Complex magma flow dynamics within fossil dykes: linking multi-method observations across scales from the Reyðarfjörður dyke swarm, eastern Iceland.

Since dykes represent the main mechanism for magma movement from the Earth’s crust to the surface, understanding how they generate a path to feed an eruption is crucial for volcanic hazard assessment. To this purpose, key information can be obtained by studying fossil dykes in extinct and eroded volcanic systems where dykes show a variety of shapes, segmentation, and propagation paths due to a suite of pre-, syn- and post-emplacement physical processes (e.g. heat transfer, host rock layering, local stress variations). To discern the factors that control these complex geometries and reveal how they affect the dynamics of magma transport, we used a multi-method approach on a N-S trending fossil dyke from the Reyðarfjörður dyke swarm (eastern Iceland). We collected meso-scale geometric data from drone photogrammetry, and rock magnetic, petrographic and microstructural laboratory analyses were conducted on oriented rock cores and samples to reveal microscopic magma flow indicators (e.g., magnetic fabrics and crystal alignment). Rock cores were sampled both across the thickness and along the breadth of the dyke segments, also recording the core position relative to different cooling surfaces (i.e. from the dyke margin to its interior). The studied dyke is exposed for ˜900 m across its breadth in ˜300 m height. It comprises several segments showing different shapes (from straight to curved paths), thickness (spanning from 0.5 to 5 m) and linkage pattern (i.e. connected or not connected segments). The photogrammetry and geological field observations show the curved segments are more frequent in the shallower and thicker portions of the dyke, whereas the amount of offset, overlap and spacing between the segments is higher in the shallower portions of the dyke exposure. Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) were used to identify magnetic fabrics that may be related to magma flow in the rock cores. These results show that the magnetic data record complex magma flow dynamics spanning from sub-horizontal to subvertical along the dyke path, which is inferred for adjacent connected segments and from the dyke margin to its interior. We relate the geometrical variability of the dyke segments to the far-field stress (controlled by regional extension) versus the near- field stress (controlled by local magma overpressure), the latter being dominant in the shallower (and thicker) portions of the dyke. This generates a mixed mode fracturing during dyke propagation, reflecting its geometrical variability, that also controls a complex magma flow pattern within the dyke. Microstructure analysis is in progress and it is expected to complement magnetic fabric analysis and fieldwork in the interpretation of magma flow dynamics. Current results already show that a multimethod approach aimed at linking observations from the mesoscale to the microscale is required to better capture small scale and complex propagation paths and magma flow patterns providing more reliable insights on dyke propagation.