INSIGHTS FROM GEOPHYSICAL MODELS OF RIFT-FILL THICKNESS IN THE ALBUQUERQUE BASIN, RIO GRANDE RIFT, NEW MEXICO

A three-dimensional geophysical model of syn-rift basin-fill thickness in the Albuquerque basin incorporates well data, seismic-reflection data, geologic cross-sections, and other geophysical data in a constrained gravity inversion. The model shows three, 3-5-km-deep, inter-connected structural depressions that increase in size, complexity, and segmentation from north to south: the Santo Domingo, Calabacillas, and Belen sub-basins. The Calabacillas sub-basin is a large east- to northeast-tilted half graben bounded to the west and east by structural benches. A southeast-trending antiform (Ziana structure) and a northwest-trending antiform (Mountainview prong) define the northern and southern boundaries of the Calabacillas sub-basin, respectively. The structure of the Belen sub-basin is more complicated, with smaller half-graben and graben segments separated by axially oriented antiforms and synforms. The east-northeast-tilted Mountainview half-graben lies southwest of the Mountainview Prong. The east-tilted Hubbell half-graben is bounded by the Hubbell Spring fault system. The southwest(?)-tilted Coyote half-graben merges with another west-tilted half-graben at the southern tip of the basin. The overall eastward tilt of the central and southeastern Albuquerque basin indicated by the geophysical model generally conforms to stratal tilts observed for the syn-rift succession, implying a prolonged eastward tilting of the basin since rifting began in late Oligocene time. The increasing southward complexity of intrabasinal structure is reflected by the transitional morphologic character of the Albuquerque basin, which sits between physiographically well-defined basins of the northern rift, and a broader zone of extension to the south. The main features of the geophysical model provide major revisions to a widely accepted structural model that was based primarily on seismic-reflection interpretations. Our model does not support the presence of a discrete, southwest-trending, scissor-like transfer zone separating oppositely tilting northern and southern parts of the Albuquerque basin. Moreover, our model suggests that sub-basins extend significantly east of the Rio Grande, contrary to the previous depiction of an eastern rift margin that generally follows the river.