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Events on August 19 2008

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Events for August 2008

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08-05-2008

Dick Heermance, U.S. Geological Survey, TOPIC: Terrace formation and climate change in southern California

Abstract

The western transverse ranges in southern California contain impressive, steep mountain fronts, abandoned fluvial terraces, and fault scarps. Although it is often assumed that these landscape features develop over geologic timespans of 10e6 or 10e7 million years, our new data indicates that this hill-and-valley topography developed over only a few tens of thousands of years, and that rapid uplift has combined with climate shifts to control the landscape evolution. New 10Be cosmogenic ages combined with geologic mapping in Ojai Valley, CA, indicate that most of the fluvial terrace straths formed either 10-12 or 25-35 thousand years ago, at times when climate was rapidly shifting from cooler (dryer) to warmer (wetter) conditions. These climatically controlled erosional periods combined with rapid uplift from the incipient faulting in the region have preserved a record of both climate change and tectonic uplift. Calculated uplift rates are twice as fast as previously published. New mapping indicates that structurally controlled micro-basins developed within the late Quaternary, but were rapidly deformed and beveled flat over the last 50 thousand years. This work underscores how the landscape can completely change over short (10e5 years) timespans due to rapid climatic and tectonic events.

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