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Abstract

Semi-brittle flow occurs when crystal plasticity and cataclastic mechanisms operate concurrently and may be common at mid-levels of the Earth’s crust. Using a Paterson gas-deformation apparatus, we performed 67 conventional triaxial deformation experiments on dry samples of Carrara marble in the temperature range of T = 20 - 800°C; confining pressures (PC) were 30, 50 100, 200, and 300 MPa, and strain rates (urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0001) were 10-3, 10-4, 10-5, and 10-6 s-1. Axial strains (ε ) were ≲ 0.12. The measured (differential) stress, urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0002, changes with strain at most applied conditions. At ε = 0.05, both stress and the hardening coefficient (h), i.e., the rate of increase of stress with strain, increase as T decreases and PC increases. At T ≲ 400°C, h is quite large and the sensitivity of urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0003 on urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0004 is low, while both are sensitive to increasing pressure. In this temperature range, the mechanical behavior of the marble is very similar to that exhibited by high-strength, high-ductility, hexagonal metals that deform by processes called twinning induced plasticity (TWIP). Twinning and dislocation motion are abundant in the samples, as are inter- and intra-crystalline micro-fractures. The concurrent activation of these deformation mechanisms leads to complex relationships of urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0005 and h with the applied T- PC - urn:x-wiley:21699313:media:jgrb55357:jgrb55357-math-0006 conditions. This behavior suggests that peak stresses for calcite rocks deforming by semi-brittle processes will occur at PC - T conditions of the middle crust, and that they might be more strongly influenced by total strain rather than by strain rate.