FEEDBACK BETWEEN HYDRATION AND DEFORMATION IN AN OCEANIC PALEOTRANSFORM (NEW CALEDONIA) FROM HIGH TEMPERATURE MYLONITIZATION TO SERPENTINIZATION

Transform faults segment mid-ocean ridges, produce considerable relief of the ocean floor and are common seismic structures of the oceanic lithosphere, yet their high-temperature ductile behavior is poorly documented. A paleotransform system exposed in the New Caledonia ophiolite (Bogota Peninsula) offers insight into the feedback between hydration and strain localization during transform shearing in nascent lithosphere. The Bogota Peninsula shear zone affects mantle peridotite, is approx. 10 km wide, and consists of subvertical foliation and shallowly plunging lineation. Deformation culminates within the Ouassé mylonite zone (approx. 1 km wide) where olivine is finely recrystallized and displays strong crystallographic preferred orientation, and orthopyroxene (opx) is highly stretched, forming opx fish that generally indicate dextral sense of shear. Geothermometry reveals T ~ 1100-1000 C (Ca-opx) and 950-850 C (Cr-Al-opx) in the least deformed rocks. In the mylonite zone a wider range of T is recorded, with minima reaching 850 C (Ca-opx) and 750 C (Cr-Al-opx). Electron microprobe analysis also reveals the presence of 20-100 micron interstitial amphibole (pargasite), with modal abundance increasing in the mylonite zone; this suggests that high-temperature pervasive fluid flow may have played a role in strain localization and mylonitization. The Ouassé mylonite zone is cut by serpentinite shear zones and veins (2 cm to m thick) with systematic orientations. Two conjugate and coeval sets of serpentinite shear zones, both at low angle to the mylonitic foliation plane, dissect the mylonite into elongate lozenges and indicate a maximum horizontal compression direction nearly normal to the faults; this suggests that the serpentinite zones were extremely weak (frictionless). A third set of pervasive subvertical serpentinite fractures, oriented nearly normal to mylonitic lineation, represents tension fractures and the likely fluid feeding system. Hydration of mantle as documented in the Bogota Peninsula paleotransform occurred from high temperature mylonitization to serpentinization and may have controlled strain localization and the rheological evolution of this transform system.