A PALEOCANYON SOLUTION FOR INTEGRATION AND CARVING OF THE 5-6 MA GRAND CANYON

We propose a resolution of “old” versus “young” Grand Canyon models by elucidating the geometry of two paleocanyon segments that became fully linked during 5-6 Ma integration of the west-flowing Colorado River. “Young” (< 6 Ma) segments are Marble Canyon , Muav Gorge, and westernmost GC. “Old” segments are the (25-15 Ma) East Kaibab paleocanyon and the Paleocene (65-50 Ma) Hualapai paleocanyon. The East Kaibab paleocanyon system breached the Kaibab uplift and likely flowed northwest. Evidence for the 25-15 Ma carving of this segment comes from thermochronology of nearby rim and river samples: rim rocks resided at temperatures of 55-65 °C from 60-30 Ma while river level rocks resided at 75-85 °C; then cooling paths converge after 15 Ma. The collective thermochronologic data indicate that eastern Grand Canyon preserved a normal geothermal gradient and that river samples were still deeply buried until after 25 Ma. Hence Grand Canyon cannot have been incised to near-modern depths in its present location at 70 nor 50 Ma. The Paleocene (65-50 Ma) Hualapai drainage system is hypothesized to have flowed north across the present Grand Canyon along the Hurricane fault system. Evidence for the Hualapai drainage system includes remnant river channels filled with Music Mountain Formation;restoring post-3 Ma west-down slip on the Hurricane and Toroweap faults raises channel heights by ~ 550 m and allows N-flowing paleochannels an outlet near Toroweap valley. We hypothesize that the 2 “old” canyon segments became linked together by integration of the west-flowing Colorado River 5-6 Ma to form the modern Grand Canyon. Hypothesized mantle-driven uplift of the western Colorado Plateau and Rocky Mountains in the last 10 Ma may have promoted downward integration of internal drainages across low topographic divides. Mechanisms of river integration (headward erosion, lake spill over, and groundwater sapping) likely operated together at regional scales in response to broad epeirogenic tilting. After 5-6 Ma, the modern Grand Canyon was carved by the Colorado River and older paleocanyon segments were widened and deepened during semi-steady incision rates of 100-200 m/Ma in the last 4 Ma.