HR: 1340h
AN: V53C-1579    [Abstracts]
TI: ``Strain Under Scrutiny": Multi-Scale Analysis of Deformation in Welded Block and Ash Flow Deposits, Meager Mountain, BC
AU: * Michol, K A
EM: kmichol@eos.ubc.ca
AF: Volcanology & Petrology Lab, Department of Earth & Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AU: Russell, J K
EM: krussell@eos.ubc.ca
AF: Volcanology & Petrology Lab, Department of Earth & Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4 Canada
AB: The 2400BP eruption of Mt. Meager has produced a variety of pyroclastic deposits including pumice fall deposits, lavas, and pumiceous pyroclastic flows. The most unique and enigmatic of these is a 100m-thick, valley-filling, welded block and ash flow deposit. Pyroclasts in this deposit range in size from ~1m blocks to fine ash, but the dominant clast size ranges from 5-15cm. Clasts mainly (85-95%) comprise angular to subrounded, variably vesicular fragments of dacitic lava. The welding intensity ranges from a densely welded, vitrophyric breccia to a weakly welded or indurated deposit, to an unconsolidated deposit of blocks and ash. The variably welded aspect of the deposit provides an excellent opportunity to study the unique conditions and processes that have contributed to the welding of this pyroclastic flow. Our approach has been to collect and analyze field data from the block and ash flow deposit, with the intent of quantifying the strain accumulated during the welding process. The main dataset derives from thirteen, hand-drawn, texture maps of 1m square areas from select outcrops of the block and ash flow deposit. These textural sketch-maps have been scanned into digital images, which have been used for image analysis purposes. For each map, image analysis provides quantitative estimates of clast numbers, sizes, orientations, eccentricities, and proportions of clasts to matrix. These metrics are then used to quantify the total strain in the deposit at the map scale and explore the degree of coupling between clasts and matrix during deformation. Future work involves parallel strain analysis on scanned images of rock slabs and thin sections to investigate how strain is partitioned at these scales. Our analysis will also elucidate the extent to which strain is accommodated as: i) volume strain, as evidenced by a reduction in porosity versus ii) pure shear strain, as indicated by more flattening than can be accommodated by porosity loss alone. Further analysis at smaller scales will serve to reinforce these trends and produce more specific values of strain, as well as determine the proportions of shear to volume strain.
DE: 8404 Volcanoclastic deposits
DE: 8434 Magma migration and fragmentation
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005