Hydrodynamic zoning of the Earth's crust and its significance

Abstract

Geological observations including observations on water regime of metamorphism and hydrodynamics of subsurface fluids in very deep boreholes, experimental results of rock deformation at high temperatures and pressures, and seismic research have made it possible to draw the following conclusions. At a depth of about 10–12 km, closure of fissures, cavities and open pores in rocks creates a dense “transition” zone impermeable to fluids. Below the dense zone, water released from thermal breakdown of hydrous minerals and arriving from the lower crust and mantle is under lithostatic pressure ( P lt). In this “lithostatic” zone, effective pressure ( P eff) is zero ( P eff = P lt− P fluid, at P fluid = P lt, P eff=0). For this reason, rock strength depends only on cohesive stresses considerably weakened by the hydrolithic effect. Considerable rock weakening under the dense transition zone favours formation of hydrofractures and accumulation of fluids. In case of deformation, a significant difference in rheological features of rocks between the upper (“hydrostatic”) and impervious transition zones, on the one hand, and the lithostatic on the other hand, causes a detachment fault (“otdelitel” in Russian) at their boundary, just below the transition zone. The paper gives brief reasons for the assumption, that both the dense transition zone and its characteristic feature—detachment faults, are recorded by seismic methods as a reflection and refraction K 1 horizon situated most frequently at depths of 7–15 km. The main geological factors influencing its position are described; among them, there are a high heat flow and gravitational load, which are responsible for a reduction in the K 1 depth. The authors compare the proposed rheological model of lithosphere with those of other researchers. The transition zone (often marked by the K 1 discontinuity) is shown to be the most important lithospheric zone. It is commonly the lowest level of a variety of tectonic structures, including the overwhelming majority of “deep faults”. Most hydrothermal vein ore deposits are associated with the transition zone. The zone has a decisive role in the formation of granitoids and regional metamorphic facies. It serves possibly as an impermeable cover to deep-seated hydrocarbon accumulations. The abbreviation: FPS (fissure-pore space) is adopted for this publication. The FPS includes fissures, connected pores and cavities.