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Analysis of strain in a welded block and ash flow deposit, Mount Meager, Southwestern British Columbia Michol, Krista A.


The 2360 BP eruption of Mount Meager, British Columbia has produced a rare welded block and ash flow deposit along with non-welded equivalents. Here, I report on this sequence of block and ash flow deposits (herein referred to as the Keyhole Falls Member) with the aim of documenting the effects of welding and the mechanisms of strain attending the welding process. Multiple texture maps are drawn at the decimeter scale (field texture maps), and at the centimeter scale (slab texture maps), and are used for image analysis purposes to quantify the transition from unconsolidated block and ash flow deposits to dense, vitroclastic breccias. Image analysis establishes a welding trajectory, whereby average clast oblateness increases and average clast orientation (relative to the horizontal) decreases with increasing welding intensity. After accounting for an original oblateness of approximately 3 0 % , estimates of strain from image analysis of field texture maps (FTMs) and slab texture maps (STMs) yield a volume strain of ~12%, or ~9% if treated as pure shear strain. An empirical experiment using image analysis of FTMs suggests that the most welded FTMs visually correspond to 30 - 4 0 % volume strain relative to the least welded portions of the deposit. Distributions of oblateness and orientation for each FTM also prove more accurate in indicating welding intensity than do the average values. Physical property measurements of the non-welded and welded block and ash flow deposits correlate well with the empirical experiments. Unconsolidated deposits reveal an average total matrix porosity of ~41% , of which less than 1% is isolated porosity. Associated clasts possess an average o f ~32% total porosity, with a maximum of 11% isolated porosity. As welding intensity increases, these values of average total porosity decrease to ~5% for clasts and ~17% for matrix. Isolated porosity is reduced to ≤1% for both components. Thus, isolated porosity is present mostly in non-welded clasts, and is lost in conjunction with connected porosity as welding progresses. These results also reveal an approximately equal amount of strain in clasts and matrix. The variations in physical property measurements suggest that both components record a maximum volume strain of ~38%, which is echoed in the results of the empirical experiment. There is also little manifestation of pure shear stress observed within the welded facies of the deposit (e.g. only occurs as rare pull-apart clasts and locally around accidental lithics), indicating that volume strain in the viscous regime is the main mechanism for welding of the Keyhole Falls Member. The deposit, as a whole, records an average of 31% strain, meaning that the lower block and ash flow deposits experienced 50 m of compaction during welding, form 162 to 112 m. In comparing the block and ash flow deposits to other volcanic deposits, it is evident that they were erupted in a more explosive manner than originally proposed. The most appropriate analogue is Soufriere Hills Volcano, Montserrat, where explosive dome collapse is triggered by a Vulcanian eruption

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