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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 in continental crust Abers et al., 1997
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Exhumation in upper crust: 25° dip on shear zones. Seismic? M W 6.8 (1985)
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Ewing 9203 MCS profile Moresby Seamount SN 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 >4% @ 2-3 GPa Vp relative to iasp91 elastic: Hacker et al. 2003 + anelasticity; no melt How Hot? 4%
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Crustal Velocities Joint inversion for Vp, Vs, hypocenters 11.5 km23.5 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 from I. Smith, multiple papers
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