ROV Exploration of Young Volcanic Rift Zones West of Baja California
We request 9 days of RIV MEL VILLE shiptime for a San Diego-San Diego research cruise in the
Mexican EEZ, primarily to explore, sample and photograph young eruptive rift zones in the southern part of the
Southern California Borderland and at adjacent abandoned spreading centers, using Scripps' new DOER H2000
ROV. Secondary objectives are to establish by a night-time dredging program the nature of the crustal basement
in which the Borderland rift zones are embedded, and to fill in gaps in multi beam sonar coverage.
Jared Kluesner, Scripps grad student (graduating November 20 II , thereafter a postgraduate researcher at
UC Santa Cruz)
James Day, assistant prof, Geosciences
Dave Stegman, assistant prof, Geophysics
Pat Castillo, professor, Geosciences
Dave Hilton, professor, Geosciences
Peter Lonsdale, professor, Geosciences
1. General Objectives
Our objectives are political, operational, educational and scientific.
(i)
Initiate a new collaborative UC-Mexico program, a follow-up to our successful
decade-long collaboration in the Gulf of California, which was seeded by UC Shiptime and UC-Mexus grants
and continued with large awards from NSF-MARGINS (to Scripps) and CONACyT (to CICESE and VNAM).
In concert with our Mexican partners, we intend to ask the US-Mexus program (in April 2012) for most of the
funding needed to analyze samples from the proposed cruise, and are planning collaborative proposals to NSF
and CONACyT for additional post-2012 field programs (for multichannel seismic profiling and for ROV
operations that will require a longer umbilical than our present 1000m. As in our work in the Gulf, this
binational collaboration will have educational as well as scientific impacts, with Scripps faculty formally and
informally advising CICESE students, and CICESE faculty doing the same for Scripps students.
(ii)
Allow UC and Mexican researchers and students to familiarize ourselves with
the capabilities and limitations of our ROV, to figure out how it could be effective in our research and teaching.
Establishing a UC user community will allow us to be better informed and more effective for recommending
improvements to the system, and more convincing when requesting extra-mural funding for future operations.
One reason our targets for the proposed cruise include recently active volcanoes at bathyal depths is that they
are likely to provide spectacular imagery, of fissures and crater walls with active mineral-depositing and
organism-supporting hydrothermal activity, providing newsworthy (La Jolla Light? Fox News?) evidence of the
value of our new vehicle.
Provide an opportunity for our STS techs and ship' s crew to gain experience
(iii)
launching, recovering and positioning the ROV, and for acquiring the expertise needed for effective driving
and sampling. None of our techs have had these skills at present (our ROV has not yet been lowered into the
ocean) and to support a Scripps ROV operation they will need to be developed quickly and safely. We hope
STS can provide more than the usual number oftechs, for on-the-job training during the proposed cruise, which
will be safer on the dynamically positioned MEL VILLE than on smaller ships with a greater potential for
wrapping the ROV umbilical around their propellers. We have also begun to explore the possibility of getting
at-sea advice from experienced local ROV operators (e.g., the team at NOAA Southwest Fisheries).
(iv)
Improve geologic knowledge of the Mexican half of the Borderland, without
which (as explained in the next section) it is difficult to interpret the much more thoroughly sampled and
surveyed U.S. half. We know of less than 40 hard-rock samples from the 55,000 km 2 Borderland area within
the Mexican EEZ, all collected by dredging, and all but a handful pre-I970 and therefore poorly located. A
notorious problem with dredge sampling in the Borderland is that the seafloor is littered with "erratic" or
"exotic" stones, dropped not from icebergs (the commonest source of marine erratics) but from the rotting
holdfasts of giant kelp (Macrocystis pyrifera) that have been swept out to sea in storms, and from the carcasses
of sealions, which ingest beach cobbles (gastroliths) to grind up food. Erratics often comprise about half of the
yield of a random Borderland dredge haul, and identifying which stones they are can be subjective.
Misidentifying dredged erratics as in situ country rocks has led to classic misinterpretations of Borderland
33"N
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Figure 1: PLAN A for proposed cruise. Circles =ROY dive sites, D =dredge site. Green =daytime activity, Red =
night-time. Numbers are night numbers (night 1, night 2, etc).
Assumptions: (i) A 9 day /8 night cruise, ideally for students convenience 2 weekends plus 5 weekdays. San
Diego ETD 0800, ETA 1600; cruising speed between stations 11.5 kts.
(ii) ROY activities restricted to daylight (may not have enough techs for round-the clocks ops;
daylight launch/recovery would be safer at least at first) . Maybe this restriction can be erased as experience
builds.
(iii) Maximum effective ROY operations depth 850-900m; average dive time for a limited-payload
ROY to these shallow depths 3 - 5 hours. Time-line at left has asterisks for dives, D for dredges
structure (e.g. Doyle and Bandy, 1972). This problem can be mitigated by limiting dredging to multi beammapped scarps and talus slopes that are rich sources of country rock, but is best solved by using an ROV, where
video inspection of outcrops and rock piles is possible before sample collection.
(v)
Lonsdale will teach his 4-unit SI0253 class "Interactions of oceanic lithosphere
with the continental margins of the Californias" during the fall of 20 12. He will propose that this cruise be
offered as a 2-unit field course supplement for the students enrolled in this class, and as a 2-unit stand-alone
field course for others. If possible, we request that the cruise be scheduled over 2 week-ends and the 5
intervening week-days, to minimize disruption of faculty teaching schedules and student attendance of other
(onshore) classes. ROV operations have proved highly educational experiences, because unlike manned sub
dives a large group of students and faculty can gather around real-time video feeds from the seafloor, and
discuss what is being seen and sampled. It would help if a separate video feed can be provided to another of the
ship's labs, remote from the inevitably overcrowded ROV control center. We shall also investigate providing
ship-to-shore video, e.g., for an Aquarium outreach program. At-sea students will also assist in sawing open and
describing the rock samples, under Day ' s and Castillo' s supervision and together with our Mexican colleagues
(who will take subsamples back to their Ensenada labs).
2. A Brief Geologic Background
20-10 Myr ago the Southern/Baja California margin was a region of intense tectonic and volcanic
activity. At the beginning of this period, a trench that had occupied this ocean/continent boundary for more than
200Myr was still subducting oceanic lithosphere that had accreted to the eastern flank of the East Pacific Rise,
and a volcanic arc was active along almost the entire length ofthe Baja California peninsula (then still attached
to the Mexican mainland). But as the westward drifting North American continent approached closer to the
crest of the East Pacific Rise, the oceanic lithosphere delivered to the California trench became younger, less
dense, and more difficult to subduct; eventually, in several episodes of "microplate formation," slabs of young
oceanic lithosphere broke away from the subducting (Cocos) plate to form fairly stagnant oceanic microplates
between the still-converging (overthrusting) continent and a slower spreading East Pacific Rise crest. Then
segments of this risecrest stopped spreading, successively from north to south; at - 18Ma at the Rodriguez axis
(Figure 3), 16.5Ma at Northeast Bank, 13.5Ma at Ferrel, and 11.5Ma at Guadalupe (Figure 3) and at a staircase
of spreading axes along the Baja California Sur margin from 27.4°N to 23.5°N (the dates come from
interpretation of marine magnetic anomalies). Cessation of spreading meant that the oceanic microplates, both
the parts exposed on the seafloor and the parts overthrust by the continent, were welded to the Pacific plate and
acquired its northwestward motion as the continental margin changed from a zone of convergence to a zone of
oblique extension, with strike-slip faulting and rifting. It is widely believed, with increasing geological and
geographical evidence, that basal drag on the continental lithosphere by the underlying northwest-moving
oceanic slab caused continental blocks to split from the North American plate and move off to the northwest.
The major 11.5Ma event probably caused the rift, 200km inland, that separated Baja California from North
America and began to open the Gulf of California. Before then, capture of smaller slabs of oceanic microplates
had torn away and translated northwestward smaller continental blocks in the region of the Southern California
Borderland. As illustrated in Fig. 4, in the northern (U.S.) part of the Borderland, northwest translation was
accompanied by a remarkable rotation, by more than 90° about a vertical axis east of Los Angeles; a slab of
"forearc rocks" equivalent to the Great Valley sequence of central California (mostly Late Cretaceous
sediments, like those of the La Jolla cliffs, on an Early Cretaceous ophiolite basement) was ripped from San
Diego County and spun around to form the Channel Islands and the Western Peninsular Ranges. Movement of
, this upper crustal block away from San Diego exposed ("exhumed") a lower crustal unit of distinctive "Catalina
Schist" rocks, some of which had been metamorphosed at depths of more than 10km in a Cretaceous subduction
zone. These weak rocks, relieved if their upper-crust overburden rose up to form diapiric hills, characteristic of
the inner parts of the northern Borderland
Also in the wake of the rotating Channel Islands block, another block of fore-arc rocks, with San
Nicolas Island in the middle of it, must have moved northwest from somewhere south of the border (it now
forms part of the southern margin of the Channel Islands, and is known from its distinctive seismic structure to
extend at least to the EEZ boundary). Figure 4, and similar reconstruction by other authors, also show a broad
expanse of Catalina Schist in the southern Borderland. However, no rocks with the distinctive metamorphic
facies of the Catalina Schist have ever been described from the poorly sampled region south of the EEZ
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Palinspastic maps of California continental borderland and adjacent regiom for past 20 m.y. Light gray areas are accretionary rocks
of Franciscan Complex or belts that are IithologicaUy similar to that complex. Medium gray areas are underlain by forearc strata. Dark gray areas
are known outcrops of forearc strata. Area in wavy lined pattern is Catalina Schist belt. White areas are floored by batholithic basement rocks. Fine
line is modem shoreline and island configuration, shown defonned and displaced for reference in earlier models. Heavy lines are major faults that
are thought to be active during time represented. Dashed double line is fault-bounded margin of Catalina Schist belt. Dashed boxed line is active margin of extending region within Catalina Schist belt. Arrows show approximate trajectories of areas with respect to North America. (A) 20 Maj time
period prior to most of the deformation. (B) 15 Ma; time period in migrating.hinge phase ofextension.(C) 5 Ma; time period in dispersed right·nor·
mal·slip phase of extension. (D) Present·day configuration.
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Geological Society of America Bulletin,June 1998
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boundary, except for blueschists reported from Islas San Benito (at the southern end of the Borderland, see Fig
2) by a Scripps Student Expedition to these islands (Cohen et aI., 1963). Only a single (unpublished, U.S.G.S.
1979) high quality (but single channel) seismic profile has been collected across the southern Borderland since
the 1963 Scripps survey reported by Moore (1969), and that one was interpreted as showing a thin sediment
cover over a "probably volcanic" basement (Crouch, 1981). The principal source of new information is derived
from multi beam sonar coverage obtained during transits to and from San Diego (Fig. 2), and this shows a
pattern of northeast-striking chains of volcanoes linking northwest striking strike-slip faults (Fig.3), a pattern
not observed in the northern (U.S.) Borderland.
Magmatic and tectonic activity in the Borderland region has diminished since spreading at all adjacent
East Pacific Rise axes was extinguished at -11.5Ma, soon followed by the principal plate boundary zone
shifting into the Gulf of California. But it has not stopped completely either at the abandoned offshore axes or in
the rifted continental crust of the Borderland. Abandoned segments of the East Pacific Rise typically continue
erupting for up to 10Myr after spreading ceases, building distinctive tall and narrow post-spreading ridges made
up of rows of coalesced volcanic peaks. The minimum duration of post spreading volcanism is estimated by
subtracting the radiometric age of the youngest lava sample from the magnetically determined date that
spreading stopped; it is 8Myr at the Davidson axis off central California (Clague et aI., 2010) and at the
Rodriguez and Northeast Bank axes (Davis et aI., 2010), located in Fig. 3. Geodetic studies (e.g. Plattner et. aI.,
2007) indicate that Baja California is moving northwest at only -90% of the local Pacific plate velocity,
implying a still-active plate boundary zone (Baja-Pacific) in the offshore region, as attested by occasional
earthquakes along the Borderlands principal strike-slip fault zones (San Clemente f.z. and San Benito f.z.). The
multibeam-mapped volcanic rift zones that link these and other northwest-trending fault zone in the southern
Borderland (Fig. 3) could be interpreted as nascent spreading centers that grew as the thinned continental crust
was stretched at 18-12Ma, then left in an arrested state of development when the tectonic action shifted to the
Gulf of California, where similar volcanic rift zones have developed into true oceanic spreading centers within
the past 4Myr. A liMa radiometric date for lava from one of the Borderland rift-zone ridges would fit this
interpretation; the only other date we have obtained (from the CICESE lab of Dr Margarita Lopez) is 3.9Ma,
demonstrating persisting volcanism. It has been suggested (Plattner et aI., 2009) that Baja California is now
slowing down as it bumps up against the "big bend" of the San Andreas fault system, increasing the rate of
shearing and rifting along the Baja-Pacific plate boundary zone .... perhaps causing a resurgence of extensional
volcanism there.
3. Proposed Science Program
Of the several structural, tectonic, petrologic and stratigraphic problems that need to be addressed in and
near the poorly known southern part of the Borderland, we have chosen understanding the age, origin,
chemistry and volcanology of eruptive rift zones to be the primary focus of a field program built around use of
an ROV with a 1000 m depth capability. There are several such features shown by existing multibeam
bathymetry and reflectivity to be shallow enough, and free of an obscuring sediment cover. None have yet been
visited by a research sub or ROV, nor have any other sites in the study area, except for the seep community
discovered at 1125 m by Barham (Webb, 1969) and the barite precipitating site found by Lonsdale (1979) ...
both of these are a few km south ofthe EEZ boundary. We give priority to examples known or likely to have
had recent volcanic eruptions, partly because they will yield the freshest, least encrusted rocks (though poorly
oxygenated California Current waters hamper deposition of Fe Mn-oxide crust shallow than 1000m,
phosphorite encrustation can be a nuisance), partly because exploration of such sites exploits the ROV's ability
to discover, photograph and sample such secondary effects of submarine volcanism as seafloor deposits of
hydrothermal minerals and clusters of vent fauna (any such sites would surely become targets for future
multidisciplinary field programs). The two sites with the best evidence for recent volcanism are the Rosa
abandoned risecrest from which Lonsdale and Castillo have dredged lava dated at 71,000 B.P., and the San
Quintin volcanic rift in the inner Borderland, close to a coastal field of subaerial volcanoes that have erupted in
the past 10,000 years (Luhr et aI., 2005). Fortunately, both these young sites have other features that make them
of special interest.
(i) Abandoned spreading axes
Rosa (Fig.5) has the largest post-spreading volcanic ridge of any of the 7 EPR axes abandoned west of Baja
California Sur at 11.5Ma (Lonsdale, I 99 I), the only one that has built up into the depth zone accessible to our
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Figure S. Looking southwest along the (incompletely surveyed) post-spreading volcanic ridge that overlies the
abandoned Rosa spreading axis. The shallowest (white capped) peak in the middle distance has a summit 4S0m
below sea level, deepest point within the 2km-diameter "explosion crater" at left foreground is 860m. lOOOm
isobath sketched on where visible. Locations of 2 existing dredge hauls indicated by red arrows.
ROV, and the only one known (from the 70,000 B.P. date on lava from one of the 2 dredge hauls we collected
from it on a 1993 UC-funded class trip) to have continued eruption into a (fairly) modern times. During a
MEL VILLE multibeam crossing in August this year (partly supported by UC Shipfunds) a remarkable young
(sediment-free) crater structure (left foreground ofFig.5) was imaged 3krn southeast of the row of peaks along
the former spreading axis. No similar submarine crater has been seen on any of the dozens of post-spreading
volcanic ridges we have examined; it seems to be a submarine analog of the subaerial crater ofBarcena (Fig.6)
a trachyte and rhyolite volcano built (to a height of Ikrn in two weeks of eruption) in 1952, or of the older
adjacent Crater Herrera, both on a small volcanic island atop another abandoned East Pacific Rise crest, and
both subjects of Richard ' s (1957) Scripps Ph.D. dissertation. We intend to make a close lithologic and structural
comparison between the submarine Rosa eruption site and Isla San Benedicto subaerial structures, with ROV
dives within the Rosa crater, up the crater walls and up one of the two volcanic peaks on its perimeter. Specific
objectives are to learn how this crater formed underwater; by explosion or by collapse of a rhyolite dome? If by
explosion, what is the nature of the possible ejecta ring that surrounds it (ash? cinders? volcanic bombs?) and
the structure and chemical composition of the peripheral peaks. A series of several older, partially collapsed
crater rims occur in deeper water (l100-1500m) down the southeast flank of this ridge, and we plan nighttime
dredge(s) here to establish their (depth-dependent?) composition. Another ROV dive will follow the crest of
the shallowest part of the post-spreading ridge (across the 450m summit labeled on Fig.5), another likely site of
most recent eruption. Castillo, Hilton and Lonsdale recently collaborated on a student-led study (Tian et
al.,2011, part of her Scripps Ph.D. dissertation) of the wide varieties oflavas collected by our two 1993 dredges
(alkali basalt, mugearite, benmoreite, basanite, phonolitic tephrite and trachy-andesite), and they are especially
interested in following this up with ROV dives that may help us understand the evident small-scale petrologic
heterogeneity; we are all interested in the poorly understood volcanology of crater-forming eruptions under
- 800m of water; and our Mexican colleague, volcanologist Dr. Arturo Martin, also wants to assess the risk that
such eruptions (plausibly tsunami-generating) may pose to Mexican coastal communities.
The Ferrel spreading axis, which intersected the east end of Guadalupe transform until it was abandoned
at 13.5Ma (the Guadalupe axis intersected the west end) has a post-spreading volcanic ridge with a - 340m
summit that is even shallower than Rosa. Multibeam coverage from transit swaths is not complete along the
entire crest (swaths get very narrow at these depths i); there are now data gaps (which we plan to fill) large
enough to contain small craters. The only known sampling was by Krause in 1960: a dredge up a side-slope
from - 840-540m recovered a single block of highly vesicular alkali basalt (major element analysis reported and
discussed by Engel and Engel, 1963 [their "PVI48"J), "a few volcanic bombs (?) ...red precipitate, .. and
volcanic ash" (Krause, 1965, and his Scripps Ph.D. dissert, 1961). Lonsdale has recently been granted
Academic Senate $$ to pay for dating any surviving igneous material from this dredge (and from one other
1960 dredge, discussed below) if they can be located in the SIO Dredge Collection and look suitable for Ar/Ar
analysis, so we may know more about the age of this ridge before the proposed ROV cruise. Because this
40krn-long ridge (with 17krn of its crest shallower than 1000m) has yielded only one sample, which may have
been completely used up in the past 50 years, and because there is dredged evidence (ash) for explosive
eruption and hydrothermal precipitation, it seems well worth targeting a couple of ROV dives along its crest, for
comparison of age, volcanology and geochemistry with Rosa.
Another attraction of Ferrel is that Krause (1965) sampled "spontaneously bursting and crepitating
vesicular basaltic glass, breaking with a sharp snapping sound when brought on deck" from a tiny satellite
volcano at a depth of3500m 10krn west of the crest of Ferrel Ridge. Similar "popping rocks" are now known
from a handful of seafloor sites, and these unusually gas-rich lavas have proven very valuable for studies of
magma volatiles and mantle degassing (e.g. , Sarda and Graham, 1990). Lonsdale and Kluesner helped relocate
the site dredged by Krause, a few krn from the D.R. position he reported, on a UCShips Student Cruise in 2006,
and dredged more material, but we were ill-prepared to capture the volatiles as they popped away on deck. We
may collect similar rocks during ROV dives along the main ridge; if not Hilton is eager to redredge the Krause
site, easy now that we know its exact location, and we will come better prepared for catching and preserving the
volatiles that he studies, geochemically and isotopically.
(ii) Intra-continental volcanic rift zones in the southern Borderland
On the continental margin immediately east of Ferrel Ridge are a group oflarge shallow volcanoes, a
couple of them with wave-planed summits now at depths of 500-600m; we have recently obtained analyses of
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Figure 6. Aerial photo (from Richards and Dietz, 1957) of Isla San Bendicto, showing
Vo1can Barcena at a late stage of its 1952 eruption, with fumaroli c steam ris ing from its
500m-diameter crater. The older structure to the north (left) of Barcena is Crater
Herrera. With its double rim, with a higher peak (partly obscured by cloud) it bears an
uncanny resemblance to the submarine crater on Rosa. Herrera is described (Richards,
1966) as "a cratered trachyte dome built within the crater of a pyroclastic cone" (the
outer rim).
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an alkali basalt that Krause dredged from one of them, and if time permits we may do additional dredgesampling in this region (Fig.7). A more inviting ROV target is a narrow sharp-crested ridge that lies along, or
alongside, the prominent northwest-striking trace of the San Benito strike-slip fault zone; it has a 23Sm-deep
summit, and a crestline shallower than SOOm for SO% of its SSkm length. Krause (l96S) reports that rocks
dredged from S70-S00m [and now being sought in the Dredge Collection so we can obtain an ArIAr radiometric
date] are basalt fragments with (volumetrically) SO% vesicles, some of the rock being completely glassy.
Lonsdale has dredged and obtained analyses for similar rocks from a similar though shorter narrow ridge along
the southward continuation of the San Benito fault zone (the Vizcaino fault zone, off the Vizcaino Peninsula,
just south of the mapped area ofFig.3). Interestingly, these two sites where basaltic magmas have apparently
intruded along this strike-slip fault trace (the only such sites along its 1000km total length) coincide with the
northern and southern boundaries of the fairly old (now 17-20Ma) underthrust part of the Guadalupe microplate
(as mapped by seismic tomography ; Wang et aI. , 2009). The onshore and offshore continental crust above this
old, thick subcrustal oceanic lithosphere has no evidence of post-Miocene eruptive volcanism. Perhaps the old,
thick subcrustal oceanic lithosphere is a barrier for melts rising from the asthenosphere, which near the fault
zone moves laterally to erupt beyond the microplates margins. In any event, the ridge along the San Benito fault
zone seems a good place to sample glassy volatile-rich lava, whose chemistries, isotopic ratios and ages should
allow better understanding of their origin, and perhaps hydrothermal sites as well.
Asthenospheric melts clearly have had no trouble rising through the thin, fractured continental crust of
the southern Borderland, north of the Guadalupe microplate, as attested by the numerous volcano rows and
volcanic ridges mapped there (Fig.3). This region must have a "slab window", a term for a region inland from
an active or inactive subduction zone where for one reason or another there is a gap or "window" in the
subducted or underthrust slab of oceanic lithosphere, through which magma can rise to the surface. There is a
large, growing, and generally controversial literature on the origins of slab windows, their role in determining
the patterns, timing and composition of continental eruptions, their value for charting the motions of subducted
slabs, etc. We wish to contribute to this literature by using new observations and analyses from this window,
which is unusually close to a recently active subduction zone. The clusters of volcanoes adjacent to the young
onshore San Quintin Volcanic Field (SQVF; Fig.S) are our favored targets. One cluster of about 30 small
volcanoes, with an overall northeast alignment, lies at the bottom of the continental slope IS-30km southwest of
the subaerial SQVF; fresh vesicular alkalic basalt with a typical SQVF chemistry has been dredged from one of
them. About half of these 30 volcanoes are in the depth range accessible by our ROV, and some of them have
suffered major collapses that seem to expose their interiors, making them good candidates for ROV fieldgeology. 10km to the west is a northeast striking row of shallow (unsampled) volcanoes, labeled "volcanic rift
zone" in Fig.9, that links a pair of strike-slip fault zones (Fig.2). Near the intersection of this rift zone with the
southern strike-slip fault zone (at a 100° angle, appropriate for a tension fracture in a broad zone of strike-slip
shearing) volcanoes have erupted on the transform ridge. The one existing sample from this cluster is a fresh
alkali basalt rich in xenoliths. The SQVF is well known to geologists and geophysicists for its xenoliths, which
include samples of the otherwise hard to access lower crust and mantle. Castillo and his students have used the
chemistry and isotopic composition ofthe SQVF as evidence for and against controversial origins for several
young volcanic fields on the Peninsula (Castillo, 200S; Maury et al.,2009); Hilton and his students are working
on volatiles from SQVF rocks, and Day and his students are conducting research on mantle xenoliths. They all
want additional samples from the submarine eruptions in this region.
The Maximinos volcanic rift zone, further north in the Borderland (Fig.3), is of interest as a dive target
because we already dredged from its deep (l4S0m) southwestern tip a dated (lIMa) sample of alkali basalt,
erupted soon after this region became a site of oblique extension, and by sampling its much shallower (400SOOm) central portion, where prolonged eruptions have built a thicker lava pile, we may obtain younger samples
that may record how magma chemistry has changed through time, perhaps to a more Mid-Ocean-Ridge-Basalt
(tholeiitic) composition. Besides, this extensive volcanic field is only SOkm from CICESE and II0km from
Scripps, so it seems poor that we know next to nothing about its age, origin and composition. The most likely
Borderland rift to have evolved into a true, tholeiite-erupting spreading center may be the Soledad rift zone
(immediately north of Ferrel Ridge), which appears to have disrupted the outer subduction complex (blue in
Fig.3); we will try to find out by dredging the ISOOm-deep crest of its axial ridge.
Figure 7. Perspective view of the San Benito fault zone
(between points A and B) and the 55km long volcanic
ridge (C - E) alongside it. X marks ROV targets, D
possible dredge targets, Dl an east-west scarp that
truncates a shallow part of the subduction complex
02 and 03 shallow volcanoes in an east-west line just
beyond the northern limit of the underthrust slab of
Guadalupe microplate oceanic lithosphere (i.e. at the
margin ofthe Borderland "slab window". Seafloor
colored red to orange is shallow enough for ROV ops.
200
450
700
950
METERS 1200
1450
1700
Figure 8. Onshore and offshore volcanic fields in the San Quintin region. The map has ai ' grid;
multi beam contours have a 20m interval; subaerial volcanoes are indicated by red circles;
Seafloor colored red to blue-green is shallow enough for ROV ops.
(iii) Nature of the suhvoIcanic basement in the southern Borderland
The proposed cruise will be an early use of an untried vehicle, with inexperienced operators. We are not
sure how easy or time-consuming it will be to launch and recover the ROV, to drive where we want to go, and
to pick up samples we select; we are not even sure of the net payload of the vehicle. This makes it difficult to
estimate the likely duration of each dive, and how many dives we will have time for. We carmot expect roundthe-clock operations, and we need to be prepared for vehicle downtimes. Our plarmed secondary uses of the
requested shiptime, in addition to transiting between dive sites, will be to collect a few additional multibeam
swaths at some incompletely surveyed sites (e.g. , Fig.S), but mainly to collect rocks by dredging. The -10
potential dredge sites on Fig.l include both volcanic rift zones and sites where non-volcanic basement appears
to crop out. So a secondary objective ofthe scientific program is to incrementally (but significantly) improve
knowledge of basement lithologic patterns (e.g. the extent of Catalina Schist in the southern Borderland). One
never-before sampled site (Fig. 9) is a small ridge in the middle of San Diego Trough, just 3 hours steam from
MarFac, that we call Legg Knoll, because its possible significance was highlighted by Scripps student Mark
Legg. In Legg (1991 , p. 301) he suggested it "may be a volcanic intrusion or a block of basement left behind"
(by the rifting that opened the Trough); our best guess from its geomorphology is that it may be a Catalina
Schist diapir, and thereby the southernmost schist outcrop exhumed by rotation ofthe Charmel Island block
away from San Diego County. We have tentatively scheduled a summit dive and a side-slope dredge to find
out; another reason for choosing this site is that it is the only relevant feature close enough to port for sampling
in daylight on the first and/or last day of the proposed cruise.
I
!!OJ
roo
700
eoo
900
1000
1200
1300
, .00
1500
Participants
(i) The 6 authors of this request, with Lonsdale acting as chief scientist (having had most experience in
ROV operations and Borderland geology; and having surveyed all proposed dive sites).
(ii) Dr. C Waters, Scripps post-doc working on intraplate magmatisn in Day' s lab.
(iii) 5 Scripps grad students working in Day' s Castillo' s and Hilton's geochemical labs (B. Peters, N.
Juda, S. Halldorsson, T. Evans and a TBN incoming Sept. 2012), and I geophysics student (R. Petersen)
who, with Stegman's mentoring , is working on geodynamic models of subducting system.
(iv) 5 other student' s registered in Lonsdale's SI0253 class.
(v) 3 researchers from CICESE (one of show will probably serve as the required Mexican observer):
Dr. M. Lopez (radiometric dating of rocks); Dr. A Martin (volcanologist), and A Hinojosa (seafloor video
specialist), plus 4 CICESE students/ post-doc
(vi) 4 STS techs (more ifpossible!), for computer EM-I22 and CHIRP sonar operation, and night-time
dredging, and daytime ROV operations.
(vii) I or 2 experienced ROV operators, who we will try to pay for if necessary (we hope for volunteers)
(viii) At least I Scripps biology student (for video identification of benthic organisms)
(ix) As many other Scripps students who are interested in the nearby seabed or in ROV operations
(until the 38 bunks on the ship are filled).
On this list, we count 2 UC post-docs, and 6 Scripps students who will use samples or insights from the
proposed cruise in their dissertation research. At least 5 more students (we hope more) will participate as
part of a formal SIO class.
Two new junior faculty with limited sea-going experience are among the requestors
Stegman' s principal interest in participating is to become more integrated into the seagoing aspects of
SIO, having now been a co-PIon a few proposals for seagoing experiments (currently under review) and
with anticipation of more in the future. He is very interested in the types of observations that are proposed
in this cruise, in particular the use of a ROV. Scientifically, the Baja peninsula and surrounding region is
an area which he is actively researching because of the locale of a former subduction zone in the
Gaudalupe Microplate and the enigmatic rotation of the Western Transverse Ranges. He thinks the
perspective that he brings from geodynamics to connect the surface tectonics to the processes deeper in the
mantle will prove valuable to this study.
Day, with his student and post-doc, wishes to address the following questions:what volcanism in the
region represents; the mantle and crustal sources that the magmas tap; the nature and age of the
lithospheric mantle itself (through study of the xenoliths from San Quentin, and any similar
xenoliths collected during the cruise); and the interplay between tectonics and volcanism. To
answer these questions, they will perform detailed petrological and geochemical studies, including
Re-Os isotope and highly siderophile element abundance analyses, to investigate the ages and trace
crust-mantle contributions to samples. Day reckons that the proposed cruise should not only
provide new insight into volcanism and tectonics in the region, but could also to provide a unusual
glimpse into the nature of volcanism and tectonics in an active continental-oceanic
destructive/transform boundary.
The expected tangible outcomes of this cruise are:
I. Significant additions to the geologic sample and seafloor-video archives at Scripps and CICESE,
for the future researchers and students use.
2. Scientific papers on the regional and thematic subjects discussed in Section 3, co-authored by some
or all of the participants in categories i-iii, plus the Mexican researcher in v, and any other
participants who bring special talents (e.g. for benthic organism identification)
3. Papers that use insights or analyses of samples from the cruise to test broader hypothesis and
models, e.g. those discussed above by Stegman and Day, and more appropriately published by a
single research group
4. Parts of 5-6 Scripps Ph.D. and M.Sc. dissertations.
5. Improved extra-mural proposals to pay for more shiptime, ROV operations, and seafloor research
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