Pleistocene to recent geomorphic and incision history of the northern Rio Grande River Gorge, New Mexico : constraints from field mapping and cosmogenic 3He surface exposure dating
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Relationships between river incision, aggradation, and widening are investigated in a ∼5 km reach of the northern Rio Grande River Gorge near the confluence with the Red River in New Mexico using detailed geomorphic mapping and cosmogenic 3He surface exposure dating. This wide (exceeding 1.5 km) and deep (∼240 m on average) stretch of the river exhibits a unique set of cohesive, stacked toreva blocks, incoherent landslides, rockfalls, and slumps developed within ∼3-5 Ma Servilleta Basalts and intercalated weak Pliocene Santa Fe Group gravels. Toreva blocks are best developed along the eastern side of the river — they exhibit coherently-dipping, patinated tops that can be reconstructed to the gorge walls, but with toes that are truncated or buried by later deposits. Located below these landslide features is a flight of fill and fill-cut terraces spanning 6 levels at elevations of 60, 45, 29, 25, 15 and 7 m above the modern river grade, on average. Terraces at 29 m can be correlated across and along the river axis, whereas others are more locally preserved. All terraces exhibit well-defined boulder levees and risers constructed from alluvium sourced from upstream, and rounded and sculpted basalt clasts on their treads. Minimum 3He surface exposure ages of multiple samples from each terrace indicate Qt6 was likely abandoned at 63.4 ± 9.8, Qt5 at 30.0 ± 4.8 ka, Qt4 at 23.8 ± 3.7 ka, Qt3 at 22.7 ± 3.6 ka, and Qt2 at 19.0 ± 3 ka, however the potential for pre- depositional inheritance is evident. Terraces are the youngest preserved deposits in the gorge and record at least three aggradation-incision geomorphic cycles over the past ∼60 ka. If initiation of gorge incision was coeval with capture of the San Luis Valley of southern Colorado at ∼440-800 ka, average incision rates prior to the for- mation of Qt6 were 0.26-0.52 mm/yr. This incision was likely coincident with toreva block formation and substantial gorge widening, as the toreva blocks and large-scale incohesive landslide deposits predate terrace development. Landsliding controls the width of the gorge along this stretch, leading to a 2-2.5x increase in width compared to immediately up- and down-river. These landsliding events were likely primarily driven by undercutting of basaltic bedrock along weaker gravel horizons. Average incision from ∼60 ka to present day appears to have been faster, with maximum average rates of ∼1.0 mm/yr from Qt6-modern river. Gorge narrowing is observed during this time period, with only minor widening accommodated by slumping and incoherent landsliding that post-date terrace treads. This period of incision was punctuated by aggradational events that roughly correlate with regional climatic events and/or late Pleistocene MIS climate cycles that increase in amplitude towards present day, with terraces incised into and abandoned during transitions from glacial to interglacial climate. Rates of surface uplift from dynamic topography and/or active slip rates along basin bounding normal faults are over an order of magnitude too small to explain the observed incision rate increase over time. These increasingly rapid incision rates mirror the phenomena seen in other western US river systems since the Pliocene.