HR: 14:10h
AN: T43C-03 [Abstracts]
TI: 3-D Crustal Structure and Variable Growth of the Mariana Island Arc from Seismic Tomography
AU: * Calvert, A J
EM: acalvert@sfu.ca
AF: Simon Fraser University, Department of Earth Sciences, Burnaby, BC V6K3N6, Canada
AU: Klemperer, S L
EM: sklemp@pangea.stanford.edu
AF: Stanford University, Department of Geophysics, Stanford, CA 94305, United States
AU: Takahashi, N
EM: narumi@jamstec.go.,jp
AF: JAMSTEC, 3173-25, Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AB:
A 3-D seismic refraction survey was acquired over the Mariana volcanic arc at 14.5° -18.5° N. First
arrival travel times from this survey, and from a separate 2-D survey acquired approximately perpendicular to the
arc, have been simultaneously inverted for a 3-D P wave velocity model using seismic tomography subject to
smoothness constraints. In the final 3-D tomographic velocity model, the 7.4 km/s isovelocity contour correlates
best with the Moho along the 2-D profile, which was inferred using both first arrivals and wide-angle reflections,
and this contour is used as a proxy for the Moho. The active arc, which initiated only 3-4 Ma ago, has an average
crustal thickness of 18 km. Approximately 40 km to the east, the inactive remnant of the rifted Eocene arc has an
average crustal thickness of 21 km, due primarily to a thicker lower crustal layer with velocities of 6.5-7.0 km/s. P
wave velocities within the upper crust of the active arc are approximately 380 m/s lower than in the remnant
Eocene arc, but are up to 280 m/s higher at a depth of 15 km. These results suggest an evolution of arc structure
with increasing age: we infer closure of fractures and porosity in the upper crust through hydrothermal circulation
and a reduction in the mafic character of the mid-lower crust as a result of intracrustal differentiation. Comparison
of averaged 1-D velocity functions with sonic velocity measurements on samples from the Tanzawa complex in
the Izu collision zone suggests that although tonalitic rocks may exist in the transition from upper to middle crust,
much of the crust is basaltic. Middle crust with velocities of 6.0-6.5 km/s, which comprises 14% of the arc crust, is
best developed beneath the Eocene arc, but varies in thickness between 2 km and 6 km along strike. Crustal
production clearly varies both temporally and spatially. Under the assumption that a 40 km wide corridor along the
modern arc has not been affected by earlier arc magmatism, we estimate that crustal production has been
116 km
DE: 8180 Tomography (6982, 7270)
DE: 8185 Volcanic arcs
SC: Tectonophysics [T]
MN: 2007 Fall Meeting