Mechanics, microstructure and AMS evolution of a synthetic porphyritic calcite aggregate deformed in torsion

• Porphyroclasts affect the mechanical properties of a porphyritic aggregate.
• We show how the grain size evolves under near-simple shear.
• Composite fabrics can form in porphyritic rocks under near simple shear.
• We show how AMS reflects the applied strain and strain evolution.
• We show how porphyroclasts deform and are destroyed by increasing strain.

In order to investigate the mechanical, microstructural and AMS evolution of porphyritic mylonites, we made a synthetic aggregate composed of 70% fine calcite (< 50 μm) and 30% coarse calcite (200–700 μm), and deformed cylindrical specimens in torsion at 300 MPa, 727 °C, a constant strain rate of 3.0E− 4 s− 1, to shear strains γ ≈ 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 γ ≈ 0, to oblique at γ ≈ 1, and finally to low angle to the shear plane at γ ≈ 5, as expected for approximate simple shear. At γ ≈ 1, stretched calcite grains > 3 mm in length defined a conspicuous foliation, and showed aligned twins. At γ ≈ 5, calcite porphyroclasts were highly stretched (aspect ratio around 20), and had rotated towards the shear plane. Between γ = 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.