New Mexico Geological Society Annual Spring Meeting — Abstracts

Historically, the ground surface above the sill-like Socorro Magma Body (SMB) has been rising on the order of 2-4 mm per year, whereas subsidence has occurred beyond its margins. For the past 25 years, oft-cited articles have reported that terraces along the Rio Grande (RG) are deformed by uplift, but the original articles did not include detailed descriptions of the terrace deposits. Ongoing geologic mapping of 7.5-minute quadrangles along the RG from Veguita to San Antonio across the southern Albuquerque Basin, the SMB, and the relatively narrow Socorro Basin shows three early to middle Pleistocene terrace deposits, local late Pleistocene terrace deposits, and top(s) of early Pleistocene fluvial basin fill. Lessons learned while tracing these discontinuous deposits southward include: 1) Original maximum fluvial aggradational elevations may be preserved only locally, especially compared to the southern Albuquerque Basin and the northern Palomas-Jornada Basin; 2) Commonly some lesser elevation of fluvial deposits is preserved by interfingering with less erodible coarse-grained valley-border fans; 3) Coarse sediments from the Rio Salado have affected the course of the RG and partially buried RG terrace deposits over several square km; 4) If the historic SMB uplift rate were constant, a 100,000 year-old terrace tread should be about 200-400 m higher, and older treads should have undergone even more uplift; 5) Short-term elastic uplift is generally viewed as domelike, but longer-term uplift could cause sufficient stretching to trigger longitudinal collapse of keystone grabens, so terrace heights might not be affected or terraces might subside in places; and 6) Multiple north-trending rift-related faults in the Socorro Basin south of San Acacia significantly affect the elevations of Bandelier ashes preserved in axial fluvial deposits (by as much as 20 m), making estimates of maximum aggradation or stream gradients uncertain.

Investigations to date show no major uplift of RG terrace treads from the southern Albuquerque basin across the northern part of SMB uplift, implying either that 1) current uplift may be too recent to be recorded in the terrace succession, or 2) current uplift is at a maximum rate, or 3) this reach is affected by both subsidence and uplift at the NE edge of the SMB, or 4) long-term uplift is matched by episodic subsidence of the surface. Within the Socorro Basin above the SMB, however, ample evidence of fault-bounded uplift, subsidence, and extensive valley-border erosion contrasts with the reaches to the north and south within the rift. These observations affect not only interpretations of the duration of uplift, but also hypotheses of a possible smaller shallow magma body and geologically reasonable rates of magma injection into the SMB.