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.

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)

Regional Map

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

[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

D’Entrecasteaux MCCs

[Baldwin et al., 2004 Nature]GPS-based velocities: Wallace et al. 2004 JGR

GPS rate

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

How can rocks be exhumed so far in extension?

• requires coupling from upper crust to mantle

Large events incontinentalcrust

Abers et al., 1997

Exhumation in upper crust: 25° dip on shear zones. Seismic?

MW 6.8 (1985)

Ewing 9203 MCS profile

MoresbySeamountS N

5 km

Earthquake Fault?• 1 plane parallel to fault

• 4-8 km depth

• 3 km water depth ODP Leg 180 (1998):fault gauge

Seismicity rates from global catalogs

Detection complete only for M>5.3

[Abers, 2001, Geol Soc Spec Pub]

Egum

BBU

WoodSeis2000

P

Ps

Receiver functions:Strong Moho arrival, varying depth

[Abers et al. 2002 Nature; Ferris, 2002 MA thesis]

Crustal Thickness Variations

[Abers et al. 2002 Nature; Ferris, 2002 MA thesis]

Flow beneath core complexes

Lower crustal flow Mantle flow

moho

Moho cross section: Thins under MCC’s

Where is missing buoyancy?

[Abers et al. 2002 Nature]

Velocity Anomaly is 30-100 km

deep beneath thin crust...

... and isostatically compensates crust

[Abers et al. 2002 Nature]

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%

Crustal VelocitiesJoint inversion for Vp, Vs, hypocenters

11.5 km 23.5 km

resolution limit

Vp[Ferris et al. 2006 in press GJI]

Velocities in the Crust

“normal” continent

low Vp below Moho

[Ferris et al. 2006 in press GJI]

Possible Compositions

Seismic Moho

Contin. Oceanic

[Ferris et al. 2006 in press GJI]

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

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

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