New Mexico Geological Society Annual Spring Meeting — Abstracts

The Socorro Magma Body (SMB) is a deep (~19 km) partly molten sill ~100-150 m thick beneath Socorro, NM that causes active surface uplift at rates up to ~3 mm/yr, as indicated by geodetic data.  The duration of uplift is not known however: Is uplift long-lived (>1 ka) due to inflation of a thicker body that is mostly crystalline or to episodic intrusion over time (Majkowski-Taylor 2008), or a recent, one-time event (<1 ka), as inferred from models of a cooling sill (Reiter et al., 2010)?  We use Rio Grande and tributary terraces as structural markers to investigate the possible longer-term history of the SMB.  Tributary terraces along the Salas Arroyo NE of La Joya, NM and an unnamed tributary W of Luis Lopez, NM were mapped and analyzed based on their elevations above their respective channels.  After further mapping, characterization, and preliminary correlation of river terraces within range of the magma body, soil carbonate development and ³⁶Cl cosmogenic dating will be used to infer ages and test correlations. A similar terrace study along the Rio Salado NW of San Acacia, NM concluded that terraces are warped by surface uplift caused by the SMB (Majkowski-Taylor, 2008).  Quaternary faults in the area were not observed to cut the young terraces from which this conclusion was drawn. Faults crossing Salas Arroyo and Luis Lopez also do not cut the terraces described here. Such faults complicate the analysis of SMB-related deformation of older terrace remnants.  Analysis of terrace elevation profiles in our two study sites does not show clear evidence of SMB-related, along-stream terrace deflections.  However, Salas Arroyo runs sub-parallel to contours of equal SMB-related uplift, so is not expected to show differential tilting.  There are, however, greater numbers of terraces preserved at Salas Arroyo than at Luis Lopez. This may indicate differential incision between the two areas caused by the SMB. Five distinct terrace levels are observed at Salas Arroyo compared to three levels at Luis Lopez. Terrace levels of 0.5 m, 1 m, and 2 m are observed at Luis Lopez while levels of 1 m, 2 m, 8-10 m, 12-13 m, and 20 m are preserved at Salas Arroyo. The 0.5 m terrace at Luis Lopez represents the modern floodplain and the youngest terrace along the tributary.  The lowest terrace at Salas Arroyo sits 1 m above the active channel and comprises the youngest surface at La Joya.  We tentatively correlate these two terraces in the absence of soil analyses. This is consistent with greater incision at La Joya, resulting from uplift rates of ~2 mm/yr, than at Luis Lopez, where uplift rates are <0.5 mm/yr (Pearse and Fialko, 2010).  Future work will consist of mapping additional terrace remnants along other tributaries and the Rio Grande, performing detailed soil analyses to characterize relative soil ages and carbonate development, which will aid terrace correlation among the various tributaries.  ³⁶Cl cosmogenic dating methods will subsequently be implemented to obtain absolute ages for terraces to test the previous correlations based on soil analyses.