Cruise Plan: Boomerang Leg 8
Dynamics of extensional convergent margins and the origin of
supra-subduction zone ophiolites: Hypothesis testing in the Tonga forearc
Sherman Bloomer and Dawn Wright, Department of Geosciences, Oregon State
University
1. Scientific Summary: The Tonga subduction zone, in the southwestern
Pacific, is the world's most active, extension-dominated convergent margin
system. We plan a geological and geophysical investigation of selected
portions of the Tonga forearc to test a number of specific hypotheses about
the evolution of these intraoceanic margins:
* The forearc is floored by arc-derived crust.. This crust is the
earliest produced in the subduction zone and is formed at rates much higher
than that of "normal" arc volcanism. Older MORB crust is non-existent or
rare in the forearc basement, having been removed by a variety of sea-floor
spreading and subduction erosion.
* These early forearc volcanics are unusually depleted, including
boninites and depleted arc tholeiites. The chemistry of these early arc
volcanics provides a unique look at the sub-arc mantle at the earliest
stages of arc evolution.
* The rock assemblages in intraoceanic forearcs are a better analog
for many ophiolitic complexes than active mid-ocean ridges.. The suites of
volcanic, gabbroic, and ultramafic rocks sampled from intraoceanic forearcs
have petrologic and geochemical features more like those of ophiolitic
suites than do similar rocks sampled from mid-ocean ridges
* The hydrothermal alteration pattern in many ophiolites is more
closely matched by the pattern in forearcs than by that in mid-ocean ridge
crustal sections.
* Tonga is the type extensional convergent margin and is undergoing
active tectonic erosion and its forearc is dominated by normal faulting and
movement of material into trench and hence down subduction zone.
* Serpentine diapirs form an important part of the crust in
intraoceanic forearcs and the fluids which form them, derived from the
downgoing plate, constitute an important source of fluid flow in this type
of convergent margins
* Intraoceanic arc volcanics show an across arc progression in
composition (corresponding to an age evolution) which indicates the
polarity of the system and provides a way to determine paleo-polarity in
collisional belts.
* The Tonga forearc was removed from both the Samoa hotspot and
Indian Ocean mantle at the time of its formation. As it has evolved and
moved, both types of mantle have been introduced into different portions of
the arc or back-arc.
The cruise will also provide essential site survey data for a proposed
Ocean Drilling Program Leg.
2. The field program: Prior to the cruise, we will synthesize all
existing geological , bathymetric and geophysical data into a single GIS
database. We have designed our shipboard experiment, which will include
swath bathymetry, side-scan sonar (both derived from the Seabeam 2000
system), selected single-channel seismic lines, gravimeter and magnetometer
measurements, and dredge sampling, around nine traverses of the outer Tonga
forearc. Swath bathymetry will also be collected over three proses ODP
sies on the eastern edge of the Tonga platform.
Transit to the first survey area will be from Suva, to the active arc at
the latitudde of Vava'u (19oS), along the western edge of the active Tofua
arc, across the proposed site for TF-1, and out to the first survey area
over ODP Site 841.
The nine detailed survey boxes will be built around Seabeam 2000 swaths and
dredge profiles from the latitude of Site 841 (to provide detailed regional
context for that site) to the northern most terminus of the trench,
providing complete and regular coverage of the forearc. Four parallel
Seabeam 2000 swaths, with gravimeter and magnetometer measurements, will be
run from the west side of the trench-slope break and back, to form survey
boxes of about 30 x 30 nm, with complete bathymetric, magnetic,
gravimetric, and side-scan coverage. Water depths will be 3000-4000 m on
the western edges of the boxes, along the trench-slope break, and from 7000
to 10000m at the eastern end, in the axis of the trench. The location of
the profiles to be run are sketched in Figure 1. At 10 kts., we estimate
that it will take 6.9 days to complete the surveys.
Transit between transects will be along the trench-slope break,
with Seabeam and geophysical data collection, as this is the region where
any serpentine diapirs should be most apparent. There is already an old
Seabeam swath covering the central axis of the Tonga Trench (Fig. 1).
Transit between sites at 10 kts. will require 2.3 days; transit from Fiji
and to Samoa will require 2.6 days.
After the surveys over the proposed sites of TF 2 and TF 5, we will
run a transit across to the Tonga platform and back out to the trench-slope
break, to collect swath bathymetry and side-scan over the proposed sites
for TF 3 and TF 4 on the Tonga platform. TF 3 is just northeast of
Tongatapu, and will require a transit just northeast of 'Eua.
Within each of the nine survey boxes a three-dredge profile will be
the basic sampling pattern. The profiles will include one deep, one
intermediate, and one shallow dredge moving up the landward slope of the
trench. Each of the dredges will be planned to have a fairly long
on-bottom track. Previous experience has shown the trench slope
assemblages to be diverse, talus samples; given the likely degree of normal
faulting and downslope movement, long dredge tracks which give a
representative sampling are of more use than short tracks used in a futile
effort to determine a stratigraphy on the landward slope. Eight additional
dredges are allocated for shallow targets along the trench slope break that
are suspected to be serpentinite diapirs. An additional nine dredges are
planned for features of particular interest--mid- or lower-slope seamounts,
to fill in particularly complex slope sampling, and to fill in for empty
dredges. A total of forty-four dredge lowerings is planned. The proposed
dredging will take 15 days of work. The dredging will be interspersed with
the surveying, as we move northwards up the forearc. We may need to
attempt one or two gravity cores, to obtain surface sedmient data for
proposed ODP rentry cones.
Gravity and magnetic data will be collected along all of the
Seabeam swaths to aid in identification of serpentinite masses and to
provide some constraints on crustal structure, to be compared with the
detailed crustal structure study across the forearc at the latitude of
Capricron Guyot (Dorman and Hildebrand, pers. comm., 1995). Single-channel
seismic surveys will be run at the western end of seven of these profiles
(1, 2, 3, 5, 6, 7, 9) to define the pattern of faulting in the distal end
of the forearc basin, to examine the geometry of the basement, to look for
discontinuities in the patterns of faulting with depth, and to determine
the general geometry of fault patterns along the forearc. A grid of lines
about 5 nm apart will be used. We have allocated 5.8 days for the SCS
surveys, as we are assuming we will need to run them at 8 kts. or less and
that we may need to do them with the Seabeam system off.
Our total estimate for the cruise as outlined is approximately 33 days
All dredged samples will be sorted by type at sea, weighed and
split for gross description. This data will provide histograms of sample
distribution each haul and will provide the data to select representative
samples for preliminary shipboard sampling. Slabs of representative
samples will be returned to the shore-based labs with the scientific party.
Shipboard data will be compiled into preliminary mosaics at sea. At sea
Wright will use techniques to filter the raw GPS navigation and to perform
nearest-neighbor weighting of SB2000 beampoint data. Each day's multibeam
data may be accessed easily by the entire shipboard scientific party with
the help of Bill Ryan's L-DEO MapMaker software package. Procedures to grid
the SB2000 sidescan data have been developed by Catherine Johnson and Dave
Sandwell at SIO, along with Dan Scheirer at UCSB. Wright will have access
to this software during the cruise.
A summary of the cruise plan and work days:
Location Type of Work Approximate Lat/Long Work included
near Vava'u turning point from 19oS, 175oW Seabeam, gravity,
Fiji magnetic survey from Fiji
TF 1 turning point 22o36'S, 176o13'W Seabeam, gravity, magnetic
survey
Site 841 detailed geophysical 23o30'S,175o30'W Seabeam 2000, gravity,
and sampling survey magnetics, dredge sampling, SCS
TF2 detailed geophysical 21o44'S,174o40'W Seabeam 2000, gravity,
and sampling survey magnetics, dredge sampling, SCS
TF3 swath bathymetry for 20o54'S, 174o59'W Seabeam 2000, gravity,
ODP site survey magnetics
Survey 3 detailed geophysical 21oS, 174oW Seabeam 2000, gravity,
and sampling survey magnetics, dredge sampling, SCS
Survey 4 detailed geophysical 20oS,173o30'W Seabeam 2000, gravity,
and sampling survey magnetics, dredge sampling
Survey 5 detailed geophysical 19oS, 173oW Seabeam 2000, gravity,
and sampling survey magnetics, dredge sampling, SCS
TF 5 detailed geophysical 17o36'S, 172o44'W Seabeam 2000, gravity,
and sampling survey magnetics, dredge sampling, SCS
TF 4 swath bathymetry 17o38'S, 174oW Seabeam, gravity, magnetic survey
for ODP site survey
Survey 7 detailed geophysical 16o30'S, 172o30'W Seabeam 2000, gravity,
and sampling survey magnetics, dredge sampling, SCS
Survey 8 detailed geophysical 15o30'S, 172o30'W Seabeam 2000, gravity,
and sampling survey magnetics, dredge sampling
TF 6 and TF 7 detailed geophysical 15oS, 173o25'W Seabeam 2000, gravity,
and sampling survey magnetics, dredge sampling, SCS
Samoa end port Seabeam 2000, gravity,
magnetics in transit
Total: Days Seabeam 2000: 33
Days digital SCS: 6
Number of dredges: 44
Days magnetics: 33
Days gravity: 20
Number ofGravity cores: 1-2?