A new 3-D resistivity formulation helps interpretation of the laminated structure in the lower crust

Deep reflection seismic data reveal that laminated structure is widely developed in the middle-to-lower crust of the continent, but no consensus has been reached for its genesis. Reflective laminated structures ranging from tens of meters to tens of kilometers can be produced by a variety of geological processes, and other physical parameters such as resistivity can constrain the nature of this laminated structure. The resistivity model of the middle-to-lower crust is usually constructed from the inversion of magnetotelluric (MT) data, but too smooth. In this study, an equivalent formula for characterizing the resistivity of three-dimensional laminated structure is derived, which can accurately consider the azimuthal anisotropy and irregular distribution. Based on this, a new flowchart of interpretation is proposed to obtain the resistivity of the reflective laminated structure in the middle-to-lower crust. We confirmed the correctness of the above idea through three-dimensional numerical simulation and applied it to the Basin and Range Province (BRP) in the western United States. We argue that the ongoing underplating with the trapped mafic melts of ~50 wt% is responsible for the formation of the conductive lamellae, supporting the end-member model of high-melt-fraction proposed before.

Battle Mtn heat flow high is caused by ongoing basaltic intrusion with high-melt-fraction. (a) The locations of two PASSCAL seismic profiles (blue lines) and EarthScope NVO08 MT site (magenta star). The red shadow covers the area of Battle Mtn heat flow high. The red polygons represent the Cenozoic calderas. DV: Dixie Valley; BVF: Buffalo Valley field. (b) Simplified interpretive model for the crust structure (to a depth of ~32 km) of the study area. The upper crust is dominated by a series of NNE-SSW normal faults. The anisotropy in the middle-to-lower crust is achieved by a series of multicyclic, subparallel lenses (orange) that tend to be flattened and elongated with depth. The lamellae in this figure were vertically exaggerated to clearly show the characteristics of the structure.

This research was supported by National Natural Science Foundation of China (NSFC) with grants 41830212 , 41974082, and 41774079.

Liu, SY;Xu, YX*;Yang, B;Guo, ZQ;Shi, Y;Liu, Y. Deciphering fine electrical conductivity structures in the crust from MT data using the equivalent conductivity formula. Journal of Geophysical Research: Solid Earth, 126(10), e2021JB022519.