Date of Award
Master of Science
D. I. Doser
K. C. Miller
The Basin and Range Province of the western United States is an unusually wide continental rift zone and its structure and evolution have been the topic of much debate. Active source seismic surveys within the Basin and Range Province have delineated a relatively uniform crustal thickness (-30 km) despite varying magnitudes of crustal extension (<10-300 km). Large scale Cenozoic crustal extension has exhumed midcrustal rocks (from 10-20 km depth) along low angle detachment faults to create metamorphic core complexes (exposed lower plate rocks). In contrast to the extreme amounts of extension and internal deformation of the Basin and Range, the Colorado Plateau has remained a relatively rigid block, with a crustal thickness of 40-50 km, apparently resistant to the deformation processes that have shaped the Basin and Range observed today. In general, geophysical surveys reveal a significant contrast in crustal structure between the Basin and Range and it's transition into the Colorado Plateau.
The debate about the origin of the Basin and Range has underscored the need for additional geophysical constraints. Thus in this study, a regional analysis of lithospheric structure in the region of southeastern California, southern Nevada, and western Arizona that includes the Basin and Rangesouthwestern Colorado Plateau was undertaken using seismic refraction/wideangle reflection, gravity, remote sensing, and geologic data.
The final lithospheric structure models show a relatively uniform crustal thickness of 27-31 km within the Basin and Range, 32-36 km in the Transition Zone, and 37-42 km in the southwesternmost Colorado Plateau. The middle crust has a velocity of -6.1 km/s from the Basin and Range into the southwestern Colorado Plateau. In the lower crust, there are thickened features in areas of greatest extension (Death Valley, Lake Mead, and Colorado River extensional corridor) with P-wave velocity of -6.4 km/sand density of 2.9 glee. Also, the long wavelength (100-400 km) component of the Bouguer gravity anomaly shows gravity highs that correlate with these mapped highly extended domains where the lower crustal thickening has been modeled. The seismic velocity and density models suggest that these lower crustal features have a felsic to intermediate composition or were intruded by 1/3 mafic material and thus, another geologic mechanism like lateral ductile flow or intrusion of intermediate composition rocks directly from the mantle is needed to account for the additional 2/3 of these features.
Seismic velocities in the upper mantle range from 7.9 km/s in the central Basin and Range, to 7.8 km/s in the southern Basin and Range and metamorphic core complex belt, to 7.9 km/s under the Colorado Plateau. Deeper mantle arrivals (P1) have been recorded and correlate across the Basin and Range, Transition zone, and southwesternmost Colorado Plateau. They have been modeled as a reflected wave off an interface at a depth of -45 km under southeastern California, and dipping toward the northern Basin and Range and Colorado Plateau to a depth greater than 50 km with a seismic velocity decrease to 7.7 km/s. The reflection off the bottom of this low velocity layer (P2) is recorded in the Transition Zone at a depth of -68 km (15 km thick) and the southwesternmost Colorado Plateau at a depth of -58 km (10 km thick) exhibiting very large amplitudes. The Basin and Range geotherm crosses the dry basalt solidus at approximately 45 km depth suggesting that this low velocity layer could be due to the melting of mafic material. The bottom of this low velocity layer is clearly present in the Transition Zone and southwesternmost Colorado Plateau, but not in the Basin and Range.
Nigel Oakley Hicks
Hicks, Nigel Oakley, "Lithospheric Structure of the Basin and Range Province-Southwestern Colorado Plateau. Southeastern California, Southern Nevada, and Western Arizona" (2001). Open Access Theses & Dissertations. 3132.