PREVIEW: The uplift history of the Nyika Plateau, Malawi, AESC 2021 - Malcolm McMillan

A 1 min preview of the upcoming Australian Earth Sciences Convention 2021 presentation by Malcolm McMillan (Thursday 11 Feb, Session 10.2) entitled The uplift history of the Nyika Plateau, Malawi: A long lived paleo-surface or a contemporary feature of the East African Rift?

McMillan, Malcolm (1), Boone, Samuel (1), Kohn, Barry (1), Gleadow, Andy (1), Chindandali, Patrick (2)

1-The University of Melbourne, School of Earth Sciences, Melbourne, Australia
2-Geological Survey of Malawi, Zomba, Malawi

Abstract:
The Malawi Rift is the southern-most expression of the magma-poor western branch of the East African Rift System. 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 Provence (RVP).

Since King (1962) 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 evidence (e.g. Njinju et al., 2019) 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 provides thermal history constraints on the upper ~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 and apatite and zircon (U-Th-Sm)/He 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 apatite fission track apparent ages with moderate mean track lengths ranging from ~11.2-12µm. Apatite helium ages are largely dispersed in all but two samples, which show consistent mid-Cretaceous apparent ages. Zircon helium ages are largely reproducible and consistently Devonian. Apatite fission track and helium ages are consistent with ages along the Livingstone Mountains (van Der Beek, 1998; Mortimer et al., 2016). Thermal history models suggest the Nyika Plateau is not a direct feature of modern-day rifting and has largely cooled slowly through the fission track partial annealing zone since at least the late Mesozoic. The youngest apatite fission track 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.