Three-dimensional anisotropic inversion and electrostratigraphic imaging of marine magnetotelluric data to understand the control of crustal deformations by pre-existing lithospheric structures in the Mexican Ridges Fold belt, Southwestern Gulf of Mexico

Abstract

SUMMARY3-D imaging of the lithosphere in the Mexican Ridges fold belt is important for understanding how the crustal deformations in this basin relate to deep tectonic processes and structures inherited from extinct Jurassic seafloor spreading. Here, we use broad-band (0.0001–0.4642 Hz) marine magnetotelluric data from the basin to reconstruct the 3-D anisotropic resistivities of the lithosphere and their spatial gradients. The resistivity gradients maxima enabled independent definition of important geological boundaries (seen on collocated seismic reflection data) and estimation of crustal thickness. We found anomalous layered zones of low resistivity and high electrical anisotropy at 5–8 km depth (coinciding with the regional detachment zone in Eocene shales in 3-D seismic data) and in the upper mantle which we interpret as indicating intense deformation and/or recent magmatic influence. We also found a banded crystalline basement structure across the fossil spreading centre comprising WSW–ENE trending, 6–10 km wide, electrically resistive subvertical sheets with conductive and anisotropic borders, which merge into a basal resistive stock-like body at 15–20 km depth. These are cut or bounded by later NNW trending major faults. These WSW and NNW structural trends correlate with the previously interpreted transform and normal faults that formed during the Late Jurassic opening of the Gulf of Mexico only if rotated clockwise by 25–30°. Surprisingly, the rugged thrust-related seabed is offset at the projected positions of the steep resistive-conductive basement sheets (which also have spatially coincident high magnetic intensity and seismicity) enabling us to infer they represent magmatic intrusions facilitated by pre-existing faults. Their conductive borders spatially coincide with possibly fluid-filled vertical fracture-sets in the overlying sediments seen in seismic data which we interpret as hydrothermal fluid pathways. We infer that a magmatic body recently intruded our study area, its ascent controlled by pre-existing basement structures, and influenced the deformation of the Neogene sequences and the seafloor topography.