CRUSTAL ACCRETION HISTORY OF THE KLAMATH MOUNTAINS PROVINCE: TRACKING TEMPORAL CHANGES IN MOHO DEPTH (CRUSTAL THICKNESS) USING BASALT TRACE ELEMENT GEOCHEMISTRY

A recent compilation of geochemical data from modern subduction zones has revealed a correlation between Moho depth and the maximum Ce/Y ratio observed in arc basalt (Mantle and Collins 2008, Geology). This correlation can be used to assess temporal variations in Moho depth in paleo-arc settings.

The Klamath Mountains province (KMP) records nearly 400 million years of convergent margin crustal growth. The Eastern Klamath terrane (EKT) provides the earliest record of convergent-margin tectonism in the Klamath province and was the nucleus (backstop) of subsequent crustal accretion. Ce/Y data from mafic rocks of the basal Trinity-Copley magmatic suite suggest a thin crust (<15 km) in Silurian-Devonian time. Late Permian basaltic rocks in the Redding section (Nosomi-Dekkas) suggest Moho depths of ~25 km; sedimentation must account for at least some of the Late Paleozoic crustal thickening. Ce/Y data for mafic rocks in post-Nevadan intrusions (TTG suite) suggest maximum EKT Moho depths ~37 km in Early Cretaceous. Ce/Y data from Quaternary Shasta basaltic rocks suggest Moho depths of 35 km, consistent with seismic data.

The accreted terranes of the western KMP record a broad range of Moho depths. Jurassic supra-subduction zone ophiolites sequences (Sexton Mtn at 177 Ma; Josephine at 164 Ma) record the shallowest Moho depths (<20 km) consistent with previous interpretations of crust generated in an SSZ back-arc setting. Ce/Y data from arc-related mafic rocks in the North Fork terrane (Sawyers Bar) and the Rattlesnake Creek terrane record the greatest Moho depths approaching 50 km, consistent with arrival of thick paleo-Pacific island arc terranes. Ce/Y data from mafic rocks associated with post-contraction plutons (Wooley Creek suite, post-Wilson Point thrusting; Western Klamath suite, post Nevadan/Orleans thrusting) indicate Moho depths of ~37 km.

Assuming that Ce/Y data are a reliable proxy for Moho depths, data from the KMP suggests that transport and underthrusting of off-board arc terranes is a major contributor to crustal thickening in Pacific-type convergent margins. Temporal variations in Moho depth indicate the KMP reached its present day values during Late Jurassic contraction events, implying a balance between subduction-related accretion and surface denudation during Cenozoic time.