A preliminary crustal model of the Oaxaca continental margin and subduction zone from magnetotelluric and gravity measurements

Abstract

A preliminary model of the conducting layer associated with the continental margin and the Cocos Plate subduction zone along two transects normal to the southern Mexico continental margin is presented. The transects are located approximately from Oaxaca City to Puerto Escondido (W-transect), and to Puerto Angel (E-transect). The study is based on fourteen magnetotelluric (MT) soundings and on regional gravity measurements. Modelling of gravity data includes spectral analysis, 2 and 2.5-D Talwani-type models and several inversion schemes with constant and exponentially variable density contrasts. One dimensional inversion of MT data using a Marquardt SVD algorithm was performed on the rotationally invariant determin ant average apparent resistivities and phases after static shift was removed with reference to a site apparently devoid of galvanic distortion. A conductive layer of varying thickness was correlated with a seismic reflecting horizon under various MT sites of the western profile. These correlated soundings were used to constrain the inversion results of the remaining MT sites. We assume that the top of the electrically conductive horizon corresponds to the top of the Phanerozoic lower continental crust, which has an approximate dipping angle of 20° to 25° and is characterized by conductivities from 10 to 90 ohm-m. Although this boundary is in general plunging towards the continent there are several sites in both transects at which it flattens and even reverses its trend. This may be an artefact of the I-D inversion in zones where the data are 2 or 3-D. This is supported by relatively high induction vectors consistently pointing SWat these sites. The crust thickens towards the central portion of both MT transects, beneath the Sierra Madre del Sur. There is a good correspondence with gravity models derived from spectral analysis and estimates of the crust/mantle boundary. There is however an apparent depth difference (8 to 10 km) between the MT- and gravity-derived crust/mantle boundary. The dip of the subducting plate estimated from the gravity model is shallow, about 13 degrees. The topography of the Moho shows a correlation with major structural features at the surface.