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AAPG Bulletin, V.
Mechanics of low-relief detachment folding in the Bajiaochang field, Sichuan Basin, China
1Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, Massachusetts 02138; [email protected]
2Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, Massachusetts 02138
3Burlington Resources, ConocoPhillips, 600 North Dairy Ashford, Houston, Texas 77079
Using three-dimensional (3-D) seismic reflection data, we apply new methods of two-dimensional (2-D) and 3-D structural restoration based on mechanical constraints to gain insights into the development of the Bajiaochang anticline, Sichuan Basin, China. This structure forms the trap for the Bajiaochang field, which is reported to contain substantial gas reserves within thick, Upper Triassic delta-front and lakeshore facies siliciclastics. The basal detachment lies in a Middle Triassic evaporite unit, above which layers are folded and faulted to various intensities based on their mechanical strengths. The new restoration methods enable us to test a variety of fault-displacement directions and to resolve that the Bajiaochang structure grew primarily by dip slip and contractional folding. Moreover, we quantify the styles and amounts of shortening within various tectonic levels of the structure. Large amounts of shortening and ductile thickening at the basal detachment level are consistent with general detachment fold models. However, we demonstrate that slip on the basal detachment is much larger than would be predicted by these models, implying that substantial slip on the basal detachment extends beyond the structure into the foreland. Moreover, structural thickening in the core of the fold is localized by a series of small ramps in the basal detachment that produce structural imbrication. This implies that the Bajiaochang detachment fold grew by a mechanism akin to duplexes, which has important implications for the manner in which shortening and strain are partitioned within different mechanical layers of the fold.
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