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Changes from dyke to sill propagation in the shallow crust are often caused by dissimilar layer properties. However, most previous studies have not considered the influence of glacial loading and unloading on dyke and sill deflection processes. Here, we attempt to collectively explore mechanical (layer stiffness) and geometrical (dyke dip, layer thickness) realistic parameters subject to two different magma overpressure values (namely 5 MPa and 10 MPa) that promote dyke-sill transitions in both non-glacial and glacial settings. To do this, we use as a field example, the Stardalur laccolith: a multiple stacked-sill intrusion located in SW Iceland. The laccolith lies near the retreating Langjökull glacier and was emplaced at the contact between a stiff lava layer and a soft hyaloclastite layer. We initially model two different stratigraphic crustal segments (stratigraphy a and b) and perform sensitivity analyses to investigate the likely contact opening due to the Cook-Gordon debonding and delamination mechanism under different loading conditions: magma overpressure, regional horizontal extension, glacial vertical load and a thin elastic layer at the stratigraphic contact. Our results show that contact opening (delamination) occurs in both non-glacial and glacial settings when the dissimilar mechanical contact is weak (low shear and tensile stress, zero tensile strength). In non-glacial settings, stiff layers (e.g., lavas) concentrate more tensile stress than soft layers (e.g., hyaloclastites/breccia) but accommodate less total (x–y) displacement than the surrounding host rock (e.g., soft hyaloclastites) in the vicinity of a dyke tip. Yet, a thicker hyaloclastite layer in the stratigraphy, subject to higher magma overpressure (Po = 10 MPa), may encourage dyke-sill transitions. Instead, in glacial domains, the stress conditions imposed by the variable vertical pressure of the ice cap result in higher tensile stress accumulation and displacement in stiff layers which they primarily control sill emplacement. [ABSTRACT FROM AUTHOR]

Volcanism in compressional tectonic settings presents several unclear relations, especially between faulting, volcano locations, and local stress fields. The Central Andes comprise one of the most important volcanic provinces in the world, where these relations can be studied. We investigate the Miocene-Quaternary faulting and the tectonic stress evolution of the Western Cordillera of Chile, between 24 and 26°S, and how they dictate the orientation of magma feeding fractures, to understand the relations between deformation and volcanism. We calculated 68 new stress tensors from faults of recognized age and reconstructed magma paths by analyzing the morphostructural characteristics of 130 monogenetic and polygenetic volcanoes of known age. Moreover, we integrated the database with previously published data from the Western Cordillera to carry out a regional comparison. Results allow us to recognize three main volcano-tectonic events. The oldest occurred in the Early Miocene and was characterized by an E-W greatest principal stress (σ1) expressed by N-S-striking reverse faults and NW–SE left-lateral strike-slip faults. Volcanoes belonging to this stage show morphometric characteristics that indicate dominant N-S magma feeding systems. The second event, active during the Upper Miocene-Pliocene, was characterized by NW–SE to NNW-SSE σ1, WNW-ESE right-lateral strike-slip faults, and NW–SE preferential direction of volcanic feeding systems. The last event was active in the Upper Pliocene–Pleistocene, mainly in the northern part of the study area, with N-S to NE-SW normal and right-lateral strike-slip faults with NNE-SSW-trending least principal stress (σ3). Volcanism in this stage is characterized by NW–SE and N-S magma feeding systems. Our results suggest that volcano distribution has been mainly controlled by variations of the stress field related to the growth and collapse of the Puna Plateau. Magma emplacement was mainly guided by fractures parallel to the compression direction, irrespectively of the horizontal stress being σ3 or the intermediate principal stress σ2. Anyway, magma emplacement occurred also through fractures oblique to σ1, along strike-slip faults, or by exploitation of inherited weak structures, such as reverse faults. [ABSTRACT FROM AUTHOR]

We describe a methodology for identifying complex rift zones on recent or active volcanoes, where structures hidden by recent deposits and logistical conditions might prevent carrying out detailed fieldwork. La Réunion island was chosen as a test-site. We used georeferenced topographic maps, aerial photos and digital terrain models to perform a statistical analysis of several morphometric parameters of pyroclastic cones. This provides a great deal of geometric information that can help in distinguishing the localisation and orientation of buried magma-feeding fractures, which constitute the surface expression of rift zones. It also allowed the construction of a complete GIS database of the pyroclastic cones. La Réunion is a perfect example where past and active volcanic rift zones are mostly expressed by clusters of monogenic centres. The data has been validated in the field and compared and integrated with the distribution and geometry of dyke swarms. Results show the presence of several main and secondary rift segments of different ages, locations and orientations, whose origin is discussed considering regional tectonics, local geomorphology, and volcano deformation. [ABSTRACT FROM AUTHOR]