New Mexico Geological Society Annual Spring Meeting — Abstracts

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Refining the age of the Reserve Graben, West-Central New Mexico, with 40Ar/39Ar dating.

Connor J Whitman1, Sam Martin1, Gary Axen1, Jolante van Wijk1, Matthew Heizler2 and Dan Koning2

1New Mexico Institute of Mining and Technology, 801 LEROY PL, SOCORRO, NM, 87801,
2New Mexico Bureau of Geology and Mineral Resources, 801 LEROY PL, SOCORRO, NM, 87801

The Reserve Graben is a small rift basin situated between three major tectonic features of the Southwestern United States: the Rio Grande Rift to the east, the Colorado Plateau to the northwest, and the Basin and Range Province to the southwest. The graben’s location at the junction of these three features suggests its formation can be tied to their relative motions (Martin et al., this meeting). However, more data are needed on the age and duration of rifting within the graben. In order to refine older, mainly K/Ar ages of graben fill, 11 samples taken from igneous intrusive and extrusive units interlayered with the basin fill have been dated using the 40Ar/39Ar radiometric method. Dated materials include groundmass from 10 basalt flows and biotite/sanidine from a quartz diorite volcanic plug. The oldest basalt, within the deepest sediments near the floor of the basin, yields an age of 16.35 ± 0.04 Ma while the youngest basalt age is 1.89 ± 0.01 Ma. The latter flow crosses the master fault of the graben without offset, providing a minimum age for fault activity, which is slightly older than the previous K/Ar age of 1 Ma (Marvin et al., 1987). Another basalt, previously dated by K/Ar at 19.2 ± 2.5 Ma (Ratté 1980), yields a precise 40Ar/39Ar age of 16.02 ± .04 Ma, consistent with other basalts in the area. These findings suggest that rifting in the Reserve Graben began significantly later than the previously estimated 21 Ma (Crews, 1994). Onset of Reserve Graben subsidence around 16 Ma corresponds closely with a rapid pulse of subsidence in the nearby Rio Grande Rift during early and middle-Miocene time (Chapin & Cather, 1994). This also occurred during the transition from southwest-directed to west-northwest-directed extension in the Basin and Range Province (McQuarrie & Wernicke, 2005). Additional samples are being dated to determine sedimentation rates and constrain better the youngest sedimentation history within the basin.


  1. Chapin, C.E., and Cather, S.M., 1994, Tectonic setting of the axial basins of the northern and central Rio Grande rift, in Keller, G.R. and Cather, S.M. eds., Basins of the Rio Grande Rift: Structure, Stratigraphy, and Tectonic Setting, Geological Society of America, Special Paper 291, p. 5–26.
  2. Crews, S.G., 1994, Tectonic control of synrift sedimentation patterns, Reserve graben, southwestern New Mexico, in Chamberlin, R.M., Kues, B.S., Cather, S.M., Barker, J.M., and McIntosh, W.C. eds., Mogollon Slope, West-Central New Mexico, New Mexico Geological Society, Fall Field Conference Guidebook 45, p. 125–134.
  3. Marvin, R.F., Naeser, C.W., Bikerman, M., Mehnert, H.H., and Ratte, J.C., 1987, Isotopic ages of post-Paleocene igneous rocks within and bordering the Clifton 1° x 2° quadrangle, Arizona—New Mexico: New Mexico Bureau of Mines & Mineral Resources Bulletin, v. 118, p. 1–63.
  4. McQuarrie, N., and Wernicke, B.P., 2005, An animated tectonic reconstruction of southwestern North America since 36 Ma: Geosphere, v. 1, p. 147–172, doi:10.1130/GES00016.1.
  5. Ratte, J.C., 1980, Geologic map of the Saliz Pass quadrangle, Catron County, New Mexico: U.S. Geological Survey Miscellaneous Field Studies.
pp. 65

2021 New Mexico Geological Society Annual Spring Meeting
April 15-16, 2021, Virtual Meeting

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