Local crustal extension at Mount St. Helens, Washington

Abstract

Geophysics Prowlram AK-50, University of Washin•lton, SeattleSeismicity and the orientation of fault planes from focal mechanisms indicate that Mount St. Helens is located at a dextral offset along the St. Helens seismic zone (SHZ): earthquake swarms occurring in this offset are related to volcanic eruptions. Because motion on the SHZ is in a right-lateral strike-slip sense, this dextral offset creates extension within a volume of the crust between the offset fault segments. This offset geometry is similar to that of geothermal areas along the San Andreas fault system. We apply a model derived from these geothermal areas to Mount St. Helens and find that the major differences between Mount St. Helens and the geothermal areas can be related to the ratio of the width of the offset between fault segments (1), to the seismogenic depth (h). At Mount St. Helens this ratio is < 1, whereas in the geothermal areas the ratio is m 1. We propose that when I/h < 1 as at Mount St. Helens, the regional minimum principal stress does not completely dominate the small volume under extension, and as a consequence, the opening geometry is poory established compared to the oblique crustal spreading that characterizes the geothermal areas where I/h m 1. Late Quaternary volcanic vents near Mount St. Helens strike northeast, similar to the strike of a set of pre-Quaternary faults and intrusive rocks that are mapped north of the volcano; in addition, the deepest earthquakes occurring within the extensional volume are aligned along a northeast striking fault. Since these northeast striking features are aligned approximately prependicular to the regional minimum principal stress, we infer that the spatial position of Mount St. Helens is controlled by the junction of the right-stepping offset of the SHZ with the older set of fractures and that these fractures are favorably aligned with respect to the contemporary regional tectonic stress directions for the transport of magma through the brittle crust. The sense of fault motions predicted by our model for local crustal extension is consistent with an apparent component of rightlateral shear measured from geodetic lines around Mount St. Helens during June and July 1980.

Deformed symmetrical volcanoes

“…The experimental model was also used to evaluate the possible generation of instability by fault movement underneath Mayon volcano in the Philippines, a volcano that has not undergone sector collapse. Mount St. Helens is a stratovolcano that is traversed by a north-northwest right-lateral strike-slip fault that was notably active in the years before and after the 1980 collapse event (Weaver et al, 1987). Previous analog modeling showed that dome intrusion caused the collapse (Donnadieu and Merle, 1998).…”

Magma system recharge of Mount St. Helens from precise relative hypocenter location of microearthquakes

“…As pressure decreased in the magma chamber during major explosive eruptions and then later increased as the chamber refilled, these faults failed in response to the changing radial stress field. Studies of regional seismicity [ Weaver and Smith , 1983; Shemeta and Weaver , 1986; Weaver et al , 1987] and geologic features [ Luedke and Smith , 1982; Evarts et al , 1987] suggest that a combination of NE striking volcanic lineaments and faults and a NNW trending seismic zone, the St. Helens Seismic zone (SHZ) control both the seismicity pattern and the location of the current volcanic rocks. These studies invoke right lateral motion on the SHZ in response to regional stresses and variable motion on bounding faults of a crustal spreading center under Mount St. Helens where the SHZ has an apparent dextral offset [ Weaver et al , 1987].…”

Tectono-Magmatic Precursors for Porphyry Cu-(Mo-Au) Deposit Formation

“…The literature contains numerous examples of the localization of plutons, volcanoes, and related ore deposits in transpressional settings within arcs (e.g., Bussell, 1976;Aydin et al, 1990;Glazner, 1991;D'Lemos et al, 1992;Tikoff and Teyssier, 1992;Bellier and Sébrier, 1994;Tommasi et al, 1994;Tobisch and Cruden, 1995;Román-Berdiel et al, 1997;Acocella et al, 1999;Brown and Solar, 1999;Benn et al, 2000;García-Palomo et al, 2000;Adiyaman et al, 2001;Gleizes et al, 2001;Hildenbrand et al, 2001;Richards et al, 2001;Chernicoff et al, 2002). Additionally, numerous studies have shown how regional stress fields and resultant crustal strain influence the orientation and structure of volcanic-plutonic systems (Robson and Barr, 1964;Pollard and Muller, 1976;Nakamura, 1977;Weaver et al, 1987;Gudmundsson, 1988;Takada, 1994;Alaniz-Alvarez et al, 1998;Román-Berdiel, 1999;Mériaux and Lister, 2002).…”