INFLUENCE OF INHERITED FEATURES ON THE BOUNDARIES AND FAULT PATTERNS OF THE EXTENSIONAL BELTS FROM THE GULF OF CALIFORNIA TO NORTHERN BASIN AND RANGE

The Basin and Range (BR) includes a continuous belt from the Gulf of California extensional province of Mexico through Southwestern USA to the northern BR. This is the core extensional to transtensional belt that lies east of the Pacific – North America (PAC-NAM) plate boundary. In simple terms, the BR is the result of the westward to northwestward, motion of the Baja California and Sierra Nevada microplates away from North America during PAC-NAM motion. The Baja California and Sierra Nevada microplates are parts of the Cretaceous batholith and the eastern edge of the batholiths define the western edge of the BR. The transtensional faulting of eastern Baja California and Walker Lane are remarkably similar and show down-to-the-east normal faults along the batholith boundary. To the east are linked normal and strike-slip faults, the latter striking from parallel to relative motion of the microplates to nearly parallel to the batholith boundary. The western BR boundary has domains of strain partitioning and other domains with northwesterly regional shear and no partitioning. Why certain areas are partitioned is not known, but secondary inherited structures may play a role. The eastern edge of BR is dominated by the Sierra Madre Occidental and the Colorado Plateau and is more variable in faulting patterns because the boundary is more variable in shape, the previous geologic history is more complex, and most extension occurred just before and in the early stage of PAC-NAM motion. There was a switch from extension to transtension and faulting localized to the western side of the BR as PAC-NAM developed. The Sierra Madre is likely underlain by a large batholith formed during caldera eruptions in the Oligocene; this batholith roughly parallels the Cretaceous batholith on the Baja California microplate and the SE BR is dominated by early extensional faults. The southern and western edges of the Colorado Plateau form the eastern boundary of the BR in the north with mainly ENE-directed core complexes in pre-17-15 Ma PAC-NAM history. Extension near Las Vegas 17 - ~10 Ma was westward and the inherited NE-oriented Wasatch line - Laramide boundary created west-facing conjugate fault wedges in an overall transtensional system. After 12 – 8 Ma, faulting moved west, and northwestward transtension dominated Walker Lane to Gulf of California.