Middle Paleocene terrane juxtaposition along the Median Tectonic Line, southwest Japan: Evidence from 40Ar/39Ar mineral ages

Abstract The Median Tectonic Line (MTL) is a regional transcurrent fault separating anadalusite-sillimanite-type metamorphic rocks and granitic rocks within the Ryoke terrane from glaucophanic-type metamorphic rocks within the Sambagawa terrane in southwest Japan. The late Cretaceous Izumi Group unconformably overlies Ryoke metamorphic rocks and is tectonically separated from the Sambagawa terrane by the MTL. Recent 40 Ar/ 39 Ar mineral ages reported for the Sambagawa terrane in this area suggest extensive Late Cretaceous uplift; however, no Sambagawa erosional debris has been described from the Izumi Group. 40 Ar/ 39 Ar ages have been determined for hornblende and biotite separated from samples systematically collected along a traverse from the Ryoke terrane into the MTL (Kashio area, Chubu district). In addition whole-rock samples of protomylonite within the MTL have been analyzed. Similar 68–70 Ma isotope correlation ages are recorded by hornblende within massive (Ikuta) and foliated (Minakata) granite and protomylonite within the MTL. Biotite from these samples records ages ranging between 66 and 68 Ma. Together the mineral ages suggest relatively rapid post-magmatic cooling through appropriate argon closure temperatures. These also indicate that temperatures maintained during protomylonite formation within the MTL were less than those required to effect even partial rejuvenation of intracrystalline argon systems within hornblende. Protomylonite samples yielded well-defined whole-rock plateau ages of 62–63 Ma which are interpreted to closely date the last phase of ductile flow within the MTL. These results suggest that a significant ductile phase of movement occurred within the MTL in the Middle Paleocene. This may have been associated with significant sinistral displacement and resultant tectonic juxtaposition of previously separated portions of the Sambagawa and Ryoke belts (with unconformably overlying Izumi Group).