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Project B2: Low-temperature thermochronology, uplift and denudation history of the Rwenzori Mountains, Uganda

In East Africa the feedback between mountain formation and decay as well as possible perturbations of the climate pattern is studied by the multidisciplinary research group - Riftlink. The focus thereby is placed on the Albertine Rift, and therein raising Rwenzori Mountains. Major questions related to the Rwenzoris refer to the timing of their formation: has their height, which exceeds more than 5.000 m a.s.l. to be entirely ascribed to movements of the rift system within the last 23 Ma (Neogene)? Or do they represent an old basement block that formed a topographic high long before? To unravel these questions the thermochronology study (Glasmacher, B2) seeks to constrain the history of the topographic growth (uplift and/or exhumation) and decay (erosion and/or denudation) of the Rwenzori Mountains. As rocks are exhumed through the Earths’ crust, they cool through the thermal field from hot sub-surface temperatures to much colder surface temperatures (cp. Reiners & Shuster 2009). A nice tool to track this cooling history is by applying low-temperature thermochronology, which gives the time the rock has passed through a certain isotherm, i.e. horizon of equal temperature (Fig. 1). The accordant information is stored in heavy minerals, like apatite and zircon. These minerals act like a mix of clock and thermometer that can be read by different analyses of low-temperature thermochronology. For this study fission-track and (U-Th-Sm)/He dating of apatite and zircon was used.

 

Fig. 1: When mountains are forming their rocks pass through different stages of topographic growth (e.g. uplift) and decay (erosion). Before reaching the Earths’ surface (and being sampled by geologists), rocks cool through the thermal field. The different thermochronological analyses are sensitive for distinct temperatures: Apatite fission-track (AFT) for example reveals temperatures of ~ 110°C, while apatite (U-Th-Sm)/He (AHe) reveals temperatures of ~70°C. Thus, an AHe-age of ~23 Ma gives the age the rock, which contained the analyzed apatite passed through the 70°C isotherm; i.e. ~23 Ma ago this rock was ~70°C hot. If the same rock-sample has an AFT age of ~120 Ma, the rock cooled about ~40°C within ~97 Ma. Thus, using the different approaches, the movement of the rock towards the surface can be deciphered.

 

Following this approach more than hundred rock samples were taken, each about four kilograms, to extract an adequate amount of apatite and zircon. In the beginning of the project research and sampling has concentrated on the eastern (Ugandan) part of the Albertine Rift, with focus placed on the Rwenzori Mountains (Fig. 2). The accordant data determined from the thermochronological analyses reveal an exhumation history (cooling) of the Rwenzoris that already started in Jurassic time with an accelerated cooling of the rocks followed by a long period of constant and slow cooling. In Neogene times, the area again was affected by processes inducing an amplified exhumation of the Rwenzori block, with differentiated erosion, and uplift movements during the last 10 Ma (Bauer et al. subm). The revealed data, furthermore, point to a rapid final uplift of the Rwenzoris in the near past, where erosion could not compensate for. Moreover, differentiated uplift movements along and across the Rwenzori Mts can be derived, with more pronounced uplift movements along the western flank, resulting in an E-W asymmetry of the Rwenzoris with a tilt to the east (Fig. 2).

 

Fig. 2: Digital elevation model of the Rwenzori Mts with location of sampled transect in the central part as well as apatite fission-track (AFT) and apatite (U-Th-Sm)/He (AHe) ages of samples from the northern part of the Rwenzoris (Bauer et al. 2008). Zoomed transect at lower right with AFT and selected AHe ages, and age-elevation plot to the left, demonstrating the elevation independence of the analyzed samples and tilt to the east.

 

The thermochronological data obtained from this study fit well into the general age pattern known from the EARS (cp. Spiegel et al. 2007, and literature cited therein), but still are on trial, as they are lacking data from the opposite side, i.e. the Democratic Republic Congo (D.R. Congo). This, however, the authors aim to cope with new samples from the Eastern D.R. Congo. Since the political situation in Eastern D.R. Congo has stabilized within the last year a joint field-work with colleagues from the Ruwenzori State University (U.O.R.) of Butembo was conducted in June/July 2009. The main intention of the expedition to the D.R. Congo was to get an overview of the geology, landscape, and morphology in the western prolongation of the Rwenzori Mountains. Focus, thereby, was placed on the area between Lake Edward and the Blue Mountains at Lake Albert, covering an entire N – S transect along the western rift shoulder. The expedition started in Butembo, SW of the Rwenzori Mts, to first cover the southern part and subsequently move northwards, including the foothills of the western Rwenzori Mts, and further north to the Blue Mountains, at Lake Albert. In the frame of this field-work, an extensive pool of data, samples, and impressions was gathered, promising to now enable a more detailed understanding of the uplift history of the Rwenzori Mts, and to achieve more sophisticated constraints on the landscape evolution of the Rwenzori Mts and the Albertine Rift, in general.

 

Fig. 3: Western flank of the Rwenzori Mts, with glaciers on Peak Margherita.

 

References

Bauer, F.U., Karl, M., Glasmacher, U.A., Nagudi, B., Mroszewski, L., subm. The Rwenzori Mountains of western Uganda - an approach to unravel the evolution of a remarkable morphological feature within the Albertine Rift. Journal of African Earth Sciences, submitted.

Bauer, F.U., Glasmacher, U.A., Foerster, A., Reiners, P., Braun, J., Nagudi, B., Schumann, A., Bechstaedt, T., 2008. The Rwenzori Mountains of Uganda, a high mountain range in a rift environment – thermochronological evidence. FT2008, 11th International Conference on Thermochronometry.

Reiners, P.W. and Shuster, D.L., 2009. Thermochronology and landscape evolution. Physics Today, 62, 9, 31-36.

Spiegel, C., Kohn, B.P., Belton, D.X., Gleadow, A.J.W., 2007. Morphotectonic evolution of the central Kenya rift flanks: Implications for late Cenozoic environmental change in East Africa. Geology 35 (5), 427-430.