A TALE OF TWO WALKER LANE PULL-APARTS

The Sierra Nevada - Walker Lane region is an archetype for early rupturing of continental lithosphere. Trantensional rifting was initially focused in thermally-weakened, partially extended crust of the Ancestral Cascades arc (~ 12 Ma), producing a large volcanic center at a releasing stepover, the Sierra Crest – Little Walker volcanic center. Here, “flood andesite” lavas erupted from fault-controlled fissures within a series of grabens that we call the “Sierra Crest graben-vent system”. This system consists of a single ~27 km long, ~ 8-10 km wide ~N-S full graben that lies along the modern Sierran crest between Sonora Pass and Ebbetts Pass, with a series of ~N-S half grabens on its western margin, and a ~30 km-wide NE transfer zone emanating from the NE boundary of the full graben on the modern range front. The N-S faults rare dextral oblique and the NE faults are sinistral oblique; the geometry and kinematics of these ~ 11 – 9 Ma faults are clearly Walker Lane (not Basin and Range) in style.

The transtensional rift tip then propagated northward, following the Mendocino triple junction, to form a younger, newly-recognized intra-arc pull-apart we call the Ebbetts Pass volcanic center. This formed at a series of right-stepping N-S dextral oblique down-to-the-east normal faults on the modern Sierra crest and range front. The oldest volcanic rocks in the pull-apart are Late Miocene, 6.305 +/-0.017 Ma (⁴⁰Ar/³⁹Ar hornblende, on a 2-pyroxene biotite quartz dacite lava flow at Elder Creek), but most of the pull-apart fill is Pliocene, and somewhat bimodal, perhaps indicating a transition from subduction to rift magmatism. The earliest volcanic rocks include a rhyolite flow at Nobel Lake (4.581 +/-0.018 hornblende) and a biotite sanidine hornblende quartz rhyolite ultra-welded ignimbrite near Silver Peak (4.636 +/-0.014 on 14 sanidine crystals). Later basaltic andesites form a large cone (4.85 +/-0.02 plagioclase), but the youngest parts of the cone are silicic, with block-and-ash-flow tuffs and central intrusions, including a hornblende biotite sanidine quartz dacite intrusion dated at 4.434 +/-0.007 (sanidine).

We predict that any <12 Ma large volcanic centers identified by future workers in the Sierra Nevada-Walker Lane region will be sited at major releasing bends or step-overs, like Lassen and Long Valley are today.