Unravelling the collapse mechanisms at a Jurassic caldera of the Chon Aike silicic LIP in Southern Patagonia (47º 15´S, 71° 40´O), Argentina

SRUOGA, P.; JAPAS, S.; SALANI, ,F. M.; KLEIMAN, L.; GRAFFIGNA, M.

Queretaro, Méjico.

Workshop; Calderas Workshop Queretaro; 2008

UNAM

La Peligrosa Caldera is located at Sierra Colorada (47º 15´S, 71° 40´ W) in the Chon-Aike silicic LIP (Pankhurst et al., 1998). This volcanic province has an extension of 1.7 x 106 km2, including the continental platform and the Antarctic Peninsula and it has anestimated volume of 235.000 km3. Middle to Late Jurassic in age (188-153 Ma, Pankhurst et al., 2000) this long-lived and widespread volcanism took place in Patagonia along the western margin of Gondwanaland. Remarkably homogeneous in composition (Sruoga, 1989; Pankhurst et al., 1998), it is dominated by voluminous ignimbritic plateaux, with granites, lava domes, minor intermediate lavas and epiclastic tuffs. However, collapse calderas have been exceptionally recognized. La Peligrosa Caldera (Sruoga, 1994, 2002) represents a unique window to understand the eruptive mechanisms which prevailed during the ignimbritic flare-up in Southern Patagonia during middle to late Jurassic times. At the northern flank of Sierra Colorada, Tertiary Andean thrusting and intense glacial erosion have helped to expose the roots of the deeply dissected La Peligrosa Caldera. Key pieces of lithologic and structural evidences are taken into account to reconstruct the volcanic structure (Sruoga et al., 2008). The sequence outcropping between C° Ghío and La Peligrosa ranch (Fig. 1) allows to unravel the collapse mechanisms leading to the caldera formation. Three units may be differentiated in this sequence, named La Salina Breccia, Los Acantilados Ignimbrite and Cerro Ghío Ignimbrite (Fig. 2). At the base, La Salina Breccia constitutes a chaotic deposit, ~100 m thick, which carries blocks up to 300 m inmersed in a pyroclastic matrix. It includes different types of lithics: Paleozoic basement schists, phyllites, quartzites and granites, Jurassic (?) andesites, rhyolites and ignimbrites, and laminated and deformed lacustrine sediments. This mega to meso breccia (Lipman, 1984) exhibits westward wedging, accompanied by strong variation in lithic size and content.  To the upper part, a transitional passage to the overlying unit may be recognized, based on lithic depletion and strong size reduction. The Los Acantilados Ignimbrite may be described as a lithic poorly to moderately welded ignimbrite.  This ~300 thick whitish and massive deposit is chraracterized by a large proportion of lithics, up to 0.50 m in size. It shows vertical zonation, with green fiamme levels developing eutaxitic textures.  In the upper part of this deposit two big lithic concentration levels are recognized (Fig. 2). The uppermost unit is the Cerro Ghío Ignimbrite. It corresponds to a ~ 400 meters thick, massive, dark green, crystal-rich and densely welded cooling unit. It shows an order of magnitude decreasing thickness at the suspected caldera margins.  South and westwards, the ignimbritic deposit thins to ~40-60 m.             The described section records at least two-post  collapse caldera event. The first one is represented by the La Salina Breccia. The megabreccia at the bottom and the vertical and lateral variation in lithic size and concentration suggest that the collapse cannot be described as a single piston-like event. The lithic concentration levels at the upper part of Los Acantilados would be representing minor local incremental adjustments during the rapid collapse. The emplacement of this large-volume lithic ignimbrite records the initial replenishment of the caldera. Shortly afterwards, due to the absence of reworked epiclastics, the collapse continued with the reactivation of normal faults and the emplacement of the Cerro Ghío Ignimbrite. Rhyolitic lava domes and flows represent the post collapse volcanism.             A prevailing transtensional regime controlled the outpouring of the silicic large-volume ignimbrites and lava flows. La Peligrosa Caldera generated after a dilatational zone related to a releasing step-over associated with a left hand en-échèlon array of transtensional N-S to NNW regional faults.  Caldera collapse is dominated by NNW sinistral and WNW dextral  transtensional faults (Fig.1). Overprinting caldera structures, later ENE transtensional faults are linked to the rhyolitic post collapse volcanism and indicate a temporal change in stress conditions. The relationship between volcanic facies (megabreccia and two intracaldera ignimbrites) and main normal faults allow us to propose a piecemeal-trapdoor type caldera, with the deepest part between the cerro Ghío and the northwestern flank of Sierra Colorada.  Although many parts are lacking to fully support this model, future studies will provide new elements for better understanding the collapse mechanisms of La Peligrosa Caldera and test its application in the rest of the Chon Aike silicic LIP.