Dyke to sill deflection in the shallow heterogeneous crust during glacier retreat: part I

Abstract

Dykes and sills occupy Mode I (extension), Mode II (shear), or hybrid mode fractures and most of the time transport and store magma from deep reservoirs to the surface. Subject to their successful propagation, they feed volcanic eruptions. Yet, dykes and sills can also stall and become arrested as a result of the crust’s heterogeneous and anisotropic characteristics. Dykes can become deflected at mechanical discontinuities to form sills, and vice versa. Although several studies have examined dyke propagation in heterogeneous and anisotropic crustal segments before, the conditions under which dykes propagate in glacial-volcanotectonic regimes remain unclear. Here, we coupled field observations with 2D FEM numerical modelling to explore the mechanical conditions that encourage (or not) dyke-sill transitions in volcanotectonic or glacial settings. We used as a field example the Stardalur cone sheet-laccolith system, which lies on the Esja peninsula, close to the western rift zone, NW of the southern part of the Icelandic rift. The laccolith is composed of several vertical dykes that transition into sills and form a unique stacked sill ‘flower’ structure. Here, we investigate whether the Stardalur laccolith was formed under the influence of stresses caused by glacial retreat due to thickness variations (0–1 km) in addition to regional and local tectonic stresses (1–3 MPa extension or compression) and varied magma overpressure (1–30 MPa), as well as the influence of the mechanical properties of the lava/hyaloclastite contact. Our results show that the observed field structure in non-glacial regimes was formed as a result of either the mechanical (Young’s modulus) contrast of the lava/hyaloclastite contact or a compressional regime due to pre-existing dykes or faulting. In the glacial domain, the extensional stress field below the ice cap encouraged the formation of the laccolith as the glacier became thinner (subject to a lower vertical load). In all cases, the local stress field influenced dyke to sill deflection in both volcanotectonic regimes.

Dyke to sill deflection in the shallow heterogeneous crust during glacier retreat: part II

“…Coupled methodologies, such as pairing geological observations with numerical modelling provide a better understanding of sill formation, showing their mechanical evolution into magma chambers (Hardee 1982;Coleman et al 2004;Barnett and Gudmundsson 2014), and their thermal evolution in the layered crust (Cashman et al 2017;Annen et al 2006). This is a compound article (Part II) with our previous work (Drymoni et al 2023c) which aim to collectively explore, using finite element method (FEM) numerical modelling, the mechanical and geometrical conditions that dictate the emplacement of sills in the shallow crust in nonglacial and glacial settings. In Part I, we explored how the magma overpressure, the local stress field (extension or compression), the presence (or not) of a very weak contact influence dyke to sill deflection.…”

“…Specifically, the contrasting stiffness (i.e., Young's modulus) between the upper and the bottom layer of a contact where E upper > E bottom can rotate the stress field ahead of the dyke tip (σ 1 becomes horizontal) and a dyke can be stalled. In rift zones, previous dyke injections or graben subsidence induce compressional regimes that make stiff (high Young's modulus) layers (e.g., lava deposits) temporary stress barriers (Gudmundsson 1995;Roman et al 2004;Geshi et al 2012;Kusumoto et al 2013;Tibaldi et al 2022;Drymoni et al 2023c). Similarly, soft (low Young's modulus) layers, such as pyroclastic deposits, tend to concentrate the horizontal compressive stress especially in areas under local extension (Gudmundsson and Brenner 2001) and become stress barriers as well.…”

“…The Stardalur laccolith is located nearby the Langjökull glacier (approximately 25 km × 50 km in size) in the Esja peninsula proximal to the western volcanic rift zone. The laccolith's structural analysis has been reviewed by previous studies (Fridleifsson 1977;Pasquarè and Tibaldi 2007;Drymoni et al 2023c). Briefly, the geology of the research area is composed of basaltic lavas, hyaloclastites, volcano/sedimentary breccias and fluvial deposits (Plio-Pleistocene), cut by several magmatic intrusions and covered by lavas younger than 0.8 Ma old.…”

“…The thin elastic layer acted as a boundary load, which had a very low Young's modulus value (E). The steady-state study is explained in detail in Part I (Drymoni et al 2023c).…”

“…When a fracture propagates from a soft to a stiff layer, it often becomes arrested or deflected at it. In our models, we explored further than before (Part I, Drymoni et al 2023c) the effect of the stiffness of a layer in dyke-sill emplacement in non-glacial and glacial settings. We modelled hyaloclastite deposits with Young's moduli over two different orders of magnitude, namely 0.5 GPa and 5 GPa, which implied soft and comparatively stiff mechanical conditions, respectively.…”