GEOLOGY SOUTH OF PUNTA GORDA, MENDOCINO TRIPLE JUNCTION REGION, CA

South of Punta Gorda rocks are highly sheared and fractured; in part the result of recent passage of the Mendocino triple junction and high coastal uplift rates. The Cooskie shear zone, a possible thrust-faulted contact which places rocks of the King Peak subterrane [to the south] over the Coastal terrane, is characterized by folded strata of the King Peak subterrane, consisting of thin-to-thick bedded turbidites, which become highly boudinage approaching a faulted contact with more competent Coastal terrane. Folds are parallel, open to tight, are commonly overturned and gently plunge to the ESE. In profile view, fold height is within two meters and the width length is slightly greater. The Coastal terrane is comprised mostly of highly brecciated and veined massive sandstone with minor pebble conglomerate and shale. The sandstone is lithified by silica cement produced during diagenesis of clays. Pumpellyite is present in sandstone from the Coastal terrane. Diagenetic chlorite cement gives the rock its green color. Laumontite veins were the latest mineral to form in the study area. This may be the result of coastal uplift or signals commencement of shearing in the Cooskie shear zone. The footwall is loaded during thrusting and with increased high pore fluid pressure there will be a decrease in effective stress leading to fracturing and allowing fluid to move between the footwall and fault zone. The hanging wall has remained unchanged with no fracturing and fluid flow. A decrease in fluid pressure may have initiated for vein precipitation and at the conclusion of shearing and or uplift.