Crustal structure of the eastern Acambay Graben, central Mexico, from integrated geophysical data

Abstract

The Acambay graben is a ∼70 km long E–W seismically active structure located within the Trans-Mexican Volcanic Belt (TMVB). Quaternary volcanism has contributed to shaping the graben morphology and reported hydrothermal activity suggests the existence of potential geothermal reservoirs. Our work aims at providing information about the unstudied subsurface structure of the graben using magnetotelluric (MT) soundings, as well as gravity and aeromagnetic data as preliminary work for near-future integrated 3D approach. We analyzed the distribution of magnetic anomalies and its relation to faulting using total field (TFM) aeromagnetic digital charts E14 and F14 (https://www.gob.mx/sgm) and through the analysis of the tilt derivative of TFM. The gravity and magnetic horizontal derivatives provided complementary results to constrain the location of the main fault and to contrast the response of the Pastores and Acambay-Tixmadejé faults. For the subsurface interpretation, we present the results of a 25 km magnetotelluric (MT) profile across the eastern tip of the Acambay graben, oriented approximately perpendicular to the azimuth of the main ∼E–W fault systems. The resulting resistivity model incorporates gravity and aeromagnetic coincident model profiles. Density and magnetic susceptibility models were constrained with field sample data, surface geology and with the resistivity structure obtained from the 2D inversion of the measured MT soundings. The dimensionality and electric strike of the 13 MT stations that compose the profile were analyzed for a period band of 0.01 to 100 s. The average electric azimuth obtained was 88.5°±10°, which was used to invert the rotated to the electric strike MT profile. The integrated geophysical model reveals important lithological contrasts at the northern and southern bounding faults of the graben. While the northern Acambay-Tixmadejé fault affects a resistive ground and is associated to a >20 mGal gravity anomaly, the southern Pastores fault corresponds to a contrasting conductive zone with relatively low gravity anomaly (∼5 mGal). In contrast, the magnetic anomaly shows the opposite behavior; at the Pastores fault the observed amplitude is of 400 nT while the amplitude across the Acambay-Tixmadejé fault is insignificant. The geophysical model shows that both the Acambay-Tixmadejé and Pastores faults have constant and opposite dipping angles of about 70°, defining a nearly symmetric graben structure. According to our results, the Acambay-Tixmadejé fault as well as the central graben fault system converge at a depth of ∼18 km that is consistent with the depth of the seismogenic crustal layer.