Some geometrical characteristics of inversion

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Abstract
Summary: The inversion of extensional fault systems results in the reversal of slip on the faults and expulsion of the synrift fill. During inversion the beds in the cover sequence shorten before the net extension at the basement level has been cancelled. Shortening of the sedimentary cover generates folding and backthrusting in the still downthrown hanging wall block. Intracratonic inverted basins in different parts of the Alpine Foreland show similar structural geometries with the major extensional faults which controlled basin development reactivated during subsequent compression. We use examples from the Western Approaches and offshore Holland (Broad Fourteens Basin) to illustrate the structural styles developed during inversion. The fundamental control on compressional structural geometry exerted by pre-existing extensional structures is also visible in more complexly deformed orogenic belts, like the Western Alps and the Pyrenees. In these areas inversion also occurs, but more commonly extensional faults which may not have inverted act as an indirect control on the location of ramps, and/or thrust orientation. Seismic data are normally required to establish these effects with certainty. However, as the body of knowledge builds up, it is possible to recognize certain geometrical characteristics which suggest the control of extensional faults in thrust belts. These include footwall shortcuts, out of sequence structures and arcuate thrust-fold traces.

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