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Gravity modeling of Cenozoic extensional basins, offshore Vietnam
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Integrating Bouguer gravity and satellite-derived free-air gravity data with published geological and geophysical data allows modeling crustal structure and estimating crustal extension for the hydrocarbon bearing Mekong and Song Hong - Yinggehai (Yinggehai) basins. Gravity modeling results provide important clues to the controversial tectonic development of Southeast Asia during the Tertiary. Combined Bouguer and free-air gravity maps and residual gravity anomaly maps were generated for the study areas. Procedures for estimating NW/SE linear crustal extension were determined from basinal geometric constraints which permitted two- dimensional gravity modeling of the Mekong basin but required three-dimensional gravity modeling of the Yinggehai basin. Crustal thickness estimates (not including sediment thickness) range from 15 to 21 km beneath the Mekong basin and 4 to 12 km beneath the Yinggehai basin. These estimates are presently unconfirmed by deep crustal seismic information. However, the idea of substantial crustal thinning beneath these basins is supported by high geothermal gradients in both basins. Formation of the Mekong basin is believed associated with a Paleocene to Oligocene episode of left-lateral movement on the NW/SE trending Mae Ping fault. Gravity modeling yields NW/SE linear crustal extension estimates ranging from 20 to 27 km for the basin. Combining these estimates with Marshall's (1991) estimates for the Malay, West Natuna, and Nam Conson basins using simple vector addition yields a total of 119 km of NNW/SSE linear crustal extension for all the major extensional basins associated with the Three Pagodas and Mae Ping fault systems. This result implies that Tertiary aged left-lateral movement on these fault systems was less than half the 300 km suggested by Tapponnier et al. (1986). The Yinggehai basin is believed to be a large pull-apart which opened near the southeast extremity of the NW/SE trending Red River fault during the Oligocene. Gravity modeling results from the Yinggehai basin yield estimates of linear crustal extension parallel to the Red River fault ranging from 133 to 191 km. If this basinopened under left-lateral motion on the Red River fault, these results suggest the magnitude of strike-slip motion was much less than the 500 km suggested by Tapponnier et al. (1986). However, the Yinggehai basin's rhombic geometry and major normal fault patterns suggest the Yinggehai basin opened under right-lateral motion on the Red River fault. In this case, crustal extension in the Yingggehai basin records a major episode of right-lateral motion on the Red River fault prior to the left-lateral motion observed by Tapponnier et al. (I 990).
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Includes bibliographical references.
Mauri, Steven Joseph (1993). Gravity modeling of Cenozoic extensional basins, offshore Vietnam. Master's thesis, Texas A&M University. Available electronically from
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