Textural evolution of synthetic anhydrite-halite mylonites |
| |
| Authors: | John V Ross Stephen J Bauer Francis D Hansen |
| |
| Institution: | Center for Tectonophysics, Texas A&M University, College Station, TX 77843, U.S.A. |
| |
| Abstract: | The rheology and textural evolution of mylonites for both crustal and mantle rocks are generally poorly understood. Stress and strain partitioning between shear zones and wall-rock, foliation development, flow mechanisms, and constitutive relations are little known. To address these problems, we have begun an experimental study in which anhydrite and halite aggregates are deformed in simple shear. Homogeneous mixes at intervals of 10 vol.% of anhydrite and halite powders of 90 to 180-μ m grain-size comprise the 1-mm-thick zone between the 35 °-precut surfaces in 2 by 4 cm right-circular cylinders of anhydrite. All specimens are confined under 200 MPa (Pc) and tested at 300 °C (T), producing an aggregate of near-zero porosity. All specimens are shortened at an axial displacement rate of 5×l0−6 cm/s to shear strains of about 2.0. Most stress-strain curves have a similar form. They are characterized by an initial linear response that is followed by a work-hardening region that leads to a plateau of constant stress whilst strain continues to accumulate. This apparent steady-state behavior is followed by a drop in stress to a second apparent steady-state stress. The strain at this stress drop and the magnitude of the drop increase with increasing anhydrite content. The second (lower) steady-state stress for all mixes approaches that of sheared pure halite. Optical examination of the fabrics shows that straining under the initial apparent steady-state is accommodated by both mineral phases. A foliation evolves as defined by mineral segregation and grain elongation of halite at 15 ° to 30 ° to the shear-zone walls. Anhydrite grains rotate until their (001)  010 slip system parallels the foliation. The anhydrite also exhibits twin lamellae, kinks and microfractures. Following the stress drop, rheologic behavior is dominated by dynamic recovery in halite, the recrystallized grain size increasing, consistent with the lower stress level. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
