CONTROLS ON SMALL-SCALE, UPPER-LEVEL DETACHMENTS BY MECHANICAL STRATIGRAPHY AND STRUCTURAL GEOMETRY OF LARGE THRUST SHEETS

Within large-scale thrust-belt structures, smaller scale structures are controlled by the geometry of the larger structures and by mechanical properties of the stratigraphy, such as upper-level detachment horizons. In the southern Appalachians, the geometry of the large-scale structures is defined by a Cambrian weak, shaly layer in the lowermost part of the cover succession and an overlying Cambrian-Ordovician stiff carbonate unit. The Cambrian shales host a regional décollement, and large frontal ramps cut through the cover strata to the surface. The Cambrian-Ordovician stiff carbonate constrains the wavelength of the large frontal ramps.

In the Appalachian thrust belt in Alabama, the Cahaba synclinorium forms the trailing part of the Jones Valley thrust sheet, the trailing cutoff of which is the Helena thrust fault and frontal ramp. The trace of the Helena fault curves abruptly at a lateral ramp, forming a small-scale (sub-regional) recess. The width of the Jones Valley thrust sheet (and Cahaba synclinorium) varies along strike across the lateral ramp that defines the small-scale recess of the Helena fault. In the small-scale recess of the Helena fault, the regional stiff layer defines a long, gently dipping frontal ramp and a wide trailing flat of the Jones Valley thrust sheet.

Stratigraphically above the regional stiff layer, a Pennsylvanian shale-coal-sandstone-conglomerate succession includes thin, alternate weak (shale-coal) and stiff (sandstone-conglomerate) layers. Within this succession, several low amplitude folds, as well as an emergent thrust fault along the hinge of a moderately tight anticline, are disharmonic with the underlying regional stiff layer. The succession is overturned in the immediate footwall of the Helena fault. The upper-level disharmonic folds and small-scale faults indicate upper-level detachments in the shale and coal succession. The large-scale, trailing thrust flat beneath the expanse of upper-level folds within the recess of the Helena fault (in the Cahaba synclinorium within the Jones Valley thrust sheet) is wider than the trailing flat of the Jones Valley thrust sheet northeast along strike from the lateral ramp of the Helena fault.