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Abstract


 
Chapter from: M 65:  Salt Tectonics: A Global Perspective
Edited By 
M.P.A. Jackson, D.G. Roberts, and S. Snelson

Authors:
Peter R. Cobbold, Peter Szatmari, L. Santiago Demercian, Dimas Coelho, and Eduardo A. Rossello

Structure, Tectonics, Paleostructure

Published 1995 as part of Memoir 65
Copyright © 1995 The American Association of Petroleum Geologists.   All Rights Reserved.
 

Cobbold, P. R., P. Szatmari, L. S. Demercian, D. Coelho, and E. A. Rossello, 1995, Seismic and experimental evidence for thin-skinned horizontal shortening by convergent radial gliding on evaporites, deep-water Santos Basin, Brazil, in M. P. A. Jackson, D. G. Roberts, and S. Snelson, eds., Salt tectonics: a global perspective: AAPG Memoir 65, p. 305-321.
Chapter 14
Seismic and Experimental Evidence for Thin-Skinned Horizontal Shortening by Convergent Radial Gliding on Evaporites, Deep-Water Santos Basin, Brazil

 

Peter R. Cobbold

Géosciences-Rennes
Université de Rennes
Rennes, France

Peter Szatmari

L. Santiago Demercian

Petrobras Research Center
Cidade Universitária
Rio de Janeiro, Brazil

Dimas Coelho

Petrobras Exploration Department
Rio de Janeiro, Brazil

Eduardo A. Rossello

Géosciences-Rennes 
Université de Rennes
Rennes, France

Present address:

Departamento de Ciencias Geológicas
Universidad de Buenos Aires
Buenos Aires, Argentina
 

Abstract


Thin-skinned gravitational gliding of sediments above a detachment layer of salt or shale is common on passive margins. Changes in surface slope result in a domain of extension upslope and a domain of contraction downslope. Contractional domains tend to occur under present-day deep water and are thus not well understood.

In the deep-water Santos Basin, Brazil, a contractional domain contains a suite of salt-cored structures. Angular folds (chevron and box folds), as well as concentric folds, are common in the upper part of the Aptian evaporite sequence, which appears to comprise alternating layers. In general, angular and concentric folds form by flexural slip during shortening of mechanically layered sequences. Their occurrence in the Santos Basin is evidence in favor of horizontal contraction. The lower part of the Aptian evaporite sequence appears to be mostly rock salt. It has been squeezed out from under synclines into spaces created by growing anticlines. In places, the layered evaporite sequence has been thickened or even repeated across thrust faults and ramp anticlines. An overlying sequence of open-marine sediments has been condensed or eroded over anticlines but forms local depocenters. These depocenters are asymmetric (of foreland style) next to isolated thrusts but symmetric in synclines or between thrusts of opposite vergence.

The structural styles have been reproduced in physical models, properly scaled for gravitational forces, in which salt is represented by silicone putty and sediments are represented by sand. The models were shortened horizontally by a screw jack. The experiments illustrate the importance of horizontal contraction and syntectonic sedimentation in shaping salt-cored structures. They have been used to establish criteria that may be diagnostic of contraction.

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