Wrench-shearing during the Namaqua Orogenesis—Mesoproterozoic late stage deformation effects during Rodinia assembly

Abstract

The Namaqua Orogeny is interpreted in terms of Rodinia assembly during Grenvillian times at 1.3–1.0Ga. The exposed terranes of the Namaqua Metamorphic Complex provide evidence of crustal thickening owing to horizontal shortening and massive silicic magmatism during amalgamation of the Kalahari Craton. This accretion stage of the Namaqua Orogeny produced coplanar and colinear LS-tectonite fabrics of sillimanite and higher metamorphic grades (the D1–D3 deformation phases, 1220–1065Ma), that are sub-parallel to the SW-directed tectonic overthrust transport direction of the terranes.The structures formed during the accretion stage are typical of major ductile shear zones in the mid-crust that are formed during continent-continent collisions. After a pause of 150Ma all of the terranes were comprehensively folded and sheared during the post-accretion stage (the D4–D6 deformation events). The D4 event produced macroscopic upright to inclined asymmetric Z-folds with fold hinges sub-parallel to the SW-directed tectonic transport direction (in terms of contemporary Gondwana coordinates). This post-accretion stage involved continued but oblique NW-directed movement of the Kalahari Craton, coupled with N–S compression.Attributes of the F4 structures are interpreted in terms of five models: the thrust-ramp-, constrictive-, strike-slip-, transtensional- and differential (wrench-shear) transport models. It is concluded that the last-mentioned is the only model that explains all of the observed characteristics. Sub-horizontal wrench-shear that caused reorientation of the incremental strain ellipsoid is superimposed on all earlier structures, resulting in the production of oblique folds in the thrusted terranes. This wrench-shear model explains the end-phase of a protracted orogenic cycle involving an indentational continent collision at 1.030Ga, possibly between the Kalahari and Laurentia Cratons. The deformation affected the entire crust and even the upper part of the underlying mantle.