Evolution of the Shimanto accretionary complex: A fission-track thermochronologic study
Published:January 01, 1993
Noriko Hasebe, Takahiro Tagami, Susumu Nishimura, 1993. "Evolution of the Shimanto accretionary complex: A fission-track thermochronologic study", Thermal Evolution of the Tertiary Shimanto Belt, Southwest Japan: An Example of Ridge-Trench Interaction, Michael B. underwood
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To place thermotectonic constraints on the evolution of the Cretaceous to Neogene Shimanto Belt, we carried out fission-track (FT) analyses of detrital apatite and zircon collected from both sandstone turbidites and blocks in mélanges on the Muroto Peninsula, Shikoku. Eight FT apatite ages show good agreement around 10 Ma, with all data except for one passing the χ2 test. These results, in conjunction with depositional ages of Cretaceous to early Oligocene, demonstrate that the Shimanto Belt was heated hotter than ∼125°C (apatite total annealing temperature) and subsequently cooled below ∼100°C at ∼10 Ma. The cooling pattern is attributed to higher thermal gradients and/or uplift caused by rapid subduction of the newly formed Shikoku Basin at ∼1 5 Ma. On the other hand, 23 zircon samples show a large range of sample ages from 150 to 17 Ma, with all failing the χ2 test except for one from a mélange. Hence, most parts of the Shimanto Belt have not been heated above the zircon partial annealing zone (ZPAZ; ∼190 to 260°C), whereas some parts of the mélanges have experienced temperatures above 260°C. FT age spectra of zircon single-grain data show that in the Northern Shimanto Belt (NSB) the youngest peak in each sample is younger than its depositional age, suggesting the maximum temperature was in the ZPAZ. In contrast, all age peaks from the Southern Shimanto Belt (SSB), are consistently older than depositional ages, providing no evidence of heating up to ZPAZ. These contrasting patterns probably reflect systematic differences in the maximum temperature reached during the evolution of the Shimanto Belt. The age-temperature paths estimated for individual tectonic units suggest successive accretion, growth, and uplift of offscraped imbricate packages as well as underplating of mélanges beneath them in an accretionary wedge.