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

摘要

(U-Th)/He in apatite dating of Franciscan Formation samples from between 39 degrees 47'N and 39 degrees 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 similar to 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 similar to 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, similar to 5 my before the MTJ passed by; these rocks have now reached the surface, similar to 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. (c) 2005 Elsevier B.V. All rights reserved.