Metamorphic expressions of transpressional deformation: the Damara Orogen, Namibia

Project Investigators

Doctor Ben Goscombe

Associate Professor Martin Hand

Doctor Jo Mawby (CERG Alumni)

Project Collaborators

Professor David Gray (The University of Melbourne)

Project details

Most studies of transpressional orogens have been structural studies, and these are principally from the upper-crust. Metamorphic studies provide an alternative dataset to explore the flow of material in transpressional Orogens. Transpressional systems show marked domainal variation in kinematic character. The competion between heat conduction and heat advection in these dynamic systems is expected to give rise to domainal variation in metamorphic character that reflects the kinematic partitioning developed. The metamorphic response in the Kaoko Belt of the Damara Orogen to transpressional systems was studied and linked with the kinematic patterns developed.

The Kaoko Belt of the Damara Orogen in Namibia, is the deeply eroded core of a sinistral transpressional Orogen that has half-flower structure geometry centred on the major, 4-5 km wide Purros Mylonite Zone. The Central Kaoko Zone represents the Barrovian-style 'Escape Zone' with large-scale nappes verging outward towards the margin. The Western Kaoko Zone experienced intense wrench-style deformation and is dissected by crustal-scale shear zones. The 'Orogen Core' is comprised of three distinct metamorphic terranes and is bound by two crustal-scale shear zones. On the east is the steep, obliquely reverse Purros Mylonite Zone. On the west side, the Three Palms Mylonite Zone has variable inclination polarity and sub-horizontal lineations, mostly with down-plunge transport, suggesting an overall extensional strike-slip shear zone. Outboard to the west is the Coastal Terrane, which experienced an earlier high-grade metamorphic cycle at 650 Ma and was subsequently reworked at low-amphibolite facies grades during the main phase transpressional Orogenesis and metamorphism of the rest of the Kaoko Belt, at 580-550 Ma.

Across-orogen metamorphic field gradients indicate a marked contrast between the Escape Zone and Orogen Core. The Escape zone experienced an inverted Barrovian-series high-P/moderate-T metamorphism and low thermal gradients of 20 ºC/km. The Orogen Core experienced moderate-P/high-T metamorphism and high thermal gradients of 30-40 ºC/km. Clockwise P-T loops are experienced in all parts of the Kaoko Belt. Tight, shallow P-T loops are experienced in the high-thermal gradient Orogen Core domains. Steep and more open P-T loops are experienced in the crustal over-thrust Escape Zone that experience low thermal gradients.

P-T and geothermal gradient variation across the Kaoko Belt.

Significantly, there is also a marked variation in the metamorphic field gradient along the length of the Orogen Core. Two high-T/moderate-P lobes or terranes are separated by a low-T/low-P metamorphic 'trough', though the thermal gradient remains high in all three domains. The extruding high-P/high-T lobes coincide with low-angle obliquely reverse stretching lineation trajectories and the low-T/low-P 'trough' coincides with downward lineation trajectories, suggesting that the stretching lineation array approximately reflects particle paths.

Developing universal models of transpression is no easy matter. Thermal gradients reflect exhumation rates, which may ultimately be linked to obliquity of convergence, offering a potential sensitive indicator of obliquity in metamorphic belts. Similarly, determining particle paths in transpressional orogens is no simple matter. Stretching lineations and kinematic indicators may record the wrench component of deformation, but this may be decoupled from the vertical component of particle paths, which is otherwise best investigated by metamorphic studies.

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