Compaction‐Driven Fluid Localization as an Explanation for Lower Crustal Electrical Conductors in an Intracontinental Setting.

We present electrical resistivity models, derived from magnetotelluric data, of the crust beneath the Bulnay region, Mongolia. They reveal that the lower crust contains a pattern of discrete zones (width of ~25 km) of low resistivity (<30 Ωm). Such features may be an effect of unaccounted‐for electrical anisotropy. However, when anisotropy is considered in the modeling, the features remain. We investigate an alternative explanation, based on a conceptual model of fluid localization and stagnation by thermally activated compaction, and demonstrate it is compatible with the observed low‐resistivity zones. The model explains the location, shape, and size of the zones, with plausible values of the activation energy for lower crustal creep (270–360 kJ/mol), and a viscous compaction length on the order of 10 km. The results imply tectonic deformation and compaction processes, rather than lithological‐structural heterogeneity, control the regional lower crustal fluid flow. Plain Language Summary: We collected magnetotelluric data in the Bulnay region, Mongolia, which is a compressive intracontinental region, by measuring electric and magnetic fields at the surface. Using these data, we generated high‐resolution electrical resistivity models. The models image the lower crust and show that it contains discrete zones of low resistivity that have a distinct pattern. Other studies have shown that such a pattern may be an effect of ignoring electrically anisotropy. But when anisotropy is considered in the modeling the features remain nearly the same. Because of this, we investigate whether an alternative explanation can cause these features. We find that a conceptual model of fluid localization and stagnation by hydromechanical compaction is compatible with the observed pattern of the low‐resistivity zones. In fact, it can explain their location, shape, and size. In addition, we use the conceptual model to determine which viscous rheology is consistent with the data. Finally, we find that estimates for hydraulic and rheological properties of the region are consistent with this explanation. This conceptual model has implications for fluid flow in the lower crust, showing that it is controlled by tectonic deformation and compaction processes, rather than lithological or structural features. Key Points: Electrical resistivity models across a compressive intracontinental region image a pattern of low‐resistivity zones in the lower crustThe pattern is consistent with hydrodynamic stagnation of crustal fluids due to thermally activated compactionThe results demonstrate that compaction processes, rather than lithological structure, control the regional lower crustal fluid flow [ABSTRACT FROM AUTHOR]