VOLCANIC RECORD AND TECTONICS OF THE BASIN OF MEXICO CITY

The Mexico City valley is a deep basin surrounded by late Pliocene to Quaternary volcanic centers. Volcanic rocks older than late Pliocene have been scarcely studied and the geometry and age of the faults that bound the basin remains only partly known. We integrated surface geologic mapping, the subsurface information provided by deep wells drilled by PEMEX, gravimetric data and new 39Ar/40Ar and K/Ar datings of both surface rocks and well cores to get a better knowledge of the basin. Gravimetric and well data, and the structure exposed between Tenango, Malinalco and Cuernavaca indicate that the basin is an E-W trending tectonic trough cut into Cretaceous limestone. The southern boundary is a system of E-W, north dipping extensional faults that underlain Nevado de Toluca and Popocatepetl volcanoes and Sierra Chichinautzin monogenetic field. Here the vertical drop of the Cretaceous limestone ranges from 2 to 2.8 km. The trough is further dissected by a two NNW-SSE trending, east dipping extensional faults, passing beneath the Sierra de las Cruces volcanic chain and the center of the city. The basin is filled by a continental Cenozoic succession in which at least 3 volcanic episodes can be recognized: 1) Oligocene lavas only found in the Texcoco well that correlate with those of the Taxco and Buenavista-Tilzapotla areas to the south. 2) Basaltic and andesitic flows of middle to late Miocene age (17 to 9 Ma) encountered in all the wells. They correlate with a belt of stratovolcanoes to the north of the basin we have dated at ~15-11 Ma (Sierra de Guadalupe, Tepotzotlan, and Villa del Carbon areas) and also are the source of the lahars and conglomerate of the Tepoztlan formation to the south of the basin. 3) Late Pliocene to Holocene mafic lavas mostly coming from the monogenetic volcanoes of Sierra Chichinautzin and flows of intermediate composion from polygenetic volcanoes of Sierra de las Cruces, Iztaccihuatl etc. Subsurface stratigraphy and exposed rocks clearly point to a gap in volcanism between ~9 and ~3.5 Ma, which is represented by several hundreds of meters of volcaniclastic deposits, sandstone and shales in the wells. The basin began to form in Oligocene. By Pliocene times the filling began to be eroded by streams flowing to the south. The emplacement of the Sierra Chichinautzin in late Quaternary time closed the basin again, forming the Tenochtitlan lake.