In order to investigate the mechanical, microstructural and AMS evolution of porphyritic mylonites, we made a synthetic aggregate composed of 70% fine calcite (<50 mu m) and 30% coarse calcite (200-700 mu m), and deformed cylindrical specimens in torsion at 300 MPa, 727 degrees C, a constant strain rate of 3.0E(-4)s(-1), to shear strains gamma approximate to 1 and 5. After peak stress, dynamic recrystallization of porphyroclasts resulted in grain size reduction and weakening till a mechanical steady state was reached. Microstructural, AMS and EBSD analyses show the consistent evolution of pre-torsion (cold-pressed) planar fabric from perpendicular to sample cylinder axis at gamma approximate to 0, to oblique at gamma approximate to 1, and finally to low angle to the shear plane at gamma approximate to 5, as expected for approximate simple shear. At gamma approximate to 1 1, stretched calcite grains >3 mm in length defined a conspicuous foliation, and showed aligned twins. At approximate to 5, calcite porphyroclasts were highly stretched (aspect ratio around 20), and had rotated towards the shear plane. Between gamma = 1 and 5, a composite fabric formed, one at low and the other at high angle to the shear plane, from which shear sense can be deduced. The AMS patterns were sensitive to increasing shearing, and tracked strain reasonably well, despite the reduced size and low susceptibility of specimens. From the CPO and the microstructure, we infer that a balance compatible with an optimal dissipation of the applied stress was achieved between grain growth and grain reduction processes.
2015. Vol. 655, no S1, 41-57 p.