OVERVIEW OF THE ANCESTRAL CASCADES ARC IN THE SIERRA NEVADA AND ITS RELATIONSHIP TO PALEOCHANNELS AND WALKER LANE FAULTS

The Sierra Nevada is a block-faulted mountain range with a gentle sloping west side and a steep, fault-bounded range front on the east side. Early workers recognized E-W oriented, Eocene to Miocene paleochannels (some with placer gold), and it was more recently recognized that these paleochannels formed on the western shoulder of a broad uplift, termed the Nevadaplano, which was disrupted by Miocene Basin and Range extension and Walker Lake transtension. Similarly, early workers inferred that the Sierran range front faults formed by Basin and Range extension, but we now know the range front is controlled by Walker Lane transtensional faults, which form the eastern boundary of the Sierra Nevada microplate.

The birth of the Walker Lane plate boundary is recorded in the 12-9 Ma Sierra Crest-Little Walker transtensional arc volcanic center, which is as areally extensive (4,000 km²) and long-lived as the modern Long Valley transtensional rift volcanic field, also in the Walker Lane. This very large Miocene pull-apart basin beheaded and deranged two Sierran paleochannels (Stanislaus and Cataract). Then, Walker Lane faulting migrated northward, in concert with the northward migration of the Mendocino Triple Junction, to produce the 6.5-4.5 Ma Ebbetts Pass arc volcanic center and pull-apart basin, a smaller system that compares well with the modern Lassen arc volcano, which also lies in a Walker Lane pull-apart basin. The Ebbetts Pass pull-apart basin beheaded a third paleochannel, the Mokelumne paleochannel, by 5 Ma.

The Walker Lane transtensional rift has been unzipping from south to north, within the axis of the Cascades arc, with rift volcanic centers forming in its wake as subduction shuts off. All of these large Miocene to Recent volcanic centers lie in releasing stepovers in the Walker Lane, which focus magmas to the surface that have formed by subduction or continental rifting.

This presentation summarizes the results of a 21-year collaboration that is ongoing, with current studies focusing on the newly-recognized Diamond Valley arc volcanic center, which lies at a releasing stepover along the Genoa fault. We are also conducting mineral composition and isotopic studies to determine magma storage levels, residence times and sources.