FAULT ZONE DEFORMATION: THE KEY TO WHAT IS A ROOTED DETACHMENT AND WHAT IS NOT

We have studied the fault zones of a wide range of brittle upper crustal detachment surfaces in an attempt to establish criteria for determining what is and what is not a rooted fault. Our underlying assumption is that large rooted normal faults ought to have characteristics associated with a prolonged history of seismically cycled displacement. In this endeavor, we have sampled the fault zones of low-angle normal faults for which rooting deep in the crust is well established. We have also examined the fault zones of rootless detachments such as large block slides in order to characterize their deformation. Rooted faults exhibit clear evidence of repeated deformation, including cross-cutting veins and pathways for cataclastic flow, secondary mineralization, multiple generations of brecciation and healed microfractures, undulose extinction and, in some cases, proximal mylonitization. Faults exhibiting these characteristics include the chlorite breccia of the Whipple Mountains and the Cave Canyon detachment of the Mineral Mountains. Non-rooted detachments studied include the massive Heart Mountain detachment as well as a number of smaller slides in the western U.S. These data were then compared with observations at detachments that proved influential in the development of the concept of active upper crustal low-angle normal faulting: the Sevier Desert detachment, documented almost exclusively from reflection seismology, and the Mormon Peak detachment, which was inferred on the basis of a series of erosional remnant blocks on low-angle surfaces. Neither detachment exhibits the characteristic deformation expected of a seismically cycled fault zone. In the case of the Sevier Desert detachment, none of the material recovered from either the upper plate or the lower plate exhibited any signs of deformation. These observations by themselves do not preclude existence of normal faults moving at inclination of less than ~20°, but they should sound a cautionary note when interpreting seismic reflection or outcrop examples of low-angle normal faults as rooted and associated with extreme crustal extension.