Michael R. Hudec1, Martin P. A. Jackson1, Leonor F. Binga2, Joaquim C. Da Silva2, Res Fraenkl3, Wytse Sikkema3
(1) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX
(2) Sonangol, Luanda, Angola
(3) Shell E&P Angola Rijswijk, Netherlands
ABSTRACT: Regional Restoration in the Offshore Kwanza Basin, Angola: Linked Zones of Extension, Translation, and Contraction
A 208-km-long dip line extending from the shelf break in Block 7 to the abyssal plain has been restored. Structural and stratigraphic patterns suggest at least 25 km of seaward translation down the continental slope, mostly in the Miocene to Recent. This estimate is consistent with timing and magnitude of extension on the continental shelf. In response to seaward translation, an allochthonous salt sheet at the downdip end of the system has advanced 25 km over the abyssal plain. We thus interpret the Angola Escarpment as the leading edge of a salt-lubricated contractional overthrust, which we term the Angola Salt Nappe.
The interpretation of allochthonous salt at the downdip end of the Kwanza Basin is based on several observations: (1) abyssal-plain sediment reflectors project under the base of the salt; (2) the width of the contractional allochthonous salt is similar to amounts of extension and translation observed farther landward; (3) the salt nappe is the only structure capable of absorbing the extension induced by raft tectonics occurring on the continental shelf.
More recently, seaward translation along the salt décollement was blocked above a basement step near the shelf break. Blocking caused diapirs landward of the step to shorten and be rejuvenated. Because translation is no longer being fed out into the downdip end of the system, the Angola Salt Nappe has ceased advancing and is now undergoing extensional collapse.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado