seabed pockmarks associated with deepwater corals off se brazilian continental slope santos basin several factors bear evidence of chemosynthetic ecosystem occurrence in the brazilian coast these motivated our investigation to sample three pockmarks located on the continental slope off se brazil 26js and 46jw at f700 m depth the crater like features were f1 km in diameter and 100 m in depth and were identified with an echo sounder ek 500 they were sampled along two transects using a rectangular dredge our main objective was to identify possible evidence of a seep related community the surrounding sediments of the pockmarks seem to be filled with coral rubble harboring a rich fauna of metazoans including epilithic and chasmolithic organisms a total of 9565 organisms were sampled the most abundant group was the ophiuroidea followed by cirripedia and polychaeta a number of new occurrences and new species were registered the polychaete worm amelinna was the first record for the atlantic ocean which has been found in reducing habitats such as wood islands the echo sounder records also showed the presence of big deepwater coral mounds associated with the edge of the pockmarks these features are f 20 m in height and from 180 to 360 m wide records of demersal fishes were also registered right above coral mounds the presence of deepwater coral mounds at the edge of pockmarks may be related to the flow of the cold oxygen rich antarctic intermediate water over the bottom or else may benefit from the fertilization of the water by the microseepage of hydrocarbons from the sediment we suggest a more thorough survey in order to map and recognize these environments furthermore our findings may shed some light on new research perspectives concerning the ecology biogeography and evolution of reducing habitats off the brazilian coast d 2004 elsevier b v all rights reserved

latest quaternary oceanographic changes in the okhotsk sea based on diatom records abstract the okhotsk sea is a locus of the north pacific intermediate water formation today the formation of the okhotsk intermediate water is linked with seasonal sea ice cover diatom records from the okhotsk sea show the following 1 during the last glacial maximum between 21 000 yr b p and 18 000 yr b p sea ice covered the western side of the sea throughout the whole year but the eastern part was open this permitted more efficient heat exchange with the atmosphere than waters and hence ventilation of the glacial okhotsk sea waters which in turn caused a greater extent of intermediate water formation through the bussol strait 2 the perennial sea ice cover extended northward and sea ice was present throughout the whole year over a great part of the sea except for the eastern part between 18 000 yr b p and 17 000 yr b p 3 the ice free area extended greatly to the south during the interval between 17 000 yr b p and 11 000 yr b p 4 there was no perennial ice cover over the whole sea and the ice free area that appeared as the seasonal ice area began to extend to the western part between 11 000 yr b p and 8 000 yr b p 5 after 8000 yr b p the ice free waters contracted to the present extent

a calculation of ionospheric currents due to semidiurnal antisymmetric tides the fully developed equivalent circuit method is used to calculate the ionospheric electric current flow due to the 2 3 and 2 5 tidal modes the 2 5 mode currents are of negligibly small amplitude the 2 3 current pattern has meridional currents in the same direction at a given local time in both north and south hemispheres there is also sometimes a minimum in the zonal current around 10 degree dip latitude and the equatorial electrojet pattern may be different in different longitude zones these features may be possible explanations of 1 the meridional current pattern observed at saint santin 2 current flows on abnormal quiet days aqds 3 the reverse equatorial electrojet 4 the seasonal phase anomaly in the lunar geomagnetic tide and 5 the invasion of one hemisphere s current pattern by that from the opposite hemisphere the recent detection of antisymmetric semidiurnal winds just below the dynamo region further demonstrates the importance of these modes

the tectonic evolution of the south atlantic from late jurassic to present an improved tectonic database for the south atlantic has been compiled by combining magnetic anomaly geosat altimetry and onshore geologic data we used this database to obtain a revised plate kinematic model starting with a new fit reconstruction for the continents around the south atlantic we present a high resolution isochron map from chron m4 to present fit reconstructions of south america and africa that require rigid continental plates result in substantial misfits either in the southern south atlantic or in the equatorial atlantic to achieve a fit without gaps we assume a combination of complex rift and strike slip movements 1 along the south american parana chacos basin deformation zone 2 within marginal basins in south america salado colorado basin and 3 along the benue trough niger rift system in africa these faults are presumed to have been active before or during the breakup of the continents our model describes a successive unzipping of rift zones starting in the southern south atlantic between 150 ma tithonian and approximately 130 ma hauterivian rifting propagated to 38 s causing tectonic movements within the colorado and salado basins subsequently between 130 ma and chron m4 126 5 ma the tip of the south atlantic rift moved to 28 s resulting in intracontinental deformation along the parana chacos basin deformation zone between chron m4 and chron mo 118 7 ma rifting propagated into the benue trough and niger rift inducing rift and strike slip motion after chron mo the equatorial atlantic began to open while rifting and strike slip motion still occurred in the benue trough and niger rift since chron 34 84 ma the opening of the south atlantic is characterized by simple divergence of two rigid continental plates

geomorphology and shallow structure of a segment of the atlantic patagonian margin abstract we study an area little known of the atlantic patagonian margin from 44 30 s to 47 40 s and from 59 w to nearly 61 w the multi beam bathymetry coupled with high resolution seismic reflection profiles has provided details on the morphology and shallow acoustic structure on this area the main morphological characteristics of the seafloor features on the shelf and middle slope are described the atlantic patagonian continental shelf north of 45 40 s is located at a depth of 170 200 m south of this latitude the shelf edge is at 128 to 200 m the shelf surface is marred by circular depression and ridges oriented oblique to the shelf edge the upper slope and upper middle slope are plowing by icebergs from antarctica in pleistocene and local reefs of cold water coral further enhance the topography of the area in the middle slope there are two terraces the 20 to 60 km wide nagera and the 15 to 60 km wide perito moreno terraces showing moats hollows pot holes sediment drifts and sediment waves the terraces may have been formed in late miocene whereas the other forms are of pleistocene age other features are a sediment swell south of 47 s and seven submarine canyons on the middle slope these incipient canyons have been developed in the middle slope by retrogressive erosion some terminating on the upper middle slope and others on the upper slope and the canyon 6 breaching the shelf edge individual seafloor features existing on the atlantic patagonian margin have been classified into two main groups according to their origin along and across slope processes these primary agents were supplemented by endogenic processes such as expulsion of gas water diapirism of high pressure mud and folding faulting the results suggest that today down slope processes on the slope are practically non existent and that the morphology of the upper and middle slope is slowly being remolded by along slope bottom currents

geological and morphological setting of 2778 methane seeps in the dnepr paleo delta northwestern black sea abstract the dnepr paleo delta area in the nw black sea is characterized by an abundant presence of methane seeps during the expeditions of may june 2003 and 2004 within the eu funded crimea project detailed multibeam seismic and hydro acoustic water column investigations were carried out to study the relation between the spatial distribution of methane seeps sea floor morphology and sub surface structures 2778 new methane seeps were detected on echosounding records in an area of 1540 km 2 all seeps are located in the transition zone between the continental shelf and slope in water depths of 66 to 825 m the integration of the different geophysical datasets clearly indicates that methane seeps are not randomly distributed in this area but are concentrated in specific locations the depth limit for the majority of the detected seeps is 725 m water depth which corresponds more or less with the stability limit for pure methane hydrate at the ambient bottom temperature 8 9 c in this part of the black sea this suggests that where gas hydrates are stable they play the role of buffer for the upward migration of methane gas and thus prevent seepage of methane bubbles into the water column higher up on the margin gas seeps are widespread but accurate mapping illustrates that seeps occur preferentially in association with particular morphological and sub surface features on the shelf the highest concentration of seeps is found in elongated depressions pockmarks above the margins of filled channels on the continental slope where no pockmarks have been observed seepage occurs along crests of sedimentary ridges there seepage is focussed by a parallel stratified sediment cover that thins out towards the ridge crests on the slope seepage also appears in the vicinity of canyons bottom flanks and margins or near the scarps of submarine landslides where mass wasting breaches the fine grained sediment cover that acts as a stratigraphic seal the seismic data show the presence of a distinct gas front which has been used to map the depth of the free gas within the sea floor sediments the depth of this gas front is variable and locally domes up to the sea floor where the gas front approaches the seafloor gas bubbles were detected in the water column a regional map of the sub surface depth of the gas front emphasises this gas front versus seep relationship the integration of all data sets indicates that the spatial distribution of methane seeps in the dnepr paleo delta is mainly controlled by the gas hydrate stability zone as well as by stratigraphic and sedimentary factors

morphology of pockmarks along the western continental margin of india employing multibeam bathymetry and backscatter data this study addresses the morphology of pockmarks along the western continental margin of india using multibeam bathymetry and backscatter data here for the first time we have utilized the application of arcgis geographical information system for understanding the morphology of pockmarks from the western continental margin of india the pockmarks observed in water depths of 145 330 m are circular elliptical or elongated in plan view with an average length and width of 157 72 m and 83 19 m respectively the average pockmark relief and perimeter are 1 9 0 9 m and 412 181 m respectively the pockmarks have average areas and volumes of 10 759 m and 15 315 m respectively spatial separation that coincides with 210 m isobath divides the pockmarks into two groups with differing distributions and morphologies these pockmarks originated from seepages of biogenic or thermogenic gas or from pore fluids from deeper sources migrated vertically along the faults besides a possible structural control the pockmark morphologies are also affected by bottom currents and or by submarine slumping the average acoustic backscatter strength from pockmark centre is higher 35 db than the average backscatter of the total area 40 db which suggests their possible linkage to the precipitation of diagenitic minerals from biodegradation of seepage material

the geological evolution of the falkland islands continental shelf the falkland islands are surrounded by four major sedimentary basins the falkland plateau basin to the east the south falkland basin to the south the malvinas basin to the west and the north falkland basin to the north the four main basins appear to have formed initially as triassic through earliest cretaceous extensional rifts associated with the break up of gondwana a valanginian end to rifting was followed by thermal sag there is evidence of cenozoic uplift in at least the north falkland basin possibly coincident with andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the scotia sea there is no evidence from offshore seismic and gravity magnetic data to support interpretations that the falkland islands have rotated clockwise through up to 180 during gondwana separation with the exception of the south falkland basin all the major basins probably underwent initially more or less east west extension and had a similar orientation to adjacent south american and western southern african basins the falkland basins probably shared a similar geological history with the offshore southern african and south american basins

speculative petroleum systems of the southern pelotas basin offshore uruguay abstract the pelotas basin of brazil and uruguay represents a frontier basin with under explored hydrocarbon potential although oil and gas accumulations have yet to be identified only 21 exploratory wells have been drilled in an area of more than 330 000 km2 20 of which are located in the brazilian portion of the basin a detailed study of the petroleum system of offshore uruguay has strong potential to contribute to a better characterization of the capacity of the basin to generate and accumulate hydrocarbons three stages have previously been recognized during the evolution of pelotas basin 1 a prerift phase which preserved paleozoic and mesozoic units of the parana basin 2 an early cretaceous volcano sedimentary synrift phase and 3 a cretaceous to cenozoic postrift phase deposited during the passive margin stage in this study we use sequence stratigraphy methodology to interpret 2d multichannel seismic sections of the southern segment of the pelotas basin in the uruguayan atlantic margin this analysis allows us to identify depositional sequences systems tracts and the distribution of the main elements of the potential petroleum systems following our analysis we propose six speculative petroleum systems sps in the pelotas basin the first sps is related to the prerift phase and is represented by a lower permian restricted marine source rock and reservoirs related to permian to upper jurassic aeolian and fluvial sandstones the second sps corresponds to the synrift phase and is constituted by a barremian lacustrine source rock with reservoirs of alluvial fluvial sandstones of the same age the other four proposed sps are associated with the postrift phase represented by marine source rocks related to aptian albian cenomanian turonian and paleocene transgressions all of which are identified in the region and interpreted in seismic lines from uruguay these postrift sps have predominantly siliciclastic reservoirs represented by early cretaceous aeolian sandstones and cretaceous to cenozoic deltaic sandstones and turbidites

un coupled thrust belt foreland deformation in the northern patagonian andes new insights from the esquel gastre sector 41 30 43 s the patagonian andes represent a unique natural laboratory to study surface deformation in relation to deep slab dynamics in the sector comprised between latitudes 41 30 and 43 s new apatite u th he ages indicate a markedly different unroofing pattern between the broken foreland area characterized by late cretaceous to paleogene exhumation and the adjacent andean sector to the west which is dominated by miocene pliocene exhumation these unroofing stages can be confidently ascribed to inversion tectonics involving reverse fault related uplift and concomitant erosion late cretaceous paleogene shortening and exhumation are well known to have affected also the thrust belt sector of the study area during a prolonged stage of flat slab subduction therefore the different ages of near surface unroofing documented in this study suggest coupling of the deformation between the thrust belt and its foreland during periods of flat slab subduction e g during late cretaceous paleogene times and dominant uncoupling during periods of steep slab subduction and rollback even when these are associated with high convergence rates i e 4 cm year as those documented in miocene times for the patagonian andes

seepage in isfjorden and its tributary fjords west spitsbergen abstract this study analyses pockmark morphology and their spatial distribution in isfjorden relative to seabed morphology bedrock geology fault systems glacial landforms and processes using multibeam bathymetric data it provides insight into the possible mechanisms of pockmark formation high density pockmark field evolution and fluid migration pathways a total of 1304 pockmarks occur in the isfjorden at water depths of 40 to 320 m varying from circular to elongate in plan view their diameter ranges from 14 to 265 m and their depths from 1 to 11 m elongate pockmarks are dominant in the isfjordbanken and outer isfjorden where the seafloor is influenced by the west spitsbergen current the west spitsbergen fold and thrust belt and other fault systems sub cropping at the isfjorden seafloor correlate spatially with the high density pockmark zones the pockmarks are preferentially located on the marine sediments draping the bedrock of isfjorden fjord system they are most abundant in the areas underlain by jurassic cretaceous and the triassic lower jurassic bedrock pockmarks found ahead of submarine slope failures may have formed due to dewatering of soft sediments as a result of rapid increase of overpressure caused by deposition of glacigenic debris lobes fault conduits potential source rock and thin postglacial sediment layers are found to be crucial for the formation of pockmarks modeling results of the near shore subsea permafrost and potential gas hydrate stability zone imply thawing permafrost and gas hydrate dissociation as additional possible mechanisms for pockmark formation in isfjorden

variations in sediment transport at the central argentine continental margin during the cenozoic 1 the construction of the sedimentary cover at most passive continental margins includes gravitational downslope transport and along slope contourite deposition which are controlled by tectonics climate and oceanography at the eastern continental margin of argentina the history of deposition and erosion is intimately linked to the evolution of the south atlantic and its water masses here we present a detailed seismic investigation of the mixed depositional system located between 41 s and 45 s the study provides a northward complement to prior investigations from the southern argentine margin and together with these may be used as background information for future ocean drilling in the region prominent features in our seismic cross sections are submarine canyons mass wasting deposits contourite channels and sediment drifts four major seismic units above regional reflector ple 65 ma are separated by distinct unconformities of regional extent using a dense grid of reflection seismic profiles we mapped the depocenter geometries of the seismic units and derived a chronology of the depositional processes during the cenozoic while the paleocene eocene 65 34 ma is characterized by hemipelagic sedimentation under relatively sluggish bottom water conditions strong antarctic bottom water aabw circulation led to widespread erosion on the slope and growth of a detached sediment drift during the oligocene and early miocene 34 17 ma after deposition of an aggradational seismic unit interpreted to represent the mid miocene climatic optimum 17 14 ma gravitational downslope sediment transport increased during the middle to late miocene 14 6 ma possibly related to tectonic uplift in south america the pliocene to holocene unit 6 ma is very heterogeneous and formed by interactions of downslope and along slope sediment transport processes as indicated by the evolution of canyons slope plastered drifts and channels

new criteria for systematic mapping and reliability assessment of monogenetic volcanic vent alignments and elongate volcanic vents for crustal stress analyses abstract linear arrays of monogenetic volcanic vent alignments represent an important source of stress strain data from volcanic regions of the earth and other planets currently however there is no standard methodology for mapping vent alignments or for assessing alignment reliability which are essential to defining a robust stress datum from vent alignments here we define a systematic method for defining monogenetic vent alignments by mapping the locations and shapes of vents elongation of vents into elliptical shapes is directly related to the trend of subsurface fissures that control alignments elongated vents therefore are critical for defining reliable regional vent alignments and also provide an independent means for assessing stress directions our reliability assessment system a b c d for vent alignments is based on the number of vents the standard deviation of vent center points from a best fit line the numbers and types of elongate vents the standard deviation of long axes of elliptical vents from a best fit line and in cases where alignments lack elongate vents by vent spacing distances we quantified morphometric parameters of well defined fissure fed vent alignments from polygenetic volcanoes and platform volcanic fields to establish appropriate threshold values for reliability assessments in this new method the combined analysis of vent locations and shapes optimizes the process of alignment mapping and the assessment of the reliability of alignments to be incorporated in stress analyses we provide quality ranking schemes for stress data derived from reliability assessed vent alignments and from elongate vents we demonstrate the utility of this new mapping and assessment methodology by analyzing monogenetic cinder cone fields within a platform volcanic field the san francisco volcanic field in arizona and within a volcanic field on the flank of a polygenetic volcano mount morning in antarctica

controls on the structural architecture and sedimentary character of syn rift sequences north falkland basin south atlantic abstract the falkland islands lie in a critical location for our understanding of gondwana supercontinent break up subsurface data obtained during recent exploration activity provide important new insights into the structure and stratigraphy of the north falkland basin s late jurassic syn rift fill history results of seismic interpretation indicate that deposition was largely controlled by prominent planar normal faults which divide the basin into a series of north south trending half grabens and minor grabens well data in the form of exploration wireline logs cores petrographic thin sections major and trace element analyses indicate that their syn rift fill was dominated by abundant volcaniclastic sediment supplied through debris flows hyperconcentrated flows and pyroclastic ash fall the geochemical affinity of the volcanic sedimentary components shows a direct link to the widespread middle jurassic to early cretaceous silicic chon aike large igneous province in southern south america and antarctica diagenesis has obscured provenance information and severely reduced porosity within the syn rift sediments causing them to have poor reservoir potential the results enable a radically new interpretation of this key period in the north falkland basin s evolution which helps modify and unify previously disparate views it is now suggested that basin development was the direct result of the chon aike back arc stretching influenced in part by the underlying palaeozoic basement trends this was followed by cretaceous recent post rift thermal subsidence of the basin which was only arrested by local structural inversion related to atlantic opening consequently it is thought that the extension that created the north falkland basin was independent of both early middle jurassic rotation of the falkland islands microplate and the evolution of the south atlantic ocean in the cretaceous and cenozoic

miocene block uplift and basin formation in the patagonian foreland the gastre basin argentina abstract the intraplate fault block mountains and intermontane deposits of the gastre basin which are recorded more than 550 km east of the andean trench in central patagonia argentina are analyzed the gastre basin is one of the largest patagonian intermontane basins limited by uplifted blocks strongly oblique to the andean chain it was originated by reverse faulting and inversion of pre existing normal faults associated with a mesozoic rift basin and defined by older crustal heterogeneities the deformational event occurred during the middle miocene related to a short contractional episode 16 1 14 86 ma probably in response to an eastward migration of the andean fold and thrust belt during pliocene to quaternary times neither younger fault block uplifts nor reconfigurations of the basin occurred similarities between the study area and other parts of the patagonian foreland such as the presence of miocene reverse or inversion tectonics as well as the accommodation of the miocene sedimentary successions suggest that the gastre basin is part of a major late early to middle miocene broken foreland system i e the patagonian broken foreland that exhumed discrete fault block mountains and generated contemporary basins along more than 950 km parallel to the andean trench i e between 40 00 and 48 00 south latitude based on recent studies on the southern andean margin this continental scale contractional episode may be the result of a flat slab subduction segment nevertheless such a hypothesis is very difficult to support when analyzing such a large flat subduction segment along the entire patagonian trench this suggests the need to consider alternative flat slab trigger mechanisms or other factors in the generation of broken foreland systems

sedimentary growth pattern on the northern argentine slope the impact of north atlantic deep water on southern hemisphere slope architecture large sedimentary deposits consisting of several major contourite drifts were studied by means of high resolution multichannel seismic data at the middle slope along the northern argentina continental margin to determine their evolutionary stages as well as to identify and assess the possible impact of northern source deep water nsdw on the slope architecture the imaged contouritic sediments allow decoding on the regional paleo oceanographic setting of the last 32 ma earliest contouritic sedimentation can be observed close to the eocene oligocene boundary based on an aggradational stacking pattern with a complex and wavy seismic facies pointing toward a hydrodynamically turbulent flow pattern this facies is most likely related to the opening of the drake passage associated with global cooling and a strengthening of surface intermediate and deep ocean currents in the southern ocean during the middle miocene plastered drift sequences with an aggradational reflection pattern were deposited their depositional style indicates weak non turbulent current conditions which are interpreted to be related to a vertical shift of water mass interfaces caused by the first formation of nsdw during the mid miocene climatic optimum on top the formation of plastered drift sequences led to the modern extent of the ewing terrace which was probably controlled by the continuous strengthening and thickening of nsdw until the final closure of the central american seaway cas during the pliocene and quaternary after the complete closure of the cas and under the influence of the full force of the nsdw mounded plastered drift sequences are built upon the ewing terrace generating the modern slope morphology therefore we suggest that deep water production in the northern hemisphere plays a significant role by controlling the shape of the continental slopes in the southwestern south atlantic since the middle miocene highlights slope of northern argentine continental margin is current controlled since 32 ma variability of northern source deep water nsdw controls sedimentary processes sedimentary processes are susceptible to changes of the brazil malvinas confluence impact of nsdw on slope processes is underestimated in the southern hemisphere

late holocene 1500 yr climatic periodicities and their implications fourier and nonlinear regression analysis of a 4000 yr paleoclimate proxy record in western canada shows strong periodicities of 1500 yr and several weaker century to millenial scale periodicities in conjunction with the 23 708 yr milankovitch periodicity these produce a model of climate fluctuation through the postglacial consistent with recognized paleoclimatic fluctuations of the past 15 000 yr in the northern mid latitudes these results suggest that postglacial climatic anomalies such as the little ice age and the younger dryas were at least in part periodic phenomena rather than the result of unique aperiodic events projecting these periodicities into the future suggests that even in the absence of anthropogenic climate forcing a natural warming trend will continue until ca a d 2400

evolution of the northern argentine margin during the cenozoic controlled by bottom current dynamics and gravitational processes a detailed seismic investigation on sediment deposition at the northern argentine margin 37 s to 42 s resolves major modifications in oceanographic circulation during the cenozoic which resulted from variations in both climatic and tectonic processes after an extensive erosional period following the onset of glaciation of antartica at 34 ma which affected all water depth levels a buried elongated mounded drift within the continental shelf was shaped by bottom current activity during the miocene this may represent the earliest deposits of the malvinas current mc that branches from the antarctic circumpolar current acc and today is part of a complex shallow water circulation system known as the brazil malvinas confluence bmc at the same time a major terrace grew to its present form on the upper slope indicating that a precursor of antarctic intermediate water aaiw was also part of the bmc after another major erosional phase represented by a seismic unconformity at 6 ma sheeted drifts mounded drifts and sediment waves formed at the continental rise during the pliocene pleistocene these extensive contourite deposits are diagnostic for a steady north setting bottom flow at the depth level of today s antarctic bottomwater aabw evidence for downslope transport mainly stems from the presence of buried turbidites and canyon related depocenters these features can be related to andean uplift during the eocene and to the activation of the canyon system during the pliocene recent mass transport is indicated by scarps and sliding blocks at the seafloor of the slope

significance of mud volcanism 1 mud volcanism and diapirism have puzzled geoscientists for 2 centuries they have been described onshore and offshore in many places on earth and although they occur in various tectonic settings the majority of the features known to date are located in compressional tectonic scenarios this paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described mud volcanoes show variable geometry up to tens of kilometers in diameter and several hundred meters in height and a great diversity regarding the origin of the fluid and solid phases gas predominantly methane water and mud may be mobilized at subbottom depth of only a few meters but in places can originate from several kilometers depth with minor crustal or mantle input the possible contribution of mud extrusion to global budgets both from quiescent fluid emission and from the extrusive processes themselves is important in regions where mud volcanoes are abundant such as the collision zones between africa and eurasia fluid flux through mud extrusion exceeds the compaction driven pore fluid expulsion of the accretionary wedge also quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and hence to greenhouse climate

distribution timing and causes of andean deformation across south america the andean orogeny in south america has lasted over 100 ma it comprises the peruvian incaic and quechuan phases the nazca and south american plates have been converging at varying rates since the palaeocene the active tectonics of south america are relatively clear from seismological and global positioning system gps data horizontal shortening is responsible for a thick crust and high topography in the andes as well as in se brazil and patagonia we have integrated available data and have compiled four fault maps at the scale of south america for the mid cretaceous late cretaceous palaeogene and neogene periods andean compression has been widespread since the aptian the continental margins have registered more deformation than the interior for the peruvian phase not enough information is available to establish a tectonic context during the incaic phase strike slip faulting was common during the quechuan phase crustal thickening has been the dominant mode of deformation to investigate the mechanics of deformation we have carried out 10 properly scaled experiments on physical models of the lithosphere containing various plates the dominant response to plate motion was subduction of oceanic lithosphere beneath continental south america however the model continent also deformed internally especially at the margins and initial weaknesses

surface and subsurface signatures of gas seepage in the st lawrence estuary canada significance to hydrocarbon exploration abstract the seabed of the st lawrence estuary is characterized by many fluid releasing features on multibeam bathymetric images these features correspond to crater like depression pockmarks predominantly found on the northwestern shoulder of the 300 m deep laurentian channel as well as on the channel floor aligned pockmarks which define segments up to 12 km long are frequent in the laurentian channel whereas they are preferentially associated with submarine landslides on the northwestern shoulder of the channel on high resolution seismic profiles pockmarks found in the laurentian channel are characterized by seismic chimneys that may be traced down to the autochthonous paleozoic rocks st lawrence platform suggesting a thermogenic origin for the gas on the northwest shoulder of the laurentian channel the seismic signature of pockmarks does not extend downward to the reflector that corresponds to bedrock st lawrence platform and or grenvillian province suggesting together with high sedimentation rate a biogenic origin for the gas these results are discussed in the light of the data collected in the onshore parts of the st lawrence platform and suggest the presence of a mature hydrocarbon source the conclusions are supportive arguments for the hydrocarbon prospectivity of both the onshore and offshore segments of the paleozoic autochthonous domain st lawrence platform

antisunward net birkeland current system deduced from the oersted satellite observation 1 from the precise magnetic field measurements by the oersted satellite at low altitudes over various local times a distinct spatial variation at middle and low latitudes was detected in the eastward component residuals b after the subtraction of a geomagnetic main field model the average b decreases on the dayside as latitude increases and it increases on the nightside that is the latitudinal structure of the b has a negative trend in the northward direction on the dayside and a positive trend on the nightside the slope of the latitudinal profile of b is steep on both dayside and nightside under geomagnetically disturbed condition and has a high correlation with geomagnetic activity index such as the ap these results strongly suggest the existence of a day night net birkeland current system that flows into the polar ionosphere on the dayside and flows out on the nightside these net currents seem to distribute in wide local time range and the center of the net currents is estimated to be around 1000 lt on the dayside and around 2200 lt on the nightside which is essentially consistent with previous theoretical predictions and ground geomagnetic observations

relict sea floor ploughmarks record deep keeled antarctic icebergs to 45 s on the argentine margin abstract this paper reports ploughmarks on the argentine continental margin up to 44 50 s the most northerly morphological evidence of giant icebergs in the southern atlantic more than 2500 ploughmarks up to 32 km long 685 m wide and more than 20 m deep have been observed at modern water depths up to 646 m taking into account these iceberg sizes and ocean current directions which appear to have remained similar from the last full glacial to the present interglacial the icebergs that produced the ploughmarks are interpreted to be calved from fast flowing ice streams draining huge basins of the west antarctic ice sheet and the western antarctic peninsula during the last glacial maximum lgm about 20 000 years ago ploughmarks at the deepest water depths correspond to icebergs more than 500 m thick and at least 2 km wide given a sea level rise of 120 m since the lgm the icebergs after having drifted northeast in the antarctic circumpolar current were transported northward by the falklands malvinas current a total distance of 2000 to 4000 km the waters north of the falkland islands in particular were probably several degrees colder than today to prevent rapid iceberg melting and deterioration

bulls eye pockmarks and polygonal faulting in the lower congo basin relative timing and implications for fluid expulsion during shallow burial abstract this study describes a new type of pockmark association from the lower congo basin offshore west africa consisting of up to 8 stacked paleopockmarks separated by intervals of drape and onlap fill the stacked paleopockmarks occur within the depocentres of polygonally faulted plio pleistocene sediments and are distributed evenly in the downslope parts of two salt mini basins the majority of the stacked pockmarks initiated synchronously in the late pliocene 3 ma with a subordinate initiation phase in the mid pliocene 4 ma the primary agents in pockmark formation are interpreted to be pore water expelled during early stage compaction together with biogenic methane bottom simulating reflections bsrs associated with free gas overlain by gas hydrates are currently found in the area it is speculated that biogenic methane accumulated within and below a clathrate cap which was repeatedly breached forming pockmarks at discrete horizons separated by intervals of draping sedimentation the mid and late pliocene pockmark initiations appear to coincide with sea level falls following periods of relatively stable highstand conditions several subsequent pockmark horizons may similarly correlate with subsequent sea level falls during the late pliocene and early pleistocene the stacked paleopockmarks are completely surrounded by polygonal faults and consistently occur within polygonal fault cells that crosscut the succession containing the stacked pockmarks early stage compaction and dewatering of the pliocene sediments thus preceded polygonal faulting providing a constraint on the conditions leading to polygonal faulting of the fine grained host sediments the relationship documented here is interpreted as due to the presence of a hydrate cap in the plio pleistocene mini basins which may have retarded the normal compaction processes and facilitated pockmark formation by allowing the build up of gas hydrate and free gas in the basin centres the relative timing and spatial relationships implies that fluids expelled due to polygonal faulting were not implicated in pockmark formation in this area

giant mounded drifts in the argentine continental margin origins and global implications for the history of thermohaline circulation abstract partially buried giant drifts are located in the southern most sector of the argentine continental margin generating a bathymetric jump at the base of the slope they are characterised as giant asymmetrical elongated mounded contourite drifts and are described in detail here for the first time this description is mainly based on the bathymetric and multichannel seismic reflection profiles data their origin and evolution in the argentine basin are decoded and their implications in an area crucial for geologic and palaeoceanographic reconstruction between the atlantic and antarctica are discussed the buried giant drifts are divided into two zones by a large seamount the major giant drift 50 km wide 300 km long and having a sedimentary thickness of nearly 1 km is located to the south of this seamount and oriented to the north where its summit outcrops at present seafloor here its asymmetric external shape is characterised by a steep western side and a gently dipping smooth eastern side with internal reflections prograding eastward based on its position morphology and internal characteristics it has been deduced that this giant drift was generated in the argentine basin by the southward branch of a confined large loop of the antarctic bottom deep water from the eocene oligocene boundary coevally with the drake passage opening until the middle miocene however after a major palaeoceanographic change in the mid to late miocene a new oceanographic scenario was established that fossilised these giant drifts these changes produced a new depositional style in the margin resulting in initiation of the present contourite depositional system and morphologic features over the slope extension of north atlantic deep water circulation into the southern hemisphere and deepening of antarctic bottom water circulation in the argentine basin are herein postulated as the factors that controlled said changes these giant drifts show strong seismic evidence of hydrates and free gas that are ripe for energy resources exploration thus demonstrating the economic potential of contourite deposits in deep marine environments

andean foothills structures in northern magallanes basin argentina the foothills of the northern segment of the magallanes basin between 47 and 49 s latitudes record the development of a complex fold and thrust belt in the patagonian cordillera during the late miocene upper paleozoic sedimentary and metasedimentary rocks and thick sequences of lower cretaceous deposits with well defined source and reservoir rocks are deformed by the andean events the structure is characterized by a triangle zone and a series of related underthrusts which define a nonemergent backthrust system at the mountain front differences in the degree of deformation from south to north reveal the evolution of the triangle zone the stratigraphy of the molasse deposits constrains the beginning of the deformation at least to the early tertiary although the final structure was developed during the late miocene these deformed deposits are covered by extensive late miocene horizontal retro arc basalts analysis of the evolving subduction zone located west of and beneath the study area shows that the formation of a subsequent volcanic gap in the magmatic arc and the development of extensive retro arc basalts are related to the collision of a segment of the chile ridge during the late miocene the final deformation and uplift of the patagonian cordillera fold and thrust belt at these latitudes are closely linked to this collision

margin segmentation and volcano tectonic architecture along the volcanic margin off argentina uruguay south atlantic abstract some 25 000 kilometers of regional multichannel seismic data acquired by bgr along the continental margins off argentina and uruguay document that the early cretaceous south atlantic continental breakup and initial sea floor spreading were accompanied by large scale transient volcanism emplacing voluminous extrusives manifested in the seismic data by huge wedges of seaward dipping reflectors sdrs the emplacement of the deeply buried 60 120 km wide sdrs was probably episodic as documented by at least three superimposed sdrs units distinct along margin variations in architecture volume and width of the sdrs wedges are probably related to margin segmentation it is suggested that the margin can be divided at least in four compartments bounded by the falkland fracture zone falkland transfer the colorado transfer the ventana transfer and the salado transfer the individual margin segments reflect in the distribution and thickness of the post rift sediments the individual transfer zones may have acted as rift propagation barriers selectively directing rift segments in left stepping patterns along the western south atlantic margin although we found extensive variations in the architecture style and extent of the seaward dipping reflector sequences a general trend is that the largest volumes are emplaced close to the proposed transfer zones and the width of the sdrs wedges decreases northward within the individual margin segments we suggest that mainly adiabatic decompression and melt generation from shallow sources can explain distinct along margin variations in the volcano tectonic architecture and volumes of extruded magmas

sediment drifts and cold water coral reefs in the patagonian upper and middle continental slope abstract the north flowing falkland malvinas current has generated sediment drifts at a depth of 1200 1600 m in the patagonian middle continental slope out of early holocene hemipelagics late pleistocene ice rafted clastics and neogene fluvial sediments possibly there may be two generations of drifts pleistocene on the outer middle slope and holocene on the inner shelf the ice rafted debris originated in antarctica at a distance of 2000 4000 km south of patagonia scattered over the upper and middle slopes at depths ranging from 300 to 1400 m are cold water coral reefs of less than a meter to about ten of meters in relief it is inferred that most of cold water coral structures flourish as a consequence of the falkland malvinas current that concentrates the food supply at the reef sites growth of cold water coral reefs documented by digital submarine photographs on the upper slope at a depth of 300 400 m may be promoted by upwelling of nutrient rich waters and associated plankton blooms created by the intrusion of falkland malvinas current onto the outer shelf

evidence of a widespread paleo pockmarked field in the orange basin an indication of an early eocene massive fluid escape event offshore south africa abstract the detailed interpretation and seismic attribute analysis of 2d and 3d seismic reflection data sets revealed the presence of a distinct seismic horizon displaying circular and elongated depressions in early eocene sediments along the northwestern slope of the orange basin on the southwest african margin their occurrence within a single seismic horizon indicates a contemporaneous formation process we interpret the depressions as paleo pockmarks which occur as single pockmarks predominantly nne ssw aligned pockmark trains and coalescing elongated pockmarks the paleo pockmarked surface covers an area of 2800 km 2 and has been mapped by integrating the 3d dataset with industrial 2d seismic grids the spatial and temporal occurrence of the paleo pockmarks is associated to two episodes of rapid sedimentation during the late maastrichtian to paleocene comprising fine grained shelf eroded material we propose that they were formed during the early eocene by the seepage of both 1 overpressured pore fluids from disequilibrium compaction of upper maastrichtian sediments and 2 thermogenic gas from underlying cretaceous mature source rocks we show that the location of the pockmarks is controlled by the underlying faults system of the paleo slope the paleocene eocene transition is known for its series of hyperthermal events characterized by negative excursions of the carbon isotope curve the early eocene widespread event of fluid and thermogenic gas escape identified in this work can be an example of the type of carbon release processes occurring in hydrocarbon prone sedimentary basins during this time

contourite depositional system on the argentine slope an exceptional record of the influence of antarctic water masses a significant contourite depositional system cds on the continental slope of the southern argentine margin is described here for the first time this system contains both erosive and depositional features that have resulted from several factors including topographic intensification of the antarctic sourced water masses the systematic northward decrease in speed of these water masses a northward increase of downslope sedimentary processes and local tectonic influences this system is an exceptional example of a cds that started to develop at the time of the eocene oligocene boundary potentially coeval with the opening of the drake passage however a new margin morphology characterized by a complex terraced slope lacking any continental rise developed after a major paleoceanographic change in the middle to late miocene we infer that this change resulted from the extension of north atlantic deep water circulation into the southern hemisphere and the deepening of antarctic bottom water circulation in the argentine basin

estimating the free gas content in baltic sea sediments using compressional wave velocity from marine seismic data a 2 d high resolution velocity field was obtained from marine seismic data to quantify free gas content in shallow muddy sediments at in situ pressure and temperature the velocities were acquired applying migration velocity analysis on prestack time migrated data compressional wave velocities are highly sensitive to free gas as very small amounts of gas can cause a significant decrease in the medium velocity the analyzed profile crosses a depression filled with organic rich holocene mud in the bornholm basin baltic sea the interval velocity field reveals two low velocity patches which extend from the reversed polarity reflections marking the top of the gassy sediment layer down to the base of the holocene mud average interval velocities within the gassy mud are lower than the seafloor migration velocity by up to 500 m s this decrease using a geoacoustic model is caused by an average 0 046 gas volume fraction the interval velocities in individual cells of the velocity field are reduced to 200 m s predicting up to 3 4 gas content the velocity field is limited in resolution due to velocity determination at and between reflections however together with the stratigraphic interpretation geological units containing free gas could be identified shallow gas occurs vertically throughout most of the holocene mud in the gassy area comparison with biogeochemical studies at other baltic sea sites suggests that the distribution of free gas is likely to be patchy in the sediment but the gas concentration may peak below the sulfate methane transition zone and gradually decrease below

iceberg scours pits and pockmarks in the north falkland basin abstract glaciation in the southern hemisphere is limited by the availability of land from which to seed ice sheets the extents of the antarctic patagonian and new zealand ice sheets at the last glacial maximum lgm are relatively well known although the rates and styles of their retreat after the lgm are poorly constrained particularly in antarctica offshore records of glaciation are relatively sparse in the southern hemisphere despite the potential ocean climate insights that can be gained from records of glaciation that are preserved offshore in this study we document the occurrence of iceberg scours and accompanying pits within the north falkland basin c 50 s and discuss their origin the cross sectional shapes of scours are u to v shaped and occur in present day water depths of 280 to 460 m individual scours are up to 38 km long 1 km wide and up to 10 m deep the scours are observed as erosional linear to curvilinear depressions showing only one point of contact between the iceberg and seafloor often with raised berms composed of excavated material identified either side of the main depression undulating width of scours is interpreted as an effect of rotation of the iceberg keel during scour excavation the elongate morphology of the scours differentiates them from asymmetrical pits interpreted to represent iceberg impact pits and symmetrical pockmarks interpreted to form due to fluid expulsion in cross section the differentiation is highly interpretative but the 3d bathymetric expression is unequivocal the sinusoidal character of the scours suggests the interaction between local tidal currents and the east falkland malvinas currents in the north falkland basin at the time of formation offshore and onshore landscape analysis is used to determine potential sources of icebergs and suggests that they were most likely sourced from the antarctic peninsula these results inform our understanding of southern hemisphere ocean climate interactions during the last glacial cycle and suggest that the east falklands malvinas current a key current in the southern hemisphere bringing cold low salinity antarctic derived waters into the south atlantic was in operation during the last glacial cycle the accumulation of icebergs west of the falkland islands would also result in further cooling from fresh meltwater perturbations enhancing the development of a potential ice bridge along the argentinian coast

slow physics of large continental ice sheets and underlying bedrock and its relation to the pleistocene ice ages very simple two component models of the earth s cryosphere lithosphere system are analyzed the models consist of a continental ice sheet coupled to a responding earth model because of relatively long response times of the bedrock at least over shorter wavelengths long period of the order of 50 000 years internal or free oscillations in the system occur over a wide range of parameters and parameterizations the oscillatory mechanism operates primarily in the ablation zone and in particular at the firn line these free oscillations could play a dynamic role in the response of the climate system to eccentricity forcing a simple analysis presented strongly suggests that for the simplest rheological models of the earth response to pleistocence ice sheets elastic response is confined to a layer considerably less than the thermal thickness of the lithosphere furthermore because of the striking lack of geological evidence for large depressions of bedrock at the leading edge of growing continental ice sheets predicted by viscoelastic models of the earth more sophisticated rheological models of the earth s uppermost layer appear necessary

geological controls on focused fluid flow associated with seafloor seeps in the lower congo basin a synthesis of backscatter imagery coupled with a large 3d seismic dataset in the lower congo basin lcb reveals a patchy distribution of features interpreted to be associated with fluid seepage from 300 m to 2500 m water depth with the exception of one region of anomalous backscatter positive relief mounds all inferred seep sites occur in negative relief pockmarks the extensive 3d seismic dataset in the lcb offers a unique opportunity to study the plumbing system that is feeding surface cold seep systems and in general to reconstruct the relationship between tectonics and fluid flow in continental margins the fluid seeps in the lcb are associated with morphologically stratigraphically or tectonically controlled focused fluid flow the integration of the geophysical datasets backscatter imagery coupled to 3d seismic clearly indicates that fluid seeps are not randomly distributed but their seabed organization reflects 1 the location of the underlying structure reservoir or trap where the fluids are coming from 2 the geometry and morphology of the reservoir trap and 3 the discontinuities in the sedimentary column along which fluids have migrated in the lcb seafloor pockmarks are always associated with underlying tectonic structures fault zones salt diapirs polygonal faults or buried sedimentary bodies turbiditic channels erosional surfaces whereas they never occur above sub horizontal parallel stratified fine grained sediments even if triggering processes can not be clearly defined here we propose a model of seafloor fluid seep organization which represents a new tool for identifying the geometry of flow pathways and the underlying buried bodies where the fluids are originating from this qualitative 3d model provides insight into the geohydrologic processes of continental margins

the late jurassic to present evolution of the andean margin drivers and the geological record 1 uncommonly long lived subduction and variable plate geometry along the south american andean plate margin resulted in diverse relationships between magmatic flux and extensional and contractional deformation as recorded by the overriding continental plate convergence velocities absolute overriding plate velocities and subducting slab ages were resolved along the trench from 170 ma to the present using a recently developed kinematic global plate model to identify any relationship between subduction conditions deformation style and magmatic features in the overriding plate key correlations reflect the dependence of macroscopic crustal strain style on subduction mechanism and relative plate vectors extensional back arc basins involving mafic oceanic crust developed only when the overriding plate velocity of south america was directed away from the trench and the modeled age of the subducting slab was older than 50 myr the development of fold and thrust belts and uplift of major plateaus was accompanied by trench normal convergence velocities in excess of 4 cm yr parameters investigated in this study revealed no correlation with the timing of major magmatic events nor was any correlation observed with the structural style of fold and thrust belts

tectonic and structural controls on neogene fluid release in the patagonian continental margin abstract analysis of high resolution multi beam bathymetry 2d multi channel seismic and high resolution seismic sub bottom profiles revealed the presence of widespread fluid escape features in the middle slope of the patagonian continental margin on the sea bottom these features correspond to pockmarks and mud volcanoes whereas in the sub surface they are represented in the seismic records by several acoustic anomalies such as chimneys acoustic blanking enhanced reflectors and reverse phase enhanced reflectors among others some of these acoustic signatures can be traced to syn rift deposits that fill a deep re activated and inverted graben analysis of pockmarks elongation and pockmarks alignments show a good correlation with inverted normal faults suggesting that faults and fractures have influenced the pockmark s shape and might have acted as pathways for upward fluid migration some of the acoustic anomalies associated with pockmarks are interpreted as evidence of gas the gas observed in the seismic data seems to be thermogenic and the seismic data suggests a deep origin associated with over pressured syn rift deposits two morphometrically different sets of pockmarks were identified an older set hosted by miocene aged rocks and a younger set hosted by quaternary deposits different potential triggers are discussed for the genesis of the miocene pockmarks in relation to the seismostratigraphy structural geology and regional tectonics it is concluded that tectonic activity associated with the neogene inversion of the graben faults due to andean compression is the most likely cause for the formation these pockmarks the presence of gas charged sediment and young pockmarks also suggest that after the middle miocene tectonic climax a more recent pulse of release of fluids occurred

patagonian broken foreland and related synorogenic rifting the origin of the chubut group basin abstract the central patagonia is characterized by prominent continental deposits that belong to the cretaceous chubut group basin whose tectonic setting remains controversial it has been interpreted as a foreland basin an extensional basin and even as a sag stage of a rift event related deposits outcrop as part of the patagonian broken foreland east of the patagonian andes particulary the san bernardo fold and thrust belt between 42 and 48 s as the main sector of the broken foreland constitutes a nnw trending intraplate belt located 450 km away from the trench coincidentally at these latitudes 1200 km east of the trench intraplate deformation has caused tectonic inversion of mesozoic basins presently located at the atlantic offshore area additionally at the san jorge gulf area this orogenic system interferes with a conspicuous e w extensional system transversally disposed to the andean strike mainly active between the late early cretaceous and late paleocene we carried out an integrated analysis from structural 2 d and 3 d seismic data at the san bernardo ftb and the adjacent rio mayo basin studying sections of the chubut group basin to unravel the different evolutionary stages thus surface and subsurface evidence of syn compressional deposition indicate the development of a broken foreland basin related to the formation of the san bernardo ftb suggesting an initial growth during the late early cretaceous and probably up to the late paleocene our results imply that the cretaceous compression acted in concert with transversal foreland extension hence we propose a syncontractional rift reactivation of a potential lithospheric anisotropy triggered by regional contraction finally the origin of compression during this time is discussed in relation to multiple processes that go from high convergence between plates and trenchward motion of south america collision of mid ocean ridges to potential dip changes in the subducted slab as suggested by arc expansion

seismic evidence for fluid escape from mesozoic cuesta type topography in the skagerrak abstract gas and fluid dynamic processes in the mesozoic to recent strata of the southern slope and center of the norwegian trench skagerrak have been analyzed by high resolution multi channel seismic data collected with a watergun as the seismic source on board of rv heincke in spring 2003 three profiles across the southern slope and the center of the trench and four profiles cover the north eastern termination of the trench the data resolve the quaternary succession and the subcropping mesozoic strata the separating unconformity shows a typical cuesta topography caused by glacial erosion on the slope contourite drifts are present above a water depth of approximately 400 m internal reflections of the quaternary are obscured by acoustic turbidity particularly phase reversed bright reflections occurring in the entire lower jurassic to upper cretaceous succession suggest the presence of fluids therein sea floor pockmarks have been observed on the southern slope and in the center of the trench both slope and trench pockmarks are vertically correlated with the sub crop of fluid charged mesozoic rocks beneath which indicate lateral fluid escape from porous mesozoic rocks fluids that migrate towards a slope pockmark are capable to wash out gas from the quaternary leaving gas free patches behind since no thermogenic gas has ever been measured in core samples from the slope and trench freshwater seepage is a possible explanation in this case the water outflow might represent artesian springs

local time distribution of net field aligned currents derived from high altitude satellite data 1 in order to explain features of geomagnetic disturbances observed at low and middle latitudes such as the dawn dusk asymmetry in the north south component an antisunward current system has been proposed the antisunward current system consists of net downward field aligned currents on the dayside antisunward auroral electrojet currents and net upward field aligned currents on the nightside however the net field aligned currents consisting in this current system have not been established by spacecraft observations yet in order to discuss whether this antisunward current system actually exists or not and whether this current system actually causes the features of geomagnetic disturbances at low and middle latitudes or not we statistically investigated the global structure of the geomagnetic disturbances generated by the field aligned currents in the inner magnetosphere by using the magnetic field data obtained by the de 1 satellite on the basis of the results local time distribution of the net field aligned currents was estimated net downward currents are found in the prenoon sector and net upward currents are found in the premidnight sector in the postnoon and the postmidnight sector net field aligned currents are rather small on an average the net downward field aligned currents in the prenoon would feed some portions of the auroral electrojets and the portions would flow from the prenoon to the premidnight to connect with the net upward field aligned currents in the premidnight these results are consistent with ground geomagnetic observations in the auroral region in addition we proposed a possible current system which consistently explains our results and many other observations it was also confirmed that the current system can actually cause the features of geomagnetic disturbances on the ground

occurrence of pockmarks on the ortegal spur continental margin northwestern iberian peninsula a total of 445 pockmarks were observed on the upper continental slope of the northwest corner of the iberian peninsula the ortegal spur area by swath bathymetric and ultrahigh resolution seismic data the pockmarks are u v and w shaped and have terraces or indentations in cross section and are dish shaped circular to oval in plan view they occur on the surface of the seabed and buried within the plio quaternary and neogene sediments four types of pockmarks were identified and mapped on the basis of their plan view and cross section morphology regular irregular asymmetric and composite the concentration of pockmarks is attributed to seepage of fluids migrating up dip from deeper parts of the sedimentary basin a linear high density concentration with a nnw to n ne and ese trend of pockmarks is observed above inferred basement faults that do not affect the quaternary succession these pockmarks are thus caused by seepage of thermogenic gas and or other pore fluids from deeper late cretaceous units and their distribution may help to improve our understanding of the fluid system and migration regime in this part of the galicia continental margin

the crustal structure of the southern argentine margin summary multichannel reflection seismic profiles combined with gravimetric and magnetic data provide insight into the crustal structure of the southernmost argentine margin at the transition from a rifted to a transform margin and outline the extent of the north falkland graben based on these data we establish a regional stratigraphic model for the post rift sediments comprising six marker horizons with a new formation in the barremian lower cretaceous our observations support that a n s trending subsidiary branch of the north falkland graben continues along the continental shelf and slope to the argentine basin during the rift phase a wide shelf area was affected by the e w extension subsequently forming the north falkland graben and the subsidiary branch along which finally breakup occurred we propose the division of the margin in two segments a n s trending rifted margin and an e w trending transform margin this is further underpinned by crustal scale gravity modelling three different tectono dynamic processes shaped the study area 1 the triassic early jurassic extensional phase resulting in the formation of the north falkland graben and additional narrower rift grabens ended synchronously with the breakup of the south atlantic in the early valanginian 2 extensional phase related to the opening of the south atlantic 3 the transform margin was active in the study area from about hauterivian times and activity lasted until late cretaceous early cenozoic both the rifted margin and the transform margin are magma poor very limited structures may have a volcanic origin but are suggested to be post rift the oceanic crust was found to be unusually thin indicating a deficit in magma supply during formation these findings in combination with the proposed breakup age in the early valanginian that considerably predates the formation of the parana etendeka continental flood basalt provinces in brazil and namibia question the influence of the tristan da cunha hotspot during the initial formation of the south atlantic

geochemical and geophysical implications of the radiocarbon calibration abstract a precise and accurate chronological framework is crucial to study the dynamics of a variety of phenomena which occurred during the last 45 000 years although the 14 c dating method has been widely applied since the 1950s it is recognized that the atmospheric 14 c 12 c ratio has not been stable during the past in order to calculate accurate ages these fluctuations have to be corrected by means of a calibration curve obtained by comparing raw 14 c measurements with true calendar ages provided by independent dating methods the calibration curve obtained so far is characterized by a long term trend with raw 14 c ages being significantly younger than calendar ages during most of the last 45 000 years abrupt 14 c shifts which occurred over centuries to millennia are superimposed on this long term trend of decreasing atmospheric 14 c 12 c ratio to a certain extent it is possible to outline the different causes of atmospheric 14 c variations by considering complementary information obtained from other cosmogenic nuclides studied at different latitudes 1 most high frequency changes in the atmospheric 14 c 12 c ratio are linked to magnetic fluctuations of solar origin as revealed by studying the last three centuries for which direct observations of the sun are available a similar conclusion is derived by comparing 14 c 12 c events with 10 be and 36 cl concentration maxima in polar ice cores 2 the long term trend shift of 14 c ages is due to a long period of decreased shielding effect of the geomagnetic dipole field which occurred over the interval between 10 000 and 40 000 years bp this interpretation is supported by paleomagnetic measurements performed on volcanic and sedimentary rocks and by 10 be and 36 cl analysed in low and high latitude records 3 a prominent and rapid atmospheric 14 c 12 c excursion occurred between 13 000 and 11 500 cal yr bp thus corresponding to the younger dryas cold period by contrast with the 14 c variability mentioned above which is linked to 14 c production changes this so called 14 c age plateau is probably due to an abrupt variation in the rates of exchange within the global carbon cycle this interpretation is supported by independent geochemical proxies and by numerical modelling of the carbon cycle

elongate depressions on the southern slope of the norwegian trench skagerrak morphology and evolution abstract multibeam echosounder surveys conducted on the southern slope of the norwegian trench between norway and denmark show that elongate depressions interpreted as current modified pockmarks are widespread in water depths of 100 500 m in the holocene recent succession the long axes of the depressions are oriented at an angle of about 15 downslope relative to the depth contours the long axes appear to almost exactly parallel the effective bottom currents in this part of the skagerrak the present current directions were probably established about 4000 years ago following a major shift in the oceanographic circulation pattern the most characteristic feature of the depressions is their large size with maximum depths of 45 m widths of 400 m and lengths of 2 km strong bottom currents continuously transport particles thrown into suspension by the seepage process away from the pockmarks and prevent further deposition continuous seepage of shallow gas at fixed pockmark sites over long time periods combined with generally high sedimentation rates are thought to be responsible for the exceptional size of many depressions the shallow gas probably has a mixed biogenic thermogenic origin however the thermogenic component is considered to be most important for the formation of pockmarks seepage from permeable layers in the jurassic cretaceous bedrock is evidenced by linear rows of depressions above these horizons while pockmarks above sub surface faults indicate gas seeps also from there in shallow water areas the shape and size of pockmarks are locally dependent on their location with respect to giant sand waves which trend sub parallel to the depth contours and are interpreted to be of late weichselian to early holocene age in deeper water areas the pockmarks have been further modified by gravity slides caused by collapse of steep pockmark walls

storm time field aligned currents on the nightside inferred from ground based magnetic data at midlatitudes relationships with the interplanetary magnetic field and substorms 1 on the basis of analysis of east west geomagnetic disturbance fields at midlatitudes we investigated the characteristics of temporal variations of field aligned currents on the nightside during geomagnetic storms the results indicate that upward field aligned currents develop on the nightside especially in the postmidnight when the interplanetary magnetic field imf is directed southward this suggests that the upward field aligned currents are principally associated with the convection electric field it is also found that the upward field aligned currents in the postmidnight can be intensified at substorm expansion while they principally depend on the southward component of the imf the intensification of the upward currents is regarded as a different physical process from the formation of a substorm current wedge at a substorm expansion we discuss some possible effects of the imf and substorms on the generation of the upward currents on the nightside especially in the postmidnight

pockmark formation by porewater expulsion during rapid progradation in the offshore taranaki basin new zealand abstract after the first discovery of seabed pockmarks in the 1960s and 1970s many examples of both modern and ancient pockmarks have been reported from sedimentary basins around the world the exact mechanisms and fluids involved in pockmark formation are still subject to debate with many studies inferring that pockmark formation is a direct indication of hydrocarbon expulsion this study provides the first description of a buried pliocene paleo pockmark field in the offshore taranaki basin new zealand the paleo pockmarks are located approximately 1 2 1 3 km below the seabed and are well imaged by three dimensional seismic data their spatial density and size increases towards the distal portions of the fan and they vary in size between 45 and 580 m in diameter and 2 35 ms twt pockmark size is inversely correlated with the fan thickness in the study area detailed analysis rules out any links with faulting while the relationship with fan variations in the depositional thickness suggests porewater expulsion during overburden progradation as the most likely cause of the paleo pockmarks a model is proposed in which rapid sediment loading generated overpressure which was greatest on the proximal fan due to the lateral gradient in the sediment load imposed by clinoform progradation fluids were consequently forced towards the distal fan where eventually the pore pressure exceeded the fracture gradient of the seal the implications of this study are that not all pockmarks can be used as evidence for hydrocarbon expulsion in frontier regions and care should be taken to examine the subsurface context plumbing systems and any associated acoustic anomalies before concluding on the pockmark origin

the morphologies and genesis of mega pockmarks near the xisha uplift south china sea pockmarks are normally regarded to be manifestations of fluids escape through the seabed kilometer wide depressions here called mega pockmarks occur as widespread features near the xisha uplift northern south china sea most of the pockmarks observed in this area are multi kilometers and much larger than normal pockmarks reported worldwide the maximum diameter observed is 3210 m and the maximum depth observed is 165 2 m based on multibeam data and 2d seismic data the pockmarks are circular elliptical and crescentic in plan view seismic profiles show that the genesis of pockmarks is related to fluid gas and or pore water escape according to the fluids pathways the pockmarks fall into four types 1 gas chimney related 2 depositional boundary related 3 gas chimney and inclined structure fault related 4 inclined structure related bottom currents are strong and complex in the study area the multibeam data and seismic profiles indicate that they may play an important role on extension maintenance and shaping of pockmarks the research of the study area is in its initial stage and the identification of these features as indicators of fluid flow is probably useful for hydrocarbon exploration c 2011 elsevier ltd all rights reserved

the argentine continental margin north of 48 s sedimentary successions volcanic activity during breakup abstract new seismic data and geophysical results from the outer argentine continental margin and from seismic flow line transects across the oceanic argentine brazil and cape angola basins are presented the results document that the early cretaceous south atlantic continental breakup and initial sea floor spreading were accompanied by large scale transient volcanism emplacing voluminous extrusive constructions on the conjugate outer continental margins of the south atlantic on the argentine margin we interpret three major tectono volcanic crustal units beneath a thick and tectonically undisturbed sedimentary succession of cretaceous and tertiary age 1 syn rift basins and even pre rift features are present on the outer shelf 2 a deeply buried 60 120 km wide and several thousand metres thick volcanic wedge occupies most of the slope the wedge is characterized by seaward dipping reflectors and is associated with the distinct magnetic anomaly g 3 the seaward adjacent oceanic crust of cretaceous age shows isochronous changes of the seismic images along our flow line transects suggesting some form of episodicity of rich magma and poor to moderate magma supply at the pre existing mid ocean ridge of the south atlantic during cretaceous time

successive oceanic and solar forcing indicated by younger dryas and early holocene climatic oscillations in the jura abstract the recent extension of 1 the residual 14 c curve back to 11 400 cal yr b p and 2 the lake level reconstruction in the jura back to ca 13 500 cal yr b p offers the opportunity of testing by proxy data the relationships between climate atmospheric 14 c the sun and the ocean recently suggested from the atmospheric 14 c record the climatic significance of the jura record is supported by correlations with climatic oscillations reconstructed in the alps from glaciers and timberline movements correspondence between the 14 c and paleoclimatic record from the jura suggests a working hypothesis two intervals within the holocene can be distinguished in the middle latitudes of western and central europe an early holocene period shows abrupt climatic oscillations linked to ocean forcing major colder climate phases developed between ca 9000 and 8800 and between ca 8000 and 7000 cal yr b c that coincide with higher 14 c values after 6000 cal yr b c a second period is characterized by smoother multicentury climatic oscillations linked to solar forcing

mud volcanoes as potential indicators of regional stress and pressurized layer depth abstract although the use of magmatic volcanoes as indicators of tectonic stress is established the application of the same principles to mud volcanoes has been overlooked here we analyse the onshore azerbaijan mud volcano province in the foreland of the greater caucasus and we present the first use of the statistical regional scale distribution of active mud volcanoes to obtain information about 1 contemporary regional tectonic stress and 2 the average depth of pressurized source layers self similar clustering suggests that mud is mobilized from a ca 5 km deep mud prone layer and mud volcano strain indicators allow the estimation of a robust present day direction of regional maximum horizontal stress s h striking ca 30 e these results are in excellent agreement with geophysical information about upper crustal structure in the region and stress data and deformation vectors we conclude that besides other fold and thrust belts mud volcano features could be used as a proxy for active stress orientation and source layer determination in inaccessible settings such as underwater or even the surface of planets where mud volcanoes are considered to be indicators of mobilized fluids

paleoceanography during the last 150 kyr off central japan based on diatom floras abstract the paleoceanographic history of the area above the northern boundary of the warm kuroshio current during the last two glacial interglacial cycles was reconstructed based on high resolution records of the diatom flora in core md01 2421 ecological categories of the diatom flora were extracted using q mode factor analysis the four most prominent factors explained 91 4 of the total variance cross spectral analyses were performed on temperature estimates derived from diatom data td the relative abundance of littoral neritic diatom associations and the oxygen isotope record of benthic foraminifera diatom derived temperatures show a 50 23 and 17 kyr periodicity with only the 23 kyr band coherent with that of benthic foraminiferal oxygen isotope values littoral neritic associations show a periodicity at 62 and 17 kyr coherent with td species composition and relative abundance of the diatom flora changed at the marine isotope stage boundaries when cold water and warm water mixed over the core site due to hydrographic disturbances by changes in transport trajectory of the kuroshio current transport and global climatic changes

u pb geochronology and magnetostratigraphy of a north patagonian syn orogenic miocene succession tectono stratigraphic implications for the foreland system configuration abstract the syn orogenic miocene deposits of the patagonian foreland system were the focus of numerous works in the latest years due to their central role in the comprehension of the tectono stratigraphy and tectonic evolution related to the patagonian andean growth in this work we present a chronostratigraphic scheme based on magnetostratigraphy and u pb geochronology for one of the syn orogenic successions of the patagonian foreland system the paso del sapo basin the sequence is constrained to an age of 14 9 to 14 9 14 6 ma for the basal alluvial systems of la pava formation with mean sedimentation rates of 4 2 cm kyr of 14 6 to 12 8 12 7 ma for the lacustrine deltaic environments of the lower collon cura formation with sedimentation rates ranging from 1 6 to 26 0 cm kyr and of 12 8 12 7 to 14 9 to

seasonal and local time dependences of the interhemispheric field aligned currents deduced from the orsted satellite and the ground geomagnetic observations 1 the precise magnetic field measurements by the orsted satellite revealed a distinct spatial variation of the eastward component residual b at mid and low latitudes on the dayside this magnetic field variation was attributed to the interhemispheric field aligned currents ihfacs at mid and low latitudes based on its comparison with ground geomagnetic field variations these ihfacs were found to flow from the summer hemisphere to the winter hemisphere in the dawn sector and from the winter hemisphere to the summer hemisphere in the noon and the dusk sectors these ihfacs flip their direction not at equinoxes but in may and november the current intensity of the ihfacs is largest not at solstices but in february and august the ground based geomagnetic field observations at 17 low latitude stations also support the seasonal dependence and local time distribution of the ihfacs