Fluids and thrusting

Abstract

Recent studies of the structure of the continental crust by COCORP and BIRPS have shown that major thrust structures in the lower crust are ubiquitous. Plate-tectonic processes of continental crust destruction or construction at sites where subduction or collision occurs (the Himalayas or the Andes) involve thrusting of continental materials or ocean crust beneath continents, and local overthrusting may accompany transcurrent motion. In all such cases massive fluid flow must occur. As first stressed by M.K. Hubbert and W.W. Rubey, thrusting mechanisms require that fluid pressures are close to, or greater than, lithostatic pressures. When thrusting occurs, compaction of the underplate must occur rapidly, expelling pore water. Crustal thickening leads to gradual heating of the underplate with prograde metamorphism near the top and even melting at the base. If the overthrust plate is thick and hot at the base, retrograde metamorphism will occur at the base of the overthrust plate and rising fluids will encounter inverted thermal gradients. In such a region veins need not occur but leaching phenomena may dominate. If an underthrust plate is hydrated, and/or contains sedimentary aquifers, large fluid volumes are expelled through shear zones into the overthrust plate. The scale of fluid-release processes can be large. Thus, in thin-skinned tectonics where the overthrust plate is 10–15 km thick, induced fluid release can easily reach 4 · 10 9 g m −2 of thrust surface. The flow of such fluids will be controlled by the thrust surface and lithology, both of which will influence hydraulic fracture mechanisms and spacing of fractures. The chemistry of thrust-derived fluids may be highly variable depending on lithology and the time constants of thrusting. Vertical thermal and redox environments will be similarly dependent. In the subduction process, impressive quantities of fluids must pass back up the thrust surface, and such fluids have recently been directly observed during drilling. Dewatering of spilites, serpentinites and sediment layers of the underthrust oceanic lithosphere must produce massive fluid flow back to the surface but some of these fluids may form hydrated minerals in the overlying mantle and, at great depth, flux mantle melting. Extreme metasomatism in blueschist belts must result from such fluids. A case of particular interest involves the cessation of subduction when a high-level slab reaches thermal equilibrium. In general, flow regimes in the thrust and fault zones associated with collisions follow a sequence from conditions of high T- P with locally derived fluids at low water/rock ratios during initiation of the structures, to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution, and at shallower crustal levels, there may be incursion of oxidizing near-surface fluid reservoirs into the faults. These fluids may have extremely low δ 18O-values, where mountain ranges form on rebound faults, and high-altitude depleted fluids penetrate down the rebound structure. Extensive mineralization may be associated with such thrust-derived fluids. Examples which will be discussed include the Mother Lode Au deposits of California, U.S.A., and the large U deposits of Lagoa Real, Brazil.