Time/Date: 3pm, 10th February 2014
Location: HUBBS 4500
Speaker: Graham Andrews
Affliation: CSU Bakerfield
Title: The emplacement of voluminous silicic lava-like tuffs and lavas
Super-voluminous (>100 km^3) silicic lavas and lava-like ignimbrites are integral components of several silicic large igneous provinces (LIPs), including the Mesozoic Etendeka-Parana (continental rift), the Eocene-Oligocene Sierra Madre Occidental (continental arc), and the Miocene Snake River Plain (continental hotspot). They are characterized by near-aphyric, anhydrous phenocryst assemblages, intense welding of pyroclasts, pervasive flow-banding and flow-folding (rheomorphism), huge areal extents, and near total absence of typical pyroclastic deposits (e.g., Plinian pumice fall deposits, non-welded ignimbrites, etc.). Rather they occur as 10s - 100s meter thick, columnar-jointed ignimbrites and lavas, "flood rhyolites", forming trap topography more typical of mafic LIPs. These volcanic products differ considerably from those of typical silicic supervolcanism where welding is weak to moderate in ignimbrites, and lavas are small: why?
We have investigated the mechanisms by which lava-like tuffs are deposited, welded, and deformed, and attempted to constrain the timescales and intensive parameters (e.g., temperature, viscosity, strain rate) associated with their emplacement. Through a combination of field observation, calorimetry, and numerical modeling, we have established the temperature-time-strain 'window' through which tuff must pass to achieve lava-like lithofacies. The high finite strain measured (1000%), and inferred high strain rate, require a significantly higher deposit temperature than the inferred magmatic temperature, in order to flow rather than break. This seemingly impossible hurdle is overcome by taking shear-heating into account: under the high-temperature, high strain rate conditions modeled, shear-heating adds 100C-200C during aggradation and rheomorphism. Recognizing the importance of shear-heating during rheomorphism, and extrapolating that to high-temperature, low strain rate lavas, explains the long-lived question of how "flood rhyolite" lavas can flow over such long distances without cooling enough to freeze.