Phanerozoic palaeo-geothermal and exhumation constraints for Tanzanian cratonic rocks: Reconciling thermal conductivity of cover rocks and apatite fission track data from borehole samples

Abstract This study presents apatite fission track (AFT) data for the 2.7 to 2.9 Ga basement rocks of the Tanzanian Craton (TC) in order to reconstruct the past geothermal gradient, exhumation and cooling histories. Measured near-surface heat flow and sedimentary crustal siliciclastic rock constraints are used in an attempt to well ascertain the thermal gradient and amount of unroofing/erosion since the Carboniferous. AFT assessment of the gradient is estimated through modelling of the data, assuming no sedimentary rocks covered the basement prior to exhumation whereas the thermal conductivity application assumes the presence of cover rocks scenario on thermal perturbation prior to uplift of the basement. In the second scenario, the geothermal gradient results into ca. 10°C/km, thus, the suggested 1 km cover rocks points to a total exhumation of approximately 6.5 km since the Carboniferous. AFT ages range between 215 ± 27.0 Ma to 310 ± 30 Ma for the deepest and shallowest samples, respectively. Mean confined track lengths decrease with depth from 12.6 ± 0.2 μm to 10.8 ± 0.3 μm, a feature which is caused by progressive annealing. Expected thermal models determined using the inverse approach on the AFT data suggest a geothermal gradient of ca. 9°C/km by neglecting the sedimentary cover rocks effect. Modelling suggests that the samples were at ca.10 km below surface during the Carboniferous (340 Ma) and a total of ∼9 km overburden was stripped-off following exhumation. The record of sedimentation within the adjacent Karoo basins reflects erosion of the cratonic rocks as evidenced by seismic data that suggest a minimum of 5 km of sediments having been deposited during the late Carboniferous − Triassic in southern Tanzania. Both estimations of palaeo-geothermal gradient results using the two approaches are indistinguishable and suggest that the gradient has been stable through the Phanerozoic era. However, possible synchronization of the two approaches’ uncertainties in ascertaining a precise geothermal gradient remains to be tested.