Graduate Thesis Or Dissertation
 

Structural interpretation of seismic reflection data from the eastern Salt Range and Potwar Plateau, Pakistan

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3n204241z

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  • Approximately 1600 km of seismic reflection profiles from the eastern Salt Range and Potwar Plateau (SR/PP) of Pakistan are integrated with available magnetostratigraphic, surface geologic and well data, to categorize structural styles, determine the timing of deformation and estimate the amount of telescoping of the sedimentary cover. The eastern SR/PP is similar to other fold-and-thrust belts underlain by evaporites in that: 1) it is part of a zone of overthrusting that extends considerably farther over the Himalyan foreland than adjacent areas not underlain by evaporites; 2) the overall thrust wedge has a narrow cross-sectional taper; 3) structures verge toward the hinterland as well as toward the foreland; and, 4) fold trends are long and continuous, consisting of tight, salt-cored anticlines separated by broad synclines. Disharmonic folding of the sedimentary section relative to the underlying basement is due to effective decoupling along the intervening salt layer. Subsurface mapping on top of a strongly reflective package of Cambrian to Eocene strata reveals that many surface folds are cored by both foreland- and hinterland-dipping, blind thrusts, and some are fault propagation folds. In some cases, intersecting thrusts result in local triangle zones; other surface folds have a pop-up geometry. The dip of the basement towards the inner part of the fold-and- thrust belt is relatively gentle in the eastern SR/PP (1°-l.5°) compared to the central SR/PP (2°-3°) . Mechanical considerations demonstrate that, unlike the relatively undeformed central SR/PP, a broad deformational zone has developed in the eastern SR/PP to provide a surface topographic slope necessary to maintain a critical taper of the thrust wedge. Furthermore, previous paleomagnetic studies indicate that deformation across much of the eastern PP preceded tectonic rotation. This implies that individual structural trends developed perpendicular to the transport direction and were then rotated into their current NE-SW alignment in response to ramping over a basement buttress in the central SR/PP. Cross-section balancing indicates that approximately 23.1 km of shortening has occurred across the foreland in the eastern SR/PP since 5.5 Ma, 17.8 km in the last 2.5 Ma. The shortening rate of 7 mm/yr for that time interval is roughly 15% of the 40-50 mm/yr convergence rate between the Indian and Eurasian plates.
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