| Abstract |
In the geothermal zone of PathÈ, located in the central part of Mexico, the first geothermal power plant on the American Continent was installed in 1959. This plant, of 3500 kilowatts, had to be abandoned because the steam was not enough. However, an important geothermal resource could be at greater depths than reached by former wells. Some hot springs and extensive alteration zones outcrop in PathÈ, with superficial temperatures higher than 35?C, while the temperatures in the old wells were in excess of 250?C. The outcropping rocks can be grouped in four units: Unit I, consisting of sedimentary and volcano-sedimentary rocks of Mesozoic to Early Tertiary ages; Unit II, composed by intrusive (granodiorite to diorite) rocks of Eocene-Oligocene ages; Unit III, made up volcanic and sedimentary (lacustrine) rocks, whose age is Late Miocene; and Unit IV, mainly composed of volcanic rocks of Plio-Quaternary age. At the geothermal zone, the most recent outcropping rocks are andesites and basalts dated at 6.2 million years (My). Thus, the zone is located out of the Mexican Volcanic Belt province. Four deformation phases were identified, mostly on rocks of Unit III. The oldest is a tensional phase producing normal faults with an almost E-W trend. The second was produced by compressive and tensional stresses, which formed some lateral faults with variable trends and some normal NE-SW faults. The third deformation event was of the distensive type, producing normal faults with a N-S to NNWSSE trend, including the formation of the Graben of PathÈ. The most recent phase was also tensional, with normal faults of an E-W direction, and took place between 6.2 and 4.2 million years ago. The Pathe geothermal zone is found at the eastern side of the Graben of PathÈ, which is oriented N-S and is some 10 km wide, aged between 7 and 6.2 My. The hot springs and alteration zones are related to the intersection of the E-W (fourth phase) and N-S (third phase) faults. The probable deep geothermal system could be related to those structural intersections, and its fluids could be contained in the fractured rocks of units III and I. |