--> Abstract: Late Quaternary Deformation of the Southern East Bay Hills, Alameda County, CA, by K. I. Kelson and G. D. Simpson; #90958 (1995).

Datapages, Inc.Print this page

Abstract: Late Quaternary Deformation of the Southern East Bay Hills, Alameda County, CA

Keith I. Kelson, Gary D. Simpson

The southern East Bay Hills provide a means to assess late Quaternary surficial deformation in the area bordered by the northern Calaveras and Hayward faults. The substantial topographic relief of the range is related, in part, to interactions between these two active, predominantly right-lateral structures. Based on mapping of fluvial terraces along Alameda Creek and analysis of range topography, we estimate the pattern, location, and rate of late Quaternary deformation within the southern East Bay Hills.

Alameda Creek, which represents the only subaerial drainage that crosses the structural grain of the range, flows westward across the range within the deeply incised Niles Canyon. The distribution of remnants of four fluvial terraces along Alameda Creek suggest broad anticlinal deformation of the range. Upstream of Niles Canyon, terrace profiles are convergent and suggest subsidence within Sunol Valley. In the eastern part of the canyon, high terraces have flat or slightly eastward (upstream) gradients likely related to uplift of the range. Terrace heights and apparent terrace spacing are greatest in the core of the range. In the western (downstream) part of the canyon, terraces converge and higher terrace-profiles have progressively steeper gradients. Based on a radiocarbon age-estim te of 20 ka for the second-highest terrace and a maximum height of 30 to 34 m, a late Pleistocene uplift rate of 1.5 to 1.7 mm/yr is estimated. A 21-m-high terrace is less than 13 ka, which provides a maximum uplift rate of 1.6 mm/yr. We estimate an average late Pleistocene uplift rate of 1.5 ± 0.5 mm/yr for the southern East Bay Hills.

Generalized topographic (envelope, subenvelope, and residual) maps suggest broad anticlinal deformation of the range as a whole, as well as two distinct loci of uplift north and south of Mission Pass. Because the rim of Niles Canyon is topographically higher than the lowest part of the range at Mission Pass (4 km to the south), deformation of the range may include development of a structural saddle at Mission Pass. The presence of discontinuous gravel remnants near the rim of Niles Canyon, which may be the middle Pleistocene upper Livermore Gravel, suggests that the ancestral Alameda Creek may have begun incising into the range during the middle Pleistocene. Thus, much of the observed deformation of the range may have occurred within the past half-million years or so.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California