VOLCANIC HISTORY OF THE ILOPANGO CALDERA, CENTRAL AMERICAN VOLCANIC ARC

Ilopango caldera is located at El Salvador, within the Central American Volcanic Arc. It is a rectangular depression, 11 by 17 km, now filled by the Ilopango Lake. Caldera´s magmatism is related to subduction between the Cocos and Caribbean plates. The development of the caldera was closely associated with regional and local fault systems that formed the El Salvador Fault Zone and the Central Graben of El Salvador. Ilopango caldera is apparently a pull-apart depression that have had several volcanic episodes; thus, it is a volcano-tectonic structure. The caldera is considered still active, as well as the related faults, such as the El Drenaje fault (San Vicente fault) with a strike-slip earthquake of Mw =6.6 at 2001, posing both a volcanic and seismic danger to the densely populated metropolitan area of capital city San Salvador, and other towns nearby to the caldera. We have carried out systematic stratigraphic work in the past two years that we have documented with new U-Pb and U-Th ages on zircon separates and AMS14C ages in the most recent deposits. The results show a long-lived magmatic system that has been active since about 1.5 Ma to Present. Just mentioning the large deposits, we have identified at least 14 ignimbrites, all separated by paleosols or erosional surfaces indicating repose times between eruptions. This means that Ilopango caldera has erupted episodically since its initial catastrophic eruption, at about 1.5 Ma, until its last large eruption, at about 1500 BP, which formed the deposit named as Tierra Blanca Joven (TBJ), an eruption with possible global clime effects that devastated the ancient populations in this part of Mesoamerica. Latest volcanism in historic times includes the emplacement of a lava dome within the Ilopango lake in 1879-1880, the Islas Quemadas dome. In addition to the zircon and ¹⁴C ages, we are currently dating all main units with ⁴⁰Ar-³⁹Ar technique at the geochronology laboratory at Oregon State University, so that we can get precise times of each eruption in order to establish the recurrence intervals for the big eruptions. This study was financed by CONACYT-CB grant 240447 and logistically supported by MARN-El Salvador.