CRETACEOUS-CENOZOIC LANDSCAPE EVOLUTION OF THE SW U.S.A.: UPLIFT AND EROSION OF THE SIERRA NEVADA

The Sierra Nevada of California is the longest, tallest mountain range in the co-terminus U.S., and has long been regarded as a very young (< 6 Ma) range; however, recent work has provided evidence that the range is very old (> 80 Ma), and represents the western shoulder of a Tibetan-like plateau (the “Nevadaplano”) that was centered over Nevada. A great deal of effort has been invested in applying modern laboratory and geophysical techniques to understanding the Sierra Nevada, yet the most unambiguous constraints on Sierran landscape evolution are derived from field studies of dateable Cenozoic strata preserved in paleochannels/paleocanyons that crossed the Sierra Nevada in Cenozoic time. Our work in the Sierra Nevada shows that neither end-member model is correct for the debate regarding youth vs. antiquity of the range. Many features of the Cenozoic paleocanyons and paleochannels reflect the shape of the Cretaceous orogen, but they were also affected by Cenozoic tectonic and magmatic events.

In the central Sierra Nevada, the inherited Cretaceous landscape was modified by three Miocene uplift and faulting events, each followed by 3 – 5 million years of subduction magmatism and sedimentation. These uplift events produced unconformities that did not cut down below the Cretaceous unconformity, because base level was raised in the Central Valley by the construction of a very thick nonmarine volcaniclastic wedge. The first uplift event, at about 16 Ma, corresponds to the westward sweep of the Ancestral Cascades arc front into the Sierra Nevada. The second uplift event, at about 11 Ma, records the birth of the “future plate boundary” by transtensional faulting and voluminous high-K volcanism at the western edge of the Walker Lane belt. The third uplift event, at about 8 Ma, is associated with renewed range-front faulting in the central Sierra. By analogy with the ∼11 Ma event, we speculate that high-K volcanic rocks in the southern part of the range mark the inception of yet a fourth pulse of range front faulting, at 3-3.5 Ma.

Taken together, field and geochemical data suggest that that the edge of the Cretaceous “Nevadaplano” decreased in elevation northward between the central and northern Sierra, and that its edge curved northeastward through northwest Nevada.