Mendocino Fracture Zone - Not So Simple!

The Mendocino fracture zone marks the southern limit of Cascadia subduction or the northern terminus of the San Andreas fault system. It currently manifests as a simple oceanic transform generating pure strike-slip earthquakes. Its long and complex tectonic history, however, includes being oriented significantly oblique to the Pacific/Juan de Fuca plate motion vector. Recent analysis of NOAA multibeam and backscatter data mapping the Mendocino fracture zone to >350 nautical miles from the northern California coast (in support of the US Extended Continental Shelf Project) combined with legacy magnetics, rock samples, seismic reflection and subbottom datasets reveals features inherited from (1) a wide, stable multi-strand transform system active 19-10 Ma (2) uplift of Mendocino Ridge west of 126W in compression shortly after 10 Ma, (3) a time of more voluminous magmatism along the Gorda spreading center abruptly ending at 7 Ma, and (4) more pronounced deformation near the Mendocino triple junction since about 6 Ma related to flexural coupling with the adjacent deforming Juan de Fuca plate. New kinematic modeling focused on the evolution of the Mendocino transform system places these events into a regional context. In particular, formation of oceanic crust within the multistrand transform system 19-10 Ma provides an explanation for observed samples of this age range found within the fracture zone and in the adjacent onshore King Range. It also explains a link between Miocene deposition of King Range sedimentary deposits and Mendocino intratransform depositional settings.