3D P-wave velocity structure of the hyper-extended Deep Galicia Margin
IPGP - Îlot Cuvier
Séminaires Géosciences Marines
University of Southampton
The Galicia 3D reflection-refraction seismic experiment was carried out in 2013 at the Galicia rifted margin in the northeast Atlantic Ocean, west of Spain. The main geological features within the 64 by 20 km (1280 km²) 3D box investigated by the survey are the peridotite ridge (PR) composed of serpentinized peridotite, a series of fault bounded, rotated basement blocks and the S reflector, which has been interpreted to be a low angle detachment fault. Forty-four short period four component ocean bottom seismometers (OBS) and 28 ocean bottom hydrophones (OBH) were deployed within the 3D box. 3D multichannel seismic (MCS) profiles sampling the whole box were acquired with two airgun arrays of 3300 cu.in. fired alternately (in flip-flop configuration) every 37.5 m. The 3D first-arrival time tomography carried out with the Galicia 3D dataset constrained the main elements of the hyper-extended zone. The final model show that the serpentinisation at the rifted continental margin was probably initiated when the whole crust cooled to become brittle, and the deformation was focussed 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 was 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. New results from 3D joint refraction-reflection traveltime tomography improve the sharpness of the P-wave velocity structure from the sediments down to the uppermost mantle, and constrain the topography of the S reflector. Results are validated by synthetic tests, by comparisons with the results of previous first-arrival traveltime tomography and with images from the processed Galicia 3D MCS volume. The 3D layered P-wave velocity structure allows a better estimation of the degree of serpentinisation below the S-reflector.