a self regulating model of bedrock river channel geometry 1 the evolution of many mountain landscapes is controlled by the incision of bedrock river channels while the rate of incision is set by channel shape through its mediation of flow the channel shape is itself set by the history of bedrock erosion this feedback between channel geometry and incision determines the speed of landscape response to tectonic or climatic forcing here a model for the dynamics of bedrock channel shape is derived from geometric arguments a normal flow approximation for channel flow and a threshold bed shear stress assumption for bedrock abrasion the model dynamics describe the competing effects of channel widening tilting bending and variable flow depth transient solutions suggest that channels may take 1 10 ky to adapt to changes in discharge implying that channel disequilibrium is commonplace if so landscape evolution models will need to include bedrock channel dynamics if they are to probe the effects of climate change

40ar 39ar geochronological constraints on the age and evolution of the permo triassic emeishan volcanic province southwest china abstract the biostratigraphically constrained permo triassic emeishan volcanic province evp extending over wide areas in southwest china has been recently considered as a large igneous province contemporaneous with the siberian traps and the siliceous tuffs at the p t boundary in south china we report the first 40ar 39ar ages on this igneous province minimum ages have been obtained on phenocrystic plagioclase of the emeishan basalt which has undergone a pervasive metamorphism most likely during subsequent tectonization as a consequence of terrane amalgamation comparison between the ar release spectra obtained on clear vs cloudy plagioclase indicates a 40 30 ma sericite resetting time a minimum apparent age of 246 4 ma for plagioclase from a plagiogranite a late differentiate of the panzhihua layered complex and an age of 254 5 ma for phlogopite from a pyroxenite near lake erhai provide the first absolute age constraint on this igneous province additional ar ar age measurements on post emeishan alkaline and mafic magmatism yielded 104 2 and 100 2 ma for an alkaline complex near panzhihua and 42 1 ma for a gabbro sill emplaced near the ertan dam further study is still needed to determine the age of the emeishan volcanic emission accurately and to test the validity of the assumed short duration of the eruption

hydraulic geometry river sediment and the definition of bedrock channels a growing literature deals with bedrock channels erosion processes within them and their role in landscape evolution formal definitions currently in use classify channels as bedrock when alluvial cover is discontinuous or thin this is equated with a physical property of the flow using up to date erosion laws and descriptions of channel dynamics we show that existing definitions are difficult to apply and may lead to conflicting classifications we propose the definition a bedrock channel cannot substantially widen lower or shift its bed without eroding bedrock applied to channels in taiwan this leads to a different classification than previous definitions by analysing at a station hydraulic geometry of these channels we show that the classification exposes different controls of sediment effects on dynamics of channels classified as alluvial and bedrock

paleomagnetism of the dewey lake formation late permian northwest texas end of the kiaman superchron in north america the remanent magnetization of red beds of the late permian dewey lake formation from northwest texas was investigated using thermal chemical and alternating field demagnetization after removal of a small present day viscous overprint the dewey lake shows univectorial decay of the remanence revealing a dual polarity characteristic direction of d 152 4 i 11 1 k 39 5 95 5 9 n 16 sites three polarity transitions were observed defining at least four polarity zones the observed polarity zones are roughly layer parallel and may correlate with the illawarra magnetozone at the end of the kiaman if this is so k ar age determinations of an interbedded ash layer kolker and fracasso 1985 suggest that the kiaman superchron ended before 251 4 ma the paleopole located at 51 4 n 126 2 e confirms the reliability of previously reported late permian poles from southwest united states which fall significantly off the late permian segment of the paleomagnetic euler pole pep track of gordon et al 1984 this implies that a more segmented set of small circles is required to model the north american apparent polar wander path diminishing the usefulness of the pep method

apparent polar wander paths of the major chinese blocks since the late paleozoic toward restoring the amalgamation history of east eurasia abstract high quality paleomagnetic poles a k a paleopoles are essential for quantitative lithospheric plate reconstruction however the current paleomagnetic database of the major chinese blocks including the north china block ncb the south china block scb and the tarim block tb since the late paleozoic era contain outdated and low quality data here we update the database by adding recently published high quality paleopoles and rejecting low quality outdated ones the database includes 288 paleopoles published 1980 2014 90 of them published after 2000 following the van der voo selection criteria 75 paleopoles each with a quality factor q smaller than 4 were rejected in the first round of selection we then removed another 59 paleopoles that have been locally rotated or shallowed since the acquisition of stable remanence eventually 154 paleopoles were selected and adopted to calculate the new apparent polar wander paths apwps for the major chinese blocks we found comparable clastic and igneous paleopoles at intervals when a quantitative comparison was available as such comparison was not possible during most periods epochs our conclusions reflect an unclear extent of inclination errors in chinese clastic paleopoles new models of the chinese apwps with and without inclination error corrections were computed from carefully selected paleopoles version 1 running mean v1rm paths were calculated at the period epoch level version 2 running mean v2rm paths were computed in a set sliding time window of 20 or 30 myr spline paths were calculated with the same time windows along with a smoothing parameter of 50 using a recent global reconstruction model and up to date geological observations new models of the chinese apwps allowed us to re evaluate the coalescence history of east eurasia since the late paleozoic era four major tectonic events were confirmed 1 the tb accreted with the kazakhstan orocline during amalgamation of the west altaids during the middle late permian period ca 265 250 ma 2 the suturing of the ncb and the scb likely occurred in a scissor like pattern and had been accomplished no later than the middle jurassic period ca 180 160 ma 3 the amalgamation between the ncb and the tb along with the microblocks between the two might have been achieved during the late jurassic early cretaceous periods ca 160 140 ma 4 the mongol okhotsk ocean should have been closed no later than the early cretaceous period ca 140 120 ma

triassic paleomagnetic results from the yangtze block s e china in an effort to contribute to the refinement of the apwp for the yangtze block samples for paleomagnetic analyses were recovered from early triassic sediments in hunan province in southeast china the samples were taken from opposite limbs of a large syncline in order to apply the fold test and rock magnetic experiments were performed to determine the carriers of remanence however instead of refining the apwp the results present some problems while the sites pass the fold test the declinations deviate clockwise with respect to the reference apwp for the block but agree well with other reported results due to the paucity of reliable triassic results from the yangtze block and to the small number of sites from this study it is difficult to determine whether these sites have been rotated or the apwp is incorrect both possibilities are explored

fluvial erosion transport equation of landscape evolution models revisited we present a mesoscale erosion deposition model which differs from previous landscape evolution models equations by taking explicitly into account a mass balance equation for the streamflow the geological and hydrological complexity is lumped into two basic fluxes erosion and deposition and two averaged parameters unit width discharge q and stream slope s the model couples the dynamics of streamflow and topography through a sediment transport length function x q which is the average travel distance of a particle in the flow before being trapped on topography this property reflects a time lag between erosion and deposition which allows the streamflow not to be instantaneously at capacity the so called x q model may reduce either to transport limited or to detachment limited erosion modes depending on x but it also may not we show in particular how it does or does not for steady state topographies long term evolution and high frequency base level perturbations apart from the unit width discharge and the settling velocity the x q function depends on a dimensionless number encompassing the way sediment is transported within the streamflow using models of concentration profile through the water column we show the dependency of this dimensionless coefficient on the rouse number we discuss how consistent the x q model framework is with bed load scaling expressions and einstein s conception of sediment motion

bedrock channel adjustment to tectonic forcing implications for predicting river incision rates we present detailed data of channel morphology for a river undergoing a transient response to active normal faulting where excellent constraints exist on spatial and temporal variations in fault slip rates we show that traditional hydraulic scaling laws break down in this situation and that channel widths become decoupled from drainage area upstream of the fault unit stream powers are 4 times higher than those predicted by current scaling paradigms and imply that incision rates for rivers responding to active tectonics may be significantly higher than those heretofore modeled the loss of hydraulic scaling cannot be explained by increasing channel roughness and is an intrinsic response to tectonic forcing we show that channel aspect ratio is a strongly nonlinear function of local slope and demonstrate that fault induced adjustment of channel geometries has reset hillslope gradients the results give new insight into how rivers maintain their course in the face of tectonic uplift and illustrate the first order control the fluvial system exerts on the locus and magnitude of sediment supply to basins

modeling the effects of bed topography on fluvial bedrock erosion by saltating bed load abrasion by saltation is an important mechanism of fluvial incision into bedrock sklar and dietrich 2004 introduced an abrasion model in which the erosion rate of an approximately planar bed is linearly dependent on the kinetic energy transferred by the vertical velocity of saltating grains however most bedrock floored channels exhibit topographic variations that yield deviations from a planar surface referred to as bed topography observations show that bed topography affects erosion here the saltation abrasion model is extended for a nonplanar bed a several centimeter high bump transverse to the flow is repeated every 50 cm the kinetic energy of grain impacts is calculated in two ways 1 impact velocity normal to bed topography and 2 vertical impact velocity by comparing the latter case with the planar model it is possible to isolate the effects of topography on the interception of saltation trajectories incorporating bed topography changes erosion in three ways first erosion is 10 to 1000 times faster depending upon transport stage and grain size enhanced erosion results from both the interception of grains by topography and the increased kinetic energy transfer associated with high angle impacts on the stoss side of bumps second erosion increases monotonically with transport stage whereas maximum erosion occurs at low to intermediate transport stage with a planar bed third erosion decreases monotonically with grain size whereas maximum erosion occurs with intermediate sized grains with a planar bed although the model is highly simplified results show that bed topography should be considered when simulating erosion of bedrock

landslides control the spatial and temporal variation of channel width in southern taiwan implications for landscape evolution and cascading hazards in steep tectonically active landscapes

magnetostratigraphy of the permo triassic boundary section at shangsi guangyuan sichuan province china continous marine sedimentation characterizes many late permian to early triassic sections on the yangtze terrane in south china the permo triassic p tr boundary section at shangsi sichuan province consists of limestones intercalated with clays and mudstones which belong to the wuchiapingian and changxingian upper permian and the griesbachian and dienerian lower triassic stages the p tr boundary is formed by a clay horizon with an unusually high iridium concentration the intensity of natural remanent magnetization is very low with a mean of 0 15 ma m 1 about 40 of the samples contain secondary or unstable magnetization components only whereas the remaining samples carry a characteristic remanent magnetization thought to reflect the polarity of the geomagnetic field during deposition with sufficient accuracy normal and reversed polarity of the characteristic magnetization constitute a pattern of at least six polarity zones the p tr boundary being situated very close to the transition from a reversed to a normal polarity zone the shangsi polarity sequence represents part of the illawarra interval of mixed polarity the exact beginning of which has still to be determined

palaeointensity studies of a late permian lava succession from guizhou province south china implications for post kiaman dipole field behaviour summary palaeomagnetic results are presented from a volcanic sequence in zhijin county guizhou province southern china the lavas and associated volcanic breccias comprising the sequence represent a southern extension of the emeishan volcanic province biostratigraphic dating of interbedded limestone units and stratigraphic constraints indicate that the section formed during the late permian 263 255 ma and is thus somewhat older than the emei stratotype section in sichuan province and close in age to reported estimates of the termination of the permo carboniferous kiaman reverse superchron rock magnetic analyses and reflected light microscopy indicate that the magnetic mineralogy of the lava units is dominated by fresh primary magnetites containing a significant fraction of single domain grains thermal demagnetization behaviour of the breccia units is poor but most lava samples have one or two components of remanence above 250 c the normal polarity characteristic remanence held by the lavas implies a post kiaman age for this succession and suggests that the termination of the kiaman occurred prior to 263 ma supporting recently published estimates according to standard criteria thellier palaeointensity results from the lavas are of good quality and reveal that the dipole field strength was comparatively low shortly after the termination of the superchron 80 per cent of samples record relative vdm values in the range 42 52 per cent of the present day value supporting recent studies of mid kiaman field intensity this suggests that a low energy dipole existed at least between 300 and 255 ma and does not appear to have been confined to the stable reverse polarity interval

preliminary phanerozoic polar wander paths for the north and south china blocks polar wander paths for the north and south china blocks suggest that 1 both were parts of gondwana in the palaeozoic 2 the north china block accreted to siberia in the late permian and 3 the south china block accreted to the north china block in the middle triassic to the early jurassic comparison of the polar wander path for the south china block with that for northern eurasia suggests that relative motion of over 4 000 km has occurred between them

consistency of scaling relations among bedrock and alluvial channels 1 this paper presents field data on channel geometry and potential control variables from 47 field settings representing a diverse range of environments these data are used to evaluate existing scaling relationships used in models of the evolution of bedrock channel geometry and to test the hypothesis that channel width w increases more slowly and depth d more rapidly in relation to discharge q or drainage area a as substrate resistance increases for this data set w a0 3 w q0 5 d a0 2 and d q0 3 the w a and w q relations are close to those found by previous investigators the d a and d q relations have not previously been reported for bedrock channels examination of trends within the data does not support the hypothesis and instead suggests that the erosional resistance of channel boundaries is not the primary control on scaling relations for channel geometry

sediment and rock strength controls on river incision into bedrock recent theoretical investigations suggest that the rate of river incision into bedrock depends nonlinearly on sediment supply challenging the common assumption that incision rate is simply proportional to stream power our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion but inhibits erosion at high supply rates by burying underlying bedrock beneath transient deposits maximum erosion rates occur at a critical level of coarse grained sediment supply where the bedrock is only partially exposed fine grained sediments provide poor abrasive tools for lowering bedrock river beds because they tend to travel in suspension experiments also reveal that rock resistance to fluvial erosion scales with the square of rock tensile strength our results suggest that spatial and temporal variations in the extent of bedrock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in rock uplift rate and climate furthermore we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedrock channel gradients and topographic relief

the triassic of the thakkhola nepal ii paleolatitudes and comparison with other eastern tethyan margins of gondwana during the triassic the thakkhola region of the nepal himalaya was part of the broad continental shelf of gondwana facing a wide eastern tethys ocean this margin was continuous from arabia to northwest australia and spanned tropical and temperate latitudes a compilation of permian triassic and early jurassic paleomagnetic data from the reconstructed gondwana blocks indicates that the margin was progressively shifting northward into more tropical latitudes the thakkhola region was approximately 55 s during late permian 40 s during early triassic 30 s during middle triassic and 25 s during late triassic this paleolatitude change produced a general increase in the relative importance of carbonate deposition through the triassic on the himalaya and australian margins regional tectonics were important in governing local subsidence rates and influx of terrigenous clastics to these gondwana margins but eustatic sea level changes provide a regional and global correlation of major marine transgressions prograding margin deposits and shallowing upward successions a general mega cycle characterizes the triassic beginning with a major transgression at the base of the triassic followed by a general shallowing upward of facies during middle and late triassic and climaxing with a regression in the latest triassic

a stability criterion inherent in laws governing alluvial channel flow the stability criterion of maximum flow efficiency mfe has previously been found inherent in typical alluvial channel flow relationships and this study investigates the general nature of this criterion using a wider range of flow resistance and bedload transport formulae for straight alluvial channels in which the effect of sediment sorting is insignificant our detailed mathematical analysis demonstrates that a flow efficiency factor e occurs generally as the ratio of sediment bedload discharge qs to stream power qs in the form of when e is maximized i e qs is maximized or is minimized maximally efficient straight channel geometries derived from most flow resistance and bedload transport formulae are found compatible with observed bankfull hydraulic geometry relations this study provides support for the use of the criteria of mfe maximum sediment transporting capacity and minimum stream power for understanding the operation of alluvial rivers and also addresses limitations to the direct application of its findings copyright 2002 john wiley sons ltd

flow resistance and hydraulic geometry in contrasting reaches of a bedrock channel assumptions about flow resistance in bedrock channels have to be made for mechanistic modeling of river incision paleoflood estimation flood routing and river engineering field data on bedrock flow resistance are very limited and calculations generally use standard alluvial river assumptions such as a fixed value of manning s n to help inform future work we measured how depth velocity and flow resistance vary with discharge in four short reaches of a small bedrock channel one with an entirely rock bed and the others with 20 70 sediment cover and in the alluvial channel immediately upstream as discharge and submergence increase in each of the partly or fully alluvial reaches there is a rapid increase in velocity and a strong decline in both n and the darcy weisbach friction factor f the bare rock reach follows a similar trend from low to medium discharge but has increasing resistance at higher discharges because of the macro roughness of its rock walls flow resistance at a given discharge differs considerably between reaches and is highest where the partial sediment cover is coarsest and most extensive apart from the effect of rough rock walls the flow resistance trends are qualitatively consistent with logarithmic and variable power equations and with non dimensional hydraulic geometry but quantitative agreement using sediment d84 as the roughness height is imperfect

bedform genesis in bedrock substrates insights into formative processes from a new experimental approach and the importance of suspension dominated abrasion bedrock channels are common in the natural environment and bedrock channel erosion sets the pace of denudation in many river catchments however in comparison to the large number of studies concerning the formation of alluvial bedforms relatively few investigations have concerned bedrock bedform genesis field based analysis of sculptured forms within bedrock channels has been restricted notably by the slow rate of bedform development in such environments furthermore only a limited number of flume scale experiments have been conducted that attempt to simulate the genesis of sculpted bedforms in bedrock channels this study demonstrates that optimisation of clay beds through analysis of clay strength enables the development of features analogous to bedrock river channel bedforms even at a scale that is orders of magnitude smaller than some natural examples three sets of suspended sediment laden experiments were carried out using hard medium and soft clay bed substrates a suite of erosive bedforms including potholes flutes and furrows developed on all experimental beds all observed erosional features have clear equivalents to those observed in natural bedrock rivers bed shear strength was found to be a significant factor for the genesis of different types of simulated bedrock bedforms in our experiments with other factors such as flow velocity bed slope and flow depth held approximately constant importantly in a subset of experiments performed with an absence of suspended sediment fluid flow did not result in the erosion and development of bedforms in the clay bed hence this work illustrates that abrasion by suspended sediments is the key process required for the formation of these simulated bedrock bedforms in our experiments in the absence of bedload abrasion other processes such as plucking cavitation and dissolution will have been negligible

geomagnetic field intensity and reversal asymmetry in late precambrian keweenawan rocks summary fifty four palaeointensity values measured by the thellier technique are used to study the cause of reversal asymmetry in late precambrian keweenawan rocks these data obtained mainly from diabase dykes carrying primary trm and their baked host rocks show that reversed palaeointensities are on average 40 per cent higher than their normal counterparts this result cannot be explained by any overall difference in remanence characteristics grain size or cooling rate dyke width between the rocks comprising the two polarity groups a significant difference in the local intensity of the geomagnetic field between times of normal and reversed polarity is thus indicated this conclusion is reinforced by positive baked contact tests for both normal and reversed thellier palaeointensities secondary component and single dipole offset models as explanations for keweenawan reversal asymmetry are in conflict with the palaeointensity results the difference in palaeointensity between normal and reversed rocks disappears when the data are reduced to the palaeoequator in accordance with a geocentric axial dipole field the palaeointensity results are therefore consistent with the idea that reversal asymmetry is caused by the north american plate occupying a lower palaeolatitude when the field had normal polarity acceptance of a geocentric axial dipole would imply that the keweenawan geomagnetic field intensity was about 45 per cent higher than at present however evidence at one locality of possibly three asymmetric reversals poses problems for a plate movement interpretation and forces consideration of a model whereby the asymmetry is caused by persistent non dipole field components rock magnetic studies show that reliable palaeointensity estimates can be obtained for rocks which have stable nrm with single domain or pseudo single domain grain sizes and with a high koenigsberger ratio baked contact rocks are the most reliable whereas hydrothermally altered lavas are unsuitable for thellier intensity determinations it is suggested that af demagnetization studies on companion specimens should be performed before thellier experiments in order to select specimens which have high values of median destructive field and which lack viscous or other overprints

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

magnetostratigraphic investigations on an emeishan basalt section in western guizhou province china abstract a magnetostratigraphic investigation has been conducted on a late permian emeishan basalt section at duge in western guizhou province china the characteristic remanent magnetization chrm directions resolved at high temperatures from the basalts pass the fold test with dual polarity and define a normal magnetozone of five sixths of the total thickness in the lower part and a reversed magnetozone of one sixth of the thickness in the top part of the section the chrm directions from the basalt clasts of pyroclastic rocks fail the conglomerate test but the direction and polarity of the chrm of the clasts from the upper reversed zone are consistent with those of the basaltic lavas in the reversed zone and the normal directions also dominate the chrm of the clasts taken from the lower normal zone this suggests that either the original remanence of the clasts was reset by cementing basaltic lavas or the chrm of the basalts was acquired late but not beyond a field reversal the magnetostratigraphic pattern n r observed at duge is as simple as that observed from some major continental flood basalt provinces in the world where very few magnetic polarity zones were observed from volcanic sequences of great thickness the emplacement of the emeishan basalts in western guizhou is estimated to be less than two million years the normal polarity magnetozone observed indicates that the extrusion of the emeishan basalts in western guizhou postdates the permo carboniferous reversed superchron pcrs the apparent shortness of the reversed magnetozone explains why far fewer reverse directions than normal ones have been so far reported from the emeishan basalts

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

the paleomagnetic record of uppermost permian lower triassic rocks from the south china block in order to constrain better the paleogeography of the blocks that now make up china we collected paleomagnetic samples from uppermost permian to middle triassic limestones from two localities in the sichuan basin south china block a low inclination component which passes both the fold test and the reversal test was found in five sites d 46 1 i 11 9 k 83 6 95 8 7 we review corresponding remanences found in other studies of permo triassic rocks from the sichuan basin and show that the component is probably characteristic of the lower triassic we then review paleomagnetic results from the uppermost permian emeishan basalt despite small local rotations the difference between the emeishan basalts and the sichuan basin poles is interpreted to be the result of motion of the whole south china block around the time of the permo triassic boundary rather than of strain within the block

emeishan large igneous province sw china in recent years there have been major advances in our understanding of the emeishan large igneous province lip of sw china following publication of a number of lip focused investigations of the terrain and associated rocks this paper reviews the current state of knowledge the volcanic and upper intrusive portion of the province is relatively small 0 3 106 km3 even when offset fragments eroded sections and buried portions are included in the volume calculation the most reliable radiometric age dates zircon u pb shrimp from an associated layered intrusive body several kilometers in area indicate generation at 259 ma consistent with the end guadalupian end middle permian stratigraphic age in addition several ar ar dating studies have been carried out mainly on the volcanic rocks with a number of reported dates 253 251 ma late permian but a consensus is emerging that these ages are problematic because they are in conflict with the stratigraphic data possibly due to a monitor standard miscalibration the ar ar investigations have also yielded a large number of secondary ages which are clustered at 175 142 98 and 42 ma and these are inferred to record sub regional tectonic events that affected the western yangtze block as east asia was assembled and later deformed by india s collision indentation into asia magnetostratigraphic data and field observations suggest that the bulk of the volcanic sequence formed within 1 2 my the geochemistry of the volcanic rocks and bio lithostratigraphic studies of the underlying maokou formation suggests a mantle plume generated the province the basalts can be classified into low and high ti groups with different parental magmas the low ti basalts are confined to the western part of the province and are overlain by the high ti basalts the low ti magmas formed at shallow mantle depths in the spinel garnet field transition zone 60 80 km whereas the high ti magmas formed by low degrees of partial melting within the deeper garnet stability field this observation suggests a deepening of the melting column as the lithosphere thickened due to under plating and a transition from peak basalt generation to the waning stage outstanding issues which might focus future studies are also outlined

self organization of river networks to threshold states numerical models in which grid cells self organize so that the shear stress in each cell equals a predetermined threshold value are remarkably successful in producing the fundamental structure of river networks such models are a reasonable approximation of networks characterized by near constant dimensionless shear stress bed shear stress7sol bed grain size with each cell s threshold value being proportional to median bed grain size previous work has shown that this is the case for channels cut into alluvium and that the characteristic narrow range of is different for gravel and sand bed channels channels in the italian apennines that are slowly incising into weak bedrock but which are covered by a veneer of gravel for most of the time are shown to be characterized by the same as alluvial gravel bed channels thus threshold models capture the fundamental behavior of many river networks even in areas of long term bedrock incision

large scale geomorphology classical concepts reconciled and integrated with contemporary ideas via a surface processes model linear systems analysis is used to investigate the response of a surface processes model spm to tectonic forcing the spm calculates subcontinental scale denudational landscape evolution on geological timescales 1 to hundreds of million years as the result of simultaneous hillslope transport modeled by diffusion and fluvial transport modeled by advection and reaction the tectonically forced spm accommodates the large scale behavior envisaged in classical and contemporary conceptual geomorphic models and provides a framework for their integration and unification the following three model scales are considered micro meso and macroscale the concepts of dynamic equilibrium and grade are quantified at the microscale for segments of uniform gradient subject to tectonic uplift at the larger meso and macroscales which represent individual interfluves and landscapes including a number of drainage basins respectively the system response to tectonic forcing is linear for uplift geometries that are symmetric with respect to baselevel and which impose a fully integrated drainage to baselevel for these linear models the response time and the transfer function as a function of scale characterize the model behavior numerical experiments show that the styles of landscape evolution depend critically on the timescales of the tectonic processes in relation to the response time of the landscape when tectonic timescales are much longer than the landscape response time the resulting dynamic equilibrium landscapes correspond to those envisaged by hack 1960 when tectonic timescales are of the same order as the landscape response time and when tectonic variations take the form of pulses much shorter than the response time evolving landscapes conform to the penck type 1972 and to the davis 1889 1899 and king 1953 1962 type frameworks respectively the behavior of the spm highlights the importance of phase shifts or delays of the landform response and sediment yield in relation to the tectonic forcing finally nonlinear behavior resulting from more general uplift geometries is discussed a number of model experiments illustrate the importance of fundamental form which is an expression of the conformity of antecedent topography with the current tectonic regime lack of conformity leads to models that exhibit internal thresholds and a complex response

experimental simulation of channel incision into a cohesive substrate at varying gradients six flume runs using a substrate of 30 bentonite and 70 fine sand were used to examine channel incision into a cohesive homogeneous substrate gradients for the various runs were 1 2 5 10 and 20 other variables remained constant discharge 2 l s sediment discharge averages 3 g s run time 15 with increasing flume gradient bed forms change from parallel longitudinal grooves to a broad shallow channel with weakly undulating bed and walls and then to a progressively deeper and more undulating inner channel downstream changes in reach scale several channel widths gradient may thus be responsible for downstream variability of erosional bed forms in the absence of changes in other controlling variables for bedrock channels it is not necessary to invoke substrate variability to explain the presence of erosional bed forms and associated variations in cross sectional area erosional bed forms may approximate an equilibrium state in which negligible change occurs in bed form dimensions or shape with time downstream alternations between solely erosion and deposition of an alluvial veneer over incisional features occur along channel reaches of constant gradient the downstream length of these alternations is inversely proportional to gradient

magnetostratigraphic constraints on two stage eruptions of the emeishan continental flood basalts temporal and spatial variations of high and low contents of ti basalts in the permian emeishan large igneous province elip of southwest china and vietnam are crucial for modeling of continental basalt formation and the nature of mantle lithosphere interaction a combined geochemical and magnetostratigraphic study was carried out at yanyuan in the inner zone of the elip geochemical results indicate that the basalts are low ti lt basalts ti y 484 2 511 4 in the luomapu section and high ti ht basalts ti y 558 2 658 9 in the gongmushan section systematic paleomagnetic sampling and stepwise thermal demagnetization demonstrate that the higher temperature component shows two distinct characteristic remanent magnetizations with northeasterly upward and sww downward directions these results indicate a simple magnetostratigraphic normal and reversed polarity pattern from the lower and upper parts of the section respectively by comparing with available magnetostratigraphic and paleomagnetic data carried out in the elip these results imply that the eruption of the emeishan basalts can be divided into early normal polarity subchron and late reversal subchron stages the early stage included lt basalts in the inner zone and most of the ht basalts in the intermediate and outer zones and the late stage contained a part of lt and ht basalts in the upper parts of the sections in the inner zone and a few in the intermediate zone of the elip correlating with available middle late permian magnetostratigraphy and the permian magnetic time scale the new magnetic polarity sequence result suggests that the elip formed over a very short time interval 1 myr

new paleomagnetic and 40ar 39ar geochronological results for the south shetland islands west antarctica and their tectonic implications

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

fragmentation of asia in the permian asia is a composite continent formed by the accretion of crustal blocks1 3 a likely sequence of events is that siberia collided with a kazakhstan block which together then collided with europe along the urals in permo triassic times2 this was followed by the sucessive accretion of various blocks of south and east asia to siberia culminating in the collision of india with asia during the cenozoic palaeomagnetic data have already demonstrated that the kolyma and sikhote alin blocks of eastern asia collided with the main continent along the late mesozoic fold belts of verkhoyansk and sikhote alin4 furthermore results from the malay peninsula5 and japan6 suggest that south east asia and japan were situated near the equator in permian times and were therefore separated from the asian continent we report here some palaeomagnetic results from the permian of the sino korean and yangtze blocks of china that show they were also situated near the equator were not in their present relationship to one another and were widely separated from siberia

experimental channel response to tectonic uplift 1 we have performed a series of experiments to investigate the relationship between channel geometry and tectonic forcing in steady state landscapes at various uplift rates the experimental setup consists of uniformly uplifted silica paste eroded by artificial rainfall in this setup erosional landscapes evolve by growth and amalgamation of incisions which organize into a drainage network whose dynamics are a function of the interaction between vertical channel cutting hillslope erosion and sediment transport high precision 0 5 mm pixel size digital elevation models were constructed using a stereogrammetric camera system from this data channel bed slope was found to be independent of discharge and position on the experimental surface and to increase linearly with uplift rate geometric parameters of the flow such as channel width or depth could not be observed directly during the experiments using laminar flow equations for the mean flow velocity these parameters were back calculated and relationships to the imposed substrate uplift rate were derived channel width cross sectional area and wetted perimeter decrease with increasing uplift rate to a limit value hydraulic radius and flow depth increase slightly and flow velocity increases approximately linearly with increasing uplift rate these results are qualitatively consistent with recent field surveys and highlight the importance of channel width and slope variations in accommodating channel response to variable rock uplift rates

palaeomagnetic constraints on the collision and rotation of north and south china the eastern half of china is dominated by the north china block ncb and south china block scb separated by a complex fold system that trends roughly east west interpretation of the geology of this fold belt has led to a variety of estimates of the age of amalgamation of these two blocks1 7 palaeomagnetic results are few and have also been subject to conflicting interpretations8 11 here we present our own palaeomagnetic results for late permian rocks for both the scb and ncb and summarize other relevant palaeomagnetic data by reinterpreting the polarity given in some previous studies we are able to synthesize the results for the widespread emeishan basalts of the scb and to suggest a new model for the collision and relative rotation of the two blocks

distribution of erosion across bedrock channels lateral erosion in bedrock rivers is an important control on the shape of channel crosssections and the coupling of channels and hillslopes recent observations link lateral erosionto the variability of flow we propose two mechanisms to explain this one is based on changing shear stress distributions within the channel with varying flood level the other on the competition between cover and tool effects in fluvial bedrock erosion we assess these processes for the liwu river taiwan and conclude that cover and tool effects dominate the partitioning of lateral and vertical erosion in this case

tectonic and lithologic controls on bedrock channel profiles and processes in coastal california 1 recent theoretical models suggest that topographic characteristics of bedrock channels are products of interactions among tectonics substrate resistance and the climatically modulated erosive ability of the river the degree to which these factors influence the form of channel profiles is poorly quantified at present here we investigate bedrock channels developed across the southern flank of the santa ynez mountains california uniform climate and systematic variations in lithology and rock uplift rate along the range allow comparison of channel morphology between 1 channels eroding rocks of uniform and nonuniform strength and 2 channels experiencing differences in tectonic forcing we combine field observations surveys and analysis of digital data to determine topographic and hydraulic characteristics of bedrock channels at a constant rock uplift rate streams flowing from resistant to less resistant bedrock exhibit highly concave profiles and increased gradients along lower reaches relative to channels developed in uniform bedrock these effects are interpreted as responses to 1 an increase in substrate resistance to channel incision in the upper reaches and 2 transport limited gradients along lower reaches comparisons of channels developed across uniform lithology but experiencing an approximately sevenfold difference in rock uplift rate reveal an approximately twofold increase in gradient and an approximately threefold decrease in width in this landscape the combined channel adjustments of gradient and width are consistent with a fluvial incision model in which channel incision rate is linearly proportional to mean bed shear stress

dynamic equilibrium between an active uplift and the sevier river utah in a zone of active tectonic uplift one expects a river to change morphology simply because valley slope deforms but by cutting through a transverse uplift the sevier river in southern utah also adapts to the tectonics by eroding and depositing these localized changes of transport are caused by varying channel geometry pattern sedimentology and slope in the uplift the river is narrow efficiently shaped coarser grained and expends a greater proportion of energy on bed particles it deposits sediments downstream in exaggerated meandering reaches with wider less efficient channels localized erosion and deposition create a dynamic equilibrium with the structural growth but this equilibrium includes nonlinear responses like knickpoints and depositional lobes in response to the required erosion and deposition the river so dramatically alters channel grain size and channel geometry that the increase of sinuosity expected in the tectonically steepened valley does not occur

weathering and abrasion of fe ti oxides during rock degradation and fluvial transport implications for sedimentary provenance studies physical and chemical degradation of individual grains of titanomagnetite and ilmenite during rock weathering and fluvial transport processes in the river eden catchment eastern scotland have been traced using backscattered electron images and electron probe microanalysis magnetic characteristics have been used to detect differences in magnetic mineralogy concentration paragenesis and grain size between rock samples showing different stages of weathering and between different sources and sediment samples results have shown higher physical and chemical durability of ilmenite during this segment of the geological cycle than titanomagnetite which is rapidly altered during progressive subaerial weathering of a dolerite the increasingly intensive and extensive oxidation and development of internal fractures in titanomagnetite are observed however under subaqueous conditions titanomagnetite was found to be progressively replaced by titanite and ti impoverished magnetite due to hydrocirculation neither the total physical nor the total compositional degradation of titanomagnetite is achieved under rock weathering and fluvial transport processes thus the importance of titanomagnetite as a provenance indicator remains despite its degradation during the geological processes studied in the river eden catchment

new facility to study river abrasion processes this technical note presents a new facility which was constructed in order to study sediment and bedrock abrasion processes during fluvial transport these processes exert an important control on long term landscape evolution but they are still poorly understood and inadequately quantified the proposed experimental device is an annular flume with four fluid injections coupled to a close water circuit and a powerful pump in order to reach hydrodynamic conditions up to whose prevailing in mountain streams fluid surface geometry and visualization experiments with a high speed camera allow us to monitor hydrodynamic variables and sediment motion during the experiments despite the circular geometry of the flume pebble trajectories are found to closely mimic the bedload behavior in straight flume based also on a direct comparison with pebble abrasion rates along a large himalayan river we hypothesize that our device simulates in a realistic way transport processes and consequently abrasion processes in mountain rivers

controls on the channel width of rivers implications for modeling fluvial incision of bedrock on the basis of the manning equation and basic mass conser vation principles we derive an expression for scaling the steady state width w of river channels as a function of discharge q channel slope s roughness n and width to depth ratio a w 5 a a1 2 2 3 3 8 q 3 8 s 23 16 n 3 8 we propose that channel width to depth ratio in addition to roughness is a function of the material in which the channel is developed and that where a river is con fined to a given material width to depth ratio and roughness can be assumed constant given these simplifications the expression emulates traditional width discharge relationships for rivers incis ing bedrock with uniformly concave fluvial long profiles more sig nificantly this relationship describes river width trends in terrain with spatially nonuniform rock uplift rates where conventional discharge based width scaling laws are inadequate we suggest that much of observed channel width variability in river channels confined by bedrock is a simple consequence of the tendency for water to flow faster in steeper reaches and therefore occupy smaller channel cross sections we demonstrate that using conventional scaling relationships for channel width can result in underestima tion of stream power variability in channels incising bedrock and that our model improves estimates of spatial patterns of bedrock incision rates

three dimensional numerical simulations of crustal systems undergoing orogeny and subjected to surface processes as several modeling studies indicate the structural expression and dynamic behavior of orogenic mountain belts are dictated not only by their rheological properties or by far field tectonic motion but also by the efficiency of erosion and sedimentation acting on its surface until recently numerical investigations have been mainly limited to 2 d studies because of the high computational cost required by 3 d models here we have efficiently coupled the landscape evolution model cascade with the 3 d thermomechanically coupled tectonics code fantom details of the coupling algorithms between both codes are given we present results of numerical experiments designed to study the response of viscous plastic crustal materials subjected to convergence and to surface processes including both erosion and sedimentation in particular we focus on the equilibration of both the tectonic structures and on the surface morphology of the orogen we show that increasing the efficiency of fluvial erosion increases the frontal thrust angle which in turn decreases the width of the orogen in addition the maximum summit elevation of the orogen during transient evolution is significantly higher in those models showcasing surface processes than those that do not this illustrates the strong coupling between tectonics and surface processes we also demonstrate that an along strike gradient of erosion efficiency can have a major impact upon the landscape morphology and the tectonic structure and deformation of the orogen in both the across strike and along strike directions overall our results suggest that surface processes by enhancing localization of deformation can act as a positive forcing to topographic building

paleomagnetic results from some permian triassic rocks from southwestern china the ailao shan fault zone in west yunnan represents a suture boundary between south china and indosinian cratons preliminary paleomagnetic results from three sites in sichuan guichou and yunnan provinces reveal a 14 northward migration of south china craton between late permian and middle triassic the two cratons were separated by about 20 latitudes during the early triassic the tectonic setting of east asia during the triassic was probably analogous to the present indonesian region

channel width response to differential uplift 1 the role of channel width and slope adjustments to differential uplift in rivers within actively deforming terrains remains contentious here high resolution topographic surveying of formerly antecedent outwash channels demonstrates marked changes in channel width as a primary response to differential uplift for five late quaternary alluvial paleochannels crossing small folds along the active ostler fault zone of southern new zealand nearly continuous measurements of paleochannel width and concomitant incision reveal abrupt narrowing of widths toward minimum values at channel positions coincident with the initial uplift when the magnitude of differential uplift is sufficiently small narrowing alone permits these channels to remain antecedent in the context of a unit stream power model for fluvial erosion observed limits on the magnitude of channel narrowing suggest that above some threshold amount of differential uplift continued incision requires concomitant changes in channel gradient thus when crossing small growing folds alluvial rivers simply narrow their channels whereas larger folds that demand greater incision prompt an initial narrowing followed by channel steepening

the role of sediment in controlling steady state bedrock channel slope implications of the saltation abrasion incision model abstract the saltation abrasion model is a mechanistic model for river incision into bedrock by saltating bedload which we have previously derived and used experimental data to constrain all parameter values here we develop a method for applying the saltation abrasion model at a landscape scale and use the model as a reference for evaluating the behavior of a wide range of alternative incision models in order to consider the implications of the saltation abrasion model as well as other models for predicting topographic steady state channel slope to determine the single valued discharge that best represents the effects of the full discharge distribution in transporting sediment and wearing bedrock we assume all runoff can be partitioned between a low flow and a high flow discharge in which all bedload sediment transport occurs during high flow we then use the gauged discharge record and measurements of channel characteristics at a reference field site and find that the optimum discharge has a moderate magnitude and frequency due to the constraints of the threshold of grain motion and bed alluviation by high relative sediment supply incision models can be classified according to which of the effects of sediment on bedrock incision are accounted for using the predictions of the saltation abrasion model as a reference we find that the threshold of motion is the most important effect that should be represented explicitly followed in order of decreasing importance by the cover effect the tools effect and the threshold of suspension effect models that lack the threshold of motion over predict incision rate for low shear stresses and under predict the steady state channel slope for low to moderate rock uplift rates and rock strengths models that lack the cover effect over predict incision rate for high sediment supply rates and fail to represent the degree of freedom in slope adjustment provided by partial bed coverage models that lack the tools effect over predict incision rate for low sediment supply rates and do not allow for the possibility that incision rate can decline for increases in shear stress above a peak value overall the saltation abrasion model predicts that steady state channel slope is most sensitive to changes in grain size such that the effect of variations in rock uplift rate and rock strength may affect slope indirectly through their possible but as yet poorly understood influence on the size distribution of sediments delivered to channel networks by hillslopes

hanging valleys in fluvial systems controls on occurrence and implications for landscape evolution 1 we document and characterize hanging valleys in a fluvially eroded landscape in eastern taiwan our conceptual model for the initiation of hanging valleys builds on a recently proposed model of bedrock incision in which erosion actually becomes less efficient on very steep channel gradients if a pulse of incision in the main stem outpaces the tributary response the gradients at tributary mouths may therefore pass a threshold value beyond which erosional efficiency declines giving rise to a mismatch between trunk and tributary erosion rates this mismatch is expected at junctions with small tributaries where a step function decrease in drainage area also leads to sharp contrasts in water and sediment flux between trunk and tributary channels the occurrence of hanging valleys in actively uplifting fluvial landscapes such as the central range of taiwan suggests that the most common parameterizations of bedrock erosion which typically assume a monotonic positive correlation between channel gradient and incision rate may be violated in very steep channels in addition hanging valleys could greatly increase the response time of landscapes to tectonic perturbations since catchments above these tributary mouths will be insulated from these perturbations until a new suite of processes e g weathering and rock mass failure wear through the hanging valley lip the results of this study underscore the need for a more complete understanding of bedrock erosion processes and the incorporation of process transitions and threshold conditions into landscape evolution models

palaeomagnetic and rock magnetic studies of cretaceous rocks in the gongju basin korea implication of clockwise rotation palaeomagnetic and rock magnetic studies have been carried out for cretaceous non marine sedimentary rocks gongju group and volcanic rocks in the gongju basin located along the northern boundary of the ogcheon belt korea k ar age dating for the volcanic rocks was also performed it is found that the gongju group was remagnetised during the tilting of the strata with the characteristic remanent magnetisation chrm direction of d i 23 9 50 6 k 95 5 9 5 3 9 at 30 per cent untilting of the strata with a maximum value of precision parameter k while the volcanic rocks are revealed to acquire primary remanence with the direction of d i 204 2 43 8 k 36 6 9 5 8 6 after the tilt correction the k ar ages of the volcanic rocks range from 81 8 2 4 to 73 5 2 2 ma corresponding to the campanian stage of the late cretaceous electron microscope observations of samples from the gongju group show authigenic iron oxide minerals of various sizes distributed along the cleavage of chlorite and in the pore spaces indicating that the strata acquired the chemical remanent magnetisation due to the formation of secondary magnetic minerals under the influence of fluids the palaeomagnetic pole positions are at lat long 69 6 n 224 3 e dp 3 5 dm 5 2 calculated for the 30 per cent tilt corrected direction of the gongju group and at lat long 67 2 n 235 3 e a 9 5 8 9 for the volcanic rocks based on the results of this study it is interpreted that the volcanic rocks acquired the primary magnetisation almost at the same time as the remagnetisation of the gongju group in the late cretaceous comparisons of cretaceous palaeomagnetic poles from the korean peninsula with those from eurasia implies that the korean peninsula underwent clockwise rotation of 21 2 5 3 for the middle early cretaceous 12 6 5 4 for the late early cretaceous and 7 1 9 8 for the late cretaceous with respect to eurasia due to the sinistral motion of the tan lu fault

a positive conglomerate test for the characteristic remanent magnetization chrm of the emeishan basalt from southwest china a positive conglomerate test has been achieved on ten basalt clasts from a pyroclastic breccia in the emeishan basaltic sequence from southwest china nine basalt samples collected from a nearby lava flow yield chrm directions similar to the northeasterly and shallow up directions reported from elsewhere in the basalt province this effectively refutes the suggestion that this group of directions represents a remagnetization of as late as jurassic or triassic age the difference between different sets of results from the emeishan basalt and the non antipodality between the northeasterly directions and the reversed southwesterly down directions are likely to be due to a combination of minor structural rotations about vertical axes between the sampled localities inaccuracy in determination of bedding attitude in massive basaltic rocks and insufficient paleomagnetic sampling

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

the mesozoic cenozoic tectonic settings paleogeography and evaporitic sedimentation of tethyan blocks within china implications for potash formation abstract potash deposits are common in the mesozoic cenozoic tethyan domain which features the giant potash deposits in the late jurassic central asian basin and the late cretaceous khorat basin of thailand laos widespread evaporitic sequences from these intervals have been found in some marine and transitional basins within the chinese continental blocks such as the yangtze simao and tarim blocks in which only one small scale potash deposit had been found in the late cretaceous simao basin these evaporites are commonly characterized by depositional cycles that are more frequent than those of other potash basins around the world it remains debated whether large or giant potash deposits could be formed in chinese blocks of tethyan domain and if so what mechanism was responsible for the potash formation to address these issues we comprehensively review the meso cenozoic drift history and collision dynamics of these chinese blocks and summarize the formation mechanism depositional characteristics and paleoclimate of the evaporite basins in these blocks these blocks drifted from the southern hemisphere and collided and welded together in the beginning of the late permian in the northern hemisphere the collisions often commenced in the east and gradually spread to the west of these blocks resulting in 1 westward seawater retreat 2 differential topographic uplift with high topography in the east and low topography in the west and 3 increasingly closed depositional environments in these basins in addition these collisions often exerted nearly n s oriented principal compressive stress which resulted in nearly e w oriented extension these forces induced a series of faulted bounded subsidence belts with n s striking extensional sub basins arranged in an e w oriented chain in these blocks in this chain like sub basins system as sea level rose and seawater flowed from west to east the western sub basins proximal to the open sea would have precipitated calcium carbonates and calcium sulfates resulting in a high degree of concentration by the time the brines reached the distal end i e eastern sub basins ultimately potash minerals were commonly deposited in the easternmost sub basins based on the above model of potash formation we predict that potash deposits may be formed in certain sub basins in the eastern part of these basins in chinese tethyan blocks which maybe lay a theoretical foundation for china s future exploration of potash deposits

fluvial archives a valuable record of vertical crustal deformation abstract the study of drainage network response to uplift is important not only for understanding river system dynamics and associated channel properties and fluvial landforms but also for identifying the nature of crustal deformation and its history in recent decades geomorphic analysis of rivers has proved powerful in elucidating the tectonic evolution of actively uplifting and eroding orogens here we review the main recent developments that have improved and expanded qualitative and quantitative information about vertical tectonic motions the effects of horizontal deformation are not addressed channel long profiles have received considerable attention in the literature and we briefly introduce basic aspects of the behaviour of bedrock rivers from field and numerical modelling perspectives before describing the various metrics that have been proposed to identify the information on crustal deformation contained within their steady state characteristics then we review the literature dealing with the transient response of rivers to tectonic perturbation through the production of knickpoints propagating through the drainage network inverse modelling of river profiles for uplift in time and space is also shown to be very effective in reconstructing regional tectonic histories finally we present a synthetic morphometric approach for deducing the tectonic record of fluvial landscapes as well as the erosional imprint of tectonic forcing sedimentary deposits such as fluvial terrace staircases are also considered as a classical component of tectonic geomorphology we show that these studies have recently benefited from rapid advances in dating techniques allowing more reliable reconstruction of incision histories and estimation of incision rates the combination of progress in the understanding of transient river profiles and larger more rigorous data sets of terrace ages has led to improved understanding of river erosion and the implications for terrace profile correlation i e extrapolation of local data to entire profiles finally planform changes in fluvial systems are considered at the channel scale in alluvial rivers and regional level in terms of drainage reorganisation examples are given of how numerical modelling can efficiently combine with topographic data to shed new light on the dis equilibrium state of drainage systems across regional drainage divides

numerical modeling of fluvial strath terrace formation in response to oscillating climate many river systems in western north america retain a fluvial strath terrace rec ord of discontinuous downcutting into bedrock through the quaternary their importance lies in their use to interpret climatic events in the headwaters and to determine long term incision rates terrace formation has been ascribed to changes in sediment supply and or water discharge produced by late quaternary climatic fluctuations we use a one dimensional channel evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences the model includes sediment transport vertical bedrock erosion limited by alluvial cover and lateral valley wall erosion we set limits on our modeling by using data collected from the terraced wind river basin two types of experiments were performed constant period sinusoidal input histories and variable period inputs scaled by the marine 18 o rec ord our simulations indicate that strath terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates straths are cut when the channel floor is protected from erosion by sediment and are abandoned and terraces formed when incision can resume following sediment cover thinning high sediment supply promotes wide valley floors that are abandoned as sediment supply decreases in contrast wide valleys are promoted by low effective water discharge and are abandoned as discharge increases widening of the valley floors that become terraces occurs over many thousands of years the transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment transport capacity formation of terraces lags by several thousand years the input changes that cause their formation our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation the incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the last glacial maximum this apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock uplift rate

oxidation processes and their effects on the magnetic remanence of early cretaceous subaerial basalts from sierra chica de cordoba argentina fil geuna silvana evangelina consejo nacional de investigaciones cientificas y tecnicas oficina de coordinacion administrativa ciudad universitaria instituto de geociencias basicas aplicadas y ambientales de buenos aires universidad de buenos aires facultad de ciencias exactas y naturales instituto de geociencias basicas aplicadas y ambientales de buenos aires argentina

cover effect in bedrock abrasion a new derivation and its implications for the modeling of bedrock channel morphology the sediment load of a bedrock river plays an important role in the fluvial incision process by providing tools for abrasion the tools effect and by covering and thereby protecting the bed the cover effect we derive a new formulation for the cover effect in which the fraction of exposed bed area falls exponentially with increasing sediment flux or decreasing transport capacity and explore its consequences for the model of bedrock abrasion by saltating bed load erosion rates predicted by the model are higher than those predicted by earlier models in a closed system the maximum erosion rate is predicted to occur when sediment supply is equal to transport capacity for a flat bed by optimizing the channel geometry to minimize the potential energy of the stream and using representative values for both discharge and grain size we derive equations for the geometry of a bedrock river and explore how predictions for width slope and bed cover vary as functions of drainage area rock uplift rate and rock strength the equations predict a dependence of channel width on drainage area similar to the relations using a simple shear stress incision law the slope area relationship is predicted to be concave up in a log log regime with a curvature dependent on uplift rate however this curvature does not deviate sufficiently from a straight line to allow discrimination between models using empirical data dependence of channel width and slope on rock uplift rate can be separated into two domains for low uplift rates channel geometry is largely insensitive to uplift rate due to a threshold effect at high uplift rates there is a power law dependence bed cover is predicted to increase progressively downstream and to increase with increasing uplift rate in our model the width to depth ratio is a function of both tectonic and climatic forcing this indicates that the scaling between channel width and bed slope is neither a unique indicator of tectonic forcing at steady state nor a signature of transience or steady state we conclude that sediment effects need to be taken into account when modeling bedrock channel morphology

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

discharge discharge variability and the bedrock channel profile long term bedrock incision is driven by daily discharge events of variable magnitude and frequency with ineffective events below an incision threshold we explore theoretically how this short term stochastic behavior controls long term steady state incision rates and bedrock channel profiles combining a realistic frequency magnitude distribution of discharge with a deterministic detachment limited incision model in which incision rate is a power function of basal shear stress above a critical shear stress our model predicts a power law relationship between steady state slope and drainage area consistent with observations the exponent of this power law is independent of discharge mean and variability while the amplitude factor which controls mountain belt relief is a power law function of mean runoff with an exponent of 0 5 and a complex function of runoff variability in accordance with evidence that incision occurs between 6 and 20 of time in rapidly incising rivers 1 mm yr our model predicts that channel steepness is virtually insensitive to runoff variability runoff variability can only decrease channel steepness for very slow incision rates and or weak lithologies the relationship between channel steepness and incision rate is always a power law whose exponent depends on the channel cross sectional geometry and runoff variability this contradicts models neglecting discharge stochasticity in which the steepness incision scaling is set by the incision law exponent our results suggest that changes in climate variability cannot explain an increase in bedrock incision rates during the late cenozoic within the context of a detachment limited model

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

a generalized exner equation for sediment mass balance 1 the advance of morphodynamics research into new areas has led to a proliferation of forms of sediment mass balance equation without a general equation it is often difficult to know what these problem specific versions of sediment mass balance leave out to address this we derive a general form of the standard exner equation for sediment mass balance that includes effects of tectonic uplift and subsidence soil formation and creep compaction and chemical precipitation and dissolution the complete equation 17 allows for independent evolution of two critical interfaces that between bedrock and sediment or soil and that between sediment and flow by eliminating terms from the general equation it is straightforward to derive mass balance equations applicable to a wide range of problems such as short term bed evolution basin evolution bedrock uplift and soil formation and carbonate precipitation and transport dropping terms makes explicit what is not being considered in a given problem and can be done by inspection or by a formal scaling analysis of the terms scaling analysis leads directly to dimensionless numbers that measure the relative importance of terms in the equation for example the relative influence of spatial versus temporal changes in sediment load on bed evolution combining scaling analysis with time averaging shows how the relative importance of terms in the equation can change with timescale for example the term representing bed evolution due to temporal change in sediment load tends to zero as timescale increases

emeishan basalts sw china and the end guadalupian crisis magnetobiostratigraphic constraints abstract a magnetostratigraphic investigation of the permian emeishan lip large igneous province was carried out on a composite section in ebian county close to the type area in sichuan sw china the main succession of twelve flows 175 m thick carries a normal polarity whilst the one reliable site from the overlying 30 m thick volcanic waning sequence is marked by a reverse polarity the data enable a correlation to be proposed with an emeishan basalt sequence in western guizhou c 400 km to the se from our knowledge of the geomagnetic field9s reversal behaviour during the permian it suggests that the main lava pile along the eastern half of the emeishan basalt outcrop belt was erupted within a half to one million years using magnetobiostratigraphic data from the adjacent south china platform the normal polarity magnetozone is correlated with the normal polarity chron associated with the upper part of the maokou limestones constrained by conodont data the main emei basalts appear to be at least two biozones older 1 1 5 ma than the mid late permian boundary it is possible however that the emei basalt waning zone sequence which represents an explosive volcanic phase might be coeval with the end guadalupian biotic crisis thus arguments implicating emei basalt volcanism as the causal mechanism behind this major global event have to accommodate the new relative age constraints

fluvial bedrock incision in the active mountain belt of taiwan from in situ produced cosmogenic nuclides the concentration of cosmogenic nuclides in rocks exposed at the earth s surface is proportional to the total duration of their exposure this is the basis for bedrock surface exposure dating and has been used to constrain valley lowering rates in the taroko gorge eastern central range taiwan taroko gorge contains a uniquely complete geomorphic record of fluvial valley lowering continuous fluvially sculpted surfaces are present in the lower 200 m of this marble gorge assuming no post fluvial erosion of the gorge wall the concentration of in situ produced cosmogenic 36 cl measured in gorge wall marbles reveals exposure ages from 0 2 ka in the active channel to 6 5 ka at 165 m above the present river these ages imply an average fluvial incision rate of 26 3m m a 1 throughout the middle and late holocene taking into account lateral gorge wall retreat after initial thalweg lowering would give rise to calculated older exposure ages without considering gorge wall retreat our estimates therefore represent maximum incision rates estimated maximum holocene incision rates are higher than the long term exhumation rates derived from fission track dating the longterm gorge development governed by tectonic uplift is superimposed by short term variations in incision rates caused by climatic or regional tectonic changes copyright 2005 john wiley sons ltd

importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem 1 fluvial erosion of bedrock occurs during occasional flood events when boundary shear stress exceeds a critical threshold to initiate incision therefore efforts to model the evolution of topography over long timescales should include an erosion threshold and should be driven by a stochastic distribution of erosive events however most bedrock incision models ignore the threshold as a second order detail in addition climate is poorly represented in most landscape evolution models so the quantitative relationship between erosion rate and measurable climatic variables has been elusive here we show that the presence of an erosion threshold when combined with a well constrained probabilistic model of storm and flood occurrence has first order implications for the dynamics of river incision in tectonically active areas first we make a direct calculation of the critical shear stress required to pluck bedrock blocks for a field site in new york second we apply a recently proposed stochastic threshold bedrock incision model to a series of streams in california with known tectonic and climatic forcing previous work in the area has identified a weak relationship between channel gradient or relief and rock uplift rate that is not easily explained by simpler detachment limited models the results with the stochastic threshold model show that even low erosion thresholds which are exceeded in steep channels during high frequency flood events fundamentally affect the predicted relationship between gradient and uplift rate in steady state rivers in a manner consistent with the observed topography this correspondence between theory and data is however nonunique models in which a thin alluvial cover may act to inhibit channel incision in the low uplift rate zone also provide plausible explanations for the observed topography third we explore the broader implications of the stochastic threshold model to the development of fluvial topography in active tectonic settings we suggest that continued field applications of geomorphic models including physically meaningful thresholds and stochastic climate distributions are required to advance our knowledge of interactions among surficial climatic and crustal processes

assessment of the debris flow susceptibility in tropical mountains using clast distribution patterns abstract channel morphometric parameters and clast distribution patterns in selected basins of the ferriferous quadrangle tropical mountains brazil were analyzed in order to assess susceptibility to debris flows median bed surface clast size d 50 in the main stream channel of these basins shows a coarsening downstream trend with drainage areas of up to 6 km 2 which is attributed to debris flow dominated channels by some authors the composition and roundness of the bed load clast sand and the presence of allochthonous large boulders throughout the channels also suggest the occurrence of past debris flow in the region luminescence optically stimulated loe dating points out that debris flow could have occurred as a consequence of climate changes in the late pleistocene and holocene and it can now be triggered by deforestation or extreme rainfall events there has not been any record of past debris flow in the study area or in other mountainous regions of brazil where debris flows have recently occurred thus the adopted approach can be useful to assess debris flow susceptibility in this and other similar areas

a mechanistic model for river incision into bedrock by saltating bed load 1 abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into bedrock here we develop a model for bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium we assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity particle impact velocity and impact frequency depend on saltation trajectories which can be predicted using empirical functions of excess shear stress the model predicts a nonlinear dependence of bedrock abrasion rate on both sediment supply and transport capacity maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the bedrock bed maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension measurements of bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter which relates rock strength to rock resistance to abrasive wear the model results suggest that grain size and sediment supply are fundamental controls on bedrock incision rates not only by bed load abrasion but also by all other mechanisms that require bedrock to be exposed in the channel bed

scaling and elevation in river networks the analysis of large river networks obtained from digital elevation models has given insight into the variation of channel slope with scale investigators have recently suggested that channel slopes are self similar with magnitude or area as a scaling parameter our data indicates otherwise in particular the variance of channel slope is larger than that predicted by simple self similarity this suggests multiscaling the scaling exponent for the standard deviation is approximately half the corresponding exponent in the relationship of the slope mean to magnitude or area a model for channel slopes based on a point process of elevation drops along the channel is presented the model reproduces observed multiscaling properties when the density of elevation increments is related to area or magnitude as a

sex that moves mountains the influence of spawning fish on river profiles over geologic timescales abstract a key component of resilience is to understand feedbacks among components of biophysical systems such as physical drivers ecological responses and the subsequent feedbacks onto physical process while physically based explanations of biological speciation are common e g mountains separating a species can lead to speciation less common is the inverse process examined can a speciation event have significant influence on physical processes and patterns in a landscape when such processes are considered such as with ecosystem engineers many studies have focused on the short term physical and biological effects rather than the long term impacts here we formalized the physical influence of salmon spawning on stream beds into a model of channel profile evolution by altering the critical shear stress required to move stream bed particles we then asked if spawning and an adaptive radiation event similar to the one that occurred in pacific salmon species could have an effect on channel erosion processes and stream profiles over geological timescales we found that spawning can profoundly influence the longitudinal profiles of stream beds and thereby the evolution of entire watersheds the radiation of five pacific salmon from a common ancestor additionally could also cause significant geomorphic change by altering a wider section of the profile for a given distribution of grain sizes this modeling study suggests that biological evolution can impact landscape evolution by increasing the sediment transport and erosion efficiency of mountain streams moreover the physical effects of a species on its environment might be a complementary explanation for rapid radiation events in species through the creation of new habitat types this example provides an illustrative case for thinking about the long and short term coupling of biotic and abiotic systems

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

a surface roughness model for predicting alluvial cover and bed load transport rate in bedrock channels partial alluvial cover in bedrock channels influences downcutting rates and mountain river morphodynamics flume experiments have demonstrated that a wide range of stable cover fractions are possible for a given ratio of sediment supply to transport capacity existing cover models impose different unique relationships between cover fraction and reach scale transport variables e g slope discharge sediment supply and grain size individually these models cannot predict the range of cover behaviors observed experimentally suggesting that additional variables may be important i propose a 1 d model in which bedrock surface roughness is a control on partial alluvial cover in the model surface roughness affects both shear stresses and thresholds of grain motion and therefore sediment transport capacity the roughness of the combined bedrock alluvial bed surface varies with the cover fraction model trends are reasonably consistent with previous experiments over ranges of roughness slope and shear stress when bedrock roughness is much greater than grain size cover increases approximately linearly with the ratio of sediment supply to transport capacity when bedrock roughness is much less than the sediment diameter the model tends to predict abrupt shifts between complete bedrock exposure and alluvial cover this runaway alluviation occurs for model solutions that are unstable to perturbations in supply which is the case when transport capacity decreases as alluvial cover increases in addition a model variant in which sediment supply is controlled by the amount of sediment on the bed suggests that a linear cover relation may be sufficient for modeling sediment flux dependent erosion at landscape scales

new paleomagnetic investigations of the emeishan basalts in ne yunnan southwestern china constraints on eruption history abstract paleomagnetic and magnetostratigraphic investigations have been conducted on two consecutive sections of the emeishan trap at shuimo sm and dadi dd in northeastern yunnan province southwestern china rock magnetic analysis indicates that the magnetic carrier of the basalts is dominantly low ti magnetite stepwise thermal demagnetization isolates a stable characteristic remanent magnetization chrm at high temperatures 400 585 c the chrms 301 323 at sm section are northeasterly upward of normal polarity with a mean direction of ds is 11 0 5 2 n 12 k 53 7 95 6 0 at dd section the chrms are southwesterly downward of reversed polarity with a mean direction of 205 9 10 0 n 5 k 78 4 95 8 7 the chrms from the basaltic gravels from the overlying conglomerate layer in late permian xuanwei formation at dd section pass the conglomerate test suggesting the primary nature of magnetization together with other magnetostratigraphic studies e g n r pattern of the emeishan basalts and the known radiometric ages ca 260 ma it suggests that the eruption period of the emeishan large igneous province is less than 3 myr

climate driven bedrock incision in an active mountain belt measurements of fluvial bedrock incision were made with submillimeter precision in the east central range of taiwan where long term exhumation rates and precipitation driven river discharge are independently known they indicate that valley lowering is driven by relatively frequent flows of moderate intensity abrasion by suspended sediment is an important fluvial wear process and channel bed geometry and the presence of widely spaced planes of weakness in the rock mass influence erosion rate and style

interpreting quaternary uplift rates at the mendocino triple junction northern california from uplifted marine terraces analysis of the altitudinal spacing of 14 flights of marine terraces indicates a spatial pattern of varying uplift rates that agrees with that determined from previously dated terraces for the past 3 81 ka and temporal changes in uplift rates from 1 m ka to 3 5 m ka that may reflect response to changes in tectonic regime during passage of the mendocino triple junction mtj a possible mechanism for regional uplift is growth of a slab window south of the mtj the region of most rapid uplift is 25 43 km south of the mtj immediately south of the northern boundary of the slab window the coastline is tilted upward to the south in the region directly above the southern edge of the subducted gorda plate at point delgada 55 km south of the present mtj where the northern edge of the slab window passes approximately 300 ka uplift rates have been 1 2 m ka for at least 330 ka more than 1 4 m y after passage of the southern edge of the subducted slab at the mendocino coast uplift rates have been less than 0 4 m ka for at least 330 ka

late permian to early triassic magnetostratigraphy a late permian to early triassic magnetostratigraphic reference section is presented the lower triassic part is based on results from marine limestone sections in south china published earlier 1 2 reliable new permian data are added here which have been collected in the nammal gorge salt range northwest pakistan where marine sediments have been deposited quasi continuously with occasional minor hiatuses during the late palaeozoic to early mesozoic about 50 of the permian samples from the nammal section contain hidden beneath a strong recent or tertiary overprint a characteristic remanent magnetization chrm which is very likely of permian age this component which was imprinted on the southern hemisphere has normal as well as reversed polarity with a normal mean direction decl 289 3 incl 50 3 95 4 3 n 113 which is in close agreement with the palaeofield direction expected for a site belonging to the indian plate as part of gondwanaland during the permian in the lower upper permian several normal polarity zones are recognized this contradicts the current assumption that rocks of this age belong to the long reversely polarized kiaman hyperzone the kiaman interval must end and the illawarra hyperzone of mixed polarity must begin in or prior to the lowermost upper permian the permian triassic boundary at nammal as well as in the chinese sections is situated very close to a transition from a reversed to a normal polarity zone the upper permian at nammal together with the lower triassic south china sections is estimated to cover about 20 ma nearly 30 polarity changes are observed which result in an average reversal frequency very similar to that observed during the early tertiary the reversal rate after the end of the long lasting reversed kiaman hyperchron apparently increases in a manner similar to that after the end of the cretaceous long normal superchron only a few polarity zones are found in the lower upper permian while progressively more frequent reversals follow in the lower triassic

transport slopes sediment cover and bedrock channel incision in the henry mountains utah 1 field data from channels in the henry mountains of utah demonstrate that abundant coarse sediment can inhibit fluvial incision into bedrock by armoring channel beds the cover effect we compare several small channels that share tributary junctions and have incised into the same sedimentary bedrock unit navajo sandstone but contain differing amounts of coarse diorite clasts owing to the spatial distribution of localized sediment sources bedrock channels that contain abundant clasts diorite rich have steeper longitudinal slopes than tributaries of these channels with smaller drainage areas and less sediment diorite poor the diorite poor tributaries have incised more deeply to lower average slopes and have more reach scale slope variability which may reflect bedrock properties longitudinal sediment sorting and incision at lower sediment supply diorite rich channels have less bedrock exposed and smoother longitudinal profiles than diorite poor channels we find that 1 coarse sediment can mantle bedrock channel beds and reduce the efficiency of incision validating the hypothesized cover effect in fluvial incision models 2 the channel slope needed to transport the sediment load can be larger than that needed to erode bedrock suggesting that the slope of incising bedrock channels can become adjusted to the sediment load 3 when abundant sediment is available transport capacity rather than thresholds of motion can be dominant in setting bedrock channel slope and 4 cover effects can be important even when moderate amounts of bedrock are exposed in channel beds

channel response to tectonic forcing field analysis of stream morphology and hydrology in the mendocino triple junction region northern california abstract an empirical calibration of the shear stress model for bedrock incision is presented using field and hydrologic data from a series of small coastal drainage basins near the mendocino triple junction in northern california previous work comparing basins from the high uplift zone huz uplift rates around 4 mm year to ones in the low uplift zone luz 0 5 mm year indicates that the huz channels are about twice as steep for a given drainage area this observation suggests that incision processes are more effective in the huz it motivates a detailed field study of channel morphology in the differing tectonic settings to test whether various factors that are hypothesized to influence incision rates discharge channel width lithology sediment load change in response to uplift or otherwise differ between the huz and luz analysis of regional stream gaging data for mean annual discharge and individual floods yields a linear relationship between discharge and drainage area increased orographic precipitation in the huz accounts for about a twofold increase in discharge in this area corresponding to an assumed increase in the erosional efficiency of the streams field measurements of channel width indicate a power law relationship between width and drainage area with an exponent of 0 4 and no significant change in width between the uplift rate zones although interpretation is hampered by a difference in land use between the zones the huz channel width dataset reveals a scaling break interpreted to be the transition between colluvial and fluvial dominated incision processes assessments of lithologic resistance using a schmidt hammer and joint surveys show that the rocks of the study area should be fairly similar in their susceptibility to erosion the huz channels generally have more exposed bedrock than those in the luz which is consistent with protection by sediment cover inhibiting incision in the luz however this difference is likely the result of a recent pulse of sediment due to land use in the luz therefore the role of sediment flux in setting incision rates cannot be constrained with any certainty to summarize of the four response mechanisms analyzed the only factor that demonstrably varies between uplift rate zones is discharge although this change is likely insufficient to explain the relationship between channel slope and uplift rate the calibrated model allows us to make a prediction of channel concavity that is consistent with a previous estimate from slope drainage area data we show that the inclusion of nonzero values of critical shear stress in the model has important implications for the theoretical relationship between steady state slope and uplift rate and might provide an explanation for the observations this analysis underscores the importance of further work to constrain quantitatively threshold shear stress for bedrock incision

rapid apparent polar wandering of south china during the latest paleozoic and early mesozoic a paleomagnetic study of upper permian limestones from guizhou province wang z and van der voo r 1993 rapid apparent polar wandering of south china during the latest paleozoic and early mesozoic a paleomagnetic study of upper permian limestones from guizhou province ec o o y c 222 165 176 the lower part of upper permian 265 ma marine platform limestones in guizhou province south china block was sampled for paleomagnetic study a total of 24 samples from 4 sites was collected progressive thermal demagnetization isolated single characteristic component magnetizations from 21 samples the samples show mixed but dominantly normal polarities the mean direction is 46 23 in situ and 53 11 tilt corrected yielding a pole position of 34n 208e irm acquisition experiments suggest that the magnetizations are carried by magnetite sem images of polished thin sections show that the magnetite has replaced pyrite indicating a secondary diagenetic origin for the magnetite considering the dominant normal polarity and by comparison with paleomagnetic results of permian and younger ages for south china we infer that these upper permian limestones in guizhou province have been remagnetized during the latest permian or early triassic a review of the paleomagnetic data reported thus far for the emeishan basalts in different localities shows that the results from these upper permian rocks in general fall into three groups one group a resembles the mean characteristic direction of our limestones the second group b has yielded directions in agreement with those of the triassic and the third group c conforms to the directions obtained from the late triassic jurassic remagnetization data in some paleozoic and triassic rocks of south china sem observations of emeishan basalts thin sections indicate that the a group directions most likely represent the primary magnetizations while the b group directions are remagnetizations a late paleozoic early mesozoic apparent polar wander path for south china can thus be constructed by connecting these poles paleopoles calculated from northerly folding associated remagnetizations c also fail on this path and are the youngest this apparent polar wander path points to larger scale rotations of the south china block during the late paleozoic and early mesozoic than previously thought

10be derived assessment of accelerated erosion in a glacially conditioned inner gorge entlebuch central alps of switzerland inner gorges often result from the propagation of erosional waves related to glacial interglacial climate shifts however only few studies have quantified the modern erosional response to this glacial conditioning here we report in situ 10be data from the 64 km2 entlen catchment swiss alps this basin hosts a 7 km long central inner gorge with two tributaries that are 100 m deeply incised into thick glacial till and bedrock the 10be concentrations measured at the downstream end of the gorge yield a catchment wide erosion rate of 0 42 0 04 mm yr 1 while erosion rates are consistently lower upstream of the inner gorge ranging from 0 14 0 01 mm yr 1 to 0 23 0 02 mm yr 1 however 10be based sediment budget calculations yield rates of similar to 1 3 mm yr 1 for the inner gorge of the trunk stream likewise in the two incised tributary reaches erosion rates are similar to 2 0 mm yr 1 and similar to 1 9 mm yr 1 moreover at the erosional front of the gorge we measured bedrock incision rates ranging from similar to 2 5 mm yr 1 to similar to 3 8 mm yr 1 these rates however are too low to infer a post glacial age 1520 ka for the gorge initiation this would require erosion rates that are between 2 and 6 times higher than present day estimates however the downcutting into unconsolidated glacial till favored high erosion rates through knickzone propagation immediately after the retreat of the lgm glaciers and subsequent hillslope relaxation led to a progressive decrease in erosion rates this hypothesis of a two to sixfold decrease in erosion rates does not conflict with the 10be based erosion rate budgets because the modern erosional time scale recorded by 10be cover the past 23 ka only these results point to the acceleration of holocene erosion in response to the glacial overprint of the landscape

paleomagnetism and electron microscopy of the emeishan basalts yunnan china abstract many normal and a few reversed characteristic directions of magnetization have been obtained by predominantly thermal demagnetization from ten sites of the late permian emeishan basalts collected near kunming yunnan province the normal magnetization directions pass a fold test at the 99 confidence level and yield declination inclination 26 12 k 46 95 6 and a paleopole at 50 n 241 e however the reversed polarity directions with declination inclination at 244 3 are not antipodal to the normal ones which is also noted in other studies of the emeishan basalts of the yangtze paraplatform of the south china block speculations about the cause of this lack of antipodality center on 1 1 local relative rotations 2 2 incomplete demagnetization 3 3 unusually large non dipole fields secular variation or asymmetric reversals in the late permian 4 4 errors in sample orientation 5 5 later remagnetization represented by one or the other polarity group the first two causes are ruled out by our observations and the third cannot be tested with data from china alone but is unlikely because it has not been observed elsewhere errors in sample orientation may be present in those studies that used a magnetic compass because sample intensities of magnetization are on the high side however for the normal polarity directions a solar compass has been used in some of the studies and all normal polarity directions observed in the area are in agreement thus sample orientation errors can be invoked only for the reversed polarity directions in order to test cause 5 we have examined the magnetic carriers in eight samples with scanning electron microscopy titanium poor to titanium rich magnetite commonly cruciform in crystal habit indicates primary igneous crystallization in two samples that have ne and wsw declinations in contrast six normal polarity samples with nne declinations show pervasive replacement of original titanomagnetite by titanium free magnetite and sphene indicating a high degree of alteration we ascribe this alteration to late hydrothermal circulation and argue that it has caused remagnetization in post permian times this study suggests therefore that the ne sw directions are more likely to be representative of the late permian paleomagnetic field than the nne directions

formation of magnetic single domain magnetite in ocean ridge basalts with implications for sea floor magnetism although magnetic data are the primary evidence for ocean floor spreading the processes by which magnetic phases in ocean floor basalts are formed remain poorly constrained scanning transmission electron microscopic observations show that magnetic single domain magnetite in sheeted dike basalts of deep sea drilling project hole 504b formed through oxidation exsolution of ilmenite exsolution of ulvospinel lamellae and recrystallization of end member magnetite by interaction with convecting fluids the data suggest that the sheeted dike basalts with single domain magnetite as an efficient and stable magnetic carrier contribute significantly to sea floor magnetism

implications of sediment flux dependent river incision models for landscape evolution 1 developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate erosion and tectonics current models of long term river network incision differ significantly in their treatment of the role of sediment flux we analyze the implications of various sediment flux dependent incision models for large scale topography in an attempt 1 to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and 2 to explain the apparent ubiquity of mixed bedrock alluvial channels in active orogens although certain forms of the various models can be discarded as inconsistent with morphological data we find that the relative intrinsic concavity indices of detachment and transport limited systems defined herein largely dictate whether the various models can be tied to distinctive steady state morphologies preliminary data suggest that no such diagnostic differences may exist and other methods must be developed to test models accordingly we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment versus transport limited channels as a function of rock uplift rate that may allow discrimination among various models further we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate in addition to general differences between detachment and transport limited systems our analysis identifies an interesting hysteresis in landscape evolution hybrid channels at the threshold between detachment and transport limited conditions are expected to act as detachment limited systems in response to an increase in rock uplift rate or base level fall and as transport limited systems in response to a decrease in rock uplift rate especially during postorogenic topographic decline the analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed