The 322 ka Tiribí Tuff: stratigraphy, geochronology and mechanisms of deposition of the largest and most recent ignimbrite in the Valle Central, Costa Rica |
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| Authors: | Wendy Pérez Guillermo E Alvarado Phillip B Gans |
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| Institution: | (1) SFB 574, University of Kiel and Leibniz Institute for Marine Science (IFM-GEOMAR), Wischhofstr. 1-3, 24148 Kiel, Germany;(2) área de Amenazas y Auscultación Sismo-volcánica, Instituto Costarricense de Electricidad. Apartado, 10032-1000 San José, Costa Rica;(3) Escuela Centroamericana de Geología, Universidad de Costa Rica, San José, Apartado, 35-2060 UCR, Costa Rica;(4) Department of Geological Sciences, University of California, Santa Barbara, 93106-9560, USA |
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| Abstract: | The Tiribí Tuff covered much of the Valle Central of Costa Rica, currently the most densely populated area in the country (∼2.4 million inhabitants). Underlying the tuff, there is a related well-sorted pumice deposit, the Tibás Pumice Layer. Based on macroscopic characteristics of the rocks, we distinguish two main facies in the Tiribí Tuff in correlation to the differences in welding, devitrification, grain size, and abundance of pumice and lithic fragments. The Valle Central facies consists of an ignimbritic plateau of non-welded to welded deposits within the Valle Central basin and the Orotina facies is a gray to light-bluish gray, densely to partially welded rock, with yellowish and black pumice fragments cropping out mainly at the Grande de Tárcoles River Gorge and Orotina plain. This high-aspect ratio ignimbrite (1:920 or 1.1×10−3) covered an area of at least 820 km2 with a long runout of 80 km and a minimum volume outflow of 25 km3 (15 km3 DRE). Geochemically, the tuff shows a wide range of compositions from basaltic-andesites to rhyolites, but trachyandesites are predominant. Replicate new 40Ar/39Ar age determinations indicate that widespread exposures of this tuff represent a single ignimbrite that was erupted 322±2 ka. The inferred source is the Barva Caldera, as interpreted from isopach and isopleth maps, contours of the ignimbrite top and geochemical correlation (∼10 km in diameter). The Tiribí Tuff caldera-forming eruption is interpreted as having evolved from a plinian eruption, during which the widespread basal pumice fall was deposited, followed by fountaining pyroclastic flows. In the SW part of the Valle Central, the ignimbrite flowed into a narrow canyon, which might have acted as a pseudo-barrier, reflecting the flow back towards the source and thus thickening the deposits that were filling the Valle Central depression. The variable welding patterns are interpreted to be a result of the lithostatic load and the influence of the content and size of lithic fragments. |
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| Keywords: | Tiribí Tuff Ignimbrite Welding facies Channelized 40Ar/39Ar geochronology Barva caldera Costa Rica |
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