Masters Theses

Date of Award

12-1992

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

William M. Dunne

Committee Members

Bob Hatcher, Charlie Onasch, Steve Driese

Abstract

The transition between southern and central Appalachian foreland thrust system is a region subjected to at least two noncoaxial deformations, where typical two-dimensional analysis such as cross section balancing or plane-strain retrodeformation cannot be applied. Three-dimensional factorization can simulate the noncoaxial strain history. Simulation results for compaction and layer-parallel shortening (LPS) were compared to finite strains of 16 quartz arenite samples from the Tuscarora Sandstone. Successful simulations indicate: in the southern Appalachians, 30-35% total compaction by volume loss including a 4.5-9% E-W horizontal shortening during lithification and 5% LPS along a 150°-330° by plane strain or axial symmetric flattening; in the central Appalachians, 30-35% total compaction by volume loss including a 3-7% N-S horizontal shortening during lithification and 10% along 120°-300'' LPS by plane strain or axial symmetric flattening; in the transition zone, 30% compaction by volume loss including a 3% EW horizontal shortening during lithification, 5% southern LPS along 150°-330° by axial symmetric flattening, and 5-10% central shortening along 130°-140° by plane strain or axial symmetric flattening at 10° to normal trend. From the simulations, horizontal shortening in the southern Appalachians during lithification in the Mississippian was greater than later Alleghanian deformation. This shortening probably represents a very early LPS developed in unlithified sand that required less deviatoric stress to deform than the cemented quartz arenites during later tectonic deformation. Also, the oblique and weaker central tectonic LPS in the transition zone probably represents behavior at the limits of central Appalachian deformation. Finally, the LPS simulations indicate that axial symmetric flattening (oblate strain) is more successful than plane strain as the representative behavior, so caution is recommended in assuming plane strain as the dominant strain behavior in orogenic forelands. Also, at the transition from the southern to central Appalachians, southern Appalachian thrust faults lose surficial displacement and terminate into a central Appalachian roof decollement in the Middle Ordovician Martinsburg Formation. These terminations are found at regional anticlines that plunge northeast. Previous workers interpreted one of these anticlines, the Sinking Creek anticline, to result from ramp formation followed by delamination of basal hanging wall stratigraphy which is placed in a footwall triangular region with concurrent folding of footwall stratigraphy and break-thrusting through the anticlinal core. This complex deformation formed the anticline by emplacement of hanging wall stratigraphy into the footwall triangular region. As such, the anticline absorbs no displacement from the terminating Saltville thrust and the break thrust absorbs all of the decreasing displacement. An alternative interpretation of the Sinking Creek anticline is a fault-tip anticline above a ramp-and-roof flat with a second-order break thrust in the anticlinal core. The anticline and break thrust absorb a portion of the displacement, but most displacement is transferred to the roof thrust. As displacement decreases, the anticline opens, the break thrust disappears from the core, and more displacement is transferred to the floor thrust in the system. Supporting evidence for this interpretation includes: (1) new detailed mapping by Bartholomew and others, showing hanging-wall cutoffs for the roof flat beneath the entire anticline; (2) seven new cross sections along the thrust termination illustrating the new mapping data downplunge into the subsurface and demonstrating the viability of the interpretation laterally along the termination; and (3) new finite strain data and mesostructural data that show a deformation sequence consistent with predictions of the new interpretation. The interpretation is applicable at the termination of another major southern Appalachian thrust, the St. Clair, which has a similar geometry to the Saltville thrust termination and Sinking Creek anticline.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS