DETRITAL SANIDINE DATING OF TERRACES OF THE COLORADO RIVER: TESTING MODELS FOR THE LEES FERRY KNICKZONE AND FOR SURFACE UPLIFT IN THE PAST 5 MA

Differential bedrock incision studies along the length of continental-scale rivers provide insights into both tectonic and geomorphic influences on river evolution. Our river incision dataset from Grand Canyon to the Gulf of California suggests the Colorado River has been graded to sea level since 5 Ma, that upstream increases in incision rates reflect movement on faults, and that cumulative upstream increases may help quantify epeirogenic surface uplift of the Colorado Plateau. We extend this record upstream of Grand Canyon across the Lees Ferry knickzone, a major convexity in the Colorado River system. Models for its origin include: a transient knickzone that originated ~ 5-6 Ma due to integration of the Colorado River across the Grand Wash cliffs, a 500 ka incision transient that is bypassing the Kaibab limestone knickpoint through softer rocks, a bedrock knickpoint that coincides with the Kaibab Limestone, and a convexity reflecting ongoing mantle-driven uplift. New detrital sanidine dating of over 5000 grains, from 27 terraces, yielded ~ 2% young (< 2 Ma) grains that give important new maximum depositional ages (MDAs) and hence minimum incision rates for 18 of the terraces. The most common youngest grains were ca. 0.63, 1.2, and 1.6 Ma. In most reaches, the terraces containing 0.63 Ma grains are ~half as high above the river as the terraces containing 1.2 Ma grains suggesting steady average incision over the past 1.2- 1.6 Ma. Above the knickzone, paradoxically within the lowest Chi-normalized gradient reach of the Colorado River, bedrock incision rates are >250 m/Ma, ~ 100 m/Ma higher over the same time period than rates below the knickzone in eastern Grand Canyon. High (>200 m/Ma) incision rates extend at least to Cataract Canyon and possibly to Glenwood Canyon on the upper Colorado River. Differential incision rate data, plus profile geometries, are difficult to explain via a headwardly eroding transient at Lees Ferry and argue against a knickpoint fixed at the Kaibab Limestone. We propose ongoing mantle-driven uplift of ~ 100 m/Ma of edge of the Colorado Plateau relative to sea level, a similar magnitude of uplift of the Rocky Mountains relative to the Colorado Plateau, and enhanced stream power in the past ~2 Ma, possibly due to downward integration of the Green River across the Uinta Mountains.