The Sensitivity of the India-Africa transform plate boundary to global plate reorganization events
IPGP - Îlot Cuvier
Séminaires Géosciences Marines
Oceanic transform plate boundaries were discovered in the 1960’s based on horizontal offsets of magnetic anomalies lineaments in the oceanic domain (Wilson, 1965). These conspicuous offsets, reaching several hundreds of kilometers in some places, reflect the segmentation of oceanic spreading centers and the relative motion of two adjacent plates in the horizontal plane. During their life time, transform boundaries mostly adjust to continuous plate motion changes. However, punctual episodes of Global Plate Reorganization Events (duration less than 5 Myrs) induce dramatic and abrupt changes in plate boundaries configuration. The response of a transform boundary to a Global Plate Reorganization Event may differ according to the intensity of the change in tectonic forces, but also the strength of the adjacent oceanic lithospheres and the length of the finite transform offset. The India-Africa transform plate boundary initiated ~90-100 Ma, in the wake of India’s travel towards Eurasia, and is still active at the Owen Fracture Zone. Its 90-100-myrs-long lifetime makes the India-Africa boundary an exceptional case-study to investigate the sensitivity of an oceanic transform system to several geodynamic changes. Using multibeam and seismic data acquired during the OWEN cruises, we reconstruct the successive configurations of the India-Africa transform boundary over the last 90-100 Ma, which are well recorded in marine sediments, especially since the emplacement of the Indus turbiditic system in Eocene. The reactions of the India-Africa transform boundary to the various global plate reorganization events involve a large variety of geological processes. Transpressive or transtensive stresses induced the uplift of the up to 2000-m-high Owen transverse ridge ~8 Ma. Several episodes of migration of the transform boundary over distances on the order of 100-km are identified at ~65 and ~47 Ma, resulting in transfer of pieces of lithosphere from one plate to another, and the emplacement of the Masirah Ophiolitic Belt along the eastern Oman margin. Conversely, the Global Plate Reorganization Event at ~24 Ma did not trigger any conspicuous response along the transform boundary. We also evidence an episode of major structural reorganization of the entire transform boundary ~2.4 Ma, unrelated to any known kinematic change, asking for a potential role of Indus sedimentation over the dynamics of the plate boundary. Migration over distance on the order of ~100-km of the India-Africa transform boundary occurred when plates were moving fast, resulting in strike-slip relative motion rates >1cm.yr-1, finite offsets > 200 km, and age contrast between the adjacent lithospheres >30 Myrs. Since the Eocene slow-down in plate motion, the age contrast between India and Arabia decreased to less than 30 Myrs. The transform boundary remained at a fixed location and has adjusted to plate motion change mainly through flexural processes. The transform plate boundary shows an increased sensitivity to local stress perturbations, such as the one related to Indus sedimentation rates.