HR: 0800h
AN: T31C-1316    [Abstracts]
TI: Progressive Basin Reorientation in the Northern Gulf of California
AU: * Sharoff, J E
EM: jes223@cornell.edu
AF: Cornell University, 6132 Cascadilla, Ithaca, NY 14853-2301 United States
AU: Smith, G
EM: gsmith1@email.arizona.edu
AF: University of Arizona, 2935 N Euclid Ave, Tucson, AZ 85719 United States
AU: Mountain, G S
EM: gmtn@rci.rutgers.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Mountain, G S
EM: gmtn@rci.rutgers.edu
AF: Rutgers, State University of New Jersey, 610 Taylor Rd, Piscataway, NJ 08854 United States
AU: Steckler, M S
EM: steckler@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AB: The northern Gulf of California is one of a few regions where evidence of active rifting can be seen in a shallow marine setting. This activity has been studied using a 150 x 200 km grid of MCS data collected by the R/V Ulloa in 1999. Four basins have begun opening to accommodate highly oblique right-lateral transtensional motion between the Pacific and N. American plates; all basins exist in different phases of development. The two northernmost basins, the Consag (CB) and Wagner Basins (WB), have axes and bounding faults striking N-S. These basins are shallower than those farther south (~200m vs. 350-900m) because of their proximity to the mouth of the Colorado River, whose sediment supply has more nearly filled the available accommodation space. The two southern basins, the Upper Delfin (UDB) (with its two sub-basins the Northern Upper Delfin (NUDB) and Southern Upper Delfin (SUDB)), and the Lower Delfin (LDB) have axes and bounding faults striking roughly NE-SW. High sedimentation rates in all four basins have provided a record of deformation that we have examined in detail. Seisworks/2D, a seismic interpretation program by Landmark Graphics, was used to identify and correlate stratigraphic layers and faults, and construct a history of basin development for the northern Gulf. This study showed that sediment currently buried as much as 1500 msec beneath the seafloor near the three southern basins accumulated in precursor basins with axes and bounding faults oriented N-S. These earlier basins were smaller and more numerous than the modern basins. The southernmost basin has reoriented so that its axis strikes NE-SW and the bounding faults now show both normal and strike-slip offsets, as opposed to the earlier faults, which were almost exclusively oblique-slip. The SUDB and NUDB are in intermediate stages of development between the smaller and N-S oriented WB and the larger, NE-SW and more mature LDB. These SUDB and NUDB sub-basins are currently undergoing reorientation accompanied by alternating areas of active faulting that we expect will eventually resolve into a single extensional basin. Throughout this period of competition for dominance, a "see-saw" pattern of sediment accumulation has developed, characterized by alternating dip directions of major reflectors and the locations of depocenters switching from one sub-basin to the other. Thus, basins of the northern Gulf appear to undergo an evolution of clockwise reorientation that is progressing from south to north. We suggest the delayed reorientation in the northernmost Gulf is due to locally more ductile crust kept warm by the thick deposit of Colorado River sediments maintaining a high geothermal gradient.
DE: 7220 Oceanic crust
DE: 8109 Continental tectonics--extensional (0905)
DE: 8158 Plate motions--present and recent (3040)
DE: 3025 Marine seismics (0935)
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting