Cataclasis and particulate flow in faulted, poorly lithified sediments

Microscopic observations of normal faults in poorly lithified sediments from the Rio Grande rift, New Mexico, USA, reveal that the mode of grain fracture within the fault zones is controlled by mineralogy and relative grain strength. Transgranular fracturing of quartz is rarely observed—quartz typically deforms by flaking of grain edges, feldspar by transgranular fracture facilitated by easy cleavage, and lithic fragments by transgranular fracture or distributed microcracking. Particle size measurements indicate that progressive deformation produces a particle size distribution that can be described by a power-law model, characterized by low D values (1.7–2.1). This indicates a preponderance of large particles with respect to cataclasite produced by constrained comminution (D∼2.6). We interpret these results in terms of cataclastic deformation by controlled particulate flow under low confining pressure, in which extensive transgranular fracturing is not necessary for strain accumulation. This style of cataclastic deformation is different from that observed in crystalline and well lithified sedimentary rocks. It is in part responsible for the characteristic internal structure and hydrologic properties of normal faults in poorly lithified sediments, and thus has implications for diagenetic processes and interpreting fault zone deformation history.