THERMAL STRUCTURE AND CRUSTAL THICKNESS BENEATH THE ALBERTINE RIFT, UGANDA FROM POTENTIAL FIELDS DATA

The thermal and crustal structure beneath the Albertine rift that forms part of the western branch of the East African Rift System have been estimated from the Curie Point Depth (CPD), crustal thicknesses and heat flow measurements. These studies were to help us understand the thermal structure beneath the rift using aeromagnetic, satellite gravity and borehole data. The top and bottom of the magnetized crust were calculated using two-dimensional (2D) power-density spectrum analyses of the total field magnetic data in overlapping windows of 0.5^o x 0.5^o while the density contrast at the crust-mantle boundary was used to determine the Moho depth from spectral analysis of satellite Bouguer gravity data. In-situ borehole temperature measurements from oil wells and geothermal borehole gradient measurements were used to correlate with heat flow results calculated from the magnetic data. The crustal thickness estimations were constrained using the existing seismic Moho depth depth. Our results suggest CPD’s (19-22± 1 km), high heat flow ~67-79±0.2 mWm-2 and a thin crust 26-33 ±2km and temperature gradient of 27-31^oc/km beneath the 40-10±2 km wide Lakes George-Edward, Albertine and the Rhino rift basins. Deeper CPD depths are found in the surrounding basement and ranges between ~24-26±1 km, lower heat flow (~57-63 ±mWm-2), thicker crust (33-39±1 km) and temperature gradient (23-27^oc/km). The heat flow estimates from CPD were constrained using in-situ borehole measurements and they show similar trends in both the Cenozoic rifts and basement areas. Additionally, these results show a good correlation between hot springs and geothermal locations. The shallow CPD’s, elevated heat flow and thinned crust are consistent with previous seismic studies that have suggested the presence of partial melt intrusions especially beneath the Rwenzori Mountains, Lakes George and Edward basin to the south of the Albertine graben. The Albertine and Rhino grabens formed within the craton and the lack of magmatic activities at surface suggest a deeper thermal anomaly that is yet to reach the surface. However, the thermal anomaly increases southwards within the Western branch of the East African rift system. The thermal anomaly beneath rifts may be responsible for strain localization during rift initiation in amagmatic rifts.