The Kalahari Basin in southern Africa, shaped by subsidence and epeirogeny, features the Okavango Rift Zone (ORZ) as a significant structural element characterized by diffused extensional deformation forming a prominent depocenter. This study elucidates the Pleistocene landscape evolution of the ORZ by examining the chronology of sediment formation and filling this incipient rift and its surroundings. Modeling of cosmogenic nuclide concentrations in surficial eolian sand from distinct structural blocks around the ORZ provides insights into sand’s residence time on the surface. Sand formation occurred from ~2.2 to 1.1 Ma, coinciding with regional tectonic events. Notably, provenance analyses of sand within ORZ’s lowermost block where large alluvial fans are found indicate different source rocks and depositional environments than those of the more elevated eolian sand. This suggests that the major phase of rift subsidence and the following incision of alluvial systems into the rift occurred after eolian dune formation. Luminescence dating reveals that deposition in alluvial fan settings in the incised landscape began not later than ~250 ka, and that a lacustrine environment existed since at least ~140 ka. The established chronological framework constrains the geomorphological effects of the different tectono-climatic forces that shaped this nascent rifting area. It highlights two pronounced stages of landscape development, with the most recent major deformation event in the evolving rift probably occurring during the middle Pleistocene transition (1.2-0.75 Ma). This event is reflected as a striking change in the depositional environments due to the configurational changes accompanying rift progression.

Optically stimulated luminescence (OSL) thermochronometry is a tool for constraining cooling histories in low-temperature domains (several tens of degree Celsius) during the past 104–105 years [1][2][3]. This method is currently applied only to rapidly denuded regions (~5mm/yr when a general geothermal gradient in is assumed to be ~0.03℃/m) because luminescence signals in slowly denuded regions saturate before the rocks are exhumated to the surface. However, cooling histories in slowly denuded regions may be constrained if unsaturated samples are obtained from deep boreholes. In addition, using deep borehole core enable to compare the results between samples at multiple depths, which is useful to isolate the denudation history from other events, such as faulting or hydrothermal activity. We applied multi-OSL-thermochronometry [2] to the deep borehole core drilled at the Rokko Mountains, Japan, where slow denudation rates (0.1-1.0 mm/yr) are expected from previous studies [4][5][6]. We used the Kabutoyama core collected by National Research Institute for Earth Science and Disaster Resilience [5][7]. The total length of Kabutoyama core is 1,313 m and we collected the samples at 408, 642, 818 and 1048 m for OSL-thermochronometry. Our results showed that useful thermal information can be extracted from the infrared stimulated luminescence signals of samples collected at depths ≥408 m. We found that the sample temperatures remained around the present ambient temperature at each depth for the last 0.1 Myr, indicating that the Rokko Mountains is topographically stable, which was consistent with previous findings. Thus, the thermal denudation history of slowly denuded regions may be constrained by multi-OSL-thermochronometry using samples from deep borehole cores. However, the denudation rates in the Rokko Mountains were too low and could not be determined by this method. Further research is required to quantify the denudation rate. This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan as part of its R&D supporting program titled “Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (Fiscal Years 2019-2021)”. References: [1] Herman et al. (2010). Earth and Planetary Science Letters, 297, 183-189; [2] King et al. (2016). Quaternary Geochronology, 33, 76-87; [3] Herman and King (2018). Elements, 14, 33-38; [4] Huzita (1968). The Quaternary Research, 7, 248-260; [5] Sueoka et al. (2010). Journal of Geography, 119, 84-101; [6] Matsuhi et al. (2014). Transactions, Japanese Geomorphological Union, 35, 165-185; [7] Yamada et al. (2012). Technical Note of the National Research Institute for Earth Science and Disaster Prevention, 371, 27p.