Gorda Escarpment

The primary objective of the Gorda project was to determine whether and when the Gorda Escarpment had been above sealevel in the past. A beach now located at 1800 m water depth had been observed earlier further to the west, where no sediments drape the Escarpment. In 1999 we mapped the sediments draping the Escarpment by collecting seismic reflection data on the R/V Ewing. We also collected bathymetric, gravity and magnetic data. In 2000 and 2001, we returned on the R/V Western Flyer to collect samples of outcropping sediments and basalts with the ROV (remotely operated vehicle) Tiburun. The map below left shows the location of the project offshore Cape Mendocino in northern California along with the boundaries between the plates in this region and the lines along which seismic data were acquired. A 3D view of the seafloor topography is shown below right.

Last Updated 20-JUL-2003 - Under Construction!

The Gorda Escarpment is the eastern end of the Mendocino Transform Fault, which separates the Gorda and Pacific Plates. The Mendocino Triple Junction, where the Gorda, North America, and Pacific plates come together is located beneath Cape Mendocino. This region was the focus of an earlier onshore/offshore seismic program in 1993-1994 (link).

An example of seismic data is shown on the above left. By bouncing sound waves off the seafloor and underlying geologic boundaries, we construct an image of the subsurface. This image shows that uplift of basement rocks has pushed sediments upward. Younger sediments filled in around the ridge and are draped over the older sediments. The photograph on the right shows a sample being recovered by the arm on the ROV. By mapping the different subsurface geologic boundaries and tying these boundaries to samples recovered along the Escarpment, we have been able to reconstruct the geologic history of the Escarpment. We have concluded that 2.7-3.7 million years ago the shallowest parts of the Escarpment were above sealevel, forming a chain of islands offshore California.

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During the cruise in 1999, we were surprised to see evidence in the seismic data of gas hydrates in this environment. Gas hydrates had not previously been reported from transform faults. The strong subsurface reflection labeled BSR in the seismic image on the right is a common fingerprint of the presence of gas hydrate. The picture on the left shows the arm on the ROV taking a sample of tube worms from a vent associated with this feature. DNA analysis of these tube worms shows that they are similar to tube worms found associated with methane vents elsewhere on the California margin.

The rock environment of the Escarpment provides habitat for a wide range of deep sea creatures (above)

Some Publications

Stakes, D., A.M. Trehu, S.K. Goffredi, T.H. Naehr, R.A. Duncan, Mass wasting , methane venting, and biological communities on the Mendocino transform fault, Geology, v. 30, p. 407-410, 2002.

Trehu, A.M., D.S. Stakes, C.D. Bartlett, J. Chevallier, R.A. Duncan, S.K. Goffredi, S.M. Potter, K.A. Salamy, Seismic and seafloor evidence for free gas, gas hydrates, and fluid seeps on the transform margin offshore Cape Mendocino, J. Geophys. Res., 108, 2263, doi:10.1029/2001JB001679, 2003. (PDF)

Leitner, B., A.M. Trehu, N. Godfrey, Crustal structure of the northwestern Vizcaino Block and Gorda Escarpment offshore northern California: Implications for the influence of plate age on internal deformation, Jour. Geophys. Res., v. 103, p. 23,795-23,812, 1998. (PDF)