The uplift of high-pressure-low-temperature metamorphic rocks

Abstract

The uplift of high-P-low-Tmetamorphic rocks has been attributed to buoyancy, diapirism, or hydrodynamically driven return flow. Buoyancy forces can return material subducted into the mantle only if subduction slows or ceases, reducing the downward traction. The buoyancy forces will be reversed within the crust, because of the increased density of high-Passemblages, and therefore can not cause the subducted material to rise beyond the base of the crust. Diapirism and hydrodynamic flow processes require a low-density, low-viscosity matrix, and can only explain the emplacement of relatively small bodies of high-Prock entrained in the flowing material. The tectonic setting of coherent regional high-P—low-Tterrains can be explained in terms of the mechanical behaviour of an accretionary wedge with negligible yield strength, where underplating is the dominant mode of accretion. Underplating thickens the wedge from beneath and increases its surface slope. This causes the upper part of the wedge to extend horizontally, even though convergence is continuing. Continued underplating beneath and extension above can allow the oldest high-Procks to rise to within reach of a moderate amount of erosion on a time scale of the order of 10 Ma. As long as subduction continues beneath the wedge, the geothermal gradient will not relax to a normal value. This process explains (a) the evidence that high-P-low-Trocks are commonly uplifted while convergence is continuing; (b) the absence in many cases of significant overprinting by higher-Tassemblages; (c) the position of the oldest and highest pressure rocks in the upper rear of orogenic wedges; (d) the lack of adequate tectonic thicknesses of overlying rock to explain the metamorphism; and (e) the common occurrence of post-metamorphic faults that excise parts of the metamorphic zonation.