EVOLVING A SKY ISLAND: SOIL PRODUCTION, CATCHMENT MEAN EROSION RATES, AND PROCESSES IN THE PINALENO MOUNTAINS, SE ARIZONA

The sky islands are mountain ranges rising thousands of feet above the desert floor in the southwestern United States. They were formed during late-stage Basin and Range extension on high-angle normal faults. In some cases their morphology is characterized by steep, escarpment-like slopes separating low relief, gently rolling hills and plains of surrounding sedimentary basins and a low-relief, gently sloped high elevation surface comparable to a plateau or mesa. Such low-relief upland surfaces are enigmatic – the processes and conditions leading to their formation are debated. We focus here on the Pinaleno Mountains of southeastern Arizona, a gneissic metamorphic core complex uplifted by high-angle faulting about 12 – 5 Ma. With 2500 m of relief, they are a stark climatic and topographic contrast from the surrounding Sonoran Desert. Previous work by Jungers et al. (2016; GSA Bulletin 128: 469-486) reported 11 cosmogenic ¹⁰Be-based average erosion rates ranging from 30 to 60 m/Ma with weak relationships to modern topography. The catchments sampled ranged in drainage area from 2 to 37 km² with mean basin slopes of 18 to 29 degrees. We build on this dataset with 9 new average erosion rates from catchments primarily from the low-relief high elevation basins, ranging from < 0.5 to 13 km² in area with mean basin slops of under 10 to 35 degrees. The combined dataset suggests that the rocky and steep side slopes of the Pinalenos are eroding about twice as fast as the high-elevation, low-relief soil mantled landscape on top of the range: about 40 m/Ma compared to about 20 m/Ma. The combined catchment average data now show a clear increase in erosion rate with mean catchment slope, mean channel steepness, and mean local relief. We explore this further through quantifying the soil production rates across the range. We report ten cosmogenic ¹⁰Be-based soil production rates from the saprolite beneath soils ranging in thickness from 5 to 90 cm and find no depth-dependence in these rates as is widely found for other soil-mantled landscapes. Soil production rates range from about 20 m/Ma to about 50 m/Ma, overlapping well with the catchment-averaged erosion rates. We use these rates, high-resolution topographic data and climate records to forward a model for the post-tectonic landscape evolution of a high-elevation, low-relief sky island.