EXHUMATION OF HIGH-PRESSURE ROCKS - A REVIEW OF CONCEPTS AND PROCESSES

The exhumation of high-pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures, or the trans port of the metamorphic rocks through the overburden. Exhumation canno t be achieved simply by thrusting or strike-slip faulting. It may be c aused by erosion of shortened and thickened crust, but this is unlikel y to be the only mechanism for exhuming rocks from depths greater than about 20 km. One or more of the following additional mechanisms may b e involved. 1 Corner flow of low-viscosity material trapped between th e 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 melange. This process does not, however, appear to be adequate to explain the exhumation of regional high-pressure terra ins. 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 sub ducted 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 t errains 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 structur al setting of many high-pressure terrains. Extension may occur in the upper part of an accretionary wedge thickened by underplating; or it m ay affect the whole lithosphere in a region of intracontinental conver gence, if surface elevation has been increased by the removal of a lit hospheric root. In the second case extension may be accompanied by mag matism and an evolution towards higher temperature during decompressio n of the metamorphic terrain.