ocean drilling program leg 112 peru continental margin part 1 tectonic history studies of ocean drilling program leg 112 cores and geophysical data define early eocene quaternary normally faulted continental sediment sequences across the peru margin to the middle slope and a 15 km wide late miocene quaternary accretionary complex beneath the lower slope the eocene shelf and upper slope sea floor were eroded during the oligocene and in middle miocene the seaward part had subsided to lower slope depths this subsidence and the missing paleogene continental slope indicate tectonic erosion during normal plate convergence subduction of the nazca ridge locally accelerated tectonic erosion leaving a scarp but no associated compressional structure nearby late miocene sediment accreted against the erosional scarp local tectonic mechanisms of forearc basin subsidence are suggested by the rapid subsidence of a late miocene forearc basin and stability of the adjacent one despite similar histories of plate convergence

seabeam and seismic reflection imaging of the tectonic regime of the andean continental margin off peru 4 s to 10 s abstract marine geophysical surveys employing seabeam multi and single channel seismic reflection gravity and magnetic instruments were conducted at two locations along the continental slope of the peru trench during the seaperc cruise of the r v jean charcot in july 1986 these areas are centered around 5 30 s and 9 30 s off the coastal towns of paita and chimbote respectively these data indicate that 1 the continental slope off peru consists of three distinct morpho structural domains from west to east are the lower middle and upper slopes instead of just two as previously reported 2 the middle slope has the characteristics of a zone of tectonic collapse at the front of a gently flexured upper slope 3 the upper half of the lower slope appears to represent the product of mass wasting 4 thrusting at the foot of the margin produces a continuous morphologic feature representing a deformation front where the products of mass wasting are overprinted by a compressional tectonic fabric 5 a change in the tectonic regime from tensional to compressional occurs at the mid slope lower slope boundary the accretionary prism being restricted to the very base of the lower slope in the paita area the andean margin off peru is an extensional active margin or a collapsing active margin developing a subordinated accretionary complex induced by massive collapse of the middle slope area

development of the accretionary prism along peru and material flux after subduction of nazca ridge results from ocean drilling program leg 112 indicated tectonic erosion of the peruvian convergent margin during subduction of the nazca ridge followed by renewed accretion against the erosional scar seismic images and swath mapping morphology show a regional geology shaped as subduction of nazca ridge migrated 800 km along the peru trench the accretionary prism is imaged at two stages of development since upper miocene time with depth migrated seismic records in which the back stop is well defined from these data material flux can be quantified after the ridge crest subducted and accretion dominated over erosion the prism grew rapidly to 10 to 15 km width during rapid growth the margin taper was large and about 60 percent of the sediment supply accreted as the prism growth slowed only 30 percent accreted despite an increased trench sediment supply since the convergence rate changed little the inverse relation between sediment supply and prism growth rate suggests that other processes allow most of the sediment input to be subducted the decreased prism growth may be partially controlled by varying the structure of the back stop the force required to activate faults cutting across the back stop is probably greater than the force allowed by basal friction across the plate boundary thus the shear stress transmitted across the plate boundary is insufficient to drive out of sequence thrust faults that would thicken the back stop or raise it beyond a limited height a constant back stop height limits accretion and the subduction window appears to open and accommodate increased trench sediment beneath the back stop rather than inducing a transport of sediment over the top of the back stop nazca ridge subduction affected the convergent margin mass flux for about 8 ma mass balancing indicates an order of magnitude increase in the current sediment volume subducted in the north compared to that near the ridge crest when growth of the accretionary prism slowed a greater terrigenous input through the trench was accommodated by sediment subduction the resulting range of material flux is estimated to increase the sediment interlayer between the plates about 500 m which may affect coupling

the pragmatic roots of american quaternary geology and geomorphology abstract h l fairchild s words from the 1904 geological society of america bulletin remain appropriate today geologists have been too generous in allowing other people to make their philosophy for them geologists have quietly followed a methodological trinity involving 1 inspiration by analogy 2 impartial and critical assessment of hypotheses and 3 skepticism of authority prevailing theoretical constraints or paradigms these methods are described in classical papers by quaternary geologists and geomorphologists mostly written a century ago in recent years these papers have all been criticized in modern philosophical terms with little appreciation for the late 19th century american philosophical tradition from which they arose recent scholarly research however has revealed some important aspects of that tradition giving it a coherence that has largely been underappreciated as 20th century philosophy of science pursued its successive fads of logical positivism critical rationalism relativism and deconstructivism for all of which science is synonymous with physics nearly all this ideology is geologically irrelevant as philosophy of science in the late 20th century has come to be identical with philosophy of analytical physics focused on explanations via ideal truths much of geology has remained true to its classical doctrines of commonsensism fallibilism and realism in contrast to the conceptualism and the reductionism of the analytical sciences geology has emphasized synthetic thinking the continuous activity of comparing connecting and putting together thoughts and perceptions the classical methodological studies of geological reasoning all concern the formulation and testing of hypotheses analysis does not serve to provide the ultimate answers for intellectual puzzles predefined by limiting assumptions imposed on the real world rather analysis in geology allows the investigator to consider the consequential effects of hypotheses the latter having been suggested by experience with nature itself rather than by our theories of nature these distinctions and methods were described in g k gilbert s papers on the inculcation of scientific method by example 1886 and the origin of hypotheses 1896 portions were elaborated in t c chamberlin s method of multiple working hypotheses 1890 and his method of the earth sciences 1904 in w m davis s value of outrageous geological hypotheses 1926 and in d johnson s role of analysis in scientific investigation 1933 american quaternary geology and geomorphology have their philosophical roots in the pragmatic tradition enunciated most clearly by c s peirce now recognized as the greatest american philosopher and considered by sir karl popper to be one of the greatest philosophers of all time quaternary geology and geomorphology afford numerous examples of peirce s method of science which might be termed the critical philosophy of common sense the most obvious influence of pragmatism in geology however has largely been conveyed by the tradition of its scientific community the elements of this tradition include a reverence for field work a humility before the facts of nature a continuing effort to discriminate the phenomena observed from the observer s inference in regard to them a propensity to pose hypotheses and a willingness to abandon them when their consequences are contradicted by reality

quantifying differential rock uplift rates via stream profile analysis despite intensive research into the coupling between tectonics and surface processes our ability to obtain quantitative information on the rates of tectonic processes from topography remains limited due primarily to a dearth of data with which to test and calibrate process rate laws here we develop a simple theory for the impact of spatially variable rock uplift rate on the concavity of bedrock river profiles application of the analysis to the siwalik hills of central nepal demonstrates that systematic differences in the concavity of channels in this region match the predictions of a stream power incision model and depend on the position and direction of the channel relative to gradients in the vertical component of deformation rate across an active fault bend fold furthermore calibration of model parameters from channel profiles argued to be in steady state with the current climatic and tectonic regime indicates that 1 the ratio of exponents on channel drainage area and slope m n is 0 46 consistent with theoretical predictions 2 the slope exponent is consistent with incision either linearly proportional to shear stress or unit stream power n 0 66 or n 1 respectively and 3 the coefficient of erosion is within the range of previously published estimates mean k 4 3 10 4 m 0 2 yr application of these model parameters to other channels in the siwalik hills yields estimates of spatially variable erosion rates that mimic expected variations in rock uplift rate across a fault bend fold thus the sensitivity of channel gradient to rock uplift rate in this landscape allows us to derive quantitative estimates of spatial variations in erosion rate directly from topographic data

hypocentral trend surface analysis probing the geometry of benioff zones a hypocentral trend surface is a continuous function of latitude and longtitude fitted by least squares to a set of hypocenters so that it predicts depth to the middle of a benioff zone in this paper we take a relatively simple approach to hypocentral trend surface analysis the hypocentral trend surface is constructed from a spherical surface harmonic expansion whose coefficients are selected so as to minimize the standard vertical deviation between hypocenters and trend surface the vertical deviation of a hypocenter from the hypocentral trend surface is called its residual consideration of a hypocentral trend surface cannot be divorced from consideration of the associated residuals and their spatial distribution any analysis of benioff zone geometry trades off presumed thickness of the zone with its presumed shape this trade off can be investigated by examining suites of candidate hypocentral trend surfaces together with their associated residual distributions generated by varying the number of degrees of freedom available to the trend surface hypocentral trend surfaces are generated for three high quality sets of hypocenters obtained by local seismic networks in honshu japan cook inlet alaska and south peru hypocenters beneath honshu generate a bimodal distribution of residuals about their trend surface the vertical separation of the upper and lower sheets of this double benioff zone averages about 33 km no double benioff zone configuration is discovered below cook inlet or south peru regional trend surfaces are established for intermediate depth teleseismic data from south and middle america in south america 0 40 s teleseismic data suggest that along strike transitions between flat and moderately steeply dipping sections of the benioff zone are achieved by flexure of a coherent slab rather than by fragmentation of the slab into tear bound flaps or separate tongues of lithosphere hypocentral trend surface analysis of teleseismic data from south peru suggests a geometry similar to that indicated by hypocentral trend surface analysis of the local data of hasegawa and sacks an eastward shallowing benioff zone beneath southwest mexico is closely associated with the geometry of the trans mexican volcanic arc the benioff zone beneath central america is more steeply dipping the flat slab steep slab transition near 96 w may relate to the difference in age of oceanic lithosphere on either side of the tehuantepec ridge while the strong flexure of the subducted nazca plate beneath ecuador is closely associated with the carnegie ridge the nazca ridge extropolates slightly north of the flexure in subducted lithosphere beneath south peru the flexure appears instead more closely associated with the pronounced inflection in the map view shape of the leading edge of the overriding south american plate

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

landscape form and millennial erosion rates in the san gabriel mountains ca abstract it has been long hypothesized that topography as well as climate and rock strength exert first order controls on erosion rates here we use detrital cosmogenic 10 be from 50 basins ranging in size from 1 to 150 km 2 to measure millennial erosion rates across the san gabriel mountains in southern california where a strong e w gradient in relief compared to weak variation in precipitation and lithology allow us to isolate the relationship between topographic form and erosion rate our erosion rates range from 35 to 1100 m ma and generally agree with both decadal sediment fluxes and long term exhumation rates inferred from low temperature thermochronometry catchment mean hillslope angle increases with erosion rate until 300 m ma at which point slopes become invariant with erosion rate although this sort of relation has been offered as support for non linear models of soil transport we use 1 d analytical hillslope profiles derived from existing soil transport laws to show that a model with soil flux linear in slope but including a slope stability threshold is indistinguishable from a non linear law within the scatter of our data catchment mean normalized channel steepness index increases monotonically though non linearly with erosion rate throughout the san gabriel mountains even where catchment mean hillslope angles have reached a threshold this non linearity can be mostly accounted for by a stochastic threshold incision model though additional factors likely contribute to the observed relationship between channel steepness and erosion rate these findings substantiate the claim that the normalized channel steepness index is an important topographic metric in active ranges

earthquake faulting as a structural process abstract structural geology is concerned with the history of movement in the earth s crust and the processes by which displacements occur in the upper one third to one half of deforming continental crust displacement is accommodated largely by seismic slip increments on existing faults it follows that earthquakes and related processes are an integral part of structural geology traditionally structural geologists have been preoccupied with the complexity of the finite deformation within fault zones and with the stress states prevailing at the initiation of faults in intact crust future structural work should be directed more towards understanding the dynamic character of fault reactivation during incremental slip and related effects questions of interest include rheological and geometrical controls on the initiation perturbation and termination of ruptures directivity effects associated with rupture propagation the recognition of structures resulting from repeated stress cycling within seismogenic crust and identification of structural features diagnostic of shear stress levels during faulting structures arising from the inter relationships between slip episodes and induced fluid flow are of special importance because these dynamic fault processes appear influential in the development of much fault hosted mineralization mesothermal gold quartz lodes hosted in high angle reverse shear zones of mixed brittle ductile character form illustrative examples of structures that arguably can only be interpreted by seismo structural analysis embodying the concepts listed above

reconstructing ice flow fields from streamlined subglacial bedforms a kriging approach the orientation of several landforms e g drumlins flutes crag and tails and mega scale glacial lineations records the direction of the overlying ice flow that created them populations of such features are used routinely to infer former ice flow patterns which serve as the building blocks of reconstructions of palaeo ice sheet evolution currently the conceptualisation of flow patterns from these flow direction records is done manually and qualitatively so the extractable glaciological information is limited we describe a kriging method with matlab code implementation that calculates continuous fields of ice flow direction convergence and curvature from the flow direction records and which yields quantitative results with uncertainty estimates we test the method by application to the subglacial bedforms of the tweed valley basin uk the results quantify the convergent flow pattern of the tweed palaeo ice stream in detail and pinpoint its former lateral shear margins and where ice flowed around basal bumps ice flow parameters retrieved by this method can enrich ice sheet reconstructions and investigations of subglacial till processes and bedform genesis

deep scientific dives in the japan and kuril trenches abstract in the summer of 1985 during the french japanese kaiko program ten dives to depths of 6000 m in the japan and kuril trenches were made in the newly launched submersible nautile the sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving seabeam mapping geomagnetic and gravimetric measurements and single channel seismic profiling the results of the dives provide new constraints on the geodynamics of these subduction zones in the japan and kuril trenches huge slump scars were observed on the landward slopes of the trenches slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material the slump scars allowed us to observe the internal structure of the margin the monoclinal structure on the northern japan trench margin deduced from the seismic profiles and dsdp drilling was confirmed several dives on kashima seamount confirmed that this volcano has recently been split into two parts by a normal fault system comparisons of lithology and paleontology on the two separated parts of the seamount were made deep sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches it can be concluded that the whole margin is venting fluids from depths of 2 3 km which is consistent with the indications of overpressure observed in drill sites on the japan trench margin the fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

the off shore continuation of an active basaltic volcano piton de la fournaise reunion island indian ocean structural and geomorphological interpretation from sea beam mapping abstract in 1984 a sea beam survey of the submarine east flank of piton de la fournaise was performed with r v jean charcot three main types of volcanic or volcano tectonic features have been identified 1 1 the subaerial ne and se volcanic rift zones of piton de la fournaise do not extend more than about five kilometers offshore unlike typical hawaiian rift zones which form narrow 2 4 km ridges extending tens of kilometers from the summit the active rift zones of piton de la fournaise widen downslope attaining more than 10 km at their front 2 2 the submarine extension of the grand brule slide is larger than the subaerial portion the entire slide forms a 7 24 km scar bounded by two ramparts to the north and south the slumped material may have moved as a debris flow forming a large talus downslope of the slide however the submarine counterpart of the south area of the grand brule seems to be composed by a slumped block whose structure is apparently not disturbed 3 3 another prominent feature is a conspicuous topographic high that occupies nearly all the center of the surveyed zone this cast flank submarine plateau cannot be associated with any active structure of piton de la fournaise its surface generally dips gently 2 3 and its northern and southern flanks are extensively cut by landslides cones of variable dimensions are observed on the plateau and farther to the east three hypotheses are examined to account for the origin of this morphology a remnant flank of an ancestral fournaise volcano associated with a large buried intrusion found by drilling beneath the grand brule b distinct volcanic massif or c material of a huge and ancient landslide the geophysical data show that the western part of this submarine plateau is reversely magnetized and associated with a moderate positive gravity anomaly this confirms that as suggested by the bathymetric analysis this part of the plateau is relatively coherent conversely the eastern portion of the plateau appears to be poorly magnetized and composed of low density material probably chaotic materials derived from landslides the reversed magnetization of the western part shows that the whole structure is older than 0 7 ma these results show that the volcanic history of piton de la fournaise is more complex than previously thought

a generalized power law approximation for fluvial incision of bedrock channels 1 sediment flux is known to influence bedrock incision rates in mountain rivers although the widely used stream power incision model lacks any explicit representation of sediment flux the model appears to work in a variety of real settings we address this apparent contradiction using numerical experiments to explore the morphology of fluvial landscapes evolved with four different incision models three of which include the influence of sediment flux on incision rate the numerical landscapes have different spatial patterns of uplift and are at steady state we analyze these landscape using the common stream power approach which views incision rates to be primarily a function of the local channel gradient s and the upstream drainage area a we find that incision rates i for these landscapes are well described by an empirical power law equation i k am sn this equation is functionally equivalent to the widely used stream power model with the important distinction that the parameters k m and n are entirely empirical these parameters take on constant values within a single landscape but can otherwise be quite different between landscapes mainly due to differences in the pattern of rock uplift within the drainage in particular the parameters m and n decrease as the rate of rock uplift becomes more focused in the upland part of a mountain belt the parameter m is particularly important in that it describes the sensitivity of a tectonically active mountain belt to changes in precipitation or tectonic accretion it also defines how incision rates will change as the discharge becomes flashier

modelling landscape evolution geomorphology is currently in a period of resurgence as we seek to explain the diversity origins and dynamics of terrain on the earth and other planets in an era of increased environmental awareness yet there is a great deal we still do not know about the physics and chemistry of the processes that weaken rock and transport mass across a planet s surface discovering and refining the relevant geomorphic transport functions requires a combination of careful field measurements lab experiments and use of longer term natural experiments to test current theory and develop new understandings landscape evolution models have an important role to play in sharpening our thinking guiding us toward the right observables and mapping out the logical consequences of transport laws both alone and in combination with other salient processes improved quantitative characterization of terrain and process and an ever improving theory that describes the continual modification of topography by the many and varied processes that shape it together with improved observation and qualitative and quantitative modelling of geology vegetation and erosion processes will provide insights into the mechanisms that control catchment form and function this paper reviews landscape theory in the form of numerical models of drainage basin evolution and the current knowledge gaps and future computing challenges that exist copyright 2010 john wiley sons ltd

modeling relative frost weathering rates at geomorphic scales abstract frost damage is a powerful agent of geomorphic change cracks can grow when the ice pressure in pores reaches a threshold that depends on matrix properties and crack geometry mineral surfaces that are preferentially wetted by liquid water rather than ice are coated by premelted liquid at a pressure that is lower than the ice pressure because this pressure difference increases as the temperature cools when the ice pressure is effectively pinned at the cracking threshold temperature gradients induce gradients in liquid pressure that draw water towards colder temperatures porosity increases and frost damage accumulates in regions where water supplies crack growth to apply this understanding over the large spatial and temporal scales that are relevant to evolving landscapes we develop a simple model that tracks porosity changes our central assumption is that frost damage is correlated with porosity increases under conditions where frost cracking takes place accordingly we account for the permeability reductions with decreased temperature that accompany ice growth along porous pathways and derive general expressions for the porosity change through time at particular depths as well as the total porosity increase through all depths beneath a point at the ground surface over the time during which cracking occurs each year to illustrate the resulting patterns of frost weathering we consider a general case in which the permeability has a power law dependence on temperature and the annual surface temperature variation is sinusoidal we find that the degree of frost damage generally decreases with depth except at localized depths where damage is elevated because the rock spends longer times near the threshold for cracking leading to enhanced water supply in comparison with neighboring regions the magnitude of the net expansion that results from porosity changes at all depths beneath the ground surface is increased for seasonal thermal cycles with larger amplitudes with a broad maximum centered on a mean annual temperature near the threshold required for crack growth warmer mean annual temperatures lead to less damage because of the reduction in time during which it is cold enough for cracking whereas colder mean annual temperatures are accompanied by reduced water supply due to the temperature dependence of permeability all of the controlling parameters in our model are tied explicitly to physical properties that can in principle be measured independently which suggests promise for informing geomorphic interpretations of the role of frost weathering in evolving landforms and determining erosion rates

a new type of active margin the convergent extensional margin as exemplified by the middle america trench off guatemala the results of dsdp legs 67 and 84 during which a transect of holes was drilled across the middle america trench off guatemala show no accretion at the front of that subduction zone since early eocene time the tectonic evolution of the trench includes extensional structure on the landward as well as the seaward trench slopes the guatemalan margin is proposed as the model of a new type of active margin the convergent extensional active margin ce margin

the importance of earthquake induced landslides to long term slope erosion and slope failure hazards in seismically active regions abstract this paper describes a general method for determining the amount of earthquake induced landsliding that occurs in a seismically active region over time this determination can be used as a quantitative measure of the long term hazard from seismically triggered landslides as well as a measure of the importance of this process to regional slope erosion rates and landscape evolution the method uses data from historical earthquakes to relate total volume of landslide material dislodged by an earthquake to the magnitude m and seismic moment m0 of the earthquake from worldwide data a linear regression relation between landslide volume v and m0 is determined as v m0 1018 9 0 13 where v is measured in m3 and m0 is in dyn cm to determine the amount of earthquake generated landsliding over time this relation is combined with data on seismic moment release for a particular region which may be derived from either earthquake history or fault slip data the form of the m0 v relation allows the rate of production of earthquake induced landslides over time to be determined from total rate of seismic moment release without regard to the distribution of individual events thus simplifying and generalizing the determination application of the method to twelve seismically active regions with areas ranging from 13 275 to 2 308 000 km2 shows that erosion rates from earthquake induced landslides vary significantly from region to region of the regions studied the highest rates were determined for the island of hawaii new zealand western new guinea and the san francisco bay region of california significantly lower rates were determined for iran tibet the sierra nevada great basin region of california and central japan for the time period from 715 ad to the present intermediate rates were determined for peru southern california onshore california turkey and central japan for the time period from 1586 ad to the present to determine the relative long term importance of seismically triggered landslides these erosion rates are compared to erosion rates calculated for other slope processes and to rates calculated from fluvial sediment discharge comparisons with other slope processes indicate that earthquake induced landslides are the predominant agents of slope erosion on the island of hawaii in the san francisco bay region and in western new guinea for hawaii the san francisco bay region and sierra nevada great basin region of california the erosion rates calculated for earthquake induced landslides also exceed the regional erosion rates calculated from fluvial sediment discharge

environmental signal propagation in sedimentary systems across timescales abstract earth surface processes operate across erosionally dominated landscapes and deliver sediment to depositional systems that can be preserved over a range of timescales the geomorphic and stratigraphic products of this source to sink sediment transfer record signals of external environmental forcings as well as internal or autogenic dynamics of the sedimentary system here we evaluate environmental signal propagation across sediment routing systems with emphasis on sediment supply qs as the carrier of up system forcings we review experimental numerical and natural examples of source to sink sediment routing and signal propagation during three timescales 1 historic which includes measurement and monitoring of events and processes of landscape change and deposition during decades to centuries 2 centuries to several millions of years referred to as intermediate timescale and 3 deep time we discuss issues related to autogenic dynamics of sediment transport transient storage and release that can introduce noise lags and or completely mask signals of external environmental forcings we provide a set of conceptual and practical tools for evaluating sediment supply within a source to sink context which can inform interpretations of signals from the sedimentary record these tools include stratigraphic and sediment routing system characterization sediment budgets geochronology detrital mineral analysis e g thermochronology comparative analog approaches and modeling techniques to measure calculate or estimate the magnitude and frequency of external forcings compared to the characteristic response time of the sediment routing systems

a giant submarine slope failure on the northern insular slope of puerto rico abstract a large amphitheater shaped scarp approximately 55 km across was imaged on the northern insular slope of puerto rico using long range sidescan sonar and bathymetric data this scarp results from the removal of more than 1500 km 3 of tertiary strata a review of seismic reflection profiles stratigraphic data and subsidence models of the northern insular margin of puerto rico were used to infer that large scale slope failure was induced by the tectonic oversteepening of the insular slope and was responsible for the formation of the scarp the oversteepening probably was caused by the most recent episode of convergence of the caribbean and north american plates which began between approximately 4 and 2 5 m y ago the tertiary strata have been tilted approximately 4 5 to the north in the last 4 m y

on the history of humans as geomorphic agents the human population has been increasing exponentially simultaneously as digging sticks and antlers have given way to wooden plows iron spades steam shovels and today s huge excavators our ability and motivation to modify the landscape by moving earth in construction and mining activities have also increased dramatically as a consequence we have now become arguably the premier geomorphic agent sculpting the landscape and the rate at which we are moving earth is increasing exponentially as hunter gatherer cultures were replaced by agrarian societies to feed this expanding population erosion from agricultural fields also until recently increased steadily this constitutes an unintended additional human impact on the landscape

reconstruction of subduction zone paleogeometries and quantification of upper plate material losses caused by tectonic erosion we propose in this paper an original method for quantifying the material losses at subduction zones mainly based on vertical movements of the active margins we first test the method using isostatic and elastic models applying it to 15 different well constrained convergent margins the main conclusion is that most of them are presently close to the isostatic equilibrium consequently we consider as a first hypothesis that such was also the case in the past and that erosion or accretion are natural mechanisms restoring the equilibrium in response to a perturbation then we apply this model to compute the geometry of the palcoslab knowing the palcotopography of the margin the comparison between present and past ocean margin lithospheric wedge geometries provides estimates of the removed material that constituted the initial wedge two cases were studied for palcoreconstructions japan and peru the results show that a considerable amount of material was removed during the neogene along both margins a detailed interpretation of the neogene history of the japan margin is proposed on the basis of our results

controls of glacial valley spacing on earth and mars abstract low order alpine glacial valleys on earth commonly have a characteristic spacing of 1 3 km here we develop analytic and numerical solutions of a coupled numerical model for alpine glacial flow and subglacial bedrock erosion to quantitatively determine the controls on glacial valley spacing assuming an initially undissected landscape an initially fluvially dissected landscape and an initially cratered landscape the characteristic spacing of glacial valleys produced by the model is controlled by a competition between the thickening of ice in incipient glacial valleys which acts to enhance flow and valley deepening and viscous sidewall drag which acts to limit flow and deepening the glacial valley spacing that represents the best compromise between these two competing effects is found to be a function of valley slope the threshold basal shear stress for ice motion the effective ice viscosity a bed friction parameter and gravity this model framework provides the preliminary basis for understanding the relative spacing of glacial valleys on earth and mars on mars montane glacial valleys have widths and spacings that are approximately 10 20 times larger than those on earth model results suggest that this difference is predominantly a consequence of lower bed slopes and larger temperature controlled ice viscosities of glacial ice on mars compared to earth

the giant submarine alika debris slide mauna loa hawaii probably represent multiple subsidence events before during and after the debris avalanches lower slopes of the slide contain distinctive lobate terraced deposits that are interpreted as having been emplaced more slowly prior to the debris avalanches estimated thicknesses of 50 200 m suggest volumes of 200 600 km 3 for the two lobes the combined volume of the entire slide and slump terrane is probably 1500 2000 km 3 the slide deposits predate a 13 ka coral reef and probably postdate the block faulted ninole basalt roughly dated as a few hundred thousand years old the alika slide or a similar deposit recognized on gloria images further north along the hawaiian ridge probably triggered a giant wave that washed 325 m high on lanai at about 100 ka slumping on mauna loa has been most intense adjacent to the large arcuate bend in its southwest rift zone as the rift zone migrated westward away from the growing kilauea volcano slumping events were probably triggered by seismic activity accompanying dike injection along the rift zone such massive slumps landslides and distal submarine turbidity flows appear to be widespread on the flanks of hawaiian volcanoes

the human role in changing fluvial systems retrospect inventory and prospect historical and modern scientific contexts are provided for the 2006 binghamton geomorphology symposium on thehuman role in changing fluvial systems the 2006 symposium provides a synthesis of research concerned with human impacts on fluvial systems including hydrologic and geomorphic changes to watersheds while also commemorating the 50th anniversary of the 1955man s role in changingthe face of the earth symposium thomas jr w l ed 1956a man srole in changing the face of the earth univ chicago press chicago 1193 pp this paper examines the 1955symposium from the perspective of humanimpacts on rivers reviews current inquiry on anthropogenic interactions in fluvial systems and anticipates future directions in this field although the 1955 symposium did not have an explicit geomorphic focus it set the stage for many subsequent anthropogeomorphic studies the 1955 conference provided guidance to geomorphologists by recommending and practicing interdisciplinary scholarship through the use of diverse methodologies applied at extensive temporal and geographical scales and through its insistence on an integrated understanding of human interactions with nature since 1956 research on human impacts to fluvial systems has been influenced by fundamental changes in why the research is done what is studied how river studies are conducted and who does the research rationales for river research are now driven to a greater degree by institutional needs environmental regulations and aquatic restoration new techniques include a host of dating spatial imaging and ground measurement methods that can be coupled with analytical functions and digital models these new methods have led to a greater understanding of channel change variations across multiple temporal and spatial scales and integrated watershed perspectives all changes that are reflected by the papers in this volume these new methods also bring a set of technical demands for the training of geomorphologists the 2006 binghamton geomorphology symposium complements the 1956 symposium by providing a more specific and updated view of river systems coupled with human interactions the symposium focuses on linkages between human land use structures and channel modification with geomorphology hydrology and ecology the emergence of sustainability as a central policy guideline in environmental management should generate greater interest in geomorphic perspectives especially as they pertain to human activities the lack of theories of anthropogeomorphic change however presents a challenge for the next generation of geomorphologists in this rapidly growing subfield

late miocene seep carbonates and fluid migration on top of the montepetra intrabasinal high northern apennines italy relations with synsedimentary folding abstract during the miocene hydrocarbon seep carbonates located atop intrabasinal highs and associated with sediment instability formed commonly at the deformation front of the northern apennine collisional orogen the parallelism between the structural trend and the distribution of seep carbonates suggests a close relationship between tectonics and gas fluid emission the montepetra intrabasinal high was formed during the closure stage of the foredeep being related to the synsedimentary growth of an anticline field geometry suggests that detachment folding was the leading mechanism of anticline growth and synsedimentary instability along the anticline flanks ten different bodies of seep carbonates occur in the tortonian early messinian sediments nine in the hinge zone and one in the southern backlimb of the anticline foraminiferal study geochemistry facies investigation and the three dimensional geometry of carbonate bodies with respect to the encasing terrigenous sediments indicate a protracted late tortonian early messinian activity of fluid migration with re mobilization and ascent of sediments from the core of the anticline stabilization of chemosynthesis related communities and in situ brecciation seepage atop the intrabasinal high was fed by different circuits one related to the compaction dewatering of shallow tortonian early messinian sediments and a deeper one related to the deformation of the anticline core and to the activity of detachment surfaces and of faults propagating through the sedimentary cover

nankai trough and zenisu ridge a deep sea submersible survey abstract eight submersible dives between 3000 and 4200 m water depth were made off southern japan in the eastern nankai subduction zone benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone at the toe of the accretionary prism a benthic community was also discovered along a zone of active compression at the foot of zenisu ridge 30 km south of nankai trough temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there studies of water samples indicate advection of methane and light hydrocarbons specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption thus the benthic communities are related to overpressure driven fluid advection along tectonic zones with active surface deformation a 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280 direction which is close to the 305 average direction of subduction in this area the dives establish further that compressive deformation is presently occurring at the foot of zenisu ridge the previous interpretation of the zenisu ridge as a zone of recent north south intraplate shortening 40 km south of the nankai trench is confirmed we conclude that tectonic evolution might well lead to future detachment of the zenisu ridge and overthrusting of this large piece of oceanic crust over the continental margin such a process might be an efficient one to emplace ophiolites over continents

vegetation modulated landscape evolution effects of vegetation on landscape processes drainage density and topography 1 topography acts as a template for numerous landscape processes that include hydrologic ecologic and biologic phenomena these processes not only interact with each other but also contribute to shaping the landscape as they influence geomorphic processes we have investigated the effects of vegetation on thresholds for channel initiation and landform evolution using both analytical and numerical approaches vegetation is assumed to form a uniform ground cover runoff erosion is modeled based on a power function of excess shear stress in which shear stress efficiency is inversely proportional to vegetation cover this approach is validated using data plant effect on slope stability is represented by additional cohesion provided by plant roots vegetation cover is assumed to reduce sediment transport rates due to physical creep processes rainsplash dry ravel and expansion and contraction of sediments according to a negative exponential relationship vegetation grows as a function of both available cover and unoccupied space by plants and is killed by geomorphic disturbances runoff erosion and landsliding and wildfires analytical results suggest that in an equilibrium basin with a fixed vegetation cover plants may cause a transition in the dominant erosion process at the channel head a runoff erosion dominated landscape under none or poor vegetation cover may become landslide dominated under a denser vegetation cover the sign of the predicted relationship between drainage density and vegetation cover depends on the relative influence of vegetation on different erosion phenomena with model parameter values representative of the oregon coast range ocr numerical experiments conducted using the channel hillslope integrated landscape development child model confirm the findings based on the analytical theory a highly dissected fluvial landscape emerges when surface is assumed bare when vegetation cover is modeled landscape relief increases resulting in hollow erosion dominated by landsliding interestingly our simulations underscore the importance of vegetation disturbances by geomorphic events and wildfires on the landscape structure simulated landscapes resemble real world catchments in the ocr when such disturbances are considered

abrupt changes in the rate of andean plateau uplift from reversible jump markov chain monte carlo inversion of river profiles abstract knowledge of the rate of surface uplift of the central andean plateau provides important boundary conditions for regional geodynamic models and paleoclimate reconstructions here we present a fully nonlinear inverse method to extract the rock uplift history of the central andean plateau from the cotahuasi river and its tributaries our approach is based on an analytical solution to the stream power model that relates elevation to rock uplift history under the assumptions of constant climatic conditions in space and time and spatially invariant rock uplift we use a bayesian framework that allows us to quantify the full state of knowledge of model parameters i e uncertainties trade offs the erosional efficiency defines the landscape response time and this must be determined using independent data therefore using thermochronometric data from the cotahuasi ocona canyon which record rapid fluvial incision at 13 10 ma we calibrate our results to infer a rock uplift history for the north central andean plateau we infer a pulse of rock uplift between 25 and 10 ma at rates as high as 0 25 km my however the rock uplift rate inferred during this pulse is not as high as is predicted if the central andean plateau grew as a result of loss of unstable lithospheric mantle therefore our results are more consistent with models requiring gradual uplift of the central andean plateau

the cenozoic tectonostratigraphic evolution of the barracuda ridge and tiburon rise at the western end of the north america south america plate boundary zone the barracuda ridge and the tiburon rise two major oceanic basement ridges lie at the western end of the diffuse north america south america plate boundary zone where they enter the subduction zone beneath the lesser antilles island arc there is a large degree of uncertainty in the motion between the north american and south american plates predicted by kinematic models of plate motion for the region of these two ridges during the cenozoic and quaternary from the analysis of new multibeam and seismic reflection profiles acquired in 2007 together with older geophysical and geological data we provide new information on the geological history of this area including the timing of the formation of the barracuda ridge and tiburon rise in their present day configurations the timing of the deformation in this region is now much better constrained through the correlation of several key seismic horizons with existing dsdp and odp holes the seafloor topography inherited from the process of formation of the crust at the mid oceanic ridge was buried by distal turbidites by the end of the paleogene beginning in the middle late miocene and then the pleistocene the tiburon rise and barracuda ridge respectively were uplifted and acquired their present day forms and elevation which is much more recent than previously believed in the quaternary the uplift was accompanied by the deposition of very large mass transport deposits the causes of uplift and deformation of the ridges have been convergence between the north american and south american plates and the flexure of these plates as they enter the lesser antilles subduction zone the zone of uplift and deformation migrated northward during the neogene and quaternary

field measurements of incision rates following bedrock exposure implications for process controls on the long profiles of valleys cut by rivers and debris flows until recently published rates of incision of bedrock valleys came from indirect dating of incised surfaces a small but growing literature based on direct measurement reports short term bedrock lowering at geologically unsustainable rates we report observations of bedrock lowering from erosion pins monitored over 1 7 yr in 10 valleys that cut indurated volcanic and sedimentary rocks in washington oregon california and taiwan most of these channels have historically been stripped of sediment their bedrock is exposed to bed load abrasion plucking and seasonal wetting and drying that comminutes hard intact rock into plates or equant fragments that are removed by higher fl ows consequent incision rates are proportional to the square of rock tensile strength in agreement with experimental results of others measured rates up to centimeters per year far exceed regional long term erosionrate estimates even for apparently minor sediment transport rates cultural artifacts on adjoining strath terraces in washington and taiwan indicate at least several decades of lowering at these extreme rates lacking sediment cover lithologies at these sites lower at rates that far exceed long term rock uplift rates this rate disparity makes it unlikely that the long profi les of these rivers are directly adjusted to either bedrock hardness or rock uplift rate in the manner predicted by the stream power law despite the observation that their profi les are well fi t by power law plots of drainage area vs slope we hypothesize that the threshold of motion of a thin sediment mantle rather than bedrock hardness or rock uplift rate controls channel slope in weak bedrock lithologies with tensile strengths below 3 5 mpa to illustrate this hypothesis and to provide an alternative interpretation for power law plots of area vs slope we combine shields threshold transport concept with measured hydraulic relationships and downstream fi ning rates in contrast to fl uvial reaches none of the hundreds of erosion pins we installed in steep valleys recently scoured to bedrock by debris fl ows indicate any postevent fl uvial lowering these results are consistent with episodic debris fl ows as the primary agent of bedrock lowering in the steepest parts of the channel network above 0 03 0 10 slope

the indication of martian gully formation processes by slope area analysis the formation process of recent gullies on mars is currently under debate this study aims to discriminate between the proposed formation processes pure water flow debris flow and dry mass wasting through the application of geomorphological indices commonly used in terrestrial geomorphology high resolution digital elevation models dems of earth and mars were used to evaluate the drainage characteristics of small slope sections data from earth were used to validate the hillslope debris flow and alluvial process domains previously found for large fluvial catchments on earth and these domains were applied to gullied and ungullied slopes on mars in accordance with other studies our results indicate that debris flow is one of the main processes forming the martian gullies that were being examined the source of the water is predominantly distributed surface melting not an underground aquifer evidence is also presented indicating that other processes may have shaped martian crater slopes such as ice assisted creep and solifluction in agreement with the proposed recent martian glacial and periglacial climate our results suggest that within impact craters different processes are acting on differently oriented slopes but further work is needed to investigate the potential link between these observations and changes in martian climate

introduction to the special issue connectivity in geomorphology abstract with the increased recent attention on understanding the complex pathways and fluxes of material energy and organisms and the multidimensional coupling between various geomorphic process domains the 47th binghamton geomorphology symposium was organized around the theme of connectivity in geomorphology six key subthemes emerged sediment connectivity hydrologic connectivity geochemical connectivity riverine connectivity landscape connectivity and modeling of connectivity papers presented at the symposium and collected in this special issue elucidate the theoretical and conceptual underpinnings of connectivity but also provide field based and analytical approaches to document the fundamental role of connectivity in geomorphic and ecological functioning among the consistent themes that emerged from the papers and from discussion during the symposium were 1 the interconnectedness of diverse forms of connectivity 2 the appropriateness or not of conceptualizing connectivity as a gradient or continuum and 3 the importance of an explicit recognition of disconnectivity as a mechanism to retain ecologically important materials and organisms within landscapes knowledge gaps identified in discussions during the symposium include identifying questions that can be usefully addressed within a framework emphasizing connectivity identifying feedbacks that enhance and limit connectivity and quantification of diverse aspects of connectivity of thresholds of connectivity and of interactions among different types of connectivity

natural experiments in landscape evolution a natural experiment in landscape evolution is a case study of landform development in which only one element varies significantly and for which the driving forces initial conditions and or boundary conditions are well constrained natural experiments provide a means of testing landscape evolution theory on the large space and time scales to which that theory applies natural experiments can involve either steady or transient conditions cases with steady conditions allow one to test predictions about the relationships among topography erosion rates and various attributes related to climate and material properties transient cases are valuable for distinguishing between models whose predictions might be similar and therefore indistinguishable under steady conditions essential ingredients of a natural experiment include minimal variation in all but one factor good constraints on timing and or rates well characterized processes and high quality topographic data other useful ingredients include information about intermediate topographic states such as a former valley profile revealed by strath terraces and knowledge of the time history of erosion rates in order to deepen our understanding of the physics and chemistry of long term landscape evolution there is a pressing need to identify natural experiments and develop the necessary databases to take advantage of them copyright 2009 john wiley sons ltd

hawaiian oral tradition describes 400 years of volcanic activity at kīlauea article i nfo culturally significant oral tradition involving pele the hawaiian volcano deity and her youngest sister hi iaka may involve the two largest volcanic events to have taken place in hawai i since human settlement the roughly 60 year long ail au eruption during the 15th century and the following development of k lauea s caldera in 1823 rev william ellis and three others became the first europeans to visit k lauea s summit and were told stories about k lauea s activity that are consistent with the pele hi iaka account and extend the oral tradition through the 18th century recent geologic studies confirm the essence of the oral traditions and illustrate the potential value of examining other hawaiian chants and stories for more information about past volcanic activity in hawai i published by elsevier b v

stratigraphic significance of siliceous microfossils collected during nautiperc dives off peru 5 6 s the geological evolution of the northern peru convergent margin can be traced using samples collected during deep sea dives of the submersible nuutile in the paita area 5 6 s the sedimentary sequence was intensively sampled along the main scarp of the middle slope area it consists of upper miocene 7 9 ma to pleistocene siltstone sandstone and rare dolostone the age distribution of these samples is the basis for a new geologic interpretation of the multichannel seismic line cdp3 siliceous microfossils both diatoms and radiolarians show influence of both cold and temperate waters local species mixed with upwelling ones diatom assemblages studied from the npl 13 and npl 15 dives bear a strong resemblance to assemblages from the pisco formation of southern peru micropaleontological data from siliceous microfossils provide evidence for two main unconformities one is at the base of the quatemary sequence and the other corresponds to a hiatus of 1 myr separating the upper miocene 7 8 ma sediments from uppermost miocene 5 6 ma sediments during the past 400 kyr a wide rollover fold developed in the middle slope area associated with a major seaward dipping detachment fault a catastrophic debris avalanche occurred as the result of an oversteepening of the landward flank of the rollover fold the gravity failure of the slope recognized by seabeam and hydrosweep mapping displaced enough material to produce a destructive tsunami which occurred 13 8 f 2 7 kyr ago

emergence and pseudo equilibrium in geomorphology abstract many geomorphic system states and behaviors often interpreted as tendencies toward establishment and maintenance of steady state equilibrium are actually emergent outcomes of two simple principles gradient selection and threshold mediated modulation the principle of gradient selection is simply that geomorphic features associated with gradient driven flows persist and grow relative to other features and pathways the principle of threshold mediated modulation reflects the inherent limits on system development along any particular pathway thresholds not only define a restricted state space for any given geomorphic system but may also result in oscillatory behavior around an intermediate condition that resembles fluctuations around a steady state equilibrium together these principles often produce outcomes that mimic steady state equilibria examples are given involving several aspects of fluvial systems channel profiles alluvial channel changes and drainage density hillslope gradients and barchan dunes steady states indeed occur in earth surface systems and are a useful simplification in some models however the assumption that such states are somehow normative or the only natural condition is incorrect and can lead to problems in geomorphic interpretations environmental restoration and management and conceptions of how earth systems work

geochemical and biogeochemical observations on the biological communities associated with fluid venting in nankai trough and japan trench subduction zones we have studied the compositions and structure of the lamellibranchiacalyptogena sp associated with fluid venting in subduction zones off japan the gills of thecalyptogena are the habitat of symbiotic bacteria which develop owing to chemoautotrophy they most probably assimilate hydrogen sulfide methane and or carbon dioxide and reduce nitrogen the isotopic budget of methane uptake enables to establish the thermal diagenetic origin of the light hydrocarbons brought by the upwelling fluids the microscopic examination of thin sections of the bacteria enables to establish that their membranes are compatible with uptake of inorganic nutrients and to establish relations with the metabolism of archeobacteria

self organized complexity in geomorphology observations and models the focus of this paper is to relate fundamental statistical properties of landforms and drainage networks to models that have been developed in statistical physics relevant properties and models are reviewed and a general overview is presented landforms and drainage networks are clearly complex but well defined scaling laws are found coastlines topography contours and lakes are classic self similar fractals the height of topography along a linear track is well approximated as a brownian walk a self affine fractal this type of behavior has also been found in surface physics for example the surface roughness of a fracture an applicable model is the langevin equation the heat equation with a stochastic white noise driver this model also reproduces the statistics of sediment deposition drainage networks were one of the original examples of self similar fractal trees an important advance in quantifying the structure of drainage networks is the application of the tokunaga fractal side branching statistics a classic problem in statistical physics is the diffusion limited aggregation the resulting tree like structures have been shown to also satisfy the tokunaga statistics a modified version of the diffusion limited aggregation model reproduces the statistics of drainage networks it is concluded that the models developed in statistical physics have direct applicability to the fundamental problems in geomorphology

the impact of humans on continental erosion and sedimentation rock uplift and erosional denudation of orogenic belts have long been the most impor tant geologic processes that serve to shape continental surfaces but the rate of geomor phic change resulting from these natural phe nomena has now been outstripped by human activities associated with agriculture con struction and mining although humans are now the most important geomorphic agent on the planet s surface natural and anthro pogenic processes serve to modify quite dif ferent parts of earth s landscape in order to better understand the impact of humans on continental erosion we have examined both long term and short term data on rates of sediment transfer in response to glacio fl u vial and anthropogenic processes phanerozoic rates of subaerial denuda tion inferred from preserved volumes of sedimentary rock require a mean conti nental erosion rate on the order of 16 m per million years m m y resulting in the accumulation of 5 gigatons of sediment per year gt yr erosion irregularly increased over the 542 m y span of phanerozoic time to a pliocene value of 53 m m y 16 gt yr current estimates of large river sediment loads are similar to this late neogene value and require net denudation of ice free land surfaces at a rate of 62 m m y 21 gt yr consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confi ned to drainage headwaters 83 of the global river sediment fl ux is derived from the high est 10 of earth s surface subaerial erosion as a result of human activity primarily through agricultural practices has resulted in a sharp increase in net rates of continental denudation although less well constrained than estimates based on surviving rock volumes or current river loads available data suggest that present farmland denudation is proceeding at a rate of 600 m m y 75 gt yr and is largely confi ned to the lower elevations of earth s land surface primarily along passive continental margins 83 of cropland erosion occurs over the lower 65 of earth s surface the conspicuous disparity between natu ral sediment fl uxes suggested by data on rock volumes and river loads 21 gt yr and anthropogenic fl uxes inferred from measured and modeled cropland soil losses 75 gt yr is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from erod ing croplands over the history of human agriculture accumulation of postsettlement alluvium on higher order tributary channels and fl oodplains mean rate 12 600 m m y is the most important geomorphic process in terms of the erosion and deposition of sedi ment that is currently shaping the landscape of earth it far exceeds even the impact of pleistocene continental glaciers or the cur rent impact of alpine erosion by glacial and or fl uvial processes conversely available data suggest that since 1961 global cropland area has increased by 11 while the global population has approximately doubled the net effect of both changes is that per capita cropland area has decreased by 44 over this same time interval 1 per year this is 25 times the rate of soil area loss antici pated from human denudation of cropland surfaces in a context of per capita food pro duction soil loss through cropland erosion is largely insignifi cant when compared to the impact of population growth

geomorphic inferences from regolith thickness chemical denudation and crn erosion rates near the glacial limit boulder creek catchment and vicinity colorado granitic regolith developed in the boulder creek catchment and adjacent areas records a history of deep weathering some of which may predate quaternary time field and well log measurements of weathering chemical denudation and rates of erosion derived from 10be cosmogenic radionuclide crn data help to quantify rates of landscape change in the post orogenic rocky mountains the density of oxidized fractured bedrock ranges from 2 7 to about 2 2 g cm 3 saprolite and grus have densities between 2 0 and 1 8 g cm 3 and 30 soil samples averaged 1 6 0 2 g cm 3 highly weathered regolith in 540 wells averages 3 3 m thick mean depth to bedrock in 1661 wells is 7 m and the weathered thickness exceeds 10 m in relatively large local areas east of the late pleistocene glacial limit thickness of regolith shows no simple relationship to rock type or structure local slope or distance from channels catchments in the vicinity of the boulder creek have an average crn erosion rate of 2 2 0 7 cm kyr 1 for the past 10 000 to 40 000 yr annual losses of cations and sio2 vary from about 2 to 5 g m 2 over a runoff range of 10 to nearly 160 cm using measured rates in simple box models shows that if a substantial fraction of void space is created by volume expansion in the weathering rock materials 7 m of weathered rock materials could form in as little as 230 kyr if density loss results mainly from chemical denudation and some volume expansion however the same weathering profile would take 1340 kyr to form rates of erosion measured by crn could be balanced by the rate of soil formation from saprolite if the annual solute loss from soil is 2 0 g m 2 and 70 of the density decrease from saprolite to grus and soil results from strain saprolite however forms from oxidized bedrock at a far slower rate and rates of saprolite formation cannot balance soil and grus losses to erosion the zone of thick weathered regolith is likely an eroding relict landscape the undulating surface marked by relatively low relief and tors is not literally a topographic surface of eocene oligocene or miocene age unless it was covered with deposits that were removed in pliocene or quaternary time

the saguenay fjord quebec canada integrating marine geotechnical and geophysical data for spatial seismic slope stability and hazard assessment in 1996 a major flood occurred in the saguenay region quebec canada delivering several km 3 of sediment to the saguenay fjord such sediments covered large areas of the until then largely contaminated fjord bottom thus providing a natural capping layer recent swath bathymetry data have also shown that sediment landslides are widely present in the upper section of the saguenay fjord and therefore should a new event occur it would probably expose the old contaminated sediments landslides in the upper saguenay fjord are most probably due to earthquakes given its proximity to the charlevoix seismic region and to that of the 1988 saguenay earthquake in consequence this study tries to characterize the permanent ground deformations induced by different earthquake scenarios from which shallow sediment landslides could be triggered the study follows a newmark analysis in which firstly the seismic slope performance is assessed secondly the seismic hazard analyzed and finally an evaluation of the seismic landslide hazard is made the study is based on slope gradients obtained from em1000 multibeam bathymetry data as well as water content and undrained shear strength measurements made in box and gravity cores ground motions integrating local site conditions were simulated using synthetic time histories the study assumes the region of the 1988 saguenay earthquake as the most likely source area for earthquakes capable of inducing large ground motions in the upper saguenay region accordingly we have analyzed several shaking intensities to deduce that generalized sediment displacements will begin to occur when moment magnitudes exceed 6 major displacements failure and subsequent landslides could occur only from earthquake moment magnitudes exceeding 6 75 8 2002 elsevier science b v all rights reserved

morphotectonic and morphometric analysis of the nazca plate and the adjacent offshore peruvian continental slope implications for submarine landscape evolution we use new swath bathymetry data acquired during the rv sonne cruise geopeco and complement them with swath data from adjacent regions to analyse the morphotectonics of the peruvian convergent margin the nazca plate is not covered with sediments and therefore has a rough surface along the entire peruvian trench the styles of roughness differ significantly along the margin with linear morphological features trending in various directions most of them oblique to the trench and roughness magnitudes of a few to several hundred meters the lower slope is locally very rough and at the verge of failure throughout the entire peruvian margin as a result of subduction erosion causing the lower slope to over steepen using curvature attributes to quantitatively examine the morphology in the yaquina and mendana areas revealed that the latter shows a larger local roughness both seaward and landward of the trench however the amplitude of morphological roughness is larger in the yaquina area we identified a 125 km2 large slump on the lima middle slope morphometric dating suggests an age of 74500 years within 35 to 40 error estimated incision rates on the upper slope are between 0 1 and 0 3 mm per year suggesting that landscape evolution on the peruvian submarine continental slope is similarly slow than that in the atacama desert

changes in the tectonic regime above a subduction zone of andean type the andes of peru and bolivia during the pliocene pleistocene this paper adresses the changes in the tectonic regime in the peruvian and bolivian andes that have occurred since the upper miocene when the present day elevation of the cordillera above sea level has been almost reached the stress patterns are deduced essentially from a field study of fault kinematics and a numerical inversion of the slip vector data measured on the fault planes the cuzco fault system in southern peru is chosen as an example to illustrate the methodology used in this region striations on both active and holocene faults are in agreement with a n s extension but faults affecting early pleistocene deposits exhibit two families of striations the younger results from the previous n s extension the older involving reverse motions results from either an e w or a n s compression faults affecting pliocene formations often show an oldest family of striations resulting from a ne sw or an e w trending extension thus three tectonic regimes are demonstrated which are also supported by regional unconformities and sedimentological data 1 a pliocene extensional regime 2 a lower pleistocene compressional regime and 3 a mid pleistocene present day extensional regime similar analyses conducted in the pacific and sub andean lowlands allow sketching of the successive pliocene pleistocene stress patterns in the central andes the quaternary and present day stress pattern is characterized by a n s extension in the high andes and in the pacific lowlands and by an e w compression in the sub andean lowlands and at the contact between the nazca and south american plates this stress pattern is interpreted at a large wavelength 100 km as an effect of compensated topography this model supposes that the vertical lithospheric stress zz increases with the topography the crustal thickness and the low density mantle beneath and that the lithospheric maximum compressional horizontal stress hmax trending e w roughly parallel to the convergence is fairly constant on both edges of the andes the tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then the e w trending hmax is 2 and hmin trending n s is 3 allowing extension to occur in this direction the pliocene stress pattern was characterized by a ne sw or an e w trending extension in the high andes in the pacific lowlands and possibly in the sub andean lowlands this stress pattern was clearly different from the present day one because the e w trending stress was hmin this required a weak push or eventually tractional boundary forces acting on the south american lithosphere it is suggested that this might result from a strong slab pull due to a long steeply dipping slab which decreased the value of the xx stress transmitted to the overriding plate the early pleistocene state of stress was compressional since the elevation of the andes had not markedly decreased during this period this required an increase of the e w trending stress value this resulted from a strong coupling between the two lithospheric plates possibly due to a rupture of a long slab under its own weight other spatial changes in the stress pattern are related to the particular situation of the forearc to the subduction of the buoyant nazca ridge and to the different dips of the slab extension in the high andes is of small magnitude of the order of 1 during the last 1 2 m y in a few basins it may have attained 40 during the pliocene 5 3 m y

the geometry and complexity of spatial patterns of terrestrial channel networks distinctive fingerprints of erosional regimes the morphology of channel networks related to long term erosion reflects the mechanisms involved in their formation this study aims to identify quantitative metrics drawn from topographic data and satellite imagery that are diagnostic of the distinctive styles of erosion by rivers glaciers subglacial meltwater and groundwater sapping from digital elevation models we identify three geometric metrics the minimum channel width channel aspect ratio longest length to channel width at the outlet and tributary junction angle we also characterize channel network complexity in terms of its stream order and fractal dimension to validate our approach we perform a principal component analysis pca on measurements of these five metrics on 70 channel networks we build understanding of these results in turn using scaling analyses of appropriate physical models we show that rivers glaciers and groundwater sapping erode the landscape in rigorously distinguishable ways whereas rivers are characterized by nearly constant minimum width variable aspect ratio and high stream orders glaciers have highly variable minimum widths and aspect ratios and much smaller stream orders erosion by subglacial meltwater remains poorly understood and we argue that we require an additional metric to fully characterize these systems our methodology can more generally be applied to identify the contributions of different processes involved in carving a channel network in particular we are able to identify transitions from fluvial to glaciated landscapes or vice versa

geomorphology complexity and the emerging science of the earth s surface the following is a white paper adapted here for print for the u s national research council s committee on challenges and opportunities in earth surface processes drafted at a national science foundation sponsored workshop associated with the 38th binghamton geomorphology symposium complexity in geomorphology held at duke university in october 2007

tectonic history of the northern peru convergent margin during the past 400 ka the late pleistocene tectonic history of the northern peru convergent margin can be retraced using data collected during deep sea dives of the submersible nautile combined with a recent hydrosweep survey conducted off peru at lat 5 6 s by the r v sonne during the past 400 ka a broad rollover fold formed in the middle slope area in association with a major seaward dipping detachment fault a catastrophic debris avalanche occurred as the result of oversteepening of the landward flank of the rollover fold the gravity failure of the slope previously recognized by seabeam mapping occurred at 13 8 2 7 ka and produced a destructive tsunami

slope instabilities at an active continental margin large scale polyphase submarine slides along the northern peruvian margin between 5 s and 6 s a mega submarine slide was evidenced along the peruvian margin during a seabeam survey of the r v charcot bourgois et al 1986 the debris slide was initially interpreted as the result of the slope failure occurring along a seaward curved scarp and extending from east to west along the line of maximum slope complementary bathymetrie data obtained from the r v sonne survey conducted to the north and to the south of the previous surveyed area has resulted in the identification of large scale polyphase submarine slides involving a total surface of about 1000 km2 between latitudes 5 15 s and 6 05 s using seabeam and hydrosweep multibeam echosounder data in combination with deep sea submersible observations three distinct slope failure types related to three main stages have been revealed the three sliding phases occurred roughly along the same trend orientated n230 and are therefore mainly controlled by the n80 orientation of the subducting nazca plate 1 the first phase of slope failure is documented by a debris avalanche deposit which extends from the lower slope down to 5 km within the trench floor the deposit originates from the northern wall of a wide valley located along the upper slope 2 the second phase of slope failure is characterized by a debris avalanche with a crescent shaped scar located along the middle slope and a hummocky deposit covering the lower slope and extending up to 10 km across the trench the volume of rock involved in this event is estimated to some 250 km3 the slope failure is assumed to be related to an oversteepening of the middle slope induced by a rollover deformation 3 the third phase of slope failure corresponds to a translational sliding block and a toppling block with volumes of 6 km3 and 13 km3 respectively the seismic energy produced during the seismic cycle has greatly increased facturation and fluids buildup along the area previously weakened by a rollover fold a restricted n s folding is observed in the vicinity of the trench to the north of the two debris avalanche deposits it may have formed in relation to the local compression limited to the subduction of the nazca plate sliding and folding thus document the paradox between the compressive regime in the lower plate and the extensional regime in the upper part of the upper plate

experimental evidence for hillslope control of landscape scale landscape evolution theory suggests that climate sets the scale of landscape dissection by modulating the competition between diffusive processes that sculpt convex hillslopes and advective processes that carve concave valleys however the link between the relative dominance of hillslope and valley transport processes and landscape scale is difficult to demonstrate in natural landscapes due to the episodic nature of erosion here we report results from laboratory experiments combining diffusive and advective processes in an eroding landscape we demonstrate that rainsplash driven disturbances in our experiments are a robust proxy for hillslope transport such that increasing hillslope transport efficiency decreases drainage density our experimental results demonstrate how the coupling of climate driven hillslope and valley forming processes such as bioturbation and runoff dictates the scale of eroding landscapes

tectonic and sedimentary evolution of the frontal part of an ancient subduction complex at the transition from accretion to erosion the case of the ligurian wedge of the northern apennines italy subduction can be associated either with accretion or with removal of material from the overriding plate these two processes can either coexist or alternate in time along the same margin their inception has the potential to change the dynamic equilibrium of a marginal wedge resulting in the development of out of sequence thrusts normal and strike slip faults or large submarine landslides in the frontal part of the subduction zone in this work we investigate the effects of the transition from frontal accretion to frontal erosion on the stability of a subduction complex through the study of an ancient example from the northern apennines ligurian subduction complex new structural data suggest that in the early neogene aquitanian the removal and underthrusting of the toe of the wedge formed by both the accreted oceanic sediments and the overlying wedge top basin fill i e the subligurian units implied a process of frontal tectonic erosion the presence on top of the subduction complex of a complete succession of mid late eocene to late miocene slope apron sediments i e the epiligurian succession facilitates a reconstruction of the sedimentary response to this event in the aquitanian large areas of the wedge were denuded from the lower slope sedimentary cover through extensive gravitational mass movements the subsequent deposition of a thick body of submarine debris flows is documented the mass wasting deposits are interpreted as the sedimentary response to the underthrusting of the frontal part of the ligurian subduction complex formed by the subligurian units

ocean drilling program leg 112 peru continental margin part 2 sedimentary history and diagenesis in a coastal upwelling environment on the shelf and upper slope off peru the signal of coastal upwelling productivity and bottom water oxygen is well preserved in alternately laminated and bioturbated diatomaceous quaternary sediments global sea level fluctuations are the ultimate cause for these cyclic facies changes during late miocene time coastal upwelling was about 100 km west of the present centers along the edge of an emergent structure that subsequently subsided to form the modern slope the sediments are rich in organic carbon and intense microbially mediated decomposition of organic matter is evident in sulfate reduction and methanogenesis these processes are accompanied by the formation of diagenetic carbonates mostly ca rich dolomites and mg calcites the downhole isotopic signatures of these carbonate cements display distinct successions that reflect the vertical evolution of the pore fluid environment from the association of methane gas hydrates burial depth and low chloride interstitial fluids we suggest an additional process that could contribute to the characteristic chloride depletion in pore fluids of active margins release of interlayer water from clays without a mineral phase change the shelf sediments also contain a subsurface brine that stretches for more than 500 km from north to south over the area drilled the source of the brine remains uncertain although the composition of the oxygen isotopes suggests dissolution of evaporites by seawater

prodigious submarine landslides on the hawaiian ridge the extensive area covered by major submarine mass wasting deposits on or near the hawaiian ridge has been delimited by systematic mapping of the hawaiian exclusive economic zone using the side looking sonar system gloria these surveys show that slumps and debris avalanche deposits are exposed over about 100 000 km2 of the ridge and adjacent seafloor from kauai to hawaii covering an area more than 5 times the land area of the islands some of the individual debris avalanches are more than 200 km long and about 5000 km3 in volume ranking them among the largest on earth the slope failures that produce these deposits begin early in the history of individual volcanoes when they are small submarine seamounts culminate near the end of subaerial shield building and apparently continue long after dormancy consequently landslide debris is an important element in the internal structure of the volcanoes the dynamic behavior of the volcanoes can be modulated by slope failure and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems the landslides are of two general types slumps and debris avalanches the slumps are slow moving wide up to 110 km and thick about 10 km with transverse blocky ridges and steep toes the debris avalanches are fast moving long up to 230 km compared to width and thinner 0 05 2 km they commonly have a well defined amphitheater at their head and hummocky terrain in the lower part oceanic disturbance caused by rapid emplacement of debris avalanches may have produced high level wave deposits such as the 365 m elevation hulopoe gravel on lanai that are found on several islands most present day submarine canyons were originally carved subaerially in the upper parts of debris avalanches subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters

estimating porosity of partially saturated sediments abstract the electrical resistivity of an unconsolidated sediment depends among other factors on the fraction of the pore space that is filled with fluid an empirical relation can be formed that relates measured resistivity and fluid content to the percent saturation the formula is applied to a core taken from a freshwater lake and the volume of gas in the sediment is estimated to a depth of 6 m into the lake bottom the procedures described can lead to improved estimates of porosity and bulk density

seabeam and deep sea submersible nautile surveys in the chiclayo canyon off peru 7 s subsidence and subduction erosion of an andean type convergent margin since pliocene times abstract a reconstruction of the subsidence history of the chiclayo canyon area off northern peru 7 s is proposed using geophysical data collected during the seaperc cruise in 1986 combined with geological data of a recent deep sea dive cruise with the submersible nautile the main results are 1 the detailed morphology and bathymetry of the chiclayo canyon 2 the recovery of samples from the palaeozoic metamorphic basement of the andean continental margin that crops out along the middle and upper slope areas at depths ranging from 2400 to 4000 m below sea level which means that the andean metamorphic basement extends seaward to a site 19 km landward from the trench axis 3 the main subaerial unconformity which extends across the margin from the upper slope to the lower middle slope boundary is of pliocene age 4 the rates of subsidence of the middle and upper slope areas are 713 and 452 m ma during the past 5 2 ma respectively and 5 the estimated amount of solid material removed by subduction erosion is between 128 and 526 km3 km 1 during the past 5 1 5 3 ma

active tectonics of the andes nearly 90 mm a 1 of relative plate convergence is absorbed in the andean plate boundary zone the pattern of active tectonics shows remarkable variations in the way in which the plate slip vector is partitioned into displacement and strain and the ways in which compatibility between different segments is solved along any traverse across the plate boundary zone the sum of relative velocities between points must equal the relative plate motion we have developed a kinematic synthesis of displacement and strain partitioning in the andes from 47 s to 5 n relevant for the last 5 ma based upon 1 relative plate motion deduced from oceanic circuits giving a roughly constant azimuth between 075 and 080 2 moment tensor solutions for over 120 crustal earthquakes since 1960 3 structural studies of deformed plio pleistocene rocks 4 topographic geomorphic studies 5 palaeomagnetic data and 6 geodetic data we recognize four neotectonic zones with subzones and boundary transfer zones that are partitioned in different ways these zones are not coincident with the classic zones defined by the presence or absence of a volcanic chain or differences in finite displacements and strains and tectonic form the long term segmentation and finite evolution of the andes may not occur in constantly defined segments in space and time in segment 1 47 39 s the slip vector is partitioned into roughly orthogonal benioff zone slip with large magnitude large slip surface earthquakes and both distributed dextral shear giving clockwise rotations of up to 50 and dextral slip in the curved liquine ofqui fault system giving 5 10 of anticlockwise fore arc rotation in segment 2 39 20 s the slip vector is partitioned into benioff zone slip roughly parallel with the slip vector andean crustal shortening and a very small component of dextral slip including that on the atacama fault system between 39 and 34 s a cross strike dextral transfer which deflects the chile trench and the volcanic arc absorbs the shortening contrast between segments 1 and 2 in segment 3 20 6 s the slip vector is partitioned into roughly orthogonal benioff zone slip crustal shortening sinistral trench parallel faulting and northeast southwest extension compatibility between segments 2 and 3 is maintained by the sinistral east southeast trending cochabamba shear zone and north trending dextral faults in segment 4 6 s to 5 n the slip vector is partitioned into roughly orthogonal benioff zone slip and dextral strike slip faulting in the fore arc and volcanic chain

structural anatomy of the ligurian accretionary wedge monferrato nw italy and evolution of superposed melanges we document in this study the internal structure of the late cretaceous late oligocene ligurian accretionary wedge in northwestern italy and the occurrence in this exhumed wedge of broken formation and three different types of melanges that formed sequentially through time the broken formation is the oldest unit in the accretionary wedge and shows bedding parallel boudinage structures which developed as a result of layer parallel extension at the toe of the internal part of the alpine wedge front during the late cretaceous middle eocene this broken formation experienced an overprint of tectonic diapiric and sedimentary processes as a result of continental collision in the late oligocene the ne vergent thrusting and associated shortening produced a structurally ordered block in matrix fabric through mixing of both native and exotic blocks forming the tectonic melange the concentration of overpressurized fluids along the thrust fault planes triggered the upward rise of shaly material producing the diapiric melange which in turn provided the source material for the downslope emplacement of the youngest late oligocene sedimentary melange the sedimentary melange units unconformably cover the collisional thrust faults constraining the timing of both this episode of contractional deformation related to continental collision and the combination and overlap of tectonic diapiric and sedimentary processes our multiscale structural analysis of the ligurian accretionary wedge shows that tectonic diapiric and sedimentary processes played a significant role in its evolution and that the interplay between and the superposition of these different processes strongly controlled the dynamic equilibrium of the accretionary wedge in the nw apennines western alps this kind of polygenetic melange development may be common in many modern and ancient accretionary complexes and the processes involved in their formation are likely to be responsible for major tsunamic events in convergent margins