Underplating of mélange evidenced by the depositional ages: U–Pb dating of zircons from the Shimanto accretionary complex, southwest Japan

Abstract

Growth of an accretionary prism is effected by frontal accretion and deep subsurface underplating at the base of the prism. A systematic oceanward and structurally downward younging of underplated sequences is expected as the prism thickens and grows. To test this hypothesis and explore the processes of underplating, the U-Pb ages of zircon grains contained in underplated mélange sequences or packages of the Late Cretaceous and early Paleogene accretionary complex of the Shimanto Belt, southwest Japan, were determined using LA-ICP-MS laser technology. The results document systematic but intermittent younging ages within a single underplated mélange package. This finding suggests that underplating took place episodically during a period of several million years and that between episodes of underplating, a large amount of sediment was subducted to depths much greater than where underplating was occurring.

Complete 40Ar resetting in an ultracataclasite by reactivation of a fossil seismogenic fault along the subducting plate interface in the Mugi Mélange of the Shimanto accretionary complex, southwest Japan

“…The zircon UePb ages of tuff layers in the upper section range from 66.2 ± 3.5 to 73.2 ± 3.6 Ma, while ages in the lower section range from 58.3 ± 3.4 to 63.8 ± 2.2 Ma (Shibata et al, 2008). These ages are consistent with the biostratigraphy (Kiminami et al, 1992).…”

“…6). The KeAr ages yielded by coarser fractions (>1.0 mm) are older than the maximum depositional age (72e69 Ma (Shibata et al, 2008);) estimated from zircon UePb dating of a tuff layer close to the KeAr sampling locality. The KeAr ages of the <1.0 mm fraction is younger than the zircon UePb age of the tuff.…”

“…Six thrust repetitions of the Mugi M elange are exposed in the Mugi area Shibata et al, 2008), and each has a total tectonic thickness of less than 200 m. The slices possibly represent a larger duplex structure Kitamura et al, 2005;Shibata et al, 2008). The m elange matrix shale consists of quartz, feldspar, and clay minerals, and has a composite planar (SeC) fabric with pressure solution seams.…”

“…The Mugi M elange is subdivided into upper (units 4e6) and lower sections (units 1e3), which differ in zircon UePb ages and maximum paleotemperature conditions Shibata et al, 2008); these unit numbers refer to the thrust slices. The zircon UePb ages of tuff layers in the upper section range from 66.2 ± 3.5 to 73.2 ± 3.6 Ma, while ages in the lower section range from 58.3 ± 3.4 to 63.8 ± 2.2 Ma (Shibata et al, 2008).…”

Geology and detrital zircon U-Pb age of the Cretaceous Mugitani Formation in the Shimanto Belt, central Kii Peninsula, Southwest Japan

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