Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125.
Proc Natl Acad Sci U S A. 2014 Jan 7;111(1):57-62. doi: 10.1073/pnas.1312251111. Epub 2013 Dec 16.
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 ((3)He) 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.
地球上和火星上的许多基岩峡谷都是由上游传播的崖壁侵蚀而成的，地貌学和行星科学的一个突出目标是从峡谷形态确定形成过程。然而，过程和形态之间的诊断联系仍然存在很大争议，需要进行野外调查，以确定峡谷形态的控制因素。在这里，我们研究了爱达荷州马拉德峡谷的起源，这是一个切入玄武岩的峡谷系统，有三个非常明显的崖壁：两个有半圆形崖壁，第三个则是活跃的伍德河，其末端是一个 7%坡度的陡坎区。半圆形崖壁的冲刷边缘、残留的急流池、泥沙输送的限制以及宇宙成因 ((3)He) 暴露年龄表明，半圆形崖壁峡谷是由大约 46ka 前的大规模洪水形成的，与南部 18 公里处的博克斯峡谷的形成以及麦金尼巴特山玄武岩的喷发同时发生，这表明在古伍德河的熔岩流改道之后，广泛的峡谷形成。陡坎区峡谷内的暴露年龄表明，在过去至少 33ka 的时间里，阶地的上游逐渐变年轻，并且陡坎区的传播速度为 2.5cm/y。研究结果表明，玄武岩中垂直半圆形崖壁与大规模洪水期间由于块状崩塌而导致的快速侵蚀之间可能存在一种潜在的诊断联系，而不是之前关于渗流侵蚀的解释，这对火星早期水圈的定量分析具有重要意义。
