Geophysical Fingerprints of Exhumed Serpentinized Mantle Domains at The Ultraslow-Spreading Southwest Indian Ridge

Abstract

Ultraslow-spreading oceanic ridges (< 20 mm/yr) comprise ~35% of the global mid-ocean ridge system, yet their lithospheric structure and accretionary process are still little understood. At these ridges, the interplay between plate- and mantle-driven processes results in large along-axis variations in the accreted lithosphere and complex relationships between intermittent volcanic seafloor and extensive nonvolcanic seafloor domains. Geological and geophysical observations acquired at exhumed mantle domains indicate a subsurface structure that differs greatly from the traditional 3-layer crystalline crust topping the uppermost mantle formed at faster-spreading rates. At the eastern Southwest Indian Ridge (<14 mm/yr; SWIR), continuous emplacement of mantle-derived peridotites at the seafloor creates the widest nonvolcanic oceanic floor documented thus far. Extensive geological sampling in this area indicates that the peridotites are variably serpentinized (hydrothermally altered) and only a very minor component of crustal rocks derived from mantle melting are present. The SISMO-SMOOTH survey collected coincident wide-angle ocean bottom seismometer (OBS) and multichannel seismic (MCS) data along two long (~150 km) orthogonal profiles across (NS profile) and along (EW profile) of the SWIR axis. This thesis discusses results based on these datasets and provides unique insights into the mantle exhumation dynamics in the subsurface and the evolution of the tectonically accreted topmost lithosphere. P-wave first arrivals recorded by OBSs are used to perform traveltime tomography and produce the first detailed 2D regional velocity models across and along an ultraslow-spreading SWIR amagmatic segment. I suggest that changes in velocity with depth are related to changes in the degree of serpentinization and interpret the subsurface structure to be composed of highly fractured and fully serpentinized peridotites at the top with a gradual decrease in pore space and serpentinization to unaltered peridotites at depth. Furthermore, a complex system of successive and alternating polarity detachment faults across-axis is imaged in the subsurface and constrained by the velocity structure for the first time in this thesis. A prestack depth migrated reflection section of the NS profile provides additional constraints on the detachment faults and serpentinization gradients. A detailed analysis of the velocity changes with time on the NS profile provides the first-ever seismic constraints on the evolution of the tectonically accreted topmost lithosphere. Comparing my results with the much better-studied magmatically accreted lithosphere, I find that the tectonically accreted lithosphere evolves much faster and in a fundamentally different way, with significant implications for continental crust genesis and geohazards associated with future subduction of tectonically accreted oceanic lithosphere. Ultimately, the results shown in this thesis are applicable to other mid-ocean ridges and other plate boundaries. Tectonically accreted lithosphere has increasingly been reported at segment ends of slow-spreading ridges (Oceanic Core Complexes) and variably serpentinized peridotites have also been inferred worldwide in the oceanic crust adjacent to magma-poor rifted margins. Here, the lack of a clear understanding of the geophysical fingerprints of these domains challenges their unequivocal identification below thick post-rift sedimentary packages. For example, the results from this thesis may allow a reevaluation of existing seismic velocity models offshore eastern Canada and may assist in further detailing their identification and characterization.