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The nature and evolution of conduit faults in the 2004-2008 Mount St. Helens lava dome eruption

University of British Columbia

Mount St. Helens reawakened 24 years after erupting in the 1980’s. This effusive eruption produced 95 million cubic meters of dacite in the form of 7 discrete, competent spines or domes of lava between September 2004- June 2008. The spines comprise low-porosity dacite that is inferred to have crystallized at a depth of about 1 km and are enveloped by a 1-3 meter carapace of fault gouge. The rate of linear extrusion of the spines peaked at 11 m/day in November 2004 and subsequently slowed to < 0.5 m/day. Dome growth was accompanied by a “drumbeat” seismicity that was sourced from 1-0.5 km below the vent. Here, field, petrographic, and microstructural observations on the nature of deformation attending the extrusion of Spines 4, 5 and 7 at Mount St. Helens (2004-2006) are presented. The enveloping fault zones provide a static view of the cumulative strain produced by shear along the conduit wall. The conduit faults narrow from Spine 4 to Spine 7 and exhibit fewer macroscopic brittle features. Strain accommodation is achieved through a scale-dependent ductility. The subsurface ascent velocities for each packet of magma are reconstructed using surface observed extrusion rates. Computed shear strain rates for the margins of the conduit range from 1x10⁻⁴ to 7.9x10⁻⁵ s⁻¹. As ascent rate decreases, fault zone width also decreases maintaining an average shear strain rate of 4.3x10⁻⁵ s⁻¹. Intense strain localization within each fault zone is expressed by 0.001 m thick slickensides implying very high (co-seismic) transient shear-strain rates of 1x10⁻¹s⁻¹ (Spines 4-5) to 2.2x10⁻²s⁻¹ (Spine 7). I conclude with a time and space model for the evolution of the fault zone as magma ascends the conduit, and how the fault zones evolve through time. The factors that contribute to the differences in conduit fault zone width and nature throughout the eruption are: 1) differences in ascent rates at the onset and origin of brittle failure, 2) variations in shear strain rates, 3) and the increasing residence time throughout the eruption that the damaged fault rocks experienced at high temperatures within the conduit.

Text

2012-02-02

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/40473

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/