COMPARING THE PATTERNS OF FAULTS IN THE NEAR-SURFACE VERSUS THE DEEP SUBSURFACE FROM THEIR GEOPHYSICAL EXPRESSION IN THE ALBUQUERQUE BASIN, NEW MEXICO

The Albuquerque basin is a Miocene extensional basin within the Rio Grande rift in New Mexico. Comparing the patterns of major faults located within a km of the basin surface (near-surface faults) to those greater than a km depth (deep faults) can determine if near-surface fault patterns within the basin accurately reflect basement fault patterns, and if not, whether the variations reflect differences in the deep (perhaps old) versus near-surface (perhaps young) stress regime of the basin.

New high-resolution aeromagnetic data for the Albuquerque basin, combined with mapped faults, give a comprehensive view of major, near-surface faults. These faults are numerous, commonly sinuous and linearly extensive, up to 50 km in length. They generally strike northerly except at significant steps in the basin boundaries and in the northern part of the basin where faults alternate between northeasterly and north-northwesterly orientations.

Deep faults expressed in gravity, seismic-reflection, and regional magnetic data primarily represent basement structures up to 5 km depth, such as major basin- or subbasin-bounding faults. Many of these structures have northerly strikes, similar to the near-surface faults, but not all. Thus, the near-surface patterns are an accurate reflection of deep faulting only in places. There are major differences in the north-central part of the basin where deep faults form northeast and northwest-trending subbasin boundaries. The near-surface faults cross these boundaries without change in orientation. In places the spacing or number of near-surface faults appears to vary near the boundaries instead, indicating influence from the deep faults. The deep faults may be the product of an older stress regime that subsequently changed orientation, leaving behind a basin morphology that affected subsequent faulting. Alternatively, the deep faults may have preferentially followed pre-existing basement inhomogeneities that are absent in the basin fill.