Thermal history modelling in extensional settings: the Rwenzori Mountains of the East African Rift

Abstract:

The formation and evolution of extensional tectonic systems continues to stimulate considerable geological debate. Low-temperature thermochronology is consistently applied in these settings to study the timing and magnitude of tectonism. However, data from extensional settings continues to test our understanding of these dating systems and how best to model thermal histories from complex datasets.

The Western Branch of the East African Rift is believed to have begun rifting in the Miocene. The Rwenzori Mountains, Western Uganda, are found in the northern extent of the Western Branch and are believed to have formed from rapid uplift from since the late Miocene to present (1.6 km/Myr). Apatite fission-track ages from the region show a variety of ages ranging from the 420 Ma to 10 Ma, while apatite (U-Th-Sm)/He ages range from 208 Ma to 3 Ma with samples presenting varying levels of ag dispersion. Previous thermal history models have supported Miocene aged uplift, however, failed to effectively resolve the late Cenozoic. New thermal history models, derived from new apatite fission-track and (U-Th-Sm)/He data, outline two distinct cooling episodes in the Eocene–Oligocene and late Miocene–Pliocene that suggest the Rwenzori’s have been exhuming since the Paleogene. Our thermal modelling approach combines thermochronology and previous published geochronology with the Bayesian approach of QTQt with a radiation damage model that incorporates differing levels of radiation damage in each grain to improve the fit of (U-Th-Sm)/He data.

In additional to thermal history modelling, uplift histories are derived by combining river profiles and thermal history modelling results to constrain the timing and rate of rock uplift. This additional step directly incorporates thermal history models into a rate of uplift across the Rwenzori’s and provides greater insight into the tectonic history. Uplift histories show the Rwenzori’s have likely been uplifting since the Palaeogene. These results provide evidence that extension in the Western Branch began in the Paleogene, which has major implications for the tectonic evolution of the East African Rift system.