Regional crustal and upper mantle structure of the Tien Shan, central Asia: Testing the application of an impedance based correction for distortion of magnetotelluric data by shallow indentifiable three-dimensional structure

In conjunction with a large interdisciplinary effort to study the Tien Shan, magnetotelluric data were collected along a transverse profile across the central Tien Shan located along 76°E. The results of 2-D inversion and modeling indicate that the northern Tien Shan consists of a discontinuous high resistivity (>5000 Ω·m) crustal section extending generally to depths of 30 to 40 km and reaching 70 km below the northern range front. The southern crustal section is composed of small high resistivity crustal blocks (∼10–15 km2) interspersed with generally less resistive (20 Ω·m to 100 Ω·m) zones. The lithosphere of the central range is characterized by a sub-vertical low resistivity (2 Ω·m to 20 Ω·m) zone extending from <10 km to >60 km and elevation of the less resistive mantle properties to depths of <30 km. A northward dipping boundary is evident in the southern section. This boundary is defined by the terminations of low resistivity zones in the lower crust and a higher resistivity mantle zone.; The electrical structure represented by the 2-D inversion of the Tien Shan data may be affected by 3-D structures. Distortion of electromagnetic fields by 3-D structures is often major obstacle in magnetotelluric data interpretation. The investigation of the Tien Shan included development of a correction process for 3-D distortion. This dissertation presents synthetic data that illustrate the correction for distortion from regional 3-D surface structures needs to be treated with a frequency dependent approach and introduces a correction based on impedances derived from modeled responses of surface geometry. The modeling studies indicate that artifacts produced from the 2-D inversion of data distorted by 3-D structures can be reduced, if not eliminated. The application of the correction to the observed data was problematic due to influences on the diagonal tensor elements from galvanic distortion and possibly the magnitude of the inductive effects. Conclusions from the corrected synthetic data set indicate that some valid adjustments were made by the correction to the Tien Shan data. In the Tien Shan modeling the 3-D artifacts consisted of isolated zones of enhanced conductivity in the middle and lower crust.