| Abstract: | The nature and development of microscopic feather fractures (mff) are investigated in experimentally deformed intact and precut cylinders of room-dry Tennessee and Coconino Sandstone. All specimens are deformed at 25° C, and at a shortening rate of 10−4 sec−1 ; the intact ones are at confining pressures from 0.5 to 2.5 kbar; and the precut specimens at 1.0 and 1.5 kbar. Mff occur in grains adjacent to induced shear fractures or faults; they are wedge-shaped and die out within one or two grain diameters from the fault; and they make acute angles with the fault such that arrows directed into the apices of these angles on either side of the fault define its sense of shear. Occurrence of mff only after slip on precut surfaces clearly demonstrates that they form as a result of shear displacement. The average angle between the mff and fault is 10° greater than that between the load axis and the fault, and it increases with increasing confining pressure in initially intact specimens. Data suggest that the abundance of mff (mean number per grain) increases with increasing normal stress across the fault and with displacement. The wedgeshaped character of many mff and their consistent orientation at 10° to the load axis are distinguishing characteristics. Mff are shown to be parallel to the local maximum compressive stress and thus are extension microfractures. They are not to be confused with precursive micro fractures developed prior to macroscopic fracture, nor to Riedel shears developed during faulting. |
