Down-plunge structural interpretation of the Placitas area, northwestern part of the Sandia uplift, central New Mexico--Implications for tectonic evolution of the Rio Grande rift
Lee A. Woodward and Barbara. Menne
The northwestern corner of the Sandia uplift involves a transition zone between the uplift and the late Cenozoic Rio Grande rift. Principal structures of the transition zone are two echelon, rift-bounding faults. the Rincon on the west and San Francisco on the east, connected by the northeast-trending Placitas fault zone. The north- trending Rincon normal fault zone is downthrown to the west along multiple high- and low-angle normal faults. Stratigraphic separation of 2400+ m at the south edge of the area decreases northward to its termination in about 3-4 km. The northerly trending San Francisco normal fault is also downthrown to the west and has 1800-2100+ m of stratigraphic separation that decreases southward along multiple splays that connect with the Placitas and Las Huertas faults. The Placitas normal fault zone is downthrown to the northwest and decreases in stratigraphic separation horn 14011-1700 in in the northeast as the fault splays to the southwest, The San Francisco and Placitas fault zones appear to be connected and form a complex listric fault system that is now tilted northward, an interpretation supported by the down-plunge view showing the Placitas fault zone dipping gently and the San Francisco fault tone with a steep dip. Additional evidence for listic geometry includes apparent rotation of bedding surfaces in the hanging wall with steep dips toward the Placitas fault and increasing stratigraphic separation northward at higher structural and stratigraphic levels along the San Francisco fault. Right slip along this margin at the Rio Grande rift is indicated by northwest-trending fold axes adjacent to the rift bounding faults. Low-angle, normal faults nearly parallel to bedding in upper Paleozoic and Mesozoic strata have minor stratigraphic separations (1 to 50m) and are inferred to have a northerly direction of tectonic transport. These limits are interpreted to be gravity-driven and formed in response to rise and northward tilting of the hanging wall of the Placitas fault. They therefore appear to be younger than the principal movement on the San Francisco and Placitas faults.
- Woodward, Lee A.; Menne, Barbara., 1995, Down-plunge structural interpretation of the Placitas area, northwestern part of the Sandia uplift, central New Mexico--Implications for tectonic evolution of the Rio Grande rift, in: Geology of the Santa Fe Region, Bauer, Paul W.; Kues, Barry S.; Dunbar, Nelia W.; Karlstrom, K. E.; Harrison, Bruce, New Mexico Geological Society, Guidebook, 46th Field Conference, pp. 127-133.