Structural and magmatic responses to spreading ridge subduction: An example from southwest Japan

Abstract

Subduction of a spreading ridge is expected to leave a geological signature upon the overriding accretionary prism distinct from that of conventional convergent plate boundaries. The late Oligocene‐early Miocene rocks of the Shimanto accretionary prism at Cape Muroto, Shikoku Island, record a tectonomagmatic fabric that is anomalous with respect to typical accretionary prisms. These accreted strata have been imprinted with an unusual late stage event that involved (1) regional cusp‐like flexing of structural trends in the prism, (2) near‐trench magmatism, and (3) pervasive faulting. This event overlaps in time with the opening of the Shikoku back arc basin (circa 26–14 Ma); the Shikoku basin spreading ridge trends perpendicular to the strike of the accretionary prism and is now located immediately offshore of Cape Muroto. The regional flexing of structural trends in the prism affects rocks as old as Cretaceous, although the flexure is most tightly developed in the late Oligocene‐early Miocene portion of the prism, where it is steeply plunging. Kinematic analysis of the flexure and the pervasive fault system indicates that the flexure most likely resulted from indentation of the prism by a rigid feature. At Cape Muroto the core of the flexure forms a locus for mafic dikes that are petrochemically equivalent to some mid‐ocean ridge basalts (MORB) from the Shikoku back arc basin. Preliminary paleomagnetic data suggest that the dikes were intruded during the later stages of flexure development. These data, combined with plate reconstructions for the area at 15 Ma, indicate that indentation of the prism most likely resulted from early Miocene orthogonal subduction of the topographically elevated Shikoku Basin spreading ridge in the Muroto area; MORB magmatic rocks in the core of the flexure appear to have emanated from the active spreading ridge during subduction.

Provenance and Dispersal Patterns of Sediments in the Turbidite Wedge of Nankai Trough

“…Relative mineral percentages (from Shipboard Scientific Party, 1991b) are based on weighted peak intensities (normalized to the quartz peak at 26.65°θ) and generally show that the clay minerals (smectite, illite, kaolinite, and chlorite) total 20% or less with respect to the sum of quartz, Plagioclase, calcite, and clay minerals Because amorphous phases (e.g., volcanic glass and biogenic silica) were not detected during the shipboard analyses, the absolute error in these estimates could be significant. lous near-trench acidic volcanic activity, granitic intrusions, and local intrusions of mafic rock during the middle Miocene (Oba, 1977;Miyake, 1985;Terakado et al, 1988;Hibbard and Karig, 1990).…”

“…When compared to other well-studied subduction margins such as Barbados (Moore et al, 1982a(Moore et al, , 1988 and the nearby Japan Trench (von Huene and Arthur, 1982;Cadet et al, 1987), the amount of terrigenous sediment influx is relatively high offshore from Kyushu, Shikoku, and the Kii Peninsula of Honshu. The Nankai stratigraphy, therefore, serves as an effective modern analog for sandstone-rich accretionary terranes such as the Franciscan Complex of California (Bachman, 1982;Underwood, 1984;Blake et al, 1985), the Kodiak Islands of Alaska (Moore et al, 1983;Byrne, 1984;Sample and Moore, 1987), and the Shimanto Belt of Japan (Taira, 1985a;Taira et al, 1988;Agar, 1990;DiTullio and Byrne, 1990;Hibbard and Karig, 1990).…”

Western Nankai Trough seismogenic zone: Results from a wide‐angle ocean bottom seismic survey

“…The Shikoku Basin located at the northern part of the Philippine Sea Plate is estimated to have opened from 26-15 Ma by backarc spreading behind the lzuBonin Arc [Kobayashi et al, 1995]. Spreading ceased at 15 Ma, leaving the Kinan Seamounts marking the extinct spreading axis [Kobayashi and Nakada, 1978;Hibbard and Karig, 1990; Okino controlling a seismic activity along the subducting Philippine Sea Plate [Sugi and Uyeda, 1984;$hiono and $ugi, 1985]. $hiono and Sugi [1985] event, however, spread with time to the southwest along the coast and landward [Kanamori, 1972;Ando, 1975].…”

Stratigraphic Synthesis of the DSDP-ODP Sites in the Shikoku Basin, Nankai Trough, and Accretionary Prism

“…These events appear to have occurred within the space of only a few million years. Geological evidence from the Shimanto belt has led Hibbard and Karig (1990) to postulate that (1) during the opening of the Shikoku backarc basin, from 26 to 15 Ma, it was separated from the Japanese margin by an extension of the Pacific Plate, and (2) the Shikoku Basin spreading center collided with southwest Japan at about 15 Ma, at which time the trench-trenchtrench (TTT) triple junction was probably initiated. In this scenario, a transform boundary is interpreted to separate the Pacific Plate from the northern edge of the Shikoku Basin, on the Philippine Sea Plate (Hibbard and Karig, 1990).…”

“…Magnetic anomaly data suggest that between 14 and 12 Ma, a late phase of north-south spreading was limited to the axial northernmost Shikoku Basin, also associated with a counterclockwise rotation of the spreading direction (Chamot-Rooke et al, 1987). Figure 2 is a summary of the plate-tectonic history of this region, using data from Taira (1986), Hibbard and Karig (1990), and Koyama (1991).…”