HR: 0830h
AN: T31E-0886    [PDF]
TI: Crustal structure and rift evolution across the Guaymas Basin, Gulf of California
AU: * Lizarralde, D
EM: danl@eas.gatech.edu
AF: Georgia Tech, School of Earth and Atm. Sci., Atlanta, GA 30332 United States
AU: Axen, G J
EM: gaxen@dino.ess.ucla.edu
AF: UCLA, Dept. of Earth and Space Sciences, Los Angeles, CA 90095 United States
AU: Fletcher, J M
EM: jfletche@cicese.mx
AF: CICESE, Dept. of Geology and Geophysics, Ensenada, BC 22860 Mexico
AU: Fernandez, A G
EM: mindundi@cicese.mx
AF: CICESE, Dept. of Geology and Geophysics, Ensenada, BC 22860 Mexico
AU: Holbrook, W S
EM: steveh@uwyo.edu
AF: University of Wyoming, Dept. of Geology and Geophysics, Laramie, WY 82071 United States
AU: Harding, A J
EM: aharding@ucsd.edu
AF: Scripps Inst. of Oceanography, IGPP, La Jolla, CA 92037 United States
AU: Kent, G M
EM: gkent@ucsd.edu
AF: Scripps Inst. of Oceanography, IGPP, La Jolla, CA 92037 United States
AU: Umhoefer, P
EM: paul.umhoefer@nau.edu
AF: Northern Arizona Univ., Dept. of Geology, Flagstaff, AZ 86011 United States
AB: A crustal-scale, active-source seismic experiment was conducted in the Gulf of California in the fall of 2002. This experiment, funded through NSF MARGINS, aimed to image crustal structure across conjugate margins of major basins throughout the gulf to determine the modes of extension, the influence of sedimentation and magmatism on breakup, and other features leading to a better understanding of the rifting process. Here we present results from Guaymas Basin, including analyses of marine MCS data recorded using the R/V Maurice Ewing's 6-km streamer and wide-angle data recorded on 39 OPSIP OBSs and 19 PASSCAL RefTeks onshore. The Guaymas Basin is heavily sedimented, with 0.5 km of sediments at the rift axis thickening to as much as 2 km at the margins. These sediments have masked the rift structure of this basin, which had seemed to have begun a true drift phase later than the basins in the south. Analysis of the seismic data show, however, that rifting proceeded to completion rapidly, and most of the crust underlying the Guaymas Basin is new igneous crust. In contrast to the southern gulf, rifting and subsequent crustal construction in Guaymas has been "magmatic", with crustal thicknesses of 12 km near the margin and 8 km near the spreading center, suggesting an along-axis gradient in either mantle temperature or dynamic upwelling that has persisted since the onset of rifting. Ingeous crustal structure generally mimics oceanic crust, with a 2- to 4-km-thick 4-5 km/s upper layer (likely intercalated sediments and sills) overlying a plutonic layer. A striking feature of the crust here is the asymmetry about the rift axis in plutonic-layer velocities and gradients, with velocities to the west of 6.4-6.9 km/s and a 6.6-7.0 km/s velocity gradient to the east. This is mimicked in the upper crust, with an average velocity of 5.0 km/s in the west and 4.0 km/s in the east. The rift axis features a 2-km-high, down-to-the east, low-angle fault(?) bounding a depression in the plutonic crust beneath the rift axis, which itself is offset from the geometric center of the basin. A combination of a rift axis offset from the locus of mantle upwelling and asymmetric tectonics at the rift may lead to evolved melts being emplaced west of the spreading center and more residual melts to the east, possibly representing a previously unknown type of tectono-magmatic asymmetry.
DE: 3025 Marine seismics (0935)
DE: 3035 Midocean ridge processes
DE: 7205 Continental crust (1242)
DE: 8105 Continental margins and sedimentary basins
SC: Tectonophysics [T]
MN: 2003 Fall Meeting