A Method to Estimate Friction Coefficient from Orientation Distribution of Meso-scale Faults: Applications to Faults in Forearc Sediment and Underplated Tectonic Mélange

Abstract:

Friction coefficients along faults control the brittle strength of the earth's upper crust, although it is difficult to estimate them especially of ancient geological faults. Several previous studies tried to determine the friction coefficient of meso-scale faults from their orientation distribution as follows. Fault-slip analysis through stress tensor inversion techniques gives principal stress axes and a stress ratio, which allows us to draw a normalized Mohr’s circle. Assuming that a faulting occurs when the ratio of shear stress to normal stress on it, i.e., the slip tendency, exceeds the friction coefficient, one can find a linear boundary of distribution of points corresponding to faults on Mohr diagram. The slope of the boundary (friction envelope) provides the friction coefficient. This method has a difficulty in graphically and manually recognizing the linear boundary of distribution on the Mohr diagram. This study automated the determination of friction coefficient by considering the fluctuations of fluid pressure and differential stress. These unknown factors are expected to make difference in density of points representing faults on the Mohr diagram. Since the density is controlled by the friction coefficient, we can optimize the friction coefficient so as to explain the density distribution.

The method was applied to two examples of natural meso-scale faults. The first example is from the Pleistocene Kazusa Group, central Japan, which filled a forearc basin of the Sagami Trough. Stress inversion analysis showed WNW-ENE trending tensional stress with a low stress ratio. The friction coefficient was determined to be around 0.66, which is typical value for sandstone. The Second example is from an underplated tectonic mélange in the Cretaceous to Paleogene Shimanto accretionary complex in southwest Japan along the Nankai Trough. The stress condition was determined to be an axial compression perpendicular to the foliation of shale matrix. The friction coefficient was about 0.23, which is extremely low indicating a weak plate boundary under the accretionary wedge.