DUCTILE DEFORMATION OF AN INTENSELY WELDED RHEOMORPHIC IGNIMBRITE, AT GREY'S LANDING, SOUTHERN IDAHO

Rheomorphic ignimbrites are welded pyroclastic deposits that undergo gravity-induced ductile shearing and flow, during and immediately after emplacement, prior to critical cooling. End member scenarios for inducing flow include: (1) the development of a syn-depositional shear zone, on and about the aggradation surface between the deposit and the over-riding current, and (2) post-depositional, gravity-controlled mass flow of the entire deposit.

Numerous rhyolitic ignimbrites were erupted from Miocene volcanic centres along the Snake River Plain volcanic province. Several are extremely intensely welded (high-grade), and some resemble lavas (“lava-like ignimbrites” of Branney & Kokelaar, 1992). They exhibit evidence of pervasive ductile deformation, including flow banding, an intense L=S fabric, d-type and (to a lesser extent) s-type pheno-clasts, together with abundant small- and medium-scale flow folds with axes variably transposed towards the elongation lineation, the latter representing the transport direction. We present a field analysis of one the largest and most extensive of deposits, the Grey's Landing ignimbrite. It has a volume exceeding 20 km³ and reaches 60 m thickness where it occupies a depression formed by a contemporary Basin and Range half-graben.

Small- and medium-scale folds and the L-S fabrics have been mapped in detail and measured along vertical traverses across the ignimbrite sheet. Results show that the styles, attitudes and orientations of the ductile deformation structures vary systematically both with height through the ignimbrite and with geographic location. A two-part vertical zonation is observed throughout the field area, defined by changes in folding style and attitude, reflecting the increased volume of material deforming with time. Progressive deformation within the deposit is shown by the re-folding of earlier sheath folds and the change in deformation style as a response to contemporaneous volatile exsolution.

We interpret the variations with height in the Grey's Landing ignimbrite to record temporal changes in deformation within a narrow, hot, upward-migrating shear-zone that developed rapidly, and tracked the rising aggradation surface of the ignimbrite-agglutinate while it progressively aggraded, followed by minor modification by mass flowage.