HR: 1340h
AN: T33D-0593    [Abstracts]
TI: Dipping Magnetic Reversal Boundaries at Endeavor Deep: Implications for Crustal Accretion
AU: * Pockalny, R A
EM: robp@gso.uri.edu
AF: Graduate School of Oceanography / University of Rhode Island, S. Ferry Rd, Narragansett, RI 02882 United States
AU: Shields, A C
EM: ACShield6166@webmail.winona.edu
AF: Winona State University, P.O. Box 5838, Winona, MN 55987 United States
AU: Larson, R L
EM: rlar@gso.uri.edu
AF: Graduate School of Oceanography / University of Rhode Island, S. Ferry Rd, Narragansett, RI 02882 United States
AU: Popham, C
EM: cpopham@gso.uri.edu
AF: Graduate School of Oceanography / University of Rhode Island, S. Ferry Rd, Narragansett, RI 02882 United States
AB: Endeavor Deep, created by ongoing rifting along the northeastern boundary of the Juan Fernandez Microplate, provides a generous 75-km long view of the upper 1-3 km of oceanic crust created ~3 Ma at a fast-spreading ridge (~80 km/Myr, half-rate). Recent near-bottom surveys with the ROV Jason collected high-resolution video, rock samples, and 3-component magnetometer data along a 5 km-wide section of the southern wall of the deep. The video and rock samples define a crustal section with 300-500 m of primarily pillows and flows overlying a 400-500 m transition zone of extrusives and dykes. Forward modeling of the total magnetic intensity calculated from the 3-component magnetometer data identifies a magnetic polarity reversal that corresponds to a reversal boundary within magnetic anomaly 2a (C2An.2r - C2AN.3n , ~3.33 Ma). The location of the modeled polarity transition suggests the reversal boundary dips downward toward the original ridge axis with shallow dips (15 degrees) in the extrusive layer becoming increasingly steeper (25 degrees) in the deeper transition zone. The dipping character of the reversal boundary has also been observed along the walls of the Blanco Fracture Zone and is consistent with evolving crustal accretion models for seafloor created at intermediate- and fast-spreading rates, which predicts the rotation of the upper extrusive layer back toward the ridge axis. As a consequence of this rotation, originally horizontal flow boundaries will dip back toward the ridge axis and the magnitude of the dip will increase with depth into the crustal section. A small reversed magnetic polarity is also observed deeper within normally magnetized C2AN.3n chron, but with a very shallow dip (3-5 degrees). We doubt this is another normal-reverse-normal polarity transition, since the anomaly suspiciously coincides with the transition from dykes to extrusives. Therefore, we believe this anomaly is either the result of an edge-effect created by the different magnetic properties of the dykes and extrusives or evidence off-axis volcanism that occurred during a more recent period of normal magnetization.
UR: http://bobbyorr.gso.uri.edu/~edeep/
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 3005 Marine magnetics and paleomagnetics (1550)
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3045 Seafloor morphology, geology, and geophysics
SC: Tectonophysics [T]
MN: Fall Meeting 2005