 |
Cruise overview |
|
|
|
R/V Roger Revelle in Pago Pago harbor (photo Davis)
Tectonic setting of the Manihiki Plateau
Bruce Luyendyk, Univ. of California, Santa Barbara,
Rob Clayton and Joann Stock, Caltech
The plate tectonic setting in the southwest Pacific
during Cretaceous time is largely unknown. Prior MG&G
mapping and satellite gravity (Sandwell and Smith, 1997)
have identified some key features that may hold clues to the
plate tectonic history in this region. These include the
Large Igneous Provinces (LIP) such as the Manihiki Plateau
east of Samoa and the Hikurangi Plateau east of New Zealand
and adjacent to and apparently in the paleotrench along the
north side of the Chatham Rise. Other features include
linear ridges and scarps that are apparent plate boundaries.
Our objectives are to map the crustal grain and marine
magnetic anomalies in the deep sea surrounding the Manihiki
Plateau in order to determine some key constraints for the
plate tectonics in this region.
The region between the Hikurangi Plateau and the Manihiki
Plateau is one of the most enigmatic sectors of seafloor in
the entire Pacific ocean basin. This region lacks identified
magnetic anomalies, except for some isolated regions which
have been interpreted individually and do not appear to make
a coherent story for the region as a whole. Engebretson et
al. (1991) reported M22-M29 east of the Tonga trench and SW
of the Manihiki Plateau trending ENE. Sharman and Mammerickx
(1990) suggested that the eastern boundary of the Manihiki
Plateau (the Eastern Scarp; see track chart) was a
propagating rift site, and that anomalies M0 to M3 lie
immediately east of the Manihiki Plateau, as part of a
sequence that is as old as M10N at 25°S. Pontoise et
al. (1986) reported E-W magnetic lineations just east of the
Tonga Trench at 25.5° S; they hypothesized that these
were M-series anomalies, but did not identify them.
The Manihiki Plateau is considered to be a LIP which formed
at or near a ridge crest in Cretaceous time, possibly near a
triple junction (Winterer et al., 1974). Its three
geomorphic plateaus (High, Western, and Northern) are
separated by linear depressions thought to be fault troughs.
It is anomalously shallow, about 3 times thicker than normal
oceanic crust, and heavily sedimented (Hussong et al.,
1979). DSDP site 317 on the Plateau reached Aptian sediments
overlying basalt (Winterer et al., 1974; Schlanger et al.,
1976); the basalt was later dated at 123±1.5 Ma (R.
Duncan, in Mahoney et al., 1993).
Joseph et al. (1993) report that the NE edge of the Manihiki
Plateau is rifted, with faults trending NW-SE. In their
model, the NE and SE sides, including the prominent Eastern
Scarp, are believed faulted by a reorganization of the
Pacific-Farallon-Phoenix triple junction (PAC-PHN-FAR), and
southward jump of the PAC-PHN ridge, at M0 time. The SW side
of the Plateau, the Suvarov Scarp, is an en-echelon
NW-striking lineament.
The scarce existing data lead to several different tectonic
models for Cretaceous tectonics in the region. Lonsdale
(1997) proposed that the Manihiki Plateau rifted from the
conjugate Hikurangi Plateau in Early Cretaceous time. The
ridge that separated these plateaus died at 105 Ma and is
now the Osbourn Trough, a linear gravity feature that
follows 25°S.
Cruise Kiwi Expedition Leg 12
The R/V Roger Revelle departed Pago Pago, American Samoa
Saturday afternoon May 9, 1998 for Kiwi expedition Leg 12.
Aboard were 17 scientists including two SIO technicians.
There were 9 scientists from UC Santa Barbara including 7
undergraduate students and 4 from Cal Tech including 3
graduate students, and a professor and student from American
Samoa College. Our objective was to map the structural and
tectonic setting of the Manihiki Plateau, a Cretaceous-age
Large Igneous Province comprising about 500,000 square
kilometers, located south and west of Manihiki Atoll (161 W,
10-23 S). Our operating area included the south flank of the
Manihiki Plateau including the Samoan Basin, the Eastern
Scarp of the Plateau, and the Penrhyn Basin east of the
Scarp. We completed a detailed Sea Beam survey of an area in
the Penrhyn Basin east of the Plateau. From there we made
several crossings of the Eastern Scarp of the Plateau, then
crossed into the deeper regions of the Plateau to the north.
Our final survey line crossed the NW-trending gravity
lineation that apparently is the north boundary of the
Manihiki Plateau near 160 W, 6 S. We ran all profiles at 8.5
knots except the Penrhyn Basin survey when we pulled the
seismic system and ran at 12 knots. We acquired underway
data enroute to Honolulu.
We used the Bathy 2000 3.5 kHz echo sounding system, a
single channel seismic reflection system employing two 150
cubic inch generator-injector airguns, the Seabeam 2100
multibeam echo sounding system, and a total field
magnetometer. We steamed out of Pago Pago at full speed
towing the magnetometer and acquiring mutibeam data. We
deployed our seismic system at 167 W, 12.7 S. We profiled
with seismics at 8 knots. All survey systems functioned well
except that the magnetometer failed near the end of our last
line. The Bathy 2000 system functioned better than expected
and penetrated almost 300 meters of section atop the
Plateau. The Manihiki survey ended with 5268 kilometers of
profiles including 4326 kilometers of single channel seismic
reflection data. We made nine sonobuoy profiles.
A program of chlorophyll sampling was undertaken by American
Samoa participants to measure primary productivity.
Scientific Results and implications from the work
Manihiki High Plateau and Nassau Step
Our expedition focused on the High Plateau region and the
Nassau Step, a complex area south of there. The southwestern
edge of the High Plateau is defined by a graben named the
Suvorov (not Suvarov as published previously) Trough. The
Suvorov Trough is asymmetric in that the seafloor depth is
about 300 m deeper than the High Plateau on the Nassau Step
southwest of this graben. From seismic interpretation,
basalt basement, the volcaniclastic (and chert?) units are
found in the bottom of the Trough. The High Plateau edge of
the Suvorov Trough shows upper chalk and ooze units
onlapping back-tilted (NE) volcaniclastic (and chert?) units
indicating that graben faulting is post Late Cretaceous (or
post-Eocene and pre-Oligocene). The Nassau Step immediately
southwest across the Suvorov Trough is comprised of a series
of terraces stepping down into the deep Samoan Basin and
includes tilted fault blocks. At the foot of the Nassau Step
apparent dips of the basement are south towards the Samoan
Basin; in two locations the volcaniclastic (and chert?)
units appear folded and onlapped by younger sediments. The
chert sequence outcrops at locations on the Step; its
surface is disrupted in an incoherent fashion. The terraces
appear to be defined by faults but Sea Beam maps show they
have variable strike. Seamounts and volcanic islands and
atolls are found on the Nassau Step.
Eastern Scarp/Manihiki Fracture Zone
The Eastern Scarp or Manihiki Fracture zone trends NNE along
the east side of the Manihiki Plateau. North of 13° S
it trends 014° and south of there 017°. It is of
variable width but approximately 100 kms. The zone is
comprised of four to five linear ridges and basins; the
ridges have 500 to 2000 m of relief and the basins are often
deeper than the adjoining deep sea floor. Within the basins
are smaller hills and valleys that trend NE to ENE, parallel
to faint bathymetric trends in the Penrhyn Basin. This may
be a result of the fracture zone being formed during two
spreading regimes or under transtension. The boundary
between the Eastern Scarp and the Manihiki High Plateau is
expressed as a ridge elevated 200 to 700 meters above the
Plateau. It has dammed the section of the Plateau behind it.
Adjacent to this ridge the volcaniclastic and chert units
are faulted. The relief on the first scarp of the zone is
about 2 kms.
Samoan and Penrhyn Basin
SeaBeam mapping in the Samoan and Penrhyn Basins detected
bathymetric grain that is E-W to ESE in the former and ENE
in the latter. Sediment thicknesses in the deep sea are very
thin - 100 msec or less. Ocean Drilling results in the
central Pacific basin suggest this section is pelagic clays
and cherts with the chert layer being smooth acoustic
basement. Magnetic anomalies do not appear to be related to
known reversal sequences. Rather, they are largely
incoherent in both basins except immediately north of Samoa
where E-W trends are clear but the anomalies cannot be
correlated to reversal sequences. In the Penrhyn basin there
is a suggestion of ENE trends in anomalies of 100 nannoTesla
amplitude and 10-20 km wavelength. In this same basin the
sea floor steps down to the north across a series of E-W
trending faults. These trends are close to, but not
precisely orthogonal to the trend of the Eastern
Scarp/Manihiki Fracture Zone, and it is unclear how to
interpret this at this time.
Primary Productivity
Sampling revealed very low primary productivity over the
Manihiki Plateau ands Samoan Basin. Immediately north of the
Plateau productivity increased to high levels just south of
the Equator.
Scientific Party
|
UCSB |
professor |
chief scientist |
|
|
Rob Clayton |
Caltech |
professor |
scientist |
|
Erika Birk |
UCSB alumni |
volunteer |
watch/data proc. |
|
Bill Keller |
Caltech |
grad student |
watch/data proc. |
|
Leo Eisner |
Caltech |
grad student |
watch/data proc. |
|
Anu Venkataraman |
Caltech |
grad student |
watch/data proc. |
|
Tracy Hoganson |
UCSB |
undergrad |
watch/data proc. |
|
Karen Powers |
UCSB |
undergrad |
watch/data proc. |
|
Carmen Alex |
UCSB |
undergrad |
watch/data proc. |
|
Marcy Davis |
UCSB |
undergrad |
watch/data proc. |
|
Annalisa Schilla |
UCSB |
undergrad |
watch/data proc. |
|
Charlotte Evans |
UCSB |
undergrad |
watch/data proc. |
|
Neil Morgan |
UCSB |
undergrad |
watch/data proc. |
|
Jennifer Aicher |
Amer. Samoa Coll. |
professor |
watch/data proc./chlorophyll |
|
Tepora Toliniu |
Amer. Samoa Coll. |
undergrad |
watch/data proc./chlorophyll |
|
Seth Mogk |
SIO |
technician |
seismics |
|
Dan Jacobsen |
SIO |
technician |
computing |
References
Engebretson, D. C., J. Mammerickx, and C. A. Raymond,
1991, Tonga Lineations - The Phoenix plate has arisen?, EOS
Trans. AGU, v. 72, no. 44 , p. 444.
Hussong, D. M., L. K. Whipperman, and L. W. Kroenke, The
crustal structure of the Ontong Java and Manihiki Oceanic
Plateaus, J. Geophys. Res., v. 84, p. 6003-6010, 1979.
Joseph, D., Taylor, B., Shor, A. N., and Yamazaki,
Toshitsugu, 1993, The Nova-Canton Trough and the Late
Cretaceous Evolution of the central Pacific, in Pringle, M.
S., Sager, et al., eds., The Mesozoic Pacific: Geology,
tectonics, and volcanism: American Geophysical Union,
Monograph 77, p. 171-185.
Lonsdale, P., 1997, An incomplete geologic history of the
southwest Pacific basin, GSA Abstracts with Programs, v. 29,
no. 5, p. 25.
Mahoney, J.J., Storey, M., Duncan, R.A., Spencer, K.J., and
Pringle, M., 1993, Geochemistry and age of the Ontong-Java
Plateau, in Pringle, M. S., Sager, et al., eds., The
Mesozoic Pacific: Geology, tectonics, and volcanism:
American Geophysical Union, Monograph 77, p. 233-262.
Pontoise, B., et al., La subduction de la ride de Louisville
le long de la fosse des Tonga: premiers résultats de
la campagne SEAPSO (Leg V). C. R. Acad. Sc. Paris, v. 303,
Série II, no. 10., p. 911-918, 1986.
Sandwell, D. T., and Smith, W. F., 1997, Marine gravity
anomaly from Geosat and ERS-1 satellite altimetry: Journal
of Geophysical Research, v. 102, p. 10,039-10,054.
Schlanger et al., Site 317, in Schlanger, S. O., Jackson,
E.D., et al., Initial Reports of the Deep Sea Drilling
Project, v. 33, Washington (US Government Printing Office),
p. 161-300.
Sharman, G. F., and J. Mammerickx, 1990, Eastern Boundary of
the Manihiki Plateau: A Propagating Rift Site, EOS Trans.
AGU, v. 71, no. 43, 1668.
Winterer, E. L., Lonsdale, P. F., Matthews, J. L., and
Rosendahl, B. R., 1974, Structure and acoustic stratigraphy
of the Manihiki Plateau: Deep-Sea Research, v. 21, p.
793-814.
