Anomalous structural evolution of the Shimanto Accretionary Prism at Murotomisaki, Shikoku Island, Japan

Abstract

The Late Oligocene-Early Miocene Nabae Sub-belt of the Shimanto Accretionary Prism was created coevally (ca 25-15 Ma) with the opening of the Shikoku back-arc basin, located to the south of the southwest Japan convergent margin. The detailed geology of the sub-belt has been controversial and the interaction of the Shimanto accretionary prism and the opening of the Shikoku Basin has been ambiguous. New structural analysis of the sub-belt has led to a new perception of its structural framework and has significant bearing on the interpretation of the Neogene tectonics of southwest Japan.The sub-belt is divided into three units: the Nabae Complex; the Shijujiyama Formation; and the Maruyama Intrusive Suite. The Nabae Complex comprises coherent units and melange, all of which show polyphase deformation. The first phase of deformation appears to have involved landward vergent thrusting of coherent units over the melange terrane. The second phase of deformation involved continued landward vergent shortening. The Shijujiyama Formation, composed mainly of mafic volcanics and massive sandstone, is interpreted as a slope basin deposited upon the Nabae Complex during the second phase of deformation. The youngest deformational pulse involved regional flexing and accompanying pervasive faulting. During this event, mafic rocks of the Maruyama Intrusive Suite intruded the sub-belt. Fossil evidence in the Nabae Complex and radiometric dates on the intrusive rocks indicate that this tectonic scheme was imprinted upon the sub-belt between -23 and -14 Ma.The timing of accretion and deformation of the sub-belt coincides with the opening of the Shikoku Basin; hence, subduction and spreading operated simultaneously. Accretion of the Nabae Sub-belt was anomalous, involving landward vergent thrusting, magmatism in newly accreted strata and regional flexing. It is proposed that this complex and anomalous structural history is largely related to the subduction of the active Shikoku Basin spreading ridge and associated seamounts.

Subduction accretion and orocline development in modern and ancient settings: Implications of Japanese examples for development of the New England Orogen of eastern Australia

“…Smaller-scale flexures occur in Shimanto belt rocks in Cape Ashizuru and Cape Muroto along the southern side of Shikoku Island and the Kii Peninsula in Honshu and are considered caused by impingement of subducting seamounts (Fig. 4) (Hibbard et al, 1992;Kimura et al, 2014).…”

“…Smaller-scale flexures are found in active convergent margins including southwest Japan (Fig. 4) where flexed structural trends on the Muroto Peninsula are interpreted to have formed from impingement of the Cenozoic Shimanto belt by subduction of the Shikoku backarc basin extinct sea-floor spreading ridge (Hibbard and Karig, 1990;Hibbard et al, 1992;Kimura et al, 2014). The Kanto Syntaxis (Fig.…”

“…7. Inset shows the flexed structural trends across Cape Muroto (after Hibbard et al, 1992). The flexure in Cape Muroto was caused by subduction of the spreading ridge in the Shikoku Basin (Hibbard and Karig, 1990).…”

Middle Miocene swift migration of the TTT triple junction and rapid crustal growth in southwest Japan: A review

“…The Muroto Peninsula has been well studied since the Shimanto Belt was recognized as an excellent example of an accretionary prism [Suzuki and Hada, 1979;Taira et al, 1980]. The north plunging antiform is interpreted as being related to a collision with the spreading axis of the Shikoku Basin (Figure 8) [Hibbard and Karig, 1990;Hibbard et al, 1992]. This collision is indicated by the appearance of MORB-type gabbros at Cape Muroto.…”

Late Cenozoic evolution of the Nankai trench-slope system: evidence from sand petrography and clay mineralogy

“…towards the Median Tectonic Line (Higashino 1990;Nakajima 1997). Unmetamorphosed forearc-basin assemblages are also preserved locally within the Outer Zone (Chijiwa 1988;Hisatomi 1988;Hibbard et al 1992). …”