EPISODIC VOLCANO GROWTH AT MT. RAINIER, WASHINGTON: A PRODUCT OF TECTONIC THROTTLING?

One of the major results of intensive geologic-geochronologic studies of Cascade volcanoes has been the demonstration that volcano growth is highly episodic. Long-term episodicity is manifest as ancestral vs. modern edifices, with time scales of 0.5 to 1 Myr. Examples are Sandy Glacier volcano vs. Mt. Hood, ancestral vs. modern Mt. Rainier, Gamma Ridge caldera vs. Glacier Peak, and Kulshan caldera vs. Mt. Baker. Shorter-term episodicity is manifest as growth stages of ~20 kyr to ~100 kyr where magmatic output can differ by perhaps as much as 10-fold. Causes of episodic behavior are unknown. Correlations with glacial cycles have not been found in the Cascades (Mt. Baker, Mt. Rainier, Mt. Adams). Variations in the flux of parental magma from the mantle wedge might be expected to appear as changes in magma composition coupled with changes in volcanic output. However, intensive geochronology (~80 high-precision K-Ar and ⁴⁰Ar/³⁹Ar ages), mapping, and geochemistry (~700 analyses) at Mt. Rainier fail to show any obvious correlation between magma composition and major eruptive stages of ~100 kyr duration. The same range and relative abundances of magma compositions have erupted since the onset of the modern edifice at 500 ka. Brief but pronounced excursions in lava trace element composition lasting less than the resolution of dating suggest that distinct magma batches transit the magmatic system in less than 10 kyr, far less than the duration of growth stages. Brief transit times argue against slow accumulation of magma in the lower crust as a factor leading to episodic growth. An intriguing possibility is that volcanic growth episodes result from tectonic causes, with stress in the crust acting to throttle the ascent of magma. The meager volcanic output in the High Cascades of Washington State, an uplifted region to the north of Mt. Rainier that is probably under compression, is consistent with tectonic throttling as an important control on magma ascent, at least in arcs like the Cascades with low overall magmatic flux. If correct, volcanic growth stages may provide information on variations in tectonic stresses over time spans not easily resolved by other methods.