New Mexico Geological Society Annual Spring Meeting — Abstracts

The Hot Springs-Walnut Canyon fault system is 50 km long and trends 20°–30°, passing under Elephant Butte Lake and along the western flank of the Fra Cristobal Range. The Hot Springs fault (HSF) links with the Caballo normal fault 4 km south of Truth or Consequences (TorC) (Jochems and Koning, 2014; Seager, 2015). On the northeastern side of Elephant Butte Reservoir, the Hot Springs fault is linked with the Walnut Canyon fault (aka Walnut Springs fault) 1.5 km to the east-northeast across a north-dipping relay-ramp structure south of the mouth of Walnut Canyon. The north end of the Walnut Canyon fault coincides with the northern tip of the Fra Cristobal Range, where the fault is generally buried by Quaternary cover. But based on gravity data, there is a 5–7 km west-step over to another concealed fault that bounds the eastern side of the San Marcial basin to the north. Previous work recognizes that the HSF experienced west-up, reverse movement in the Laramide orogeny (Nelson, 1993; Seager, 2015) and notable west-down normal motion during rifting (Lozinsky, 1986). Workers have also recognized a component of right-lateral slip on the fault, ranging from 460 m (Lozinsky, 1986) to 24 km (Harrison and Cather, 2004, who interpret the strike-slip motion as being Laramide in age).

Our study focuses on sites that constrain the displacement history of the Hot Springs-Walnut Canyon fault system. Near the mouth of Mescal Canyon, 2 km northeast of TorC, there is a small butte cored by the Permian San Andres Formation. The west side of the butte is bounded by a west-down normal fault and the east side by a west-up reverse fault, the latter displacing Permian San Andres Formation over Cretaceous Mancos Shale. To the north and south of this butte, exposures indicate angular unconformities in basal strata of the Rincon Valley Formation (middle–late Miocene) and drag folding in the younger lower Palomas Formation. Thus, notable normal movement occurred during the middle(?) – late Miocene and early Pliocene. We map two west-striking folds in Cretaceous rocks in between two strands of the HSF, west of Elephant Butte Dam, that are consistent with a component of Laramide age right-lateral slip. Although we have not observed apparent piercing points, the near-horizontal attitude of associated normal-drag folds, modest scale of shearing and brecciation, and apparent absence of en echelon folds and faults, suggest that maximum allowable strike-slip, probably Laramide age, is in the range of several hundred meters. Cross-section construction and our study of basal Santa Fe Group strata west of the Elephant Butte Dam indicates exhumation of 0.5–1.5 m of Cretaceous strata occurred west of the HSF prior to rifting, probably due to Laramide uplift west of the HSF, and 0.7–1 km of west-down throw during rifting.

In three places, the Hot Springs-Walnut Canyon fault system is overlain by basaltic flows or maar deposits that are clearly not offset by the fault. Therefore, dating these basaltic features provides a minimum age for the last movement on this fault. Two dikes intruding maar deposits at Rattlesnake Island, in the southern part of Elephant Butte Reservoir, yield indistinguishable ⁴⁰Ar/³⁹Ar ages of 2.468 ± 0.014 Ma and 2.447 ± 0.057 Ma. A cinder cone overlying the fault at Red Cliff, in the central part of the eastern shoreline, returned an age of 2.44 ± 0.04 Ma. Approximately 2 km north of Hellion Canyon, located in the central Fra Cristobal Range, a basalt flow extends down slope across the Walnut Canyon fault without being offset. Where sampled near the fault, this flow returned a somewhat complicated age spectrum consistent with the age of basalts at the other sites (ca. 2.5 Ma) but possibly as old as 2.9 Ma; a likely correlative of the flow 100 m to the east returned an age of 2.47 ± 0.007 Ma. Thus, widespread, ca. 2.5 Ma basaltic volcanism in the Cutter Sag and Engle basin appears to coincide with termination of fault motion along the Hot Springs-Walnut Canyon fault system. We propose a scenario in which fault sealing and local compression associated with subsurface dike injection counteracted extension in the Engle Basin and caused subsequent extensional strain to shift 20 km westward to the Cuchillo Negro fault system and Willow Draw fault.

References:

1. Jochems, A.P., and Koning, D.J., 2014, Geologic map of the Williamsburg 7.5-minute Quadrangle, Sierra County, New Mexico: New Mexico Bureau of Geology and Mineral Resources, Open-file Geologic Map 250, scale 1:24,000.
2. Lozinsky, R.P., 1986, Geology and late Cenozoic history of the Elephant Butte area, Sierra County, New Mexico: New Mexico Bureau of Geology and Mineral Resources, Circular 187, 40 p. and 2 plates.
3. Nelson, E.P., 1993, Thick- and thin-skinned Laramide deformation, Fra Cristobal Range, south-central New Mexico: Geological Society of America, Special Paper 280, p. 257–270.
4. Seager, W.R., 2015, Geology of the Palomas Gap 7.50-Minute Quadrangle, Sierra County, New Mexico: New Mexico Bureau of Geology and Mineral Resources, Open-file Geologic Map 260, scale 1:24,000.