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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 construction.

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