Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
2004 GSA Annual Meeting, Denver, Colorado, November 7–10, 2004
Preliminary Analysis of Thrust Kinematics in the Lower Congo Basin, Deepwater Southern Gabon
Richard E. Kilby, Martin P. A. Jackson, and Michael R. Hudec
Toe-of-slope fold-and-thrust belts detach on Aptian salt along much of the Atlantic passive margin off West Africa. However, little is known about their timing and kinematics. These topics have been investigated using a high-quality seismic volume in the Anton Marin and Astrid Marin lease blocks of the Lower Congo Basin. A 27 x 14km study area straddles the transition between updip extension and downdip thrusting. Proximal structures underwent initial extension or halokinesis overprinted by later shortening. Distal structures comprise regularly spaced thrust faults verging southward. The strong seaward vergence of the thrusts is attributed to decoupling over salt that had been greatly thinned by expulsion into passive diapirs before thrusting began.
Thrust kinematics and associated salt tectonics are recorded by thickness variations in synkinematic sediments. These variations were quantified by time-thickness jumps in 9 stratigraphic intervals along 39 traverses across 7 dominant thrust faults. Expansion indices (time-thickness ratios across thrust faults) identify the initiation and cessation of faulting for a specific fault and qualitatively record fault slip rates through time. Additionally, variations in expansion indices along strike reveal differential fault slip and yield qualitative slip vectors for each time interval. Older thrusts in the distal region decrease in fault slip towards a central point along each thrust. This decrease implies convergent slip, a pattern typical of a coastal re-entrant. Additionally, identification of an oblique-slip lateral ramp nearby suggests crowding during convergent slip. Comparison of expansion indices across this lateral ramp and proximal younger thrusts show that distal thrusting ended as proximal thrusting culminated in the Late Cretaceous or early Paleogene.
Proximal thrusts have relatively continuous expansion indices along strike implying parallel rather than convergent slip.