EFFECTS OF HIGH PORE-FLUID PRESSURE INDUCED HYDRAULIC FRACTURING AND FLUIDIZATION IN SECONDARY SEDIMENTARY STRUCTURE DEVELOPMENT IN SUBDUCTION-ACCRETION SYSTEMS: FRANCISCAN COMPLEX ON THE SAN SIMEON COAST, CENTRAL CALIFORNIA

Liquefaction widely occurred in a radius of 600 km around the epicenter of the 2011 Great Tohuku earthquake (Mw=9.0), and strongly affected the ancient channel deposits and recent man-made buried sites close to Tokyo. The 1964 Niigata earthquake (Mw=7.5) also caused widespread liquefaction that resulted in tilting and damaging of apartment complexes. High pore-fluid pressure induced by strong earthquake shaking is commonly responsible for liquefaction. Other pore-fluid pressure effects include hydraulic fracturing and fluidization, which are common in sandstones in ancient subduction zones. During our mapping of the coastal areas of San Simeon in central California we have observed many outcrops of fluidized and hydraulically fractured turbidite beds as a result of paleo-liquefaction events that formed conglomeratic strata. These conglomeratic bodies commonly consist of breccias with pebbles and cobbles of monolithic sandstone and rare oceanic rocks such as basalt and chert, and have been mistakenly interpreted as mélange bodies in previous studies. Our field observations indicate, however, that these rocks formed due to hydraulic fracturing that triggered mud injections into oceanic rocks and mélange bodies. The “Cambria Slab” in the San Simeon area is another example of a high pore fluid pressure effect that developed fluidized turbidites with water escape structures, such as dewatering vein (pillar), dish and web structures. We infer that conglomerate pebbles sank downward within the fluidized bodies of the Cambria Slab during its formation. We have identified the way-up directions in the Cambria Slab by using such structures, in addition to relict graded bedding features, to map the small- and large-scale isoclinal folds and thrust faults. Fluidization occurs as the pore-fluid pressure increases significantly during compaction and deformation. Our work in the Franciscan Complex in the San Simeon area shows that high pore-fluid pressures and fluidization are among the most important processes in subduction-accretionary prisms that produce widespread deformation and secondary sedimentary structures.