Indian Monsoonal Variations During the Past 80 Kyr Recorded in NGHP-02 Hole 19B, Western Bay of Bengal: Implications From Chemical and Mineral Properties

金沢大学理工研究域地球社会基盤学系Detailed reconstruction of Indian summer monsoons is necessary to better understand the late Quaternary climate history of the Bay of Bengal and Indian peninsula. We established a chronostratigraphy for a sediment core from Hole 19B in the western Bay of Bengal, extending to approximately 80 kyr BP and examined major and trace element compositions and clay mineral components of the sediments. Higher δ 18 O values, lower TiO 2 contents, and weaker weathering in the sediment source area during marine isotope stages (MIS) 2 and 4 compared to MIS 1, 3, and 5 are explained by increased Indian summer monsoonal precipitation and river discharge around the western Bay of Bengal. Clay mineral and chemical components indicate a felsic sediment source, suggesting the Precambrian gneissic complex of the eastern Indian peninsula as the dominant sediment source at this site since 80 kyr. Trace element ratios (Cr/Th, Th/Sc, Th/Co, La/Cr, and Eu/Eu*) indicate increased sediment contributions from mafic rocks during MIS 2 and 4. We interpret these results as reflecting the changing influences of the eastern and western branches of the Indian summer monsoon and a greater decrease in rainfall in the eastern and northeastern parts of the Indian peninsula than in the western part during MIS 2 and 4. © 2018. American Geophysical Union. All Rights Reserved

Progressive illitization in fault gouge caused by seismic slip propagation along a megasplay fault in the Nankai Trough

International audienceThe question of whether coseismic ruptures along megasplay faults in accretionary prisms (i.e., large landward-dipping thrust faults branching from the plate boundary) reach the seafl oor is critical for assessing the risk of tsunami disaster. However, samples from active megasplay faults have not previously been available. Here we present geochemical and mineralogical data of megasplay fault samples obtained from the shallow (<300 m below seafl oor) portion of the megasplay fault that coincides with the rupture area of the A.D. 1944 Tonankai earthquake in the Nankai Trough. The megasplay fault zone is characterized by localized shear zones of brecciated host rocks. A prominent slip zone, here termed "dark gouge," was discovered within one of the shear zones. Compared to the surrounding breccias, the dark gouge is chemically enriched in Al and K, and depleted in Ca and Sr. It is also characterized by higher illite content in illite-smectite mixed-layer clays. These chemical and mineralogical features may refl ect a transformation in clay mineralogy caused by frictional heating, and suggest that the seismic slip can propagate to very shallow levels along megasplay fault system