evaluating knickpoint recession along an active fault for paleoseismological analysis the huoshan piedmont eastern china abstract ground rupturing earthquakes can generate tectonic knickpoints within upstream reaches of streams across active fault zones these knickpoints are characteristic of upstream propagation of time related process once generated by an earthquake so analysis of knickpoint series in streams which cross fault zones can be used to infer paleoearthquake events we studied the knickpoints along the huoshan piedmont fault hpf which is an active normal fault in the shanxi faulted basin zone china and demonstrate that analysis of knickpoints shows evidence for two paleoearthquakes in the hpf first we identified knickpoints in bedrock reaches upstream of the hpf using high resolution dems derived from irs p5 stereo images and the stream gradient method after excluding non faulting knickpoints 47 knickpoints were identified in 23 bedrock reaches upstream from the hpf analysis of the most recent knickpoints caused by the 1303 ce hongdong earthquake allowed for local calibration of the retreat rates applying these retreat rates across the study area allows for the estimation of the age of other knickpoints and constrains the age ranges of two knickpoint groups to be 2269 3336 a bp and 4504 5618 a bp these ages constrain the ages of two paleoearthquake events at 2710 102 and 4980 646 a bp the knickpoints along the hpf obey the parallel retreating model in which knickpoint morphology was roughly maintained during retreat so the heights of knickpoints represent the coseismic vertical displacements generated by the earthquakes along the hpf the vertical offsets for these three earthquake events are similar and are approximately 4 m which indicates that the ruptures on the hpf obey a characteristic slip model with a similar slip distribution for several successive earthquakes these results provide additional evidence of paleoearthquakes on the hpf and show that analysis of knickpoint recession along an active fault is a valuable tool for paleoseismology

knickpoints within the suoshui watershed and the implication for the age of sandstone pillars in zhangjiajie china using dem and topographic and geological maps alongside field surveys this study investigated the characteristics and retreat processes of the knickpoints along the mainstream and major tributaries of the suoshui watershed seven disequilibrium transient knickpoints are found along the mainstream and 0 6 are on the trunk of each of the major tributaries the knickpoints are labeled from kp0 to kp7 from the lowest to the highest according to their spatial position the average drop across each knickpoint is in the range of 9 19 m and the cumulative fall from kp7 to kp0 is about 107 m using a brute force two parameter search the coefficient m in the stream power incision model for the suoshui watershed is found to be 0 525 and the coefficient k is variable with an inverse relationship with channel steepness using the stream power incision model with these parameters the time for the highest knickpoint kp7 to retreat from the mouth of the suoshui river to its present location is found to be ca 0 454 ma based on the relationship between the cumulative fall and migration time of knickpoints the average downcutting rate of the suoshui river is estimated to be 0 224 mm a in the past 0 454 ma by comparing the downcutting rates with the height of typical sandstone pillars in the suoshui watershed the formation of these pillars might commence in the early pleistocene c 2014 elsevier b v all rights reserved

waterfalls on the eastern side of rocky mountain national park colorado usa abstract we examined 30 waterfalls on the eastern side of rocky mountain national park in colorado usa to evaluate whether drainage area or bedrock properties as reflected in joint characteristics correlate more strongly with the location and characteristics of individual waterfalls longitudinal profiles tend to be more concave for larger drainages to have a smaller proportion of total elevation loss in waterfalls and to have vertical drops rather than angled or ramp waterfalls we interpret these trends to indicate greater overall incisional capability for larger catchments shape of individual waterfalls and height of drop correlate more strongly with bedrock properties waterfalls in bedrock lacking prominent vertical joints perpendicular to flow are more likely to have a single drop rather than multiple drops and taller waterfalls correlate with more widely spaced horizontal joints waterfalls also noticeably correspond to resistant bedrock outcrops that form steep segments along hillslopes adjacent to the channel we interpret these results to indicate that the location and characteristics of waterfalls along headwater streams in the study area reflect primarily a limited ability to incise through more resistant segments of the underlying bedrock

interpretation and downstream correlation of bedrock river terrace treads created from propagating knickpoints 1 derivation of an analytical model for the slope of a strath terrace created following an upstream propagating wave of incision reveals that for detachment limited river incision the exponent on channel slope n governs terrace tread slope terrace elevations can increase upstream n 1 downstream n 1 or can be horizontal n 1 numerical modeling confirms these results for temporally evolving knickpoint geometries and for incision due to sudden base level fall and an increase in rock uplift rate except in the case of bedrock river incision with a slope threshold only exceeded during knickpoint propagation a terrace created from transient headward incision contains no information about the paleo channel gradient gradients in rock uplift rate along channels potentially complicate the interpretation of terraces by altering the primary tilt on strath terraces in particular monotonic gradients in rock uplift can produce terrace treads that are apparently folded because gradients in rock uplift rate are common where terraces are not longitudinally traceable care is warranted in terrace correlation in simple tectonic settings where terraces are longitudinally traceable the slope of a strath terrace created from headward incision may provide a means of estimating the dependency of river incision rate on channel slope terraces in four locations argued to have formed from headward incision are parallel or close to parallel with the active channel implying a slope exponent on river incision rate that is much greater than one or a threshold slope for incision that is only exceeded during knickpoint propagation

numerical model predictions of autogenic fluvial terraces and comparison to climate change expectations terraces eroded into sediment alluvial and bedrock strath preserve an important history of river activity river terraces are thought to form when a river switches from a period of slow vertical incision and valley widening to fast vertical incision and terrace abandonment consequently terraces are often interpreted to reflect changing external drivers including tectonics sea level and climate in contrast the intrinsic unsteadiness of lateral migration in rivers may generate terraces even under constant rates of vertical incision without external forcing to explore this mechanism we simulate landscape evolution by a vertically incising meandering river and isolate the age and geometry of autogenic river terraces modeled autogenic terraces form for a wide range of lateral and vertical incision rates and are often paired and longitudinally extensive for intermediate ratios of vertical to lateral erosion rate autogenic terraces have a characteristic reoccurrence time that scales with the time for relief generation there is a preservation bias against older terraces due to reworking of previously visited parts of the valley evolving spatial differences in bank strength between bedrock and sediment reduce terrace formation frequency and length favor pairing and can explain sublinear terrace margins at valley boundaries age differences and geometries for modeled autogenic terraces are consistent in cases with natural terraces and overlap with metrics commonly attributed to terrace formation due to climate change we suggest a new phase space of terrace properties that may allow differentiation of autogenic terraces from terraces formed by external drivers

erosion of an active fault scarp leads to drainage capture in the amazon region brazil far from the continental margin drainage basins in central amazonia should be in topographic steady state but they are not abandoned remnant fluvial valleys up to hundreds of square kilometers in size are observed throughout amazonia and are evidence of significant landscape reorganization while major late miocene drainage shifts occurred due to initiation of the transcontinental amazon river local landscape change has remained active until today driven either by dynamic topography tectonism and or climatic fluctuations drainage captures in amazonia provide a natural experiment for assessing the geomorphic response of low slope basins to sudden capture related base level falls this paper evaluates the timing of geomorphic change by examining a drainage capture event across the baependi fault scarp involving the cuieiras and taruma mirim river basins northwest of the city of manaus in brazil a system of capture related knickpoints was generated by base level fall following drainage capture through numerical modeling of their initiation and propagation the capture event is inferred to have occurred between the middle and late pleistocene consistent with other studies of landscape change in surrounding areas in low slope settings like the amazon river basin base level fall can increase erosion rates by more than an order of magnitude and moderate to large river basins can respond to episodes of base level fall over timescales of tens to hundreds of thousands of years copyright 2013 john wiley sons ltd

fluvial landscape response time how plausible is steady state denudation whether or not steady state topography and denudation are probable states depends on the timescale of system response to tectonic and climatic perturba tions relative to the frequency of those perturbations this paper presents analytical derivations of algebraic relations for the response time of detachment limited fluvial bedrock channel systems both to tectonic and climatic perturbations detachment limited fluvial erosion is described by the stream power incision model and the derivations are limited to the applicability of that model all factors likely to influence system response time that are not adequately captured by the stream power incision model will tend to increase the response time the calculations presented thus provide minimum estimates of landscape response time and therefore over predict the prob ability of attaining and sustaining steady state topography and denudation the central range of taiwan is used as a case study to estimate response times in a landscape often argued to be in steady state model parameters are fit to modern stream profiles by assuming that the topography represents a quasi steady state form estimated response times generally range from 0 25 to 2 5 ma depending on the non linearity of the incision rule and the magnitude and type of perturbation thus it may be reasonably argued that steady state topography and denudation are likely to prevail during periods of climatic stability response time is sufficiently short compared with plate tectonic timescales however rapid climatic fluctuation in the quaternary appears to preclude the attainment of steady state conditions in modern orogens motivation

particle dynamics the continuum of bedrock to alluvial river segments abstract particle dynamics refers to production erosion transport and storage of particulate material including mineral sediment and organic matter particle dynamics differ significantly between the end members of bedrock and alluvial river segments and between alluvial river segments with different grain size distributions bedrock segments are supply limited and resistant to change with relatively slow linear adjustments and predominantly erosion and transport particle dynamics in alluvial segments in contrast are transport limited and dominated by storage of mineral sediment and production of organic matter alluvial segments are resilient to change with relatively rapid multidirectional adjustments and stronger internal influences because of feedbacks between particles and biota bedrock segments are the governors of erosion within a river network whereas alluvial segments are the biogeochemical reactors fundamental research questions for both types of river segments center on particle dynamics which limit network scale incision in response to base level fall bedrock segments and habitat biogeochemical reactions and biomass production alluvial segments these characterizations illuminate how the spatial arrangement of bedrock and alluvial segments within a river network influence network scale resistance and resilience to external changes in relative base level climate and human activities

a kuroko type polymetallic sulfide deposit in a submarine silicic caldera manned submersible studies have delineated a large and actively growing kuroko type volcanogenic massive sulfide deposit 400 kilometers south of tokyo in myojin knoll submarine caldera the sulfide body is located on the caldera floor at a depth of 1210 to 1360 meters has an area of 400 by 400 by 30 meters and is notably rich in gold and silver the discovery of a large kuroko type polymetallic sulfide deposit in this arc front caldera raises the possibility that the numerous unexplored submarine silicic calderas elsewhere might have similar deposits

implications of radiocarbon ages of organic and inorganic carbon in coastal lakes in florida for establishing a reliable chronology for sediment based paleoclimate reconstruction coastal lake sediments are valuable paleoclimate archives provided that they can be accurately dated here we report radiocarbon ages of bulk sediment organic matter om plants shells particulate om and dissolved om from coastal lakes in florida bulk sediment om yielded ages that are consistently older than contemporaneous plants and shells indicating significant radiocarbon deficiencies in sedimentary om in these lakes the data show that the om radiocarbon deficiency varies over time and with location making it impossible to determine a proper correction factor for radiocarbon ages of bulk sediments from these lakes as a result we consider ages obtained from bulk sediment om from these lakes unreliable the age reversals in bulk sediment om observed in the sediment cores are likely caused by rapid increases in erosion and sedimentation resulting from large storm events the data also show that sedimentation rate can vary considerably within a given lake implying that an age depth model established for one core cannot be directly applied to other cores despite their close proximity analyses of shells from one of the lakes suggest that fresh brackish water shells may serve as a good substrate for radiocarbon dating owing to a small reservoir effect on inorganic carbon

subduction modified mantle derived triassic high mg andesites in the sanjiang tethys eastern tibet abstract andesites in the yangla district of the sanjiang tethys eastern tibet are characterized by relatively high mgo and with rich large ion lithophile elements liles indicating similarities to typical high mg andesites formed in continental collisional zones zircon u pb age of 232 1 ma for these andesites shows that they occurred synchronously with the regionally widespread late middle triassic granitoids and bimodal volcanic rocks primitive mantle normalized spidergrams for these andesites show depletion in high field strength elements hfses with sr nd pb isotopic compositions that are comparable to both arc and morb like high mg rocks generated from subduction modified sources modelling results suggest that an input of 20 sediment derived melts into mantle derived melts could be possibly responsible for the sr nd isotopic characteristic of these andesites morb and arc like compositions likely indicate a metasomatized mantle wedge modified by subduction processes during the closure of the paleo tethys ocean and subsequent continental collision it is speculated that the late middle to late triassic intra continental re activation associated with regional tectonic collapse and thermal relaxation triggered the melting of this previously subduction modified mantle leading to the formation of the high mg andesites in the yangla district

sanjiang tethyan metallogenesis in s w china tectonic setting metallogenic epochs and deposit types abstract tectonically the sanjiang tethyan metallogenic domain stmd is located within the eastern himalayan tibetan orogen in the sanjiang tethys southwestern china although this metallogenic domain was initiated in the early palaeozoic extensive metallogenesis occurred in the late palaeozoic late triassic and himalayan tertiary epochs corresponding tectonic settings and environments in the domain are an arc basin system related to the subduction of the palaeo tethyan oceanic slabs a post collision crustal extension setting caused by the lithospheric delamination or slab breakoff underneath the sanjiang tethys during the late triassic large scale strike slip faulting and thrusting systems due to the indo asian continent collision since the palaeocene in this metallogenic domain important gold copper base metals rare metals and tin ore belts incorporating a large number of giant deposits were developed the main types of deposits include 1 porphyry copper deposits controlled by a large scale strike slip fault system 2 vhms deposits mainly occurring in intra arc rift basins and post collision crustal extensional basins 3 shear zone type gold deposits in the ophiolitic melange zone along the thrusting shearing system 4 hydrothermal silver polymetallic deposits in the triassic intra continental rift basins and tertiary strike slip pull apart basins and 5 himalayan granite related greisen type tin and rare metallic deposits within the metallogenic epochs of the late palaeozoic to cenozoic the styles and types of the ore deposits changed from vhms types in the late palaeozoic through exhalative sedimentary type deposits in the late triassic to porphyry type copper deposits shear zone type gold deposits hydrothermal vein type silver polymetallic deposits greisen type tin and rare metal deposits in the cenozoic correspondingly ore forming metals also changed from a pb zn cu ag association through ag cu pb zn fe ag pb and ag au hg associations to ag cu pb zn cu mo au sn and li rb cs nb zr hf y ce sc associations

lithology and the evolution of bedrock rivers in post orogenic settings constraints from the high elevation passive continental margin of se australia understanding the role of lithological variation in the evolution of topography remains a fundamental issue especially in the neglected post orogenic terrains such settings represent the major part of the earth s surface and recent modelling suggests that a range of interactions can account for the presence of residual topography for hundreds of millions of years thereby explaining the great antiquity of landscapes in such settings field data from the inland flank of the se australian high elevation continental margin suggest that resistant lithologies act to retard or even preclude the headward transmission of base level fall driven by the isostatic response to regional denudation rejuvenation be it episodic or continuous is caught up on these resistant lithologies meaning in effect that the bedrock channels and hillslopes upstream of these stalled knickpoints have become detached from the base level changes downstream of the knickpoints until these knickpoints are breached therefore catchment relief must increase over time a landscape evolution scenario that has been most notably suggested by crickmay and twidale the role of resistant lithologies indicates that detachment limited conditions are a key to the longevity of some post orogenic landscapes whereas the general importance of transport limited conditions in the evolution of post orogenic landscapes remains to be evaluated in field settings non steady state landscapes may lie at the heart of widespread slowly evolving post orogenic settings such as high elevation passive continental margins meaning that non steady state landscapes with increasing relief through time are the rule rather than the exception

knickpoint retreat and transient bedrock channel morphology triggered by base level fall in small bedrock river catchments the case of the isle of jura scotland a sudden drop in river base level can trigger a knickpoint that propagates throughout the fluvial network causing a transient state in the landscape knickpoint retreat has been confirmed in large

new constraints on the late cenozoic incision history of the new river virginia abstract the new river crosses three physiogeologic provinces of the ancient tectonically quiescent appalachian orogen and is ideally situated to record variability in fluvial erosion rates over the late cenozoic active erosion features on resistant bedrock that floors the river at prominent knickpoints demonstrate that the river is currently incising toward base level however thick sequences of alluvial fill and fluvial terraces cut into this fill record an incision history for the river that includes several periods of stalled downcutting and aggradation we used cosmogenic 10be exposure dating aided by mapping and sedimentological examination of terrace deposits to constrain the timing of events in this history 10be concentration depth profiles were used to help account for variables such as cosmogenic inheritance and terrace bioturbation fill cut and strath terraces at elevations 10 20 and 50 m above the modern river yield model cosmogenic exposure ages of 130 600 and 600 950 ka respectively but uncertainties on these ages are not well constrained these results provide the first direct constraint on the history of alluvial aggradation and incision events recorded by new river terrace deposits the exposure ages yield a long term average incision rate of 43 m my which is comparable to rates measured elsewhere in the appalachians during specific intervals over the last 1 ma however the new river s incision rate reached 100 m my modern erosion rates on bedrock at a prominent knickpoint are between 28 and 87 m my in good agreement with rates calculated between terrace abandonment events and significantly faster than 2 m my rates of surface erosion from ancient terrace remnants fluctuations between aggradation and rapid incision operate on timescales of 104 105 year similar to those of late cenozoic climate variations though uncertainties in model ages preclude direct correlation of these fluctuations to specific climate change events these second order fluctuations appear within a longer term signal of dominant aggradation until 2 ma followed by dominant incision a similar signal is observed on other appalachian rivers and may be the result of sediment supply fluctuations driven by the increased frequency of climate changes in the late cenozoic

geochemistry and s pb isotope of the yangla copper deposit western yunnan china implication for ore genesis abstract the yangla copper deposit situated in the middle section of jinshajiang tectonic belt between zhongza zhongdian block and changdu simao block is a representative and giant copper deposit that has been discovered in jinshajiang lancangjiang nujiang region in recent years there are coupled relationships between yangla granodiorite and copper mineralization in the yangla copper deposit five molybdenite samples yielded a well constrained 187 re 187 os isochron age of 233 3 3 ma the metallogenesis is therefore slightly younger than the crystallization age of the granodiorite s pb isotopic compositions of the yangla copper deposit indicate that the ore forming materials were derived from the mixture of upper crust and mantle also with the magmatic contributions in the late early permian the jinshajiang oceanic plate was subducted to the west resulting in the formation of a series of gently dipping thrust faults in the jinshajiang tectonic belt meanwhile accompanied magmatic activities in the early late triassic which was a time of transition from collision related compression to extension in the jinshajiang tectonic belt the thrust faults were tensional it would have been a favorable environment for forming ore fluids the ascending magma provided a channel for the ore forming fluid from the mantle wedge after the magma arrived at the base of the early stage yangla granodiorite the platy granodiorite at the base of the body would have shielded the late stage magma from the fluid the magma would have cooled slowly and some of the ore forming fluid in the magma would have entered the gently dipping thrust faults near the yangla granodiorite resulting in mineralization

erosion in the juniata river drainage basin pennsylvania abstract previously calculated erosion rates througouth the appalachians range from 1 2 to 203 m myr 1 calculation of erosion rates has been accomplished by 1 evaluation of riverine solute and sediment load in either large or small drainage basins 2 estimation from the volume of derived sediments and 3 methods involving either 10be or fission track dating values of specific conductance and suspended sediment collected at the juniata river gauging station at newport pennsylvania are used with corrections along with a bedload estimate to determine the total amount eroded from the 8687 km2 drainage basin during the water years 1965 1986 the amount eroded is used to calculate a present erosion rate of 27 m myr 1

the influence of bedrock geology on knickpoint development and channel bed degradation along downcutting streams in south central indiana geologic controls on knickpoint development and the mechanics of channel bed degradation were eluci dated by comparison of downcutting streams in south central indiana developed in lithologically homoge neous carbonate units and layered siliciclastic strata knickpoints formed in a single lithologic unit are stepped each step consists of a knickpoint face and tread the height of each step is equal to and controlled by bed thickness spatially knickpoints in homogeneous terrain are limited to jointed strata that dip 1 upstream or 2 downstream at angles less than the channel gradient knickpoints observed in lithologically layered strata are characterized by a single step in which resistant jointed rocks overlie and protect weaker units total knickpoint relief is influenced by thickness of the weaker strata knickpoints in layered siliciclastic strata are spatially limited to bedrock reaches produced by differential rates of channel incision knickpoint distribution is also influenced by joint

relict landscape resistance to dissection by upstream migrating knickpoints expanses of subdued topographies are common at high elevation in mountain ranges they are often interpreted as relict landscapes and are expected to be replaced by steeper topography as erosion proceeds preservation of such relict fragments can merely reflect the fact that it takes time to remove any preexisting topography however relict fragments could also possess intrinsic characteristics that make them resilient to dissection we document here the propagation of a wave of dissection across an uplifted relict landscape in puerto rico using 10be 26al burial dating on cave sediments we show that uplift started 4 ma and that river knickpoints have since migrated very slowly across the landscape modern detrital 10be erosion rates are consistent with these long term rates of knickpoint retreat analysis of knickpoint distribution combined with visual observations along the streambeds indicates that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat the studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks earlier studies have shown that vegetation over the relict topography efficiently limits erosion allowing for the formation of a thick saprolite underneath such slow erosion reduces streambed load fluxes delivered to the knickpoints as well as bed load grain size both processes limit abrasion compounding the effect of slow abrasion wide joint spacing in the bedrock makes plucking infrequent thus the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream reducing the celerity at which the relict subdued landscape is dissected we conclude that similar top down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies

wave train model for knickpoint migration rivers respond to a drop in their base level by incising the topography the upstream propagation of an incision as usually depicted by a knickpoint migration is thought to depend on several parameters such as the drainage area lithology and the amplitude of the base level drop we first investigate the case of the messinian salinity crisis that was characterized by the extreme base level fall 1500 m of the mediterranean sea at the end of the miocene the response of drainage areas of three orders of magnitude 103 to 106 km2 highlights the dominant role of the drainage area with a square root relationship in controlling the knickpoint migration after a base level fall a compilation of mean rates of knickpoint propagation for time durations ranging from 102 to 107 years displays a similar relationship indicating that successive wave trains of knickpoint can migrate in a river first wave trains linked to the release of the alluvial cover and then wave trains related to the bedrock incision which correspond to the real time response of rivers wave trains with very low retreat rates long lived knickpoints 1 my rather correspond to the response time of regional landscape

fluvial incision into bedrock insights from morphometric analysis and numerical modeling of gorges incising glacial hanging valleys western alps france bedrock gorges incising glacial hanging valleys potentially allow measurements of fluvial bedrock incision in mountainous relief using digital elevation models topographic maps and field reconnaissance we identified and characterized 30 tributary hanging valleys incised by gorges near their confluence with trunk streams in the romanche watershed french western alps longitudinal profiles of these tributaries are all convex and have abrupt knickpoints at the upper limit of oversteepened gorge reaches we reconstructed initial glacial profiles from glacially polished bedrock knobs surrounding the gorges in order to quantify the amount of fluvial incision and knickpoint retreat from morphometric analyses we find that mean channel gradients and widths as well as knickpoint retreat rates display a drainage area dependence modulated by bedrock lithology however there appears to be no relation between horizontal retreat and vertical downwearing of knickpoints assuming a postglacial origin of these gorges our results imply high postglacial fluvial incision 0 5 15 mm yr 1 and knickpoint retreat 1 200 mm yr 1 rates that are however consistent with previous estimates numerical modeling was used to test the capacity of different fluvial incision models to predict the inferred evolution of the gorges results from simple end member models suggest transport limited behavior of the bedrock gorges a more sophisticated model including dynamic width adjustment and sediment dependent incision rates predicts present day channel geometry only if a significant supply of sediment from the gorge sidewalls 10 mm yr 1 is triggered by gorge deepening combined with pronounced inhibition of bedrock incision by sediment transport and deposition

zircon u pb ages hf o isotopes and whole rock sr nd pb isotopic geochemistry of granitoids in the jinshajiang suture zone sw china constraints on petrogenesis and tectonic evolution of the paleo tethys ocean the jinshajiang suture zone located in the eastern part of the tethyan tectonic domain is noticeable for a large scale distribution of late jurassic to triassic granitoids these granitoids were genetically related to the evolution of the paleo tethys ocean beiwu linong and lunong granitoids occur in the middle zone of the jinshajiang suture zone and possess similar geochemical features indicating that they share a common magma source sims zircon u pb dating reveals that the beiwu linong and lunong granitic intrusions were emplaced at 233 9 1 4 ma 2 233 1 1 4 ma 2 and 231 0 1 6 ma 2 respectively all of these granitoids are enriched in abundances of si sio2t 65 2 73 5 wt and large ion lithophile elements liles but depleted in high field strength elements contents hfses e g nb ta and ti in addition they have low p2o5 contents 0 06 0 11 wt a cnk values molecular al2o3 cao na2o k2o mostly 1 1 and 10 000 ga al ratios 1 7 2 2 consistent with the characteristics of i type granites in terms of isotopic compositions these granitoids have high initial 87sr 86sr ratios 0 7078 0 7148 pb isotopic compositions 206pb 204pb t 18 213 18 598 207pb 204pb t 15 637 15 730 and 208pb 204pb t 38 323 38 791 zircon 18o values 7 3 9 3 and negative end t values 5 1 to 6 7 suggesting they were predominantly derived from the continental crust their nb ta ratios average value 8 6 are consistent with those of the lower continental crust lcc however variable ehf t values 8 6 to 2 8 and the occurrences of mafic microgranular enclaves mmes suggest that mantle derived melts and lower crustal magmas were involved in the generation of these granitoids moreover the high pb isotopic ratios and elevated zircon 18o values of these rocks indicate a significant contribution of the upper crustal composition we propose a model in which the beiwu linong and lunong granitoids were generated under a late collisional or post collisional setting it is possible that this collision was completed before late triassic decompression induced mantle derived magmas to be underplated and provided the heat for the anatexis of the crust hybrid melts including mantle derived and the lower crustal magmas were then generated the hybrid melts thereafter ascended to a shallow depth and resulted in some degree of sedimentary rock assimilation such three component mixing magma source and subsequent fractional crystallization could be responsible for the formation of the beiwu linong and lunong granitoids

knickpoint evolution in a vertically bedded substrate upstream dipping terraces and atlantic slope bedrock channels we conducted flume experiments to investigate the nature of knickpoint and bedrock channel longitudinal profile evolution for a channel reach underlain by a vertically bedded substrate of alternating resistance this research was motivated by the presence behavior and apparent persistence of impressive knickpoints formed in steeply dipping or steeply foliated bedrock in atlantic slope drainages of the appalachian mountains the experiments were carried out in a 10 m long wooden box flume filled with a very fine to medium grained sand alluvial substrate and a single 30 cm wide ridge of vertically oriented lacustrine silt clay varves designed to simulate a bedrock channel reach numerous control experiments established a stable meandering channel form and transport gradient from which knickpoints were produced by a single instantaneous base level fall at the channel mouth knickpoint evolution in the alluvial material downstream of the bedrock reach was dominated by inclination of the knick point face this resulted in a rapid transition from a waterfall at the point of base level fall to a broad convex zone spanning the entire lower alluvial channel reach a waterfall and plunge pool reformed at the contact between the lower alluvial reach and the simulated bedrock ridge where the knickpoint shortened and steepened the knickpoint then migrated upstream through the bedrock reach by a combination of parallel retreat and vertical channel incision the knickpoint evolution process resulted in the formation of upstream dipping strath terraces an uncommon landform possibly present in the holtwood gorge of the susquehanna river as base level fall effects were transmitted to the alluvial channel above the bedrock reach a complex channel response accompanied by pulses of sediment alternately buried and excavated the bedrock reach these results illustrate the complex behavior associated with knickpoint evolution the unsteadiness of the bedrock channel erosion process and the significant lag times that may exist in natural fluvial systems that are in the process of adjusting to base level fall

rapid late pleistocene incision of atlantic passive margin river gorges the direct and secondary effects of rapidly changing climate caused large rivers draining the atlantic passive margin to incise quickly into bedrock beginning about 35 000 years ago measured in samples from bedrock fluvial terraces 10 beryllium shows that both the susquehanna and potomac rivers incised 10 to 20 meter deep gorges along steep convex lower reaches during the last glacial cycle this short lived pulse of unusually rapid down cutting ended by 13 000 to 14 000 years ago the timing and rate of downcutting are similar on the glaciated susquehanna and unglaciated potomac rivers indicating that regional changes not simply glacial meltwater initiated incision

substrate controls on the longitudinal profile of bedrock channels implications for reach scale roughness 1 in this paper we examine the relationships among bedrock properties and hydraulics in shaping bedrock channel morphology at the reach scale the ocoee river and four other bedrock streams in the blue ridge province of the southeastern united states which have reach scale differences in bedrock erodibility controlled by lithologic and structural variation are the focus of this study we describe a simple conceptual model for concentrated erosion in bedrock channels and test three hypotheses in order to investigate the interactions among rock erodibility characteristics of undulating rib like bed forms reach scale gradient and hydraulic roughness and energy dissipation substrate differences correlate with variation in reach morphology i e gradient bed form orientation and amplitude such that less erodible substrates are associated with steeper reach gradient and with transversely oriented ribs of greater amplitude one dimensional modeling in hec ras indicated that in the reach with the least erodible substrate and greatest bed slope and rib amplitude the reach averaged hydraulic roughness was the greatest increased hydraulic roughness in steeper reaches points to the importance of positive and negative feedbacks in these systems greater substrate erosional resistance limits profile lowering which likely creates steeper bed slopes and greater stream power creating a self enhancing feedback this local increase in stream power is balanced by increased roughness resulting from the erosional processes that produce bedrock ribs which represents a self regulating feedback the overall result reflects quantifiable adjustments between substrate resistance and hydraulic driving forces in bedrock channels

characteristics of focal mechanisms and the stress field in the southeastern margin of the tibetan plateau crustal earthquake focal mechanisms are investigated in the southeastern margin of the tibetan plateau where the tibetan plateau and stable south china block merge an updated database of focal mechanisms has been compiled by selecting 132 global centroid moment tensor solutions and by adding the 173 new solutions 3 5 ms 7 4 estimated by waveform inversion in this study a total of 305 mechanisms are included in this database these solutions show regionally specific distributions with dominant strike slip faulting and some normal and reverse faulting focal mechanism solutions have also been inverted for the stress tensor orientation to obtain the principal stress axes over the study region results show that the horizontal maximum principal 1 axes rotate clockwise with a wider range than the geodetically measured surface motion in the east which is not limited to the xianshuihe xiaojiang fault but has some overlap with the zhaotong lianfeng fault localized normal faulting stress regimes are observed in the jinshajiang litang fault areas and the baoshan sub block the minimum principal axes are oriented with a gradually changing trend from north south to northwest southeast from north to south indicating diverse compression stress patterns significant changes in the crustal stress field after the wenchuan earthquake are preliminarily observed in the baoshan sub block where orientations of two principal axes have changed and in the jinggu ximeng sub block areas where the strike slip faulting stress pattern has transformed to normal faulting

fluvial terrace formation and controls in the lower river danube se romania abstract the black sea climatically driven level change is the main control of the lower danube incision and aggradation in the long term subsiding setting of the eastern romanian plain the interglacial episodes of aggradation suggest a decreasing fluvial accommodation while t4 terrace fill partially buried the cut terrace t5 in the upstream part of the studied area lower danube arges d mbovi a younger deposition did not compensate the previous incisions black sea level fallings during glaciations were followed by lower danube incisions which propagated upward through knickpoints loess deposits can be correlated with these periods five episodes of incision were recognized based on terrace levels t5 to t1 their ages and of the covering loess were obtained through optically stimulated luminescence osl and were correlated with mis 6 mis 3 and classic glacial chronological events latest rissian middle wurmian the maximum limiting age of danube oldest terrace t5 in the studied area is 141 25 kyr while of the youngest t1 is 50 6 kyr the equivalent terraces on tributaries are younger the time delays between terrace incisions on the lower danube system were estimated the 32 7 15 9 kyr danube floodplain deposits and the 5 kyr forests buried up to 7 m under arges floodplain suggest that younger than 44 40 kyr terraces may be buried

drainage network morphometry and evolution in the eastern lesotho highlands southern africa abstract present day drainage patterns in mountain regions are strongly affected by antecedent tectonic and geologic factors in the mountains of eastern lesotho southern africa bedrock comprises flat lying jurassic basalts which have given rise to a relatively uniform initial land surface here we present key morphometric measurements of river drainage patterns from this region from six 4th order river basins including principal river sinuosity river long profiles and valley cross profiles in order to evaluate geological controls on drainage evolution and the potential ages of river elements results show that there are highly variable spatial patterns of river morphometric properties even between adjacent basins despite identical geologies and altitudes across the region however northerly catchments show knickpoints variable long profiles and have steep sided and v shaped valley cross profiles southerly catchments have much smoother long profiles no knickpoints and concavo convex valley cross profiles there are also differences in river sinuosity values both for the catchments as a whole and for reaches of different lengths within the catchments these spatially varying river morphometric and drainage properties most likely are the long preserved outcomes of epeirogenic forcing on the land surface including tectonic uplift mantle relaxation and rebound during the cenozoic and subtle long term variations in climate from north to south across the region including in precipitation and weathering

evaluation of arsenic sorption and mobility in stream sediment and hot spring deposit in three drainages of the tibetan plateau abstract enrichment of arsenic as in sediment 12 227 mg kg in the upstream tributaries of the indus and brahmaputra rivers originating from the tibetan plateau where hot springs were abundant has been found sandy sediment samples from the nu salween river lantsang mekong river and jinsha yangtze river also originating from the tibetan plateau were collected in july 2012 and were found to contain 6 8 mg kg to 30 5 mg kg as average17 3 6 5 mg kg n 12 deposits collected within 1 m of two hot springs in changdu tibet displayed significantly higher as levels 263 7 mg kg in ld 2 with 65 quartz and 101 8 mg kg in nud 1 with 82 calcite to evaluate the valence states of as and also the phases responsible for sorption x ray absorption spectroscopy xas was employed to analyze the two hot spring deposits and three river sediment samples coarse sand nud 6 as 14 6 mg kg fine sand nud 4 as 17 8 mg kg and silty sand ld 1 as 30 5 mg kg the x ray absorption near edge spectrum xanes data indicate that 70 of as from 3 samples in the nu salween river drainage nud 1 nud 4 and nud 6 is as v or arsenate with the rest being as 0 or as fe sulfides the proportion of as v is 90 for 2 samples in the lantsang mekong river drainage ld 1 and ld 2 linear combination fit of the iron extended x ray fine structure spectrum exafs show that 3 samples from the nu salween river contain 20 ferrihydrite and 10 goethite without any hematite being detected but 2 samples from the lantsang mekong river contain

late quaternary fluvial landform evolution and controlling factors along the yulin river on the northern tibetan plateau abstract the yulin river in the northern tibetan plateau consists of multiple geomorphological features including alluvial fans fluvial terraces and river profile knickpoints collectively these provide a natural laboratory for exploring the late quaternary fluvial landform evolution in response to both climatic change and tectonic activities in the region of the altyn tagh fault in this study we investigated the distribution sequences and sediment characteristics of the fluvial landforms along the yulin river based on high precision 0 5 m resolution worldview satellite images and field observations mid pleistocene alluvial fans formed the highest fluvial landform surface into which ten major inset fluvial terraces were developed distributed along three stream segments real time kinematic global positioning system rtk gps measurement of terrace surfaces and the modern river longitudinal profiles was conducted across the dongbatu shan to show fluvial landform geometry and tectonic deformation fluvial terrace surfaces were deformed due to fault uplift as illustrated by spatially variable surface geometries the chronological sequences of terraces along the yulin river since 160 ka were reconstructed based on 10be terrestrial cosmogenic nuclides tcns dating results indicated that river incision occurred mainly during periods of climate transition suggesting that terrace evolution was partly controlled by climate changes moreover terrace exposure ages showed an upstream younging trend along the river demonstrating a probable response to knickpoint retreat combined with terrace deformation and ages fault uplift rates were constrained which were lower than the river incision rates during the late quaternary indicating that crustal shortening was mainly absorbed by the growth of the regional anticline collectively our results suggest that the evolution of fluvial landforms along the yulin river is a function of interplay between regional tectonic activities and climatic changes which is significant to understand the nw outward splay of the altyn tagh fault

erosion of an ancient mountain range the great smoky mountains north carolina and tennessee analysis of 10be and 26al in bedrock n 10 colluvium n 5 including grain size splits and alluvial sediments n 59 including grain size splits coupled with field observations and gis analysis suggest that erosion rates in the great smoky mountains are controlled by subsurface bedrock erosion and diffusive slope processes the results indicate rapid alluvial transport minimal alluvial storage and suggest that most of the cosmogenic nuclide inventory in sediments is accumulated while they are eroding from bedrock and traveling down hill slopes spatially homogeneous erosion rates of 25 30 mm ky 1 are calculated throughout the great smoky mountains using measured concentrations of cosmogenic 10be and 26al in quartz separated from alluvial sediment 10be and 26al concentrations in sediments collected from headwater tributaries that have no upstream samples n 18 are consistent with an average erosion rate of 28 8 mm ky 1 similar to that of the outlet rivers n 16 24 6 mm ky 1 which carry most of the sediment out of the mountain range grain size specific analysis of 6 alluvial sediment samples shows higher nuclide concentrations in smaller grain sizes than in larger ones the difference in concentrations arises from the large elevation distribution of the source of the smaller grains compared with the narrow and relatively low source elevation of the large grains large sandstone clasts disaggregate into sand size grains rapidly during weathering and downslope transport thus only clasts from the lower parts of slopes reach the streams 26al 10be ratios do not suggest significant burial periods for our samples however alluvial samples have lower 26al 10be ratios than bedrock and colluvial samples a trend consistent with a longer integrated cosmic ray exposure history that includes periods of burial during down slope transport the results confirm some of the basic ideas embedded in davis geographic cycle model such as the reduction of relief through slope processes and of hack s dynamic equilibrium model such as the similarity of erosion rates across different lithologies comparing cosmogenic nuclide data with other measured and calculated erosion rates for the appalachians we conclude that rates of erosion integrated over varying time periods from decades to a hundred million years are similar the result of equilibrium between erosion and isostatic uplift in the southern appalachian mountains

channel incision in the rio atenguillo jalisco mexico defined by 36cl measurements of bedrock abstract the jalisco block of western mexico has undergone uplift in the tertiary in response to subduction tectonics the atenguillo river interior to the jalisco block has incised the bedrock units of the jalisco block including cretaceous ash flow tuff granite as well as younger volcanic lava flows to study incision rates knickpoint propagation and the uplift rate of the region samples from nine different points along the atenguillo channel were collected for 36 cl exposure age dating exposure ages along the length of the channel are high near the head of the basin where the river meanders in a broad plain ages are younger at the knickpoints and downstream where the river cuts deeply and forms a canyon incision rates measured at different points along the channel also correlate with channel slope and age low incision rates are measured in the oldest part of the channel near the head station b 2 5 mm year and just above the knickpoints station c 0 6 mm year and station g 1 4 mm year the incision rates are highest just below the knickpoints station f 2 4 mm year and farther downstream stations h and i 5 6 and 2 9 mm year respectively the high incision rates can be attributed to subduction related uplift of the jalisco block other features attributable to uplift are emergent paleoshorelines and uplifted marine sediments along the coast and relatively lower incision rates were measured north of the jalisco block

the effect of the wenchuan earthquake on the fluvial morphology in the longmen shan eastern tibetan plateau discussion and speculation abstract although large earthquakes can have significant impacts on the geomorphology of mountain rivers the consequences of such impacts remain poorly understood the ms 8 0 wenchuan earthquake of 12 may 2008 created knickpoints waterfalls in the river systems of the longmen shan eastern tibet plateau in addition to numerous landslides along rivers in this study we use high resolution remote sensing images and field surveys to examine how the baisha and jianjiang river channels responded to the wenchuan earthquake the morphological response along the baisha river was a knickpoint with a height of 4 5 m which was studied from 2008 to 2012 however the river did not show rapid incision via knickpoint migration during the post earthquake period in fact post seismic flooding destroyed the face of the co seismic knickpoint the rapid removal of the knickpoint suggests that the abrupt vertical displacement associated with the earthquake did not exceed the adjustment threshold of the fluvial dynamic equilibrium and so was unable to drive rapid morphological change at the local scale along the jianjiang river the average meander migration rate of the beichuan reach was 2 8 m yr between 2001 and 2008 but increased to 51 1 m yr during the five years following the 2008 wenchuan earthquake this rapid increase in the meander migration rate is attributed to excess sediment supplied to the river as a consequence of the earthquake these two case studies indicate that the wenchuan earthquake caused channel aggradation rather than a significant increase in river incision by locally generating relief across the river

a record of appalachian denudation in postrift mesozoic and cenozoic sedimentary deposits of the u s middle atlantic continental margin abstract the complex interplay between source terrain uplift basin subsidence paleoclimatic shifts and sea level change left an extensive sedimentary record in the contiguous offshore basins of the u s middle atlantic margin salisbury embayment baltimore canyon trough and hatteras basin isopach maps of 23 postrift lower jurassic to quaternary a allostratigraphic units coupled with a revised stratigraphic framework reveal that tectonism by regulating sediment supply accumulation rate dominated the interplay of forcing mechanisms tectonic pulses are evidenced by abruptly accelerated sediment accumulation marked latitudinal shifts in the location of depocenters and regional changes in lithofacies relatively rapid tectonic subsidence during the early and middle jurassic history of the basins may have enhanced sediment accumulation rates beginning in the late jurassic however subsidence rates decreased significantly though occasional short pulses of subsidence may have effected relative sea level rises sea level change heavily influenced the distribution and redistribution of sediments one they reached the basins and paleoclimate regulated the relative abundance of carbonates and evaporites in the basins we conclude that source terrains of the central appalachian highlands were tectonically uplifted intensely weathered and rapidly eroded three times since the late triassic 1 early to middle jurassic aalenian to callovian 2 mid early cretaceous barremian and 3 late cenozoic middle miocene intervals of tectonic quiescence following these three tectonic pulses provided conditions suitable for the formation of regional erosion surfaces geomorphic features commonly reported to characterize the central appalachian highlands this series of three irregularly spaced tectonic quiescent cycles does not however match the traditional four cycle concept of post triassic appalachian peneplanation

australian volcanic hosted massive sulfide deposits features styles and genetic models the cambro ordovician period has yielded the major development of volcanic hosted massive sulfide vhms deposits in australia resulting in about 12 million metric tons of contained metal copper lead and zinc concentrated in the mount read volcanics tasmania and the mount windsor volcanics queensland the archean 4 million metric tons of metal and the silurian 3 5 million metric tons of metal constitute the next important episodes while the devonian and permian have produced isolated deposits the deposits range from cu type to zn cu type to zn pb cu type the zn cu type deposits are restricted to the archean and silurian whereas the cu type and zn pb cu type occur sporadically throughout the time span from early archean to permian in general terms the major vhms bearing districts are calc alkaline in character and display a thick basal pile of rhyolitic volcanics 1 3 km thick including lavas epiclastics and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite dacite andesite basalt and sediments the major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence there is a wide range in variability of styles of australian vhms deposits including mounds pipes sheets layered deposits stacked deposits stockwork and disseminated deposits distal reworked deposits and cyclic layered deposits although these various styles show a range of features there is a consistent theme across the spectrum of metal zonation alteration mineralogy alteration chemistry sulfur isotopes macrotextures microtextures and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit the classic mound style deposits such as hellyer have a series of key features subsets of which are represented in the other deposit styles the mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures leading to zone refining and lead zinc silver gold enrichment in the outer and upper parts of the mound departure from the classic mound style of deposit is related principally to three key factors the chemistry of the ore fluid salinity temperature f02 and ah2s the nature permeability and chemistry of the volcanic pile and the sea hoor environment sea hoor topography and seawater depth these factors control the aspect ratio of the deposit the extent of stringer zone development the degree of subsea floor replacement mineralization the nature and spatial development of footwall alteration and the development and style of related distal mineralization sulfur isotope studies of australian vhms deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks this conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences the input of metals directly from volcanic magma chambers is not precluded by the available data and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume rising to mix with convective seawater fluids which contribute lead zinc silver and gold leached from the footwall volcanics and basement rocks the relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles arid their spatial relationship to volcanic centers or adjacent sedimentary basins

autogenic incision and terrace formation resulting from abrupt late glacial base level fall lower chippewa river wisconsin usa abstract a paucity of research exists regarding the millennial scale response of inland alluvial streams to abrupt base level fall studies of modern systems indicate that over short time scales the response is a diffusion like process of upstream propagating incision in contrast evidence from the lower chippewa river lcr located in the upper midwest of the usa suggests that autogenic controls operating over time scales of several millennia can overwhelm diffusion resulting in incision that is prolonged and episodic during the last glacial maximum the lcr drained the chippewa lobe of the laurentide ice sheet to the glacial upper mississippi river umr as a meltwater stream it aggraded and filled its valley with glacial outwash as did its largest tributaries which were also meltwater streams its nonglacial tributaries aggraded too filling their valleys with locally derived sediment during deglaciation the umr incised at least twice abruptly lowering the lcr s base level 15 m at 16 ka or earlier and an additional 40 m at ca 13 4 ka each of these base level falls initiated incision of the lcr led by upstream migrating knickpoints the propagation of incision has however been a lengthy process the optically stimulated luminescence osl ages of terrace alluvium indicate that by 13 5 ka incision had advanced up the lcr only 15 km and by 9 ka only 55 km the process has also been episodic resulting in the formation of fill cut terraces inferred from gpr surveys and exposures of terrace alluvium that are younger and more numerous in the upstream direction autogenic increases in sediment load and autogenic bed armoring the result of periodic tributary stream rejuvenation and preferential winnowing of fines by the incising river may have periodically caused knickpoint migration and incision to slow and possibly stop allowing lateral erosion and floodplain formation to dominate a decline in sediment flux from stabilizing incised tributary stream systems would have led to renewed knickpoint migration and incision when floods of sufficient magnitude to breach the channel armor occurred minimal floodplain development along the upper section of the present day lcr along with the channel morphology of an unstable wandering gravel bed river immediately downstream from it suggest that the river is still responding to the base level falls that happened many millennia ago the autogenic controls on the lcr s response to umr incision are a direct consequence of the thick fills of noncohesive sediment that accumulated in its valley and the valleys of its tributary streams during the late wisconsinan making the lcr a prime example of a former proglacial river that remains a paraglacial fluvial system

lithologic control of bedrock meander dimensions in the appalachian valley and ridge province marked differences in bedrock meander dimensions in the appalachian valley and ridge province at times in adjacent reaches of a single stream are related to differences in relative erodability of the bedrock meanders cut in thick bedded to massive lithologies typically carbonates are distinctly smaller overall than meanders cut into shaly lithologies other factors that could affect bedrock meander dimensions were considered and none appeared to offer any help in explaining the dimensional differences observed above in plateau regions underlain by essentially horizontal strata the meander form characteristic of one lithology may become superimposed upon another as incision progresses producing anomalous relationships between bedrock meander dimensions and lithology empirical relations developed from meander geometry measurements and estimated bedrock erodability for 78 bedrock meander reaches containing a total of 1089 individual meander loops show that meanders cut in shaly lithologies ml 105 q0 50f where ml meander length and qf most probable annual flood are about twice the length of meanders cut in non shaly lithologies ml 39 30 56f the valley floor width of the meanders cut in shaly bedrock vf 28 q0 43f is two to three times wider than the valley floor width of the meanders cut in the more resistant non shaly bedrock vf 8 q0 43f the mean and median meander length values for individual reaches typically differ by less than 10 per cent

late cenozoic flexural deformation of the middle u s atlantic passive margin despite the century long recognition of regional epeirogeny along the middle atlantic passive margin relatively few studies have focused on understanding postrift uplift mechanisms here we demonstrate that epeirogenic uplift of the central appalachian piedmont and subsidence of the salisbury embayment represent first order flexural isostatic processes driven by continental denudation and offshore deposition our results show that regional epeirogenic processes present on all atlantic type passive margins are best resolved by specific stratigraphic and geomorphic relationships rather than topography a simple one dimensional geodynamic model constrained by well dated baltimore canyon trough coastal plain and lower susquehanna river piedmont stratigraphy simulates flexural deformation of the u s atlantic margin the model represents the passive margin lithosphere as a uniformly thick elastic plate without horizontal compressive stresses that deforms flexurally under the stress of strike averaged vertically applied line loads model results illustrate a complex interaction among margin stratigraphy and geomorphology the isostatic response to denudational and depositional processes and the modulating influence of exogenic forces such as eustasy the current elevation with respect to modern sea level of fluvial terraces and correlative coastal plain deposits or unconformities is successfully predicted through the synthesis of paleotopography eustatic change and margin flexure results suggest that the middle u s atlantic margin landward of east coast magnetic anomaly is underlain by lithosphere with an average elastic thickness of 40 km flexural rigidity d 4 1023 n m the margin experiences an average long term denudation rate of approximately 10 m m y and the piedmont has been flexurally upwarped between 35 and 130 meters in the last 15 m y long term isostatic continental uplift resulting from denudation and basin subsidence resulting from sediment loading are accommodated primarily by a convex up flexural hinge physiographically represented by the fall zone our results elucidate an inherent danger in using topography alone to constrain late stage passive margin deformation mechanisms only through careful synthesis of field stratigraphic and geomorphic elements such as fluvial terraces coastal plain deposits and offshore stratigraphy can age control be extended from the offshore depositional setting to the erosionally dominated continent this study demonstrates that despite a relatively subdued topography the middle u s atlantic margin experiences progressive flexural isostatic deformation similar to that proposed for high relief margins characterized by great escarpments thus margin topographic diversity remains a function of other factors such as lithospheric composition and or structure supracrustal stratigraphy and structure degree of drainage integration drainage divide migration and climate

neogene rejuvenation of central appalachian topography evidence for differential rock uplift from stream profiles and erosion rates abstract the persistence of topography within ancient orogens remains one of the outstanding questions in landscape evolution in the eastern north american appalachians this question is manifest in the outstanding problem of whether topographic relief is in a quasi equilibrium state decaying slowly over many millennia or whether relief has increased during the late cenozoic here we present quantitative geomorphic data from the nonglaciated portion of the susquehanna river drainage basin that provide insight into these end member models analysis of channel profiles draining upland catchments in the northern valley and ridge appalachian plateau blue ridge and piedmont provinces reveals that a large number of streams have well defined knickpoints clustered at 300 600 m elevation but not systematically associated with transitions from weak to resistant substrate cosmogenic 10 be inventories of modern stream sediment indicate that erosion rates are spatially variable ranging from 5 30 m myr above knickpoints to 50 100 m myr below knickpoints overall channel gradients normalized for drainage area scale linearly with catchment averaged erosion rates collectively regionally consistent spatial relationships among erosion rate channel steepness and knickpoints reveal an ongoing wave of transient channel adjustment to a change in relative base level reconstructions of relict channel profiles above knickpoints suggest that higher rates of incision are associated with 100 150 m of relative base level fall that accompanied epierogenic rock uplift rather than a change to a more erosive climate or drainage reorganization channel response timescales imply that the onset of relative base level change predates 3 5 ma and may have begun as early as 15 ma we suggest that adjustment of the channel network was likely driven by changes in mantle dynamics along the eastern seaboard of north america during the neogene

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

post collisional crustal extension setting and vhms mineralization in the jinshajiang orogenic belt southwestern china abstract the jinshajiang orogenic belt job of southwestern china located along the eastern margin of the himalayan tibetan orogen includes a collage of continental blocks joined by paleozoic ophiolitic sutures and permian volcanic arcs three major tectonic stages are recognized based on the volcanic sedimentary sequence and geochemistry of volcanic rocks in the belt westward subduction of the paleozoic jinshajiang oceanic plate at the end of permian resulted in the formation of the chubarong dongzhulin intra oceanic arc and jamda weixi volcanic arc on the eastern margin of the changdu continental block collision between the volcanic arcs and the yangtze continent block during early middle triassic caused the closing of the jinshajiang oceanic basin and the eruption of high si and al potassic rhyolitic rocks along the permian volcanic arc slab breakoff or mountain root delamination under this orogenic belt led to post collisional crustal extension at the end of the triassic forming a series of rift basins on this continental margin arc significant potential for vhms deposits occurs in the submarine volcanic districts of the job mesozoic vhms deposits occur in the post collisional extension environment and cluster in the late triassic rift basins

the role of waterfalls and knickzones in controlling the style and pace of landscape adjustment in the western san gabriel mountains california bedrock rivers set the pace of landscape adjustment to tectonic and climatic forcing by transmitting signals of base level change upstream through the channel network and ultimately to hillslopes river incision is typically modeled as a monotonic function of bed shear stress or stream power modulated by sediment tools and cover effects but these models do not apply in channels with steep or vertical bedrock reaches due to changes in flow dynamics hydraulic geometry and bed cover here we investigate how such knickzones oversteepened channel reaches often containing waterfalls influence the propagation of slope break knickpoints that separate relict from adjusting topography and thus the response times of landscapes to external forcing we use a conceptual long profile model to explore the consequences of waterfalls and knickzones on channel response and compare predictions to light detection and ranging lidar topography field observations and cosmogenic radionuclide data from big tujunga creek a 300 km2 watershed in the san gabriel mountains california three prominent knickzones along big tujunga creek characterized by numerous waterfalls show contrasting behavior for the upper knickzone waterfalls align with bands of harder rock exposed on adjacent hillslopes and between waterfalls the channel is mantled by large 2 m boulders indicating knickzone retreat is slow compared to predictions of slope break knickpoint retreat from stream power models enhancing the preservation of an upstream relict landscape the middle knickzone shows evidence for both fast and slow knickzone retreat as well as significant deviations from predictions of uniform tributary knickpoint elevations derived from stream power models the lower knickzone is characterized by a waterfall and knickzone within an incised inner gorge that provide evidence of rapid retreat relative to background channel incision overall we find a pattern of decreasing knickzone and waterfall retreat rate with distance upstream of the range front beyond decreases predicted by simple area dependent celerity models our results highlight that waterfalls and knickzones can both enhance and inhibit landscape adjustment leading to divergent controls on the pace of landscape evolution

an automated method for producing synoptic regional maps of river gradient variation procedure accuracy tests and comparison with other knickpoint mapping methods abstract the study of abrupt changes in longitudinal river profiles or knickpoints is currently approached through an empirical power law the slope area relationship results based on digital elevation model dem analyses and stream extractions are generally intended to determine crustal uplift rates and identify transient landscape conditions in this article we present an alternative geomorphometric method for locating knickpoints and knickzones based on local slope gradient and curvature attributes intended as a rapid regional scale automated knickpoint detection technique the accuracy of this slope curvature method is tested on two digital elevation grids nasa s srtm ground resolution of 90 m resampled here to 75 m and the aster dem 15 m in the sierra nacimiento new mexico usa a basement cored mountain range recently exhumed by waves of headward drainage integration in response to remote tectonic deformation in the adjacent rio grande rift out of every 10 gradient anomalies detected by the srtm derived numeric routine up to 8 are certifiable knickpoints recognized among a population of georeferenced occurrences surveyed in the field an independent comparison with the slope area method provided a further accuracy test which was particularly useful at sites that could not be validated in the field for practical reasons given the low tectonic activity of the study area the majority of knickpoints was also found to coincide with lithologic boundaries making it difficult without further geomorphological data to single out dynamic knickpoints directly caused by the upstream propagation of channel instabilities relating to base level change

the longmenshan tectonic complex and adjacent tectonic units in the eastern margin of the tibetan plateau a review abstract the longmenshan tectonic complex lstc along the eastern margin of the tibetan plateau the site of devastating earthquakes such as the magnitude 8 0 wenchuan earthquake on 12 may 2008 preserves an exceptionally complete history of the tectonic evolution of the yangtze block and its relations to adjacent tectonic units due to sequential tectonic superposition and tectonic reactivation the tectonic nature of the lstc and in particularly the older history has been profoundly debated and many different tectonic models have been proposed herein we summarize the current understandings of the major tectonic events that have shaped this important tectonic complex highlighting problems left to be solved by future work including 1 the nature and constraints for at least 6 regional tectonic events i e building of the metamorphic basement art the columbia nuna supercontinent pt1t the rodinia supercontinent pt3t the paleozoic passive continental margin pzt the paleotethys orogeny mzt and the neotethys orogeny kzt 2 metamorphic basement exposures and their tectonic implications including rock types and geochronological constraints for the archean paleoproterozoic and neoproterozoic basements 3 nature of the present lstc and its affinity with adjacent tectonic units 4 consideration of the ne striking longmenshan thrust belt and arcuate shape yanyuan muli thrust belt as parts of a single tectonic feature 5 mountain basin coupled systems recording past tectonic eposides we draw the following conclusions and tectonic models based on published research combined with our own recent studies 1 the well preserved archean yudongzi gneiss group in the lstc has a genetic affinity with the kongling group and thus belongs to the yangtze block 2 the paleoproterozoic hejiayan group juxtaposed adjacent to the archean yudongzi group may represent a 2000 1800 ma orogenic belt which corresponds to the supercontinent nuna columbia amalgamation event 3 a neoproterozoic trench arc basin system which is reconstructed based on identification of a neoproterozoic ophiolite complex arc type magmatic rock assemblages and volcaniclastic basinal deposits along the western margin of the yangtze block and the lstc may represent the record of eastward subduction of the neoproterozoic mozambique oceanic lithosphere beneath the yangtze block during the assembly of the rodinia supercontinent 4 a complete bidirectional wilson cycle was reconstructed by the formation of the late permian to the middle late triassic back arc ganze litang rift and ocean following the early paleozoic mianlue continental rift and ocean and subsequent closure of the ocean basin by simultaneous bidirectional northward and southwestward subduction and later collision this relatively uncommon bidirectional wilson cycle might be attributed to the formation of the three armed rift system in the eastern paleotethys associated with the late permian eemeishan large igneous province in the lstc 5 a three stage tectonic sequence of in sequence imbricate thrust in the lstc during india eurasian collisional orogeny at 55 15 ma extrusion from 15 to 5 ma and the plateau uplift since 5 ma resulting from lower crustal channel flow is proposed for the formation of the present lstc

postglacial recession of niagara falls in relation to the great lakes abstract the recessional history of niagara falls in the present niagara gorge during the postglacial period has been a focus of study throughout this century radiocarbon ages of clam shells suggest that niagara falls migrated very slowly around the narrowed gorge section at niagara glen from 10 500 to 5500 yr b p when upper great lakes water bypassed lake erie and flowed to the ottawa river via the outlet at north bay ontario prior to that interval river discharge and recession rates were similar to those at present and similar rates resumed after 5200 yr b p by about 4500 yr b p the present gorge had intersected a buried gorge of the pre late wisconsinan niagara river st davids gorge the sediment derived from the excavated buried valley fill may be present as a distinct marker horizon in the sediments in southwestern lake ontario

zircon u pb geochronology and elemental and sr nd hf isotopic geochemistry of the daocheng granitic pluton from the yidun arc sw china abstract the garze litang suture zone located in the eastern part of the tethyan tectonic domain is notable for widespread late triassic granitic plutons which are genetically associated with the evolution of the paleo tethys ocean the daocheng granitic pluton in the eastern yidun arc sw china is located in the middle of the garze litang suture zone and has an outcrop area of 2800 km 2 in the present study we report zircon u pb ages and elemental and sr nd hf isotopic data for the daocheng granitic intrusion secondary ion mass spectrometry and laser ablation inductively coupled plasma mass spectrometry u pb analyses on zircons yield consistent ages of ca 216 ma for three samples from the pluton all the daocheng granitic rocks are enriched in si sio 2 68 1 76 4 and large ion lithophile elements but depleted in high field strength elements e g nb ta and ti mineralogical and geochemical features indicate that these are high k and calc alkaline and i type granites they are characterized by relatively variable initial 87 sr 86 sr ratios 0 7059 0 7102 negative e nd t values 5 7 to 7 8 wide ranging e hf t values 9 8 to 3 4 and two stage hf model ages of 1 04 to 1 88 ga these isotopic signatures indicate that the source for the daocheng granite was probably derived from partial melting of a late paleoproterozoic to early mesoproterozoic mafic intermediate lower crust with a variably minor addition

petrogenesis and tectonic implications of the triassic volcanic rocks in the northern yidun terrane eastern tibet abstract triassic paleo tethyan subduction related volcanic rocks are extensively distributed in the xiangcheng and changtai regions northern yidun terrane eastern tibet the 228 ma volcanic rocks in the xiangcheng region have intermediate felsic composition 60 1 to 64 7 wt sio 2 with relatively low y 14 18 ppm and high sr 630 1830 ppm yielding high sr y ratios 43 94 they have lree enriched and hree depleted ree patterns la yb 19 26 typical of adakites sr nd isotopic compositions of these volcanic rocks are similar to those of the contemporaneous high silica adakitic rocks in the southern yidun terrane the 231 230 ma volcanic rocks in the changtai region are composed of basalts andesites dacites and rhyolites with sio 2 ranging from 47 7 to 81 0 wt basalts in the changtai region have initial 87 sr 86 sr ratios from 0 7052 to 0 7058 and e nd t from 1 1 to 1 5 they display oib like geochemical signatures and were likely derived from low degrees of partial melting of an oib like mantle source with subordinate input of subduction components indicated by increasing th content andesites and dacites show elemental and isotopic compositions that evolved from the basalts indicative of derivation via crustal contamination and fractional crystallization afc of basaltic magmas rhyolites have lower initial 87 sr 86 sr ratios and higher e nd t values than andesites dacites inconsistent with an afc process and are suggested to have been derived from anatexis of crustal materials emplacement of volcanic rocks in the changtai and xiangcheng regions could be attributed to the subduction of the ganzi litang ocean a branch of the paleo tethys ocean volcanic rocks in the changtai and xiangchang regions have zircon u pb ages about 4 6 myr older than that of arc granites in the eastern yidun terrane the spatio temporal distribution of the volcanic rocks indicates that the subduction was initiated under the southern yidun terrane in the middle triassic and extended to the northern yidun terrane at the beginning of the late triassic

analysis and modeling of gully headcut dynamics north american high plains gullies are dynamic fluvial features that can be the primary driver for landscape dissection and sediment production in many settings this research exploits a well constrained field area near west bijou creek colorado u s in order to develop a natural experiment in which we explore gully headcut erosion rates the controls on gully headcut height and the morphology of gully longitudinal profiles analysis of headcut retreat using aerial photography and airborne lidar imagery indicates that headcut retreat rates correlate with the square root of drainage area approximately we investigate how a drainage area control on headcut retreat translates into the longitudinal profile morphology over time using a simple numerical model the model combines fluvial erosion deposition and headcut retreat to identify the necessary and sufficient conditions needed to reproduce longitudinal profiles observed in the field field profiles are typically concave upward predominantly aggradational channel profiles with retreating headcuts whose height varies with catchment position systematic variation of environmental parameters in the model showed that the most successful model was achieved when highly resistant vegetation is applied throughout the channel excluding a bare soil zone downstream of the headcut this model scenario maintained an abrupt headcut over hundreds of model years and produced a realistic longitudinal profile that aggrades downstream of the headcut over time the vegetation pattern used in the best model fit is observed at the field site where easily erodible sparsely vegetated soil downstream of the headcut grades into a more resistant grassy channel downstream

tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the sanjiang region sw china abstract the sanjiang region in se tibet plateau and nw yunnan is known to have formed by amalgamation of gondwana derived continental blocks and arc terranes as a result of oceanic subduction followed by continental collision from paleozoic to mesozoic in this paper we provide a synthesis of tectonic evolution magmatism and metallogeny in the region based on data from literatures early paleozoic ophiolites 473 439 ma in the changning menglian belt indicate the existence of a proto tethys ocean in this region two episodes of subduction related magmatism in the early paleozoic one occurred in the baoshan and tengchong blocks at 502 455 ma and the other occurred in the simao block at 421 401 ma are regarded as evidence for two different events of subduction of the proto tethys ocean at different locations the proto tethys was succeeded in early devonian by the paleo tethys which comprised the main ocean and three branches ailaoshan jinshajiang and garze litang the changning menglian main ocean existed from middle devonian to middle triassic the remnants of the oceanic crust are preserved in a few places in the longmu tso shuanghu suture as well as in the changning menglian ophiolite belt the eastward subduction of the main oceanic plate from early permian to early triassic formed a prominent arc terrane stretching 1500 km from yunnan to eastern tibet from the waning stage of subduction to post subduction numerous s type granite plutons with ages varying between 230 and 219 ma such as the lincang batholith in yunnan were emplaced at or close to the suture this event produced several hydrothermal w sn deposits in the region the tectonic evolution and associated magmatism of the jinshajiang and ailaoshan branch oceans are generally comparable to those of the main ocean however the branch oceans were subducted westward instead the garze litang branch ocean also underwent westward subduction from middle devonian to late triassic arc related high sr y porphyry intrusions and associated porphyry skarn cu mo au deposits are common in the jinshajiang ailaoshan region especially in the yidun arc which formed prior to jurassic the vms deposits in the sanjiang region formed in diverse tectonic settings including middle silurian back arc basins carboniferous oceanic islands paleozoic subduction zones and triassic post subduction rifting environments the mesozoic and early cenozoic evolution of the baoshan and tengchong blocks was largely influenced by eastward oceanic subduction of the meso and neo tethys from late permian to middle cretaceous and from late cretaceous to 50 ma respectively abundant early cretaceous granitoids and associated skarn type pb zn and sn fe deposits in the baoshan and tengchong blocks were produced in the background of the shan boundary oceanic slab subduction to the west and the break off of the nujiang bitu oceanic slab to the north the subduction of the neo tethys oceanic plate beneath the tengchong block from late cretaceous to paleogene formed abundant s type granitoids and many skarn type and greisen type sn w deposits granitoids formed at 105 to 81 ma and contemporaneous hydrothermal w mo ag and au deposits which temporally coincided with the subduction of the neo tethys are common in the yidun arc terrane

a mechanistic model of waterfall plunge pool erosion into bedrock landscapes often respond to changes in climate and tectonics through the formation and upstream propagation of knickzones composed of waterfalls little work has been done on the mechanics of waterfall erosion and instead most landscape scale models neglect waterfalls or use rules for river erosion such as stream power that may not be applicable to waterfalls here we develop a physically based model to predict waterfall plunge pool erosion into rock by abrasion from particle impacts and test the model against flume experiments both the model and experiments show that evolving plunge pools have initially high vertical erosion rates due to energetic particle impacts and erosion slows and eventually ceases as pools deepen and deposition protects the pool floor from further erosion lateral erosion can continue after deposition on the pool floor but it occurs at slow rates that become negligible as pools widen our work points to the importance of vertical drilling of successive plunge pools to drive upstream knickzone propagation in homogenous rock rather than the classic mechanism of headwall undercutting for a series of vertically drilling waterfalls we find that upstream knickzone propagation is faster under higher combined water and sediment fluxes and for knickzones composed of many waterfalls that are closely spaced our model differs significantly from stream power based erosion rules in that steeper knickzones can retreat faster or more slowly depending on the number and spacing of waterfalls within a knickzone which has implications for interpreting climatic and tectonic history through analysis of river longitudinal profiles

a case of rapid rock riverbed incision in a coseismic uplift reach and its implications abstract during the 1999 chi chi earthquake m w 7 6 in taiwan the coseismic displacement induced fault scarps and a pop up structure in the taan river the fault scarps across the river experienced maximum vertical slip of 10 m which disturbed the dynamic equilibrium of the fluvial system as a result rapid incision in the weak bedrock with a maximum depth of 20 m was activated within a decade after its armor layer was removed this case provides an excellent opportunity for closely tracking and recording the progressive evolution of river morphology that is subjected to coseismic uplift based on multistaged orthophotographs and digital elevation model dem data the process of morphology evolution in the uplift reach was divided into four consecutive stages plucking is the dominant mechanism of bedrock erosion associated with channel incision and knickpoint migration the astonishingly high rate of knickpoint retreat kpr as rapid as a few hundred meters per year may be responsible for the rapid incision in the main channel the reasons for the high rate of kpr are discussed in depth the total length of the river affected by the coseismic uplift is 5 km 1 km in the uplift reach and 4 km in the downstream reach the downstream reach was affected by a reduction in sediment supply and increase in stream power the kpr cut through the uplift reach within roughly a decade further significant flooding in the future will mainly cause widening instead of deepening of the channel

self formed waterfall plunge pools in homogeneous rock waterfalls are ubiquitous and their upstream propagation can set the pace of landscape evolution yet no experimental studies have examined waterfall plunge pool erosion in homogeneous rock we performed laboratory experiments using synthetic foam as a bedrock simulant to produce self formed waterfall plunge pools via particle impact abrasion plunge pool vertical incision exceeded lateral erosion by approximately tenfold until pools deepened to the point that the supplied sediment could not be evacuated and deposition armored the pool bedrock floor lateral erosion of plunge pool sidewalls continued after sediment deposition but primarily at the downstream pool wall which might lead to undermining of the plunge pool lip sediment evacuation and continued vertical pool floor incision in natural streams undercutting of the upstream pool wall was absent and our results suggest that vertical drilling of successive plunge pools is a more efficient waterfall retreat mechanism than the classic model of headwall undercutting and collapse in homogeneous rock

henty a shallow water gold rich volcanogenic massive sulfide deposit in western tasmania the mount read volcanics of western tasmania host a number of polymetallic volcanogenic massive sulfide deposits a cluster of disseminated cu au deposits at mount lyell and a high grade gold deposit avg grade 27 ppm au at henty the henty deposit occurs at the base of the tyndall group a sequence of rhyolite lavas and crystal rich volcaniclastics fossiliferous sediments and welded ignimbrites within this group are indicative of a local shallow water environment mineralization occurs in a conformable package of altered rocks in the footwall of a major fault the host rocks were strongly deformed during a later compressional event the deposit has a distinct asymmetry in its alteration and metal distribution an exhalite unit is zoned outward from massive pyrite with minor lenses of massive sulfide to massive calcite with red jasper to a chlorite calcite rock gold occurs with pyrite chalcopyrite and galena in late fractures in an intensely silicified zone in the footwall of the massive pyrite this zone is surrounded by silica sericite pyrite altered rocks with disseminated base metals during the deformation the silicified rocks were repeatedly fractured whereas the sericite rich rocks became high strain zones pressure solution of sulfides enhanced gold transport into late brittle fractures high gold values are confined to the footwall of proximal parts of the exhalite indicating that gold was introduced during the volcanogenic massive sulfide event and that remobilization was a localized process

new insights into the mechanics of fluvial bedrock erosion through flume experiments and theory river incision into bedrock drives the topographic evolution of mountainous terrain and may link climate tectonics and topography over geologic time scales despite its importance the mechanics of bedrock erosion are not well understood because channel form river hydraulics sediment transport and erosion mechanics coevolve over relatively long time scales that prevent direct observations and because erosive events occur intermittently and are difficult and dangerous to measure herein we synthesize how flume experiments using erodible bedrock simulants are filling these knowledge gaps by effectively accelerating the pace of landscape evolution under reduced scale in the laboratory we also build on this work by providing new theory for rock resistance to abrasion thresholds for plucking by vertical entrainment sliding and toppling and by assessing bedrock analog materials bedrock erosion experiments in the last 15 years reveal that the efficiency of rock abrasion scales inversely with the square of rock tensile strength sediment supply has a dominant control over bed roughness and abrasion rates suspended sediment is an efficient agent of erosion and feedbacks with channel form and roughness strongly influence erosion rates erodibility comparisons across rock concrete ice and foam indicate that for a given tensile strength abrasion rates are insensitive to elasticity the few experiments that have been conducted on erosion by plucking highlight the importance of block protrusion height above the river bed and the dominance of block sliding and toppling at knickpoints these observations are consistent with new theory for the threshold shields stress to initiate plucking which also suggests that erosion rates in sliding and toppling dominated rivers are likely transport limited major knowledge gaps remain in the processes of erosion via plucking of bedrock blocks where joints are not river bed parallel waterfall erosion by toppling and plunge pool erosion feedbacks between weathering and physical erosion erosional bedforms and morphodynamic feedbacks between channel form and erosion rates despite scaling challenges flume experiments continue to provide much needed tests of existing bedrock erosion theory force development of new theory and yield insight into the mechanics of landscapes

modeling energy dissipation and hydraulic jump regime responses to channel nonuniformity at river steps 1 the river step is an important driver for geomorphic evolution in bedrock rivers but the effect that variations in channel geometry upstream and downstream of a river step have on hydraulic jump regime and energy dissipation has not previously been investigated the associated hydraulic jump is inherent to a river step and its regime is a primary control on step morphodynamics in turn the hydraulic jump regime is controlled by several variables as detailed in a new conceptual framework herein also in this study a parsimonious semianalytical numerical model of step hydraulics is developed to quantify energy dissipation and delineate hydraulic jump regimes accounting for discharge jump submergence and nonuniform channel geometry through a step despite remaining limitations in step theory the model simulates how natural steps respond to a wide range of conditions the model shows that hydraulic jump regime and energy dissipation exhibit greater sensitivity to channel nonuniformity as discharge increases and or step height decreases also channel conditions that create greater jump submergence lead to decreased energy dissipation regardless of the discharge regime the model also reinforces the common observation about gully erosion that downstream channel widening enhances upstream knickpoint migration the new algorithm may be used to aid river engineering involving steps and could be useful for landscape evolution modeling

sediment yield from disturbed earth systems changes of sediment yield rates through time reflect evolutionary changes within a landscape when a drainage basin is perturbed significantly by base level climatic or tectonic change sediment yields increase dramatically but with no further disturbance they decline rapidly these sediment yield changes have been documented at all scales from small experimental studies to incised channels to the colorado river basin and to the himalaya mountains thus the shape of the sediment yield curve can be used to estimate future sediment yields and to interpret past tectonic events

seismic reflection identification of susquehanna river paleochannels on the mid atlantic coastal plain abstract land based high resolution seismic reflection methods were used to image quaternary paleochannels of the susquehanna river system using a portable 12 channel signal enhancing seismograph 12 accelerometers as receivers and a 4 54 kg sledge hammer struck against an aluminum plate as a source a sixfold multichannel seismic profile 2 5 km long was acquired at taylors island maryland on the processed seismic profile pronounced high amplitude seismic reflections delineate the unconformity between quaternary and underlying tertiary sediments and the disconformable contact separating miocene and eocene deposits subsurface seismic stratigraphic relationships that clearly indicate the presence of two paleochannels were observed one believed to be the exmore paleochannel projected to underlie northern taylors island based on marine seismic data an overlapping sequence of fill sediments was observed on the eastern margin of the exmore paleochannel the second paleochannel may be a tributary of the exmore or possibly the western edge of the younger eastville paleochannel results from this study indicate that land based shallow high resolution seismic reflection data can be used to delineate subsurface geomorphology successfully in coastal plain environments this technique of defining erosional surfaces and depositional units beneath present land areas when integrated with chronostratigraphic data is a powerful tool for developing a better understanding of the quaternary record

cave sedimentation genesis and erosional history in the cheat river canyon west virginia a model of cave sedimentation and genesis is used to gain greater resolution and accuracy in the calculation of an incision rate for cheat river west virginia maze caves along the river and their primary sediments were created and deposited beneath base level single conduit caves are largely unrelated to base level and their sediments are derived from overlying strata a magnetostratigraphic record is reported for cave sediments within the canyon the magnetostratigraphy of each sample is plotted versus elevation relative to base level and depositional environment vadose or phreatic the resulting chart accurately depicts the range of error associated with using cave sediments as indicators of previous base level positions this technique can be applied within any future studies using cave sediments for deriving incision rates of rivers the calculated incision rate of cheat river within the study area is between 56 0 and 63 2 mm k y

formation of waterfalls by intermittent burial of active faults waterfalls commonly exist near bounding faults of mountain ranges where erosional bedrock catchments transition to depositional alluvial fans we hypothesize that aggradation on alluvial fans can bury active faults and that the faults accumulate slip in the subsurface to produce a bedrock scarp following entrenchment of the alluvial fan the scarp can be exposed as a waterfall to explore this hypothesis we derived a geometric model for waterfall height that depends on alluvial fan length and the relative time scales of 1 tectonic uplift 2 a forcing mechanism for cycles of fan aggradation and incision and 3 a response of fan aggradation to changes in sediment flux we find that the model is consistent with observations at gower gulch death valley california where a man made drainage capture event in 1941 caused rapid fan incision and exposed a waterfall at the canyon fan transition we also compared the model to 62 waterfalls in 18 catchments of the death valley area and found that at least 15 of the waterfalls are best explained by the fault burial mechanism using field measurements of grain size and channel geometries we show that the fault burial mechanism can produce the observed waterfall heights measuring 4 19 m under a uniform climatic forcing scenario requiring variations of 20 in precipitation during the late pleistocene the fault burial mechanism through the creation of upstream propagating waterfalls may allow catchment fan systems to experience frequent cycles of enhanced erosion in catchments and deposition on fans that likely convolve tectonic and climatic signals

development of longitudinal profiles of alluvial channels in response to base level lowering degradation of alluvial channels in cohesive sediments was studied in 15 m and 20 m long flumes with a slope of 0 01 cm cm degradation was initiated by lowering base level to a fixed position and the development of the longitudinal profile of the channel is analysed through a model formulated as a heat diffusion equation it is based on the equation of sediment continuity combined with an assumption regarding sediment transport namely that sediment discharge is linearly proportional to the channel slope in accordance with the boundary and initial conditions imposed by the experimental setup and procedure the basic equation is amenable to an analytical solution which defines bed elevation at any distance and time as a function of the amount of base level lowering and a diffusion coefficient additional problems arising from bank erosion and channel armouring are also treated successfully within the framework of the same model the results show that in homogeneous alluvial sediments not subject to armouring the ultimate result of base level lowering by a certain amount is degradation all along the channel by the same amount the main impact of erosion is felt in the early stages after initiation of the process and mainly near the mouth the rate of degradation at any station along the channel reaches a peak and then slowly decreases with time and the peak rate is attenuated with distance from the outlet the model permits the prediction of intermediate stages of profile development at any distance from the outlet and at different times

formation of fluvial hanging valleys theory and simulation 1 although only recently recognized hanging tributary valleys in unglaciated tectonically active landscapes are surprisingly common stream power based river incision models do not provide a viable mechanism for the formation of fluvial hanging valleys thus these disequilibrium landforms present an opportunity to advance our understanding of river incision processes in this work we demonstrate that thresholds apparent in sediment flux dependent bedrock incision rules provide mechanisms for the formation of hanging valleys in response to transient pulses of river incision we simplify recently published river incision models in order to derive analytical solutions for the conditions required for hanging valley formation and use these results to guide numerical landscape evolution simulations analytical and numerical results demonstrate that during the response to base level fall sediment flux dependent incision rules may create either temporary or permanent hanging valleys these hanging valleys form as a consequence of 1 rapid main stem incision oversteepening tributary junctions beyond some threshold slope or 2 low tributary sediment flux response during the pulse of main stem incision thus limiting the tributary s capacity to keep pace with main stem incision the distribution of permanent and temporary hanging valleys results from four competing factors the magnitude of base level fall the upstream attenuation of the incision signal the lag time of the sediment flux response and the nonsystematic variation in tributary drainage areas within the stream network the development of hanging valleys in landscapes governed by sediment flux dependent incision rules limits the transmission of base level fall signals through the channel network ultimately increasing basin response time

inner gorge slot canyon system produced by repeated stream incision eastern alps significance for development of bedrock canyons abstract many inner bedrock gorges of the alps show abrupt downstream changes in gorge width as well as channel type and gradient as a result of epigenetic incision of slot canyons many slot canyons also are associated with older gorge reaches filled with quaternary deposits the age of slot canyons and inner bedrock gorges however commonly is difficult to constrain for the inner bedrock gorge system of the steinberger ache catchment eastern alps active slot canyons as well as older abandoned gorge reaches filled with upper wurmian proglacial deposits record three phases of gorge development and slot canyon incision a 234u 230th age of cement of 29 7 1 8 ka in fluvial conglomerates onlapping the flank of an inner gorge fits with late wurmian valley bottom aggradation shortly before pleniglacial conditions in addition the age indicates that at least the corresponding canyon reach must be older during advance of ice streams in the buildup of the last glacial maximum lgm the catchment was blocked and a proglacial lake formed bedrock gorges submerged in that lake were filled with fluviolacustrine deposits during the lgm the entire catchment was overridden by ice during post glacial reincision streams largely found again their preexisting inner bedrock canyons in some areas however the former stream course was missed and a slot canyon formed the distribution of pleistocene deposits the patterns of canyon incision and the mentioned u th cementation age however together record a further discrete phase of base level rise and stream incision well before the lgm the present course of steinberger ache and its tributaries is a patchwork of 1 slot canyons incised during post glacial incision 2 vestiges of slot canyons cut upon an earlier middle to late wurmian cycle of base level rise and fall 3 reactivated reaches up to 200 m in width of inner bedrock gorge that are watershed at present and more than at least 30 ka in age and 4 abandoned sediment filled reaches of bedrock canyons that also must be older than 30 ka and that are exposed alongside the active streams multi cyclic bedrock canyon systems composed of reaches of markedly different ages may be common in mountain ranges subject to glaciations and or mass wasting

delamination in collisional orogens during continental collision the weight of the downgoing slab may lead to extensional shearing in the descending plate permitting separation of the downdip dense oceanic lithosphere from the adjacent lighter continental lithospher separation may occur by means of a lithosphere penetrating normal sense shear zone by means of pure shear necking of the lithosphere or by some combination of these mechanisms during separation the continental crust attached to the lower plate may undergo an episode of extensional tectonics the proximity of asthenosphere to crust may result in syntectonic regional metamorphism and plutonism if the continents continue to converge following delamination a compressive tectonic regime will be reestablished in the crust the subsequent cooling of asthenosphere at the base of the crust may lead to the establishment of a continuous flat moho the likely imbrication of crustal and mantle lithologies during delamination and during the subsequent compressive tectonic regime as well as underplating associated with syntectonic plutonism may account for the seismic complexities observed at moho depths beneath some mountain belts

mountain building along a passive margin late neogene tectonism in southeastern victoria australia the hoddle ranges maximum elevation of 750 m above sea level lie along the southeastern australian passive margin detailed geological geomorphological studies of the southern margin of the ranges focusing on a fault block of oligocene miocene sedimentary rocks have constrained the landscape evolution in the mid cretaceous this area changed from a subduction zone accumulating volcanogenic sediments to a passive low relief margin which was crossed by north flowing rivers in the paleocene the eruption of basaltic volcanics formed a low divide which diverted these rivers towards the northeast so that sediments deposited on either side of the divide contain different heavy mineral assemblages in the middle late miocene early pliocene the area was subjected to a period of relatively rapid mountain building the kosciuszko uplift as broadly nw se oriented compression created the hoddle ranges at an uplift rate of 0 15 mm a uplift was not uniform a small southern block was uplifted only 200 m and its surface has acted as a local base level for the agnes river which flows across it with a major knickpoint on the southern side the southeastern highlands to the north were uplifted by up to 700 1000 m during the kosciuszko uplift similar to the maximum increase in elevation of the hoddle ranges 600 m the kosciuszko uplift tectonism occurred at rates greater than typical of passive margins and belies australia s reputation as a tectonically stable continent

experimental study of knickpoint and longitudinal profile evolution in cohesive homogeneous material a series of experiments were conducted in homogeneous bedrock to study knickpoint and longitudinal profile evolution knickpoints are created by successive drops in base level which simulate intermittent uplift channel morphology and flow characteristics vary systematically along a knickpoint reach as the knickpoint lip is approached width decreases but depth velocity and bottom shear stress increase as a result knickpoints are rapidly destroyed through the process of 1 knickpoint replacement and 2 subsequent knickpoint inclination knickpoint replacement consists of two morphologically distinct erosional reaches the knickpoint face below the knickpoint lip which decreases in slope and an incising reach above the knickpoint lip which increases in slope knickpoint replacement is a significant process in the evolution of knickpoints in homogeneous bedrock within experimental limits this study supports the notion that knickpoints do not undergo parallel head ward retreat in homogeneous bedrock given intermittent uplift except in those possible cases of extensive and pervasive bedrock jointing only the erosional sediment transport discontinuity above the knickpoint is developed in cohesive sediments below the knickpoint a steep bedrock channel slope low sediment load and rapid decrease in grain size of sediment load preclude development of the aggradational sediment transport discontinuity knickpoint evolution can be characterized by three general models parallel retreat replacement and inclination these models are based on the following criteria within the knickpoint reach 1 relationship between bottom shear stress 0 and critical shear stress needed to initiate motion c 2 nature of the bed load transport discontinuities and 3 spatial variability of bedrock resistance to fluvial erosion

stability analysis of waterfall cliff face at niagara falls an implication to erosional mechanism of waterfall although recession of waterfalls or knickpoints in bedrock rivers is a common geomorphological process detailed mechanics of waterfall recession has only been examined in a few cases caprock recession model at niagara falls in which gravitational collapse of caprock induced by undercutting notch plays a significant role has been one of the well known models describing the waterfall erosion but the validity of the model has hardly been examined in a quantitative context here we assess the stability of the cliff of waterfall face of niagara falls in terms of the strength of bedrock and the length of undercutting notch the result of a cantilever model analysis shows that the caprock remains stable until the undercut reaches tens to over a hundred meters however the actual length of undercutting notch of waterfall face is up to 10 m and such a long notch to cause gravitational collapse of the caprock can hardly be formed the recession of the waterfall could therefore be caused by gradual detachment of the rock of the waterfall face induced by fluvial erosion of surface water flow rather than by elongation of undercutting notch and episodic gravitational collapses of the caprock

an episode of rapid bedrock channel incision during the last glacial cycle measured with 10be we use 10 be to infer when how fast and why the susquehanna river incised through bedrock along the u s atlantic seaboard one of the world s most prominent and ancient passive margins although the rate at which large rivers incise rock is a fundamental control on the development of landscapes relatively few studies have directly measured how quickly such incision occurs either in tectonically active environments or along passive margins exposure ages of fluvially carved bedrock strath terraces preserved along the lower susquehanna river demonstrate that even along a passive margin large rivers are capable of incising through rock for short periods of time at rates approaching those recorded in tectonically active regions such as the himalayas over eighty samples collected along and between three prominent levels of strath terraces within holtwood gorge indicate that the susquehanna river incised more than 10 meters into the appalachian piedmont during the last glacial cycle beginning 36 ka incision rates increased dramatically and remained elevated until 14 ka the northern half of the susquehanna basin was glaciated during the late wisconsinan however similar rates and timing of incision occurred in the unglaciated potomac river basin immedi ately to the south the concurrence of incision periods on both rivers suggests that glaciation and associated meltwater were not the primary drivers of incision instead it appears that changing climatic conditions during the late pleistocene promoted an increase in the frequency and magnitude of flood events capable of exceeding thresholds for rock detachment and bedrock erosion thus enabling a short lived episode of rapid incision into rock although this study has constrained the timing and rate of bedrock incision along the largest river draining the atlantic passive margin the dates alone cannot explain fully why or by what processes this incision occurred however cosmogenic dating offers compelling evidence that episodes of rapid incision into bedrock are tied to glacial cycles and changes in global climate these results and the methods we employ provide valuable insights into the nature of bedrock channel incision not only along the susquehanna river and passive margins but also across a wide range of settings around the globe because river incision into bedrock transmits the effects of changing climate and tectonics through fluvial networks to hillslopes comprehending when where and why rivers incise has important implications for the evolution of land scapes

apparent increasing rates of stream incision in the eastern united states during the late cenozoic stream incision rates were obtained from 16 studies of terraces or stream associated cave levels in the eastern united states a log log regression of age against height above modern river level arl shows that age increases approximately as the square of height arl this relation may indicate an increasing rate of stream incision through time alternatively the relation may simply reflect an inherent effect of measured time interval on process rate as described by gardner et al who found that data from hundreds of studies showed a tendency for geologic process rates to decrease for increasing measurement intervals a regression equation for landscape denudation rates showed that the thickness removed increased as the 0 82 power of the time interval over which the denudation was measured gardner et al suggested that this bias could be removed by the use of statistically derived scaling functions however a similar regression equation for the data in this study shows a slope of 0 38 representing a much more rapid decrease in rate with increasing measurement interval than found by gardner et al even after applying the scaling function calculated by gardner et al my data show a 19 fold decrease in incision rate for a 103 increase in the measured time interval this result suggests that the observed decrease in stream incision rates with greater time spans may be at least partly real

geochemistry of the cambrian volcanic hosted massive sulfide rich mount read volcanics tasmania and some tectonic implications the cambrian mount read volcanics belt in western tasmania hosts five major gold rich massive sulfide deposits and remains the focus of intense exploration activity and geologic research a geochemical study of least altered lavas and shallow intrusions with emphasis on basaltic to andesitic compositions was carried out to determine the primary magmatic affinities and tectonic setting of eruption of the mount read volcanics to aid in internal correlations within the belt and to provide starting material estimates for mineralization related alteration studies three calc alkaline suites one extending to shoshonitic compositions and two tholeitiic suites have been distinguished within the mount read belt suite i is voluminous and includes the eastern sequence central volcanic complex tyndall group the intrusive quartz feld spar porphyries and granitoids and the andesitic lavas of the que hellyer footwall sequence suite i andesites are transitional medium to high k calc alkaline rocks la yb n 5 12 avg 8 1 and are the least light ree enriched calc alkaline lavas in the mount read volcanics suite ii comprises intrusive and extrusive often hornblende porphyritic andesites and dacites mainly from the upper part of the southern central volcanic complex they are more p 2 o 5 and light ree enriched la yb n 10 26 avg 16 7 than suite i and have high k calc alkaline affinities suite iii includes basaltic and andesitic lavas from the que hellyer hanging wall sequence in the northern part of the belt the lynch creek basalts near queenstown and intrusive basalts in the howards plains area northwest of queenstown suite iii rocks show a striking compositional range from low tio 2 0 4 0 5 low p 2 o 5 0 1 basalts with la yb n values from 8 to 12 and almost flat heavy ree patterns to low tio 2 0 4 0 8 but strongly to exceptionally p 2 o 5 and light ree enriched basalts with up to 350 times chondritic la and la yb n values up to 34 the latter are regarded as shoshonites tholeiitic pillow basalts of the henty fault wedge and the closely related dikes of the henty dike swarm constitute suite iv these have tio 2 contents from 1 0 to 1 6 percent very low nb contents 3 ppm and only slightly light ree enriched ree patterns la yb n 1 4 3 4 they are considered to be most similar to suprasubduction zone basalts erupted during the early phase of arc splitting and back arc basin development and indicate a period of tension and rifting of the mount read belt in post central volcanic complex but pre tyndall group time suite v is constituted by the miners ridge basalts and includes low tio 2 0 7 basalts with strong light ree depletion la yb n 0 5 and higher tio 2 to 2 6 lavas with only slight light ree depletion la yb n 0 9 suite v basalts are tentatively correlated with rift tholeiite sequences associated with the crimson creek formation and therefore are regarded as part of the pre mount read volcanics basement the three calc alkaline suites occur in a general stratigraphic order with suites i and ii being more or less coeval near queenstown they indicate a magmatic evolution from transitional medium to high k rocks through high k rocks to strongly enriched shoshonitic rocks closest modern analogues are postcollisional volcanics in regions of arc continent collision e g east papua and the roman province the occurrence of boninite derived chromites probably derived from the allochthonous mafic ultramafic complexes outcropping just west of the mount read belt in two sedimentary units within the mount read volcanics sequence below suite iii basalts supports a postcollisional origin for at least the major part of the mount read volcanics other similar postcollisional sequences particularly if formed in a submarine setting must be considered highly prospective for vhms deposits

hillslope response to knickpoint migration in the southern appalachians implications for the evolution of post orogenic landscapes the southern appalachians represent a landscape characterized by locally high topographic relief steep slopes and frequent mass movement in the absence of significant tectonic forcing for at least the last 200 ma the fundamental processes responsible for landscape evolution in a post orogenic landscape remain enigmatic the non glaciated cullasaja river basin of south western north carolina with uniform lithology frequent debris flows and the availability of high resolution airborne lidar dems is an ideal natural setting to study landscape evolution in a post orogenic landscape through the lens of hillslope channel coupling this investigation is limited to channels with upslope contributing areas 2 7 km2 a conservative estimate of the transition from fluvial to debris flow dominated channel processes values of normalized hypsometry hypsometric integral and mean slope vs elevation are used for 14 tributary basins and the cullasaja basin as a whole to characterize landscape evolution following upstream knickpoint migration results highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network of the cullasaja basin and adjacent hillslopes metrics of topography relief slope gradient and hillslope activity landslide frequency exhibit significant downstream increases below the current position of major knickpoints the transient effect of knickpoint driven channel incision on basin hillslopes is captured by measuring the relief mean slope steepness and mass movement frequency of tributary basins and comparing these results with the distance from major knickpoints along the cullasaja river a conceptual model of area elevation and slope distributions is presented that may be representative of post orogenic landscape evolution in analogous geologic settings importantly the model explains how knickpoint migration and channel hillslope coupling is an important factor in tectonically inactive i e post orogenic orogens for the maintenance of significant relief steep slopes and weathering limited hillslopes copyright 2011 john wiley sons ltd

disequilibrium profile of the potomac river near washington d c a result of lowered base level or quaternary tectonics along the fall line longitudinal profiles of major rivers in maryland and virginia are generally closely adjusted to the resistance of the rocks over which they flow but they all show major discontinuities at or just above the fall line the amount of discrepancy at the fall line between the actual profiles and the projection of the profiles of the upstream reaches increases from about 27 m at the james river to about 50 m at the susquehanna river at the potomac river it is about 45 m detailed study of the terraces along the potomac in the piedmont above washington d c suggests that the discontinuity in longitudinal profile is the result of quaternary downcutting in response to a lowered base level changes in base level during sea level fluctuations in the pleistocene must have been an important factor but strong circumstantial evidence suggests that the discontinuities in river profiles may in part be due to differential uplift of the piedmont with respect to the coastal plain along a zone of flexure or distributed faulting near the fall line

evidence of transient topographic disequilibrium in a landward passive margin river system knickpoints and paleo landscapes of the new river basin southern appalachians the upper new river basin of the southern appalachian mountains a major tributary of the modern ohio river represents the unglaciated headwaters of the tertiary teays river system of eastern north america dating of relict fluvial gravels have suggested that new river incision may be outpacing lowering of the surrounding uplands but physical evidence of transient topographic disequilibrium has yet to be identified we use focused topographic analysis of the upper new river basin to delineate a perched low relief paleo landscape that is experiencing transgressive dissection due to incision by the new river and its tributaries accelerated incision has decoupled hillslopes from the drainage network generating knickpoints which represent the boundary between remnants of the paleo landscape and actively adjusting topography downstream steepening of hillslopes downstream of knickpoints suggests dynamic headward migration which along with knickpoint occurrence throughout the drainage network is inconsistent with the development of fixed stream profile convexities atop strike extensive geologic contacts in the absence of tectonic forcing we favor a climatically forced drop in external base level as driver of the incision pattern we observe plio pleistocene glacial damming and diversion of the teays river to form the modern ohio river lowered regional base level for the study area potentially forcing the paleo landscape developed during the teays era to adjust to the modern drainage pattern the upper new river may therefore represent the potential for glacially driven drainage rearrangement to drive transient topographic evolution hundreds of kilometers away from the ice margin long after the disappearance of ice sheets copyright 2013 john wiley sons ltd

mesozoic plutons of the yidun arc sw china u pb geochronology and hf isotopic signature abstract the yidun arc is a triassic volcanic arc located between the songpan garze fold belt and the qiangtang block southwest china to constrain the age of a number of the major granitic plutons from the yidun arc laser ablation icp ms u pb analysis of zircon was conducted hafnium isotope data was also acquired through laser ablation multicollector icpms analysis of zircon with the aim of gaining insight into the age and nature of the source region of the plutons three age groups have been identified from seven granite samples early middle triassic 245 to 229 ma late triassic 219 to 216 ma and cretaceous 105 to 95 ma hafnium analysis shows the triassic granites to have negative and variable e hf values and mesoproterozoic 1 6 ga depleted mantle model ages which is interpreted to reflect derivation from an isotopically heterogeneous largely crustal source the cretaceous granite shows higher and less variable e hf values and slightly younger model ages 1 3 ga and is interpreted to be derived from melting of a more homogeneous crustal source a depleted mantle model age of 1 5 ga is calculated from the pooled triassic and cretaceous samples the source region for these magmas may be tentatively correlated with mesoproterozoic material of the yangtze craton which has been suggested to underlie the yidun arc however further work is necessary to demonstrate this suggestion