LONG- AND SHORT-TERM INTRUSION RATES IN THE THREE SISTERS REGION, CENTRAL OREGON CASCADE RANGE

Ongoing (1998 to present) deformation in the upper Separation Creek watershed, west of South Sister, has prompted renewed interest in Cascade Range magmatism. A decade ago, a heat-budget approach was used to infer long-term intrusion rates of 9-33 cubic km/km-arc-length/Ma for the central Oregon Cascades (Ingebritsen et al., Science, 1989; USGS PP 1044-L, 1994). Local intrusion rates for the Separation Creek watershed, which subtends about 8 km of arc length, can be inferred on three independent bases: hydrothermal chloride discharge, hydrothermal heat discharge, and magmatic carbon discharge. First-order calculations indicate that current rates of chloride (10 g/s), heat (16 MW), and carbon (21 tonnes/day) discharge could be supported by intrusion rates of 16, 33, and 60 cubic km/km-arc-length/Ma, respectively. These calculations assume that the chloride originates from magmatic volatiles; ~270 kJ/kg of latent heat of crystallization and ~500 degrees Celsius of post-emplacement cooling; and that the chloride and carbon dioxide contents of the magma are about 0.1 and 0.65 wt. percent. They are subject to uncertainties of about a factor of two. The chloride- and heat-based intrusion-rate estimates are similar to the long-term central Oregon rate inferred from the regional heat budget, whereas the carbon-based rate is marginally larger. The current deformation detected by satellite-radar interferometry and other geodetic data (Wicks et al., GRL, 2002) equates to a much higher intrusion rate of 300 cubic km/km-arc-length/Ma or more over the 4-year period since deformation began. The USGS is currently monitoring hydrothermal chloride, heat, and carbon dioxide discharge in the Three Sisters area in order to detect any increases related to the ongoing deformation.