Exhumation, rift-flank uplift, and the thermal evolution of the Rwenzori Mountains determined by combined (U-Th)/He and U-Pb thermochronometry

Abstract

Rising over 5 km along the border of Uganda and the Democratic Republic of the Congo, the Rwenzori Mountains represent an extreme example of basement rift-flank uplift in the western branch of the East African Rift, a phenomenon common throughout the East African Rift System and characteristic of continental rift systems in general. A thermochronologic study combining (U-Th)/He and U-Pb analysis of apatite, titanite, and zircon separated from crystalline basement rocks was conducted across the Rwenzori block to characterize the timing and rate of rift-flank exhumation related to continental extension in east-central Africa. The thermochronologic data coupled with field and remote sensing observations make the case for recent and non-steady state uplift of the massif. Uranium-lead thermochronology indicate that, prior to Upper Neogene rifting, the rocks of the Rwenzori experienced a protracted history of slow cooling without major tectonothermal perturbation since at least the Paleoproterozoic (ca. 1900 Ma).

(cont.) Stream channel steepness profiles and thermochronometry along the western slope of the range show it to be the main active scarp that accommodates uplift. Relatively old (U-Th)/He zircon and apatite dates (>400 Ma, >70 Ma respectively) along the high peaks and eastern slope of the range reflect a transient lag period resulting from yet-insufficient exhumation to remove the inherited pre-rift cratonic thermal structure. This non-steady state condition of rapid uplift outpacing erosion has resulted in preservation of relict landsurfaces, truncated spurs, hanging valleys, uplifted river terraces, and vast stranded bogs at high elevation. Given the low cooling rate and geothermal gradient prior to rifting implied by U-Pb thermochronometry we determine that no more than 1.7 km of erosion could have accompanied uplift on the order of at least 5 km in the Rwenzori region. Biostratigraphic evidence suggests the range rose from beneath local baselevel within the last 2.5 Ma. This requires a minimum average uplift rate of 1.6 km/Myr. Regardless of the active rock uplift rate of the Rwenzori, net exhumation cannot yet have exceeded the depth of the (U-Th)/He closure isotherm in apatite (<1.7 km). These results highlight the danger of modeling young orogenic systems using the simplifying assumption of topographic steady state.