6th Colloquium on historical earthquakes & paleoseismology studies

Cacahuamilpa Cave, Mexico: a unique paleo-speleo-sismological and paleoclimate archive

25 Oct 2018, 15:30

20m

Han-sur-Lesse

Oral Preference Session 4

Speaker

Prof. Víctor-Hugo GARDUÑO-MONROY (UMSNH-INICIT)

Description

Paleospeleosismology is a discipline, which studies fallen stalagmites and stalactite as archives of past seismic events (Quinif, 1996; Gilli, 2005). Damaged or fallen speleothems (named seismothems) are often observed in caves, and first recognized at Han-sur-Lesse karst complex. There are several physical causes that can damage or break stalagmites: one is the reaction of the stalagmite – as an inverse pendulum – at the passage of a seismic wave (Szeidovitz et al., 2008). Here we present the Cacahuamilpa Cave karst complex, one of the largest in Mexico but which formation and evolution is still poorly documented. This karstic complex contains numerous fallen stalagmites, several one of giant dimensions, that could have been broken during large past seismic events. Cacahuamilpa Cave is located in the Guerrero state, at midway between Mexico City and Acapulco in one of the regions of Mexico with the higher seismicity caused by the subduction of the Cocos plate under the North American one. A few studies in the 60’ and 70’ suggested that the cave formed end of Pliocene as a suspended river channel into Cretaceous carbonates of the Morelos Fm. (Enjalbert, 1964; Bonet, 1971). Between 80kyrs and 50kyrs, one of the entrances of the karstic tunnel was sealed by lahar products of the nearby Nevado del Toluca stratovolcano (Capra et al., 2002). The karstic tunnel is disseminated of stalagmites with size variable from 1-2 meters longs to 70 m high and 300 tons weight. Some of them are broken and oriented following precise patterns and orientations. Because of the total lack of geological ages, an ongoing study was initiated in 2013 to obtain a clear chronological framework of the construction of the speleothems and obtain reliable dates of the broken surfaces, using U/Th dating methods. First results published in Méjean et al. (2015) showed that three groups of stalagmites were broken between 80kyrs and Holocene (1000 years ago). Theoretical calculations suggested seismic events with magnitude of M 7-8 and epicentral distances of 50-100 km from the cave. A recent survey in February 2018 was focused on two groups of stalagmites, one including the Calendario Azteca (The Aztec Calendar), a giant broken speleothem of 13 meters length and 3.5 m diameter. New U/Th dating showed that these stalagmites grow extremely fast and most of them in two short periods corresponding to the interstadials of the Marine Isotopic Stage 5, between 75kyrs and 90kyrs (5a, 5b) and 110 to 120kyrs (5d) (Liesecky and Raimo, 2005). These periods are characterized by high rate soft humidity and rainfall (Bar-Matthews et al.2000), which could have favored the precipitation of large amounts of carbonates creating the giant speleothems in the Cacahuamilpa cave. Speleothems of modest dimensions have been probably broken by seismic events but large ones are difficult to explain and alternatively hypotheses, such as sediment creeping when the cave was still opened on both sides and invaded regularly by volcanoclastic sediments, should be take into account. Quinif, Y., 1996. Ann. Soc. Geol. Belg. 119, 1-13 ; Gilli, E., 2005. C.R. Geosci. 337, 1208-1215 ; Szeidovitz, G. Et al., 2008. J. Seismol. 12, 21-33; Enjalbert, H., 1964. Bull. Ass. Géograph. Franc. 324-325, 30-58; Bonet, F., 1971. UNAM, Inst. Geol. Boletín 90, 98; Méjean, P. et al., 2015. J. South Amer. Earth Sci. 57, 23-31; Capra, L. et al., 2002. J. Volcanol. Geotherm.Res. 113, 81-110; Lisiecki, L.E. et Raymo, M.E., 2005. Paleocean. 20; Bar-Matthews, M. et al., 2000. Chem. Geol. 169, 145-156.