Autori: Cecil, M.R., Ducea, M.N., Reiners, P.W., and Chase, C.G.

Editorial: Geological Society of America Bulletin, 118, p.1481-1488, 2006.

Rezumat:

Apatite and zircon (U-Th)/He ages from a 100-km long range-perpendicular
transect in the northern Sierra Nevada, California, are used to constrain the exhumation
history of the range since ca. 90 Ma. (U-Th)/He ages in apatite decrease from 80 Ma
along the low western range flanks to 46 Ma in the higher elevations to the east. (U-
Th)/He ages in zircon also show a weak inverse correlation with elevation, decreasing
from 91 Ma in the west to 66 Ma in the east. Rocks near the range crest, sampled at
elevations of 2200-2500 m, yield the youngest apatite helium ages (46 – 55 Ma), whereas
zircon helium ages are more uniform across the divide. These data reveal relatively rapid
cooling rates between ~ 90 and 60 Ma that are consistent with relatively rapid
exhumation rates of 0.2 – 0.8 km/My, followed by a long period of slower exhumation
(0.02 – 0.04 km/My) from the early Paleogene to today. This is reflected in the low-
relief morphology of the northern Sierra Nevada, where an Eocene erosional surface has
long been identified. A long period of slow exhumation is also consistent with well-
documented, widespread lateritic paleosols at the base of Eocene depositional units.
Laterites preserved in the northern Sierra Nevada are the product of intense weathering in
a (sub)tropical environment and suggest an enduring, soil-mantled topography. We
interpret this exhumation history as recording a late Cretaceous to early Cenozoic period
of relatively rapid uplift and unroofing followed by tectonic quiescence and erosional
smoothing of Sierran topography through the Neogene. Well-documented recent incision
appears to have little effect on (U-Th)/He ages, suggesting that less than ~ 3 km has been
eroded from the Sierra Nevada since the early Pliocene.

Cuvinte cheie: Sierra Nevada, tectonics, exhumation, paleomorphology, thermochronology