Characteristics of the George V and Tasman Transform Fault systems, South-East Indian Ridge, and implications for mantle dynamics.

The George V and Tasman Transform Fault Systems (TFS) are major, right-stepping offsets of the South-East Indian Ridge between 140°E and 148°E. The George V TFS (~140°E) has an offset of about 300 km, and the Tasman TFS (~148°E) an offset of about 600 km. These TFS have multiple shear zones with intra-transform ridge segments (ITRS), mostly unmapped yet. We present the results of the analysis of geophysical and petrological data collected during the STORM cruise (South Tasmania Ocean Ridge and Mantle), completed with global data sets including satellite-derived gravity and bathymetry, and earthquake distribution. The swath bathymetry data cover some parts of the shear zones and only a few of ITRSs. They reveal a complex interaction between tectonic processes at the plate boundary and near-axis volcanic activity along and across the transform faults. In both the George V and Tasman TFS the western ITRS are shallower than the eastern ones, and they appear to receive a lot more magma supply. These western ITRS display off-axis volcanism observed on swath bathymetry or suspected from free-air gravity anomaly highs. In both TFS also, the western shear zone consists of two segments separated by a tectonic massif which we interpret to represent a push-up resulting from transpression along the transform. The mechanism involved in generating the transpression is a lengthening of the western ITRS to the west due to its high magma supply, leading to an overlap between the ITRS and the ridge segment immediately to the west of the TFS, that is in a mechanism similar to the processes currently uplifting the mylonitic massif along the St. Paul TF in the Equatorial Atlantic. The bathymetric and backscatter maps of the western George V TFS also reveal a series of recent off-axis oblique volcanic ridges. Rocks dredged on one of these ridges consist of picrites (i.e. basalts rich in olivine phenocrysts). These observations suggest that both TFS are not magma starved like many mid-ocean ridge transforms, but are the locus of significant primitive melt supply. Such an unexpected production of high-Mg melt might be related to the presence of a mantle thermal anomaly beneath the easternmost SEIR, the result of regional extension following clockwise rotations of the spreading direction, and/or to a western flow of mantle across the TFS. Some of the ITRS actually appeared after changes in the Australia-Antarctic plate motion.