neogene caribbean plate rotation and associated central american tectonic evolution during the neogene the caribbean plate moved eastward with respect to the north and south american plates reconstructing this motion by closing the cayman trough along its bounding transform faults creates a gore of ocean between northern central america and southern mexico we present instead a reconstruction which juxtaposes these two continental areas by strike slip motion using geological evidence from around the cayman trough the strike slip fault zones of northern central america and the continental margin of southern mexico as our constraints we propose that seafloor spreading began in the cayman trough at 30 5 ma and that subsequently there was 1100 km of relative plate displacement at an average rate of 3 7 0 6 cm yr this neogene plate motion cannot be achieved by rigid plate rotation about a single pole which implies that central america must have rotated anticlockwise relative to the rest of the plate there is evidence for internal plate deformation in central america consistent with such a sense of rotation our suggestion of an originally easterly trending arc in southern central america is more consistent with the timing and geometry of the collision of panama with northern colombia and the resultant deformation than were earlier models that depicted collision of a northerly trending arc

an inverse problem for the determination of the stress tensor from polyphased fault sets and earthquake focal mechanisms abstract a new method for determining the stress tensor from fault slip data is presented the solution is obtained by minimizing the deviation between the stress ratio and a value corresponding to the maximum of a probability density function the method can be used to separate successively different tectonic phases and is developed for calculating the stress tensor from earthquake focal mechanisms

chronology of cenozoic tectonic events in western venezuela and the leeward antilles based on integration of offshore seismic reflection data and on land geology newly acquired deep penetration broadband onshore offshore lithospheric investigation of venezuela and the antilles arc region seismic reflection data from offshore western venezuela bonaire basin and around the leeward antilles are combined with existing geologic and geophysical data sets to examine the chronology of late cretaceous cenozoic tectonic events in this part of the caribbean south american plate boundary zone these tectonic events have controlled the maturation and structural trapping of known hydrocarbons in the offshore bonaire basin and the adjacent onland falcn basin we infer three tectonic phases that are constrained using these combined data sets 1 the late eocene early oligocene north south opening of the 3 6 km 1 8 3 7 mi thick falcn bonaire basin occurred along east west striking normal fault systems that have locally been inverted by later tectonic phases these paleogene normal faults rifted the upper cretaceous arc crust and paleogene marine depositional sequences within the offshore bonaire basin 2 northwest striking normal faults crosscut the older normal faults of the bonaire basin and leeward antilles and form deep submarine rifts that contain up to 4 km 2 5 mi of sedimentary fill and form deep water channels between islands of the leeward antilles offshore well data and age of onshore sediments in the falcn basin indicate that this second phase of rifting occurred mainly during the late oligocene to early miocene and remains active to the present 3 inversion of the subaerial falcn basin commenced during the middle miocene this inversion phase is reflected in the present day pattern of an east northeast trending fold thrust belt that can be traced over 200 km 124 mi along strike in the falcn basin a second offshore fold thrust belt la vela can be traced over a distance of 175 km 108 mi along strike and parallel to the northeast trending falcn basin coast restoration of imbricate thrusts seen on seismic lines perpendicular to the la vela fold thrust belt indicates a minimum of 7 km 4 3 mi of northeast southwest directed thin skinned shortening geochemical work indicates that source rocks for scattered occurrences of hydrocarbons in the falcn basin and its coastal zone are paleogene and miocene marine shale reservoir rocks are tertiary marine sandstone and shale deposited in paleogene rifts formed during the first tectonic phase in the late eocene to early oligocene structural traps were formed by thrusting during the second tectonic phase in the late oligocene to early miocene

the 1900 mw 7 6 earthquake offshore north central venezuela is la tortuga or san sebastian the source fault abstract a new high resolution shallow marine seismic survey provides further insights on the fault source of the october 29th 1900 earthquake which affected a large region of north central venezuela both offshore islands and mainland this mw 7 6 to 7 7 seismic event has been indistinctly ascribed to either the la tortuga or san sebastian fault which has installed a live debate among different authors these two faults allegedly run roughly east west offshore along the coastal range the san sebastian fault ssf bounds this range to the north being responsible for its linearity and the steep slope of its northern flank whereas the la tortuga fault ltf would do about the same but some 20 30 km farther north based on acoustic surveys carried out in the late 70 s and early 80 s we herein bring strong evidence to support that the ssf is the source fault of this major offshore earthquake mainly based on the freshness of its submarine scarp and recent fault throw preserved at the chuspa bay close to cabo codera eastern end of the coastal range in central venezuela as depicted by one of the profiles in addition it is shown that most of the previous extent ascribed to ltf is not so the ltf besides exhibiting mainly normal slip is much shorter than originally proposed this precludes that ltf may have the seismic potential for an earthquake as big as the 1900 event finally available reflection seismic data reveal the existence of a major fault system bounding the la guaira shelf on the north which exhibits tectonic inversion of pre existing extensional faults shortening and strong syn tectonic deformation during the plio quaternary deposition sedimentation in the bonaire basin during the plio quaternary time is strongly controlled by this system activity

seismic evidence for blind thrusting of the northwestern flank of the venezuelan andes surface geology and seismic and well data from the northwestern flank of the venezuelan andes indicate overthrusting of andean basement rocks toward the adjacent maracaibo basin along a blind thrust fault the frontal monocline is interpreted as the forelimb of a northwestward verging fault related fold deformed over a crustal scale ramp the andean block has been thrust 20 km to the northwest and uplifted 10 km on a ramp that dips about 20o 30 o southeastward the thrust fault ramps up through crystalline basement rocks to a decollement horizon within the shaly units of the cretaceous colon mito juan formations backthrusts in the monocline produce a wedge geometry and reduce the amount of blind slip required on the decollement northwest of the andes the rigid andean uplift was caused by northwest southeast compressive tectonic forces related to the convergence of the caribbean plate the panama volcanic arc and northwestern south america the thick up to 6 km molasse deposits accumulated in the foredeep basin indicate that the venezuelan andes started to rise as early as the early miocene however a late miocene intramolasse unconformity marks the beginning of the formation of the monocline and the greatest uplift the crustal scale fault related fold model may explain structural features seen in other areas of basement involved foreland deformation the regional geologic setting of the area can be described as a mountain front adjacent to a foredeep basin the mountain front represents a precambrian paleozoic igneous metamorphic complex partially covered with sediments of mesozoic and cenozoic ages the foredeep is an asymmetric structural basin that contains up to 12 km of sedimentary section the early miocene to pleistocene molasse deposits up to 6 km in thickness dominate the sedimentary section the oldest sedimentary rocks that crop out in the mountain front are jurassic red beds which are unconformably overlain by a cretaceous sequence up to 2 km in thickness from early cretaceous to middle eocene sediments were deposited in an extensive passive continental margin setting figure 3 coarse clastic sediments of the rio negro formation bioclastic carbonates of the apon formation and siliciclastic sediments of the aguardiente formation were deposited first in the initial stage of the cretaceous transgressive event limestones calcareous shales and black shales of the capacho and la luna formations represent anoxic facies deposited during the maximum marine transgressiolof late cretaceous cenomanian campanian age regression began in the latest cretaceous maestrichtian with marine deposition of the shaly colon formation and the sandier mito juan formation this

gps estimate of relative motion between the caribbean and south american plates and geologic implications for trinidad and venezuela global positioning system gps data from eight sites on the caribbean plate and five sites on the south american plate were inverted to derive an angular velocity vector describing present day relative plate motion both the caribbean and south american velocity data fit rigid plate models to within 1 2 mm yr the gps velocity uncertainty the caribbean plate moves approximately due east relative to south america at a rate of 20 mm yr along most of the plate boundary significantly faster than the nuvel 1a model prediction but with similar azimuth pure wrenching is concentrated along the approximately east striking seismic el pilar fault in venezuela in contrast transpression occurs along the 068 trending central range warm springs fault in trinidad which is aseismic possibly locked and oblique to local plate motion

oblique collision and accretion of the netherlands leeward antilles to south america 1 accretion of the netherlands leeward antilles volcanic island arc to northern south america offshore venezuela was accompanied by deformation and clockwise block rotation structural analysis of aruba curacao and bonaire indicates that they were deformed during at least one ductile event d1 and three brittle events f1 f3 the first deformation phase d1 and f1 began in the early cretaceous at approximately 110 ma deformation features are consistent with displacement partitioning at the obliquely convergent caribbean atlantic proto caribbean plate boundary this deformation was accompanied by an approximate 90 clockwise rotation of the forearc terrane the onset of the second phase of deformation f2 occurred in the eocene 55 ma also accompanied by a small amount of rotation and structures are consistent with wrench tectonics in a pull apart regime this coincided with the collision of the caribbean plate with the great bahama bank and the initiation of convergence between north and south america the youngest deformation structures and rotation f3 are also associated with wrench faulting likely motion along the oca fault and the caribbean plate transform fault initiated a major restraining bend in the late paleogene beginning 12 ma the northeastward escape of the maracaibo block further increased the complexity of tectonism at the caribbean south american plate boundary deformation structures are consistent with a total of 135 clockwise rotation of the leeward antilles islands and regional diachronous deformation from west to east since the early cretaceous

current block motions and strain accumulation on active faults in the caribbean the caribbean plate and its boundaries with north and south america marked by subduction and large intra arc strike slip faults are a natural laboratory for the study of strain partitioning and interseismic plate coupling in relation to large earthquakes here we use most of the available campaign and continuous gps measurements in the caribbean to derive a regional velocity field expressed in a consistent reference frame we use this velocity field as input to a kinematic model where surface velocities results from the rotation of rigid blocks bounded by locked faults accumulating interseismic strain while allowing for partial locking along the lesser antilles puerto rico and hispaniola subduction we test various block geometries guided by previous regional kinematic models and geological information on active faults our findings refine a number of previously established results in particular slip rates on the strike slip faults systems bounding the caribbean plate to the north and south and the kinematics of the gonave microplate our much improved gps velocity field in the lesser antilles compared to previous studies does not require the existence of a distinct northern lesser antilles block and excludes more than 3 mm yr of strain accumulation on the lesser antilles puerto rico subduction plate interface which appears essentially uncoupled the transition from a coupled to an uncoupled subduction coincides with a transition in the long term geological behavior of the caribbean plate margin from compressional hispaniola to extensional puerto rico and lesser antilles a characteristics shared with several other subduction systems

historical seismicity of the southeastern caribbean and tectonic implications we have relocated the twenty eight largest magnitude 4 3 ms 7 3 historical 1922 1963 earthquakes of the southeastern caribbean we also present new focal mechanisms for seven of these events the relocations are based on reported issp ands arrival times that we analyzed using generalized linear inversion techniques the new focal mechanisms were constrained by first motionp polarities as reported by the iss and as picked by us where records were available and by the polarities and ratios ofsh andssh andsv andssv arrivals that we determined from seismograms the results of the relocations are commensurate with the distribution of seismicity observed in the recent era hypocenters are shallow and intermediate in depth 0 200 km and the events occur almost exclusively in areas known to be currently seismic the frequent seismic activity in the vicinity of the paria peninsula venezuela is clearly a persistent feature of the regional earthquake pattern intermediate depth earthquakes indicative of subduction beneath the caribbean plate occur here and along the lesser antilles arc the grenadines seismic gap is confirmed as an area of low seismic moment release throughout the historical era trinidad and the eastern gulf of paria were also largely quiescent

miocene to recent cariaco basin offshore venezuela structure tectonosequences and basin forming mechanisms abstract the cariaco basin located 40 km off the central part of the coast of venezuela is the largest 4000 km 2 and bathymetrically deepest 1400 m bsl neogene fault bounded basin within the right lateral strike slip plate boundary zone that separates the caribbean and south american plates using subsurface geophysical data we test two previously proposed tectonic models for the age distribution and nature of east west striking strike slip faults and basin forming mechanism for the two main depocenters of the cariaco basin the earliest interpretation for the opening of the twin cariaco depocenters by schubert 1982 proposes that both depocenters formed synchronously by extension along transverse north south normal faults at a 30 km wide rhomboidally shaped pull apart basin between the right lateral east west striking and parallel san sebastian and el pilar fault zones a later model by ben avraham and zoback 1992 proposes that both depocenters formed synchronously by a process of transform normal parallel extension or rifting in a north south direction orthogonal to the east west striking and parallel strike slip faults we use more than 4000 km of 2d single and multi channel seismic data tied to 11 wells to map 5 tectono stratigraphic sequences and to produce a series of structural and isopach maps showing how the faults that controlled both cariaco depocenters evolved from paleogene to the present comparison of fault and isopach maps for dated horizons from paleogene to late neogene in age show three main phases in basin development 1 from middle miocene to pliocene the west cariaco basin formed as a rhomboidally shaped pull apart at a 30 km wide stepover between the northern branch of the san sebastian fault and the el pilar fault zone 2 during the early pliocene a new strike slip fault transected the west cariaco basin southern branch of the san sebastian fault and caused extension to cease and 3 during the early pliocene to recent a lazy z shaped pull apart formed along the curving connection between the southern branch of the san sebastian and el pilar fault zones

chlorine isotopic geochemistry of salt lakes in the qaidam basin china abstract the isotopic compositions of chlorine in salt lake brine coexisting with halite oil field water and river water from the qaidam basin qinghai china have been examined using high precision measurement of chlorine isotopes based on thermal ionization mass spectrometry of the cs 2 cl ion 37 cl values vary from 2 05 to 2 94 with average values of 2 94 for hot spring water 1 35 for river water 0 38 for oil field brine 0 40 for saline water and 0 63 for salt lake brine the preliminary results show isotopic fractionation of chlorine in salt lake brine during salt precipitation the variation of the 37 cl value in these salt lakes is mainly associated with brine evaporation and halite precipitation and with the hydrochemistry of the input water the chlorine isotopic composition may reflect the degree of evaporation of the salt lake brine and may be used to distinguish brines that are in equilibrium with precipitating salts and brines that have dissolved earlier evaporates

tectonic inheritance crustal architecture and contrasting structural styles in the venezuela andes surface and subsurface data image contrasting structural styles for the two flanks of the venezuela andes in the north a flexural basin developed in neogene times between the andes and the lake maracaibo north verging thrusts are mainly detached in the pre cretaceous substratum and form a deeply buried antiformal stack secondary decollement levels occur both in the upper cretaceous and tertiary strata accounting for the passive roof thrust of a conventional frontal triangle zone in the south the barinas basin hardly compares with a flexural basin it is largely dominated by both north and south verging basement involving structures paleogene normal faults are locally inverted and caribbean nappes are frequently reactivated or refolded by younger oblique neogene andean structures seismic profiles in this area also attest to the strong neogene structural inversion of upper jurassic lower cretaceous grabens hercynian or more likely early paleozoic structures account for local preexisting crustal heterogeneities reactivated during both the tethyan rifting and the andean deformations two trans andean regional profiles have been balanced and constrained by an inversion of the gravimetric data they imply a progressive deepening of the northern moho and a south dipping subduction of the infracontinental lithospheric mantle of the maracaibo block the shortening for both sections averages 60 km palinspastic restorations assume a relative cylindricity for the deep crustal architecture of the andes and minimize the possible effects of a progressive right lateral escape of the maracaibo block with respect to stable south america along the bocono fault strain partitioning during neogene oblique convergence induced surficial thrust fronts parallel to the plate boundary strike slip motion in the allochthon along the bocono fault and an asymmetric subduction wedging at depth

contrasted metamorphic evolutions in thrusted cover units of the brianconnais zone french alps a model for the conservation of hp lt metamorphic mineral assemblages the evolution of organic matter silicate and fluid phases in cover units of the three structural zones of the vanoise area allows one to distinguish differentp t cooling paths these paths depend on whether the cover has been transported western and northern vanoise or not southern vanoise all units first underwent a common high pressure low temperature hp lt metamorphic stage 300 c 6 kbar of blueschist type fe mg carpholite chloritoid facies the cover units transported on the extermal colder zone coal measure series metamorphosed in albite chlorite facies preserved their hp mineralogy fe mg carpholite lawsonite and organic matter content oils wet gases and kerogen while the unit which remained in contact with its more thermally conductive basement polymetamorphic now shows extensive greenschist facies overprinting breakdown of fe mg carpholite and lawsonite appearance of chlorite pyrophyllite chloritoid and clinozoisite absence of oils and wet gases these two tectonic evolutions might serve as a more general model for the preservation of hp lt mineral assemblages

effective stress history and the potential for seismicity associated with hydraulic fracturing of shale reservoirs the influence of effective stress history on reservoir stability and the response to hydraulic fracturing are simulated for two hypothetical shale reservoirs it is numerically demonstrated that the effective stress history influences the present day stability of faults and natural fractures any assumption of a homogeneous stress state is shown to be unrealistic in the majority of fractured shale reservoirs the simulations demonstrate that depending upon the effective stress history shear displacements can be induced by stress changes associated with stimulation without a change of fluid pressure in the destabilized fractures elastoplastic shale and other fractured reservoirs are shown to have a memory of past geomechanical states these numerical findings demonstrate the value of interpreting reservoir effective stress history when planning fluid injection a previous case history is summarized to illustrate the overwhelming dependence of stress history analyses on a wide spectrum of earth science disciplines it also illustrates the application of stress history analyses to activities other than hydraulic fracturing the current inexperience of effective stress history description means that new approaches must be identified to optimize the necessary multidisciplinary investigations supplementary material details of the simulations are available at www geolsoc org uk sup18743

microseismicity evidence for subduction of the caribbean plate beneath the south american plate in northwestern venezuela over 1100 microearthquakes with body wave magnitude mb 4 have been located in western venezuela and the southwestern caribbean region since the installation in 1980 of the venezuelan seismological array together with 120 events of mb 4 one of them with surface wave magnitude ms 6 this tectonically complex region is part of the boundary between the caribbean and the south american plates the main seismically active feature inland in western venezuela is the northeast striking 600 km long 100 km wide right lateral strike slip bocono fault zone along the venezuelan andes about 80 of the earthquakes located in the entire region in the period 1980 mid 1995 have occurred on this fault zone at focal depths 20 km microearthquake activity at lower rates also occurs northwest of the venezuelan andes both in the continental and caribbean sea regions part of this activity takes place at depths down to 150 km northwest oriented seismicity depth profiles show the existence of a benioff zone dipping to the southeast beneath northwestern venezuela and northern colombia this indicates the presence of a northeast striking southeast dipping subducted slab of the caribbean plate beneath the south american plate hypocentral locations show that the northeastern end of this subduction occurs northwest of the curacao aruba region in the vicinity of a northwest trending right lateral strike slip fault zone that joins up with the northeastern end of the bocono fault zone this latter place turns out to be the western end of the east west striking san sebastian fault along the venezuelan coast

an inverse problem in microtectonics for the determination of stress tensors from fault striation analysis abstract a method is proposed to compute some characteristics of a possible stress tensor related to striations measured in a given faulted area if several tectonic phases are responsible for the striations the method separates and determines the successive stress tensors the iterative algorithm consists of alternatively a sorting the data attributed to a tectonic phase and b computing the characteristics of the stress tensor of this phase the method is tested using an application to synthetic and actual data

the 1967 caracas earthquake fault geometry direction of rupture propagation and seismotectonic implications the fault plane orientation of the july 30 1967 caracas earthquake mw 6 6 has been a source of controversy for several years this strike slip event was originally thought to have occurred on an east west oriented fault plane reflecting the relative motion between the caribbean and south american plates more recently however the complex seismic radiation from this event was interpreted as being indicative of a north south striking fault that ruptured along three en echelon segments in this study we synthesize evidence based on the intensity and damage reports the distribution of aftershocks and the results of a joint formal inversion of the p and sh waves and show that these data clearly indicate that the rupture of the 1967 earthquake occurred on an east west trending fault system using a master event technique the largest aftershock which occurred 40 min after the main event is shown to lie 50 km east of the epicenter of the mainshock the epicentral distances of small aftershocks registered in caracas based on the s p arrival time differences and the polarizations of the p waves are also consistent with these events occurring on an east west oriented fault system north of caracas a joint inversion of the teleseismic p and sh waves recorded on long period seismographs of the world wide standardized seismographic network shows that in a time frame of 65 s four distinct bursts of seismic moment release subevents occurred with a total seismic moment of 8 6 1018 n m the first three subevents triggered sequentially from west to east in a direction that is almost identical to the east west trending nodal planes of the source mechanisms the average depth of these three subevents is 14 km the fourth and last identifiable subevent of the sequence shows a reverse faulting mechanism with the nodal planes oriented roughly east west it occurred at a 21 km depth about 50 km to the north of the fault zone defined by the strike slip subevents this fourth subevent appears to reflect compressional deformation of the southern caribbean possibly related to underthrusting along the proposed curacao trench the complexity of the fault system causing the 1967 earthquake suggests that the relative motion along the caribbean south america plate boundary in central venezuela is taken up over a broad highly faulted and highly stressed zone of deformation and not by a simple major throughgoing fault

plate motions in the north andean region repeated geodetic measurements with the global positioning system gps provide direct measurements of displacements due to plate motions and active crustal deformation in central america and northern south america an area of complex interaction of the nazca cocos caribbean and south american plates the displacement rates for the period 1988 1991 obtained from the results of the first three central and south america casa gps campaigns are in general agreement with the predictions of the nuvel 1 plate motion model but there are differences in detail between the observations and the model the nazca north andes convergence rate vector measured by gps is different from the nuvel 1 vector at 95 confidence the difference implies that the north andes are moving northward relative to south america the measured convergence between the caribbean plate and the north andes suggests that the southern margin of the caribbean plate is located in the south caribbean deformed belt the april 1991 costa rica earthquake and the cocos caribbean convergence rate determined by gps suggest the possibility of significant ongoing deformation between central america and the stable interior of the caribbean plate our gps results are consistent with deformation of the overriding plates at the convergent margins of central and south america and confirm that active convergence is occurring around much of the southern margin of the caribbean plate from colombia west to costa rica costa rica and panama are not part of the stable caribbean plate instead the south caribbean deformed belt and the north panama fold belt probably represent the southern margin of the caribbean plate

subduction of the caribbean plate and basement uplifts in the overriding south american plate the new tectonic interpretations presented in this paper are based on geologic field mapping and gravity data supplemented by well logs seismic profiles and radiometric and earthquake data the present caribbean south american plate boundary is the south caribbean marginal fault where subduction is indicated by folding and thrusting in the deformed belt and a seismic zone that dips 30 to the southeast and terminates 200 km below the maracaibo basin the caribbean south american convergence rate is estimated as 1 9 0 3 cm yr on the basis of the 390 km length of the seismic zone and a thermal equilibration time of 10 m y the caribbean south american convergence has produced a northwest southeast maximum principal stress direction 1 in the overriding south american plate the mean 1 direction for the maracaibo santa marta block is 310 10 based on earthquake focal mechanism determinations and structural and gravity data on the overriding south american plate basement blocks have been uplifted 7 12 km in the last 10 m y to form the venezuelan andes sierra de perija and the colombian santa marta massif crystalline basement of the venezuelan andes has been thrust to the northwest over tertiary sediments on a fault dipping about 25 and extending to the mantle in the sierra de perija mesozoic sediments have been thrust 16 26 km to the northwest over tertiary sandstones along the cerrejon fault a thrust fault dipping 15 10 to the southeast is consistent with field mapping and gravity and density data the santa marta massif has been uplifted 12 km in the last 10 m y by northwest thrusting over sediments the basement block overthrusts of the perijas venezuelan andes and the santa marta massif are pliocene pleistocene analogs for laramide orogenic structures in the middle and southern rocky mountains of the united states the nonmagmatic basement block uplifts along low angle thrust faults reveal horizontal compression in the overriding plate over 500 km from the convergent margin present day east northeast west southwest 080 compression is indicated by earthquake focal mechanisms and strike slip motion on the bocono fault these earthquakes are intraplate deformation associated with east west 080 nazca south american convergence

crustal structure of the caribbean northeastern south america arc continent collision zone 1 we present the results of a 568 km long regional wide angle seismic profile conducted in the southeastern caribbean that crosses an active island arc a remnant arc two basins possibly floored by oceanic crust an allochthonous terrane of forearc affinity and the passive margin of northern south america the velocity structures of the late cretaceous aves ridge remnant arc and miocene and younger lesser antilles arc are remarkably similar which implies that magmatic processes have remained moderately steady over time crustal thickness is 26 km at the aves ridge and 24 km at the lesser antilles arc in comparison to the izu bonin and aleutian arcs the lesser antilles arc is thinner and has no evidence for a lower crustal cumulate layer which is consistent with the estimated low magma production rates of the lesser antilles arc crustal thickness beneath the grenada and tobago basins is 4 10 km and the velocity structure suggests that these basins could be floored by oceanic crust a decrease of 1 km s in average seismic velocity of the upper crust is observed from nw to se across the north coast fault zone we argue that this marks the suture between the far traveled caribbean arc and the passive margin of the south american continent current strike slip motion between the caribbean and south american plates is located 30 km to the south and thus material originally deposited on the south american passive margin has now been transferred to the caribbean plate

basin formation along the bocono moron el pilar fault system venezuela within the boundary zone between the caribbean south america plates which includes the bocono moron el pilar right lateral fault system that has been active since the late tertiary there are several sedimentary basins in the venezuelan andes and the caribbean mountains these were formed in zones of crustal depression where faults diverge and as pull apart basins in zones of en echelon stepovers or along releasing bends in the faults in addition there are pull apart basins along the avila and la victoria faultzones in the caribbean mountains which are associated with the plate boundary the basins are filled with alluvial lacustrine fluvial and marine sediments mainly of quaternary age on the basis of model studies dimensions of the basins and age of their sedimentary fill it is estimated that right lateral offset necessary to form them along the bocono moron el pilar fault system varies from 2 to 125 km along the avila and la victoria fault zones this right lateral offset is estimated to range from 2 to 30 km on this basis the maximum right lateral offset along the southern caribbean plate boundary that can be documented along the bocono moron el pilar fault system has not exceeded 100 125 km if there has been larger offset it has to be documented by other means or it has taken place along other fault zones within the system

earthquake focal mechanisms and tectonic processes along the southern boundary of the caribbean plate the focal mechanisms of five small earthquakes 4 9 mb 5 6 that occurred in northwestern venezuela have been determined using teleseismically recorded rayleigh waves the rayleigh wave data were also used to constrain the depths of the five events studied these results suggest that the southern boundary of the caribbean plate is presently characterized by a broad zone of internal deformation where the eastward displacement of the caribbean plate with respect to the south american plate is being accommodated along variously oriented strike slip faults this strike slip faulting appears to occur along pre existing zones of weakness such as the bocono fault and the earthquakes studied here are all consistent with an east west trending compressive stress field being released along these zones of weakness these focal mechanisms therefore reflect the tectonic stress field generated by the eastward motion of the caribbean plate as opposed to relative plate motion per se thus the southern boundary of this plate cannot be classified as a simple plate boundary i e transform fault spreading center or subduction zone instead relative plate motion in this region appears to be accommodated by internal deformation within a broad zone extending from the interior of the caribbean plate into northern south america

quaternary tectonics and present stress tensor of the inverted northern falcon basin northwestern venezuela abstract the tertiary falcon basin in northwestern venezuela has a privileged position in the geodynamic puzzle of northwestern south america occurring in a region where several major plates caribbean south america and nazca and minor lithospheric blocks maracaibo bonaire and western colombia are interacting a combination of good exposures due to aridity and a near continuous sedimentary record in a now inverted basin helps to unravel the neogene and quaternary geodynamic evolution of this region a neotectonic and microtectonic investigation of the plio quaternary sedimentary rocks of the northern falcon basin reveals that this region is subject to a compressive to transpressive regime at present this regime is characterized by a nnw sse oriented maximum horizontal stress and a ene wsw trending intermediate or minimum horizontal stress as is confirmed by focal mechanism solutions this stress field is in agreement both with the nne directed extrusion of the maracaibo and bonaire blocks in western venezuela where the falcon basin is located and present day transpression along the caribbean south america plate boundary zone

structure of the merida andes venezuela relations with the south america caribbean geodynamic interaction for over 50 years several models based on diverse geologic concepts and variable quality of data have been proposed to explain the major structure and history of the merida andes ma in western venezuela lately this chain growth and associated flexural basins deepening have been related to incipient type a subductions of either polarity accounting for the across chain asymmetry however these recent models have not well integrated the present tectonically active setting driven by neighboring major plate interactions at present this chain exhibits ongoing strain partitioning where cumulative right lateral slip along chain axis is as much as half of or about the same as the transverse shortening since late miocene thus implying that the nne directed maracaibo block extrusion with respect to the south america sa plate is not a secondary feature consequently this paper discusses some limitations exhibited by the se directed continental subduction models maracaibo crust underthrusting the merida andes in the light of available geological and geophysical data besides it is herein proposed that the merida andes structuration is related to a nw directed gently dipping incipient type a subduction where chain growth and evolution are similar to those of a sedimentary accretionary wedge i e barbados but at crustal scale and with ongoing strain partitioning this continental subduction is the se portion of a major orogenic float that also comprises the perija range and the santa marta block d 2002 elsevier science b v all rights reserved

the caribbean plate pulled pushed or dragged 1 mechanical coupling between the lithosphere and the asthenosphere remains a controversial topic in the geosciences beneath the caribbean plate shear wave splitting measurements indicate ew strain in the asthenosphere which can be interpreted as mantle flow driving or resisting motion of the overlying lithosphere here we constrain the average shear traction on the base of the caribbean plate by balancing all torques these torques result from body forces that act on the caribbean slab pull ridge push lateral density variations from plate boundary friction and from basal shear tractions we obtain a range of physically realistic torque solutions which we examine further by computing the corresponding stresses and rotations within the caribbean plate for comparison with observations the deformation field for the caribbean is particularly sensitive to the amount of friction on intraplate faults representative models have a good fit with observations and are characterized by 1 a near zero basal shear traction 0 3 mpa 2 lithosphere averaged plate boundary friction 10 mpa 3 local forces due to indenters and trench pull 4 a net pull by the caribbean slab and 5 intraplate fault shear stresses on the order of tens of megapascals we conclude that the mechanical coupling of the caribbean plate to the underlying asthenosphere is small

the spanish portuguese central system an example of intense intraplate deformation and strain partitioning abstract the intraplate deformation of iberia during the cenozoic produced a series of ranges and deformation belts with a wide variety of structural trends the spanish portuguese central system is the most prominent feature crossing over the whole of central iberia it is a large thick skinned crustal pop up with ne sw to e w thrusts however the 500 km long left lateral strike slip messejana plasencia fault also ne sw oriented bends these thrusts to produce ne sw local paleostresses close to the fault which seems to be consistent with a common deformational arrangement this is also supported by the similar sedimentary infilling characteristics found in the surrounding cenozoic basins the moment of the maximum intraplate deformation is registered at the same time in all these basins during the upper priabonian lower chattian as there are two possible sources for the intraplate compressive stresses the pyrenean n s shortening orogen to the north and the betic nw se shortening orogen to the south neither can simply explain both simultaneous movements ne sw strike slip and ne sw thrusting the deduced age of the main deformation indicates a pyrenean origin in contrast the concept of strain partitioning between the two types of faults gives as a result an overall north trending compression existing data do not support crustal detachment from the betics neither from the pyrenees but are consistent with a crustal uplift related to lithospheric folding the subsequent betic related stress field only slightly reworked previously pyrenean related structures except for the portuguese sector where tectonic activity occurred mainly in the upper miocene

semi periodic sequences and extraneous events in earthquake forecasting ii application forecasts for japan and venezuela in order to analyze observed seismicity in central japan and venezuela we applied a new method to identify semi periodic sequences in the occurrence times of large earthquakes which allows for the presence of multiple periodic sequences and or events not belonging to any sequence in the time series we also explored a scheme for diminishing the effects of a sharp cutoff magnitude threshold in selecting the events to analyze a main four event sequence with probability p c 0 991 of not having occurred by chance was identified for earthquakes with m 8 0 in central japan venezuela is divided from west to east into four regions for each of these the magnitude ranges and identified sequences are as follows region 1 m 6 0 a six event sequence with p c 0 923 and a four event sequence with p c 0 706 region 2 m 5 6 a five event sequence with p c 0 942 region 3 m 5 6 a four event sequence with p c 0 882 region 4 m 6 0 a five event sequence with p c 0 891 forecasts are made and evaluated for all identified sequences having four or more events and probabilities 0 5 the last event of all these sequences was satisfactorily aftcast by previous events whether the identified sequences do in fact correspond to physical processes resulting in semi periodic seismicity is of course an open question but the forecasts properly used may be useful as a factor in seismic hazard estimation

orogenic float of the venezuelan andes abstract the venezuelan or merida andes are a ne trending intracontinental orogen that started to rise from the middle miocene due to the e w far field convergence between the maracaibo block to the northwest and the guyana shield to the southeast oblique convergence is responsible for strain partitioning with thrusting along both foreland basins and right lateral strike slip faulting along the ne sw bocono fault cutting the venezuelan andes along strike the central part of the belt is also cut by the n s left lateral strike slip valera fault that connects the bocono fault both faults bounding the trujillo block that escapes towards the nne even though the regional geology of belt is well known its structure at depth remains a matter of debate our work based on the integration of geological and geophysical data aims to better constrain the deep geometry of faults and the tectonic evolution of the mountain belt we used the orogenic float model to construct two nw se trans andean crustal scale balanced sections the late neogene quaternary shortening varies from 40 km in the south to 30 km in the north across the trujillo block indicating that a quarter of the deformation seems to be absorbed by the tectonic escape process more importantly a major reorganization in the crust took place in the early pliocene it is characterized by the imbrication of the maracaibo crust into the guyana crust this resulted in the subduction of the guyana lower crust and the formation of a nw vergent basement thrust propagating upwards and surfacing along the las virtudes thrust rapid uplift of the northern flank of the belt subsequently occurred together with massive deposition of the plio quaternary coarse grained betijoque formation in the northwestern foreland basin

paleostress inversion of fault slip data using the shear stress solution of means 1989 abstract means 1989 presented a graphical solution for shear and normal stress orientations on any generally oriented plane this solution can be used to solve for paleostress orientations for a population of faults where slip direction and slip sense are known a grid search method is used which tests a range of stress orientations and stress ratios attempting to find the tensor which minimizes the angle between measured and predicted slip directions minimizes resolved normal stresses and maximizes resolved shear stresses comparison with previously published tensor search algorithms indicates that computation speed is increased with no loss of accuracy test analysis of data from the yuli aftershock sequence indicates that this method produces results similar to previously published solutions analysis of geologic faults from the piedmont of south carolina indicates a pre jurassic period of normal faulting and extensional deformation dominated by a northwest trending 3

paleoseismicity studies on the oca ancon fault system northwestern venezuela abstract the e w trending right lateral oca ancon fault system extends eastward for 650 km from the colombian caribbean coast near the city of santa marta to the town of boca de aroa located on the eastern coast of falcon state northwestern venezuela across the goajira peninsula the outlet of lake maracaibo the coastal plains of buchivacoa and the central falcon range this system is very conspicuous west of the city of maracaibo where its single trace sharply truncates the northern ends of the santa marta block and the perija range fault related landforms along the system trace e g displaced beach strandlines at sinamaica and fault scarplets in the buchivacoa plains suggest that it may have been active during holocene times a first trenching attempt carried out north of maracaibo at sinamaica in 1969 only confirmed its holocene tectonic activity on the western coastal plains of falcon state and east of lake maracaibo the system is less simple as it is composed of two parallel fault strands but they are clearly evidenced by holocene paleoseismic scarplets recently this system has been paleoseismically evaluated by means of two trenches placed across each of the two parallel branches known as the oca and the ancon faults dimensions of these bulldozer dug trenches are 80 m long and 8 m deep various lines of evidence obtained from these exploratory trenches indicate that a three surface rupture events dated slightly prior to 7755 320 6240 39 and 1945 630 yr b p have occurred along the oca fault b one surface rupture event has occurred within the past 3125 185 yr on the ancon fault c the holocene slip rate of the system is close to 2 mm yr and d either of the faults can generate a maximum event of magnitude 7 4 7 5 recurrence of such earthquakes on the ancon fault is close to 1900 years while it is about 4300 years on the oca fault

calcite and quartz microstructural geothermometry of low grade metasedimentary rocks northern range trinidad abstract the northern range of trinidad is a mountainous exposure of deformed metasedimentary rocks with mesozoic depositional ages and tertiary metamorphic ages located in the caribbean south american plate boundary zone these rocks lack precise mineralogical indicators of metamorphic grade using temperature sensitive quartz and calcite microstructures and fission track data we identify describe quantify and map a nearly continuous spectrum of relatively low l50 c to relatively high 250 400 c deformation temperatures from east to west across the range average estimated rock exhumation rates also increase systematically along this trend the analysis illuminates a sharp east west trending major thermal discontinuity i e a fault along the southern boundary of the range kinematic analysis of faults and shear bands adjacent to this boundary and modeling indicate that the fault probably dips southward at 80 85 and accommodated normal dip slip at rates that decreased from west to east earlier experimental studies at high strain rates illustrated a strong strain rate dependency for quartz microstructural transitions our work indicates a possible strong temperature control for quartz microstructural transitions at natural strain rates

neotectonics of bocono fault western venezuela abstract neotectonic morphologic evidence along the bocono fault with a mapped length of 500 km consists of the typical features found along strike slip faults offset alluvium and drainage shutterridges closed depressions sag ponds and marshes fault scarps and trenches triangular facets and zones of mylonite and fault gouge evidence on fault planes such as slickensides suggests a predominant strike slip displacement and morphologic evidence suggests that this offset is right lateral with a magnitude of 60 100 m during the holocene and of several kilometers during the quaternary calculations based on different empirical relationships suggest maximum expected richter magnitudes of 7 2 7 9 for earthquakes along the fault using rupture length estimates and probable intervals of less than 200 years for events of magnitude 8 using observed total displacement during the holocene

oblique collision and tectonic wedging of the south american continent and caribbean terranes our studies of the surface structures seismicity and gravity anomalies of the caribbean south american plate boundary zone of northeast venezuela and trinidad show that the crustal and upper lithospheric structure and kinematics of this zone are complex and differ markedly along strike we propose a model of the zone9s lateral transition from near trinidad west to caracas emphasizing three concepts 1 the steepening detachment and then sinking away of atlantic oceanic lithosphere that is was attached to the northern edge of continental south america 2 the wedging seaward and imbricate thickening of the northern margin of the continent in the region of the detaching atlantic slab the continental wedge overrides the slab and is overridden by terranes attached to the caribbean plate and 3 that the structural and kinematic transitions are a consequence of the progressive oblique collision between continental south america and the overriding caribbean terranes

kinematic indicators on active normal faults in western turkey quaternary normal fault zones in western turkey comprise multiple slip planes and zone parallel layers of fault breccia they also contain several little known kinematic indicators that are probably typical of many formed at shallow levels in extending terrains the recent exhumation by contractors of about 2000 m2 of slip planes in a sse dipping fault zone separating quaternary colluvium from bedrock carbonates at yavansu 7 km se of kusadasi permits an unusually complete inventory of the indicators to be compiled the most spectacular indicators are metre scale 69 w pitching corrugations in slip planes and recemented breccia sheets underlying them corrugations characterized by sinusoidal profiles normal to their long axes and less commonly culminations and depression along their axes possibly developed as a result of upwards propagating slip planes seeking undemanding pathways through heterogeneous fault precursor breccias that formed in advance of tip lines parallel to corrugation long axes are those of gutters flat floored sleep sided channels a few centimetres wide probably related to the abrasion of subslip plane breccia sheets centimetre scale tool tracks scored in the uppermost subslip plane breccia sheet by resistant colluvial clasts are irregular at their proximal ends but distally they swing into alignment with corrugation axes frictional wear striae centimetres long but only a few millimetres wide and pitching 78 w are superimposed on the other slip parallel lineations comb fractures nearly perpendicular to slip planes define an intersection lineation which is normal to corrugation axes fault plane solutions of earthquakes on sse dipping active faults in the west anatolian extensional province indicate that mainly normal combined with minor dextral slip is the dominant mode a conclusion in accord with the sense of slip inferred from the indicators exposed on the yavansu slip planes

intermediate depth seismicity in northern colombia and western venezuela and its relationship to caribbean plate subduction the recent intermediate depth seismicity in northern colombia and western venezuela was analyzed to understand its origin and its presumed relationship to a subducted lithospheric slab in northwestern south america the area included in this study is located to the north and east of the bucaramanga nest which is a particular region beneath bucaramanga northern colombia that presents a high concentration of intermediate depth earthquakes to the north of the nest the seismicity of the area is sparse and most of the events are of low magnitude mb 5 l thus only 23 earthquakes were large enough to be investigated using teleseismic data the focal parameters of the two largest events mb 5 4 were obtained from the formal inversion of long period body waves recorded at teleseismic distances the focal mechanisms of 10 more events were determined from first motion data in total the focal mechanisms of 12 events were determined from both the inversion of p and s h waveforms and the polarities of first arrivals for the smaller earthquakes the focal depths were calculated by measuring the observed pp p interval time and comparing it to the theoretical travel time tables the isodepth curves reflect a slab striking in a nne ssw direction and dipping approximately at 25 32 to the southeast this observation is corroborated by the direction and plunge of the t axes of the focal mechanisms which are generally parallel to the gradient of the slab defined by the spatial distribution of hypocenters these results indicate that the intermediate depth earthquakes in western venezuela and northern colombia are apparently related to the presence of a continuous lithospheric slab subducted near the northern coast of colombia the two largest earthquakes located at a significant distance from the bucaramanga nest present similar fault plane solutions moreover they also agree with those of the two largest earthquakes reported inside the nest this similarity suggests that the bucaramanga nest lies on the same subducted slab where the other earthquakes occur there is not enough shallow seismicity to define the location where the caribbean lithosphere subducted beneath the south american plate however the extension of the slab toward the surface inferred from the intermediate depth seismicity suggests that the subducted lithosphere may still be attached to the caribbean plate

late cretaceous miocene diachronous onset of back thrusting along the south caribbean deformed belt and its importance for understanding processes of arc collision and crustal growth 1 the venezuelan basin is obliquely subducted to the east southeast beneath the continental south american plate at the east west trending south caribbean deformed belt scdb a 50 100 km wide wedge of accreted sedimentary rocks derived both from offscraping of the 0 5 to 1 km thick sedimentary cover of the subducting venezuelan basin and from incorporation of clastic sediments derived from the continental margin of south america in this paper we describe the structure and sequence stratigraphy of five late cretaceous to recent sedimentary sequences overlying a 230 000 km2 area of the southern venezuelan basin and beata ridge the data set includes 5900 km of 2d seismic reflection data that are tied to lithologic and age data taken from dsdp sites 146 149 and 150 in the central part of the venezuelan basin we use observed patterns of wedging in mapped and dated sedimentary sequences adjacent to the scdb to determine the age of the diachronous west to east initiation of thrusting and subduction at the scdb the onset of subduction in the scdb corresponds with the cessation of thrusting from west to east along the zone of collision between the intraoceanic caribbean arc and the south american continent this correlation in the age of deformation indicates that collision between the caribbean arc and the south american continent led to subduction polarity reversal a process that is commonly observed at various stages of tectonic evolution in other areas of active arc continent collision including japan and sunda

paleomagnetism and magnetic fabric of the eastern cordillera of colombia evidence for oblique convergence and nonrotational reactivation of a mesozoic intracontinental rift we report the paleomagnetic and magnetic fabric results of 58 sites from cretaceous miocene marine and continental strata from the eastern cordillera ec and the cucuta zone at the junction between the santander massif and the merida andes of colombia the ec is an intracontinental doubly vergent range inverting a triassic to early cretaceous rift zone twenty three sites reveal nonsystematic tectonic rotations including unrotated areas of the ec range with respect to stable south america our data show that the ec inverted a nne oriented rift zone and that the orientation of the mesozoic rift and the mountain chain roughly correspond interestingly magnetic lineations from anisotropy of magnetic susceptibility analysis do not trend parallel to the chain but rather are oblique to the main orogenic trend by also considering gps evidence of a 1 cm yr ene displacement of central western colombia accommodated by the ec we suggest that the miocene recent deformation event of this belt arises from ene oblique convergence reactivating a nne oriented rift zone oblique shortening was likely partitioned into pure dip slip shear characterizing thick skinned frontal thrust sheets well known along both chain fronts and by range parallel right lateral strike slip faults which have not been identified yet but likely exist in the axial part of the ec finally the 35 9 clockwise rotation observed in four post miocene magnetically overprinted sites from the cucuta zone reflects late cenozoic and ongoing right lateral strike slip displacement occurring along faults parallel to the bocono fault system possibly connected with the right lateral faults inferred to exist along the axial part of the ec

spatial and temporal relationships between compression strike slip and extension in the central venezuelan andes clues for plio quaternary tectonic escape abstract the geometry of tectonic structures attributed to the neogene quaternary time interval is described in the active setting of the venezuelan andes our methodology is based on the analysis of radar satellite and digital elevation model imagery complemented by structural fieldwork and the compilation of seismotectonic data to make a structural analysis on a regional scale radar images provide first class data for morphostructural analysis in areas of dense vegetation and frequent cloud covering like the venezuelan andes we focused our analysis in the burbusay rio momboy and bocono faults corner located in the central part of the belt we have described three stages of deformation during the neogene quaternary the first one mio pliocene in age is a nw se compression responsible for the uplift of the venezuelan andes the second tectonic stage corresponds to a strike slip regime of deformation marked by shearing along the bocono burbusay and valera faults which separates two triangular wedges in the larger trujillo block this strike slip faulting dominated compressional extensional tectonic regime allowed the trujillo crustal block to move towards the ne wrenching has therefore started at some point between the pliocene and the quaternary these two tectonic events are consistent with ongoing strain partitioning in the venezuelan andes the third stage corresponds to extensional deformation limited to the trujillo block and is still active today extension is associated with the motion of crustal blocks moving relative to each other probably above the upper lower crust boundary such extensional deformation can be understood considering that the crust extends and stretches at the same time as it moves towards the ne the combination of both horizontal lateral motion and extension is characteristic of a tectonic escape process the northeastward escape of the trujillo block which belongs to the larger north andes block occurs as a result of the combination of the nw se intracontinental convergence between the south american plate and the maracaibo block and the presence to the north of the caribbean oceanic plate considered as a free boundary we have showed that the kinematics of the caribbean plate offers not only a favorable environment but may also be considered as the driving force of the tectonic escape of the north andes block

present day tectonics of the southeastern caribbean and northeastern venezuela four telemetered seismic arrays were operated in northeastern venezuela during the summer of 1979 an analysis of the data collected has resulted in accurate locations for about 100 microearthquakes and four new focal mechanism solutions on the basis of these new data an in depth analysis of teleseismic data and geologic evidence we propose a new plate tectonic framework for the southeastern caribbean in this model underthrusting of the atlantic seafloor along the lesser antilles is extended to the southeast of trinidad the subducted slab is shown to dip northwesterly beneath trinidad and the caribbean sea penetrating depths of at least 150 km this subduction appears to terminate abruptly in the vicinity of the los bajos el soldado fault zone which trends wnw nw and is located in the gulf of paria west of trinidad geologic evidence shows right lateral strike slip rlss motion in this fault zone probably initiated in the late pliocene this wnw nw trending fault zone joins up with the e w trending el pilar casanay fault system in northeastern venezuela which also moves right laterally the el pilar fault is apparently offset by nnw trending faults that exhibit rlss motion as well as normal faulting this complex motion on an en echelon series of faults is interpreted to be the result of wnw motion of south america relative to the caribbean plate in our interpretation the e w component of relative motion is accommodated by rlss motion on e w faults and results in normal faulting on nnw sse faults the smaller component of n s convergence is reflected by rlss motion on nnw trending faults and is accommodated by some internal deformation the timing of the initiation of rlss motion on the los bajos fault zone and the known total amount of displacement on it as well as other geologic indicators lead us to suggest that this complex mode of strike slip motion began only a few m y ago and that the average rate of relative plate motion in this region is probably 0 9 0 2 cm yr

subduction of the eastern panama basin and seismotectonics of northwestern south america spreading along the cocos nazca plate boundary since the breakup of the farallon plate in the miocene has resulted in the formation of the panama basin and a complex interaction of plates in and near northwestern south america current plate boundaries have been defined and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes considering only well located events and through focal mechanism determinations the existence of relict plate boundaries bathymetric features and the panamanian isthmus has affected the subduction process of the nazca plate beneath south america and determined the present day configuration of the subducting lithospheric plate there is no single triple junction separating the caribbean south american and nazca plates instead the panamanian isthmus and surrounding areas are accommodating east west compression and a lesser degree of north south compression along a series of thrust faults striking nw to ne and the andean ranges of ecuador colombia and venezeula are moving as a block nne relative to the rest of the south american plates along a system of faults following the front of the eastern cordillera the subducted portions of the panama basin and old farallon plate have become segmented into three pieces recognized in this study from north to south they are 1 a bucaramanga segment continuous with the caribbean seafloor northwest of colombia 2 a cauca segment continuous with oceanic crust nazca plate currently being subducted beneath south america at the colombia ecuador trench and 3 an ecuador segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern peru the segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted

quaternary fault kinematics and stress tensors along the southern caribbean from fault slip data and focal mechanism solutions deformation along the southern caribbean coast as confirmed by the compilation of stress tensors derived from fault plane kinematic indicators microtectonics and further supported by focal mechanism solutions herein presented results from a compressive strike slip transpressional senso lato regime characterized by a nnw sse maximum horizontal stress 1h 11 and or an ene wsw minimum 1h 13 or 12 horizontal stress which is responsible for present activity and kinematics of six sets of brittle features east west right lateral faults nw se right lateral faults synthetic riedel shears ene wsw to east west dextral faults p shears nnw sse normal faults almost north south left lateral faults antithetic riedel shears and ene wsw reverse faults sub parallel to fold axes and mostly in the subsurface the latter ones being associated to ene wsw trending folding in this particular region the brittle deformation obeys the simple shear model although not all the deformation can be accounted for it since partitioning is also taking place regional folding and thrusting is essentially due to the normal to structure component of the partitioned maximum horizontal stress conversely the maximum horizontal stress on

surface rupture of the cariaco july 09 1997 earthquake on the el pilar fault northeastern venezuela abstract this paper discusses the surface rupture of the cariaco july 09 1997 ms 6 8 earthquake in northeastern venezuela located at 10 545 n and 63 515 w and about 10 km deep the field reconnaissance of the ground breaks confirms that this event took place on the ene wsw trending onshore portion of the dextral el pilar fault between the gulfs of cariaco and paria which is part of the major wrenching system within the caribbean south america plate boundary zone dextral slip along this fault was further supported by the structural style of this rupture en echelon right lateral r shears connected by mole tracks at restraining stepovers and by larger geometric complexities pop ups at las manoas and guarapiche as well as by the focal mechanism solutions determined for the event by several authors this 1997 surface ruptre comprised two distinct sections from west to east a a main very conspicuous continuous 30 km long rather straight 075 n trending alignment of en echelon surface breaks with a rather constant purely dextral coseismic slip of about 25 cm but reaching a maximum value of 40 cm slightly northwest of pantono and b a secondary discontinuous 10 km long boomerang shaped rupture with a maximum coseismic slip of 20 cm at guarapiche the onshore extent of the surface rupture totalled 36 km but may continue westward underwater as suggested by the very shallow aftershock seismicity this aftershock activity also clearly defined the steep north dip of the fault plane along the western rupture suggesting tectonic inheritance on this major fault from many locals accounts the rupture seems to have propagated from pantono to the west highly asymmetric bidirectionality this suggests that earthquake nucleation happened at or near the casanay guarapiche restraining bend and rupture quickly propagated westward allowing only a small fraction to progress eastwards beyond the bend additionally the large fraction of after slip or creep released is to be related to such restraining bend which seems to have partly locked slip during rupture