Resistivity imaging of Mendocino Fracture Zone using marine magnetotelluric data

Seafloor fracture zones are the inactive extensions of transform faults that represent a discontinuity in seafloor age, temperature, and bathymetry. Their structures and tectonic features provide important information about lithospheric evolution. The age contrast across the fracture zone produces differential subsidence resulting in lithospheric flexure with uplift on the young side and subsidence on the old side. There is still debate on whether a fracture zone is significantly weaker than the surrounding lithosphere.  Some models predict small-scale convection beneath the FZ due to the sharp thermal contrast. A magnetotelluric (MT) survey over the Mendocino fracture zones (MFZ) was carried out 600 km away from the US west coast in 2018. The primary objective is to determine whether changes in the electrical resistivity across the MFZ are caused by temperature variations or other factors at the LAB. There is a strong crust age contrast at different sides of the MFZ. The northern side, younger with a crust age of 6.5 Ma, is formed at the Gorda Ridge with depths of about 3 km. The southern side, older with a crust age of 33 Ma, is part of the Pacific plate with depths of about 4 km. On both sides of MFZ, seamounts are observed in the study region, which indicates off-axis magma upwelling. Two-dimensional (2D) inversion of MT data was done to construct a 2D resistivity model across MFZ. The inversion can fit the data to an RMS of 1.8 with an error floor of 5 % on the MT impedance tensor. The model shows strong resistivity contrasts at the depths of 30-80 km on both sides of MFZ. The resistivity contrast at such depths is likely attributed to partial melt and temperature since composition is most likely similar at both sides of MFZ. Other conductive anomalies are also observed at shallow depths along the profile, which collocates with seamounts at the study region, especially near the southern end of our MT profile. Therefore, the conductive anomalies at the shallow depths likely present partial melts driven by upwelling magma.