Constraints on the amount and pattern of ground deformation induced by dike emplacement are important for assessing potential eruptions. The vast majority of ground deformation inversions made for volcano monitoring during volcanic unrest assume that dikes are emplaced in either an elastic half-space (a homogeneous crust) or a crust made of horizontal layers with different mechanical properties. We extend these models by designing a novel set of two-dimensional finite-element method numerical simulations that consider dike-induced surface deformation related to a mechanically heterogeneous crust with inclined layers, thus modeling a common geometry in stratovolcanoes and crustal segments that have been folded by tectonic forces. Our results confirm that layer inclination can produce localized ground deformation that may be as much as 40× higher in terms of deformation magnitude than would be expected in a non-layered model, depending on the angle of inclination and the stiffness of the rock units that host and are adjacent to the dike. Generated asymmetrical deformation patterns produce deformation peaks located as much as 1.4 km away from those expected in non-layered models. These results highlight the necessity of accurately quantifying both the mechanical properties and attitude of the geology underlying active volcanoes.
Constraints on the amount and pattern of ground deformation induced by dike emplacement are important for assessing potential eruptions. The vast majority of ground deformation inversions made for volcano monitoring during volcanic unrest assume that dikes are emplaced in either an elastic-half space (a homogeneous crust) or a crust made of horizontal layers with different mechanical properties. Here, we extend these models by designing a novel set of two-dimensional Finite Element Method numerical simulations that consider dike induced surface deformations related to a mechanically heterogeneous crust with inclined layers, thus modelling a common geometry in stratovolcanoes and crustal segments that have been folded by tectonic forces. Our results confirm that layer inclination can produce localized ground deformations which may be up to 30 times higher in terms of deformation magnitude than would be expected in a purely homogeneous model, depending on the angle of inclination and the stiffness of the rock units that host and are close to the dike, generating asymmetrical deformation patterns with peaks located as much as 1.4 km away from the expected in the homogeneous model. These results highlight the necessity to accurately quantify both the mechanical properties and attitude of the geology underlying active volcanoes.
<p>In the Atacama Desert, at the Precordillera of northern Chile, a series of Paleocene-Eocene caldera deposits and ring-faults are exceptionally well-preserved<sup>1</sup>. Here we aim to build on previous mapping efforts to consider the location, timing and style of pre, syn and post caldera volcanism in the region. We focus on the partially nested caldera complexes of Lomas Bayas and El Durazno<sup>2,3</sup> where deposits record several stages of caldera evolution (pre-collapse, collapse/intra-caldera and extra-caldera, resurgence and post-collapse eruptive deposits). The pre-caldera basement is a thick sequence of early Paleocene mafic lavas<sup>4, 5</sup>. The caldera complex formed between around 63 and 54 Ma<sup>4, 5</sup>. Both calderas constitute subcircular structures approximately 13 km in diameter and are cut by several NNW to NNE-trending felsic dikes which are spatially related to felsic domes interpreted as resulting from post caldera formation unrest<sup>1,</sup><sup>4</sup>. These calderas have been interpreted as part of the Carrizalillo megacaldera complex<sup>2 </sup>. We combine field observations, such as the attitude of dikes, as well as information on their dimension and composition, the size, location and composition of domes and lava flows, as well as the evidence of the regional stress field operating during the caldera evolution from measurements of fault kinematics. This data will be used as the input to finite element method models to investigate the effect of nested caldera geometry, ring-faults and crustal heterogeneities on the location of domes and eruptive centers generated during caldera unrest. The results will be potentially useful for constraining models of eruption forecasting during periods of unrest in calderas and ore deposition models which have been shown to be linked to caldera structure and magma emplacement.</p><p><strong>References</strong></p><p><sup>1 </sup>Rivera, O. and Falc&#243;n, M. (2000). Calderas tipo colapso-resurgentes del Terciario inferior en la Pre-Cordillera de la Regi&#243;n de Atacama: Emplazamiento de complejos volcano-plut&#243;nicos en las cuencas volcano-tect&#243;nicas extensionales Hornitos y Indio Muerto: IX Congreso Geol&#243;gico Chileno, v. 2.&#160;Soc. Geol. de Chile, Puerto Varas.</p><p><sup>2 </sup>Rivera, O., and Mpodozis, C. (1994). La megacaldera Carrizalillo y sus calderas anidadas: Volcanismo sinextensional Cret&#225;cico Superior-Terciario inferior en la Precordillera de Copiap&#243;, paper presented at VII Congreso Geol&#243;gico Chileno. Acad. de Cienc. del Inst. Chilecol. de Geol. de Chile, Concepci&#243;n.</p><p><sup>3 </sup>Rivera, O. (1992). El complejo volcano-plut&#243;nico Paleoceno-Eoceno del Cerro Durazno Alto: las calderas El Durazno y Lomas Bayas, Regi&#243;n de Atacama, Chile. Tesis Departamento de Geolog&#237;a, Universidad de Chile, 242. (Unpublished).</p><p><sup>4 </sup>Ar&#233;valo, C. (2005). Carta Los Loros, Regi&#243;n de Atacama. Servicio Nacional de Geolog&#237;a y Miner&#237;a, Carta Geol&#243;gica de Chile, 92, 1(100.000), 53 p.</p><p><sup>5 </sup>Iriarte, S., Ar&#233;valo, C., Mpodozis, C. (1999). Mapa Geol&#243;gico de la Hoja La Guardia, Regi&#243;n de Atacama. Servicio Nacional de Geolog&#237;a y Miner&#237;a. Mapas Geol&#243;gicos, 13, 1(100.000).</p>
<p>For magma chambers to form or volcanic eruptions to occur magma must propagate through the crust as dikes, inclined sheets and sills. The vast majority of models that investigate magma paths assume the crust to be either homogeneous or horizontally layered, often composed of rocks of contrasting mechanical properties. In subduction regions that have experienced orogenesis, like the Andes, the crust has been deformed over several million years, resulting in rock layers that are commonly folded and steeply dipping. The assumption of homogeneous properties or horizontal layering then does not capture all of the potential magma path crustal interactions. Here we tackle this problem by determining the effect of a crust made of steeply inclined layers in which sills and inclined sheets are emplaced. We combine field observations from a sill emplaced in the core of an anticlinal fold at El Juncal in the Chilean Central Andes, such as lithologies, sill and fold limbs attitude, sill length and layers and sill thickness, with a suite of finite element method models to explore the mechanical interactions between inclined layers and magma paths. Our results demonstrate that the properties of the host rock layers as well as the contacts between the layers and the crustal geometry all play an important role on magma propagation and emplacement at shallow levels. Sill propagation and emplacement through heterogeneous and anisotropic crustal segments changes the crustal stress field promoting sill arrest, deflection or propagation. Specifically, sills are more likely to be deflected when encountering shallow dipping layers rather than steeply dipping layers of a fold. Mechanically weak contacts encourage sill deflection due to the related rotation of the maximum principal compressive stress and this effect is attenuated when the fold layers are more steeply dipping. This processes may change the amount and style of surface deformation recorded, with significant implications for monitoring of active volcanoes.</p>
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