EXHUMATION, RIFT-FLANK UPLIFT, AND THE THERMAL EVOLUTION OF THE RWENZORI MOUNTAINS DETERMINED BY COMBINED (U-TH)/HE AND U-PB THERMOCHRONOMETRY

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, a phenomenon common throughout the East African Rift System and characteristic of continental rift systems in general. A thermochronologic study combining U-Pb and (U-Th)/He analysis of apatite, titanite, and zircon was conducted within the Rwenzori to characterize the timing and rate of rift-flank exhumation related to continental rifting in east-central Africa. New thermochronologic work coupled with field and remote sensing observations makes the case for recent and non-steady state uplift of the massif. Uranium-lead results indicate that, prior to Neogene rifting, the rocks of the Rwenzori experienced a protracted history of slow cooling without major tectonothermal perturbation since at least the Paleoproterozoic. Stream channel steepness profiles and thermochronometry along the western slope of the range show it to be the main active scarp accommodating uplift. Relatively old (U-Th)/He zircon and apatite ages (>400 Ma, >70 Ma respectively) along the eastern slope of the range reflect a transient lag period resulting from yet-insufficient exhumation to remove the inherited pre-rift thermal structure. This non-steady state condition of rapid uplift outpacing erosion has resulted in preservation of relict landsurfaces, truncated spurs, hanging valleys, vast stranded bogs and uplifted river terraces at high elevation. Given the typical continental geothermal gradient prior to rifting implied by U-Pb thermochronometry, no more than 2 km of erosion could have accompanied uplift on the order of 5 km in the Rwenzori region. This requires a minimum average uplift rate of 1.6 km/Myr based on biostratigraphic evidence suggesting the range rose from beneath local baselevel within the last 2.5 Ma. 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 (~2 km). These results highlight the danger of modeling young orogenic systems using the simplifying assumption of topographic steady state.