PALEOSEISMIC INVESTIGATIONS IN THE RIO GRANDE RIFT, USA, 1978-2003

The Rio Grande rift trends north-south for 1000 km between northern Chihuahua, Mexico, and central Colorado, USA. In the past 25 years most of the major rift-bounding faults have been subjected to paleoseismic study. At the northern end , late Quaternary fault scarps were first discovered in 2002 on the Williams Fork Mountains fault, on the east side of the Williams Fork Mountains. Discontinuous late Pleistocene fault scarps exist on the western side of the Upper Arkansas Graben (Sawatch Fault), which was trenched in 1978. South of Poncha Pass the symmetry of the rift reverses, and the 120 km-long Northern Sangre de Cristo fault bounds the large San Luis Graben on its eastern side, and is tentatively divided into 3 segments. Trenches on this fault in 1980 and 2002 document the MRE at about 7.6 ka, and the PE between 10-15 ka. South of Taos, New Mexico the symmetry of the rift reverses again, with major displacement shifting to the western margin (Pajarito Fault). The accommodation zone is occupied by the sinistral Embudo Fault, which displaces Quaternary but not late Quaternary deposits. The Pajarito Fault forms a 75-130 m-high escarpment in a plateau formed by the 1.2 Ma Bandelier Tuff, yielding a long-term slip rate of about 0.1 mm/yr. Since 1997 at least 20 trenches have been dug in this fault zone, which bounds the Los Alamos National Laboratory. Recurrence on the Pajarito Fault tends to be about 20 ka, with the latest event in the mid-Holocene. Normal faults in the Albuquerue-Belen Basin are typified by large (25-100 m-high) fault scarps across old (500-1200 ka) geomorphic surfaces west of the Rio Grande, and by much smaller scarps at the base of the Sandia Mountains. South of Albuquerque the Hubble Springs fault bounds the eastern rift margin and was trenched in 2000. At the latitude of Socorro the Socorro Mountain fault bounds the western side of the rift. Trenches in 1999 showed a Holocene MRE. Farther west the longer La Jencia fault was extensively trenches in the 1980s, and a recurrence interval of 20 ka was estimated. In southern New Mexico the Alamogordo and Organ Mountains faults have been trenched in a preliminary fashion. Finally, the southernmost part of the rift at El Paso is composed of the East Franklin Mountains fault, which was trenched in 1996. The MRE there was ca. 10ka, but there are questions about slip rate and recurrence.