Paleoseismologic studies of the Pajarito fault system, western margin of the Rio Grande rift near Los Alamos, NM

Conference · Thu Apr 01 04:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5130219

Kelson, K I ^[1]; Hemphill-Haley, M A; Wong, I G ^[2]; Gardner, J N; Reneau, S L ^[3]

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1. Wm. Lettis Associates, Oakland, CA (United States)
2. Woodward-Clyde Federal Services, Oakland, CA (United States)
3. Los Alamos National Lab., NM (United States)

As in much of the Basin and Range province, low levels of historical seismicity in the Rio Grande rift (RGR) are inconsistent with abundant geologic evidence for large-magnitude, late Pleistocene and Holocene earthquakes. Recent trenching and surficial mapping along the 40-km-long, north-trending Pajarito fault system (PFS) near Los Alamos provide evidence for multiple surface-rupture events during the late Pleistocene and Holocene. Near Los Alamos, the Pajarito fault (PAF) exhibits an east-facing scarp up to 120 m high that has had at least four surface-rupture events in the past few hundred thousand years. Four trenches across the base of the highest, easternmost fault scarp show that the most-recent rupture occurred prior to about 9 ka, and possible prior to deposition of the 100- to 150-ka El Cajete Pumice. The long-term (post-1.1 Ma) slip rate on the PAF is about 0.1 mm/yr. The down-to-the-west Rendija Canyon (RCF) and Guaje Mountain (GMF) faults both have had at least two surface ruptures since the middle Pleistocene, including most-recent events at about 7.4 ka along the RCF and about 4 to 6 ka along the GMF. Slickensides and other indirect evidence suggest right-oblique normal slip on the RCF and GMF. Long-term (post-1.1 Ma) slip rates on these two faults are approximately an order of magnitude less than that on the PAF. Based on the observed spatial and temporal variations in activity, the subparallel PAF, RCF, and GMF apparently act as independent seismic sources, although they are located only about 1 to 3 km apart. Nevertheless, the average recurrence interval for faults within the PFS is probably comparable to intervals of 10[sup 4] yr estimated along the eastern rift margin near Taos.

• OSTI ID: 5130219
• Report Number(s): CONF-9305259--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:5
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
CENOZOIC ERA
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DISPLACEMENT RATES
GEOLOGIC AGES
GEOLOGIC FAULTS
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GEOLOGIC HISTORY
GEOLOGIC STRUCTURES
NEW MEXICO
NORTH AMERICA
QUATERNARY PERIOD
RIFT ZONES
SEISMICITY
SEISMOLOGY
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USA