the denver earthquakes of 1967 1968 abstract a detailed study of the earthquakes associated with the rocky mountain arsenal disposal well is presented long period surface wave studies are used together with p wave first motions to show that the 10 april 9 august and 27 november 1967 earthquakes occurred at depths of 3 to 5 km and were characterized by normal faulting along a northwest striking fault plane a joint hypocenter relocation of 103 microearthquakes of a data set of 279 recorded between 1967 and 1968 shows a hypocenter pattern striking n50 w with most of the events located about 5 km northwest of the disposal well at depths between 3 to 8 km a fault plane dipping southwest is tenuously suggested by those earthquakes with depths less than 5 km modeling of near field seismoscope observations lend support to the focal mechanisms derived

earthquake nests as natural laboratories for the study of intermediate depth earthquake mechanics article the physical mechanism of intermediate depth earthquakes is still under debate in contrast to conditions in the crust and shallow lithosphere at temperatures and pressures corresponding to depths 50 km rocks ought to yield by creep or flow rather than brittle failure some physical process has to enable brittle or brittle like failure for intermediate depth earthquakes the two leading candidates for that are dehydration embrittlement and ther mal shear runaway given their great depth intermediate depth earthquake processes can t be observed direct ly insteadwe must relyona combination ofseismologyand thestudy of laboratory analogs tounderstand them earthquake nests are regions of highly concentrated seismicity that are isolated from nearby activity in this paper we focus on three intermediate depth earthquake nests vrancea hindu kush and bucaramanga and what they reveal about the mechanics of intermediate depth earthquakes we review published studies of tectonic setting focal mechanisms precise earthquake locations and earthquake source physics at these locations with an emphasis on the bucaramanga nest all three nests are associated with subducting litho sphere and at least two of the nests have consistently larger stress drops compared to shallow seismicity in contrast the bucaramanga nest has a larger b value larger variability of focal mechanisms and shows no evidence of aftershock sequences unlike the other two we also report for the first time finding a signifi cant number of repeating earthquakes some with reverse polarity

sea level control on the submarine fan architecture in a deepwater sequence of the niger delta basin abstract submarine fan architecture has long been considered much complicated as controlled by both allogenic and autogenic mechanisms however to which extent the allogenic sea level change controls the submarine fan architecture is still unclear this study uses integrated 3 d seismic well log and core data to characterize the submarine fan architecture in a deepwater sequence of the niger delta basin correlation of the timing of architecture units to high frequency sea level curve helps to investigate the sea level control on architecture distribution and evolution of submarine fan systems the studied sequence is a 3rd order sequence that formed during 12 5 10 5 ma and contains a lowstand systems tract lst and a transgressive highstand systems tract tst hst we find that the lst develops a single channel system that is terminated by a lobe system whereas the tst hst develops two mutually incised channel systems and mudflow deposits which are distinct from the conventional condensed sections and interpreted to result from the repeated 4th order sea level cycles during the tst hst of the 3rd order sequence single submarine fan systems which last for 0 3 0 6 myr are assumed to form in response to single 4th order sea level cycles since they are well correlated with each other each single submarine fan system contains several vertically stacked complex sets which are well correlated with 5th order sea level cycles that last for 0 1 0 2 myr suggesting their potential genetic links different complex sets in a single submarine fan system exhibit variable architectural features which are interpreted to result from the evolution of gravity flow types as the 4th order sea level fluctuates thus for the niger system at time scales 0 1 myr the allogenic sea level change is considered to have played a major role in controlling the submarine fan architecture at the scale of submarine fan systems and complex sets whereas autogenic effects on the submarine fan architecture may increase and become dominant on time scales

geometry of the subducted plate near kodiak island and lower cook inlet alaska determined from relocated earthquake hypocenters abstract using observations from 28 seismograph stations in southern alaska we have used the jhd method to relocate 341 well recorded events with focal depths beneath about 40 km which occurred between 56 4 n and 60 5 n and between 151 6 w and 156 5 w these hypocenters show that the wadati benioff zone has a dip of about 45 however its strike changes by about 15 beneath lower cook inlet at about 59 n near where the volcanic line also bends northward near this bend there is also a distinctly higher level of seismic activity than in the area to the southwest of 59 n a few events are situated about 20 km beneath the main wadati benioff zone in cook inlet and a focal mechanism we have determined for one of these differs from other mechanisms for events in this area focal mechanisms available from previous work show that at intermediate focal depths the area between 59 n and 60 5 n is controlled by horizontal north south compression and as such differs from the region to the southwest relying primarily on the observed change in strike and apparent thickness of the wadati benioff zone we interpret these data to suggest that the subducted lithophere separates into two distinct segments as it subducts beneath cook inlet because the direction of subduction is not perpendicular to the volcanic arc the events situated beneath the wadati benioff zone in cook inlet may be part of the southwestern plate segment although the geometry and available focal mechanisms for events beneath cook inlet are similar to what would be expected in a double wadati benioff zone we suggest instead that these observations result because we are at the boundary between two adjacent segments of the subducting plate

seismicity of the unalaska region alaska the wadati benioff zone beneath unalaska island in the eastern aleutian arc is shown to dip at approximately 48 between the depths of 50 and 150 km above this depth the seismic zone appears to dip at a more shallow angle 12 the depth of the wadati benioff zone beneath makushin volcano on unalaska island is about 100 km events located within the upper plate near the wadati benioff zone indicate that the maximum depth of seismic coupling between the subducting and overriding plates is about 30 km this suggests that the maximum width of seismic coupling between the downgoing and overriding plates is about 75 km earthquake locations based on the local network data and 20 yr of teleseismic data pde define similar seismically active and quiescent regions a major seismicity gap exists for events of magnitudes greater than at least 4 6 in the forearc region near unalaska island this seismicity gap correlates well with the eastern edge of the tsunami source area of the great mw 8 6 1957 earthquake and a large forearc basin 75 km south of unalaska island suggesting a similar process may be controlling the location of all three phenomena only one intermediate sized mb 6 1 earthquake has occurred within the seismicity gap during the past 20 yr this event is shown to be a high stress drop 690 bars intraplate event within the subducting slab at a depth of 35 km that may be related to lateral compressive stress induced by a change in slab curvature with depth a review of the historic record from about 1770 reveals that the unalaska region may have ruptured in a great earthquake in 1878 and a large earthquake in 1902 if this region is currently locked an earthquake of magnitude 8 2 could rupture this section of the alaska aleutian arc estimates of the recurrence interval for events of this size vary from between 40 and 70 yr

shumagin seismic gap alaska peninsula history of great earthquakes tectonic setting and evidence for high seismic potential the shumagin seismic gap a segment of the plate boundary along the eastern aleutian arc has not ruptured during a great earthquake since at least 1899 1903 because at least 77 years have elapsed since the shumagin gap last ruptured in a great earthquake and repeat times for the 1938 rupture zone and part of the shumagin gap are estimated to be 50 to 90 years a high probability exists for a great earthquake to occur within the shumagin gap during the next one to two decades reconsideration of the rupture zones of the aleutian earthquakes of 1938 1946 and 1948 suggests that those events did not break the interplate boundary beneath the shumagin islands thus the shumagin seismic gap extends from the western end of the 1938 rupture zone to the eastern end of that of 1946 these boundaries also coincide with transverse structural features at least the eastern half of the shumagin gap broke in great earthquakes in 1788 and 1847 and possibly in 1898 1903 the shumagin gap is probably not the result of aseismic slip rather plate motion is accommodated there seismically and episodically and can be expected to produce large earthquakes in the future although there is no definitive evidence of long term precursors of a possible future earthquake several observations suggest that the shumagin gap is in an advanced stage of the earthquake cycle both teleseismic and local network data indicate a near absence of seismic activity m 2 above a depth of 30 km along the main thrust zone within the gap this is in strong contrast to adjacent portions of the arc where seismic activity is scattered across most of the main thrust zone two earthquakes with high stress drops 600 900 bars which occurred at the base of the main thrust zone may indicate the accumulation of a considerable amount of strain energy within the gap a possible seismic gap at the eastern end of the aftershock zone of the aleutian earthquake of 1957 has been identified near unalaska island an earthquake that nucleates in the shumagin gap could also rupture the possible unalaska gap to the west the 1938 aftershock zone to the east or both with resultant magnitude up to mw 9 0 alternatively the shumagin gap alone or in one of the above combinations could rupture in a series of very large earthquakes instead of a single great shock past alaska aleutian earthquakes including those of 1788 1938 1946 1957 1964 and 1965 have generated very large tsunamis future large earthquakes in the shumagin islands region could generate wave heights of several tens of meters along shorelines near the rupture areas the shumagin gap is one of two major gaps along the united states portion of the alaska aleutian plate boundary and is one of the few areas in the united states where processes leading to a great earthquake are likely to be observed within a reasonable span of time

progressive multiple event location pmel abstract this paper introduces a new method of using earthquake arrival times to simultaneously determine earthquake hypocenters and station corrections progressive multiple event location pmel forms a solution in three major steps 1 hypocenters are located by conventional single event methods using current station correction estimates 2 an annulled data set pavlis and booker 1980 is formed and accumulated using householder transformations and 3 station correction perturbations are calculated from a pseudoinverse solution of the annulled data set in practice pmel converges rapidly we analyze the non uniqueness of pmel estimates in terms of resolution and covariance matrices one of the most important features of pmel is the computer algorithm that we describe which requires only a modest amount of computer memory that is independent of the number of earthquakes to be processed consequently this algorithm can process a virtually unlimited number of earthquakes we apply pmel to a set of 8049 arrival times from 652 earthquakes and 4 explosions recorded by a network in the coso range inyo county california pmel converges after four adjustments to the station corrections the final rms misfit of 0 05 sec is remarkably good and the station correction estimates correlate strongly with surface geology

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

analysis of the kodiak shelf basin using 1d and 2d fluid flow compaction models the kodiak shelf basin is one of the undeveloped basins in alaska in surrounding areas hydrocarbon accumulations have been found it is of interest to investigate whether there might also be hydrocarbons generated in the kodiak shelf basin using both one and two dimensional fluid flow compaction models together with geological geophysical and geochemical data from the kodiak shelf basin we simulate the development with time and depth of sedimentary compaction porosity permeability pressure palaeoheat flux variation and palaeotemperature variation we then evaluate quantitatively the maturation of organic matter in the basin the model results indicate that there was only a low likelihood of hydrocarbon generation in the basin due mainly to the combined effects of a cool palaeothermal history and low organic carbon content

seismicity of the wrangell and aleutian wadati benioff zones and the north american plate along the trans alaska crustal transect chugach mountains and copper river basin southern alaska the configuration of the aleutian and wrangell wadati benioff zones in southern alaska between longitudes 142 w and 148 w is determined from regionally recorded earthquakes from 1971 to 1986 the earthquakes are located with a simple velocity model calibrated with seismic refraction measurements conducted for the trans alaska crustal transect tact west of the transect meridian approximately 145 20 w and beneath the chugach mountains shocks in the depth range 25 60 km compose the shallow part of the northwest to north dipping aleutian zone this zone can be traced updip and seaward beneath prince william sound east of the transect meridian and north of 61 n latitude the weakly active north northeast dipping wrangell zone extends to a depth of at least 100 km beneath the wrangell volcanoes and along strike for at least 150 km the two zones appear to be continuous at least in the depth range 20 45 km and may define adjacent limbs of a buckle in the subducted oceanic lithosphere north of 61 n focal mechanisms in the shallow aleutian zone are compatible with a uniform stress field characterized by in plane least compressive stress directed toward the steeply dipping deeper part of the aleutian zone north of cook inlet stress orientation in the wrangell zone is unresolved because of the low level of seismicity no focal mechanisms corresponding to slip on a gently northward dipping megathrust have been observed along the transect meridian a change of focal mechanism with depth near the top of the dipping seismic zone is inferred to reflect the interplate boundary beneath the chugach mountains this boundary coincides with a band of strong reflectors imaged by tact vertical seismic profiling the overriding plate appears to be aseismic beneath the northern chugach mountains but minor shallow seismicity occurs beneath the copper river basin first motion patterns for these shallow shocks suggest the most compressive stress within the overriding plate is oriented subhorizontally and approximately north south

a high resolution local network study of the nazca plate wadati benioff zone under western argentina this paper presents a precise determination of the fine structure of a wadati benioff zone based on analysis of analog and digital seismic data recorded by a local network in the san juan province of western argentina the network is located above one of the subhorizontal segments of the subducted nazca plate without the complicating effects of a dipping slab on the locations of earthquakes the nearly flat geometry of the subducted plate provides an excellent opportunity to determine the thickness and structure of the wadati benioff zone we found that the depth of the wadati benioff zone beneath the network is 107 5 km for a set of digitally recorded events the distance between the shallowest and the deepest event gives a thickness of 20km with 90 of the data concentrated in a zone 12 km thick the spatial distribution of hypocenters shows no evidence of complex internal structure such as a double wadati benioff zone the local network data also suggest that in the region near 32 s where the dip of the wadati benioff zone changes from flat to steep the nazca plate flexes rather than tears at depths less than at least 125 km the intermediate depth seismicity is separated from an active zone of crustal seismicity by an aseismic region between depths of about 40 and 95 km the upper surface of the subducted plate inferred to be at about 90 km provides an upper limit to the thickness of the south american plate in this region an apparent westward dip of the wadati benioff zone as determined by the local network data can be accounted for by a 6 westward dip of the moho beneath the san juan region a result that is in agreement with regional gravity and topographic data

new evidence for segmentation of the alaska subduction zone the purpose of this article is to provide additional evidence for segmentation of the subducted plate in alaska and to introduce a catalog of the relocated earthquakes for future studies we used the joint hypocenter determination method to relocate 14 099 subduction zone earthquakes that occurred from july 1988 to july 1998 and were located between 58 n and 65 n latitude the earthquake data were taken from the alaska earthquake information center catalog the selected earthquakes were divided into 16 blocks on the basis of their hypocentral locations and each block was relocated separately average epicenter shift was 3 8 km and average upward and downward depth shifts were 4 1 and 4 4 km respectively roughly the same number of earthquakes shifted upward 47 and downward 53 the overall change with respect to the initial locations is that the seismicity became more compact revealing details about the fine structure of the wadati benioff zone in particular we were able to identify more precisely the boundary between the kenai and mckinley blocks of the subducting plate in addition there is evidence for plate segmentation within the mckinley block manuscript received 15 november 2000

crustal detachment during south atlantic rifting and formation of tucano gabon basin system a series of major sedimentary basins of mesozoic to tertiary age are associated with the brazilian continental margin between 8 and 14 s fig 1 this passive continental margin formed when africa and south america split apart with the initiation of seafloor spreading in the aptian albian3 the offshore basins which border the continental margins on both sides of the atlantic show evidence of both rifting and crustal attenuation before break up and thermal subsidence after it in contrast a combined stratigraphical and gravity study of the brazilian onshore basins which are separated from the offshore basins by a northward widening strip of precambrian suggests that up to 7 km of non marine sediments were deposited during the rifting stage of the south atlantic with no apparent extension of the lower crust beneath these basins and no subsequent thermal subsidence a mechanism for the linked development of these onshore and offshore basins is proposed here along the lines suggested by wernicke4 5 in which upper crustal extension in the onshore region is connected to and balanced against deeper lithospheric extension beneath the incipient passive margin by intracrustal detachments

intermediate and deep seismicity and lateral structure of subducted lithosphere in the circum pacific region in this paper we present a region by region review of the wadati benioff zone structure of most of the world s seismically active subduction zones focusing primarily on the intermediate and deep seismicity lateral changes in wadati benioff zone structure are common in every major subduction zone in this study we use these changes to define possible boundaries between portions or segments of lithosphere with differing subduction geometries although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate rather than tearing we found the strongest evidence for the existence of tears where tears are geometrically necessary such as where a transform boundary terminates a trench as at the new hebrides tonga and south sandwich trenches weak evidence suggesting other tears does exist in some regions such as taiwan the japan izu bonin corner and the philippines the causes of these changes in structure are in most cases unclear only about 34 of the possible segment boundaries coincide with subducting bathymetric features some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry we have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and wadati benioff zones to estimate the lateral strain in subduction zones throughout the circum pacific region although no observed subduction zone has a perfectly strain free geometry there is a broad range of geometries for which the lateral strain is small indeed the observed geometry of most subduction zones involve relatively little lateral strain comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain the mechanisms of intermediate and deep earthquakes show no effects of lateral stress and downdip stresses are clearly dominant in regions such as the mariana arc where the model predicts very large lateral extension lateral tension is very evident in the focal mechanisms in regions such as the hokkaido corner where the modeling predicts large compressional strains the plate appears to buckle and bending stresses parallel to the trench are evident in general this study finds that subducted lithosphere is remarkably cohesive and rigid and only rarely deforms by breaking or stretching

evidence for late messinian seismites nijar basin south east spain the feos formation of the nijar basin comprises sediments deposited during the final stage of the messinian salinity crisis when the mediterranean was almost totally isolated levels of soft sediment deformation structures occur in both conglomeratic alluvial sediments deposited close to faults and the hyposaline lago mare facies a laminated and thin bedded succession of whitish chalky marls and intercalated sands alternating with non marine coastal plain deposits deformation structures in the coarse clastics include funnel shaped depressions filled with conglomerate liquefaction dykes terminating downwards in gravel pockets soft sediment mixing bodies chaotic intervals and flame structures evidence for soft sediment deformation in the fine grained lago mare facies comprises syndepositional faulting and fault grading sandstone dykes mixed layers slumping and sliding of sandstone beds convolute bedding and pillar and flame structures the soft sediment deformed intervals resemble those ascribed elsewhere to seismic shaking moreover the study area provides the appropriate conditions for the preservation of deformation structures induced by seismicity such as location in a tectonically active area variable sediment input to produce heterolithic deposits and an absence of bioturbation the vertical distribution of soft sediment deformation implies frequent seismic shocks underlining the importance of seismicity in the betic region during the late messinian when the nijar basin became separated from the sorbas basin to the north the presence of liquefied gravel injections in the marginal facies indicates strong earthquakes m 7 the identification of at least four separate fissured levels within a single lago mare interval suggests a recurrence interval for large magnitude earthquakes of the order of millennia assuming that the cyclicity of the alternating lago mare and continental intervals was precession controlled this suggestion is consistent with the present day seismic activity in se spain

middle to late miocene oxygen isotope stratigraphy of odp site 1085 se atlantic new constrains on miocene climate variability and sea level fluctuations abstract the middle miocene 18o increase represents a fundamental change in earth s climate system due to a major expansion and permanent establishment of the east antarctic ice sheet accompanied by some effect of deepwater cooling the long term cooling trend in the middle to late miocene was superimposed by several punctuated periods of glaciations mi events characterized by oxygen isotopic shifts that have been related to the waxing and waning of the antarctic ice sheet and bottom water cooling here we present a high resolution benthic stable oxygen isotope record from odp site 1085 located at the southwestern african continental margin that provides a detailed chronology for the middle to late miocene 13 9 7 3 ma climate transition in the eastern south atlantic a composite fe intensity record obtained by xrf core scanning odp sites 1085 and 1087 was used to construct an astronomically calibrated chronology based on orbital tuning the oxygen isotope data exhibit four distinct 18o excursions which have astronomical ages of 13 8 13 2 11 7 and 10 4 ma and correspond to the mi3 mi4 mi5 and mi6 events a global climate record was extracted from the oxygen isotopic composition both long and short term variabilities in the climate record are discussed in terms of sea level and deep water temperature changes the oxygen isotope data support a causal link between sequence boundaries traced from the shelf and glacioeustatic changes due to ice sheet growth spectral analysis of the benthic 18o record shows strong power in the 400 kyr and 100 kyr bands documenting a paleoceanographic response to eccentricity modulated variations in precession a spectral peak around 180 kyr might be related to the asymmetry of the obliquity cycle indicating that the response of the dominantly unipolar antarctic ice sheet to obliquity induced variations probably controlled the middle to late miocene climate system maxima in the 18o record interpreted as glacial periods correspond to minima in 100 kyr eccentricity cycle and minima in the 174 kyr obliquity modulation strong middle to late miocene glacial events are associated with 400 kyr eccentricity minima and obliquity modulation minima thus fluctuations in the amplitude of obliquity and eccentricity seem to be the driving force for the middle to late miocene climate variability

additional evidence for down dip tension in the pacific plate beneath central alaska abstract small intermediate depth earthquakes cluster in a volume about 20 km in diameter beneath gold king a landing strip on the northern flanks of the alaska range these events almost 600 km from the aleutian trench are the northern most which can be identified as being associated with the benioff zone extending inland from cook inlet west of anchorage first motion observations of the gold king events at local stations can be combined to determine a tensional axis dipping steeply to the north at an azimuth just west of north this composite mechanism suggests that the slab is subjected to down dip tensional forces probably as a result of gravitational sinking

contourite drift molded by deep mediterranean outflow since its inception in early pliocene time the deep mediterranean outflow has eroded and sculptured the southern iberian margin the 50 km long 300 m thick faro drift constructed by this powerful bottom current has been the subject of detailed study sandy silty and muddy contourite facies can be clearly characterized and a distinctive vertical arrangement or sequence of these facies can be related to hydrodynamic fluctuation these results are important for the identification of ancient contourites the reconstruction of paleocirculation patterns and documentation of the effects of quaternary climates on the mediterranean outflow

abyssal current character determined from sediment bedforms on the nova scotian continental rise abstract bottom photographs across the nova scotian continental rise show effects of an intense bottom current at 4800 5000 m that decreases in strength upslope to tranquil near bottom conditions at 3200 4000 m apparently weak currents are present above 3200 m long term geologically significant flow parallels the bathymetric contours toward the southwest but significant short term variability in direction is indicated by orientations of small scale bedforms the western boundary undercurrent documented elsewhere along the continental margin at depths less than 4000 m has relatively little effect in modifying the sea floor on the nova scotian rise but the deep currents 4000 m have a significant effect repeated photography of small seafloor areas suggests that many bedforms especially those of smaller scale are constructed and destroyed by strong variably directed current events benthic storms on time scales of 3 6 months or less thus a significant part of the geologic response of the seabed to currents may be event controlled on time scales not very different from those in coastal environments

the 1964 prince william sound earthquake joint inversion of tsunami and geodetic data the 1964 prince william sound alaska earthquake mw 9 2 ruptured a large area beneath the continental margin of alaska from prince william sound to kodiak island a joint inversion of tsunami waveforms and geodetic data consisting of vertical displacements and horizontal vectors gives a detailed slip distribution two areas of high slip correspond to seismologically determined areas of high moment release the prince william sound asperity with average slip of 18 m and the kodiak asperity with average slip of 10 m the average slip on the fault is 8 6 m and the seismic moment is estimated as 6 3 1022 n m or over 75 of the seismic moment determined from long period surface waves

identification of ancient sandy contourites proposed criteria for the identification of ancient sandy contourites include occurrence thin beds reworked sandy turbidites coarse lag structure bioturbated no regular vertical sequence orientation of cross lamination perpendicular to regional downslope paleocurrents grain size silt or sand little mud low or negative skewness fabric grain orientation and composition downslope and along slope provenance concentrations of heavy minerals a suggested three stage approach to recognition of sandy contourites in the field involves work on a progressively increasing scale with particular emphasis on paleocurrent data the various deep water settings in which disparate current directions may occur are summarized and those in which sandy contourites may be formed and recognizably preserved are indicated a broad definition of the term contourite is now possible because of the widespread documentation of contourites even in freshwater environments along slope flow that is persistent in time and space is a requirement

structural styles in the deep water fold and thrust belts of the niger delta the deep water niger delta includes two large fold and thrust belts products of contraction caused by gravity driven extension on the shelf that exhibit complex styles of thrusting these contractional structures formed above multiple detachment levels in the overpressured shales of the akata formation using the patterns of growth sedimentation fold shapes fault plane seismic reflections and combined conventional and shear fault bend folding theories we describe and model the structural styles and kinematics of the fault related folds and imbricate thrust systems that compose these belts individual fault related folds involving both forethrusts and backthrusts are characterized by long planar backlimbs that dip less than the associated fault ramps with upward shallowing of dips in growth strata above the backlimbs suggesting components of progressive limb rotation forelimbs are short compared to backlimbs but growth strata show more consistent dips that suggest a component of folding by kink band migration thus we employ a combination of classic and shear fault bend fold theories to describe these structures including the influence of a weak basal detachment zone in the overpressured shales we expand upon these theories to model the kinematics of imbricate thrust systems which display a complex history of thrusting related to spatial and temporal variations in deposition across the delta regional patterns of folded growth strata are used to define break forward break backward and coeval thrusting involving single and multiple detachment levels we define two main types of imbricate thrust systems type i system with a single basal detachment level and type ii imbricate system with multiple basal detachment levels which cause massive structural thickening of the akata formation and refolding of shallow thrust sheets through the sequential restoration of two regional cross sections across these systems we resolve the structural styles the timing and sequences of thrusting as well as the regional amounts of shortening all of which have important implications for hydrocarbon maturation and charge in the deep water niger delta

double seismic zones and stresses of intermediate depth earthquakes data from japanese local seismograph networks suggest that the stresses in double seismic zones are in plate compression for the upper zone and in plate tension for the lower zone the stresses do not necessarily appear to be down dip it may therefore be possible to identify other double seismic zones on the basis of data which indicate that events with differing orientations of in plate stresses occur in a given segment of slab a global survey of published focal mechanisms for intermediate depth earthquakes suggests that the stress in the slab is controlled at least in part by the age of the slab and the rate of convergence old and slow slabs are under in plate tensile stresses and the amount of in plate compression in the slab increases with increasing convergence rate or decreasing slab age young and fast slabs are an exception to this trend all such slabs are down dip tensile since these slabs all subduct under continents they may be bent by continental loading double seismic zones are not a feature common to all subduction zones and are only observed in slabs which are not dominated by tensile or compressive stresses unbending of the lithosphere and upper mantle phase changes are unlikely to be the causes of the major features of double zones although they may contribute to producing some of their characteristics sagging or thermal effects possibly aided by asthenospheric relative motion may produce the local deviatoric stresses that cause double zones

effect of cenozoic ice sheet fluctuations in antarctica on the stratigraphic signature of the neogene stratigraphic successions from the gulf of mexico offshore alabama northeast java indonesia ross sea antarctica and several other continental margins have been examined all are characterized by very similar neogene stratal geometries in seismic profiles and well log cross sections from these areas a large mid oligocene i e latest early oligocene earliest late oligocene basinward shift in coastal onlap is followed by a major early miocene transgression and aggradation which is in turn followed by early middle miocene transgressions and late middle and late miocene progradational episodes the succession culminates in plio pleistocene high frequency progradations and transgressions the interregional character of the neogene stratal signature and its similarity to the stratal geometry found in seismic data from the ross sea continental shelf antarctica suggest that the neogene stratal signature is a manifestation of glacioeustatic fluctuations a review of the literature and an analysis of recently acquired and published data indicate that the first major ice sheet grounding event in the ross sea occurred during middle to late oligoce time the ross sea is the repository for ice flowing from a major portion of the continental interior thus the glacial record of the ross sea should serve as a gage of ice volume changes on the continent that were large enough to influence global eustasy the ice advance onto the ross sea continental shelf during middle to late oligocene time may have been the result of a decrease in the rate of shelf subsidence as rifting in the ross sea slowed or ceased advance of the ice sheet resulted in widespread erosion of the continental shelf and shelf overdeepening it is hypothesized that metastable marine based ice sheets have waxed and waned on the antarctic continental shelf since at least the oligocene and that the these waxing and waning events were responsible for the development of a global neogene stratigraphic signature

details of stress directions in the alaska subduction zone from fault plane solutions we used the cumulative misfit method to divide a data set with heterogeneous stress orientations into subsets with homogeneous stress directions we propose that slope changes of the cumulative misfit as a function of earthquake number pinpoint the boundaries of homogeneous stresses using a synthetic data set of 50 fault plane solutions composed of two halves corresponding to two incompatible stress tensors we tested the validity and efficacy of the cumulative misfit method our acceptance criteria for results of stress tensor inversions are 1 the directions must be well constrained that is the 95 confidence regions of the greatest and least principal stresses do not overlap and 2 there must be evidence for homogeneity in the sample that is the average misfit of the inversion is less than 6 we estimated stress directions in the alaska wadati benioff zone wbz using 470 fault plane solutions which we determined for earthquakes with ml 3 and 67 published fault plane solutions for earthquakes with ms 5 we succeeded in subdividing the data set of the small and large earthquakes into 25 and 3 subsets with the average misfit ranging from 3 2 to 5 5 the average misfits of the subsets we accept as satisfying the assumption of homogeneity are smaller than the average misfit of the overall data set f 10 by factors of 2 to 3 we estimated the stress fields at two scales along the alaska wbz the stress directions measured by the large earthquakes ms 5 were homogeneous over large volumes with extension downdip and the direction of greatest compression along strike this unusual orientation of the greatest principal stress is attributed to the bend of the slab under central alaska which generates compressive stresses along strike the stress orientations revealed by small earthquakes in contrast exhibited a great deal of heterogeneity as a function of space although their general trend confirms the overall stress directions obtained from the large events we propose that the ratio of the dimensions of the stress field sensed by earthquakes to the rupture dimensions is about 20 to 50 the estimated stress directions of the crustal earthquakes corresponded to the following mechanisms 1 strike slip faulting with the greatest principal stress oriented n s near fairbanks and 2 thrusting with the greatest principal stress oriented nw se near mount mckinley

earthquake location in island arcs abstract a comprehensive data set of selected teleseismic p wave arrivals and local network p and s wave arrivals from large earthquakes occurring at all depths within a small section of the central aleutians is used to examine the general problem of earthquake location in island arcs reference hypocenters for this special data set are determined for shallow earthquakes from local network data and for deep earthquakes from combined local and teleseismic data by joint inversion for structure and location the high velocity lithospheric slab beneath the central aleutians may displace hypocenters that are located using spherically symmetric earth models the amount of displacement depends on the position of the earthquakes with respect to the slab and on whether local or teleseismic data are used to locate the earthquakes hypocenters for trench and intermediate depth events appear to be minimally biased by the effects of slab structure on rays to teleseismic stations however locations of intermediate depth events based on only local data are systematically displaced southwards the magnitude of the displacement being proportional to depth shallow focus events along the main thrust zone although well located using only local network data are severely shifted northwards and deeper with displacements as large as 50 km by slab effects on teleseismic travel times hypocenters determined by a method that utilizes seismic ray tracing through a three dimensional velocity model of the subduction zone derived by thermal modeling are compared to results obtained by the method of joint hypocenter determination jhd that formally assumes a laterally homogeneous velocity model over the source region and treats all raypath anomalies as constant station corrections to the travel time curve the ray tracing method has the theoretical advantage that it accounts for variations in travel time anomalies within a group of events distributed over a sizable region of a dipping high velocity lithospheric slab in application jhd has the practical advantage that it does not require the specification of a theoretical velocity model for the slab considering earthquakes within a 260 km long by 60 km wide section of the aleutian main thrust zone our results suggest that the theoretical velocity structure of the slab is presently not sufficiently well known that accurate locations can be obtained independently of locally recorded data using a locally recorded earthquake as a calibration event jhd gave excellent results over the entire section of the main thrust zone here studied without showing a strong effect that might be attributed to spatially varying source station anomalies we also calibrated the ray tracing method using locally recorded data and obtained results generally similar to those obtained by jhd

the bouma sequence and the turbidite mind set abstract conventionally the bouma sequence bouma a h 1962 sedimentology of some flysch deposits a graphic approach to facies interpretation elsevier amsterdam 168 pp composed of ta tb tc td and te divisions is interpreted to be the product of a turbidity current however recent core and outcrop studies show that the complete and partial bouma sequences can also be interpreted to be deposits formed by processes other than turbidity currents such as sandy debris flows and bottom current reworking many published examples of turbidites most of them hydrocarbon bearing sands in the north sea the norwegian sea offshore nigeria offshore gabon gulf of mexico and the ouachita mountains are being reinterpreted by the present author as dominantly deposits of sandy debris flows and bottom current reworking with only a minor percentage of true turbidites i e deposits of turbidity currents with fluidal or newtonian rheology in which sediment is suspended by fluid turbulence this reinterpretation is based on detailed description of 21 000 ft 6402 m of conventional cores and 1200 ft 365 m of outcrop sections the predominance of interpreted turbidites in these areas by other workers can be attributed to the following 1 loose applications of turbidity current concepts without regard for fluid rheology flow state and sediment support mechanism that result in a category of turbidity currents that includes debris flows and bottom currents 2 field description of deep water sands using the bouma sequence an interpretive model that invariably leads to a model driven turbidite interpretation 3 the prevailing turbidite mind set that subconsciously forces one to routinely interpret most deep water sands as some kind of turbidites 4 the use of our inability to interpret transport mechanism from the depositional record as an excuse for assuming deep water sands as deposits of turbidity currents 5 the flawed concept of high density turbidity currents that allows room for interpreting debris flow deposits as turbidites 6 the flawed comparison of subaerial river currents fluid gravity flows dominated by bed load transport with subaqueous turbidity currents sediment gravity flows dominated by suspended load transport that results in misinterpreting ungraded or parallel stratified deep sea deposits as turbidites and 7 the attraction to use obsolete submarine fan models with channels and lobes that require a turbidite interpretation although the turbidite paradigm is alive and well for now the turbidites themselves are becoming an endangered facies

stress directions along the alaska wadati benioff zone from inversion of focal mechanism data abstract the focal mechanisms of 137 intermediate depth 60 180 km earthquakes in the alaska wadati benioff zone wbz were derived from p wave first motions the study focused on three earthquake clusters referred to as iliamna talkeetna and denali in all three clusters the orientations of the principal stresses pressure p null b and tension t axes are similar the p axes for most of the events show nne ssw trends with shallow plunge conforming to the geometry of the wbz in the study area the rest of the events yielded e w oriented p axes the t axis for most of the events plunge at a steeper angle than p axes but parallel the dip of the wbz we interpret this feature as indicting the predominance of gravitational sinking of the associated underthrust lithospheric plate the regional stress direction was estimated from the focal mechanisms using the inversion method of gephart and forsyth 1984 the results show that the maximum regional stress in the 60 90 km depth range plunges shallowly in the iliamna 14 and talkeetna 5 areas and approximately in the direction of convergence n163 e of the pacific and north american plates in this depth range similar results for the denali area could not be obtained due to a lack of data however the inversion of the focal mechanism data for all three clusters for depths greater than 100 km yielded large values for both misfit and 95 percent confidence area compared with those in the depth range of 60 90 km this may indicate the stress directions for depths greater than 100 km along the alaska wbz are more heterogeneous than in the depth range of 60 90 km

stress pattern in the double seismic zone beneath cook inlet south central alaska abstract relocation of south central alaskan earthquakes recorded by the regional seismograph network using the joint hypocenter determination method revealed a double seismic zone in the underthrusting pacific plate beneath the cook inlet area the lower zone is separated from the upper one by a decrease in seismic activity within a 5 10 km wide band and extends from a depth of 50 km to about 90 km stress tensor inversion for earthquakes in the upper and lower zones shows different stress regimes in the upper zone minimum principal stress is oriented in the down dip direction and maximum principal stress is normal to the wadati benioff zone wbz in the lower zone both maximum and minimum principal stresses are horizontal and rotated by 40 60 from the down dip direction of the wbz below 80 90 km depth stress tensor inversion results show down dip orientation of the minimum principal stress and the maximum stress aligned in the direction of convergence of pacific plate

double seismic zone beneath the mariana island arc a double zone of seismicity has been found beneath the northern marianas in the depth range 80 120 km the benioff zone undergoes a pronounced bend with radius of curvature about 200 km before straightening and descending nearly vertically to a depth of more than 600 km thus this is an ideal area in which to search for evidence of stresses associated with unbending of the subducted oceanic plate we relocated all teleseismic events from 1963 through 1975 by using a regional composite master event technique which significantly decreased the scatter of the hypocenters the three events of the lower seismic zone are separated from the upper zone by about 30 35 km the true thickness of the upper seismic zone is probably 20 km or less the limited focal mechanism data indicate that there is downdip tension in the lower zone and downdip compression in the upper zone the seismicity patterns could be generated either by thermal stresses or by unbending a plate which was originally deformed anelastically we show that an elastic perfectly plastic model of the rheology of the plate can account for the separation of the two seismic zones the focal mechanisms and the greater activity of the upper seismic zone

fine structure of the dipping seismic zone and subduction mechanics in the shumagin islands alaska microearthquake data from a permanent telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the shumagin islands alaska the shallow microseismicity is characterized by active interplate thrusting in the 25 45 km depth range arcward of this interplate activity microearthquakes in the overlying plate have a strike slip mechanism the near horizontal p axis of this mechanism is oriented in the direction of plate convergence suggesting that the plate interface at shallow depths is currently efficiently coupled below 45 km depth the dipping seismic zone appears to be double planed the upper plane dips at 32 from 45 to 100 km depth where it exhibits a kneelike bend below the volcanic front the lower plane begins at about 65 km depth where it is separated from the upper plane by some 25 km and appears to converge with the upper plane at about 120 km depth the inferred geometry of the plate interface suggests that the double planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate a composite fault plane solution for events in the upper plane indicates however that downdip tension is currently present rather than the downdip compression that would be expected from unbending it is proposed that because of locking of the plate interface at shallower depths slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked this model is consistent with previous interpretations that there is a high probability of a great earthquake in the shumagin islands seismic gap within the next decade or two

ichnological analysis of contourites past present and future abstract contourite deposits are significant facies in deep water sedimentary environments because they record critical paleoclimatological and paleoceanographic data contourite research also informs slope stability geological hazard assessment and petroleum exploration scientific consensus has not converged on several key aspects of contourite interpretation however some researchers interpret bioturbation as a diagnostic feature that differentiates contourites from associated facies e g turbidites while other researchers consider bioturbation to be of second order importance this work reviews existing literature and relevant field data to better define the role of ichnological analysis in contourite research review of significant contributions by earlier studies including formalization of terminology bioturbation burrow etc can help constrain wider usage of ichnological concepts e g ichnofacies and ichnotaxonomic analysis this paper also revisits ichnological data in paleoenvironmental interpretations reservoir characterization and the rare example of ichnofacies and ichnofabric analyses designation of diagnostic criteria can promote the use of ichnological information in contourite research regardless of consensus concerning bioturbation as a diagnostic criterion in contourite characterization ichnological data can still inform interpretations and integrate with other techniques to advance understanding of contourite sedimentary facies for a broad range of applications

punctuated evolution of global climate cooling during the late middle to early late miocene high resolution planktonic foraminiferal and oxygen isotope records from the mediterranean high resolution planktonic foraminiferal and oxygen isotope records are presented from a mediterranean deep marine succession dated astronomically between 12 12 and 9 78 ma planktonic and benthic oxygen isotope records are punctuated by two episodes of 18o increase which have astronomical ages of 11 4 and 10 4 ma and correspond to the mi5 and mi6 events of miller et al 1991a these ice growth events coincide with low amplitude variations in the 1 2 myr obliquity cycle and are accompanied by significant faunal changes in the mediterranean such as the arrival of neogloboquadrinids the increase in abundance of the g apertura g obliquus group and the areal differentiation between n atlantica and n acostaensis short term variations in the planktonic foraminiferal and oxygen isotope records correspond to dominantly precession controlled sedimentary cycles features of the sapropel grey marl layers indicate that the short term astronomically controlled circum mediterranean climate changes remained basically the same over the last 12 myr

correlation of himalayan exhumation rates and asian monsoon intensity although most data suggest that the india eurasia continental collision began 45 55 myr ago the architecture of the himalayan tibetan orogen is dominated by deformational structures developed in the neogene period 23 myr ago the stratigraphic record and thermochronometric data indicate that erosion of the himalaya intensified as this constructional phase began and reached a peak around 15 myr ago it remained high until 10 5 myr ago and subsequently slowed gradually to 3 5 myr ago but then began to increase once again in the late pliocene and pleistocene epochs here we present weathering records from the south china sea bay of bengal and arabian sea that permit asian monsoon climate to be reconstructed back to the earliest neogene these indicate a correlation between the rate of himalayan exhumation as inferred from published thermochronometric data and monsoon intensity over the past 23 myr we interpret this correlation as indicating dynamic coupling between neogene climate and both erosion and deformation in the himalaya although the india eurasia collision initiated 50 myr ago major deformation and exhumation of the himalaya did not begin until the early neogene 23 myr ago this coincides with the increased intensity of the asian monsoons as indicated by weathering records from the south china sea bay of bengal and arabian sea and hints at a dynamic coupling between climate and both erosion and deformation in the himalaya

the relationship between submarine canyon fill and sea level change an example from middle miocene offshore gabon west africa abstract a series of middle miocene submarine canyons has been mapped in offshore gabon west africa using a combination of seismic facies analysis and well information three distinct seismic facies are recognized in the canyon fill the evolution of these facies is inferred to be related to sea level change during sea level lowstand deposition was characterized by lateral accretion on the southern canyon wall whilst progressive erosion occurred on the northern wall this depositional pattern was controlled by a local coast parallel current that flowed northwestwards along west africa during the middle miocene as relative sea level rose the erosion on the northern wall ceased and the canyons were filled by sediments showing an aggradational stratal pattern finally during the early part of the transgression the canyons were filled with horizontal or subhorizontal sedimentary strata the sedimentary fill is thought to consist of alternating sands and mud within its lower part but as relative sea level rose clayey sediments gradually dominated the canyon fill

tomographic imaging of the alaska subduction zone the alaska subduction zone is characterized by the pacific plate descending beneath the north american plate causing abundant seismic activity in the crust and along the wadati benioff zone down to a depth of approximately 200 km we have used 142 908 p wave arrival times from 12 237 shallow and intermediate depth earthquakes recorded by the alaska earthquake information center jointly run by the geophysical institute university of alaska fairbanks and u s geological survey in the period from january 1977 to november 1991 to investigate the three dimensional 3 d p wave velocity structure beneath central and southern alaska travel times and ray paths are accurately calculated by using an efficient 3 d ray tracing technique the nonlinear tomographic problem is solved by iteratively conducting linear inversions and the velocity structure and hypocentral locations are simultaneously determined we conducted two types of inversions one is an inversion with a laterally homogeneous starting model the others are what we call slab inversions in which we introduce into the initial model the high velocity subducting pacific plate as a priori information we found that the slab inversions gave a seismologically more plausible result and a final root mean square travel time residual significantly smaller than that of the inversion with the homogeneous starting model detailed p wave tomographic images are obtained for the crust and upper mantle down to a depth of 200 km with spatial resolutions of 30 60 km the tomographic image of the upper crust correlates well with the major surface geological features such as slow sedimentary basins and fast ultramafic bodies prominent low velocity anomalies exist in the crust and upper mantle beneath active volcanoes in the mantle wedge the low velocity anomalies dip toward the continental side and extend to a depth of about 150 km which are considered to be associated with the active volcanism in the alaska subduction zone the results suggest that the subducting pacific plate has a thickness of 45 55 km and a p wave velocity 3 6 higher than that of the surrounding mantle

seismicity and tectonics of the subducted cocos plate we have examined teleseismic earthquake locations reported by the international seismological centre isc for the middle america region and selected 220 as the most reliable these hypocenters and other data are used to delineate the deep structure of the subducted cocos plate the results indicate that the subducted plate consists of three major segments segment i extends from the panama fracture zone to the nicoya peninsula the structure of this segment is poorly defined segment ii is the largest and best defined segment this segment consists of two parts iia and iib part iia extends from the nicoya peninsula to western guatemala and is very well defined and continuous in structure its strike follows the curvature of the trench and dips at about 60 part iib extends from western guatemala to orizaba mexico the dip of this part of the segment decreases slightly toward the northwest and its strike is more northward than that of the trench segment iii extends from orizaba to the rivera fracture zone and is not well defined due to a lack of earthquake activity beneath about 100 km its orientation differs markedly from segment ii and strikes somewhat more westward than the trench between parts iia and iib of segment ii the subducted plate seems to be continuous bending smoothly to accommodate the change in geometry local network data from costa rica suggest there may be a tear between segments i and ii between segments ii and iii there is a gap in the hypocenters which makes it difficult to define the boundary the change in geometry between these two segments indicates that there may be a tear and two strike slip focal mechanisms in the region support this conclusion we find no convincing evidence supporting the existence of segments smaller than the three described above if there is smaller scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths there is no evidence for any major tear in the subducted plate associated directly with either the tehuantepec ridge or the orozco fracture zone the shallow subduction at the northwestern end of segment ii may be related to the bouyancy of the tehuantepec ridge the cocos ridge is probably directly responsible for the change in geometry between segments i and ii and may even be slowing or stopping subduction in segment i the structure of the subducted plate in segment ii and the changes in the character of volcanism along the arc can be related to the relative motion of the north american and caribbean plates the present geometry of part iib of segment ii is more consistent with the probable configuration of the trench about 7 ma ago than with the present configuration indicating that the north american plate is overriding the subduction zone appendices 2 3 and 4 are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 009 2 50

soft sediment deformation structures interpreted as seismites in middle late pleistocene aeolian deposits apulian foreland southern italy abstract soft sediment deformation structures in middle late pleistocene deposits of torre san gennaro brindisi southern italy have been analysed from a morphologic point of view the deformational structures are represented by deformed cross bedding and large injection structures they are restricted to aeolian sediments and the mechanism of deformation is related to liquefaction and or fluidization processes the liquefaction and or fluidization processes have been interpreted as seismic induced after consideration of all other possible trigger agents the discussion about the possible source areas of the palaeoseismic event was carried out through the analysis both of the general magnitude distance relations and the main structural features of the study zone in conclusion we consider the key criteria for trigger mechanism interpretation of soft sediment deformation structures in aeolian deposits and the actual feasibility of using aeolian sediments for the palaeoseismic studies

possible seismic reflections from the downgoing pacific plate 275 kilometers arcward from the eastern aleutian trench multichannel seismic reflection data collected near the southern kenai peninsula alaska show deep events that could stem either from a linear diffractor or from a curved reflector whichever is the cause it must be at a minimum depth of 12 km if a linear diffractor causes the most obvious events it strikes at a 60 angle across the surface trend of mesozoic rocks thus the diffractor could be associated with a transverse tectonic boundary that on the basis of other evidence is thought to strike through the area of this study alternatively these deep events could be reflections from rocks associated either with a detached piece of oceanic crust or with the downgoing pacific plate these associated rocks could be porous and overpressured sedimentary rocks that are squeezed between indurated or metamorphosed sedimentary rocks in the overthrust plate and igneous rocks in the downgoing oceanic crust

sea valleys and related depositional systems in the gulf of lion and ebro continental margins 1 four main types of sea valleys submarine canyons gullies channels of turbidite systems and a mid ocean channel have been characterized in the ebro and the gulf of lion distal margins in the catalan sea together with three main types of depositional bodies pyrenean canyon deep sedimentary body pcdsb apron deposits and channel levee complexes related to the development of these sea valleys erosion and deposition mainly during the quaternary have formed the majority of these sea valleys although some structurally controlled sea valleys also exist as an example a messinian entrenchment has been confirmed in the petit rhone canyon the evolution of the drainage patterns is established from the detailed analysis of the sedimentary structure this analysis revea s the presence of buried sedimentary bodies and older drainage systems differing from those observed in the bathymetric charts specific factors determine the evolutionary differences between both margins including the absence or presence of underlying evaporitic messinian layers and the regional geodynamic evolution during and after the alpine orogeny we propose a dual evolutionary model from the end of the miocene to the quaternary

diagenesis of cretaceous sandstone reservoirs of the south gabon rift basin west africa mineralogy mass transfer and thermal evolution abstract the off shore gabon basin contains large hydrocarbon reserves in lower cretaceous argillaceous sandstones deposited in the rifting stage of the opening of the south atlantic the study area 5 10 km includes a n30 e trending fault system a few kilometers wide in the rocks of this fault system the diagenetic mineralogy consists of early siderite and abundant calcite cements followed by late quartz minor dolomite and chlorite the abundant calcite cement occludes most of the porosity in the region adjacent to the fault system the diagenetic mineralogy consists of early siderite and minor calcite followed by late quartz abundant dolomite and kaolinite amounts of diagenet c cement in both regions range from ten to more than forty percent of the bulk rock volume mass balance based on petrographic evidence and point counting indicates that such quantities of cement cannot have been generated locally which thus implies considerable large scale mass transfer 18o values for coexisting though not clearly cogenetic dolomite and kaolinite from the kaolinite zone suggest temperatures 100 c in late diagenesis published evidence from fission tracks in apatites and from fluid inclusion homogenization temperatures indicates that the temperature during early diagenesis 130 120 ma was 200 250 c or even higher we hypothesize that both the required diagenetic mass transfer and the observed distribution of diagenetic minerals were brought about by hot water flow this circulation perhaps convective was driven by the high early geothermal gradients directly related to the opening of the south atlantic and was especially focused along the tensional fracture systems involved in the rifting the abundant early calcite cement that occurs in the fracture system within the study area represents the diagenetic mineral signature of the high geothermal gradient that prevailed early in the history of the gabon rift basin

detection and location of earthquakes in the central aleutian subduction zone using island and ocean bottom seismograph stations a network of eight university of texas ocean bottom seismographs obs operated for 6 weeks in 1978 about 50 km offshore of adak island alaska and nearby islands in 1979 a similar network of nine instruments was deployed for 7 weeks farther offshore within and up to 100 km seaward of the aleutian trench for shallow earthquakes on the outer trench slope for shallow earthquakes in the thrust zone and for intermediate depth events we have analyzed the obs and island based network data and evaluated the island network s capabilities for earthquake detection and location and for focal mechanism determination our three major conclusions are presented the first concerns shallow earthquakes on the outer trench slope in 1979 about 30 earthquakes occurred within the aleutian trench and up to 60 km seaward of the trench axis the island network located none of these events and detected p phases for only three of them ray tracing shows that the islands lie in a geometric shadow zone for events on the outer trench slope the best located events are shallower than 20 km and exhibit first motions consistent with normal faulting several authors have suggested that these events are caused by bending of the oceanic lithosphere at the outer rise prior to subduction if so then the event locations reported here show that the bending stresses exceed the strength of lithosphere only in a narrow zone extending about 10 km landward and 60 km seaward of the trench axis the second conclusion concerns shallow earthquakes in the thrust zone epicenters determined by island stations alone are virtually identical to epicenters determined using data from both island and obs stations the locations are similar whether they are determined using flat layered velocity models or a more realistic model and a ray tracing scheme nearly all the earthquakes detected by the obs network are also recorded by island stations three composite focal mechanism solutions that use data from both island and obs stations have p axes parallel to the direction of plate convergence the thrust zone events are separated from events on the outer trench slope by a gap in seismicity about 50 km wide situated on the inner trench slope the third conclusion concerns earthquakes deeper than 70 km epicenters determined using island network stations alone lie 10 to 80 km south of those determined using obs and island stations with the differences between epicenters depending both on event depth and on the velocity model used ray tracing with a realistic velocity model shows that for events at 200 km depth locations are about 80 km north of locations determined from island data alone these results do not support the observed increase in the dip of the benioff zone beneath 100 km depth that is suggested by locations determined from island network data alone using a flat layered velocity model

measurement of horizontal motions in alaska using very long baseline interferometry we have analyzed 6 years of very long baseline interferometry vlbi data beginning in 1984 from a network of sites in alaska the yukon and the conterminous united states the analysis of these 53 data sets includes constrained continuous piecewise linear estimation of the site atmospheres and clocks application of values and correlations from a self consistent earth orientation series derived from other vlbi data and simultaneous estimation of site positions and velocities from a solution that includes 21 007 observations with a fit of 40 ps in delay and 64 fs s in delay rate we have determined the horizontal velocities of seven sites with precisions of 0 5 2 mm yr 1 standard deviation the three vlbi sites within the alaska aleutian subduction boundary zone yakataga kodiak and sand point have higher instantaneous velocities relative to eastern north america than the interior sites of alaska fairbanks and sourdough the estimated velocity of yakataga relative to fairbanks is 37 8 1 3 mm yr at n27 w 2 modeling of the yakataga data suggests that the observed motion is the result of elastic straining of the overriding plate due to a locked main thrust zone with a component of oblique slip the estimated velocity of kodiak relative to fairbanks is 13 6 0 8 mm yr at n41 w 3 this result is lower than the rate predicted on the basis of simple elastic dislocation models the estimated velocity of sand point relative to fairbanks is 6 4 0 9 mm yr at n65 w 8 the component of this velocity in the plate direction 3 5 1 1 mm yr is not consistent with either an aseismic slip model i e no strain accumulation in the overriding plate or a model in which the site is undergoing compressional strain 10 9 mm yr at n28 w due to simple subduction at the predicted orientation of pacific plate convergence in a north america fixed reference frame the estimated velocities of fairbanks 1 5 0 5 mm yr at s12 e 20 and sourdough 3 9 1 0 mm yr at s13 w 13 are both southward these results may imply motion of this region relative to stable north america but more observations are required to reduce the uncertainties in the azimuths the time evolution of the estimated site positions from vlbi in alaska are in general consistent with constant velocity however in 1988 the yakataga site showed a significant departure from the position predicted from a constant velocity model obtained from the previously estimated position this departure may be static displacement associated with two major strike slip earthquakes that occurred in the gulf of alaska between the 1987 and 1988 measurement sessions

significance of the fine drainage pattern for submarine canyon evolution the foix canyon system northwestern mediterranean sea abstract submarine gullies are relatively small valleys that occur in a variety of submarine slopes they are very common in continental slopes and in submarine canyon heads and flanks where they often form tributary networks gullies are smaller than submarine canyons though there is no standardised size criterion to distinguish between them gullies and gully networks have been often viewed as initial steps in the development of larger submarine canyons and more mature drainage networks the shelf incising submarine foix canyon system fcs is located in the northwestern mediterranean sea numerous well developed and morphologically diverse gullies carve its two heads and flanks the objective of this study is to analyse the drainage network of the fcs and decipher the role of gullies in its evolution submarine gully and canyon networks were extracted from swath bathymetry data of 50 m grid size using geographic information systems gis a systematic morphometric analysis was carried out on drainage network of the fcs by using the horton strahler method our results show that the fcs is formed by 1660 streams 1000 km in total length which can be classified to six stream orders to detect relevant morphological changes along valley sections the drainage density the stream frequency and the drainage area relief parameters were applied furthermore a branching index i b was developed to characterise the geometry of the submarine drainage network in the canyon heads i b values are 1 7 which correspond to a dendritic network whereas i b in the canyon branches displays values of 2 2 corresponding to a pinnate one at a finer scale we have identified two types of canyon flank gullies namely rim gullies and toe gullies 1 rim gullies form large dendritic networks that extend from the canyon thalweg up to the canyon rim and 2 toe gullies form smaller pinnate networks restricted to the lower part of the canyon flanks the formation and development of rim gullies are interpreted as the main process responsible for canyon head growth and across flank transport of material from the continental shelf toe gullies on the other hand are the morphological expression of the rejuvenation of rim gullies and the canyon itself axial incision and toe gully formation are interpreted as the result of bottom to top erosive processes acting at different scales in response to regional and local changes of the factors controlling the system respectively this study helps to identify a variety of gullies and the processes they result from and provides a morphometric methodology to characterise submarine drainage networks at fine resolution scales

earthquakes plate subduction and stress reversals in the eastern aleutian arc plate subduction beneath the 1500 km long segment of the eastern aleutian arc between kodiak and atka islands 154 w and 176 w longitude is studied with observations from teleseismic data the primary data base consists of hypocenters of earthquakes for the period 1965 1975 carefully selected from the bulletins of the international seismological centre and of 44 new focal mechanism solutions the principal results of this study are that hypocenters of intermediate depth earthquakes in the eastern aleutians appear to define a weakly developed double seismic zone at depths between 70 and 170 km additional evidence for a double seismic zone comes from focal mechanisms which generally show downdip directed p axes for earthquakes in the upper zone and downdip directed t axes in the lower zone major features of the double zone can be explained by thermoelastic stresses in the downgoing plate the observed predominant downdip stress polarity at intermediate depths in the descending plate reverses along strike of the arc this stress reversal coincides in map view with a change from a continental to an oceanic arc the coincidence may result from spatial differences either in the coupling between the plates at shallow depths or in the rheology of the surrounding oceanic versus continental mantle alternatively the stress reversal may be related to the time since the last great earthquake if the observed stress polarity relates to the elapsed time since the last great earthquake it may be a significant observation for long to intermediate term prediction of great thrust earthquakes in subduction zones portions of the eastern aleutian arc where downdip tension predominates contain one or more seismic gaps that appear to have a high probability for great earthquakes in the next few decades a moderate sized bathymetric feature in the pacific plate the amlia fracture zone modifies subduction and produces a bend or offset in the downgoing plate as well as an offset in the line of active volcanoes in the upper plate subduction of the fracture zone appears also to cause formation of summit basins on the arc platform and some aseismic behavior of a 100 km long segment of the main thrust zone

new insights into deformation mechanisms in the gravitationally driven niger delta deep water fold and thrust belt we use two and three dimensional seismic reflection data from the deep water niger delta fold and thrust belt to document evidence for two discrete postfaulting deformation mechanisms an early phase of thrust propagation folding is followed by folding caused by thickness changes within the basal shale detachment unit the later phase of folding is caused by a lateral redistribution of the strata within the basal detachment unit this example of late deformation occurred over a 4 5 m y period as a result of the displacement of approximately 590 km3 370 mi3 of the underlying strata within the detachment unit in another deformation event in the basal detachment unit about 160 km3 100 mi3 moved laterally creating a synform in the overburden and parallel onlap fill indicative of the relatively rapid creation of accommodation space on the basis of seismic reflection data from the delta and a consideration of the volumes and rates of movement of sedimentary rock we conclude that the poorly imaged succession commonly referred to as mobile shale cannot deform solely by ductile mechanisms as interpretations of shale tectonic provinces have commonly suggested but instead by brittle processes that involve thickening by thrust faulting and subseismic accommodation structures processes such as liquefaction where a complete loss of shear strength is observed had a minimal function therefore the term mobile shale in this setting is widely exaggerated an awareness of postfaulting deformation mechanisms will be important for the successful exploration of gravitationally driven fold and thrust belts

relocation of earthquakes in the cook inlet area south central alaska using the joint hypocenter determination method abstract the joint hypocenter determination jhd method was used to relocate 1604 earthquakes that occurred during 1988 to 1993 in the wadati benioff zone wbz beneath the cook inlet in south central alaska the study utilizes both p and s wave arrivals recorded by a recently improved regional seismic network and it provides an improvement over previous studies the jhd locations of the events allowed improved delineation of the geometry of the subducted slab and the detection of a double seismic zone within the wbz in the 40 to 100 km depth range the jhd station corrections obtained correlate with the major surface geological features in the study area slow velocity sedimentary basins as well as with crustal and mantle structure high velocity subducting slab and low velocity anomalies beneath the active volcanoes to investigate the dependence of the jhd relocation procedure on factors such as the hypocentral depths of the events used the velocity model the p s ratio and random and systematic errors in travel time readings synthetic travel times were calculated using an exact raytracing technique a three dimensional tomographic p wave velocity model the present network configuration and the actual seismicity distribution then the synthetic events were located with the jhd algorithm the synthetic experiments showed that the jhd procedure is rather insensitive to the choice of plane layered velocity model and p s ratio preserves relative hypocenter locations well and can handle systematic and large random reading errors effectively

the nature of deep focus earthquakes wadati 1928 first proved convincingly that some earthquakes occur at depths well beneath the earth s crust he essentially proposed the nomenclature used today by the international seismological centre isc i e earthquakes with focal depths exceeding 300 km are deep earthquakes and those with depths between 70 and 300 km are intermediate earth quakes however when there is little reason to distinguish between the two groups both types are called simply deep earthquakes to avoid confusion in this paper we refer to those with focal depths exceeding 300 km as very deep earthquakes and those with focal depths between 70 and 300 km as intermediate earthquakes deep earthquakes are of interest for at least four reasons first they are exceedingly common between 1964 and 1986 they constituted no less than 22 of all earthquakes having mb greater than 5 0 in the isc catalog second they most often occur in association with deep ocean trenches and volcanic island arcs in subduction zones one of the great achievements of twentieth century geophysics was the recognition that their occurrence in inclined planar groups or wadati benioff zones apparently delineates the cold down going cores of convection cells in the uppermost mantle third seismologists use body waves of deep earthquakes dispro portionately more often than those of shallow earthquakes to inves tigate core mantle and crustal structure this is because a deep earth quakes often possess relatively more impulsive sources b their body phases traverse the heterogeneous uppermost mantle only once on the ray path from hypocenter to station and c surface waves do not contaminate these body phase arrivals on seismograms finally deep earthquakes are

model calculations of regional network locations for earthquakes in subduction zones algorithms for locating earthquakes recorded by local networks commonly assume a flat layered velocity structure even in island arc regions where the presence of a dipping lithospheric slab introduces large lateral velocity variations moreover because seismic stations occur along the narrow island arc the resulting linear network geometry degrades earthquake location to model this situation we trace rays in a spherical earth from hypothetical earthquakes through appropriate velocity models to networks of various geometries to isolate the effect of the slab most of the velocity models consist simply of a planar dipping constant velocity region embedded in a sphere with a constant lower seismic velocity we then use the calculated p and s travel times to relocate the hypothetical earthquakes with a flat layered velocity structure we perform calculations of this kind in a variety of models varying the dip of the slab the velocity contrast between the slab and the mantle the station geometry and the distance between the earthquakes and the slab mantle boundary we also perform calculations in more realistic models with geometry closely resembling two seismic networks in alaska these model calculations emphasize the great care that seismologists should exercise when interpreting local network locations in island arc regions in the present study all the relocations incorporate p and s arrivals at all stations and include no random or picking errors nevertheless the relocated hypocenters differ from the actual model hypocenters by up to 100 km or more and produce significant distortions in the wadati benioff zone the most prominent spurious feature is a clear increase in the apparent dip of the wadati benioff zone beneath a certain depth furthermore the relocation reduces the width or apparent thickness of the wadati benioff zone e g they produce an apparent thickness of 5 km or smaller for a zone of earthquakes with an actual thickness of 20 km however if different combinations of stations report different events it is possible to determine an apparent thickness which exceeds the actual thickness for local networks near subduction zones it is useful to estimate the mislocation and rms residual expected for each event recorded and located this can be accomplished by performing model calculations in a simple model with a constant velocity dipping slab embedded in a half space for such a simple model there exists only one ray path from each earthquake within the slab to each station and the travel time for this path can be calculated as a part of the routine network location process a relocation using the routine network location algorithm and these calculated travel times provides an indication of the expected mislocation and rms residual for the actual recorded event

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

mid pleistocene climate transition drives net mass loss from rapidly uplifting st elias mountains alaska erosion sediment production and routing on a tectonically active continental margin reflect both tectonic and climatic processes partitioning the relative importance of these processes remains controversial gulf of alaska contains a preserved sedimentary record of the yakutat terrane collision with north america because tectonic convergence in the coastal st elias orogen has been roughly constant for 6 my variations in its eroded sediments preserved in the offshore surveyor fan constrain a budget of tectonic material influx erosion and sediment output seismically imaged sediment volumes calibrated with chronologies derived from integrated ocean drilling program boreholes show that erosion accelerated in response to northern hemisphere glacial intensification 2 7 ma and that the 900 km long surveyor channel inception appears to correlate with this event however tectonic influx exceeded integrated sediment efflux over the interval 2 8 1 2 ma volumetric erosion accelerated following the onset of quasi periodic 100 ky glacial cycles in the mid pleistocene climate transition 1 2 0 7 ma since then erosion and transport of material out of the orogen has outpaced tectonic influx by 50 80 such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out of sequence fault patterns the 1 2 my mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response millions of years the st elias range provides a key example of how active orogenic systems respond to transient mass fluxes and of the possible influence of climate driven erosive processes that diverge from equilibrium on the million year scale

comment on ichnological analysis of contourites past present and future by francisco j rodriguez tovar and f javier hernandez molina earth science reviews 182 2018 28 41 abstract by their impressive paper title ichnological analysis of contourites past present and future rodriguez tovar and hernandez molina 2018 have implied that the paper is about a comprehensive review of the topic in space i e global and time i e past present and future but in reality the paper has covered only three case studies in europe in the real world bioturbation and burrows are not unique to contourites they are equally abundant in turbidites hyperpycnites tempestites and even in deformed seismites therefore the immediate challenge is in distinguishing contourite facies from other facies e g hyperpycnites with intense bioturbation disappointingly the review has neglected to address this fundamental issue without resolving the basic issue of distinguishing contourites any ichnological analysis of contourites is purely academic and has no relevance in advancing contourite research

shelf margin architecture variability and its role in sediment budget partitioning into deep water areas abstract a review of 127 shelf margin accretion segments from 24 margins worldwide suggests that sediment flux can be low or high that shelf accommodation can be low medium or high and that climatic conditions can be icehouse or greenhouse the interplay of these three variables yields four main greenhouse margins and six main icehouse margins and thus has a fundamental control on shelf margin architectural styles providing new tools for predicting internal architecture of deep water systems in greenhouse settings there is commonly a strong supply drive because of higher temperatures more variable precipitation and higher sediment discharge but only modest shelf accommodation because of low frequency and low amplitude eustatic sea level changes low shelf accommodation is able to deliver significant volumes of sand sourced from either low or high sediment supply areas to deep water environments and residence time for deltas at greenhouse margins is likely to be relatively long because of the low amplitude of greenhouse sea level changes medium accommodation in contrast may overwhelm any low sediment supply but could be partly overwhelmed by high sediment supply which in turn can inhibit fans or encourage highstand fans in icehouse settings there is a strong cross shelf accommodation drive created by high frequency and high amplitude eustatic sea level changes thus causing significant and frequently repeated transits of river mouths and deltas across the shelf sandy basin floor fans created by low accommodation are predicted with either high or low sediment flux both medium and high accommodation can be volumetrically overwhelmed by high sediment supply which in turn cause greater volumes of sediment storage on the shelf resulting in volumetrically less significant highstand or transgressive submarine fans however a low supply may barely allow deltas to reach the shelf edge causing the deep water areas to be dominantly muddy

crustal uplift in the south central alaska subduction zone new analysis and interpretation of tide gauge observations we have examined tide gauge measurements of apparent sea level height in south central alaska to determine the history of crustal uplift subsequent to the 1964 prince william sound earthquake there are spatial and temporal variations in the uplift rate since the 1994 earthquake that depend on the location of the tide gauge relative to the coseismic rupture features at seward on the eastern side of the kenai peninsula we find slow uplift that is consistent with elastic strain accumulation at the locked north american pacific plate boundary conversely at seldovia and nikiski on the western side of the kenai peninsula we find persistent rapid uplift of 10 mm yr 1 that may be longterm transient response to the earthquake but that cannot be sustained over the entire several hundred year recurrence interval for a great earthquake farther to the southwest at kodiak the rate of uplift is several millimeters per year but has slowed significantly over the past three and a half decades to the east of the kenai peninsula we find subsidence at cordova and an uncertain behavior at valdez at cordova and to a lesser extent valdez there is a mathematically significant time dependence although the evidence for the time dependence is less compelling than at kodiak at anchorage there is little evidence of vertical motion since the earthquake the along strike spatial variability in the relaxation time of the rates of vertical motion since the 1964 earthquake may be related to variations in the updip coseismic slip during the megathrust event

structure of the benioff zone beneath the shumagin islands alaska relocation of local earthquakes using three dimensional ray tracing seismic rays are traced through a prescribed three dimensional inhomogeneity that simulates the subducted slab below the shumagin islands region to calculate local station delays for a given hypocenter and a slab model hypocenters determined by the shumagin seismic network are then relocated using the station delays a flat layered velocity structure and a standard earthquake location computer program station delays are calculated for 12 hypocenters with respect to six different slab models to identify the slab model that is most consistent with the available arrival time and waveform data a set of path corrections that is calculated for each grid point station pair on a preliminary grid of 36 points in the depth range from 60 to 300 km is used to recalculate the hypocenters for all of the 1982 earthquakes with depths greater than 50 km application of this method to data from 1982 for the digitally recording shumagin seismic network shows the following results 1 a previously observed apparent increase in dip of the subducted slab at depths of 100 km disappears 2 the subducted slab can be modeled as a dipping structure that dips at a constant angle of 45 toward north northwest at depths between 80 and 250 300 km and has a 7 higher velocity than the surrounding mantle 3 hypocenters determined from shumagin network data are located only 10 20 km south of high quality hypocenters determined from teleseismic data alone 4 qualitative comparison of digitally recorded seismograms with calculated ray paths shows enrichment of high frequency coda possible converted phases and low amplitudes of first p arrivals for rays that travel mostly along the slab conversely for rays that travel almost vertically through the upper plate the seismograms show a high amplitude of first p arrivals that are followed by an insignificant coda and low amplitude s waves

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

structural evolution and sequence formation offshore south gabon during the tertiary abstract the present investigation of the tertiary succession offshore south gabon is based on seismic surveys of 1985 1989 vintage and wells covering the southern part of gabonese waters in general the tertiary basin off south gabon evolved during two structural stages during the paleocene and early middle eocene a trough like basin subsided slowly towards the west during this period there was a cyclic development of the succession which consists of up to four sequences mostly bounded by type 2 sequence boundaries the sequential changes were induced by eustatic sea level changes this pattern changed during the neogene when the evolution was characterized by continental crustal flexuring in early middle miocene times this resulted in strong subsidence of the western part of the basin and uplift of the eastern part consequently the eastern part was exposed to strong erosion and a thick middle miocene succession was deposited west of the shelf approximately the present day shelf especially on the slope the clinoformal and lens shaped middle late miocene succession which consists of four sequences bounded by type 1 sequence boundaries has been controlled by eustatic changes in sea level in addition to the overall subsidence pattern salt tectonics has modified the external and internal architecture of the sequences on a local scale canyon formation in connection with slumping results in the establishment of well developed fan systems on the lower part of the slope and on the basin floor area

crustal deformation and the seismic cycle across the kodiak islands alaska 1 the kodiak islands are located 120 to 250 km from the alaska aleutian trench and are within the southern extent of the 1964 prince william sound mw 9 2 earthquake rupture and aftershock zone here we report new campaign gps results 1993 2001 from northeastern kodiak and reprocessed gps results 1993 1997 from southwestern kodiak the rate and orientation of the horizontal velocities relative to a fixed north america range from 29 7 1 7 mm yr at n30 3 w 3 3 located 120 km from the deepest point of the trench to 8 0 1 3 mm yr at n62 4 w 9 3 located 230 km from the trench we evaluated alternate models of coseismic and interseismic slip to test the importance of the mechanisms that account for surface deformation rates near the gulf of alaska coastal region of kodiak the horizontal velocity can be accounted for primarily by the viscoelastic response to plate motion and a locked main thrust zone mtz downdip creep and to a lesser extent slip in the 1964 earthquake farther inland the dominant mechanisms that account for post 1964 uplift rates are time dependent downdip creep and a locked mtz for the horizontal velocity component southwest translation of western kodiak may be important as well on the basis of the pre 1964 and post 1964 earthquake pattern of interseismic earthquakes we suggest that between the occurrences of great earthquakes like the 1964 event more moderate to large earthquakes occur in the southwestern kodiak region than near northeastern kodiak