ON THE ASYMMETRY OF TURTLE-BACK GROWTH ANTICLINES

Turtle-back anticlines are common structures that develop between faci ng listric growth faults rooted in a flat lying decollement. Seismic e xamples from the South Atlantic margins show that whereas their extern al shape can by symmetric, their internal structure is more generally asymmetric. Laboratory experiments on layered sand silicone models des igned to study gravity gliding are used to examine the mode of develop ment of turtle-back anticlines. The results provide information on the ways turtle-back asymmetry develops. The comparison between nature an d experiments leads to the following conclusions. The fact that turtle -back anticlines often start symmetric and progressively become asymme tric is due to an increase in coupling between overlying brittle sedim ents and ductile (salt) layers. The increase in coupling is also respo nsible for the transition in time and space from dominant seaward faul ting to landward faulting for the turtle-back anticlines which never p ass through a symmetric stage. The salt diapirs, which separate blocks of the sedimentary cover rafted above the basal decollement at early stages of gravity gliding can play a significant role in the developme nt of turtle-back anticlines. As suggested by previous authors, the di apirs first rise and then fall. Our experiments confirm that diapir fa ll favours the wrapping of rafted blocks by synkinematic layers. (C) 1 998 Elsevier Science Ltd. All rights reserved.