Seismic velocity structure at Deep Sea Drilling Project Site 504B, Panama Basin; evidence for thin oceanic crust

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doi: 10.1029/JB094iB07p09283
Author(s): Collins, John A.; Purdy, Michael G.; Brocher, Thomas M.
Author Affiliation(s): Primary:
Woods Hole Oceanogr. Inst., MIT/WHOI Joint Program Oceanogr., Woods Hole, MA, United States
U. S. Geol. Surv., United States
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 94(B7), p.9283-9302. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. Woods Hole Oceanogr. Inst., Contrib. No. 7068. 60 refs.; illus. incl. 2 tables, sketch maps
Summary: We present an analysis of wide-angle reflection/refraction data collected in the immediate vicinity of Deep Sea Drilling Project hole 504B in the Panama Basin, currently the deepest drill hole (1.288 km) into oceanic crust. The data were acquired with a 1785 inch3 air gun array and fixed-gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths. Near-normal-incidence, multichannel seismic (MCS) reflection data were acquired simultaneously. Observed P and S wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness. Comparison of the travel times and amplitudes of the P and S wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method. Forward modeling shows that in contrast to standard oceanic velocity models, a velocity-depth profile that better explains the observed data is characterized by high-velocity gradients (up to 0.6 km/s/km) in the middle crust, a 1.8-km-thick low-velocity zone (Vp=7.1-6.7 km/s) immediately above Moho, and a total crustal thickness of only 5 km. Interpretation of the high-velocity gradients in the middle crust is constrained by the observation of P wave amplitude focusing at ranges of 16-19 km. Although not as well developed in comparison to the P wave arrivals, S wave arrivals show similar focusing. Total crustal thickness is constrained by the combined interpretation of a P wave, wide-angle reflection event observed at a range of 16-28 km and an MCS reflection event with a crustal travel time of 1.4-1.5 s. Although these events cannot be directly correlated, their travel times are consistent with the assumption that both have a common origin. Amplitude modeling of the wideangle event demonstrates that these events are generated at the Moho. Copyright 1989 by the American Geophysical Union.
Year of Publication: 1989
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; 20 Geophysics, Applied; Acoustical methods; Crust; DSDP Site 504; Deep Sea Drilling Project; East Pacific; Geophysical methods; Geophysical profiles; Geophysical surveys; IPOD; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Panama Basin; Seismic methods; Seismic profiles; Surveys; Tectonophysics; Thickness; Velocity structure
Coordinates: N010000 N020000 W0833000 W0840000
Record ID: 1989063673
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.