Basement Geometry Research Articles

Gravity inversion predicts the nature of the Amundsen Basin and its continental borderlands near Greenland

The high-Arctic Eurekan Orogeny was caused by a northward movement of Greenland relative to North America and Eurasia during the latest Cretaceous to late Eocene. While the Eurekan N–S shortening is well-documented in Ellesmere Island, North Greenland and Svalbard, the nature of the event is largely unknown in the ice-covered Arctic Ocean to the north of Greenland. In this contribution, we show that the tectono-physiographic evolution of the oceanic Amundsen Basin, the continental Lomonosov Ridge and the Morris Jesup Rise were all affected by significant Eurekan compression. We present the results of 3-D gravity inversion for predicting the sediment thickness and basement geometry within the Amundsen Basin and along its borderlands. We use the recently published LOMGRAV-09 gravity compilation and adopt a process-oriented iterative cycle approach that minimizes misfit between an Earth model and observations. The sensitivity of our results to lateral variations in depth and density contrast of the Moho is further tested by a stochastic inversion. Within their limitations, the approach and setup used herein provides the first detailed model of the sediment thickness and basement geometry in the Arctic Ocean north of Greenland. Our preferred result, using a C25 breakup scenario of the Amundsen Basin, correlates well with seismic observations along existing and several new marine seismic profiles. Breakup-related rift basins are predicted along the Lomonosov Ridge and a broad depocentre is predicted above high-relief basement in the central Amundsen Basin. Significantly, an up to 7 km deep elongated sedimentary basin is predicted along the northern edge of the Morris Jesup Rise. This basin continues into the Klenova Valley south of the Lomonosov Ridge and correlates with an offshore continuation of the Eurekan Mount Rawlinson Fault in Ellesmere Island. We compute the anomalous basement topography and show evidence of deformed oceanic and continental crust in relation to this fault zone (LKFZ), suggesting that pronounced Eurekan crustal shortening took place here.

Tomographic imaging of sub-basalt Mesozoic sediments and shallow basement geometry for hydrocarbon potential below the Deccan Volcanic Province (DVP) of India

Summary A shallow 2D tomographic P-wave velocity image is obtained in the Deccan Volcanic Province (DVP) of India using first-arrival seismic data along a 90 km long N-S trending profile in the Deccan Syneclise region. The tomographic image depicts smooth velocity variations of Quaternary (2.0-2.5 km/s) sediments, basalt (5.0-5.5 km/s) and sub-basalt Mesozoic sediments (4.0-4.5 km/s) as well as basement (5.8-6.0 km/s) configuration down to a maximum depth of 5.0 km. Due to late Cretaceous volcanism with outpouring of basaltic lava flows, this region is affected by numerous dyke intrusions and thick basaltic trap forming sills with the presence of horst and graben structures due to deep basinal faults. Although imaging below the basalt is a major challenge for the oil industry, with the help of tomographic imaging technique of long-offset seismic refraction data, we are able to image sporadic sub- and intra-trappean Mesozoic sediments (<0.75 km) deposited below thick columns (0-3 km) of basalt flows above the basement. The imaged Mesozoic sediments are expected to contain hydrocarbon due to their wide extension and suitable trapping mechanism of basaltic sills. The robustness of the velocity image is assessed through numerous tests like velocity perturbations, χ 2 estimates, RMS residuals of traveltime fit, resolution and uncertainty estimates through computation of ray-density or Hits.

Distribution of Paleogene and Cretaceous rocks around the Nazko River belt of central British Columbia from 3-D long-offset first-arrival seismic tomography

Across the Nechako–Chilcotin plateau of British Columbia, the distribution of Cretaceous sedimentary rocks, which are considered prospective for hydrocarbon exploration, is poorly known due to the surface cover of glacial deposits and Tertiary volcanic rocks. To constrain the subsurface distribution of these Cretaceous rocks, in 2008 Geoscience BC acquired seven long, up to 14.4 km, offset vibroseis seismic reflection lines across a north-northwest-trending belt of exhumed sedimentary rocks inferred to be part of the Taylor Creek Group. P-wave velocity models, which are consistent with sonic logs from nearby wells, have been estimated using three-dimensional first-arrival tomography to depths ranging from 1 to 4 km. Igneous basement can be identified on most lines using the 5.5 km/s isovelocity contour, which locates the top of the basement to an accuracy of ∼400 m where its depth is known in exploration wells. There is no general distinction on the basis of seismic velocity between Cretaceous sedimentary and Paleocene–Eocene volcanic–volcaniclastic rocks, both of which appear to be characterized in the tomographic models by velocities of 3.0–5.0 km/s. The geometry of the igneous basement inferred from the velocity models identifies north-trending basins and ridges, which correlate with exposed rocks of the Jurassic Hazelton Group. Identified Cretaceous sedimentary rocks occur beneath less negative Bouguer gravity anomalies, but the original distribution of these rocks has been disrupted by later Tertiary extension that created north-trending basins associated with the most negative gravity anomalies. We suggest that Cretaceous sedimentary rocks, if deposited, could be preserved within these basins if the rocks had not been eroded prior to Tertiary extension.

Combination of Geophysical Methods to Support Urban Geological Mapping

Urban geological mapping is a key to assist management of new developed areas, conversion of current urban areas or assessment of urban geological hazards. Geophysics can have a pivotal role to yield subsurface information in urban areas provided that geophysical methods are capable of dealing with challenges related to these scenarios (e.g., low signal-to-noise ratio or special logistical arrangements). With this principal aim, a specific methodology is developed to characterize lithological changes, to image fault zones and to delineate basin geometry in the urban areas. The process uses the combination of passive and active techniques as complementary data: controlled source audio-magnetotelluric method (CSAMT), magnetotelluric method (MT), microtremor H/V analysis and ambient noise array measurements to overcome the limitations of traditional geophysical methodology. This study is focused in Girona and Salt surrounding areas (NE of Spain) where some uncertainties in subsurface knowledge (maps of bedrock depth and the isopach maps of thickness of quaternary sediments) need to be resolved to carry out the 1:5000 urban geological mapping. These parameters can be estimated using this proposed methodology. (1) Acoustic impedance contrast between Neogene sediments and Paleogene or Paleozoic bedrock is detected with microtremor H/V analysis that provides the soil resonance frequency. The minimum value obtained is 0.4 Hz in Salt city, and the maximum value is the 9.5 Hz in Girona city. The result of this first method is a fast scanner of the geometry of basement. (2) Ambient noise array constrains the bedrock depth using the measurements of shear-wave velocity of soft soil. (3) Finally, the electrical resistivity models contribute with a good description of lithological changes and fault imaging. The conductive materials (1–100 Ωm) are associated with Neogene Basin composed by unconsolidated detrital sediments; medium resistive materials (100–400 Ωm) correspond to Paleogene, and resistive materials (600–1,000 Ωm) are related with complex basement, granite of Paleozoic. The Neogene basin-basement boundary is constrained between surface and 500 m depth, approximately. The new geophysical methodology presented is an optimized and fast tool to refine geological mapping by adding 2D information to traditional geological data and improving the knowledge of subsoil.

Tectonic models for the origin of regional transverse structures in the Grenville Province of SW Quebec interpreted from regional gravity

Analysis of regional gravity data over the Grenville Province of SW Québec, Canada, has identified hitherto unrecognized transverse deep-crustal structures, in addition to lineaments that correspond to mapped upper crustal, orogen-parallel tectonic boundaries. Lineaments that trend NW–SE, i.e. transverse to the orogen, are interpreted as reactivated Palaeoproterozoic faults in Archaean basement that are controlled by structures in Archaean sub-crustal lithospheric mantle (SCLM). A prominent lineament (the Mont-Laurier lineament) that extends from the SE Superior Province across the SW Grenville Province is identified on both gravity and seismic tomographic imagery, the latter confirming it represents a lithospheric-scale structure. Baltica is placed at the Laurentian margin in tectonic reconstructions, and the Mont-Laurier lineament correlates with the extension towards the Superior Craton of the Belomorian Province between the Karelia and Kola cratons. It is proposed that this structure developed as a normal fault zone, and was subsequently reactivated as a transfer fault, during two orthogonal Palaeoproterozoic rifting events. The ensuing irregular basement geometry subsequently influenced sedimentation and deformation during the Grenvillian orogenic cycle and the Mont-Laurier lineament again acted as a transfer fault zone during Grenvillian orogenesis and post-orogenic collapse. Identification of tranverse structures has implications in understanding the architecture of this part of the Grenville Province. The role of pre-existing basement structures and rigid SCLM on the formation and preservation of long, linear structures transverse to an orogen throughout a complex sequence of rifting, back-arc basin formation, collisional orogenesis, and post-orogenic collapse in the Grenville Province of SW Quebec is also documented in New Guinea, with which comparisons are made.

Insights from the Jan Mayen system in the Norwegian-Greenland Sea-II. Architecture of a microcontinent

SUMMARY We study the formation and evolution of the Jan Mayen microcontinent (JMMC) in the framework of conjugate margin evolution in the Norwegian–Greenland Sea. The JMMC structural map and crustal architecture have been constrained by seismic mapping and potential field modelling, supplemented by published seismic refraction transects. The sedimentary and basement geometries have been further studied together with their conjugate to refine our knowledge on the less explored microcontinent. Structurally, the JMMC main ridge is characterized by a platform and/or terrace architecture flanked by sag type basins, similar to those described on the mid-Norwegian conjugate margin, while its southern part is marked by windows exposing crustal and/or mantle material that was exhumed during higher degrees of extension. The eastern side of the JMMC broke up in a magma-rich fashion, while the western side is magma poor. The limited amount of magma involved in the JMMC formation suggests that changes in far field forces were the driving mechanism to initiate the isolation process and that a mantle plume had a lesser influence, but might have helped the final mid-ocean ridge establishment between the western Jan Mayen and Greenland margin.

Timing of fault reactivation in the upper crust of the St. Lawrence rift system, Canada, by K–Ar dating of illite-rich fault rocks1GEOTOP Contribution 2012-0002.

Formed during late Proterozoic – early Paleozoic, the St. Lawrence rift system of eastern North America hosts an active seismic zone, also displaying evidence of subsequent tectonic reactivation in Paleozoic and Mesozoic times. However, lack of a detailed reconstruction of the basement geometry and limited geochronological constraints limit our understanding of its structural evolution. Late Silurian – Early Devonian fault reactivation is demonstrated here in typical fault sites of the St. Lawrence rift system by combining structural observations of basement structures and a mineralogical, morphological, and K–Ar isotopic study of clay-rich fault material (&lt;0.4 µm et 1–2 µm). The K–Ar data of clay-rich size fractions from gouges of varied rock types from Saint-Laurent and Montmorency faults define two isochron ages at 436 ± 45 and 406 ± 22 Ma that are within analytical uncertainty and give an average at 421 ± 15 Ma. However, these two faulting episodes could also picture a single long-lasting phase of foreland subsidence of the Appalachian orogen starting during the Late Ordovician – Early Silurian deformation in the external Humber zone and continuing during Late Silurian – Early Devonian subsidence related to a normal faulting. This interpretation is based on a different mineral composition of the younger size fractions that consist of smectite-enriched clay minerals and could, therefore, correspond to the end of a reactivation event that was episodically active between 436 ± 45 and 406 ± 22 Ma. The faults were selectively reactivated depending on their location relative to pre-existing metamorphic and tectonic fabrics of the Grenvillian basement. The identified Late Silurian – Early Devonian reactivation of the Saint-Laurent and Montmorency faults was contemporaneous with the subsidence of the adjacent Appalachian foreland, resulting in SE-dipping faults in the crystalline basement of the St. Lawrence rift system and NW-dipping faults in the Appalachian cover.

Structural style variation and its impact on hydrocarbon traps in central Fars, southern Zagros folded belt, Iran

The Fars area is the main target for Permian gas exploration in the Zagros fold belt. It contains approximately 15 percent of the world’s proven gas reserves. The geometrical characteristics of the folded structures change dramatically across the N–S trending Gavbandi High. We used seismic profiles, well data, magnetic survey information and field observations to show that thickness variation of the sedimentary pile inherited from basement geometry is the main reason behind structural style variation in this area which occurred during the Zagros folding. Differences in thickness were more significant in Early-Middle Paleozoic time and decreased considerably upward in time. The total thickness of the Lower Paleozoic succession in the eastern side of the Gavbandi High is approximately 40–50% thicker than on the summit of this basement high. Sedimentary pinch-outs through Cretaceous and Tertiary times indicate that the activity of the basement faults decreased but did not stop. The impact on hydrocarbon traps of the pre-folding basin architecture and the differences in the behavior of the sedimentary cover after Miocene folding is discussed and documented.

Mafic intrusions east of Svalbard imaged by active-source seismic tomography

A seismic refraction and reflection tomography experiment was performed across the igneous province east of Svalbard which is a part of the Cretaceous High Arctic Large Igneous Province. Seismic travel times from 12 ocean bottom seismometers/hydrophones deployed along a 170km line are inverted to produce smooth 2D images of the crustal P-wave velocity and geometry of the acoustic basement and Moho. The inversion of travel times was complemented by forward elastic wave propagation modeling. Integration with onshore geology as well as multichannel seismic, magnetic and gravity data have provide additional constraints used in the geological interpretation. The seismic P-wave velocity increases rapidly with depth, starting with 3km/s at the sea floor and reaching 5.5km/s at the bottom of the upper sedimentary layer. The thickness of this layer increases eastward from 2km to 3.5km. On average the P-wave velocity in the crystalline crust increases with depth from 5.5km/s to 6.8km/s. The crustal thickness is typical for continental shelf regions (30–34km). Finger-shaped high-velocity anomalies, one reaching 12% and two of 4–6% velocity perturbation, are obtained. These velocity anomalies are concomitant with Lower Cretaceous basaltic lava flows and sills in the shallow sediments and elongated gravity and magnetic highs, traced towards the northern Barents Sea passive continental margin. We interpret the obtained velocity anomalies as signatures of dikes emplaced in the basement during breakup and subsequent spreading in the Arctic Amerasia Basin.

Basement Geometry, From Magnetic and Gravity Data in Gastre Trough, Chubut, Argentina

A gravimetric and magnetometric study at Gastre Trough, province of Chubut, allowed to reveal its basement geometry. Maps of anomalies show a fractured basement with high and sunk blocks inside the Trough. The minimum anomalous are associated to graben-like depocenters possibly inverted during the Tertiary, while the maximum are linked to basement blocks. Some of these blocks outcropping at the edges of the trough and some others covered by sediments inside. A 3D analytical signal and a gravity horizontal gradient were applied in order to make an appropriate interpretation, associating the maximum magnetic susceptibility changes and density to contacts between different lithologies. Finally, outcropping density values as well as seismic density values allowed to make 2D density models across to the Trough, which showed the presence of a high basement as central Gastre Trough.

History and Geologic Overview of the Yanacocha Mining District, Cajamarca, Peru

Discovery and recognition of the Yanacocha district as a belt of northeast-trending, near-surface, high sulfidation-type epithermal gold deposits with strong supergene oxidation was made over a four-year period (1984–1988) through a joint-venture alliance operated by Newmont Mining Corporation with partners Compania de Minas Buenaventura and CEDIMIN (BRGM). Open-pit mining began in the district in late 1993 with initial full-year gold production in 1994 of 300,000 oz and a reported end-of-year reserve of 4 Moz. The production growth curve at Yanacocha over 15 years (1994–2008) has been about 10 times the first full year of production in 1994. In February 2007, Minera Yanacocha (Newmont 51%; Buenaventura 44%; International Finance Corp 5%) poured its 20-millionth gold ounce life-of-mine. Reported 2008 end-of-year oxide reserves within the operating district are >13 Moz gold. Since exploration began in the district, over 20 hard-rock deposits and two gravel fan deposits (La Quinua, La Quinua Sur) have been developed. Cumulative historical discovery-to-reserve conversion costs average <$10/oz Au. The estimated oxide plus sulfide drill-indicated gold endowment within the district exceeds 70 Moz (2,170 tonnes). The high-sulfidation epithermal gold deposits are hosted by volcanic rocks that occur at the southern terminus of the northern Peruvian volcanic belt, a continuous sequence of a north-northwest–trending Miocene-Pliocene suite of bimodal andesite to rhyolite volcanic rocks that extend into southern Ecuador. In the Yanacocha district, the volcanic pile has been subdivided into three groups: (1) the lower andesite sequence, consisting of an intercalated sequence of block and ash flow tuffs, flow sequences with rare, associated flow domes, and an upper zone dominated by ignimbrites and fine-grained, laminated epiclastic sequences that show a transition into the overlying Yanacocha pyroclastic sequence; (2) the Yanacocha pyroclastic sequence, a variable sequence of lithic to lithic crystal tuffs, extensively altered in the central portion of the district and primary host to the majority of gold deposits within the district; (3) the upper andesite-dacite sequence, consisting of intercalated units of andesite to dacite flows, dominated by multiple flow dome complexes in its upper portion. Ar-Ar age dating within the district has yielded ages ranging from 19 Ma (basal lower andesite) to >12 Ma (upper andesite sequence). The entire volcanic pile has been crosscut by multiple phases of phreatic (vapor phase dominant), phreatomagmatic (intrusive component) and hydrothermal breccias, and intruded by multiple late-stage phases of andesite dikes and dacite to quartz dacite plugs, dikes and stocks (10–8 Ma), the latter of which are associated with shallow Au-Cu porphyry-style mineralization that underlies the high-sulfidation epithermal deposits. Alteration in the district consists of multiple stages of advanced argillic assemblages, typical in nature, that are zoned outward from a central core of massive to vuggy quartz to broader envelopes of advanced argillic alteration (alunite-pyrophyllite) to still broader zones of intermediate argillic and propylitic alteration halos. On a district scale, a generally southwest to northeast progression of multiple alteration centers with various timing has been documented through Ar-Ar dating of hydrothermal alunite, with ages ranging from 11.5 to 8.5 Ma. In places the advanced argillic alteration assemblage is overprinted by a later event consisting of creamy colored cryptocrystalline chalcedony plus barite associated with intermediate sulfidation type feeders. Gold mineralization is a late-stage event that was superimposed over all alteration types, most commonly occurring in late-stage fractures with massive, vuggy, and granular quartz, and especially where higher grade gold mineralization (>3.0 g/t Au) is associated with indurated replacement zones of creamy chalcedony. Mineral deposits are commonly localized within pyroclastic host rocks and phreatic breccias that envelop the margins of less permeable phreatomagmatic and within hydrothermal breccia pipes and andesite-dacite dome margins. Geophysical surveys have been employed as part of an integrated approach from the earliest exploration stages in the district. Resistivity and magnetic and potassium radiometric response anomalies are spatially associated with gold on a district-wide and deposit scale. Gravity surveys have added to understanding of basement geometry and have outlined volcanic subbasins. The east-northeast alignment of the Yanacocha gold deposits is interpreted to have been caused by the intersection of a northeast-trending trans-arc crustal break (Chicama-Yanacocha structural corridor). This is superimposed over older, north-northwest to northwest Andean-parallel folded and thrusted Cretaceous-age sedimentary basement terrane (Paleocene-Eocene). Both structural orientations dominate in the Yanacocha district, controlling emplacement of breccias and intrusions as well as formation of gold mineralization. Discontinuous east-west–oriented faults and fracture zones are interpreted to be extensional and control gold mineralization on a deposit scale. The transition from near-surface, gold-dominant mineralization to underlying copper-dominant sulfide mineralization associated with shallow quartz eye-bearing monzonite to tonalite intrusions has been documented by deep (500–1,200 m) diamond drilling beneath the Yanacocha complex (Sur-Norte-Oeste), the largest single gold deposit in the district with an endowment exceeding 18 Moz.

Accommodation zones and tectono-stratigraphy of the Gulf of Suez, Egypt: A contribution from aeromagnetic analysis

ABSTRACT This paper starts with an up-to-date literature review of the pre-rift, syn-rift and post-rift stratigraphy of the Gulf of Suez. The geometry and depth of the Proterozoic basement is not generally known due to poor seismic images below the Upper Miocene evaporites (including massive rock salt) and clastics. The pre-rift Paleozoic to Early Oligocene succession shows that several local basins (c. 10s of km in extent) occur in the Gulf, with thick sedimentary sections (e.g. c. 3,000 m for Paleozoic and 1,000 m for Jurassic and Lower Cretaceous). The origin and distribution of these basins is not well understood and the presence of similar pre-rift basins in the southern Gulf is not known to occur. The syn-rift Late Oligocene to Middle Miocene and post-rift Late Miocene – Pliocene successions are widely distributed within the rift basin and reach a thickness in excess of 5,000 m. In order to visualize the grain and relative relief of the Proterozoic basement, a series of aeromagnetic images are shown in this paper. The images include Total Magnetic Intensity (TMI), Reduced-to-Pole (RTP), filtered regional and structural RTP, and Second Vertical Derivative (SVD). The paper also shows a three-dimensional visualization image of the magnetic basement that highlights the distribution of the basins in the Gulf. The magnetic lows do not generally trend along the Suez (NNW-trending Clysmic) Fault, but instead show highly variable orientations attributed to a complex pattern of criss-crossing faults. In particular, two areas were selected to interpret the geometry and depth of the basement. The first area covered the northern Zaafarana Accommodation Zone and involved modeling five aeromagnetic profiles. The Zone was interpreted as an EW-trending basement plateau bounded by basins that are c. 8,000 m deep. The second modeled area (four profiles) covered the southern Morgan Accommodation Zone. This zone was interpreted as an ENE-trending plateau of similar relief to the Zaafarana Zone. The Morgan Zone is terminated in the eastern Gulf by the 8,000-m-deep Morgan Basin. The very deep basins surrounding the two plateaus may contain both pre-rift and syn-rift source rocks, from which the numerous surrounding petroleum fields were sourced.

Bathymetry and active geological structures in the Upper Gulf of California

Bathymetric surveys made between 1994 and 1998 in the Upper Gulf of California revealed that the bottom relief is dominated by narrow, up to 50 km long, tidal ridges and intervening troughs. These sedimentary linear features are oriented NW-SE, and run across the shallow shelf to the edge of Wagner Basin. Shallow tidal ridges near the Colorado River mouth are proposed to be active, while segments in deeper water are considered as either moribund or in burial stage. Superposition of seismic swarm epicenters and a seismic reflection section on bathymetric features indicate that two major ridge-troughs structures may be related to tectonic activity in the region. Off the Sonora coast the alignment and gradient of the isobaths matches the extension of the Cerro Prieto Fault into the Gulf. A similar gradient can be seen over the west margin of the Wagner Basin, where in 1970 a seismic swarm took place (Thatcher and Brune, 1971) overlapping with a prominent ridge-trough structure in the middle of the Upper Gulf. It is believed that these major elongated features can be linked to active geological structures reflecting the geometry of the deep basement in the Wagner Basin.

O método transiente eletromagnético (tem) aplicado ao imageamento geoelétrico da bacia de resende (RJ, Brasil)

Este trabalho aborda a aplicacao do metodo geofisico Transiente Eletromagnetico (TEM - no dominio do tempo) na regiao da Bacia de Resende, Rio de Janeiro, Brasil. Nesta bacia, foi efetuado em 2001, um levantamento geofisico de 88 sondagens visando contribuir para o conhecimento do potencial hidrogeologico da regiao. Como esta area e densamente povoada e industrializada, a presenca de ruido eletromagnetico tende a dificultar a utilizacao do metodo. Os dados foram interpretados utilizando um algoritmo de inversao 1-D, que exprime o decaimento da voltagem induzida em termos de resistividade com a profundidade. Os valores da resistividade mostram a existencia de camadas superficiais altamente condutivas (~5Ω.m) e o valor de 200Ω.m para uma resistividade de transicao que corresponde ao embasamento geoeletrico, sendo adotada com base em dados litologicos e estratigraficos da bacia com um ajuste fino calibrado na inversao 3-D de dados gravimetricos. A partir das inversoes 1-D foi construida uma imagem geoeletrica da geometria do embasamento. Apesar das propriedades fisicas envolvidas (densidade e resistividade) nao terem nenhuma relacao fisica entre si, a topografia do embasamento gravimetrico foi utilizada para obter, por interpolacao, profundidades que nao foram atingidas pelo sinal produzido no metodo TEM. A superficie geoeletrica final tem uma topografia que pode ser considerada como representativa do embasamento da bacia. Deste modo, a utilizacao do metodo TEM na Bacia de Resende permitiu estimar os limites laterais e propor tambem limites em profundidades para o embasamento geoeletrico. Alem disso, o imageamento geoeletrico resultante mostra distintamente a existencia de tres areas: um alto estrutural central ladeado por dois baixos estruturais (depocentros), um ao leste e outro ao oeste da bacia.

Emplacement, growth, and gravitational deformation of serpentinite seamounts on the Mariana forearc

SUMMARY Serpentinite seamounts, representing some of the first material outputs of the recycling process that takes place in subduction zones, are found on the outer Mariana forearc. Multichannel seismic (MCS) and bathymetric data collected in 2002 image the large-scale structures of five seamounts, as well as the pre-seamount basement geometry and sediment stratigraphy. We present data from three edifices that provide insights into seamount growth and internal deformation processes and allow us to support the interpretation that serpentinite mud volcanoes are formed by the episodic eruption of mud flows from a central region. The presence of thrust faulting at the base of Turquoise and Big Blue Seamounts, along with the low surface slopes (5°–18°) of all the seamounts studied, lead us to infer that these edifices spread laterally and are subject to gravitational deformation as they grow. Numerical simulations using the discrete element method (DEM) were used to model their growth and the origins of features that we see in MCS sections, such as basal thrusts, inward-dipping reflections and mid-flank benches. The DEM simulations successfully reproduced many of the observed features. Simulations employing very low basal and internal friction coefficients (∼0.1 and ∼0.4, respectively) provide the best match to the overall morphology and structures of the serpentinite seamounts. However the simulations do not capture all of the processes involved in seamount growth, such as withdrawal of material from a central conduit leading to summit deflation; compaction, dewatering and degassing of mud flows; mass wasting in the form of sector collapse and growth upon a dipping substrate. A strong reflection beneath the summit of Big Blue, the largest serpentinite seamount on the Mariana forearc, represents the floor of a summit depression that has been partially in-filled by younger muds, supporting the idea that serpentinite seamounts grow by episodic mud volcanism. Boundaries of mud-flow units are visible in bathymetric data and as normal polarity, subhorizontal reflections on seismic profiles. Big Blue Seamount displays complex nesting relationships as it merges with other seamounts to form a large, composite edifice. Flank flows of serpentinite muds on Big Blue and Celestial Seamounts downlap pre-existing forearc substrate. The interface between serpentinite seamounts and the underlying forearc sediments is represented by a reverse polarity reflection beneath Big Blue and Celestial Seamounts, suggesting that the substrate is undercompacted/overpressured and may be a zone of fluid migration. DEM simulations suggest that this boundary represents a distinct decollement along which the seamounts slide laterally. In contrast, Turquoise Seamount grows laterally, not by stable sliding along the top of forearc sediments, but by incorporating them into large basal thrusts.

Delimitation of ground failure zones due to land subsidence using gravity data and finite element modeling in the Querétaro valley, México

Sedimentary basins of arid and semiarid zones are often subject to regimes of intense ground-water withdrawal as it is normally the only source of water for development of communities. An associated phenomenon is land subsidence, which can develop to ground failures, and consequently, damage to infrastructure. Aquifer deformation can be analyzed using a stress–strain or a flux–force approach depending on the aquifer material (compact or loose) and on whether the water withdrawal forms a predominant flow direction toward a cone of depression. Geometry of the aquifer system also plays an important role as uneven thickness induces differential compaction and hence, tensional and shear stresses on the ground mass. In this work we present a stress–strain approach to analyze subsidence for an unconfined aquifer of varying thickness; this is done in two steps, namely when the aquifer is in equilibrium, and when it is totally depleted. Using this scheme in a region where ground failure is evident, a portion of the aquifer system of the Querétaro valley is analyzed. The geometry of the hydrologic basement is first modeled using gravity measurements properly correlated with wells and field data. Then a stress analysis is implemented using the finite element method in order to identify probable zones of ground weakness, which are calibrated with known ground failures. The results indicate that both, tensional and shear stress are present, which induce ground failure in the form of surface faults.

Gravity anomalies of sedimentary basins and their mechanical implications: Application to the Ross Sea basins, West Antarctica

In general, sedimentary basins are characterized by negative free-air and Bouguer gravity anomalies. However, the extensional basins of the Ross Sea are paradoxical in that positive gravity anomalies overlay the Victoria Land Basin, Northern Basin, Central Trough and Northern Central Trough while basement highs are associated with negative gravity anomalies. Measured basement densities from DSDP basement cores give values between 2600–2800 kg/m 3 while bulk sediment densities range from 1210–2200 kg/m 3, indicating a normal density relationship between basement and sediment infill. In contrast, the relatively young and narrow Terror Rift is associated with negative free-air and Bouguer gravity anomalies, but has a different geological history as compared to the larger Ross Sea basins. Process-oriented gravity modeling indicates that magmatic underplating and crustal intrusions are inconsistent with the observed gravity and basement geometry of the Ross Sea basins. The magma volume necessary to account for the distribution and amplitude of the positive gravity anomaly of the Central Basin and be isostatically balanced would need to be comparable to the tholeiitic flood basalt volume of the Columbia River province—it is thus unlikely that the volume of Neogene volcanics of the Ross Sea region is sufficient to explain the observed gravity relationship by modifying the bulk density of the crust. We demonstrate that positive free-air and Bouguer gravity anomalies over extensional basins are the consequence of a relatively low flexural strength of the lithosphere during rifting being contrasted by higher flexural strengths later during sedimentation. As the difference between the rigidity of the lithosphere during sedimentation increases relative to the rigidity of the rifted lithosphere, the gravity over the basin becomes progressively more positive but only for a limited range of wavelengths. The narrow width of the Terror Rift precludes it from having a positive gravity anomaly while the opposite is true for the large Ross Sea basins. For the Ross Sea region, such a loading scenario requires a significant delay between extension and the timing of sediment infilling of the basins, consistent with the late Cretaceous extension of the Ross Sea region and the sedimentary succession being dominated by large-scale late Eocene–Neogene glaciogenic progradational sequences. Sediment source was presumably from the denudation of the Transantarctic Mountains, which commenced in the late Paleogene. The time delay between the late Cretaceous formation of the Transantarctic Mountains, late Paleogene exhumation, and the generation of significant Paleogene paleobathymetry requires either the Ross Sea region to be sub-aerial and sediment starved for most of the Paleogene and/or the Paleogene climate was ineffective in producing clastics until the onset of glaciation in the late Eocene–early Oligocene.

The Hospital coal basin (Massif Central, France): relay on the left-lateral strike-slip Argentat fault in relation to the Variscan postorogenic extension

Abstract Structural and microstructural analyses of the Argentat fault, combined with sedimentological and structural analyses of the associated Hospital basin allow us to discuss the tectonic control of coal basins by crustal-scale faults during the late Palaeozoic evolution of the Variscan lithosphere in the French Massif Central. The brittle Argentat fault zone consists of first- and second-order strike-slip faults, with dominant NNW-sinistral faults, NNE-dextral or sinistral faults and secondary ENE-dextral faults. Several experimental and theoretical models explain the observed fault patterns, like en echelon faults, A-type secondary faults, conjugate faults and Riedel shears. Strike-slip faulting is responsible for folding of the metamorphic formations characterized by N-S to NE-SW-trending axis. The regional-scale geometry of brittle faults and associated folds corresponds to a positive flower structure centered on the brittle Argentat fault, combined to a negative flower structure centered on the coal basin. Using tectonic inversion software, we show that these structures result from a left-lateral movement of the brittle Argentat fault in relation to a tectonic regime intermediate between extension and strike-slip, with a horizontal NE-SW to NNE-SSW-trending maximum stretching axis. Detailed map and cross-sections, and sedimentological interpretations of the late Stephanian Hospital basin show the occurrence of intra-basin syn-sedimentary strike-slip faults and progressive overlaying, indicating that sedimentation occurs during left-lateral strike-slip faulting and folding of basement along the Argentat fault. These data are consistent with a model of N-S to NE-SW-trending postorogenic extension proposed to account for the late Carboniferous evolution of the Variscan lithosphere. They also point out the complexity and the variety of structures developed along a regional brittle strike-slip fault zone and the necessity to take into account all the structures and the resulting geometry of the basement in order to better constrain the tectonic setting of intra-continental deposits.

Structure of the Transkei Basin and Natal Valley, Southwest Indian Ocean, from seismic reflection and potential field data

Marine geophysical data from the southern Natal Valley and northern Transkei Basin, offshore southeast Africa, were used to study the structure of the crust and sedimentary cover in the area. The data includes seismic reflection, gravity and magnetics and provides information on the acoustic basement geometry (where available), features of the sedimentary cover and the basin's development. Previously mapped Mesozoic magnetic anomalies over a part of the basin are now recognized over wider areas of the basin. The ability to extend the correlation to the southeast within the Natal Valley further confirms an oceanic origin for this region and provides an opportunity to amplify the existing plate boundary reconstructions. The stratigraphic structure of the southern Natal Valley and the northern Transkei Basin reflects processes of the ocean crust formation and subsequent evolution. The highly variable relief of the acoustic basement may relate to the crust formation in the immediate vicinity of the continental transform margin. Renewed submarine seismicity and neotectonic activity in the area is probably related to the diffuse boundary between the Nubia and Somalia plates. 2.5-D crustal models show that a 1.7–3.2-km-thick sediment sequence overlies a 6.3±1.2-km-thick normal oceanic crust in the deep southern Natal Valley and Transkei Basin. The oceanic crust in the study area is heterogeneous, made up of blocks of laterally varying remanent magnetization (0.5–3.5 A/m) and density (2850–2900 kg/m 3). Strong modifications of accretionary processes near ridge/fracture zone intersections may be a reason of such heterogeneity.

Modeling of 3D Basin Structures for Seismic Wave Simulations Based on Available Information on the Target Area: Case Study of the Osaka Basin, Japan

Practical procedures for modeling basin structure are proposed. One is to construct a smooth 3D basin structure model under and around the target site for low-frequency strong motion simulation. The other is to make a 1D velocity structure model just under the target site for high frequencies. These procedures were applied to the Osaka basin, Japan, in which numerous exploration studies were conducted before and after the 1995 Hyogoken-Nanbu (Kobe) Earthquake. The published results of those explorations in addition to those from our several investigations were compiled to construct the model. A two-dimensional third-order B-spline function was used to establish a smooth structure model from available depth data. Microtremor array observations were used to model the velocities and densities of the sedimentary layer structures. A four-layer model well explains the dispersion characteristics of Rayleigh waves that make up the microtremors. By adding a detailed shallow velocity structure to the top of the deeper four-layer model, adequate site responses for a wide frequency range of ground motions can be obtained. This proposed procedure can be used to determine the basement geometry of other sedimentary basins.