Development of asymmetric shale pull-aparts in evaporite shear zones

Abstract

Antithetically rotated lozenge-shaped shale pull-aparts are quite common in the shallow domains of the evaporite shear zones of the Jura in northern Switzerland. The pull-aparts are formed by synthetic shear fractures related to a transpressive strain field that also induces the antithetic rotation of the pull-aparts immediately after their formation. A simple analytical model shows that the acute angle of fracture, α, and the antithetic rotation, δ, are determined by structural and rheological parameters and the direction of maximum compressive stress. The pull-aparts are commonly separated by sulphate veins, as the elongation resulting from antithetic rotation does not compensate for the bulk elongation. Despite this, some pull-aparts are bordered by synthetic drag zones. Antithetically rotated pull-aparts are indicative either of compression normal to shear zone boundaries in décollement-type shear zones or, in shear zones deformed by simple shear, of an initial obliquity of the dismembered layers relative to the shear zone boundary. Resemblance of the shale pull-aparts to type 1 asymmetric pull-aparts and book-shelf structures, both of which rotate synthetically, demonstrates that rotated and asymmetric pull-aparts may be ambiguous kinematic indicators.