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Other Tectonic Rifts: the Woodlark-D'Entrecasteaux Rift, Papua New Guinea
Geoffrey Abers, Boston University
Thanks to:A. Ferris (BU), S. Baldwin (Syracuse), B. Taylor (Hawaii), H. Davies (UPNG), many others.
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Some Recent US Projects
• Marine geophysical surveys (1990s)
• Leg 180 Drilling: active fault system (1998)
• WoodSeis2000 Passive seismic experiment
• Core complex P-T-t-D thermochronology
• Raised/drowned coral survey (2002-4)
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Regional Map
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Woodlark Basin Sea floor
• Rift follows weak, thickened continental crust• Magnetic lineations constrain past 4-6 Ma motion
[Taylor et al., 1999 JGR]volcano
72 mm/yr
57 mm/yr
32 mm/yr
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[Taylor et al., 1999 JGR]
Sea-floor history shows stretching
• 200 ± 40 km stretching before breakup (130-300% strain)
• 100-200 km extension in MCC region
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D’Entrecasteaux MCCs
[Baldwin et al., 2004 Nature]GPS-based velocities: Wallace et al. 2004 JGR
GPS rate
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Exhumation Constraints
• 2-2.4 Ma, 4-5 Kb granitoids
• 4-5 Ma, 7-11 Kb gneisses– [Hill & Baldwin, 1993]
• 4.3 Ma, > 70 km eclogites– [Baldwin et al., 2004]
2.2Ma
4Ma
5-20 km/Ma exhumation
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How can rocks be exhumed so far in extension?
• requires coupling from upper crust to mantle
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Large events incontinentalcrust
Abers et al., 1997
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Exhumation in upper crust: 25° dip on shear zones. Seismic?
MW 6.8 (1985)
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Ewing 9203 MCS profile
MoresbySeamountS N
5 km
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Earthquake Fault?• 1 plane parallel to fault
• 4-8 km depth
• 3 km water depth ODP Leg 180 (1998):fault gauge
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Seismicity rates from global catalogs
Detection complete only for M>5.3
[Abers, 2001, Geol Soc Spec Pub]
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Egum
BBU
WoodSeis2000
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P
Ps
Receiver functions:Strong Moho arrival, varying depth
[Abers et al. 2002 Nature; Ferris, 2002 MA thesis]
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Crustal Thickness Variations
[Abers et al. 2002 Nature; Ferris, 2002 MA thesis]
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Flow beneath core complexes
Lower crustal flow Mantle flow
moho
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Moho cross section: Thins under MCC’s
Where is missing buoyancy?
[Abers et al. 2002 Nature]
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Velocity Anomaly is 30-100 km
deep beneath thin crust...
... and isostatically compensates crust
[Abers et al. 2002 Nature]
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Vp-T relations at 2 GPa
-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06
500 750 1000 1250 1500
T, °C
dlnVp
pyrolitedunite
>4% @ 2-3 GPa
Vp relative to iasp91elastic: Hacker et al. 2003 + anelasticity; no melt
How Hot?
4%
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Crustal VelocitiesJoint inversion for Vp, Vs, hypocenters
11.5 km 23.5 km
resolution limit
Vp[Ferris et al. 2006 in press GJI]
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Velocities in the Crust
“normal” continent
low Vp below Moho
[Ferris et al. 2006 in press GJI]
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Possible Compositions
Seismic Moho
Contin. Oceanic
[Ferris et al. 2006 in press GJI]
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What is seismic Moho good for?• Marker of density
boundary? probably
• Structural marker indicating strain?– not clear what is
below– UHP exhumation
requires crust, mantle exchange
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Summary
• Up to 200 km extension in 4 Ma (how?)• Exhumation of 70 (100) km in 4.3 (8) Ma• Crustal unroofing on faults dipping 25°-
33°• Thinned crust underneath, mantle flow• Compensated by hot mantle• Magmatism mostly at base of crust
before rifting
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syn-rift volcanics: progression from andesitic to bimodal at high extension
0
2
4
6
8
10
12
40 50 60 70 80
SiO2 (wt%)
Na2O + K2O (wt%)modern DEntrecasteaux
Pap Pen
older Trob Platf
from I. Smith, multiple papers