The papers in this thematic set arise from a discussion meeting sponsored by the Tectonic Studies Group and held at Burlington House on 6-7 March 1996. About 170 people heard 19 formal oral presentations and studied 17 posters of which 15 were briefly introduced with five minute talks. Sixteen of these contributions are published in this thematic set.

Pre-existing heterogeneities in the continental lithosphere are thought to influence markedly its response to subsequent deformation. A major reason for this thinking is the long held view (e.g. Hills 1956) that fault zones are inherently weak. Virtually all regions of continental lithosphere are riddled with faults and shear zones. Thus any subsequent deformation might be expected to utilize these pre-existing weak zones (as reviewed by White et al. 1986). This type of reactivation, where faulting activity is localized onto pre-existing faults, can be contrasted with more general cases of deformation reworking broader volumes of lithosphere. There need not be complete reactivation of individual faults and shear zones. The distinc­tion is nicely illustrated by the concepts of basin inversion, a topic that has been well aired in recent years (e.g. Cooper & Williams 1989;Coward 1994). In many sedimentary basins there is evidence of contractional reactivation of segments of originally extensional faults (e.g. Letouzey 1990). Yet for the contrasting tectonic setting of collisional mountain belts, recent work has emphasized that, in general, thrusts do not reactivate pre-existing extensional faults (e.g. Gillchrist et al. 1987). However, the sites of almost all the shortened continental

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