CASCADE ARC AND WALKER LANE INTERACTION IN THE LASSEN VOLCANIC REGION, CALIFORNIA

The Cascade Volcanic Arc in the Lassen Region is fundamentally driven by subduction of the Gorda Plate beneath the North American (NA) Plate but is strongly influenced by tectonic motions among the Sierra Nevada Block (SNB) to the S, the Basin and Range (BR) to the E, and the Oregon Block (OB) to the N. The SNB is separated from the BR by the Walker Lane (WL), which accommodates ~20% of the dextral movement between the NA and Pacific Plates and is migrating N parallel to movement of the Mendocino triple junction.

Subduction-zone volcanism has built a regional platform of short-lived, small-volume, calc-alkaline volcanoes composed of mafic to intermediate rocks. The volcanic axis migrated westward from the CA-NV border at 12 Ma to its present position and simultaneously narrowed. Major, long-lived, large-volume volcanic centers that evolve to dacite and rhyolite are intercalated within the regional, more mafic calc-alkaline rocks. A separate suite of low-K olivine tholeiite is related to the BR. Vent alignments show that the lesser horizontal stress is ENE.

A broad gravity low encompasses all volcanism <825 ka in the region and is centered on the <300 ka silicic dome field of the Lassen Volcanic Center (LVC). Matched filtering discriminates the crustal sources of the anomaly. At 3.5 km depth, a pronounced gravity low coincides with the youngest volcanism at LVC, and gravity lineaments are associated with all significant tectonic structures. At 12.7 km depth, a broad triangular low is evident, which we interpret to reflect relatively low-density material in the mid crust and a pull-apart basin in the upper crust. We speculate that the source of the Lassen gravity low is a combination of intrusive rocks, shallow low-density volcanic rocks, high temperatures at depth, and restricted volumes of partial melt.

The orientation and shape of the Lassen gravity anomaly suggest that in the last ~825 ka the upper and mid crust were influenced by long-term interaction between the WL and the Cascade arc. The pull-apart basin containing the LVC originated by dextral shear of the WL superimposed on dip-slip movement related to BR extension. The vigorous Lassen Volcanic Center is thus the product of subduction-related, mantle-level processes of the Cascade Volcanic Arc and crustal-level tectonic processes related to the Walker Lane.