In rocks deformed by natural orogenic processes it is usual to find that the finite strain state varies from locality to locality. In some deformed rocks high strain states are localized within approximately planar zones commonly known as "shear belts".The general relationships that exist between variable displacement and variable strain state are established, and these general equations are solved for particular types of strain within shear zones. Only a limited number of types of solution are possible. Using these solutions the geometric forms of the structures found in shear zones in several regions are analyzed. Methods for computing the finite strain through these zones are described, and these finite strains are integrated to determine the total displacements across these zones. Schistosity is developed in some of the shear zones described. It is not parallel to the walls of the shear zone and is therefore not parallel to the dominant displacement (shear) directions. The schistosity appears to be formed perpendicular to the principal finite shortening (i.e. perpendicular to the shortest axis of the finite strain ellipsoid). Variations of the schistosity planes represent variations in the finite strain trajectories of XY planes in the strain states ([Formula: see text] ellipsoid axes). The intensity of development of the schistosity is correlated with the values of the principal finite strains.

A detailed structural traverse across the basement rocks of the Eastern Desert of Egypt shows that they consist, apart from intrusions, of four broadly recumbent tectonic units. The lowest, of arkosic metasediments of continental shelf facies, is exposed in a dome. This unit is overlain by an allochthonous ophiolitic mélange containing complete and dismembered ophiolitic masses in a matrix of deep-oceanic graphitic pelites and turbidites. A near horizontal, schistose to mylonitic fabric, most intense near and below the base of the mélange, indicates extensive lateral tectonic transport of the ophiolitic mélange over the shelf metasediments, but the mélange originated as an olistostrome before being tectonically transported. The mélange extends over an area of at least 10 000 km 2 . It is locally overlain by calc-alkaline volcanics, in which deformation is less intense but increases, and schistosity flattens downwards, indicating some translation over the mélange. Unconformable on the mélange and calc-alkaline volcanics is a molasse-facies series, itself also locally strongly deformed. Late tectonic granites preceded, and locally post-date, the molasse-facies sediments. Still later, diapiric peralkaline riebeckite granites locally up-domed the recumbent structures. The tectonic evolution is related to a late Proterozoic subduction zone to the SE of the region.

In the sub-Alpine chains of Haut Provence, SE France, a very well-exposed Mesozoic sequence showing rapid thickness and facies changes associated with Jurassic and Cretaceous extension on the margin of the Ligurian Tethys has been deformed by ‘Alpine’ compression which occurred from the Late Cretaceous to the Pliocene. Although the geology has been very well known for decades, aspects of the structure remain enigmatic and cannot be explained by either Mesozoic extension or Alpine shortening alone. We infer that some deformation resulted from salt tectonics. A completely overturned, highly condensed Jurassic section near Barles village resembles the elevated roof of a Triassic salt body in a deep-marine basin. This carapace became overturned as a flap in the Middle Jurassic when salt broke out at the seafloor and overran the inverted flap as an allochthonous extrusion, comparable to those in the deepwater Gulf of Mexico or Angola. Later, Alpine compression exploited the weakness of the salt sheet as the Digne Thrust moved over the inverted flap. Although the flap is in the footwall of the thrust, evidence of soft-sediment deformation and other anomalous structures within the flap suggest that it did not originate as an overturned footwall syncline.

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.

Interpretation of long-offset 2D depth-imaged seismic data suggests that outer continental margins collapse and tilt basinward rapidly as rifting yields to seafloor spreading and thermal subsidence of the margin. This collapse post-dates rifting and stretching of the crust, but occurs roughly ten times faster than thermal subsidence of young oceanic crust, and thus is tectonic and pre-dates the 'drift stage'. We term this middle stage of margin development 'outer margin collapse', and it accords with the exhumation stage of other authors. Outer continental margins, already thinned by rifting processes, become hanging walls of crustal-scale half grabens associated with landward-dipping shear zones and zones of low-shear strength magma at the base of the thinned crust. The footwalls of the shear zones comprise serpentinized sub-continental mantle that commonly becomes exhumed from beneath the embrittled continental margin. At magma-poor margins, outer continental margins collapse and tilt basinward to depths of about 3 km subsea at the continent-ocean transition, often deeper than the adjacent oceanic crust (accreted later between 2 and 3 km). We use the term 'collapse' because of the apparent rapidity of deepening (<3 Myr). Rapid salt deposition, clastic sedimentation (deltaic), or magmatism (magmatic margins) may accompany collapse, with salt thicknesses reaching 5 km and volcanic piles 1525 km. This mechanism of rapid salt deposition allows mega-salt basins to be deposited on end-rift unconformities at global sea level, as opposed to deep, air-filled sub-sea depressions. Outer marginal collapse is 'post-rift' from the perspective of faulting in the continental crust, but of tectonic, not of thermal, origin. Although this appears to be a global process, the Gulf of Mexico is an excellent example because regional stratigraphic and structural relations indicate that the pre-salt rift basin was filled to sea level by syn-rift strata, which helps to calibrate the rate and magnitude of collapse. We examine the role of outer marginal detachments in the formation of East India, southern Brazil and the Gulf of Mexico, and how outer marginal collapse can migrate diachronously along strike, much like the onset of seafloor spreading. We suggest that backstripping estimates of lithospheric thinning (beta factor) at outer continental margins may be excessive because they probably attribute marginal collapse to thermal subsidence.