PALEOTOPOGRAPHY TIED TO LATE BREAKUP OF RODINIA IN THE GRAND CANYON REGION

The Grand Canyon Chuar Group and other Neoproterozoic sedimentary successions in western North America have often been correlated with late Rodinian breakup c. 700 Ma. Although these sedimentary rocks are preserved only as isolated occurrences, it has been proposed that they were once continuous, reflecting deposition in an intra-continental seaway that deepened to the west. Here, we present new zircon (U-Th)/He thermochronologic data from the Upper and Lower Granite Gorges of the Grand Canyon that indicate that these two closely-adjacent areas experienced very different thermal histories, thus implying very different burial and exhumation histories during the Neoproterozoic. These results cast doubt on the intra-continental seaway hypothesis. The Lower Granite Gorge gives reproducible, consistent dates of exhumation as old as 750 ± 10.65 Ma, whereas samples within the Upper Granite Gorge yield more heterogeneous results with a range of maximum dates from 360 ± 5.60 Ma to 832 ± 8.12 Ma. Resetting during Phanerozoic burial can explain the distribution of younger dates in these samples, but it cannot account for the spatial heterogeneity in the older ZHe dates because Phanerozoic burial did not vary abruptly over the short geographic wavelengths of those data. We interpret the difference in thermochronologic data between the two gorges and within the Upper Granite Gorge to reflect small-scale paleotopography that formed as a result of patchwork faulting during the break-up of Rodinia.