A 3D GEOLOGICAL MODEL OF THE BUCKSKIN – HARCUVAR LOW-ANGLE NORMAL FAULT AND RELATED METAMORPHIC CORE COMPLEX STRUCTURES BASED ON SEISMIC REFLECTION PROFILES, WESTERN ARIZONA

We created a three-dimensional model of the down-dip projection of the low-angle normal fault exposed in the Harcuvar and Buckskin metamorphic core complexes (MCC’s) of western Arizona. The model is based on reprocessed and extend-correlated industry seismic reflection profiles obtained by CALCRUST in the 1980’s, and was created using EarthVision software by Dynamic Graphics, Inc. Like other profiles in the highly extended terrane, these show distinctive reflection character domains in the crust that define parts of the metamorphic core complex system. The domains include a zone of high-amplitude, discontinuous reflections in the middle crust; dipping reflections at the top of the highly sheared low-angle normal fault (LANF) footwall; high-amplitude subhorizontal reflections in the LANF hanging wall called the Bagdad Reflection Sequence (BRS); a relatively transparent upper crust, and the Moho. A reflective lower crust is notably absent here, consistent with refraction data indicating granitic crust all the way to Moho. The LANF forms a regionally extensive “corrugated” or antiformal-synformal shape like that exposed in the MCC’s and present beneath the valleys between them. The corrugation crests plunge northeast at about 15 degrees, and merge into mid-crustal reflectivity at about 18 km. The BRS projects to the surface in the Weaver Mountains, where it is exposed as Proterozoic mafic dikes that are tens to over a hundred meters thick, intruding Proterozoic granites. Reflectivity in the LANF lower plate mimics the mylonitic gneiss fabrics in the MCC’s, and are tangential to and merge with the LANF reflections. We interpret the subhorizontal attitude of the BRS to indicate that the plunging LANF corrugations are a primary feature and indicate no rotation of the LANF upper plate during MCC development. Lower plate reflectivity suggests progressive extraction and freezing of mobile middle crust to the rising, tilting lower plate. The top of this reflectivity is the mylonitic front exposed in the Harcuvar MCC.