Abstract
Seismological and geodetic data from modern volcanic systems strongly suggest that magma is transported significant distance (tens of kilometres) in the subsurface away from central volcanic vents. Geological evidence for lateral emplacement preserved within exposed dykes includes aligned fabrics of vesicles and phenocrysts, striations on wall rocks and the anisotropy of magnetic susceptibility. In this paper, we present geometrical evidence for the lateral emplacement of segmented dykes restricted to a narrow depth range in the crust. Near-total exposure of three dykes on wave cut platforms around Birsay (Orkney, UK) are used to map out floor and roof contacts of neighbouring dyke segments in relay zones. The field evidence suggests emplacement from the WSW towards the ENE, and that the dykes are segmented over their entire vertical extent. Geometrical evidence for the lateral emplacement of segmented dykes is likely more robust than inferences drawn from flow-related fabrics, due to the prevalence of ubiquitous ‘drainback’ events (i.e. magmatic flow reversals) observed in modern systems.
Highlights
1.1 BackgroundIgneous dykes are the frozen remnants of magma conduits and preserve evidence of major Earth processes such as magma transport from mantle to crust and the rifting of continents and oceans [Burchardt 2018]
This paper describes three well-exposed segmented dykes in Orkney (UK) and uses their geometrical form in outcrop to infer the likely direction of dyke emplacement
We describe the geometry of the dykes in relation to the uniformly dipping sedimentary host rocks, with a particular focus on the relays —or bridges—between adjacent segments
Summary
1.1 BackgroundIgneous dykes are the frozen remnants of magma conduits and preserve evidence of major Earth processes such as magma transport from mantle to crust and the rifting of continents and oceans [Burchardt 2018]. Threedimensional (3D) models of dyke nucleation, propagation and arrest are in their infancy, and our current understanding remains rooted in two-dimensional (2D) models [Kavanagh et al 2018; Pollard and Townsend 2018; Rivalta et al 2015; Townsend et al 2017]. Magma derived from the mantle must undergo a significant vertical component of movement to be emplaced in the upper crust. Note that a key corollary of this model is that the segment tip-lines are steeply plunging. This conceptual model is derived from a linear elastic fracture mechanics approach to dyke propagation in tensile cracks [Delaney and Pollard 1981].
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