Pali Aike, a history of volcanoes, glaciers and wind in southernmost South America

CORONATO, A., H. CORBELLA, B. ERCOLANO, P. TIBERI, J. RABASSA

Malargue, Mendoza. Argentina

Workshop; IAVCEI 3rd International Maar Conference; 2009

IAVCEI 3rd International Maar Conference

IAVCEI – CVS – IAS 3IMC Conference Malargüe, Argentina, 2009 Pali Aike, a history of volcanoes, glaciers and wind in southernmost South America Andrea Coronato 1, Hugo Corbella 2, Bettina Ercolano 3, Pedro Tiberi 3 and, Jorge Rabassa 1 1, Hugo Corbella 2, Bettina Ercolano 3, Pedro Tiberi 3 and, Jorge Rabassa 1 1 CONICET-CADIC-UNPSJB. B. Houssay 200. 9410 Ushuaia, Argentina. acoro@cadic.gov.arCONICET-CADIC-UNPSJB. B. Houssay 200. 9410 Ushuaia, Argentina. acoro@cadic.gov.ar 2 UNPA-MACN Av.Angel Gallardo 470. 1405 Buenos Aires, Argentina. UNPA-MACN Av.Angel Gallardo 470. 1405 Buenos Aires, Argentina. 3 UNPA-UARG. .Lisandro de la Torre 1070. 9400 Río Gallegos, Argentina. UNPA-UARG. .Lisandro de la Torre 1070. 9400 Río Gallegos, Argentina. Keywords: fissural volcanic field, Pleistocene glaciations, eolian erosion. : fissural volcanic field, Pleistocene glaciations, eolian erosion. The Pali Aike volcanic field extends from the northern side of the Río Gallegos valley to the area of the Magellan Straits (51º - 52º S) within the Magellan Basin, 300 km southeast of the present Andean volcanic front. It is a landscape controlled by tablelands of diverse origin and smoothly undulated in some areas, for which volcanic structures emerge, eroded by internal and external geodynamic processes. Circular or semi-circular depressions of volcanic and hydro-eolian origin are common, some of them occupied by lakes with their associated landforms. Southern Patagonia was extensively faulted and rifted in a NW direction during the break-up of Gondwanaland. Since late Cretaceous time, the area has been under E-W and ENE-WSW compressional stress due to the Nazca plate high rate oblique subduction (9 cm/yr) under the South America plate. In contrast, during late Tertiary time, after the collision of the Chilean ridge, the low frontal convergence (2 cm/yr) of the Antarctic plate caused a N-S stretch, due to a new strength field in the southern flank of the Magellan Basin. The NW structures allowing the eruption of the Pali-Aike volcanic rocks are located approximately on the vertical of the underlying Jurassic Rift zone. Seismic data show that 27 sills, 30 to 60 meters thick, intruded at around 1000 and 2000 meters depth are also aligned along the NW structures (Corbella 2004). Along the southern Patagonian Andes, several rift systems sub-perpendicular to the Cordillera form transversal valleys, nowadays occupied by glacial lakes. All these valleys formed since 3.5 Ma suggest a possible N-S stretch all around the area. In Pali-Aike, the E and ENE structures are parallel or sub-parallel to the faults that channeled the large lakes and marine bays. Pali-Aike is part of a Pliocene-Holocene (3.8 to 0.01 Ma) back-arc volcanism (Corbella, 2004). The predominant fault system has a NW direction, followed by less frequent faults of E and ENE trend. Two principal fracture systems controlled the outpouring of the lavas and the close alignment of the scoria and ash cones and maars. Basalts and basanites crop out in a belt of NW direction 50 by 150 km long. Holocene volcano-tectonic activity was concentrated in the southeast. There, most of the youngest lavas reached the surface and two parallel fractures of ENE direction limit a graben 600 m wide. The deviation of the lava flows toward the graben is evidence of active faulting and the central block sinking during this Holocene eruption. The abundance of maars and phreatomagmatic rings characterizes the volcanic morphology. Around 100 maars crop out along the belt, forming craters 500 to 3000 m wide. Maars, cinder, lapilli and spatter cones and lava fields are relevant features of this fissural volcanism landscape. Most of the maars are seated along faults. Many of them have coalesced forming "paternoster" design alignments up to 15 km long. One of the biggest, the Potrok Aike maar (3 km in diameter) still preserves a lake 100 m deep. Due to the scarce precipitation, many maars are occupied by a lagoon in their central areas or filled by sediments due to the high wind enhanced evaporation rate. Several maar depressions have spatter cones inside them as a consequence of the post-maar eruptive activity; others have scoria spatter cones in the periphery of the phreatomagmatic ring (Corbella 2004). If the volcanic processes had not happened in the area, the Pleistocene continental-type glaciation landscape would have prevailed over the landscape. Several glaciations took place westwards from this area since late Miocene and Pliocene times (Rabassa, 2008 and previous authors). Most of the glacial sedimentary evidence is found interbedded with lava flows. The Pali Aike volcanic field area displays not only glacial deposits but also impressive glacial morphology. Till -forming basal and fluted moraines and glaciofluvial plains- are spreaded from the Andes foothills up to the Atlantic coast and covered by younger lava flows in several places. The "Estancia Bella Vista" basalt offered the maximum 40Ar/39Ar age for the Great Patagonian Glaciation (GPG) in southernmost Patagonia (see Coronato et al., 2004, and Rabassa, 2008, for a complete list of references). To the SW and W, frontal moraines, intermorainic channels, outwash 40Ar/39Ar age for the Great Patagonian Glaciation (GPG) in southernmost Patagonia (see Coronato et al., 2004, and Rabassa, 2008, for a complete list of references). To the SW and W, frontal moraines, intermorainic channels, outwash Coronato et al. plains and glaciofluvial terraces are exposed. The ancient glacial lobes flowed repeatedly along the Río Gallegos valley in a W-E direction, and also along the Skyrring and Otway sounds and the Magellan Straits in a SW-NE direction (Caldenius, 1932; Meglioli, 1992), thus showing similar age but different provenance. The ice lobes flowed from the paleo-icecap developed over the Patagonian and Fuegian Andes during Pleistocene time. At least, five glaciations occurred in southernmost South America since middle Pleistocene: the Great Patagonian Glaciation (GPG, Sierra de los Frailes Glaciation: 1.1-1.0 Ma), the Post GPG-I (or Cabo Vírgenes Glaciation: <1.0 >0.6 Ma), the Post GPG-II (or Punta Delgada Glaciation: ca. < 600 >300 ka), the GPG-III (Primera Angostura Glaciation, ca.: 200 ka) and the Last Glacial Maximum (LGM, Segunda Angostura Glaciation: 25 ka). Evidence of several Pre-GPG glacial advances have been found to the northwest (Coronato et al., 2004). Also, the occurrence of two advancing stages during the GPG or even one of them corresponding to a former glaciation has been postulated recently, southwest of the city of Río Gallegos, based upon the finding of three different drift units intruded by several sand-wedge casts sets (Bockheim et al., in press). In the Pali Aike volcanic field area, glacial and glaciofluvial features and deposits have been mostly interpreted as belonging to the piedmont-type glaciers, which advanced during the GPG up to the present Atlantic Ocean platform. Otherwise, the influence of the younger glaciations should be also remarked. During the Post GPG I and II, the ice lobes were channeled along the main regional SW-NW structures carving them as fiords, sounds or channels. They were shorter than in their previous advance, so the young glacial topography remains nested into the oldest one. Frontal and lateral morainic arcs as well as the outwash and intermorainic channels of the Otway and Magellan paleoglaciers seems to be coincident with many of the NW-SE structures which controlled the fissural eruptions occurred after 1 Ma. While the younger glaciation occurred, the non-glaciated surrounding lands were under tundra conditions supporting temperatures 11 to 15º C colder than today (Bockheim et al., in press). Deglaciation processes forced by climatic amelioration included not only glacier retreat but also tundra defrosting. This means that a huge volume of surface and subsurface waters was certainly available within a substratum mainly composed of impervious till and highly-permeable gravels. Melt-out water would have been channeled along most of the NW structures allowing aquifer recharge, thus offering appropriate conditions for the occurrence of phreatomagmatic processes along the ancient Gondwanic alignments. No evidence of subglacial volcanism has been found yet. The water availability would have been much larger during the ice front retreat periods than in the maximum glacial expansions. The definitive Miocene uplifting of the Southern Andes allowed the formation of a rain-shadow desert along the eastern side of the mountain ranges, generating the present arid to semi-arid steppe environment. The westerlies acted as a continuous landscape agent since 1 Ma filling and eroding cryogenic structures and soils, forming lag deposits over the gentle slope surfaces, over-deepening depressions and promoting deflation hollows. During the relatively dry interglacial periods, eolian deflation would have contributed to the enlarging and sedimentary-cover erosion of the depressions while they were water-free. It could be expected to find discontinuous sedimentary deposits in present maar lakes due to the accumulation interruption while tundra conditions were effective during glacial periods or due to eolian erosion during relatively dry, shorter interglacial times. The occurrence of crustal, inner geodynamic processes in relation to simultaneous alternation in the availability of very large melt water volumes in a semi-arid environment, as a result of glaciation/interglacial period successions since at least the younger early Pleistocene epoch, is the objective of on-going research by the authors. References Bockheim, J., Coronato, A., Rabassa, J., Ercolano, B., Ponce, J. In press. Relict sand wedges in southern Patagonia and their stratigraphic and paleo-environmental significance. Quaternary Science Reviews. Caldenius, C. 1932. Las Glaciaciones Cuaternarias en Patagonia y Tierra del Fuego. Ministerio de Agricultura de la Nación. Dirección General de Minas y Geología. 95:1-148. Buenos Aires. Corbella, H. 2002. El Campo volcano-tectónico de Pali Aike. In: Haller (Ed.) Geología y Recursos Naturales de la Provincia de Santa Cruz. Relatorio del XV Congreso Geológico Argentino, I-18:285-301. El Calafate - Buenos Aires. Coronato, A., Martínez, O,. Rabassa, J. 2004. Pleistocene Glaciations in Argentine Patagonia, South America. In: Ehlers, J. and Gibbard, P. (Eds.) Quaternary Glaciations, Extent and Chronology. Part III. Quaternary Book Series: 49-67. Elsevier. Meglioli, A. 1992. Glacial Geology of Southernmost Patagonia, the Strait of Magellan and Northern Tierra del Fuego. Ph. D. Dissertation, Lehigh University, Bethlehem, U.S.A. Unpublished. Rabassa, J. 2008. Late Cenozoic glaciations in Patagonia and Tierra del Fuego. In: Rabassa, J. (Ed.) The Late Cenozoic of Patagonia and Tierra del Fuego. Developments in Quaternary Science, 11: 151-203. Elsevier.