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Many bedrock canyons on Earth and Mars were eroded by upstream propagating headwalls, and a prominent goal in geomorphology and planetary science is to determine formation processes from canyon morphology. A diagnostic link between process and form remains highly controversial, however, and field investigations that isolate controls on canyon morphology are needed. Here we investigate the origin of Malad Gorge, Idaho, a canyon system cut into basalt with three remarkably distinct heads: two with amphitheater headwalls and the third housing the active Wood River and ending in a 7% grade knickzone. Scoured rims of the headwalls, relict plunge pools, sediment-transport constraints, and cosmogenic (3He) exposure ages indicate formation of the amphitheater-headed canyons by large-scale flooding ∼46 ka, coeval with formation of Box Canyon 18 km to the south as well as the eruption of McKinney Butte Basalt, suggesting widespread canyon formation following lava-flow diversion of the paleo-Wood River. Exposure ages within the knickzone-headed canyon indicate progressive upstream younging of strath terraces and a knickzone propagation rate of 2.5 cm/y over at least the past 33 ka. Results point to a potential diagnostic link between vertical amphitheater headwalls in basalt and rapid erosion during megaflooding due to the onset of block toppling, rather than previous interpretations of seepage erosion, with implications for quantifying the early hydrosphere of Mars. [ABSTRACT FROM AUTHOR]

Abstract: 3He concentrations were measured in zircon, apatite and pyroxene separates from rhyolite surfaces along the Snake River near Twin Falls, Idaho. These fluted and potholed surfaces were presumably scoured by the Bonneville outburst flood ∼17.5ka. Several of the samples contain inherited 3He from prior exposure, reflecting the complex incision history of the Snake River canyon and suggesting earlier flood events. Each individual mineral from the remaining surfaces yielded 3He concentrations that are within error of each other. 3He produced by neutron capture on 6Li was established from shielded samples and a simple neutron production model. The remaining 3He is spallogenic and was used to calibrate the production rate of cosmogenic 3He in zircon, apatite and pyroxene by assuming minimal erosion since the Bonneville flood. In all three phases the resulting production rates are ∼7–12% lower than previous 3He production rate estimates obtained by reference to various production rates for 10Be in quartz. This disagreement is partially reconciled here by recalculating previous results with a 10Be production rate of 4.51 at g−1 a−1 in quartz. Adopting this revised rate brings three independent 3He production rate estimates for zircon and apatite into agreement within 5%, with grand means of 103±3 at g−1 a−1 in zircon and 133±6 at g−1 a−1 in apatite. The major source of uncertainty in cosmogenic 3He dating of these phases is now correction for 3He produced by neutron capture on 6Li. Calculations are presented for assessing the amount of uncertainty introduced by this correction as a function of Li concentration, cosmic-ray exposure age, and He closure age. [ABSTRACT FROM AUTHOR]