FAULT FRAMEWORK FOR THE ALBUQUERQUE BASIN, RIO GRANDE RIFT

The composite Albuquerque basin lies within the central part of the Rio Grande rift in New Mexico. It formed by extension during Miocene to Holocene time. We have used a GIS compilation of faults obtained from field mapping, 10-m digital elevation models, high-resolution aeromagnetic surveys, and gravity data to construct a framework of major faults for this rift basin. These coverages reveal hundreds of faults that offset the shallow (<1 km) basin-filling Santa Fe Group sediments. The near-surface faults generally strike north, but they are commonly sinuous, branch with other faults, and locally display en echelon map patterns. Northeast-striking faults, typically short, merge with north- to north-northwest-striking faults where the basin steps eastward at its northern end. Evidence for large stratigraphic throw, long strike length, and strong expression on multiple GIS coverages was used to trace major faults throughout the basin. These major faults are the ones most responsible for shaping three deep sub-basins within the composite Albuquerque basin, as defined by isostatic residual gravity data. The major faults define both asymmetric and symmetric grabens as deep as 4 km. The gravity data also define buried, northwest-striking faults that partly bound the central sub-basin. These northwest-striking faults probably formed during an early rift phase of southwest-directed extension.

Field examination indicates that most basin faults dip steeply (50-80°) and have a dominant dip-slip normal sense of offset. Locally, these dip-slip faults are overprinted by moderate- to low-rake slickensides. Paleostress inversions from fault-slip data indicate that least-principle stress directions within the basin ranged between southwest and northwest during basin formation. This varying extension direction probably reflects general clockwise rotation from southwest to northwest extension that has been recognized over much of the Basin and Range province during middle to late Miocene time. Nonetheless, field relations of relative fault-slip chronology in the Albuquerque basin suggest that the stress variations were locally more complex.