Thermal Perturbations Beneath the Incipient Okavango Rift Zone, Northwest Botswana

We use aeromagnetic and Bouguer gravity data from the incipient Okavango Rift Zone (ORZ), the surrounding Archean cratons and Proterozoic mobile belts in northwestern Botswana to estimate the depth to the Curie isotherm and the crustal thickness beneath the rift. Imaging the depth to the base of magnetic sources is important in understanding and in constraining the thermal structure of the continental crust in zones of incipient continental rifting where no other data are available to image the crustal thermal structure. Our objective was to determine if there are thermal perturbations within the lithosphere under this young rift that might be suggestive of the presence of magmatic processes associated with rift initiation. The top and bottom of the magnetized crust were calculated using the two dimensional (2D) power-density spectrum analyses and three dimensional (3D) inversion of the total field magnetic data in overlapping windows of 1o x 1o. We found out that the Curie Point Depth (CPD) estimates vary between ~7 and ~23 km. The deepest CPD values (16 to 23 km) occur within the cratons which have lithospheric thickness greater than 200 km. Within the orogenic mobile belts, the CPD estimates range between 15 and 17 km. The shallowest CPD values of 7 - 14 km occur beneath the ORZ. In the northeastern part of ORZ with more developed rift structures, hot springs are known to occur. The shallowest CPD values, high heat flow and thin crust occur within a zone of ~60 km along the southeastern boundary of the ORZ and coincide with a strong NE-trending basement fabric, possibly acting as conduits for upward migration of hot fluids. These observations suggest the existence of a possible thermal anomaly within the lithosphere at shallow crustal depths beneath ORZ. We interpret the thermal anomaly as resulting from an infiltration front of melts/fluids due to ascending asthenosphere. Additional detailed tomographic imaging to be obtained as part of the Project for Rift Initiation, Development and Evolution experiments is needed to confirm this proposition...Geolog

Thermal Perturbations beneath the Incipient Okavango Rift Zone, Northwest Botswana

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-2) 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 ( \u3c 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-2), 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

Thermal Perturbations beneath the Incipient Okavango Rift Zone, Northwest Botswana

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-2) 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 ( \u3c 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-2), 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

Thermal perturbations beneath the incipient Okavango Rift Zone, northwest Botswana

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-2\u3e) 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 (\u3c\u3c30 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-2), 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