Magmatism, ash-flow tuffs, and calderas of the ignimbrite flareup in the western Nevada volcanic field, Great Basin, USA

Abstract

The western Nevada volcanic fi eld is the western third of a belt of calderas through Nevada and western Utah. Twenty-three calderas and their caldera-forming tuffs are reasonably well identifi ed in the western Nevada volcanic fi eld, and the presence of at least another 14 areally extensive, apparently voluminous ash-fl ow tuffs whose sources are unknown suggests a similar number of undiscovered calderas. Eruption and caldera collapse occurred between at least 34.4 and 23.3 Ma and clustered into fi ve ~0.5-2.7-Malong episodes separated by quiescent periods of ~1.4 Ma. One eruption and caldera collapse occurred at 19.5 Ma. Intermediate to silicic lavas or shallow intrusions commonly preceded caldera-forming eruptions by 1-6 Ma in any specifi c area. Caldera-related as well as other magmatism migrated from northeast Nevada to the southwest through time, probably resulting from rollback of the formerly shallow-dipping Farallon slab. Calderas are restricted to the area northeast of what was to become the Walker Lane, although intermediate and effusive magmatism continued to migrate to the southwest across the future Walker Lane. Most ash-fl ow tuffs in the western Nevada volcanic fi eld are rhyolites, with approximately equal numbers of sparsely porphyritic (≤15% phenocrysts) and abundantly porphyritic (~20-50% phenocrysts) tuffs. Both sparsely and abundantly porphyritic rhyolites commonly show compositional or petrographic evidence of zoning to trachydacites or dacites. At least four tuffs have volumes greater than 1000 km 3 , with one possibly as much as ~3000 km 3. However, the volumes of most tuffs are diffi cult to estimate, because many tuffs primarily fi lled their source calderas and/or fl owed and were deposited in paleovalleys, and thus are irregularly distributed.

Eruptive history, geochronology, and post-eruption structural evolution of the late Eocene Hall Creek Caldera, Toiyabe Range, Nevada

“…Best and others (2009;2013) interpreted this barrier as the western edge of a high continental plateau coincident with a crustal-scale, east-vergent thrust fault; the edge of the plateau was enhanced by precipitation and consequent erosion and isostatic uplift on the westward (Pacific Ocean) side compared to the dry eastern side (for example, Masek and others, 1994). Henry and John (2013) interpreted several tuffs sourced from calderas in western Nevada to have flowed east through the area of this proposed topographic barrier and suggested a drainage divide further east ( fig. 1).…”

“…Their distribution thus informs knowledge of regional topography prior to formation of the modern Basin and Range Province, when the region was thought to be a broad, high orogenic plateau, much of which drained to the Pacific Ocean (for example, DeCelles, 2004;Cassel and others, 2009;Henry and John, 2013). others (2009, 2013) interpreted a major north-south topographic high between Austin and Tonopah, Nevada ( fig.…”

“…Silicic magmatism was broadly time transgressive ( fig. 1), sweeping southwest across the Basin and Range from northern Nevada in the Eocene to south central Nevada in the early Miocene (for example, Armstrong and others, 1969;Henry and John, 2013). Extensional faulting locally began during this time and has continued episodically up to the present, but the relation between extension and magmatism-both locally and at the province scale-has been debated for several decades as new studies gradually unraveled the relative timing of volcanism and extensional faulting.…”

“…Both silicic and intermediate post-caldera lavas were locally erupted within 400 thousand years of the main eruption, and for the next approximately 10 million years sedimentary rocks and distal tuffs sourced from calderas farther west ponded in the caldera basin surrounding low areas nearby. Patterns Henry and John (2013) and proposed middle Tertiary drainage divides from Best and others (2009) and Henry and John (2013) (Ma, mega-annum).…”

“…Colgan_IP-075500_fig 01 Inferred "Toiyabe uplift" of Speed and others (1988) Proposed paleodivide of Henry and John (2013) Proposed paleodivide of Best and others (2013) Shaded-relief base derived from U.S Geological Survey National Elevation Dataset 30-meter, digital elevation model, 1999 Universe Transverse Mercator projection…”