NEW INTERPRETATION OF FRANCISCAN MÉLANGE AT SAN SIMEON COAST, CALIFORNIA: TECTONIC INTRUSION INTO AN ACCRETIONARY PRISM

Many concepts and interpretations on the formation of the mélanges at San Simeon, California, have been proposed, but yet the topic remains a subject of debate since its first introduction by Ken Hsu followed by detail researches by Cowan, Cloos, Ernst, Ukar, and others. We aim here to show that the internal structures and textures of the chaotic rocks are characterized by various kinds of blocks from blue-or greenschist, oceanic rocks, including bedded chert, limestone, and particularly various kinds of sandstones. Interrelationships between the mélange blocks and the surrounding matrix materials suggest that all the features were produced by typical accretionary prism processes in which turbidites (both in the blocks and matrices) and oceanic rocks are in tectonic or diapiric (not sedimentary) contact with each other. The new interpretation is similar to that of Cloos, Becker, or Ukar, but different in a sense of the surrounding turbiditic formation (Cambria Slab) to be of an accretionary prism, not of the slope basin sediments. The turbiditic rocks display various kinds of structures associated with layer-parallel shortening or stretching by folds and faults. Way-up directions in turbiditic rocks are used to map to verify recumbent folds and thrust stacks, and repetitions of the slump beds for duplex structures. The “mélanges” are commonly developed along the thrust faults between turbidite slices. We conclude that the entire Franciscan Complex at San Simeon consists of the Cambria Slab into which the mélange bodies were intruded. The exhumation of the blueschist blocks is still controversial, but the common extensional fractures and brecciation in most of the mélange blocks and further mixture of many fragments into one block, which is further enclosed into mélange matrices, must lead a story that once deep buried blocks were exhumed from considerable depths to the accretionary prism body, probably by return flow, to intrude either by thrusting or diapirism, or both, during when retrograde deformation occurred with retrograde metamorphism. The Recent examples of the accretionary prism and its gravitational collapse observed by submersible research in the Nankai trough of Japan must play the role in comparative study on mélange processes.