Abstract

The King Range is a rugged coastal mountain range that parallels the San Andreas transform fault system just south of the Mendocino triple junction. Point Delgada is a small coastal headland that projects into the Pacific Ocean just southwest of the King Range. Apatite fission-track ages from parts of the King Range are remarkably young, averaging 1.2 Ma, indicating that a minimum of 2-5 km of uplift and unroofing have occurred in the past 1.2 m.y. In contrast, ages from Point Delgada are about 12 Ma, and fission-track length data indicate that rocks there have resided at low temperatures (≤50 °C) and thus at shallow depths since soon after 12 Ma. Therefore Point Delgada has experienced relative vertical stability. The contrast in uplift histories indicates that the two areas are separated by a major fault with a minimum of ∼1 km of Quaternary vertical offset. The fault is probably part of the San Andreas system and so may also have undergone major Quaternary strike-slip offset. The uplift in the King Range seems too great and too localized to have resulted from isostatic effects accompanying passage of the Mendocino triple junction and development of a slab-free window; rather, it is probably a local response to space problems among the various moving crustal blocks around the triple junction and San Andreas fault.

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