Exhumation of high‐pressure rocks: a review of concepts and processes

Abstract

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.

Extensional tectonics in the Caledonides of southern Norway, an overview

“…This process would have increased the syn-orogenic surface slope, and it could also have reduced the dip of the sole thrust in the eastern parts of the orogen. Both geometrical adjustments, combined with reduced rate of convergence, would favour gravitational spreading of the mountain belt (England and Houseman, 1988;Platt, 1993). After initial extension possibly related to gravitational spreading, belt-wide reversal in the polarity of deformation, from top-east to top-west, occurred in the Scandinavian Caledonides with exception of the thin-skinned foreland fold and thrust belt.…”

Pan-African extension and near-isothermal exhumation of a granulite facies terrain, Dronning Maud Land, Antarctica

“…Late-orogenic extensional denudation can explain rapid exhumation of deeper crustal rocks (e.g. Platt, 1993). Decompression driven by surficial or tectonic processes may also trigger partial melting by crossing phase boundaries during uplift (Teyssier & Whitney, 2002).…”

Kinematics and geometry of structures in the southern limb of the Paraíba do Sul divergent structural fan, SE Brazil: a true transtensional shear

“…Extensional shear zones are a common structural feature in modern and ancient convergent zones (Coward et al 1987, Malavieille 1993, Platt 1993. Such features have been described in the Caledonides (Marten and Dewey 1998, Fossen 2000, Stra-374 NOLAN M. DEHLER et al chan et al 2001, in the Himalaya (Herren 1987, Pêcher 1991, in the Andes (Dalmayrac and Molnar 1981), in the western Cordillera (Crittenden et al 1980, Wernicke 1981, Davis 1983, Spencer 1984, in the Alps (Selverstone 1988, Ratschbacher et al 1989, and elsewhere.…”