dynamics the observed event consisted of multiple surges each with clear variation of flow properties along the length of the surge steep highly resistant surge fronts of coarse grained material without measurable pore fluid pressure were pushed along by relatively fine grained and water rich tails that had a wide range of pore fluid pressures some two times greater than hydrostatic surges with larger nonequilibrium pore fluid pressures had longer travel distances a wide range of travel distances from different surges of similar size indicates that dynamic flow properties are of equal or greater importance than channel properties in determining where a particular surge will stop progressive vertical accretion of multiple surges generated the total thickness of mapped debris flow deposits nevertheless deposits had massive vertically unstratified sedimentological textures

stochastic processes of soil production and transport erosion rates topographic variation and cosmogenic nuclides in the oregon coast range landscapes in areas of active uplift and erosion can only remain soil mantled if the local production of soil equals or exceeds the local erosion rate the soil production rate varies with soil depth hence local variation in soil depth may provide clues about spatial variation in erosion rates if uplift and the consequent erosion rates are sufficiently uniform in space and time then there will be tendency toward equilibrium landforms shaped by the erosional processes soil mantle thickness would adjust such that soil production matched the erosion previous work in the oregon coast range suggested that there may be a tendency locally toward equilibrium between hillslope erosion and sediment yield here results from a new methodology based on cosmogenic radionuclide accumulation in bedrock minerals at the base of the soil column are reported we quantify how soil production varies with soil thickness in the southern oregon coast range and explore further the issue of landscape equilibrium apparent soil production is determined to be an inverse exponential function of soil depth with a maximum inferred production rate of 268 m ma 1 occurring under zero soil depth this rate depends however on the degree of weathering of the underlying bedrock the stochastic and large scale nature of soil production by biogenic processes leads to large temporal and spatial variations in soil depth the spatial variation of soil depth neither supports nor rejects equilibrium morphology our observed catchment averaged erosion rate of 117 m ma 1 is however similar to that estimated for the region by others and to soil production rates under thin and intermediate soils typical for the steep ridges we suggest that portions of the oregon coast range may be eroding at roughly the same rate but that local competition between drainage networks and episodic erosional events leads to landforms that are out of equilibrium locally and have a spatially varying soil mantle copyright 2001 john wiley sons ltd

styles of crustal deformation in compressional orogens caused by subduction of the underlying lithosphere abstract crustal scale deformation is calculated for models in which the driving mechanism corresponds to the asymmetric detachment and underthrusting of the underlying mantle lithosphere the plane strain finite element model results provide indications of the styles of deformation to be expected in small compressional orogens in particular these styles occur where shortening of the mantle lithosphere is achieved by the nearly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other the crust is modelled using coulomb plastic frictional and thermally activated power law viscous rheologies and the effects of compositional layering and variable geothermal gradients are included results are presented for a range of models in which the strength of the coupling between the model crust and its basal boundary surface denudation partial and total subduction of the crust and compositional layering are examined the results show the development of inclined step up shear zones which are a consequence of conjugate thrusting in regions of coulomb controlled rheology these zones link to sub horizontal shear zones which occur where composition and temperature render the crust viscously weak the model strain fields are interpreted in terms of deformation on discrete planes and the seismic reflectivity fabric that may be associated with this discrete deformation finally we ask whether similar seismic reflectivity fabric can be recognized in observations from small compressional orogens

polyphase formation and exhumation of high to ultrahigh pressure rocks in continental subduction zone numerical modeling and application to the sulu ultrahigh pressure terrane in eastern china 1 high to ultrahigh pressure hp uhp metamorphic rocks commonly form and exhume during the early continental collision and many questions related to their origin still remain unresolved we focus our study on explaining the poly metamorphic origins of many hp uhp terranes composed of tectonic units having strongly variable ages peak metamorphic conditions and p t paths these features are especially well characterized for the sulu uhp terrane in eastern china which we have chosen therefore as the reference case we conducted 2 d thermomechanical numerical modeling of continental subduction associated with formation and exhumation of hp uhp rocks our experiments suggest existence of several consequent episodes of u hp rocks exhumation related to the inherently cyclic origin of continental crust subduction detachment exhumation processes three major phases of these processes are identified in our reference model for the sulu uhp terrane 1 first and 2 second exhumation episodes of hp rocks originated in the subduction channel at lithospheric depths and 3 exhumation of uhp rocks originated at asthenospheric depths numerical models also suggest that subducted uhp rocks which are positively buoyant compared to the mantle may detach from the slab forming a flattened plume underplating the overriding lithosphere this sublithospheric plume may exist for several million years being heated to 800 900 c by the surrounding hot mantle at the later stage upward extrusion of hot partially molten rocks from the plume may exhume high temperature ht uhp complexes toward the surface

the variability of root cohesion as an influence on shallow landslide susceptibility in the oregon coast range decades of quantitative measurement indicate that roots can mechanically reinforce shallow soils in forested landscapes forests however have variations in vegetation species and age which can dominate the local stability of landslide initiation sites to assess the influence of this variability on root cohesion we examined scarps of landslides triggered during large storms in february and november of 1996 in the oregon coast range and hand dug soil pits on stable ground at 41 sites we estimated the cohesive reinforcement to soil due to roots by determining the tensile strength species depth orientation relative health and the density of roots 1 mm in diameter within a measured soil area we found that median lateral root cohesion ranges from 6 8 23 2 kpa in industrial forests with significant understory and deciduous vegetation to 25 6 94 3 kpa in natural forests dominated by coniferous vegetation lateral root cohesion in clearcuts is uniformly 10 kpa some 100 year old industrial forests have s

pre collisional high pressure metamorphism and nappe tectonics at active continental margins a numerical simulation the evolution of a subduction channel and orogenic wedge is simulated in 2d for an active continental margin with p t paths being displayed for selected markers in our simulation subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years with four zones from the trench into the forearc i an accretionary complex of low grade metamorphic sedimentary material ii a wedge of nappes with alternating upper and lower crustal provenance and minor interleaving of oceanic or hydrated mantle material iii a megascale melange composed of high pressure hp and ultra high pressure uhp metamorphic rocks extruded from the subduction channel and iv the upward tilted frontal part of the remaining lid the p t paths and time scales correspond to those typically recorded in orogenic belts the simulation shows that hp uhp metamorphism of continental crust does not necessarily indicate collision but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction

storm rainfall conditions for floods and debris flows from recently burned areas in southwestern colorado and southern california abstract debris flows generated during rain storms on recently burned areas have destroyed lives and property throughout the western u s field evidence indicate that unlike landslide triggered debris flows these events have no identifiable initiation source and can occur with little or no antecedent moisture using rain gage and response data from five fires in colorado and southern california we document the rainfall conditions that have triggered post fire debris flows and develop empirical rainfall intensity duration thresholds for the occurrence of debris flows and floods following wildfires in these settings this information can provide guidance for warning systems and planning for emergency response in similar settings debris flows were produced from 25 recently burned basins in colorado in response to 13 short duration high intensity convective storms debris flows were triggered after as little as six to 10 min of storm rainfall about 80 of the storms that generated debris flows lasted less than 3 h with most of the rain falling in less than 1 h the storms triggering debris flows ranged in average intensity between 1 0 and 32 0 mm h and had recurrence intervals of two years or less threshold rainfall conditions for floods and debris flows sufficiently large to pose threats to life and property from recently burned areas in south central and southwestern colorado are defined by i 6 5 d 0 7 and i 9 5 d 0 7 respectively where i rainfall intensity in mm h and d duration in hours debris flows were generated from 68 recently burned areas in southern california in response to long duration frontal storms the flows occurred after as little as two hours and up to 16 h of low intensity 2 10 mm h rainfall the storms lasted between 5 5 and 33 h with average intensities between 1 3 and 20 4 mm h and had recurrence intervals of two years or less threshold rainfall conditions for life and property threatening floods and debris flows during the first winter season following fires in ventura county and in the san bernardino san gabriel and san jacinto mountains of southern california are defined by i 12 5 d 0 4 and i 7 2 d 0 4 respectively a threshold defined for flood and debris flow conditions following a year of vegetative recovery and sediment removal for the san bernardino san gabriel and san jacinto mountains of i 14 0 d 0 5 is approximately 25 mm h higher than that developed for the first year following fires the thresholds defined here are significantly lower than most identified for unburned settings perhaps because of the difference between extremely rapid runoff dominated processes acting in burned areas and longer term infiltration dominated processes on unburned hillslopes

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

how steady are steady state landscapes using visible near infrared soil spectroscopy to quantify erosional variability although topographic steady state is often used as a simplifying assumption in sediment yield studies and landscape evolution models the temporal and spatial scales over which this assumption applies in natural landscapes are poorly defined we used visible near infrared visnir spectroscopy to measure the weathering of hilltop soils and quantify local erosional variability in two watersheds in the oregon coast range united states one watershed appears adjusted to base level lowering driven by rock uplift in the cascadia forearc while the other is pinned by a gabbroic dike that locally slows river incision and hillslope erosion models for uniformly eroding hillslopes imply uniform soil residence times instead we observe significant variability around the mean value of 18 8 k y 31 2 11 8 k y for our adjusted watershed the magnitude of erosional variability likely reflects the time scales associated with stochastic processes that drive bedrock weathering soil production and soil transport e g tree turnover the residence time distribution for our pinned watershed has a mean value of 72 9 k y 165 6 50 6 k y and is highly skewed with a substantial fraction of long residence time soils we speculate that this pattern results from the lithologic control of base level and lateral divide migration driven by erosional contrasts with neighboring catchments our novel and inexpensive methodology enables us to quantify for the first time the magnitude of erosional variability in a natural landscape and thus provides important geomorphic context for studies characterizing regolith development more generally we demonstrate that soils can record catchment scale landscape dynamics that may arise from lithologic controls or forcing due to climate or tectonics

tectonostratigraphic subdivisions of the himalaya a view from the west 1 indian plate rocks in the central himalaya have traditionally been divided into orogen parallel fault bound tectonostratigraphic zones a straightforward westward extrapolation of these zones has proved problematic in part because of a lack of consensus on the existence or significance of major faults within the metamorphic zone of the indian plate in pakistan where more than 10 locations for the main central thrust mct have been proposed we address this ambiguity by systematically tracing established central himalayan tectonostratigraphy around the western himalayan syntaxis and across pakistan this exercise reveals the following stratigraphic and structural relationships 1 there is a westward decrease in neogene shortening across the himalayan fold and thrust belt such that there is no age equivalent thrust in pakistan with displacement and metamorphic juxtaposition equivalent to the central himalayan mct 2 shortening across the fold and thrust belt in western pakistan is concentrated in the unmetamorphosed foreland as opposed to the metamorphic zone in the central himalaya 3 lesser himalayan higher himalayan and tethyan rocks are in stratigraphic order within the metamorphic zone of pakistan which appears to be the metamorphic equivalent of kashmir tethyan stratigraphy 4 the combination of early paleozoic and late paleozoic tectonism in pakistan has locally eliminated upper proterozoic higher himalayan rock and lower to middle paleozoic tethyan rock from the metamorphic zone of pakistan 5 late cretaceous and or early paleocene proto himalayan deformation in the pakistan foreland telescoped and eroded stratigraphy prior to the main phase of himalayan orogeny 6 tectonostratigraphic zones are offset in eastern pakistan by the transverse jhelum balakot fault 7 there is no evidence within the indian plate of pakistan for a large scale normal fault system comparable to the south tibetan detachment system 8 stratigraphy as well as the age and tectonic setting of deformation and metamorphism must be taken into account when drawing tectonostratigraphic zones

stochastic forcing of sediment routing and storage in channel networks the stochastic field of sediment supply to the channel network of a drainage basin depends on the large scale interactions among climatically driven processes such as forest fire and rainstorms topography channel network topology and basin scale during infrequent periods of intense erosion large volumes of colluvium are concentrated in parts of a channel network particularly near tributary junctions the rivers carry bed material and wash load downstream from these storage sites at different rates the bed material travels slowly creating transient patterns of sediment transport sediment storage and channel morphology along the channel network as the concentrations of bed material migrate along the network their waveforms can undergo changes by diffusion interference at tributary junctions and loss of mass through temporary sediment storage in fans and terraces and through particle abrasion which converts bed material to wash load we investigated how these processes might influence the sediment mass balance in channels of third and higher order in a 215 km 2 drainage basin within the oregon coast range over a simulated 3000 year period with a climate typical of the late holocene we used field measurements and a simulation model to illustrate interactions between the major controls on large scale processes functioning over long periods of time in complex drainage basins