Twinning Strains in Synfolding Calcite, Proterozoic Sinian System, China

Abstract

Synfolding calcite was precipitated between layers of Neoproterozoic sandy dolomite and striated parallel to the fold axis of an open anticline with a shallow plunge during folding. The fold had limb dips of 45° and plunged 20° to the south. The synfolding calcite had sub-horizontal grooves that trended parallel to the fold. Limb-hinge-limb calcite samples (3 samples; n = 100 grains) preserved a layer-parallel shortening strain that trended at an acute (45°) angle to the trend of the fold axis and fold lineations. Extension axes were vertical and there was no strain overprint (low NEVs). Shortening strain magnitudes were −2.9% and the differential stress responsible for twinning was −38 MPa. The commonly observed structures were layer-parallel slip striations normal to the fold axis: sub-horizontal interlayer slip surfaces parallel to a fold axis (parallel to bedding strike) were unreported; as was a sub-horizontal shortening strain at an acute angle to the axis of a plunging fold.

Keywords

• structural geology
• folding dynamics
• strain analysis