Seismic anisotropy, mantle fabric, and the magmatic evolution of Precambrian southern Africa

The observed seismic anisotropy of the southern African mantle from both shear-wave splitting and surface wave observations provides important constraints on modes of mantle deformation beneath this ancient continent. We find that the mantle anisotropy beneath southern Africa is dominated by deformational events in Archean times occurring within the lithosphere, rather than present-day processes in the sublithospheric mantle. Consequently, the distribution and magnitude of anisotropy provide valuable data to constrain the mantle’s role in the tectonic evolution of this region. The pattern of mantle anisotropy reveals several noteworthy characteristics. First, mantle anisotropy is closely associated with the Great Dyke of the Zimbabwe Craton, with values of the splitting fast polarization direction, , parallel to the Dyke. This correspondence with the Great Dyke is likely not due to the present-day Dyke structure but instead is most probably due to the emplacement of the Dyke parallel to pre-existing mantle fabric within the Zimbabwe craton. This deformation thus predates dike emplacement and is no younger than Neo-Archean in age. Second, there is a spatially continuous arc of mantle anisotropy extending from the western Kaapvaal Craton to the northeastern Kaapvaal and Limpopo Belt. All along the arc, is subparallel to the trend of the arc. Given the crust/mantle chronology associated with these regions, the anisotropy likely represents deformation that occurred at ~2.9 to ~2.6 Ga during collisional accretion of both the western Kimberley and northern Pietersburg blocks onto the seismically isotropic eastern shield of the Kaapvaal, with accretion on the northern ramparts of the Kaapvaal ultimately culminating in the Neo-Archean Limpopo orogen. The anisotropy-inferred arc of deformation reveals diverse zones of both strong and weak coupling between the crust and mantle, as measured by the coherence between mantle deformation and geologically-inferred surface deformation. In particular, there is high coherence between surface and mantle deformation at the southwestern and northeastern ends of the arc, which implies strong crust-mantle coupling in these regions. Conversely, apparent decoupling exists in the northwestern portion of the arc, where northeast to southwest trending anisotropy cuts across north to south trending structures, such as the surface outcrop and aeromagnetic expressions of the Kraaipan Greenstone Belts. Independent seismic evidence from seismic reflection profiling supports the conclusions that these north-south-trending crustal features are superficial and confined to the upper crust. We present evidence that the mantle fabric producing seismic anisotropy constitutes fossil structure in the mantle that is subsequently reactivated, much like the more commonly acknowledged reactivation of crustal structures. In particular, we argue that Neo-Archean collisional orogenesis imparted a mechanical anisotropy to the mantle that controlled the subsequent magmatic history of cratonic southern Africa. We furthermore suggest that four major Precambrian magmatic events: the Great Dyke, the PAUL G. SILVER, MATTHEW J. FOUCH, STEPHEN S. GAO, MARK SCHMITZ AND KAAPVAAL SEISMIC GROUP SOUTH AFRICAN JOURNAL OF GEOLOGY, 2004, VOLUME 107, PAGE 45-58 45