Inelastic behavior and mechanical strength of the shallow upper crust controlled by layer-parallel slip in the high-strain zone of the Niigata region, Japan.

We investigated the relationship between contractional deformation of sedimentary mass in the upper crust and the geodetic strain rate in a high-strain zone (part of the Niigata–Kobe Tectonic Zone) of the mid-Niigata region, Central Japan. We observed numerous examples of layer-parallel slip (bedding-plane slip) generated by folding. The slip layers have an average spacing of ~ 3.6 m (measured normal to bedding planes) at the Katagai site. In shallow sedimentary units of the upper crust, numerous bedding-plane slip events act to reduce the mechanical strength (effective elastic thickness) of sedimentary rocks under contractional deformation. The results support an existing model, based on geodetic observations, of mechanical decoupling between the weak sedimentary layers and basement. On the layer-parallel slip, we measured the friction coefficient of gouge generated by bedding-plane slip and of mud around non-slip surfaces using double-direct shear tests, and found no difference in frictional coefficient between slip and non-slip surfaces. [ABSTRACT FROM AUTHOR]