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

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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.