LAUSR

In this study, we investigated the geomorphic and incision history for an ~5 km reach of the northern Rio Grande gorge in New Mexico using field and LiDAR-based geomorphic mapping and cosmogenic 3He surface exposure dating. This wide (>1.5 km) and deep (~240 m) section of the gorge exhibits Toreva blocks, incoherent landslides, rock falls, and slumps developed within Servilleta Basalts and intercalated weakly consolidated Pliocene Santa Fe Group gravels. Located deeper in the gorge topographically below the landslides is a flight of six fill and fill-cut terraces (Qt6–Qt1) at 50, 40, 28, 21, 10, and 8 m above the modern river. 3He surface-exposure ages (1-sigma) of multiple samples from each terrace indicate Qt6 was likely abandoned at 69.0 +8.4/−9.2 ka, Qt5 at 36.7 +13.4/−9.0 ka, Qt4 at 26.9 +5.5/−4.2 ka, Qt3 at 25.3 +3.1/−3.2 ka, and Qt2 at 24.3 +7.6/−6.7 ka. We interpret the terraces to record three aggradation-incision cycles during the past ~70 k.y. The most prominent terrace surface (Qt4) falls within MIS 2 and appears to closely track incision associated with Pinedale ice retreat. Previous work suggests that the initiation of gorge incision occurred between ca. 440–800 ka, which suggests average incision rates prior to the formation of the highest terrace (Qt6) of 260–512 m/m.y. Average incision from ca. 70 ka to present appears faster, with maximum rates of ~752 m/m.y. Compared to incision rates for nearby river systems, rates along the Rio Grande are nearly twice as fast over both middle and late Pleistocene to Holocene timescales, suggesting a persistent driving force for incision that is unique to this river system. Rates of dynamic surface uplift and/or slip along basin-bounding normal faults associated with the Rio Grande rift are over an order of magnitude too small to explain the fast incision; thus we suggest the most probable driver of incision is drainage basin (re-)integration and transient knickpoint migration due to the capture of the northern San Luis Basin during the middle Pleistocene, superimposed on a strong climatic signature in the late Pleistocene. ■