17:15 〜 19:15
[SCG44-P01] Landscape Evolution Archived in Detrital Cosmogenic 10Be in Source-to-Sink Systems: Cases in the Western Margin of the Kinki Tectonic Triangle
★Invited Papers
キーワード：宇宙線生成核種、地形発達史、河川縦断形、遷急点の遡上、隆起速度、侵食速度
We reconstructed long-term landscape evolution of fault-block mountains in response to changes in uplift rate, using cosmogenic nuclides in fluvial sediment from both sources and sinks. We focused on the Rokko and Hira mountain ranges located in the western margin of a tectonically active zone in central Japan, so-called Kinki Triangle. Transient landscapes have developed in these ranges reflecting tectonic regime shift known as Rokko Movement during the late Quaternary. The topography of the mountains is characterized by elevated low-relief erosional surfaces, and high-relief flanks that are distinctly bounded by knickpoints along their channel profiles. Sediments sourced from the Rokko and Hira Mountains are inferred to have been deposited in Osaka Bay and Lake Biwa, respectively.
We measured cosmogenic 10Be in quartz extracted from the outlets of the source watersheds and from deep boring cores in the sinks. Higher 10Be concentrations were observed in the hilltop low-relief watersheds, while lower concentrations were obtained in steep, fault-facing, high-relief basins. Current watershed-wide erosion rates based on these 10Be data varied across one order of magnitude depending on the topographic conditions. The 10Be concentration profile from the boring core in Osaka Bay showed a clear decreasing trend toward shallower depths, reflecting the landscape evolution in the Rokko Mountain in response to the Rokko Movement, which involved progressive steepening accompanying with knickpoint migration. We conducted inversion analysis of channel long-profiles using a general stream power model to reconstruct the history of uplift rate changes and topographic evolution of the responding watersheds. Through this approach, the 10Be concentrations simulated from the reconstructed landscape evolution in the source area were consistent with the 10Be profiles observed in the sink core. In contrast, 10Be data from the offshore deep boring core in Lake Biwa indicated a stable concentration from ca. 1.2 Ma to 0.4 Ma, reflecting a steady-state landscape condition in the source area before the uplift onset of the Hira Mountains. On the other hand, 10Be data from the shallow coastal boring core showed significantly lower concentrations than the offshore data, reflecting the active erosion of the fault-facing flank of the Hira range during the last 40 ka. The 10Be data also indicated short-term variations in erosion rates corresponding to glacial-to-interglacial climate shift. The detected variability in erosion rate under the influence of climatic perturbations was significantly smaller compared to that of tectonically derived temporal changes in erosion rate, suggesting that long-term tectonic forcing serves the primary control on landscape evolution in the Kinki Tectonic Triangle.
We measured cosmogenic 10Be in quartz extracted from the outlets of the source watersheds and from deep boring cores in the sinks. Higher 10Be concentrations were observed in the hilltop low-relief watersheds, while lower concentrations were obtained in steep, fault-facing, high-relief basins. Current watershed-wide erosion rates based on these 10Be data varied across one order of magnitude depending on the topographic conditions. The 10Be concentration profile from the boring core in Osaka Bay showed a clear decreasing trend toward shallower depths, reflecting the landscape evolution in the Rokko Mountain in response to the Rokko Movement, which involved progressive steepening accompanying with knickpoint migration. We conducted inversion analysis of channel long-profiles using a general stream power model to reconstruct the history of uplift rate changes and topographic evolution of the responding watersheds. Through this approach, the 10Be concentrations simulated from the reconstructed landscape evolution in the source area were consistent with the 10Be profiles observed in the sink core. In contrast, 10Be data from the offshore deep boring core in Lake Biwa indicated a stable concentration from ca. 1.2 Ma to 0.4 Ma, reflecting a steady-state landscape condition in the source area before the uplift onset of the Hira Mountains. On the other hand, 10Be data from the shallow coastal boring core showed significantly lower concentrations than the offshore data, reflecting the active erosion of the fault-facing flank of the Hira range during the last 40 ka. The 10Be data also indicated short-term variations in erosion rates corresponding to glacial-to-interglacial climate shift. The detected variability in erosion rate under the influence of climatic perturbations was significantly smaller compared to that of tectonically derived temporal changes in erosion rate, suggesting that long-term tectonic forcing serves the primary control on landscape evolution in the Kinki Tectonic Triangle.