ROOF UPLIFT IN THE MID-CRUST - A CASE STUDY FROM THE DAMARA BELT, NAMIBIA

Space creation during the emplacement of sheet-like granite plutons under high confining pressures in the mid-crust is commonly thought to be tectonically or structurally assisted, or as being accomplished by the downward translation of the wall rocks beneath the pluton floor.

The 180 km² Kubas granite is intrusive into mid-crustal (11-13 km), amphibolite facies rocks of the Pan-African (550-500 Ma) Damara Belt, Namibia. This late- to post-tectonic granite pluton is emplaced into the southern limb of a northeasterly trending regional-scale anticline, where the exposed upper parts of the pluton display a laccolithic geometry defined by a flat, shallowly dipping roof and relatively steeply outward dipping sidewalls. The displacement and steepening of the overlying metasediments document uplift of the roof rocks by between 1.4 and 3km, suggesting a growth model where the emplacement of an initial sill is followed by predominantly vertical space creation by uplift of the roof rocks. Despite this displacement, the wall rock stratigraphy is continuous across the pluton. The translation of the wall rocks has been achieved by a combination of lithologically controlled, rheology-dependent deformation of the wall rocks. Deformation is mostly brittle in competent units where fractures open in response to the sidewall-parallel stretch induced during emplacement. The displacement is accommodated in the weaker layers by ductile thinning and pervasive fabric development.

The Kubas granite illustrates that the emplacement of large (>100 km3) granitic plutons can be driven solely by the injection of roughly lithostatically pressured magmas and accommodated by the combined brittle-ductile displacement of hundreds of cubic kilometers of wall rocks in the crustal column overlying the emplacing pluton. These results suggest that structural controls such as dilational sites or shear zones need not necessarily be invoked during space creation in the mid crust.