IGNIMBRITE STRATIGRAPHY OF THE “VOLCANIC” WESTERN CORDILLERA AND ADJACENT ALTIPLANO IN THE REGION OF THE PUCHULDIZA GEOTHERMAL AREA (19°15'S TO 19°25'S), NORTHERN CHILE: IMPLICATIONS FOR ALTIPLANO UPLIFT

The “Volcanic” Cordillera lies between the Western Andean Thrust Belt to the west, and the Altiplano to the east. Since the 1970’s, uplift of the western Altiplano has been attributed to west-vergent thrusting along its western edge, but over the past decade, some workers have attributed uplift to lower crustal flow. Ignimbrites are key for evaluating the relative importance of these two mechanisms, because they form strain markers and record crustal thickening events, but they are poorly known here. We construct a new ignimbrite stratigraphy, through mapping, petrographic and geochemical analysis, and U-Pb zircon single crystal ion probe dating, which is ideal for rocks altered by protracted arc magmatism. K/Ar dating by previous workers yielded inconsistent results and prevented correlation. Ignimbrites include:

(1) Late Oligocene (25 Ma) rhyolite ignimbrites, previously mapped as Cretaceous; these are equivalent to the “strongly-folded Belén ignimbrites” [1] at 17°20’S.

(2) Early Miocene rhyolite ignimbrites, including 18.8±0.4 Ma Utayane Formation and 18.9±0.7 Ma Condoriri Formation. These are age-correlative with the Oxaya ignimbrites, which extend 300 km N-S from Peru to Chile, and 130 km E-W from the Altiplano to the coast [2]. This age cluster also includes previously unrecognized, distinctive pyroxene trachydacite/dacite ignimbrites (18.3±0.3 Ma), interstratified with lavas of the same composition previously referred to as “andesites” of Lower Puchuldiza Formation.

(3) Middle Miocene (14.6±0.4 Ma to 13.0±0.3 Ma) rhyolite ignimbrites, including “Upper Puchuldiza Formation”, and previously unrecognized rhyolite ignimbrites: a composite ignimbrite sheet overlain in angular unconformity by a distinctive lava-like ignimbrite, also folded.

(4) Flat-lying Pliocene dacite ignimbrite (3.02±0.15 Ma), similar to the 2.7 Ma “post-tectonic” Lauca-Perez ignimbrite [1], which covers wide areas to the north in Chile, Peru & Bolivia, and reaches the coast.

Our chronostratigraphy reveals an episode of wide-spread east-vergent thrusting, whose contemporaneity with the onset of Altiplano uplift places important constraints on existing geodynamic models.

[1] Schröder W., Wörner G., 1996. Symp. Int. Géodynam. Andine, Saint-Malo (FRA), 645-648. [2] Wörner et al., 2002, Tectonophysics 345:183-198.