415 PROGRESS REPORT ON TRACKING RIO GRANDE TERRACES New Mexico Geological Society Guidebook, 60 th Field Conference, Geology of the Chupadera Mesa Region, 2009, p. 415-424. PROGRESS REPORT ON TRACKING RIO GRANDE TERRACES ACROSS THE UPLIFT OF THE SOCORRO MAGMA BODY D.W. LOVE 1 , D. J. MCCRAW 1 , R. M. CHAMBERLIN 1 , M. REITER 1 , S. D.CONNELL 1 , S. M. CATHER 1 , AND L. MAJKOWSKI 2 1 NM Bureau of Geology & Mineral Resources, NM Institute of Mining & Technology, 801 Leroy Place, Socorro, NM 87801, dave@gis.nmt.edu 2 Earth & Environmental Science, New Mexico Institute of Mining & Technology, Socorro, NM 87801 ABSTRACT—Although the ground surface above the Socorro magma body (SMB) is rising at rates as high as 3 mm per year his- torically, 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 34.50 o N (Veguita) to 33.75 o 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 coarse- grained 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 suc- cession, 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. INTRODUCTION The Socorro magma body (SMB) is interpreted to be a large, pancake-shaped body of magma that is uplifting the ground surface in the Socorro region of New Mexico (Sanford and Long, 1965; Reilinger and Oliver, 1976; Reilinger et al., 1980; Reinhart and Sanford, 1981; Larsen et al., 1986; Ake and Sanford, 1988; Schlue et al., 1996; Balch et al., 1997; Fialko and Simons, 2001; http:// www.ees.nmt.edu/Geop/magma.html; Finnegan and Pritchard, 2009). This body extends over an area of 3,400 km 2 at a depth of 18.75 km (Figures 1, and 2), and is estimated to be 100-150 m thick. The rate of inflation at the center of the uplift near San Acacia has been estimated using conventional geodetic surveys and resurveys yielding an uplift value of 1.8 mm/yr (Larsen et al., 1986) and using Interferometric Synthetic Aperture Radar (InSAR) from satellites yielding a value of 2.5-4 mm/yr (Fialko and Simons, 2001; http://igpp.ucsd.edu/~fialko/research5-6.html; Finnegan and Pritchard, 2009). A recent GPS survey (Newman et al., 2004) indicated short-term uplift of 20 mm/yr, a possible small, shallow magma body, and the possibility of magmatic “breathing.” The region surrounding the magma body appears to be subsiding. Uplift is accompanied by increased seismic activity known as the Socorro Seismic Anomaly (Sandford et al., 1995; 2002). Bachman and Menhert (1978) suggested that uplift by the SMB had affected the attitude of Pliocene lava flows and sedi- ments on the east side of the Rio Grande between San Acacia and Socorro. Ouchi (1983, 1985) investigated the effects of uplift on stream behavior and interpreted the gradient of the Rio Grande and its adjacent Pleistocene terraces to show that the SMB is affecting the present stream gradient. He suggested that uplift has continued for some time and has warped a least one of the ter- races. Ouchi adopted the terrace nomenclature of Denny (1941) and mapped some surfaces along the Rio Grande from La Joya to San Acacia, New Mexico. He interpreted uplift of one terrace as much as 36 m in the reach he studied. Ouchi’s conclusions have been used for the past 25 years in discussions of magma body age and emplacement behavior. The area including the SMB has a complex geologic history and several on-going and interrelated geologic processes that should be considered under multiple working hypotheses as par- tial explanations for the attitudes and elevations of terraces. Tec- tonic, erosional, and sedimentologic processes may play roles. Not only is this area undergoing uplift, it is also being extended and broken into multiple tilted blocks of the Rio Grande rift where the Southern Albuquerque structural basin merges with the Socorro and La Jencia basins, and the northern Jornada Basin (Fig. 2). Older tectonic fabrics such as the Socorro accommoda- tion zone continue to influence local tectonics. Two major tributary streams join the Rio Grande in this area, the Rio Puerco (the most sediment-laden stream in the US) and