GEOLOGIC CHARACTERIZATION OF A PRESUMED COLLAPSE CALDERA IN SOUTH CENTRAL HIDALGO, MEXICO

The Trans Mexican Volcanic Belt is a 1000 km long east-west trending volcanic province that bisects the Mexican Republic. This province hosts volcanic calderas, many of which have been identified by indirect methods (satellite images and aerial photographs) and more recently, by digital elevation models and visualization tools inherent to geographic information systems.

In this work we present our findings regarding a presumably volcanic collapse caldera, informally called the Amajac Caldera, which is located in the south central region of the State of Hidalgo, Mexico. Geologically, this caldera is located near the boundary between the Mexican Volcanic Belt and the Faults and Folds Province. The Amajac caldera is semicircular, with a northeast-southwest trending 8.5 km long axis, and a northeast-southwest trending 8.0 km long short axis.

The identified stratigraphic column in the area consists on Mesozoic marine sediments as the dominant older rocks, and unconformably overlying younger Plio-Pleistocene volcanic and sedimentary rocks. The Mesozoic marine sediments, that constitute the basement of the Amajac Caldera, have been deformed during the Laramide Orogeny and now define a northwest-southeast trending structural grain. We associate the orientation of the axes of the Amajac Caldera with the reminder of the compressional stresses that prevailed during the last stages of the Laramide Orogeny. Younger Plio-Pleistocene rocks include volcanic rocks of the Atotonilco el Grande Formation, and fossiliferous lacustrine sediments that preserve seismites.

Our work indicates that the geologic evolution of the Amajac Caldera include stages of instability due to: seismicity, as recorded by the lacustrine sediments; collapse, as denoted by the tilted lacustrine sediments near the rim; volcanic resurgence; and due to the activity of presently active extensional faults. Moreover, along the area of influence of the Amajac Caldera, polygenetic mineralization and active hydrothermalism are present.