AN: T72A-1225
TI: Low-fold, ultra-deep MCS Profiling of the Mariana Arc
AU: * Tidwell, T
EM: ttidwell@stanford.edu
AF: Stanford University, Department of Geophysics, Stanford, CA 94305 United States
AU: Klemperer, S L
AF: Stanford University, Department of Geophysics, Stanford, CA 94305 United States
AU: Kerr, B C
AF: Stanford University, Department of Geophysics, Stanford, CA 94305 United States
AU: Goodliffe, A M
AF: University of Hawaii, SOEST, Honolulu, HI 96822 United States
AB: We collected 2600 km of 12- and 15-fold MCS profiles in April 2002, using the R/V Ewing 6-km 240-channel streamer and a tuned 20-airgun array (10,810 cu. in.). The unusually low fold resulted because our primary focus was OBS wide-angle recording (Kerr et al., this session), so airguns were fired at a shot-interval of 90 to 110 s (shot-spacing of 200 m or 250 m) to allow water-column noise to decay between shots. As a positive aspect we recorded 60 s record lengths (the Syntrak-480 recording system could not record longer traces), so potentially imaging structure to over 200-km depth. We acquired 4 arc-parallel profiles: along the back-arc margin of the Mariana arc 30 km west of the main arc; along the volcanic line from 14$°$30'N (near Rota) to 18$°$00'N (latitude of Pagan), 30 km east of the arc along the uplifted forearc high; and along the forearc 80 km east of the arc but 140 km west of the trench. We also shot 6 arc-perpendicular lines to link the arc-parallel lines. All profiles were processed through post-stack migration during the cruise, but to only 20 s travel-time. Despite the low fold, the data provide excellent images of shallow basins between the volcanoes that shed light on the volumes and relative timings of eruptions along the arc, and of normal faults particularly along our eastern fore-arc line. Large and rapid topographic variations along the volcanic arc and uplifted forearc cause significant problems of sideswipe (Gunther et al., this session); and strong water-bottom multiples make recognition of deep structure almost impossible on the preliminarily processed data. However, beneath the shallowest, smoothest bathymetry (the guyot extending north of Farallon de Mendenilla in the uplifted forearc high) clear reflections are seen to 6 s (c. 15 km). In principle our low-frequency high-power source should penetrate deep into the mantle, and our streamer was towed at 12-m depth to minimize swell noise and enhance low-frequency signals. Currently we are processing the data to 60 s to search for reflections from the subducting Pacific Ocean crust. Trench-crossing 15 s profiles in this region have been able to trace the subducting slab 50 to 60 km arc-ward of the trench.