venezuela basin crustal structure velocity depth profiles derived from six two ship expanding spread experiments in combination with other geophysical data define the characteristics of two distinct types of venezuela basin crust and the boundary between them each two ship common midpoint reflection refraction profile is automatically transformed into the p plane picked and interpreted to provide v z functions with appropriate confidence bounds the results together with gravity magnetic and near vertical incidence reflection data reveal a 50 000 km2 triangle of venezuela basin crust which resembles normal oceanic crust in a magnetic quiet zone north and west of this triangle lies the previously defined thick caribbean crust having two distinct layers above the m discontinuity acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at deep sea drilling project sites 146 149 sills and 150 flows also depths to mantle are greater than normal interpretations of near vertical and wide angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal the boundary between the two crustal areas has a ne sw trend which parallels the dominant structural and magnetic lineations this boundary coincides in position though not in trend with the previously defined central venezuela basin fault zone further study is required to determine whether this boundary is of tectonic origin or if it represents a change in style of crustal production

opening of the grenada back arc basin and evolution of the caribbean plate during the mesozoic and early paleogene abstract geological and geophysical data indicate that the grenada basin was presumably created during the paleocene by sundering of a proto eastern caribbean arc into a remnant arc to the west aves swell and an active arc to the east lesser antilles ridge grenada basin spreading is thought to have been penecontemporaneous with the creation of the yucatan basin located at the opposite side of the caribbean sea i suggest that a continuous mesozoic caribbean arc m c a including the greater antilles the aves lesser antilles system and the aruba blanquilla chain netherland venezuelan antilles was initiated in the pacific probably about 130 120 ma ago its arrival in front of and its subsequent motion inside the central atlantic tethyan seaway caused the opening of both yucatan and grenada basins which occurred at the two initial points of contact with the north and south american cratons in contrast to the style of many other island arcs this back arc spreading event occurred only once in the long history of the m c a the lesser antilles appear to be the oldest currently active intra oceanic island arc

magnetism in rocks rock magnetism is the study of induced and remanent magnetization of ferrimagnetic mineral grains in rocks sediments soils and organisms its applications include environmental magnetism magnetic anisotropy sources of continental and oceanic magnetic anomalies records of geomagnetic field variations and polarity reversals and the paleomagnetic record of plate motions and the wilson cycle this paper reviews the beginnings of rock magnetism and then traces the development of six particularly interesting areas pseudo single domain behavior magnetic domains and micromagnetic structures diagnostic tests of the type and stability of remanent magnetization magnetic microanalysis thermoviscous remagnetization and chemical remanent magnetization other areas including sediment and soil magnetism are covered in a companion paper by verosub and roberts on environmental magnetism

seabed morphology and shallow sedimentary structure of the storfjorden and kveithola trough mouth fans north west barents sea abstract this study aims to present an overview of the seafloor morphology and shallow sedimentary structure of the storfjorden and kveithola trough mouth fans tmfs on the northwestern barents sea continental margin data have been compiled from two international polar year ipy cruises svais of the bio hesperides and eglacom of the r v ogs explora that yielded 15 340 km2 of multi beam bathymetry and 9500 km of sub bottom seismic profiles in this area the continental shelf edge defines three wide and subdued sedimentary lobes forming storfjorden tmf one single lobe on kveithola tmf and three inter tmf areas on the continental slope the two northernmost lobes of storfjorden tmf lobes i and ii are composed by thick up to 50 m sequences of glacially derived debris flow deposits interbedded with thin a few metres de glacial and interglacial deposits a network of upper slope gullies incises these debris flow deposits as a consequence of subglacial meltwater release at or near the shelf break gullies evolve into channels whose morphologic evidence disappears midslope leaving place to a subdued chevron like morphological pattern inherited by the preceeding glacial maximum debris flow deposits a drastic change occurs on the continental slope of storfjorden tmf lobe iii and kveithola tmf where are several translational submarine landslides mostly originated in the upper slope the majority of which detach at the contact between middle weishelian glacigenic debris flows and the overlying acoustically laminated plumites dendritic canyon systems only develop in inter tmf areas the data suggest that tmf continental slope progradation depends on short lived episodes of extreme sedimentation during glacial maxima and during the early deglaciation phase and that an important controlling factor is the mechanism of ice stream retreat from the continental shelf edge we suggest that the two northern storfjorden sub ice streams were composed of thicker and perhaps faster ice progressively draining a distal and larger ice source mainly located on svalbard conversely the southernmost storfjorden sub ice stream and the kveithola ice stream were fed by a local smaller marine based ice dome grounded on spitsbergenbanken the ice dome persisted after the lgm maintaining a local ice drainage system close to the shelf edge whose sedimentary evidence can be found on the continental slope of the southern lobe of storfjorden tmf and kveithola tmf the high degree of lateral variability in the style of sedimentation on tmf slopes suggests that ice stream dynamics may vary considerably within the same glacial trough and that such variability affects the long term development of the architecture of tmfs

back arc rifting in the okinawa trough abstract geological and geophysical data reveal that the okinawa trough shows incipient continental rifting and crustal separation started from about 2 ma the early extensional movements in the trough are probably of miocene age in addition to the miocene phase two main periods of extension are recognized a pleistocene phase between 1 9 and 0 5 ma and the present day phase during the stage short central rifts central grabens were formed the opening however may have occurred only in the southern part of the trough basin having an average half spreading rate of 2 cm yr 1 since early pleistocene time producing its present width of several tens of kilometres these activities were well represented by igneous intrusions sedimentary facies and sedimentary structures in and around the okinawa trough the width of the zone affected by back arc extension defined as greater okinawa trough is larger than the present okinawa trough whose width is 200 250 km the present form of the greater okinawa trough started to form at the same time as that of the okinawa trough

tectono magmatic evolution of the west philippine mariana region and the origin of boninites re examination of published data on the tectomagmatic evolution of the west philippine mariana region indicate that arc magmatism and back arc extensional pulses are not synchronous but are largely asynchronous arc volcanism ceases within a few million years of the development of a back arc basin and recommences oceanward on a new arc during the final stages in the development of the back arc basin following karig s model we believe that ascent beneath the arc axial chain of a line of diapirs of morb source mantle interferes with processes of arc magma generation and arc magmatism wanes partial melting of the diapirs produces voluminous morb type tholeiites which split the arc and form the crust of a widening back arc basin we show for both the west philippine mariana region and occurrences of boninite like lavas in ophiolites that boninites appear to be erupted after arc magmatism and immediately before eruption of morb type lavas passage of a diapir of hot morb source mantle through refractory sub arc peridotite which has been enriched in lile and h2o will result in limited partial melting of the hydrous peridotite and the restricted production of boninitic magmas these are more likely to erupt modified or only slightly modified by crystal fractionation through the thin forearc crust continued ascent of the morb source mantle diapir will result in it partially melting to yield morb tholeiites which erupt shortly after and in considerably more voluminous amounts than the boninites finally we suggest that forearcs regions are likely to be incorporated into foldbelts and that boninite or low ti lava bearing ophiolites may characterize such regions

mid eocene early oligocene sinistral transcurrent faulting in puerto rico associated with formation of the northern caribbean plate boundary zone left lateral transcurrent wrench faulting through the southern puerto rico fault zone sprfz in central southern puerto rico is indicated by fault orientations and motion direction indicators the sprfz is a west northwest trending 3 8 km wide diffuse system of faults and folds that within the study area bounds paleocene and eocene volcaniclastic rocks the eocene belt and is flanked by cretaceous volcanic and volcaniclastic rocks structural data show that much of central southern puerto rico is cut by strike and oblique slip faults whereas locally in the southeastern part of the eocene belt rocks are pervasively thrust northeastward over cretaceous rocks of central puerto rico structural data and the distribution of fault types support two local evolutionary scenarios 1 different structural horizons of the transpressional sprfz i e upper thrust dominated horizon and lower strike slip dominated horizon were juxtaposed along post transpressional normal faults that border the ponce basi

the coulomb critical taper theory applied to gravitational instabilities the critical coulomb wedge theory has been widely applied to the structural evolution of accretionary wedges and fold and thrust belts but it also predicts the spreading of a wedge under gravity this solution may be applied to the formation of gravitational spreading and gliding along passive margins where elevated pore fluid pressure is common following the initial hypotheses of the theory we provide an alternative expression of the exact solution for a noncohesive wedge better suited to slope instabilities our formulation allows a direct calculation of the fluid pressure required for the system to deform and predicts two kinds of gravitational deformation shallow slumping and or deep gravitational spreading rooting on the basal detachment to verify the predictions of the model we performed scaled experiments with pore pressure under conditions close to the critical taper hypotheses the good agreement between our experimental results and the theory confirms the applicability of the critical taper model to the formation of gravitational structures in sedimentary wedges subjected to pore fluid pressure

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

structure and deformation history of the northern range of trinidad and adjacent areas conflicting models have been proposed for both the evolution of northern south america and the neotectonics of the south caribbean plate boundary zone the trinidadian portion of the margin is particularly controversial but surprisingly it has been little studied we present a structural analysis of trinidad s northern range pertinent updates of the island s stratigraphy and sedimentology and new zircon fission track age determinations and use them to constrain trinidad s geologic history and to better understand the controlling tectonic processes in our interpretation trinidad s three e ene striking ranges which are separated by late neogene recent depocenters expose 1 the northern range group generally greenschist metamorphosed upper jurassic to cretaceous north facing continental slope sediments of the northern range deposited on the northern south american passive margin 200 400 km to the wnw and 2 the trinidad group cretaceous paleogene shelf slope sediments of the central and southern trinidad deposited less than 100 km wnw of their present location a small allochthon composing the sans souci group cretaceous tholeiitic volcaniclastic basaltic and gabbroic rocks sans souci formation and sediments toco formation now in the northeastern northern range has been transported hundreds of kilometers from the west with the caribbean plate despite earlier references to cretaceous orogenesis all deformation in trinidad is of cenozoic age the first deformation in the northern range d1 formed north vergent nappes and induced greenschist metamorphism probably in the late eocene or oligocene the nappes developed either by the underthrusting of the proto caribbean crust beneath south america due to convergence between north and south america or as gravity slides caused by oversteepening induced by this convergence and or the passage of the caribbean plate s peripheral bulge and arrival of its foredeep northern range d2 deformation is south vergent and represents the incorporation of northern range metasediments into the caribbean accretionary prism the transition to d3 brittle transpressive right lateral strike slip faulting is interpreted to be due to the uplift and east southeastward transpressive emplacement of northern range caribbean prism rocks onto the south american stepped shelf this emplacement formed the miocene transpressive thrust belts and foreland basin in central and southern trinidad in the final phase of northern range deformation d4 e w normal faults and shear zones and conjugate nnw sse and ne sw normal faults developed and displacement on preexisting e w right lateral strike slip faults continued the 11 ma northern range zircon fission track ages suggest rapid uplift from the late miocene to recent late miocene subsidence of the tobago platform immediately to the north of the northern range and greater than 3 km of normal down to the north displacement indicated for the north coast fault zone separating the northern range and tobago platform leads us to postulate that the rapid uplift of the northern range was in response to the northward detachment of the tobago platform from above the northern range along the north dipping transtensional north coast fault zone this late miocene change in deformation style can be explained by a change from caribbean south american right lateral transpression to right lateral strike slip generally striking 080 this has generally induced a component of extension on pre existing faults striking at greater than 080 and a component of compression on faults striking at less than 080

the mineralogy and geochemistry of the cerro matoso s a ni laterite deposit montelibano colombia the cerro matoso s a ni laterite deposit in northwest colombia is an important producer of ferronickel expanded production of ferronickel is planned to be 55 000 mt by mid 2004 the deposit is developed over a peridotitic protolith that is exposed in the form of an elongated hill the deposit s weathering profile is variable both vertically and laterally and 10 distinct lithostratigraphic units have been characterized two typical sections through the weathering profile were sampled from an area of the mine with high pit 1 and lower pit 2 ni grades bench mapping has shown that pits 1 and 2 have distinctly different weathering profiles from bottom to top the profile in pit 1 is weakly serpentinized peridotitic protolith saprolitized peridotite green saprolite main ore horizon tachylite used by mine geologists to describe an enigmatic fe oxide horizon black saprolite yellow laterite red laterite the sequence is then capped by a magnetic to nonmagnetic ferricrete known locally as canga the succession in pit 2 is from serpentinized peridotite saprolitized peridotite brown saprolite yellow laterite red laterite and lacks the green saprolite ore horizon all the units in pit 2 have currently uneconomic ni grades the thickness of the units is highly variable but most of the major horizons have maximum thicknesses of the order of tens of meters both pits contain abundant fault and joint related silicate veins sometimes in stockworks in the lower part of the sequence these veins contain the distinctive green mineral known as garnierite actually pimelite a form of nickeliferous talc as well as quartz and chalcedony and they can have a ni content of up to 30 to 40 wt percent the bulk geochemistry in most units of both profiles shows a fairly typical ni laterite pattern in which mgo and sio2 are depleted toward the top of the sequence whereas feo increases mineralogic studies confirm that the protolith in both pits is a partly up to 50 serpentinized harzburgite and that in pit 1 the main ni bearing phases in the weathering profile are ni sepiolite ni serpentines and other hydrous silicates the garnierites in cerro matoso have been identified as pimelite in which various amounts of ni have substituted for mg the upper part of the sequence is dominated by amorphous and crystalline fe oxide phases the magnetic canga is composed mainly of maghemite that may have been produced by oxidation of magnetite rich units the mineral content of pit 2 is dominated by poorly structured fe oxides or goethite and by subordinate clay minerals and quartz the geochemistry and mineral content of the deposit suggest that as in many other ni laterite deposits ore genesis is strongly controlled by local climate topography and drainage mass balance calculations indicate that the profiles in pits 1 and 2 had different weathering histories because the degree of profile collapse and residual enrichment in pit 1 is far more extreme than that in pit 2 this difference may be the result of different degrees of serpentinization of the protolith in the two pits and potential dilution of the ore in pit 2 by input from an exotic unit ni in the deposit has also undergone supergene enrichment resulting from the leaching of ni from the upper part of the lateritic profile and its transport to the green saprolite unit where the ni was fixed in silicate minerals

palaeoceanography of organic rich rocks of the loma chumico formation of costa rica late cretaceous eastern pacific the late cretaceous late cenomanian campanian loma chumico formation consists of laminated to massive bedded siliceous shales cherts ash fall tuffs volcanic glass beds and volcaniclastic sandstones the loma chumico formation is at least 630 m thick core data in the tempisque basin and can be subdivided in the subsurface into organic rich and organic lean total organic carbon 1 facies rock eval total organic carbon toc data show that toc in the organic rich facies averages 15 5 with values as high as 33 inorganic geochemical petrological and sedimentological analyses indicate that all the lithologies of the loma chumico formation were deposited in middle shelf to outer slope environments adjacent to an active subaerial island arc the organic geochemistry and palaeontology of the loma chumico formation help constrain the factors that controlled biological productivity in the region during the late cretaceous biomarkers identified in organic rich facies by gas chromatography mass spectrometry gc ms show a high abundance of total steranes relative to hopanes in the morote 1 core indicating algal dominated organic input the hopane to sterane ratio of the extract from the manzanillo 1 core indicates a relatively high bacterial component to the organic matter upwelling and extrusive volcanism were the most important factors that controlled nutrient supply including silica flux and biological productivity within the loma chumico formation depositional system episodic radiolarian dinoflagellate or diatom blooms instigated by rapid infusions of volcanogenic silica may have resulted in expansion of the oxygen minimum zone omz enhancing the preservation of organic matter productivity appears to have been uniform even during deposition of the organic lean facies the findings of this study suggest that ocean wide anoxia in the eastern pacific was unlikely during the late cretaceous high toc contents of organic rich radiolarites from central caribbean deep sea drilling project dsdp sites may be related to upwelling produced in the eastern pacific by convergence of the easterly trade winds eastern pacific equatorial upwelling zone or may be part of a more complex pacific atlantic ocean interaction the end of deposition of organic rich rocks began near the end of the latest santonian earliest campanian in the central caribbean and costa rica palaeoceanographic models suggest that improved circulation and ventilation of bottom waters due to early opening of the north atlantic south atlantic seaway may explain this observation this may represent the influx of a previously unrecognized source of colder oxygenated intermediate bottom water proto antarctic intermediate water into the late cretaceous central atlantic and eastern pacific oceans

differences between magnetic domain imaging observations and theory a quasi two dimensional model is developed to calculate domain structures in magnetite grains containing two to 20 domains the calculation shows that domains do not consist of uniformly magnetized regions consistent with the conclusions previously carried out on sub micron grains but that they become more uniformly magnetized for grains containing several domains consistent with domain imaging results in spite of improvement over previous calculations there remains a significant difference between the number of domains observed and calculated

cenozoic rift formation in the northern caribbean rifts form in many different tectonic environments where the lithosphere is put into extension we outline the distribution orientation and relative ages of 16 cenozoic rifts along the northern edge of the caribbean plate and suggest that these structures formed successively by localized extension as the caribbean plate moved eastward past a continental promontory of north america evidence leading to this conclusion includes 1 recognition that the rifts become progressively younger westward 2 a two phase subsidence history in a rift exposed by upthrusting in jamaica 3 the absence of rifts east of jamaica and 4 the observation that removal of 1400 km of strike slip displacement on the cayman trough fault system places the paleogene rifts of jamaica in an active area of extension south of yucatan where the rifts of honduras and guatemala are forming today

hot regions and the origin of marginal basins in the western pacific abstract volcanic provinces showing no genetic relationship to plate boundaries may be classified into two categories hot spots which are a few hundred kilometers across and may be nearly fixed in position with respect to deep mantle and hot regions which are a few thousand kilometers across and can migrate more freely the mantle upwellings which presumably give rise to them may originate at different depths by different causes it is observed that the marginal basins older than 15 ma of the western pacific become progressively younger northward with the exception of the south fiji basin this leads to the hypothesis that the spreading of marginal basins was caused by the mantle upwelling associated with a migrating hot region and that a hot region which started from the australia region in the late cretaceous migrated northward successively creating the tasman coral sea caroline south china parece vela shikoku and japan basins through paleocene eocene oligocene and early miocene time this hot region presently underlies northern china mongolia and adjacent areas another hot region is assumed to have caused the spreading of the south fiji basin and all the currently spreading basins in the southwestern pacific spreading of individual marginal basins occurred during the short period when a hot region was passing the pertinent area the main merit of this hypothesis is that it can explain the relationship between the location and age as well as the short duration of spreading of western pacific marginal basins marginal basins are formed commonly though not always in the overriding plates near subduction zones because in such a setting a spreading marginal basin is readily accommodated either by an oceanward movement of the nearby subduction zone or by a continentward movement and subduction of the overriding oceanic plate there are a few marginal basins that were formed in other tectonic settings and accommodated by some other mechanism of compensative subduction presumably the splitting of the supercontinent pangea was also caused mainly by the effect of the mantle upwelling associated with the same hot region that later migrated northward creating marginal basins in the western pacific the main basis for this presumption is the fact that the splitting of pangea was completed in the australia region in middle cretaceous time whereas the spreading of marginal basins in the western pacific began in the same australia region in the late cretaceous immediately following

two modes of back arc spreading back arc spreading is initiated with rifting along the volcanic zone of an island arc under extensional tectonics the width of the volcanic zone of the arc which is closely related to the dip angle of the subducting slab should cause a variation in the style of initial rifting and subsequent back arc spreading two modes of back arc spreading single rift type and multirift type are introduced from the comparative study of the bonin arc and the japan sea divergent tectonics in an island arc above a steeply dipping slab will cause a single rift system in a narrow volcanic zone but will cause a multirift system in a broad volcanic zone which is related to a shallow dipping angle of subduction

acoustic imaging of gas hydrate and free gas at the storegga slide 1 analysis of p wave velocities of an ocean bottom cable obc data set demonstrates the existence of gas hydrates and free gas on the northern flank of the storegga slide the distribution and concentration of gas hydrates and free gas show significant variation over the length of the obc average gas hydrate saturation of pore space is 5 when modeled by effective medium theory with hydrates as a sediment frame component average gas saturation is 0 45 assuming homogeneous distribution the bottom simulating reflector bsr is generally identified as termination of enhanced reflection but in some places appears as a reflection proper amplitude anomalies related to the bsr are primarily caused by the gas and not the hydrates the gas hydrates at the storegga slide develop from gas rich fluids migrating into the gas hydrate stability zone ghsz from the sediments below development of polygonal faults and the related expulsion of formation water might drive the fluid flow in the area the physical and geological properties of the sediments control the distribution and concentration of gas which migrates predominantly along strata rather than along the base of the ghsz this migration mechanism controls the flux of gas rich fluids into the ghsz and hence the distribution of gas hydrates and the variation in gas hydrate concentration

dolomite formation in caribbean islands driven by plate tectonics abstract postdepositional dolomite occurs in the shallow subsurface of several caribbean islands in tertiary and pleistocene carbonates e g on barbados grand cayman jamaica and st croix recent investigations have interpreted these dolomites as relatively early diagenetic formations from chemically modified seawater mainly on the basis of petrography stratigraphic association and stable isotope values interpretations of the hydrologic regimes during dolomitization vary and usually include freshwater seawater mixing thermal or density driven convection of seawater or a combination thereof this paper offers two alternative interpretations based on the relatively low sr isotope ratios of the dolomites 0 7085 to 0 7092 and their proximity to active plate margins dolomitization may have been driven by plate tectonics i e either by thermal convection of pore fluids at active spreading centers or by tectonic expulsion of fluids from accretionary prisms the dolomitizing solution was seawater that was enriched in 86sr by magmatic hydrothermal solutions or by interaction with siliciclastics in an accretionary prism respectively

thermal modeling of the geometry of subduction with implications for the tectonics of the overriding plate the path which subducting lithosphere follows can be broken into three zones zone i a subhorizontal region of nearly constant dip near the trench zone ii a region of rapidly increasing dip and zone iii a region generally deeper than 125 km again of nearly constant dip we have modeled the thermal evolution of slabs as they pass through these three zones using the method of lines a numerical technique well suited to the solution of time dependent partial differential equations by use of appropriate initial conditions and time dependent boundary conditions we can simulate subduction with variations in age of subducting lithosphere slab velocities and paths followed by the slab thermal modeling results indicate that the geometry of the slab in region ii can be empirically related directly to the thermal structure of the slab in that region and thus indirectly to age and velocity it follows that low angle subduction can extend several hundred kilometers inland only in cases of high convergent velocities 15 cm yr 1 and old 120 m y lithosphere we further suggest that rapid variations in the convergence velocities can provide an enabling mechanism for back arc spreading while slow changes in convergence velocities and or age variations in the subducting lithosphere lead to migration of volcanic arcs

galicia bank sediment transport activity in response to continuous sedimentary instability dynamics a geotechnical perspective the sedimentary instability dynamics occurring over time throughout the isolated galicia bank atlantic ocean nw iberian peninsula have been studied using a sedimentological and geotechnical approach featuring a quantitative assessment of slope stability under different scenarios including earthquake activity the erosion of the scarps in the galicia bank includes the continuous development of small instabilities evacuated along gullies and channels or deposited on slopes these deposits may be subsequently mobilized evolving into new sedimentary gravity flows e g turbidity flow the studied sediments consist mostly of poorly to very poorly sorted sands and silts transported by turbidity currents and occasionally by debris flow processes the sediments in the study area identified as normally consolidated and located on gently sloping areas gradients less than 5 may become unstable if low magnitude seismic events occur pga 10 without any trigger other than oversteepening in contrast overconsolidated sediments may remain stable under static conditions and may become unstable on slope gradients 10 when earthquakes occur with the maximum peak ground acceleration pga 0 34 the sedimentological and geotechnical models presented herein are complementary approaches that can be utilized to understand the long term sedimentary instability dynamics observed within the study area such results are critical for better understanding sedimentary models of the dismantling processes of deep seamounts located far away from continental sedimentary inputs

terrestrial style slow moving earthflow kinematics in a submarine landslide complex abstract morphometric analysis of simrad em300 multibeam bathymetric dems reveals details of deformation patterns in a 145 km2 submarine landslide complex that are commonly associated with slow moving earthflows in terrestrial settings this mode of failure where existing landslide debris is remobilised repeatedly along discrete shear boundaries and is progressively conveyed through the complex has not previously been recognised in the submarine environment the kinematics contrast with the more traditional models of submarine landslide complex development in which repeated catastrophic failures each mobilise new source material to form a composite stacked landslide deposit in our study of the tuaheni landslide complex on the hikurangi margin of new zealand remobilisation has formed boundary shear zones imaged at the seafloor surface in multibeam data and at depth in multichannel seismic reflection data a significant amount of internal deformation has occurred within the debris streams phases of deformation appear to be partitioned longitudinally as extensional and contractional zones rooted into a basal decollement and laterally with strike slip shears partitioning discrete debris streams while slow moving terrestrial earthflows are activated by fluctuating piezometric levels typically controlled by precipitation different processes cause the equivalent mobility in a submarine earthflow elevated pore pressures in submarine earthflows are produced by processes such as earthquake generated strong ground motion and or gas fluid release earthflow movement in submarine settings is prolonged by slow dissipation in pore pressure

paleoseismicity along the southern kuril trench deduced from submarine fan turbidites abstract large m 8 damaging interplate earthquakes occur frequently in the eastern hokkaido region northern japan where the pacific plate is subducting rapidly beneath the okhotsk north american plate at approximately 8 cm yr 1 with the aim of estimating the long term recurrence intervals of earthquakes in this region seven sediment cores were obtained from a submarine fan located on the forearc slope along the southern kuril trench japan the cores contain a number of turbidites some of which can be correlated among the cores on the basis of the analysis of lithology chronology and the composition of sand grains foraminiferal assemblages and the composition of sand grains indicate that the upper middle slope 1000 m water depth is the source of the turbidites the deep sea origin of the turbidites is consistent with the hypothesis that they were derived from slope failures initiated by strong shaking associated with earthquake events the recurrence intervals of turbidite deposition are 113 439 yr for events that occurred over the past 7 kyr the short intervals are recorded in the cores obtained from levees on the middle fan although many large earthquakes 150 cm s 2 of peak ground acceleration at the inferred slump sources occurred during the 19th and 20th centuries the pilot core from the upper fan contains only three turbidites located stratigraphically above layers of 17th century volcanic ash the results of 210pbex and 137cs dating combined with simulations of the ground accelerations of historical earthquakes enable correlation of the three turbidites with known historical earthquakes the 1952 tokachi oki and the 1961 and 1973 nemuro oki earthquakes the turbidites within the sampled cores potentially record about half of the large earthquakes known to have occurred over the interval covered by the cores the fact that any single core records only a portion of the known seismic events suggests that the recurrence interval of earthquakes in this region is less than 113 yr during holocene

slope stability assessment of the helland hansen area offshore the mid norwegian margin abstract numerous large scale submarine slides affected the continental slope offshore mid norway during the holocene one of them the storegga slide is located south of the voring plateau stretching from the north sea fan to the helland hansen arch with a volume of 3200 km 3 and an influenced area of 95 000 km 2 the storegga slide which occurred about 8250 years before present bp represents one of the largest submarine slides worldwide the slope stability assessment concentrates on the northern sidewall helland hansen area where gas hydrates are suspected from the presence of a bottom simulating reflector bsr on reflection seismic data and where few slope stability investigations have been performed compared to the ormen lange area the limit equilibrium and finite element methods fem were used for the evaluation of static and dynamic seismic stabilities of the dipping seabed to account for the uncertainties in the soil parameters a probabilistic approach was applied by coupling the limit equilibrium model with the first and second order reliability methods form and sorm and the monte carlo simulation the finite element simulation for the seismic loading indicates that a strong earthquake 6 5 ms could be a potential trigger for slope failure but only down to 30 m subsurface obviously one needs a significant pore pressure build up and loosening of the sediments to explain the slide on such gentle slopes more critical preconditions to failure in the deeper sediments should exist to explain the thickness of the sliding slab knowing that the northern sidewall is approximately 150 m high cyclic loading due to a series of earthquakes could explain the slide affecting the shearing resistance in a marine clay unit weak layer by excess pore pressure generation drained conditions the degradation of undrained shear strength with cyclic loading plays a major role in the instability process

fluidization of buried mass wasting deposits in lake sediments and its relevance for paleoseismology results from a reflection seismic study of lakes villarrica and calafquen south central chile abstract a dense grid of very high resolution seismic profiles on lake villarrica provides a quasi 3d view on intercalated lenses of low amplitude reflections which are connected by acoustic wipe out patches and fractures to an underlying voluminous mass wasting deposit the lenses are interpreted as being created by earthquake triggered liquefaction in this buried mass wasting deposit and subsequent sediment fluidization and extrusion at the paleo lake bottom these sediment volcanoes are mapped in detail they have a rather uniform circular geometry and show a linear relationship between apparent width and maximum thickness on a seismic section the largest sediment volcanoes are up to 80 m wide and 1 9 m thick their slope angles designate a syn to post depositional sagging of most sediment volcanoes sediment volcano detection and mapping from nearby lake calafquen further strengthen the revealed geometrical relationships locally some of the sediment fluid escape structures extend to a higher position in the stratigraphy which points to a polyphase escape process associated with multiple multi century spaced strong earthquakes thickness and morphology of the source layer seem to exert a dominant control in the production of sediment fluid extrusions this study shows that reflection seismic profiling allowed recognizing 4 different seismic events in the studied stratigraphic interval which are evidenced by mass wasting deposits and or fluidization features

subsidence of the shikoku back arc basin abstract sources of basaltic tephras found in miocene cores at dsdp sites 443 and 444 in the eastern side of shikoku basin are likely to be the kinan seamounts which were emergent during middle to early late miocene abnormally high vesicularity of dredged and dsdp pillow basalts together with existence of a thin layer of limestone or chalk overlying the basalts seem to indicate that the axial zone of shikoku basin was much shallower than the present depth immediately after solidification of the basement and that it subsided quickly shallow water depths of the back arc basins are characteristic of the present lau basin and okinawa trough whereas the mariana trough possesses water depths about 1000 m deeper than normal oceans of the same age most of inactive back arc basins have depth age relations consistent with that of mariana trough very shallow water depths of the axial zone of back arc basins similar to those of the shikoku basin axis during their opening period appear to be due to non isostatic uplift caused by excess mass and to isostatic elevation by buoyancy of hot partially molten materials accumulated above the subducting slab

cenozoic plate tectonic history of the northern venezuela trinidad area geological and geophysical data coupled with recent plate tectonic reconstructions suggest that the cenozoic geologic history of the northern venezuela trinidad area has been dominated by strike slip displacement of discrete crustal blocks allochthonous terranes within the area include metavolcanic rocks of the cretaceous villa de cura group and metamorphic rocks of the precambrian to cretaceous cordillera de la costa a relatively competent crustal block margarita block is defined by an outline around the metamorphic basement of margarita island the araya paria peninsula the northern range of trinidad and tobago island reconstruction of the margarita block to its original position requires at least partial closure of the falcon basin closure of the bonaire and cariaco basins and restoration of about 50 km of motion on both the oca and bocono faults post middle eocene eastward translation of the caribbean plate caused eastward motion of the margarita block a minor change in relative plate motion during the late oligocene or early miocene produced a right step in the moron fault forming the cariaco pull apart basin and el pilar fault zone maximum offset on el pilar fault is estimated to be no more than 125 km though displacement along the entire fault zone may have been greater transpressional stresses between the caribbean plate and northern south america caused folding of the serrania del interior of venezuela and the central range of trinidad eastward migration of transpressional stresses at the southeastern corner of the caribbean south american plate boundary is being accommodated by formation of oblique thrusts transpressive anticlines and downwarping of the crust bouguer gravity data suggest that jurassic aged atlantic oceanic crust is being depressed as the caribbean plate expands into the demerara plateau area this study suggests that the faults and transtensional transpressional compressional structures identified in this study are the result of stresses produced during the large eastward translation of the caribbean plate since the paleocene and are not the product of a shear couple

analysis of slope failures in submarine canyon heads an example from the gulf of lions to improve understanding of evolution of submarine canyons a three dimensional slope stability model is applied to bourcart canyon in the western gulf of lions in the mediterranean sea the model builds on previous work by chen and others and it uses the upper bound theorem of plasticity to calculate the factor of safety of a kinematically admissible failing mass examples of three dimensional failure surfaces documented in the literature were used to test the model formulation model application to bourcart canyon employed the results of a detailed stratigraphic analyses based on data acquired by swath bathymetry sub bottom profiling high resolution seismic reflection surveys and piston coring the sediment layers were also characterized using in situ geotechnical measurements and laboratory tests the effects of three loading scenarios were analyzed 1 earthquake shaking 2 hemipelagic sedimentation and 3 axial incision these three mechanisms influenced the predicted volumes and shapes of slope failures along the flanks of bourcart canyon and comparison of these predictions with failure geometries inferred from seafloor morphology showed that mass failures could account for the observed morphology along the canyon walls as well as a mechanism of canyon widening

subduction zones and back arc basins a review in the framework of plate tectonics the basic mechanism operating in the trencharc back arc systems is believed to be the subduction of oceanic lithosphere this model however was developed more or less as a logical consequence of sea floor spreading the question then is whether or not the subduction model can explain all the major features of the trench arc back arc systems many features such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained but the occurrences of extentional spreading and high heat flow in the back arc region and arc volcanism are difficult to explain an approach to solve these problems may be found in what may be called comparative subductology there seem to exist two basically different modes of subduction one chilean type causing compressional stress regime in the arc and back arc regions the other mariana type causing tensional world s subduction zones are classified in terms of the postulated two modes and some tectonic implications and causes of the two modes are discussed

effect of gas hydrates melting on seafloor slope instability we present a theoretical study of the thermodynamic chemical equilibrium of gas hydrate in soil by taking into account the influence of temperature pressure pore water chemistry and the mean pore size distribution the model uses a new formulation based on the enthalpy form of the law of conservation of energy the developed model shows that due to a temperature and pressure increase hydrates may dissociate at the top of the hydrate occurrence zone to ensure a chemical equilibrium with the surrounding bulk water this original result confirms what has been already shown through experiments the second part of the paper presents an application of the model through a back analysis of the giant storegga slide on the norwegian margin two of the most important changes during and since the last deglaciation hydrostatic pressure due to the change of the sea level and the increase of the sea water temperature were considered in the calculation simulation results show that melting of gas hydrate due to the change of the gas solubility can be at the origin of a retrogressive failure initiated at the lower part of the storegga slope once again the developed model leads to predictions which are supported by laboratory experiment results but contradictory to previous interpretations and beliefs considering that hydrate dissociation occurs only at the bottom of the gas hydrate stability zone

theoretical and experimental studies of magnetic domains the primary purpose of this paper is to review recent contributions from north american scientists to theory and observations of magnetic domain structures in magnetic minerals common to rocks although rock magnetism deals with all the magnetic properties of rocks a major goal of rock magnetists has been to understand the origins and properties of remanent magnetization rm a remanent magnetization is defined as the magnetization that exists in the absence of an external magnetic field the analyses of rms of which there are more than 20 kinds have led to the numerous and well known successes of paleomagnetism

deep sea current flow in the late cretaceous caribbean measurements on st croix u s virgin islands abstract new measurements of sedimentary structures in rocks on st croix u s virgin islands record deep ocean currents in the northeastern part of the caribbean during the late cretaceous flow was directed toward the west a direction parallel to the surface circulation in this region paleocurrent analysis of marine volcaniclastic sections of late cretaceous age reveals a two phase depositional pattern primary emplacement by turbidity currents flowing toward the south and the subsequent reworking of these layers by w flowing bottom currents sands were first transported to lower slope aprons and these were then reworked along bathymetric contours by bottom water flow probably driven by thermohaline processes at the end of the cretaceous the paleo atlantic was much narrower and the caribbean configuration considerably different than at present the abundance of current reworked turbidites and bioturbation structures however provide clear evidence of active deep water circulation in the tectonically active island arc setting and the maintenance of oxygenated conditions on the sea floor as at present

a cretaceous and jurassic geochronology an integrated geomagnetic polarity and geologic time scale for the jurassic and cretaceous periods is presented based on various methods according to the availability of definitive isotopic ages an age calibrated sea floor spreading model is used to interpolate the ages of the kimmeridgian to barremian and the campanian to maestrichtian stages numerical age estimates for the aptian to santonian stage boundaries follow published isotopic age determinations the hypothesis of equal duration of ammonite zones is employed as a vernier to apportion time for the hettangian to oxfordian stages the new scale results in ages of 208 ma for the base of the jurassic 144 ma for the jurassic cretaceous boundary and 66 5 ma for the top of the cretaceous the integrated biostratigraphic magnetostratigraphic and geochronometric record serves as a working hypothesis for geologic correlation of jurassic and cretaceous strata

accretionary margin of north western hispaniola morphology structure and development of part of the northern caribbean plate boundary abstract broad range side scan sonar gloria images and single and multi channel seismic reflection profiles demonstrate that the margin of north western hispaniola has experienced compression as a consequence of oblique north american caribbean plate convergence two principal morphological or structural types of accretionary wedges are observed along this margin the first type is characterized by a gently sloping 4 sea floor and generally margin parallel linear sets of sea floor ridges that gradually deepen towards the flat hispaniola basin floor to the north the ridges are caused by an internal structure consisting of broad anticlines bounded by thrust faults that dip southwards beneath hispaniola anticlines form at the base of the slope and are eventually sheared and underthrust beneath the slope in contrast the second type of accretionary wedge exhibits a steeper 6 16 sea floor slope characterized by local slumping and a more abrupt morphological transition to the adjacent basin the internal structure appears chaotic on seismic reflection profiles and probably consists of tight folds and closely spaced faults we suggest that changes in sea floor declivity and internal structure may result from variations in the dip or frictional resistance of the decollement or possibly from changes in the cohesive strength of the wedge sediments the observed pattern of thickening of hispaniola basin turbidites towards the insular margin suggests differential southwards tilting of the hispaniola basin strata probably in response to north america caribbean plate interactions since the early tertiary based upon indirect age control from adjacent parts of the northern caribbean plate boundary we infer a late eocene to early miocene episode of transcurrent motion i e little or no tilting an early miocene to late pliocene period of oblique convergence i e increased tilt during which the accretionary wedge began to be constructed and a late pliocene to recent episode of increased convergence i e twice the miocene to pliocene tilt which has led to rapid uplift and erosion of sediment sources on the margin and on hispaniola generating a submarine fan at the base of the insular slope

four phase tectonostratigraphic development of the southern caribbean abstract although the stratigraphy of northern south america and the southern caribbean has been studied in detail in many areas a clear understanding of the sequence of tectonic events and their relation to regional stratigraphy has not been forthcoming since northern south america collided with southern north america in the late palaeozoic we identify four discrete tectonic phases from the stratigraphy of the area that is now the southern caribbean these phases derived from a synthesis of regional stratigraphy were 1 panama arc collistion with colombia in the late miocene early pliocene which drove a complex triangular wedge bounded by strike slip faults northward into the caribbean 2 right lateral strike slip faulting associated with post eocene eastern motion of the caribbean ocean floor into the gap between the separating americas 3 island arc development and collision associated with the entry of the present caribbean ocean floor an area of buoyant ocean crust into the gap between the separating americas and 4 continental rifting and passive margin development in the late jurassic early cretaceous associated with the separation of north and south america approximately along the palaeozoic suture zone overprinting of younger structures on inherited structures from previous phases is responsible for the overall complexity of the region

strike slip deformation within the colombian andes the colombian andes are characterized by a dominant ne structural trend which is offset by ene trending right lateral and nw trending left lateral structures ne trending faults are either dip slip or oblique thrusts generated as a result of a transpressive regime active since at least palaeogene times nw trending faults are considered to be reactivated pre cretaceous extensional structures right lateral shear on ene trending faults has resulted from oblique convergence between the nazca plate and the northern andes major changes in the geometry of the oblique plate convergence between the nazca and south american plates have generated the northward escape of the northern andes and stress strain partitioning within the mountain belt these strike slip structures have exerted important controls on sedimentation source rock distribution fluid flow and ore mineralization during cenozoic times the interpretation of the northern andes as a mountain belt affected by strike slip deformation provides a structural context in which to reassess the exploration plays each of these cordilleras has a different composition and evolution as a result of different tectonic processes that have affected the region since mesozoic times the central cordillera originated in response to triassic subduction and consequent volcanic and igneous activity barrero et al 1969 barrero vesga 1976 bartok et al 1981 triassic and jurassic backarc rifting generated a predominantly ne sw structural trend that influenced the sedimentation in the adjacent magdalena valley and eastern cordillera the western cordillera is composed of oceanic crust and deformed deep marine sediments representing an accretionary complex established in the late cretaceous de freitas et al 1997 the eastern cordillera represents the inversion of thick mesozoic and tertiary sedimentary basins see taboada et al 2000 eastward subduction of the nazca plate beneath the northern andes occurs at c 60 mm a 1 trenkamp et al 2002 along the western margin of ecuador and colombia fig 1 additionally north of latitude 8 n the caribbean plate is moving ese at an average velocity of 20 mm a 1 trenkamp et al 2002 as a result of the oblique convergence of both the nazca and the caribbean plates with the northern andes the andean block fig 1 is moving northeastwards relative to the south american plate pennington 1981 kellogg et al 1985 the colombian andes have been interpreted as an assemblage of terraines that have been accreted to south america with a dominant nne structural trend etayo serna et al 1983 lopez barrero 2003 this interpretation has led to the assumption of plane strain deformation orthogonal to the strike of the main structures which in colombia is almost parallel to the continental margin fig 1 for this reason the colombian andes have been described as a classical fold and thrust belt mojica franco 1990 schamel 1991 cooper et al 1995 however oblique ene and nw strike slip deformation has been recognised in the region feininger 1970 boinet et al 1986 diederix et al 1987 cuervo 1995 velandia komuro 1998 montes 2001 ujueta 2001 branquet et al 2002 acosta et al 2004 and the importance of these structures in the evolution of the belt has for the most part been neglected in this paper a variety of geological geophysical geodetic and geodynamic information has been integrated to examine the origin and evolution and of some of these strike slip fault systems within the colombian andes

strain degradation of saturated clay under cyclic loading soil degradation is studied from the point of view of cyclic axial strain through stress controlled triaxial tests on hangzhou normally consolidated clay different influence factors on strain such as cyclic stress ratio overconsolidation ratio and frequency are studied degradation index is redefined according to the tests a mathematical model for strain degradation is presented and verified key words cyclic loading saturated clay strain degradation mathematical model

progress in stability analysis of submarine slopes considering dissociation of gas hydrates gas hydrates have the potential to be a new energy source and a submarine geohazard though researchers generally agree about the association between gas hydrate dissociation and submarine slope failures the processes and mechanism of submarine slope failure caused by gas hydrate dissociation are not clearly understood in the last few years some authors have tried to analyse submarine slope stability by considering the existence and dissociation of gas hydrate and a few researchers have presented quantitative models this paper presents a review of the various causes of submarine slope failures associated with gas hydrate dissociation also analysis models of submarine slope stability associated with gas hydrate dissociation that are documented from the literatures including the infinite slope model wedge model slump and retrogressive failure model are interpreted and illustrated respectively

la desirade island lesser antilles revisited lower cretaceous radiolarian cherts and arguments against an ophiolitic origin for the basal complex la desirade island in the guadeloupe archipelago is a keystone of the structural framework of the lesser antilles island arc hypothetical ophiolitic series and wide discrepancy in the radiometric datings of its igneous basement have raised considerable debate concerning the structural significance of this island and the geodynamic constraints for the caribbean plate recent detailed field work leads to a reconsideration of the problem the volcanic basement of la desirade can be subdivided into three independent units the cherts interbedded in the unit 2 basalts contain hauterivian barremian radiolarians that represent the oldest biostratigraphic assemblage found in the lesser antilles this affirms the jurassic radiometric age of the unit 1 trondhjemite outcrop observations and analyses allow us to discard the concept of an ophiolitic origin for la desirade whose basement was generated like the northern half of the lesser antilles arc by island arc processes during the mesozoic

giant earthquakes in south central chile revealed by holocene mass wasting events in lake puyehue abstract very high resolution reflection seismic profiling 3 5 khz revealed nine holocene mass wasting events in lake puyehue south central chile these events are made up of numerous coeval mass wasting deposits and some homogenites which are radiocarbon dated the two youngest mass wasting events could be attributed to the giant ad 1960 and ad 1575 valdivia earthquakes the most extensive event took place around 1660 cal yr bp evaluation of all possible slope failure processes led us to infer that giant earthquakes roughly comparable to the ad 1960 valdivia earthquake mw 9 5 are the most likely trigger mechanisms of all mass wasting events in lake puyehue these occurred with a mean recurrence rate of 1000 yr although relatively aperiodically ranging between 500 and 2000 yr quantitative comparison of mass wasting events related to historically reported earthquakes ad 1960 and ad 1575 showed significant differences although these earthquakes are assumed to have had a comparable strength a lowered background sedimentation rate could be responsible for this variable earthquake recording which highlights the importance of a thorough assessment of the depositional history before using lacustrine records for quantitative paleoseismic analysis

seismic volcanostratigraphy of the norwegian margin constraints on tectonomagmatic break up processes voluminous volcanism characterized early tertiary continental break up on the mid norwegian continental margin the distribution of the associated extrusive rocks derived from seismic volcanostratigraphy and potential field data interpretation allows us to divide the more voring and lofoten vesteralen margins into five segments the central more margin and the northern voring margin show combinations of volcanic seismic facies units that are characteristic for typical rifted volcanic margins the lofoten vesteralen margin the southern voring margin and the area near the jan mayen fracture zone show volcanic seismic facies units that are related to small volume submarine volcanism the distribution of subaerial and submarine deposits indicates variations of subsidence along the margin vertical movements on the mid norwegian margin were primarily controlled by the amount of magmatic crustal thickening because both the amount of dynamic uplift by the icelandic mantle plume and the amount of subsidence due to crustal stretching were fairly constant along the margin thus subaerial deposits indicate a large amount of magmatic crustal thickening and an associated reduction in isostatic subsidence whereas submarine deposits indicate little magmatic thickening and earlier subsidence from the distribution of volcanic seismic facies units we infer two main reasons for the different amounts of crustal thickening 1 a general northward decrease of magmatism due to increasing distance from the hot spot and 2 subdued volcanism near the jan mayen fracture zone as a result of lateral lithospheric heat transport and cooling of the magmatic source region furthermore we interpret small lateral variations in the distribution of volcanic seismic facies units such as two sets of inner seaward dipping reflectors on the central voring margin as indications of crustal fragmentation

caribbean tectonics and relative plate motions during the last century three different ways of interpreting the tectonic evolution of the gulf of mexico and the caribbean have been proposed taking into account the bailey willis school of a permanent pre jurassic deep sea basin the edward suess school of a subsided continental terrain and the alfred wegener school of continental separation the present investigation is concerned with an outline of an interpretation which follows that of pindell and dewey 1982 an attempt is made to point out ways in which the advanced hypotheses can be tested the fit of africa north america and south america is considered along with aspects of relative motion between north and south america since the early jurasic attention is given to a framework for reconstructing caribbean plate evolution the evolution of the caribbean the plate boundary zones of the northern and southern caribbean and the active deformation of the caribbean plate

the ophiolitic bath dunrobin formation jamaica significance for cretaceous plate margin evolution in the north western caribbean two neighbouring areas of tholeiitic basalts dolerites isotropic gabbros and tonalites in eastern jamaica are recognized as the upper section 2 5 km thick of an ophiolite limestones associated with this bath dunrobin formation contain a fauna of upper campanian and lower maestrichtian age the basalts are overlain by a thick sequence of maestrichtian volcaniclastic rocks and are faulted against a blueschist terrain since its emplacement the ophiolite has been subjected to uplift and major left lateral strike slip tectonism beginning in the eocene recognition of this subduction zone complex and regional considerations of island arc polarity necessitates that ocean crust must have been consumed by a southward dipping subduction zone the overlying late cretaceous palaeogene arc stretched from guatemala through jamaica to oriente province cuba this arc separated the yucatan basin to the n where active spreading accompanied subduction from the colombian basin and the southern nicaraguan rise to the se where tholeiitic magmatism b penetrated older ocean crust the bath dunrobin formation probably represents ocean crust from the yucatan basin obducted during the maestrichtian the major ophiolite terrains of guatemala and oriente cuba are by inference also derived from a proto yucatan basin

geomechanical issues in the exploitation of natural gas hydrate abstract as a clean fossil fuel with great reserves natural gas hydrate ngh is widely regarded as an important future alternative energy source ngh is widely distributed in onshore tundra and shallow sedimentary layers in the deep sea these sedimentary layers typically exit shallow burial depth poor diagenesis and low strength characteristics moreover the decomposition of ngh can also greatly reduce reservoir strength therefore ngh development can easily causes many geomechanical problems including reservoir instability sand production and seabed landslides etc which may further trigger a series of environmental disasters such as tsunamis natural gas leakage and the acceleration of global warming this study mainly reviews the research progress regarding geomechanical issues in ngh development including mechanical properties of ngh bearing sediments borehole stability hydraulic fracturing sand production reservoir settlement and seabed landslides in addition to previous research achievements regarding geomechanical problems in ngh exploitation the limitations and challenges are also discussed and several questions and insightful suggestions are put forward for future research from our point of view

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

interrelationship of the terranes in western and central cuba abstract present day cuba is a complex of terranes in the western and central parts of the island the sierra de los organos and the escambray terranes which are genetically related and the volcanosedimentary cretaceous sequences of the greater antilles arc are distinguished the escambray and sierra de los organos massifs are terranes that were detached from their source areas near the yucatan a rift separating the yucatan from the florida bahama plate was involved separation of the escambray sequence from that of the sierra de los organos took place in the early cretaceous along a ne sw trending transform fault the escambray sequence was transported together with the greater antilles island arc in front of the farallon plate about 80 ma ago the volcanosedimentary cretaceous sequence of the greater antilles island arc was thrust over the ophiolitic association in this way the zaza zone was formed at that time some overthrusting and metamorphism took place in the escambray massif which was then overthrust and pressed into the zaza zone at about 45 50 ma ago detachment and nappe overthrusting took place in the sierra de los organos and overthrust units reached the margin of the margin of the florida bahama plate various structures are involved in the zaza zone including 1 the volcanosedimentary cretaceous sequences of the greater antilles arc which is a tectonized and overthrust terrane and 2 the ophiolitic association the greater antilles arc terrane is thrust over this association both of these tectonic units were folded together the sierra de los organos and the escambray terranes were thrust onto the zaza zone and then deformed together with it during later phases