Geochronological, morphometric and geochemical constraints on the Pampas Onduladas long basaltic flow (Payún Matrú Volcanic Field, Mendoza, Argentina)

Abstract

The Pampas Onduladas flow in southern Mendoza, Argentina, is one of the four longest Quaternary basaltic flows on Earth. Such flows (>100km) 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 viscosity. This study investigates the rheological and geochemical characteristics of the ~170km long Pampas Onduladas flow, assessing conditions that facilitated its exceptional length. The study also reports the first geochronological results for the Pampas Onduladas flow. 40Ar/39Ar step-heating analyses of groundmass reveal an eruption age of 373±10ka (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 elevation 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 determined 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 controlled by a steep slope, combined with an insulating mechanism, which helped in providing optimal conditions for a travel length of almost 170km.