Cataclasis and the Generation of Fault Gouge

Cataclasis, a friction-dependent mechanism of deformation involving fracture and rigid-body rotation, occurs during faulting of sandstone in the upper crust. Two stages of cataclasis of a sandstone are represented by (1) mildly deformed cataclastic sandstone that is shattered but contains many original grains that have not been fragmented and (2) gouge so severely deformed that the few surviving grains are almost surrounded by a fine-grained matrix of crushed grains. Gouge is restricted to shear fractures and larger slip planes within the natural fault zones, whereas cataclastic sandstone usually pervades the fault zone. Grain size and sorting of gouge decrease as a function of increasing confining pressure and increasing displacement along a shear fracture. In cataclastic sandstone associated with gouge, microfractures ate strongly oriented parallel to the maximum principal compressive stress. These microfractures, which are extension fractures, may be used to determine the orientation of the stress field during faulting. Rigid-body rotation is important during Cataclasis, and grains rotate in a manner that agrees with the sense of shear on the fault plane.