TECTONICALLY CONTROLLED, TIME-PREDICTABLE BASALTIC VOLCANISM FROM A LITHOSPHERIC MANTLE SOURCE, SOUTHWESTERN NEVADA VOLCANIC FIELD, USA

Understanding the evolution of basaltic volcanic fields is critical to concepts of basaltic magmatism and to volcanic risk assessment. Volcanological, geochemical, and time-volume characteristics of the Plio-Pleistocene part of the Southwestern Nevada Volcanic Field (SNVF) suggest it is an example of an extremely low volume-flux end member of basaltic volcanic fields. The SNVF has produced 17 volcanoes of dominantly trachybasaltic composition over the past 5 Myr with a total volume of slightly less than 6 km3. Eruptive volumes decreased between Pliocene- and Pleistocene-age volcanoes. Major element data suggest that most of the magmas underwent similar degrees of fractionation during ascent, and trace element compositions indicate a decrease in the degree of partial melting of the lithospheric mantle source since ~3 Ma. Isotopic data support an interpretation wherein magmas ascended relatively quickly from their source regions with little if any interaction with crustal rocks. Relationships between age and cumulative erupted volume indicate that the repose interval between eruptive episodes is determined by the volumes of prior episodes and, since ~3 Ma, an average eruption rate of ~0.5 km3/Myr, i.e., the field is time-predictable. All of these features support a model wherein magmatism is a passive result of regional tectonic strain. Partial melt resides in pockets of lithospheric mantle that are relatively enriched in hydrous minerals; slow deformation focuses melt, occasionally resulting in sufficiently high melt pressure to drive dikes upward and feed eruptive episodes. Larger source volumes result in larger eruptive volumes and wider dikes that relieve relatively more strain in the crust than smaller volume events, and therefore are followed by longer repose intervals required for recovery of crustal stresses. We suggest that time-predictability may be a fundamental property of tectonically controlled basaltic fields, where melt accumulation and ascent are controlled by tectonic strain rate. This behavior contrasts with magmatically controlled fields where magma flux is sufficiently high to overwhelm local tectonic strain, eruptions are primarily caused by magmatic processes that build pressure in reservoirs, and the systems are more likely to be volume-predictable.