Peat deposits as records of quaternary tectonics and landscape dynamics in the semi-volcanic highlands of Madagascar

Peat in the highlands of Madagascar is widespread in semi-enclosed tectonic basins that occur close to the continental drainage divide. Here, the lithostratigraphy of multiple cores and sections displaying successive generations of radiocarbon-dated peat was studied in Ifanja Basin and Antsirabe Basin as a means of characterising and quantifying Quaternary landscape dynamics. The bedrock floor of the fill sequences was encountered at depths of only 16–25 m, with sediment ages documenting opportunities for peat and sediment accumulation between 40 ka and the Present at rates of 0.5 to 4.25 mm/a. Vertical offsets on extensional faults cross-cutting the stratigraphy, consistent with modern seismicity in the broader region recorded by earthquake catalogues, suggest continuing basin subsidence. The absence of pre-40 ka deposits points to imbalances between tectonic subsidence rates and sediment accumulation rates, contrastingly affecting sediment preservation potential before and after ∼40 ka. Trunk streams exit the basins via bedrock thresholds that control catchment sediment yields before descending into bedrock gorges. The gorges themselves function as high-energy knickzones likely to propagate remote base-level changes upstream across the basin floors. Given similarities in the chronostratigraphic records of other basins such as Vinaninony, Ambohibary and Antananarivo, which are all situated, like Ifanja and Antsirabe, within a radius of 100 km from the Ankaratra volcanic dome summit (elevation: 2642 m), catchment-wide sediment yields and incision by streams have fluctuated in response to regionally consistent drivers of base-level change. The latter involve a combination of asthenosphere-driven crustal uplift and drainage expansion and piracy.