A synthesis of the structural geology of a major tectonic boundary between a 1000 m.y. mobile belt and a 3000 m.y. craton

Abstract

Abstract On the boundary between the 3000 Ma Kaapvaal Craton and the ±1100 Ma Gordonia sub-province of the Namaqua Province situated west of the craton lies the Areachap Group, which mainly comprises amphibolitic and calc-silicate rich rocks. Various studies investigating the structural relationships between the adjacent terranes all reveal an intensely deformed area, where at least four phases of deformation can be defined, developing complex interference patterns which in turn have been displaced by a large number of shear zones. Geochemical evidence shows that the Areachap Group may represent a volcanic island arc. In order to explain the different orientations of the various phases of deformation, an oblique collision model with a rigid indentor is suggested. The model suggests that the initially pointed indentor caused thrusting in what is now described as the ± 2000 Ma Kheiss Province, situated on the edge of the craton. This event could have changed the shape of the indentor from pointed to flat, resulting in a differently oriented stress field thus causing the interference structures typical of this area. The structures resulting from this collision have been interpreted as the lateral ramp of a N-directed collision zone, causing thrusting to take place obliquely towards the northeast, so that the NW-trending shear zones can be considered frontal ramps. The NNW-trending shear zones have consequently been interpreted as oblique ramps. Careful analysis of fabric data and joint systems has led to the interpretation that σ1 was sub-perpendicular to the strike-slip zone, which together with compressive conditions is interpreted as a transpressive environment. Differential transport and the consequent rotation of σ1 caused the differential stresses responsible for the development of the en-echelon fault pattern with zones of reversed sense shearing.