Tag team tectonics: mantle upwelling and lithospheric heterogeneity ally to rift continents (Invited)
Abstract
The configuration of continents we know today is the result of several billion years of active Wilson Cycle tectonics. The rifting of continents and subsequent development of ocean basins is an integral part of long-term planetary-scale recycling processes. The products of this process can be seen globally, and the East African Rift System (EARS) provides a unique view of extensional processes that actively divide a continent. Taken together with the adjoining Red Sea and Gulf of Aden, the EARS has experienced over 40 Ma of volcanism and ~30 Ma of extension. While early (pre-rift) volcanism in the region is attributed to mantle plume activity, much of the subsequent volcanism occurs synchronously with continental rifting. Numerous studies indicate that extension and magmatism are correlated: extension leads to decompression melting while magmatism accommodates further extension (e.g. Stein et al., 1997; Buck 2004; Corti 2012). Evaluation of the entire EARS reveals significant geochemical patterns - both spatial and temporal - in the volcanic products. Compositional variations are tied directly to the melt source(s), which changes over time. These variations can be characterized broadly by region: the Ethiopian plateau and Turkana Depression, the Kenya Rift, and the Western Rift. In the Ethiopian plateau, early flood basalt volcanism is dominated by mantle plume contributions with variable input from lherzolitic mantle lithosphere. Subsequent alkaline shield volcanism flanking the juvenile Main Ethiopian Rift records the same plume component as well as contributions from a hydrous peridotitic lithosphere. The hydrous lithosphere does not contribute indefinitely. Instead, young (< 2 Ma) volcanism taps a combination of the mantle plume and anhydrous depleted lithospheric mantle. In contrast, volcanism in the Kenya Rift and the Western Rift are derived dominantly from metasomatized lithospheric mantle rather than mantle plume material. These rifts lie in the mobile belts flanking the Archean Tanzanian craton, suggesting the lithosphere in these regions has a complex geodynamic history that provides an avenue of weakness for rift development around the craton. In the Tanzanian portion of the Kenya Rift, highly sodic mafic and carbonatitic lavas dominate both the rift valleys and rift shoulders. Mantle xenoliths from this region show petrographic and isotopic evidence for ancient and recent metasomatic events. In the Western Rift, highly potassic mafic lavas dominate the volcanic landscape in the rift valley and rift margins. The lithospheric mantle underlying this region is highly metasomatized, as evidenced by the presence of clinopyroxenite, websterite and glimmerite xenoliths and the absence of peridotite. The highly fusible metasomatized mantle beneath both the Kenya and Western rifts enables lithospheric melting with comparatively minor P-T perturbation. Regardless of which process begot the other, lithospheric thinning promotes melting while volcanism promotes lithospheric weakness and enables rift propagation. When viewed holistically, both mantle plume activity and lithospheric heterogeneities have played a vital role in the initiation and subsequent evolution of the rifting throughout eastern Africa.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T13G..01N
- Keywords:
-
- 8415 VOLCANOLOGY Intra-plate processes;
- 8109 TECTONOPHYSICS Continental tectonics: extensional;
- 1038 GEOCHEMISTRY Mantle processes