Sedimentologic and geomorphic evidence for seesaw subsidence of the Santo Domingo accommodation-zone basin, Rio Grande rift, New Mexico

Basin analysis of facies patterns and basin geomorphology reveals a complex subsidence pattern for part of the Rio Grande rift in north-central New Me xico, United States, The Santo Domingo basin study site is located in the a ccommodation zone between the east-tilted northern Albuquerque basin and th e west-tilted Espanola basin half-graben. Upper Miocene through lower Pleis tocene basin-fill sediment records a longitudinal, south-flowing, ancestral Rio Grande and flanking piedmont stream systems. Chronology of deposition is provided by Ar-40/Ar-39 geochronological study of lava flows and pyrocla stic deposits interbedded with the sediment, Down-to-the-east faults concen trated in the western basin illustrate increasing displacement toward the n orth and merge with structures forming the western margin of the Espanola b asin. Down-to-the-west faults in the central and eastern basin exhibit sout hward-increasing displacement and become the principal structures along the eastern margin of the northern Albuquerque basin. The latitudinal overlap of these fault systems in the Santo Domingo basin gives the appearance of a symmetrical graben with anticlinal bedding attitudes, Analysis of the dist ribution of sedimentary facies, high-level geomorphic surfaces, and Pleisto cene terraces indicates, however, that the basin subsided in a seesaw fashi on rather than as a symmetrical graben. During the late Miocene and early P liocene, the course of the ancestral Rio Grande was deflected to the west, suggesting strong westward tilting of the basin. In late Pliocene and Pleis tocene time, however, eastward tilting brought the river into the eastern b asin, At that time, an east-sloping pediment was eroded across west-dipping strata in the western basin. Pediment gravel and tuff present as unconform ity-bounded strata in the western basin correlate with continuously aggrade d conformable basin-fill strata to the east. A return to westward tilting s ince middle Pleistocene time is suggested by unpaired terraces consistent w ith net westward migration of the Rio Grande, lack of post-middle Pleistoce ne motion on faults in the eastern basin, and numerous fault displacements of the pediment gravel in the western basin.