Geochronological constraints on multiple deformations of the Honam Shear Zone, South Korea and its tectonic implication

The NE-trending Honam Shear Zone, South Korea, is a tectonic-scale shear zone, consisting of several parallel mylonite zones: Jeonju, Sunchang and Yecheon shear zones. Here we report the sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages from the mylonitic Precambrian orthogneiss and hornblende-biotite granitoid near the Myeongho area within the Yecheon Shear Zone, a representative segment of the Honam Shear Zone. The core and overgrowth parts of the zircons from the foliated hornblende-biotite granitoid yielded ca. 187 Ma and ca. 178 Ma, respectively. The former is interpreted as a timing of emplacement, whereas the latter as a timing of subsequent mylonitization, which is similar to zircon overgrowth age of ca. 179 Ma obtained from mylonitized quartz-feldspar orthogneiss. Muscovite and biotite K-Ar ages of mylonitized Precambrian rocks and foliated granitoids range from 162-168 Ma, 148-155 Ma, and 136 Ma. Our geochronological results, combined with those of previous geochronological and structural studies, suggest that the Honam Shear Zone is a tectonic element created during Middle Jurassic Daebo (or early Yanshanian) Orogeny and lasted possibly as late as at Early Cretaceous in relation to the subduction of the Izanagi Plate under the Asian continent.