SEAMOUNT ARRIVAL INTO THE FRANCISCAN SUBDUCTION COMPLEX AT 100 MA: MARIN HEADLANDS, SAN FRANCISCO BAY, CA

In the Marin Headlands area of the Franciscan Complex, pillow basalt overlain by layered radiolarian chert form a ~8 km wide mass imbricated by thrust faulting. The 80 m chert section ranges in age from Pliensbachian to early Cenomanian (~185 to 95 Ma). Two locations have been found where the chert is depositionally overlain by greywacke. Shale-matrix mélange is mapped around and intruding the sequence.

Depositional contacts of greywacke on chert are present at Rodeo Cove and Blacksand Beach. LA-ICP-MS analysis was used to obtain U/Pb ages for 120+ detrital zircons from the sands to constrain source areas and the maximum age of deposition. More than 95% of the zircons from four samples near the contact are of Mesozoic age and probably sourced from the Sierra Nevada region. For the three samples collected within 20 centimeters of the contact, the three youngest zircons yield an average age of 100 ± 1 Ma for the maximum age of deposition. As the Sierra Nevada volcanic arc is nearby, the youngest zircons are probably syndepositional and thus record the age of deposition. Intrusions of shale-matrix mélange with blocks of greywacke, greenstone, and chert are also exposed at Rodeo Cove and Blacksand Beach. From both locations, one 25-50 cm wide greywacke block was processed to determine the U/Pb zircon ages. The mélange block from Rodeo Cove is distinctly different as about 20% of its zircons have Precambrian ages. The sample from Blacksand Beach has a zircon population nearly identical with the bedded greywacke. In both mélange blocks, the three youngest zircons are 100±1 Ma.

The simplest explanation for the Marin Headlands pillow basalt/chert sequence is that it is a fragment of a seamount formed on the Farallon plate far out in the Pacific Ocean basin. The seamount entered the Franciscan subduction zone after ~90 m.y. of pelagic sedimentation. As the plate bent down to subduct, normal faulting formed scarps. At 100 ± 1 Ma, the seamount impinged upon the landward trench wall and sand-rich flows sourced from the Sierran arc scoured fragments of chert from fault scarps and deposited them nearby. The topmost part of the seamount became imbricated as it detached from the descending plate and accreted to the front of the Franciscan prism. As this occurred, fingers of shale-matrix mélange intruded the seamount and blanketing greywacke.