Imbricate thrust spacing: experimental and theoretical analyses
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Compressional features observed as thrust faults occur in the Gulf of Mexico's upper slope area. A compressional feature manifested as a thrust can be seen in the Ewing Bank area in Block 988. This toe structure is initiated by upslope tensional forces via a type of large-scale slump. Wells that penetrate this thrust given no direct indications of repeat sections; however, well correlations tied via paleo and seismic data demonstrate the reverse nature of the fault's throw. The thrust feature can be seen clearly on a set of 3-D migrated extracted lines, which allow enhanced definition and increased confidence in the interpretation. Time slices through the area allow a plan view of the thrust. The Ewing Bank thrust fault zone trends east-west. Paleoreconstruction indicates the main thrust grew through time and suggests salt withdrawal beneath the decollement, which allowed the funneling of sediment. The compressional force apparently originated from the north due to normal faulting located at the front of a salt sheet; another possible origin is a more regional transmission of stress. The top and bottom of the salt sheet is appropriately imaged by 3-D data. By restoring salt thicknesses to depth, the interpreter can better appreciate the area'smore » depositional and thrusting histories.« less
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A review of the timing and displacement evidence of the major structures of the western Wyoming Overthrust belt and foreland shows there is a progression in thrust displacement, apparent duration of motion, and palinspastic position of thrust traces from west to east. Those toward the west moved farther for an apparently longer period of time and are more widely spaced in their restored positions than those toward the east. However, average thrust velocities are all on the order of 0.5 ± 0.5 cm/yr (0.2 in./yr). Foreland events are in part synchronous with thrust belt events and had an effect on them. Although dating precision varies widely on major normal faults, present evidence does not contradict the generally held view that all normal faults postdate the youngest thrusting.
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The Monte Mountain thrust, a newly identified thrust exposed in the Timpahute Range, east central Nevada places porous Devonian reservoir rocks over rich Mississippian source rocks at the peak oil generating window. The thrust provides insurmountable evidence of a thrust model that may lead to discovery of giant oil and gas fields along the 400-mi long central Nevada thrust belt. The Timpahute Range lies a little over 50 mi on strike to the south of the prolific Grant Canyon field. Scattered remnants of the north-trending thrust belt are obscured by parallel valleys of Tertiary valley fill and volcanics. The fact that the east-west-trending Timpahute Range could contain exposures of the north-south-trending central Nevada thrust belt attracted them to the range, Familiarity with the stratigraphic section led to the discovery of the thrust. As much as 750 ft of Devonian Guilmette sandstones, in the hanging wall just above the thrust contact have been erroneously mapped as Mississippian Scotty Wash sandstones. These Devonian sandstones could be excellent reservoir rocks. Sandstones in the Guilmette increase in thickness westward. East-vergent thrusting has juxtaposed plates of thicker Guilmette sandstones with plates of thinner sandstones, Reconstruction of Devonian paleogeography provides a clue to the amount ofmore » displacement along thrust boundaries.« less
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This study investigates the influence of fault geometry, kinematics, and displacement on the exhumation history of the central Himalaya using geologic mapping constraints, a new interpreted cross-section, and a suite of 176 thermochronometer ages through the Marsyangdi valley in central Nepal. Guided by the cross-section, we integrate a forward model of fold-thrust belt evolution with a 2D thermokinematic model. Model-predicted and measured thermochronometer ages were compared to evaluate the sensitivity of thermochronometer ages to the geometry and location of structures, and their rates of deformation. Results indicate 84% of the measured data can be reproduced with a largely in-sequence system of faulting where displacement occurs on the Main Central thrust (MCT) from 23-16 Ma, the Ramgarh-Munsiari thrust (RMT) from 16-7.5 Ma, the Trishuli thrust (TT) from 7.5-6 Ma and the Main Boundary thrust (MBT) from 6-3 Ma. Our cross-section solution shows the development of a duplex that initiates at 4 Ma with the TT and MBT as the roof thrust. The duplex is translated over the Main Himalayan thrust (MHT) ramp, concurrent with forward propagation of faulting in the synorogenic Siwaliks from 3 Ma to present. Notably, the 2-0.5 Ma apatite fission-track ages between the MCT and Tethyan strata 45 km north of the MCT are not reproduced by the wide range of in-sequence deformation scenarios we explored. Instead, these data are consistent with simulations that include significant displacement on two out-of-sequence (OOS) faults in the last ∼1 Ma: 10 km of OOS faulting near the MCT followed by 5 km of displacement on a fault 15 km south of the MCT. Modeled OOS thrusting both builds significant topography in the hinterland and flexurally suppresses topography in the foreland. Consequently, our preferred kinematic solution has ∼10 km of final fault motion in the Siwaliks to rebuild topography at the MBT and the active MFT from ∼0.5 Ma to the present. Modeled exhumation rates are highly variable through time, and are highest during translation of rocks over high (∼10 km) ramps and during significant changes in architecture, such as duplex formation and OOS faulting. Notably high exhumation rates include 10.5 mm/yr during the first 4 Myr of MCT motion, 4-6 mm/yr at ∼7 Ma, due to the translation over an ∼10 km high TT ramp, and 7-12 mm/yr from 4 Ma to present driven by the development of the duplex and its translation over the MHT ramp, followed by OOS thrusting. The high (7-12 mm/yr) exhumation rates documented here from 4 Ma, correlate with a time of distinct climate change, including strengthening of the monsoon, and northern hemisphere glaciation, lending support to potential climate-tectonic feedbacks.
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