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Cruise History and Purpose |
Mendocino Fracture Zone Cruise, August 2001 |
(Skip to history and background
information) EquipmentDesc: Participants: RegionDesc: Dive2-Anne1 Dive3-Debra2 Dive4-Anne2 Dive5-Bob1 Dive6-Bob2 Dive7-Anne3 |
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The mid ocean ridge is the source of the
driving force behind plate tectonics. At the ridge, which wraps
itself around the world's oceans much like the seams on a baseball,
volcanic activity slowly releases magma from the earth's mantle into
the bottom of the ocean which forms new oceanic crust. Click here
for a moving model showing the motion of the earth's plates. Read more about plate tectonics in this article by
Ken Macdonald
and Paul Fox: Mid Ocean Ridge
Article This effort on the Mendocino Fracture Zone is collaborative with R. Duncan and A. Trehu of Oregon State University. Gabbros collected from the MR were provided by M.Fisk for study in 1999. Multichannel seismic reflection data collected from the GE and the eastern side of the MR show extensive outcrops of bedrock. For the MR these outcrops are likely slabs of Gorda Plate ocean crust accreted to the Pacific Plate. The ROV dives are necessary to resolve the structural continuity of the slabs. The GE outcrops could be similar slabs of ocean crust, or pieces transported northward from the continental margin such as Salinian or Franciscan basement. Alternatively, the steep sided basement slabs imaged by the MCS suggest a diapiric structure such as serpentine. Thus these outcrops have both structural and geochemical interest. In addition, bottom simulating refectors (BSR’s) are observed along the crest of the GE and appear to be truncated in the face of the escarpment, just above a series of vertical (dewatering??) channels obvious on the EM300 bathymetry. Thus these are attractive targets for a diversity of scientific goals. The Mendocino Transform
Zone (MTZ) separates 28-30 my lithosphere of the Pacific
plate from younger (< 25 my) lithosphere of the Gorda plate and
is characterized by a two distinct tectonic components
(image). West of
126 W., the Mendocino Ridge rises to a
depth of 1200 m and is composed of slabs of
ocean crust slivered from the Gorda Plate and accreted
onto the Pacific Plate. East of 126 W., the north facing Gorda Escarpment
separates the Gorda Plate from the anomalously shallow Vizcaino
Block to the south. The Vizcaino Block likely represents accretionary
complex of the North American plate formed prior to subduction
of the Farallon Ridge and subsequently captured by the Pacific
plate after initiation of the San Andreas transform fault. Thus
the Vizcaino Block is the northernmost extension of the
California Borderland tectonic terranes, such as
we have been studying in the Monterey Bay area
(Stakes et al., 1999; Stakes et al., 1998 and in prep).
Previous studies have examined the crustal structure of the Mendocino
triple junction using seismic reflection and refraction techniques.
Submersible and ROV studies have collected basalts from the summit
and gabbros from the north face of the Mendocino Ridge. ODP Site
1022 was drilled 30 km south of the Gorda Escarpment to study the
paleoceanography of the California Current. In May 1999
Debra Stakes and Karen
Salamy participated in an NSF-funded program lead by Anne
Trehu (Oregon State University), that
collected high resolution multichannel seismic reflection
data combined with Hydrosweep multibeam bathymetry data. |
