Abstract

K-Ar cleavage dating and fission track cooling ages have been combined with existing stratigraphic data to constrain the evolution of the Shimanto Belt, a Cretaceous-Neogene accretionary complex. Cleavage ages record two major phases of accretion during the late Cretaceous and late Eocene-early Oligocene. The minimum time periods between sedimentation and deformation are constrained to 9, 5, 9 and 0Ma intervals within four tectonic units containing youngest sediments of Maastrichtian, late Eocene, late Eocene and early Oligocene ages respectively. One phase of middle to late Miocene uplift is indicated for the whole study area by the fission track cooling ages. Frontal underplating combined with out-of-sequence thrusting are suggested as mechanisms for this regional uplift.

The Cretaceous-Neogene Shimanto Belt (Fig. la) forms the outermost, onshore exposure of accreted material in SW Japan. The boundaries of constituent accreted units of the Shimanto Belt are defined by a detailed stratigraphy (Taira et al. 1980Taira et al. 1988) providing an opportunity to test the application of cleavage dating and fission track studies to accretionary complex evolution. The Butsuzo Tectonic Line (Fig. la) juxtaposes the Shimanto Belt to the Jurassic Sanbosan Belt in the north. Younger accreted material extends offshore to the south as far as the present-day boundary of the Eurasian Plate, the Nankai Trough (Fig.la). Continued accretion seaward of the Shimanto Belt has back-rotated older accreted units to their present-day, steeply dipping orientations.

The type section of the Shimanto Belt (Taira et al. 1980) lies in the eastern Hata Peninsula (Fig. 1b) which contains the study

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