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

neotectonic and paleoseismicity studies on the urumaco fault northern falcon basin northwestern venezuela the northern falcon basin in northwestern venezuela is affected by several small active faults subordinated to the major right lateral east west trending oca ancon fault system a set of prominent nw se right lateral faults synthetic shears such as the urumaco rio seco lagarto and la soledad faults stands out among those the urumaco fault located between the lagarto and mitare rivers in the urumaco trough west of coro presents a rather complex active fault trace that comprises two nw se fault segments linked by an ene wsw reverse echelon all showing a restraining stepover geometry its western segment seems to continue to the north at sea conversely the eastern one dies out on land and its northern tip ends in a transtensive horse tail structure that disrupts an early pleistocene conglomerate this same unit is flexed and upheaved some 30 m at the restraining overlap the kinematics and present stress tensor the latest activity and the seismogenic potential of the eastern segment of the urumaco fault have been assessed at a set of three river cuts of an ephemeral tributary stream of the urumaco river 3 km north of the urumaco village where the urumaco formation is truncated by a late pleistocene terrace 14 cd ate of 20 700 950 yr bp at the base of the urumaco river on the one hand one of these outcrops features the urumaco fault affecting the late miocene urumaco formation which comprises two prominent fault planes disposed as a wedge the southwestern bounding plane juxtaposes two different sequences whereas the northeastern one does not implying different slip behavior in fact the northeastern plane shows oblique slip striations 29on normal dextral whereas the other one shows perfectly horizontal striations right lateral on the other hand both updip plane prolongations in the overlying alluvial unit are not so sharp if the 17 cm throw of the erosive bottom of such terrace measured at the lowermost part of the southwestern plane is regarded as an artifact however a mudflat deposit within this unit is bent with a 14 cm throw right above the northeastern fault plane clearly affecting the underlying miocene unit the estimated total offset per event allows to infer the occurrence of two individual events of magnitude ranging between ms 5 8 and 6 4 on this strand of the eastern segment of the urumaco fault in the last 20 000 yr 1999 elsevier science b v all rights reserved

p wave velocity tomography of the venezuelan region from local arrival times arrival times from the local seismological network of venezuela were used to estimate a three dimensional p wave velocity model for the region between longitude 60 74 w and latitude 6 14 n to a depth of more than 80 km the inversion was carried out by damped least squares describing the media by homogeneous velocity blocks the resolved lateral velocity variations in the first layer 0 30 km depth showed a correlation with the main stratigraphic features of the area while second layer 30 50 km depth showed the influence of moho depth variations through the region generating a pattern well correlated to the bouguer anomaly map lithospheric seismic velocities below the moho appear to be influenced by the major crustal fault systems an important low velocity zone is present below the triple junction of the fault systems of oca bocono and moron in northwestern venezuela farther south a similar low velocity zone is present below the junction of the bocono and the santa marta fault systems those are the two continental corners of the triangular maracaibo block below 80 km depth the fourth layer the low velocity zones show a connected pattern that follows or is adjacent to the crustal fault zones the presence of subducted atlantic lithosphere below the eastern venezuelan basin could explain the high velocity zone at this location a similar interpretation emerges from the tectonic wedging model previously proposed to explain the pronounced minimum of the gravity anomaly

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

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

the interaction of two closely spaced cracks a rock model study the influence of preexisting fractures on fracture development and the geometric patterns of crack growth have been investigated experimentally rectangular specimens of berea sandstone with two parallel thin cuts were compressed along their longest dimension at a confining pressure of 60 mpa a pore pressure of 20 mpa an average displacement rate of 2 10 4 cm s and room temperature the precuts are inclined 35 to the loading direction they are en echelon with each other each extending only part way from the specimen boundary when the sense of shear left or right lateral overlap left or right stepping and the amount of separation are changed significant difference of fracture interaction occurs an extensional fracture zone forms between left stepping precuts subjected to left lateral shear or right stepping precuts under right lateral shear secondary fractures tend to develop from one precut and toward the other precut the sequence of the fracture development is as follows a fractures which are about 60 to the precuts fractures which are about 30 to the precuts and fractures for large overlap pattern which are about 70 to the precuts but opposite in direction the fracture zone is confined by precuts and fractures therefore the larger the overlap the larger the fracture zone that develops for large overlap patterns a set of fractures forms nearly parallel and equally spaced within the fracture zone in cross sections fractures are normal faults with surface depression microscopic study shows that microfractures are highly concentrated in the area immediately adjacent to the macrofractures and at angles less than 30 to the macrofracture grains away from macrofractures remain unfractured for right stepping precuts subject to left lateral shear or left stepping precuts under right lateral shear a compression zone forms between the precuts the fractures propagate at precut tips and diverge away the fractures develop within the compression zone and are bounded by precuts and fractures the fractures develop within the compression zone and are bounded by precuts and fractures the fractures have reverse offsets in cross section

timing of glaciation during the last glacial cycle evaluating the concept of a global last glacial maximum lgm abstract it has long been known that mountain glaciers and continental ice sheets around the globe reached their respective maximum extent at different times during the last glacial cycle often well before the global last glacial maximum lgm c 23 19 ka which is formally defined by peaks in global sea level and marine oxygen isotope records however there is increasing evidence from around the world that it was not only mountain glaciers which were asynchronous with the global lgm but also some regions of the large continental glaciers the barents kara ice sheet in northern eurasia together with a majority of ice masses throughout asia and australasia reached their maximum early in the last glacial cycle a few thousand years before the global lgm period the east antarctic ice sheet also reached its maximum extent several millennia before the global lgm in numerous mountainous regions at high mid and low latitudes across the world glaciers reached their maximum extent before marine isotope stage mis 2 in mis 5 4 and 3 this is in contrast to most sectors of the laurentide ice sheet the cordilleran ice sheet the se sector of the fennoscandinavian ice sheet and the alpine ice sheet in central europe which appear to have reached their maximum close to the global lgm in mis 2 the diachronous maximum extents of both mountain glaciers and continental ice sheets during the last glacial cycle means that the term and acronym last glacial maximum lgm has limited chronostratigraphical meaning when correlating glacial deposits and landforms

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

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

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

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

pleistocene plateau ice fields in the high atlas morocco large ice fields 25 km2 formed over the tazaghart and iouzagner plateaux of the high atlas morocco during the late pleistocene the plateau ice fields were drained by large valley glaciers forming a series of moraine assemblages four moraine units have been mapped and subdivided on the basis of morphostratigraphy and the degree of soil weathering soil profile development index pdi values indicate that the moraine units are widely separated in time with the oldest moraines deeply weathered and degraded whereas soils are absent on the youngest moraines the highest moraine unit was formed by a small niche glacier which was present as recently as the mid 20th century pleistocene glaciers are likely to have been associated with wetter conditions than today in addition to colder air temperatures combined with ice in neighbouring areas such as the toubkal massif the sw high atlas supported some of the largest glaciers in africa during the pleistocene the extent of glaciation with ice exploiting and breaching drainage divides has major implications for landscape development the evolution of the high atlas is therefore strongly shaped by glaciation and this is closely intertwined with tectonic fluvial and slope processes

timing of the younger dryas event in east africa from lake level changes the last deglaciation was interrupted by an abrupt cooling event the younger dryas at 11 000 ndash 10 000 yr bp uncalibrated radiocarbon timescale 1 originally recognized in climate records from northwest europe the younger dryas has now been identified in marine and ice core records worldwide2 6 in the tropics a broadly contemporaneous change in climate is recorded by decreases in water levels and increased salinity of lakes7 9 14 indicating a period of arid climate caused by a reduction in ocean to land moisture flux the exact timing of these changes in relation to the younger dryas event in high latitude records has remained unclear however here we present climate records based on analyses of diatom assemblages geochemistry and magnetic mineralogy of radiocarbon dated sequences of laminated lake sediments from lake magadi in the east african rift these records provide a detailed record of climate change in lowland equatorial africa throughout the last deglaciation 12 800 ndash 10 000 14c yr bp we find that lake level and humidity maxima coincide with the most rapid phases of ice melting in the northern hemisphere and that the climate changes including the younger dryas event were synchronous at low and high latitudes thus the effects of abrupt climate change appear to be felt at both high and low latitudes without a significant time lag

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

southeastern caribbean sea level variability and viscoelastic relaxation viscoelastic modeling of earthquakes in the southeast caribbean shows that distant dip slip earthquakes in the lesser antilles and the geometry of the subduction zone play a significant role in vertical deformation observed along the southern caribbean plate margin we find that dip slip earthquakes yield vertical displacements that are 1 2 orders of magnitude greater than those generated by strike slip earthquakes along the caribbean south american plate boundary in addition postseismic vertical displacements are at least as great or greater at distances of several hundred kilometers than the coseismic counterpart depending upon the geometry of the source tide gage data from venezuela suggest that these vertical displacement rates may still be observed several decades after the occurrence of large subduction zone earthquakes ms 7 or greater in addition the uplift rate observed at the tide gages largely follows the anticipated flexure of the caribbean plate due to the subduction process

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

pollen derived rainfall and temperature estimates from lake tanganyika and their implication for late pleistocene water levels abstract palaeoclimatic estimates of mean annual temperature and rainfall in the southern tanganyika basin between 25 000 and 9000 yr b p have been established from two pollen sequences based on the best analogue method the results give evidence of a mean temperature decrease of about 4 2 c during the last glaciation a value consistent with that previously obtained in the catchment area on the burundi highlands this cooling was synchronous with a decrease of mean annual precipitation of about 180 mm yr postglacial climatic conditions were established by 12 700 yr b p with warming and wetness continuing to increase from this date onward these new palaeoclimatic data will be useful for hydrological reconstructions of lake tanganyika particularly during the last glacial age for which the magnitude of water level fall has been a controversial issue our rainfall estimates are more consistent with low values 250 to 300 m fall than with high ones 600 m previously proposed

anisotropy of magnetic susceptibility in relation to source vents of ignimbrites empirical observations the anisotropy of magnetic susceptibility ams of young undeformed ashflow deposits e g bishop tuff california bandlier tuff new mexico show alignment of the maximum axis of magnetic susceptibility kmax with inferred flow lines the sense of flow is deduced from the predominant plunge bias of kmax axes these principles are discussed in relation to four deformed settings of ashflow deposits in the great basin of the western united states the magnetic susceptibility data provide valuable but not always unambiguous insight into probable source areas various complications arise in interpreting the ams data in terms of flow directions but these can be reduced by integration with geologic observations

role of a ductile decollement in the development of pull apart basins experimental results and natural examples abstract traditional models describe pull apart basins as graben or half graben basins with normal or normal oblique slip master faults analogous to death valley california yet many pull aparts are characterized by asymmetric basins with strike slip master faults indicating that not all pull apart basins conform to the simple death valley models we present analogue modelling results that show developmental sequences and structural styles of pull aparts are dramatically different when overburden rides over a ductile horizon and that thickness of the ductile horizon exerts control on basin development in our models synthetic and antithetic strike slip faults control basin geometries while localized normal faulting and local oblique slip on strike slip faults accommodate basin subsidence faults evolve from initial strike slip to normal oblique and normal dip slip to form a system of either isolated sub basins in the case of thick ductile layers or coalescing sub basins in the case of thin ductile layers these results demonstrate distinct differences between non ductile and ductile decollement pull apart structures basin boundaries dominated by normal faults suggest a decollement within or at the base of a non ductile layer similar to death valley california basin bounding faults dominated by strike slip and oblique slip faults indicate basin formation over a ductile layer similar to the gulf of elat aqaba or gulf of paria venezuela and trinidad

geodetic control of neotectonics in venezuela abstract henneberg h g 1983 geodetic control of neotectonics in venezuela in p vyskocil a m wassef and r green editors recent crustal movements 1982 tectonophysics 97 1 15 net installations for measurements of movement started in 1973 along the bocono fault which is the main fault in the venezuelan andes investigations of horizontal movements are made on three locations along this fault mucubaji at 3700 m height mitisus at 1700 m and yacambu at 700 m at mucubaji there are two nets with 8 stations each and at mitisus we have three nets represented by 29 stations each station has a concrete pillar with a forced centering device on top and has a 1 4 m3 concrete plaform these nets were measured several times giving resulting difference vectors of a range of several cm one of these nets is also a control net of a hydroelectrical power dam installed near the fault trace at yacambu we have a 26 km extended tunnel net which is crossing the fault this net was measured twice with difference vectors up to 6 cm in the maracaibo basin area the vertical component of subsidence due to oil extraction reaches 5 m as a result of continuous movements since 1925 gravimetric research is also carried out over all the mentioned locations in cooperation with the universities of hannover and stuttgart and the german geodetic research institute a programme was started to determine the geoid and deflections of the vertical in the high andes mountains especially around the mucubaji fault site parallel to and in combination with these geodetic investigations seismological studies are carried out by a geophysical group from merida university venezuela

paleopedology of middle wisconsin weichselian paleosols in the merida andes venezuela abstract buried spodosols podzols within a thick lithostratigraphic section in the northern venezuelan andes are interbedded with tills and a succession of multicyclic lacustrine clays silts and sands from the early and middle stades of the wisconsinan weichselian glaciation the peats dated by ams radiocarbon allow calculations of the time required for pedogenesis these spodosols were studied to determine compositional changes from initial to full soil development during lake low water stands peats of variable thickness developed along with soils belonging to the spodosol order on a relative time scale the initial stage of soil genesis produced peat with a weathered mineral zone bs initial spodic horizon of a few centimeters thickness with a longer time for soil genesis of a few centuries a thin spodosol developed with an e bs horizon sequence with still longer time spans of a few thousand years between 55 and 60 ka more mature spodosols with e bs cox cu profiles formed beneath very thick peat in conjunction with groundwater fluctuations the lowermost peat vii in the section varies from 15 to 26 5 cm in thickness the paleosols are sandy with little evidence of either clay transformations or silt accretion from airfall influx apparent from the particle size analysis xrd analysis of the clay fraction shows minor weathering of illite to vermiculite and chlorite in the bs horizons chemically extractable forms of fe and al show increases of both soluble and insoluble minerals from the e to the bs horizons in the buried spodosols a considerably different trend compared with the surface soils entisols geochemical trace element analysis shows a moderate degree of translocation of soluble chemical elements from the e to the bs horizons

catastrophic rock avalanches in a glaciated valley of the high atlas morocco 10be exposure ages reveal a 4 5 ka seismic event surface modification processes leading to large debris accumulations in high relief mountain areas are important for understanding landscape evolution especially in some of earth s most active orogens the arroumd rock avalanche at the foot of the nw face of mount aksoual 3912 m above sea level masl in the jebel toubkal area of the high atlas morocco represents one of the largest mass movement landforms in north africa the age and origin of this extensive feature have been contested for over a century late pleistocene moraines are also present in the same valley adjacent to the avalanche debris the mean of six 10 be cosmogenic exposure ages shows that a series of catastrophic rock slope failures occurred at 4 5 0 5 ka while a set of eight exposure ages from two of the three mapped moraines has a far larger spread from 1 5 to 7 5 ka this suggests that the avalanche events were effective agents in modifying the true surface exposure age of the pleistocene moraines in the arroumd valley this has resulted in similar mean 10 be apparent exposure ages for the preexisting late pleistocene moraine surfaces and holocene catastrophic rock slope failures similar rock avalanche deposits are present in other glaciated valleys in the high atlas we conclude that the trigger for collapse was seismic activity related to proximity of the major tizi n test fault these findings have important implications for interpreting and dating glacial landforms in tectonically active settings

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

cenozoic plate tectonic history of the northern venezuela trinidad area geological and geophysical data coupled with recent plate tectonic reconstructions suggest that the cenozoic geologic history of the northern venezuela trinidad area has been dominated by strike slip displacement of discrete crustal blocks allochthonous terranes within the area include metavolcanic rocks of the cretaceous villa de cura group and metamorphic rocks of the precambrian to cretaceous cordillera de la costa a relatively competent crustal block margarita block is defined by an outline around the metamorphic basement of margarita island the araya paria peninsula the northern range of trinidad and tobago island reconstruction of the margarita block to its original position requires at least partial closure of the falcon basin closure of the bonaire and cariaco basins and restoration of about 50 km of motion on both the oca and bocono faults post middle eocene eastward translation of the caribbean plate caused eastward motion of the margarita block a minor change in relative plate motion during the late oligocene or early miocene produced a right step in the moron fault forming the cariaco pull apart basin and el pilar fault zone maximum offset on el pilar fault is estimated to be no more than 125 km though displacement along the entire fault zone may have been greater transpressional stresses between the caribbean plate and northern south america caused folding of the serrania del interior of venezuela and the central range of trinidad eastward migration of transpressional stresses at the southeastern corner of the caribbean south american plate boundary is being accommodated by formation of oblique thrusts transpressive anticlines and downwarping of the crust bouguer gravity data suggest that jurassic aged atlantic oceanic crust is being depressed as the caribbean plate expands into the demerara plateau area this study suggests that the faults and transtensional transpressional compressional structures identified in this study are the result of stresses produced during the large eastward translation of the caribbean plate since the paleocene and are not the product of a shear couple

atmospheric circulation patterns during late pleistocene climate changes at lake malawi africa abstract the climate of tropical africa transitioned from an interval of pronounced orbitally paced megadroughts to more humid and stable conditions approximately 70 000 years ago scholz et al 2007 the regional atmospheric circulation patterns that accompanied these climatic changes however are unclear due to a paucity of continental paleoclimate records from tropical africa extending into the last interglacial we present a new 140 kyr record of the deuterium hydrogen isotopic ratio of terrestrial leaf waxes d wax from drill cores from lake malawi southeast africa that spans this important climatic transition d wax shifts from highly variable and relatively d depleted to more stable and d enriched around 56 ka contemporary with the onset of more humid conditions in the region moisture source and transport history dominate the d wax signal at lake malawi with local rainfall amount playing a secondary role for much of the paleorecord analysis of modern moisture sources for lake malawi suggests that d depletion of waxes during the megadroughts may have been caused by an enhanced contribution of the drier d depleted air mass currently located in central southern africa to the lake malawi catchment this d depleted air mass is associated with the descending limb of the hadley cell which implies significant changes in the hadley circulation during the megadroughts and related changes in the position of the intertropical convergence zone over africa these findings demonstrate the ability of d wax to serve as an atmospheric tracer when used in conjunction with additional proxy records for moisture balance and elucidate potential mechanisms for pronounced hydrological change in southeast africa during the late pleistocene

effect of aridity and rainfall seasonality on vegetation in the southern tropics of east africa during the pleistocene holocene transition fossil pollen analyses from northern lake malawi southeast africa provide a high resolution record of vegetation change during the pleistocene holocene transition 18 9 ka recent studies of local vegetation from lowland sites have reported contrasting rainfall signals during the younger dryas yd the lake malawi record tracks regional vegetation changes and allows comparison with other tropical african records identifying vegetation opening and local forest maintenance during the yd our record shows a gradual decline of afromontane vegetation at 18 ka around 14 5 ka tropical seasonal forest and zambezian miombo woodland became established at 13 ka drier more open formations gradually became prevalent although tropical seasonal forest taxa were still present in the watershed during the yd this drought intolerant forest type was likely restricted to areas of favorable edaphic conditions along permanent waterways the establishment of drought tolerant vegetation followed the reinforcement of southeasterly tradewinds resulting in a more pronounced dry winter season after 11 8 ka the onset of the driest most open vegetation type was coincident with a lake low stand at the beginning of the holocene this study demonstrates the importance of global climate forcing and local geomorphological conditions in controlling vegetation distribution

14 c and th u dating of pleistocene and holocene stromatolites from east african paleolakes during recent humid episodes stromatolites were built along paleolake margins some 60 m above the modern water level of lakes natron and magadi southern gregory rift valley three generations of stromatolites are observed the more recent ones frequently covering pebbles and boulders eroded from the older ones the youngest one yielded 14c ages ranging from approximately 12 000 to 10 000 yr b p their 13c values 2 6 suggest isotopic equilibrium between the paleolake total inorganic dissolved carbon and the atmospheric co2 thereby lending credence to the reliability of the 14c an initial 230th232th ratio in the detrital component was determined by thu measurements on the 14c dated stromatolites using this value a 230th234u chronology for the older stromatolites was calculated ages of 240 000 and 135 000 10 000 yr were obtained for the first and second generations respectively a humid episode apparently characterized eastern africa during each glacial interglacial transition 18o and 13c measurements on stromatolites when compared to values on modern waters and carbonates provide paleohydrological information long residence time of the paleolake waters and less seasonally contrasted regimes are inferred

climatic and hydrological instability in semi arid equatorial east africa during the late glacial to holocene transition a multi proxy reconstruction of aquatic ecosystem response in northern tanzania this paper reports new multi proxy palaeoenvironmental data on the late glacial holocene transition c 14 8 9 3 ka in equatorial east africa in the form of microfossil assemblages chironomids

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

response of the himalayan glacial cycles to multiple equilibrium of climate system a review abstract the astronomical theory of milankovitch supports the waxing and waning of ice sheets and polar glaciers by orbital forcing excluding the fact that small changes in solar radiation flux are difficult to reconcile with the amplitude of glacial cycles the numerical model explaining the 100 kyr problem has now revealed that the phase locking mechanism of orbital forcing has started to modulate the climatic transitions since 1 2 myr back and remained locked till present climatic state rial et al 2013 however the true evidence to confirm it is not present the present study is an attempt to comprehend the concept of synchronization of summer insolation modulation and internal climate forcing responsible for glacial interglacial termination present in and around the himalaya tibet and worldwide we present geographical evidences of glaciations from the himalayan tibet region pointing out that responses i e 18 24 kyr and 60 70 kyr bp glaciation events are similar to the progression in climatic data and glaciations cycles for the last 100 kyr the compelling evidence presents a possible explanation towards synchronized modulation of 100 kyr orbital cycles with 41 kyr obliquity cycles coincidently resulting in unprecedentedly large glaciation around the last glacial cycle 60 70 kyr bp and the last glacial maxima 18 24 kys bp by considering the influence of internal and external factors on glacier mechanism we underline the most probable cause of 41 kyr cycle obliquity as the foremost driver of glaciations in the himalaya and tibet on longer timescales we provide several lines of evidence to establish the possible connection of glaciation events and orbital cycles with the help of geographical and geological records consistent with the fact that resonance might be faded through time but the effect of frequency modulation still persists in the system

the lake natron footprint tuff northern tanzania volcanic source depositional processes and age constraints from field relations the engare sero footprint site situated on the southern shore of lake natron in northern tanzania has been reported to host one of the best preserved sets of fossilized hominid footprints in the

climatic changes in eurasia and africa at the last glacial maximum and mid holocene reconstruction from pollen data using inverse vegetation modelling in order to improve the reliability of climate reconstruction especially the climatologies outside the modern observed climate space an improved inverse vegetation model using a recent version of biome4 has been designed to quantitatively reconstruct past climates based on pollen biome scores from the biome6000 project the method has been validated with surface pollen spectra from eurasia and africa and applied to palaeoclimate reconstruction at 6 cal ka bp calendar years the climate was generally wetter than today in southern europe and northern africa especially in the summer winter temperatures were higher 1 5 c than present in southern scandinavia northeastern europe and southern africa but cooler in southern eurasia and in tropical africa especially in mediterranean regions summer temperatures were generally higher than today in most of eurasia and africa with a significant warming from 3 to 5 c over northwestern and southern europe southern africa and eastern africa in contrast summers were 1 3 c cooler than present in the mediterranean lowlands and in a band from the eastern black sea to siberia at 21 cal ka bp a marked hydrological change can be seen in the tropical zone where annual precipitation was 200 1 000 mm year lower than today in equatorial east africa compared to the present a robust inverse relationship is shown between precipitation change and elevation in africa this relationship indicates that precipitation likely had an important role in controlling equilibrium line altitudes ela changes in the tropics during the lgm period in eurasia hydrological decreases follow a longitudinal gradient from europe to siberia winter temperatures were 10 17 c lower than today in eurasia with a more significant decrease in northern regions in africa winter temperature was 10 15 c lower than present in the south while it was only reduced by 0 3 c in the tropical zone comparison of palaeoclimate reconstructions using lgm and modern co2 concentrations reveals that the effect of co2 on pollen based lgm reconstructions differs by vegetation type reconstructions for pollen sites in steppic vegetation in europe show warmer winter temperatures under lgm co2 concentrations than under modern concentrations and reconstructions for sites in xerophytic woods scrub in tropical high altitude regions of africa are wetter for lgm co2 concentrations than for modern concentrations because our reconstructions account for decreased plant water use efficiency

paleocurrent analysis for the late pleistocene holocene incised valley fill of the yangtze delta china by using anisotropy of magnetic susceptibility data abstract anisotropy of magnetic susceptibility ams analysis has been conducted on samples from borehole core cm 97 from the yangtze river changjiang incised valley fill china to determine the paleocurrent directions to help in reconstructing sedimentary paleoenvironments borehole cm 97 consists of fluvial unit 1 estuarine units 2 6 and deltaic units 7 9 sediments after the last glacial maximum in ascending order the ams results show that the paleocurrent directions for the tide dominated estuarine and deltaic sediments were westerly or northwesterly directed flood tide dominated but give no definite trend for the fluvial sediments comparison between the paleocurrent directions inferred from primary sedimentary structures and in situ ams data shows that they are in good agreement confirming the applicability of ams as a good paleocurrent indicator for sediments deposited in coastal tide dominated environments considering that these sediments were strongly tide influenced and the tidal pattern since 12 kyr bp has not changed significantly we think that the westerly or northwesterly current direction most probably resulted from flood tidal currents and the sedimentary paleoenvironment was a flood tide dominated estuary or delta additionally it has been found that downhole changes of some ams parameters including the mean magnetic susceptibility k the corrected anisotropy degree p j and the magnetic foliation f clearly mark the defined stratigraphic boundaries in the borehole this further extends the validity of ams as a good stratigraphic marker in addition to its long known credibility as a sensible paleocurrent recorder

earthquakes of the holocene geologic studies of earthquakes involve mapping of coseismic surface faulting and secondary deformation from historical events trenching and geomorphic analysis to define the timing and size of past earthquakes and investigations of fault zone structure and geometry in both unconsolidated sediments and bedrock this research is now being referred to as paleoseismology seismic geology and earthquake geology since the mid 1970s it has led to some of the most exciting and important contributions to the understanding of earthquake behavior in space and time hanks 1985 allen 1986 the present report is the first of what is hoped will become a regularly contributed summary of the geologic aspects of the study of earthquakes

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