Hydration of mantle peridotite to form serpentinite occurs at specific temperature and pressure conditions and yields changes in physical properties of the mantle rocks. Serpentinized rocks are found beneath the seafloor at slow- to ultraslow-spreading mid-ocean ridges and are thought to be present at about half the world's rifted margins. They are also inferred to be present in the downgoing plate at the subduction zones. The formation of serpentinite requires a supply of fluid to the mantle. The crustal rocks have low bulk permeabilities and the fault zones with higher permeabilities are believed to play an important role in the migration of fluids to the mantle depths. We present the results from 3D first-arrival time tomography carried out with the Galicia 3D (2013) dataset that covered a 64 by 20 km (1280 km²) 3D box at the at the hyper-extended zone of the Deep Galicia Margin (off-shore Spain). The P-wave velocity model is obtained using more than 150,000 first arrival times. It constrains the main elements of the hyper-extended zone, including: the peridotite ridge (PR); the fault bounded, rotated basement blocks; and the S reflector, which has been interpreted to be a low-angle detachment fault. The final model shows that the serpentinization at the rifted continental margin was probably initiated when the whole crust cooled to become brittle, and the deformation was focused along crust-cutting normal faults. We map the distribution of serpentinite below the hyper-extended crust and show that the amount of the serpentinite formed at the bottom of each fault is directly proportional to the displacement on that fault, which in turn is closely related to the duration of the fault activity. We also estimated the average rate at which seawater entered the mantle through the faults, and the rates are comparable to those estimated for water circulation in hot rock at mid-ocean ridges. We present also further analysis of the tomographic dataset using joint reflection-refraction tomography, which yields improved resolution of velocity variations above and below the S reflector

Bayrakci, G. ... et al.-- American Geophysical Union Fall Meeting, 12-16 December 2016, San Francisco

Peer Reviewed