Asymmetrical structure, hydrothermal system and edifice stability: The case of Ubinas volcano, Peru, revealed by geophysical surveys

Abstract

Ubinas volcano, the historically most active volcano in Peru straddles a low-relief high plateau and the flank of a steep valley. A multidisciplinary geophysical study has been performed to investigate the internal structure and the fluids flow within the edifice. We conducted 10 self-potential (SP) radial (from summit to base) profiles, 15 audio magnetotelluric (AMT) soundings on the west flank and a detailed survey of SP and soil temperature measurements on the summit caldera floor. The typical "V" shape of the SP radial profiles has been interpreted as the result of a hydrothermal zone superimposed on a hydrogeological zone in the upper parts of the edifice, and depicts a sub-circular SP positive anomaly, about 6 km in diameter. The latter is centred on the summit, and is characterised by a larger extension on the western flank located on the low-relief high plateau. The AMT resistivity model shows the presence of a conductive body beneath the summit at a depth comparable to that of the bottom of the inner south crater in the present-day caldera, where intense hydrothermal manifestations occur. The lack of SP and temperature anomalies on the present caldera floor suggests a self-sealed hydro-thermal system, where the inner south crater acts as a pressure release valve. Although no resistivity data exists on the eastern flank, we presume, based on the asymmetry of the basement topography, and the amplitude of SP anomalies on the east flank, which are approximately five fold that on the west flank, that gravitational flow of hydrothermal fluids may occur towards the deep valley of Ubinas. This hypothesis, supported by the presence of hot springs and faults on the eastern foot of the edifice, reinforces the idea that a large part of the southeast flank of the Ubinas volcano may be altered by hydrothermal activity and will tend to be less stable. One of the major findings that stems from this study is that the slope of the basement on which a volcano has grown plays a major role in the geometry of the hydrothermal systems. Another case of asymmetrical composite cone edifice, built on a steep topography, is observed on El Misti volcano (situated 70 km west of Ubinas), which exhibits a similar SP pattern. These types of edifices have a high potential of spreading and sliding along the slope owing to the thicker accumulation of low cohesion and hydrothermally altered volcanic products.

Magma extrusion during the Ubinas 2013–2014 eruptive crisis based on satellite thermal imaging (MIROVA) and ground-based monitoring

“…The phreatic explosions of September 2013 mark the beginning of the Phase I and possibly reflect the first interaction between a new ascending magma batch and the large hydrothermal system located under the Ubinas crater (Gonzales et al, 2014).…”

“…The edifice lies along the western margin of the Rio Tambo graben (E-W extensional regime) with a 2000 m altitude gradient between the low-relief high plateau, to West, and the Ubinas valley, to E and SE (cf. Lavallée et al, 2009;Byrdina et al, 2013;Gonzales et al, 2014).…”

“…The recent eruptions occurred within the southern pit crater, where fumarolic areas, emitting volcanic gas and steam have been commonly observed during inter-eruptive periods (cf. Gonzales et al, 2014). Before the 2013-2014 episode, the last eruption of Ubinas refers to the 2006-2009 crisis (Rivera et al, 2010(Rivera et al, , 2014.…”

“…Rivera et al, 2014). According to Rivera et al (2014) the 2006-2009 activity represents an "archetype of Ubinas's eruptions" in the last 500 years: repeated ascent of small-volume magma batches (b10 Mm 3 ) from a shallow reservoir (4-7 km) that interact during their ascent with sectors of the hydrothermal system in the shallowest portion of the conduit (Gonzales et al, 2014).…”

“…1; Gonzales et al, 2014), has been measured every 5 min by a data logger (Tinytag Aquatic 2). In this survey, we plot the temperatures measured every day at 4.00 AM (local time) in order to avoid any fluctuations due to variations of solar radiation (Fig.…”