quantitative seismostratigraphic inversion of a prograding delta from seismic data quantitative stratigraphy predicts the geometry of geologic strata and their petrophysical properties based on mathematical models of geological processes and parameters which affect the formation and evolution of geologic strata such processes include erosion sediment transport and deposition quantitative stratigraphic inversion estimates the process parameters from a priori data by systematic modeling until the predictions are compatible with the observations in the case of quantitative seismostratigraphic inversion the a priori data are predominantly of the seismic kind to evaluate feasibility we designed a preliminary algorithm which we applied in a prograding delta environment we found that by fixing the sediment influx rate subaerial transport rate and the initial topography we were able to reproduce the basic features of the delta with only three to five parameters which included subaqueous transport rate and its depth dependent decay rate base level and sediment source location and spatial extent

the effect of sparse vegetation on the transport of dune sand by wind the precise effect of a sparse plant cover on the transport of dune sand by wind is a critical factor in understanding the morphology evolution and global distribution of aeolian dunes1 3 it is also of considerable practical importance in sand stabilization rehabilitation and restoration ecology here i report the first detailed experimental quantification of this effect using living plants in patterns approximating natural vegetation for wind velocities v at 0 5 m elevation up to 15 m s 1 and plant covercup to 17 the rate of aeolian sand transport q g cm 1 s 1 is closely approximated by q b v 1 kc vt 3 where b is bagnold s constant4 vt is a threshold velocity and k is a constant dependent on plant geometry for typical small erect or spreading herbaceous dune plants k 0 018 for small rounded stemless plants k 0 046

structure of the subduction system in southern peru from seismic array data the subduction zone in southern peru is imaged using converted phases from teleseismic p pp and pkp waves and pwave tomography using local and teleseismic events with a linear array of 50 broadband seismic stations spanning 300 km from the coast to near lake titicaca the slab dips at 30 and can be observed to a depth of over 200 km the moho is seen as a continuous interface along the profile and the crustal thickness in the back arc region the altiplano is 75 km thick which is sufficient to isostatically support the andes as evidenced by the gravity the shallow crust has zones of negative impedance at a depth of 20 km which is likely the result of volcanism at the midcrustal level of 40 km there is a continuous structure with a positive impedance contrast which we interpret as the western extent of the brazilian craton as it underthrusts to the west v p v s ratios estimated from receiver function stacks show average values for this region with a few areas of elevated v p v s near the volcanic arc and at a few points in the altiplano the results support a model of crustal thickening in which the margin crust is underthrust by the brazilian shield

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

lithospheric scale structure across the bolivian andes from tomographic images of velocity and attenuation for p and s waves we have developed a three dimensional lithospheric scale model across the bolivian andes at 20 s based on tomographic images of velocity and attenuation for both p and s waves observations of travel time and attenuation for this study are from regional mantle earthquakes in the subducted nazca plate recorded on a portable broadband seismic array broadband andean joint experiment and seismic exploration of the deep andes in bolivia and chile the shallow mantle under the altiplano from 18 s to 21 s is high velocity and moderately high q vp 8 3 vs 4 7 qp 500 and qs 200 suggesting lithospheric mantle high velocity material in the altiplano extends to a depth of 125 150 km the shallow mantle of the western cordillera is characterized by high vp vs 1 83 suggesting a correlation between vp vs and arc volcanism seismic velocity in the western cordillera mantle is on average only slightly reduced from global averages however velocity and attenuation anomalies are locally strong vp 7 8 vs 4 3 qp 200 and qs 100 consistent with partial melt conditions under the los frailes volcanic field in the eastern cordillera shallow mantle velocity and q decrease drastically from the neighboring altiplano vp 7 8 vs 4 3 qp 300 qs 100 however high vp vs is not as pervasive as it is in the western cordillera we believe that slab derived water and perhaps other volatiles strongly influence the western cordillera while the eastern cordillera low velocity region is more affected by partial melt and or compositional changes average velocity and q in the shallow mantle across the bolivian andes where the tomographic images are best resolved are significantly higher than in most mantle wedge environments where corresponding images are available this is likely the result of a compressional back arc setting in the andes this implies that lithospheric shortening and thickening associated with the formation of the andes has profoundly influenced the shallow mantle structure across the range shallow mantle structure is locally influenced by the subduction processes particularly under the western cordillera however the differing volcanism and seismic character under the two cordilleras suggest that the volcanic process in the eastern cordillera may be distinct from arc volcanism tertiary volcanism in the eastern cordillera is located in the region where mantle shortening is suspected to be greatest both the timing and location of volcanism are consistent with upward migration of mantle wedge asthenosphere following the removal of over thickened lithosphere

surface rupture associated with a 5 3 mb earthquake the 5 april 1986 cuzco earthquake and kinematics of the chincheros quoricocha faults of the high andes peru abstract the shallow mb 5 3 1986 earthquake that affected the cuzco region reactivated the 18 km long chincheros qoricocha faults in the central part of the cuzco fault system although the event has a moderate magnitude clear surface ruptures were observed to total 3 km in length with maximum 10 cm normal displacements fault kinematics related to the earthquake analyzed where the qoricocha fault had a maximum surface fault displacement is in agreement with roughly n s extension the normal fault mechanism proposed for this earthquake is compatible with the surface kinematic analysis seismic activity prior to the 1986 earthquake is deduced from analysis of holocene scarps trench exposures give evidence for at least three fault displacements suggesting an average recurrence interval of the order of a few thousand years for surface rupturing seismic events these fault reactivations with vertical displacement ranging from 0 6 to 2 m were related to seismic events of mw magnitude probably ranging from 5 8 to 6 9 structural analyses of holocene faults indicate that the chincheros qoricocha fault activity was related to roughly n s trending extension characterized by a horizontal n s trending 3 axis and a vertical 1 axis the n s extensional tectonics that controlled the holocene and 1986 chincheros qoricocha fault activity are the typical deformational pattern of the peruvian high andes and seem to be in agreement with compensated high topography

numerical modelling of drainage basin evolution and three dimensional alluvial fan stratigraphy abstract a forward numerical model is presented to study the effects of bedrock erodability thrust displacement rate pulsating tectonic activity and sea level fluctuations on drainage basin morphology and stratigraphy of large alluvial fan systems a low value of the bedrock erodability coefficient k b 0 5 10 4 year 2 3 vs thrust rate 2 0 m ky is associated with a time lag between cessation of tectonic activity and maximum sedimentation rates observed on the alluvial fans applying higher k b values ranging between 1 0 and 8 0 10 4 year 2 3 results in higher sediment yields due to more rapid headward catchment erosion and elaborate sideways branching of drainage networks pulsating tectonic activity is reflected in a stratigraphic alternation of prograding and retrograding alluvial fan gravels during tectonic activity fan gravel fronts and coastlines retreat because catchment yields are insufficient to fill the accommodation space created by the flexural response due to thrust loading phases of tectonic quiescence and cessation of flexural subsidence are indicated by progradation of the gravel front depending on the position in the basin the lag time for arrival of the gravel associated with tectonic cessation is several tens to hundred thousands of years a combination of pulsed tectonic activity with sinuous sea level fluctuation leads to a more complex stratigraphic pattern in that case stratigraphic response to tectonic pulses is masked by a similar but higher order frequency stacking pattern especially in the more distal parts

three dimensional p and s wave velocity structures of southern peru and their tectonic implications in 1981 an 18 station regional array was operated in southern peru above the area where the dip of the subducted nazca plate changes from virtually horizontal in the northwest to about 30 in the southeast arrival times of compressional p and shear s waves from microearthquakes recorded by this array are used in this paper to investigate the three dimensional velocity structure of the crust and upper mantle in the region between the coast and the cordillera occidental the results suggest a crustal thickness of about 40 km beneath the coast increasing to about 70 km beneath the cordillera occidental this change in thickness occurs abruptly in the northwest but is gradual in the southeast the inverse correlation between the dip of the moho and the dip of the slab suggests a broad scale causal relation between the two at the same time free air gravity anomalies suggest that the steepening of the moho in the northwest may occur only in the region under the array because the crust is thicker in the southeast than originally surmised a suite of earthquakes previously thought to occur in the mantle now appears to be confined to the crust the association of these earthquakes with a shallow dipping moho is suggestive of ongoing crustal deformation to the west of the andes however other seismological and geological observations make the role of these earthquakes in crustal deformation unclear s wave velocities in the mantle between 70 and 130 km depth above the 30 dipping slab are low suggesting the presence of partially melted asthenosphere that may be responsible for the magmatic activity observed in southern peru

hyperconcentrated sand water mixture flows over erodible bed field surveys and experiments in a tilting flume on hyperconcentrated sand water mixture swm flows were conducted at slopes slightly smaller than the equilibrium slope a rapid development of sand bars propagating upstream was observed these bars consist of an almost horizontal terrace with subcritical flow a steep lee side slope with supercritical flow and a hydraulic jump between subsequent bars the erosion processes on the bar are limited by the high sand concentrations and flow velocities on the steep slopes as a result the erosion rate is more moderate than predicted by the pickup functions used in classical sediment transport theory a simple one dimensional model is presented to study the behavior of triangular schematized bars from an analysis of the equations it is concluded that an increase in erosion rate requires an increase in bed slope to maintain equilibrium flow conditions finally simulations with the model are compared with observations in the field and in the tilting flume t

downstream fining in contrasting reaches of the sand bedded yellow river compared to downstream fining of a gravel bedded river little field evidence exists to support the process of downstream fining in large fine sand bedded rivers in fact the typically unimodal bed sediments of these rivers are thought to produce equal mobility of coarse and fine grains that may discourage downstream fining to investigate this topic we drilled 200 sediment cores in the channel beds of two fine grained sand bedded reaches of the yellow river a desert reach and a lower reach and identified a fine surface layer fsl developed over a coarse subsurface layer csl in the 3 m thick bed deposits in both reaches downstream the thickness of the fsl increased while that of the csl decreased comparison of the depth averaged median grain sizes of the csl and the fsl separately in both reaches shows a distinct downstream fining dependence to the median grain size which indicates that at a large scale of 600 800 km the csl shows a significant downstream fining but the fsl shows no significant trends in downstream variations in grain size this result shows that fine sediment supply 0 08 mm median grain size from wind borne sediments and cross desert tributaries can interrupt the fsl and coarsen the channel bed surface locally copyright 2011 john wiley sons ltd

composition of the crust and upper mantle in the central andes 19 30 s inferred from p wave velocity and poisson s ratio the present day crustal structure of the andean chain results from deformation processes and magmatic activity since the onset of subduction one line of investigation to resolve questions about the mechanism of crustal thickening is to get information about the composition of the crust and upper mantle by imaging the structures in velocity and poisson s ratio or vp vs a high density seismic network was operated during six months in 1994 across the whole andean chain the p and s travel times of the best located local earthquakes mainly situated in the subduction zone have been inverted for vp and vp vs using thurber s 3d iterative simultaneous inversion method the resulting models giving short scale information about the crustal and upper mantle composition provide new limits on the respective involvement of crustal shortening and magmatism in the thickening of the andean crust beneath the western cordillera the seismic properties are well explained by typical active volcanic arc processes and the thickening of the crust is explained by magmatic addition conversely beneath the central altiplano basin and the eastern cordillera our results preclude significant volume of magmatic addition from the mantle and reinforce crustal shortening as the main thickening process moreover cratonic lower crust and subcontinental lithospheric upper mantle are observed under the eastern cordillera up to 67 w evidence of the underthrusting of the brazilian craton which drives the crustal shortening beneath the central altiplano basin a thin lithospheric mantle is observed above a well marked asthenospheric wedge which extends in depth below the western part of the eastern cordillera thus partial melt is observed in the shallow mantle west of the western cordillera up to the brazilian craton

three dimensional numerical modeling of deltaic depositional sequences 2 influence of local controls abstract we investigate the influence of three local controls sediment supply subsidence and physiography on delta development and sequence variability using the three dimensional numerical model of deltaic deposition presented in a companion paper ritchie et al 2004 independently varying a single local control within geologically constrained limits produces marked differences in three dimensional morphology cross sectional stratal geometry and delta evolution during a cycle of sea level change sediment supply strongly influences not only the timing of transgressive and maximum flooding surface development during sea level rise and hence the diachroneity of lowstand transgressive and highstand systems tracts but also the timing of onset of fluvial incision and the characteristics of incised valleys and forced regressive wedges low sediment supply leads to early drowning of forced regressive and lowstand prograding wedges high magnitude transgressions and the late development of maximum flooding surfaces during relative sea level fall low sediment supply results in early initiation of fluvial incision and development of few but major incised valleys that route sediment to elongate forced regressive lobes at their mouths in contrast high sediment supply leads to early onset of normal regression and late transgression during relative sea level rise and numerous poorly developed incised channels and a broad laterally continuous forced regressive apron during relative sea level fall on basin margins with a shelf slope morphology base of slope deepwater deposits may occur at times of sea level highstand due to sediment bypass across the slope during relative sea level fall fluvial incision is enhanced by the relatively steep gradient of the slope leading to the early onset of incision and rapid headward propagation as a result major deep incised valleys develop that capture sediment and feed it directly to thick shelf edge forced regressive and lowstand prograding wedges the greater water depth ahead of the shelf edge and the steep slope gradients result in limited forced and normal regression compared to a ramp setting in contrast to other controls tectonic subsidence or uplift leads to modification of accommodation so that relative sea level change may vary significantly along a basin margin in high subsidence settings relative sea level may continue to rise even at times of high rates of background sea level fall in such settings deltas lack incised valleys forced regressive delta lobes and prominent sequence boundaries furthermore normal regression and even transgression in high subsidence locations can be contemporaneous with forced regression and incised valley development in adjacent lower subsidence locations the results of this sensitivity analysis suggest that systems tracts and their bounding surfaces are likely diachronous along many basin margins and may locally be absent more than one control can produce a similar stratigraphic response making interpretation of controlling processes from the stratigraphic product equivocal the models provide a framework for understanding the stratigraphy of natural basin fills but further research on the interplay between the controls is required in order to understand the climatic tectonic and sea level signals concealed in the stratigraphic record

new data on the geodynamics of southern peru from computerized analysis of spot and sar ers 1 images abstract several geological structures are extracted from spot panchromatic and ers 1 radar images of the central andes arequipa area southern peru our new methodology of numerical image processing is an aid for identification of the structures this numerical approach helps to locate characterize and determine the amplitude of the vertical offset of structures the results show that the nw se trending major faults are principally normal with a small sinistral strike slip component the open ne sw faults are tension gashes extension in the studied area trends ne sw and the major nw se striking faults form an extensional en echelon pattern including an active volcanic zone these new kinematic results lead to a regional geodynamic model which takes into account oblique convergence between pacific and south america plates within the wadati benioff zone of the peru trench

extension related to a high topography results from a microearthquake survey in the andes of peru and tectonic implications summary a microearthquake study performed in 1985 october november in the cordillera blanca western cordillera of northern peru allowed us to make a precise mapping of its shallow seismicity 160 local events are accurately located in the vicinity of an active normal fault system showing a vertical displacement of 4500 m over the last 7 myr part of the seismic activity can be related to this west dipping system but a great number of events indicate a new seismic zone situated under the highest peaks of the chain by using an inversion algorithm of p wave polarities we show a regional deviatoric stress tensor which defines an extensional axis in a direction approximately orthogonal to the andean structures this result is in good agreement with the theoretical distribution of stresses expected in an elongated high topography bordered by tectonic convergent zones moreover we suggest that an absolute uplift of this young batholith occurs on a normal fault system and that this phenomenon is partly due to buoyancy forces originating in its crustal root

nonuniform surface uplift of the andean plateau revealed by deuterium isotopes in miocene volcanic glass from southern peru abstract proposals for rapid late miocene surface uplift driven by large scale lithospheric removal beneath the central andean plateau have been based largely on temperature sensitive paleoaltimeters both the magnitude and mechanism of this proposed pulse of uplift have been challenged first climatic general circulation models support protracted uplift with predicted temperature and isotopic shifts enhanced by attainment of threshold elevations second tectonic models in which surface elevations are compensated by regional contraction and crustal thickening question the need for lithospheric removal and predict broadly coeval uplift of the entire plateau we present hydrogen isotope data using a novel temperature insensitive volcanic glass proxy from continuous well dated lower to middle miocene basin fill in the western cordillera of the northern plateau that show a rapid decrease in d values 62 8 at 19 16 ma with extremely negative values continuing into the pliocene we propose that the basin reached its current elevation by 16 ma 6 myr earlier than proposed for the central plateau the rapid decrease in d values is consistent with punctuated surface uplift of 2 2 3 7 km between 19 and 16 ma whereas the 3 7 km upper estimate assumes a static climate similar to modern the 2 2 km lower estimate conservatively incorporates modeled changes in the isotopic composition of precipitation associated with elevation change comparison of these results to existing paleoelevation estimates from the andean plateau facilitates a tentative reconstruction of earliest middle miocene paleotopography showing a central depression with flanking hinterland and thrust belt highlands this apparent pattern of nonuniform plateau uplift contradicts shortening proportional topographic growth over tens of myr we propose that temporally and spatially irregular surface uplift may be linked to pronounced local variability in crustal shortening and or piecemeal removal of dense mantle lithosphere insofar as these irregularities reflect the scale of heterogeneity for surface uplift processes available paleoelevation estimates point to a principal uplift mechanism that varies over relatively short

fluvial aeolian interactions part i modern systems two modern fluvial aeolian depositional systems great sand dunes national monument colorado and the mojave river wash california are remarkably similar in spite of different climates sizes fluvial sediment textures and relative directions of aeolian and fluvial transport dune growth and migration and deflation of blowouts create 8 10 m of local relief in unflooded aeolian landscapes there are six prominent fluvial aeolian interactions 1 fluvial flow extends into the aeolian system until it is dammed by aeolian landforms 2 interdune areas overbank interdunes upstream of aeolian dams and alongside channels are flooded 3 water erodes dunes alongside channels and interdunes 4 flood waters deposit sediment in interdune areas 5 fluvially derived groundwater floods interdunes interdune playas 6 wind erodes fluvial sediment and redeposits it in the aeolian system unique and characteristic sediments are deposited in overbank interdunes and in interdune playas reflecting alternate fluvial and aeolian processes and rapidly changing flow and salinity conditions these fluvial aeolian interdune deposits are characterized by irregular concave up bases and flat upper surfaces containing mudcracks or evaporite cement characteristic low relief surfaces form in aeolian systems as an effect of flooding fluvial deposits are resistant to aeolian deflation aeolian sand is preserved when flood sediments are deposited around the bases of the dunes thus repetitive fluvial and aeolian aggradation tends to be stepwise as interdune floors are suddenly raised during floods the effects of flooding should be easy to recognize in ancient aeolianites even beyond the area covered with overbank muds

the contribution of aeolian processes to fluvial sediment yield from a desert watershed in the ordos plateau china in arid zones many active aeolian dunes terminate at ephemeral and perennial desert rivers the desert rivers show very high rates of sediment transport that cause deleterious downstream effects on the river system and ecology high sediment loading has been attributed to severe water erosion of sparsely covered watersheds during infrequent but heavy rainfall although aeolian erosion is known to lead to high rates of wind blown sand transport direct confirmation of whether the aeolian processes accelerate or inhibit fluvial sediment loss is lacking here we show that an aeolian fluvial cycling process is responsible for the high rate of suspended sediment transport in a sudalaer ephemeral desert channel in the ordos plateau of china frequent aeolian processes but low frequency once every 3 years on average flooding occur in this region wind blown saltating grains appeared to be unable to cross the desert channel because of interruption of channel induced recirculating air flow and therefore tended to settle in the channel during the windy seasons leading to channel narrowing during flooding this narrowed channel was found to yield a threefold increase in suspended sediment loading and a 3 4 fold increase in the weight percentage of the 0 08 0 2 mm sediment fraction on 18 july 2012 loss of stored aeolian sand due to channel erosion accounted for about half of the total sediment yield in this watershed these findings show that aeolian processes play an essential role in accelerating the sediment yield from a watershed characterized by aeolian fluvial interplay and also suggest that the drier the region and the greater the aeolian process the more the aeolian process contributes to fluvial sediment yield copyright 2013 john wiley sons ltd

out of our depth on the impossibility of fathoming eustasy from the stratigraphic record the evolution of a sedimentary basin s gross morphology depends upon 1 basement movement 2 sediment accumulation and compaction and 3 variations of eustasy with time thus if an accurate history of eustasy could be derived it would aid in our understanding of basin stratigraphy and help in the prediction of sediment geometry from described areas techniques which attempt to determine the magnitude of eustatic sea level excursions include the measurement of 1 the amount of sedimentary onlap onto the continental margins with and without the use of hypsometric curves 2 the thickness of marine sedimentary cycles and the elevation and distance between indicators of old strandlines 3 the perturbations on individual thermo tectonic subsidence curves and stacked crustal subsidence curves 4 the variations in deep ocean oxygen isotopes found in sediments and 5 the size of variables used in graphical and numerical simulations of basin fill in terms of tectonic behavior rates of sediment accumulation and eustasy which invert the problem to date a combination of the use of relative sea level charts derived from sediment onlap of the continental margin with dimensioning by oxygen isotopes responding to glacial events offers the best potential for relative tectono eustatic sea level curves but even this method can not produce a unique solution for absolute eustatic variations mathematical modelling shows that at best it is possible to obtain only the sum of tectonic basement subsidence and sea level variations from the above methods and at worst not even that simple a combination thus every proposed scheme to measure eustatic sea level excursions assumes some behavior for two of the three underlying processes tectonic movement of the basement sedimentary accumulation and eustatic sea level each scheme then determines the third process relative to the assumed model behavior of the other two it would seem that like king canute we cannot command the sea though we can still use undifferentiated relative tectono eustatic sea level curves to generate a family of solutions which are the product of a variety of absolute sea level curves and tectonic models each solution has then to be assessed in terms of geologic setting and the hypotheses it generates

tectonic implications of the microearthquake seismicity and fault plane solutions in southern peru using recordings from 15 portable instruments and six permanent stations operated during two field investigations of microearthquakes in southern peru we determined locations of 888 shallow and intermediate depth earthquakes and 56 fault plane solutions a series of tests evaluating the effects of elevation corrections erroneous velocity structure and random errors in arrival times on calculated locations suggest that the locations of most of the events are uncertain by less than 7 km at shallow depths 60 km the seismicity is concentrated in three belts one offshore one beneath the coast and one beneath the western cordillera there is also scattered but infrequent shallow activity near the altiplano the belts offshore and beneath the coast are apparently associated with slip along the nazca south american plate boundary although the style of deformation is complicated one can infer that beneath the coast the active boundary between the nazca and south american plates is within the mantle rather than at the base of the south american crust the belt beneath the western cordillera is about 30 km in width and dips southwest from near the surface to a depth of 60 km fault plane solutions suggest that crustal shortening is taking place in a northwest southeast direction in this region the significance of this activity is difficult to explain but these earthquakes provide no clear evidence of tectonic erosion in this area a composite fault plane solution from the few events near the altiplano shows normal faulting which is typical of the active tectonics of that region the results of the investigation of intermediate depth earthquakes shows that the downgoing slab in southern peru is warped and not torn in the region where the dip changes from nearly horizontal to a dip of about 30 they also suggest that the axis of the contortion is not perpendicular to the coast but parallel to the direction of plate motion moreover the volcanic line ends at the contortion which suggests that a relatively thick wedge of aesthenospheric material is necessary for the formation of subduction related volcanism fault plane solutions for events between 60 and 100 km depth show a large scatter but some suggest a detachment of a sliver of the downgoing slab solutions for deeper events are more consistent with one another with the t axes for most events being nearly horizontal and generally normal to the projections of the seismic zone

quantitative models of sedimentary basin filling summary quantitative modelling of the filling of sedimentary basins was begun in earnest in the 1960s dozens of themes and variations have been proposed since then and have yielded an abundance of idealized stratigraphic patterns as functions of both imposed changes and basin properties post plate tectonic modelling began with rigid lid models which show the stratigraphic signature of subsidence variation this work introduced the connection between stratigraphy and the rheology of the lithosphere rigid lid models are the simplest type of geometric model in which the sediment surface is assigned prescribed geometries usually corresponding to different depositional environments these can reproduce many aspects of overall stratal geometry but are formally restricted to relatively long timescales for which quasi steady surface topography can be assumed so called dynamic models attempt to represent the morphodynamics of the sediment surface by abstracting and averaging short term transport processes most of the dynamic models proposed to date can be seen as special cases of a single general morpho dynamic equation the most important result of the first wave of quantitative basin filling models is that even relatively simple models can produce reasonable stratal patterns we now have a wide array of tools for exploring scenarios searching for general behaviours and effects and making initial quantitative predictions we have also learned that basin response to external forcing as recorded in stratigraphy can be as sensitive to the characteristics of the basin as to the forcing the main brake on the development of basin modelling is not computing power but lack of methods and data for testing the models we have already developed physical experiments which are only just beginning are one means of doing this experimental stratigraphy is a bridge to quantitative field tests which will require collaboration among academic researchers from a wide range of areas and between academia and industry on projects of greater scale and degree of integration than we have seen so far the advancement of quantitative sedimentary geology will also require significant changes in the way the subject is taught at all levels

automatic calibration of stratigraphic forward models for predicting reservoir presence in exploration understanding and predicting reservoir presence and characteristics at regional to basin scales is important for evaluating risk and uncertainty in hydrocarbon exploration simulating reservoir distribution within a basin by a stratigraphic forward model enables the integration of available prior information with fundamental geologic processes embedded in the numerical model stratigraphic forward model predictions can be significantly improved by calibrating the models to independent constraints such as thicknesses from seismic or well data a three dimensional basin scale stratigraphic forward modeling tool is coupled with an inversion algorithm the inversion algorithm is a modification of the neighborhood algorithm a type of genetic algorithm which is designed to sample complex multimodal objective functions and is parallelized on computer clusters to accelerate convergence the process generates a set of representative geological models that are consistent with prior ranges for uncertain parameters calibration constraints and associated tolerance thresholds the workflow is first demonstrated on two data sets a synthetic example based on a clastic passive margin and a real hydrocarbon exploration example for slope and basin floor stratigraphic traps in the neocomian lower cretaceous of the west siberian basin the analysis of calibrated models provides constraints on stratigraphic controls and allows prediction of locations with higher potential to develop stratigraphic traps these locations are related to complex interactions between paleobathymetry subsidence eustatic fluctuations characteristics of sediment input sources and sediment transport parameters results show the potential of stratigraphic forward modeling combined with inverse methods as an additional tool to support conventional play based exploration and reservoir presence prediction

velocity structure of the andes of central peru from locally recorded earthquakes arrival times of local events recorded during two short term seismic fied investigations in central peru are used to determine the p and s velocity structures above the slab in a segment of the andes where the subduction is horizontal the velocities in the upper crust can be related to the sedimentary cover the moho depth is estimated to be about 50 km below the western part of the sub andean zone and deepens gently to the west reaching 60 km under the western cordillera an uplift of high velocity material is observed under the coastal zone associated with a cluster of earthquakes in and above the slab the asymmetrical structure of the crust below the eastern cordillera reflects the convergence process in this part of the andes the shortening occurs along active reverse steeply west dipping faults involving the crust in its entirety on the eastern border of the cordillera the compression then decreases toward the east

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

three dimensional interplay of advective and diffusive processes in the generation of sequence boundaries quantitative stratigraphic models have been generated using either geometric constructs or diffusion algorithms to simulate the development and timing of sequence boundaries and how they vary in response to tectonics eustasy climate and sediment supply while diffusion models provide a physical basis for the generation of clinoforms the resulting model predictions appear to do little more than track the transgressive and regressive movements of the shoreline on the other hand geometric models do a relatively good job in simulating the observed stratigraphy but offer little in actually ascertaining the physics controlling the processes of deposition and erosion all these stratigraphic models fail to predict the most fundamental response of depositional systems namely the development of a downward shift in coastal onlap during a fall in relative sea level this basinward shift and the development of an onlap surface are the critical observations that define chronostratigraphic sequences and are not simply a consequence of shoreline transgressions and regressions a major assumption in these two dimensional models is that the observed stacking patterns are solely the consequence of deposition processes and sediment transport within the modelling section in marked contrast to the observation that the major sediment transport direction along margins is sub parallel to the margin we present a variation on the diffusion theme in which across margin transport and deposition are dominated by advection type processes and along margin erosion and deposition processes are dominated by diffusion with this combination of advection and diffusion it is possible to generate a lowstand system tract and sequence boundaries i e onlap surfaces in particular the generation of the lowstand system tract is in large part the result of depositional processes operative along the margin i e out of the section

aeolian and fluvial evidence of changing climate and wind patterns during the past 100 ka in the western simpson desert australia abstract sediments near finke in central australia provide evidence of late quaternary evolution and the interaction of aeolian and fluvial systems in response to changing climate in the western simpson desert thermoluminescence tl dating is used to develop a chronology of aeolian and alluvial activity and to identify differences in sand provenance quaternary alluviation in the finke valley at this location occurred at or prior to c 90 ka and with no surviving evidence of subsequent activity until the holocene in contrast to rivers in the eastern part of the lake eyre basin substantial alluviation took place here in the early to mid holocene probably due to reactivation of the northern monsoon and its partial penetration into central australia the regional dunefield near finke consists of linear dunes largely reworked and aligned during the last glacial 30 12 ka as part of the great anticlockwise whorl of dunes in central australia the oldest dated source bordering dunes from the finke river are bright red in colour were deposited at c 100 ka and are now buried beneath paler source bordering dunes the latter consist of two exposed units a lower one of unknown orientation that dates at 17 9 ka and an upper unit aligned to the northwest and parallel to the prevailing winds that dates at 5 0 ka the tl signature and sediment texture of the oldest source bordering dunes show them to have probably been derived from weathered alluvial red beds of similar or greater age nearby and to have contributed abundant sand to the regional dunefield immediately north of the river in contrast sand from the younger paler source bordering dunes appears to be from a different source and to have only recently extended into the regional field the finke region would seem to be in a pivotal position for the study of palaeowind patterns that have created and modified australia s whorl of continental dunes the 30 18 ka regional linear dunes near finke are oriented almost due north their cross sectional asymmetry steeper eastern slopes suggests a response to southwesterly or westerly sand transporting winds between 18 and 10 ka these winds appear to have shifted to their present southeast orientation during the past 5 ka from dune ages alignments and asymmetry it is proposed here that there was a northward shift in the wind pattern at finke by about 100 150 km 1 1 5 of latitude during the last glacial but that through this period the regional dune pattern has remained remarkably stable

local earthquake tomography of the altiplano and the eastern cordillera of northern bolivia the velocity structure in the crest beneath the central andes of northern bolivia is obtained by the simultaneous inversion of local earthquake arrival time data for velocity and hypocentral parameters the data were recorded between october 1990 and february 1991 by the french lithoscope network the 34 vertical short period seismic stations were installed along a 320 km long profile crossing two main morphological units of the andean chain the altiplano and the eastern cordillera the arrival times of seismic waves generated by earthquakes located in the subduction zone of the nazca plate underneath the south american plate were inverted using a modified version of thurber s 3 d iterative simultaneous inversion code the results are consistent with the inversion of teleseismic travel time residuals from the same network and closely correlate with the mapped surface geological features the altiplano basin is characterized by a low velocity zone down to 20 km enclosed by the san andres and laurani faults at the surface the cordillera real fault zone crfz bounds to the west the high p wave velocity anomaly lying under the axial zone of the eastern cordillera the velocity discontinuity dips steeply to the southwest to the bottom of the crust from the velocity model the depth of the moho is estimated to be 66 km under the altiplano moreover the different behaviour of p and s waves found below the western cordillera and above the slab is consistent with the presence of partially melted asthenosphere that may be responsible for the magmatic activity finally the seismicity defines between 90 and 225 kin a part of the slab dipping to the north northeast with a low angle of about 30

evidence for the subduction and underplating of an oceanic plateau beneath the south peruvian margin during the late cretaceous structural implications abstract uring late cretaceous times santonian to maastrichtian the southern peruvian and northernmost chilean andes show a series of tectonic magmatic and sedimentological features which appear to be specific to this area when compared with northern and southern parts of the andes a gap in the magmatic activity in the coastal range and the western cordillera between 84 and 70 ma huge overthrusting faults involving the precambrian basement in the arequipa region and the syntectonic filling of the cuzco and sicuani foreland continental basins these particular features may be regarded as consequences of the subduction and underplating of a paleo oceanic plateau the mollendo ridge which would have occurred in this area between 85 and 70 ma b p with an almost n s direction of convergence between the farallon and south american plates this inferred mollendo ridge would have been an eastern equivalent of the present day manihiki plateau of the western pacific ocean the proposed model provides new elements for the interpretation of the subsequent structural and magmatic evolution of the central andes it permits the explanation of the particular location and the petrochemical and metallogenetic features of the andahuaylas yauri batholith the segment of abnormally thick crust created by the underplating of the mollendo ridge and by overthrusting tectonics will act as an accumulation point for further cenozoic deformation along the andean margin then the subduction of the mollendo ridge would mark the outset of the development of the oroclinal bending of the central andes the arica elbow and of the associated altiplano and its anomalous thick crust

aeolian process induced hyper concentrated flow in a desert watershed ephemeral desert channels are characterized by very high rates of sediment transport during infrequent flood events here we show that aeolian process induced hyper concentrated ahc flows occur in the sudalaer desert watershed in the ordos plateau of china which primarily transport 0 08 0 25 mm non cohesive aeolian sand and have a peak suspended sediment concentration of 1 1 1 4 x 10 6 mg l 3 aeolian sand supply and storage in the channel play a crucial role in causing hyper concentrated flow our results indicate that non cohesive aeolian sand can be entrained from the bed and suspended in the turbulent flow when the channel bed slope exceeds a critical threshold 0 0003 we also show that if the frequency ratio of wind blown sandstorms to rainstorms t w t p exceeds beta gamma gamma 0 alpha p v 3 a l where alpha is the wind blown sand transport coefficient beta is the runoff coefficient gamma gamma 0 is the increase in suspension concentration caused by addition of aeolian sands p is the density of rainstorms v is the wind speed of sandstorms a is the runoff generating area l is the aeolian sand filled channel length an ahc flow occurs during the passage of a flood in a desert channel since high frequency aeolian processes provide an adequate quantity of transportable sediment and promote ahc flow most of the infrequent rainfall induced floods occurring in arid zones can develop as ahc flows c 2014 elsevier b v all rights reserved

three component three dimensional seismic data acquisition three component three dimensional seismic data is acquired by locating a plurality of receivers off the seisimic shot line of a seismic source the angle between the seismic shot line and a line extending between the seismic source and each receiver is used to normalize the signals generated by the two shear wave detectors in each receiver by mathematically rotating the orientation of the receiver and the orientation of the seismic source to produce horizontal shear wave signals and vertical shear wave signals generally equivalent to those that would have been produced if each receiver and seismic source were aligned along a seismic shot line

the simulation of the sedimentary fill of basins an interactive computer simulation which erects models of sedimentary geometries by filling in a two dimensional basin from both sides with a combination of clastic sediment and or in situ and transported carbonate sediments is described the simulation program is implemented in c using graphical plotting functions data entry including the initial basin configuration the local tectonic behavior the sea level curves the amount and source direction of clastic sediment and the growth rates of carbonates as a function of water depth is performed interactively geometries of clastic and carbonate sediments evolve through time and respond to tectonic movement eustasy and sedimentation clastic modeling includes sedimentary bypass and erosion and sedimentation in alluvial and coastal plains marine shelf basin slope and basin floor settings carbonate modeling includes progradation the development of hard grounds downslope aprons keep up catch up back step and drowned reef settings as well as lagoonal and epeiric settings also included in the model are extensional vertical faulting of the basin sediment compaction and isostatic response to sediment loading sediment geometries are plotted on a graphics terminal as they are computed so the user can immediately view the results based on these observations parameters can be changed and the program rerun until the user is satisfied with the resultant geometry this paper includes output showing the simulation s sensitivity to eustatic sea level variations and variations in sedimentation and subsidence rates also shown are outputs demonstrating the modeling of some examples from the geologic section in the permian basin of west texas and the upper permian of the sichuan basin

a seismological study of the 1835 seismic gap in south central chile we study the possible seismic gap in the concepcion constitucion region of south central chile and the nature of the m 7 8 earthquake of january 1939 from 1 march to 31 may 1996 a seismic network of 26 short period digital instruments was deployed in this area we located 379 hypocenters with rms travel time residuals of less than 0 50 s using an approximate velocity distribution using the velest program we improved the velocity model and located 240 high precision hypocenters with residuals less than 0 2 s the large majority of earthquakes occurred along the wadati benioff zone along the upper part of the downgoing slab under central chile a few shallow events were recorded near the chain of active volcanos on the andes these events are similar to those of las melozas near santiago a few events took place at the boundary between the coastal ranges and the central valley well constrained fault plane solutions could be computed for 32 of the 240 well located events most of the earthquakes located on the wadati benioff zone had slab pull fault mechanism due to tensional stresses sub parallel to the downgoing slab this slab pull mechanism is the same as that of eight earthquakes of magnitude around 6 that are listed in the cmt catalog of harvard university for the period 1980 1998 this is also the mechanism inferred for the large 1939 chilean earthquake a very small number of events in the benioff zone had slab push mechanisms that is events whose pressureaxis is aligned with the slab these events are found in double layered wadati benioff zones such as in northern chile or japan our spatial resolution is not good enough to detect the presence of a double layer but we suspect there may be one 2002 elsevier science b v all rights reserved

central andean crustal structure from receiver function analysis abstract the central andean plateau 15 27 s is a high plateau in excess of 3 km elevation associated with thickened crust along the western edge of the south america plate in the convergent margin between the subducting nazca plate and the brazilian craton we have calculated receiver functions using seismic data from a recent portable deployment of broadband seismometers in the bolivian orocline 12 21 s region and combined them with waveforms from 38 other stations in the region to investigate crustal thickness and crust and mantle structures results from the receiver functions provide a more detailed map of crustal thickness than previously existed and highlight mid crustal features that match well with prior studies the active volcanic arc and altiplano have thick crust with moho depths increasing from the central altiplano 65 km to the northern altiplano 75 km the eastern cordillera shows large along strike variations in crustal thickness along a densely sampled sw ne profile through the bolivian orocline there is a small region of thin crust beneath the high peaks of the cordillera real where the average elevations are near 4 km and the moho depth varies from 55 to 60 km implying the crust is undercompensated by 5 km in comparison a broader region of high elevations in the eastern cordillera to the southeast near 20 s has a deeper moho at 65 70 km and appears close to isostatic equilibrium at the moho assuming the modern day pattern of high precipitation on the flanks of the andean plateau has existed since the late miocene we suggest that climate induced exhumation can explain some of the variations in present day crustal structure across the bolivian orocline we also suggest that south of the orocline at 20 s the thicker and isostatically compensated crust is due to the absence of erosional exhumation and the occurrence of lithospheric delamination

plio quaternary geodynamic evolution of a segment of the peruvian andean cordillera located above the change in the subduction geometry the cuzco region abstract the cuzco region which is located above a change in subduction geometry appears to be characterized by a variable plio quaternary tectono sedimentary evolution essentially located along the major fault system that separates the high plateaux from the eastern cordillera after the higher surface formation of the high plateaux a set of neogene basins were filled by miocene fluvio torrential series and by plio pleistocene fluvio lacustrine deposits the neogene series have been affected by compressional tectonic forces attributed to the late miocene this compression is followed by roughly e w trending syn sedimentary extensional tectonics attributed to the pliocene it is related to reactivation of the pre existing major faults basin evolution and volcanic activity concentrated along the faults in the early pleistocene fluvio lacustrine deposits are affected by syn and post sedimentary compressional tectonism it is characterized by shortening that trends both n s and e w and produces folding and faulting of the sedimentary cover extensional tectonism trending roughly n s has been taking place from the middle pleistocene to the present it is coeval with shoshonitic volcanic activity and with sedimentation of fluvio lacustrine terraces torrential fans and moraines quaternary and active normal faults due to this tectonism are located in a narrow zone more than 100 km long between the high plateaux and the eastern cordillera and two 15 km long fault sectors in the eastern cordillera characteristic pleistocene scarps 400 m or more high are due to the cumulative normal offset and there are also little scarps with heights ranging between 2 and 20 m which are related to holocene fault reactivations recent fault reactivation on the cuzco fault system during the april 5 1986 earthquake m b 5 3 is due to the n s trending extension this state of stress located at a mean elevation of roughly 3730 m is generally homogeneous to different scales the active cuzco normal faults may be a consequence of adjustment between the compensated western cordillera and the undercompensated eastern cordillera this latter being uplifted higher than its isostatic equilibrium due to compression acting on its eastern edge the variation of the state of stress during the plio quaternary is in agreement with the variations of the compressional boundary forces it may be explained by variation of the convergence rate or by the variation of pull slab forces

mathematical modelling of sedimentary basin processes abstract mathematical models of various sedimentary basin processes including hanging wall deformation evaporite deposition carbonate platform sedimentation and sequence development on active domino style fault blocks are presented many of these processes may be formulated using the equation of continuity of an open system as a starting point sedimentary and tectonic processes may be combined into a single mathematical formulation provided that an eulerian coordinate system is used and this ensures that tectonics and sedimentation are modelled as simultaneous rather than sequential processes the resulting algorithms are fast robust and applicable to many geologically very different situations but may be limited to relatively simple examples the conditions under which the models are numerically stable are also easily found the abstract nature of the mathematical models requires that simply determined properties such as palaeo water depth and palaeo slope must be used as proxy facies markers the main limitation of the method is that it is restricted to modelling processes which change slowly with time and space and the method can therefore not cope properly with episodic or chaotic processes

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

relative geologic time age volumes relating every seismic sample to a geologically reasonable horizon the age volume described in this paper can significantly increase the amount of useful geologic information extracted from a seismic data volume this paper has three goals the first is to introduce and describe the relative geologic time volume referred to herein as either an rgt volume or an age volume every seismic sample in an age volume contains an estimate of the relative geologic time for that sample which automatically relates every sample to a geologic horizon the second goal is to discuss some of the applications and advantages of using a rgt volume to store and access seismic interpretation for example it can be used to sculpt a seismic volume along a desired horizon or view depositional changes as a function of geologic time the third goal is to describe two methods for generating such a volume the first employing standard interpretation techniques and the second relying on unwrapping instantaneous phase unwrapping the instantaneous phase is a new holistic approach to interpreting seismic data that can produce a very detailed seismic interpretation the goal of building and using an rgt volume is to extract significantly more geologic information from the seismic data than is available from conventional methods the rgt volume is both a way of storing and retrieving seismic horizons to generate such a volume each seismic sample needs to be assigned an estimate of relative geologic time which in essence connects it to a geologically reasonable horizon in normal sections these relative geologic time values will increase with depth or traveltime high gradients in the rgt volume could be indicative of faults unconformities fluid contacts or interpretation errors while a constant value surface in a rgt volume is a seismic horizon figure 1 contains a portion of the seismic data used to illustrate the

towards process based geological reservoir modelling obtaining basin scale constraints from seismic and well data abstract forward stratigraphic modelling aims at representing the spatial distribution of lithology as a function of physical processes and environmental conditions at the time of deposition so as to integrate geological knowledge into the reservoir modelling workflow thus increasing predictive capabilities of reservoir models and efficient exploitation of hydrocarbons application of process based models in inverse mode is not yet well established due to our limited insight into the information content of common subsurface data and the computational overhead involved in this paper we examine inverse modelling of stratigraphy by using a typical dataset acquired in the hydrocarbon industry which consists of seismic data and standard logs from a limited number of wells the approach is based on the use of a forward model called simclast developed at delft university of technology to generate facies distribution and architecture at the regional scale three different goodness of fit functions were proposed for model inversion following an inference approach a synthetic reservoir unit was used to investigate the impact of the uncertainty affecting the input parameters and the information content of seismic and well data the case study showed that the model was more sensitive to the initial topography and to the location of the sediment entry point than to sea level the depth of the seismic reflector corresponding to the top reservoir surface was the most informative data source the initial and boundary conditions of the simulation were constrained by evaluating the depth of this reflector across the whole basin area in the reservoir area where the seismic to well tie was established the depth of the reservoir top does not give enough information for constraining the model parameters our results thus indicate that evaluation of basin scale data permits reduction of uncertainty in geostatistical reservoir models relative to the current workflow in which only local data are used effective use of well data to generate reservoir models conditioned to basin scale scenarios requires post processing methods to downscale the output of the forward model used in the experiments

three dimensional numerical modeling of deltaic depositional sequences 1 influence of the rate and magnitude of sea level change abstract a three dimensional numerical model of deltaic deposition is used to investigate the influence of sea level changes on delta development and sequence variability results illustrate the three dimensional morphology of key stratal surfaces and architecture of stratal units systems tracts and highlight the importance of the rate and magnitude of sea level change in controlling the evolution of deltaic depositional sequences high rates of sea level fall lead to the development of a limited number of major incised channels that focus sediment supply to a few elongate finger like forced regressive lobes separated by large areas of nondeposition in contrast low rates of sea level fall cause only minor channel incision which occurs late during sea level fall as a result sediment is supplied more uniformly to the delta front leading to an attached laterally continuous forced regressive apron during lowstand and subsequent sea level rise the delta morphology and internal geometry are strongly controlled by the rate of rise high rates lead to i poorly developed lowstand wedges that are drowned early ii high magnitude transgressions and iii the late development of maximum flooding surfaces the stratigraphy developed during sea level rise is also strongly influenced by the incised valley system created during the preceding sea level fall if deep major valleys developed that captured most of the sediment supply the resultant stratigraphy has well developed lowstand wedges that are flooded relatively late during sea level rise even within a single delta systems tracts and key stratal surfaces show three dimensional variability and two dimensional sections often lack significant elements of the stratigraphy as a result analysis of two dimensional sections can often lead to miscorrelation and erroneous interpretations of the controlling processes

turbidity current with erosion and deposition the equations which govern the movement of two dimensional gradually varied turbidity currents in reservoirs and over beaches are derived and solved numerically turbidity currents are sediment laden gravity currents that exchange sediment with the bed by erosion or deposition as the flow travels over the downslope turbidity currents derive this driving force from the sediment in suspension they experience a resisting shear force on the bed and entrain water from above turbidity currents can be eroding or depositive accelerating or decelerating dependent on the combination of initial conditions bed slope and size of sediment particles they can be controlled from upstream supercritical or downstream subcritical gravity currents with and without erosion and deposition are examined in order to understand the effects of sediment exchange on the flow

structure of the earth mantle and core the deep interior structure of the earth has been extensively analyzed using a wide variety of seismic phases and techniques during the last four years most studies have emphasized quantitative three dimensional mapping of the lateral velocity heterogeneity of the mantle and core these aspherical velocity variations are believed to be direct manifestations of thermal and compositional heterogeneity associated with convective processes a first generation of global models for the lateral velocity variations in the deep earth has been produced providing tantalizing images of large scale structures suggestive of a non steady state thermal convection system the upper 400 km of the mantle has the strongest lateral velocity variations of up to 10 for shear velocity surface wave analyses that do not require a priori regionalizations have demonstrated that there is a strong association between surface tectonic provinces and the uppermost mantle velocity variations thus the thermal and convective state of the upper 200 km of the mantle can be reliably interpreted in the context of plate tectonics the upper mantle models support the contention that continents have deep roots with differences in velocity structure from oceanic and active tectonic regions extending as deep as 400 km very long wavelength lateral velocity variations of a few percent have been detected in the transition zone at depths from 400 to 670 km as well as throughout the lower mantle these deep seated variations have little correspondence to surface tectonics and efforts to interpret their nature are just beginning the lowermost 200 km of the mantle d region has lateral velocity fluctuations comparable to those in the upper mantle and evidence has been presented for the presence of a sizable velocity discontinuity at the top of the d layer a combined thermal and compositional boundary layer roughly mirroring the lithosphere is a likely explanation for this anomalous zone the core mantle boundary appears to have significant 10 km long wavelength topography presumably sustained by dynamic stresses from deep mantle convection the inner core may have strong lateral heterogeneity or axially symmetric anisotropy suggesting a complex thermal and compositional state significant progress has been made in characterizing the frequency dependence of anelastic attenuation in the mantle in the short period body wave band models for teleseismic p wave attenuation operators have converged with t values of 0 7 1 0 s appropriate at 1 hz and t values of 0 4 0 6 s appropriate at 4 hz regional variations of attenuation are slowly being mapped out as well

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

comparison of crustal thickening budget and shortening estimates in southern peru 12 14 s implications for mass balance and rotations in the bolivian orocline two fundamental questions of interest with regard to the andean plateau are the mass balance of material needed to create and sustain a 3 4 km high plateau and the relationship between the plateau and the bending of the bolivian orocline the link be tween these two questions is the distribution of shortening through the central andes is crustal shortening suffi cient to support an isostatically compensated 60 70 km thick crust is differential shortening between the hinge of the orocline and its limbs suffi cient to account for both physiographic curvature and measured rotations three dimensional 3 d models of deformation within curved orogens require in addition to paleomagnetic data viable two dimensional estimates of dis placements to this end we present new map data and use those data to derive estimates of shortening across the northern margin of the andean plateau the cross section extent from the eastern edge of the volcanic arc to the foreland basin is approximately one half of the physiographic width of the andean plateau in peru cross sectional shortening estimates in southern peru 12 14 s provide a preferred estimate of 123 km or 40 short ening with a minimum estimate of 58 km or 25 shortening and maximum estimate of 333 km or 65 shortening the largest con trolling factor on the magnitude of shorten ing is where basement becomes involved in thrust belt deformation using our preferred shortening estimate the differential shorten ing between our cross section on the northern margin of the bolivian orocline 12 14 s and cross sections across the axis of the bolivian orocline 17 18 s is 177 km corresponding to 23 of rotation the differential shortening for the young younger than 15 ma defor mation of the altiplano and subandean re gion between 12 14 s and 17 18 s is 88 km correlating to 11 5 of rotation while dif ferential shortening can account for young younger than 15 ma vertical axis rotations it cannot completely account for early 45 35 ma rotations 37 15 this suggests that forearc rotations refl ect both bending due to differential shortening 12 as well as block rotations 25 30 which facilitated defor mation in a tightening core and transported material toward the center of the orocline the preferred estimate of shortening is well short of the required 240 300 km of shorten ing needed to account for a 60 70 km thick crust under the entire plateau this suggests that for an isostatically equilibrated crust either 1 there is a signifi cant amount of shortening 100 150 km in the western half of the plateau that is hidden by the volcanic arc or 2 crustal material is being added to the peruvian section of the andean plateau most likely through lower crustal fl ow

velocity structure of the pamir hindu kush region possible evidence of subducted crust the arrival times of compressional p and shear s waves from approximately 580 microearthquakes recorded by a temporary array in the pamir hindu kush region in central asia are used to deduce one and three dimensional velocity structures of this region the results for one dimensional structures imply that the moho is at 70 5 depth also there is a velocity reversal near 160 km depth which is inferred to be the beginning of the low velocity zone this reversal continues to depths of approximately 230 km below 230 km velocities are somewhat higher than those of normal mantle at similar depths 9 3 km s versus 8 4 km s for p waves the outstanding feature of the results for three dimensional velocity structures is a broad 40 km centrally located region with 8 to 10 lower velocities than those in the surrounding regions this low velocity region envelopes the seismic zone at depths between 70 and 150 km the region may actually extend beyond these depths but the results for shallower and deeper structure lack sufficient resolution to decide several tests using both hypothetical and real data were performed to estimate the reliability of the three dimensional solutions the results of these tests suggest that the inferred velocities are reasonably accurate representations of the average velocities in the blocks although one must be cautious of the effects of averaging in interpreting the solution the low velocity region is inferred to be a manifestation of substantial quantities of subducted continental crust therefore while subduction has occurred in the pamir hindu kush the results of the three dimensional inversions suggest that continental rather than only oceanic lithosphere has been subducted to depths of at least 150 km appendices are available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document j81 013 1 00 payment must accompany order

lithospheric stress and deformation the study of the mechanisms magnitudes and modes of lithospheric stress and deformation occupies a central position in the discipline of tectonophysics it is therefore difficult in assembling a comprehensive review to place limits on what should be included for example one cannot discuss stress and deformation apart from considerations of the rheological models of the lithosphere which link the two and ultimately control the behavior of plates at their margins and in their interiors once admitting to the importance of rheological considerations in discussing stress and deformation one is then compelled to include information relating to the thermal state of the lithosphere since temperature is one of the foremost environmental variables controlling rheology furthermore lithospheric processes cannot be considered in isolation from those affecting the asthenosphere since surely there exists some coupling between the motion of the lithosphere and convection in the earth s interior in fact an understanding of lithospheric stress and deformation can be considered a prerequisite to solving problems in geodynamics because it is through the lithospheric filter that one detects the consequences of thermal and compositional anomalies at greater depth the topic of this report cannot even be cleanly separated from reviews in other disciplines such as seismology and geodesy the magnitude and orientation of lithospheric stress is often indicated by earthquakes while lithospheric strain can be observed geodetically the philosophy i have adopted here is that some overlap with other review papers in this series is unavoidable and probably desirable from the standpoint of emphasizing how recent work n other areas of geophysics has contributed to the advancement in our understanding of the lithosphere s state of stress and mechanisms of deformation i have however resisted the temptation to include extraterrestrial studies presumably covered in the planetary section despite the similar causes for deviatoric stress and deformational mechanisms operating on other terrestrial planets certainly ideas of how silicate materials behave and how lithosphere responds to internal forces benefit from the perspective of omparative planetology finally in defining the scope of this portion of the u s national report to the iugg i did not consciously exclude papers by non u s citizens work done at non u s institutions projects not funded by the u s government or however else foreign might be defined topics in geophysical research cannot be divided along political boundaries however i did not attempt to seek out articles written in languages other than english or published in journals not readily available in the united states and i apologise for this obvious shortcoming

the state of stress in an overriding plate situated above a flat slab the andes of central peru in the andes of central peru that are situated above a flat dipping slab analysis of both structural data collected in the field and available focal mechanisms show that the quaternary and recent state of stress is characterized by n s trending extension in the western cordillera and along the pacific coast whereas e w trending compression prevails in the sub andean lowlands and at the contact between the nazca and south american plates the eastern cordillera deformations suggest an intermediate state of stress i e strike slip faulting characterized by e w trending compression and n s trending extension thus major fault zones striking nw se i e parallel to the central peruvian andes are normal with a sinistral component in the western cordillera e g cordillera bianca and reverse sinistral in the eastern cordillera e g cordillera de huaytapallana this state of stress displays similarities with that of southern peru and may be also interpreted as an effect of compensated high topography in this model the vertical stress zz increases with the topography and hmax is considered as fairly constant trending e w roughly parallel to the convergence on both edges of the andes tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then hmax is 2 allowing hmin striking n s to be 3 along the pacific coast tectonics is not compressional as expected and a nearly neutral state of stress may be due to a topographical effect related to the nearby deep peru chile trench however a conspicuous difference occurs in the andes of central peru in respect to the southern peru andes compressional strike slip tectonics is observed in the eastern cordillera indeed the mean elevation of the eastern cordillera is 3 7 km whereas that of the western cordillera is 4 2 km allowing an intermediate state of stress to occur therefore in central peru a lower mean andean topography and a stronger coupling consequence of the flat subduction seems to explain that compressional tectonics is distributed in a more widespread area than in southern peru superficial deformations related to the subduction of the aseismic nazca ridge are restricted to the coastal domain opposite to this ridge they support the fact that submarine topography of large extension may subduct without producing compressional deformation in the overriding plate which can be reminiscent of a collisional setting

structure of the subduction transition region from seismic array data in southern peru data from three seismic arrays installed in southern peru were analysed using receiver functions from p pp and pkp wave phases in order to image the subducted nazca slab the arrays cover the transition region from flat slab subduction in central peru to normal subduction with an angle of about 30 further south a previous study used data from the first array in the normal subduction region to image the moho depth and slab and showed the existence of a mid crustal structure at 40 km depth that is suggested to be a possible underthrusting of the brazilian shield here we discuss new observations from the other two arrays that span the transition between the two subduction regimes and the flat subduction region the results provide an image of the flattened slab from the coast to approximately 300 km inland and also across the transition region from flat to 30 subduction which appears to be a bend rather than a tear in the slab in the flat slab region the slab is well defined near the coast and flattens out at 100 km depth beneath the altiplano the slab appears to start flattening some 400 km in advance of the subduction of the nazca ridge and the flattening is maintained for 1300 km after its passage the moho begins at a depth of around 30 km near the coast and has a maximum depth of 75 km beneath the altiplano consistent with the results of the other arrays both arrays also show a positive impedance mid crustal structure at 40 km depth which if explainable by underthrusting of the brazilian shield would add further support to the observations from the normal subduction region and show the northward and westward extent of the signal the underthrusting hypothesis would explain the missing crust from the shortening budget needed to support the altiplano the vp vs ratios for both arrays exhibit average values between 1 73 and 1 75 indicating a lesser likelihood that there is a high degree of partial melting or magma bodies at depth in this region the receiver function results provide new imaging of the flat slab and transition from normal to flat slab subduction which allows for comparison of different subduction regimes