HISTORY OF QUATERNARY VOLCANISM IN WESTERN GRAND CANYON BASED ON LIDAR ANALYSIS, 40AR/39AR DATING, AND FIELD STUDIES: IMPLICATIONS FOR FLOW STRATIGRAPHY, TIMING OF VOLCANIC EVENTS, AND LAVA DAM STABILITY

Intra-canyon basalt flows in western Grand Canyon provide one of the best records in the world of the interactions among volcanism, river incision, and faulting. A synthesis of geochronology indicates that at least four major episodes of volcanism produced intra-canyon basalt flows: 725-475 ka, 400-275 ka, 225-150 ka, and 150-75 ka. A longitudinal river paleo-profile created from LIDAR data shows well-defined flow tops and bottoms whose slopes can be quantified and used to correlate the now-dissected flow remnants. The combined results of new geochronology, spatial correlation of lava flow remnants, and field studies provide a better understanding of the spatial and temporal distribution of volcanism in Grand Canyon, and leads to new models for the nature and stability of lava dams. 725-475 ka volcanism was most voluminous near the Toroweap fault and produced dike-cored cinder cones on both rims and within the canyon. Reconstruction of the resulting 700-m-high composite edifice created by flows and cinder cone fragments suggests porous dams that would allow infiltration and create local dam instabilities. Large volume eruptions (i.e. Prospect) during this period resulted in far-traveled basaltic lavas (Black Ledge) which flowed down-canyon about 120 km. The inferred presence of lava tubes during their emplacement may have not only allowed the flows to travel long distances, but also could facilitated rapid, post-emplacement, erosion. 400-275 ka volcanism was most voluminous near the Hurricane fault and produced flow stacks that filled Whitmore Canyon and flowed into Grand Canyon near river mile 188. Ar-Ar dating and spatial analysis of remnants has established the resulting Whitmore dam as one of the best-defined in the canyon, with a clear source, downstream limit, and internal structure. Gravels and volcanic debris found under, within and on top of the Whitmore flows indicate prolonged interactions between river aggradation and dam-forming volcanism. While new data indicates that basalt dams varied in structure and longevity, even the most stable blockages seem to have had minor effect on the Colorado River's long term incision rates (50-75 m/Ma in western Grand Canyon). Dam failure models will remain inconclusive until the source, timing, and structure of dam-forming flows is unraveled by ongoing Ar-Ar dating.