DEFORMATION PATTERNS & EXHUMATION PROCESSES IN CONVERGENT MARGIN ACCRETIONARY MÉLANGES: CRETACEOUS FRANCISCAN (USA) AND TERTIARY MINEOKA (JAPAN) COMPLEXES

We examined two representative examples of convergent margin accretionary mélanges, one in the Cretaceous Franciscan Complex exposed along the San Simeon coast of central California & the other in the Tertiary Mineoka belt exposed in the Boso Peninsula of central Japan. The Franciscan Complex has a prolonged history of subduction & associated high-P metamorphism & accretion along the active margin of the Mesozoic western North America. Blocks of clastic rocks in the Franciscan are composed mainly of continentally derived turbidites. The Mineoka belt currently occurring at the intersection of the Honshu & Izu forearcs has experienced a complex tectonic history involving oblique subduction accretion & triple junction migration. Blocks of its clastic rocks consist of island arc derived volcaniclastics. Both mélanges have similar lithological units & structures, including blocks of metamorphic rocks, serpentinite (harzburgitic), plutonic rocks (tonalite predominant in Mineoka), basalt (affinities ranging from MORB, IAT to WPB) and bedded radiolarian chert in a lower-grade shale and/or serpentinite matrix. Some blocks, initially described as knockers in the mélange matrix, likely represent discontinuous fault zones in these accretionary complexes. These blocks occur as phacoidal bodies having tails of axial symmetry/asymmetry, and their contacts with the surrounding matrix are faulted displaying Riedel shear patterns. Internally, the blocks are faulted & folded displaying cataclastic deformation features developed in two stages, first under ductile conditions & subsequently in a brittle regime. We interpret both mélanges as transpressional fault zone assemblages developed first in a forearc setting, and incorporated later into the accretionary prisms of their respective active margins. Exhumation mechanisms of the metamorphic & plutonic blocks of the Mineoka belt in a trench slope setting are similar to those documented from the Sofugan tectonic zone in the southern part of the modern Izu island arc. Although the geodynamic evolutionary paths of the Franciscan & Mineoka accretionary complexes differ from each other, both complexes appear to have experienced similar structural, sedimentological and tectonic processes during their accretionary histories.