HR: 14:25h
AN: GP23B-04    [Abstracts]
TI: Anatomy of an oceanic mantle shear zone deduced from high-field magnetic anisotropy: the Humboldt corridor, New Caledonia
AU: * Ferre, E C
EM: eferre@geo.siu.edu
AF: Southern Illinois University at Carbondale, Department of Geology - MC4324, Carbondale, IL 62901 United States
AU: Belley, F
EM: francebelley@yahoo.fr
AF: Southern Illinois University at Carbondale, Department of Geology - MC4324, Carbondale, IL 62901 United States
AU: Tikoff, B
EM: basil@geology.wisc.edu
AF: University of Wisconsin - Madison, Department of Geology and Geophysics, 1215 W Dayton Street, Madison, WI 53706 United States
AU: Martin-Hernandez, F
EM: fatima@geo.uu.nl
AF: Utrecht University, Paleomagnetic Laboratory Fort Hoofddijk, Utrecht, CD 3584 Netherlands
AU: Nzokwe, G Y
EM: gerry01@siu.edu
AF: Southern Illinois University at Carbondale, Department of Geology - MC4324, Carbondale, IL 62901 United States
AU: Ward, C
EM: chad422@hotmail.com
AF: Southern Illinois University at Carbondale, Department of Geology - MC4324, Carbondale, IL 62901 United States
AB: The Humboldt corridor is located in the Massif du Sud of the New Caledonia ophiolite. This NW-SE trending corridor (40 x 20 km), exhibits tectonic peridotites with steep foliations and strong penetrative fabrics that grade into horizontal foliation outside the shear zone. Five N130ø trending zones have been identified from SW to NE: 1) a 3-4 km-thick zone of harzburgites with pyroxenite dikes, 2) a 1-2 km-thick zone of harzburgites with gabbro dikes, 3) a 1-4 km-thick zone of harzburgites without dikes displaying a prominent centimeter-scale layering, 4) a zone similar to zone 2; 5) a zone similar to zone 1. The mineral lineation remains subhorizontal throughout the corridor. The dikes throughout the corridor are normal to the mineral lineation. Some porphyroblasts in zone 3 have an aspect ratio up to 10. The harzburgites display porphyroclastic microstructures while olivine LPOs indicate plastic flow with activation of the (010) [100] high temperature slip system. The origin of such a zonation in mantle peridotites remains uncertain. This corridor is oblique to the regionally dominant N-S mineral lineation of Massif du Sud. Although the origin of this corridor is still unclear, olivine LPO data indicates that deformation occurred at high temperature in the mantle. The large width and the orientation of the corridor with respect to the spreading ridge framework seem incompatible with a transform zone interpretation. Magnetic measurements have been performed on 40 specimens (20 mm cubes) from 10 stations. The low field magnetic susceptibilities and AMS, representing the ferromagnetic contribution of secondary magnetite, vary between 700 and 7000 $\mu$SI. The magnetite is formed during serpentinization and therefore the low field AMS does not reflect mantle deformation. The high field susceptibilities and AMS, measured using a vibrating sample magnetometer, represent the contribution of paramagnetic minerals only (olivine and orthopyroxene) and varies from 230 to 350 $\mu$SI. The magnetic saturation, measured using a vibrating sample magnetometer, was reached before 200 mT. Torsion magnetometry tests conducted at fields up to 1.8 T indicate that the saturation conditions were met and no high coercivity phases. Preliminary results show that the low field AMS and the high field AMS principal axes (K1 and K3) are statistically distinct. The degree of magnetic anisotropy correlates with the intensity of mineral fabric and suggests that mantle deformation in these peridotites is very heterogeneous at scales from a few centimeters up to 1 km.
DE: 8159 Rheology--crust and lithosphere
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 1518 Magnetic fabrics and anisotropy
DE: 1527 Paleomagnetism applied to geologic processes
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting