Abstract: Analyses of normal faults in mechanically layered strata reveal that material properties of rock layers strongly influence fault nucleation points, fault extent (trace length), failure mode (shear v. hybrid), fault geometry (e.g. refraction through mechanical layers), displacement gradient (and potential for fault tip folding), displacement partitioning (e.g. synthetic dip, synthetic faulting, fault core displacement), fault core and damage zone width, and fault zone deformation processes. These detailed investigations are progressively dispelling some common myths about normal faulting held by industry geologists, for example: (i) that faults tend to be linear in dip profile; (ii) that imbricate normal faults initiate due to sliding on low-angle detachments; (iii) that friction causes fault-related folds (so-called normal drag); (iv) that self-similar fault zone widening is a direct function of fault displacement; and (v) that faults are not dilational features and/or important sources of permeability.

The southeastern margin of the Colorado Plateau (CP) lies in southwestern New Mexico and southeastern Arizona. It is defined as the boundary between the CP, the Rio Grande rift, and the Basin and Range (BRP) provinces. Along its western and southern margins, the CP is physically distinguishable from the BRP by major escarpments. Because of a thick cover of volcanic material associated with the Mogollon-Datil volcanic field (MDVF), no such demarcation exists in the study area. To accurately determine lithospheric structure in this area, two seismic refraction lines that intersect within the MDVF were studied. In addition, gravity data along these lines were simultaneously analyzed. Based on these data, velocity and density profiles were created via an iterative forward modeling processes. Major crustal boundaries for each paired gravity/refraction profile were constrained to agree at the point of intersection and with other geological and geophysical control. Structurally, the CP margin appears as an abrupt northward deepening of the Moho just south of Datil, New Mexico. This lies beneath a major low density-low velocity upper crustal body, interpreted to be a plutonic complex approximately 230 by 150 by 6 km. It is very likely that this complex provided the source material for the MDVF. Stoping appears to be the vehicle for emplacement of this body, implying a major crustal reorganization in middle Tertiary time.

The region of central New Mexico where the Rio Grande rift begins to widen has been investigated via an integrated geological and geophysical analysis. Existing seismic refraction profiles and two seismic reflection profiles, when combined with gravity and drilling data, provide a good picture of basin geometry deep crustal structure in the region. The Palomas Basin is shown to be a half graben bounded by a listric-normal fault imaged by the seismic reflection data. The Engle Basin is also associated with a half graben. The structure of the Tularosa Basin is more symmetric with two grabens. An analysis of deeper structures indicates that the crust is 28 to 33 km thick in the rift with the greatest thickness being in the northern portion of the area.