Browsing by Autor "Matthew J. Comeau"

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Crustal structure of the Lazufre volcanic complex and the Southern Puna from 3-D inversion of magnetotelluric data: Implications for surface uplift and evidence for melt storage and hydrothermal fluids
(Geological Society of America, 2023) Martyn Unsworth; Matthew J. Comeau; Daniel Díaz; Heinrich Brasse; Benjamín Heit; A. Favetto; Cristina Pomposiello; Hernán Barcelona; Giselle Peri; Faustino Ticona
Abstract The Central Andes are unique in the global system of subduction zones in that a significant, high-altitude plateau has formed above a subduction zone. In this region, both subduction and the associated magmatism have been shown to vary in both space and time. Geophysical data have been invaluable in determining the subsurface structure of this region. Extensive seismic studies have determined the regional-scale distribution of partial melt in the crust and upper mantle. Magnetotelluric studies have been effective in providing independent constraints on the quantity and composition of partial melt in the crust and upper mantle. Geodetic studies have shown that a small number of volcanic centers exhibit persistent, long-term uplift that may indicate the formation of plutons or future eruptions. This paper describes a detailed study of the Southern Puna using magnetotelluric (MT) data. This region is located at the southern limit of the Central Andes in a region where a recent transition from flat-slab subduction to normal subduction has caused an increase in magmatism, in addition to hypothesized lithospheric delamination. It is also a region where an extensive zone adjacent to the volcanic arc is undergoing surface uplift, located near Volcán Lastarria and Cordon del Azufre (collectively called Lazufre). The main goals of the work are to define the crustal structure and to investigate processes that may cause surface uplift of relatively large regions not associated with active volcanism. As part of the PLUTONS project, MT data were collected on an east-west transect (approximately along 25°S) that extended across the Southern Puna, from Lazufre to north of Cerro Galan. The data were combined with previously collected MT data around Lazufre and inverted to give a 3-D resistivity model of the crust. The low resistivity of the crust resulted in limited sensitivity to mantle structure. A number of major crustal conductors were detected and included (1) a mid-crustal conductor extending eastward from the volcanic arc as far as the Salar de Antofalla; (2) an upper- to mid-crustal conductor located north of Cerro Galan; and (3) a conductor that rises westward from (1) and terminates directly beneath the region of surface uplift at Lazufre. These conductors are broadly coincident with the location of crustal low shear-wave anomalies. The conductive features were interpreted to be due to zones of partial melt stored in the crust, and petrological data were used to estimate melt fractions. Below Lazufre, it is likely that aqueous fluids contribute to the high conductivity, which is observed within the depth range of the inflation source, giving evidence that the surface uplift may be associated with both magmatic and hydrothermal processes.
Dissecting a Zombie: Shallow Volcanic Structure Revealed by Multiple Geophysical Data Sets at Uturuncu Volcano, Bolivia
(2021) Patricia MacQueen; Joachim Gottsmann; M. E. Pritchard; Nicola Young; Faustino Ticona J.; Ruben Tintaya; Thomas Hudson; Ying Liu; J. M. Kendall; Matthew J. Comeau
Uturuncu volcano in southern Bolivia is something of a “zombie” volcano – presumed dead, but showing signs of life. The volcano has not erupted in 250 kyr, but is exhibiting unrest in the form of ground deformation, seismicity, and active fumaroles. Elucidating the subsurface structure of the volcano is key for interpreting this recent unrest. Magnetotelluric measurements revealed alternating high and low resistivity anomalies at depths <10 km beneath the volcano, with a low-resistivity anomaly directly beneath Uturuncu. A key question is, what is the nature of this anomaly? To what extent is it partial melt, a hydrothermal brine reservoir, or a mature ore body? Knowing the density of this anomaly could distinguish between these scenarios, but existing density models of the area lack sufficient resolution. To address this issue, we collected additional gravity measurements on the Uturuncu edifice with 1.5 km spacing in November 2018. Gradient analysis and geophysical inversion of these data revealed several features: a 5 km diameter, high density anomaly beneath the summit of Uturuncu (1 – 3 km elev.), a 20 km diameter ring-shaped negative density anomaly around the volcano (-3 – 4 km elev.), a NNE trending, positive density anomaly northwest of the volcano (0 – 4 km elev.), and a NW trending, negative density anomaly to the southeast. These structures often (but not always) align with resistivity anomalies, features in new seismic tomography models, and relocated earthquake hypocenters. Based on a joint analysis of these data, we interpret the positive density anomaly as a crystallizing dacite pluton, and the negative density ring anomaly as a zone of hydrothermal alteration. Earthquakes around the edges of the crystallizing pluton may represent escaping fluids as the magma cools. The high density anomaly to the northwest likely represents a solidified pluton, and the low density anomaly to the southeast may represent a fractured fault zone. We posit that the alternating zones of high and low resistivity anomalies represent zones of low and high fluid/brine content, respectively. Based on this analysis we suggest that the unrest at Uturuncu is unlikely to be pre-eruptive. This study shows the value of joint analysis of multiple types of geophysical data in evaluating volcanic subsurface structure.
Magnetotelluric images of magma distribution beneath Volcán Uturuncu, Bolivia: Implications for magma dynamics
(Geological Society of America, 2015) Matthew J. Comeau; Martyn Unsworth; Faustino Ticona; M. Sunagua
Research Article| March 01, 2015 Magnetotelluric images of magma distribution beneath Volcán Uturuncu, Bolivia: Implications for magma dynamics Matthew J. Comeau; Matthew J. Comeau 1Department of Physics, University of Alberta, Edmonton, Alberta T6E 2E1, Canada Search for other works by this author on: GSW Google Scholar Martyn J. Unsworth; Martyn J. Unsworth * 1Department of Physics, University of Alberta, Edmonton, Alberta T6E 2E1, Canada *E-mail: unsworth@ualberta.ca Search for other works by this author on: GSW Google Scholar Faustino Ticona; Faustino Ticona 2Universidad Mayor de San Andrés, Casilla 10077 Correo Central, La Paz, Bolivia Search for other works by this author on: GSW Google Scholar Mayel Sunagua Mayel Sunagua 3Empresa Kawsaqi, Correo Central, La Paz, Bolivia Search for other works by this author on: GSW Google Scholar Author and Article Information Matthew J. Comeau 1Department of Physics, University of Alberta, Edmonton, Alberta T6E 2E1, Canada Martyn J. Unsworth * 1Department of Physics, University of Alberta, Edmonton, Alberta T6E 2E1, Canada Faustino Ticona 2Universidad Mayor de San Andrés, Casilla 10077 Correo Central, La Paz, Bolivia Mayel Sunagua 3Empresa Kawsaqi, Correo Central, La Paz, Bolivia *E-mail: unsworth@ualberta.ca Publisher: Geological Society of America Received: 09 Sep 2014 Revision Received: 19 Dec 2014 Accepted: 09 Jan 2015 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2015 Geological Society of America Geology (2015) 43 (3): 243–246. https://doi.org/10.1130/G36258.1 Article history Received: 09 Sep 2014 Revision Received: 19 Dec 2014 Accepted: 09 Jan 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Matthew J. Comeau, Martyn J. Unsworth, Faustino Ticona, Mayel Sunagua; Magnetotelluric images of magma distribution beneath Volcán Uturuncu, Bolivia: Implications for magma dynamics. Geology 2015;; 43 (3): 243–246. doi: https://doi.org/10.1130/G36258.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Altiplano-Puna volcanic complex in the central Andes records a history of major caldera-forming eruptions over the past 10 m.y. Geophysical and geodetic data indicate that magma is still present, and perhaps mobile, in the crust. Broadband magnetotelluric data were used to generate two-dimensional and three-dimensional electrical resistivity models of the Altiplano-Puna magma body (APMB) with a focus on the zone of inflation around Volcán Uturuncu in southern Bolivia. Low electrical resistivities (<3 Ωm) at a depth of ∼15 km below sea level are interpreted as being due to the presence of andesitic melts of the APMB and require a melt fraction >20%. The upper surface of the APMB is shallowest beneath Uturuncu and the geometry is consistent with geodynamic models that require the upward movement of a melt layer at this location. The shallower resistivity structure is characterized by discrete electrically conductive bodies, oriented east-west near sea level (depth of 5 km), which are interpreted as a combination of partial melt and fluids. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.