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Subduction-Related Accretionary Wedges (B-type Subduction) |
1 Institut für Geowissenschaften, Johannes Gutenberg-Universität, Becherweg 21, 55099 Mainz, Germany ring{at}mail.uni-mainz.de
2 Kline Geology Laboratory, Yale University, P.O. Box 208109, New Haven, CT 06520-8109, USA
Deformation measurements from 64 sandstone samples collected in three study areas from the Eastern Belt of the Franciscan Complex are used to evaluate how the high-pressure metamorphic interior of the Franciscan wedge was exhumed. Pressure estimates indicate 25–30 km of exhumation in this part of the Franciscan Complex. Much of the Eastern Belt has a semi-penetrative cleavage that formed by solution mass transfer (SMT) while the rocks were moving through the wedge. Individual samples have absolute principal stretches of SX = 1.00–1.52, SY = 0.60–1.21, and SZ = 0.33–0.81. Strain magnitudes and directions are quite variable at the local scale. The deformation at the regional scale is estimated by calculating tensor averages for groups of measurements. The three study areas, which are spaced over a distance of c. 500 km along the Franciscan margin, give remarkably similar averages, which indicates that the deformation of the Eastern Belt is consistent at the regional scale. The tensor average for all data indicates a nearly vertical Z direction with SX = 0.96, SY = 0.92, and SZ = 0.70. SX and SY are near one because at the local scale, the X and Y directions vary considerably in orientation, which means that their stretch contributions are averaged out at the regional scale. This unusual strain type, consisting of both plane strain and uniaxial shortening, results from the fact that shortening in Z was balanced by a pervasive mass-loss volume strain, averaging about 38%. The geometry of directed fibre overgrowths was used to measure internal rotations. These data indicate that in sandstones, SMT deformation was nearly coaxial (mean kinematic vorticity number is 0.05 at the regional scale and generally <0.4 for individual samples). A simple one-dimensional steady-state model indicates that ductile thinning accounted for only c. 10% of the overall exhumation. Ductile shortening across the Franciscan wedge was very slow, at rates <8 x 10–17 s–1 (<0.3% Ma–1). Assuming that this strain was active in an across-strike zone <200 km wide, we estimate that horizontal ductile flow would have accounted for <0.25% of the total convergence across the Franciscan margin. We conclude that the SMT mechanism operated slowly as a background deformation process, and that the dislocation glide mechanism was completely inactive down to depths of 25–30 km. Thus, the stability of the Franciscan wedge was probably better defined by the Coulomb wedge criterion than by a viscous wedge criterion. No definitive normal faults have been found in or adjacent to the Eastern Belt. Therefore, we infer that wedge taper was mainly controlled by deep accretion and erosion of an emergent forearc high.
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