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, trachyandesite, and andesite. Climactic eruptions at its southern end produced the Little Walker caldera and its Stanislaus Group trachydacite ignimbrites. In contrast, the Ebbetts Pass volcanic center erupted much less basalt, and an approximately equal mixture of basaltic andesite, andesite, dacite, and rhyolite. By virtue of its higher SiO2, on average, it shows geochemical evidence for higher extents of crustal contamination. All of the volcanic rocks except the Stanislaus Group are compositionally identical to Mesozoic plutonic rocks of the Sierra Nevada Batholith, indicating a similar petrogenesis by shallow-level fractional crystallization with no evidence for deep-seated pyroxenite cumulates. Only the high-K2O Stanislaus Group requires a significant degree of fractionation of clinopyroxene and Fe-Ti oxides. The major-oxide trends and clinopyroxene-rich modes of the Stanislaus Group may be the volcanic expression of processes that produced pyroxenite cumulates. The basal and most voluminous unit of the Stanislaus Group, the Table Mountain Latite, is a likely parent to succeeding units of the Stanislaus Group (Eureka Valley Tuff and Dardanelles Formation), but this particular liquid line of descent is otherwise rare in the Mesozoic and Cenozoic history of the Sierra Nevada.
Volcano-tectonic activity continued for a longer period (ca. 12–6 Ma) in the northern half of the Sierra Crest–Little Walker volcanic center than it did in the southern half (ca. 12–9 Ma), overlapping for ~1 m.y. with the onset of volcano-tectonic activity at the Ebbetts Pass volcanic center to the north. This provides local-scale confirmation of the regional-scale interpretation that a transtensional rift tip propagated northward with time within the arc axis, in concert with northward migration of the Mendocino triple junction. Transtensional rift magmatism followed in its wake and continues today at various points along the length of the Walker Lane. “Tectono-stratigraphic recycling” was a key geologic process throughout the development of these ancestral Cascades arc pull-apart basins, and it consisted of the transfer of megaslide slabs, up to 2 km long and tens to hundreds of meters thick, from footwall to hanging-wall blocks in the transtensional rift. A time-slice series of block diagrams is used to illustrate the structural controls on arc volcanism in the early stages of Walker Lane transtensional faulting (ca. 12–4.6 Ma).