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

seep carbonates in a thrust related anticline at the leading edge of an orogenic wedge the case of the middle late miocene salsomaggiore ridge northern apennines italy abstract seep carbonates are important indicators of submarine fluid expulsion and dewatering of sediments along convergent margins and accretionary prisms in the northern apennine compressional wedge seep carbonates usually occur in pelitic successions from different settings of the middle late miocene foreland basin system in serravallian foredeep deposits of the salsomaggiore area seep carbonates cemented and or encrusted coarse grained sandstones and conglomerates fluid expulsion strictly related to variable intensities in tectonic stresses generated seep carbonate facies and sediment instabilities depending on the position of the fluid pathway feeders within the salsomaggiore ridge a middle late miocene intra basinal high at the leading edge of the apenninic orogenic wedge this site provides a unique opportunity for analyzing depositional characters and the three dimensional distribution and evolution of a seepage system in a thrust related anticline the resulting evolutionary model shares common features with recent ridges in other accretionary wedges in this model a strict relationship between tectonics seepage and sediment instability during a major tectonic phase is outlined

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

clay minerals as indicators of sources of terrigenous sediments their transportation and deposition bering basin russian alaskan arctic abstract cluster analysis of clay mineral abundances in surficial sediments from 52 stations revealed the presence of four station cluster groups within the bering basin and contiguous eastern aleutian trench forearc these results can be explained using two end members of terrigenous clay mineral sources and their admixtures the first end member cluster group i consists of the highest average concentration of illite 48 and chlorite plus kaolinite 40 and lowest of expandable minerals 12 in contrast the second end member cluster group iv has the highest concentration of expandable minerals 43 and lowest concentrations of illite 30 and chlorite plus kaolinite 27 the group i assemblages come from two regions the eastern aleutian trench forearc and the northwestern margin of the bering basin off koryak mountains east siberia clays in the former region are derived primarily from the meta sedimentary and meta volcanic rocks of the chugach mountains with infiltrations from alaska peninsula and are transported from the southeastern kodiak shelf of the gulf of alaska to the trench forearc by the alaska current alaskan stream similar clays are derived from the koryak mountains group iv clays located within the komandorsky and bowers basins and unmak plateau originated from the mio pliocene andesites and basalts of the eastern kamchatka peninsula and aleutian islands respectively it is postulated that group ii clays in the southcentral aleutian basin and bowers ridge basin area and group iii clays in the northern aleutian basin are mixtures obtained primarily from terrigenous sources of group i and iv the distribution patterns and boundaries of the cluster groups reflect the dispersal pathways of clays from the primary sources and the extent of their mixing by regional and local surface currents and mesoscale gyres eddies presumably the minor infiltration of anadyr river clays into group iii is a result of clay transport by episodic turbidity currents most of the clays derived from the alaska mainland and russia proper do not reach the bering basin as these clays are trapped within the bering sea shelf

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

source and depositional processes of the surface sediments and their implications on productivity in recent past off mahanadi to pennar river mouths western bay of bengal abstract spatial distribution and depositional processes of surface sediments along 4 transects from 27 to 2825 m water depth off mahanadi godavari krishna and pennar river mouths from western part of bay of bengal were studied using grain size clay minerals organic carbon nitrogen phosphorus calcium carbonate and metal concentration grain size distribution of surface sediments has revealed that overall the sediment size decreased with increasing water depth away from the coast which indicates a transport direction from west to east and also north to south in the study area high sand content and tio 2 values at shallow water depth indicate their terrigenous source and relatively high energy conditions prevailed near the shore off mahanadi river mouth illite was the dominant clay mineral while off godavari krishna pennar river mouths smectite was the dominant clay mineral indicating input from felsic and mafic rocks respectively slightly higher ti off godavari krishna rivers as compared to mahanadi also suggested mafic source for the sediments large illite concentration in the northern part of study area supports its major supply from himalayas furthermore the decreased illite concentration southwards indicates dilution of ganges borne clays from river inputs of peninsular region possible loss of energy of ganges waters during transport inhibits the southward movement of sediments along transect off mahanadi the c n ratio varied from 6 53 to 9 43 indicating a mixed source for organic matter derived from terrestrial as well as marine along godavari krishna and pennar river mouth regions the c n ratio varied between 8 19 and 13 54 indicating increased terrestrial organic matter input supplied through the rivers all transects showed higher c n values in shallower regions c n ratio in most of the sediments was higher than redfield ratio except in deeper areas off mahanadi where they are closer to redfield ratio however n p and c p ratios were lower than the redfield ratio indicating significant limitation of p in the area calcium carbonate concentration in this region was low due to increased clay loads which cause dilution of carbonates the turbidity occurring near river mouths due to suspension of fine particles creates oxidizing conditions under such oxygenated waters preservation of organic carbon and phosphorus is less possible thereby estimating reduced productivity than actual mn excess values observed in this region may also represent oxic waters prevalent in the region high fe excess values off mahanadi godavari and pennar indicate additional iron input from terrigenous source in addition ferric hydroxide minerals from the red beds off visakhapatnam must have contributed to fe excess the al ti ratios were distinctly different off the mouth of mahanadi godavari and krishna at 9 06 6 35 and 3 97 respectively suggesting that they are possibly provenance signatures of these three rivers and could be useful as typical end members

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

submarine landslides at subduction margins insights from physical models abstract submarine landslides which commonly develop along subduction margins at the front of accretionary prisms have recently been recognized as one of the most serious geohazards to investigate the mechanical processes involved in the development of these landslides a series of sandbox experiments were performed and the results were analyzed using the digital image correlation dic technique numerous slope failures which may be analogous to natural submarine landslides were produced on the model surface the slope failures can be classified as follows type i failures which are relatively small but are frequently developed on the lower parts of the slope and type ii failures which are relatively large and cover the entire slope but occur less frequently detailed observations reveal that the failure process can be divided into four stages 1 pre failure 2 steepening of the lower slope 3 type i failure and 4 type ii failure the minor type i slides act as precursors of major type ii failures each slope failure produces a topographic undulation triggering events in adjacent areas and a lateral migration of discrete slope failures although such experiments provide useful insight into slope failure processes a detailed 3d analysis of the failed sediment is vital if we are to understand the precise history of thrust activity at subduction margins recent and miocene accretionary prisms along the nankai trough japan display the geometric and lithologic characteristics of type i and type ii failures in their slide deposits

contribution of detrital materials from the yukon river to the continental shelf sediments of the bering sea based on the electron spin resonance signal intensity and crystallinity of quartz abstract bering sea sediments contain detrital materials from the yukon river these materials may contain records of past climate changes in the arctic area such as the melting of glaciers around the drainage basin of the yukon river which help to understand hydrological cycle in this area in the bering sea however the spatial extent and pattern of the detrital materials supplied from the yukon river is not yet fully understood here we developed a method to identify clay to sand sized detrital materials derived from the yukon river based on electron spin resonance esr intensity of the e 1 center and the crystallinity of quartz we then estimated the spatial pattern of quartz contributed by the yukon river on the bering sea shelf by applying the method to core top samples from the continental shelf and slope of the eastern bering sea the results showed a large contribution of sand sized quartz from the yukon river to wide areas of the continental shelf and slope whereas contributions of clay to silt sized quartz from the yukon river were small except on the northeastern shelf these spatial distribution patterns suggest that sand sized quartz was repeatedly reworked and transported by processes such as storm surges to the outer continental shelf whereas the clay to silt sized quartz on the northeastern shelf was supplied as suspended materials directly from the yukon river

the giant ruatoria debris avalanche on the northern hikurangi margin new zealand result of oblique seamount subduction despite convergent margins being unstable systems most reports of huge submarine slope failure have come from oceanic volcanoes and passive margins swath bathymetry and seismic profiles of the northern hikurangi subduction system new zealand show a tapering 65 30 km wide by 65 km deep margin indentation with a giant 3150 630 km3 blocky debris avalanche deposit projecting 40 km out across horizontal trench fill and a debris flow deposit projecting over 100 km slide blocks are well bedded up to 18 km across and 1 2 km high the largest being at the avalanche deposit s leading edge samples dredged from them are mainly miocene shelf calc mudstones similar to those outcropping around the indentation cores from cover beds suggest that failure occurred 170 40 ka possibly synchronously with a major extension collapse in the upper indentation however the northern part of the indentation is much older the steep straight northern wall is close to the direction of plate convergence and probably formed around 2 0 0 16 ma as a large seamount subducted leaving in its wake a deep groove obliquely across the margin and an unstable triangle of fractured rock in the 60 angle between groove and oversteepened margin front the triangle collapsed as a blocky avalanche leaving a scalloped southern wall and probably causing a large tsunami tentative calculations of compacted volumes suggest that the indentation is over 600 km3 larger than the avalanche supporting a two stage origin that includes subduction erosion since failure convergence has carried the deposits 9 km back toward the margin causing internal compression the eventual subduction accretion of the ruatoria avalanche explains the scarcity of such features on active margins and perhaps the nature of olistostromes in fold belts

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 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

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

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

clastic components in quaternary sediments of the northwest pacific and their paleo oceanic significance abstract ninety seven mineral analyses for the total 0 05 0 1 mm fraction and its heavy subfraction are given of late pleistocene to holocene sediments from the northwestern subarctic pacific including the kuril and aleutian trenches and bering sea heavy mineral assemblages indicating ice rafted debris and its main provenance were defined the assemblages are derived mainly from the northeastern bering sea shelf where large scale generation of sea ice is presumed temporal and spatial changes of the mineral suites also suggest intermediate to deep water flow along the eastern flank of the emperor chain during glaciation the flow might penetrate the northwest pacific basin to the south of 45 n it was traced by the fine and medium grained terrigenous debris in a core from 38 n and 165 e

wisconsin holocene paleoenvironment of the bering sea evidence from diatoms pollen oxygen isotopes and clay minerals abstract a core from the southeastern bering sea contains a depositional record of the wisconsin glacial interval and the holocene four lithologic units show distinctive changes in diatom species palynomorphs oxygen isotopes in diatom silica and clay minerals unit 4 the oldest probably represents the glacial advance of the early wisconsin and records extensive sea ice cover and a soggy herbaceous tundra on the exposed continental shelf unit 3 represents the stades and interstades of the middle wisconsin including an early period of high seasonal variability and a later period with expansion of winter sea ice in the basin and of tundra onshore unit 2 represents the late wisconsin glacial maximum and subsequent deglaciation with a tundra covered shelf sea ice cover increased to a maximum during which the surface waters were probably covered by sea ice throughout the year subsequent to the sea ice maximum the isotope ratios record a major meltwater event unit 1 represents the holocene with expansion of shrub tundra or scrub forest in the coastal upland and waters of increased temperature and salinity in the bering sea

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

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

westward intensification of the mean circulation on the bering sea shelf abstract most of the water that eventually flows northward through bering strait originates about 500 km south seaward of the shelfbreak in the bering sea cumulative observational evidence supports the idea that most of this northward flow across the gently shoaling eastern bering sea continental shelf occurs as a western boundary current along the siberian coast a homogeneous rotating laboratory model and a barotropic numerical model each demonstrate this westward intensification of the mean flow the intensification results from the well known topographic effect the combination of rotation and the depth decrease in the direction of flow acts in a similar fashion to the meridional gradient of the coriolis parameter for reasonable values of bering strait transport and shelf bottom friction current speeds of 10 20 cm s 1 and a current width of 50 km are predicted

sources and dispersal patterns of clay minerals in surface sediments from the continental shelf areas off alaska the three mineral suites were recognized in the gulf of alaska the glaciomarine sediments of fjords and the eastern shelf consist typically of the primary phyllosilicates micas and chlorites derived locally from argillite graywacke and metavolcanics under conditions of mild chemical weathering more glycol expandable clay minerals gecm in the north gulf substantiate clay supply to that region primarily from the yakataga and poul creek formations and to a lesser extent from the copper river the northern gulf clay mineral suite is transported to central prince william sound and northeast kodiak shelf by the alaska coastal current in the western gulf clay minerals adjacent to the alaska peninsula are derived principally from the susitna river via cook inlet whereas those of the outer shelf presumably have their provenance in kodiak island and the katmai ash submarine deposit in the southeast bering sea a progressive northwestward decrease in gecm from 65 to 11 is caused by a gradually decreased sediment export from the alaska peninsula and the aleutians with a possible minor increase in illite from the kuskokwin river the yukon river is the predominant clay source for the north bering sea as signified by an abrupt increase in that area of gecm 20 to 35 and of kaolinite chlorite ratios clay mineralogy indicates that the central chukchi sea is the major repository of the yukon clays suggesting that clay mineral stratigraphic studies have implications for interpretation of the quaternary paleogeography of beringia in the beaufort sea nearshore influence of different fluvial inputs and their dispersals by currents are sharply defined the lack of a definite pattern to the distribution of clay minerals in the outer continental shelf may be due to haphazard transport of clays by ice rafting and or reworking and redistribution of sediments resulting from ice gouging of the sea substrate movement of sediment to the west in the alaskan portion of the inner shelf of beaufort sea is evident an apparent seaward increase in illite gecm ratio in the colville delta is tentatively ascribed to regeneration of degraded illite micas by k fixation in saline waters an isolated area of sediment rich in expandable clay minerals in the central beaufort shelf presumably is relict

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

discovery of two new large submarine canyons in the bering sea abstract the beringian continental margin is incised by some of the world s largest submarine canyons two newly discovered canyons st matthew and middle are hereby added to the roster of bering sea canyons although these canyons are smaller and not cut back into the bering shelf like the five very large canyons they are nonetheless comparable in size to most of the canyons that have been cut into the u s eastern continental margin and much larger than the well known southern california canyons both igneous and sedimentary rocks of eocene to pliocene age have been dredged from the walls of st matthew and middle canyons as well as from the walls of several of the other beringian margin canyons thus suggesting a late tertiary to quaternary genesis of the canyons we speculate that the ancestral yukon and possibly anadyr rivers were instrumental in initiating the canyon cutting processes but that due to restrictions imposed by island and subsea bedrock barriers cutting of the two newly discovered canyons may have begun later and been slower than for the other five canyons

changes and influencing factors in biogenic opal export productivity in the bering sea over the last 4 3 ma evidence from the records at iodp site u1340 we reconstructed changes in biogenic opal export productivity boep in the southern bering sea bs over the last similar to 4 3 ma based on mass accumulation rate mar of biogenic opal from integrated ocean drilling program iodp site u1340 the results show that the boep in the bs was high and variable between similar to 4 3 and similar to 1 9 ma extremely low and relatively stable from similar to 1 9 to similar to 1 1 ma and then fluctuated frequently generally high during interglacials and low during glacials during the last similar to 1 1 ma one interval of enhanced boep from similar to 4 3 to similar to 3 2 ma is a response to the late miocene early pliocene biogenic bloom event another interval from similar to 2 8 to similar to 1 9 ma correlates with global opal burial shifting from high latitude oceans to upwelling influenced regions following the intensification of the northern hemisphere glaciation nhg whereas the increase in bs opal export productivity during the last similar to 1 1 ma tends to be a local phenomenon overall the boep shows a similar trend and good correspondence to the input of the alaskan stream as which can be traced using the na2o k2o ratio we thus conclude that the as may be the direct and primary factor on boep variability in the bs during the last similar to 4 3 ma in addition although the poor correlation between opal mar and volcanic glass suggests that boep variability was not controlled by longterm variations in the volcanism or ash abundance increased ash abundance indicated by high contents of volcanic glasses was also a possible reason for enhanced boep during the period from similar to 4 3 to similar to 3 2 ma and the last similar to 0 5 ma

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

clay mineralogy and chemistry as indicators of hemipelagic sediment dispersal south of the aleutian arc abstract near surface hemipelagic sediments from across the aleutian arc trench system were analyzed for clay mineral composition by x ray diffraction in addition oriented aggregates of clay sized fractions were analyzed for their chemical compositions by energy dispersive x ray analysis eds in conjunction with scanning electron microscopy the main clay minerals are illite chlorite and expandable clay minerals principally smectite smectite is diagnostic of sediment derived from the aleutian volcanic arc in contrast continental sources on the alaskan mainland produce mainly illite and chlorite backarc samples adjacent to the central arc segment contain an average of 71 smectite in the central forearc however smectite values are generally less than 30 and decrease progressively eastward only on meiji sediment tongue in the far western aleutian segment do smectite contents begin to dominate the eds data correlate well with clay mineral abundances al 2 o 3 mgo and feo all increase in abundance with increasing chlorite content and al 2 o 3 and k 2 o increase in illite rich mud silica is the only oxide which increases in abundance with increasing smectite content the compositional and chemical disparity between forearc and backarc sediments is due to dilution of arc derived mud by suspended sediment derived from mainland alaska the westward transport of mud for thousands of kilometers occurs both through isobath parallel bottom currents and by westward flowing turbidity currents these results show that interpretations of tectonic provenance cannot be based on elemental concentrations or mineralogy without also taking into account the possibility of long distance transport

diffuse spectral reflectance of surficial sediments indicates sedimentary environments on the shelves of the bering sea and western arctic abstract visible diffuse spectral reflectance dsr has been described by previous workers as an efficient tool for extracting lithological compositions clay minerals iron oxyhydroxides carbonates and diatoms in sedimentary material the spatial patterns of these components are essential to understanding the processes driving their distribution and accumulation in the marine environment some of these processes include sea ice variability nutrient supply current flow patterns current strength sediment supply provenance and bathymetry the dsr derivatives and varimax rotated principal component analysis vpca of the spectral data collected from 234 core samples identified five leading components that account for variations in sediment lithology of the surface samples from the bering sea and western arctic these spectral components were then matched to spectral standards to identify the mineral assemblages responsible for them the components responsible for the vpca trends are chlorite muscovite goethite phycoerythrin phycocyanin smectite calcite dolomite and illite chlorophyll a the spatial pattern of the inorganic components is indicative of provenance and processes controlling their transportation and depositional patterns while the organic components are indicative of areas of primary productivity and driving mechanisms the chlorite muscovite assemblage which is sourced dominantly from the yukon river has maximum values on the bering sea shelf this is the source of the chlorite muscovite transport to the chukchi sea the chlorite muscovite assemblage also has a main transport route to the aleutian basin in the northern bering sea shelf the chukchi sea on the western arctic ocean shelf is a region of highly reducing condition this is based on the lowest values for the goethite phycoerythrin phycocyanin assemblage supported by the high carbonate concentrations at this location the carbonate distribution pattern on the bering sea shelf also appears to be tidally influenced this is because their spatial pattern divides the bering shelf into three domains which are similar to the three hydrographic domains on the bering shelf a function of tidal influence the smectite assemblage reaches maximum values along the coast of the chukotka peninsula in the chukchi sea denoting the chukotka peninsula as a major source area of smectite and supporting the notion for volcanically derived sediment to the chukchi sea another likely source of smectite to the chukchi sea is via the bering strait the illite in the illite chlorophyll a assemblage shows maximum values and distribution on the western arctic ocean shelf indicating the western arctic ocean shelf to be a main province of illite all the high values of the illite chlorophyll a component on the bering sea shelf are chlorophyll a from diatoms our results support the usefulness of the dsr method as an effective and efficient tool for provenance analysis and understanding the processes controlling sediment and biologic distribution in this case in the marine environment

kelvin waves and the quasi biennial oscillation an observational analysis a method is derived for estimating the kelvin wave contribution to the vertical flux of westerly momentum in the equatorial stratosphere which is based on temperature and geopotential perturbations this method is used to estimate the momentum transfer due to kelvin wave activity as derived from the cryogenic limb array etalon spectrophotometer claes temperature data set for the onset of the westerly phase of the qbo during 1992 and the first few months of 1993 that is during the first part of the upper atmosphere research satellite uars mission the results are compared with the zonal winds as observed by the high resolution doppler imager hrdi also flown on board uars and the united kingdom meteorological office ukmo data assimilation model product a correlative data set to the uars mission the analysis shows that although the kelvin wave momentum flux convergence is occasionally sufficient to account for the observed qbo westerly acceleration the observed flux is sporadic in nature and virtually disappears during the second half of the sample when the westerly vertical shear zone approaches the 100 hpa level an estimate of the total westerly momentum flux necessary to produce the observed descent of the westerly phase of the qbo is made using the transformed eulerian mean tem formalism the results suggest that kelvin waves are not sufficient to force the descent of the westerly phase of the qbo there appears to be a need for enhanced westerly forcing throughout the descent of the westerly phase of the qbo this enhanced forcing is most likely provided by gravity waves that are unresolved by the satellite observations these results are in agreement with the results derived from general circulation models

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

reconstructing the origin and trajectory of drifting arctic sea ice recent studies have indicated that drifting arctic sea ice plays an important role in the redistribution of sediments and contaminants here we present a method to reconstruct the back ward trajectory of sea ice from its sampling location in the eurasian arctic to its possible site of ori gin on the shelf based on historical drift data from the international arctic buoy program this method is verified by showing that origins derived from the backward trajectories are generally con sistent with other indicators such as comparison of the predicted backward trajectories with known buoy drifts and matching the clay mineralogy of sediments sampled from the sea ice with that of the seafloor in the predicted shelf source regions the trajectories are then used to identify regions where sediment laden ice is exported to the transpolar drift stream from the new siberian islands and the central kara plateau calculation of forward trajectories shows that the kara sea is a major contributor of ice to the barents sea and the southern limb of the transpolar drift stream

enhanced particle scavenging in deep water of the aleutian basin revealed by 210po 210pb disequilibria the high sedimentation rate but low primary production in surface ocean raised a question whether particles from local upper water column could support high deposition in the aleutian basin here we first present large 210po 210pb disequilibria in deep water of the aleutian basin dissolved 210po and 210pb were depleted relative to 210pb and 226ra respectively in deep water below 1000 m as well as decreased with depth suggesting enhanced particle scavenging in the deep water the 210po residence times 1 2 a in deep water were comparable to those in the upper water column indicating that 210po scavenging rates were high in deep water of the aleutian basin the export fluxes of 210po from the upper 100 m were estimated to be 0 2 0 8 bq m2 d much lower than those in the deep water 7 8 bq m2 d similarly poc export fluxes in deep water 24 80 mmolc m2 d were higher than those in the upper 100 m 1 mmolc m2 d such a large discrepancy between the upper and deep water suggested that particles from local upper water column could not totally meet the enhanced scavenging in the deep water based on mass balance calculations the extra fluxes of 210pb and poc imported to deep water were estimated to be 8 12 bq m2 d and 22 79 mmolc m2 d respectively the ratio of poc to particulate 210pb i e poc ppb in the extra source was estimated to be 6 5 mmol bq which was lower than that in the bering shelf with a mean poc ppb ratio of 10 9 mmol bq implying that particles in the bering shelf could be a potential source for the enhanced particle scavenging in deep water of the aleutian basin however quantitative and detailed role of ridges and manganese from sediments in particle scavenging in the deep water was unclear and further studies are necessary

circulation in the bering sea basin observed by satellite tracked drifters 1986 1993 abstract from 1986 through april 1993 86 satellite tracked buoys were deployed in the north pacific and bering sea most of the buoys were drogued at 40 m a composite current pattern is derived using these data the two principal currents the alaskan stream and kamchatka current are clearly evident eddy kinetic mean kinetic energy ratios are low in the stream and along the western bering sea basin an eastward flowing current occurred along the north flank of the aleutian islands this flow was modified by inflow at the passes westward flow occurred north of 56 n its source was the bering slope current the kamchatka current originated near 175 e along the russian coast numerous eddies and meanders were observed in the kamchatka current eddies were also present on the eastern side of the basin

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

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

clay mineralogy of a glacially dominated subarctic continental shelf northeastern gulf of alaska abstract glaciation is the single most important process responsible for contributing sediment to the northeastern gulf of alaska continental shelf over 75 percent of the northeastern gulf s drainage basin is presently ice covered or only recently has emerged from beneath holocene glacial ice many glaciers terminate in rivers that drain into the gulf or terminate in tidewater bays this study examines the mineralogy of the northeastern gulf of alaska continental shelf clay minerals in light of the glacially dominant nature of the area analyses of 110 bottom samples from the gulf of alaska continental shelf between icy point and prince william sound show an average clay mineral assemblage of 61 percent kaolinite chlorite 37 percent illite and 2 percent smectite over most of the shelf deviations from the average composition are generally 10 percent one significant exception is an area south of the copper river where illite content is as high as 65 percent and kaolinite chlorite ranges from 35 to 48 percent suspended sediment and bottom samples collected in successive years from the copper river the largest sediment source to the gulf have variations in clay mineralogy suggesting that different tributaries and hence different source areas control the composition of the river s mineralogy during different years upper river sources are rich in kaolinite chlorite whereas illite rich sediment is introduced in the lower 100 km transportation of sediment south of the copper river across the shelf and into prince william sound is documented by a decrease in illite from a continental shelf point source south of the river s mouth in addition to modern fluvial input a second source of the modern shelf sediment appears to be the glacially derived miocene to quaternary yakataga formation older formations such as the poul creek and katalla formations show fewer compositional similarities to nearby shelf sediment

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

circulation in the bering sea 1982 83 based on satellite tracked drifter observations abstract a satellite tracked drifting buoy deployed unintentionally in the bering sea in 1982 completed a circuit of that basin in about one year during its cyclonic passage around the bering sea it experienced many different flow regimes ranging from steady alongshelf motion at the shelf break to highly variable tidal flow on the shelf itself the buoy trajectory differs from previous descriptions of the deep bering sea circulation because it moved southwestward in the central bering sea rather than along the western boundary as other depictions have suggested many of the bering sea mesoscale eddies reported earlier were evident indicating that possibly these eddies are permanent features the forces which caused the buoy to move between the various flow regimes are unclear but the data suggest an annual period for the deep bering sea circulation

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

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

mineralogical and biogenic composition of the zanzibar channel sediments tanzania sediments from the zanzibar channel have been analysed for mineralogical and biogenic composition the main objective of the study was to describe the depositional environment of the sediments based on these parameters the study was also conducted to give an insight into sediment provenance of the sediments quartz feldspar both microcline and plagioclase and hornblende are the main mineralogical components while zircon muscovite biotite and augite occur as accessory minerals the quartz grains are generally angular to sub angular and highly strained indicative of immature sediments probably derived from highly metamorphosed rocks the presence of hornblende also supports the conclusion that the sediments are immature a possible source is the metamorphic rocks of the mozambique belt located about 25 50 km from the coastline benthic foraminifera are the dominant biogenic constituents other common biogenic constituents are molluscs pelecypods and gastropods and corals bryozoans and ostracods are relatively less common while sea urchins are accessory biogenic components the composition and abundance of these biogenic components and morphological sedimentological and mineralogical data permit construction of a simple model which distinguishes between three bio physiographic settings in the zanzibar channel using depth contours 1 the coastal zone 0 10 m depth 2 the reef platform patch reefs zone 10 20 m depth and 3 the central channel zone 20 cm depth in the coastal zone the sediments are dominated by fine sand sized with a very prominent fine sand mode the proportion of benthic foraminifera is lower than in the other two zones 25 two lithogenic mineral components quartz and feldspar occur in significant proportions in the reef platform patch reefs zone the sediments are coarse to medium sand sized with no distinct mode the proportion of benthic foraminifera is highest 80 85 there and so is the biogenic quartz ratio 10 but that of lithogenic components is lowest 5 in the central channel zone the sediments are medium to fine sand sized and often bimodal with a prominent fine mode and a less prominent coarse mode the proportion of benthic foraminifera 70 75 is also higher than in the coastal zone but slightly lower than in the reef zone the proportion of lithogenic components 5 20 is higher than in the reef zone but lower than in the coastal zone and the biogenic quartz ratios is intermediate

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

clay mineral dispersal patterns in the north bering and chukchi seas abstract characterization of the clay mineralogic constituents of the m e s d equivalent spherical diameter size fraction of contemporary marine and fluvial sediments from the north bering sea chukchi sea region and adjacent land areas has resulted in delineation of suites dominated by broadly defined illitic and expandable group components with lesser amounts of chlorite and kaolinite distribution patterns elucidated indicate net northward transport of sediments relatively enriched in the expandable group with predominant terrigen ous source from the yukon river system this material is distributed across the study area in a medial zone along a general north south trend with peripherally decreasing relative amounts of this expandable group component and concomitant relative increases in the illitic components these patterns are consistent with known physical oceanographic and regional geologic relationships in terms of sediment sources transport processes and depositional mechanisms the foregoing suggest further that consideration of the stratigraphic relationships of these clay mineral component types as a function of time in relevant marine sediments north and south of the present bering strait should be informative with respect to regional quaternary paleogeography as related to global sea level fluctuations during glacial and interglacial episodes

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

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

clay mineral stratigraphy during the last 2 4ma at iodp exp 323 site u1343 in the bering sea abstract the clay mineral composition at iodp exp 323 site u1343 in the bering sea was analyzed so as to unravel their provenance over glacial interglacial cycles for the last 2 4 ma smectite was negatively correlated with the sum of illite and chlorite therefore their ratio s i c was used as an indicator of clay mineral composition changes in general the s i c ratio was rather similar for glacial and interglacial periods during most of the last 2 4 ma in addition these results overlap with those of surface sediments in the modern east aleutian basin which suggests that smectite rich clay particles are delivered from the aleutians by the northward bering slope current bsc rather than from rivers in western alaska however some clay mineral compositions of the glacial periods after the mid pleistocene transition mpt 1 25 0 7 ma were characterized by low illite and relatively high smectite during this period extensive ice rafting might have transported the smectite rich clay particles to site u1343 from the glacial shelf off alaska and or from glacial rivers from that area

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