Clast-based kinematic vorticity gauges: The effects of slip at matrix/clast interfaces

Abstract

Clast-based methods for estimating the mean kinematic vorticity number W m are compromised by strain localization at the clast margins. Localization increases with modal matrix mica content as determined with samples from the Sandhill Corner mylonite zone – a crustal-scale, high-strain, strike-slip shear zone in Maine. Using these samples, we estimate W m with the oblique quartz shape-preferred orientation and rigid-clast rotation methods. The rigid-clast rotation method yields much lower values for W m than the quartz method. To investigate whether or not slip at the matrix/clast interface can explain the discrepancy in calculated W m, we conducted numerical modeling of rigid clasts enveloped by a low viscosity layer, both embedded within a shearing viscous matrix. Within this dynamic framework, we carried out numerical sensitivity analyses in which we varied the viscosity ratio between the lubricating layer and the surrounding matrix, the thickness of the lubricating layer, and the kinematic vorticity number of the bulk flow. Our data and numerical results succeed in explaining why W m estimates from clast-based rotational methods are typically lower than estimates from other methods, and this has implications for testing hypotheses related, for example, to vorticity partitioning in oblique convergent settings, crustal-scale extrusion or channel flow, and exhumation of ultra-high pressure rocks, all of which rely on robust estimates of W m. The relation between the shape preferred orientations of clasts and modal mica content lead to the hypothesis that mica is the cause of the lubrication at clast/matrix interfaces. If so, then we surmise that mica fish should be self-lubricating and would therefore form an end-member shape preferred orientation, regardless of matrix modal mica content. The unique role of mica allows us to speculate about the bounds on viscosity contrast between the matrix and lubricated clast interfaces.