Progress report on tracking Rio Grande terraces across the uplift of the Socorro Magma Body
D. W. Love, D. J. McCraw, R. M. Chamberlin, M. Reiter, S. D. Connell, S. M. Cather, and L. Majkowski
Although the ground surface above the Socorro magma body (SMB) is rising at rates as high as 3 mm per year historically, three discontinuous early- to middle-Pleistocene terrace deposits, local late-Pleistocene terrace deposits, and top(s) of early-Pleistocene fluvial basin fill (upper Santa Fe Group) show no major uplift of terrace treads from the southern Albuquerque basin across the northern part of SMB uplift. These terraces and basin fill surfaces can be traced along the Rio Grande from 34.50o N (Veguita) to 33.75o N (south of San Antonio) across the southern Albuquerque Basin, the Socorro magma body, and the relatively narrow Socorro Basin. Complications of interpreting surfaces 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 coarsegrained valley-border alluvium; 3) coarse sediments as terraces from the Rio Salado have affected the course of the Rio Grande and partially buried Rio Grande terrace deposits over several square km; 4) if the historic SMB uplift rate were constant (2-4 mm/yr), a 100,000 year-old terrace tread should be about 200-400 m higher than its present elevation, and older treads should have undergone even more uplift—they have not; 5) 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 (20 m displacement locally confirmed), making estimates of maximum aggradation or stream gradients uncertain; 6) Short-term elastic uplift is generally viewed as domelike, but longer-term uplift could cause sufficient extension to trigger longitudinal collapse of keystone grabens, so terrace heights might not be affected or terraces might subside in places; and 7) The upper crust may be uplifted over short time episodes, but over longer periods, the mid crust may deform ductilely so that the ground surface returns to equilibrated elevations. Implications for the processes of the SMB are A) current uplift may be too recent to be recorded in the terrace succession, or B) current uplift is at a maximum rate, or C) this reach is affected by both subsidence and uplift of the SMB, or D) 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 basins to the north and south within the rift, suggesting that this local area is more active both tectonically and erosionally than the adjacent broad basins. 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.
- Love, D. W.; McCraw, D. J.; Chamberlin, R. M.; Reiter, M.; Connell, S. D.; Cather, S. M.; Majkowski, L., 2009, Progress report on tracking Rio Grande terraces across the uplift of the Socorro Magma Body, in: Geology of the Chupadera Mesa, Lueth, Virgil W.; Lucas, Spencer G.; Chamberlin, Richard M., New Mexico Geological Society, Guidebook, 60th Field Conference, pp. 415-424.