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The Lípez region of southwest Bolivia is the locus of a major Neogene ignimbrite fl areup, and yet it is the least studied portion of the Altiplano-Puna volcanic complex of the Central Andes. Recent mapping and laserfusion 40 Ar/ 39 Ar dating of sanidine and biotite from 56 locations, coupled with paleomagnetic data, refi ne the timing and volumes of ignimbrite emplacement in Bolivia and northern Chile to reveal that monotonous intermediate volcanism was prodigious and episodic throughout the complex. The new results unravel the eruptive history of the Pastos Grandes and Guacha calderas, two large multicyclic caldera complexes located in Bolivia. These two calderas, together with the Vilama and La Pacana caldera complexes and smaller ignimbrite shields, were the dominant sources of the ignimbriteproducing eruptions during the ~10 m.y. history of the Altiplano-Puna volcanic complex. The oldest ignimbrites erupted between 11 and 10 Ma represent relatively small volumes (approximately hundreds of km 3 ) of magma from sources distributed throughout the volcanic complex. The fi rst major pulse was manifest at 8.41 Ma and 8.33 Ma as the Vilama and Sifon ignimbrites, respectively. During pulse 1, at least 2400 km 3 of dacitic magma was erupted over 0.08 m.y. Pulse 2 involved near-coincident eruptions from three of the major calderas resulting in the 5.60 Ma Pujsa , 5.65 Ma Guacha, and 5.45 Ma Chuhuilla ignimbrites, for a total minimum volume of 3000 km 3 of magma. Pulse 3, the largest, produced at least 3100 km 3 of magma during a 0.1 m.y. period centered at 4 Ma, with the eruption of the 4.09 Ma Puripicar, 4.00 Ma Chaxas, and 3.96 Ma Atana ignimbrites. This third pulse was followed by two more vol canic explosivity index (VEI) 8 eruptions, producing the 3.49 Ma Tara (800 km 3 dense rock equivalent [DRE]) and 2.89 Ma Pastos Grandes (1500 km 3 DRE) ignimbrites. In addition to these large calderarelated eruptions, new age determinations refi ne the timing of two little-known ignimbrite shields, the 5.23 Ma Alota and 1.98 Ma Laguna Colorada centers. Moreover, 40 Ar/ 39 Ar age determinations of 13 ignimbrites from northern Chile previously dated by the K-Ar method improve the overall temporal resolution of Altiplano-Puna volcanic complex development. Together with the updated volume estimates, the new age determinations demonstrate a distinct onset of Altiplano-Puna volcanic complex ignimbrite volcanism with modest output rates, an episodic middle phase with the highest eruption rates, followed by a decline in volcanic output. The cyclic nature of individual caldera complexes and the spatiotemporal pattern of the volcanic fi eld as a whole are consistent with both incremental construction of plutons as well as a composite Cordilleran batholith.

The Neogene ignimbrite flare-up of the Altiplano Puna Volcanic Complex (APVC) of the Central Andes produced one of the best-preserved large silicic volcanic fields on Earth. At least 15 000 km3 of magma erupted as regional-scale ignimbrites between 10 and 1 Ma, from large complex calderas that are typical volcano-tectonic depressions (VTD). Simple Valles-type calderas are absent. Integration of field, geochronological, petrological, geochemical and geophysical data from the APVC within the geodynamic context of the Central Andes suggests a scenario where elevated mantle power input, subsequent crustal melting and assimilation, and development of a crustal-scale intrusive complex lead to the development of APVC. These processes lead to thermal softening of the sub-APVC crust and eventual mechanical failure of the roofs above batholith-scale magma chambers to trigger the massive eruptions. The APVC ignimbrite flare-up and the resulting VTDs are thus the result of the time-integrated impact of intrusion on the mechanical strength of the crust, and should be considered tectonomagmatic phenomena, rather than purely volcanic features. This model requires a change in paradigm about how the largest explosive eruptions may operate.

[1] The geologic origin of the Medusae Fossae Formation (MFF) has remained a mystery despite three decades of research. To better constrain its formation, an in-depth analysis of observations made in the literature was combined with a new survey of over 700 Mars Orbiter Camera narrow-angle images of the MFF to identify morphologic characteristics and material properties that define this formation as a whole. While previous work has identified clear agreement on some characteristics, our analysis identifies yardangs, collapse features, and layering as pervasive features of the MFF. Whereas collapse features and layering may implicate several different physical and chemical processes, yardangs provide vital information on material properties that inform about mechanical properties of the MFF lithology. Aspect ratios of megayardangs range from 3:1 to 50:1, and slope analyses reveal heights of up to 200 m with cliffs that are almost vertical. Other yardangs show lower aspect ratios and topographic profiles. These characteristics coupled to the presence of serrated margins, suggest that MFF lithology must be of weakly to heavily indurated material that lends itself to jointing. The characteristics and properties of the MFF are inconsistent with those of terrestrial pyroclastic fall deposits or loess, but are in common with large terrestrial ignimbrites, a hypothesis that explains all key observations with a single mechanism. Yardang fields developed in regionally extensive ignimbrite sheets in the central Andes display morphologic characteristics that correlate with degree of induration of the host lithology and suggest an origin by pyroclastic flow for the MFF.

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