Mantle Anisotropy, Collisional Rifts and the Magmatic Evolution of Southern Africa: Old (Mantle) Fabric Never Dies

Abstract

The observed seismic anisotropy of the southern African mantle, made possible by data from the Southern African Seismic Experiment, provides important constraints on modes of mantle deformation beneath this ancient continent. We find that the mantle anisotropy is dominated by deformational events in Archean times occurring within the lithosphere, and can thus be used to assess the mantle's role in the tectonic evolution of this region. The pattern of mantle anisotropy derived from shear wave splitting observations reveals two noteworthy characteristics. First, there is a spatially continuous arc of mantle anisotropy extending from the western Kaapvaal and Zimbabwe Cratons to the northeastern Kaapvaal. All along the arc, the splitting fast polarization direction, 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 ca. 2.9 to 2.7 Ga during collisional accretion of both the western Kimberley and northern Pietersburg blocks onto the ca. 3.2 Ga eastern shield of the Kaapvaal, with accretion on the northern ramparts of the Kaapvaal ultimately culminating in the Neo-Archean Limpopo orogen. Second, there is mantle anisotropy closely associated with the Great Dyke with values of Φ parallel to the Dyke, just north of and distinct from this arc of anisotropy. We present evidence that the mantle fabric that generates seismic anisotropy also constitutes preexisting structure in the mantle that is subsequently reactivated, much like the more commonly cited reactivation of crustal structures. In particular, we argue that the Neo-Archean collision imparted a mechanical anisotropy to the mantle that controlled the subsequent magmatic history of cratonic southern Africa (old fabric never dies!). Study of mechanical anisotropy of olivine aggregates suggests that failure is most likely parallel to the olivine b-plane, which for transcurrent deformation is a vertical plane striking parallel to the splitting fast polarization direction. In particular, we suggest that three major Precambrian magmatic events: the Great Dyke, the Ventersdorp, and Bushveld all represent extensional failure along planes oriented parallel to the local splitting fast polarization direction. In each case, the rift orientations associated with these magmatic events are locally subparallel to values of Φ, with the Great Dyke being the most dramatic example. In addition, each of these magmatic events is likely a collisional rift, similar to the Baikal rift of northern Eurasia, where the stress field associated with a collision produces extension and rifting for orientations at a high angle to the belt of the collision. Precise crustal geochronology associates the Ventersdorp and Great Dyke with the earliest and latest phases of the Neo-Archean Limpopo collision, respectively, whereas the Bushveld is temporally linked to the Proterozoic Magondi orogen, which is also responsible for the reactivation of Neo-Archean structures in the Limpopo and surrounding areas. In all cases the rifts are at a high angle to the collisional belt as predicted for a collisional rift.

Meeting Name

AGU Fall Meeting (2003: Dec. 8-12, San Francisco, CA)

Department(s)

Geosciences and Geological and Petroleum Engineering

Keywords and Phrases

Continental tectonics: general; Stresses: crust and lithosphere

Document Type

Article - Conference proceedings

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2003 American Geophysical Union (AGU), All rights reserved.

Publication Date

01 Dec 2003

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