I.A. Petrinovic

Late Cenozoic tectonism, collapse caldera and plateau formation in the central Andes

Abstract The evolution of Andean volcanism including the formation of late Miocene to Recent collapse calderas on the Puna plateau is generally interpreted in terms of the kinematic framework of the Nazca and South American Plates. We present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first-order NW–SE-striking zones of left-lateral transtension on the local and regional scales. Consequently, ages of collapse calderas indicate activity of these fault zones which initiated at about 10 Ma on the Puna plateau. The onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen-parallel stretching upon reaching maximum crustal thickness and critical surface elevation. Horizontal magma sheets that formed at mid-crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub-vertical faults. This accounts well for the observed tectono-magmatic phenomena at surface. It follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the Andean plateau rather than to changes in the geometry of the Wadati–Benioff zone or plate boundary kinematics.

Morphometry and evolution of arc volcanoes

Volcanoes change shape as they grow through eruption, intrusion, erosion, and deforma- tion. To study volcano shape evolution we apply a comprehensive morphometric analysis to two contrasting arcs, Central America and the southern Central Andes. Using Shuttle Radar Topography Mission (SRTM) digital elevation models, we compute and defi ne parameters for plan (ellipticity, irregularity) and profi le (height/width, summit/basal width, slope) shape, as well as size (height, width, volume). We classify volcanoes as cones, sub-cones, and massifs, and recognize several evolutionary trends. Many cones grow to a critical height (~1200 m) and volume (~10 km 3 ), after which most widen into sub-cones or massifs, but some grow into large cones. Large cones undergo sector collapse and/or gravitational spreading, without sig- nifi cant morphometry change. Other smaller cones evolve by vent migration to elliptical sub- cones and massifs before reaching the critical height. The evolutionary trends can be related to magma fl ux, edifi ce strength, structure, and tectonics. In particular, trends may be controlled by two balancing factors: magma pressure versus lithostatic pressure, and conduit resistance versus edifi ce resistance. Morphometric analysis allows for the long-term state of individual or volcano groups to be assessed. Morphological trends can be integrated with geological, geophysical, and geochemical data to better defi ne volcano evolution models.

Upper-Crustal Structure of the Central Andes Inferred from Dip Curvature Analysis of Isostatic Residual Gravity

The relationship between Bouguer gravity, isostatic residual gravity and its dip curvature, first-order structural elements and distribution of Neogene volcanic rocks was examined in the southern Altiplano and Puna Plateau. In the southern Altiplano, strong positive Bouguer gravity corresponds to areas affected by late Cenozoic faulting and large-scale folding of upper crustal rocks. Dip curvature analysis of isostatic residual gravity shows that elongate zones of maximum curvature correspond remarkably well with the structural grain defined by first-order folds and faults. Similarly, isostatic residual gravity in the Puna is largely controlled by prominent, upper-crustal structures and also by the distribution of Miocene and younger volcanic rocks. In particular, the Central Andean Gravity High, one of the most prominent features of the residual gravity field, corresponds with domains of low topography, i.e., internally- drained basins, which are surrounded by zones of Neogene faults and abundant felsic volcanic rocks. Dip curvature analysis of the isostatic residual gravity field shows that elongate zones of maximal curvature correlate with the strike of prominent Neogene faults. Our study suggests that such analysis constitutes an important tool for imaging upper-crustal structures, even those that are not readily apparent at surface. For example, upper-crustal faults in the Salar de Atacama area, the presence of which is suggested by the dip curvature of residual gravity, offers a plausible explanation for the pronounced angular departure of the volcanic belt from its overall meridional trend and its narrowing south of the salar. In contrast to previous interpretations, our study suggests that gravity anomalies of the Central Andes are largely controlled by the distribution of late Cenozoic volcanism and tectonism. Dip curvature analysis of gravity fields bear great potential for elucidating first-order structural elements of deformed, upper-crustal terrains such as the modern Andes.

Pucarilla-Cerro Tipillas volcanic complex: the oldest recognized caldera in the southeastern portion of central volcanic zone of Central Andes?

We recognize the most eastern and oldest collapse caldera structure in the southern portion of the Central Volcanic Zone of the Andes. A description of Middle-Upper Miocene successions related to explosive- effusive events is presented. The location of this centre close to Cerro Galn Caldera attests a recurrence in the volcanism between 12 and 2 Ma in this portion of the Altiplano - Puna Plateau.

Construction and degradation of a broad volcanic massif: The Vicuña Pampa volcanic complex, southern Central Andes, NW Argentina

The Vicuna Pampa volcanic complex, at the SE edge of the arid Puna Plateau of the Central Andes, records the interplay between volcanic construction and degradational processes. The low-sloping Vicuna Pampa volcanic complex, with a 1200-m-deep, southeastward-opening depression, was previously interpreted as a collapse caldera based on morphological considerations. However, characteristic features associated with collapse calderas do not exist, and close inspection instead suggests that the Vicuna Pampa volcanic complex is a strongly eroded, broad, massif-type composite volcano of mainly basaltic to trachyandesitic composition. Construction of the Vicuna Pampa volcanic complex occurred during two distinct cycles separated by the development of the depression. The first and main cycle took place at ca. 12 Ma and was dominated by lava flows and subordinate scoria cones and domes. The second cycle, possibly late Miocene in age, affected the SW portion of the depression with the emplacement of domes. We interpret the central depression as the result of a possible sector collapse and subsequent intense fluvial erosion during middle to late Miocene time, facilitated by faulting, steepened topography, and wetter climate conditions compared to today. We estimate that ∼65% of the initial edifice of ∼240 km 3 was degraded. The efficiency of degradation processes for removing mass from the Vicuna Pampa volcanic complex is surprising, considering that today the region is arid, and the stream channels within the complex are predominantly transport limited, forming a series of coalesced, aggraded alluvial fans and eolian infill. Hence, the Vicuna Pampa volcanic complex records the effects of past degradation efficiency that differs substantially from that of today.