Fabric development in the mantle section of a paleotransform fault and its effect on ophiolite obduction, New Caledonia

Abstract

The Bogota Peninsula shear zone has been interpreted as a paleotransform fault in the mantle section of the New Caledonia ophiolite. New, detailed fi eld measurements document the rotation of foliation, lineation, and pyroxenite dikes across a 50-km-wide region. Deformation intensity recorded by folding and boudinage of dikes increases toward a central, 3-km-wide mylonitic zone. We used several additional methods to characterize fabric patterns across the shear zone. The shape-preferred orientation of orthopyroxene grains, computed from outcrop tracings, closely parallels fi eld fabrics, with increased alignment and fl attening near the center of the shear zone. The lattice-preferred orientations of olivine are consistent with high-temperature fabrics; the a axes within the mylonitic core were used to constrain the orientation of shear zone boundaries. Seismic anisotropy calculations, based on the lattice-preferred orientation of olivine, indicate 5%-11% shear-wave anisotropies, with increased values in the center of the shear zone. The magnetic silicate fabric in the rocks, determined from anisotropy of magnetic susceptibility techniques, broadly matches fi eld fabrics but provides less consistent information across the shear zone than other fabric methods. This suite of fi eld and laboratory data provides a unique and detailed view of strain and fabric patterns across a shear zone in oceanic mantle lithosphere. Because the primary mantle fabrics seem to be related to the present distribution of ophiolitic rocks in New Caledonia, we propose that ophiolite obduction and Neogene extension may have been controlled by preexisting fabrics and structures in the oceanic lithosphere.

Heterogeneity and anisotropy in the lithospheric mantle

“…In ophiolites, mylonitic and ultramylonitic zones are related to transform faults or to the obduction process Boudier and Coleman, 1981;Boudier et al, 1988;Suhr, 1993;Dijkstra et al, 2002;Michibayashi and Mainprice, 2004;Titus et al, 2011). In most cases, deformation occurred under retrogressive conditions, in which grain size reduction is associated with progressive strain localization.…”

The emplacement of the Peridotite Nappe of New Caledonia and its bearing on the tectonics of obduction

“…In addition, only the northwestern half of the island includes a domain of Cenozoic HP‐LT rocks (Figure ). These two features are best explained by assuming that the axis of the Norfolk Ridge and the zone of intraoceanic subduction that eventually led to the obduction of the Peridotite Nappe were not parallel, as in most geodynamic models of the region [e.g., Crawford et al ., ; Titus et al ., ], but significantly oblique [ Cluzel et al ., ]. Figure a shows a possible configuration for this obliquity.…”

Tectonics: 50 years after the Revolution

“…ocean basin to the north. Subsequent ophiolite emplacement on the Norfolk Ridge, a narrow continental ridge east of Australia, possibly was controlled by these preexisting fractures (Titus et al, 2011;Nicolas, 1989).…”

“…See also Moores and Twiss (1995, p. 138-141). (D) New Caledonia, showing the Bogota Peninsula and the Belep shear zones, two possible oceanic transform fault zones (after Titus et al, 2011;Nicolas, 1989;Prinzhofer and Nicolas, 1980). on the Earth.…”