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1
Hussein, Musa Jad. "Integrated and comparative geophysical studies of crustal structure of pull-apart basins the Salton Trough and Death Valley, California regions /." To access this resource online via ProQuest Dissertations and Theses @ UTEP, 2007. http://0-proquest.umi.com.lib.utep.edu/login?COPT=REJTPTU0YmImSU5UPTAmVkVSPTI=&clientId=2515.
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Jordan, Tom A. R. M. "Gravity anomalies, flexure, and the long-term rigidity of the continental lithosphere." Thesis, University of Oxford, 2007. http://ora.ox.ac.uk/objects/uuid:9f803b42-522e-442b-9849-bb8e6c2a5494.
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The cause and distribution of spatial variations in the mechanical properties of the continental lithosphere are fundamental questions for modern geology. In this study variations in long-term lithospheric rigidity have been investigated. These investigations used profile- and grid-based flexural models of the lithosphere’s response to geologically imposed topographic, or buried, loads. These models were constrained by topographic and gravity data allowing recovery of best fitting rigidity values. In Oman a Cretaceous ophiolite acts as a significant load on the continental crust. Flexural models along profiles orthogonal to the ophiolite strike show that the observed gravity data can be best modelled by an elastic beam with standard thickness (Te) of 30 km. Along strike there is shown to be significant variation in the foreland shape and the observed gravity signal. This, it is proposed, relates to the complex tectonic processes which occurred as the ophiolite was obducted. The Himalayan foreland has been the focus of controversy over the recovered long-term rigidity of the continents, with recovered Te values ranging from 40 to over 90 km. Both profile- and grid-based techniques show that Te is high (>70 km) in the foreland region. Across the India-Eurasia collisional system as a whole Te values are variable. Beneath the Tibetan plateau recovered values are generally low (<10 km), while the plateau margins are marked by regions of higher rigidity. Recovered Te values across the Arabia-Eurasia collisional system range from over 60 km in the foreland region to close to zero beneath the high Zagros mountains. In the eastern part of the foreland, flexural models match the gravity data; however, they disagree with sediment thickness data for the material infilling the foreland. This discrepancy is interpreted in terms of de-coupling of the flexural lithosphere from the shallower crustal levels, caused by the presence of significant salt deposits in this region. Application of grid-based techniques to South America, North America and Europe recover a broad range of Te values from ∼0 to over 90 km. The low Te values are explained in active orogenic belts in terms of current processes acting to weaken the lithosphere, and in the continental interiors as the relics of past orogenic events. High Te values in the continental interiors correlate with ancient cratonic cores which have undergone little deformation since their formation in the Archean. This study shows that Te variations have a critical influence on the development of large compressional orogenic belts. In the Himalayan and Andean orogens there is a correlation between the over-thrusting of the orogenic belt and high Te foreland regions. Where lower Te regions are seen, less over thrusting is apparent, and in the case of the India-Eurasia collisional system out-flow of lower crustal material may be occurring.
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Hernandez, Orlando. "Tectonic analysis of northwestern South America from integrated satellite, airborne and surface potential field anomalies." Columbus, Ohio : Ohio State University, 2006. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1158512351.
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Guo, Bin. "An integrated geophysical investigation of the Tamworth Belt and its bounding faults." Phd thesis, Australia : Macquarie University, 2005. http://hdl.handle.net/1959.14/13240.
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Thesis (PhD)--Macquarie University, Division of Environmental & Life Sciences, Department of Earth and Planetary Sciences, 2005.Bibliography: leaves 202-224.
Introduction -- Geological setting of the New England Fold Belt -- Regional geophysical investigation -- Data acquisition and reduction -- Modelling and interpretation of magnetic data over the Peel Fault -- Modelling and interpretation of magnetic data over the Mooki Fault -- Gravity modelling of the Tamworth Belt and Gunnedah Basin -- Interpretation and discussion -- Conclusions.
This thesis presents new magnetic and gravity data for the Southern New England Fold Belt (SNEFB) and the Gunnedah Basin that adjoins to the west along the Mooki Fault in New South Wales. The SNEFB consists of the Tamworth Belt and Tablelands Complex that are separated by the Peel Fault. The Tablelands Complex to the east of the Peel Fault represents an accretionary wedge, and the Tamworth Belt to the west corresponds to the forearc basin. A total of five east-north-east trending gravity profiles with around 450 readings were conducted across the Tamworth Belt and Gunnedah Basin. Seven ground magnetic traverses of a total length of 60 km were surveyed across the bounding faults of the Tamworth belt, of which five were across the Peel Fault and two were across the Mooki Fault. The gravity data shows two distinct large positive anomalies, one over the Tamworth Belt, known as the Namoi Gravity High and another within the Gunnedah Basin, known as the Meandarra Gravity Ridge. All gravity profiles show similarity to each other. The magnetic data displays one distinct anomaly associated with the Peel Fault and an anomaly immediately east of the Mooki Fault. These new potential field data are used to better constrain the orientation of the Peel and Mooki Faults as well as the subsurface geometry of the Tamworth Belt and Gunnedah Basin, integrating with the published seismic data, geologic observations and new physical properties data. --Magnetic anomalies produced by the serpentinite associated with the Peel Fault were used to determine the orientation of the Peel fault. Five ground magnetic traverses were modelled to get the subsurface geometry of the serpentinite body. Modelling results of the magnetic anomalies across the Peel Fault indicate that the serpentinite body can be mostly modelled as subvertical to steeply eastward dipping tabular bodies with a minimum depth extent of 1-3 km, although the modelling does not constrain the vertical extent. This is consistent with the modelling of the magnetic traverses extracted from aeromagnetic data. Sensitivity analysis of a tabular magnetic body reveals that a minimum susceptibility of 4000x10⁻⁶cgs is needed to generate the observed high amplitude anomalies of around 2000 nT, which is consistent with the susceptibility measurements of serpentinite samples along the Peel Fault ranging from 2000 to 9000 x 10⁻⁶ cgs. Rock magnetic study indicates that the serpentinite retains a strong remanence at some locations. This remanence is a viscous remanent magnetisation (VRM) which is parallel to the present Earth's magnetic field, and explains the large anomaly amplitude over the Peel fault at these locations. The remanence of serpentinite at other localities is not consistent enough to contribute to the observed magnetic anomalies. A much greater depth extent of the Peel Fault was inferred from gravity models. It is proposed that the serpentinite along the Peel Fault was emplaced as a slice of oceanic floor that has been accreted to the front of the arc, or as diapirs rising off the serpentinised part of the mantle wedge above the supra subduction zone.
Magnetic anomalies immediately east of the Mooki Fault once suggested to be produced by a dyke-like body emplaced along the fault were modelled along two ground magnetic traverses and three extracted aeromagnetic lines. Modelling results indicate that the anomalies can be modelled as an east-dipping overturned western limb of an anticline formed as a result of a fault-propagation fold with a shallow thrust step-up angle from the décollement. Interpretation of aeromagnetic data and modelling of the magnetic traverses indicate that the anomalies along the Mooki Fault are produced by the susceptibility contrast between the high magnetic Late Carboniferous Currabubula Formation and/or Early Permian volcanic rocks of the Tamworth Belt and the less magnetic Late Permian-Triassic Sydney-Gunnedah Basin rocks. Gravity modelling indicates that the Mooki Fault has a shallow dip ( ̃25°) to the east. Modelling of the five gravity profiles shows that the Tamworth Belt is thrust westward over the Sydney-Gunnedah Basin for 15-30 km. --The Meandarra Gravity Ridge within the Gunnedah Basin was modelled as a high density volcanic rock unit with a density contrast of 0.25 tm⁻³, compared to the rocks of the Lachlan Fold Belt in all profiles. The volcanic rock unit has a steep western margin and a gently dipping eastern margin with a thickness ranging from 4.5-6 km, and has been generally agreed to have formed within an extensional basin. --The Tamworth Belt, being mainly the product of volcanism of mafic character and thus has high density units, together with the high density Woolomin Association, which is composed chiefly of chert/jasper, basalt, dolerite and metabasalt, produces the Namoi Gravity High. Gravity modelling results indicate that the anomaly over the Tamworth Belt can be modelled as either a configuration where the Tablelands Complex extends westward underthrusting the Tamworth Belt, or a configuration where the Tablelands Complex has been thrust over the Tamworth Belt. When the gravity profiles were modelled with the first configuration, the Peel Fault with a depth extent of around 1 km can only be modelled for the Manilla and Quirindi profiles, modelling of the rest of the gravity profiles indicates that the Tablelands Complex underthrust beneath the Tamworth belt at a much deeper location.
Mode of access: World Wide Web.
xi, 242 leaves ill., maps
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Ussami, Naomi. "Interpretation of the gravity anomalies of Bahia state Brazil." Thesis, Durham University, 1986. http://etheses.dur.ac.uk/6828/.
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This thesis describes the gravity interpretation of Bahia State, Brazil, which comprises the northern Sao Francisco craton, the Upper Proterozoic fold belts and the basins adjacent to the continental margin. The study centres on the isostatic analysis of the region and on the interpretation of large and high amplitude negative anomalies which occur over the Precambrian and the sedimentary basins. The isostatic analysis of the northern Sao Francisco craton was carried out using the isostatic response function technique. Taking into account subsurface loads, an elastic plate with a minimum effective thickness of 20-40 km explains the observed isostatic response function. The subsurface loads are (l) a slight thickening of the crust under the Espinhaco Fold System and (2) five to ten kilometres of low density rocks in the upper crust. A large and high amplitude ( - 50 mGal ) negative anomaly of shallow origin, centred near the western border of the Paramirim complex and parallel to the Espinhaco fold belt, is interpreted as caused by a large and mainly unexposed granite batholith. The granite substantially underlies the fold belt and extends towards the centre of the Paramirim complex. The minimum density contrast between the granite and the country rocks is estimated to be -0.06 g cm(^-3). The thickness of the granite is 8 to 13 km for density contrasts of -0.15 g cm(^-3) to -0.10 g cm(^-3). A series of high amplitude negative anomalies (50 to 100 rrGal), without flanking positive anomalies, characterizes the onshore Reconcavo, Tucano and Jatoba basins, which were ail formed in connection with the South Atlantic opening. The gravity interpretation indicates up to 7 km of sediments infilling these basins and no significant Mo ho upwarp beneath. In contrast, the gravity anomalies over the offshore Jacuipe and Sergipe-Alagoas basins are explained by a thick accumulation of sediments on a strongly attenuated crust. The onshore basins show short-lived subsidence ( < 25 Ma) with little, if any thermal subsidence. Syn-rift and post-rift (thermal) sedimentation is observed only in the continental margin basins. A mechanism in which upper crustal extension in one region (onshore basins) is compensated and balanced against lower extension in another region (offshore basins), through a detachment fault, may explain the way these basins formed.
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Eskamani, Philip K. "Seafloor spreading in the eastern Gulf of Mexico| New evidence for marine magnetic anomalies." Thesis, Colorado State University, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=1564450.
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Possible sea-floor spreading anomalies are indentified in marine magnetic surveys conducted in the eastern Gulf of Mexico. A symmetric pattern of lineated anomalies can be correlated with the geomagnetic time scale using previously proposed opening histories for the Gulf of Mexico basin. Lineated magnetic anomalies are characterized by amplitudes of up to 30 nT and wavelengths of 45-55 km, and are correlatable across 12 different ship tracks spanning a combined distance of 6,712 km. The magnetic lineations are orientated in a NW-SE direction with 3 distinct positive lineations on either side of the inferred spreading ridge anomalies. The magnetic anomalies were forward modeled with a 2 km thick magnetic crust composed of vertically bounded blocks of normal and reverse polarity at a model source depth of 10 km. Remnant magnetization intensity and inclination are 1.6 A m-1 and 0.2° respectively, chosen to best fit the magnetic observed amplitudes and, for inclination, in accord with the nearly equatorial position of the Gulf of Mexico during Jurassic seafloor spreading. The current magnetic field is modeled with declination and inclination of and 0.65° and 20° respectively. Using a full seafloor spreading rate of 1.7 cm/yr, the anomalies correlate with magnetic chrons M21 to M10. The inferred spreading direction is consistent with previous suggestions of a North-East to South-West direction of sea-floor spreading off the west coast of Florida beginning 149 Ma (M21) and ending 134 Ma (M10). The opening direction is also consistent with the counter-clockwise rotation of Yucatan proposed in past models.
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Hegmann, Mary Jane. "Gravity and magnetic surveys over the Santa Rita Fault System, southeastern Arizona." Thesis, The University of Arizona, 1998. http://hdl.handle.net/10150/278675.
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Gravity and magnetic surveys were performed in the northeast portion of the Santa Rita Experimental Range, in southeastern Arizona, to identify faults and gain a better understanding of the subsurface geology. A total of 234 gravity stations were established, and numerous magnetic data were collected with portable and truck-mounted proton precession magnetometers. In addition, one line of very low frequency electromagnetic data was collected together with magnetic data. Gravity anomalies are used to identify two normal faults that project northward toward a previously identified fault. The gravity data also confirm the location of a second previously interpreted normal fault. Interpretation of magnetic anomaly data indicates the presence of a higher-susceptibility sedimentary unit located beneath lower-susceptibility surficial sediments. Magnetic anomaly data identify a 1-km-wide negative anomaly east of these faults caused by an unknown source and reveal the high variability of susceptibility in the Tertiary intrusive rocks in the area.
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Bennett, Randall. "Gravity Investigation of a Normal Fault in Southern St. Landry Parish, Louisiana." Thesis, University of Louisiana at Lafayette, 2019. http://pqdtopen.proquest.com/#viewpdf?dispub=10981215.
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Previous work conducted by Kushiyama (2010) identified a relative gravity profile with an abnormal anomaly across a normal fault. The relative gravity should have decreased when crossing from the upthrown side to the downthrown side. Additional relative gravity data were collected and incorporated with the existing data to create an improved gravity anomaly map. The map shows that the gravity generally increases from the southwest to the northeast in the study area. In two areas where profiles cross the fault at nearly a perpendicular angle, the fault is clearly visible and interpretable from the gravity data. However, along Chris Road, that is not the case. This is most likely caused by an underlying salt ridge (Varvaro, 1958). The mobilization of this salt upwards through more dense strata might be the cause of the low gravity effect of the upthrown side of the fault along Chris Road.
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Wyer, Paul Patrick Andrew. "Gravity anomalies and segmentation of the Eastern USA passive continental margin." Thesis, University of Oxford, 2003. http://ora.ox.ac.uk/objects/uuid:cefa0dff-a009-4511-a530-c3e3d3b2da1e.
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A compilation of high quality post-rift sediment isopach data has been used in conjunction with the observed free-air gravity anomaly to determine segmentation of the long term mechanical properties of the lithosphere at the Eastern USA passive margin. This segmentation is represented by a process-oriented analysis in which the flexural response of the margin to post-rift sediment loading is controlled by spatial variations in effective elastic thickness (Te) of the underlying lithosphere. Existing Eastern USA margin Te estimates range from less than 10km to more than 30km. In this study it is shown that high strengths of 10 - 40km Te are confined to structural arches dividing the broadest marginal basins, while low strengths of less than 10km Te are typically found in structural embayments and beneath the deep basins. The hinge zone, across which the degree of continental thinning increases rapidly, marks the transition between high and low strength. Yield strength envelope models support an argument that regions of low strength were created by lithospheric thinning during rifting, and sustained by thermal insulation and flexural curvature associated with voluminous post-rift sediment deposition. Along-strike Te variations - reflected in the alternation of basement platforms and embayments - are attributed to inheritance of lithospheric segmentation from earlier tectonic events. Along-strike segmentation of the margin has previously been observed as a 300 - 500km wavelength spectral energy peak in the shelf break Airy isostatic gravity anomaly (IGA) high. That this segmentation is explained by variations in the underlying lithospheric strength is demonstrated by a flexural IGA high in which the equivalent spectral peak is absent. The spectral energy of the along-strike Te distribution peaks in the same waveband. Removal of process-oriented components from the observed free-air gravity anomaly reveals other contributions that were not resolved in earlier studies. In particular, the (previously unknown) Carolina Trough Isostatic Gravity Anomaly, has been identified and attributed to an extrusive (syn-rift) volcanic source. Detailed study of this anomaly suggests that the margin is segmented in terms of its volcanic character, and argues against recent estimates of the volume of new igneous material emplaced during rifting.
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Huang, Ou. "Terrain Corrections for Gravity Gradiometry." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1339698991.
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Yale, Mara M. "Modeling upper mantle rheology with numerical experiments and mapping marine gravity with satellite altimetry /." Diss., Connect to a 24 p. preview or request complete full text in PDF format. Access restricted to UC campuses, 1997. http://wwwlib.umi.com/cr/ucsd/fullcit?p9823700.
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Fagbola, Olamide Olawumi. "Integrated study of basins in the Four Corners Region." To access this resource online via ProQuest Dissertations and Theses @ UTEP, 2007. http://0-proquest.umi.com.lib.utep.edu/login?COPT=REJTPTU0YmImSU5UPTAmVkVSPTI=&clientId=2515.
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Chan, Mei-ki. "Gravity and aeromagnetic modelling of the Longmenshan Fold-and-Thrust Belt, SW China." Click to view the E-thesis via HKUTO, 2008. http://sunzi.lib.hku.hk/hkuto/record/b4020330x.
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Jones, Michael Bryan. "Correlative Analysis of the Gravity and Megnetic Anomalies of Ohio and their Geologic Significance." The Ohio State University, 1988. http://rave.ohiolink.edu/etdc/view?acc_num=osu1392823447.
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Tozer, Brook. "Crustal structure, gravity anomalies and subsidence history of the Parnaíba cratonic basin, Northeast Brazil." Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:90ce8bb0-e55d-4b3c-87e1-aab60084ef42.
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Cratonic basins cover more than 10% of Earth's continental surface area, yet their origin remains enigmatic. In this thesis a suite of new and legacy geophysical and geological data are integrated to constrain the origin of the Parnaíba basin, a cratonic basin in Northeast Brazil. These data include a 1400 km long, deep (20 s two-way travel time) seismic reflection profile, five +/- 110 km offset wide-angle split-spread receiver gathers, gravity anomaly, and well data. In the centre of the basin, the depth to pre-Paleozoic basement is ~ 3.3 km, a zone of midcrustal reflectivity (MCR) can be traced laterally for ~ 250 km at depths between 17-25 km and Moho depth is ~ 42 +/- 2 km. Gravity and P-wave modelling suggests that the MCR represents the upper surface of a high density (2985 kg m3) and Vp (6.7 - 7.0 km s-1) lower crustal body, likely of magmatic origin. Backstripping of well data shows a concave up decreasing tectonic subsidence, similar in form to that commonly observed in rift-type basins. It is shown, however, that the seismic and gravity data are inconsistent with an extensional origin. It is shown that an intrusive body in the lower crust that has loaded and flexed the surface of the crust, combined with sediment loading, provides a satisfactory fit to the observed gravity anomaly, sediment thickness and basin shape. A buried load model is also consistent with seismic data, which suggest that the Moho is as deep or deeper beneath the basin centre than its flanks and accounts for at least part of the tectonic subsidence through a viscoelastic stress relaxation that occurs in the lithosphere following load emplacement. Comparative analysis of the Michigan and Congo basins shows gravity data from these basins is also consistent with a lower crustal mass excess, while subsidence analysis shows viscoelastic stress relaxation may also contribute to their early subsidence histories. However, unlike Parnaíba, both of these basins appear to have been subjected to secondary tectonic processes that obscure the primary 'cratonic basin' subsidence signals. Parnaíba basin, therefore, offers an excellent record for the investigation of cratonic basin formation.
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Harbi, Hussein M. "2-D MODELING OF SOUTHERN OHIO BASED ON MAGNETIC FIELD INTENSITY, GRAVITY FILED INTENSITY AND WELL LOG DATA." University of Akron / OhioLINK, 2005. http://rave.ohiolink.edu/etdc/view?acc_num=akron1125523809.
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Asgharzadeh, Mohammad Forman. "Geodynamical analysis of the Iranian Plateau and surrounding regions." Columbus, Ohio : Ohio State University, 2007. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1173126914.
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Chan, Mei-ki, and 陳美琪. "Gravity and aeromagnetic modelling of the Longmenshan Fold-and-Thrust Belt, SW China." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2008. http://hub.hku.hk/bib/B4020330X.
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Hayward, Nathan. "Marine geophysical study of the Eurasian-African plate boundary in the vicinity of Gorringe Bank." Thesis, University of Oxford, 1996. http://ora.ox.ac.uk/objects/uuid:e2ca90ad-c2e3-4495-97da-f5cc8bcf1e74.
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The Gorringe Bank region is located at the eastern end of the Azores-Gibraltar plate boundary, which plate kinematic studies show to progressively change from extension at the Azores, through pure right lateral strike slip at the Gloria fault to compression at Gibraltar. The region is dominated by high relief (4-5 km), highly deformed, uncompensated, ENE-WSW trending seamounts and intervening abyssal plains with basin sediment thicknesses in excess of 4 km and minimal surface deformation. Gorringe Bank, which was formed by overthrusting of the African plate upon the Eurasian plate at about 10 Ma along the plate boundary, is supported in part by exure of the Eurasian plate, as indicated by pre-loading sediments and basement to the north which are tilted towards Gorringe Bank. Broken plate models show the Eurasian plate to have an elastic thickness of about 35 km which is in agreement with that expected for the crustal age (130-135 Ma) at the time of loading. Coral Patch Ridge was formed by a combination of thrust faulting and whole crustal buckling resulting from the past 20 Ma compression and was partially uplifted before deposition of an olistostrome in the Middle Miocene. Recent compressional deformation is distributed over a wide region, as indicated by the dispersed shallow seismicity and has a trend which rotates from approximately N45oE to N70oE from west to east across the region, near perpendicular to westward verging plate motion vectors. The majority of extensional and strike-slip deformation is explained by a regional strike-slip strain ellipse model, including an antithetic NNE-SSW strike slip fault between Gettysburg and Hirondelle seamounts which marks the boundary between the Eastern and Western Horseshoe Basins. Isostatic models for the Madeira-Tore Rise, which initially formed at the Mid Atlantic Ridge, give an elastic thickness of approximately 15 km indicating that significant material was added to the Rise as it moved away from the Ridge.
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Tavarez, Samantha Catherine. "Geophysical Evidence for Mid-crustal Magma Reservoirs in the Lassen Volcanic Region, California." Scholar Commons, 2015. http://scholarcommons.usf.edu/etd/6038.
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Regional-scale complete Bouguer gravity anomalies underlying the Lassen and Shasta -Medicine Lake regions in northern California and southern Oregon are associated with subduction of the Gorda plate beneath North America. These generally negative anomalies reflect where underplating has deepened to form the mantle wedge, and where subduction has given rise to a series of Quaternary volcanoes comprising the southernmost end of the Cascade range. Multiple conductive bodies were identified by Park and Ostos (2013) in their magnetotelluric (MT) study of the broader Lassen volcanic region. Their broadband and long period measurements were conducted along a 250 km profile spanning from the California-Nevada border, to just west of the Great Valley in California. Utilizing their MT conductor geometries as a starting point, a forward gravity model was generated along the same profile, and agrees well with what they interpret to be the locations and depths of mid-crustal magma bodies in the Lassen and surrounding regions. The excess mass and volume of modeled anomaly (a) - most closely attributed to underlying Lassen Peak - were estimated at -2 x 1014 kg and 7 x 1011 m3, respectively.
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Scully, Malcolm E. "Modeling of Critically-Stratified Gravity Flows: Application to the Eel River Continental Shelf, Northern California." W&M ScholarWorks, 2001. http://www.vims.edu/physical/projects/CHSD/publications/reports/S2001%5FMS.pdf.
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van, den Bremer T. S. "The induced mean flow of surface, internal and interfacial gravity wave groups." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:e735afe7-a77d-455d-a560-e869a9941f69.
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Although the leading-order motion of waves is periodic - in other words backwards and forwards - many types of waves including those driven by gravity induce a mean flow as a higher-order effect. It is the induced mean flow of three types of gravity waves that this thesis examines: surface (part I), internal (part II) and interfacial gravity waves (part III). In particular, this thesis examines wave groups. Because they transport energy, momentum and other tracers, wave-induced mean flows have important consequences for climate, environment, air traffic, fisheries, offshore oil and other industries. In this thesis perturbation methods are used to develop a simplified understanding of the physics of the induced mean flow for each of these three types of gravity wave groups. Leading-order estimates of different transport quantities are developed. For surface gravity wave groups (part I), the induced mean flow consists of two compo- nents: the Stokes drift dominant near the surface and the Eulerian return flow acting in the opposite direction and dominant at depth. By considering subsequent orders in a separation of scales expansion and by comparing to the Fourier-space solutions of Longuet-Higgins and Stewart (1962), this thesis shows that the effects of frequency dis- persion can be ignored for deep-water waves with realistic bandwidths. An approximate depth scale is developed and validated above which the Stokes drift is dominant and below which the return flow wins: the transition depth. Results are extended to include the effects of finite depth and directional spreading. Internal gravity wave groups (part II) do not display Stokes drift, but a quantity analogous to Stokes transport for surface gravity waves can still be developed, termed the “divergent- flux induced flow” herein. The divergent-flux induced flow it itself a divergent flow and induces a response. In a three-dimensional geometry, the divergent-flux induced flow and the return flow form a balanced circulation in the horizontal plane with the former transporting fluid through the centre of the group and the latter acting in the opposite direction around the group. In a two-dimensional geometry, stratification inhibits a balanced circulation and a second type of waves are generated that travel far ahead and in the lee of the wave group. The results in the seminal work of Bretherton (1969b) are thus validated, explicit expressions for the response and return flow are developed and compared to numerical simulations in the two-dimensional case. Finally, for interfacial wave groups (part III) the induced mean flow is shown to behave analogously to the surface wave problem of part I. Exploring both pure interfacial waves in a channel with a closed lid and interacting surface and interfacial waves, expressions for the Stokes drift and return flow are found for different configurations with the mean set-up or set-down of the interface playing an important role.
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Sepehrmanesh, Mahnaz. "APPLICATION OF THE KALMAN FILTER ON FULL TENSOR GRAVITY GRADIOMETRY DATA AROUND THE VINTON SALT DOME, LOUISIANA." UKnowledge, 2014. http://uknowledge.uky.edu/ees_etds/26.
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Full tensor gravity (FTG) data are known for their high resolution but also for higher noise in its components due to the dynamic nature of the platform used for data acquisition. Although a review of the literature suggests steady increase in the success of gravity gradiometry, we still cannot take advantage of the full potential of the method, mostly because of the noise with the same amplitude and wavenumber characteristics as the signal that affects these data. Smoothing from common low pass filters removes small wavelength features and makes it difficult to detect structural features and other density variations of interest to exploration. In Kalman filtering the components of the FTG are continuously updated to calculate the best estimation of the state. The most important advantage of the Kalman filter is that it can be applied on gravity gradiometry components simultaneously. In addition, one can incorporate constraints. We use the Laplace’s equation that is the most meaningful constraint for potential field data to extract signal from noise and improve the detection and continuity of density variations. We apply the Kalman filter on the FTG data acquired by Bell Geospace over the Vinton salt dome in southwest Louisiana.
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Saballos, Jose Armando. "Short and Long Term Volcano Instability Studies at Concepción Volcano, Nicaragua." Scholar Commons, 2013. http://scholarcommons.usf.edu/etd/4757.
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Concepción is the most active composite volcano in Nicaragua, and is located on Ometepe Island, within Lake Nicaragua. Moderate to small volcanic explosions with a volcanic explosivity index (VEI) of 1-2 have been characteristic of this volcano during the last four decades. Although its current activity is not violent, its volcanic deposits reveal stages of violent activity involving Plinian and sub-Plinian eruptions that deposited vast amounts of volcanic tephra in the Atlantic Ocean. These observations, together with the 31,000 people living on the island, make Concepción volcano an important target for volcanological research.
My research focuses on the investigation of the stability of the volcano edifice of Concepción, using geophysical data such as gravity, geodetic global positioning system (GPS), sulphur dioxide (SO2) flux, real-time seismic amplitude (RSAM), and satellite remotely-sensed data. The integration of these data sets provides information about the short-term behavior of Concepción, and some insights into the volcano's long-term behavior.
This study has provided, for the first time, information about the shallow dynamics of Concepción on time scales of days to weeks. I furnish evidence that this volcano is not gravitationally spreading in a continuous fashion as previously thought, that its bulk average density is comparable to that of a pile of gravel, that the volcano edifice is composed of two major distinctive lithologies, that the deformation field around the volcano is recoverable in a matter of days, and that the deformation source is located in the shallow crust. This source is also degassing through the relatively open magmatic conduit. There are, however, several remaining questions. Although the volcano is not spreading continuously there is the possibility that gravitational spreading may be taking place in a stick-slip fashion. This has important implications for slope stability of the volcano, and the
associated hazards. The factors influencing the long term slope stability of the volcano are still not fully resolved, but internal volcanic processes and anthropogenic disturbances appear to be the major factors.
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Ohl, Derek Robert. "Rock formation characterization for carbon dioxide geosequestration: 3D seismic amplitude and coherency anomalies, and seismic petrophysical facies classification, Wellington and Anson-Bates fields, Sumner County, Kansas, USA." Thesis, Kansas State University, 2012. http://hdl.handle.net/2097/13637.
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Master of ScienceDepartment of Geology
Abdelmoneam Raef
Amid increasing interest in geological sequestration of carbon dioxide (CO2), detailed rock formation characterization has emerged as priority to ensure successful sequestration. Utilizing recent advances in the field of 3D seismic attributes analysis, offers improved opportunities to provide more details when characterizing reservoir formations. In this study, several post-stack seismic attributes integrated with seismic modeling for highlighting critical structural elements and petrophysical facies variation of rock formations at Wellington and Anson-Bates fields, Sumner County, Kansas. A newly acquired 3D Seismic data set and several geophysical well logs are also used to achieve the objectives of this study. Results sought in this study are potentially important for understanding pathways for CO2 to migrate along. Seismic amplitude, coherency, and most negative curvature attributes were used to characterize the subsurface for structural effects on the rock formations of interest. These attributes detect multiple anomaly features that can be interpreted as small throw faults. However, in this study, there is a larger anomalous feature associated with the Mississippian formation that can be interpreted as a small throw fault or incised channel sand. Determining which of the two is very important for flow simulation models to be more exact. Modeling of the seismic was undertaken to help in the interpretation of the Mississippian amplitude anomaly. An artificial neural network, based on well log porosity cross-plots and three seismic attributes, was trained and implemented to yield a seismic petrophysical facies map. The neural network was trained using three volume seismic waveform attributes along with three wells with difference in well log porosity. A reworked lithofacies along small throw faults has been revealed based on comparing the seismic structural attributes and the seismic petrophysical facies. Arbuckle formation characterization was successful to a certain degree. Structural attributes showed multiple faults in the northern half of the survey. These faults are in agreement with known structure in the area associated with the Nemaha uplift. Further characterization of the Arbuckle was hindered by the lack of well data. This study emphasizes the need for greater attention to small-scale features when embarking upon characterization of a reservoir for CO2 based geosequestration.
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Pešek, Michal. "Měření vertikálního tíhového gradientu na lokalitě TS-73 "Polom", Trutnov - Babí." Master's thesis, Vysoké učení technické v Brně. Fakulta stavební, 2015. http://www.nusl.cz/ntk/nusl-390186.
Full textAbstract:
This master´s thesis deals with the determination of vertical gravity gradient in the shaft below the infantry bunker TS - 73 "Polom" located in village Babí. After performing position, elevation and gravity measurements detailed points, representing the two vertical profiles were calculated gravity, vertical gravity gradient, block densities ang gravity anomalies depending on the depth. Other outputs work as a topographic map and a map of full Bouguer anomalies for the wider area of the location and interpretive geologic profile.
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Durham, Rachel Lauren. "POTENTIAL FIELD MODELING ACROSS THE NEODYMIUM LINE DEFINING THE PALEOPROTEROZOIC-MESOPROTEROZOIC BOUNDARY OF THE SOUTHEASTERN MARGIN OF LAURENTIA." UKnowledge, 2017. http://uknowledge.uky.edu/ees_etds/53.
Full textAbstract:
A zone of high magnetization along the SE margin of Paleoproterozoic Laurentia in the United States is indicated by magnetic anomaly data. The SE edge corresponds to the geochemical Neodymium mantle derivation model age (TDM) boundary and the entire anomaly overlies the Paleoproterozoic Mazatzal Province. Two-dimensional gravity and magnetic models across the Nd boundary are created with Moho constrained from receiver functions with gravity, sedimentary thickness and the base of the crustal magnetization. Upper crustal magnetization does not show strong variation across this boundary and much of the strong magnetization appears to lie in the middle crust. Using magnetic modeling of several potential geologic scenarios, we estimate magnetization, depth extent, and width of this zone of high magnetization. The anomaly has variable width (~ 300 km) with amplitude of approximately 200 nT. Pre-1.55Ga Paleoproterozoic mid crustal blocks have significantly higher average effective susceptibility (0.06 SI) than those of the post-1.55Ga Mesoproterozoic (0.01 SI). In two of the three profiles, the Paleoproterozoic zone of high magnetization has the highest average susceptibility indicating the Mazatzal province is innately highly magnetic. The zone may have formed either by magmatism associated with westward subduction or from highly magnetic terranes wedging between accreting island arcs.
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Koehl, Daniel Grant. "Investigating an Apparent Structural High in Seismic Data in North Terre Haute, Indiana, Through First-Arrival Traveltime Tomography and Gravity Analysis." Wright State University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=wright1559120344838085.
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Bassett, Daniel Graham. "The relationship between structure and seismogenic behaviour in subduction zones." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:dd284a79-d94a-4732-8dec-cb38c78fca73.
Full textAbstract:
The largest earthquakes on Earth take place on the megathrusts of subduction zones, but the slip behavior of megathrusts is variable. This thesis considers why by conducting local, regional and global studies of the interrelationships between the structure and seismogenic behavior of subduction zones. New marine geophysical data collected from the collision zone between the Louisville Ridge seamount chain with the Tonga-Kermadec trench constrain overthrusting and subducting plate structure. Mo'unga seamount is identified beneath the outer-forearc, which calibrates the association of residual bathymetric anomalies and subducting relief, implies an E-W geometry for the subducted ridge and suggests the 200 km wide Louisville seismic gap is modulated by the sediment filled flexural moat. Spectral averaging is then applied along the Tonga-Kermadec margin and along strike variations in overthrusting plate structure are verified by wide-angle seismic transects. The remnant Tonga-Ridge occupies the inner fore-arc and residual free-air gravity anomalies constrain its latitudinal extent (north of 30.5°S), width (110±20 km) and strike (~005° south of 25°S). Plate tectonic reconstructions suggest the Lau Ridge is unmodified by subduction related erosion, <200 km of the Tonga Ridge has been eroded, and neither ridge ever occupied the southern Kermadec arc. Crustal thickness variations are thus inherited, reflecting the Cenozoic tectonic evolution of the Tonga-Kermadec-Hikurangi margin. Spectral averaging is finally applied to all subduction zones on Earth. Part one develops a global catalogue of subducting relief, which is compared with seismological and geodetic inferences of fault-slip behavior. Most seamounts are aseismic, relatively undeformed and observations are not consistent with mechanical models proposing full-decapitation. Aseismic ridges are also associated with megathrust complexity, but are of a larger wavelength and contrasting mode of isostatic compensation. Part two shows almost all intra-margin along-strike transitions in seismogenic behavior are related to pre-existing crustal structure. A paired forearc anomaly is interpreted consisting of a trench-parallel ridge landward of the deep-sea-terrace basin. The ridge crest correlates with the down-dip limit of coseismic slip and strong interplate coupling, the up- dip limit of tremor epicentres, and is interpreted as defining the boundary between the velocity-weakening and seismogenic portion of the subduction interface and the down-dip frictional transition zone. Paired anomalies may be attributed to unrecovered interseismic elastic strain, the preferential subduction erosion of the outer-forearc and/or underplating beneath the inner forearc.
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Hunter, Shannon K. "Geologic and Paleomagnetic Study of the Miocene Haycock Mountain Tuff: Markagunt Plateau, Southwest Utah." Kent State University / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=kent1543652864307195.
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Gac, Sébastien. "Modélisation numérique 3D des caractéristiques physiques des segments de dorsale lente." Phd thesis, Université de Bretagne occidentale - Brest, 2002. http://tel.archives-ouvertes.fr/tel-00008837.
Full textAbstract:
J'ai développé des modèles numériques 3D de l structure thermiques et des caractéristiques géophysiques résultants des segments de dorsale lente pour vérifier si la présence d'une zone chaude sous les centres de segment rend compte des observations géophysiques axiales et hors-axe et pour déterminer les caractéristiques d'une telle zone chaude. Les observations sur la dorsale Médio-Atlantique montrent des variations le long de l'axe des caractéristiques géophysiques des centres de segment vers leurs extrémités. Les observations hors-axe montrent l'existence de formes rhomboédriques suggérant qu'un segment s'allonge puis se raccourcit au cours du temps sur une période de 3 à 9 Ma. Dans mon modèle, la structure thermique est contrôlée par la géométrie et les dimensions d'une zone chaude permanente impose sous le centre du segment, dans laquelle est simulée la remontée adiabatique de matériel mantellique. Les processus physiques à l'origine d'une telle zone chaude ne sont pas considérés. A partir de la structure thermique, les paramètres géophysiques (anomalies gravimétriques, anomalies magnétiques, profondeur maximale des microséismes et variation axiale de l'épaisseur crustale) sont simulées et comparées à ceux observés pour identifier les dimensions et géométrie particulières de la zone chaude qui reproduisent les observations géophysiques. Les structures thermiques de différents types de segment, caractérisés par des longueurs différentes, sont déterminées de cette façon. Les modèles montrent que, les autres paramètres restant constants, l'allongement de la zone chaude est suffisant pour rendre compte de la variation des caractéristiques des différents types de segment. Cette simple évolution de la zone chaude suggère que les différents types de segment représentent différentes phases dans l'évolution d'un type unique de segment. Pour tester si une telle évolution peut rendre compte des observations hors-axe j'ai modélisé cette évolution de la zone chaude et les structure et évolution thermiques résultantes. Une telle évolution rend compte des observations hors-axe si la zone chaude s'allonge progressivement pendant 3 Ma puis se raccourcit pendant 3 Ma.
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Khurshid, Akbar. "Crustal structure of the Sulaiman Range, Pakistan, from gravity data /." 1991. http://hdl.handle.net/1957/9234.
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Soofi, Muhammad Asif. "Crustal structure of the northwestern continental margin of the Indian subcontinent from gravity and magnetic data." Thesis, 1991. http://hdl.handle.net/1957/29074.
Full textAbstract:
The continental margin off the coast of Pakistan between the
Murray ridge and the Gulf of Cambay has been studied in this
work using gravity, magnetic and bathymetric data. Two dimensional
gravity and magnetic models based on free-air
gravity and residual magnetic data are developed along a north-south
profile off the coast of Karachi. The purpose was to
interpret the gross crustal structure of the region. A magnetic
map has also been developed for the region between latitudes 20°N and 27°N and between longitude 60°E and 70°E.
The gravity model extends to a distance of about 1200 km
seaward south of Karachi. The seaward end of the gravity model
is constrained by seismic refraction data which suggest the
presence of typical oceanic crust. The Moho depth at this end of
the profile is about 12 km. At the landward end of the profile A-A'
the Moho depth is not constrained by seismic data. The gravity
model suggests 27 to 17 km as the possible range of the depth of
the Moho and a gradual thinning of the crust from land to sea. In
addition, the gravity models as interpreted in this study show
grabens at the distances of 350 and 450 km along the profile. If
the graben-like structures are rift grabens formed during the
rifting of India from Africa then transitional crust can be expected
to extend to the 500 km mark along the profile A-A'.
Two dimensional models for the magnetic data along the
profile were also developed. These anomalies can be interpreted
as due to oceanic crust or magnetic bodies embedded in
transitional crust. The possibility that the observed magnetic
anomalies are due to oceanic crust has been studied in detail in
this work. The location of the observed magnetic anomalies with
respect to marine magnetic anomaly (28) observed earlier on the
Indian Ocean floor, were compared to a marine magnetic time
scale. To get a reasonable correlation between the observed and
theoretical anomalies requires a considerable amount of
adjustment in the spreading rate of individual magnetic blocks.
Also on the magnetic map the trend of the lineation of these
anomalies is perpendicular to the continental margin instead of
being parallel to the continental margin as expected for a rifted
continental margin. The presence of horst-and-graben structures
in the inland region suggests the rifted nature for the continental
margin off Karachi than the sheared nature. This indicates that
the lineations should be parallel to the margin. But the magnetic
lineations are perpendicular to the continental margin and if they
are from oceanic crust then they would suggest that the margin is
a sheared margin, which contradicts the extensional structures
observed inland. This makes it very unlikely that the source of
these anomalies is oceanic crust. However, it is quite possible that
the magnetic lineations observed in the map were parallel to the
continental margin initially but later on the continent rotated
clockwise along a fault landward of the magnetic lineation. This
rotation is perhaps responsible for making the lineation
perpendicular to the continental margin.
One objective of this study was to locate the continent-ocean
boundary, but with the available amount of data it is not possible
to decide on the most appropriate source for the observed
magnetic anomalies. Hence it was not possible to decide exactly
on the location of continent-ocean boundary. However, on the
basis of gravity and magnetic data it can be said that the
continent-ocean boundary lies at a distance of 500 km or greater
along the profile.Graduation date: 1992
34
Peavy, Samuel Thomas. "A gravity and magnetic interpretation of the Bay St. George carboniferous subbasin in western Newfoundland /." 1985. http://collections.mun.ca/u?/theses,122576.
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Diehl, Theresa Marie 1981. "Gravity analyses for the crustal structure and subglacial geology of West Antarctica, particularly beneath Thwaites Glacier." 2008. http://hdl.handle.net/2152/18359.
Full textAbstract:
The West Antarctic Ice Sheet (WAIS) is mostly grounded in broad, deep basins (down to 2.5 km below sea level) that are stretched between five crustal blocks. The geometry of the bedrock, being mostly below sea level, induces a fundamental instability in the WAIS through the possibility of runaway grounding line retreat. The crustal environment of the WAIS further influences the ice sheet’s fast flow through conditions at the ice-bedrock boundary. This study focuses on understanding the WAIS by examining the subglacial geology (such as volcanoes and sedimentary basins) at the icebedrock boundary and the continent’s deeper crustal structure- primarily using airborne gravity anomalies. The keystone of this study is a 2004-2005 aerogeophysical survey over one of the most negative mass balance glaciers on the continent: Thwaites Glacier (TG). The gravity anomalies derived from this dataset- as well as gravity-based modeling and spectral crustal boundary depth estimates- reveal a heterogeneous crustal environment beneath the glacier. The widespread Mesozoic rifting observed in the Ross Sea Embayment (RSE) of West Antarctica extends beneath TG, where the crust is ~27 km thick and cool. Adjacent to TG, spectrally-derived shallow Moho depths for the Marie Byrd Land (MBL) crustal block can be explained by thermal support from warm mantle. I assemble here new compilations of free-air and Bouguer gravity anomalies across West Antarctica (from both airborne and satellite datasets) and re-interpret the extents of West Antarctic crustal block and their boundaries with the rift system. Airy isostatic gravity anomalies reveal that TG is relatively sediment starved, in contrast to the sediment-rich RSE. TG’s fast flow velocities could be sustained in this sediment poor environment if higher heat flux in MBL was providing an ample source of subglacial melt water to the glacier. The isostatic anomalies also indicate that TG’s outlet rests on a bedrock sill that will impede future grounding line retreat (up to ~100 km) and temporarily stabilize the glacier.text
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Macario, Ana L. G. "Frequency response function analysis of the equatorial margin of Brazil using gravity and bathymetry." Thesis, 1989. http://hdl.handle.net/1957/29232.
Full textAbstract:
The overall objective of this study is to address questions concerning the long-term
mechanical strength of the lithosphere across the equatorial margin of Brazil. The
approach used in this study consists of calculating the frequency response function
estimates, also called admittance, using gravity and bathymetry data. These
experimental estimates are then compared to theoretical admittance curves for Airy and
thin elastic plate models for which estimates on the flexural rigidity or, equivalently,
effective elastic thickness may be made.
Twelve profiles, each 256 km long, were extracted from gridded gravity and
bathymetry data (data sources: project EQUANT, Defense Mapping Agency, National
Geophysical Data Center files and GEOS 3/SEASAT altimeter data). Three profiles
were specifically used for testing truncation errors introduced by four different data
treatment procedures (before Fourier transforming the data) : detrending, applying 10%
cosine tapering, mirror imaging and the use of the first derivatives. The method I
adopted is similar to the one used by McNutt (1983) and consists of testing how
reliably a given admittance estimate can be recovered as a function of the data treatment
procedure. A "predicted" gravity anomaly was obtained by convolving each
bathymetric profile with a theoretical admittance filter. The edges of this anomaly are
then submitted to the same treatment as the corresponding bathymetric profile before
Fourier transforming both profiles and calculating admittance. The stability of the
long-wavelength admittance estimates, in the presence of noise, was also investigated
by introducing Gaussian noise, in the range of -50 to +50 mGals, in the "predicted"
gravity signal. The results indicate that relatively unbiased long-wavelength admittance
estimates can be obtained by using the first derivative of the data sets. In addition, it is
shown that the mirroring technique, used in previous admittance studies across
Atlantic-type margins, leads to overestimated admittance values and, therefore,
overestimated flexural rigidities.
Neither the theoretical curves for the Airy model nor the plate flexure model can
explain the experimental admittance estimates. Not only are the experimental
admittance estimates higher than the predicted values but they also have a narrower
peak than the theoretical curves. This raises the question of the applicability of highly
simplified isostatic models for tectonic provinces such as Atlantic-type continental
margins. The following reasons may explain the discrepancies between the
experimental and theoretical admittance estimates:
(1) The abrupt nature of the transition between oceanic and continental crust
controlled by the Romanche Fracture Zone - Unlike the eastern North American
continental margin which was formed as a result of extensive rifling and pulling apart,
the obliquely-rifled equatorial margin of Brazil has undergone a complex tectonic
evolutionary process, where additional components such as shear and right-lateral
wrenching were present. Therefore, representing the margin as a thin homogeneous
elastic plate might be reasonable when the transition is gradual (for which the uniform
flexural rigidity assumption seems reasonable) but is probably not a good
approximation when it is as abrupt as the equatorial margin of Brazil
(2) Presence of subsurface loads - Previous studies have shown that estimates
of the average flexural rigidity of continental lithosphere using the admittance approach
are biased when subsurface loads are present. In principle, the proximity of the
Romanche Fracture Zone and associated volcanism suggest that shallow buried loads,
caused by intrusive bodies, might be present in the area. This could partially account
for the mismatch between theoretical curves and experimental admittance estimates.
(3) "Masked" estimates - The admittance estimates presented here are likely to
reflect the combination of two different signals: one related to the compensation of the
Barreirinhas/Piaui-Camocim sub-basin which has no topographic/bathymetric expression
and the other one related to the topography/bathymetry and its compensation which is of
interest in the admittance studies. Since the wavelengths of these signals do not differ by
much (around 80-100 km for the basin) it is possible that in the averaging process some
overlapping occurs. The combination of these signals could yield anomalous results
masking the admittance estimates in the diagnostic waveband.
In addition, I present a two-dimensional cross section obtained by forward
modelling the gravity anomaly along a profile using the line integral method. The uniform
sedimentary infill of the Barreirinhas/Piaui-Camocim basin is enough to account for
the gravity low over the inner shelf and no Moho topography is required. A plausible
explanation for this "rootless" basin structure is that the lithosphere is capable of
supporting the sediment infill load, and thus, has finite flexural rigidity (basin is locally
uncompensated).Graduation date: 1990
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Enriquez, Kelly D. "Three-dimensional gravity analysis of the Pacific-Antarctic east Pacific rise at 36.5°S, 49.8°S and 54.2°S." Thesis, 1994. http://hdl.handle.net/1957/29143.
Full textAbstract:
Three-dimensional gravity analysis is the process of removing the predictable
components from the free-air gravity anomalies and has proven to be useful for
interpreting the subsurface structures and active processes at mid-ocean ridges. The
three-dimensional effects of the seafloor and Moho topography, assuming a constant
crustal thickness and constant crust and upper mantle densities, are subtracted from the
free-air anomalies, yielding the mantle Bouguer anomalies. Mantle Bouguer anomalies
at mid-ocean ridges are believed to be largely due to the three-dimensional thermal
structure, which can be predicted using a simple passive flow model. When the gravity
contribution from the predicted thermal structure is removed from the mantle Bouguer
anomalies, the residual mantle Bouguer anomalies are created, which represent lateral
variations in the crustal thickness and/or density variations from the assumed model.
Three-dimensional gravity analysis has been carried out over three areas along
the Pacific-Antarctic East Pacific Rise (EPR): (1) the eastern intersection of the Menard
transform with the EPR, (2) the overlapping spreading center (OSC) at 36.5°S and, (3)
the western intersection of the Raitt transform with the EPR. This geophysical analysis
provides an essential tool for understanding the subsurface crustal/upper mantle structure of the fast spreading EPR, and more specifically at transform and nontransform offsets along the EPR.
Several interesting features were observed at the eastern intersection between the
Menard transform and the EPR. The continuous nature of the residual mantle Bouguer
anomalies along the ridge axis suggests that the 60 km of ridge axis surveyed here has a
fairly uniform crustal/upper mantle structure. Significant features are not observed in
the residual mantle Bouguer anomalies at the ridge-transform intersection or along the
eastern 75 km of the Menard transform. At the ridge-transform intersection, fresh lavas
from the observed overshot ridge have filled in the transform valley and have
subsequently thickened the crust, eliminating any crustal thinning that is occurring there.
The large OSC at 36.5°S has a left-stepping offset of approximately 34 km. The
most significant feature in the gravity data from this study area is the observed low in
the mantle Bouguer anomalies which extends from the northern ridge segment, eastward
to the "inactive" rift and continued along the southern ridge segment. This gravity low
suggests that this region is underlain by thicker crust and/or hotter, less dense material.
No significant features are observed in the residual mantle Bouguer anomalies
associated with the overlap basin or the two smaller basins that border the "inactive" rift.
The western ridge-transform intersection (RTI) between the Raitt transform and
the EPR significantly differs from the Menard transform study area. A transform valley
is not observed at this RYE and neither is an overshot ridge. Instead, a transformparallel
median ridge is observed east of the RTI, and a fossil transform valley is
observed north of the RTI. A low in the residual mantle Bouguer anomalies is
associated with the fossil transform valley and the median ridge, suggesting that these
areas are underlain by thicker crust and/or less dense material. Positive residual mantle
Bouguer anomalies observed at the inside corner of the RYE suggest that this area is
underlain by thinner crust and/or colder, more dense material; while at the outside corner of the ridge-transform intersection, a residual anomaly low is observed which suggests
that the outside corner is underlain with thicker crust and/or hotter, less dense material.Graduation date: 1995
Figures in original document are black and white photocopies. Best scan available.
38
Braga, Luiz F. S. "Isostatic evolution and crustal structures of the Amazon continental margin determined by admittance analyses and inversion of gravity data." Thesis, 1991. http://hdl.handle.net/1957/29265.
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Wiseman, Ronald. "Potential field modelling and interpretation along the Lithoprobe East onshore seismic reflection transects across the Newfoundland Appalachians /." 1994. http://collections.mun.ca/u?/theses,41270.
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Scheidhauer, Maren. "Crustal structure of the Queen Charlotte Transform Fault Zone from multichannel seismic reflection and gravity data." Thesis, 1997. http://hdl.handle.net/1957/37218.
Full textAbstract:
The Queen Charlotte Fault system is a segment of the North America - Pacific
plate boundary. From 45 Ma - 5 Ma, plate motion has been primarily translational.
Since 5 Ma, transpression has been the dominant mode of interaction. The plate
boundary west of the Queen Charlotte Islands is characterized by an approximately 30-
km wide terrace, flanked to the west by a topographic trough and to the east by the
seismically active Queen Charlotte Fault. At 53.4°N the fault bends eastward and the
terrace becomes wider and discontinuous, forming triangular shaped highs and
intervening lows.
Approximately 300 km of multichannel seismic reflection and gravity data along
and across the Queen Charlotte Fault off Dixon Entrance were collected as part of the
ACCRETE experiment in 1994. Structural interpretation of the five new profiles
reveals the presence of faults and folds within the terrace, which form an angle of 20°
to the strike of the Queen Charlotte Fault. The direction of these structures
corresponds to the trend of the plate boundary south of the bend and west of the Queen
Charlotte Islands, implying that through complex compression and shear, material must
have been carried from south to north along the margin during oblique plate motion.
Based on this observation and on forward gravity modeling, which places limits on the
possible plate configuration at depth, a four-dimensional model has been developed to
explain the temporal and spatial evolution of structural styles in this region.
Considering the amount of shortening that must be accommodated within the
past 5 Ma (a maximum of 100 km), a model of an underthrusting Pacific plate is
preferred over one of pure upthrusting. About 5-6 Ma ago, when transpression began,
oceanic crust was flexed and thrust upward at the plate boundary to eventually reach a
steady-state configuration of a subducting slab. Fractured basement rock and
consolidated, deformed sediments underlie the terrace and form its foundation. As a
result of strain partitioning, the terrace is now decoupled and moves both parallel to the
continent and perpendicular to the underthrusting Pacific plate. North of the bend in the
Queen Charlotte Fault, underthrusting north of it occurs obliquely along preexisting
fractures at the base of the terrace. The repetitive pattern of triangular terrace slivers is
the result of continuing uplift and shear along these trends. Active tectonism influences
sediment dispersal and creates traps.
A N-S trending fault was also identified in the trough segment and possibly
involves oceanic basement. Its origin is thought to be due to distributed shear that was
transmitted across the plate boundary. Sea-floor spreading magnetic anomalies trend
north-south as well. Along these zones of weakness, synthetic strike-slip faults of a
transpressional strain ellipse could has been initiated during early stages of subduction.
Reactivation of such faults may occur when oceanic crust approaches the outer terrace
boundary, as is the case in the study region.
Gravity modeling confirmed the existence of thin (24 km) continental crust and
an increase in oceanic Moho dip beneath the terrace, which is topped by unconsolidated
sediments and underlain by material of near-basement densities. It could not be
determined using gravity modeling whether oceanic crust exists beneath the continent,
but if it does, it must be welded to the North American plate in shallow subduction.Graduation date: 1998
41
Doguin, Pierre. "Crustal structure and faulting of the Gulf of California from geophysical modeling and deconvolution of magnetic profiles." Thesis, 1989. http://hdl.handle.net/1957/29154.
Full textAbstract:
Using gravity, magnetic, bathymetric and seismic
refraction data, I have constructed a geophysical cross-section
of the central part of the northern Gulf of California. The
section exhibits a crustal thickness of 18 km and features an
anomalous block of high density lower basement (3.15 g/cm³)
which probably resulted from rifting processes during the
opening of the Gulf. The magnetization of the upper
basement ranges from 0.0005 to 0.0030 emu/cm³. Three
different layers of sediments are modeled, ranging from
unconsolidated (1.85 g/cm³) to compacted (2.50 g/cm³).
I present a deconvolution method for automated
interpretation of magnetic profiles based on Werner's (1953)
simplified thin-dike assumption, leading to the linearization
of complex nonlinear magnetic problems. The method is
expanded by the fact that the horizontal gradient of the total
field caused by the edge of a thick interface body is
equivalent to the total field of a thin dike. Statistical decision
making and a seven point operator are used to insure good
approximations of susceptibility, dip, depth, and horizontal
location of the source. After using synthetic models to test
the inversion method, I applied it to the Northern Gulf of
California using data collected in 1984 by the Continental
Margins Study Group at Oregon State University. Fault traces,
computed by the deconvolution, are plotted on a map. The
faulting pattern obtained is in good agreement with that
proposed by other workers using other methods. The depths
to the top of the faults range from 4 to 5 km in the eastern
part of the Gulf, where they may be interpreted as the top of
the structural basement. Deeper estimates are obtained for
the western part of the Gulf.Graduation date: 1990
42
Stephenson, Andrew. "Crustal velocity structure of the Southern Nechako Basin, British Columbia, from wide-angle seismic traveltime inversion." Thesis, 2010. http://hdl.handle.net/1828/3145.
Full textAbstract:
In the BATHOLITHSonland seismic project, a refraction - wide-angle reflection survey was shot in 2009 across the Coast Mountains and Interior Plateau of central British Columbia. Part of the seismic profile crossed the Nechako Basin, a Jurassic-Cretaceous basin with potential for hydrocarbons within sedimentary rocks that underlie widespread volcanics. Along this 205-km-long line segment, eight explosive shots averaging 750 kg were fired and recorded on 980 seismometers. Forward and inverse modelling of the traveltime data were conducted with two independent methods: ray-tracing based modelling of first and secondary arrivals, and a higher resolution wavefront-based first-arrival seismic tomography. Gravity modelling was utilized as a means of evaluating the density structure corresponding to the final velocity model.
Material with velocities less than 5.0 km/s is interpreted as sedimentary rocks of the Nechako Basin, while velocities from 5.0-6.0 km/s may correspond to interlayered sediments and volcanics. The greatest thickness of sedimentary rocks in the basin is found in the central 110 km of the profile. Two sub-basins were identified in this region, with widths of 20-50 km and maximum sedimentary depths of 2.5 km and 3.3 km. Such features are well-defined in the velocity model, since resolution tests indicate that features with widths greater than ~13 km are reliable. Beneath the sedimentary rocks, seismic velocities increase more slowly with depth – from 6.0 km/s just below the basin to 6.3 km/s at ~17 km depth, and then to 6.8-7.0 km/s at the base of the crust. The Moho is interpreted at a depth of 33.5-35 km along the profile, and mantle velocities are high at 8.05-8.10 km/s.