A tale of two Walker Lane pull-apart basins in the ancestral Cascades arc, central Sierra Nevada, California

We integrate new geochronological, petrographic, and geochemical data with previously published (Sierra Crest-Little Walker volcanic center) and new (Ebbetts Pass volcanic center) structural and stratigraphic data to describe two deeply dissected, spectacularly well-exposed Walker Lane pull-apart basins in the ancestral Cascades arc. The Miocene (ca. 12-5 Ma) Sierra Crest-Little Walker arc volcanic center and the Miocene-Pliocene (ca. 6-4.6 Ma) Ebbetts Pass arc volcanic center formed in pull-apart basins in the Walker Lane, a NNW-trending zone of dextral strike-slip and oblique normal faults at the western edge of the Basin and Range Province. The Sierra Crest-Little Walker arc volcanic center is a transtensional arc volcanic field that is as areally extensive (∼4000 km2) and as long-lived (∼3 m.y.) as the Pliocene to Holocene Long Valley transtensional rift volcanic field, which is also in the Walker Lane. The Ebbetts Pass volcanic center is the next major volcanic center to the north within the ancestral Cascades arc. It formed within a smaller pull-apart basin. Its estimated original volume (∼270 km3) is comparable to the estimated volume for the Lassen volcanic center in the last 825 k.y. (200 km3), a major magmatic focus in the northernmost Walker Lane pull-apart basin. Postvolcanic faulting and erosion provide spectacular three-dimensional time-integrated views of the structure and stratigraphy of the Miocene to Pliocene volcanic centers described herein. The volcanic stratigraphy of this region is divided into four unconformity-bounded sequences: (1) Oligocene silicic ignimbrites erupted in central Nevada (Valley Springs Formation), (2) Miocene arc volcanic and volcaniclastic rocks (Relief Peak Formation), (3) ca. 11.5-9 Ma high-K arc volcanic rocks (Stanislaus Group), and (4) Miocene-Pliocene arc volcanic rocks (Disaster Peak Formation). We present 19 new 40Ar/39Ar ages and integrate them with previous 40Ar/39Ar ages to erect a detailed stratigraphic and intrusive chronology. For the first time, we present a systematic description of all of the volcanic and intrusive map units, using outcrop photos, photomicrographs, modal analyses, and geochemical data. Geochemistry and modal analyses show that volcanism at the younger, smaller Ebbetts Pass volcanic center is more evolved than that of the Sierra Crest-Little Walker volcanic center. The Sierra Crest-Little Walker volcanic center is dominated by Stanislaus Group basalt, trachybasaltic andesite, trac