The Evolution of the Lau Basin, FijiTonga

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The Evolution of the Lau Basin, Fiji-Tonga

• Andrew M. Goodliffe
• Department of Geological Sciences
• University of Alabama

DGS Brown Bag Seminar, April 7, 2004
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• Outline
• Location of the Lau Basin
• Brief explanation of backarc basins
• Data types involved
• Data collection
• Explanation of the evolution of key parts of the
  Lau Basin
• Overview and conclusions

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• A simple model for the growth of a backarc basin
• Formation of arc
• Arc breakup with some magmatism
• Initiation of seafloor spreading
• Karig 1970 proposed that the Lau Basin was a
  young ocean basin formed by the splitting of an
  island arc.
• Lawver et al. 1976 proposed that rather than
  forming by conventional seafloor spreading, the
  Lau Basin formed by diffuse deformation and
  magmatism.
• Taylor et al. 1996 showed that the growth of
  the Lau Basin was formed by conventional seafloor
  spreading.

From Taylor, 1993
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• Three main data types will be included in this
  presentation
• Bathymetry (detailed kinematics from abyssal hill
  fabric)
• Swath, multibeam, wide beam, predicted, digitized
  charts
• Magnetic (seafloor age, kinematics)
• Magnetic anomaly, magnetization
• Acoustic Imagery (where is the neovolcanic zone,
  active faulting)
• Sidescan, beam amplitude

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• R/V Moana Wave
• Seafloor mapping (shallow and deep towed)
• Dredging, coring
• Magnetic and gravity surveys
• Single channel seismics
• Physical oceanography
• Good food

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• R/V Hakurei-Maru No. 2
• Similar facilities to the R/V Moana Wave
• Additional seafloor imaging facilities (next
  slide)
• Food subject to interpretation

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An example of data types collected aboard a
typical research vessel.
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Bathymetry
(b) Narrow beam echosounder
(a) Conventional widebeam echosounder
From SeaBeam operators manual
(d) Multibeam
(c) Unstabilized narrow beam echosounder
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• Transmit array is long in fore-aft direction,
  creating a narrow beam in the port-starbord
  direction.
• Receive array is long in the port-starboard
  direction, creating a series of beams that are
  long in the fore-aft direction.
• In addition to bathymetry, the amplitude of the
  returning signal is recorded, giving an
  indication of seafloor hardness.
• Susceptible to heavy seas

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• The HAWAII-MR1 (Hawaii Wide-Angle Acoustic
  Imaging Instrument) is a towed interferometric
  swath mapping system.
• Both sidescan and bathymetry data are recorded.

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• Towed behind a depressor weight which is in turn
  attached to the ship -- decouples the system from
  the ships motion.
• The system is towed beneath the mixed layer
  (60-100 m)
• Operates better in heavy seas than most scientists

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• In a conventional sidescan system a single
  transducer array measures traveltime after the
  transmission of a signal.
• In the HAWAII-MR1 system two arrays are arranged
  in parallel
• The phase difference between the same signal
  received at each array indicates the angle from
  which that signal came.
• This permits the calculation of depth in addition
  to traveltime and amplitude.
• Pairs of arrays are mounted on each side of the
  system. The port pair operates at 11 kHz, and the
  starboard pair at 12 kHz
• Bathymetry swath width is in excess of 3.4 times
  water depth (water depth 2.5 km, swath width
  8.5 km).
• Sidescan swath width is in excess of 7.5 times
  water depth.

Courtesy of Mark Rognstad, University of Hawaii.
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• Marine magnetic data is typically collected using
  a towed proton precession magnetometer.
• Consists of a coil wrapped around a hydrogen rich
  liquid (water, oil)
• When a current is applied to the coil the protons
  align themselves with the induced magnetic field.
• When the current is switched off the protons
  revert to the direction of the earths magnetic
  field by the process of precession.
• This generates a sinusoidal current in the coil,
  the frequency of which is directly related to the
  strength of the magnetic field.
• Sharks love them

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(a)
(b)

• As the Lau Basin is close to the magnetic equator
  and the seafloor lineations trend approximately
  N-S (a), the amplitude of the magnetic anomalies
  is low.
• Skewness complicates interpretation (ie peak of
  anomaly is not over the source).
• Reduction to the pole (b) or the calculation of
  the seafloor magnetization (if bathymetry is
  available) helps in the interpretation of
  magnetic anomalies.
• Early interpretations of the evolution of the Lau
  Basin were limited due to the failure to
  interpret the magnetic anomalies correctly, and
  led to the erroneous interpretation that there
  was no organized seafloor spreading in the basin.

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Total field magnetic anomaly
Seafloor magnetization
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Overview of the Evolution of the Lau Basin

• The basement is Middle Eocene to Early
• Oligocene arc terrain.
• Beginning at 10 Ma, the Fiji platform
• underwent a counterclockwise rotation of
• 21o to 13517.5o James and Falvey, 1978
• Inokuchi, 1992 Taylor et al., 2000.
• Rotation had stopped by 3 Ma, probably
• coinciding with the start of E-W spreading
• on the ELSC at gt 3.6 Ma.
• All reconstructions i.e. Taylor et al.,
• 2000 ignore the SE motion of the Tonga
• platform along the Peggy Ridge (PR)
• transform fault. The magnitude of this
• motion is uncertain.

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• Present day plate motion relative to a fixed
  Australian plate.
• Present day plate boundaries.

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WEP Western Extensional Province. ELSC East
Lau Spreading Center. VFR Valu Fa Ridge. PR
Peggy Ridge. MTJ Mangatolu Triple
Junction. FOSC Fonualei Spreading
Center. NWLSC Northwest Lau Spreading
Center. FSC Futuna Spreading Center. LETZ
Lau Extensional Transform Zone
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• Parson et al. 1992 proposed a rifted arc origin
  for the WEP based on ODP results and deep graben
  seen in sparse profiles.
• The more detailed morphology available today is
  consistent with a spreading origin.
• 3-D magnetization solution shows distinct and
  continuous magnetic anomalies.
• 3.5 Ma MORB basement was recovered at site 835.
• This graben was formed by a propagating spreading
  center.
• This age is consistent with that predicted by the
  magnetization solution (2An.3n, 3.33-3.58 Ma).
• 5.6 Ma MORB recovered at site 834 is consistent
  with seafloor spreading origin.
• Crustal thickness from refraction is consistent
  with a seafloor spreading origin (Crawford et
  al., 2002).

Western Extensional Province
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East Lau Spreading Center and Valu Fa Ridge

• ELSC originated in the north prior to 3.5 Ma and
  propagated south.
• The northern segments have progressively died.
• The only currently active segments are in the
  south

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• The depth and cross section of the spreading
  center changes dramatically with latitude.
• The rock types change from MORB in the north to
  andesites in the south as the spreading center
  gets closer to the arc.
• Similar changes with distance from axis.
• Multiple segments, rapid dualing propagators

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Black and white smokers, andesites, mussel beds,
bacterial mats, shrimps, barnacles.
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Fonualei Spreading Center and Mangatolu Triple
Junction

• Crust as old as Chron 2A (2.58-3.58 Ma) is
  identified on either side of the FOSC and MTJ.
• The present spreading center, as identified in
  acoustic imagery, cross-cuts Brunhes Chron crust
  (0-0.78 Ma) in the north, showing that this part
  of the spreading center has recently reoriented.
• The FOSC is currently propagating in the the
  active arc and creating a 1000 m relief graben.

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Central Lau Spreading Center and Peggy Ridge

• PR initiated prior to the CLSC, probably linking
  the ELSC with a spreading center to the
  northwest.
• The CLSC initiated between 0.78 and 1.77 Ma, but
  probably closer to 0.78 Ma.
• The LETZ formed by 0.78 Ma. It cut the corner
  formed by the CLSC and PR, shortening the active
  section of the PR.
• The CLSC is currently propagating south at the
  expense of the ELSC (up to 320 km/Myr).

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• Highly complex seafloor.
• The relationship of the many spreading centers is
  unclear.

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Overview and Conclusions
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R/V Moana Wave in Pago Pago harbor, American
Samoa.
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