andean tectonics as a cause for changing drainage patterns in miocene northern south america newdatafromneogenestratainnorthernsouthamericasuggestthatmiocenetectonisminthenortheasternandeswasresponsible for the genesis of the amazon river and changes in the drainage patterns of other major rivers such as the magdalena and the orinoco here we present a new model for the paleogeographic evolution of northern south america during the miocene in the early miocene a large part of the drainage of northwest amazonia was directed northward along the paleo orinoco river system to a deltainlakemaracaibo upliftoftheeasterncordillerainthelate middle miocene caused thefirst development of the amazon river however no connection with the atlantic was established and the amazon fed the paleo orinoco river system which drained toward the caribbean substantial andean uplift in the late miocene resultedinmajorchangesinpaleogeography theorinocochangedits course the amazon established a connection to the atlantic causing the drowning of carbonate platforms and the amazon caribbeanconnectionwasclosed thusthedrainageandpaleogeography ofnorthernsouthamericainthemiocenewerestronglycontrolled by tectonic movements in the northeastern andes

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

tectonic controls of the right lateral burro negro tear fault on paleogene structure and stratigraphy northeastern maracaibo basin the northeastern maracaibo basin in western venezuela was deformed by paleogene thrusting and an associated tear fault burro negro right lateral strike slip fault zone related to paleogene oblique collision between the caribbean and south american plates two different tectonic models have been previously proposed for the thick paleogene depocenter located along the northeastern margin of the maracaibo basin the first model proposes that the depocenter is a foreland basin controlled by southwestward directed overthrusting during a late paleocene middle eocene oblique collision between the caribbean and south american plates the second model supported by data presented in this study proposes that the asymmetric paleogene maracaibo sedimentary wedge was controlled by strike slip displacement along a right lateral tear fault separating southeast directed thrust sheets to the east lara nappes from a more stable platform area to the west maracaibo basin regional seismic lines recorded to 5 s two way traveltime reveal the structure of the asymmetric paleogene depocenter in the northeastern part of the maracaibo basin the approximately 100 km 62 mi long burro negro fault is a right lateral strike slip fault separating less deformed inner to outer shelf rocks of the western maracaibo basin from highly deformed deep marine rocks of the eastern maracaibo basin seismic lines northeast of the burro negro fault zone show elongate subsurface basins bounded by partially inverted reverse and strike slip faults filled with about 3 km 1 8 mi of oligocene and miocene clastic marine sedimentary rocks structural highs of eocene rocks are characterized at depth on seismic reflection lines by chaotic seismic reflections that underlie the more coherently stratified oligocene and miocene subbasins we interpret these structural highs as steeply dipping fault zones and shale diapirs activated during eocene oligocene oblique plate convergence the geology and overall structural configuration of the northeastern maracaibo basin and the burro negro fault zone support its origin as a right lateral tear fault in our model the burro negro fault zone accommodated southeastward migration of the thrust front in the deep water area east of the fault in the present day falcn region the paleogene clastic wedge of the maracaibo basin exhibits many common features of a classic foreland basin including onlap onto an arch or forebulge located near the center of the present day lake maracaibo this paleogeographic coincidence of the burro negro fault zone and the maracaibo shelf edge suggests that the paleotrend of the south american passive margin prior to collision was serrated in map view west to east migration of the caribbean south america oblique collision formed progressively younger parallel tear faults to the east of the maracaibo basin that may have formed by the same tectonic process as the burro negro fault zone

provenance studies of esie sculptural soapstone from southwestern nigeria a group of soapstone statues found near the village of esie in southwestern nigeria is one of the largest collections of prehistoric sculpture found in sub saharan africa the statues were found at esie by people of the igbomina sub grouping of the yoruba ethnic group when they settled in the esie area some 300 years ago the origin of the statues remains unknown several areas of soapstone in the nigerian schist belt are considered as potential source areas for the esie statues mineral assemblages textures and compositions of talc cummingtonite and chlorite from some of the statues suggest a source for the statue soapstone from the agbonda area the closest proposed source area to esie however mineralogic data from one of the statues are unlike any of the proposed source areas the result of our study together with earlier studies of stevens hambolu and onabajo provides strong evidence that the soapstone statues were carved from local ultramafic rocks from esie area

investigating high zircon concentrations in the fine fraction of stream sediments draining the pan african dahomeyan terrane in nigeria sixteen hundred stream sediments 150 m fraction collected during regional geochemical surveys in central and sw nigeria have high median and maximum concentrations of zr that exceed corresponding zr concentrations found in stream sediments collected from elsewhere in the world with similar bedrock geology x ray diffraction studies on a sub set of the analysed stream sediments showed that zr is predominantly found in detrital zircon grains however the main proximal source rocks pan african older granites of nigeria and their proterozoic migmatitic gneiss country rocks are not enriched in zircon or zr nevertheless u pb la icp ms dating with cathodoluminescence imaging on detrital zircons both from stream sediment samples and underlying pan african older granites confirms a local bedrock source for the stream sediment zircons a combination of tropical chemical weathering and continuous physical weathering both by wet season flash flooding and dry season unidirectional winds are interpreted to have effectively broken down bedrock silicate minerals and removed much of the resultant clay phases thereby increasing the zr contents in stream sediments the strong correlation between winnowing index th al and zr concentration across the study area support this interpretation therefore anomalous high values of zr as well as other elements concentrated in resistant heavy minerals in nigeria s streams may not reflect proximal bedrock concentrations of these elements this conclusion has important implications for using stream sediment chemistry as an exploration tool in nigeria for primary metal deposits associated with heavy minerals

stress field and seismotectonics of northern south america we have integrated in situ stress neotectonic and global positioning system data to investigate the complex interactions among the south american caribbean and nazca plates and the costa rica panama microplate and to examine different seismotectonic models that have been proposed for the region the resulting data set was used to generate an integrated stress map of the region that shows that the stress field in northern south america varies systematically in both orientation and relative magnitude in the southwestern part of the study area the ecuadorian andes stress province exhibits strong e w compression resulting from the subduction of the nazca plate beneath the south american plate in the north andes stress province the observed nw se compression may result from the convergence between the caribbean and the south american plates and or the negative buoyancy of the already subducted caribbean plate beneath northwestern south america possible convergence between the costa rica panama microplate with respect to northwestern south america may also be a source of compression in this region in north and northeastern venezuela normal and strike slip faulting with a ne sw direction of extension characterizes the san sebastian el pilar stress province

the pan african reactivation of eburnean and archaean provinces in nigeria structural and isotopic data eastern nigeria experienced high grade metamorphism during the pan african collision event marked by a pervasive migmatization structural evidence demonstrates that anatexis was contemporaneous with regional deformation and emplacement of calc alkaline granite diorite plutons new pb evaporation data on single zircons from the migmatites confirm that partial melting and plutonism occurred synchronously around 580 10 ma the pan african reactivation is almost ubiquitous and therefore the protolith age is still debated however sm nd analysis of migmatitic gneisses from eastern nigeria yielded eburnean model ages of the protolith between 1 7 and 2 0 ga in agreement with previous model ages of 1 3 and 2 0 ga from granitoids and gneisses of the same area these results suggest that the protoliths in eastern nigeria are distinct from those from the western archaean province furthermore recent work has shown that further north in the tuareg shield the pan african belt consists of amalgamated terranes thus a terrane model could explain the contrast between eastern nigeria characterized by eburnean protoliths and calc alkaline granitic rocks and western nigeria characterized by archaean protoliths and units having tonalite trondhjemite granodiorite and greenstone affinities the contrasted terranes also correspond to a gold province in the west and to a tin province in the east of nigeria

early and middle miocene depositional history of the maracaibo basin western venezuela the uplift of the sierra de perij and mrida andes and the marine connection with the falcn basin ultimately controlled the distribution of shallow marine coastal and nonmarine sedimentary rocks in the maracaibo basin during the early and middle miocene these rocks contain the most important shale top seal in the basin and nearly three quarters of the produced plus proven reserves of the supergiant bolvar coastal fields the maracaibo basin has been isolated from extrabasinal drainage systems since the late oligocene and sediments derived from the surrounding highlands were either deposited in the basin or delivered into the neighboring falcn basin through a narrow marine passage the westward extension of the falcn channel four unconformity bounded sequences mapped in the northeastern sector of the maracaibo basin help recreate its regional paleogeography as it was flooded from the northeast through this passage in the early miocene part of the basin became a semi enclosed shallow marine gulf and wave and tide modified deltas prograded across the temporarily inactive lama icotea fault system as sea level dropped the shoreline advanced eastward of the falcn channel and valleys were incised and subsequently filled by transgressive estuarine sediments in the next sea level highstand tidal bar complexes of a tide dominated delta system prograded and filled all available accommodation space in the middle miocene relative sea level dropped into the falcn basin and the maracaibo area became a mixed load fluvial drainage basin by late middle miocene the two basins were separated and the maracaibo basin became an intermontane fluvial lacustrine depression

geometry of flexural uplift by continental rifting in corinth greece understanding early rifting of continental lithosphere requires accurate descriptions of up bended rift margins and footwalls that correlate in space and time with the elastic flexural uplift that produces them here we characterize the geometry of elastic flexural uplift produced by continental rifting at its spatiotemporal scale in nature 10s km 10 4 10 6 yr using geomorphic evidence from the uplifting margin of the corinth rift greece our geomorphic analyses of space borne topography novelly outline the coherent elastic flexure of continental lithosphere along and across the rift margin and throughout faulting 10 6 yr as defined by the distribution of footwall uplift south of the active bounding fault topography and river drainages outline an elastic flexure signal that increases exponentially towards the bounding fault across the footwall for 50 km and changes in amplitude along the footwall following a parabola that decays from the rift center and has a 60 km wavelength such continental lithosphere up bend correlates with the scale of the rift and appears maximum in the center of the rift where drainage reversal of large catchments suggests rapid slip rates at the bounding fault this is consistent with the growth of a new rift scale high angle normal fault the coherency of elastic flexure in space and time implies highly localized strain in the rift bounding fault and suggests that the fault transects continental lithosphere with long term strength the unparalleled record of flexural uplift and highly localized strain in the landscape of corinth suggest these processes are intrinsic to early continental rifting elsewhere

pan african post collisional ferro potassic granite and quartz monzonite plutons of eastern nigeria abstract three pan african hypersthene bearing monzogranitic and quartz monzonitic plutons from the eastern terrane of nigeria have been investigated in detail new major trace and ree data used to constrain their origin and nature indicate that they display chemical features of ferro potassic trans alkaline affinity further trace element discrimination suggests i production of calc alkaline medium k diorite magmas by partial melting of fluid metasomatised mantle wedge possibly combined with melts from the dehydration partial melting of altered oceanic crust ii simultaneously production of the granite quartz monzonite ferro potassic magmas from partial melting of hornblende bearing granodioritic crustal sources iii mixing of the two magmas sr initial ratios of 0 707 to 0 711 witness that the source of the granite magmas is the lower crust ages of the lower crustal granulitic protoliths is bracketed by nd model ages between 1 9 and 2 2 ga pb evaporation ages on single zircons constrain the emplacement of the three plutons around 580 ma 40 ar 39 ar ages of amphiboles at about 560 ma suggest cooling rates around 15 c ma extensive field work has established that pluton emplacement occurred during a regional north south dextral strike slip tectonics following the 630 610 ma stage of oblique continent continent collision in this part of west africa

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

palaeogeographical reconstructions of the pan african brasiliano orogen closure of an oceanic domain or intracontinental convergence between major blocks abstract a reconstruction of the pan african brasiliano orogen has become possible thanks to new data from the southern pan african belt and by using published data geological mapping which included structural thermobarometric and geochemical studies in togo and neighbouring countries have enabled to define the existence of a preorogenic continental rift whose closure followed soon after its opening continental crust convergence induced an oblique collision that caused nappe stacking this involved external nappes thrust over the west african craton intermediate nappes including metabasic rocks with eclogitic assemblages and internal nappes composed of highgrade anatectic gneiss and intrusive granite and charnockite comparison with data from other regions hoggar adrar des iforas northeastern brazil nigeria central africa has led to the recognition of a pre orogenic continental rift that bordered the west africa sao francisco congo cratons in the southern part of the chain farther north in the hoggar and adrar des iforas this continental rift opened on a wider oceanic domain which involved a passive margin along the west african craton and an active margin to the east after these rifting and subduction stages continental collision followed that was dominated by strike slip movement oblique to the plate boundaries the general organization of the pan african brasiliano belt can be considered as the result of a confrontation between three major continental domains these were the west african and sao francisco congo cratons and a reworked shield composed in its inner part of polycyclic basement rocks and bounded by a fringe of monocyclic metasedimentary thrust belts that faced the cratons

upper proterozoic schist belts in the nigerian sector of the pan african province of west africa abstract schist belts in nigeria occur in a 400 km wide zone which trends nne parallel to the boundary between the pan african province and the west african craton a polycyclic migmatite gneiss granite terrain ranging in age from archaean to lower palaeozoic flanks the schist belt zone to east and west and separates the individual belts within the zone the schist belts fall into two age groups the earlier group contains assemblages of mafic igneous rocks pelitic schists and phyllites banded iron formation and locally coarse grained clastics and carbonate rocks these belts show complex structural styles and are extensively invaded by granitic plutons belonging to the widely distributed pan african magmatic suite schists and gneisses within and adjacent to some of these belts have been dated at 1100 ma and this group is thought to belong to the kibaran orogeny similar kibaran rock associations can be recognised elsewhere in the pan african province in the hoggar and cameroun the kibaran orogeny in this region is interpreted as a largely ensialic event although with possible minor ocean basin formation that led into the more intense and widespread pan african orogeny the later group of schist belts is characterised by coarse to fine grained clastics insignificant mafic igneous rocks a simple straight structural style and an absence of internal granite plutons these schist belts are believed to belong to the pan african orogeny their sediments may have been deposited in a back arc basin related to an e dipping subduction zone at the western margin of the pan african province

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

where are the eburnian transamazonian collisional belts the reconstruction of early proterozoic crustal evolution and geodynamic environments in africa and south america is incomplete if cratonic areas alone are studied if the presence of high grade

a mechanism to explain rift basin subsidence and stratigraphic patterns through fault array evolution rift basin stratigraphy commonly records an early stage of slow subsidence followed by an abrupt increase in subsidence rate the physical basis for this transition is not well understood although an increase in extension rate is commonly implied here a numerical fault growth model is used to investigate the influence of segment linkage on fault displacement rate patterns along an evolving normal fault array the linkage process we describe is controlled by a stress feedback mechanism which leads to enhanced growth of optimally positioned faults model results indicate that even with constant extension rates slow displacement rates prevail during an initial phase of distributed extension followed by an increase in displacement rates as strain becomes localized on linked fault arrays this is due to the dynamics of fault interactions rather than mechanical weakening comparison of model simulations with rift basin subsidence and stratigraphic patterns in the gulf of suez and north sea suggests that the occurrence and timing of rapid basin deepening can be explained by the mechanics of fault zone evolution without invoking a change in regional extension rates

chemical characterization of some potsherd pavements from parts of yorubaland in southwestern nigeria several samples of potsherd pavement were recovered during excavation and archaeological reconnaissance of six sites along the trade and migration route of the yoruba of southwestern nigeria a significant feature of these pavements is the similarity to ife in design which consists of a herringbone design with a ritual pot embedded in their middle microscopic analysis of samples shows the presence of high proportions of non plastic grains mainly quartz micas minor plagioclases and traces of ferruginous grains tourmaline and amphibole interpreted as being produced by low temperature firing major trace and rare earth elements composition of 21 samples of sherds was determined using inductively coupled plasma icp ms technique the chemistry of the potsherd samples is generally similar to regolith derived from granitic rocks which are abundant in the area principal component analysis pca was used to reduce the number of variables and model based cluster analysis was used to find clusters in the pca scores model based clustering found one major cluster in the data set which corresponds to the local weathered granitic pegmatitic raw materials near the sites our work shows that the technology for the manufacture of these pavements might have originated from ile ife but the production was geologically controlled

campanian miocene tectonostratigraphy depocenter evolution and basin development of colombia and western venezuela abstract the position of the central axis of deposition over colombian and venezuelan continental crust has varied markedly through time the axis migrated from west to east from late cretaceous to oligocene but at times secondary drainage divides were established by local uplift events in oligocene times with initial inversion of the eastern cordillera the central axis of deposition was divided into two main axes the proto magdalena and the proto orinoco systems the west to east migration of the central axis of deposition had a tectonic origin and it occurred in combination with tectonically driven changes in accommodation space depocenter evolution can be divided as follows 1 the axis of the campanian and early maastrichtian depocenter was located few km east of the present position of the central cordillera of colombia it migrated east with gradual uplift of the central cordillera 2 the central axis of late maastrichtian deposition is positioned approximately over the present day western foothills of the eastern cordillera possibly crosses the eastern cordillera over the santander massif and continues into the maracaibo lake in western venezuela accommodation space decreased from campanian to maastrichtian times in cretaceous tertiary boundary times parts of the eastern margin of the eastern cordillera were uplifted by an initial phase of inversion of deeply rooted jurassic and early cretaceous normal faults 3 in paleocene times the central axis of deposition was located along the spine of the eastern cordillera and extended into the maracaibo basin and accommodation space continued to decrease 4 in latest paleocene times the central axis of deposition shifted to eastern regions of the eastern cordillera and accommodation space decreased 5 the early eocene central axis of deposition was located along the present day eastern foothills of the eastern cordillera accommodation space continued to decrease and the regional middle eocene unconformity began to develop in middle eocene times a regional unconformity developed marking the climax of the pre andean orogeny deposition during these times was dominant in the maracaibo basin area where large amounts of sediment derived from vast exposed areas accumulated 6 the late eocene central axis of deposition was confined to the present position of the llanos foothills late eocene deposition reflects a regional increase in accommodation space in oligocene times the initial uplift of the eastern cordillera divided the main depocenter into two central axes accommodation space diminished in uplifted regions but continued to increase in the depocenters allowing sporadic marine ingressions into the present position of the llanos foothills as uplift of the eastern cordillera continued the eastern depocenter axis proto orinoco migrated east and the western depocenter axis proto magdalena migrated west this process continued through the rest of the cenozoic

tourmalinization related to late proterozoic early palaeozoic lode gold mineralization in the bin yauri area nigeria three types of tourmaline occurrence have been identified in the area of bin yauri nigeria mesothermal lode gold mineralization these are 1 stratabound tourmalinites in pelitic metasediments 2 tourmaline in a hydrothermal alteration assemblage within hornfelsed wall rocks 3 tourmaline in auriferous quartz and quartz carbonate veins although the tourmaline occurrences are all within or close to the contact aureole of a granodiorite intrusion geochemical characteristics of the tourmalines are broadly similar and reflect a common metasedimentary source two stages of tourmalinization are envisaged the earlier ca 1100 ma involved syngenetic diagenetic formation of tourmalinites while the later ca 500 ma involved epigenetic hydrothermal tourmaline gold mineralization possibly derived by dehydration and devolatilization of metasedimentary sequences containing tourmaline rich rocks or tourmalinites electron microprobe analyses indicate that the tourmalines are intermediate members of the schorl dravite solid solution series plots involving feo mgo and or al2o3 from these analyses are used to constrain the sources and processes of tourmalinization two metallogenic implications are derived from this study one is that although the tourmalinites are barren of gold and base metal mineralization their occurrences nevertheless encourages exploration for syngenetic exhalative massive sulphide deposits in the region the other implication applies to the potential use of tourmaline in deciphering the physico chemical conditions of gold mineralizing fluids in the bin yauri area

chronology of nappe assembly in the pan african dahomeyide orogen west africa evidence from 40ar39ar mineral ages abstract the pan african dahomeyide orogen represents the southwestern segments of the eastern tectonic zone along which the west african craton was incorporated into gondwana orogenic contraction produced nappe complexes comprised of passive margin sediments and accreted exotic magmatic rocks in ghana and togo external nappes include the atacora composed of lower amphibolite facies quartzite and mica schist which were structurally imbricated with mylonitic gneiss derived from c 2 0 ga foreland basement the suture zone is represented by high pressure granulite facies garnet homblende gneiss we have analyzed muscovite and homblende concentrates from external and suture zone nappes in incremental heating 40 ar 39 ar experiments muscovites which define the penetrative foliation in the atacora nappes and a shear foliation in the basement parautochthon record 40 ar 39 ar plateau ages of 579 4 0 8 ma and 578 1 0 5 ma respectively in contrast c 600 km to the north muscovite in quartz schist near the base of the atacora nappes displays a spectrum which gives a minimum age of 608 1 1 2 ma whereas muscovite in the overthrust kara nappes yields a plateau age of 633 8 0 5 ma the c 30 m y difference between the atacora nappes in the northern dahomeyides and the younger dates of the southern external nappes indicate out of sequence thrust imbrication however forward imbricating thrusting is also suggested by the older age of kara nappes relative to atacora nappes in the northern segment the youngest ages are inferred to date orogen parallel nappe transport at c 575 ma hornblende from the structurally lower sections of the suture zone nappes record a 36 ar 40 ar versus 39 ar 40 ar isotope correlation age of 587 4 3 ma hornblende from a tonalitic vein in the host garnet mafic gneiss yields an isotope correlation age of 581 9 2 4 ma these ages are interpreted to date the exhumation of the suture zone nappes and thus provide a minimum age of crustal thickening in the dahomeyide orogen the age data are consistent with coeval forward and hindward propagating thrusting and nappe imbrication and constrain the amalgamation in northwest gondwana to have occurred in the latest proterozoic rather than cambrian

regional geologic and tectonic setting of the maracaibo supergiant basin western venezuela this special issue contains eight topical studies on the structure stratigraphy and petroleum system of the maracaibo basin a supergiant basin in western venezuela most of the work reported in this special issue is the product of thesis related research by master s and doctoral level students at the jackson school of geosciences of the university of texas at austin during a collaborative relationship with the venezuelan national oil company petrleos de venezuela s a that was initiated in the late 1980s this introductory article presents a regional overview of the tectonic setting and geology of the maracaibo basin with a cumulative oil production of more than 30 billion bbl since the first production well was drilled in 1914 and estimated ultimate oil reserves of more than 44 billion bbl the maracaibo basin is the most prolific hydrocarbon basin in the western hemisphere unlike the more extensive gulf of mexico giant hydrocarbon provinces the relatively small size 50 000 km2 19 305 mi2 relative simplicity in its structure and stratigraphy and wealth of surface and subsurface data make the maracaibo basin an attractive target for basinwide synthesis the objective of this article is to present a regional compilation of two dimensional 2 d and three dimensional 3 d seismic data wells and outcrop data at a basinwide scale to reveal the basin s 3 d structure and stratigraphy moreover we show regional tectonic reconstructions regional geologic maps and basin subsidence history to better constrain four major tectonic events that affected the basin and that are critical for understanding the timing and distribution of major unconformities and clastic wedges the distribution of the reservoir rocks the reactivation of older fault trends and the timing of maturation for underlying source rocks many of these topics are discussed in greater detail in the other eight articles in this special issue

an overview of the petroleum system of maracaibo basin the geologically complex maracaibo basin in northwestern venezuela is one of the most prolific hydrocarbon basins in the world having a basinal area of 50 000 km2 19 300 mi2 the basin has produced more than 30 billion bbl of oil with estimated recoverable oil reserves of more than 44 billion bbl the central elements of the petroleum system of the basin include 1 a world class source rock upper cretaceous la luna formation deposited on a shelf to slope environment under anoxic conditions and modified by intermittent oxygenated periods and tectonic events 2 high quality clastic reservoir rocks deposited in eocene and miocene fluviodeltaic settings 3 two main periods of rapid tectonic subsidence responsible for two pulses of voluminous hydrocarbon generation first during paleogene caribbean south american oblique plate collision and second during the neogene uplift of the sierra de perij mrida andes and 4 lateral and vertical migration of oil along strike slip normal and inverted faults as well as a regional unconformity of late eocene oligocene age the maturation migration and trapping of hydrocarbons were closely controlled by the tectonic evolution of the maracaibo basin during the paleogene the development of a foredeep along the northeastern margin of the basin and the strike slip reactivation of the rift related jurassic faults on the maracaibo platform controlled the early structural setting of the source and reservoir rocks hydrocarbons migrated updip from source rocks beneath the north northeastern margin of the basin along north south strike slip faults and into overlying eocene clastic reservoirs in the south central parts of the basin the second period of the maracaibo basin petroleum system developed during subaerial exposure of most of the maracaibo basin during oligocene miocene uplift of the adjacent sierra de perij and mrida andes uplift of mountain ranges surrounding the basin folded and depressed the interior of the basin to form the extensive maracaibo syncline because of the formation of the maracaibo syncline oil generation began in the central and southern parts of the synclinal basin and migrated northward hydrocarbons migrated up the flanks of the maracaibo syncline along reactivated strike slip faults and into miocene rocks adjacent to the uplifted mountain ranges escaping oil has formed numerous surface seeps along the edges of the maracaibo basin

tectonics basin subsidence mechanisms and paleogeography of the caribbean south american plate boundary zone abstract using a mega regional dataset that includes over 20 000 km of on and offshore 2d seismic lines and 12 wells we illustrate three different stages of fault formation and basin evolution in the caribbean arc south american continent collisional zone transpressional deformation associated with oblique collision of the caribbean arc migrates diachronously over a distance of 1500 km from western venezuela in paleogene time 57 ma to a zone of active deformation in the eastern offshore trinidad area each diachronous stage of pre syn and post collisional basin formation is accompanied by distinct patterns of fault families we use subsidence histories from wells to link patterns of long term basinal subsidence to periods of activity of the fault families stage one of arc continent collision initial collision is characterized by overthrusting of the south and southeastward facing caribbean arc and forearc terranes onto the northward subducting mesozoic passive margin of northern south america northward flexure of the south american craton produces a foreland basin between the thrust front and the downward flexed continental crust that is initially filled by clastic sediments shed both from the colliding arc and cratonic areas to the south as the collision extends eastward towards trinidad this same process continues with progressively younger foreland basins formed to the east on the overthrusting caribbean arc and forearc terranes north south rifting adjacent to the collision zone initiates and is controlled by forward momentum of southward thrusting arc terranes combined with slab pull of the underlying and subducting north dipping south american slab uplift of fold thrust belts arc continent suture induces rerouting of large continental drainages parallel to the collisional zone and to the axis of the foreland basins stage two this late stage of arc continent collision is characterized by termination of deformation in one segment of the fold thrust belt as convergent deformation shifts eastward rebound of the collisional belt is produced as the north dipping subducted oceanic crust breaks off from the passive margin inducing inversion of preexisting normal faults as arc continent convergence reaches a maximum strain partitioning also begins to play an important role as oblique convergence continues accommodating deformation by the formation of parallel strike slip fault zones and backthrusting southward subduction of the caribbean plate beneath the south caribbean deformed belt as subsidence slows in the foreland basins sedimentation transitions from a marine underfilled basin to an overfilled continental basin offshore sedimentation is mostly marine sourced by the collided caribbean terranes localized islands and carbonate deposition stage three this final stage of arc continent collision is characterized by 1 complete slab breakoff of the northward dipping south american slab 2 east west extension of the caribbean arc as it elongates parallel to its strike forming oblique normal faults that produce deep rift and half grabens 3 continued strain partitioning strike slip faulting and folding the subsidence pattern in the caribbean basins is more complex than interpreted before showing a succession of extensional and inversion events the three tectonic stages closely control the structural styles and traps source rock distribution and stratigraphic traps for the abundant hydrocarbon resources of the on and offshore areas of venezuela and trinidad

geology of the northern borborema province ne brazil and its correlation with nigeria nw africa the borborema and benin nigeria provinces of ne brazil and nw africa respectively are key areas in the amalgamation of west gondwana by continental collision during the brasiliano pan african orogenies both are underlain by complex basement nigeria has c 3 05 ga archaean crust but no known palaeoproterozoic rocks 2 0 ga in ne brazil 2 6 3 5 ga archaean rocks form small cores within palaeoproterozoic gneiss terrains affected by plutonism at c 2 17 ga both regions exhibit late palaeoproterozoic c 1 8 ga rift related magmatism and metasedimentary sequences overlying the basement the serido group of ne brazil 0 65 ga is similar to the igarra sequence in sw nigeria the ceara group which may date back to c 0 85 ga is a passive margin deposit on crust thinned during initiation of an oceanic domain in both provinces basement and sedimentary cover were involved in tangential tectonics that resulted in crust thickening by nappe stacking associated with closure of this ocean frontal collision between c 0 66 and 0 60 ga later evolved to an oblique collision generating north south continental strike slip shear zones at c 0 59 ga in ne brazil the main pan african suture is probably buried beneath the parnaiba basin the transbrasiliano lineament interpreted as the prolongation of the kandi 4 50 lineament in hoggar may represent a cryptic suture

erosion and tectonics at the margins of continental plateaus the topography across the eastern margin of the central andean plateau north of 18 s beni region bears a strong resemblance to the topography of the southern margin of the tibetan plateau nepal himalaya with both regions featuring a steep frontal slope and high peaks at the plateau edge in contrast the topography of the eastern margin of the central andean plateau south of 18 s pilcomayo region tapers toward the foreland more gently and has no line of high peaks at the margin both the himalayan and the beni regions have been the sites for large amounts of denudation and in both regions geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the neogene we hypothesize that the steep frontal slope and high peaks of the beni region and himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid pilcomayo region reflects a tectonic landform only slightly modified by erosion we propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope eroding back the plateau while enhancing or maintaining the steep long wavelength slope a numerical model coupling orographic precipitation erosion and tectonic uplift demonstrates the plausibility of this hypothesis the erosional efflux in both the beni and nepal himalaya have been considerable and simple mass balance calculations for the himalaya suggest that during the neogene the erosional mass efflux has generally outpaced the tectonic mass influx this contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period and or increased late cenozoic erosion rates

localization of quaternary slip rates in an active rift in 105 years an example from central greece constrained by 234u 230th coral dates from uplifted paleoshorelines mapping dating and modeling of paleoshorelines uplifted in the footwall of the 1981 gulf of corinth earthquake fault greece ms 6 9 6 7 are used to assess its slip rate history relative to other normal faults in the area and study strain localization the 234u 230th coral ages from cladocora caespitosa date uplifted shoreface sediments and paleoshorelines from glacioeustatic sea level highstands at 76 possibly 100 125 175 200 216 240 and 340 ka uplifted quaternary and holocene paleoshorelines decrease in elevation toward the western tip of the fault exhibiting larger tilt angles with age showing that uplift is due to progressive fault slip since 125 ka uplift rates varied from 0 25 to 0 52 mm yr over a distance of 5 km away from the fault tip tilting was also occurring prior to 125 ka but uplift rates were lower because the 125 ka paleoshoreline is at 77 of the elevation of the 240 ka paleoshoreline despite being nearly half its age comparison of paleoshoreline elevations and sedimentology with the quaternary sea level curve shows that slip rates increased by a factor of 3 2 0 2 at 175 75 ka synchronous with cessation of activity on a neighboring normal fault at 382 112 ka we suggest that the rapid localization of up to 10 15 mm yr of extension into the narrow gulf 30 km wide resulted from synchronous fault activity on neighboring faults followed by localization rather than sequential faulting with consequences for the mechanism controlling localization of extension

rapid spatiotemporal variations in rift structure during development of the corinth rift central greece the corinth rift central greece enables analysis of early rift development as it is young 5 ma and highly active and its full history is recorded at high resolution by sedimentary systems a complete compilation of marine geophysical data complemented by onshore data is used to develop a high resolution chronostratigraphy and detailed fault history for the offshore corinth rift integrating interpretations and reconciling previous discrepancies rift migration and localization of deformation have been significant within the rift since inception over the last circa 2 myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift but this transition did not occur synchronously along strike isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620 340 ka characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at 30 m kyr since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin demonstrated by lateral growth of discrete depocenters at 40 m kyr a single central depocenter formed by circa 130 ka indicating full fault linkage these results indicate that rift localization is progressive not instantaneous and can be synchronous once a rift border fault system is established this study illustrates that development processes within young rifts occur at 100 kyr timescales including rapid changes in rift symmetry and growth and linkage of major rift faults

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

numerical models of the inversion of half graben basins 1 we investigate the dynamic evolution of fold and thrust structures which form by compression and inversion of a sequence of half graben basins the choice for a half graben geometry is motivated by seismic studies and reconstructions of preinversion geometry of inverted regions which show that rifting often leads to a series of half grabens to examine the deformational structures which result from basin inversion we use a two dimensional viscous plastic numerical model and start our experiments from a preexisting extensional geometry we find that synrift and postrift sediments are uplifted in the initial stages of basin inversion this uplift is accompanied by rotation of the basement blocks beneath the basins in the postrift sequence new shear zones form which are a continuation of basin bounding faults with continuing shortening further inversion is more difficult owing to relative strengthening of the half graben region significant surface erosion facilitates inversion back thrusts mainly develop in association with listric basin bounding faults and less for planar domino array faults weak sediments such as salt or shales at the base of the basins promote the development of basement shortcut faults the presence of a postrift decollement layer tends to decouple deformation of the postrift sediments from the material below it in our model preexisting weak basin bounding shear zones are a requirement for substantial amounts of basin inversion to occur our numerical model results display many first order characteristics of examples from nature and analogue studies

slip acceleration on normal faults due to erosion and sedimentation results from a new three dimensional numerical model coupling tectonics and landscape evolution abstract although it is well accepted that mass redistribution on earth s surface due to erosion and sedimentation affects the rate of crustal deformation the influence of surface processes on the slip behaviour of individual faults remains poorly understood here we use the new tool casqus which combines landscape evolution modelling with three dimensional tectonic models to investigate how erosion and sediment deposition affect fault behaviour in extensional tectonic regimes a systematic set of experiments with models consisting of an isostatically responding elastic upper crust containing a single normal fault shows that the slip accumulation on the fault is accelerated by erosion and sedimentation depending on fault geometry extension rate and parameters controlling erosion and sediment deposition the slip rate of the normal fault is up to 15 higher when surface processes are taken into account the slip acceleration can be explained by an increase in the differential stress caused by erosion of the fault footwall and sedimentation in the hanging wall which alter the faulting induced flexure of the crust we observe a similar slip rate increase in a more complex model of a growing horst bounded by three faults of different length our results show that a positive feedback exists between surface processes and tectonics and imply that geologically derived fault slip rates that integrate over several earthquake cycles may contain a contribution from surface processes

the origin and significance of non aqueous co2 fluid inclusions in the auriferous veins of bin yauri northwestern nigeria non aqueous co2 and co2 rich fluid inclusions are found in the vein quartz hosting mesothermal gold sulphide mineralization at bin yauri northwestern nigeria although mineralizing fluids responsible for gold mineralization are thought to be co2 rich the occurrence of predominantly pure to nearly pure co2 inclusions is nevertheless unusual for a hydrothermal fluid system many studies of similar co2 rich fluid inclusions mainly in metamorphic rocks proposed preferential loss leakage of h2o from h2o co2 inclusions after entrapment in this study however it is proposed that phase separation fluid immiscibility of low salinity co2 rich hydrothermal fluids during deposition of the gold mineralization led to the loss of the h2o phase and selective entrapment of the co2 the loss of h2o to the wallrocks resulted in increasing oxidizing effects there is evidence to suggest that the original co2 rich fluid was intrinsically oxidized or perhaps in equilibrium with oxidizing conditions in the source rocks the source of the implicated fluid is thought to be subducted metasediments subjected to dehydration and devolatilization reactions along a transcurrent anka fault shear system which has been described as a pan african 450 750 ma crustal suture

the tectonic evolution of the southern alps new zealand insights from fully thermally coupled dynamical modelling summary we present here the results of numerical models of the southern alps in which the thermal and dynamical developments of the orogen are fully coupled to one another this interlinkage allows the use of a wide range of thermal and physical features of the orogen as constraints on the applicability of our models in particular the thermal aspect of this method enables the prediction of the distribution of thermochronological ages at the surface of the model perturbation of the geothermal structure due to rapid uplift and exhumation causes an intrinsic weakening and concentration of strain along the equivalent of the alpine fault zone in these models this thermal weakening of the crust also produces a zone of high strain antithetic to the alpine fault in the upper crust that is comparable to the main divide fault zone within the southern alps the introduction of orographic rainfall and erosional processes into the model leads to the development of high asymmetric topography comparable to that of the southern alps this topographic profile results from the capture of available precipitation by the windward side of the orogen and the resultant rain shadow on the leeward side due to the time required to accumulate suycient topography for this rain capture evect to become significant the establishment of this high asymmetric topography lags the initiation of surface uplift by several million years comparison of the observed thermal history of the southern alps with that seen in models based on divering hypotheses of the tectonic evolution of the south island shows that the present tectonic regime of the orogen most probably developed in a single rapid reorganization of plate motions at approximately 5 ma with relative stability of the tectonic regime since that time the variation in isotopic ages along the southern alps is consistent with that expected from variation in accumulated uplift and exhumation along the orogen arising from the obliquity of convergence of the australian and pacific plates

late proterozoic schist belts and plutonism in nw nigeria the deformed low grade metasedimentary volcanic schist belts of nw nigeria and the voluminous granitoid plutons which invaded them are expressions of late proterozoic early phanerozoic activity in the terrain separating the w african and congo cratons recent interpretations of nigerian geotectonic evolution have invoked two generations of schist belt one a product of kibaran c 1100 ma ensialic processes the other due to pan african 700 450 ma marginal basin development the detailed histories of the anka belt pan african maru belt kibaran and birnin gwari belt unknown age and the plutons emplaced in them are documented here on the basis of new field chemical and isotopic data each belt represents a dominantly quiet water sedimentary environment but volumetrically minor lithologies reveal important differences between the belts the volcanic rocks and early minor intrusions have strong affinities with those of destructive plate margins the schist belts were deformed congruently and simply before c 750 ma following the development of flat lying possibly thrust related structures in the maru and birnin gwari belts subsequent plutonism beginning c 750 ma ago has the calcalkaline i type characteristics of subduction zones but younger plutons have mildly peralkaline compositions due to thickening crust

geology and geochemistry of crude oils bolivar coastal fields venezuela the bolivar coastal fields bcf are located on the eastern margin of lake maracaibo venezuela they form the largest oil field outside of the middle east and contain oil which is mostly heavy with a gravity less than 22 api lake maracaibo is now in an intermontane basin enclosed on three sides by the andes mountains the area has a complex history and tectonic movement continues today in the cretaceous the area was part of the platform of a large geosyncline but by the eocene it was near a coast where a series of large sandy deltas was deposited with terrestrial sediments on the south and thick marine shales on the north at this time conditions for oil generation in the shales and migration to the sands were established but the subsequent oligocene faultin uplift and erosion may have allowed meteoric water to penetrate into reservoirs during the miocene and pliocene the basin was tilted first west and then south and filled with continental sediments from the rising andes tilting is still continuing and oil is moving up along the oligocene unconformity forming surface seeps most oil fields are located in sands above the unconformity or in fault blocks immediately below it thirty crude oils from the bcf were collected along two parallel and generally southwest northeast trends these oils were characterized by their api gravity percent saturates aromatics nso and asphaltic compounds gas chromatograms for whole oils c4 c7 fractions and aromatics also 24 associated waters were sampled and analyzed for ca mg na hco3 co3 so4 ph and total dissolved solids tds the oils show the classic sequence of biodegradation and range from green 40 api oils with a full suite of n alkanes and isoprenoids to black heavy oils with a gas chromatogram that is an unresolved hump in many respects the oils are chemically simi ar and appear related possibly sharing the same source rock the miocene l 5 reservoir contained two oil types which did not appear to fit the main trend one type is depleted in n alkanes in the range c8 c14 whereas the other type is depleted in n alkanes above c17 benzene and toluene values for these oils were normal in general oils in the eocene reservoirs below the unconformity are less degraded than those in the miocene sands above it the formation waters range from very salty 62 000 mg l to quite dilute 3 000 mg l those associated with the degraded oils are typically meteoric in chemical composition with considerable bicarbonate 20 to 90 meq l small quantities of chloride 2 to 25 meq l and extremely low amounts of magnesium and calcium mostly less than 1 meq l if the amount of bicarbonate is taken as an indicator of the meteoric character of the water then the more meteoric the water the more degraded the oil the presence of at least four classes of waters with different compositions in the area of the bcf suggests that there is no through going flow at present many of the fields have oil water contacts descending toward the south showing that continued tilting southward occurred during the mio ene and pliocene after the oil was emplaced in the reservoir it seems likely that there was large scale secondary migration of oil from south to north probably along the unconformity surface which still leaks oil where it is exposed in the shallower miocene reservoirs along the northeast margin of the field heavy asphaltic oil overlies lighter oil downdip to the west and south this suggests that the oil became more degraded the farther it moved finally becoming viscous and asphaltic possibly even gelled in this immobilized form it acted as a seal trapping the less degraded oil which followed

sequence stratigraphic analysis of eocene clastic foreland basin deposits in central lake maracaibo using high resolution well correlation and 3 d seismic data eocene clastic rocks of the maracaibo basin were deposited in an asymmetrical foreland basin formed during the oblique paleogene collision between the caribbean and south american plates in this study we use more than 300 wells and 2000 km2 772 mi2 of seismic data in the central maracaibo basin to produce a detailed sequence stratigraphic interpretation of the eocene maracaibo foreland basin the base of the eocene stratigraphic succession in the central maracaibo area is characterized by an approximately 250 m 820 ft thick aggradational succession of fluviodeltaic sandstone overlain by an approximately 600 m 1968 ft thick retrogradational succession of shallow marine shale and sandstone containing minor progradational units the upper part of the foreland basin sequence is marked by an approximately 100 m 328 ft thick aggradational succession of fluviodeltaic sandstone in the approximately 1000 m 3280 ft thick eocene section we interpreted 17 parasequence sets 6 genetic sequences and 1 depositional sequence only one classic sequence boundary was interpreted within the eocene section that marks the boundary between the retrogradational shallow marine section and the overlying aggradational fluviodeltaic succession based on the stratigraphic architecture and thickening trends of several of the parasequence sets we conclude that the main source of clastic sedimentation was located on the south america craton south of the maracaibo basin instead of along the thrusted north northeastern margin of the basin as proposed by previous workers a lack of recognition of classic sequence boundaries suggests that eocene clastic rocks of the central maracaibo foreland basin were not subaerially exposed during most of the eocene and that their stratigraphic architecture was controlled by tectonic subsidence related to thrusting along the northeastern edge of the foreland basin eustasy was not an important control on the stratigraphic evolution of the foreland basin until its middle eocene aggradational period that marked the end of foreland basin subsidence well logs and three dimensional seismic data show that depositional environments on the eocene delta plain and shelf of the central maracaibo foreland basin were dominated by fluvial and tidal processes that are similar to modern depositional processes of the orinoco delta in eastern venezuela

internal fabric and strike slip emplacement of the pan african granite of solli hills northern nigeria the solli hills pluton 250 km2 in northern nigeria is an example of a pan african granite emplaced circa 580 ma ago in the jos adamawa block its micro structures and internal fabrics have been studied in order to help constrain the kinematics of the pan african orogeny in this block we used field observations and measurements petrographic examinations and measurements of the magnetic susceptibility and anisotropy of magnetic susceptibility of 124 regularly spaced stations most samples are ferromagnetic i e belong to the magnetite series granites and present predominantly magmatic microstructures particularly remarkable are the asymmetrical zoning of the petrographic types the susceptibility magnitudes and the planar linear magnetic fabrics the magnetic foliations and lineations are very consistent in the pluton around n 20 e 60 w and n 205 e 10 respectively field and microscopic observations reveal that deformation occurred everywhere in the pluton and immediate country rock envelope during a dextral strike slip synplutonic episode it is suggested that the solli hills pluton was emplaced in a late pan african dextral tear fault system the striking fabric asymmetry of the pluton strongly suggests a shear zone termination emplacement model

the changing course of the orinoco river during the neogene recent studies have revealed that the varied fresh water vertebrate fossil fauna of the urumaco formation in northwestern falcon of late miocene age belongs biogeographically to the orinoco river system this makes possible to chart the changing course of the orinoco river through the neogene the misoa delta of lake maracaibo of middle eocene age was probably constructed by a proto orinoco river that ran in a south north direction draining the central cordillera of colombia and the guayana highlands the late eocene uplifting of western venezuela changed the paleogeographic setting with no evidence of delta building during the oligocene in the earliest miocene the falcon basin was the site of marine shale sedimentation in the second half of the early miocene a deltaic sequence was initiated in northwestern falcon that continued except for a transgressive episode until the end of the middle miocene depositing a total thickness of over 2 5 km the urumaco overlies this sequence and its vertebrate fauna indicates that the river that built the previous delta was the proto orinoco the initial uplifting of the northern part of the andes and the western edge of the caribbean mountains can thus be dated to occur at the end of the middle miocene more causing the distal course of the river to move to a west east direction the sedimentary record of the orinoco river delta appears in the maturin basin of eastern venezuela at the end of the miocene and is especially evident in trinidad and the columbus basin during the pliocene and pleistocene less

geology geography and humans battle for dominance over the delivery of fluvial sediment to the coastal ocean abstract sediment flux to the coastal zone is conditioned by geomorphic and tectonic influences basin area and relief geography temperature runoff geology lithology ice cover and human activities reservoir trapping soil erosion a new model termed bqart in recognition of those factors accounts for these varied influences when applied to a database of 488 rivers the bqart model showed no ensemble over or underprediction had a bias of just 3 across six orders of magnitude in observational values and accounted for 96 of the between river variation in the long term 30 years sediment load or yield of these rivers the geographical range of the 488 rivers covers 63 of the global land surface and is highly representative of global geology climate and socioeconomic conditions based strictly on geological parameters basin area relief lithology ice erosion 65 of the between river sediment load is explained climatic factors precipitation and temperature account for an additi

modeling fluvial incision and transient landscape evolution influence of dynamic channel adjustment 1 channel geometry exerts a fundamental control on fluvial processes recent work has shown that bedrock channel width depends on a number of parameters including channel slope and is not solely a function of drainage area as is commonly assumed the present work represents the first attempt to investigate the consequences of dynamic gradient sensitive channel adjustment for drainage basin evolution we use the channel hillslope integrated landscape development child model to analyze the response of a catchment to a given tectonic perturbation using as a template the topography of a well documented catchment in the footwall of an active normal fault in the apennines italy that is known to be undergoing a transient response to tectonic forcing we show that the observed transient response can be reproduced to first order with a simple detachment limited fluvial incision law transient landscape is characterized by gentler gradients and a shorter response time when dynamic channel adjustment is allowed the differences in predicted channel geometry between the static case width dependent solely on upstream area and dynamic case width dependent on both drainage area and channel slope lead to contrasting landscape morphologies when integrated at the scale of a whole catchment particularly in presence of strong tilting and or pronounced slip rate acceleration our results emphasize the importance of channel width in controlling fluvial processes and landscape evolution they stress the need for using a dynamic hydraulic scaling law when modeling landscape evolution particularly when the relative uplift field is nonuniform

three dimensional structural architecture and evolution of the eocene pull apart basin central maracaibo basin venezuela abstract three dimensional seismic mapping of interpretative sub surface time slice sections incorporating age data and detailed structural observations places constraints on the structural architecture and stratigraphic evolution of the icotea pull apart basin in the central maracaibo basin venezuela all four fault bounded sides of the icotea basin its 3 km thick eocene basin fill of clastic sediments and faults pre dating the opening of the basin can be mapped in three dimensions the development of the icotea basin provides an excellent observational basis for understanding the structural history of three sets of regionally extensive faults and testing two models for the formation of pull apart basins in general three main fault trends produced at different periods in the formation of the cretaceous recent maracaibo basin are mapped using 3d seismic data in the central part of the basin 1 n ne striking normal faults including the icotea fault these faults formed originally during late jurassic early cretaceous rifting and reactivated as left lateral strike slip faults during late paleocene eocene convergent deformation between the caribbean and south american plates 2 nw striking faults formed by late paleocene eocene overthrusting of the caribbean plate and downward flexure of the south american plate to create a major foreland basin depocenter and 3 ne striking normal faults of pre cretaceous age were reactivated during late paleocene eocene plate convergence three dimensional architecture of the icotea basin interpreted from five time slices through the basin and its flank areas supports the simple pull apart model for the eocene opening of the icotea basin the amount of extension across the suite of normal faults ranges between 0 8 and 2 25 km this range of offset is consistent with some previous estimates of minor left lateral displacement along the icotea fault but is inconsistent with either low angle or high angle thrusting during eocene time as inferred from previous interpretations of widely spaced two dimensional seismic lines the normal faults that formed the pull apart basin reactivated pre existing faults due to plate flexure

cretaceous to holocene structural and stratigraphic development in south lake maracaibo venezuela inferred from well and three dimensional seismic data the subsurface of the southern part of the maracaibo basin has been studied in much less detail than the central and northern basin because hydrocarbon rich rocks of cretaceous and paleogene age are deeply buried to a depth of 4 5 km 2 7 mi by a thick neogene clastic section eroded off of surrounding mountain ranges in this article we describe the results of the interpretation of 1600 km2 617 mi2 three dimensional 3 d seismic reflection data centered on this horizontal to monoclinally southwestward dipping middle cretaceous to holocene section adjacent to a zone of northwestward overthrusting along the mountain front of the mrida andes to establish the structural and stratigraphic history of the 3 d seismic study area a series of time slices were constructed at key horizons through the 3 d seismic volume the ages of units in these time slices were correlated to stratigraphic formations known from outcrops at the basin edges well data from the 3 d seismic study area and subsurface correlation with better studied areas in the northern basin the type and orientations of structures along each time horizon were mapped to constrain the successive structural and stratigraphic events that affected the different tectonosequences in the southern maracaibo basin the main tectonic events recorded both by structures and unconformity bound tectonosequences include the following events from youngest to oldest 1 pliocene holocene shortening produced by the uplift of the mrida andes and overthrusting at the mountain front has produced an approximately 6 km 3 7 mi deep localized foredeep basin the axial traces of these folds trend northeast southwest and parallel the strike of a set of normal faults all structural features are consistent with regional northwest southeast shortening a low angle thrust fault responsible for the formation of the main northeast southwest fold is inferred to continue downdip as a flat lying thrust that may be continuous with low angle thrust faults observed along the northern mrida andes mountain front the level of thrust detachment occurs at the cretaceous coln shale and accommodates a negligible amount of shortening 500 m 1640 ft 2 late miocene pliocene shortening caused by uplift of the sierra de perij and the mrida andes produced an inversion of north south striking normal faults and folds in a parallel orientation the axial traces of these folds and the trend of inverted structures coincide with the trends of the most prolific eocene sandstone reservoirs in the central and northern lake maracaibo areas 3 late paleocene middle eocene formation of a foreland basin is related to the southward overthrusting at the northeastern margin of the basin normal faults with an average trend of s37e record flexure of the basin during this event faults also include the southern splayed termination of the left lateral icotea strike slip fault 4 the termination of the icotea fault in the south lake area is consistent with the small amount of left lateral strike slip displacement previously inferred from the geometry of the eocene icotea pull apart basin in central lake maracaibo

trace element geochemistry and petrogenesis of some meta ultramafites in apomu and ife ilesa areas of southwestern nigeria some high magnesian rocks mgo 23 32 having chemical similarity with rocks from komatititic suites were subjected to a detailed geochemical study which lead to a discussion of their petrogenesis all rocks were subjected to regional metamorphism of upper greenschist to amphibolite facies consequently the discussions on petrogenesis have relied heavily on the abundance of the less mobile elements such as rare earth elements and some transition elements e g ni and cr in general the meta ultramafites show almost flat heavy rare earth element and extensive enrichment in light rare earth elements la sm n 1 3 6 3 and ce yb n 0 8 28 ni 1700 4400 ppm and cr 1400 5200 ppm are enhanced which is consistent with their ultramafic parentage the komatiitic meta ultramafic rocks of southwestern nigeria are highly enriched in most trace elements relative to komatiites from well known areas of the world this extensive enrichment is considered to be mostly a characteristic of the source region of these rocks

new constraints on bedrock erodibility and landscape response times upstream of an active fault abstract considerable progress has been made in modelling the response of rivers to tectonic perturbation in order to decode the tectonic signals embedded in river long profiles and planform geometry whilst studies have showed the importance of rock type on the morphology of rivers responding to tectonics on a local scale these effects are often not captured in landscape evolution models in fact current models of fluvial response to tectonic perturbation such as active faulting require carefully collected data sets to fully constrain or calibrate key parameters including the effect of bedrock lithology on substrate erodibility and timescales for tectonic signal propagation in bedrock river systems here we constrain the role of bedrock in controlling fluvial incision for a 240 km2 catchment draining into the gulf of corinth which has excellent tectonic constraints and a variety of bedrock lithologies an active normal fault at the downstream end of the catchment the east eliki fault is known to have initiated at 0 7 ma with average quaternary uplift and incision rates of 1 00 1 25 mm yr the initiation of the east eliki fault is recorded in the river as a prominent knickzone 7 16 km upstream of the fault detailed field data collected within this catchment at 500 m intervals along the main channel length at this tectonically well constrained field site show an order of magnitude increase in river channel slopes y x 0 02 to 0 18 and stream powers 1 kwm 2 to 27 kwm 2 at the lithological boundary between weak and resistant bedrock the weak conglomerates and strong limestones have schmidt hammer compressive strengths of ca 30 and 50 respectively based on in situ rock strength measurements consequent erodibility values of 1 8 0 3 10 14 and 5 6 2 10 15 ms2kg 1 show a factor of three to four decrease in erodibility for a two fold increase in rock strength a simple simulation of tectonic signal propagation through the river catchment based on these erodibility values indicates that the observed plan view position of the knickpoint is consistent with our calculated knickpoint positions based on erodibility values derived from incision rate and stream power data however the tectonic signal associated with faulting would have propagated completely through the catchment within 1 ma if it were entirely composed of weak rock for similar climatic and tectonic conditions indicating that lithology has a major control on landscape response times our results help constrain the effect of lithology on river channel geometry and allow erosional parameters to be derived that are crucial for effective modelling of tectonic rates from topography

gravity domains and assembly of the north american continent by collisional tectonics trends of geophysical anomalies have made important contributions to local and regional structural studies within the north american continent1 2 particularly in examining the precambrian basement underlying the relatively undeformed phanerozoic sediments that cover 45 of north america3 5 here we present a gravity trend map of north america based on a horizontal bouguer gravity gradient map6 produced from gravity data for canada and the conterminous united states and use it to define a continental mosaic of gravity trend domains akin to structural domains contrasting trend characteristics at gravity domain boundaries support the concept of the outward growth of the continent primarily by accretionary collisional tectonics gravity patterns however indicate a different style of tectonics dominated in the development of now buried proterozoic orogenic belts in the south central united states supporting a view that these belts formed along the leading edge of a southward migrating proterozoic continental margin7

the togo benin nigeria shield evidence of crustal aggregation in the pan african belt abstract the importance of suspect or exotic i e allochthonous terranes as a major element in collisional orogenic belts is becoming well established we propose that the southern pan african domain in west africa is an aggregation or mosaic of island arcs interarc ensimatic basins and continental fragments a fracture zone in northwestern nigeria already identified as a possible crustal suture is shown to separate two contrasted basement terranes elsewhere in the shield are flat lying structures characteristic of those associated with crustal convergence lying within or near to major fractures many small ultramafic mafic bodies occur in the shield and some of them may be remnants of ophiolites caught up in suture zones an aggregation of allochthonous terranes island arcs sedimentary basins and continental blocks would help to explain 1 a the great width of the pan african belt 2 b the spread of ages within the pan african range c 750 450 ma also the relict liberian and eburnian ages c 2700 and 2000 ma respectively and the enigmatic kibaran event c 1100 ma 3 c the contrasted volcano sedimentary characteristics of the different supracrustal belts

soapstone as a locally used and limited sculptural material in remote area of northern moravia czech republic in contrast to nordic countries talc rich rocks massive and schistose talc bearing rocks talcite magnesite talc schists chlorite talc schists and or chlorite actinolite talc schists often loosely classified as soapstone represented practically an unknown natural building and sculptural stone in the czech republic this extraordinary material was quarried and used in the only region of the czech republic where sobotin amphibolite massif sumperk district northern moravia outcrops metamorphosed basic igneous rocks contain small lenses of soapstone that were exploited in the past and used not only for refractory purposes but also as a construction and sculptural material occurrences of sobotin soapstone show distinct zone macrofabric marked by the occurrence of massive soapstone in the core that gradually changes to talc dominant schists later to actinolite tremolite rich schists and finally to chlorite schists at the contact with surrounding amphibolites the most typical sobotin soapstone exhibits high density up to 2 900 kg m 3 low water uptake maximum 0 68 wt favorable dynamic elastic properties p wave velocity 5 340 5 840 m s 1 s wave velocity 2 900 3 300 m s 1 and mechanical properties tensile splitting strength 5 2 6 9 mpa uniaxial compressive strength 17 5 24 1 mpa sobotin soapstone is relatively resistant to weathering action although schistose varieties show typical delamination when exposed to frequent freeze thaw cycling in harsh mountainous environment field survey of monuments architectural and building artefacts made from sobotin soapstone contributed to our knowledge on duration of exploitation of sobotin soapstone that was for sure utilised from the second half of seventeen century till the beginning of the twenty century the most characteristic feature of sobotin soapstone is its strict local use on the territory of about 20 villages surrounding previously exploited soapstone lenses

spatial and temporal variations in growth rates along active normal fault systems an example from the lazio abruzzo apennines central italy abstract the geometry kinematics and rates of active extension in lazio abruzzo italian apennines have been measured in order to gain a better understanding of the spatial and temporal variations in fault growth rates and seismic hazards associated with active normal fault systems we present fault map traces throws throw rates and slip directions for 17 parallel en echelon or end on active normal faults whose 20 40 km lengths combine to form a soft linked fault array ca 155 km in length and ca 55 km across strike throw rates derived from observations of faulted late glacial features and holocene soils show that both maximum throw rates and throw rate gradients are greater on centrally located faults along the strike of the array total throws and throw gradients show similar spatial variations but with weaker relationships with distance along strike when summed across strike throw rates are increasingly high towards the centre of the array relative to summed throws we interpret the above to suggest that throw rates have changed in the recent past ca 0 7 ma from spatially random fault growth rates initiating at 2 5 3 3 ma to growth rates that are greater on centrally located faults we interpret this as evidence for fault interaction producing throw rate variations that drive throw profile readjustment on these crustal scale soft linked faults the results are used to discuss seismic hazards in the region which are quantified in a second paper in this issue

growth of faults by accumulation of seismic slip the accommodation of large strains in the upper crust is largely achieved by the accumulation of displacement on faults observation shows that as a fault accumulates displacement it grows in size i e its surface area and its length increase here we address the question for an increase in the amount of displacement on a fault by how much would the length of the fault change it is argued by cowie and scholz 1992a that the displacement on a fault is linearly related to the length of the fault this simple result is expanded upon in this paper by assuming that a fault accumulates displacement by repeated earthquakes two different approaches are presented the first approach considers the balance that must be achieved between the energy available for deformation and the work involved in creating new fault surface area as the fault grows the available energy is provided by changes in strain energy when the fault slips the second approach is to construct a geometrical model for fault growth using the scaling relationship between the slip during a single earthquake and the length of the rupture the total displacement on a fault is the sum of the slips contributed by many earthquakes the usefulness of these two approaches is that the growth of a fault over geologic time can be described by parameters that can be obtained from earthquake and fault data the models presented here predict that 1 the maximum amount a fault can grow in a single earthquake that ruptures the entire fault is of the order of 1 of its previous length and 2 the work of faulting may account for 10 of the total energy available during an earthquake the energy lost from the system is accounted for by work done against friction and seismic radiation consequences of fault growth for the segmentation and thus seismogenic potential of a fault over geologic time are discussed using predictions of the theory

the organic geochemistry of oil seeps from the sierra de perija eastern foothills lake maracaibo basin venezuela the organic geochemistry of samples from 11 oil seeps was studied the samples were collected from the cachiri area carboniferous region of tule lake maracaibo basin venezuela associated with the tigre fault biomarkers hopanes steranes n alkanes acyclic isoprenoids and aromatic steroids were analyzed using gas chromatography mass spectrometry gc ms these hydrocarbon rich fluids have undergone biodegradation 2 6 on the peters and moldowan scale showing both the partial loss of n alkanes and the microbial degradation of isoprenoids and steranes these oil seeps were generated from a mature calcareous source rock that was deposited in a marine paleoenvironment under reducing conditions moreover these seeps are likely derived from the cretaceous la luna formation that reached a level of maturity near the peak of oil generation in the study area the nature of the studied oil seeps together with the oil generation models reported for this rock unit in the study area suggests that these oils are a mixture of an initially heavy altered oil and a second migrated light crude oil resulting from two generation pulses from the la luna formation evidence for the presence of light oil trapped in the study area should prompt re exploration in the northwestern coast of lake maracaibo in shallow reservoirs previously discarded because they usually demonstrated a lack of light oils