Large scale intrusion, lateral magma reservoir growth and stress field changes at Lazufre volcano region, Chile; Code: LAzufre MAgmatic Systems (LAMAS)
Final Report Abstract
The voluminous ash-flow eruptive products in the Central Andes imply that there are major magma bodies; however, how these magma bodies develop and change in time and space are not understood. The stress regime and structural fabric in tectonically active areas control the shape of magmatic reservoirs and the orientation of dikes. Hence sub-volcanic structures provide potentially good indicators to assess regional geodynamic processes through time. In the LAMAS project, we analyzed two major intrusive bodies by a satellite radar interferometry (InSAR) data set: Lazufre volcano and Uturuncu volcano system. We analyze the spatiotemporal evolution of the Lazufre volcanic area in the central Andes, where a regional-scale elliptical area of surface deformation commenced in 1997. The deformation signal observed by satellite radar interferometry (InSAR) can be explained by a magmatic intrusion located at approximately 10 km depth. We compare this elongated region of deformation with several nearby volcanic structures. We find that a higher concentration of volcanic vents is located within the deforming area compared to the surrounding region. Furthermore, the mean alignment of volcanic structures is NW–SE (N129°), almost perpendicular to the mean elongation of the deformation ellipticity (N28°). Applying the borehole breakout concept, we propose a unifying model that explains magmatic activity both at the short-term (InSAR) and long-term (volcano morphometric parameters): a maximum horizontal compressive stress (SH) oriented NW–SE is found to be constant in time and in space, and is dominant both at the depth of the intrusion and near the surface. Important implications arise, such as the temporal stability of the principal stress directions and the significant role of topographic loading in the stress field analysis. Based on the excellent seismic data recorded in the planning period of the Lamas project, we further determine for the first time the exact geometry and location of the hydrothermal and magmatic reservoirs in the Lazufre volcanic area. This furthers the understanding of the origin of one of the largest worldwide volcanic uplift regions, both in space and amplitude. The location and shape of the sources generating a double-wide uplift region in the Lazufre found by past deformation data (InSAR and GPS) have not been well-delimited. The exact locations of the sources generating hydrothermal and magmatic fluids found by geochemical gas analysis have neither been precisely determined. In this study, we use seismological data to perform a 3-D high-resolution S-wave velocity model, a regional P-wave traveltime tomography and PS receiver function study. This allows defining better the locations of the sources of the deformations and the hydrothermal and magmatic reservoirs. Two of them (with S-wave velocity of about 1.2-1.3 km/s) are located below the Lastarria volcano. One is between 0 and 1 km below its base, has a funnel-like shape, and indicates a hydrothermal reservoir. The other is located between a depth of 3 and 6 km beneath the volcano crater. Its shape and depth suggest a magma reservoir that could feed the shallow hydrothermal system. This double hydrothermal and magmatic source is in agreement with previous geochemical and magneto-telluric studies. Both anomalies can explain the small uplift deformation of about 1 cm/yr deduced from InSAR/GPS data at Lastarria volcano. The third S-wave low-velocity zone (with S-wave velocity of about 2.7 km/s) is located below 5 km depth and beneath the center of the main uplift deformation of about 3 cm/yr in the Lazufre zone. This S-wave anomaly lies on top of a strong, major P-wave anomaly reaching down to 15 km below sea level. Both P- and S-wave anomalies very likely indicate the bulk and the top of a large magma chamber. Note that the position and size of the large magma chamber is roughly consistent to the one previously modeled by InSAR/GPS data. At Uturuncu we find that the strength and the pattern of the present deformation can be explained by a pressurized source, such as an inflating flat-topped magma body at ∼22 ± 9 km depth below the surface. Furthermore, we examine the optical remote sensing data to perform a lineament analysis, which shows in a geographic information system (GIS) that a girdle of river streams and faults encircle the volcano at radial distance of approximately 15 km. Using numerical stress models, we locate a magma body beneath the volcano and find that the lineaments are best explained by a deflating flat-topped magma body at approximately 18 ± 2 km depth, which is consistent with the InSAR study. Thus, both the independent analysis of InSAR and lineament data suggest the presence of a horizontally extended, flat-topped magma body beneath Uturuncu. The location depth is in agreement with, or just above, a prominent seismic low velocity zone. Consequently, although the sign of deformation caused by the herein constrained magma body differs, the similar geometry and similar location suggest them to be similar, possibly indicating longevity of a magma storage region.
Publications
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(2010) The relationship between current uplift geometry revealed by InSAR, structural framework, and the present-day stress field at Lazufre volcanic area, central Andes. Tectonophysics, 492, 133–140
Ruch J, Walter TR
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(2011) Doming of Lazufre (in German: Die "Beule" von Lazufre). System Erde, 1,1
Walter TR, Ruch J, Manconi A, Shirzaei M, Motagh M, Anderssohn J
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(2012) Salt lake deformation detected from space. Earth Planet Sc Lett 331:120-7
Ruch J, Warren JK, Risacher F, Walter TR, Lanari R
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. (2014): Deflation and inflation of a large magma body beneath Uturuncu volcano, Bolivia? Insights from InSAR data, surface lineaments and stress modelling. Geophysical Journal International, 198, 1, p. 462-473
Walter, T. R., Motagh, M.
