New Mexico Geological Society Annual Spring Meeting — Abstracts

During the summer of 2022, we conducted 0.05 mGal resolution relative gravity surveys along two W-E lines in the western Sevilleta National Wildlife Refuge. Gravity data can provide a test of rift models, which include deep-seated listric faulting (Ricketts et al., 2015) or multiple generations of cross-cutting, planar-rotational normal faults, initially formed at steep dips (Chamberlin and Love, 2016).
Our survey had 74 new relative gravity stations. At each station, GPS coordinates were collected with a Trimble RTX to a 10-cm horizontal and 5-cm vertical accuracy. We used a ZLS Burris Gravity Meter with 5 µGal resolution for relative gravity measurements. Stations along each profile were at a spacing of 0.25 to 1 km. We re-occupied a subset of stations multiple times to correct for drift and to tie to NGA absolute benchmarks. Data was reduced to the mean measured gravity and manually drift corrected. The total Bouguer anomaly was calculated assuming a crustal density of 2.65 g/cm³.
The northern survey, from the base of Ladron Peak to Bernardo along the AT&T road, reveals some unexpected variations in gravity anomalies. The western end, at the foot of Ladron Peak, shows a gentle eastward gradient above the low-angle Silver Creek fault. At two km from the mountain front, a steep eastward gradient (40 mGal/km) offsets this gentle gradient. The gravity profile is then again gently eastward sloping for the next 9 km. A small eastward gradient (2 Mgal) is observed down dip of the Loma Pelada (aka. Coyote) fault trace. This is much less than expected, since seismic reflection data indicate over 2 km of east-down displacement on the Coyote fault. Three km further east the flat gradients steepen to 10 mGal/km eastward and then reverse to 15 mGal/km westward for the final 2 km.
The southern profile along Alamillo Road begins at a gravity low in the westerly tilted Plio-Pleistocene La Jencia Basin. From this low, gravity observations increase eastward at about 5 mGal/km toward the crest of the west-tilted basin block where it is truncated by the low-angle Silver Creek fault. A gentle and narrow eastward gradient (1mGal) is observed 1 km east of the Silver Creek fault trace. This gradient then turns westward above another west tilted basin block floored by volcanic rocks at Cerritos de las Minas. The profile maximum occurs on the south flank of Cerrito de las Minas. At the south end of the Loma Pelada fault, basin-fill strata reverse dip direction to 30º east. Here the gravity gradient also changes direction and slopes uniformly at 5 mGal/km eastward across a possible relay ramp structure.

Geologic map plus gravity data demonstrate that the Miocene low-angle Silver Creek fault is displaced 400m down-to-the east by a Pliocene rift fault that dips 70º to the SE.