Browsing by Subject "Plate tectonics"
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Item The Arequipa-Antofalla Basement, a tectonic tracer in the reconstruction of Rodinia(2002-12) Loewy, Staci Lynn; Connelly, James N.The Arequipa-Antofalla Basement (AAB), an anomalous Proterozoic block along the central Andean margin of South America, may be critical to reconstruction of the Meso-Neoproterozoic supercontinent, Rodinia. Dalziel (1994) proposed that the AAB is an allochthonous block, transferred to Amazonia from northeastern Laurentia as a result of ca. 1.0 Ga collision between the two continents during the amalgamation of Rodinia. New U/Pb geochronology and whole-rock isotope geochemistry from the AAB and northeastern Laurentia are used to test this proposed correlation. Based on U/Pb ages, growth polarity, sequence of age provinces, and whole-rock Pb isotopic signatures I conclude: 1) the AAB is a single basement block that formed through progressive growth within a larger continent, 2) neither Amazonia nor Laurentia were its parent craton, and 3) the AAB accreted to Amazonia during the ca. 1.0 Ga Sunsás Orogeny. Thus, the data do not support the proposed collision between Amazonia and northeastern Laurentia. Instead, the characteristics of the AAB are consistent with derivation from Kalahari. Moreover, critical examination of existing Pb data also reveals that the Pb signature of the isotopically distinct ca. 1.1 Ga basement of the southern and central Appalachians matches that of southwestern Amazonia. Based on these potential correlations, I propose an alternate position for Amazonia within Rodinia, adjacent to southeastern Laurentia and western Kalahari. Evaluation of Pb isotopic signatures from juvenile rock units of large Precambrian age-provinces suggests that they were derived from two isotopically distinct mantle reservoirs. These reservoirs may be the precursors of those that exist today, a normal reservoir in the northern hemisphere and the DUPAL reservoir in the southern hemisphere. If true, the Pb isotopic composition of juvenile rocks in Precambrian age-provinces may indicate the hemisphere in which the province formed. Coherency of Pb isotopic signatures across juvenile age provinces and the existence of two isotopically distinct reservoirs permit the use of Pb isotopes to correlate transferred terranes with potential parent cratons. If Pb isotopic compositions of juvenile Precambrian rocks are indicative of the hemisphere in which they formed, Pb isotopes may also be used to resolve ambiguity in polarity of paleomagnetic data.Item Continental extension in orogenic belts : modes of extension, origin of core complexes, and two-phase postorogenic extension(2016-05) Wu, Guangliang, Ph. D.; Lavier, Luc LouisContinental extension principally occurs in orogenic belts, however, most of numerical simulations use uniform crust that cannot represent an orogenic belt. We simulate lithospheric extension in an orogenic hinterland approximated by a crustal wedge. We first show that the presence of a preexisting weak mid-crustal shear zone dipping at low angle exerts a critical control on whether crustal and mantle deformation are decoupled or coupled. When the lower crust and the mid-crustal shear zone are weak, decoupling occurs and crustal deformation is compensated by lower crustal flow. When the lower crust is strong or a weak shear zone is absent, coupling occurs and crustal deformation is compensated by flow in the mantle. By varying the strength of the lower crust and the weak shear zone in numerical lithospheric extension experiments, we examine structures developed and compare them with structures observed in extended and collapsed orogenic belts. In models with a weak mid-crustal shear zone, we find that decoupling is particularly effective. In these models, we distinguish three modes of extension: 1) localized, asymmetric crustal exhumation in a single metamorphic massif with a weak lower crust, 2) the formation of rolling-hinge normal faults and the exhumation of lower crust in multiple metamorphic core complexes with an intermediate strength lower crust, and 3) distributed domino faulting over the weak mid-crustal shear zone with a strong lower crust. In models without a mid-crustal shear zone, extension is coupled and structures similar to those observed in continental margins form. We further analyze my model to better explain and understand the core complexes and low-angle normal faults which develop when a preexisting weak mid-crustal shear zone is present. We define three types of detachment systems and present four models which produce core complexes that bear striking resemblance to natural examples: 1) bivergent core complexes, 2) metamorphic core complexes, 3) boudinage structures, and 4) flexural core complexes. We also discuss intracrustal isostasy and the thermal history of material particles sampled in modeled detachment. Finally, based on a geological and geophysical synthesis and using numerical simulations, we propose a two-phase postorogenic extensional scenario that approximates the evolution and the structures observed in the South China Sea margins.Item Crustal Structure of the Central High Plains of Texas from Rayleigh Wave Dispersion(Texas Tech University, 1972-12) Stanton, James CliffordNot Available.Item Deformation mechanisms along active strike-slip faults : SeaMARC II and seismic data from the North America-Caribbean plate boundary(1992-05) Tyburski, Stacey Ann; Muehlberger, William R.The northwest part of the North America-Caribbean plate boundary zone is characterized by active, left-lateral strike-slip faults that are well constrained seismically and are corroborated by on- and offshore geologic mapping. The onshore plate boundary zone comprises the Motogua and Polochic fault systems of southern Guatemala which join and continue offshore as the Swan Islands fault zone along the southern edge of the Cayman trough. At the Mid-Cayman spreading center in the central Caribbean Sea, the fault motion is transferred at a 100 km wide left-step in the fault system to the Oriente fault zone. A third system, the Walton fault zone, continues east from the Mid-Cayman Spreading center to define the Gonave microplate. Seafloor features produced by strike-slip faulting along the Swan Islands and Walton fault zones have been imaged and mapped using the SeaMARC II side-scan sonar and swath bathymetric mapping system, single-channel seismic data, multichannel seismic data and 3.5 kHz depth profiles. Structures mapped along the Swan Islands and Walton fault zones include: 1) twenty-six restraining bends and five releasing bends ranging in size from several kilometers in area to several hundred kilometers in area; 2)en echelon folds which occur only within the restraining bends; 3) straight, continuous fault segments of up to several tens of kilometers in length; 4) restraining and releasing bends forming in "paired" configurations; and 5) a fault-parallel fold belt fold and thrust belt adjacent to a major restraining bend. The features observed along the Swan Islands and Walton fault systems are compared to other features observed along other strike-slip fault systems, from which empirical models have previously been derived. Based on the features observed in these strike-slip systems, a rigid plate scenario is envisioned where the geometry of the fault and the direction of plate motion have controlled the types of deformation that have occurred. In a related study, microtectonic features in an area of Neogene extension within the northwestern Caribbean plate were investigated in order to provide insight on the nature of intraplate deformation related to the motion along the plate boundary. Microtectonic features were measured in the Sula-Yojoa rift of northwestern Honduras with the intention of inverting the data to estimate stress states responsible for the observed strains. Data inversion for the estimation of stress states could not be undertaken with the available measurements, however, the observations made can be used to support several existing models for the intraplate deformation as well as to encourage the elimination of other models.Item Geological Modeling of Dahomey and Liberian Basins(2010-01-16) Gbadamosi, Hakeem B.The objective of this thesis is to study two Basins of the Gulf of Guinea (GoG), namely the Dahomey and the Liberian Basins. These Basins are located in the northern part of the GoG, where oil and gas exploration has significantly increased in the last 10 years or so. We proposed geological descriptions of these two Basins. The key characteristics of the two models are the presence of channels and pinch-outs for depths of between 1 km and 2 km (these values are rescaled for our numerical purposes to 600- m and 700-m depths) and normal faults below 3 km (for our numerical purposes we use 1 km instead of 3 km). We showed that these models are consistent with the plate tectonics of the region, and the types of rocks and ages of rocks in these areas. Furthermore, we numerically generated seismic data for these two models and depth-migrated them. We then interpreted the migrated images under the assumption that the geologies are unknown. The conclusions of our interpretations are that we can see clearly the fault systems in both models. However, our results suggest that seismic interpretations of the channels and pinch-outs associated with the geology of the Dahomey and Liberian Basins will generally be difficult to identify. In these particular cases, we missed a number of channels and pinch-outs in our interpretations. The limited resolution of seismic images is the key reason for this misinterpretation.Item High-temperature deformation in the Josephine Peridotite: linkage between crustal and mantle spreading structure in a supra-subduction zone ophiolite(Texas Tech University, 2003-08) Salisbury, MichaelThe Josephine peridotite is a tectonized harzburgite that represents the upper mantle beneath a paleo-spreading ridge (Dick, 1976; Loney and Himmelberg,1976; Dick and Sinton, 1979). Structures in the peridotite such as layering domains, foliations, isoclinal folds, and lower shear strain LPOs related to foliations which cross cut modal layering, provide a framework for a model of upper mantle flow beneath a spreading ridge. Layers within the Josephine peridotite have ubiquitously shallow dips and can be divided into three distinct domains. Plastic foliations within the Josephine peridotite are divided into four types on the basis of geometry with respect to modal layering: (1) parallel to modal layering (2) in layer domain boundaries (3) cross cutting layering within layer domains and (4) adjacent to tabular dunites. The foliation types can be further distinguished from each other on the basis of microstructural observations, LPO pattern and lineation type. On the basis of lineation data and cross cutting relationships, vertical shortening and horizontal extension occurred first during upwelling. Shear on steeper dipping lower temperature foliation planes occurred probably during spreading. Group one foliations and lineations are shallowly dipping/plunging in comparison with groups two and three foliations/lineations. Layer domains are bounded by high-angle shear zones. Domain boundaries are also characterized by large dunite bodies and may have provided pathways for melt migration. At least two temperature regimes are documented in the olivine LPO. Imbricate olivine neoblasts are the result of high temperature (a1200°C), high shear strain (a1.4) deformation (Zhang et al., 2000). Samples with olivine ribbons have LPO patterns that support lower temperature (si 100°C), lower shear strain (sO.8) deformation (Zhang et al., 2000). The observations from this work support the interpretation that km-sized blocks of mantle harzburgite were vertically shortened and horizontally extended at temperatures a 1200°C. Boundaries behween blocks are defined by dunite shear zones that accommodated the movement of individual blocks in relation to each other and acted as zones of accumulation and transport of basalt through the upper mantle. Lithosphere-scale 'boudinage' of the upper mantle may have occurred beneath the ridge axis during plate separation. Lineation patterns broadly reflect north south extension in the mantle, consistent with extension directions in the crustal section.Item Mass transport processes and deposits in offshore Trinidad and Venezuela, and their role in continental margin development(2007-05) Moscardelli, Lorena Gina, 1977-; Wood, Lesli J.Mass transport complexes (MTC) form a significant component of the stratigraphic record in ancient and modern deep water basins. One such basin, the deep marine margin of eastern offshore Trinidad, situated along the obliquely converging boundary of the Caribbean and South American plates and proximal to the mouth of the Orinoco River, is characterized by catastrophic shelf margin processes, intrusive and extrusive mobile shales, active tectonics and possible migration and sequestration of hydrocarbons. Major structural elements found in the deep water slope regions include: large transpressional fault zones along which mobile shales extrude to form seafloor ridges; fault-cored anticlinal structures overlain by extrusive seafloor mud volcanos; shallow-rooted sediment bypass grabens near the shelf break; and normal and counterregional faults. A data volume consisting of 10,708 km2 of several merged 3D seismic data volumes enable subseafloor interpretation of several mass transport event deposits and the erosional surfaces that form their boundaries. The data shows numerous mass transport complexes which are characterized by chaotic, mounded seismic facies and fanlike geometries. Their extent (up to 2017 sq. km) and thickness (up to 250 m) is strongly influenced by seafloor topography. Depositional and erosional architectures identified with these units includes: large magnitude lateral erosional edges, thrust faulting, linear basal scours, side-wall failures, flow geometries, possible displaced blocks and chaotic matrix material. Active tectonism in the region, high sedimentation rates associated with the Orinoco Delta System, and abundant unstable gas hydrates suggest the presence of higher frequency mechanisms at work for MTCs generation than sea-level fluctuations alone. Three types of mass transport complexes are identified in offshore Trinidad; shelfattached systems that were fed by shelf edge deltas whose sediment input is controlled by sea level fluctuations, slope- attached systems which occur when upper slope sediments catastrophically fail due to gas hydrate disruptions, earthquakes and/or storm activity, and locally detached systems formed when local instabilities in the sea floor trigger small collapses. Such classification of the relationship between slope mass failures and the sourcing regions enables an understanding of the nature of initiation, length of development history, petrography and petrophysics of MTC’s. In addition, a collection of morphometric parameters of MTCs from different continental margins are analyzed in order to better understand their causal mechanisms, and to establish whether systematic morphometric parameters characterize these deposits across different tectonic settings. Observations suggest that there is a clear relationship between morphometric parameters of MTC and their causal mechanisms.Item Mass transport processes and deposits in offshore Trinidad and Venezuela, and their role in continental margin development(2007) Moscardelli, Lorena Gina; Wood, LesliMass transport complexes (MTC) form a significant component of the stratigraphic record in ancient and modern deep water basins. One such basin, the deep marine margin of eastern offshore Trinidad, situated along the obliquely converging boundary of the Caribbean and South American plates and proximal to the mouth of the Orinoco River, is characterized by catastrophic shelf margin processes, intrusive and extrusive mobile shales, active tectonics and possible migration and sequestration of hydrocarbons. Major structural elements found in the deep water slope regions include: large transpressional fault zones along which mobile shales extrude to form seafloor ridges; fault-cored anticlinal structures overlain by extrusive seafloor mud volcanos; shallow-rooted sediment bypass grabens near the shelf break; and normal and counterregional faults. A data volume consisting of 10,708 km2 of several merged 3D seismic data volumes enable subseafloor interpretation of several mass transport event deposits and the erosional surfaces that form their boundaries. The data shows numerous mass transport complexes which are characterized by chaotic, mounded seismic facies and fanlike geometries. Their extent (up to 2017 sq. km) and thickness (up to 250 m) is strongly influenced by seafloor topography. Depositional and erosional architectures identified with these units includes: large magnitude lateral erosional edges, thrust faulting, linear basal scours, side-wall failures, flow geometries, possible displaced blocks and chaotic matrix material. Active tectonism in the region, high sedimentation rates associated with the Orinoco Delta System, and abundant unstable gas hydrates suggest the presence of higher frequency mechanisms at work for MTCs generation than sea-level fluctuations alone. Three types of mass transport complexes are identified in offshore Trinidad; shelfattached systems that were fed by shelf edge deltas whose sediment input is controlled by sea level fluctuations, slope- attached systems which occur when upper slope sediments catastrophically fail due to gas hydrate disruptions, earthquakes and/or storm activity, and locally detached systems formed when local instabilities in the sea floor trigger small collapses. Such classification of the relationship between slope mass failures and the sourcing regions enables an understanding of the nature of initiation, length of development history, petrography and petrophysics of MTC’s. In addition, a collection of morphometric parameters of MTCs from different continental margins are analyzed in order to better understand their causal mechanisms, and to establish whether systematic morphometric parameters characterize these deposits across different tectonic settings. Observations suggest that there is a clear relationship between morphometric parameters of MTC and their causal mechanisms.Item Mechanics of lithospheric delamination in extensional settings.(2015-03-23) Jex, Jeffrey A. 1988-; Dunbar, John A., 1955-Delamination, the foundering of the lower crust and sub-crustal lithosphere, is one of the most important geodynamic processes that is still poorly understood. Geodynamic modeling has constrained conditions and likely outcomes of delamination in orogenically-thickened crust. In this study, I do the same for delamination in extensional settings by using finite element models of young passive margins. Delamination in these models may occur as melt beneath oceanic crust intrudes between the lower continental crust and sub-crustal lithosphere, driven by buoyancy. When sufficient melt is available and the lower crust is weak, the melt wedges between the lower crust and sub-crustal lithosphere, initiating delamination of the sub-crustal lithosphere. The speed of delamination is strongly dependent on weakness of the lower crust followed by the amount of melt present.Item Rayleigh wave model of crustal structure of northeaster Mexico(Texas Tech University, 1978-08) Pinkerton, Roger ParrishNot available