WHICH SCALE(S) OF STRAIN ANALYSIS BEST CHARACTERIZES AN OVERPRINTING “BULK” DEFORMATION? A CASE STUDY FROM THE CAPRICORN RIDGE SHEAR ZONE, CENTRAL AUSTRALIA

Exposures of deformed deeper continental crust are often multiply-deformed and poorly suited for strain analyses, yet structural studies of such exposures are often used as evidence of large scale conceptual models for lithospheric deformation during orogenesis. In the Mount Hay block, central Australia, ~1710 Ma penetrative deformation fabrics (~800°C, ~8 kb) are cross-cut and overprinted by the >7 km thick, ~1551 Ma Capricorn Ridge shear zone (CRSZ; >798 ± 33°C; >7.6 ± 0.7 kb). Foliation intensity mapping and mesoscale structures indicate the S-side-up CRSZ also records flattening.

Fabric type and intensity data – proxies for strain type and intensity – from 4 litho-fabric domains were collected from 8 stations along a ~0.5 km traverse crossing mapped strain gradients. The data are: shape axes of feldspar porphyroclasts (in charnockitic and gabbroic domains), felsic blebs (“blebby” gabbroic), and quartz ribbons (quartzofeldspathic and charnockitic); the shape-preferred orientation of clinopyroxene and opaque mineral grains (select gabbroic gneiss samples); and magnetic fabric defined by the anisotropy of susceptibility (AMS) and of anyhysteretic remanence (AARM; all 4 domains). The results demonstrate increasing fabric intensity across the strain gradients, but different fabric types. Blebs and ribbons record increasing oblate and prolate fabric, respectively. Feldspar porphyroclast and grain shapes record slightly increasing plane-strain fabric intensity. The alignment of magnetic fabric axes as defined by AMS and AARM, within the sample set for a given litho-fabric domain, and between litho-fabric domains at a given station, become more aligned with each other and with the field foliation and lineation. Magnetic fabric axis orientation distribution functions suggest the overprinting deformation involved flattening (stretching in the plane of foliation highly oblique to the lineation) before alignment of the maximum (k₁) and minimum (k₃) axes with the field lineation and pole to foliation, respectively. Overall, these results suggest that m- to cm-scale proxies for strain intensity reflect heterogeneity in pre-existing fabric and/or strain partitioning, whereas fabric mapping and mesoscale structures delineate a consistent “bulk” deformation across the CRSZ.