Faulting within the Mount St. Helens conduit and implications for volcanic earthquakes

The 2004–2008 eruption of Mount St. Helens produced seven dacite spines mantled by cataclastic fault rocks, comprising an outer fault core and an inner damage zone. These fault rocks provide remarkable insights into the mechanical processes that accompany extrusion of degassed magma, insights that are useful in forecasting dome-forming eruptions. The outermost part of the fault core consists of finely comminuted fault gouge that is host to 1- to 3-mm-thick layers of extremely fine-grained slickenside-bearing ultracataclasite. Interior to the fault core, there is an ∼2-m-thick damage zone composed of cataclastic breccia and sheared dacite, and interior to the damage zone, there is massive to flow-banded dacite lava of the spine interior. Structures and microtextures indicate entirely brittle deformation, including rock breakage, tensional dilation, shearing, grain flow, and microfaulting, as well as gas and fluid migration through intergranular pores and fractures in the damage zone. Slickenside lineations and consistent orientations of Riedel shears indicate upward shear of the extruding spines against adjacent conduit wall rocks.

Paleomagnetic directions, demagnetization paths, oxide mineralogy, and petrology indicate that cataclasis took place within dacite in a solidified steeply dipping volcanic conduit at temperatures above 500 °C. Low water content of matrix glass is consistent with brittle behavior at these relatively high temperatures, and the presence of tridymite indicates solidification depths of