McMillan, Malcolm1, Boone, Sam1, Kohn, Barry1, Gleadow, Andy1, Chindandali, Patrick2
1The University of Melbourne, Melbourne, Australia, 2Geological Survey of Malawi, Zomba, Malawi
The Malawi Rift is the southern-most expression of the magma-poor western branch of the East African Rift. A noticeable feature belonging to Malawi is the Neogene Lake Malawi (Nyasa), occurring directly above the main locus of the Malawi Rift. Lake Malawi hosts a series of half-grabens of alternating polarity with elevated shoulders. In the northern region of Lake Malawi, just ~10km west of the largest border fault system, the Nyika Plateau rises ~2100m above the surrounding landscape. Nyika is a Paleoproterozoic (Ubendian) granitic intrusion, surrounded by Neoproterozoic Pan-African metamorphic complexes with sparse Permian Karoo sediments outcropping to the north and east of the plateau. Similar elevations to Nyika occur in the area, on the Mozambiquan side of Lake Malawi above the Livingstone border fault and to the north of Lake Malawi, in the Oligocene Rungwe Volcanic Province (RVP).
Since King1 it has been contentiously hypothesized that such high-relief plateaux, like Nyika and the Livingstone mountains, are long-lived “Gondwana surfaces” and are largely unrelated to the modern-day Malawi Rift. However, recent seismic evidence2 suggests a zone of thinned lithosphere beneath the RVP persists broadly beneath the Nyika Plateau and the Livingstone Mountains, and may indicate that higher elevations in the surrounding region are actively supported by rising asthenospheric mantle, resulting in relatively rapid, recent, tectonic uplift.
Low-temperature thermochronology typically provides thermal history constraints on the upper ~3-7km of the crust, using radiometric dating techniques sensitive to changes in the thermal regime, from events such as rapid uplift causing denudation or changes in the geothermal gradient. Here, we use apatite fission track (AFT), apatite (U-Th-Sm)/He (AHe), and zircon (U-Th)/He (ZHe) thermochronology to further investigate the uplift history of the Nyika Plateau and the surrounding region from 25 samples collected with help from the Geological Mapping and Mineral Assessment Project (GEMMAP) and Malawi’s Geologic Survey Department.
We found consistently Permo-Triassic AFT apparent ages with moderate mean track lengths ranging from ~11.2-12µm. AHe ages are largely dispersed in all but two samples, which show consistent mid-Cretaceous apparent ages. ZHe ages are consistently Devonian. AFT and AHe ages are consistent with ages previously reported along the Livingstone Mountains3, 4. Thermal history models suggest that the Nyika Plateau is not a direct feature of modern-day rifting and has largely cooled slowly through the partial annealing zone since at least the late Mesozoic. The youngest AFT age (~70Ma) occurs off the plateau to the north, directly adjacent to a Permian Karoo sequence. Using this Karoo deposition as a thermal history constraint indicates the area may have been completely covered by Karoo deposition, up to ~2km (considering moderate geothermal gradients), that may have blanketed Nyika and the surrounding region in the Permo-Triassic.
1King, 1962. The Morphology of the Earth, Oliver and Boyd, Edinburg and London, 29.
2Njinju et al, 2019. Lithospheric Structure of the Malawi Rift: Implications for Magma-Poor Rifting Processes. Tectonics 38, 3835-3853.
3van der Beek et al, 1998. Denudation history of the Malawi and Rukwa Rift flanks (East African Rift system) from apatite fission track thermochronology. J. African Earth Sciences 26, 363-385.
4Mortimer et al, 2016. Spatio-temporal trends in normal-fault segmentation recorded by low-temperature thermochronology: Livingstone fault scarp, Malawi Rift, East African Rift System. Earth and Planetary Science Letters 45, 65-72.
Malcolm is a PhD student at the University of Melbourne. His research focuses on investigating dynamic topography related to active rifting in Malawi using low temperature thermochronology.