Geologic map of the Caetano caldera, north-central Nevada: One of the world's best-exposed and most informative calderas
Abstract
The 34.0 Ma Caetano caldera formed during eruption of ~1100 km3 of crystal-rich rhyolite (72-78% SiO2). Miocene extension cut and tilted the originally 20 x 12-18 km caldera into a set of ~40° east-dipping fault blocks that expose the entire caldera to its floor, including minor pre-caldera volcanic rocks, two units of intracaldera Caetano Tuff up to 4 km thick, ash-flow tuff feeder dikes, caldera collapse breccias, post-collapse resurgent intrusions, and the caldera structural margin and topographic wall. Geologic mapping at scales ranging from 1:6,000 to 1:75,000 allows the volcanic and structural history of this caldera system to be reconstructed at a level of detail available for few calderas worldwide. The caldera was not built on an older volcanic edifice, although it was preceded by ~6 Ma of regional intrusive magmatism, including 35.7 Ma rhyolite dikes emplaced during formation of nearby Carlin-type gold deposits. Only a very small volume of andesitic lava erupted locally prior to formation of the caldera itself (about 1.2 Ma before). The caldera floor subsided asymmetrically as an apparently coherent block during the eruption. Early eruption and subsidence was likely rapid, allowing the lower intracaldera tuff to pond as a single cooling unit up to 3 km thick in the eastern part of the caldera. Later subsidence was greater in the western part, where the upper intracaldera tuff is up to 2 km thick but thins to <0.5 km to the east. This phase of the eruption was episodic enough that fine-grained sediments accumulated between ash-flow cooling units, but extensive 40Ar/39Ar dating indicates that the entire volcanic interval lasted <100 ka. Collapse occurred dominantly along a single ring fracture, but a composite ring fracture is preserved on the northeastern margin. Here, greatest collapse (up to 4 km) occurred along an inner fracture that transitions upward from a discrete fault zone to a topographic wall. The topographic wall here (and elsewhere in the caldera) is only ~1 km outboard of the structural margin, and megabreccia constitutes no more than a few % of intracaldera fill. Lesser collapse occurred along an outer ring fracture, which preserves a 10-40 m wide, flow-banded to lithic and pumice-rich ash-flow tuff dike that fed a very early phase of the eruption. Where exposed, caldera-margin faults dip 60-70° inward to ~4 km depth. Geometric accommodation of caldera collapse thus requires either significant pre-collapse doming of surrounding Paleozoic rocks (for which there is no direct evidence), or an inner zone of outward dipping thrust faults within the caldera floor itself, which may now be concealed by intracaldera tuff. Eruption was followed by emplacement of two large resurgent granitic intrusions into intracaldera tuff. The first (Redrock Canyon porphyry) drove a large hydrothermal system that strongly altered much of the tuff in the western part of the caldera. The emplacement mechanism for this intrusion is not clear, but it did not deform the surrounding tuff. The later resurgent intrusion (Carico Lake pluton) strongly deformed the intracaldera tuff, indicating forceful emplacement.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V13C2608C
- Keywords:
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- 8109 TECTONOPHYSICS / Continental tectonics: extensional;
- 8178 TECTONOPHYSICS / Tectonics and magmatism;
- 8440 VOLCANOLOGY / Calderas;
- 8486 VOLCANOLOGY / Field relationships