Date/Time: 2 May 2011 (3 PM)
Location: DMS 104 (Davidson Mathematics & Science Center, UNR)
Title: Spatial patterns of internal strain and deformation temperature in the Himalayan fold-thrust belt, Bhutan: Implications for the evolution of strain in thrust sheets
Quartz microstructural analysis together with quantification of 3-D crystal-plastic strain of quartz clasts constrains deformation temperature ranges and internal strain magnitude/orientation within Himalayan thrust sheets in Bhutan. Frontal thrust sheets exhibit low-magnitude (~7%), layer-parallel shortening (LPS) strain. Further to the hinterland, thrust sheets exhibit higher-magnitude, layer-normal flattening (LNF) strain, with median strain ellipsoids of ~2.0:2.0:1.0. We propose that LPS strain developed foreland-ward of the thrust deformation front, and that LNF strain resulted from later tectonic loading by thick hinterland thrust sheets. The LPS to LNF transition occurs at minimum temperatures for quartz plasticity (ca. 250-270ºC), illustrating the first-order control of rheology on strain patterns. LNF ellipsoids are folded to the same degree as strata within thrust sheets, indicating that internal strain preceded thrust imbrication and translation. At the scale of 10’s of meters structural distance, strain magnitude of hanging wall rocks does not increase as basal thrusts are approached, which supports the existence of discrete faults with large translations, as generally depicted in balanced cross-sections, even in thrust sheets that involve low- to moderate-grade metasedimentary rocks.
Sean Long’s staff page – http://www.nbmg.unr.edu/staff/Long.html