Thursday, May 14, 2015

Notes on a Few West Coast Bathymetry Features

Taking a look at bathymetry off the west U.S. coast one can see and contemplate the ocean floor features some of which have been side scanned to remarkable resolution.

Ocean floor off northwest Washington Coast
 
Strait of Juan de Fuca in the northeast of image has a clear link to a current pathway that funnels water currents down to submarine canyon. Other submarine canyons pull sediment via density currents to the deep ocean floor to the west. The ridges on the lower slope above the deep ocean floor are the result of the under thrusting of the ocean floor under the continent. These ridges are the expression of the mega earthquakes of the past as well as a telling indicator of what we might expect in the future. 
 
Ocean floor off southwest Washington coast
 
The estuary of the Columbia River is on the southeast part of the image with Willapa Bay to the north. Several prominent canyons and even a fan on the deep ocean floor from the Columbia. The off shore thrust ridges are also prominent.
 
Northern California coast

There are still prominent canyons off the California coast, but note the ridges are not present. No subducting plate here. California has a different fault regime - strike slip along the San Andreas system.

Mendocino Fracture Zone is the sharp east-west ridge
 
The Mendocino Fracture Zone marks the boundary between the convergent plate boundary to the north and the strike-slip boundary on the south. The north-south ridge extending from the fracture zone is the ocean spreading ridge off the northern California coast - the Gorda Ridge with the Gorda ocean plate to the east of the ridge and the Pacific Plate to the west. The Gorda ocean plate is decidedly not smooth reflecting internal deformation within the plate perhaps the result of its being in a bit of bind between larger plate motions.

One of the striking things of the Mendocino Fracture Zone is the ocean floor to the south is much deeper. This is a significant factor in how tsunamis are transmitted to the coast that creates a rather unfortunate circumstance for Crescent City, California by focusing wave energy towards the California coast at that location.

A bit a field from the Washington coast but something to consider in the concept of accreted terranes (of which Washington is full of) is a set of ocean seamounts off the central California coast - Taney Seamounts. This is a set of basalt volcanoes described by Clague, Reynolds and Davis (2000). The studies have produced some relatively high resolution bathymetry of these seamounts as well as a few other volcanic seamounts not directly associated with spreading ridges or hot spots seamounts.

This set of peaks would present a bit of a problem if they encountered a subduction zone. Something to consider when contemplating accretion complexes.

1 comment:

nwgeology said...

Dan and friends,
I discuss many of these toics in my new geology book, Geology Underfoot in Western Washington. The problem of seamount subduction is in the chapter on Olympic Peninsula geology; sea floor off the mouth of the Columbia is in a chapter devoted to coastal change. Order the book from your book store. Dave Tucker nwgeology.wordpress.com