evolution of tectonic compaction in the barbados accretionary prism estimates from logging while drilling abstract resistivity and bulk density logs acquired while drilling are used to document the evolution of porosity volumetric loss and effective stress in the upper 300 m of the barbados accretionary prism the computed profiles across a thrust fault enable the separation of pre syn and post accretion components total volume loss is divided into normal consolidation pre accretion tectonic volume loss syn accretion and thrust fault loading post accretion in the footwall quantitatively the tectonic volume loss in the barbados accretionary prism estimated from the normal consolidation in a reference section is as large as the pre accretion volume loss the compaction history of the accretionary prism is essentially controlled by the vertical displacement of the thrust and the predicted maximum volume loss throughout the prism may be extrapolated from the volume loss trend in the hanging wall the porosity and effective stress profile at the present time is consistently larger than its corresponding syn accretion trend and less than the extrapolated maximum from the hanging wall the present profile is approximately equal to the average of the two curves the remaining difference in the accretionary prism above the inferred syn accretion trend is due to post accretion compaction the post accretion volume loss is a physical feedback process due to the superposition of thrust sheets in an accretionary prism in the case of multiple thrusts developing in a prism the porosity and effective stress profiles are saw toothed and approach the maximum tectonic volume loss gradient with depth the cumulative post accretion compaction increases with depth and becomes increasingly greater than the pre and syn accretion compaction with thickening of the accretionary prism post accretionary loading is the dominant mechanism of volume loss and dewatering in an accretionary prism during its early stages of growth

paleomagnetic results from honduras and their bearing on caribbean tectonics paleomagnetic results have been obtained from 37 sites in central and western honduras and one site in north western nicaragua the rocks are limestones sandstones and granites and range in age from 140 to 60 m y regional fold tests demonstrate that the rocks carry a prefolding pre laramide magnetization the data indicate a very large clockwise rotation of the sampling area during the early cretaceous followed by a counter clockwise rotation in the late cretaceous the changes in latitude are minor during this time period it is highly unlikely that these large rotations apply to the entire caribbean plate rather they reflect the motion of a smaller area known as the chortis block this interpretation is supported by paleomagnetic data from neighboring areas the results also suggest that the chortis block sutured onto southern mexico guatemala in latest cretaceous time in agreement with geochemical data and several plate tectonic models these new data together with published paleomagnetic data from around the caribbean imply that a rigid caribbean plate did not exist until well into the tertiary

anisotropy and transverse isotropy a number of authors in the exploration literature have written about anisotropy but have restricted their discussions to wave propagation through rock having transverse isotropy with a vertical symmetry axis this note shows that there are fundamental differences between transverse isotropy when the symmetry axis is vertical normal to the free surface and more general anisotropy with an azimuthal variation of properties these differences are important now that effective azimuthal shear wave anisotropy resulting from aligned cracks and pores is becoming recognized as a significant property of crustal rocks

the sub himalayan fold thrust belt in the 1905 kangra earthquake zone a critical taper model perspective for seismic hazard analysis 1 we investigate the broader epicentral area of the m 7 8 1905 kangra earthquake s north india affecting the sub himalayan hills the tectonics of the area is characterized by two major rentrants kangra and dehradun interposed by the nahan salient the first order topography between the himalayan frontal thrust and the main boundary thrust shows a marked lateral variation along strike of the mean gradient characterized by a very small mean slope angle 1 in correspondence with the reentrants and higher values 3 in the salient these tectonic and topographic features also show a good correspondence with the peculiar macroseismic field of the 1905 event s which is characterized by two distinct intensity maxima separated by a distance of 150 km clearly overlapping the two major tectonic reentrants in this paper based on available geological and geophysical information and a critical analysis of the general mechanical constraints the seismogenic volume of the external sector of the chain is investigated in terms of critical taper model attempting to clarify the possible correlations between tectonics topography and seismicity in the sub himalayan belt based on different assumptions three possible seismotectonic scenarios are explored in order to constrain their likelihood and therefore to suggest a potential seismic gap in the area corresponding to the nahan salient which may experience an event of significant magnitude in the future

elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the nankai trough subduction megathrust 1 recent seismic reflection and ocean bottom seismometer obs studies reveal broad regions of low seismic velocity along the nankai subduction plate boundary megathrust offshore sw japan these low velocity zones lvz s extend 55 km landward from the trench corresponding to depths of 10 km below sea floor here we estimate the in situ pore pressure and stress state within these lvz s by combining p wave velocities obtained from the geophysical surveys with new well constrained empirical relations between p wave velocity porosity and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment we document excess pore pressures of 17 87 mpa that extend 55 km into the subduction zone indicating that trapped pore fluids support 45 91 of the overburden stress along the base of the upper plate and surrounding major fault zones the resulting effective stresses in the lvz are limited to 1 3 of the values expected for non overpressured conditions these low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary the region of lowest effective stress coincides with precisely located very low frequency earthquakes providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure

the triple junction of the north america cocos and caribbean plates seismicity and tectonics the triple junction of the north america cocos and caribbean plates is ambiguously defined mainly because the north america caribbean plate boundary does not clearly continue beyond its known surface trace the motagua fault zone in western guatemala to intersect the middle america trench well located regional shallow earthquakes h 70 km show that there is no intermediate or large magnitude seismic activity associated with a presumed extension of the north america caribbean plate boundary to the west beyond its well defined surface trace there is however a clear zone of shallow seismic activity from the western section of the fault system through southern mexico fault plane solutions for these events indicate a left lateral strike slip displacement which is in good agreement with surface faulting we suggest that these strike slip faults together with the salina cruz fault in the isthmus of tehuantepec mark the boundaries of a broad zone of deformation in southern mexico and northern central america which takes up the interactions of the three plates in this sense no single point constitutes the triple junction the geologic record suggests that the motagua fault zone developed because the westernmost portion of the caribbean plate became locked against north america

the transpressive left lateral sierra madre de chiapas and its buried front in the tabasco plain southern mexico the sierra madre de chiapas evolved in the vicinity of the triple junction between the cocos north america and caribbean plates the sierra madre de chiapas tectonics reflects positive topographic growth along its main core and a northwards directed collapse through a free border related to the gulf of mexico major exhumation and topographic growth occurred during the middle late miocene 16 10 ma evidence for this deformational event is provided by fault activity major stratigraphic unconformities along the sierra madre de chiapas and the tabasco coastal plain i e southern gulf of mexico major salt related motion northward progradation of the sedimentation and northward migration of the buried deformational front during the neogene strike slip deformation and its related exhumation migrated landwards from the western edge of the chiapas massif complex to the chiapas sierra horizontal displacement along the main strike slip faults in the chiapas sierra has been indirectly estimated to be between 30 and 43 km during the last 6 5 ma implying 0 5 0 8 cm a 1 of lateral accommodation these values suggest that a significant amount of the motion transferred by the caribbean and north american plates is currently being accommodated along the chiapas area supplementary material a geological map of the sierra madre de chiapas is available at www geolsoc org uk sup18507

active seismic deformation in the grabens of northern central america and its relationship to the relative motion of the north america caribbean plate boundary abstract the active seismic deformation along the grabens of northern central america is calculated using a method which relates relative plate velocity with the seismic strain rate tensor the latter is in turn obtained from seismic moment tensors we calculate the typical or average moment tensor from available centroid moment tensors and fault plane solutions this average tensor is then used to obtain the sum of moment tensors of historical and modern earthquakes reported in the literature both historical and modern earthquakes yield an average extension rate of 8 mm yr along the grabens of northern central america while the relative motion between the north america and caribbean plates expressed along the motagua polochic fault system is 20 mm yr this result suggests that part of the seismic deformation related to the plate boundary is taken up as extension along the grabens which play the role of fault termination structures for the motagua polochic system

mineralization of breccia pipes in northern arizona the paleozoic sedimentary rocks on the colorado plateau of northern arizona are host to hundreds ofbreccia pipes the uranium and copper deposits in these breccia pipes transgress formation boundaries from the mississippian redwall limestone to the triassic chinle formation they are not classic breccia pipes in that there is no volcanic rock associated with them in time or space they are the result of solution collapse within the redwall limestone and stoping of the overlying strata the karst development in the redwall limestone began in the mississippian and apparently either continued to the triassic or was at least once again active during that time the mineralization apparently occurred shortly thereafter sometime during the mesozoic mining activity in breccia pipes of the grand canyon region began during the nineteenth century and continues today with the operation of the hack i ii and iii mines although the exploited commodity has changed from cu to u although small in size these pipes contain samples with up to 55 percent u 3 o 8 and can yield ore averaging between 0 30 and 0 60 percent u 3 o 8 mineralization at the surface commonly occurs within nodules and concretions associated with pyrite and goethite and along fractures while the primary ore of the unoxidized zones is commonly within a comminuted sandstone matrix surrounding breccia fragments of overlying formations the ore mineral is uraninite although associated with it are sphalerite galena chalcopyrite tennantite millerite siegenite and molybdenite some of the surface nodules are encrusted with malachite and are exceptionally enriched in ag pyrite is abundant and the organic carbon content of some rocks is high enough to suggest that it along with the pyrite may be a reductant for uranium in contrast it is possible if uranium were transported as a bicarbonate or carbonate complex that only a conduit of brecciated rock was necessary to release co 2 thus disrupting the equilibrium and allowing uraninite to precipitate an extensive suite of elements is significantly enriched in the mineralized rock ag as ba cd co cr cs cu hg mo ni pb sb se sr u v zn and the rare earth elements of these cu pb zn ag and particularly as appear to be the best geochemical indicators of mineralized pipes at present the origin of the mineralizing fluids is not known the lack of extensive silification within the breccia along with the 80 degrees to 173 degrees c fluid inclusion filling temperatures on sphalerite dolomite quartz and calcite suggests relatively low temperature mineralizing fluids although heated in excess of what would be expected from the normal geothermal gradient on the colorado plateau with the exception of the u mineralized rock the mineral assemblage and geochemistry is similar to mississippi valley type deposits

stress induced smectite dehydration ramifications for patterns of freshening and fluid expulsion in the n barbados accretionary wedge abstract presented is new experimental evidence that smectites can partially dehydrate from 18 5 a to 15 4 a hydrates when they are subjected to effective mineral framework stresses above 1 3 0 3 mpa we show here that this process can substantially freshen saline pore fluids and impact our understanding of the hydrogeologic processes in convergent margins where smectites are abundant the general anomalous freshening of pore fluids in the n barbados accretionary wedge was previously proposed to relate to the widespread lateral expulsion of fluids derived from mineral dehydration reactions at great depth in contrast our analysis suggests that a significant part of the widespread anomalous freshening relates to stress induced smectite dehydration that occurred during the ocean drilling program odp pore water extraction process clear evidence for long distance lateral flow appears to be limited to localized chloride and isotopic anomalies developed in a 20 m thick locally mineralized region at the top of decollement horizon at site 948 our reinterpretation is consistent with the fluid flow in the decollement zone being 1 highly transient on time scales of several hundreds of years and 2 channeled laterally into heterogeneous finger like flow paths that are apparently not intersected and or sampled by drilling in most regions at the toe of the wedge the incipient stress induced dehydration of smectite may also be buffering in situ fluid pressures in the wedge maintaining the effective stresses at 1 3 mpa

rotation of the chortis block causes dextral slip on the guayape fault the chortis block is undergoing deformation as a direct result of large magnitude sinistral strike slip faulting on the north america caribbean plate boundary we have studied the chortis block using geologic mapping remote imagery and earthquake seismology to evaluate the effect that large magnitude strike slip faulting has on an adjacent microcontinental block in the eastern part of the chortis block recent geologic mapping has shown that the northeast striking guayape fault is dextral instead of sinistral as previously assumed because this is a major fault that cuts most of the central america isthmus the new evidence for dextral slip necessitates a reinterpretation of the active tectonics of the chortis block the presence of several north trending rifts in the western part of the chortis block has led previous workers to propose that the chortis block is divided into smaller independently rotating blocks we use earthquake seismology and remote imagery data to extend this interpretation to the central portion of the chortis block however we have modified their model to account for dextral slip on the guayape fault we believe that rotation of the chortis block causes dextral slip on the guayape fault even though central america is undergoing large sinistral displacement with respect to north america

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

geologic evaluation of plate kinematic models for the north american caribbean plate boundary zone heubeck ch and mann p 1991 geologic evaluation of plate kinematic models for the north american caribbean plate boundary zone tectonophysics 191 l 26 four published plate kinematic models for the present day north american caribbean plate boundary predict significantly different fault behavior along this dominantly strike slip plate boundary zone pbz using a computer graphics system we compare the deformational pattern predicted by the four kinematic models with geologic and seismic observations along the entire length of the pbz from central america to the northern lesser antilles our results indicate that none of the kinematic models predict all the geologic and seismic observations across the entire pbz however all four models predict mapped fault character and observed earthquake focal mechanisms along certain segments of the pbz we conclude that the present day deformation within the north american caribbean plate boundary zone cannot be described by rotation about a single pole instead the caribbean plate appears to be segmented into at least three major sub plates or blocks each of which requires an individual pole of rotation to describe its motion relative to the north american plate sub plate boundaries correspond to two poorly studied zones of diffuse deformation within the caribbean plate honduras depression of central america and beata ridge of the caribbean sea

genesis of the hydrothermal gold deposits in the canan area lepaguare district honduras the canan area honduras is characterized by a gold bearing ore deposit that is associated with quartz veined shear zones gold mineralization occurs in low to medium grade metamorphic host rocks graphitic and sericitic schists hydrothermal fluids which are associated with the emplacement of cretaceous tertiary granodioritic intrusions are responsible for the formation of quartz veins and the hydrothermal alteration of wall rocks three main altered zones have been detected in the wall rocks as far as 150 cm from the quartz veins the distal zone up to 50 cm thick contains quartz chlorite and illite the intermediate zone is the thickest up to 80 cm and is marked by quartz muscovite sulphides kaolinite and native elements such as au and ag the proximal zone which is close to the quartz veins is rather thin up to 25 cm and contains clay minerals al oxides hydroxides and sulphides the transition from the distal to the proximal zone is accompanied by the enrichment of sio2 and the depletion of all other major elements except for fe2o3 tot precious metals occur in the highest concentrations in the intermediate zone au up to 7 6 ppm and ag up to 11 ppm we suggest that gold was transported as a reduced sulphur complex and was precipitated from the hydrothermal solution by the reaction of the sulphur complexes with fe2 from the alteration of the mafic minerals of the host rock fluid wall rock interactions seem to be the main cause of gold mineralization genetic relationships with a strike slip fault system hydrothermal alteration zones within the metamorphic wall rocks and an entire set of geochemical anomalies are consistent with orogenic type gold deposits of the epizonal class

pore pressure distribution of a mega splay fault system in the nankai trough subduction zone insight into up dip extent of the seismogenic zone we use the pore pressure distribution predicted from a waveform tomography wt velocity model to interpret the evolution of the mega splay fault system in the nankai trough off kumano japan to map pore pressure around the mega splay fault and plate boundary decollement we integrate the high resolution wt velocities with laboratory data and borehole well log data using rock physics theory the predicted pore pressure distribution shows that high pore pressures close to lithostatic pressure along the footwall of the mega splay fault extend seaward to the trough region and the normalized pore pressure ratio is nearly constant over that extent this continuity of the overpressured zone indicates that a coseismic rupture can potentially propagate nearly to the trough axis we interpret a high pressure belt within an accretionary wedge on the landward side of the present mega splay fault as evidence of the ancient mega splay fault because the ancient mega splay fault soles into the active mega splay fault the active mega splay fault may function as a basal detachment fault and is directly connected to the seaward plate boundary decollement

seismic reflection and tomographic velocity model constraints on the evolution of the tofino forearc basin british columbia summary the tofino basin is a sedimentary forearc basin that overlies the continental shelf of the cascadia margin to the southwest of vancouver island the basin which contains up to 4k mo fmarine clastic sedimentary rocks formed following accretion in the early eocene of the crescent and pacific rim terranes and subsequent accretionary wedge basement subduction of the juan de fuca plate has since been the primary tectonic driving force in the development of the basin s structure investigations using coincident seismic reflection profiles tomographic velocity models and recently reassessed biostratigraphic well data show that basement composition has largely controlled deformation of the overlying tofino basin sediments anticlinal folds overlying the accretionary wedge exhibit low p wave velocities at the apex of the fold which may be related to fracturing of older more lithified sediments accompanied by fluid expulsion from the accretionary wedge in contrast the velocity variation across folds over the crescent terrane mimics the fold geometry and does not appear anomalous a sub basin containing up to 3k m ofoligocene to holocene sediment has developed in the central part of the tofino basin at the boundary between the crescent and pacific rim ter ranes seismic interpretation suggests that deposition has increased more rapidly in the late miocene to holocene subsidence within the sub basin is likely to have been controlled by sediment loading flexure and regional tectonic forces localized by pre existing zones of weakness such as the tofino fault the development of the sub basin may also have been influenced by the displacement landward of part of the lower forearc crust during subduction erosion diapiric structures along the axis of the sub basin suggest that fluid expulsion into the tofino basin from the deeper accreted terranes is localized by the terrane bounding fault further seaward fluid expulsion from the accretionary wedge may be more pervasive the seismic data demonstrate that subsidence of the sub basin beneath the inner continental shelf has been occurring relatively late in the history of the tofino basin and that the cascadia accretionary complex cannot be viewed as growing gradually seaward with no inboard deformation

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

interpretation of synthetic common depth point gathers for a single anisotropic layer analysis of synthetic traveltime gathers shows that anisotropy may have a large enough effect on p sh and sv propagation to alter significantly the interpretation of the subsurface below the anisotropic layers consequently if anisotropy exists below a seismic line it is important to estimate the anisotropic parameters correctly we discuss the effects of anisotropy on seismic waves and present a method for estimating the elastic constants of a transversely isotropic layer from p and sh arrival time gathers the technique may be extended to more general anisotropic symmetries by analyzing gathers from several azimuths to illustrate the possible effect of anisotropy on exploration surveys p sh and sv velocity variations are calculated for several types of anisotropic sedimentary fabrics alignments due to bedding shale lithology and dry parallel cracks may have similar velocity variations fabrics with other configurations of cracks may still possess overall transversely isotropic symmetry but t

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

western mediterranean ridge mud belt correlates with active shear strain at the prism backstop geological contact a high resolution swath mapping survey conducted in the deep waters of the eastern mediterranean sea allowed mapping of active faults and mud volcanism along a sizable portion of the mediterranean ridge active shear is localized at the prism backstop con tact a major dextral flower structure and a site of massive mud expulsion we investigate the relationship between the mud output rate and horizontal strain rate by combining the mud volume estimate from sea bottom reflectivity with kinematic modeling based on far field global positioning system data and local fault and strain patterns we find a direct correlation between maxima of mud output and maxima of the shear component of strain at the backstop contact mud volcanism may reflect the abundance of solid mud and fluid methane sources combined with a favorable tectonic regime established at the prism backstop contact in post pliocene time in relation to plate tectonic changes

the saqisan mine a paleokarst uranium deposit south china abstract the saqisan uranium deposit located in guangxi province south china is hosted mainly by a dark gray limestone solution breccia unit and paleocavern fill sedimentary rocks the deposit is the largest known breccia hosted uranium deposit in china and was initially described as of synsedimentary shallow marine origin however recent open pit operations at the saqisan mine have uncovered new exposures that necessitate a thorough change in the genetic interpretation of the deposit the new exposures suggest that the occurrence of saqisan ores in sediment filled solution breccia and paleokarst cavities of limestone is in part similar to that of solution collapse breccia pipe type uranium deposits in arizona united states the interconnected paleokarst passages and caverns were filled with consolidated mineralized clastic sedimentary rocks most of the orebodies were deposited in the interconnected paleokarst passages and caverns two stages of primary uranium mineralization are recognized the early stage i mineralization is the most important for uranium as it accounts for 60 of saqisan s total uranium tonnage stage i mineralization formed low grade ore the late stage ii mineralization which accounts for the remaining 40 of saqisan s total uranium tonnage is hosted by strongly silicified foliated and mylonitized host limestones the uranium in stage i ores predominantly occurs as micro sized pitchblende in the clay rich breccia matrix the uranium in stage ii ores mainly occurs as pitchblende veinlets 0 1 2 mm wide and as disseminated pitchblende grains ranging from less than 0 1 to 1 mm in size sulfur isotope data indicate that biogenic pyrite in the host carbonate may have been the major source of sulfur in the mineralizing fluids oxygen and carbon isotope data show that most of the participated co 2 and hco 3 during stage i mineralization was derived from bicarbonate dissolved out of the marine host carbonate oxygen and hydrogen isotope data combined with determinations of temperature and salinity for the inclusion fluids of stage ii veinlet quartz show that the ore bearing fluids of stage ii mineralization are apparently devoid of co 2 and ch 4 and were originally saline meteoric ground water these basinal brines which were from connate waters trapped in the paleozoic carbonates are believed to have flowed along aquifers of the basinal sequence and up preexisting subvertical faults to produce the observed style of mineralization by chemical interaction with host carbonates it is postulated that the high grade uranium ore of stage ii is the product of multiple superimposed reconcentration of uranium from the early synpaleokarstic ore stage i by low to middle temperature 110 251 c uranium enriched hydrothermal brines driven by a regional heating event such as the late yanshanian tectonism the heat source for the saqisan hydrothermal system could have been related to conversion of mechanical energy into heat during the late yanshanian regional tectonic deformation

wide angle seismic data reveal extensive overpressures in the eastern black sea basin summary we present new data that explores the link between pore pressure and seismic velocity to estimate the magnitude of the overpressure within the deep sediments of the eastern black sea basin new wide angle seismic data combined with coincident reflection data have been modelled simultaneously using the seismic tomography code jive3d to provide a wellconstrained seismic velocity model of the sediments our models reveal a widespread lowvelocity zone at the depth of 5 5 8 5 km which is characterized by a velocity decrease from 3 5 to 2 5 km s 1 using two separate methods that relate changes in seismic velocity to changes in effective stress we estimate pore pressures of at least 160 mpa within the lowvelocity zone these pore pressures give values of 0 8 0 9 within the centre of the basin and above the mid black sea high the low velocity zone occurs within the maikop formation an organic rich mud layer identified as the source of mud volcanism in the black sea and south caspian sea

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

effective stress and pore pressure in the nankai accretionary prism off the muroto peninsula southwestern japan 1 we developed a theoretical method for predicting effective stress and pore pressure based on rock physics model we applied the method to reveal the pore pressure distribution within the nankai accretionary prism off southwestern japan and to investigate variations in pore pressure associated with evolution of the plate boundary decollement from the crack aspect ratio spectrum estimated from laboratory and well log data we calculated a theoretical relationship between acoustic velocity and mean effective stress by using differential effective medium theory by iteratively fitting the theoretically calculated velocity to the seismic velocities derived from 3d tomographic inversion we estimated in situ mean effective stress within the accretionary prism pore pressure is then the difference between the effective stress and the confining stress when we calculated pore pressure we considered compressive state of stress in the accretionary prism our results confirm that pore fluid pressure is high within the subducting sedimentary sequence below the decollement we determined a normalized pore pressure ratio of 0 4 0 7 abnormal pore pressures develop in the under thrust sequence as a result of the increase in overburden load because of the thickened overlying prism and a low permeability barrier across the decollement overpressuring within the accreted sequence is initiated at the deformation front and proceeds landward the increase in horizontal compaction within the accreted sequence may raise pore pressures within the accreted sequence and the pore pressure mean effective stress contrast at the decollement becomes smaller landward of the deformation front

seismic velocity anisotropy and fluid pressure in the barbados accretionary wedge from an offset vertical seismic profile with seabed sources 1 the state of compaction and fluid pressure in the barbados accretionary wedge near its toe at ocean drilling program site 949 were investigated by modeling travel times of seismic waves from ocean bottom shots to a borehole geophone array the model constrained by a three dimensional seismic survey and well logs shows 1 a velocity gradient of about 1 1 25 s 1 in the uppermost 180 230 m of the wedge 2 a zone of variable but no net change in velocity between 230 and 350 m depth 3 a low velocity zone 40 50 m thick just above the decollement at 391 m and 4 a displacement of the low velocity zone by thrust faults pore fluid pressure sections derived from p wave velocity show that the upper half of the wedge is normally pressured while the lower half is overpressured the 160 m thick underconsolidated basal zone shows anisotropy which increases downward the lowest 40 50 m has velocity varying 1 azimuthally 3 being fastest in the direction of plate convergence and 2 in the vertical plane 2 5 horizontal faster than vertical after correction for the effect of anisotropy in the derivation of effective stress from seismic velocity the calculated pore fluid pressure ratio does not exceed 0 9 and in the lowest 40 50 m of the basal zone is between 0 71 and 0 82 with fluid pressure hydrostatic lithostatic pressure hydrostatic between 0 5 and 0 65 in accordance with in situ measurements of fluid pressure in the decollement zone beneath these indicate that the accretionary wedge is stronger and less overpressured than was previously supposed

plumbing accretionary prisms effects of permeability variations fault zones focus fluid expulsion in the muddy northern barbados ridge accretionary prism with fault parallel permeabilities about 1000 times greater than intergranular permeabilities in the adjacent sediment in the oregon prism the low bedding perpendicular permeability due to mudstones inhibits intergranular dewatering however intergranular flow is concentrated where submarine erosion breaches high permeability sandy layers even so faults can capture fluid flow from these exposed sandy layers suggesting the faults have a still higher permeability such observations coupled with laboratory measurements permeabilities suggest that faults off oregon may have fault parallel permeabilities at least 10 10000 times greater than the adjacent sediments results from barbados and oregon suggest fluid flow is concentrated along the most active faults at the toe of prisms the fault zones are being progressively loaded by the thickening wedge and are undergoing compaction preliminary experiments show that permeability decreases relative to the surrounding wall rocks along faults within this compactive deformation regime we believe that these faults must undergo dilation perhaps linked to transient increases in pore pressure if they are to be preferential fluid conduits farther upslope erosion exposes rocks that are more consolidated commonly more cemented and generally of lower intergranular permeability than rocks of equivalent burial further seaward because of their lithification and overconsolidation these rocks dilate during faulting locally enhancing fracture permeability in such dilative regimes faults become evermore focused zones of fluid expulsion relative to occluded intergranular pathways

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

hydrogeochemistry of the northern barbados accretionary complex transect ocean drilling project leg 110 detailed studies of the major element geochemistry oxygen and hydrogen isotope ratios of pore fluids and the 87sr 86sr ratio of dissolved strontium have made it possible to unravel physical and chemical processes that affect the pore fluid chemistry in a transect of drill holes across the northern barbados accretionary complex these processes include 1 alteration of volcanic ash buried in the pleistocene pliocene sediment column 2 alteration of underlying basalts of layer 2 of the oceanic crust 3 movement of fluids from deep in the accretionary complex along fault zones particularly the decollement and minor permeable layers these fluids from deeper in the complex are characterized by low chloride concentrations and increased 18o h2o values presumably as a result of dehydration of smectite interlayers and 4 mixing processes involving the migrating fluids cause incongruities in the geochemical anomalies of these fluids

pine point orebodies and their relationship to the stratigraphy structure dolomitization and karstification of the middle devonian barrier complex mississippi valley type ore field on the south side of great slave lake pine point has produced 58 2 million tons of 3 0 percent pb and 6 7 percent zn with present reserves of 25 7 million tons of 2 7 percent pb and 6 3 percent zn most pine point orebodies were deposited in interconnected paleokarst networks that developed at the lower limits of coarse dolomite alteration of upper barrier limestones karstic structures which exhibit the largest cavity development and consequently the greatest amounts of infilling internal sediments host the best grades of mineralization the following conclusions are suggested the barrier developed due to subtle epeirogenic crustal movements and or eustatic sea level changes affecting normal sedimentary processes deformation of the barrier is possibly due to differential sediment compaction or warping of underlying evaporites coarse dolomitization and paleokarstification metals introduced by chloride rich brines brines expelled by overburden pressures migrated eastward along the barrier and precipitated metals in paleokarst structures upon encountering reduced sulfur species modified journal abstract

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

porosity corrections for smectite rich sediments impact on studies of compaction fluid generation and tectonic history porosity is a fundamental parameter that must be correctly determined in order to relate physical property hydrologic and chemical flux studies to natural systems traditional porosity determinations generated from physical property or seismic data can greatly overestimate the true porosity of sediments in which hydrated minerals such as smectite are abundant to produce a true porosity distribution such data must be corrected to account for the h 2 o residing in smectite interlayers which can make up to 25 of the total hydrated mineral mass such h 2 o is easily removed from the mineral by oven drying and or exposure to low humidities standard physical property measurements can be corrected provided the weight percent smectite in the sediment is known and the interlayer h 2 o content of the mineral can be estimated we illustrate the significant consequences of this correction by comparing profiles of reported and corrected porosities for smectite rich barbados abyssal plain and accretionary wedge sediments from ocean drilling program leg 110 sites 671 and 672

geologic fluid inclusion and stable isotope studies of the gays river lead zinc deposit nova scotia canada the gays river deposit is the largest carbonate hosted lead zinc deposit known in the mississippian windsor group of atlantic canada the orebody mainly occupies one flank of a porous dolomitized carbonate bank that unconformably overlies a basement ridge of the metasedimentary lower paleozoic megurea group the orebody consists of stratiform disseminated sulfides as open space fillings in primary and secondary pores of the dolomite and fault controlled veinlike bodies of high grade massive sphalerite and galena the carbonate bank is overlain by evaporite deposits of mississippian age underground mapping and laboratory studies suggest four main stages of evolution for gays river 1 deposition of the visean carbonate bank preore evaporite deposition pervasive dolomitization and growth of diagenetic marcasite 2 deposition of sphalerite galena chalcopyrite and calcite as a result of influx of hydrothermal metal rich saline brines into fractures and porous dolomite 3 postore deposition of calcite fluorite barite marcasite pyrite and selenite and 4 uplift localized dedolomitization of the mineralized carbonates and exposure by erosion in cretaceous times microthermometry of fluid inclusions yields the following mean homogenization temperatures sphalerite 215oc ore stage calcite 173oc postore calcite 142oc fluorite 142oc and barite 137oc pressure corrections should be minimal for these temperatures no evidence of boiling was observed in the inclusions salinity determinations from the ice melting temperatures of inclusions in postore fluorite and calcite yield a value of ca 20 4 equivalent wt percent naci for the hydrothermal fluid during the waning stages of mineralization isotopic analyses indicate that anhydrite and gypsum in the overlying evaporites and barite within the orebody are enriched in heavy sulfur t 34s 13 1 16 5 0 suggesting a mississippian seawater source ore stage sphalerite galena and chalcopyrite probably derived their sulfur b34s 8 0 13 65 0 from sulfates or possibly a homogenized basement source postore marcasite and pyrite are characterized by light probably biogenic sulfur ba4s 9 7 to 46 0 mineralized carbonates are significantly depleted in the heavier isotopes of oxygen and carbon with respect to their unmineralized equivalents preliminary lead isotope data for galena suggest that the metasedimentary basement was a source of lead this study disputes an earlier interpretation that the stratiform lead zinc ores are an early low temperature diagenetic cement of the carbonate bank the high grade zones are not residual concentrations related to sinkholes as originally thought instead both disseminated and massive ores are probably the result of the same mineralizing episode involving deeply convecting brines or brines expelled during basin dewatering under high geothermal gradients in the late paleozoic

relative motion between the caribbean and north american plates and related boundary zone deformation from a decade of gps observations global positioning system gps measurements in 1986 1994 and 1995 at sites in dominican republic puerto rico cuba and grand turk define the velocity of the caribbean plate relative to north america the data show eastward motion of the caribbean plate at a rate of 21 1 mm yr 1 standard error in the vicinity of southern dominican republic a factor of 2 higher than the nuvel 1a plate motion model prediction of 11 3 mm yr independent measurements on san andres island and an euler vector derived from these data also suggest a rate that is much higher than the nuvel 1a model available data combined with simple elastic strain models give the following slip rate estimates for major left lateral faults in hispaniola 1 the north hispaniola fault offshore the north coast of hispaniola 4 3 mm yr 2 the septentrional fault in northern dominican republic 8 3 mm yr and 3 the enriquillo fault in southern dominican republic and haiti 8 4 mm yr the relatively high plate motion rate and fault slip rates suggested by our study combined with evidence for strain accumulation and historical seismicity imply that seismic risk in the region may be higher than previous estimates based on low plate rate low fault slip rate models and the relatively low rate of seismicity over the last century

geomorpho tectonic evolution of the jamaican restraining bend this work applies recent advances in tectonic geomorphology in order to understand the geomorphic evolution of the jamaican restraining bend located along the caribbean gon ve north american plate boundary we propose a classification of landscapes according to their erosional stages the approach is mainly based on the combination of two dem based geomorphic indices the hypsometric integral which highlights elevated surfaces and the surface roughness which increases when the relief is incised by the drainage network river longitudinal profiles were also analyzed as the drainage network responds quickly to base level change triggered by external forcing such as tectonics anomalies in river profiles knickpoints and convex segments were mapped using stream length gradient sl and normalized steepness ksn indices the results provide new insights for understanding the complex evolution of the jamaican restraining bend three main morphotectonic regions were identified in jamaica 1 the blue mountain wagwater unit located at the eastern tip of the island 2 the jamaican highlands plateau which covers most of the northern and central areas and 3 the tilted block province located along the southern part of jamaica each region has a specific morphological signature which marks a different stage in the late miocene to present evolution of the jamaican restraining bend the evolution of the bend is mainly associated with the western propagation of major e trending strike slip faults and nw trending thrusts in the western and central parts of jamaica the present day motion between the caribbean plate and the gon ve microplate is broadly distributed along several structures while in the easternmost part of the island this motion seems to be almost completely accommodated along the blue mountain range and the plantain garden fault

fluid pressure in the frontal thrust of the oregon accretionary prism experimental constraints seismic reflection profiles of many accretionary prisms including the oregon prism exhibit high amplitude reversed polarity reflections from the decollement and other thrusts it has been suggested that these reflectors image fault zones with enhanced fluid content due to dilation by very high fluid pressure we present measurements of velocity as a function of effective stress on samples of the oregon frontal thrust that show that velocity decreases by up to 15 as fluid pressure rises from hydrostatic to lithostatic conditions under constant confining stress in fault zone and wall rock alike synthetic seismic modeling shows that the frontal thrust reflector at this location is the result of a thin low velocity zone in the plane of the fault 100 300 m s slower than the walls combining models with experimental results we conclude that fluid pressure of 86 to 98 of lithostatic stress reduces velocity enough to generate the reflections

relation between permeability and effective stress along a plate boundary fault barbados accretionary complex in situ bulk permeability was measured in a borehole that intersected the decollement zone a low angle detachment fault between the north american and caribbean plates permeability measurements were made at a variety of fluid pressure conditions defining a quantitative relation between bulk permeability and effective stress for this plate boundary fault zone the bulk permeability in this zone changed by several orders of magnitude as a consistent function of fluid pressure this relation may help to explain the dynamics of fluid fault interactions and the transient nature of hydrologic processes during deformation at convergent margins

porosity formation factor and porosity velocity relationships in barbados prism ocean drilling program leg 171a collected logging while drilling lwd data to investigate the porosity and velocity structure of barbados accretionary complex porosity and velocity measurements were needed for interpretation of prism dynamics however the lwd tool string did not measure velocity and previous velocity data from barbados prism are scarce and generally of low quality furthermore most density based porosities are subject to errors associated with variations in smectite content to determine porosities more accurately we use smectite corrected density based porosities to calibrate a porosity formation factor ff transform then calculate porosities from resistivity logs we used cation exchange capacity cec measurements and lwd logs to evaluate the possibility that resistivity is affected by clay conduction the cec data warn of potential clay conduction especially in smectite rich zones but lwd data indicate that clay conduction is minor in these high porosity sediments instead tortuosity is the secondary control of ff after porosity by comparing resistivity based porosities to velocities from vertical seismic profiles we determine a porosity velocity transform applicable to barbados we then use this transform to calculate velocity logs from resistivity based porosity logs

shear wave velocities and anisotropy in the barbados accretionary complex s wave velocities at the base of the barbados accretionary wedge were determined from mode converted s waves in two vertical seismic profiles in ocean drilling program boreholes at hole 949c the profile extends from 92 to 397 m below seafloor mbsf ending 17 m into the decollement zone separating the wedge from the underthrust sediment on the subducting plate beneath at hole 948d the profile extends from 105 to 471 mbsf ending 19 m above the decollement zone s wave velocities are 303 549 m s at hole 949c and 453 679 m s at hole 948d the profiles show that the lowest 70 m of the accreted sediment just above the decollement has lower s wave velocity 379 26 m s at hole 949c and 453 11 m s at hole 948d and higher ratio of p wave to s wave velocity vp vs 4 59 0 31 at hole 949c and 3 99 0 10 at hole 948d than in the overlying layer this coincides with high clay content that has a high proportion of smectite shear wave splitting observed in records from the lowest 100 m of the accretionary wedge at both sites indicates little or no anisotropy in the low velocity zone a single measurement at hole 949c of the horizontal component of polarization of the leading split s wave from an ocean bottom shot is n68 e 20 at hole 949c split shear wave delays in the downward propagating s wave originating at a fault at 275 mbsf indicate strong anisotropy in the range 275 306 mbsf and little or no anisotropy in the range 306 380 mbsf the rate of increase of delay between 275 and 306 mbsf 0 52 0 16 ms m can be modeled by either a hudson cracked medium with crack density 0 186 0 054 or preferably a hornby model of shale with 44 of clay particles aligned within 5 of the symmetry plane this strong anisotropy is attributed to scaly fabric alignment of clay particles by pervasive shear in the fault zone which is observed in core the low anisotropy in the remainder of the section shows that there is very little overall alignment of pore space in the lower part of the accreted sediments supporting the hypothesis that the sediments are undercompacted rather than hydrofractured the low s wave velocities and high vp vs ratios indicate undercompaction and low effective stress despite the tectonic burial of the lowermost accreted sediments

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

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

evolution of fluid expulsion and concentrated hydrate zones across the southern hikurangi subduction margin new zealand an analysis from depth migrated seismic data identification of methane sources controlling hydrate distribution and concentrations in continental margins remains a major challenge in gas hydrate research lack of deep fluid samples and high quality regional scale seismic reflection data may lead to underestimation of the significance of fluid escape from subducting and compacting sediments in the global inventory of methane reaching the hydrate zone the water column and the atmosphere the distribution of concentrated hydrate zones in relation to focused fluid flow across the southern hikurangi subduction margin was investigated using high quality long offset 10 km streamer pre stack depth migrated multichannel seismic data analysis of low p wave velocity zones bright reverse polarity reflections and dim amplitude anomalies reveals pathways for gas escape and zones of gas accumulation the study shows the structural and stratigraphic settings of three main areas of concentrated hydrates 1 the opouawe bank dominated by focused periodic fluid input along thrust faults sustaining dynamic hydrate concentrations and gas chimneys development 2 the frontal anticline with a basal set of protothrusts controlling permeability for fluids from deeply buried and subducted sediments sustaining hydrate concentrations at the crest of the anticline and 3 the hikurangi channel with buried sand dominated channels hosting significant amounts of gas beneath the base of the hydrate zone in sand dominated channels gas injection into the hydrate zone favors highly concentrated hydrate accumulations the evolution of fluid expulsion controlling hydrate formation offshore southern hikurangi is described in stages during which different methane sources in situ buried and thermogenic have been dominant

tectonic controls on carbonate sequence formation in an active strike slip setting serranilla basin northern nicaragua rise western caribbean sea abstract the serranilla basin is a flat floored semi circular bathymetric depression 100 100 km 1100 1200 m deep at the western end of the northern nicaragua rise nnr in the caribbean sea it is bound to the north by the cayman trough an area of active sea floor spreading and is part of the northern caribbean plate boundary zone ncpbz single channel high resolution seismic data were calibrated to rock dredges and odp site 1000 to define the geologic evolution and attempt to tie sequence formation within the basin to tectonic developments in this part of the caribbean five seismic sequences were identified within the basin the two lower sequences a and b are interpreted as neritic and shallow periplatform deposits which infill three distinct basins that make up the early to late miocene serranilla basin the three upper sequences c through e are interpreted as periplatform and pelagic deposits interspersed with turbidites and in some areas megabreccias faulting is prevalent in sequences a through c in the central basin and becomes progressively younger toward the south disrupting the seafloor in places and perhaps indicating renewed activity along the pedro fracture zone the timing of sequence boundary formation has been correlated to tectonic activity along the ncpbz and closure of the central american seaway possible mechanisms of sequence boundary formation include tectonic tilting within the basin in conjunction with increased turbidite deposition carbonate platform drowning and subsequent back stepping associated with circulation changes resulting from tectonic gateway closure and megabreccia deposition associated with bank demise although a direct genetic relationship is not proven regional tectonic changes are considered more important than eustatic sea level changes in controlling depositional sequence formation in the serranilla basin

transverse intra arc rifting palaeogene wagwater belt jamaica abstract the wagwater belt in eastern jamaica is an exceptionally well exposed example of a transverse intra arc rift formed during an abrupt transition from convergent to strike slip tectonics the belt consists of the uplifted wagwater half graben containing at least 5 6 km of coarse clastic terrestrial sedimentary rocks which is overlain and flanked by a much wider basin filled with about 1 2 km of upward fining marine sedimentary rocks neogene strike slip movements structurally inverted the half graben and exposed thick sections of both the half graben and overlying basin early palaeocene early eocene rifting preceded the middle eocene formation of oceanic crust in the cayman trough pull apart basin and affected a 50000 km 2 area near the eastern distal end of the pull apart fifteen half grabens identified in jamaica and in offshore areas using seismic profiling range in strike from north east to north west rifts in jamaica are presently oblique to the generally east west strike of plutons and sedimentary facies belts within a maestrichtian palaeocene island arc based on their apparent relationship to the strike of the enclosing palaeo arc we interpret the jamaican rifts as transverse intra arc rifts following the deposition of palaeocene redbeds and evaporites wagwater formation in the half graben subsidence was widespread in a marine sedimentary basin this marine basin was filled in early eocene time by marine conglomerates port maria and albany members of the richmond formation sandstone and siltstones roadside member and finally in the latest early eocene to earliest middle eocene time by a calcareous mudstone langley member a bimodal suite of volcanic rocks newcastle and nutfield volcanics was erupted during the deposition of the wagwater formation and the roadside member the structure and tectonic setting of transverse intra arc basins in jamaica is somewhat similar to rifts of the neogene pannonian basin in hungary in both areas an abrupt transition between convergent and strike slip tectonics resulted in transverse intra arc rifts striking almost perpendicular to a dying volcanic arc transverse intra arc rifts may be common features of small arc fringed plates such as the caribbean and pannonian plates that respond quickly to collision by strike slip motion towards an unopposed oceanic free face

accretionary prisms in subduction earthquake cycles the theory of dynamic coulomb wedge 1 we expand the theory of critically tapered coulomb wedge for accretionary prisms by considering stress changes in subduction earthquake cycles building on the coulomb plasticity of the classical theory we assume an elastic perfectly coulomb plastic rheology and derive exact stress solutions for stable and critical wedges the new theory postulates that the actively deforming most seaward part of an accretionary prism the outer wedge overlies the updip velocity strengthening part of the subduction fault and the less deformed inner wedge overlies the velocity weakening part the seismogenic zone during great earthquakes the outer wedge is pushed into a compressively critical state with an increase in basal and internal stresses and pore fluid pressure after the earthquake the outer wedge returns to a stable state the outer wedge geometry is controlled by the peak stress of the updip velocity strengthening part of the subduction fault achieved in largest earthquakes the inner wedge generally stays in the stable regime throughout earthquake cycles acting as an apparent backstop and providing a stable environment for the formation of forearc basins the new theory has important implications for the studies of the updip limit of the seismogenic zone the evolution of accretionary prisms and forearc basins activation of splay faults and tsunami generation evolution of the fluid regime and mechanics of frontal prisms at margins dominated by tectonic erosion

stratigraphic implications of palaeomagnetic data from honduras summary palaeomagnetic results from mainly cretaceous rocks in honduras delineate a distinctive apparent polar wander path apwp which is well suited for magnetostratigraphy the apwp is based on palaeomagnetic data from palaeontologically dated units in the established stratigraphic column and three radiometrically dated intrusions the ages and stratigraphic positions of sedimentary units whose stratigraphic assignments have been debated can be inferred by comparing their palaeornagnetic pole postion with the known apwp the results demonstrate the usefulness of magnetostratigraphy for resolving stratigraphic problems in the absence of fossils or marker beds and for areas of discontinuous exposures for regional interpretations the most important conclusion is that strata in honduras that have been assigned to the todos santos formation were not deposited on the same tectonic plate as the todos santos formation of guatemala and southern mexico there is no evidence that the honduran todos santos strata are depositionally related to the true todos santos formation the name todos santos formation should be abandoned in honduras and for the entire chortis block

consolidation patterns during initiation and evolution of a plate boundary decollement zone northern barbados accretionary prism borehole logs from the northern barbados accretionary prism show that the plate boundary decollement initiates in a low density radiolarian claystone with continued thrusting the decollement zone consolidates but in a patchy manner the logs calibrate a three dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid and which portions have consolidated the seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid rich conduit extending down the decollement surface fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch

barbados ridge hydrogeologic tests implications for fluid migration along an active decollement hydrogeologic tests were conducted at a sealed borehole penetrating the decollement of the barbados ridge accretionary complex at low excess pore pressures p p p h p l p h 0 0 to 0 36 where p p pore pressure p h hydrostatic pressure and p l lithostatic pressure estimated permeabilities were comparable to those of similar unfractured sediment these tests complement shipboard packer tests completed at higher fluid pressures 0 5 to 1 0 during ocean drilling program odp leg 156 together the test results suggest a 4 to 5 order of magnitude permeability increase as fluid pressure varied from hydrostatic 0 to lithostatic 1 however unlike the results of the shipboard packer tests the test results presented here exhibit no evidence of a relationship between permeability and pore pressure the combined findings from the two sets of hydrogeologic tests indicate that significant permeability increases can occur within the decollement at pore pressures below lithostatic pressure

neotectonics of the caribbean a compilation of earthquake focal mechanism solutions together with the interpretation of inclined intermediate depth seismic zones neogene strike slip faults fault controlled basins and volcanism is used to test three existing models for the neotectonic i e neogene or post oligocene development of the caribbean the current models although based on different hypotheses using different data sets are similar and suggest varying degrees of convergence between the caribbean and either north or south american plates or convergence between the caribbean plate and continental fragments of the northwestern corner of south america using model 1 previous workers have interpreted the disposition of fracture zones and magnetic anomalies in the central and south atlantic as indicating that the caribbean plate is being compressed by north south convergence between the americas with the result that it is being driven eastward relative to the americas along strike slip faults while producing east west trending subductionlike convergent zones along northern and southern plate margins using model 2 previous workers have interpreted the azimuths of active strike slip faults and focal mechanism solutions and closure of three plate velocity circuits as indicating that the south american plate converges in a west northwesterly direction on the southern margin of the caribbean plate and has produced compressional features while strike slip motion dominates along the northern boundary using model 3 previous workers have interpreted focal mechanism solutions and the pattern of strike faulting and basin development in northwestern south america to suggest right lateral displacement of the northwestern corner of south america relative to the craton with convergence and compression in the south central caribbean our integration of geologic information on faults basins and volcanism considerably extends the limited record of plate interactions known from historical reports of the past few hundred years and recorded earthquakes and suggests that 1 relative motion between the north american and caribbean plates is dominantly east west strike slip motion although areas of complex internal deformation associated with restraining bend or compressional strike slip fault segments have been identified in jamaica hispaniola and central america 2 relative motion between the south american and caribbean plates is dominantly east west strike slip motion although an area of restraining bend deformation has been identified in the araya peninsula of venezuela 3 relative motion between a triangular continental fragment of northwestern south america the maracaibo block ranges from northwest to northeast along the south central margin of the caribbean and appears to be driven mostly by the late miocene pliocene collision of the panama arc although additional driving forces may include the collision of the carnegie ridge and or oblique subduction to the northeast in ecuador and 4 relative motion internal deformation may be occurring at a very slow rate between fault bounded blocks within the caribbean plate as a result of the convergence of the maracaibo block and or eastward motion of the caribbean plate relative to a continental promontory of north america in northern central america because the caribbean margins are composed mainly of anisotropic easily deformed continental and arc lithosphere plate boundaries are zones of diffuse deformation and it is difficult to assess the degree of convergence between the americas although data from the caribbean itself suggest that it may be smaller than the amount inferred by some workers using model 1 fracture zone and magnetic anomalies in the atlantic some of the compressional features cited as evidence for the relative convergence of the americas may be instead related to the convergence between the maracaibo block relative to the southern margin of the caribbean many aspects of caribbean neotectonics remain less than completely understood but more integration of seismic and geologic studies particularly exploratory fault trenching of onshore faults and seismic profiling of offshore faults should lead to rapid advances

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

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

evidence for quaternary convergence across the north america south america plate boundary zone east of the lesser antilles new seismic and bathymetric data reveal that the plate boundary zone between the north american and south american plates east of the caribbean has occupied a 200 km wide zone of localized basement uplift and folding and of faulting of the sedimentary cover during the early pleistocene to holocene this zone which includes the barracuda ridge and tiburon rise exhibits north south compressional structures and continues eastward up to 500 km from the boundary of the caribbean plate a sequence of young turbiditedominated sediments fi lls the basin between these two ridges and shows synsedimentary deformation the base of the sequence is of early pleistocene age dated by biostratigraphic correlation with deep sea drilling project site 27 recent deformation in the turbidite fi lled trough north of the barracuda ridge is shown by uplift of the seafl oor at the crests of anticlines across which the shallowest sediment layers thin or pinch out

u pb dating of uranium deposits in collapse breccia pipes of the grand canyon region two major periods of uranium mineralization are indicated by u pb isotope dating of uranium ores from collapse breeeia pipes in the grand canyon region northern arizona the hack 2 and 3 kanab north and ez 1 and 2 orebodies apparently formed in the interval of 200 or 20 ma similar to ages inferred for strata bound late triassic hosted uranium deposits in southern utah and northern arizona samples from the grand canyon and pine nut pipes however indicate a distinctly older age of about 260 ma the pigeon orphan and arizona 1 deposits were apparently mineralized before 220 186 and 169 ma respectively but no useful upper age limits can be inferred there is no evidence in the u pb isotope data for uranium mineralization related to laramide tectonism mid tertiary volcanism or late tertiary uplift the clustering in ages for a variety of uranium deposits at about or slightly younger than the age of the lower part of the chinle formation late triassic suggests that uranium in these deposits may have been derived by leaching from volcanic ash in the chinle and mobilized by ground water movement resulting from changing hydrologic gradients caused by regional uplift to the southwest pb isotope ratios of galenas in mineralized pipes are more radiogenic than those of sulfides from either uranium poor pipes or occurrences away from pipes this isotopic contrast suggests that fluids which passed through the pipes had interacted with the proterozoic basement possibly through the vertical fractures which influenced the location and evolution of the pipes themselves regardless of the source of the common pb in the uranium bearing pipes the generally distinct pb isotope composition of their galenas compared to those of barren pipes and nonpipe sulfides in the region may provide a useful exploration guide

heterogeneous hydrofracture development and accretionary fault dynamics ocean drilling program observations at many accretionary wedges indicate that fluid flow in low permeability sediments is focused along faults rich in clay minerals recent ring shear experiments show that shear failure generally reduces permeability in muddy faults to values well below the surrounding sediments and that very little increase occurs in the fault permeability even when continued failure proceeds under steadily reduced effective loads it is thus unlikely that observed focused flow occurs through the matrix of muddy faults and we suggest that focused flow can only be explained by the development of open fracture systems the lithostatic fluid pressures necessary for hydrofracturing in weak sediments cannot however exist throughout the imbricate thrust system and basal decollement zone because a zero basal shear stress condition would result in regional instability and accretionary wedge collapse we propose that the regional hydrofracture networks must propagate heterogeneously leaving relatively strong asperity regions at sublithostatic fluid pressures to balance forces in the wedge in three dimensions

seismic activity along the central america volcanic arc is it related to subduction of the cocos plate we determine seismic strain rate of tectonic earthquakes along the central america volcanic arc we then compare this result to those obtained from earthquakes related to the convergence of the cocos and caribbean plates and to earthquakes in the back arc region of northern central america the seismic strain rate tensor for shallow focus earthquakes along the central america volcanic arc since 1700 has a compressive eigenvector with a magnitude of 0 7 10 8 year 1 and oriented in a 3578 azimuth the extensive eigenvector is oriented in a 868 azimuth with a magnitude of 0 82 10 8 year 1 when only centroid moment tensor solutions cmt are considered the respective eigenvectors are 1 2 10 8 year 1 and 1 0 10 8 year 1 the compressive eigenvector from the seismic strain rate tensor for earthquakes along the cocos caribbean convergent margin is 2 0 10 8 year 1 plunging at 258 and oriented in a 298 azimuth its magnitude and direction are similar to those of the compressive eigenvector for earthquakes along the volcanic arc the extensive eigenvector along the convergent margin on the other hand has a large vertical component the compressive and extensive eigevenvectors are 4 9 10 8 year 1 and 4 6 10 8 year 1 using only cmts as the database earthquakes along the grabens of northern central america yield a seismic strain rate tensor whose extensive eigenvector has a magnitude of 2 4 10 8 year 1 oriented in a 1098 azimuth magnitude and direction are similar to those of the extensive eigenvector for earthquakes along the volcanic arc the compressive eigenvector along the grabens is practically vertical similarities in magnitudes and directions for compressive and extensive eigenvectors suggest to us that the strain field along the central america volcanic arc is the result of compression along the convergent cocos caribbean margin and extension in the back arc region along the grabens of northern central america this field is resolved as strike slip faulting along the arc d 2005 elsevier b v all rights reserved

slow wavespeeds and fluid overpressure in a region of shallow geodetic locking and slow slip hikurangi subduction margin new zealand abstract travel times recorded onshore from active source marine seismic surveys were used to determine hikurangi forearc wavespeeds ray path midpoints sample forearc crust above the shallow v p from the seabed to 4 km depth consistent with rapidly deposited clastic sediments and near constant v p 5 0 0 2 km s at depths of 4 10 km within the forearc the northern region has slow slip events at shallow depths of c 5 15 km beneath the volume sampled by our analysis average travel times at offsets of 20 80 km are 1 s more than at equivalent offsets in the south and v p increases with depth within the forearc from 3 5 0 1 km s at 4 km depth to 4 5 0 2 km s at 10 km depth we interpret the low wavespeeds in terms of compaction disequilibrium and compare seismic inferences of anomalously high porosity 10 with hydrostatic reference compaction models to show effective stress is low in both the north 27 10 mpa and south 36 14 mpa in the south pore space and conduits have a high chance of being localized on faults or sand layers the wedge is clearly compressionally critical and we suggest the higher seismic velocities recorded primarily reflect higher effective stress levels transmitted through the rock framework the observed wedge geometry and our estimate of fluid pressure within the wedge 0 87 0 05 suggest a weak overpressured subduction thrust in the north we suggest high fluid pressure is maintained by a large fluid inventory from subducting sediment a pore space geometry characterized by pervasively fractured rock and mudstone and a lower mean stress due to its stable and non critical wedge geometry we consider both end member assumptions of convergent and tensile failure the latter of which reveals the possibility of near lithostatic fluid pressure on the north hikurangi subduction interface although this remains unproven our observations are consistent with high fluid pressures encountered in petroleum wells at depths

s wave velocities and anisotropy in sediments entering the nankai subduction zone offshore japan summary p and mode converted s waves from airgun shots recorded along a line of ocean bottom seismometers were used to construct a 2 d velocity section across the nankai trough and the toe of the accretionary complex offshore southeast japan the site was chosen because of plans to drill into the seismogenic zone of the main subduction thrust which has a history of m 8 earthquakes the section is constrained by well log data from three ocean drilling program boreholes s wave velocities of the sediments over the entire section down to 1 1 km below seafloor range from 107 to 915 m s 1 shear wave splitting indicates anisotropy in all the sediments except possibly the distal trench fill if interpreted conventionally as being due to vertical stress aligned cracks the anisotropy in both the trench and the toe of the accretionary wedge is consistent with crack orientation nw se parallel to the direction of maximum tectonic compression crack density is 0 01 0 03 consistent with closeness to fracture criticality

a hierarchical tectonic model of the gulf of mexico and caribbean region abstract a quantitative computer graphic methodology hierarchical tectonic analysis has been used to model the tectonic evolution of the gulf of mexico and caribbean region this method employs a highly structured hierarchical description of the relative motions of pairs of tectonic elements in order to quantify the timing and style of plate interactions the main features described in this model are 1 1 a revised fit of pangea 2 2 the opening of the gulf of mexico in the middle late jurassic 3 3 the formation of a proto caribbean ocean due to the separation of north and south america during the early cretaceous 4 4 the formation of the greater antilles arc along the western margin of the proto caribbean ocean 5 5 the insertion of the farallon plate between north and south america during the middle cretaceous resulting in the northeastward advance of the greater antilles arc and the subduction of proto caribbean oceanic crust 6 6 the complete subduction of the proto caribbean ocean and the collision of the greater antilles arc with the bahamas platform in the late cretaceous 7 7 the truncation of the caribbean section of the farallon plate from its pacific counterpart and the formation of the panama arc latest cretaceous and 8 8 the westward movement of north and south america with respect to the caribbean plate resulting in the opening of the yucatan basin paleocene and subsequently in the evolution of the cayman trough and e w zone of strike slip

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

seismically inferred dilatancy distribution northern barbados ridge decollement implications for fluid migration and fault strength a 5 x 25 km three dimensional seismic survey of the lower part of the northern barbados ridge accretionary prism creates a three dimensional image of a major active decollement fault the fault is usually a compound negative polarity reflection modeled as a low velocity high porosity zone less than 14 m thick this thickness is significantly less than that defined by drilling of a 40 m zone of deformation at ocean drilling program odp site 671b located within the surveyed area we infer that the seismically defined fault is a thin high porosity zone and is thus an undercompacted high fluid pressure dilatant section if these inferences are correct then map view variations in seismic reflection waveform and amplitude illustrate complex patterns of fault zone fluid content and fluid migration paths the amplitude map suggests kilometre wide channels of locally high porosity and thus focused fluid flow these paths are only subparallel to the expected minimum head as inferred from the shape of the overlying sediment wedge other factors must modify fluid concentrations and ultimately migration several areas of positive polarity fault reflections define square kilometre sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault one coincident with site 671b may explain the success of drilling through the fault here all other holes drilled in the area were within the negative polarity regions and were unsuccessful in penetrating through the entire fault zone possibly because of instability associated with high fluid pressures and a weak fault odp leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures

late cretaceous cenozoic sequence stratigraphic and paleogeographic controls on petroleum system elements of the nicaraguan platform western caribbean sea late cretaceous to present day mixed carbonate clastic deposition along the nicaraguan platform western caribbean sea has evolved from a tectonically controlled rifted upper eocene shallow to deep marine carbonate siliciclastic shelf to an upper miocene to present day tectonically stable shallow marine carbonate platform and passive margin by integrating subsurface data of 287 two dimensional seismic lines and 27 wells we interpret the cenozoic stratigraphic sequence as 3 cycles of transgression and regression beginning with an upper eocene rhodolitic algal carbonate shelf that interfingered with marginal siliciclastic sediments derived from exposed areas of central america bordering the margin to the west during the middle eocene a carbonate platform was established with both rimmed reefs and isolated patch reefs a late eocene forced regression produced widespread erosion and subaerial exposure across much of the platform and was recorded by a regional unconformity the oligocene upper miocene sedimentary record includes a southeastward prograding delta of the proto coco river which drained the emergent area of what is now northern nicaragua the late miocene to present day period marks a period of strong subsidence with the development of small pinnacle reefs we describe favorable petroleum system elements of the nicaraguan platform that include 1 eocene fossiliferous limestone source rocks documented as thermally mature in vintage exploration wells and seen as active gas chimneys emanating from inferred carbonate reservoirs 2 upper to middle eocene reservoirs in patch and pinnacle reefs middle eocene calcareous slumps and oligocene fluvial deltaic facies documented in wells and 3 regional seal intervals that consist of both regional unconformities and eocene oligocene intraformational shale

investigation of late quaternary fault block uplift along the motagua swan islands fault system implications for seismic tsunami hazard for the bay of honduras abstract uplifted and warped coastal landforms fossil coral reef and beachrock wave cut and beach terraces on the western part of roatan island off the northern honduran coast record at least two late holocene earthquakes that we estimate to have had magnitudes of m7 uplift has been primarily related to a fault that follows the southern coast of western roatan herein termed the flowers bay fault a subsidiary fault of the motagua swan islands fault system which marks the boundary between the north american and caribbean plates using electron spin resonance esr and radiocarbon ages of calcium carbonate samples and a late quaternary sea level elevation curve that is compatible with caribbean sea level data we constrain the ages and long term uplift rates of the displaced landforms on roatan caused by the vertical component of slip on the flowers bay fault the fossil reef that is uplifted along the fault grew between 43 and 34 ka and the beachrock horizon and lowest uplifted terrace along the southern and western coasts developed between 1000 and 1700 ad we describe evidence of one earthquake that raised the south coast 3 m as much as 5 m locally and that post dates 1700 ad we interpret this event to be the great earthquake of august 1856 that generated a tsunami which ran as much as 24 km onto the mainland another earthquake circa 900 ad produced a similar amount of uplift as the 1856 event and likely generated a similar tsunami the age and elevation of the fossil reef suggest a long term uplift rate of 3 mm year consistent with a recurrence interval of 1000 years for these large earthquakes

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

western caribbean intraplate deformation defining a continuous and active microplate boundary along the san andres rift and hess escarpment fault zone colombian caribbean sea the san andres rift sar located on the lower nicaraguan rise is a previously poorly studied active 015 trending bathymetric and structural rift basin that is 11 27 km 7 17 mi wide and extends for 346 km 215 mi across the western flank of the caribbean plate in this study we integrate bathymetric maps potential field data and high resolution two dimensional 2 d seismic lines to understand the crustal structure tectonic history and tectonic origin of the sar which is one of the active areas within the otherwise stable caribbean plate we compiled regional gravity and magnetic data that revealed a negative gravity anomaly and positive magnetic anomaly that we interpret as a result of crustal thinning and an elevated moho along the main rift axis of the sar forward models of gravity data show four possible interpretations for the origin of the crust underlying and surrounding the sar interpretations of 2 d seismic reflection data show structural features within the upper crust and sedimentary sections typical of other active rift systems including a sar parallel north south alignment of earthquakes with the larger events showing normal and strike slip focal mechanisms sequential kinematic restorations based on 2 d seismic profiles reveal three major phases of sar opening 1 the initial early eocene rifting stage 2 middle eocene extension and 3 a rapid middle miocene to early pliocene extension accompanied by emergence of the san andres island as a rift shoulder we propose slab rollback and intraplate extension as main tectonic mechanisms to explain all rift phases and neogene volcanism found in the western caribbean region

seismic modeling of the decollement zone at the base of the barbados ridge accretionary complex wide aperture seismic reflection profiling of the northern barbados ridge accretionary complex at 16 12 n has imaged the decollement horizon across more than 100 km of the accretionary complex revealing prominent changes in the amplitude of the decollement reflection synthetic seismograms from models of the variation of velocity and density with depth were compared with selected seismic traces between 2 5 km seaward of the toe of the accretionary complex and 20 km landward of the toe to determine the changes in velocity density properties along the decollement thrust zone modeling requires a thin low velocity layer to develop landward along the decollement beneath the complex while no such layer exists at the corresponding stratigraphic boundary in the undeformed sediments seaward of the complex two dimensional modeling that incorporated lateral variations in topography velocity and density was performed on a 7 km section of the decollement 20 km landward of the deformation front the best fitting model includes a 20 m thick low velocity layer at the decollement horizon with a relative velocity decrease in the layer of 0 2 km s changes in the reflection amplitude along this 7 km section could be produced by either a change in the velocity anomaly within the layer or a thinning of the decollement layer to less than 4 m the shape of the reflected wavelet from the decollement indicates that the principal cause of the amplitude variation is a change in the velocity contrast rather than a change in thickness the low velocity layer can be explained by a decollement shear zone that has high porosity maintained by elevated pore fluid pressure high fracture permeability within the decollement permits dewatering of sediment attached to the oceanic crust and thrust beneath the accretionary complex

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

velocity anisotropy in shales a petrophysical study using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content clay mineralogy and porosity at a wide range of effective pressure we find that elastic anisotropy of shales increases substantially with compaction the effect is attributed to both porosity reduction and smectite to illite transformation with diagenesis a means of kerogen content mapping using velocity versus porosity crossplot for shales is shown matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0 4 at total organic carbon toc 15 20 a strong chemical softening effect was found in shales containing even minor amounts of swelling smectite clay when saturated with aqueous solution this effect results in a significant p wave anisotropy reduction as compared to dry and oil saturated shales since mature black shales are normally oil wet this effect can only have a local significance restricted to the wellbore wall accurate measurements of phase velocities including velocities at a 45 direction to the bedding plane allow us to immediately calculate elastic stiffnesses and anisotropic parameters intrinsic high pressure properties of shales display an e 0 relation introduction of the bedding parallel microcracks in overpressured shales results in a decrease when fully fluid saturated and a increase when partially gas saturated with a characteristic effect on the shape of the p wave velocity surface at small angles of incidence filtering the contribution of the intrinsic anisotropy of shales it is possible to estimate the pore fluid phase microcrack density and aspect ratio parameters using seismic anisotropy measurements

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

evolution of hydrothermal fluids in the ashanti gold belt ghana stable isotope geochemistry of carbonates graphite and quartz the oxygen and carbon isotope systematics of ankerite siderite calcite quartz and graphite from the bogosu and prestea mining districts of the ashanti gold belt ghana are used to speculate on the processes leading to hydrothermal alteration and gold deposition the geologic environment of this turbidite and graywacke hosted gold system was a sediment dominated accretionary complex the gold ores are localized within the ashanti structural belt which we suggest became a conduit for deep seated fluids during uplift and dilatancy ankerite and siderite in mineralized sedimentary rocks are broadly similar in oxygen and carbon isotope compositions to corresponding minerals in the metasedimentary country rocks evidence of alteration that preceded gold mineralization is best preserved in spatially associated altered mafic dikes the compositions of fluids responsible for the gold mineralization and associated alteration have been reconstructed using temperatures from carbonate mineral geothermometry alteration of country rocks occurred under rock dominant greenschist facies conditions the isotopic composition of the least altered mafic dikes was influenced by interaction with fluids generated from birimian greenschist facies metasedimentary rocks later extensive carbonate alteration of mafic dikes adjacent to the structural conduit occurred during slow upward migration of fluids lateral diffusion of hydrogen into the structural conduit permitted partial conversion of co 2 to methane resulting in 13 c enrichment of both the residual co 2 and the deposited carbonate minerals rapid even explosive expulsion of deep seated fluids and co 2 h 2 o phase separation occurred episodically throughout the evolution of the hydrothermal system this caused precipitation of sulfides arsenides and gold at relatively high crustal levels and low ambient pressure and temperature during mineralization fluid ascent was too rapid for isotopic and chemical reequilibration between the deep seated ore fluid and adjacent country rocks the ore fluid had chemical and isotopic compositions consistent with deep seated metamorphic fluids that were subsequently modified by preferential partitioning of 18 o and 13 c into a gas phase the composition of the deep seated metamorphic fluid determined using chemical equilibria co 2 h 2 o rich 7 25 mole co 2 is in approximate agreement with that required to produce the isotopic depletion of the residual fluid via phase separation

elevated fluid pressure and fault zone dilation inferred from seismic models of the northern barbados ridge decollement in 1992 a large volume of three dimensional seismic reflection data were acquired in a 5 25 km area across the toe of the barbados accretionary complex that covers the deep sea drilling project leg 78a and ocean drilling program legs 110 and 156 drilling sites these data are used to examine the acoustic character of the decollement seismic reflection and to qualitatively and quantitatively characterize fluid pressures within the fault zone seismic models have been constructed across a 6 km region of the decollement where it has been mapped as a moderate to bright polarity reversed reflection the models show that this segment of the decollement reflection is caused by a low velocity interval usually 12 16 m thick the top of the low velocity interval appears to be a sharp boundary that requires a decrease in velocity from 1 8 km s to between 1 7 and 1 65 km s with some localized bright reflections with an even lower velocity of 1 6 km s the base of the low velocity layer is less certain from modeling the base consists of either a velocity increase that is usually approximately half the velocity contrast at the top of the layer or the velocity increase is equal to the contrast at the top of the layer but distributed over a 10 m thick interval comparison of these results to laboratory experiments on the relationship between fluid pressure and seismic velocity indicates that in this interval of the decollement fluid pressure is at or near lithostatic furthermore the reflection coefficients of the decollement are sufficiently large that some dilation of the fault zone is required the dilation should lead to high fracture zone permeability and explain the observation of a laterally consistent decollement reflection along a 5 km segment of the decollement it is within these segments of the fault that fluid pressure approaches lithostatic and significantly reduces fault strength

crustal structure of the peruvian continental margin from wide angle seismic studies keine zusammenfassung in englischer sprache vorhanden no summary in english language