T23B-0576 1340h
Applications of Crustal Velocity Models to Tectonic Problems
A velocity model has been produced for NW Europe using available 2-D wide-angle seismic profiles. By using these data estimates of velocity structure and depth to major velocity transitions can be determined. By extrapolating the velocity structure between adjacent profiles it is possible to produce a 3-D model of the velocity structure of the crust, and by simple translation a model of the density variations. This model is being used to study a variety of tectonic problems. It has been used to examine the relationship between crustal thickness and topography. This analysis of the data demonstrates that there must be significant lateral changes in the density of the upper mantle (previously inferred from modelling of gravity data) in order to support the observed topography. Thus it is possible to accurately determine the degree of continental dynamic support. The model reveals the extent of magmatic underplating beneath the continental margin of NW Europe and reveals a major discrepancy between estimates of exhumation and the distribution and amount of magmatic underplating. From this study it is possible to show the extent to which the velocity/density structure controls the topography and the present surface geology and to make predictions about the future evolution of surface elevation.
T23B-0577 1340h
Geodynamical model of development the gold ore deposit Muruntau in Central Kyzylkums /Western Uzbekistan/
Recent advances in geology and sciences dealing with planets resulted in ever-growing information on the internal structure and composition of upper and lower mantle and its role in the development of mineral deposits. Thus, the analysis of geologic-geophysical velocity sections on DSS-MOVZ profiles, that cross Central Kyzylkum, has revealed a number of features, which are characteristic of the upper mantle rocks, related to morphology of bodies, their physical properties, consisting mainly in their contrasting values for contiquous blocks, and general increased velocity and density of the rocks they contain. Petrologic-geophysical analysis of the same profiles proved the previous conclusions, and besides it allowed us to create the geodinamical model of development of a complicated continental lithosphere within Central Kyzylkum Ore Magmatic Concenter (CKOMC). CKOMC is a part of Western Uzbekistan's territory, that has maximum concentration of gold ore deposits of different scale, including the gigantic one- Muruntau. There are two main factors, that condition the area of the concenter: 1.Obduction of crust of the ocean to continental one during the Upper Paleozoic collision of Kyrgiz-Kazakh and Karakum-Tadjik microcontinents; 2.Geochemical gold specialization of Turkestan paleo-ocean's lithosphere. As a result of obduction of doubled, and sometimes even trebled crust of the ocean to continental, the front of rock granitization shifted to the sphere of ocean sediments development, and granite-gneiss layer of the continental crust went down to granulite zone. The latter factor provoked mass penetration of fluids from the former granite-gneiss and sedimentary-metamorphic layers, which due to their uprush to the surface, formed fluid-hydrothermal system , enriched with gold, in the new upper crust. This, in its turn, conditioned the formation of numerous gold ore deposits of different facies and formations observed in Central Kyzylkums. Abrupt increase in the crust's thickness has broken the balance of the crust and mantle, melts of which also rushed upwards along the rift zones and faults, replacing the ancient and the newly formed granite-gneiss layer. Such a mechanism of the continental lithosphere formation explains the formation of blocks of crust, conditioned by the abrupt change of rocks having different density (1), and the presence single parts waveguides in lower parts of the upper crust, which are considered to be relics of the primary granite-gneiss layer enriched with fluids. Only the wedge of crust of the ocean thrusted over the continental crust could cause formation of rock blocks of unusual shape and content, but of the same metallogenic gold ore trend. The studied profiles allowed us to determine one more magmatogene structure of mantle formation. It has funnel shape, with its basement directed to the surface. Complex ring plutons and tectonic ring-shaped disturbances these can be seen during the analysis of space images. The narrowed part of the structure goes down to the lower mantle, and can form a shape of sand-glass, the surface of which served as a conductor of mantle plumes to the upper crust. Evidence of this are bodies of alkaline mafites and ultramafites in the upper crust of Central Kyzylkums, and presence of non-coherent elements containing gold (such as platinum and rare earth elements) at gold ore deposits.
T23B-0578 1340h
The basement terrain configuration in the Western Barents Sea-Svalbard region from integrated geophysical analysis
A large geophysical database has been compiled for the western Barents Sea-Svalbard region comprising a regional grid of deep crustal MSC-data, expanded spread profiles and modern ocean bottom seismic wide-angle profiles, satellite altimetry and marine gravity data, and magnetic data. Gravity modelling has been carried out along the regional lines to further constrain the crustal structure. The configuration of several basement terrains have been outlined in the western Barents Sea, each characterized by a specific seismic reflectivity and velocity/density structure. Terrain boundaries are associated with major changes in the velocity/density structure, or appear as strong lineaments in the potential field data. High reflectivity lower crust often correlates with increased velocity/density, as well as high magnetization, interpreted as predominantly mafic terrains. One such terrain is the Loppa High, forming a structural high during the post-Caledonian extensional events, where the deep MCS data also show a distinct crustal root. These mafic terrains appear to provide the underlying framework for episodic rift events and basin formation in the south-western Barents Sea from Late Paleozoic to Late Mesozoic/Early Tertiary times. Within these extensive tectonic regimes sedimentary basins evolved that can exceed more than 20 km depth (S{\o}rvestsnaget Basin), in strong contrast to adjacent basement highs. Preliminary results show that most of these high-standing crustal blocks are characterized by a common magnetic signature, or by high lower crustal velocity. It is suggested that rheological differences between the mafic terrains being stronger, and surrounding terrains with higher lower crustal quarts content being weaker, in interplay with the regional stress field, nucleated the major faults at the terrain boundaries. The north-western Barents Sea including Svalbard is characterized by north-south trends. South of Svalbard the deep seismic data reveal a relatively thick and reflective crystalline crust. East of this terrain, depth to basement increases and reflectivity and higher velocities/densities are associated with the lower crust.
T23B-0579 1340h
The Precambrian Structure of the Estancia Basin, Central New Mexico: New Seismic Images of the Mazatzal Province
The Estancia Basin, located between the Manzano Mountains and Pedernal Hills, in central New Mexico, provides an excellent location for studying the effects of Proterozoic structural grain on subsequent Phanerozoic tectonic events. The Estancia Basin lies within the Proterozoic Mazatzal province. In recent years, the National Science Foundation Continental Dynamics Program within the Rocky Mountains Project, (CD-ROM) group has been examining the boundary between the two broad northeast-trending tectonically-mixed Paleoproterozoic terranes in New Mexico: the Yavapai province to the north and the Mazatzal province to the south. Reflection data collected as part of the CD-ROM effort image a portion of the Mazatzal province at a location 100 km east of the Estancia Basin. In an effort to contribute to a deeper understanding of the CD-ROM seismic image and regional Precambrian geology, we are analyzing ten seismic reflection profiles, well-logs, magnetic and gravity data from the Estancia basin area. The seismic data show numerous dipping reflections within the Precambrian basement that may represent prominent Precambrian ductile shear zones similar to those exposed in the adjacent Manzano Mountains and Pedernal Hills. An earlier study that focused on the Paleozoic evolution of the Estancia Basin, by Barrow and Keller (1994) also noted these same reflectors and that a prominent gravity low observed in the vicinity of the basin could not be fully explained by the Paleozoic geology. We present a new interpretation of these data.
T23B-0580 1340h
Mesozoic inversion in southeastern parts of the Neuqu\'{e}n Basin, west-central Argentina: Implications for tectonic deformation and stratigraphic development across the Andean foreland of Argentina
The Neuqu\'{e}n Basin of west-central Argentina is a segment of the Andean foreland that has significant structural complexity due to protracted intraforeland deformation between Late Triassic to Recent time. Some structural features in the Neuqu\'{e}n Basin predate the main phases of the Andean orogeny although these structures were reactivated later and influenced basin configuration during the foreland-basin stage. The most conspicuous of these structures is the Huincul Arch, a 200-km-long right-lateral shear zone that was most active during Jurassic to Cretaceous time. Inversion structures along the Huincul Arch are associated with a restraining bend along the main east-west trending shear zone that cuts across the Argentine foreland. An extensive seismic and borehole data set was analyzed to evaluate the styles and intensity of Mesozoic foreland deformation in an approximately 10,000 sq km area north of the Huincul Arch. Transpressional and transtensional deformation is broadly distributed across the study area and other parts of the Neuqu\'{e}n foreland, although a series of inversion structures (e.g., Sierra Barrosa and Aguada Toledo anticlines) reflect more intense, localized deformation. These structures are the result of inversion of Late Triassic half-grabens and produced fault-propagation folds that affected the post-rift fill up to the Upper Jurassic (Tithonian). The most significant reactivation along the Huincul Arch south of our study area occurred during Kimmeridgian (Late Jurassic) time along the main displacement zone. To the north, however, significant inversion was slightly younger and occurred during and after deposition of the Tithonian to Berriasian (latest Jurassic-earliest Cretaceous) Vaca Muerta and Quintuco formations. Thus, early phases of tectonic inversion across the Argentine foreland were diachronous and likely reflect an accumulation of strain along the Huincul Arch was necessary before additional strain could propagate northward into our study area. Seismic-stratigraphic analyses also show that the growing inversion structures created bathymetric perturbations that affected sediment dispersal and stratigraphic development north of the Huincul Arch during deposition of the Vaca Muerta-Quintuco interval. This study contributes to the understanding of Pre-Andean deformation in the Neuqu\'{e}n Basin. 3D seismic data across the study area also provides a unique opportunity to investigate the geometries and kinematic history of inversion across the Andean foreland, as well as to evaluate tectonic controls on Mesozoic stratigraphic development in the poorly understood Northern Sub-basin of the Neuqu\'{e}n Basin.
T23B-0581 1340h
Gravity Analysis of the Jeffera Basin, Tunisia
Southern Tunisia consists of two main tectonic provinces: 1) the Saharan Platform and 2) the folded Atlasic domain, separated by the North Saharan Flexure. The Saharan Platform, which contains the Ghadames Basin and the Telemzane Arch, consists of gently dipping Paleozoic strata overlain by Triassic to Cretaceous sediments. The Atlasic domain consists of a thicker sequence of mainly Mesozoic and younger rock with less complete sequences of Paleozoic strata. Within the Atlasic domain are the still actively subsiding Chotts and Jeffera basins. The Jeffera basin, which occurs to the east of the Telemzane Arch contains at least eight kilometers of Paleozoic and younger sediment that were formed during numerous subsidence episodes since Carboniferous time. The Jeffera basin is dominated by tilted fault blocks that were formed during numerous tectonic episodes. Several unpublished seismic reflection profiles and well data exist for the Jeffera basin, however a deep structural analysis of the basin has not been published. We examined the existing gravity data in conjunction with available well and geologic data to determine structural features within the basin. The Bouguer gravity anomaly map shows that the Jeffera basin is dominated by a narrow northwest-trending gravity minimum. However, a more detailed analysis consisting of wavelength filtering and edge enhancements indicate that the structure of the basin is more complicated than indicated by the Bouguer gravity anomaly map. A residual gravity anomaly map indicates that the Jeffera basin consists of at least three and maybe four subbasins. Additionally, the Jeffera Fault marks the boundary between northwest-trending gravity anomalies to its northeast and east-trending anomalies over the Saharan Platform. The above observation is amplified by the construction of the enhanced horizontal derivatives (EHG) of both the complete Bouguer gravity and the residual gravity anomaly maps. The EHG maps highlight the lateral boundaries of subsurface density contrasts and emphasizes that the Jeffera basin is dominated by northwest-trending anomalies while the Saharan Platform consists of a series of northeast- and east-trending anomalies. The final interpretation of the gravity data will consist of constructing a series of two and one-half dimensional gravity models across the Jeffera basin.
T23B-0582 1340h
Mechanics of a contracting reservoir in an elastic half-space with an intervening visco-elasto-plastic layer
The extraction of fluids from porous rocks within the Earth's crust leads to localised volume strains. These in turn induce stress changes and displacements in the surrounding rock mass. The relationships between these processes are governed by the constitutive properties of the rocks. For the case of a poroelastic fluid reservoir in a linear-elastic matrix the mechanics are relatively well known and understood. In this study we extend these models by investigating the case of a contracting rock body (caused by declining pore pressure) embedded within a linear-elastic half space, but with the addition of a visco-elasto-plastic layer between the contracting reservoir and the free surface. The problem is of growing importance as the exploitation of hydrocarbon reservoirs beneath salt bodies occurs at ever greater depths in the deepwater Gulf of Mexico. This is because the creep properties of salt are strongly temperature dependent, so that as depths increase, and hence ambient temperatures, creep can occur at a rate that is impossible to ignore over the reservoir lifecycle. The models are explored using a finite element approach and make use of sophisticated salt constitutive models and large-deformation three-dimensional geomechanical simulation codes; the reservoir deformations are governed by either poro-elastic or cap plasticity constitutive laws. However, a general behaviour pattern can be observed: the visco-elasto-plastic salt layer tends to decouple the deformation fields from the free surface with stress and displacements accentuated below the salt. The magnitude of the increased horizontal displacements below the salt layer are relatively independent of the layer thickness. The accentuated vertical displacements though are more strongly dependent on the thickness of the salt layer. This work was performed at Sandia National Laboratories funded by the US DOE under Contract DE-AC04-94AL85000. Sandia is a multiprogam laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration.
T23B-0583 1340h
Inverse Modelling of Continental Margins and Sedimentary Basins
The wealth of data available from the hydrocarbon industry provides us with detailed information about the subsidence histories of extensional sedimentary basins and passive margins. This resource is often exploited in forward models of basin and margin evolution although little attempt has been made to invert such data. We are interested in developing an inverse methodology in order to constrain the spatial and temporal variation of strain rate in these regions. Any inversion scheme which searches the possible movements of the lithosphere over geological time requires a fast forward model at its heart. We present a new kinematic model for use in such an inversion. Our finite-difference model is capable of simulating the thermal and subsidence effects of basins and margins that have undergone differential stretching with both depth and distance across the stretching area. Speed is achieved by a modular design and optimisation of the code for the architecture on which it is running. The model can simulate fifty million years of extension in around a second on a desktop computer. Currently there is much interest in cold continental margins such as the Newfoundland/Iberia system where crust has been thinned to zero but lithospheric mantle has been exhumed without extension. We believe this is not possible without differential thinning and will be testing this hypothesis with our new model. The inversion scheme is also being used to investigate flanks of actively rifting regions, such as those around Lake Baikal and the Albertine rift, and older extensional systems such as those in the Northern North sea.
T23B-0584 1340h
The Post-Permian evolution of the Northern Part of the North German Basin
In the frame of the Priority Program 1135 of the German Research Foundation (DFG) "Dynamics of sedimentary systems under varying stress conditions by example of the Central European Basin System", the scientific goal of the NeoBaltic project is to describe the post-Permian to recent geological evolution of the entire western Baltic Sea region, with a special emphasis on neotectonic activity and it relation to salt dynamics. The western Baltic Sea comprises the northern part of the North German Basin (NGB), a part of the Central European Basin System (CEBS), and the transitional zone between the NGB and the Baltic Shield. In order to investigate these scientific goals the Universities of Aarhus (Denmark) and Hamburg (Germany) has since 1998 completed seven marine campaigns in the western Baltic Sea, collecting 2D high resolution seismic (HRS), gravity and magnetic data in the entire region during different projects. Since 2003 all these data has been available for the NeoBaltic project. All together the data pool have more than 7000 km HRS, 5000 km gravity and 4000 km magnetic data. Until now the project work has been focused on the completion of the data processing and the digital interpretation of important Mesozoic and Cenozoic markers on the seismic sections from the Bays of Kiel and Mecklenburg. Furthermore, several maps have been completed from the potential field data (gravity and magnetic). As a result of the digital interpretation of the HRS data, the overall geological evolution of the northern part of the NGB can be subdivided into four distinct periods. During the Triassic and the Early Jurassic, E-W extension and the deposition of clastic sediments initiated the movement of the underlying Zechstein evaporites. This is seen by the presence of several salt pillows in the region. The deposition ceased during the Middle Jurassic, when the entire area was uplifted, due to the Mid North Sea Doming. The uplift resulted in a pronounced erosion of Upper Triassic and Lower Jurassic strata. This event is marked by a clear unconformity on the seismic sections. The region remained an area of non-deposition until the end of the Early Cretaceous. The sedimentation resumed in the late part of the Early Cretaceous and continued throughout the Late Cretaceous. No pronounced halokinesis has been detected during this period. Towards the end of the Late Cretaceous, the Alpine Collision caused the reactivation of salt structures seen on a thinning of the Cenozoic sequence across the different structures. As a result of the different Pleistocene glaciasations, several buried valleys have been detected on the seismic sections, especially in the Bay of Kiel and the Danish Lillebelt region. Some of these buried valleys contain biogenic gas, which results in a sufficient pull-down of the underlying markers on the seismic sections.
T23B-0585 1340h
In the Footsteps of Charles Darwin: Patterns of Coastal Subsidence and Uplift Associated with Seamount Subduction, Basal Fore-arc Erosion and Seamount Accretion in Latin America
In Geological Observations on South America (1846), Charles Darwin described beds of late Cenozoic marine seashells that were uplifted to elevations as much as several hundred meters above some localities on the western coastline of South America and implied that the whole coast was uplifting at geologic time scales. We know now that such evidence is generally restricted to coastal embayments above fore-arc basins where offshore seamounts are colliding with the South American fore arc (e.g., the Juan Fernandez seamount chain, Valpariso Basin and Valpariso Bay). We suggest that the phenomena of basal fore-arc erosion and basin formation and coastal uplift are closely related to effects of seamount subduction. Marine multibeam sonar images and multichannel seismic reflection surveys by others demonstrate that seamounts, although locally cut by normal faults in the outer-rise/near-trench region, initally subduct intact and the primary interaction with the toe of the fore arc is plowing, with material eroded from the fore arc that accumulates above and on the margins of the seamount. Submarine landslides above such regions over-steepened by plowing can lead to coastal embayments far upslope of the plowing. Such plowing interaction can therefore lead to the formation of large forearc basins and coastal embayments such as those at Valpariso, Chile, or narrow corridors of subsidence in the wake of subducting seamounts in Costa Rica. It is also known that the transition between interplate thrust seismicity, representing mechanical coupling between the plates, and aseismic slip occurs at depths of typically 30-60 km and often geographically near coastlines that mark the boundary between outer fore-arc subsidence and inner fore-arc uplift. We suggest that decoupling can occur at the base of seamounts (i.e., the originally sedimented seafloor on which the seamount lavas are laid down) and that such seamounts can be accreted to the fore arc above and lead to coastal uplift. Such basal decoupling is known to occur under active volcanic islands in the open ocean in connection with rifting and gravitational spreading, such as beneath the island of Hawaii. The spatial and temporal patterns of coastal uplift and subsidence on active margins can therefore record the local history of seamount subduction. This conceptual model explains the spatial patterns of offshore subsidence and coastal uplift in Chile and Costa Rica and also has implications for patterns of seismicity along the interplate thrust boundary.
T23B-0586 1340h
Vertical Motions and Drainage Evolution in the Congo River Catchment
Africa's large-scale physiographical features are thought to be dynamically supported by mantle convection. The resultant "basin and swell" topography correlates closely with long wavelength free-air gravity anomalies, which can be used as a proxy for the convective pattern. In addition, seismic tomographic studies reveal a region of slow seismic velocity beneath sub-equatorial Africa. This region broadly correlates with a zone of anomalously high elevation, known as the "African Superswell" that stretches from the South Atlantic Ocean across Africa to Afar. Recently, dynamical modelling has been used to predict dynamic topography and uplift rates from the convective pattern inferred by seismic tomography. In this study, high resolution topographic data are used to further constrain spatial and temporal variation of vertical motions in Africa. The Congo Basin is of particular interest as its uplifted southern margin makes up a significant proportion of the catchment. Digital elevation models are used to construct detailed profiles of the rivers that drain Congo basin. River profiles to the north-east of the Congo Basin are concave upwards and appear to have reached equilibrium. In contrast, the elevated region of the southern Congo Basin shows convex upwards river profiles and major knickpoints, indicating that the rivers in this region have not yet reached equilibrium. These knickpoints do not generally correlate with geological boundaries and are more likely the result of a recent change in base level related to mantle convection. By gaining an understanding of the relationship between uplift and surface processes in this region, geomorphology can be used as a further test of uplift rates predicted from seismic tomographic data.
T23B-0587 1340h
Late Palaeocene Mantle Plume Uplift on The Fugloy Ridge, NE Faroes
The Fugloy ridge is a large (~100 km wide) anticlinal structure situated to the NE of the Faroe Islands separating the Norwegian Ocean basin from the Faroe-Shetland trough. Flexural backstripping and post-breakup thermal subsidence modelling has been performed on a profile crossing the Fugloy Ridge to provide an estimate of mantle plume uplift at the end of the Palaeocene (~55 Ma). The modelling is carried out on a 370 km Q-marine multi-streamer swath reflection profile acquired by the M/V Geco Topaz during the summer of 2002 as part of the iSIMM (integrated Seismic Imaging and Modelling of Margins) project seismic acquisition programme. The profile provides good resolution of post-breakup sediment structure across the margin and also of deeper sub-basaltic structure along the profile. Flexural backstripping and reverse post-breakup thermal subsidence modelling is a 2D (or 3D) technique which is used to restore present day stratigraphic cross sections to earlier post-breakup times. The method removes units of stratigraphy from the top-downwards and calculates isostatic and sediment decompaction responses to this unloading. Thermal subsidence arises from the cooling of stretched continental lithosphere and the recently formed oceanic lithosphere, and may be predicted from the lithosphere beta stretching factor (McKenzie, 1978). Two approaches have been used to determine beta stretching estimates for the profile, the first approach uses beta stretching factors from crustal thinning estimates derived from a gravity anomaly inversion technique (Hurst et al., 2004). The second approach uses palaeo-bathymetric constraints to determine the beta stretching estimates for the profile. Results from the modelling show that the Fugloy Ridge present day stratigraphy flattens out progressively as the 2D cross section is restored to breakup (55 Ma) using beta stretching factor estimates derived from gravity anomaly inversion. The Fugloy Ridge has been proposed as a possible compressional fold structure, however the results from this modelling show that its present anticlinal structure can be explained purely as a result of a combination of differential sediment loading and post-breakup thermal subsidence. Modelling results show a discrepant bathymetry of ~500m when restored to breakup. Our preferred interpretation is that this discrepant bathymetry is due to ~500 m of transient Palaeocene uplift, for which a likely mechanism is dynamic uplift by the early Iceland mantle plume. This plume uplift estimate is consistent with values from previous work for the northern North Sea Basin (Nadin et al., 1997) and the Faroe-Shetland Basin (Jones & White, 2003). This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Schlumberger Cambridge Research & Badley Geoscience, supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, Conoco-Phillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, R Spitzer, NW Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, D Healy & V Tymms.