Temperatures at the Base of the Seismogenic Crust Beneath Long Valley Caldera, California, and the Phlegrean Fields Caldera, Italy. In- Volcanic Seismology

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Book Section: Temperatures at the Base of the Seismogenic Crust Beneath Long Valley Caldera, California, and the Phlegrean Fields Caldera, Italy. In- Volcanic Seismology

Abstract
In seismically active volumes of the crust, the base of the seismogenic zone commonly corresponds with the temperature at which the first mineral species in the crustal rock enters the quasi-plastic domain. This sub-solidus temperature marks the onset of the transition from brittle (seismogenic) failure to plastic (aseismic) flow for a given rock type, and it varies with both strain rate and water content. Earthquake activity and deformation accompanying recent unrest in Long Valley caldera, California, and the Phlegrean Fields caldera, Italy, provide an opportunity to compare predictions of laboratory-based constitutive relations for the brittle-plastic transition in regions of differing rock type (quartzo-feldspathic in Long Valley and feldspathic in the Phlegrean Fields), for which strain rates (3 _ 10_14 and 5 _ 10_12s_1 for Long Valley and the Phlegrean Fields, respectively) and earthquake focal depths are well determined. Magma underlies sections of both calderas at depths somewhere between 4 to 10 km. Results indicate temperatures in the range 250 to 350 °C for the base of the seismogenic zone in the granitic crust beneath Long Valley caldera, the depths to which vary from less than 5 km beneath the resurgent dome and Mammoth Mountain to 8 or 9 km beneath the south moat. Temperatures could approach 500°C at depths of 5 km beneath the resurgent dome, however, if strain rates increase significantly with depth toward the underlying inflation center and rocks in the plastic domain are dry. The higher temperatures (600 to 700 °C) predicted for the base of the seismogenic zone at depths of 4 to 5 km beneath the Phlegrean Fields caldera reflect a higher strain rate and a feldspathic (trachyte) composition of the crustal rocks. These temperatures are consistent with a linear extrapolation of geothermal gradients measured in adjacent, 3-km-deep wells. If, however, strain rates increase significantly with depth toward the inflation center at a depth between 3 and 5 km and rocks in the plastic domain are dry, the temperature at the base of the seismogenic zone could approach 800 °C beneath the central section of the Phlegrean Fields caldera.

Author 
David P. Hill








Published 
Springer-Verlag Berlin Heidelberg, 1992





DOI 
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Online 
Internet link for Temperatures at the Base of the Seismogenic Crust Beneath Long Valley Caldera, California, and the Phlegrean Fields Caldera, Italy. In- Volcanic Seismology

Citation

David P. Hill. 1992. Temperatures at the Base of the Seismogenic Crust Beneath Long Valley Caldera, California, and the Phlegrean Fields Caldera, Italy. In- Volcanic Seismology. New York, NY: Springer-Verlag Berlin Heidelberg. 432-461p.