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The exhumation of high‐pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures, or the transport of the metamorphic rocks through the overburden. Exhumation cannot be achieved simply by thrusting or strike‐slip faulting. It may be caused by erosion of shortened and thickened crust, but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km. One or more of the following additional mechanisms may be involved. 1 Corner flow of low‐viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock, and may explain some occurrences of high‐pressure tectonic blocks in mélange. This process does not, however, appear to be adequate to explain the exhumation of regional high‐pressure terrains. 2 Buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings. This is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle, but cannot explain the emplacement of coherent tracts of high‐density metamorphic rock into shallow crustal levels. Some high‐pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low‐density mud or serpentinite. 3 Extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high‐pressure terrains. Extension may occur in the upper part of an accretionary wedge thickened by underplating; or it may affect the whole lithosphere in a region of intracontinental convergence, if surface elevation has been increased by the removal of a lithospheric root. In the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain.

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Ab~'aet-A distinct class of structures can form as a result of extension along a plane of anisotropy (foliation). The effect of the foliation is to decrease the ductility of the material in this orientation so that brittle fractures or shearbands develop. Foliation boudinage is caused by brittle failure; extensional fractures cause symmetric boudinage, and shear fractures cause asymmetric boudinage. Extensional crenulation cleavage is defined by sets of small-scale ductile shear-bands along the limbs of very open microfolds in the foliation. The sense of movement on the shear-bands is such as to cause a component of extension along the older foliation. Conjugate cleavage sets indicate coaxial shortening normal to the foliation; the shortening axis bisects the obtuse angle between the sets. A single set indicates oblique or non-coaxial deformation. Extensional crenulation cleavage is microstructurally and genetically distinct from other types ofcleavage. It does not occur as an axial plane structure in folds, and has no fixed relationship to the finite strain axes. It is common in mylonite zones, and may be favoured by crystal-plastic and cataclastic deformational mechanisms. These cause grain-size reduction, and hence softening, which favour the development of shear-bands.

The Betic-Rif arc is one of the smallest and tightest orogenic arcs on Earth, and together with its extensional hinterland, the Alborán Domain, it formed between two colliding continents. The region provides examples of a range of tectonic processes that are not predictable from the rules of rigid-plate tectonics. The Alborán Domain reveals two stages of subduction and accretion, with different thermal histories and mechanisms of exhumation. The external Betic-Rif thrust belt illustrates four processes that create an arcuate orogen and a strongly divergent pattern of slip vectors: (a) the interaction between the westward moving Alborán Domain and the converging African and Iberian margins, (b) divergence in relative motion due to extension within the Alborán Domain, (c) slip partitioning onto strike-slip faults within the arc, and (d) vertical-axis rotations resulting from oblique convergence on the limbs of the arc.

The Betic Cordillera of southern Spain provides a clear example of a collisional orogen that has undergone large‐scale extensional collapse while convergent motion of the bounding plates continued. Extension was accommodated by coeval shortening in thin‐skinned fold and thrust belts around the periphery of the system, and much of the region has now subsided to form a large marine basin. The thermal and deformational record of these processes is preserved in rocks from the upper mantle, crystalline crust, and sedimentary cover. Upper mantle peridotites record evidence for exhumation in several stages from asthenospheric depths to the surface. Early stages of exhumation probably occurred during Mesozoic rifting. Cooling at midlithospheric depths reflects continental convergence, and subsequent heating indicates loss of most of the underlying lithosphere and ascent of asthenosphere, whilst the final stages of exhumation in early Miocene time reflect extensional collapse. Crustal rocks in the internal zone of the Betic Cordillera were metamorphosed down to 50 km depth and are now exposed beneath major low‐angle normal detachment zones that separate them from heavily faulted low‐grade rocks above. Cooling ages of associated mylonites indicate that these detachments were active during the early to middle Miocene. Fault‐bounded intramontane basins, developed during the early to middle Miocene, contain coarse continental sediments heavily affected by normal fault systems, followed by a less deformed late Miocene marine succession. All of these phenomena can be explained by convective removal of the lithospheric root beneath a Paleogene collisional orogen, leading to large‐scale extension followed by thermal subsidence of the center of the system.