HR: 14:14h
AN: GP33B-03 INVITED     [Abstracts]
TI: Toward a minimum change model for recent plate motions: Calibrating seafloor spreading rates for outward displacement
AU: * DeMets, C
EM: chuck@geology.wisc.edu
AF: University of Wisconsin-Madison, Department of Geology and Geophysics 1215 W Dayton St, Madison, WI 53706 United States
AU: Wilson, D S
EM: dwilson@geol.ucsb.edu
AF: University of California - Santa Barbara, Department of Geological Sciences, Santa Barbara, CA 93106 United States
AB: We use seafloor spreading distances derived from dense magnetic surveys of young magnetic anomalies flanking seven seafloor spreading centers and the velocities of 398 continuous GPS sites on the plates bordering these spreading centers to study outward displacement, a phenomenon in which seafloor spreading magnetic lineations are displaced outward from their idealized locations as a consequence of extrusive and intrusive emplacement of new magma across a several-km-wide zone centered on the spreading axis and outward sloping reversal boundaries. Linear regressions of age-opening distance series derived from crossings of magnetic reversals 1n-3An.2 (0.78 Ma-6.72 Ma) yield positive Y-intercepts for 42 out of 53 seafloor spreading segments, corresponding to displacement of reversals outward from the seafloor spreading axis. The improvement in the least-squares fit of a model that allows for outward displacement relative to a model in which outward displacement is assumed to be zero is significant at a very high confidence level. Separate inversions of 13 age-distance series derived from magnetic anomaly crossings grouped by plate boundary yields 12 estimates of outward displacement that range from 0.5-3 km and unusually wide outward displacement of 6.1+-0.4 km along the Reykjanes Ridge. Detailed analysis of numerous crossings of Anomaly 1n from the Southeast Indian ridge suggests there is a correlation between axial morphology and the magnitude of outward displacement; however, too few data are available from axial rise segments along other seafloor spreading centers to confirm whether this correlation is characteristic of other seafloor spreading centers. Our results corroborate previous estimates of magnetic polarity transition zone widths derived from near-bottom magnetic measurements, which range from 1-8 km and average 2 km. Statistical tests of our age-distance series indicate that all but two are consistent within errors with a globally averaged value for outward displacement of 1.9+-0.2 km. Seafloor spreading rates corrected for outward displacement agree better with instantaneous rates estimated from GPS-derived plate angular velocity vectors than do uncorrected long-term rates, underscoring the need to correct seafloor spreading rates for outward displacement before attempting to interpret differences between geodetic and geologic estimates of plate motions.
DE: 1209 Tectonic deformation (6924)
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 8158 Plate motions: present and recent (3040)
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005