Jackson, Martin P. A.¹, Michael R. Hudec¹, David C. Jennette¹ 
(1) Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

ABSTRACT: Progressive Effects of Shortening Superposed on Formerly Extensional Diapirs and Faults: Insights from Serial Profiles in Deep-Water Lower Congo Basin, Gabon

Gravity-driven shortening in deep-water settings of passive margins can propagate landward into formerly extensional zones. Uplift of shelf and continental slope can also shorten previously extensional structures. Superposed shortening creates hybrid structures that are complex and difficult to interpret in isolation. However, the process is graphically illustrated by along-strike variations in 3-D seismic data. Because these variations range from barely visible shortening to extreme shortening, they provide helpful analogs for seismic interpreters working on highly shortened cover in which preshortening structures are difficult to recognize or analyze. Shortening increases nearer salt bodies because these form the weakest links in the system. Normal faults near a salt diapir can be highly inverted, unlike parts of the same faults only slightly farther away from the diapir. As a salt roller is squeezed, both hangingwall and footwall of the normal fault rise and rotate away from each other. The formerly downthrown hangingwall is thrust upward to override the squeezed salt roller. Grabens can be laterally squeezed near a rejuvenating diapir; the gentle syncline in the graben tightens as the graben is thrust onto the roof of an adjoining salt wall. Lateral squeezing rejuvenates diapirs by arching their roofs and steepening flanking strata. Flanks of triangular passive walls can become thrust surfaces during shortening. Complex structural bridges can link salt diapirs. Such zones contain structures produced by coeval extension (forming a salt roller), shortening (forming a buckle fold), and strike slip.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.