Paleo-Geomorphology of the Sierra Nevada, California, from (U-TH)/He Ages in Apatite

Abstract

New apatite (U-Th)/He ages from the central Sierra Nevada, California, place limits on the morphology and evolution of longitudinal profiles of major transverse river drainages developed in the Late Cretaceous. Helium ages from a new orogen-parallel, constant-elevation sample transect are relatively uniform (∼60 Ma) and not correlated with topography, unlike those from a similar, lower elevation transect approx 15 km to the west. We interpret the marked difference in the two orogen-parallel profiles to reflect a headward decrease in long-wavelength, transverse relief at the time of cooling, consistent with either a concave-up stream gradient, typical of those observed near the headwaters of modern Sierran trunk streams, or a convex gradient like those found along slope breaks of eroding plateau edges. From the San Joaquin drainage we obtained a new helium age-versus-elevation profile, which has a comparable slope to those previously reported for the Yosemite Valley, Kings Canyon, and Mt. Whitney areas. This new profile yields slightly older ages at a given elevation as expected from its position adjacent to the largest major drainage. The age versus elevation profiles imply that the mean denudation rate of the region in the Cenozoic was about 0.04 to 0.05 mm/yr. We show that long-wavelength (λ = 70 km) relief inferred from longitudinal helium age variations is a strong function of erosion rate, such that even relatively subdued relief is detectable with helium age profiling provided that denudation rates were low. Using the rate implied by Sierran age-versus-elevation profiles, we infer long-wavelength relief of 1500 ± 500 m in the Late Cretaceous. By analogy with modern orogenic plateaus, this value of long-wavelength relief suggests a Cretaceous interior Cordilleran plateau lay at an elevation of at least 3000 m.

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