New Mexico Geological Society Annual Spring Meeting — Abstracts

The elliptical pancake-shaped Socorro magma body (SMB) is inferred to extend over 3,400 km² at a depth of ~19 km with an estimated thickness of 0.1 km: thus yielding a volume of about 340 km³ (Balch et. al., 1997). Since 1909 the rate of historic uplift near the center of the SMB at San Acacia has been between 1.8 mm/yr and 4 mm/yr (Larsen et al., 1986; Fialko and Simmons, 2001). Using vertical uplift rates from previously published studies, we calculate average tilt rates on the flanks of the uplift to be ~0.12 microradians (µ rad) per year (~1^º in 145 ka). Above the SMB, fault blocks of the early Rio Grande rift are numerous, strongly extended, and variably tilted. However, the long
axis of the SMB is approximately coincident with the active axis of the rift as defined by Quaternary grabens that outline the southern end of the Belen Basin and the much narrower graben of the Socorro Basin.

With matching funds from the New Mexico Department of Public Safety, we installed tiltmeters on three margins of the historic uplift. The solar-powered meters are capable of measuring 0.1 µ-rad differences in tilt for extended periods. We anticipate that monitoring over several years might provide insight into the uplift process and interaction of magmatic stresses with long-term tectonic stresses as defined by upper crustal fault blocks. We selected tiltmeter locations to the north (Sevilleta 1) and south (Tech 1) that were away from Quaternary scarps and within graben blocks in order to minimize riftrelated block rotation. The western station (Silver Creek 1), however, is on a large active west-tilting half graben (La Jencia Basin) where tectonic tilt and magmatic tilt should be additive. Available equipment limited the installation to depths of 3-4 m in alluvium. Dataloggers record tilts, temperatures, and precipitation (only at Silver Creek) every 30 seconds and store averaged data every 20 minutes. The tilt data show diurnal fluctuations in the range of tenths of µ-rad that apparently are related to Earth tides. Longer-term trends of tens of µrad are contrary to expectations and may be due to several factors, including settling after installation. Surface waves from distant earthquakes as small as magnitude 5.1 temporarily affect the 20-minute averages and show that some tiltmeter rotation matches Rayleigh-wave motions. Temporary reversals in tilt direction follow precipitation events at Silver Creek. Sevilleta 1 continued to tilt southeast throughout the year. Tech 1 tilted northwest until the end of October, when it began to tilt northeast. Silver Creek 1 tilted southeast from March until August, then tilted briefly to the southwest and northeast, and tilted west and northwest in November and December.