Geomorphic surfaces of the Rio Grande Valley in the vicinity of Socorro, New Mexico

A bstrAct — The sides of the Rio Grande Valley north and south of Socorro, New Mexico, exhibit readily apparent, sloped, quasi-planar surfaces (“terraces”). These have only recently been systematically mapped, characterized, and dated. The dating confirms that the terraces decrease in age with decreasing height above the Rio Grande. The highest terrace, the Las Cañas surface, represents the land surface at the time of maximal aggradation of the Rio Grande, at about 800 ka. At this time, the top of the basin fill was about 100 m higher than the modern Rio Grande. The surface about 20 m below it, the Tio Bartolo, stabilized at ca. 610 ka, near the 621 ka termination (Lisiecki and Raymo, 2005) of a very intense global glaciation (MIS 16). Forty meters below it, the Valle de la Parida surface formed ca. 135 ka, also at the end of an exceptionally intense glacial interval. Below the Valle de la Parida are three more terraces, two of which formed due to fluvial incision at the start of a glacial paleoclimate and the youngest within an interglacial. This pattern supports a model originally advanced by John Hawley, which proposed glacial/interglacial transitions as a control on terrace formation. The Rio Grande rift in the vicinity of Socorro is currently undergoing uplift as a result of injection of magma into a mid-crustal sill. A longitudinal profile of the terraces along the river shows two features that may be related to earlier magma injections. The first is an apparent ‘bulge’ of about 50 m in the two highest terraces, near San Acacia, the current area of most rapid uplift. The second is a more subtle rise in terrace elevations, amounting to 10 m, across the southern boundary of the current magma body. These both suggest that magma injections in the geological past may have produced upward topographic deflections, but additional data and examination of alternative explanations are needed before they can be considered solid evidence of such events.

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