Monterey Bay Aquarium Research Institute

Mendocino Fracture Zone Cruise
August 24 - 31, 2000
Over 300 km off the California-Oregon Coast
Cruise History and Purpose


(Skip to history and background information)

Cruise Dates: Aug 24, 2000 - Sept 2, 2000
Cruise Location: Mendocino Fracture Zone/Gorda Escarpment
Ship: R/V Western Flyer
Chief Scientist: Debra Stakes
Scheduled Start Time: Thursday 24 August 2000 000 UTC
Scheduled End Time : Saturday 2 September 2000 0300 UTC
Planned Transit: Mendocino Fracture Zone (at a latitude of approximately 40'23"N, and between longitudes of 126' 30" W to 125' 00" W). Specific dive tracks already provided to DMO.
Required Equipment: First three to six dives will use the Tiburon drillsled with the sediment pushcorers attached. Remaining dives will use standard toolsled with drawer, two manipulators, sediment pushcorers, and low-temperature vent temperature probe. Night Ops will be limited.
Participants: MBARI: Debra Stakes, Karen Salamy, Bethany Schaarschmidt-Ames, Paul McGill. Oregon State: Bob Duncan, Anne Trehu, Martin Fisk, Pete Kalk, Allen Throop, sedimentologists TBA.

 

Purpose

This cruise is a collaborative effort to study the tectonic history and structure of the northernmost extent of the San Andreas System, as exposed along the Mendocino Fracture Zone. The MBARI program, lead by Debra Stakes, will focus on the lithology of uplifted oceanic crust. The NURP-funded Oregon State Program, led by Robert Duncan joined by Martin Fisk and Anne Trehu, will focus on the tectonic history of the Gorda Escarpment, and its relationship to the plate margin evolution. The western portion of the Mendocino Fracture Zone (the Menocino Ridge) contains uplifted pieces of oceanic crust and some evidence of past subareal exposure. The eastern portion of the Mendocino Fracture Zone is called the Gorda Escarpment. At this contact, a recent seismic cruise imaged uplifted pieces of basement of unknown lithology capped by deformed sediments of unknown age. To the east, drainage channels carved into the face of the Gorda Escarpment appear to begin at the same depth where layers of buried hydrate disappear from the seismic record. The seven dive days of this cruise will explore the deformed ocean crust of the Mendocino Ridge, the uplifted contact between the Pacific and Gorda plates and the enigmatic channels on the easternmost Gorda Escarpment

The North American, Pacific, and Juan de Fuca plates are the three major tectonic plates of the Pacific Northwest. The boundaries between these plates are the San Andreas Fault, the Cascadia Subduction Zone, and the Mendocino Transform Fault (MTF) which meet at the Mendocino Triple Junction near Cape Mendocino, CA. Complex plate boundary processes appear to have created a thick block of crust south and west of the triple junction. Rocks from this block are the key to deciding between alternative hypotheses about the tectonic motion in this area, but evidence from a single dredge collected in 1960 is ambiguous. This region may also provide a laboratory for understanding the effect of crustal movements on methane hydrates and the impact of methane hydrate dissociation on sediment diagenesis. Methane hydrates are currently of great international scientific interest because of their potential for affecting the climate and because they may be a rich source of fossil fuel in the future. 

The Gorda Escarpment exposes more than 1 km of the thick crustal block along the MTF between the Pacific and Gorda plates. Our major scientific objective is to collect samples from this escarpment to answer the following questions: (1) What is the origin of the Gorda Escarpment? Is it the eastern end of the Mendocino Ridge, an exposed part of the oceanic crust thrust up by the collision of the Pacific and Gorda Plates, or the northernmost extent of continental crust isolated by the eastward- stepping San Andreas Fault? (2) How did it get there? Was it added to the Pacific plate from the Gorda plate by compression, or is it the leading edge of a slice of the continent? and (3) Did this part of the MTF reach sealevel like portions further west? If so, when and for how long? (4) Did this region experience a major episode of hydrate dissociation in response to tectonic uplift, leaving a distinctive signature in the sediment record? The answers to these questions are important for understanding the geologic history of the San Andreas Fault and MTF and may also link changes in west coast climate to vertical tectonics along the MTF.

This project is a multidisciplinary collaboration between scientists at MBARI and at Oregon State University (OSU), with funding from MBARI, OSU, the National Science Foundation, the National Undersea Research Program. We are using the Monterey Bay Aquarium Research Institute's (MBARI) deep ROV Tiburon to survey and sample in order to test hypotheses about the origin and composition of the Gorda Escarpment and about the relationship between tectonics, fluid flow and gas hydrate dissociation. We used high-resolution seafloor bathymetry and reflectivity data collected by MBARI in 1998, seismic profiles collected during an NSF-funded program in May 1999, and results from a deep hole drilled by the Ocean Drilling Program (ODP) in 1996 to plan the current diving program. The seismic data provide images of subsurface sediment and basement structure and reveal a complicated history of uplift, erosion and subsidence on the southern flank of the Gorda Escarpment. They also reveal the presence of gas hydrates beneath the eastern end of the escarpment and of an enigmatic reflection that cuts across the sediment reflections beneath the central part of the area. However, they do not tell us the composition of basement rocks. Composition and age of sediments are also poorly known in critical parts of seismic survey because of the large distance to the one existing ODP deep drill hole. 

In situ rock samples can be obtained only with the ROV Tiburon. We plan to use the Tiburon drillsled to collect cores from the vertical walls of the Gorda Escarpment. We have strapped four sediment push cores to the frame of the drillsled to collect whatever softer material we may encounter. We also put a small basket on the other side for grab samples. At this point, we really don't know what rock types to expect so we want to be prepared for everything. After initial reconnaisance using the Tiburon, sediment samples will be recovered by gravity coring using the OSU gravity coring system during times when the Tiburon crew resting and performing maintenance on the vehicle. Samples thus obtained will permit us to better estimate the composition of the basement rocks imaged in the seismic data and the age and depth of deposition of the sediments, permitting us to refine and revise our reconstruction of the geological history of this region.

 

Cruise History and Background

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

Three tectonic plates, the Pacific plate, the North American plate, and the Juan de Fuca plate, all converge in a single spot in cape Mendocino, CA. The Mendocino Transform Fault is one of the boundaries between these plates. The tectonic activity at this junction of plates has created an exposed, uplifted section of oceanic crust which lies on the Gorda escarpment. We will be conducting research on the Gorda Escarpment in order to test hypothesis about the origin and makeup of this area. Dive locations using the ROV Tiburon will be along the upper gabbro exposures on the north side of the Medocino Ridge. The prospect of sampling deep ocean crust where it has become exposed will be an exciting part of this cruise.

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.

The purpose of this cruise was to fill in the data gaps in both bathymetry and seismic reflection data, examine the tectonic development and uplift history of the Gorda Escarpment, and look for clues regarding the structural relationship between the Gorda Escarpment and the Mendocino Ridge. This new multibeam data, when combined with the EM300 data collect in 1998 by MBARI and Seabeam 2000 data collected by NOAA provides the first complete bathymetry of this important tectonic region (image). The MCS data recovered from this cruise illuminated a sedimentary diagenetic front proposed by Trehu and Stakes (submitted to Science, June 1999) as a "paleo-BSR". The distribution and isotopic composition of carbonates from this horizon may provide new insights into the cause of the broad isotopic variation observation in carbonates from Monterey Bay (Stakes et al., 1999; Naehr and Stakes, 1999 and in prep). In addition, samples of carbonate from ODP Site 1022C taken from the "paleo-BSR" horizon are the same age and may be analogous to exposures near Santa Cruz (Aiello et al., 1999 and in prep. The MCS data also revealed several important targets for Tiburon dives where the bedrock beneath the sediments is exposed in the Gorda Escarpment and the eastern Mendocino Ridge. If the Gorda Escarpment is formed from uplifted Vizcaino Block material, the Escarpment provides a window into the deeper portion of the accretionary complex. Sampling the Escarpment would provide constraints on whether the Vizcaino Block contains different terrains assembled through strike-slip tectonics, implying eastward migration of the San Andreas fault system. Alternatively, if the rocks exposed in the Escarpment are basaltic, similar to those forming the Mendocino Ridge further west, and are separated from the rest of the Vizcaino Block by an E-W trending fault (observed on the new data), then the history of the Gorda Escarpment is closely related to the history of the Mendocino Ridge. Systematic sampling of outcrops along the MTZ combined with the new seismic stratigraphic evidence of uplift, faulting, tilting and erosion will provide new constraints for the tectonic history of both the Mendocino Ridge and the Gorda Escarpment which can be linked to the evolution of the California margin. The carbonate horizons recovered from the ODP drillholes as well as new samples collected from escarpment exposures will also be valuable extensions of diagenetic models developed for seep sites in Monterey Bay to the south and Eel River and Cascadia to the north (Naehr and Stakes, 1999 and in prep; Stakes et al., 1999).

 

Click on the links to the left to find out more about this exciting cruise!

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