Recovering ascent rates and eruption dynamics from the properties of sintered volcanic shear zones: A case study from Chaos Crags, Lassen Volcanic Center (USA)

Abstract:

The properties and microstructures of exhumed volcanic shear zones record information about magma ascent and eruption processes. Here we describe the shear zone developed in Dome C, part of the Chaos Crags eruption within the Lassen Volcanic Center (California, USA). The shear zone comprises crystal-rich, glass-poor volcanic fault gouge and variably densified cataclasites. These materials evidence (1) deep-seated production of fault gouge within the conduit followed by (2) solid-state sintering that converts unconsolidated gouge to dense cataclasite on the timescale of ascent through the conduit.

Dome C erupted 1103 ±13 years B.P. so there are no direct or indirect determinations of ascent (m/s) or extrusion (m³/s) rates for the magma plug that produced Dome C. To constrain these rates, we used a new volcanic geospeedometer based on the properties of the cataclasites. Using measured values of residual porosity as a proxy for the extent of sintering, estimates of in-conduit temperatures and stresses acting on the gougeand a time-dependent model for densification by solid-state sintering, we solved for the minimum in-conduit residence times necessary to produce the observed cataclasites. These minimum residence times can only be achieved if Dome C magma and the corresponding shear zone were ascending at ≤ 10 m/d. Assuming a cylindrical conduit geometry (diameter: 250 m, the width of the Dome C remnant) this linear ascent rate corresponds with volumetric extrusion rates of ≤ 5.7 m³/s. For comparison, measured extrusion rates during the 2004-2008 eruption of Mount St. Helens (USA) and the 1991-1995 eruption of Mount Unzen (Japan) were ≤ 6 m³/s and ≤ 7 m³/s respectively.

In addition, we evaluate the consequences of sintering of the shear zone for the eruption of other crystal-rich, glass-poor magmas (e.g. Mount St. Helens, Mount Unzen, Mount Pelée (Martinique)). Chaos Crags cataclasites preserve evidence of multiple cycles of fracturing, cataclasis and (re-)sintering, suggesting these cycles may be responsible for rhythmic seismicity commonly observed during dome-building eruptions as well as transitions between effusive and explosive activity.