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

Abstract

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, ϕ, 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, ϕ 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 Ventersdorp, Bushveld, and the Soutpansberg, all represent extensional failure along planes oriented parallel to the local splitting fast polarization direction. Each of these events is interpreted to be a collisional rift, similar to the Baikal rift of northern Eurasia, where the stress field associated with collision produces extension and rifting for orientations at a small angle to the direction of the collision. Precise crustal geochronology associates both Ventersdorp and Great Dyke magmatism with the earliest and latest phases of the Limpopo collision, respectively. Similarly, the Bushveld magmatic event is temporally linked to the ~ 2.0 Ga reactivation of Neo-Archean structures in the Limpopo and surrounding areas by the Magondi Orogen, and the Soutpansberg is related to the ~1.9 Ga Kheis Orogen. Since the timing of these basaltic intrusions is controlled by temporal variations in lithospheric stress associated with orogenesis, it implies either that the melting process is genetically related to the evolution of the far-field collision, or that there was a semi-permanent reservoir of basaltic magma residing in the sublithospheric mantle during the ~1 billion-year time period spanned by these magmatic events. The existence of an extensive magma reservoir would argue for elevated temperatures just beneath the lithosphere during this time. Splitting delay times, δ t , a measure of the magnitude of anisotropy, reveal geologically controlled variations in the strength of anisotropy. In particular, the Meso-Archean Kaapvaal shield, the area that was not exposed to ~2.9 Ga and later deformational events, is effectively isotropic. We observe two areas where the anisotropic/isotropic transition is relatively sharp. The north-south boundary appears to coincide with the east-west trending Thabazimbi-Murchison Lineament. In the west, the boundary has been observed in the vicinity of Kimberley, South Africa, near the Colesberg Magnetic Lineament. The Eastern Shield has been relatively devoid of the kind of rifting and magmatic events seen elsewhere in cratonic southern Africa since the Meso-Archean, suggesting that the Eastern Shield lithosphere is mechanically stronger than surrounding areas. This relative strength difference may in part be due to the absence of the mechanical anisotropy inferred for the surrounding areas.