Using Quaternary fluvial terraces as structural indicators of magma body-related uplift, Socorro, NM

The Socorro Magma Body (SMB) is a deep (~19 km) partly molten sill ~100-150 m thick with an area of ~3400 square kilometers beneath Socorro, NM. It causes domal surface uplift of 0 to ~2.5 mm/yr over a diameter of ~60 km. The age and history of the SMB are poorly known, but thermal modeling suggests an age < 1 ka. Over the time span of geodetic data (1911-2013), at current maximum rates, ~25 cm of uplift is expected. Several arguments suggest that the SMB may be significantly older than ~1 ka. If so, terraces of the Rio Grande and major tributaries may record lateral elevation gradients and differential tilting. Terrace deformation over the SMB is complicated by active N-S striking normal faults. Terrace elevations are taken from a commercial, airborne radar-based DEM with ~0.5-1 m of point-to-point scatter, projected to a valley-center line and smoothed using a moving average approach, resulting in ~1 m precision. Deformed terraces of the Rio Salado suggest the SMB is older than ~1 ka. The Rio Salado is a major E-flowing tributary ~25 km north of Socorro, that crosses the N flank of the uplift (rates of 1.0-2.0 mm/yr) and three Quaternary fault zones. Five Middle(?) Pleistocene to Holocene terrace levels are preserved in a 17-km alluvial reach W of the confluence with the Rio Grande. The lowest terrace (Qtb) is ~0.5-3 m above the streambed and is degraded in most areas. No deflection is apparent in Qtb. Older, higher terraces (~17m Qtc; 30 m Qte; 60 m Qtf; and 90 m Qtg) are cut by the E-down Loma Blanca fault and are W-tilted and offset as expected. Qtc, however, is E-tilted over the E flank of the uplift (E-ward change in uplift of ~ -0.4 mm/yr) and merges E-ward with Qtb, consistent with an episode of inflation older than Qtb but younger than Qtc. This tilting cannot be explained by slip on the older W-down Cliff fault, which is overlapped by Qte. Qte is tentatively correlated with the 122 × 18 ka "airport surface". A constant long-term slip rate approach, using the age and displacement of Qte across the Loma Blanca fault, yields an age range of 39-53 ka for Qtc. This is consistent with published soil descriptions that suggest a Late Pleistocene age based on weakly developed calcium carbonate-enriched C horizon (Cca) in Qtc. A constant long-term net incision rate approach indicates an age of 44-75 ka. The presence of reddening in the B horizon of Qtb implies an early stage of soil development, suggesting that Qtb should be older than 2-3 ka. Application of constant net incision rate approach yields an age range of 8-14 ka for Qtb. We conclude that the magma body caused deformation before ~2-3 ka ago, requiring a longer, and more complex history than a single intrusive event within the last 1 ka. Future mapping, characterization and correlation of river terraces over the SMB should allow us to distinguish fault-vs.-SMB related deformation and better constrain the space-time history of SMB inflation. Key terraces will be dated using 36Cl cosmogenic isotopes to provide numerical ages.