HR: 16:00h
AN: V54B-01    [Abstracts]
TI: Fragmentation and Cataclasis of Lava Domes: Field Evidence of Conduit-Margin Faulting and Cryptodome Unloading at Mount St. Helens
AU: * Pallister, J S
EM: jpallist@usgs.gov
AF: USGS Cascades Volcano Observatory, 1300 SE Cardinal Court, Vancouver, WA 98683, United States
AU: Hagstrum, J
EM: jhags@usgs.gov
AF: USGS, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Cashman, K
EM: cashman@uoregon.edu
AF: Dept. Geological Sciences, University of Oregon, Eugene, OR 97403, United States
AU: Tuffen, H
EM: h.tuffen@lancaster.ac.uk
AF: Lancaster University, Environmental Science Dept., Lancaster, LA14YQ, United Kingdom
AB: Structures and textures preserved in dome rocks reveal much about ascent history, seismicity, and dynamics of eruptions. The current eruption of Mount St. Helens (MSH) produced dacite spines mantled by fault gouge and breccia. Flow-banded spine interiors attest to early degassing and ductile deformation; micro-textures and structures in the spine margins indicate entirely brittle shear, rock breakage, grain-flow and gas-escape along fractures. Paleomagnetic pole positions and demagnetization data constrain cataclasis to the sub-vertical volcanic conduit at temperatures above 500°-570°C. Low water content of matrix glass and presence of tridymite require nearly complete decompression-driven solidification at depths <1 km, coincident with the eruption's seismogenic zone. 1-3 m thick cataclastic breccia of spine margins contains multiple Reidel shears in a conjugate set formed by shear between the vertically extruding spines and conduit walls. This breccia is overlain by a thin (<10 cm) outer mantle of finely comminuted gouge with 1-3 mm-thick, surface-parallel layers of slickenside-bearing ultracataclasite, forming through-going fault planes. Slickenside lineations and direction indicators are consistent with upward transport of the spines. These relations document two dominant modes of brittle failure in the spine margins, similar to the brittle S-C fabrics seen in tectonic fault zones. The Reidel shears represent limited-slip planes (S-shears), which are inclined relative to the primary bounding fault planes (C-surfaces). We infer that the Reidel shears formed as multiple, domino-like episodes of fracture, prior to transfer of slip to the bounding C-surfaces. Because the depth of deformation is the same as the depth of the seismogenic zone, and because there are two distinct modes of brittle fracture (S and C fabrics) as well as two distinct types of earthquakes (volcano-tectonic and longer-period hybrids) it is logical to infer that these structures are sources for many of the earthquakes at MSH. Additionally, grain-flow textures in the bounding ultracataclasite fault zones suggest that observed periods of aseismic extrusion were accommodated by creep on these faults. In contrast, Pine Creek age (~2.5 ka) dome dacite exposed over a large area at the mouth of the MSH crater is pervasively fractured over paleo-depths of >300m. Like the dacite of the current eruption, this rock has a microcrystalline groundmass, indicative of extensive shallow crystallization and subsurface solidification. However, the extent and character of fragmentation is unrelated to shear along conduit margins. Instead, a multi- stage history of brittle deformation consists of: 1) early 10-20cm thick subhorizontal zones of sandy cataclasite that repeat at intervals of meters to tens of meters within pervasively shattered dacite, 2) early cross-cutting high- angle faults with slickensides and <1mm pseudotachylite seams, and 3) later cross-cutting planar faults. We suggest that early cataclasis was produced by rapid unloading of the still-hot dome during a Pine-Creek age sector collapse and that the early high-angle faults represent boundaries of large mega-blocks that had begun to detach within the source dome. Together with mapped Pine Creek avalanche deposts nearby (Hausback, 2000), these features suggest that a transition from avalanche to still-intact Pine-Creek age cryptodome is exposed in the mouth of the MSH crater.
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8429 Lava rheology and morphology
DE: 8434 Magma migration and fragmentation
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting