Episodic intra-continental reactivation during collapse of a collisional orogen: The Damara Belt, Namibia

Abstract

Collision, high-angle contraction, crustal thickening and heating at 555–516 Ma, primed the Damara Belt ready for crustal collapse, which was triggered by a transition to ENE–WSW contraction along the length of the belt in response to orogenic events in east Gondwana at 516–505 Ma. Along-orogen shortening reworked and thickened the high-grade core of the belt, increasing gravitational instability, and establishing an NW–SE extension direction across the belt that was conducive to reactivation of pre-existing structures and eventual collapse. This extension direction persisted, and the switch to vertical σ1 and collapse was signalled by decompression melting at ∼502 Ma and subsequent rapid cooling. Collapse was focused on the high-grade core of the belt that was exhumed as a ∼170 km wide, semi-coherent massif-type metamorphic core complex with steep extensional shear zones and faults in the marginal flanks. During exhumation the core complex was reactivated by oblique-slip extensional shear zones that responded to external transient stress fields. Reactivation by middle- to lower-amphibolite facies dextral-normal shear zones at ∼500–495 Ma and ∼495–490 Ma, involved E–W to ENE–WSW shortening consistent with accretionary events in the west Gondwana margin during the Pampean Orogeny. Reactivation by greenschist facies sinistral-normal shear zones at ∼485 Ma, involved N–S shortening consistent with accretionary events in the south Gondwana margin during the Famatinian Orogen. Early stages of exhumation involved decompression melting, flattening folds and ductile ultramylonite zones within carbonate that formed by NW–SE extension. Late stage exhumation in the brittle field from ∼480 Ma onwards, involved a stress-switch to radial extension directions dominated by NE–SW. This stage involved flat-lying breccia, inclined faults, vertical fractures, and oxidizing fluids partitioned into the top of the lower-levels of the massif. Ongoing exhumation of the core complex drove localized NW–SE shortening within the flanking margins and hanging-wall, and produced low-strain reverse structures that straddle the ductile to brittle transition. The pressure difference between exhumed massif (4.8–5.5 kbar) and hanging-wall margins (3.9–4.2 kbar), indicate that ∼3.2–4.6 km of crust was stripped from above the core complex.