THE VALLE DE BRAVO VOLCANIC FIELD. A NEW REPORT OF A QUATERNARY MONOGENETIC FIELD IN THE MEXICAN VOLCANIC BELT

The Valle de Bravo Volcanic Field (VBVF) is located at the southern front of the central Mexican Volcanic Belt, at 100°29'40'' - 99°50'50'' long.W, and at 19°28'45'' - 18°48'35'' lat. N. It lies to the SW of Nevado de Toluca stratovolcano. The VBVF accounts for approximately 3,703 km2, and includes at least 122 cinder cones, some shield volcanoes, a few lava domes, and a volcanic complex. The VBVF overlies Mesozoic metamorphic rocks (schists, metalimestones and pillow-lavas), and Paleocene granitic rocks. Cinder cones’ dimensions are 0.4 - 3.3 km (basal diameter) and heights of 80 - 360 m; the shield cones are bigger, with basal diameters up to 5.1 km and heights of up to 1,120 m. The 7 domes identified range in size from 1.5 x 1.4 km to 3.6 x 2.7 km and have heights of 70 to 1120 m; three other domes are coulee type and have sizes of 7.5 x 2.5 km to 3 x 1.3 km and heights of 160 to 420 m. A polygenetic volcanic complex composed of cones and domes is located at the western part of the VBVF. It is uncertain its age relationship with respect to the VBVF, but apparently was coeval with the initiation of the VBVF. We also identified several mafic lava flows that lack a cone source, suggesting that they are fissure-type events. Geomorphology of the cones indicates a relatively young age for most of them, since craters are still evident and flanks are little eroded. Many lava flows still show levees and some of them are little vegetated and lack soil. This is confirmed with age reports as young as 5 ka in some of the mafic lavas. The VBVF is mainly composed of olivine-rich basalts, basaltic andesites, and andesites. Shoshonites are also reported in this field