Evolution of a Tectonically Active Coastline: Lessons from Santa Cruz, California

By

Robert S. Anderson

University of California at Santa Cruz, Santa Cruz, CA

Lesley A. Perg

University of Minnesota, Minneapolis, MN

Peter N. Adams

University of California at Santa Cruz, Santa Cruz, CA

Michael G. Loso

University of California at Santa Cruz, Santa Cruz, CA

Nan Rosenbloom

National Center for Atmospheric Research, Boulder

Kirsten M. Menking

Vassar College, Poughkeepsie, NY

The marine terraced geomorphology of an actively uplifting coastline can be used both to deduce the rates and pattern of uplift, and the rates and processes involved in landscape evolution. I will summarize our work in one such landscape, near Santa Cruz, California, making use of the marine terraces in many ways. Using the elevational pattern of the flight of five well-developed terraces, we deduce the activity on two faults that appear to be primarily responsible, the Loma Prieta and San Gregorio faults. While the pattern of uplift is well explained by repeating earthquakes on these faults, constraint on the rates of uplift and hence the rates of slip on the faults requires a knowledge of the terrace ages.

I address the degradation of the marine terraces subsequent to their removal from the littoral zone. This occurs by both decay of the seacliffs and by incision of stream channels into bedrock. Evolution of the seacliffs from nearly vertical cliffs into rounded forms higher in the landscape is not a simple diffusive process. Not only must debris be freed by weathering from the bedrock in order to be transported, but the transport processes in this fully vegetated landscape are dominated by the activity of rodents. While these animals move material downslope, making their role in regolith transport to first order diffusive, their occupation of the landscape is not uniform; it varies with both slope and regolith thickness. Again, while the general pattern of evolution can be reproduced in a numerical model, constraint on the rates requires knowledge of the ages of the seacliffs.

I summarize our work on establishing the ages of the marine terraces using profiles of cosmogenic radionuclides (CRNs) in cover deposits on the terrace flats.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming