3D P-wave velocity structure of the deep Galicia rifted margin

Abstract

Galicia 3D, a reflection-refraction and long offset seismic experiment was carried out in 2013, at the Galicia rifted margin, in the northeast Atlantic Ocean, west of Spain. The survey covered a 64 km by 20 km (1280 km2) zone where the main geological features are: the fault bounded, rotated, continental crustal blocks, the S reflector which has been interpreted to be a low angle detachment fault, and the Peridotite Ridge (PR) thought be upper mantle exhumed to the seafloor during rifting. In the 3D box, two airgun arrays of 3300 cu.in. were fired alternately (in flip-flop configuration) every 37.5 m. All shots are recorded by 44 short period four component ocean bottom seismometers and 26 ocean bottom hydrophones. We present the P-wave velocity volume derived by the 3D first arrival time tomography. Tomographic results are validated by synthetic tests and by the comparison with published multi-channel and wide-angle seismic lines. The main outcomes are as follows: 1) The top of the syntectonic sediments imaged by MCS matches with the tomographic 3.5 km/s iso-velocity contour. Block bounding faults are imaged as lateral velocity contrasts. Velocities within the basement blocks vary between 3.5 km/s and 6.5 km/s. Lower crustal velocities are only observed within the middle and westernmost basement blocks. 2) The PR rises up to 5.5 km depth on the southernmost profiles. Twenty kilometers north, the basement deepens 1 km, and the basement expression of PR dissipears. 3) The 6.5 km/s isovelocity contour matches the MCS retrieved S reflector. Beneath the S reflector, the velocities increase from 6.5 to 7.5 km/s within less than 1 km depth interval. Low velocity zones (~ 7 km/s) are observed at the intersections between the block bounding faults and S reflector suggesting that the faults act as conduits for the water flow in the upper-mantle