DETACHMENT FOLDING WITHIN THE FORELAND BASIN OF FOLD-THRUST BELTS: AN EXAMPLE FROM THE SICHUAN BASIN, SOUTH CHINA

Although cryptic, finite strain deformation has been widely documented in the protothrust zone within the foreland basin of fold-thrust belts. Such shortening strains are frequently accommodated by thrust faults, detachment folding, and growth strata and thus carry useful information on the deformation history and basin evolution of the fold-thrust belt. The Bajiaochang detachment fold is an E-trending structure in the north-central Sichuan Basin, South China, about 100 km in the foreland basin of the Micang Shan fold-thrust belt to the north. The conventional area-depth analysis would give an erroneous estimate of the depth of the detachment, indicative of an open-system behavior. Our seismic interpretation shows multiple detachments, a plastically deformed core, duplex structures, a roof ramp, and growth strata, as well as a set of normal faults at its crest. We conducted bed-by-bed structural restoration to investigate its history. Two episodes of deformation can be identified. In the first episode, thrusts, backthrusts, duplex, and a gentle detachment fold developed during the Late Triassic to the Middle Jurassic, characterized by the presence of a roof ramp atop the detachment fold. After the Late Jurassic and most likely in the Cretaceous, this structure is reactivated during the second episode, and the detachment folding intensified with local flow in the basal layers and minor extension in the shallow layers.

We further examined the geologic maps of the entire Sichuan Basin and mapped more than 50 broad, gentle folds that can be interpreted as detachment folds. Three sets of folds can be identified on the map: the E-trending folds are superimposed by the NW-trending ones, which are further overprinted partially by the NE-trending ones. The Bajiaochang detachment fold, along with other E-trending folds, demonstrates the existence of a 185-km-wide thrust wedge within the Sichuan foreland basin during the Mesozoic Micang Shan intracontinental orogeny. This wedge is then partially superimposed by the Mesozoic Daba Shan intracontinental orogeny from the northeast. The youngest orogens, Longmen Shan and Huaying Shan, further superimpose some of the older fold-and-thrust structures, culminating in the complex fold geometry across the entire basin at present.