Investigating the relationship between pluton growth and volcanism at an active intrusions in the central Andes

Lead Research Organisation: University of Bristol
Department Name: Earth Sciences

Abstract

How magma is emplaced and interacts with its surrounding rock is of central interest in the Earth Sciences. The intrusion of magma into the Earth's crust plays a major role in the dynamics and the evolution of continental crust. In many cases magmas are funnelled upwards and erupt at volcanoes that dot the Earth's surface, particularly in areas where tectonic plates collide. The Andes are part of such a collision zone where large magma bodies (batholiths) form, due to chemical evolution of intruding deeper magma as well as partial melting of surrounding rocks. A common style of volcanic activity in the Andes is the catastrophic eruption of many hundreds to thousands of cubic kilometres of magma in the form of ignimbrites (volcanic rock containing ash and pumice) which often results in the collapse of the magma chamber roof upon eruption, leaving behind more or less ring-shaped surface depressions with diameters of many kilometres. The project is motivated by results obtained from space-borne satellites indicating ground deformation and significant uplift at in the central Andes at Uturuncu volcano, Bolivia, where magma may be accumulating for 270 thousand years. It is suspected that this inflation is caused by the growth of a large magma body at depth. If this interpretation is correct then these anomalies provide an outstanding opportunity to answer questions such as how large magma bodies are assembled in the crust to form plutons, how they evolve, how they relate to volcanism in general and how they manifest at the Earth's surface, potentially before eruption. We aim to find answers to these questions via a coordinated, integrated approach across various disciplines of the Earth Sciences. Central to this ambitious project lies the amalgamation of geodesy, geophysics, geology, petrology and mathematical modelling to document pluton growth in real time. The implications of the proposed work include assessing the role of plutons in continental dynamics and the potential for large volcanic eruptions. We requests funds for the UK component of a collaborative UK-US project, which also involves partners from Spain, Chile and Bolivia. We propose an integrated investigation of the Uturuncu uplift to test the hypothesis that pluton growth is occurring, to document the dynamics of growth, and to explore the links between plutonism, volcanism and tectonics. The core of the study will be a geophysical experiment over a 4-year period to study the ground deformation, mass changes and seismicity, and to image the sub-surface structure beneath the volcano. The geophysical experiment will be complemented by geological and petrological investigations as well as mathematical modelling to set the geophysical experiment in the context with igneous processes and the long-term magmatic evolution. A key outcome of the research will be a new generation of mathematical models to inform on how large magma chambers grow and which geodetic or geophysical signals we might expect to record at the Earth's surface. We will quantify the nature of the sources responsible for ground inflation by separating the contributions of shallow migration of (hot) water and gases, and deep magma replenishment and ponding, to geophysical signals. We are also interested to find out where these reservoirs are located, how many there are and how they relate to the depth of magma chambers that have led to eruptions in the volcano's past. For the latter, lava morphology studies and petrology will give insights onto the conditions in these magma chambers. We aim at developing advanced models of magma systems embedded in continental crust incorporating complexities such as variable mechanical properties of the crust, plastic deformation of deeper crust as well as the influence of crystallization of gas-saturated magma and shallow hydrothermal systems on ground deformation.

Publications

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Description The large ground deformation anomaly centred around Uturuncu volcano in Bolivia resides within a large negative gravity anomaly. The gravity anomaly is one of the largest documented in continental crust.



The low density anomaly can be explained by the presence of a large partially molten magma body (the Altiplano-Puna Magma body; APMB) at depths of around 17 km and below.



The local Bouguer gravity anomaly can best explained by vertically elongated bodies of lower density with respect to surrounding rocks.





The bodies extend from the depth of the APMB to shallower levels and reveal vertical dimension of about 10 km.

Petrological and geophysical constraints on the density of the crust and the magmatic systems suggest the presence of a melt phase to account for the observed Bouguer anomaly and density contrast within the anomalous bodies.



Our observations are consistent with ongoing ascent of partially molten structures from mid-crustal source regions.



Deformation models to explain the ongoing ground inflation demonstrate that heterogenous properties of the continental crust in the Altiplano-Puna region may play an important role in modulating the subsurface stress and strain evolution.



Our modelling shows that the observed time-dependent ground deformation can be explained by an inelastic mechanical behaviour of the crust. Invoking a viscoelastic rheology the numerical models demonstrate that the presence of a low-rigidity, partially molten APMB significantly controls strain partitioning and hence the observed surface displacement patterns by buffering subsurface strains induced by pressure variations.



Our preferred deformation model suggests that pressurisation of a magma source extending upward from the Altiplano-Puna magma body is causing the observed surface uplift and alludes to a continued increase in this pressure to explain both the spatial and temporal deformation patterns.
Exploitation Route Will help interpretation of seismic as well as electromagnetic subsurface structure beneath deformation anomaly
Sectors Environment

 
Description EC FP7
Amount € 3,500,000 (EUR)
Funding ID 282759 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 10/2011 
End 09/2015
 
Description EC FP7
Amount € 6,000,000 (EUR)
Funding ID 308665 
Organisation European Commission 
Sector Public
Country European Union (EU)
Start 06/2013 
End 05/2016