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

from transtension to transpression along the northern caribbean plate boundary off cuba implications for the recent motion of the caribbean plate abstract marine geophysical surveys using seabeam single channel seismic reflection gravimetric and magnetic measurements have been conducted along a segment of the northern caribbean transcurrent plate boundary seacarib ii cruise the data allow a better definition of the geometry and the tectonic regime of this major strike slip area they support the following results 1 1 along the southern cuban margin the oriente fault displays a discontinuous trace mainly composed of dextral offset en echelon segments some pull apart basins are located between fault segments cabo cruz basin chivirico and baitiquiri basins in the windward passage area the plate boundary enters into the tortue channel and is not connected with the subduction front off northern hispaniola 2 2 the eastern part of the oriente deep and the santiago promontory are characterised by active compressional tectonics they form the santiago deformed belt described here for the first time this deformed belt can be divided longitudinally into three main segments each one characterised by a particular tectonic style its development is related to a transpressional mechanism along the left lateral oriente strike slip fault our observations suggest that a tectonic and kinematic reorganisation occurred recently in this area probably in the late pliocene which may be compared with the recent geological events recorded on land in the northern caribbean domain the precise knowledge of both geometry and structures along the oriente strike slip fault south of cuba provides new constraints for the recent kinematic evolution along the northern caribbean transcurrent plate boundary it leads us to infer the existence of a convergence component associated with the slip component along the oriente transform fault

the 1946 hispaniola earthquakes and the tectonics of the north america caribbean plate boundary zone northeastern hispaniola we have determined focal mechanisms for the largest earthquake ms 7 8 recorded instrumentally in the caribbean basin the august 4 1946 hispaniola earthquake and three of its large magnitude ms 6 1 aftershocks we also relocated 63 aftershocks and one foreshock of the event series the aftershock series is elongate trends wnw and is centered on the samana peninsula of northeast hispaniola shallow aftershocks are in a 75 km wide linear zone and intermediate depth 70 to 130 km aftershocks apparently delineate a moderately south or ssw dipping slab it is not clear however whether these events indicate active subduction of north american atlantic ocean lithosphere or are strike slip events on the interface between subducted but no longer sinking slab and caribbean mantle we constrained focal mechanisms of the main shock and three aftershocks by combining observed p and s polarities and amplitude ratios and also by waveform modeling the two methods yield consistent results the mechanisms include strike slip and thrust displacements on nw striking nodal planes fault dip is variable ne or sw the nw striking fault planes parallel mapped terrane boundaries and faults in the north america na caribbean ca plate boundary zone and are also parallel to the aftershock series trend we interpret the events to be motions on a wnw trending restraining bend segment of the na ca plate boundary in eastern hispaniola we have calculated magnitudes for eight of the earthquakes in the series for the three events including the main shock for which data are available our magnitudes are systematically less than the previously published magnitude estimates given the high magnitude and large aftershock area of the august 4 1946 event these earthquakes probably represent the true long term interplate motions between north america and the terranes in this portion of the plate boundary zone

the effect of fluorine boron and excess sodium on the critical curve in the albite h 2 o system experiments to define the critical curve for a series of silicate melts in equilibrium with a hydrous fluid were carried out in a hydrothermal diamond anvil cell silicate compositions studied were albite with several wt excess na2o b2o3 and f2o 1 complete miscibility between melt and water was observed at lower pressure and temperature conditions compared to pure albite for all compositions for albite excess na2o the critical curve had been lowered by 143 and 247 c at 10 kbar for 5 and 10 wt excess na for albite 5 and 10 wt f the difference at 10 kbar was 147 and 246 c respectively and for albite 5 and 10 wt b differences of 168 and 262 c were found these results are likely to be additive with the presence of more than one of the components depressing the critical curve to even lower temperatures and pressures the results suggest that in complex pegmatitic systems complete miscibility between melt and fluid may be important in the final stages of crystallisation the unusual properties of fluid phases under conditions close to the critical curve in a silicate melt water system may be essential for the enrichment of trace elements in pegmatites as well as for the formation of typical pegmatite textures

morphologic and geologic effects of the subduction of bathymetric highs morphologic and geologic observations suggest that subduction of bathymetric highs such as aseismic ridges chains of seamounts and fracture zones are important in the development of many forearc features and that those features form during relatively brief episodes of intense tectonism a bathymetric high obliquely entering a subduction zone tends to compress sediments along its leading edge resulting in arcward compression of the accretionary wedge a landward deflection of the trench axis and a steepened inner wall result from this deformation if a significant component of oblique slip occurs along the subduction zone then along strike movement of the accretionary wedge may also occur stresses resulting from subduction of bathymetric features with sufficient buoyancy or high relief extend farther landward than in the case of smaller less buoyant features inducing uplift of the leading edge of the overriding plate tectonic erosion of the base of the overriding plate and along strike transport of are material may also occur the accelerated tectonism observed along several convergent margins can be attributed to the consumption of bathymetric irregularities on the seafloor rather than temporally abrupt changes in rates and directions of plate motions or other episodic events in the accretionary prism

structure and tectonics of the upper cenozoic puerto rico virgin islands carbonate platform as determined from seismic reflection studies the puerto rico virgin islands carbonate platform was deposited over an area of 18 000 km2 from early oligocene to holocene on top of an inactive and subsiding cretaceous earliest oligocene island arc regional single channel and multichannel seismic reflection lines presented in this study provide the first information on the regional stratigraphy and structure of this platform that has previously been known mainly from onshore stratigraphic sections of a relatively small 2250 km2 portion of the platform exposed by late neogene tectonic uplift along the north coast of puerto rico seismic reflection lines are used to map the thickness of the carbonate platform strata and to correlate this thickness with onshore outcrop and well data from northern and southern puerto rico st croix u s virgin islands and the saba bank limestone thickness variations from a little over 2 km to almost zero are used to subdivide the puerto rico virgin islands platform into five distinct carbonate provinces 1 north puerto rico area including the onshore exposures 2 virgin islands area 3 st croix and saba bank area 4 south puerto rico area and 5 mona passage area carbonate thickness and structural information from each area are used to test five previously proposed models for the deformation and vertical movements of the platform the most prominent feature of the platform in the puerto rico virgin islands area is a large east west trending arch the northern limb of this arch exhibits a smoother more uniform dip than the steeper more abruptly faulted southern limb the core of the arch is responsible for the exposure of arc basement rocks on puerto rico the origin of this arch which occurs over a 300 km wide area is best explained by north south shortening and arching caused by interaction at depth of subducted slabs of the north america and caribbean plates other important evidence for this model can be found in the benioff zones observed in the earthquake profiles loading of the caribbean plate results in downward flexing of the north america plate and causes the 4 km subsidence of the carbonate platform north of puerto rico

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

neotectonics of hispaniola plate motion sedimentation and seismicity at a restraining bend abstract the pattern of neogene faulting volcanism and sedimentation in hispaniola helps to resolve a problem that arises in attempting to determine the direction of caribbean plate motion from earthquakes alone namely how well do earthquake mechanisms define plate motion hispaniola occupies a compressional north step or restraining bend in the generally east west trending motagua cayman trough puerto rico trench fault system which marks an active plate boundary zone pbz of left lateral strike slip motion between the north america and caribbean plates four areas are distinguishable in hispaniola from field mapping and from the interpretation of satellite imagery and conventional aerial photographs two of these areas consist of active left lateral strike slip fault systems that roughly define the northern and southern coasts of the island the structure of both fault systems suggest a roughly east west trending direction of relative plate motion consistent with previous findings the intervening area consists of en echelon mountain ranges thrust up at the restraining bend from the early miocene a pleistocene volcanic province within this area is interpreted as defining a diffuse extensional fault termination of the southern strike slip fault zone the fourth area in eastern hispaniola is interpreted as being underthrust by discontinuous fragments of lithosphere in response to convergence at the restraining bend it appears possible that thinner young island arc lithosphere may be more prone to subduction than to escape or flaking processes that are more important in older more buoyant lithosphere

from transcurrent faulting to frontal subduction a seismotectonic study of the northern caribbean plate boundary from cuba to puerto rico recent marine and onland geophysical and geological investigations along the northern caribbean plate boundary call for a review of its tectonic interpretation in the light of a new compilation of the seismological data avalaible from cuba to puerto rico we show that the shallow seismicity in the northeastern caribbean is concentrated along an east west trending lineament corresponding to the trace of the major strike slip fault system the most intense seismicity is located around restraining bends such as southern cuba and northern hispaniola the stress and strain distribution deduced from focal mechanisms and microtectonic analysis lead us to infer a small n s convergence component associated with the major eastward strike slip motion of the caribbean plate versus north america earthquake distribution and focal mechanisms suggest the existence of a lithospheric slab inherited from the frontal subduction under the lesser antilles diping down under puerto rico and eastern hispaniola we propose a model in which this slab is disconnected from the atlantic oceanic lithosphere by transcurrent faulting along the plate boundary

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

muertos trough subduction microplate tectonics in the northern caribbean the relative plate motion between the north american and caribbean plates has been commonly assumed to take place in the form of oblique subduction along the puerto rico trench1 west of hispaniola this plate boundary becomes pure strike slip nevertheless it has been suggested that this boundary is more diffuse with part of the plate motion taken up by subduction along the muertos trough mt south of hispaniola2 5 fig 1 but there has been no direct seismological evidence of subduction to confirm this here we present the results of a study of the 24 june 1984 earthquake beneath mt the focal mechanism shows thrusting at a depth of 32 km which together with reports of a great earthquake in 1751 confirms that mt is an active plate boundary this implies that plate motion is accommodated by several tectonic boundaries delineating microplates caught between the caribbean and north american plates fig 4

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

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

equivalent strike slip earthquake cycles in half space and lithosphere asthenosphere earth models by virtue of the images used in the dislocation solution the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike slip fault in an earth model consisting of an elastic plate lithosphere overlying a viscoelastic half space asthenosphere can be duplicated by prescribed slip on a vertical fault embedded in an elastic half space for the case in which each earthquake ruptures the entire lithosphere thickness h the half space equivalent slip rate is as follows depth interval 0 h slip identical to that in lithosphere asthenosphere model i e abrupt coseismic slip and no subsequent slip depth interval 2n 1 h to 2n 1 h n 1 2 slip rate uniform in space and dependent upon time as fn t exp t a where fn is a n 1 degree polynomial in t a is twice the asthenosphere relaxation time and t is measured from the instant after the preceding earthquake the slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion for reasonable values of a slip rates below 5h do not vary much from that mean value and can be treated as constant thus the surface deformation due to the earthquake cycle in the lithosphere asthenosphere model can be calculated very simply from the half space model with time dependent slip in the two depth intervals h 3h and 3h 5h and uniform slip at a rate equal to the secular relative plate velocity below depth 5h inversion of 1973 1988 geodetic measurements of deformation across the segment of the san andreas fault in the transverse ranges north of los angeles for the half space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm yr below 30 km one equivalent lithosphere asthenosphere model would have a 30 km thick lithosphere and an asthenosphere relaxation time greater than 33 years but other models are possible

slip rate and earthquake recurrence along the central septentrional fault north american caribbean plate boundary dominican republic 1 the septentrional fault zone sfz is the major north american caribbean strike slip plate boundary fault at the longitude of eastern hispaniola the sfz traverses the densely populated cibao valley of the dominican republic forming a prominent scarp in alluvium our studies at four sites along the central sfz are aimed at quantifying the late quaternary behavior of this structure to better understand the seismic hazard it represents for the northeastern caribbean our investigations of excavations at sites near rio cenovi show that the most recent ground rupturing earthquake along this fault in the north central dominican republic occurred between a d 1040 and a d 1230 and involved a minimum of 4 m of left lateral slip and 2 3 m of normal dip slip at that site our studies of offset stream terraces at two locations rio juan lopez and rio licey provide late holocene slip rate estimates of 6 9 mm yr and a maximum of 11 12 mm yr respectively across the septentrional fault combining these results gives a best estimate of 6 12 mm yr for the slip rate across the sfz three excavations two near tenares and one at the rio licey site yielded evidence for the occurrence of earlier prehistoric earthquakes dates of strata associated with the penultimate event suggest that it occurred post a d 30 giving a recurrence interval of 800 1200 years these studies indicate that the sfz has likely accumulated elastic strain sufficient to generate a major earthquake during the more than 800 years since it last slipped and should be considered likely to produce a destructive future earthquake

a gps estimate of relative motion between north and south america gps velocity data are used to estimate the euler vector describing rigid body motion of north america relative to south america assuming the boundary between the north and south american plates is located near the fifteen twenty fracture zone in the equatorial atlantic the euler vector predicts extension across the royal trough up to 1 mm yr and convergence across the barracuda ridge at about 2 mm yr in agreement with geological estimates averaged over tens of millions of years further west convergence between north and south america at rates up to 8 mm yr may contribute to deformation of the caribbean plate along its southwest boundary with south america

paleomagnetic evidence for late miocene counterclockwise rotation of north coast carbonate sequence puerto rico a paleomagnetic study of the essentially undeformed middle tertiary carbonate sequence along the north coast of puerto rico reveals statistically significant pre pliocene discordance of characteristic component directions against those expected from cratonic north america for much of the section despite generally weak to moderately weak magnetic intensities confirmation of the magnetization as primary in origin comes from the presence of two distinct components of magnetization intrasite bipolarity and or the reproducibility of measurements the mean geographic direction for the upper oligocene to middle miocene strata is 335 2 degree 32 9 degree and the corrected mean paleomagnetic pole is 207 6 degree 66 5 degree n 3 alpha 95 4 3 degree this suggests a counter clockwise ccw block rotation of puerto rico and its microplate of 24 5 degrees plus minus 5 8 degrees during the late miocene using a width of 250 km for the northern caribbean plate boundary zone ncpbz between the north american plate and caribbean plate the mean left lateral displacement implied is 1 8 to 2 4 cm yr which agrees fairly well with published relative motion rates for the two plates average rotation rate for 50 ma to 20 ma was 0 7 degree my but perhaps as great as 4 degree my in the miocene resolution of mean paleolatitude indicates northward motion of a degree or lessmore during the period of rotation causes of this short lived rotation may include 1 tectonic escape from the inhibiting presence of the bahama banks and beata ridge during eastward motion of puerto rico along the sinistral transpressive puerto rico trench and muertos trough fault systems or 2 changes in relative plate motions of the caribbean and north american plate during the late miocene less

oblique collision in the northeastern caribbean from gps measurements and geological observations 1 previous caribbean gps studies have shown that the rigid interior of the caribbean plate is moving east northeastward 070 at a rate of 18 20 3 mm yr relative to north america this direction implies maximum oblique convergence between the island of hispaniola on the caribbean plate and the 22 27 km thick crust of the bahama carbonate platform on the adjacent north america plate we present a tectonic interpretation of a 15 site gps network which spans the hispaniola bahama oblique collision zone and includes stable plate interior sites on both the north america and caribbean plates measurements span the time period of 1994 1999 in a north america reference frame gps velocities in puerto rico st croix and the lesser antilles indicate that these areas move as a single block in an east northeast direction 070 at a rate of 19 20 mm yr consistent with the movement of the rigid interior of the caribbean plate gps velocities at six sites in central and eastern hispaniola dominican republic show drastically different behavior with more eastwardly strikes 080 and much slower rates 4 17 mm yr than areas of the stable caribbean plate the boundary between the relatively slower moving hispaniola collisional zone and the relatively faster moving uncollided puerto rico virgin islands area is the mona passage where late neogene rifting occurs in a broad zone elastic modeling favors strain partitioning with oblique slip on the outer low angle submarine thrust faults north hispaniola muertos and strike slip on the inner subvertical subaerial strike slip faults septentrional enriquillo

subduction of aseismic ridges beneath the caribbean plate implications for the tectonics and seismic potential of the northeastern caribbean normal seafloor entering the puerto rico and northern lesser antillean trenches in the northeastern caribbean is interrupted by a series of aseismic ridges on the north and south american plates these topographic features lie close to the expected trend of fracture zones created about 80 110 m y ago when this seafloor was formed at the mid atlantic ridge the northernmost of the ridges that interact with the lesser antillean subduction zone the barracuda ridge intersects the arc in a region of high seismic activity some of this seismicity including a large shock in 1974 occurs within the overthrust plate and may be related to the deformation of the caribbean plate as it overrides the ridge a large bathymetric high the main ridge is oriented obliquely to the puerto rico trench and intersects the subduction zone north of the virgin islands in another cluster of seismic activity along the inner wall of the trench data from a seismic network in the northeastern caribbean indicate that this intersection is also characterized by both interpolate and intraplate seismic activity magnetic anomalies bathymetric trends and the pattern of deformed sediments on the inner wall of the trench strongly suggest that the main and barracuda ridges are parts of a formerly continuous aseismic ridge a segment of which has recently been overridden by the caribbean plate reconstruction of mid miocene to recent plate motions also suggest that at least two aseismic ridges and possibly fragments of the bahama platform have interacted with the subduction zone in the northeastern caribbean the introduction of these narrow segments of anomalous seafloor into the subduction zone has segmented the arc into elements about 200 km long these ridges may act as tectonic barriers or asperities during the rupture processes involved in large earthquakes they also leave a geologic imprint on segments of the arc with which they have interacted a 50 km landward jump of the locus of island arc volcanism occurred in late miocene time along the northern half of the lesser antilles we postulate that the subduction of a segment of seafloor of anomolously thick crust being more buoyant than adjacent seafloor resulted in a marked shoaling in the dip of the descending slab and therefore a shift of the locus of volcanism in the region near western puerto rico and eastern hispanolia plio pleistocene interaction with a similar feature in this case a part of the bahama platform about 3 4 m y ago led to a jump in the locus of subduction as evidenced by a gap in the downgoing seismic zone that segment of the bahama platform interferred with the subduction process and was subsequently sutured onto the caribbean plate when the boundary jumped about 60 km to the northeast the maximum size of historic shallow earthquakes along the lesser antillean arc varies from about 7 0 7 5 in the center of the arc where the dip of the shallow part of the plate boundary is steep to 8 0 8 5 along the northern part of the arc where the dip is shallow the interaction of anomalous seafloor as along the northern lesser antilles can lead to the development of a wider than normal zone of interplate contact and hence to earthquakes that are larger than those associated with more typical seafloor entering subduction zones major seismic gaps and regions of high seismic potential currently exist along the northern lesser antilles and to the north of puerto rico both gaps are bounded by anomalous features on the downgoing plate the intersection of these features with the plate boundary created large asperities that may be good places to search for precursors to future large earthquakes a great shock in 1787 may have ruptured an existing seismic gap north of puerto rico between 65 and 67 w thus that gap can be expected to eventually rupture again in a great shock and not to accommodate plate motion by totally aseismic processes

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

deformation of the caribbean region one plate or two new deep penetrating high resolution multichannel seismic reflection data collected in the eastern caribbean during r v ewing cruise ew9501 imaged both the crustal structure and overlying stratigraphic successions on the basis of this new multichannel seismic data we define the geologic development of the beata ridge and venezuelan basin the caribbean crust was formed by seafloor spreading in late jurassic early cretaceous time prior to the senonian widespread and rapid eruption of basaltic flows began in concert with extensional deformation of the caribbean crust thick volcanic wedges characterized by divergent reflectors are observed along the boundary that separates rough from smooth oceanic crust are coincident with an abrupt shallowing of the moho and appear to be bounded by a large northwest dipping fault system the locus of major extensional deformation migrated through time from the venezuelan basin to the western flank of the beata ridge extensional unloading of the beata ridge footwall caused uplift and rotation of the ridge sediment thicknesses and stratal geometry observed across the venezuelan basin and beata ridge suggest that the majority of the deformation in this region occurred during and soon after the emplacement of the volcanics minor fault reactivation in the neogene along the eastern flank of the beata ridge is associated with an accommodation zone i e tear fault that records a change in the deformation style from bending and subduction of the caribbean plate along the muertos trough south of puerto rico to compressional deformation and obduction of the caribbean plate south of hispaniola we propose that this difference in deformational style is in part a consequence of the thicker crust on the beata ridge which is more resistant to subduction

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

a dynamic model of the curvature of the mariana trench oceanic trenches are prominent features of the earth s surface they represent the convergent plate boundaries and generally possess convex curvatures towards the subducting plates based on this observation many static geometric models have been proposed for the origin of trench curvatures1 6 unfortunately many of these models have limited applicability7 alternatively a dynamic model has been proposed which suggests that trench curvatures are evolved from collisions between aseismic ridges and the trench axis8 we have developed a flow model to quantitatively evaluate this dynamic process for the mariana trench system the model is able to yield an excellent fit to the present trench curvature and determine past rotations for back arc volcanic islands such as saipan and guam with surprising accuracy when compared with palaeomagnetic observations