Dating topography of the Sierra Nevada, California, using apatite (U-Th)/He ages

The upward motion of rock masses relative to the Earth's surface has been documented for most of the main mountain belts using thermochronological and petrological techniques. More fundamental to the physical processes of mountain building, however, is the motion of the Earth's surface itself, which remains elusive(1). Here we describe a technique for estimating the age of topographic relief by mapping the low-temperature thermal structure imparted by river incision using the ages of apatites determined from their uranium, thorium and helium contents. The technique exploits horizontal variations in temperature in the shallow crust that result from range-normal river drainages(2,3), because cooling beneath ancient river valleys occurs earlier than beneath inter vening ridges. Our results from the Sierra Nevada, California, indicate that two of the modern transverse drainages, the Kings and the San Joaquin, had developed deep canyons by the Late Cretaceous period, suggesting that the high topography of the, range is similar to 50-60 million years older than generally thought(4-6).