DUCTILE AND BRITTLE DEFORMATION STRUCTURES IN THE BLOCKS OF FRANCISCAN (CALIFORNIA) AND MINEOKA (JAPAN) MÉLANGES: IMPLICATIONS FOR RETROGRADE DEFORMATION DURING EXHUMATION OF DEEP-BURIAL ROCKS

Tectonic blocks of metamorphic, igneous and oceanic rocks, in both the Franciscan (California) and Mineoka (Japan), show widespread cataclastic and mylonitic structures. The mélanges, formerly interpreted as debris flow deposits of a sedimentary origin, display systematic fracture patterns with certain orientations as below, so that not of sedimentary in origin. The boundaries between clasts and the matrix materials are generally pulverized, and Riedel shears form sharp planes in the rocks. The fracture planes are plotted in great or small circles on a stereographic projection. Such structural features indicate a tectonic origin, rather than sedimentary for their formation. We interpret that all these brittle to ductile structures formed during the exhumation of the rocks. The metamorphic mineral phases suggest their crystallization at 10-20 to 50 km within subduction zones with low to intermediate thermal gradients. The rock exhumation involved volumetric expulsion via dilatant shear deformation, and formation of quartz, calcite, zeolite and other low-pressure minerals, in most cases associated with quartz, calcite, zeolites or other low-pressure or low-temperature minerals. Block shapes are commonly phacoidal with fish tails, or turtle-like, surrounded by pelagic matrices or serpentinite. Some blocks are metamorphosed into either blueschist (Franciscan case), or amphibolites (Mineoka case), superimposed by later veins or rinds of epidote-amphibolite or greenschist facies assemblages, suggesting retrograde metamorphism. Serpentine minerals, lizardite-chrysotile and rare antigorite, indicates vast amount of volume increase by hydration that may have played a significant role in exhumation through diapiric or intrusive emplacement of the retrograde metamorphic blocks. Tonalitic igneous bodies have similar deformation features of retrograde metamorphism. These blocks within the both mélanges strongly support the concept of subduction channel of return flow for the exhumation processes, whose modern analogues occur in the Paleogene Izu forearc.