SEAMOUNT SUBDUCTION: MELANGE FORMATION IN THE FRANCISCAN COMPLEX OF CALIFORNIA AND SEISMOGENIC BEHAVIOR

The Pacific basin is dotted by more than 50,000 mafic seamounts taller than 1 km tall. Almost all seamounts taller than 2 km were generated from hot spot or arc volcanism, but most shorter ones were generated at ocean ridges. The Franciscan Complex that underlies the California Coast Ranges is a large accretionary prism that formed since about 155 Ma. Many thousands of seamounts must have encountered the Franciscan trench as more than 10,000 km of Farallon plate was consumed.

It is observed that where plate movement has conveyored seamounts to the subduction environment, fragmentation by normal faulting can occur as the descending plate bends downwards. As they move arcwards, tall ones bulldoze the leading edge of the overriding block. This causes local oversteeping of slope and mass movements. Deeper-seated processes must be inferred. Short seamounts can move downwards with abrasive movements in the shear zone detaching pieces. Tall seamounts that abut the leading edge of the accretionary prism above the shear zone can become detached and accreted as relatively intact fragments. Most seamounts that entered the Franciscan trench subducted, as pieces that are km+ wide are rare. Two of the larger examples are the imbricated chert/basalt sequence that underlays the Marin Headlands area north of the entrance to San Francisco Bay and the Calera limestone/basalt sequence to the south. The Franciscan Central Belt is dominated by shale-matrix mélange that contains about 5% mafic greenstone blocks with varied pinch-and-well accommodated by cataclasis and metamorphism that developed during tectonic shearing. It is probable that most, if not all, of these greenstones are fragments of seamounts.

Subduction channel shear zones thin downdip when underplating thickens an overriding accretionary prism. When this occurs, mafic seamounts can subduct, shedding pieces along the way from an intact core that moves with the descending plate. Seamounts taller than the shear zone become abraded as they scrap the base of the overriding block. Tall and strong seamount cores can become jammed. When this occurs, elastic strains develop near the seamount asperity that are released during earthquake-generating ruptures. Initial earthquake magnitudes from seamount underplating by decapitation should be proportional to seamount size.