Three-dimensional geometry and kinematic evolution of the Pine Mountain thrust system, southern Appalachians

Abstract

The Pine Mountain thrust system is the westernmost major structure in the southern Appalachian Valley and Ridge in Virginia, Tennessee, and Kentucky. Surface, subsurface, and seismic reflection data have been integrated to construct a series of balanced cross sections and interpret the three- dimensional geometry and kinematic evolution of the structure. The structure is subdivided into three main areas on the basis of structural style. In the southwestern and northeastern areas, the structure consists of a simple ramp- related anticline, the Powell Valley anticline, related to a ramp in the Pine Mountain thrust from the Cambrian Rome to the Devonian Chattanooga Formation. In the central oil-producing area, the Pine Mountain thrust is exposed in several erosional fensters, as a result of folding by one or more subthrust imbricates. Here, the Pine Mountain thrust develops a double-ramp geometry, with an intermediate flat in the Cambrian Maynardville Formation. The structure varies from a system of overlapping anticlines in the Brooks and Martin Creek fenster sections, to two anticlines separated by an intermediate syncline in the Bethel and Sulphur Springs fenster sections, to a partially overlapping duplex- imbricate system in the Big Stone Gap section. The total shortening associated with the Pine Mountain thrust system decreases from about 70,000 ft (21.3 km) at the southwest end to less than 10,000 ft (3.0 km) at the northeast end. In the central fenster area, the total shortening is distributed between the Pine Mountain thrust and its subthrust imbricates. The structural elevations in both the Pine Mountain and Bales I sheets are greatest in the central fenster area. The greatest structural elevations in the Bales I sheet are in the Rose Hill and Ben Hur oil fields. Palinspastic restorations of the thrust system and the construction of detailed fault maps indicate that variations in structural geometry and shortening along strike are accommodated by interactions between thrust faults and oblique ramps or tear faults. The variations in three-dimensional geometry can be explained in terms of combinations of three main mechanisms: unequal hanging-wall displacements over a single footwall ramp, equal displacements over a footwall ramp displaced along an oblique ramp, and displacement of part of a footwall ramp due to movement on one or more frontal thrusts.