Experimental deformation of a quartz mylonite

Abstract

Samples of a pure quartz mylonite have been axially shortened up to 60% in nickel capsules with water, at temperatures of 600–900°C, strain rates of 10 −5–10 −7 s −1, and a confining pressure of 1.64 GPa in a solid medium deformation apparatus. The mylonite was loaded in four orientations relative to the initial foliation and lineation: PSL, parallel to the foliation and lineation; 45SL, 45° to the foliation and lineation; NSL, normal to the foliation and lineation; and PSNL, parallel to the foliation, normal to the lineation. The strength of the mylonite varies with the orientation of the specimen with respect to the loading direction, as well as with temperature and strain rate. Specimens deformed PSL are weakest, with those deformed 45SL slightly stronger, while specimens deformed NSL and PSNL are generally 200–400 MPa stronger. All experiments exhibit strain softening at strains greater than 15%. Strongly flattened grains with sub-basal deformation lamellae are developed in all specimens, with the amount of recrystallization increasing as temperature is increased and/or strain rate is decreased. The initial c-axis preferred orientation, typical of quartz mylonites, is an asymmetric girdle normal to the mylonitic foliation and lineation. In specimens deformed NSL and PSNL strengthening of the c-axis maximum parallel to the shortening direction is the major change in preferred orientation. The c-axes in these specimens are inferred to be in stable orientations with respect to the shortening direction. In specimens deformed PSL a new girdle develops parallel to the initial foliation, with subsidiary girdles running from the shortening direction to a position normal to the initial foliation and lineation. More complex fabrics observed in specimens deformed 45SL are associated with the triclinic symmetry of the overall deformation. The fabrics develop by initial kinking of grains, which have c-axes at high angles to the shortening direction, causing them to rotate into orientations more suitable for dislocation slip. Slip occurs on both basal and prism planes. Rhomb slip may have a minor influence. In all cases slip parallel to ( a) is consistent with the observed fabrics. The complexity of the fabrics indicates that even at these high strains (60%) quartz can still show a memory for fabrics developed in previous deformations.