Middle Miocene forearc alkaline magmatism in Amami-Oshima Island, central Ryukyu Arc: implications for paleoreconstruction of Shikoku Basin

Abstract During the middle Miocene, forearc alkaline magmatism occurred in Outer Zone of Southwest Japan and northern Ryukyu Arc, resulting formation basaltic rocks with ocean island basalt (OIB)-like chemical signatures. In contrast, subduction-related was present central Arc. However, southwestern margin poorly constrained We examined two dikes Chichibu accretionary complex Amami-Oshima Island, The cut massive basalt, reddish chert, varicolored shale, grey chert. compositions are characterized by enrichment incompatible trace elements, possibly representing a low degree partial melting from deep mantle source. 40 Ar/ 39 Ar dating analyses indicate that yield ages 16.37 ± 0.14 Ma 16.51 0.10 Ma. paleomagnetic on stepwise thermal demagnetizations allowed extracting direction stable magnetizations unblocking temperatures 450–575 °C for [(Dec, Inc) = (138.1°, − 13.3°), (124.0°, 24.9°)]. magnetization could be primary, acquired either as part secular variation or geomagnetic excursion during reversed polarity chron/subchron. directions within error range imply they intruded almost simultaneously C5Cn.2r (16.532–16.434 Ma). age element patterns comparable to those Miocene rocks, which resulted initial subduction young warm Shikoku Basin. discovery Island suggests distribution may extend Hence, end Kyushu-Palau Ridge (i.e., southern Basin) have been located south around 16.5–16.4 Ma, then moved eastward current location. Graphical