Abstract Subaqueous mass-transport deposits (MTDs) can be important elements in hydrocarbon systems, forming potential reservoirs or seals. Most research has targeted outcrops or moderately to deeply buried MTDs and, therefore, the petrophysical properties of near-seafloor MTDs, and their influence in the trapping and release of shallow fluids, is poorly studied. Here, we investigate shallow MTDs in Lake Villarrica (Chile) by combining sub-bottom profiles, free-fall penetrometer data, pore pressure dissipation tests and geotechnical properties of sediment cores. Low undrained shear strength under a surficial MTD indicates underconsolidation caused by sudden loading and rapid sealing. Larger, buried MTDs show acoustic signatures of free gas at their base, indicating effective sealing. This is supported by degassing core gaps just below MTDs and by excess pore pressure ratios c . 30–70% within MTDs. Acoustic windows below rafted blocks suggest local fluid escape. MTDs exhibit elevated undrained shear strength and reduced porosity compared to surrounding sediments, but are comparable to upslope source sequences. This suggests that MTD sealing capacity in Villarrica relates to the apparently overconsolidated nature of the slope sequence, leaving a minor role for shear densification. This study shows that shallow MTDs can form a relatively rapid seal for fluid migration, locally degraded by rafted blocks.

Abstract We examine 150 volcanoes and geothermal areas in the central, southern and austral Andes for thermal anomalies between the years 2000 and 2010 from two different spaceborne sensors: (1) those automatically detected by the MODVOLC algorithm from MODIS; and (2) manually identified hotspots in night-time images from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer). Based on previous work, we expected to find eight volcanoes displaying thermal anomalies (Ubinas, Villarrica, Copahue, Láscar, Llaima, Chaitén, Lonquimay and Chiliques). We document 35 volcanic areas with pixel-integrated temperatures of 4 up to more than 100 K above background in at least two images, and another 16 areas that have questionable hotspots with either smaller anomalies or a hotspot identified in only one image. Most of the thermal anomalies are related to known activity (i.e. lava and pyroclastic flows, growing lava domes, fumaroles, and lakes) while others are of unknown origin or reflect activity at volcanoes that were not thought to have surface activity. A handful of volcanoes exhibit temporal variations in the magnitude and location of their temperature anomalies that can be related to both documented and undocumented pulses of activity. Our survey reveals that low-amplitude volcanic hotspots detectable from space are more common than expected, based on lower spatial resolution data, and that these features could be more widely used to monitor changes in the activity of remote volcanoes. We find no evidence from ASTER or MODVOLC that the thermal anomalies were affected by six earthquakes with M w above 7 in our study area from 2000 to 2010, although the observations may not have been optimal to detect such anomalies. Supplementary material: Supplementary tables of data and figures for the volcanoes studied are available at http://www.geolsoc.org.uk/SUP18581 .

INTRODUCTION The 1:5,000,000 map of Paraguay gives a fairly acceptable picture of the distribution of the systems of rocks in the region east and south of Asunción. It is, however, unsatisfactory in the northern half of eastern Paraguay. Figure 1 shows the correct generalized distribution of the outcrops according to our present knowledge. In addition, a tiny exposure of fossiliferous Lower Devonian rocks should be plotted in the Chaco plains near Fortín Aroma (Lat. 20°12’ S., Long. 60°28’ W.). The following account of the geology of Paraguay is mainly a summary of the writer’s paper (1950) on the eastern half of the country, with the addition of some information on the Chaco region, made available through the courtesy of the Union Oil Company of California. The geologic literature on Paraguay is very meager. The most important papers are Milch’s (1894) and Goldschlag’s (1913) descriptions of some Mesozoic eruptive rocks, Beder’s (1923) discovery of Mesosaurus- bearing beds near Villarrica, Beder and Windhausen’s (1918) report on the discovery of Lower Devonian fossils, and Boettner’s (1947) brief account of the Precambrian and Lower Paleozoic rocks exposed south of the Rio Apa. STRATIGRAPHY PRECAMBRIAN (epC) Two main areas of Precambrian rocks are known in Paraguay, one in the northern and the other in the southern part of the country. The northern exposures are mainly gneiss, micaschist, and phyllite intruded by biotite granite and crossed by granite pegmatite dikes many of which bear tourmaline and beryllium. These rocks were briefly described by Boettner (1949) between Puerto Fonciere and Toldo-cué, south of the Rio Apa. The metasedimentites appear to be a southern extension of the Cuibá group of Mato Grosso (Brazil) and are best regarded as middle Pre-cambrian. Metasedimentites are scarce in the southern exposures. They crop out in small patches between Villa Florida and Paso Pindó on the Rio Tebicuary and consist of foliated gneiss, micaschist, and chlorite quartzite with occasional thin intercalations of marble. These rocks are intruded by biotite granite and granite porphyry. Extensive exposures of rhyolite and keratophyre seem related to the granite. They are little weathered and almost devoid of metamorphism and may be tentatively correlated with the late Precambrian Aiguá extrusives of Uruguay and the Castro and Maricá “porphyries” of southern Brazil. Two small outcrops of pink biotite granite are known a little east and southeast of Asunción, one at San Bernardino on the east shore of Lake Ypacaraí (Fig. 2) and the other near Itá.