Quaternary Basalts in Grand Canyon: Correlation of Flows Using Lidar, 40Ar/39Ar Dating, Geochemical Correlation, Neotectonic Studies, and History of Lava Dams

In western Grand Canyon, basalts flowed into the already existing canyon from at least 719 ka to present. These basalts provide a key for deciphering Quaternary rates of incision, neotectonic slip rates on the Toroweap and Hurricane faults, and the history of lava dams. Stratigraphy and/or inset relationships between basalt flows is exceedingly complex because of the multiple eruptions, extreme topography, long transport distance, and incomplete preservation. Correlation of flows using 40Ar/39Ar dating, LIDAR data, orthophotography, and geochemical analysis, as well as structural and geomorphic studies, lead to a working hypothesis for four major episodes of basaltic eruptive activity. From 719 to 484 Ma major volcanoes erupted near the Toroweap fault zone. The extent of the remnants and presence of 527 ka dikes indicates that cones where built within Grand Canyon during this phase. These flows had the longest outflow (719 ka flow at mile 246). At 349 to 296 ka flows seem to have entered the canyon at Whitmore Wash, north rim. The two remaining episodes, dated at 199-193 ka and 109-97 ka are interpreted to have cascaded into the canyon at and upstream from Whitmore Wash. LIDAR/orthophotography interpretation of the tops and bottoms of the flows and geochemical analysis of phenocrysts aid in correlation of undated remnants and reconstructing the shape of volcanic edifices. Flows dated from 720-450 ka include Prospect, Black Ledge, D-Dam, and Toroweap flows, thus Black Ledge flows are considerably older than previously thought. The 350 to 300 flows include Whitmore, Layered Diabase, Massive Diabase, and 177-mile flows. All the dated 200 and 100 ka flows have been called Grey Ledge flows, suggesting that the Grey Ledge represents two distinct events. Basalt data indicate an interaction of canyon incision and Quaternary fault slip. Bedrock incision rates are calculated using dated flows that overly bedrock straths. Rates vary across active faults indicating fault dampening of apparent river incision rates. Incision rates for eastern Grand Canyon are 127 m/my over 387 ka. Similar rates just east of the Toroweap faults (136 m/my over 349 ka and 153 m/my over 484 ka) suggest that a fairly uniform regional rate of ~ 140 m/my can be considered the average rate for Grand Canyon incision east of the Toroweap fault. This is subequal to the sum of apparent incision rate just below the Toroweap fault (56 m/my over 484 ka) plus fault slip rate (90 m/my over 550 ka). Similarly, apparent incision rates below the Hurricane fault (66 m/my over 527 ka and 76 m/my over 604 ka, near Granite Park) plus fault slip rate (~80±20 m/my over 185 ka) is subequal to the far field incision rate. Accumulating data suggests that apparent incision rates are lowest adjacent to faults in the hanging wall and highest adjacent to faults in the footwall, with rates varying systematically across fault blocks. This suggests that faulting is taking place by domino rotation of blocks bounded by normal faults with mild listric character. These new empirical data help constrain physical models for Quaternary fault slip across the active Colorado Plateau- Basin and Range bounding structures.