new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

lessons from a joint interpretation of vibroseis wide angle and near vertical reflection data in the northeastern yilgarn western australia abstract a wide angle reflection seismic experiment was carried out in the eastern goldfields granite greenstone terrane of the archaean yilgarn craton during 2001 this was the first time in australia that wide angle data were collected using a vibrator source and with a high density of observations unlike other wide angle surveys carried out in other parts of the world our survey used both a smaller number of sweeps and shorter sweeps we recorded three sweeps each with its own frequency range at each vibration point the experiment demonstrated that the sum of three 12 s sweeps using 3 large vibrators provides enough energy to record signal at offsets up to up to 60 70 km a comparison of individual shot gathers from near vertical data and receiver gathers from wide angle data demonstrated higher reflectivity in near vertical data this may be due to differences in the frequency bands of the recording equipment the after stack section obtained from dense wide angle data is different from that obtained from conventional near vertical reflection data the conventional reflection section provides higher quality image of the crust compared to the wide angle section this could be explained by the low fold in wide angle data and differences in the acquisition and processing methodology the wide angle survey which was coincident with a regional vibroseis seismic reflection transect was focused on the leonora laverton region the survey was designed to supplement the deep seismic reflection studies with velocity information this also created an opportunity to compare velocity model derived from wide angle reflection seismic data with a structural image obtained from the deep common mid point seismic reflection data and thus refine our geological understanding of the area a high velocity body reaching a maximum thickness of 2 km was identified exclusively from the seismic velocity model derived from wide angle study this body is interpreted as mafic rocks within the archaean granite greenstone belt the joint interpretation also shows that structural boundaries do not always follow lithological boundaries in our study area the combination of wide angle reflection and near vertical reflection data has facilitated a more complete geological interpretation of the seismic data

stratigraphic relations and lithologic variations in the jemez volcanic field new mexico over 100 radiometric dates and recent detailed geologic mapping allow some refinements of the stratigraphic relations of major units and generalization of temporal lithologic variations in the jemez volcanic field volcanism had begun in the area by about 16 5 ma with episodic eruptions of alkaline basalts by 13 ma alkaline volcanism had been replaced with eruptions of more voluminous olivine tholeiite high silica rhyolite derived from melts of lower crust also was erupting by about 13 ma basalt and high silica rhyolite continued to be erupted until about 7 and 6 ma respectively but effusions of dominantly andesitic differentiates of basalt that began as early as about 12 ma volumetrically overshadowed all other eruptive products between 10 and 7 ma from 7 to 3 ma the dominant erupted lithology was dacite which appears to have been generated by mixing of magmas whose compositions are approximated by earlier andesites and high silica rhyolites less than 4 3 ma volcanism was dominated by eruption of rhyolitic tuffs field relations geochemistry and dates specifically indicate the following with regards to stratigraphie relations 1 distinctions among basalt of chamisa mesa paliza canyon formation basalts and lobato basalt for other than geographic reasons are artificial basaltic volcanism was continuous in volcanic field from 13 to 7 ma 2 canovas canyon and bearhead rhyolites form a continuum of high silica rhyolite volcanism from 13 to 6 ma 3 hypabyssal and volcanic rocks of the cochiti mining district probably represent the exhumed interior of a keres group volcano s 4 temporal overlaps exist among the major stratigraphie groups which may imply some genetic relations and 5 the tewa group formation cerro rubio quartz latite may more appropriately be considered part of the tschicoma formation of the polvadera group preliminary analysis of hydrothermal alteration in the context of the volcanic stratigraphy suggests at least three distinct hydrothermal events have occurred in the volcanic field s history

geological setting of the wassa gold deposit sw ghana including past production current indicated and inferred resources wassa is a 5 moz poly deformed early orogenic gold deposit located on the eastern flank of the ashanti belt in southwest ghana it is hosted by metamorphosed volcanic intrusive and sedimentary rocks of the sefwi group ca 2260 2160 ma early mineralization has an eoeburnean age 2164 22 ma re os on pyrite and is characterized by quartz veins by a carbonate alteration of the host rocks and by deformed gold bearing pyrite remobilization of this gold occurred during the late stages of the eburnean orogeny 2 1 ga and is associated with quartz carbonate veins with visible gold and euhedral pyrites

late mesozoic gravity sliding and oxfordian hydrocarbon reservoir potential of the northern yucatan margin abstract the stratigraphy and structural evolution of the western florida margin is much better understood than its northern yucatan conjugate because previous hydrocarbon exploration has been more extensive on the florida margin in the deep water northeastern gom the post salt section records late jurassic cretaceous gravity sliding of rafted blocks along a basinward dipping layer of callovian salt this study uses approximately 31 000 line km of 2d seismic data to describe a coeval area of widespread gravity sliding along the less studied northern yucatan margin we define three structural domains based on their distinctive salt structures and associated deformation 1 a region in the northeastern study area consists of relatively undeformed late jurassic cretaceous section underlain by 0 300 m thick salt 2 areas in the central and southwestern study area contain late mesozoic gravity slides defined by 300 600 m thick salt rollers and normal faults rooted onto a 1 4 basinward dipping salt detachment and 3 the distal margin of the western and central study area exhibits large salt diapirs up to 6 km tall that deform quaternary stratigraphic units in the central yucatan study area containing late mesozoic gravity slides a stratigraphic unit equivalent to the productive oxfordian norphlet sandstone of the deep water northeastern gom is identified based on its similar seismic character reconstructing the extent of this norphlet equivalent unit along the yucatan margin to its location during oxfordian deposition places it adjacent to an extensive area of norphlet sandstone mapped in the deep water northeastern gom by previous authors our reconstruction suggests a 48 000 km2 fairway of potential aeolian late jurassic reservoirs on the yucatan and campeche margins that includes productive reservoirs in the southeastern bay of campeche which previous authors correlated with the norphlet formation

an overview of the petroleum systems of the northern deep water gulf of mexico the northern deep watergulf ofmexico is one of themost active deep water petroleum provinces in the world this paper introduces the regional geologic setting for the northern deep water gulf of mexico and briefly discusses the importance of technology in developing the area s resources exploration has focused on four major geologic provinces basins subsalt fold belt and abyssal plain these provinces formed from the complex interactions between mesozoic cenozoic sedimentation and tectonics improved understanding of the geology of these provinces has largely been accomplished by improvements in seismic acquisition and processing in addition advances in drilling technology have permitted drilling and field development in increasingly greater water depths the 226 oil and gas fields and discoveries in the northern deep water gulf of mexico are summarized in terms of their exploration and development history producing facility ages of reservoirs upper jurassic upper paleocene lower eocene oligocene lower miocene upper pleistocene and trap type structural combined structural stratigraphic and stratigraphic in addition the interpreted regional distribution of upper jurassic and possible lower cretaceous source source rocks is shown in part based on the 26wells that have penetrated these source rocks the eight papers in this special issue review the geology of the mississippi canyon and northern atwater valley protraction areas the first five papers review the subregional structural setting and the evolution of its tectonics and petroleumsystems the final three papers summarize the geologic evolution of two economically important intraslope basins thunderhorse andmensa in terms of their stratigraphy structural evolution and petroleumsystems these two basins contain two of the larger oil and gas fields respectively in the northern deep water gulf of mexico copyright 2017 the american association of petroleum geologists all rights reserved

structural styles of albian rafts in the espirito santo basin se brazil evidence for late raft compartmentalisation on a passive continental margin in recent years hydrocarbon exploration offshore se brazil has been focusing on lower cretaceous strata deformed by gravity gliding above aptian salt a three dimensional 3d seismic volume from the espirito santo basin se brazil is here used to a test the parameters considered to control raft tectonics on a margin tectonically reactivated in the cenozoic and b investigate the impact of prolonged halokinesis on raft deformation offshore espirito santo the combined effects of halokinesis and multiple andean tectonic phases are expressed by local collapse fault reactivation and late segmentation of albian rafts as a result of this deformation we observe four main raft geometries a rolled over rafts b tabular rafts c collapsed rafts and d folded and tilted rafts on the flanks of salt rollers this work shows that salt rollers formed buttresses to moving albian cenomanian rafts with withdrawal of salt from underneath some of the rafts leading to their collapse and welding onto pre salt strata this process occurred in the studied part of the espirito santo basin with minimum control of post raft overburden thickness on raft compartmentalisation salt withdrawal from underneath the rafts is an important phenomenon as it enhanced connectivity between pre salt and post salt units potentially promoting the migration of hydrocarbons from syn rift source units into post salt reservoirs

modern style plate subduction preserved in the palaeoproterozoic west african craton the timing of onset of modern style plate tectonics on earth is debated analysis of rocks in the west african metamorphic province which is more than 2 gyr old reveals that some minerals formed under conditions similar to those in modern day subduction zones suggesting that subduction occurred on the palaeoproterozoic earth

origin of oil in the sureste basin mexico geochemical and biological marker analyses of oils and rock samples from the sureste basin of mexico were effective in identifying and geographically limiting four major oil families related by age and source rock depositional environment oxfordian tithonian early cretaceous and tertiary the source rocks giving rise to the jurassic and cretaceous oils are associated with marine carbonate environments in contrast the source rocks giving rise to the tertiary oils are associated with a marine deltaic siliciclastic depositional setting biomarker and isotope differences observed in the oils derived from marine carbonate environments can be interpreted in terms of salinity clay content and oxygen depletion variations these differences provide diagnostic criteria for recognizing and differentiating five distinct organic rich depositional regimes as the sources for these oil types an anoxic hypersaline marine carbonate environment associated with a narrow and shallow semirestricted sea oxfordian age family 1 oil an anoxic marine carbonate environment associated with a silled basin geometry tithonian age family 2 oils subtype 2a an anoxic marine carbonate environment associated with a shallow gentle broad marine carbonate ramp in a distal position tithonian age family 2 oils subtype 2b a clay rich suboxic anoxic marine carbonate environment associated with a carbonate platform in a proximal position tithonian age family 2 oils subtype 2c and an anoxic marine evaporitic environment early cretaceous age family 3 oils the tertiary oils family 4 are derived from bacterially reworked terrigenous and marine organic source materials deposited in a marine deltaic environment the tithonian related oils in the mexican southern side of the gulf of mexico accumulated both offshore and onshore and throughout the stratigraphic column from kimmeridgian to pleistocene reservoirs suggesting vertical pathways as the principal secondary migration mechanism the lateral variations of these oils can be interpreted to reflect the tithonian paleogeography in the area and could be useful in predicting differences in the oil compositions

three dimensional kirchhoff migration imaging of the jemez volcanic field using teleseismic data 1 we computed a reflectivity image for the earth beneath the jemez volcanic field new mexico using a novel adaptation of petroleum exploration seismic imaging this image was obtained by applying the kirchhoff wave field imaging method to digitally recorded teleseismic data the volume imaged has a lateral extent of 30 km extends to 45 km depth and lies beneath the valles caldera to the west of los alamos new mexico the derived picture of the earth is a three dimensional 3 d map of the locations of impedance changes within the crust and upper mantle below the jemez volcanic field with a spatial resolution on the order of a kilometer significant features seen in the image include the base of the caldera fill several reflectors in the crust we interpret to be associated with intrusions coming from the mantle and or other crystallized chambers such as a low velocity zone seen in the tomographic image two strong reflectors coincident with the crust mantle interface and a zone of layered reflections at the base of the crust which may be basaltic underplating this method proved to be a powerful tool for imaging the earth s crust and mantle when combined with other methods such as tomography

jurassic evolution of the gulf of mexico salt basin we present a new hypothesis for the jurassic plate tectonic evolution of the gulf of mexico basin and discuss how this evolution influenced jurassic salt tectonics four interpretations some based on new data constrain the hypothesis first the limit of normal oceanic crust coincides with a landward dipping basement ramp near the seaward end of the salt basin which has been mapped on seismic data second the deep salt in the deep water gulf of mexico can be separated into provinces on the basis of position with respect to this ramp third paleodepths in the postsalt sequence indicate that salt filled the gulf of mexico salt basin to near sea level fourth seismic data show that postsalt sediments in the central louann and the yucatan salt basins exhibit large magnitudes of late jurassic salt detached extension not balanced by equivalent salt detached shortening in our hypothesis callovian salt was deposited in preexisting crustal depressions on hyperextended continental and transitional crust after salt deposition ended rifting continued for another 7 to 12 m y before sea floor spreading began during this phase of postsalt crustal stretching the salt and its overburden were extended by 100 to 250 km 62 155 mi depending on location sea floor spreading divided the northern gulf of mexico into two segments separated by the northwest trending brazos transform the eastern segment opened from east to west leaving the walker ridge salient in the center of the basin as the final area to break apart in some areas salt flowed seaward onto new oceanic crust first concordantly over the basement as a parautochthonous province then climbing up over stratigraphically younger strata as an allochthonous province

jemez mountains volcanic field new mexico time term interpretation of the cardex seismic experiment and comparison with bouguer gravity the jemez mountains volcanic field jvf is centered astride the western flank of the rio grande rift in north central new mexico near the center of the volcanic field are the valles and toledo calderas the sites of two cataclysmic eruptions of silicic tephra and subsequent caldera collapses these calderas are among the largest and most recent 1 1 and 1 4 ma of the world s giant volcanic calderas the valles caldera especially has associated highly developed hydrothermal systems and may still contain residual magma within a cooling compositionally zoned pluton at shallow crustal depths the main goal of our study was to estimate the complex shallow crustal structure beneath the jemez volcanic field using data from a seismic refraction experiment that encompassed the central part of the field a time term technique was used to process first arrival time data from the network of temporary seismic stations of the caldera and rift deep seismic experiment cardex refraction experiment the data set consisting of 290 time distance pairs was processed to yield time terms at 84 locations 78 recording stations and 6 explosion sites the p wave velocity of the precambrian basement refractor was estimated to be 5 86 0 01 km s station time terms are highly correlated to bouguer gravity anomalies and thus tend to corroborate models for which the crystalline basement outcrops along the western margins of the rift and western flank of the jvf and dips generally eastward to depths of at least 600 m below sea level beneath sediment filled axial basins of the rift

tectonostratigraphic development in the eastern lower congo basin offshore angola west africa abstract as in all aptian salt basins off western africa the post rift evolution of the lower congo basin offshore angola was greatly influenced by raft tectonics we suggest that rafting in the lower congo basin took place during relatively short periods 7 10 ma characterized by high strain rates separated by longer periods 15 35 ma characterized by low strain rates the high strain rate periods are dated as aptian late cretaceous late eocene late oligocene and late miocene recent with respect to the tertiary development we have obtained a positive correlation between sedimentary thickness and cumulative stretching suggesting sedimentary loading as an important driving mechanism for raft tectonics during the tertiary two different types of depocentres developed these are i broad first order depocentres governed by regional subsidence and ii narrower elongated second order depocentres governed by growth on active faults within the study area the depocentres seem to have migrated from the west towards the east

a low velocity zone in the basement beneath the valles caldera new mexico we present quantitative results of forward modeling applied to a suite of travel time delays observed for teleseismic p wave first arrivals recorded in the valles caldera new mexico in 1987 we used a ray tracing method to construct a two dimensional model of the caldera s subsurface p wave velocity structure for the vertical cross section defined by the linear seismometer array geometry used to collect the data our initial trial models were based on observations of a systematic pattern in the recorded p wave delays which indicated that an isolated low velocity zone lvz exists in the precambrian basement beneath the caldera s surface layers thus the primary goal of the forward modeling was to determine the size shape location and velocity contrast of the lvz due to incomplete ray coverage we could not uniquely constrain all parameters defining the lvz and our final model thus represents a first look at the caldera s midcrustal structure we placed upper and lower bounds on most model parameters by investigating the ranges of models that fit the data well we determined that the lvz must be lens shaped pinching out laterally on each side and is located beneath the center of the caldera s resurgent dome its width is approximately 17 km from tip to tip and the depth from the surface to its center is 10 13 km we could not determine an upper bound for the lvz s thickness but we found it could be no smaller than 8 km from top to bottom for this thickness the p wave velocity was constrained to lie between 3 5 and 4 2 km s compared with a homogeneous half space model the best lvz model reduced the rms of the residual delays for 21 events by 70 equivalent to a 91 decrease in the residual variance the low p wave velocity suggests that the lvz could be a region of partial melt possibly related to the bandelier ignimbrite or postcaldera rhyolite eruptions

3d modeling of the ashanti belt southwest ghana evidence for a litho stratigraphic control on gold occurrences within the birimian sefwi group abstract a three dimensional model of the gold rich paleoproterozoic ashanti greenstone belt has been built with a hybrid geological modeling method using geomodeller and gocad then inverted using vpmg software the model helps to identify the main regional scale controls leading to abundant gold mineralization in the southeast of the belt by highlighting the lithological distribution and geometry of geological structures the three dimensional geological model shows that 85 of gold occurrences in the ashanti greenstone belt are proximal to a specific tholeiitic metabasaltic horizon bv1 this result is noteworthy given that the relatively thin 300 m thick bv1 layer represents only 10 by volume of the modeled sefwi group stratigraphy a map of distance between the topography and the bv1 layer is produced and identifies areas showing a high probability to host gold deposits in the southeast of the ashanti belt possible structural controls performed by the bv1 tholeiitic metabasaltic rocks on the location of gold occurrences are discussed and we hypothesize that the bv1 layer acted as a mineralization trap or source of gold both hypotheses agree with examples from the literature and explain the observed distribution of the mineralized sites

the cretaceous paleogene boundary deposit in the gulf of mexico large scale oceanic basin response to the chicxulub impact hydrocarbon exploration in the last decade has yielded sufficient data to evaluate the gulf of mexico basin response to the chicxulub asteroid impact given its passive marine setting and proximity to the impact structure on the yucatan peninsula the gulf is the premier locale in which to study the near field geologic effect of a bolide impact we mapped a thick decimeter to hectometer scale deposit of carbonate debris at the cretaceous paleogene boundary that is ubiquitous in the gulf and readily identifiable on borehole and seismic data we interpret deposits seen in seismic and borehole data in the deepwater gulf to be predominately muddy debrites with minor turbidites based on cores in the southeastern gulf mapping of the deposit in the northern gulf of mexico reveals that the impact redistributed roughly 1 05 105 km3 of sediment therein and over 1 98 105 km3 gulfwide deposit distribution suggests that the majority of sediment derived from coastal and shallow water environments throughout the gulf via seismic and megatsunamic processes initiated by the impact the texas shelf and northern margin of the florida platform were significant sources of sediment while the central and southern florida platform underwent more localized platform collapse the crustal structure of the ancestral gulf influenced postimpact deposition both directly and indirectly through its control on salt distribution in the louann salt basin nevertheless impact generated deposition overwhelmed virtually all topography and depositional systems at the start of the cenozoic blanketing the gulf with carbonate debris within days

sediment source regions and paleotransport of the upper jurassic norphlet formation eastern gulf of mexico the upper jurassic norphlet formation is an eolian sandstone and important hydrocarbon reservoir that overlies the louann salt in the gulf of mexico gom because the sand was concentrated into dunes formed by late jurassic winds determining the source areas and paleotransport direction of the sand can improve predictions of the distribution of the dune facies around the gom paleo wind blown sediment transport into the proto gom was controlled by wind direction and magnitude and the extant topography of the basin and adjacent uplands analysis of the norphlet formation in the eastern gom shows that wadis and alluvial fans controlled by the location of highs were the primary route for introducing sediment of varied provenance into the eolian erg eolian transport directions interpreted from dip log analyses are south directed in southern alabama and west to northwest directed in western florida interpretations of regional two dimensional prestack depth migrated seismic data show that erosional incision of the middle ground arch occurred prior to and during the time of norphlet deposition this as well as preexisting lows in the basement topography may have facilitated basinward sand transport of sediment that fed the norphlet formation erg preserved in the deep water subsurface eastern gom

heterogeneous structure around the jemez volcanic field new mexico usa as inferred from the envelope inversion of active experiment seismic data summary we analyse active experiment seismic data obtained by the 1993 jemez tomography experiment jtex programme to elucidate the heterogeneous structure of the jemez volcanic field which is located at the boundary between the colorado plateau and the rio grande rift using a single isotropic scattering assumption we first calculate the envelope green s functions for the upper and lower crust and the uppermost mantle by fitting the theoretical envelopes with the observed three component data we estimate depth dependent features of the scattering coefficients around valles caldera we estimate the ratios of scattering coefficients rather than scattering coefficients themselves because of the uncertainty of the seismic efficiency of the explosive sources and knowledge of absolute site amplification factors the strongest scattering coefficients are observed at a shallow depth beneath the valles caldera this is considered to be related to the complex structure caused by two episodes of caldera formation and the ensuing resurgent uplift in the caldera etc the depth dependent characters of the scattering coefficients for the colorado plateau and the rio grande rift are similar to each other a transparent upper crust and a heterogeneous lower crust small and large scattering coefficients respectively however the scattering coefficients beneath the rio grande rift are several times larger than those beneath the colorado plateau depths of the lower crust and the moho boundary beneath the rio grande rift are shallower than those of the colorado plateau from their geological settings and other geophysical results around the region we infer that the larger scattering coefficients of the rift are associated with rift formation and volcanic activity such as magma ascent from the upper mantle to the crust

explosive rhyolitic volcanism in the jemez mountains vent locations caldera development and relation to regional structure the jemez volcanic field straddles the western margin of the rio grande rift where the rift is intersected by the jemez lineament in north central new mexico the field has a record of volcanism extending back to before 13 ma initial basaltic activity was related to active rifting with minor rhyolitic eruptions occurring along n s rift bounding faults between 10 and 7 ma voluminous andesitic volcanism took place in the central jemez mountains area overwhelming contemporaneous basaltic and rhyolitic magmatism an apparent tectonic lull took place from 7 to 4 ma accompanied by lower eruption rates during this interval dacitic magmas were erupted to form the tschicoma volcanic center but mafic and rhyolitic volcanism virtually ceased since 4 ma accompanying resumption of rifting a growing silicic magma system has been present under the central part of the jemez mountains ultimately evolving to the magma body that produced the voluminous rhyolitic bandelier tuffs explosive rhyolitic eruptions from this large magma body have occurred many times since 3 to 4 ma early eruptions 3 6 2 8 ma produced high silica rhyolite ignimbrites restricted to the southwest part of the jemez mountains formation of these units may have been accompanied by caldera collapse these events were followed by the two caldera forming bandelier tuff ignimbrite eruptions 1 45 and 1 12 ma post caldera explosive and effusive rhyolite eruptions have also tapped the magma body from vents generally located along ring fractures after both bandelier events vent and caldera locations for the rhyolitic eruptions during 3 4 ma have been inferred from grain size characteristics dispersal patterns and facies variations in the plinian deposits and ignimbrites lithic breccia zones of the pre bandelier ignimbrites indicate possible caldera sources in the southwest part of the present valles caldera during the eruption of both bandelier tuffs initial plinian falls and early pyroclastic flows emanated from vents centrally located in the jemez mountains in the lower bandelier tuff eruption a transition to ring fracture vents occurred before the emission of later pyroclastic flows but there is no strong evidence to suggest such a transition occurred during the upper bandelier eruption calderas associated with the lower and upper bandelier tuffs toledo and valles respectively are almost identical in location as are the plinian vent sites for these two large eruptions the toledo embayment northeast of the valles caldera contains lava domes from up to 3 6 ma and may be a caldera or crater associated with early explosive dacitic volcanism in the tschicoma volcanic center post 1 4 ma lava domes also fill this depression the main volcanic features of the jemez mountains field including the valles caldera complex eruption vents and the apical graben of the post valles redondo resurgent block appear to be aligned along the ne sw trending jemez fault zone this zone the local expression of a precambrian basement feature the jemez lineament has exerted strong control on the location and style of eruptions from the bandelier rhyolitic magma system

vibroseis recording techniques and data reduction from the jemez tomography experiment abstract the jemez tomography experiment jtex is a multidisciplinary study focused on the valles caldera and the jemez mountains new mexico the objectives of the project are to create a high resolution crustal model of the subsurface structure of this silicic volcanic system and to develop an interpretation of its volcanic evolution use of vibroseis sources in the acquisition of refraction wide angle reflection seismic data provided challenges beyond conventional explosive source data processing of the jtex vibroseis data is an involved procedure consisting of sorting cross correlating filtering and stacking numerous individual seismograms in the production of final record sections however excellent results high signal to noise seismograms at relatively small spacings are obtainable with coherent arrivals at source receiver distances of more than 60 km the primary drawback in this approach lies in the massive volume of data that is necessary to produce record sections one benefit of the vibroseis source used during jtex was a method to decrease effective seismogram spacing this technique dubbed a source offset technique provides smaller overall seismograph station spacing by moving the vibroseis sources during acquisition and leaving deployed seismographs stationary after station corrections this method effectively decreases station spacings and increases detail in resulting record sections various shallow crustal heterogeneities create travel time advances and delays that affect the source offset data differently than single source data synthetic modeling demonstrates small travel time discrepancies associated with the source offset technique however the addition of traces with smaller station intervals clarifies secondary arrivals within record sections and aids in interpretation of these arrivals with a minimum amount of field effort required

compilation of west african mineral deposits spatial distribution and mineral endowment abstract the west african craton is highly endowed in minerals and their spatial and temporal distribution varies from single to multi phase mineralization events they are broadly related to three major tectono metallogenic elements and formed during distinct mineral epochs 1 in both archean shields kenema man and reguibat and paleoproterozoic domains baoule mossi eglab these are characterized by giant iron ore deposits that formed between ca 2 5 2 3 ga nearly all gold porphyry copper lead zinc and sedimentary manganese ore that developed between 2 2 and 2 1 ga and primary diamonds that formed between two intervals at ca 2 2 2 0 ga and in the mesozoic 2 across pan african and variscan belts these are distinguished by major precambrian iocg s copper gold that formed at ca 2 1 ga and approximately 680 ma and neoproterozoic sedimentary iron ore and phosphate deposits 3 within intracratonic and coastal basins these include the development of cenozoic lateritic bauxites over mesozoic dolerites tertiary quaternary mineral sands deposits oolitic iron ore and sedimentary phosphate deposits geological spatial and temporal correlations using the multi commodity west african mineral deposit database highlight that gold and non gold commodities formed in multiple phases this commenced in the liberian orogeny 2 9 2 8 ga with the enrichment of iron ore nickel sulphides diamonds and gold in the earth s crust the pre eburnean or tangaean eoeburnean eburnean i event yielded gold and the major eburnean orogeny yielded gold iron ore manganese diamonds magmatic nickel sulphides copper gold lead zinc and ree minerals throughout the pan african event sedimentary manganese deposits lead zinc ree minerals sedimentary phosphates and again gold were formed primary diamonds and magmatic nickel sulphides are related to the break up of gondwana followed by an intense lateritic weathering period that formed bauxite deposits along the craton margin

age constraints on gold mineralization and paleoproterozoic crustal evolution in the ashanti belt of southern ghana abstract u pb isotopic dating in the region of the ashanti belt in ghana has been carried out to improve understanding of crustal evolution and gold mineralization in this paleoproterozoic terrane a u pb zircon age of ca 2174 2 ma for a belt type granitoid from sekondi accords with previous ages on belt type plutons in ghana but metamorphic titanite within this granitoid gives an age of 2092 3 ma coeval with intrusion of the late basin type granitoids the within belt granitoid at banso village has an age close to 2097 3 ma which falls into the age range typical for basin type granitoids 2116 2088 ma these results indicate that widespread plutonism in the birimian sedimentary basins also produced magmatic and metamorphic effects within the volcanic belts most dated detrital zircons from birimian metasediment samples at the ashanti gold mine agree in age at 2155 2 ma with others that are slightly older this gives a maximum estimate for the depositional age of the sediments and for gold mineralization at the ashanti gold mine hydrothermally altered and auriferous granitoid plutons along the western flank of the ashanti belt at anyankyerim nhyiaso yaomensakrom and esuajah ayanfuri all have intrusion ages within error of 2105 2 ma this is also the age of metamorphic titanite in an unmineralized basin type granitoid at kwapia rutiles from the hydrothermally altered yaomensakrom and the mineralized esuajah ayanfuri granitoid were dated at 2098 7 and 2086 4 ma respectively and probably provide minimum ages on gold mineralization these data indicate a close temporal association among auriferous granitoid plutons near the western margin of the ashanti belt at 2105 ma gold mineralization may be only slightly younger than magmatic emplacement of the plutons granitic plutonism in the sedimentary basins metamorphism and hydrothermal gold mineralization in southern ghana are all part of a major crust formation event the eburnean orogeny that can be constrained to the time range 2120 2080 ma

late kinematic timing of orogenic gold deposits and significance for computer based exploration techniques with emphasis on the yilgarn block western australia abstract orogenic gold deposits are a widespread coherent group of epigenetic ore deposits that are sited in accretionary or collisional orogens they formed over a large crustal depth range from deep seated low salinity h2o co2 ch4 n2 ore fluids and with au transported as thio complexes regional structures provide the main control on deposit distribution in many terranes first order faults or shear zones appear to have controlled regional fluid flow with greatest ore fluid fluxes in and adjacent to lower order faults shear zones and or large folds highly competent and or chemically reactive rocks are the most common hosts to the larger deposits focusing of supralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes normally within d3 or d4 in a d1 d4 deformation sequence reactivation of suitably oriented pre existing structures during a change in far field stress orientation is a factor common to many deposits and repeated reactivation may account for multiple mineralization episodes in some larger deposits absolute robust ages of mineralization support their late kinematic timing and in general suggest that deposits formed diachronously towards the end of the 100 to 200 m y long evolutionary history of hosting orogens for example in the yilgarn block a region specifically emphasised in this study orogenic gold deposits formed in the time interval between 40 and 90 m y with most about 60 to 70 m y after the youngest widespread basic ultrabasic volcanism and towards the end of felsic magmatism the late timing of orogenic gold deposits is pivotal to geologically based exploration methodologies this is because the present structural geometries of i the deposits ii the hosting goldfields and iii the enclosing terranes are all essentially similar to those during gold mineralization at least in their relative position to each other thus interpretation of geological maps and cross sections and three dimensional models can be used to accurately simulate the physical conditions that existed at the time of ore deposition it is particularly significant that the deposits are commonly related to repetitive and predictable geometries such as structural heterogeneities within or adjacent to first order structures around rigid granitoid bodies or in specific locked up fold thrust structures importantly the two giant greenstone hosted goldfields kalgoorlie and timmins show a remarkably similar geometry at the regional scale computer based stress mapping and gis based prospectivity mapping are two computer based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provide important geological aids in exploration both in broad regions and more localized goldfields both require an accurate and consistent solid geology map stress mapping requires knowledge of the far field stresses during mineralization and the empirical prospectivity mapping requires data from a significant number of known deposits in the terrane the kalgoorlie terrane in the yilgarn block meets these criteria and illustrates the potential of these methodologies in the exploration for orogenic gold deposits low minimum stress anomalies interpreted to represent dilational zones during gold related deformation coincide well with the positions of known goldfields rather than individual gold deposits in the terrane and there are additional as yet unexplained anomalies the prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are i proximity to and orientation and curvature of granitoid greenstone contacts ii proximity to segments of crustal faults which strike in a preferred direction iii proximity to specific lithological contacts which have similar preferred strike iv proximity to anticlinal structures and v the presence of preferred reactive host rocks e g dolerite the prospectivity map defines a series of anomalous areas which broadly conform to those of the stress map 78 correspondence the most prospective category on this map covers less than 0 3 of the greenstone belts and yet hosts 16 of the known deposits which have produced 80 of known gold thus it discriminates in favour of the larger economically more attractive deposits in the terrane the successful application of stress mapping and prospectivity mapping to geology based exploration for orogenic gold deposits indicates that more quantitative analysis of geological map data is a profitable line of research the computer based nature of these methodologies is ideal for the production of an ultimate integrated deposit target map which can be compared to other more conventional targeting parameters such as geophysical and geochemical anomalies such an integrated strategy appears the way forward in the increasingly difficult task of cost effective global exploration for orogenic gold deposits in poorly exposed terranes

crust and upper mantle p wave velocity structure beneath valles caldera new mexico results from the jemez teleseismic tomography experiment new results are presented from the teleseismic component of the jemez tomography experiment conducted across valles caldera in northern new mexico we invert 4872 relative ital p wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km the three principle features of our model for valles caldera are 1 near surface low velocities of minus 17 percent beneath the toledo embayment and the valle grande 2 midcrustal low velocities of minus 23 percent in an ellipsoidal volume underneath the northwest quadrant of the caldera and 3 a broad zone of low velocities minus 15 percent in the lower crust or upper mantle crust shallower than 20 km is generally fast to the northwest of the caldera and slow to the southeast near surface low velocities are interpreted as thick deposits of bandelier tuff and postcaldera volcaniclastic rocks lateral variation in the thickness of these deposits supports increased caldera collapse to the southeast beneath the valle grande we interpret the midcrustal low velocity zone to contain a minimum melt fraction of 10 percent while we cannot rule out the possibility that this zone is the remnant 1 2 ma bandelier magma chamber the eruption history and geochemistry of the volcanic rocks erupted in valles calderamore following the bandelier tuff make it more likely that magma results from a new pulse of intrusion indicating that melt flux into the upper crust beneath valles caldera continues the low velocity zone near the crust mantle boundary is consistent with either partial melt in the lower crust or mafic rocks without partial melt in the upper mantle in either case this low velocity anomaly indicates that underplating by mantle derived melts has occurred copyright 1998 american geophysical union less

teleseismic p wave image of crust and upper mantle structure beneath the valles caldera new mexico initial results from the 1993 jtex passive array teleseismic p wave relative arrival time data collected from a temporary array during the 1993 jemez tomography experiment jtex have been inverted to image velocity anomalies beneath the valles caldera in northern new mexico instruments were deployed in two 30 km long profiles one of 8 and one of 9 stations these profiles crossed the caldera trending at azimuths of n46 w and n60 e respectively two dimensional teleseismic relative arrival time inversion of the 1993 data set supplemented with data from an overlapping 1987 profile confirms the existence of a mid crustal low velocity region 30 beneath the valles caldera in the depth range of 8 to 13 km below sea level with about a 6 km horizontal extent this feature is interpreted to be the seismic expression of the remnant magma chamber a shallow low velocity anomaly beneath san antonio mountain coincides with the region of highest thermal gradient values in the caldera a lower crust upper mantle low velocity anomaly is imaged but is not as well constrained due to the limited length of the profile we tentatively correlate this anomaly with the thermal effects of basaltic magmas ponded at the crust mantle boundary

re os geochronological evidence for multiple paleoproterozoic gold events at the scale of the west african craton the archean proterozoic craton of west africa hosts numerous gold deposits which are spatially and temporally related to the eburnean orogeny that took place between 2250 and 1980 ma and included multiple deformation events the majority of these gold deposits are located along shear zones the structural history is relatively well established for most gold deposits but absolute timing of the mineralization is commonly lacking five deposits hosted in the baoule mossi domain located in the southwestern part of the west african craton were studied to better constrain timing of gold events nassara and kiaka in burkina faso and wassa damang and obuasi in southern ghana gold mineralization was structurally characterized at each deposit and dated by the re os method on pyrite arsenopyrite and pyrrhotite grains that were coeval with the gold deposition combined structural and re os geochronological constraints allow two groups of gold deposits to be distinguished early orogenic gold formed during the eoeburnean orogeny i e between 2190 and 2125 ma as represented by kiaka1 at 2157 24 ma and wassa1 at 2164 14 ma these early gold occurrences are expressed as early disseminated gold enrichment 1 3 g t au at kiaka and gold hosted in folded and boudinaged quartz veins that are transposed along the primary s foliation at wassa these re os ages on the early sulfides at wassa and kiaka are significant because they definitively provide the first direct age constraints on gold only eoeburnean mineralization in the birimian of west africa late orogenic gold hosted by brittle structures and with higher gold grades up to 60g t at kiaka formed during late eburnean deformation d d events between 2120 and 2000 ma as represented by the re os dating of these ores at wassa at 2055 18 ma and obuasi at 2045 40 ma similar style ores dated at nassara at 2131 99 ma and damang at 2080 135 ma but both with large uncertainties and an undated younger event at kiaka are also suggested to be part of this later mineralizing episode the high uncertainties on the nassara and damang ages may be directly linked to the low rhenium and osmium contents of the studied samples these results highlight the polyphase character of the widespread gold mineralization in the west african craton although the late stages of the eburnean orogeny constitute a prolific period for the formation of high grade gold mineralization identification of less well studied early stage gold deposits which can also contain large quantities of gold is critical for mineral exploration in the west african craton

geologic framework of norphlet and pre norphlet strata of the onshore and offshore eastern gulf of mexico area abstract hydrocarbon accumulations have been discovered in the jurassic norphlet formation in the onshore and offshore eastern gulf of mexico area an understanding of the regional geologic framework of the norphlet and pre norphlet stratigraphic succession in the study area is crucial to successful exploration for and development of norphlet hydrocarbon reservoirs in the eastern gulf region these strata include jurassic sedimentary rocks and pre jurassic sedimentary and crystalline rocks of these strata only the norphlet formation has produced economic quantities of hydrocarbons from reservoirs which include eolian dune interdune wadi and marine sandstones however the pre norphlet geology and paleotopography of the area controlled to a large degree norphlet depositional patterns and subsequent structural development especially important were the locations of pre jurassic paleohighs and the distribution and thickness patterns of the underlying jurassic louann salt norphlet hydrocarbon traps are generally associated with structures that have resulted from halokinesis of the louann traps include salt anticlines faulted salt anticlines and extensional faults associated with salt movement in addition to these salt related structural traps traps resulting from pre salt basement paleotopography may exist in the study area these traps might include norphlet structural highs associated with preexisting basement highs and norphlet stratigraphic terminations along the flanks of basement highs where norphlet reservoir facies are present the key factor controlling hydrocarbon accumulation potential is structural setting in the mississippi alabama florida mafla region that is underlain by the norphlet formation four structural settings have been identified types a and b structural settings are characterized by norphlet sediments overlying basement or thin salt no norphlet fields have been established in these settings types c and d structural settings are characterized by structures related to movement of thick louann salt all norphlet fields established in the mafla region occur in these settings four geographic regions have been defined in the study area for the purpose of characterizing norphlet petroleum potential region 1 has excellent potential for future norphlet discoveries region 2 has low to moderate potential and regions 3 and 4 have very little to no potential

national geophysical data center candidate for the world digital magnetic anomaly map marine and airborne magnetic anomaly data have been collected for more than half a century providing global coverage of the earth furthermore the german champ satellite is providing increasingly accurate information on large scale magnetic anomalies the world digital magnetic anomaly map project is an international effort to integrate all available near surface and satellite magnetic anomaly data into a global map database teams of researchers were invited to produce candidate maps using a common pool of data sets here we present the national geophysical data center ngdc candidate to produce a homogeneous map the near surface data were first line leveled and then merged by least squares collocation long wavelengths were found to agree surprisingly well with independent satellite information this validates our final processing step of merging the short wavelength part of the near surface data with long wavelength satellite magnetic anomalies

ek balam field a structure related to multiple stages of salt tectonics and extension the ek balam field is a major oil and gas field in the offshore campeche bay mexico the structure was formed as a result of multiple episodes of salt tectonics and associated extension salt movement into the core of the structure initiated in the jurassic early cretaceous and continued until the miocene the most prominent normal faults are of jurassic early cretaceous age and detach in the jurassic salt these faults mostly dip to the west although in the vicinity of the ek balam structure some east dipping faults are also present some of these faults were subsequently reactivated in the late cretaceous and tertiary separating the ek and balam structures is a set of upper cretaceous faults which drop the eastern part of the structure down tertiary faults are related to continued uplift of the salt and are symmetrically distributed with respect to the central uplift seismic time maps three dimensional depth models of the top of the cretaceous breccia and the kimmeridgian and balanced structural cross sections provide accurate details of the structural geometry and fault patterns in the ek balam structure at the level of the cretaceous breccia many of the faults have small throws and are discontinuous along trend the top of the kimmeridgian however is cut by faults of different ages many of which have significant throw and extend for longer distances the detailed structural geometry and fault patterns will be useful for identifying compartments within the ek balam field and for identifying secondary traps within kimmeridgian and oxfordian reservoirs