New Mexico Geological Society Annual Spring Meeting — Abstracts

Fault scarp morphology and soil-geomorphic studies of a major N-S trending fault scarp (VLA scarp) offsetting late Quaternary surficial deposits in the northeastern San Agustin Plains west-central New Mexico define the amounts, timing and rates of Quaternary displacements. A soil chronosequence (c.f., Ritter and others, this volume) developed on geomorphic surfaces near the scarp suggests a mid-early Pleistocene age for Q1 pediment fans, a older late Pleistocene age for Q2 inset fill terraces, and Holocene to late Pleistocene ages for Q3 valley fill terraces. Topographic profiles of the VLA scarp indicate that the Q1 and Q2 surfaces have been offset along 20-30 m composite scarps and 2-5 m single rupture scarps, respectively, whereas Q3 surfaces extend unruptured across the fault trace, thereby constraining the age of most recent rupture to a late Pleistocene interval. Linear regression analyses of the log of scarp heaight (H) vs. maximum scarp slope angle 2
(A;7-12°) for single-event scarps (A=3 .78 + 8.9 log H; r²=0.68; n =5) further indicate an older late Pleistocene age (10⁵y. B.P. ) for the most recent rupture, an est.imate supported by the nearby independently-dated La Jencia Fault scarp (Machette and McGimsey, 1983). The soil observed on the VLA Fault scarp slope exhibits a morphol ogy (eg . St . II-III calcic horizon) intermediate between Q2 and Q3, but closer to the former, thereby supporting the morphologic estimate for the most recent rupture. Comparisons between the heights of composite and single rupture VLA scarps indicate 5-10 events since the formation of the Q1 surface, implying a probable 10⁵ yr recurrence interval. These rates/ magnitudes of Quaternary activity are less than estimated for the Great Basin and northern Rio Grande Rift (RGR), but do resemble those reported for southeastern Arizona and southwestern New Mexico (including southern RGR).