an alternative interpretation of the cayman trough evolution from a reidentification of magnetic anomalies magnetic data were collected during the wilkes 1973 and seacarib ii 1987 cruises to the cayman trough a new interpretation of magnetic data is carried out an isochron pattern is drawn up from our anomaly identifications an early eocene age 49 ma ypresian for cayman trough opening is proposed instead of the late oligocene or middle eocene ages suggested by previous studies our plate tectonic reconstruction is simpler and fits the on land geology jamaica and cuba and the tectonics our reconstruction shows a southward propagation of the spreading centre between magnetic anomalies 8 and 6 26 and 20 ma the trough width increases by 30 km in this period the southward propagation of the cayman spreading centre from the middle oligocene to the early miocene induced the development of the restraining bend of the swan islands the formation of a 1 km high scarp on the eastern trace of the cayman trough transform fault walton fault and the formation of a pull apart basin hendrix pull apart magnetic anomalies and magnetization maps give information about the deformation and the rocks the proposed evolutionary model of the cayman trough from the inception of seafloor spreading to the present configuration is presented in relation to the tectonic escape of the northern boundary of the caribbean plate from the maastrichtian to the present

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

a full coverage high resolution topographic model of greenland computed from a variety of digital elevation data in this paper a new high resolution elevation model of greenland including the ice sheet as well as the ice free regions is presented it is the first published full coverage model computed with an average resolution of 2 km and providing an unprecedented degree of detail the topography is modeled from a wide selection of data sources including satellite radar altimetry from geosat and ers 1 airborne radar altimetry and airborne laser altimetry over the ice sheet and photogrammetric and manual map scannings in the ice free region the ice sheet model accuracy is evaluated by omitting airborne laser data from the analysis and treating them as ground truth observations the mean accuracy of the ice sheet elevations is estimated to be 12 13 m and it is found that on surfaces of a slope between 0 2 and 0 8 corresponding to approximately 50 of the ice sheet the model presents a 40 improvement over models based on satellite altimetry alone on coastal bedrock the model is compared with stereo triangulated reference points and it is found that the model accuracy is of the order of 25 35 m in areas covered by stereo photogrammetry scannings and between 200 and 250 m elsewhere

paleogene continental margin truncation in southwestern mexico geochronological evidence the reasons for and mechanisms of continental margin truncation in sw mexico where mesozoic cenozoic plutons are situated directly on the pacific coast are not yet well understood large scale dextral and or sinistral displacements of the continental margin terranes now forming parts of baja california or the chortis block have been proposed the well defined along coast nw se decreasing granitoid intrusion age trend 1 2 cm yr in the 100 ma 40 ma time interval between puerto vallarta and zihuatanejo is interpreted by us to be a geometric artifact of oblique continental margin truncation rather than the consequence of a sinistral offset of the chortis block from those latitudes toward the se changes in the dip and velocity of the nnw sse trending cretaceous tertiary subduction zone resulted in a landward migration of the magmatic arc taking into account certain stratigraphic affinities of chortis and the oaxaca and mixteca terranes together with the known displacement rates along the north america caribbean plate boundary the northwesternmost paleoposition of the chortis block with respect to sw mexico was near zihuatanejo in contrast between zihuatanejo and the isthmus of tehuantepec the cessation of the tertiary magmatism decreased more rapidly 7 7 cm yr although the trend is not so obvious starting in the late eocene chortis moved about 1100 km to the se along a transform boundary associated with the opening of the cayman trough based on our geochronological data and structural relationships between mylonite zones and plutons in the acapulco tehuantepec area we propose an approximately 650 km se movement of chortis from about 40 25 ma with a velocity of 6 5 4 3 cm yr since this is considerably slower than the decreasing age trend obtained by us using the geochronological data we consider batholith formation in this segment to predate and postdate the offshore passage of the north america farallon caribbean triple junction geological observations and paleomagnetic data do not give strong support for large scale right lateral displacements of crustal blocks like the baja california given the isotopic data presented the continental margin truncation in sw mexico seems to be the consequence of an interaction of mechanisms of these we regard tectonic erosion associated with the subduction process to be the most important in the northwestern segment on the other hand the lateral removal of material associated with the displacement of chortis is more important in the southeastern segment

over winter persistence of supraglacial lakes on the greenland ice sheet results and insights from a new model we present a newly developed 1 d numerical energy balance and phase transition supraglacial lake model glacierlake glacierlake incorporates snowfall in situ snow and ice melt incoming water from the surrounding catchment ice lid formation basal freeze up and thermal stratification snow cover and temperature are varied to test lake development through winter and the maximum lid thickness is recorded average wintertime temperatures of 2 to 30 circ rm c and total snowfall of 0 to 3 45 m lead to a range of the maximum lid thickness from 1 2 to 2 8 m after sim 250 days with snow cover exerting the dominant control an initial ice temperature of 15 circ rm c with simulated advection of cold ice from upstream results in 0 6 m of basal freeze up this suggests that lakes with water depths above 1 3 to 3 4 m dependent on winter snowfall and temperature upon lid formation will persist through winter these buried lakes can provide a sizeable water store at the start of the melt season expedite future lake formation and warm underlying ice even in winter

collapse basin a new type of extensional sedimentary basin from the devonian of norway pure shear and simple shear models provide interpretations of sedimentary basins formed by extension of crust and lithosphere of normal thickness devonian basins of western norway developed during collapse of the thickened caledonian crust and return to a crust and lithosphere of normal thickness because extension did not thin the lithosphere to less than its normal thickness no thermal subsidence phase followed the initial subsidence

a model study for estimating optimum upward continuation height for gravity separation with application to a bouguer gravity anomaly over a mineral deposit jilin province northeast china upward continuation can be used to separate a regional gravity anomaly resulting from deep sources from the observed gravity we present a practical method based on model studies to derive an optimum upward continuation height for regional residual gravity separation using this method we can calculate an optimal height for upward continuation although mathematically there is no optimum height this method provides an objective procedure to calculate a best height for upward continuation we initially use a 2d model to calculate an optimum separation height as given by the maximum crosscorrelation between the upward continuation of the observed gravity and a known regional anomaly for an unknown regional field we calculate a series of crosscorrelations between the upward continuations at two successive heights the average height of the maximum deflection of these crosscorrelation values yields the optimum height for regional residual separation the method was applied to the bouguer gravity anomaly over a mineral deposit in the jilin province in northeast china when we subtract the estimated regional anomaly obtained in this manner from the bouguer anomaly we can obtain a residual anomaly that clearly shows the location of two known iron bodies

actively evolving microplate formation by oblique collision and sideways motion along strike slip faults an example from the northeastern caribbean plate margin abstract the pattern of folding faulting and late quaternary coral reef uplift rates in western and central hispaniola haiti and dominican republic suggest that the elongate gonave microplate a 190 000 km2 area of the northeastern caribbean plate is in the process of shearing off the caribbean plate and accreting to the north american plate late cenozoic transpression between the southeastern bahama platform and the caribbean plate in hispaniola has inhibited the eastward motion of the northeastern corner of the plate transpression is manifested in western and central hispaniola by the formation of regional scale folds that correspond to present day anticlinal topographic mountain chains continuous with offshore anticlinal ridges areas of most rapid quaternary uplift determined from onland coral reefs 125 ka and younger coincide with the axial traces of these folds offshore data suggest recent folding and faulting of the seafloor onshore reef data do not conclusively require late quaternary folding but demonstrate that tectonic uplift rates of the axial areas of the anticlines decrease from the northwest peninsula of haiti 0 37 mm yr to to the central part of the coast of western haiti 0 19 mm yr to the south central part of western haiti 0 mm yr formation of the 1200 km long enriquillo plantain garden walton fault zone as a bypass strike slip fault has isolated the southern edge of the gonave microplate and is allowing continued unimpeded eastward motion of a smaller caribbean plate past the zone of late neogene convergence and quaternary uplift of coral reefs in hispaniola offshore seismic reflection data from the jamaica passage the marine strait separating jamaica and haiti show that the enriquillo plantain garden fault zone forms a narrow but deep active fault bounded trough beneath the passage the active fault is continuous with active faults mapped onshore in western haiti and eastern jamaica the bathymetric deep is present because the jamaica passage fault segment represents a 50 km wide transtensional left step of the fault trace between haiti and jamaica onshore satellite imagery and field observations suggest that the enriquillo plantain garden fault forms a continuous trace extending from central hispaniola east of lake enriquillo dominican republic to the westernmost end of the southern peninsula of haiti the regional lineament corresponds to a recent fault scarp in quaternary alluvium of the clonard pull apart basin in the central part of the southern peninsula of haiti and suggests that at least this part of the lineament has undergone recent slip calmus 1983 has suggested a total offset of 30 50 km of the enriquillo plantain garden fault zone in the southern peninsula of haiti using the apparent offset of lithologic units seismic reflection data from lake enriquillo document recent deformation of quaternary lake sediments where the lineament crosses the lake leveling of the crest of a late holocene coral reef and associated algal tufa around lake enriquillo demonstrate late holocene vertical movement and tilting in a 1500 m wide zone parallel to the fault trend lateral offset is difficult to show in the enriquillo valley area because of rapid recent sedimentation into the valley the pattern of inactive strike slip faults and fold belts of cenozoic age in cuba and the yucatan basin suggest that two elongate microplates were sheared off the proto caribbean plate and accreted to the north american plate by a similar process in paleocene and eocene times age of terminal deformation in western central and eastern cuba is consistent with southeastward younging and migration of arc collision the similarity of the size and sequence of events in cuba suggests that the process of oblique collision and sideways motion of the plate along a new strike slip fault towards a free face may be an important process of microplate formation and interplate transfer in other areas

extension and rotation of crustal blocks in northern central america and effect on the volcanic arc sinistral displacements across the north american caribbean plate boundary in northern central america are distributed among major arcuate faults that have been active in neogene time south of the jocotan and motagua faults in guatemala extensional tectonics has accompanied rotation of the trailing edge of the caribbean plate around these faults segmentation of the volcanic arc in northern central america hitherto attributed to transverse breaks in the subducting cocos plate may instead be a result of this block rotation accompanying changes along the arc are observed in seismicity gravity anomaly patterns volume and composition of volcanic products and topography therefore the complex volcano tectonic geology south of the main boundary faults may be explained by interaction and rotation of crustal blocks in the overriding caribbean plate above the magma production zone along the downgoing cocos slab

the nature of the crust under cayman trough from gravity abstract considerable crustal thickness variations are inferred along cayman trough a slow spreading ocean basin in the caribbean sea from modeling of the gravity field the crust to a distance of 50 km from the spreading center is only 2 3 km thick in agreement with dredge and dive results crustal thickness increases to 5 5 km at distances between 100 and 430 km west of the spreading center and to 3 5 6 km at distances between 60 and 370 km east of the spreading center the increase in thickness is interpreted to represent serpentinization of the uppermost mantle lithosphere rather than a true increase in the volume of accreted ocean crust serpentinized peridotite rocks have indeed been dredged from the base of escarpments of oceanic crust rocks in cayman trough laboratory measured density and p wave speed of peridotite with 40 50 serpentine are similar to the observed speed in published refraction results and to the inferred density from the model crustal thickness gradually increases to 7 8 km at the far ends of the trough partially in areas where sea floor magnetic anomalies were identified basement depth becomes gradually shallower starting 250 km west of the rise and 340 km east of the rise in contrast to the predicted trend of increasing depth to basement from cooling models of the oceanic lithosphere the gradual increase in apparent crustal thickness and the shallowing trend of basement depth are interpreted to indicate that the deep distal parts of cayman trough are underlain by highly attenuated crust not by a continuously accreted oceanic crust

guayape papalutla fault system a continuous cretaceous structure from southern mexico to the chortis block tectonic implications recent papers have opened the debate over whether the chortis block was located off the coast of southern mexico or in a more outboard position and this led me to explore whether correlations with older structures could be established to determine the evolution of the southwest corner of the north america plate in this paper i hypothesize that the papalutla fault of mexico and the guayape fault system of honduras both considered to be terrane boundaries were roughly continuous in the cretaceous extending from southern mexico to the chortis block they influenced early cretaceous clastic sedimentation of the zicapa formation on the guerrero morelos platform southern mexico and the tepemechin formation of the central chortis terrane honduras the units accumulated in fault bounded basins associated with left lateral slip the basins developed in the central chortis terrane and guerrero morelos platform both geological entities appear to represent a continuous cretaceous geologic province characterized by a trangression that occurred earlier in honduras than in southern mexico this nonrotational hypothesis for the location of the chortis block adjacent to southern mexico during the cretaceous is consistent with northeastward displacement of the caribbean plate during cretaceous time and implies 1500 km of chortis block displacement during the cenozoic

precipitation over greenland retrieved by a dynamic method and its relation to cyclonic activity abstract in order to calculate the vertical motion over some high mountain regions such as greenland an equation without the quasigeostrophic approximation in coordinates has been developed a dynamic method for retrieving precipitation over greenland is based on this equation the retrieved annual mean precipitation distribution for 1987 and 1988 is in very good agreement with the observed annual accumulation pattern over the greenland ice sheet the major weather system producing precipitation over greenland is the frontal cyclone based on the precipitation characteristics greenland can be divided into five subregions precipitation over the north coastal and central interior regions primarily occurs in summer for the three other subregions if the composite monthly mean sea level pressure charts for high and low monthly precipitation amounts are constructed a clear relationship between precipitation and cyclonic activity emerges if a mean cyclone exists in the labrador sea heavy precipita

seismic stratigraphy and structure of the caribbean igneous province multichannel seismic reflection profiles reveal the tectonic and stratigraphic complexity of the caribbean province a smooth reflector b forms the top of widespread cretaceous volcanic flows drilled during dsdp leg 15 and odp leg 165 in addition we also show other seismic evidence of volcanism sills and dipping horizons the southern parts of the venezuela and colombia basins are underlain by rough basement that shows many characteristics of oceanic crust several thick volcanic plateaus separated by deep basins with thin crust comprise the caribbean volcanic province in the venezuela basin only normal oceanic crust is present but in other basins this crust is underlain by underplated volcanic material the crust of the venezuela basin is less than 5 km thick and was probably formed in the pacific during the jurassic refraction data indicate that a thin layer 2v is formed by original oceanic crust overlain by a thin volcanic layer an intra basement reflection sub b marks the top of original oceanic crust that is sandwiched between an upper volcanic layer and lower underplated material this underplated layer forms a very thick layer 3v beneath the beata and nicaragua volcanic plateaus the upper part of layer 3v is gabbroic and outcrops on the beata ridge the thickening occurs mainly in the high velocity lower part of layer 3v and is attributed to the presence of magnesian rocks picrites or ultramafic cumulates a highly reflective horizon r is located at the top of this layer we present a model where the galapagos plume is 2500 km wide but has narrow heads that formed the caribbean volcanic plateaus separated by deep basins the caribbean igneous province is compared to similar volcanic features in the indian ocean kerguelen plateau and in the western pacific ocean ontong java plateau nauru and pigafetta basins the thick plateaus of the caribbean province resemble the kerguelen and ontong java plateaus whereas the deep basins present some similarities with the nauru and pigafetta basins the caribbean igneous province is younger 90 75 ma than the indian and pacific plateaus 110 130 ma however several geological evidences from land and geophysical offshore observations indicate that the onset of volcanic activity in the caribbean province could be as old as 115 ma

seamarc ii mapping of transform faults in the cayman trough caribbean sea seamarc ii maps of the southern wall of the cayman trough between honduras and jamaica show zones of continuous well defined fault lineaments adjacent and parallel to the wall both to the east and west of the cayman spreading axis these lineaments mark the present active traces of transform faults which intersect the southern end of the spreading axis at a triple junction the swan islands transform fault to the west is dominated by two major lineaments that overlap with right stepping sense across a large push up ridge beneath the swan islands the fault zone to the east of the axis named the walton fault is more complex containing multiple fault strands and a large pull apart structure the walton fault links the spreading axis to jamaican and hispaniolan strike slip faults and it defines the southern boundary of a microplate composed of the eastern cayman trough and western hispaniola the presence of this microplate raises questions about the veracity of caribbean plate velocities based primarily on cayman trough opening rates

tectonic evolution and mantle structure of the caribbean we investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic p wave velocity structure under the caribbean region in the upper mantle positive seismic anomalies are imaged under the lesser antilles and puerto rico these anomalies are interpreted as remnants of atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the lesser antilles island arc during the past 45 myr the imaged lesser antilles slab consists of a northern and southern anomaly separated by a low velocity anomaly across most of the upper mantle which we interpret as the subducted north america south america plate boundary the southern edge of the imaged lesser antilles slab agrees with vertical tearing of south america lithosphere the northern lesser antilles slab is continuous with the puerto rico slab along the northeastern plate boundary this results in an amphitheater shaped slab and it is interpreted as westward subducting north america lithosphere that remained attached to the surface along the northeastern boundary of the caribbean plate at the muertos trough however material is imaged until a depth of only 100 km suggesting a small amount of subduction the location and length of the imaged south caribbean slab agrees with proposed subduction of caribbean lithosphere under the northern south america plate an anomaly related to proposed oligocene subduction at the nicaragua rise is absent in the tomographic model beneath panama a subduction window exists across the upper mantle which is related to the cessation of subduction of the nazca plate under panama since 9 5 ma and possibly the preceding subduction of the extinct cocos nazca spreading center in the lower mantle two large anomaly patterns are imaged the westernmost anomaly agrees with the subduction of farallon lithosphere the second lower mantle anomaly is found east of the farallon anomaly and is interpreted as a remnant of the late mesozoic subduction of north and south america oceanic lithosphere at the greater antilles aves ridge and leeward antilles the imaged mantle structure does not allow us to discriminate between an intra americas origin and a pacific origin of the caribbean plate key points tectonic reconstructions for the caribbean validated by seismic tomography cenozoic lesser antilles and mesozoic greater antilles subduction confirmed tomography gives insight into geodynamics of lesser antilles

crustal deformation in northern central america abstract evaluation of the seismic moment tensor for earthquakes on plate boundary is a standard procedure to determine the relative velocity of plates which controls the seismic deformation rate predicted from the slip on a single fault the moment tensor is also decomposed into an isotropic and a deviatoric part to discover the relationship between the average strain rate and the relative velocity between two plates we utilize this procedure to estimate the rates of deformation in northern central america where plate boundaries are seismically well defined four different tectonic environments are considered for modelling of the plate motions the deformation rates obtained here compare well with those predicted from the plate motions models and are in good agreement with actual observations deformation rates on faults are increasingly being used to estimate earthquake recurrence from information on fault slip rate and more on how we can incorporate our current understanding into seismic hazard analyses

modeled variations of precipitation over the greenland ice sheet abstract a parameterization of the synoptic activity at 500 hpa and a simple orographic scheme are used to model the spatial and temporal variations of precipitation over the greenland ice sheet for 1963 88 from analyzed geopotential height fields produced by the national meteorological center nmc model coefficients are fitted to observed accumulation data primarily from the summit area of the ice sheet all major spatial characteristics of the observed accumulation distribution are reproduced apart from the orographic accumulation maximum over the northwestern coastal slopes the modeled time averaged total precipitation amount over greenland is within the range of values determined by other investigators from surface based observations a realistic degree of interannual variability in precipitation is also simulated a downward trend in simulated ice sheet precipitation over the 26 years is found this is supported by a number of lines of evidence it matches the accumulation trends during this peri

multiyear accumulation and temperature history near the north greenland ice core project site north central greenland this paper presents a comparison of two independent methods of estimating subseasonal accumulation across the interior of greenland these methods high resolution snow pit studies and atmospheric modeling have differing spatial and temporal resolution but both can estimate net accumulation for subseasonal and shorter periods the snow pit approach is based on a documented relationship between high resolution snow pit profiles of oxygen stable isotope ratio 18o and multiyear special sensor microwave imager ssm i 37 ghz brightness temperature records comparison of ssm i data to profiles obtained during the 1995 alfred wegener institut north greenland traverse field season shows that 18o data from snow in north central greenland are a reliable high resolution temperature proxy this enables determination of accumulation amount rate and timing from approximately july 1991 through june 1995 across this 220 km long transect of the ice sheet precipitation estimates derived from early modeling based on european centre for medium range weather forecasts data show a similar average seasonal pattern but a diminished magnitude of accumulation 56 for these sites the slope of the multiyear t versus correlation was evaluated for each site on the basis of the observed and calculated temperature history from the nearby north greenland ice core project ngrip site automatic weather station these data should assist interpretation of the paleoclimatic record in the ngrip deep core

greenland ice sheet surface mass balance variability 1991 2003 the polar mm5 mesoscale atmospheric model was run for 13 years 1991 2003 over greenland at 24 km horizontal resolution box and others 2004 the model physics were driven by satellite station and weather balloon observational data assimilation i e european centre for medium range weather forecasts ecmwf operational analysis the analysis in this study focuses on the response of the surface mass balance to its primary controls temperature and precipitation the results indicate coherent spatial patterns of variability and statistically significant links with temperature and precipitation and the north atlantic oscillation precipitation trends have the same spatial pattern and sign as temperature suggesting an association of precipitation and temperature variability increasing temperatures contribute to an increasing ablation trend and expansion of the ablation zone despite increasing accumulation trends the pinatubo philippines volcanic cooling in the early 1990s enhances this apparent warming trend only in the northeast does precipitation appear to dominate the surface mass balance where both temperature and precipitation have decreased there is little evidence for a total ice sheet surface mass balance trend although the meltwater runoff has a positive trend and combined with iceberg discharge and basal melting estimates suggests the ice sheet as a whole is in a state of net mass loss over this period

rapid thinning of parts of the southern greenland ice sheet aircraft laser altimeter surveys over southern greenland in 1993 and 1998 show three areas of thickening by more than 10 centimeters per year in the southern part of the region and large areas of thinning particularly in the east above 2000 meters elevation the ice sheet is in balance but thinning predominates at lower elevations with rates exceeding 1 meter per year on east coast outlet glaciers these high thinning rates occur at different latitudes and at elevations up to 1500 meters which suggests that they are caused by increased rates of creep thinning rather than by excessive melting taken as a whole the surveyed region is in negative balance

mesozoic stratigraphy of central honduras geologic mapping near lake yojoa in central honduras has shown the need to modify the published mesozoic stratigraphic column the ilama formation is not a useful unit for regional mapping inasmuch as ilama type strata are discontinuous and in various stratigraphic positions the formal name ilama formation should be abandoned a new formal name the jaitique formation is introduced to designate a major cenomanian limestone unit that is stratigraphically about midway in the valle de angeles group in the santa barbara lake yojoa region the jaitique formation consists of up to 140 m of shallow marine strata that can be divided into two members 1 a lower member of thick bedded shelf carbonate strata and 2 a distinctive upper member of thin bedded laminated nonfossiliferous limestone which may represent a shallow possibly hypersaline lagoon or intertidal mud flat environment because the lower member of this cenomanian limestone was formerly misidentified as the atima formation of the yojoa group the upper thin bedded member was originally defined as the guare formation of the yojoa group the name guare is retained but the unit is downgraded to the guare member of the newly defined jaitique formation within the valle de angeles group although the jaitique formation is similar in stratigraphic position and age to the esquias formation in the minas de oro area on the east it is lithologically distinct and probably slightly older the two units are not thought to be lithostratigraphically contiguous recognition of major cenomanian limestone units within the valle de angeles group has led to reappraisal of the hydrocarbon potential of certain structural zones in honduras

age and spreading history of the cayman trough as determined from depth heat flow and magnetic anomalies the opening of the cayman trough records aspects of relative motion between the north american and caribbean plates essential to accurate reconstruction of caribbean evolution assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates depths to basement show that the crust has subsided away from the present axis in two stages a younger stage characterized by substantial and asymmetric greater to the east subsidence preceded by an older stage of slight increases of depth with distance depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm yr since 25 30 ma and at 20 30 mm yr prior to 30 ma marine magnetic anomalies show lineations perpendicular to the trough including a distinct central anomaly but the pattern is neither well defined nor symmetrical about the present axis however partial anomaly sequences can be identified on both sides of the axis and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps mostly eastward the overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm yr since and prior to 26 ma both subsidence and magnetics indicate that the trough opened by at least 45 50 ma

asiras airborne radar resolves internal annual layers in the dry snow zone of greenland 1 we present data from the european space agency s airborne sar interferometric radar altimeter system asiras flown during the cryovex 2004 field calibration validation campaign and new high resolution depth profiles of snow density measured in the field by neutron scattering we combine these data to calculate the depth of internal reflecting horizons in the asiras data the high resolution density data allow us to identify annual layers in the snow density profile and correlate their peaks with the reflecting horizons we use the thickness of the annual layers combined with the density profile to determine the spatial and temporal pattern of snow accumulation along the radar track for a period of 6 years from 1995 2002 our mean annual accumulation rate is 0 47 0 09 ma 1 water equivalent in agreement with the value determined from a core taken in this location in 1992 similarly our inter annual variability shows the same trends as recent model estimates over the entire ice sheet because asiras was designed to mimic as closely as possible the characteristics of the sar interferometric radar altimeter siral the principal payload of cryosat the detection of internal layering with asiras illuminates the possibility of detecting internal layers with a space borne radar and thus the possible application of this technique to the dry snow zones of antarctica greenland and smaller ice bodies

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

neogene development of the swan islands restraining bend complex caribbean sea we present seamarc ii and multichannel seismic data describing deformation at an active restraining bend complex formed along a left lateral strike slip fault in the western caribbean sea the bend complex consists of three distinct strike slip fault traces that form two right stepping sharp restraining bends or push ups at its western end and form a right stepping gentle restraining bend at its eastern end deformation at the sharp restraining bends is characterized by local folding between overlapping strike slip faults deformation at the much larger gentle restraining bend consists of a large anticline adjacent to a gently curving strike slip fault a small active accretionary wedge forms the northern flank of the bend complex we speculate that the 30 km wide right stepping bend complex formed as a consequence of southward propagation of the mid cayman spreading center during oligocene to early miocene time active faults along the southern edge of the bend complex may have formed after the initial uplift and folding of the complex and may currently serve to straighten the strike slip fault by bypassing the bend complex

tectonic interpretation of the morphology of honduras the morphotectonics of honduras have been studied using existing geologic and topographic maps and imaging done from satellites and the following interpretations have been made during the late oligocene to early miocene a wide strike slip boundary existed in western honduras and eastern guatemala and produced east west trending basins containing the coarse clastic rocks of the subinal formation marine geophysical studies of the continental shelf of honduras show that during the rest of the miocene and the early pliocene the boundary was inactive but motion resumed in the late pliocene to pleistocene and produced horsts on the north coast of honduras in northwestern honduras the east northeast trending alignment of the jocotan copan and charnelecon river valleys may result from crustal flexuring rather than from faulting as previously proposed because the postulated fault the jocotan chamelecon fault cannot be reconciled with available geologic maps the northwest and northeast flowing rivers of central and eastern honduras define domains of strike slip faulting that have probably rotated 30 from an original set of conjugate fractures widespread deposition of conglomerates suggests that strike slip movements began in the mid cretaceous and reoccurred at intervals culminating in the development of pull apart basins in western honduras during the late pliocene at the present time no further strike slip movement is possible as the faults defining each domain lie parallel and perpendicular to the middle america trench new conjugate fractures however cut honduras along north south and east northeast trends and have also separated into domains in the domain of north south fractures easterly directed ductile extension of the lithosphere beneath western honduras and guatemala is opening up rifts but the domain of east northeast fractures is characterized by strike slip motion that has produced left lateral pull aparts in central honduras early workers identified no rift valley extending from the gulf of fonseca to the caribbean and there is no fresh evidence upon which to base the existence of one today the notion of the honduras depression which has persisted over the last half century stems from misinterpretation of the maps and writings of karl sapper

modeled precipitation variability over the greenland ice sheet on the basis of the evaluation of recent greenland precipitation studies some of the deficiencies in the modeled precipitation are probably related to the topographic data employed in modeling in this paper the modern digital elevation data of ekholm 1996 is used if the horizontal pressure gradient force in coordinates is separated into its irrotational and rotational parts which are expressed by the equivalent geopotential and geo stream function respectively the topographic effect on the precipitation can be accurately modeled the equivalent geopotential and geo stream function are implemented in a fully consistent manner in the generalized equation in this paper a simplified large scale condensation without evaporation of condensate is also used these improvements are combined to yield an improved dynamic method two aspects of the precipitation distribution are refined by the improved dynamic method one is the 10 cm yr 1 contour near summit greenland and the other is a relative large precipitation area centered near the point 70 n 49 w extensive comparisons are made between the retrieved precipitation and the observed annual accumulation time series from 11 ice core sites on the ice sheet the modeled precipitation from the original method must use sealers to have a high degree of interannual correspondence between the measured accumulation and the retrieved precipitation but the retrieved precipitation from the improved method increases at all ice core sites and a good correspondence is obtained without any sealer being required the spatial average of multiyear mean error e j is 11 5 cm yr 1 for the modeled precipitation from the improved method while that for p from era 15 is 14 5 cm yr 1 the total mean error em is 3 0 cm yr 1 for the improved method while em for the p from era 15 is 4 0 cm yr 1 these two errors show that the precipitation modeled by the improved method is better than the p from era 15 thus the distribution of precipitation over the 11 sites retrieved by the improved dynamic method is considerably refined large downward trends in annual amounts are present in the precipitation retrieved by the improved dynamic method for all of greenland and its southern and central west coastal regions the modeled precipitation from the improved dynamic method and observed accumulation from ice cores are all in agreement with the thomas et al 1999 result that the southern greenland ice sheet above 2000 m is approximately in balance it also shows that local thickening and thinning areas of the ice sheet derived by airborne laser altimetry from 1993 to 1999 over the entire greenland above 2000 m krabill et al 2000 are approximately consistent with precipitation change retrieved by the improved dynamic method

greenland ice sheet high elevation balance and peripheral thinning aircraft laser altimeter surveys over northern greenland in 1994 and 1999 have been coupled with previously reported data from southern greenland to analyze the recent mass balance of the greenland ice sheet above 2000 meters elevation the ice sheet is in balance on average but has some regions of local thickening or thinning thinning predominates at lower elevations with rates exceeding 1 meter per year close to the coast interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet sufficient to raise sea level by 0 13 millimeter per year approximately 7 of the observed rise

changes in greenland ice sheet elevation attributed primarily to snow accumulation variability the response of grounded ice sheets to a changing climate critically influences possible future changes in sea level recent satellite surveys over southern greenland show little overall elevation change at higher elevations but large spatial variability using satellite studies alone it is not possible to determine the geophysical processes responsible for the observed elevation changes and to decide if recent rates of change exceed the natural variability here we derive changes in ice sheet elevation in southern greenland for the years 1978 88 using a physically based model of firn densification and records of annual snow accumulation reconstructed from 12 ice cores at high elevation our patterns of accumulation driven elevation change agree closely with contemporaneous satellite measurements of ice sheet elevation change and we therefore attribute the changes observed in 1978 88 to variability in snow accumulation similar analyses of longer ice core records show that in this decade the greenland ice sheet exhibited typical variability at high elevations well within the long term natural variability our results indicate that a better understanding of ice sheet mass changes will require long term measurements of both surface elevation and snow accumulation

modeled antarctic precipitation part ii enso modulation over west antarctica the impacts of the el nino southern oscillation enso on the antarctic region are of special importance in evaluating the variability and change of the climate system in high southern latitudes in this study the enso signal in modeled precipitation over west antarctica since 1979 is evaluated using forecast precipitation from several meteorological analyses and reanalyses additionally a dynamical retrieval method drm for precip itation is applied over the last two decades the southern oscillation index soi has an overall anticorrelation with precipitation over the west antarctic sector bounded by 758 908s 1208w 1808 while it is positively correlated with precipitation over the south atlantic sector bounded by 658 758s 308 608w decadal variations are found as the relationship between the soi and west antarctic precipitation is stronger in the 1990s than that in the 1980s the polar front jet stream west antarctic precipitation and the soi show a well ordered correspondence during the 1990s as the jet zonal speed is negatively correlated to the soi and positively correlated to west antarctic precipitation these relationships are weaker during the 1980s consistent with the change in sign of the correlation between the soi and west antarctic precipitation the decadal variations are apparently related to changes in the quasi stationary eddies that determine the local onshore and offshore flow over west antarctica

snowmelt on the greenland ice sheet as derived from passive microwave satellite data abstract the melt extent of the snow on the greenland ice sheet is of considerable importance to the ice sheet s mass and energy balance as well as arctic and global climates by comparing passive microwave satellite data to field observations variations in melt extent have been detected by establishing melt thresholds in the cross polarized gradient ratio xpgr the xpgr defined as the normalized difference between the 19 ghz horizontal channel and the 37 ghz vertical channel of the special sensor microwave imager ssm i exploits the different effects of snow wetness on different frequencies and polarizations and establishes a distinct melt signal using this xpgr melt signal seasonal and interannual variations in snowmelt extent of the ice sheet are studied the melt is found to be most extensive on the western side of the ice sheet and peaks in late july moreover there is a notable increasing trend in melt area between the years 1979 and 1991 of 4 4 per year which came to an abrupt halt in 1

emag2 a 2 arc min resolution earth magnetic anomaly grid compiled from satellite airborne and marine magnetic measurements 1 a global earth magnetic anomaly grid emag2 has been compiled from satellite ship and airborne magnetic measurements emag2 is a significant update of our previous candidate grid for the world digital magnetic anomaly map the resolution has been improved from 3 arc min to 2 arc min and the altitude has been reduced from 5 km to 4 km above the geoid additional grid and track line data have been included both over land and the oceans wherever available the original shipborne and airborne data were used instead of precompiled oceanic magnetic grids interpolation between sparse track lines in the oceans was improved by directional gridding and extrapolation based on an oceanic crustal age model the longest wavelengths 330 km were replaced with the latest champ satellite magnetic field model mf6 emag2 is available at http geomag org models emag2 and for permanent archive at http earthref org cgi bin er cgi s erda cgi n 970

origin and provenance of basement metasedimentary rocks from the xolapa complex new constraints on the chortis southern mexico connection abstract the u pb la mc icpms geochronology of xolapa metasedimentary rocks from tierra colorada guerrero to puerto angel oaxaca in southern mexico reveals that their protoliths accumulated in two distinctive cycles of sedimentation one of early jurassic age and another of late cretaceous age these ages are younger than thought and demonstrate that xolapa metasedimentary rocks are not rocks from the acatlan or oaxacan complexes or their paleozoic sedimentary covers as claimed however detrital zircon ages indicate that xolapa sediments received contemporaneous detritus most likely from these assemblages suggesting a probably para autochthonous origin for the xolapa terrane xolapa rocks record two major tectonothermal events of 64 59 ma and 34 ma the first event produced the high grade metamorphism and widespread migmatization that characterize xolapa and the second event is likely related to extended heating produced by coeval arc plutonism pre jurassic assemblages of the chortis block of central america contain zircon populations that significantly coincide with those recorded in both the acatlan and oaxacan complexes and their paleozoic sedimentary covers as well as with those recorded in xolapa metasediments which suggests a spatial connection among these petrotectonic assemblages during much of the mesozoic it is proposed that xolapa was generated in a basin floored by permian rocks flanked on one side by southern mexico terranes and on the other side by the chortis block contraction of the basin tied to the approach and accretion of the guerrero terrane arc assemblages during late cretaceous time produced crustal thickening and high grade metamorphism and migmatization at mid crustal levels diachronic exhumation of xolapa began during early paleogene time very likely promoted by the detachment and migration of the chortis block the migration would additionally produce slicing of xolapa assemblages generating its elongated and juxtaposed structure and the margin truncation that characterizes southern mexico

the padre miguel ignimbrite suite central honduras paleomagnetism geochronology and tectonic implications article ar 39 ar geochronology the padre miguel group in western honduras is a silicic volcanic sequence that forms part of the central american miocene volcanic arc built on the chortis continental fragment we report new 40 ar 39 ar data of 16 1 0 2 ma and 14 42 0 08 ma for the padre miguel group and present paleomagnetic data for 36 cooling units mainly ignimbrites from localities north and northeast of the capital city of tegucigalpa these rocks are characterized by univectorial characteristic magnetization carried mainly by low ti titanomagnetites or two component magne tizations with a minor secondary overprint dual polarity magnetizations suggest that it is a primary thermo remanence and the obtained data scatter can be straightforwardly explained by paleosecular variation the overall mean of 33 selected sites 14 39 n 87 14 w defines a paleomagnetic pole at lat 80 0 n lon 142 9 e k 24 5 a95 5 2 the overall mean compared with the north america expected direction indicates sta tistically insignificant rotation and inclination anomaly r 4 1 6 1 and an f 8 6 11 3 restoring 255 km of sinistral slip in the polochic motagua fault system as suggested by cayman trough reconstructions since 15 ma bringsthemid miocene arcs ofsouthernmexico oaxaca andthecentralamericaignimbrite province into alignment this is consistent with a derivation of the chortis block from southern mexico our paleomagnetic and geochronological studies hence support models that explain the offset of the miocene ignimbrite belt of central america by post middle miocene displacement of the chortis block relative to north america

late cretaceous cenozoic tectonic transition from collision to transtension honduran borderlands and nicaraguan rise nw caribbean plate boundary

interannual variations of snow accumulation on the greenland ice sheet 1985 1996 new observations versus model predictions newly measured time series of net water equivalent accumulation from ice cores are reported for 11 sites located near the 2000 m contour of the greenland ice sheet many of these sites are located in regions where accumulation has not been previously measured we compared these new time series of annual accumulation with previously reported modeled precipitation covering the same time range because ice core accumulation records include both regional and local accumulation components we used adjacent ice core records at six multiple core sites to estimate the local component and extrapolated these results to the five single core sites we investigated the impact of this short scale spatial variability in accumulation on statistical comparisons between the observed accumulation and model precipitation although sealers were required to match the 12 year mean modeled precipitation to the mean observed accumulation a high degree of correspondence between observed interannual accumulation and scaled modeled precipitation was found at 7 of 11 locations the need for scaling and the low correspondence between model simulations and observations at some sites clearly indicate that model improvements are needed although very encouraging a more spatially distributed array of ice core accumulation measurements spanning the same time period as the precipitation modeling is necessary for continued model improvement and validation in agreement with earlier studies the large temporal variability in snow accumulation predicted by modeling and confirmed by ice cores indicates that the ice sheet elevation varies by tens of centimeters from year to year simply because of changing accumulation

carbon cycle and sea water palaeotemperature evolution at the middle late jurassic transition eastern paris basin france abstract a very high resolution carbon and oxygen stable isotope analysis bulk carbonate of a biostratigraphically well constrained callovian oxfordian series is provided here for the first time the homogeneity of the clayey series and the weak diagenetic alteration allow the isotopic signal variations to be considered as primary in origin a prominent and brief negative excursion in the 13c curve 2 occurring at the start of the middle callovian jason zone obductum subzone and correlated regionally suggests a possible methane release the increasing 13c values thereafter up to the early oxfordian concomitant with a warming episode highlight the burial of carbon in organic rich layers which in return may have triggered a decrease in atmospheric pco2 at higher frequencies observed fluctuations of the 13c and 18o values are orbitally driven 405 kyr and 100 kyr eccentricity cycles and may correspond to the salinity and temperature variability recorded in sea water the 18o isotopic measurements from well preserved diagenetically screened belemnites and bivalves along the series compared to available data from tethyan domains agree with the scenario of a global cooling at the middle late jurassic transition the well dated 18o isotopic curve suggests that the onset of this cooling event occurred at the end of the coronatum zone middle callovian

a comparison of quasigeostrophic and nonquasigeostrophic vertical motions for a model simulated rapidly intensifying marine extratropical cyclone abstract large scale departures from quasigeostrophic vertical motions are diagnosed for a model simulation of the qe ii storm 9 11 september 1978 the simulation was performed by the limited area mesoscale prediction system lamps initialized at 1200 utc 9 september 1978 the model cyclone intensified from a central pressure of 1003 mb to 976 mb in 24 h considerably short of the 59 mb 24 h 1 observed deepening but reasonable in comparison to other model simulations of this storm this diagnosis centers on a hydrostatic generalized omega equation which scales to the quasigeostiophic omega equation for small rossby number vertical motions were computed both from this generalized omega equation and the quasigeostrophic omega equation in order to examine the importance of nonquasigeostrophic effects the high correlation of vertical motions from a control experiment using most of the terms in the generalized omega equation with the vertical motions predicted by the model establishes the validity o

evaluation of recent precipitation studies for greenland ice sheet the retrieval of an accurate spatial and temporal record of contemporary greenland precipitation is a uniquely challenging task because of the extreme variability in both atmospheric processes and the resulting precipitation distribution over relatively small spatial scales a comparison of precipitation data sets composed of monthly mean values from recent studies shows a convergence on the general features of the long term spatial patterns but substantial disagreement on the temporal variability both regionally and for all of greenland there is general agreement on a long term greenland average of about 35 cm yr 1 and on long term values for regional scales although differences for outlying data sets exceed 50 of the observed glaciological estimate for particular regions a fundamental problem is the inadequate topographic representation of greenland in the numerical analyses nearly all of the data sets are overly dry for high elevation areas as seen from comparisons with glaciological observations from summit the east central region of greenland is found to be particularly susceptible to the temporal discontinuities in data sets which employ operational analyses in contrast there is strong agreement among all methods on the temporal variability for the west central region over a 15 year period from the comparison it is concluded that none of the data sets is able to capture all of the regional scale features in general however the deficiencies of each data set are readily identifiable from comparison and evaluation in the context of circulation features agreement among the methods on particular regions and timescales gives increased confidence in drawing conclusions related to aspects of greenland s precipitation climatology in particular an enhanced precipitation retrieval method is found to be less susceptible to data artifacts than other methods using operational analyses in the north anomalously high precipitation is associated with cyclonic development near the fram strait for west central greenland the close agreement among methods is related to the dominant contribution of the mean circulation

tectonic evolution of the caribbean the development of the caribbean is discussed in terms of modern tectonic theory the nature of the site on which the caribbean formed is examined and the development of the rifted margins of the caribbean is described constraints on caribbean evolution from the relative motions of north and south america are briefly examined and the caribbean oceanic plateau is discussed the great island arc system of the caribbean is addressed in detail emphasizing the way the great arc of the caribbean was segmented the role of central america in caribbean history is briefly considered

precipitation over greenland and its relation to the north atlantic oscillation the equation method based on an equivalent isobaric geopotential height in coordinates has been used to retrieve the precipitation over greenland this approach is designed to accurately represent the topographic effects of the greenland ice sheet on atmospheric motion and precipitation the 11 year mean precipitation from 1985 to 1996 over all of greenland is 376 mm yr 1 which is close to the long term mean precipitation of 346 mm yr 1 estimated from glaciological data the precipitation over all of greenland shows that the largest value in 1986 is 472 mm yr 1 and the smallest value in 1995 is 309 mm yr 1 the major interannual variability of the atmospheric circulation in the north atlantic can be represented by the variation of the north atlantic oscillation nao index which is most pronounced during winter it is found that if the nao index increases the total precipitation over greenland decreases and vice versa the correlation coefficient between these two series for 1985 1995 is 0 75 the mean precipitation over southern greenland where the majority of precipitation falls is more closely related to the nao index in winter and their correlation coefficient is 0 80 this relationship can be understood from the composite maps of sea level pressure and greenland precipitation for the high and low index months during months of high nao index values the icelandic low is strong during months of low nao index values the monthly mean low is located to the southwest of greenland over the labrador sea precipitation amounts over the southeast coast of greenland are about 100 mm larger during the low nao index months than the high nao index months precipitation over all of greenland during the low nao index months is higher there are significant downward trends in annual precipitation from 1985 1995 for all of greenland and its southern and central west coastal regions amounting to about 3 per year

mass changes of the greenland and antarctic ice sheets and shelves and contributions to sea level rise 1992 2002 changes in ice mass are estimated from elevation changes derived from 10 5 years greenland and 9 years antarctica of satellite radar altimetry data from the european remote sensing satellites ers 1 and 2 for the first time the dh dt values are adjusted for changes in surface elevation resulting from temperature driven variations in the rate of firn compaction the greenland ice sheet is thinning at the margins 42 2g t a 1 below the equilibrium line altitude ela and growing inland 53 2g t a 1 above the ela with a small overall mass gain 11 3g t a 1 0 03 mm a 1 sle sea level equivalent the ice sheet in west antarctica wa is losing mass 47 4g t a 1 and the ice sheet in east antarctica ea shows a small mass gain 16 11 gt a 1 for a combined net change of 31 12 gt a 1 0 08 mm a 1 sle the contribution of the three ice sheets to sea level is 0 05 0 03 mm a 1 t he antarctic ice shelves show corresponding mass changes of 95 11 gt a 1 in wa and 142 10 gt a 1 in ea thinning at the margins of the greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate the marked thinnings in the pine island and thwaites glacier basins of wa and the totten glacier basin in ea are probably ice dynamic responses to long term climate change and perhaps past removal of their adjacent ice shelves the ice growth in the southern antarctic peninsula and parts of ea may be due to increasing precipitation during the last century

the middle devonian basins of western norway sedimentary response to large scale transtensional tectonics abstract the devonian basins of western norway were formed during late to post orogenic extension of overthickened caledonian crust the basins are situated in the hanging wall of the extensional nordfjord sogn detachment zone nsdz and display extensional half graben geometries in sections parallel to the local direction of principal extension based on overall facies configurations paleocurrent patterns and intrabasinal structures we infer an anticlockwise rotation of the syndepositional extension direction from nw se in the south solund basin to wsw ene in the north hornelen basin the axes of folds that are roughly parallel to the local extension direction are rotated correspondingly the kvamshesten basin is located between the solund and hornelen basins sedimentological and structural data show evidence of an early southeastwards tilt direction followed by a more eastwards tilt and associated e w flowing paleodrainage correspondingly nw se trending folds and reverse faults are superposed by e w trending ones at low to intermediate stratigraphic levels the variations in apparent tilt direction for the basins together with variations in intrabasinal structure is interpreted to reflect an anticlockwise rotation of the regional syndepositional strain field the above observations and inferences indicate that the devonian basins in western norway formed in a strain field dominated by regional transtension accommodated by extension along the nsdz and sinistral strike slip along orogen parallel shear zones and faults to the north of the basins alternatively nw directed extension preceded the introduction of a sinistral strike slip component the models are in accordance with recent work carried out in the footwall of the nsdz and illustrates the tectono sedimentary response to a complex interplay between extension and strike slip that appears to have been fundamental in the late stage disintegration of the caledonian orogen

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin

regional gravity investigation of honduras central america abstract a regional gravity study of honduras was performed as part of a major study of the geothermal resource potential of honduras this study was conducted by los alamos national laboratory in cooperation with the honduras government regional offshore free air and onshore bouguer gravity maps and residual isostatic gravity maps of honduras and surrounding regions were produced from these data several regional crustal and upper mantle models were produced these models pass through two local geothermal sites platanares and san ignacio the regional geologic and tectonic implications of the models and their relevance to the geothermal potential of honduras and to six well known geothermal sites in particular are examined no obvious regional structures observed in the gravity data can explain the thermal enhancement in general or the specific geothermal sites more local tectonic or structural conditions must control the distribution of the thermally enhanced areas