Geochronological, morphometric and geochemical constraints on the Pampas Onduladas long basaltic flow (Payún Matrú Volcanic Field, Mendoza, Argentina) Venera R. Espanon a,b, ⁎, Allan R. Chivas a , David Phillips c , Erin L. Matchan c , Anthony Dosseto a,b a GeoQuEST Research Centre, School of Earth & Environmental Sciences, University of Wollongong, NSW 2522, Australia b Wollongong Isotope Geochronology Laboratory, School of Earth & Environmental Sciences, University of Wollongong, NSW 2522, Australia c School of Earth Sciences, The University of Melbourne, Parkville, VIC 3010, Australia abstract article info Article history: Received 9 August 2014 Accepted 28 October 2014 Available online 10 November 2014 Keywords: Pampas Onduladas Long lava flow Rheology Payún Matrú Volcanic Field The Pampas Onduladas flow in southern Mendoza, Argentina, is one of the four longest Quaternary basaltic flows on Earth. Such flows (N 100 km) are relatively rare on Earth as they require special conditions in order to travel long distances and there are no recent analogues. Favourable conditions include: a gentle topographic slope, an insulation process to preserve the melt at high temperature, and a large volume of lava with relatively low vis- cosity. This study investigates the rheological and geochemical characteristics of the ~170 km long Pampas Onduladas flow, assessing conditions that facilitated its exceptional length. The study also reports the first geochronological results for the Pampas Onduladas flow. 40 Ar/ 39 Ar step-heating analyses of groundmass reveal an eruption age of 373 ± 10 ka (2σ), making the Pampas Onduladas flow the oldest Quaternary long flow. The methods used to assess the rheological properties include the application of several GIS tools to a digital el- evation model (DEM) to determine the length, width, thickness, volume and topographic slope of the flow as well as algorithms to determine its density, viscosity and temperature. The slope of the Pampas Onduladas flow deter- mined from the initial part of the flow on the eastern side of La Carbonilla Fracture to its end point in the province of La Pampa is 0.84% (0.29°), the steepest substrate amongst long Quaternary flows. The rheological properties, such as density viscosity and temperature from the Pampas Onduladas flow are similar to values reported for other long Quaternary flows. However, the minimum volume calculated is relatively low for its length compared with other long Quaternary flows. Therefore, the extension of the Pampas Onduladas flow was probably con- trolled by a steep slope, combined with an insulating mechanism, which helped in providing optimal conditions for a travel length of almost 170 km. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Long basaltic flows (N 100 km) produced in a single volcanic erup- tion are unusual on Earth (but common on Mars), as they require rela- tively large lava volumes and steep slopes (Keszthelyi and Self, 1998; Keszthelyi et al., 2004). For the Quaternary (b 2.6 Ma), only four flows have been reported to be longer than 100 km, and there are no historic analogues of long flows. The four long Quaternary flows recognised are: the Toomba and Undara flows in Queensland, Australia (Stephenson et al., 1998); the Thjorsa flow in Iceland (Vilmundardottir, 1977); and the Pampas Onduladas flow in Mendoza, Argentina (Pasquarè et al., 2005). These have reported volumes greater than 12 km 3 and a pahoehoe character. Some of the basic requirements for long basaltic flows are: (1) an insulating mechanism to maintain the lava at high temperature; and (2) a large volume of erupted lava (Pinkerton and Wilson, 1994). The four long Quaternary basaltic flows exhibit inflation structures such as lava rises and/or tumuli and in some cases lava tubes such as in the Toomba and Undara flows (Stephenson et al., 1998) that insulate the lava, thereby reducing its cooling by b 50 °C/100 km accord- ing to the models of Keszthelyi and Self (1998). Of special interest is the Pampas Onduladas flow as it has been described as the longest on Earth during the Quaternary (Pasquarè et al., 2008). It has a relatively narrow (~5 km) tongue-like structure that dominates for more than 70% of its length and lacks lava tube structures. Despite the significance of the Pampas Onduladas flow, rheological, geochemical and geochronological analyses are lacking. Previous inves- tigations mainly dealt with recognising and describing this flow from a morphological view point (Pasquarè et al., 2005, 2008). The purpose of this investigation is to assess some of the physical parameters and Journal of Volcanology and Geothermal Research 289 (2014) 114–129 ⁎ Corresponding author at: Wollongong Isotope Geochronology Laboratory, School of Earth & Environmental Sciences, University of Wollongong, NSW 2522, Australia. Tel.: +61 24221 5899; fax: +61 242214250. E-mail addresses: vre981@uowmail.edu.au (V.R. Espanon), toschi@uow.edu.au (A.R. Chivas), dphillip@unimelb.edu.au (D. Phillips), ematchan@unimelb.edu.au (E.L. Matchan), tony_dosseto@uow.edu.au (A. Dosseto). http://dx.doi.org/10.1016/j.jvolgeores.2014.10.018 0377-0273/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores