THE WESTERN MARGIN OF THE RIO-GRANDE RIFT IN NORTHERN NEW-MEXICO - ANABORTED BOUNDARY

The northwestern margin of the Espanola basin, part of the Rio Grande rift in northern New Mexico, is characterized by a zone >17 km wide of oblique-slip faults that off-set upper Paleozoic and Mesozoic strata of the eastern Colorado Plateau from Eocene and younger sedimentary ro cks of the rift. Along this margin, a reasonably complete section of p re- and synrift Tertiary sediments is exposed. Combined interpretation s of seismic reflection, seismic refraction, gravity, and geologic dat a acquired along a profile perpendicular to this boundary define the g eometry of faulting, possible rotation of sedimentary units, and strat igraphy of rift fill. Vertical separation on the westernmost major fau lt, assumed to be the bounding fault between the rift and the Colorado Plateau, is <500 m; separation on other faults in the zone is <200 m. Thus the northwestern part of the Espanola basin (''Abiquiu embayment '') is a shallow platform rather than a deep rift basin. The embayment is separated from the main Espanola basin by the east-northeast-strik ing Embudo transfer fault, which appears to act as the northern boundi ng fault of the main basin. Although Tertiary units are progressively faulted downward toward the axis of the rift, depth to inferred Precam brian crystalline rocks becomes shallower and the stratigraphic thickn ess of the intervening Paleozoic and Mesozoic units decreases toward t he axis. We interpret pinching out of these units toward the east as e rosional thinning on the western flanks of the Laramide-age Sangre de Cristo/Brazos geanticline, which underlay much of the present rift bas in. Imprecise age constraints suggest that faulting of the rift margin began 10-7 Ma, but was not active after 7 Ma. Extension was apparentl y transferred to the Embudo fault zone, which remained active until at least 2.5 Ma and possibly into Quaternary time. The Embudo transfer z one effectively decoupled the Abiquiu embayment from the main Espanola basin. Thus the boundary at Abiquiu preserves an early stage in the f ormation of the rift boundary. The shift in activity may have resulted from a change in regional stress field, or from increasing magnitude of strain, or both. The change in locus of extension reflects a narrow ing of rift basins through time and an integration of main bounding st ructures between adjacent basins. Although we are uncertain whether th e Abiquiu region, which uniquely preserves an early stage of deformati on, is representative of other areas of continental extension, our res ults indicate that the initial formation of rift basins may occur as h igh-angle, planar normal faults distributed over a broad zone. No evid ence from seismic data or from rotation of beds exists to indicate tha t faults become listric with depth, which is compatible with the small amount of extension (3.5%) inferred at this boundary.