T103 Rheological Behavior of Ductile Lithosphere during Strain Localization

Monday, 2 November 2015
8:00 AM - 12:00 PM
Room 308

MANTLE DEFORMATION AND STRAIN LOCALIZATION AT AN OCEANIC PALEOTRANSFORM FAULT: THE BOGOTA PENINSULA SHEAR ZONE

Paper number: T103-14
Program: Technical Sessions
Time: 11:30 AM – 11:45 AM

Authors

Abstract

The assumption that oceanic transform faults extend into the lithospheric mantle as shear zones raises questions about the nature of mantle deformation in this setting: 1) What is the width of the zone of localized ductile deformation; 2) What is the 3D deformation pattern; and 3) What is the spatial variation of mantle strain and rheology.

The Bogota Peninsula shear zone (New Caledonia) comprises the exhumed mantle portion of an oceanic paleotransform fault. The shear zone has a width of 10 km, and contains an ~1 km wide mylonitic zone (Ouassé mylonites). High-temperature deformation in the shear zone is heterogeneous in terms of strain magnitude and geometry. From the borderland to the shear zone, we record rotation of the high-temperature fabric (foliation and lineation) toward parallelism to the mylonitic zone strike. The mantle fabric, determined from the 3D shape of the spinel grains, is characterized by an increase of the octahedral shear strain from 0.5 in the borderland to a value of at least 1.2 in the center of the shear zone. The fabric geometry varies from being dominantly prolate in the borderland and the main part of the shear zone, as opposed to oblate fabrics preserved in the mylonites. Harzburgites within the shear zone show a strong olivine crystallographic texture (mainly A-type and E-type) suggesting that dislocation creep is the dominant deformation mechanism. The development of the E-type olivine texture suggests moderate water content and low flow stress. Based on olivine recrystallized grain size paleopiezometers, the estimated differential stress in the shear zone is 15-19 MPa, which is consistent with the differential stress in the upper mantle portion of continental transform faults (e.g., San Andreas fault). The strain rate in the shear zone is 10-12 s-1. Strain localization in the mantle portion of the paleotransfrom fault involves increase of strain, stress, and strain rate, accommodated within a complex 3D deformation framework. The new data could provide critical constraints for models of oceanic transform faults development and evolution.