POST-PLIOCENE UPLIFT OF THE SIERRA NEVADA, CALIFORNIA

Abstract

Matthes determined the position of three ancestral stages of the upper Merced River by projecting the gradients of hanging-valley tributaries to points above the present main stream. From the gradients of the two older stages he concluded that the Sierra Nevada range had been tilted westward, resulting in 9000 feet of uplift of the range crest at the head of the river, since the establishment in late Miocene time of the oldest, broad-valley, stage. Axelrod's work, based on paleobotany, indicates that at Granite Chief, on the summit of the range, about 100 miles north of the Merced River, the uplift has been about 5505 feet since late Miocene time. In a redetermination of the old Merced channels by Matthes' method, the broad-valley stream is traced to the western edge of the range and thence into the ancestral San Joaquin Valley, by way of a pediment. A new calculation of the amount of tilting undergone by the range since broad-valley time, now considered early Pleistocene, is based on the assumption that certain well-graded stretches of the ancient stream had pretilt slopes equal to those of subjacent, well-graded parts of the modern river. The amount of tilting in the pediment region is determined by conventional geologic methods. The calculations indicate that the summit of the range, at the head of the Merced River, has been raised only 3930 feet since broad-valley (= early Pleistocene) time. The same methods applied to the Middle Fork of American River indicate that the contemporaneous uplift at Granite Chief was 2054 feet. Results of calculations made by other physical methods indicate the uplift at this place to have been between 2289 and 2745 feet since middle Eocene time and 1946 feet since the early Pliocene.