Denudation Process of Crystalline Nappes in a Continental Collision Zone Constrained by Inversion of Fission‐Track Data and Thermokinematic Forward Modeling: An Example From Eastern Nepalese Himalaya

Thermochronological methods were applied to the Higher Himalayan Crystalline (HHC) nappe and the underlying Lesser Himalayan Sequences (LHS) to elucidate the denudation process for the middle- and upper-crust of eastern Nepal over millions of years. Thermochronological inverse modeling was undertaken for new results of fission-track (FT) age and FT length data of zircon and apatite in order to reconstruct the time-temperature (t-T) paths in the temperature range of 60–350°C. Eight t-T paths calculated along the across-strike section show that the cooling process of the HHC nappe in this study area is characterized by the following three aspects: (a) gradual cooling followed by rapid cooling and subsequent gradual cooling, (b) northward-younging of the timing of the rapid cooling, and (c) gradual cooling followed by <2 Myr rapid cooling in the frontmost part of the HHC nappe. The observed FT ages and t-T paths were then compared with those predicted by forwarding thermokinematic modeling. The results of the thermokinematic modeling for the “Flat-Ramp-Flat MHT model”, in which the HHC and the underlying LHS are denudated in direct proportion to the uplift of rocks transported along the Main Himalayan Thrust (MHT), reproduced the observed t-T paths and FT ages in eastern Nepal. This indicates that the observed FT ages and t-T paths reflect a denudation process driven by the movement of the MHT with a flat-ramp-flat geometry and that the denudation rate and its spatial distribution have roughly been constant in eastern Nepal since ca. 9 Ma

Edad por Huellas de Fision de las Piedras Pomes Incluidas en la Formacion Gigante, Norte de Neiva, Colombia

Reports by Grant-in-Aid for Overseas Scientific Survey1982 Grant-in-Aid for Scientific Research (Grant-in-Aid for Overseas Scientific Survey) Reports of Research Project (Number of Project 1981: 56041034, 1982: 57043030)Head Investigator 1981: Kyoto University, Primate Research Institute, Professor, Shiro KONDO, 1982: Kyoto University, Primate Research Institute, Associate, Tsuyoshi WATANABE南米大陸における広鼻猿類の系統・進化に関する研究 = Phylogenetical and Evolutionary Studies of New World Monkeys in South Americ

Fission-Track Dating of the Upper Part of Miocene Honda Group in La Venta Badlands, Colombia

Reports by Grant-in-Aid for Overseas Scientific Survey1986 Grant-in-Aid for Scientific Research (Grant-in-Aid for Overseas Scientific Survey) Reports of Research Project (Number of Project 1984: 59041040, 1985: 60043041)Phylogenetic Studies of South American MonkeysHead Investigator 1984, 1985: Kyoto University, Primate Research Institutc, Professor, Yasuo NOGAM

Fission-Track Age of Pumices Included in the Gigante Formation, North of Neiva, Colombia

Reports by Grant-in-Aid for Overseas Scientific Survey1982 Grant-in-Aid for Scientific Research (Grant-in-Aid for Overseas Scientific Survey) Reports of Research Project (Number of Project 1981: 56041034, 1982: 57043030)Head Investigator 1981: Kyoto University, Primate Research Institute, Professor, Shiro KONDO, 1982: Kyoto University, Primate Research Institute, Associate, Tsuyoshi WATANABE南米大陸における広鼻猿類の系統・進化に関する研究 = Phylogenetical and Evolutionary Studies of New World Monkeys in South Americ

Fission Track Dating of the Volcanic Ash Layers in Continental Deposits at Salla, Bolivia: Re-examination

Reports by Grant-in-Aid for Overseas Scientific Survey1986 Grant-in-Aid for Scientific Research (Grant-in-Aid for Overseas Scientific Survey) Reports of Research Project (Number of Project 1984: 59041040, 1985: 60043041)Phylogenetic Studies of South American MonkeysHead Investigator 1984, 1985: Kyoto University, Primate Research Institutc, Professor, Yasuo NOGAM

Zircon sensitive high mass-resolution ion microprobe U-Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implications

The Late Miocene-Pliocene Taitao ophiolite is composed of a complete sequence of classic oceanic lithosphere and is exposed approximately 50 km southeast of the Chile triple junction, where the Chile Ridge subducts beneath the South American Plate. Gabbros and ultramafic rocks are folded into a complex pattern, but only evidence for block rotation has been reported in the overriding sheeted dyke complex. In the present study, sensitive high mass-resolution ion microprobe U-Pb and fission-track dating methods were applied to zircon crystals separated from gabbros and sheeted dykes. Two sets of radiometric ages of gabbros range between 5.9 ± 0.4 and 5.6 ± 0.1 Ma. These ages coincide within their error ranges and imply rapid intrusion and cooling of gabbros. The U-Pb age of a dacite dyke intruded into the sheeted dyke complex was determined to be 5.2 ± 0.2 Ma. These data indicate that the magmas of the Taitao ophiolite were formed during the 6 Ma Chile Ridge collision event and emplaced in a shorter period than previously thought. A short segment of the Chile Mid-oceanic Ridge must have been emplaced during the 6 Ma event