Arrested tectonic development: Preservation of high-temperature fabrics in the sinistral Caiçara shear zone (Borborema province, northeastern Brazil)

Abstract

The protracted duration of shearing in large-scale shear zones and/or their reactivation may obscure the metamorphic conditions under which the mylonites initially formed. Here, we describe the case of the high-temperature Caiçara shear zone (Borborema Province, northeastern Brazil), which shows only limited overprint by low temperature fabrics. The NE-trending Caiçara shear zone is ∼35 km long and comprises a 2 to 5 km-wide mylonitic belt. The shear zone was characterized by using kinematic analysis, including field and microstructural observations and measurement of quartz c-axis. The mylonitic foliation is steeply NW-dipping and contains a shallowly SW-plunging stretching lineation. Kinematic criteria (e.g., asymmetric porphyroclasts, S–C fabrics, asymmetric folds, C′-type shear bands) are consistent with sinistral shear. The dominant microstructures (e.g., chessboard extinction in quartz, grain boundary migration and subgrain rotation recrystallization in quartz, and polygonal aggregates of feldspar around porphyroclasts) and application of the opening angle thermometer indicate deformation dominantly at temperatures higher than ∼ 600 °C. In contrast to the common situation in the central Borborema Province, the Caiçara shear zone is unconnected to other shear zones. We attribute preservation of high-T fabrics to cessation of shearing in an early stage of development, preventing continuous fabric development at decreasing temperature conditions.