Geological study of Juchipila graben, western Mexico, and radioactive heat production of ignimbrites: an example of unconventional geothermal reservoir
Abstract
The NNE-SSW trending Juchipila graben, located the southeastern part of the Sierra Madre Occidental (SMO) of Mexico formed in the initial stage of the lithosphere extension that led to the opening of the Gulf of California. Despite no silicic volcanic activity occurred after 23 Ma, the graben is characterized by hot springs with temperatures between 35 and 74 °C and elevated heat flow values of 90-200 mW/m2. This suggests that possible existence of an unconventional geothermal reservoir related to heat producing isotopes (U, Th y K) in the subsurface geologic units.
We carried out a geologic and U-Pb and Ar-Ar geochronology study, measured abundances of heat producing isotope and estimated heat production in ignimbrites and rhyolitic lavas of the graben. Our study indicates the presence of ten units which can be grouped into: 1) pre-extensional units (48.1-32.71 Ma) including andesitic lava flows, siliciclastic sandstones and ignimbrites from distal sources; 2) syn-extensional units, consisting of pyroclastic fall and flow deposits, previously dated at ~25.5 Ma (fission track in zircons), for which we obtained 7 U-Pb and Ar-Ar ages in the range 30.3-29.0 Ma. Capping these pyroclastic units are Na-alkaline basaltic lavas with Ar-Ar age of 25.9 Ma that were vented from the graben border faults; 3) post-extensional rocks consisting of ash flows deposited inside the NE part of the graben (~24 Ma), intra-graben sediments (max age of deposition ~20 Ma) and sub-alkaline basaltic lava flows (13-11 Ma) that filled the southern part of the graben damming the drainage and causing the formation of a lake whose sediments with U-Pb ages of 6.95- 5.59 Ma. Minor faulting and tilting of the lacustrine sediments in the southern end of the graben point to a tectonic reactivation in this area, which allows the reactivation of the drainage and the partial erosion of the graben filling. The distal ignimbrites of the first group shows the highest values of heat production (5.02-3.6 μWm-3) followed by the syn-extensional ignimbrites (1.3-3.7 μWm-3). Once corrected for the rock density, these values are comparable with those reported in the literature for heat-producing granies of Australia, Saudy Arabia and Slovakia. Post-extensional rocks show relatively low values (0.9-2.4 μWm-3).- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H11K1653M
- Keywords:
-
- 0915 Downhole methods;
- EXPLORATION GEOPHYSICS;
- 1822 Geomechanics;
- HYDROLOGY;
- 3616 Hydrothermal systems;
- MINERALOGY AND PETROLOGY;
- 8135 Hydrothermal systems;
- TECTONOPHYSICS