Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana

Abstract

We used aeromagnetic and gravity data to investigate the thermal structure beneath the incipient Okavango Rift Zone (ORZ) in northwestern Botswana in order to understand its role in strain localization during rift initiation. We used three-dimensional (3-D) inversion of aeromagnetic data to estimate the Curie Point Depth (CPD) and heat flow under the rift and surrounding basement. We also used two-dimensional (2-D) power-density spectrum analysis of gravity data to estimate the Moho depth. Our results reveal shallow CPD values (8-15 km) and high heat flow (60-90 mW m À2 ) beneath a~60 km wide NE-trending zone coincident with major rift-related border faults and the boundary between Proterozoic orogenic belts. This is accompanied by thin crust (<30 km) in the northeastern and southwestern parts of the ORZ. Within the Precambrian basement areas, the CPD values are deeper (16-30 km) and the heat flow estimates are lower (30-50 mW m À2 ), corresponding to thicker crust (~40-50 km). We interpret the thermal structure under the ORZ as due to upward migration of hot mantle fluids through the lithospheric column that utilized the presence of Precambrian lithospheric shear zones as conduits. These fluids weaken the crust, enhancing rift nucleation. Our interpretation is supported by 2-D forward modeling of gravity data suggesting the presence of a wedge of altered lithospheric mantle centered beneath the ORZ. If our interpretation is correct, it may result in a potential paradigm shift in which strain localization at continental rift initiation could be achieved through fluid-assisted lithospheric weakening without asthenospheric involvement.

Acquisition of a Unique Onshore/Offshore Geophysical and Geochemical Dataset in the Northern Malawi (Nyasa) Rift

“…Analyzing and integrating these data will enable the development of a complete picture of this early-stage rift and extract valuable constraints on continental rifting processes. We expect that our study will strongly complement previous, ongoing, and future studies in this and other parts of the EAR, including the Project for Rift Initiation Development and Evolution (PRIDE) (Bufford et al, 2012;Leseane et al, 2015;Yu et al, 2015).…”

Is the Okavango Delta the terminus of the East African Rift System? Towards a new geodynamic model: Geodetic study and geophysical review

“…Some authors add the OG as the terminus of the southwestern extension branch (e.g. Scholz et al, 1976;Modisi et al, 2000;Leseane et al, 2015). As this southwestern extension branch is poorly defined, we name the region from the EARS in the east, Lake Upemba in the north and the OG in the southwest, the South-Western Extension Area (SWEA).…”

“…Crust models for the OG area are proposed by two local studies based on seismic receiver functions (Yu et al, 2015b) and aeromagnetic and gravity data (Leseane et al, 2015). Both studies indicated a thinned crust beneath the OG relative to the adjacent crust although they suggest different magnitudes for the thinning (4 to 5 km and 10 to 12 km respectively).…”

Heterogeneity and anisotropy in the lithospheric mantle

“…Advective heating by migrating magmas might affect larger volumes of the lithosphere in case of continued magmatic activity. Heating of the lithospheric mantle and crust by migration of hot mantle fluids focused in Precambrian lithospheric shear zones has been invoked, for instance, to explain the shallow Curie temperatures, which imply temperatures of >500°C at depths of 8-15 km, and high heat flows (60-90mWm -2 ) observed in the incipient Okavango Rift Zone in southern part of the East African rift (Leseane et al, 2015). However, though it may be important atop plumes and in arcs, the actual contribution of heat advection by migrating melts in heating the lithosphere is still poorly constrained.…”