5 Neotectonics, Seismology and Paleoseismology Laura Perucca 1,2 and Hugo Bastias 2 1 CONICET 2 Gabinete de Neotecto ´nica – INGEO – FCEFN – UNSJ. Av. Ignacio de La Roza y Meglioli. 5400 San Juan, Argentina 1. Introduction The purpose of the present chapter, dedicated to the neotectonics of Argentine–Chilean Patagonia, is to com- plete an outlook of active tectonics of the southernmost region of South America, which constitutes a field of relatively recent development on a national standard. The present state of our knowledge can be summarized in Fig. 2, where the main faults active during the Late Neogene and the main Neogene geologic units, sediment cover and Miocene–Holocene volcanic cover have been represented. The main regional alignments related to out- standing morphological features are shown in Fig. 1. Based upon the interpretation of satellite images, at least 70% of the moderately preserved Neogene volcanic structures were identified (Fig. 2). In Figs 3 and 4, earthquakes of magnitude over 4 that have been recorded in the region since the mid-twentieth century have been represented. In these figures, it can be seen that there is a marked lack of uniformity in the distribution of seismic activity, in the localization as well as in the epicenter depth. Nevertheless, some regio- nal patterns governing the major morphostructural domains have been found. Apparently, there is a certain correspondence between these great structures and the location of seismic events, such as it has been observed with earthquakes related to the Liquin ˜e–Ofqui fault sys- tem (Figs 1 and 2), and to the Magallanes–Fagnano fault system (Fig. 2). It may also be observed that the major structures set the boundaries of the volcanic environments, especially during the Late Neogene; such is the case of the Liquin ˜e– Ofqui fault system in western South America and its assumed northern continuation into the province of Neu- que ´n. This is the main fault system in western Patagonia, in which the Chilean territory is called the ‘‘Liquin ˜e– Ofqui fault system’’ (Figs 2 and 3), a N–S trending intraarc fault that extends over hundreds of kilometers and described by Lavenu and Cembrano (1999a) as a dextral strike-slip duplex. In the province of Chubut, southern Argentine Pata- gonia, a megafracture with a NW–SE trend is located; this is known as the ‘‘Gastre Megafault’’ (Rapela et al., 1991) which has been described by Rapela (1997) as an intra-continental fault with a dextral strike-slip displace- ment. However, other authors, such as Franzese and Martino (1998) or von Gosen and Loske (2004), claimed that there is insufficient evidence to assert the presence of an intra-continental fault system with a dextral heading. Instead, they suggested that there is evidence of sinistral displacement all along the fault areas in this part of the northern Patagonian Massif. At 46° S, in the central-southern part of the Chubut Province, a NW–SE trending megafracture is located, which can be identified by an extensional depression with evidence of recent tectonic activity. In this area, one can distinguish a series of aligned springs and a shear zone that is part of the Genoa megafracture (Fig. 3). Another megafracture, conjugated with the previous one and bear- ing a NE–SW trending, reveals as well a shearing zone with a strike-slip displacement. In Pampa de Agnia, in mid-western Chubut Province, abundant evidence of modern tectonic activity has been found, with south trending faults that affect not only the piedmont but also the Neogene lava flows. The Rı ´o Chubut displays a marked structural control along this section. The Magallanes–Fagnano fault is located in Isla Grande de Tierra del Fuego (Fig. 2), trending WNW– ESE over more than 600 km from the Atlantic to the Pacific. All along this active transcurrent fault, with a sinistral movement, the South American continent has a slow westward displacement in relation to the Andean region in Tierra del Fuego. This displacement is shown by strong earthquakes and low seismicity, mainly on the Chilean side. In Argentina a classic model considering, from west to east, two distinctive seismotectonic environments has prevailed for many years. The Andean western portion is described as a seismic area with a present active tectonic zone, and an eastern section that extends from the Andean front eastward with a non-seismic regime. Studies of neotectonic activity are scarce due to the lack of seismographic equipment and a low population density, and thus the absence of historical records. How- ever, the presence of remarkable morphotectonic features associated with active faulting fronts in several of the defined seismotectonic regions indicates modern tectonic activity for this region of Patagonia. This evidence should be completed and studied in detail with associated disciplines like paleoseismology and the morphological evolution of landscapes with special climatic conditions and high erosion rates. The present work does not analyze individual features of Neogene faulting, but rather tries to define environ- ments or regions where Neogene faulting apparently has similar characteristics. From the point of view of seismic risk studies, it is logical to assume that these regions have DEVELOPMENTS IN QUATERNARY SCIENCES Ó 2008 ELSEVIER B.V. VOLUME 11 ISSN 1571-0866 ALL RIGHTS RESERVED 73