Cenozoic structural and paleogeographic evolution of the Transition Zone, central Arizona

The evolution of the modern landscape in central Arizona was examined from a geological and geophysical perspective. A central theme centers on the structural and topographic inversion between the high Colorado Plateau to the north and the low Basin and Range to the south, when crustal thickening during the Laramide Orogeny gave way to mid-Tertiary crustal thinning and the resultant reversal of stream systems across the Transition Zone.; In the Maricopa Mountains region southwest of Phoenix, the interpretation of four seismic reflection lines provides a three-dimensional perspective of highly extended crust beneath widespread basin fill. Six mid-Tertiary fault-bounded basins were later down-faulted into the subsurface by Basin and Range high-angle faults. An extensive unconformity caps the mid-Tertiary basins and merges laterally with broad granitic pediments surrounding the Maricopa Mountains. Concave-upward faults merge at depth into dense zones of subhorizontal reflectors that are interpreted as metamorphic core complexes. Two deeply penetrating, northwest-trending, moderately dipping shear zones are reactivated crustal weaknesses that date to the Proterozoic.; In the upper Salt River region between the White Mountains and the Tonto Basin, the architecture of the Tertiary elastic rocks was mapped and analyzed in relation to major faults that bisect the Transition Zone. The ancestral geography is synthesized from a fairly complete geologic column, which provides constraints on the timing and magnitude of Tertiary tectonics. A deep paleocanyon, incised into the underlying Proterozoic rocks by a northeast-flowing Laramide river, was utilized by the Salt River following topographic inversion and drainage reversal. West-up Laramide offset of many faults in the Transition Zone is cancelled by later opposite-sense reactivation during middle to late Tertiary extension, causing faults to show minimal offset.; This work provides insights on the uplift history and landscape evolution of the southern Colorado Plateau. Five Cretaceous to Tertiary unconformities contribute to the modern landscape. Denudation of Phanerozoic rocks from central Arizona initially occurred during Early Cretaceous rift-shoulder uplift. Extensive canyon cutting into this stripped surface was caused by Laramide northeast tilting and monoclinal folding. Mid-Tertiary extension and southward flexure slowly induced regional drainage reversal, with some modern streams of the Colorado and Salt Rivers, re-occupying Laramide paleocanyons.