HR: 1340h
AN: T13B-1367 [Abstracts]
TI: Poisson_s Ratios in The Crust and Mantle North of The Kane Fracture Zone From 87-147 Ma
AU: * Kim, S D
EM: dkim@eas.gatech.edu
AF: Georgia Institute of Technology,EAS, 311 Ferst Drive, ES&T building, Atlanta, GA 30308
United States
AU: Lizarralde, D
EM: danl@eas.gatech.edu
AF: Georgia Institute of Technology,EAS, 311 Ferst Drive, ES&T building, Atlanta, GA 30308
United States
AU: Gaherty, J B
EM: gaherty@eas.gatech.edu
AF: Lamont Doherty Earth Observatory, P.O Box 1000 61 Route 9W, Palisades, NY 10964-1000
United States
AU: Collins, J A
EM: jcollins@whoi.edu
AF: Woods Hole Oceanography Institution, 93 water street MS#16, Woods Hole, MA 02543
United States
AU: Hirth, G J
EM: ghirth@whoi.edu
AF: Woods Hole Oceanography Institution, 93 water street MS#16, Woods Hole, MA 02543
United States
AB:
We present preliminary results of modeling mode-converted S-wave arrivals recorded by 14 Ocean Bottom Seismometers (OBS)
deployed during the 2001 Far-Offset Active Source Imaging of the Mantle (FAIM) experiment in the Western Atlantic.
Wide-angle seismic data were acquired along two transects: Line 1, an 800-km transect extending along a plate-kinematic flow
line on 87-147 Ma lithosphere, and Line 2, orthogonal to Line 1. Previously, we demonstrated that crustal thickness along
Line 1 decreases by ~1.5 km as paleo-spreading rate decreases from slow (13 mm/yr) to ultraslow (8 mm/yr). Results of FAIM
mantle refraction and gravity modeling studies suggest that the equivalent of ~1.5 km thickness of gabrroic inclusions are
distributed within the upper 30 km of the slow-spreading mantle, thus balancing the crustal thickness deficit and implying
that spreading rate controls melt extraction rather than melt production. Here, we investigate whether some part of the
lower crust consists of serpentinized peridotites. We identified PSP type mode-converted S-wave arrivals Sg, SmS, and Sn,
and modeled those travel-times to constrain Poisson_s ratio. The range of measured Poisson_s ratio within the lower crust
is 0.27-0.29 (_0.01) at all instrument locations, including crust created during ultraslow spreading, and the range of
lower-crustal Vp is 6.8-7.0 km/s. Peridotite has a Poisson_s ratio of 0.29_0.02 and peridotites containing ~30%
serpentinite have Poisson_s ratios of 0.29-0.31. A mixture of 30% serpentinite and peridotite with Vp of 8.2 km (the
measured uppermost-mantle velocity beneath Line 1), would have Vp of 7.30 km/s at 2 kb. Thus, measured lower-crustal Vp and
Vs are marginally consistent with a partially serpentinized peridotite (30-40% serpentinite) at the base of the crust, but
are more consistent with a gabbroic (Vp ~6.8, Poisson_s ratio 0.27-0.30) lower crust. This consistency, coupled with the
observation that the sharpness of the crust/mantle boundary and the lower-crustal Poisson_s ratio are both laterally uniform
along Line 1 and Line 2, suggest that the øcrustñ along the FAIM transects is primarily melt-derived igneous crust.
DE: 8180 Tomography
DE: 7218 Lithosphere and upper mantle
DE: 7220 Oceanic crust
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting