MEASUREMENT OF THE DEFORMATION OF THE SIERRA NEVADA, CALIFORNIA, SINCE MIDDLE EOCENE
The main channel of the middle Eocene Yuba River is known from its mouth, a short distance east of the western edge of the Sierra Nevada, to the present range crest, immediately northwest of Donner Pass. Abrupt changes of gradient between variously directed reaches of this channel led Lindgren to conclude that the range had been tilted 60 feet per mile west-southwest as a rigid block. Faulting east of the summit seemed insufficient to account for such increase of slope, so he proposed uplift of an area greater than the present range with local sinking of moats. A corollary, not stated by him, is that both moats and mountains suffered absolute uplift. The aggregate downthrow of faults east of the crest is actually 3550 feet, an amount more than enough to account for the 3300-foot uplift of the summit caused by a simple tilt of 60 feet per mile. A study of Lindgren9s basic data and new information shows that uplift at Donner Pass was less than 2000 feet, or little more than half the sum of the downthrows on faults east of the pass. The deeper grabens east of the range must have suffered absolute depression since middle Eocene. The uplift of the Sierra Nevada is not due to simple tilting of a rigid block. Gradient calculations indicate five zones of deformation on the western slope. Field studies confirm two of these, the more important being a north-trending fault zone situated about midway between the summit and the western edge of the range and extending 12 miles, from Forest Hill to near Dutch Flat.