SMALL-SCALE CURVATURES IN THRUST BELTS: LINKING SUB-DECOLLEMENT BASEMENT STRUCTURES WITH THIN-SKINNED

The assumption of a low-angle, hinterlandward-dipping basal slope of the critical wedge eliminates consideration of the effects of an irregularly shaped basal detachment surface in the evolution of thrust belts. Geometry and kinematics of structural styles in the thin-skinned, unmetamorphosed Appalachian thrust belt of Alabama and Georgia may be directly and indirectly related to sub-décollement basement structures and the pre-deformational stratal architecture of the unit that hosts the regional décollement. In the leading and intermediate imbricates of Georgia and the leading imbricates in Alabama, low-amplitude fault-related anticlines form where depth to basement is shallow. Folds and faults in intermediate imbricates have local strike deviations which are not transmitted to either leading or trailing thrust sheets. In the intermediate imbricates in Alabama, high-amplitude fault-related anticlines form where the regional décollement is deep within the Birmingham basement graben; detachment folds nucleate above down-to-southeast basement faults with moderate vertical separation; and a mushwad (ductile duplex) evolves above a broad, deep graben containing a large volume of weak strata (i.e., thick shale-dominated succession). Small-scale curvatures of the thrust belt are also related to differences in basement elevation across basement transverse faults. Small-scale salients of the Helena fault and Gadsden mushwad are very abrupt, and they are related to a transverse basement fault that separates a narrow Birmingham graben on the northeast from a wider and deeper graben on the southwest. This observation suggests that a transverse basement fault primarily controlled the geometry of small-scale salients of the Rome, eastern Coosa, and Talladega faults. Abrupt curvatures are confined in transverse zones suggesting that distribution of northwest-striking basement faults and related differences in elevation of the top of basement exerted a primary role in the location of transverse structures in the thrust belt.