Fluvial terraces in the lower Rio Salado valley: correlations, estimated ages, and implications for Quaternary faulting and for surface uplift above the Socorro Magma Body

A bstrAct — The Rio Salado, southwest of Belen, is a large western tributary of the Rio Grande with a valley that is flanked by six major terrace levels. The presence of several Quaternary, rift-related normal faults and a mid-crustal magma layer beneath the Rio Salado valley provide an unusual opportunity to investigate the effects of various modes of tectonic deformation of the terraces. In this study, we mapped Rio Salado terraces using a commercial high-resolution DEM and digital color stereophotographs on a GIS workstation. The terraces were projected onto a vertical plane to construct longitudinal profiles. We employed the terrace nomenclature of existing 1:24,000-scale geologic maps, but divide Qte into two distinct terraces (Qte1 and Qte2). We estimated terrace ages of 346±123 ka (Qtg), 235±105 ka (Qtf), 160±86 ka (Qte1), 95±36 ka (Qtd), 52±39 ka (Qtc), and 7±5 ka (Qtb) using a net incision rate of 0.30±0.10 m/kyr, inferred from the correlation of Qte2 to the 122±18 ka Airport surface, ~25 km south of the Rio Salado valley. Terraces in the Loma Blanca fault (LBF) hanging wall are back-tilted relative to the footwall, suggesting a listric geometry for the LBF. Two exceptions are terrace levels Qtf and Qtc, which are east-tilted relative to their footwall counterparts. Both Qtf and Qtc merge eastward with the next youngest terrace in the flight. The Qtc terrace is arched, possibly reflecting surface uplift due to the Socorro magma body (SMB). Qtc is not offset by the LBF, suggesting that fault activity ceased in the valley prior to tread abandonment. This study is a preliminary report on the configuration and correlation of Rio Salado terraces. Future work will involve cosmogenic 36 Cl surface exposure dating to obtain a quantitative chronology for the Rio Salado terraces and enable the determination of incision rates and improve correlation with terraces regionally. Surface exposure dates will also provide constraints on slip rates of Quaternary faults and the geologic history of the SMB.