Metamorphic response and crustal architecture in a classic collisional orogen: The Damara Belt, Namibia

Abstract

The Damara Belt is a well-exposed mid-crustal section through a collisional orogen of Cambrian age that closed the Khomas Ocean basin between passive margins on the Congo and Kalahari Cratons. Collision resulted in a bi-vergent orogen with a distinct paired metamorphic pattern of foreland-vergent high-P/low-T orogenic margins and a broad high-grade, low-P/high-T orogen core. Spatial and temporal patterns of the metamorphic response to collision have been characterized for all parts of the belt using; a large dataset (n~240) of internally consistent quantitative PT determinations, evolution of mineral parageneses and semi-quantitative P-T paths, metamorphic mapping and quantitative metamorphic field gradients. Integration with deformation history, structural profiles, metamorphic chronology, magmatic history and stratigraphy, constrains a dynamic model of crustal architecture during peak metamorphic events. The pattern of zonal metamorphic response is demarcated by three major metamorphic discontinuities (MD) with steep pressure gradients, inferring crustal-scale structures that accommodated lateral exhumation of crustal wedges. Discontinuities are confirmed by deformation features in the field, and metamorphic mineral growth indicates that vertical flattening at the peak of metamorphism progressing through ductile to brittle extensional structures. Crustal wedges along the orogenic margins experienced steep clockwise P-T paths with peak-PT conditions terminated by isothermal decompression during rapid exhumation in isostatic response to deep burial. Metamorphic chronology and over-printing metamorphic fabrics constrain a sequence of foreland propagating out-wedging of crustal thrust wedges that resulted in telescoping of the orogenic margins. Peak metamorphism at a geothermal gradient of 20–25°C/km and 8–9kb in the Southern Zone (Wedge I) was attained between ~517–530Ma, followed by south-directed out-wedging at the Uis-Pass Suture, accommodated by relative extension (MD1) at high structural levels near the boundary with the high-grade orogen core. Out-wedging of the Southern Zone, further buried the Southern Margin Zone (Wedge II) in the footwall below the Uis-Pass Suture. Peak metamorphism at 17°C/km and 9.5–11.5kb in the Southern Margin Zone was attained at 517±4Ma and followed by out-wedging on basal thrusts, accommodated by vertical flattening and extensional reactivation of the Uis-Pass Suture (MD2). Peak metamorphism at 17°C/km and 10.5kb in a high-P/low-T crustal wedge in the northern margin (Wedge III) occurred at 510±4Ma. Peak metamorphism in this wedge was terminated by isothermal decompression during north-directed thrusting, accommodated by extension at higher structural levels near the high-grade orogen core (MD3). Granulite facies metamorphism in the high-grade orogen core, which formed the upper plate to the deeper thrust wedges occurred at significantly lower pressures (4.3–6.0kb), higher T/depth ratios (≥38–47°C/km) and low ∆P/∆T clockwise P-T paths indicating only moderate burial and protracted high-grade conditions. Metamorphic chronology confirms high-grade conditions (540–505Ma) persisted beyond isostatic adjustment of the high-P/low-T orogenic margins. High-heat flow conditions and long thermal lag are best explained by lithospheric breakoff during collision. In contrast to the high-P/low-T margins that experienced lateral exhumation in a convergent system, the high-grade orogen core was exhumed vertically as a broad core complex during extensional collapse of the orogen. Peak metamorphic conditions in the central part of the orogen core record higher pressures than marginal lower-grade zones to both the north and south, indicating vertical exhumation as a broad massif. This post-peak vertical exhumation of the central core occurred after ~505 Ma was accommodated by brittle extensional detachments at high stratigraphic levels with km-scale omission of crustal section.