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					Eakins and Vukajlovich: multibeam surveying and rock dredging in the Baja California Borderland,
Oct 5–10, 2005


Field Work

Overview: We will conduct a 6-day cruise onboard the R/V Roger Revelle, departing from San Diego,
CA at ~8am, Wednesday, October 5th, and returning at ~4pm on Monday, October 10th (Fig. 1). Berthing
will be available onboard the ship the evening before departure (Tuesday, Oct 4th) and we recommend
that cruise participants stay onboard that night (we will not delay departure on Wed for late arrivers);
berthing will not, however, be available after the cruise (on Monday, Oct 10th). We anticipate
performing some 10 to 20 dredge hauls over the course of just a few days, which will require
coordination, some fair amount of (dirty) deck work at all hours of the day and night, and careful
attention to handling of recovered rock samples. We expect ~25 students (grads and undergrads in
geology, from Scripps, San Diego State, Centro de Investigación Científica y de Educación Superior de
Ensenada, and Indiana State) to be participating,

Objectives:
(i) San Quintin submarine volcanic field: Seafloor mapping and rock dredge sampling (6+ dredges) of a
     shallow-water, unsampled (previously unknown and unexplored) submarine volcanic field
     immediately offshore of the young San Quintin volcanic field in Baja California for comparison with
     already-collected onshore samples from the extensively-studied subaerial field (Site 1, Fig. 2). The
     subaerial San Quintin volcanics (late Pleistocene to Recent: 180,000 to 3,000 years before present)
     are the only Quaternary lavas in Baja Mexico that contain lower-crustal and upper-mantle xenoliths.
     The submarine sampling will provide an opportunity to collect and analyze seawater-quenched
     volcanic glass that can be used to understand the geochemistry of the erupted material and would
     complement analyses of phenocrysts and upper-mantle xenoliths found in the subaerial field.
(ii) Borderland spreading axis: Rock dredge sampling (1–2 dredges) of an adjacent, morphologically-
     young volcanic ridge (Site 2, Fig. 2) that likely formed during recent extensional opening and
     shearing of the California Borderland; basalts dredged from a similar ridge (Fig. 1) have returned an
     age of 4.9 Ma. Age dates from this ridge will help constrain the tectonic history of the borderland.
(iii)Popcorn Ridge: Seafloor mapping and deep-water rock dredge sampling of Popcorn Ridge (2–3
     dredges; Site 3, Fig. 3). A rock dredge in the early 1960s recovered basaltic lavas that exploded on
     deck as trapped volatiles escaped from highly pressurized vesicles within the very young rocks; only
     one other site (in the mid-Atlantic) has such ‘popping rocks,’ which are thought to contain
     undifferentiated mantle volatiles. As such, this site could be exceedingly valuable for constraining
     mantle geochemistry and provide insight into mantle processes that occur during oceanic crustal
     formation. A prior UC-funded excursion to Popcorn Ridge in 1984 was unsuccessful, largely due to
     lack of navigational fixes to properly locate the dredge target.
(iv) Guadalupe landslide: Seafloor mapping, rock dredge sampling (2–3 dredges) and perhaps sediment
     gravity coring (1–2 cores) of an apparent landslide deposit east of Guadalupe Island (Site 4, Fig. 4).
     Specific goal is to constrain the age of the likely-tsunamigenic collapse and determine whether the
     collapse and post-shield volcanism, located principally within the caldera, are related. The high
     alkali content of the shield lavas should allow for determination of the ages of rocks included in the
     landslide, while biostratigraphic dating of sediment overlying the landslide blocks (and estimation of
     sediment accumulation rate) may provide an upper age limit—echo-sounder profiles may also help
     constrain landslide age by determining the thickness of landslide-covering sediment.
Timeline:
   • 12-hour transit to San Quintin submarine volcanic field, arriving ~8pm Wednesday
   • ~34 hours of combined rock dredging of known volcanic cones within the submarine field and
       seafloor mapping in shallower water to identify other potential targets, and two dredge hauls of
       the borderland spreading axis, until ~6am Friday
   • 6-hour transit to Popcorn Ridge to conduct 2–3 deep-water dredge hauls, until ~10am Saturday
   • 4-hour transit to Guadalupe landslide, arriving ~2pm Saturday
   • ~32 hours of combined rock dredging and sediment coring of landslide debris, and multibeam
       surveying of the submarine flanks of the two shield volcanoes that make up Isla Guadalupe, until
       ~10pm Sunday
   • 18-hour return transit to San Diego, arriving 4pm Monday

Equipment to be used:
Multibeam sonar: The primary tool for mapping the sea floor will be the EM-120 multibeam sonar
system mounted on the hull of Revelle. The system emits a high-frequency (12 kHz) sound pulse into
the water column and records the returning echo to measure the bathymetry and backscatter orthogonal
to the ship track at high resolution (a few tens of meters, depending upon water depth). It will be used to
help interpret seafloor geology and to identify appropriate sites for dredging: steep slopes of high
backscatter that represent bare rock not buried by sediment (an issue for older volcanoes and those near
the coast, where subaerial erosion supplies massive quantities of sediment).

Expendable bathythermograph (XBT): Because the multibeam sonar system actually measures the travel
time between the sound pulse and the returning echo, it is mandatory that we know how fast sound
travels in water (a function of temperature, salinity and pressure). Pressure varies systematically with
depth, salinity not much at all (though it can be an issue in isolated seas) and therefore temperature
variations (mostly vertically in the ocean) supply the biggest unknown in determining sound speed. We
will thus be taking daily measurements of ocean temperature using an XBT, which is attached to a thin
copper wire and dropped over the side (not recovered), with the resulting temperature profile used to
calculate a sound velocity profile that is fed into the sonar system.

Rock dredge: A ‘dredge’ is an open-bottomed steel box (3 feet by 18 inches in size) that it is attached to
a thick cable, with “teeth” along its rim and a large-mesh chain-link bag hanging underneath for
catching rocks. Once sites have been chosen for dredging, we will bring the ship onto position, lower the
dredge slowly to the bottom, drive the ship a short distance, then reel in the dredge, dragging it along the
bottom (always upslope) and monitoring tension (how strongly it grips, and hopefully detaches, rocks on
the sea floor). Once enough cable has been reeled in to bring the dredge off the bottom, we will then
recover it, collect the rocks on deck and move them quickly into the rock lab, and proceed to the next
dredge site.

Gravity core: As time permits, we may collect a sediment gravity core from the Guadalupe landslide. A
gravity core is a long metal tube attached to a steel cable that is lowered to just above the sea floor, then
“dropped”, allowing penetration into the seafloor sediments, which it encases and retains inside. It will
promptly be recovered (it’s still attached to the cable), brought onto deck and stored in the lab for
analysis back at SIO. Microfossils within the sediment may be able to place upper limits on the
emplacement age of the landslide.
Towed magnetometer: While transiting between the research regions, we will deploy and tow a
magnetometer that measures Earth’s total magnetic field, which is why it is deployed away from the
steel hull of Revelle. The intensity and orientation of Earth’s magnetic field are recorded in volcanic
rocks as they cool, thus providing a record of past variations, especially of known and well-dated
reversals. Removal of Earth’s present field from the collected magnetic data produces magnetic
anomalies that may, in oceanic crust, be identified and used to “date” said crust. While part of the
borderland region is not strictly oceanic crust, the magnetic data may still prove useful and is collected
routinely for future research benefit.

3.5 kHz echo-sounder: Another hull-mounted sonar system is the 3.5. It emits a lower frequency sound
pulse that, under certain circumstances, can penetrate the seafloor and return subbottom profiles,
especially in regions of thick sediment cover. It can thus be quite useful in interpreting stratigraphic
relations.

Sample handling
The principal rock samples from each dredge will need to be examined, labeled, described and
catalogued, and then cut with an onboard rock saw to allow distribution of duplicate sample sets to SIO
and CICESE for archiving and analysis. A primary concern will be ensuring proper sample labeling (to
facilitate the sharing of rock analyses data between researchers) and, very important, the prevention of
mixing of rock samples from separate dredge hauls — once control is lost of which dredge haul a
particular rock came from it becomes almost worthless, as we won’t be able to confidently associate it
with a known dredge site, and it calls into question the parentage of other samples.

At-sea seminar
Dr. Peter Lonsdale, SIO, will be daily teaching a class on ‘the tectonic and volcanic history of the
southern California Borderland and Guadalupe crenel’ that cruise participants are expected to attend,
and SIO students register for.

Relevant papers
Batiza, R., Petrology and chemistry of Guadalupe Island: an alkalic seamount on a fossil ridge crest,
   Geology, 5, 760–764, 1977.
Lonsdale, P., and B.R.T.e. Simoneit, Structural patterns of the Pacific floor offshore of Peninsular
   California, Gulf and Peninsular Province of the Californias, 87–125, 1991.
Luhr, J.F., et al., San Quintin Volcanic Field, Baja California Norte, Mexico: Geology, Petrology and
   Geochemistry, Journal of Geophysical Research, v. 100, no. B7, 10,353–10,380, 1995.
Sarda, P. The popping rocks of the ocean ridges. Endeavor, v. 14, no. 3, pg 144, 1990.

Marine life
October is a good time of year for birdwatching offshore Baja California, especially around Isla
Guadalupe. We can also expect to observe some marine mammals (Guadalupe fur seal) so it might be
worthwhile to bring along some binoculars.

				
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