Magma flow and palaeo-stress deduced from magnetic fabric analysis of the Álftafjörður dyke swarm: implications for shallow crustal magma transport in Icelandic volcanic systems

Abstract

Neogene regional mafic dykes extending north of the Á lftafjörður central volcano in east Iceland are studied to test models of dyke swarm emplacement at spreading ridges. This is accomplished by using anisotropy of magnetic susceptibility to define fossilized magma flow regimes. The imbrication of the foliation plane, defined by the minor susceptibility axis, is used as an indicator of the flow direction. Contemporaneous shear resolved on the dyke walls may modify a pure flow-induced fabric and such shear regimes are therefore retracted. The magma flow and palaeo-stress resolved on the dykes are determined in 13 of 24 dykes. The magma flow is interpreted as subhorizontal and northwards directed away from the central volcano for nine dykes, and found to be vertical in three cases. The preferentially subhorizontal magma flow in the Á lftafjörður swarm suggests that dyke propagation in this type of Icelandic volcanic system originates in shallow crustal magma chambers. The regional tectonic palaeo-stress field is deduced to cause oblique spreading across the Á lftafjörður dyke swarm and govern a subhorizontal dextral shear component on the dyke planes during propagation. This interpretation is not in conflict with the left-stepping en echelon trend distribution of individual dykes relative to the trend of the swarm.

Flow dynamics in mid-Jurassic dikes and sills of the Ferrar large igneous province and implications for long-distance magma transport

“…These sill complexes comprise a stacked series of mafic intrusions(e.g., the Golden Valley Sill Complex, Karoo LIP, and sill complexes in the North Atlantic igneous province, seeMagee et al, 2016 for a review),Wright et al, 2012; e.g., magmatic rift segments of Iceland and East Africa:Muirhead et al, 2015;Urbani et al, 2015). AMS studies addressing magma flow within the intrusive systems of sill-dominated LIPs are rare compared to studies investigating sub-parallel swarms of dikes (e.g.,Delcamp et al, 2014;Eriksson et al, 2014 and references therein). Below we discuss magma transport dynamics within dikes and sills of the Ferrar LIP.…”

Magma transport in sheet intrusions of the Alnö carbonatite complex, central Sweden

“…1) which we combine with field observations and microscopic data. In connection with flow-test calculations25 and a compilation of geologically plausible syn- and post-emplacement scenarios78910111214152728, we use the AMS results to explain sheet emplacement in the Alnö ring complex.…”

“…Away from the sheet tip, laminar magma flow during sheet propagation can produce a slight imbrication of phenocrysts, producing magnetic fabrics that reflect magma flow (Fig. 8b, ‘laminar flow plus wall rock friction’; cf 21045…”

The use of palaeomagnetism and rock magnetism to understand volcanic processes: introduction

“…Magma movement in this case is very complicated and does not follow a specific trend, suggesting that the dykes reflect variations in the local stress field across the rift or evidence of storage during eruption events. Eriksson et al (2014) study magma flow in dykes and associated stress fields in a palaeo-rift of east Iceland. They suggest that the subhorizontal magma movement is conditioned by the shear component on the dyke planes during propagation.…”