Mount St. Helens Fault Gouge - Textural Constraints on Deformation Mechanisms

An unusual feature of the 2004-2006 eruptive activity of Mount St. Helens (MSH) has been the continuous growth of successive spines that are mantled by a zone 1-3 m thick composed of breccia, cataclasite and fault gouge. Individual fragments are holocrystalline, indicating that crystallization of the ascending magma preceded gouge formation. Here we examine both the transformation of solid dacite to cataclasite, and the further textural modifications that accompany deformation and disaggregation of cataclasite to form gouge, by detailed analysis of 8 oriented samples. We then compare textural features of the gouge with those of gouge from creeping and stick-slip segments of the San Andreas fault to better constrain the deformation mechanisms by which holocrystalline magma ascends to the surface. Initial disruption of solid dacite occurs by distributed dilation (porosity increase) and fracture. Fractures tend to follow phenocryst margins but also traverse the matrix and some larger crystals. Further deformation produces cataclasite with a wide range of grain size, a fine-grained (< 10μm) matrix, and rounding of larger fragments (commonly armored by powdered matrix). The outer, unconsolidated gouge zone contains finely comminuted shear zones (slickensides); crystals and lava fragments adjacent to these zones are shattered into small angular fragments that are entrained into shear trains to create a well-developed foliation. By comparison, microstructures in gouge from a creeping segment of the San Andreas fault record distributed shear deformation and porosity reduction without accompanying grain breakage; strain is accommodated by compaction, grain rotation, and possibly grain sliding and/or rolling. In contrast, fault gouge formed during the 1906 slip event contains deformation bands characterized by grain comminution and strongly localized slip, as reflected in abundant fractured and broken grains, and preferred grain orientations ~ 30° counterclockwise from the fault. From these observations we suggest that particle rolling and sliding may dominate when the grain size variation is large; this type of behavior seems to characterize shear in much of the MSH cataclasite. Extreme shear localization in the gouge, with accompanying extensive grain shattering and crushing, may reflect the onset of stick-slip behavior.