Experimental deformation of crystalline rocks

Abstract

Eighteen typical igneous or metamorphic “basement” rocks have been experimentally deformed at about 1% main under “shallow” conditions (1 kbar confining pressure, 150°C) and, together with two important rock-forming minerals — biotite and plagioclase — udner “deep” conditions (5 kbar, 500°C). Under shallow conditions (about 15,000 ft.) the rocks are essentially brittle and fail after a few percent shortening on shear fractures of faults inclined at 21–33° to the maximum principal compressive stress. In these short-time tests, at least, the predominant mode of deformation even under deep conditions (about 11 miles) is still faulting except in favorably oriented micaceous rocks: gneiss, schist, and slate. The fault angles lie between 27° and 38° in the nonfoliated rocks and are well predicted by the Coulomb criterion. Only one good correlation between ultimate strength and composition (or texture) is obvious. Gneiss and slate are appreciably weaker when the load is applied at 45° to the planar anisotropy defined by the statistical parallelism of the micas, and the faults lie close to the foliation and the 45° position of maximum shear stress. This is probably associated with the very low critical resolved shear stress for slip, less than 100 bar in biotite. Petrofabric studies have revealed certain deformation mechanisms that may serve as clues to the principal stress directions in naturally deformed rocks: 1. (1) Biotite single crystals deform by bend glidingon {001} when that plane is initially subparallel to the maximum principal compression σ 1. Kinking occurs, and the boundaries of the bands tends to form perpendicular to σ 1. 2. (2) In gneiss with s planes delineated by the strong preferred mica orientation, the biotites also deform by bend gliding when s lies at low angles to σ 1. This leads to gross kinking of the rock in bulk, and again the bands tend to develop normal to σ 1. Kindking does not occur when the angle σ 1 Λ s exceeds about 25°. Faulting is parallel to s when s lies at 45° to σ 1. 3. (3) In the schist and slate, kinking occurs when s is subparallel to σ 1. In the latter, structures develop which resemble chevron folds. The boundaries may be inclined as little as 30° to σ 1; however, one can still identify σ 1 from the sence of external rotation of s within the kink bands. 4. (4) In the pyroxenite, intracrystalline gliding on {100} in enstatite leads to the displacive inversion to clinoenstatite.