Southward-younging apatite (U–Th)/He ages in the northern California Coast Ranges due to a northward-migrating crustal welt

(U–Th)/He in apatite dating of Franciscan Formation samples from between 39°47′N and 39°08′N in the northern California Coast Ranges (NCCR) reveals a southward- and downward-younging pattern of ages (21.8 ± 0.6 Ma to 7.9 ± 0.9 Ma). These ages are interpreted to represent a northward or northeastward dipping fossil He partial retention zone that was tilted and exhumed by a northward-migrating pulse of crustal thickening that begins ∼150–200 km ahead of the migrating Mendocino Triple Junction (MTJ) [K.P. Furlong, R. Govers, Ephemeral crustal thickening at a triple junction: the Mendocino crustal conveyor, Geology 27 (1999) 127–130). The ∼8 Ma ages from the southern end of the sample transect are interpreted to come from rocks that were immediately below the He partial retention zone when crustal thickening began, ∼5 my before the MTJ passed by; these rocks have now reached the surface, ∼3 my after passage of the MTJ. The older ages from the central and northern portions of the sample transect are interpreted to represent rocks that were within the He partial retention zone prior to crustal thickening ahead of the MTJ, and which therefore record ages transitional from older subduction-related cooling and exhumation to more recent transform-related cooling and exhumation. In addition to providing support for the model of a migrating crustal welt associated with the MTJ, the existence of an essentially intact, gently tilted fossil partial retention zone in the northern California Coast Range raises the possibility of using isochronous surfaces as neotectonic structural markers in this geologically complex and heavily vegetated region.