Transtensional flanking structures

Abstract

Flanking structures are deflections of an existing planar fabric (e.g., foliation) alongside a cross-cutting element (e.g., a vein) that can develop in a wide range of rock types and glacier ice. Nearly all published examples of flanking structures are interpreted to have formed either under simple shear or transpressional general shear, although theoretically they should also form under transtensional general shear. This paper describes the geometry and development of transtensional flanking structures in glacial ice of the Pasterze, Austria's largest alpine valley glacier. The cross-cutting elements are a few metres long and are interpreted as fractures that rotate into the shear flow and consequently accommodate anti- and synthetic offset, forming a- and s-type flanking folds. However, shear bands, with co-shearing cross-cutting elements inclined against the shear flow, are absent. The geometries of the mapped structures are successfully reproduced with a semi-analytical modified Eshelby solution for a frictionless cross-cutting element embedded in a linear viscous medium deforming under a remote transtensional sub-simple shear. The geometry of the mapped flanking folds, the absence of shear bands, the spatial variation of cross-cutting element orientations and the geometry of the glacier's splaying crevasses are consistent with two-dimensional transtensional sub-simple shear caused by down-glacier valley widening.