THE ANOMALOUS LITHOSPHERIC STRUCTURE UNDER THE TEHUANTEPEC ISTHMUS, INSIGHTS FROM TWO MAGNETOTELLURIC TRANSECTS AND SURFACE GEOLOGY
We present the results of two ~200 km magnetotelluric (MT) profiles aimed to study the lithosphere under the Tehuantepec Isthmus. The broadband MT soundings were gathered along two NE-SW profiles that go from the Pacific to the Gulf coasts. The MT profiles across the isthmus are located close the triple junction of the North America, Cocos, and Caribbean plates. It is a region of great complexity, where diverse tectono-stratigraphic terrains and regional fault systems converge into a narrow land space.
The data were inverted using 2D and 3D algorithms in order to evaluate differences. The resulting model shows sharp electrical resistivity contrast along the profiles, defining dissimilar rock assamblages and successions in lateral contact. The spatial correlation of the resistivity model with the gravity and magnetic anomaly maps suggests a highly fragmented upper lithosphere, which seems to be a characteristic of the region. From this comparison we get a robust structural model across the isthmus that is consistent with the surface geology.
The main geophysical contrast divides the Chiapas Massif Igneous and Metamorphic complex, a Proterozoic-Paleozoic uplifted basement in the center of the isthmus, from the Mazateco Complex to the south, a large and deep body that extends to lower crustal depths (~50 km). The deeply rooted resistive (>1,000 Ohm-m) massif basement includes an upper section of Jurassic-Cretaceous marine crustal rocks in the upper 4-5 km. Farther south, two upper crustal terrains can be distinguished, the Oaxaquia and the Xolapa complexes, overly the more conductive accretionary prism associated with the subduction of the Cocos plate. North of the Chiapas Massif lies the Campeche Basin, a flatland largely covered by a Pleistocene sedimentary sequence whose basement configuration is unknown. Under the Campeche Basin a more conductive lithosphere can be observed.
The seismic moho is consistent with the resistivity structure of the lithosphere in that it is deeper under more resistive and thicker crust than under thinner, more conductive crustal blocks. The comparison of the conductivity distribution with preexisting deep seismic tomographic data points to a highly fractured and probably a more hydrated lithosphere in the region. Other regional features such as the uneven sea levels and subducting rates of the segmented Cocos plate at the trench, the change in subduction angles of the segments over short distances, and the reported mantle windowing under the isthmus suggest that the observed anomalous lithosphere under the Tehuantepec Isthmus could have been a consequence of the Chicxulub meteorite impact. Although the impact zone is over 600 km from this tectonic complex region, our results, and the evidence from other studies point to the possibility that the impacting meteorite could have penetrated the upper mantle, which affected not only the crust but the thin marine lithosphere modifying the prevailing physical conditions farther and deeper than previously thought.
PAPIIT UNAM IG100922.