Block Glides Offshore Newport Beach, Southern California Continental Margin

H. Gary Greene, Samuel H. Clarke, Jr., Michael P. Kennedy

The continental slope offshore Newport Beach, California, is characterized by a relatively gentle (approximately 1°) grade and is dissected by numerous channels and canyons, of which the most conspicuous is Newport Canyon. An unusual series of block-glide landslides have developed on this slope adjacent to many of these channels. Locally, secondary channels that develop along pull-apart fractures between the slide blocks may service as conduits for downslope sediment movement.

A detailed seismic-reflection survey of the area shows that the slope is underlain by soft water-saturated unstable sediment of Quaternary age. The block-glides lie wholly within this sediment; displaced blocks appear to have moved only a short distance downslope and are preserved as intact masses that exhibit downward increasing internal deformation. This deformation reaches a maximum near the front of the displaced mass and in basal beds nearest the slip surface. The morphology of the blocks and their intervening channellike erosional scarps is similar to that of glacial blocks and their associated bergschrunds.

The formation of new scarps and the widening of channels formed as pull-aparts by the ongoing process of block movement may contribute to headward erosion and widening of Newport Canyon and its tributaries. Slope failure might be greatly enhanced by strong ground motion associated with nearby earthquakes. We suspect that renewed movement occurs on these blocks during major seismic events on the nearby Newport-Inglewood fault (e.g., 1933 M_L 6.3 event).

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.