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[SCG57-P01] Can the faulting age of brittle fault rocks be determined using fission track dating?
Keywords:slickenfibre, calcite, zircon, apatite, fission track, brittle fault
Zircon fission-track (ZFT) or apatite fission-track (AFT) dating has been used to estimate fault age for ductile deformed rocks or pseudotachylite that are covered relatively high temperature. Dating for brittle fault rocks, however, are few. We tried to estimate a fault age for a brittle deformed fault rock using the ZFT and the AFT dating. The fault that we targeted is considered as one of the bookshelf faults resulting a basin block rotation in ~17–16 Ma (2023). No direct evidence was obtained to support the faulting age.
The fault is found in volcaniclastic rocks that are ~17 Ma. These host rocks contain density volcanic clasts and clasts of Jurassic basement sandstone and mudstone. Fault gouge and calcite slickenfibers are developed with widths of a few millimeters. The samples containing a calcite slicken fibers were taken from the fault surface, which was less than 5mm width, for zircon U–Pb, ZFT, and AFT dating.
The dating results provided the following insights: 1) The sample yielded pre-Neogene and Neogene zircons with ages determined by U–Pb and FT dating. 2) Both zircon U–Pb and ZFT ages for the youngest zircon age cluster for zircon grains were consistent with the ages of the host rocks. 3) All apatite grains yielded young ages with low FTs indicated that their grains ages were likely of the same age origin, in contrast to the zircon grains. These results suggest that the sample either lacked old apatite grains or that all apatite grains, including those of old age, underwent sufficient heating to reset their ages. The AFT age is likely to be the faulting age, which is very close to the age of the host volcanic rocks, if the apatite ages indicate a complete reset due to later heating.
The fault is found in volcaniclastic rocks that are ~17 Ma. These host rocks contain density volcanic clasts and clasts of Jurassic basement sandstone and mudstone. Fault gouge and calcite slickenfibers are developed with widths of a few millimeters. The samples containing a calcite slicken fibers were taken from the fault surface, which was less than 5mm width, for zircon U–Pb, ZFT, and AFT dating.
The dating results provided the following insights: 1) The sample yielded pre-Neogene and Neogene zircons with ages determined by U–Pb and FT dating. 2) Both zircon U–Pb and ZFT ages for the youngest zircon age cluster for zircon grains were consistent with the ages of the host rocks. 3) All apatite grains yielded young ages with low FTs indicated that their grains ages were likely of the same age origin, in contrast to the zircon grains. These results suggest that the sample either lacked old apatite grains or that all apatite grains, including those of old age, underwent sufficient heating to reset their ages. The AFT age is likely to be the faulting age, which is very close to the age of the host volcanic rocks, if the apatite ages indicate a complete reset due to later heating.