are the venezuelan fault systems part of the southern caribbean plate boundary three major fault systems have been recognized in venezuela the oca bocono and el pilar fault zones the bocono el pilar system is an active late pleistocene holocene fault system which extends for over 1300 km between the venezuela colombia border through the venezuelan andes and along the northern venezuelan coast to the east of trinidad recent tectonic evidence suggests that until the late tertiary or early quaternary the caribbean plate south america plate boundary included the el pilar and oca fault systems since then it has included the bocono fault system right lateral offset along these fault systems is not sufficient to derive the caribbean plate from pacific crust alternatively the caribbean plate may have been a part of the south america plate until comparatively recent geologic time

erosion of the laurentide region of north america by glacial and glaciofluvial processes collection of seismic reflection data from continental margins and ocean basins surrounding north america makes it possible to estimate the amount of material eroded from the area formerly covered by laurentide ice sheets since major glaciation began in north america a minimum estimate is made of 1 62 106 km3 or an average 120 m of rock physically eroded from the laurentide region this figure is an order of magnitude higher than earlier estimates based on the volume of glacial drift cenozoic marine sediments and modern sediment loads of rivers most of the sediment produced during laurentide glaciation has already been transported to the oceans the importance of continental glaciation as a geomorphic agency in north america may have to be reevaluated evidence from sedimentation rates in ocean basins surrounding greenland and antarctica suggests that sediment production sediment transport and possibly denudation by permanent ice caps may be substantially lower than by periodic ice caps such as the laurentide low rates of sediment survival from the time of the permo carboniferous and precambrian glaciations suggest that predominance of marine deposition during some glacial epochs results in shorter lived sediment because of preferential tectonism and cycling of oceanic crust versus continental crust

cenozoic collision of the lesser antilles arc and continental south america and the origin of the el pilar fault it is proposed that the cenozoic tectonic record of the southern lesser antilles arc and northeastern continental south america can be explained by ongoing right oblique collision between the arc and continent the collision has proceeded by the transport of the leading edge of the arc across the slope and outer shelf of a former north facing passive margin of the south american continent the overriding began in the study region near the gulf of cariaco in eastern venezuela in late eocene or oligocene time and has migrated with a generally se vector suturing has occurred between the arc and continent after the attainment of a critical distance of overlap today s point of suturing lies in the paria peninsula east of there overriding continues major tectonic elements engaged in or created by the collision are the southern lesser antilles magmatic arc forearc basin the araya tobago terrane a south american foreland thrust and fold belt and a foreland basin the araya tobago terrane is thought to consist of sediments of south american provenance that were accreted to the lesser antilles forearc during its transit of an ocean basin and the continental slope and outer shelf the emplacement of the magmatic arc and the araya tobago terrane caused tectonic imbrication of shelf strata to propagate ahead of the arc front as a foreland thrust and fold belt tectonic loading of the shelf also caused subsidence of a major foreland basin on the continentward side of the thrust belt it is proposed the el pilar fault exists between the gulf of cariaco and the paria peninsula as an active right slip fault but not east of paria it is not a throughgoing transform fault between the south american and caribbean plates the el pilar fault exists where the overlapping arc and the continent are sutured and takes up a suture parallel component of convergence between arc and continent the eastern tip of the fault propagates east with the point of suturing reconstructions of the cenozoic collision of the lesser antilles arc and the south american continent suggest that the arc lay somewhat north and west of its present position in the eocene this conclusion differs from that of plate reconstructions that assume that the arc was the leading edge of a caribbean plate that has moved east from pacific longitudes since the eocene

south america and a few grains of sand part 1 beach sands to study correlations between petrography and provenance of modern sands and thus contribute to improved interpretation of ancient sandstones this study reports on the petrography of 218 modern south american beach sands obtained from a shoreline of 25 300 km judged by quartz feldspar rock fragment ratios there are five major associations pacific 24 16 60 argentina 28 14 58 brazil 91 4 5 caribbean east 59 8 23 and caribbean west 54 17 29 study of potash feldspar volcanic rock fragments and the sio 2 al 2 o 3 ratio supplement q f rf ratios tectonics are the dominant control on the mineralogy both the pacific and brazilian associations which form 75 of south america s coastline identify correctly tectonic setting that is active margin composition for the pacific and passive margin for the brazilian however the argentine association would yield a serious error because sand with active margin composition overprints a passive margin variation of total quartz and feld

glaciation erosion and uplift over part of east antarctica isostatic rebound effects after the removal of large glacial loads are well known from several parts of the world but isostatic uplift during glaciation and caused by glacial erosion is less well documented this is because preglacial landscapes which provide a datum against which glacial erosion and related uplift is measured are generally either destroyed by glaciation or not recognized remnants of such a preglacial land surface are exposed in the prince charles mountains of east antarctica we now report that the amount of glacial erosion here is 1 5 km measured by the difference in level between the preglacial land surface and the present mean mostly subglacial rock surface the isostatic uplift caused by this erosion is 0 8 km both amounts are thought to be well above average for east antarctica and glaciated areas in general because the region is traversed by east antarctica s largest outlet glacier system parts of which are believed to surge periodically

velocity field across the southern caribbean plate boundary and estimates of caribbean south american plate motion using gps geodesy 1994 2000 global positioning system gps observations between 1994 and 2000 at twenty two sites in the lesser antilles and northern south america indicate that the caribbean plate along its southern boundary slips at a rate of 20 5 2 mm a with an azimuth of n 84 2 e at 65 w relative to the south american plate east of 68 w 80 of the dextral slip is contained within a 80 km wide shear zone centered on the el pilar san sebastian fault system west of 68 w the plate boundary broadens to more than 300 km with dextral shear shared between the northeast trending bocono fault 9 11 mm a in western venezuelan and an offshore system near the northern coast

origin of cariaco basin southern caribbean sea abstract the cariaco basin an anoxic basin in the central eastern venezuelan continental borderland is located in the area of overlap of two en echelon right lateral strike slip fault zones the moron and el pilar fault zones on the basis of 28 acoustic reflection profiles across it the basin is interpreted as a pull apart basin due to east west tension in the area of the right step between the two fault zones 125 km of overlap and 35 km separation the offset along the fault zones is less than 100 km and at least 25 km its age is most probably quaternary as suggested by the thickness of the sedimentary fill and the estimated rate of deposition and because strike slip displacement along the master faults probably began no earlier than the late tertiary the cariaco basin is the largest pull apart basin along the bocono moron el pilar fault system which is a part of the caribbean south america plate boundary

a test of alternative caribbean plate relative motion models two contrasting models have been proposed to describe the present day motions between the caribbean plate and neighboring plates one model based on both north america caribbean and cocos caribbean data assumes that north america caribbean motion is reflected by the spreading rate approximately 2 cm yr inferred from magnetic anomalies at the cayman spreading center and the azimuths of nearby transforms jordan 1975 this geometry was used by minster and jordan 1978 in deriving global plate motion model rm2 the other model based only on north america caribbean data uses rates and azimuths inferred from the geometry of the lesser antilles wadati benioff zone sykes et al 1982 the cayman spreading center data were discounted assuming that they underestimate the full north america caribbean motion owing to the complex tectonics of the greater antilles the latter model assumes plate convergence at the lesser antilles at a rate about twice as fast as and an azimuth differing by 25 from the jordan model the two models also differ in their predictions of south america caribbean motion the jordan model predicts oblique convergence whereas sykes et al predict oblique divergence we use the nuvel 1 global relative motion data set which incorporates recent data not used in earlier studies to discriminate between the alternative models we find that the jordan geometry provides a better fit to the direction of north america caribbean motion moreover we find that models based on this geometry better fit the direction of cocos caribbean motion observed in earthquake slip vectors along the middle america trench since the prediction of this direction depends strongly on the rate of north america caribbean motion we conclude that the jordan model better describes caribbean plate motions the discrepancy between the two models results from the different methods of estimating plate motions sykes et al 1982 report that they determined a caribbean north america convergence rate of approximately 4 cm yr from the length of the lesser antilles wadati benioff zone we employ a similar procedure and obtain a rate closer to 2 cm yr we also find that the method they report using to determine the convergence direction is unable to distinguish between the alternative azimuths we thus conclude that estimates of the rate and direction of plate motion from wadati benioff zone configurations are insufficiently robust and that such estimates are better derived using rates from magnetic anomalies and azimuths from transform faults and slip vectors we further conclude that successful estimates of relative motion require internal consistency between the euler vectors of all relevant plates as demonstrated by the powerful cocos caribbean constraint not incorporated in the sykes et al model

bacterial mineralization patterns in basaltic aquifers implications for possible life in martian meteorite alh84001 to explore the formation and preservation of biogenic features in igneous rocks we have examined the organisms in experimental basaltic microcosms using scanning and transmission electron microscopy four types of microorganisms were recognized on the basis of size morphology and chemical composition some of the organisms mineralized rapidly whereas others show no evidence of mineralization many mineralized cells are hollow and do not contain evidence of microstructure filaments either attached or no longer attached to organisms are common unattached filaments are mineralized and are most likely bacterial appendages e g prosthecae features similar in size and morphology to unattached mineralized filaments are recognized in martian meteorite alh84001

within plate deformations in the maracaibo and east zulia basins western venezuela abstract subsurface data help in understanding the complex geometry of major structures of the maracaibo basin and surrounding areas during extensional and compressional phases decoupling levels were activated in the upper cretaceous black shales and the paleogene flysch sequences thus inducing frequent disharmony between a rigid basement and thin skinned cover structures two main extensional episodes are widespread across the maracaibo basin the first episode late jurassic in age accounts for the development of north northeast trending basement horsts and asymmetrical grabens the second episode more probably a transtensive one occurred during paleogene times it reactivated older north and northwest trending basement involving faults i e the ceuta and pueblo viejo faults but also generated listric normal faults and negative flower structures in the sedimentary cover by contrast compressional deformations are localized and reactivate older basement faults inducing the frequent inversion of former jurassic or lower to middle eocene grabens shortcuts in the basement and coeval wedging or fishtails in the sedimentary cover thus the major inversion episode in the maracaibo basin is upper eocene in age and probably relates to the emplacement of the caribbean nappes further to the northeast it was followed by an episode of tectonic quiescence and erosion during the oligocene neogene deformations have a limited extent beneath lake maracaibo but become widespread in the west towards the perija foothills or in the southeast near the north andean thrust front where they account for the development of frontal triangle zones east of the lake the east zulia basin is characterized by extremely thick paleogene flysch and a post middle eocene continental molasse superimposed on late jurassic or early paleogene troughs and normal vertical offsets of the basement the upper eocene to oligocene basin fill is largely detached from its substratum along a regional detachment in the eocene mud pile thus these thin skinned extensional structures display a piggyback attitude with progressive migration of depocenters overtime and gentle gliding of the sedimentary cover over underlying basement blocks the faulted borders of these late eocene oligocene basins were subsequently reactivated transpressively during late neogene times accounting for the frequent inversion of former oligocene depocenters

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

cenozoic landscape evolution of an east antarctic oasis radok lake area northern prince charles mountains and its implications for the glacial and climatic history of antarctica abstract ice free areas antarctica reveal a multi million year history of landscape evolution but most attention up to now has focused on the transantarctic mountains the amery oasis in the northern prince charles mountains borders the lambert glacier amery ice shelf system that drains 1 million km2 of the east antarctic ice sheet and therefore provides a record of fluctuations of both local and regional ice since the ice sheet first formed in early oligocene time this glacial record has been deciphered by i geomorphological mapping from aerial photographs and on the ground ii documenting the relationship between thick well dated uplifted glaciomarine strata and the underlying palaeolandscape iii examining surficial sediment facies and iv surface exposure dating using 10be and 26al the se amery oasis records at least 10 million years of landscape evolution beginning with a pre late miocene phase of glacial erosion followed by deposition of glaciomarine strata of the battye glacier formation pagodroma group in late miocene time a wet based ice sheet next expanded over the se amery oasis following which deposition of the glaciomarine pliocene bardin bluffs formation pagodroma group took place both formations were uplifted by at least 500 and 200 m respectively their tops are characterised by geomorphological surfaces upon which intensive periglacial activity took place higher level bedrock areas were subjected to deep weathering and tor formation early pleistocene time was characterised by expansion of a cold based ice sheet across the whole area but it left little more than patches of sandy gravel and erratic blocks late pleistocene expansion of local ice the battye glacier saw deposition of moraine mound complexes on low ground around radok lake and ice dammed lake phenomena subglacial drainage of the lake escaped to the east exhuming the sediment filled gorges holocene landscape modification has been relatively superficial overall the landscape of the amery oasis evolved primarily under the influence of wet based probably polythermal glaciers in miocene and pliocene times whereas the quaternary period was characterised mainly by cold based glaciers that had comparatively little impact on the landscape

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

evidence of rapid cenozoic uplift of the shoulder escarpment of the cenozoic west antarctic rift system and a speculation on possible climate forcing the cenzoic west antarctic rift system characterized by cenozoic bimodal alkalic volcanic rocks extends over a largely ice covered area from the ross sea nearly to the bellingshausen sea it is bounded on one side by a spectacular 4 to 5 km high rift shoulder scarp maximum bedrock relief 5 to 7 km from northern victoria land queen maud mountains to the ellsworth whitmore horlick mountains jurassic tholeiites crop out with the late cenozoic volcanic rocks along the section of the transantarctic mountains from northern victoria land to the horlick mountains the cenozoic rift shoulder diverges here from the jurassic tholeiite trend and the tholeiites are exposed discontinuously along the lower elevation 1 2 km section of the transantarctic mountains to the weddell sea various lines of evidence no one of which is independently conclusive lead the authors as others have also suggested to interpret the following the transantarctic mountains part of the rift shoulder and probably the entire shoulder has been rising since about 60 ma at episodic rates of approximately 1 km m y most recently since mid pliocene time rather than continuously at the mean rate of 100m m y uplift rates vary along the scarp which is cut by transverse faults the authors speculate that this uplift may have climatically forcedmore the advance of the antarctic ice sheet since the most recent warm period they suggest a possible synergistic relation between episodic tectonism mountain uplift and volcanism in the cenozoic west antarctic rift system and waxing and waning of the antarctic ice sheet beginning about earliest oligocene time less

paleogeographic and tectonic implications of quartzose sandstones of barbados the provenance of paleogene sandstones on the island of barbados is relevant to various models of the tectonic evolution of the southern caribbean area modal point counts of 26 sandstones from barbados show that the constituent grains most likely were derived from a composite craton interior and orogenic margin abundant detrital polycrystalline quartz grains have affinities to a spectrum of low to high rank metamorphic plutonic crystalline source rocks microprobe data suggest that feldspars were derived from metamorphic or plutonic source terrances the complex heavy mineral suite of the terrigenous sandstones including glaucophane chloritoid titanite zircon and aluminosilicates supports derivation from orogenic and cratonal terranes due to the complex history of the south american caribbean plate boundary and poor age constraints on timing of deposition widely contrasting models have been proposed concerning the provenance of the sandstones convergence between the aruba orchila arc and continental south america during the paleogene resulted in the formation of an e w trending foreland fold thrust belt and foredeep dominant sediment dispersal was to the east northeast along tectonic strike the present study shows that the terrigenous sandstones on barbados probably were deposited in deep sea fans to the north of present day araya peninsula during and shortly after the late middle eocene probable source terranes include the guayana shield the caribbean mountain system interior foreland fold thrust belt and possibly to a lesser extent the lesser antilles magmatic arc or predecessor arc

emergence of the shackleton range from beneath the antarctic ice sheet due to glacial erosion abstract this paper explores the long term evolution of a subglacial fjord landscape in the shackleton range antarctica we propose that prolonged ice sheet erosion across a passive continental margin caused troughs to deepen and lower the surrounding ice sheet surface leaving adjacent mountains exposed geomorphological evidence suggests a change in the direction of regional ice flow accompanied emergence simple calculations suggest that isostatic compensation caused by the deepening of bounding ice stream troughs lowered the ice sheet surface relative to the mountains by 800 m use of multiple cosmogenic isotopes on bedrock and erratics 26al 10be 21ne provides evidence that overriding of the massif and the deepening of the adjacent troughs occurred earlier than the quaternary perhaps this occurred in the mid miocene as elsewhere in east antarctica in the mcmurdo dry valleys and the lambert basin the implication is that glacial erosion instigates feedback that can change ice sheet thickness extent and direction of flow indeed as the subglacial troughs evolve over millions of years they increase topographic relief and this changes the dynamics of the ice sheet

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

landscape evolution of the dry valleys transantarctic mountains tectonic implications there are different views about the amount and timing of surface uplift in the transantarctic mountains and the geophysical mechanisms involved our new interpretation of the landscape evolution and tectonic history of the dry valleys area of the transantarctic mountains is based on geomorphic mapping of an area of 10 000 km2 the landforms are dated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution following a lowering of base level about 55 m y ago there was a phase of rapid denudation associated with planation and escarpment retreat probably under semiarid conditions eventually downcutting by rivers aided in places by glaciers graded valleys to near present sea level the main valleys were flooded by the sea in the miocene during a phase of subsidence before experiencing a final stage of modest upwarping near the coast there has been remarkably little landform change under the stable cold polar conditions of the last 15 m y it is difficult to explain the sirius group deposits which occur at high elevations in the area if they are pliocene in age overall denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland which is in good agreement with the results of existing apatite fission track analyses it is suggested that denudation reflects the differences in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy most crustal uplift 2 4 km is inferred to have occurred in the early cenozoic with 400 m of subsidence in the miocene followed by 300 m of uplift in the pliocene

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

limited modification of mid latitude landscapes by ice sheets the case of northeast scotland the paper uses a case study in scotland to examine the amount and processes of landscape modification by quaternary ice sheets there is an inverse correlation between the distribution of landforms of glacial erosion and pre glacial landscape remnants in northeast scotland the implication is that in places ice sheets can preserve a pre glacial landscape unscathed while elsewhere they remove the pre glacial weathered rock the location of glacial protection or erosion is strongly influenced by the topography and its influence on former ice sheet flow and basal thermal regime the classic glacially eroded landscape of areal scouring can be produced by the removal of only 10 50 m of weathered rock furthermore rock basins often regarded as the hallmark of glacial erosion may be directly inherited from the pre glacial pattern of deep weathering