Kimberlite (U-Th)/He dating links surface erosion with lithospheric heating, thinning, and metasomatism in the southern African Plateau

Abstract

Documenting the surface response to processes at depth is a central challenge in continental tectonics. Kimberlites bear an unusualrecord of both surface and deep processes through their downrafted crustal clasts, pipe erosion levels, and mantle xenolith suites. Here we combine new apatite (U-Th)/He (AHe) data from four South Africa kimberlites ranging in emplacement age from ca. 143 Ma to ca. 74 Ma with a wealth of other geologic information from the pipes o resolve the timing, patterns, and causes of erosion across ~200 km of the southern African Plateau. Comparison of the AHe dates with the kimberlite eruption ages constrains a strong regional unroofing pulse ca. 117-90 Ma, which was most intense from ca. 100 to 90 Ma. This nroofing phase is synchronous with warming, thinning, and metasomatism of the mantle lithosphere recorded by xenocrysts and peridotite xenoliths within these same kimberlites. These thermochemical changes could be related to still deeper dynamic mantle processes. Our AHe data appear to directly document the erosional response to surface uplift induced by these various events. The AHe results also detect as much as 1.5 km of Cenozoic unroofing that is spatially centered on a proposed mid-Tertiary paleotributary to the Orange River, suggesting rainage patterns as the cause. Our results not only tightly refine the surface history of the interior of the southern African Plateau, but how that kimberlite AHe dating can tie cratonic erosion phases withcontemporaneous thermochemical changes in the mantle and therebypoint to causative links. © 2013 Geological Society of America.