3D Structural Analysis and Visualization of Oblique-Ramp Accommodation Structures in Thrust Sheets

Peter H. Hennings, Bruce H. Prine, and John S. Wickham

We have observed numerous examples of complex 3D deformation associated with oblique ramps near abrupt thrust terminations. The structures are restricted to hanging-walls of thrust sheets and are associated with zones of rapid loss in stratigraphic separation and/or large displacement gradients along strike. These structures produce a complex but characteristic pattern of folds and/or backthrusts and tear faults that intersect the main thrust at angles up to 90°. We call these features oblique-ramp accommodation structures.

Conventional 2D restoration techniques fail to accurately model oblique-ramp accommodation structures because their transport direction often differs significantly from that of the main thrust. We have analyzed a suite of these structures by constructing 3D computer models using GeoSec 3D. Palinspastic restoration was performed using an equal area, directional flattening algorithm that is based on the azimuth of local structural vergence. The three-dimensionally restored hanging-walls were used to determine the pre-thrust geometry of their corresponding footwalls. In each case the resulting footwall contains an abrupt bend in the strike of the main thrust adjacent to the oblique-ramp accommodation structures in the hanging-wall.

Oblique-ramp accommodation structures are the manifestation of local out-of-plane motion in the hanging-wall as it passes over segments of the footwall ramp that are oblique to the overall transport direction. They indicate that the termination of the main thrust is probably nearby. They can significantly affect trap geometry by compartmentalizing thrust closures, and can produce local structural highs along main hanging-wall anticlines.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California