Cataclasis and deformation-band formation in unconsolidated marine terrace sand, Humboldt County, California

Abstract

Zones of deformation bands occur in unconsolidated late Pleistocene marine terrace sand in the footwall of the active McKinleyville thrust fault in Humboldt County, California. Individual deformation-band shear zones are as much as 8 cm wide and accommodate ∼50 cm of reverse-dip separation. Like deformation bands described in Mesozoic sandstone of the Colorado Plateau, these structures formed in fine-grained, well-sorted, porous sand. We assessed the relative importance of compaction, grain breakage, and grain rotation during shear-zone development by measuring grain size, grain shape, grain orientation, porosity, and bulk strain both within and outside deformation-band shear zones. Sand grains within shear zones are smaller, more compacted, and have stronger preferred orientations and more elongate shapes than grains outside of deformation-band shear zones. Bulk strain analyses of sand within shear zones give strain ellipses that are compatible with dextral shear strains of ∼0.1–0.5 and volume loss of 5%–15%. On the basis of these observations, we conclude that compaction, grain rotation, and extensive cataclasis all contribute to deformation-band shear-zone formation in these unconsolidated sands, despite very low confining pressures. In addition, the position of these deformation-band shear zones adjacent to an active fault with a history of episodic slip during large earthquakes suggests that they may form in conjunction with slip events.