5:15 PM - 6:45 PM
[HQR05-P11] Landslide rock clasts found from pseudo-moraine surfaces at Yaridaira in the upper Migimatadani Valley, the northern Japanese Alps
Keywords:glacial landforms, landslide phenomena in alpine zone, pseudo-moraine, landscape changes in the Holocene epoch
The origin of moraines in the Japanese Alps has been reexamined, and it has been pointed out that some of them originated not from glaciation but from large to medium-scale rockslides. In this study, we investigated the distribution and lithology of large rock clast (LRC; > 2 m diam.) covering the ground surface of two pseudo-moraine features at Yaridaira in the upper Migimatadani Valley (Y-H and Y-L; ca. 2000m ASL), near the main ridge of the Yari-Hotaka Range.
The slopes of both Y-H and Y-L are covered by LRC which are mostly composed of dacitic welded-tuff (Wm) and thermally recrystallized Wm and also include diorite porphyry (Dp). Well-defined cirques are formed at the headwaters of Migimatadani Valley, 3 km from Yaridaira, where volcanic conglomerate (Ya), crystalline schist (My), and granodiorite (Gdt/Gt) are exposed. Thus, if late Pleistocene glaciers were directly involved in the movement and transport, LRC on Y-H and Y-L derived from Ya, My, and Gdt/Gt should be found. However, such clasts have not yet been discovered. Wm, which accounts for most of LRC on Y-H and Y-L, is widely exposed on the slopes east of Yaridaira, including the drainage basin of Minamisawa Valley, the left branch of Migimatadani Valley.
LRC on Y-H and Y-L would be moved and emplaced by landslide occurred east of Yaridaira. A steep slab without vegetation cover exists on the right bank of the lower Minamisawa Valley (700 m ENE from Yaridaira Hut) and this slab is one of strong candidates for a landslide source area. Further investigation for evaluating volumetric magnitude of landslide and age as well as exploring subsurface geology are required.
The slopes of both Y-H and Y-L are covered by LRC which are mostly composed of dacitic welded-tuff (Wm) and thermally recrystallized Wm and also include diorite porphyry (Dp). Well-defined cirques are formed at the headwaters of Migimatadani Valley, 3 km from Yaridaira, where volcanic conglomerate (Ya), crystalline schist (My), and granodiorite (Gdt/Gt) are exposed. Thus, if late Pleistocene glaciers were directly involved in the movement and transport, LRC on Y-H and Y-L derived from Ya, My, and Gdt/Gt should be found. However, such clasts have not yet been discovered. Wm, which accounts for most of LRC on Y-H and Y-L, is widely exposed on the slopes east of Yaridaira, including the drainage basin of Minamisawa Valley, the left branch of Migimatadani Valley.
LRC on Y-H and Y-L would be moved and emplaced by landslide occurred east of Yaridaira. A steep slab without vegetation cover exists on the right bank of the lower Minamisawa Valley (700 m ENE from Yaridaira Hut) and this slab is one of strong candidates for a landslide source area. Further investigation for evaluating volumetric magnitude of landslide and age as well as exploring subsurface geology are required.