ABSTRACT

Ephemeral channels incise into the piedmonts (both alluvial fans and pediments) of the northeastern Sonoran Desert, USA. Located around metropolitan Phoenix, this tectonically quiescent region experienced only aggradation in endorheic structural basins throughout the Pliocene. A wave of aggradation then followed Salt and Gila river integration at the start of the Pleistocene. Aggradation of piedmont base levels continued throughout the rest of the Quaternary. This paper explores two hypotheses to explain piedmont incision despite rising base levels. The classic explanation is that incision is part of the evolution of desert mountain ranges as they decrease in size. A new alternative we propose here involves a lateral shift in base level from Pliocene endorheic basin playas to positions kilometers closer to range fronts in response to river integration. We present a thought exercise of modeling a pediment longitudinal profile as a 1D diffusive system, and we also analyze incision into alluvial fans of the Sierra Estrella range. While our 1D modeling results for pediments are consistent with both explanations for range-front incision, Sierra Estrella bajada incision is best explained by the sudden relocation of the base level to the toe of desert piedmonts.

摘要

短暂通道切入美国索诺兰沙漠东北部的山前（包括冲积扇和山墙）。这个构造静止的区域位于凤凰城周围，仅在整个上新世的内流构造盆地中经历了加聚。在更新世开始时，盐和吉拉河合并之后出现了一波加积。在整个第四纪的其余时间里，山前基线水平的加积持续。本文探讨了两种假设来解释山前切口尽管基础水平上升。经典的解释是，随着沙漠山脉尺寸的减小，切口是沙漠山脉演变的一部分。我们在这里提出的一个新的替代方案涉及从上新世内流盆地平原向距离山脉前沿数公里的位置横向移动，以响应河流整合。我们提出了将前冲纵向剖面建模为一维扩散系统的思想练习，我们还分析了 Sierra Estrella 范围冲积扇的切口。虽然我们对山墙的一维建模结果与对范围前切口的两种解释一致，但 Sierra Estrella bajada 切口的最佳解释是基础水平突然重新定位到沙漠山前的脚趾。