Stressed Out at the Border: Geological Observations and Models of Elevated Stresses along the Boundaries of Strong Lithologies in Shallow and Deep Mélanges

Plate boundary faults are often characterized by broad zones of distributed deformation, within which boudinage of strong layers and plucking of wallrock creates a block-in-matrix fabric (e.g. a mélange). Stress concentrations around strong blocks are known to occur, however models of elevated stress are infrequently tied to geological observations of deformation distribution in natural shear zones. We use geological observations (maps, thin section analysis, XRD, FTIR, microprobe) to constrain the distribution of lithologies and the deformation mechanisms active in both blocks and matrices during shear. These data are used as inputs to finite-element models of stress heterogeneity during deformation, and model outputs are compared with the distribution of observed deformation microstructures (fractures, faults, recrystallized grain sizes). Two end-member mélanges are mapped and modelled: a shallow subduction mélange (Mugi mélange, Japan, T = 190ºC) and a deep crustal shear zone (Lower Fish River-Onseepkans Shear Zone (LFROSZ), Namibia, T = 700ºC). At both sites localized deformation occurs along the boundaries of blocks. At Mugi, cataclasites and pseudotachylyte form within altered basaltic blocks at the contact with the shale mélange matrix. At the LFROSZ, finely recrystallized metapelites (matrix) occur along the margin of a gabbro intrusion. The highest stresses in models occur along the margins of blocks, particularly when the boundaries are at an angle to the foliation of the matrix. At shallow crustal levels localized brittle failure occurs within strong blocks, as crystal plastic deformation operates too slowly to relieve the local increase in stress. At depth deformation is predominantly accommodated by increased strain-rates in the weak matrices. The transition from localized deformation in blocks to an increase in shear strain-rate in the matrix can occur by increasing temperature or decreasing strain rate. Geological environments where localized deformation in strong blocks is favorable may be more prone to microseismicity and tremor.