Abstract
Sub-volcanic intrusive networks, of which cone sheets are recognised as a major constituent, partially control volcano growth and eruption style. The accepted cone sheet model is that these confocally dipping intrusions originate from an unexposed central magma chamber through dip-parallel magma flow. However, the emplacement mechanism of cone sheets remains poorly understood. The ∼58 Ma cone sheet swarm of Ardnamurchan, NW Scotland, offers an excellent opportunity to further resolve the emplacement dynamics of cone sheets, through studying magma flow, and their importance in volcanic edifice construction. Structural measurements and anisotropy of magnetic susceptibility (AMS) analyses have constrained a lateral magma flow pattern, consistently oriented NW–SE, in the majority of the Ardnamurchan cone sheets. Field observations also highlight the importance of host rock structure and interference between competing local and regional stress fields in controlling intrusion geometry. Our observations suggest cone sheet formation may be linked to laterally propagating NW–SE trending regional dykes, sourced from laterally adjacent magmatic systems (likely the Palaeogene Mull Central Complex), which are deflected around the central complex by stress field interference. Implicitly, edifice construction and potential eruption precursors observed at a volcano may instigate, or result from, magmatic activity in laterally adjacent volcanic systems.
Highlights
Discerning the emplacement mechanisms of sheet intrusions is key to understanding the transport, delivery and accommodation of magma in the upper crust
We suggest that the magnetic lineations measured from the inclined sheets reflect the primary magma flow pattern because: 1) they are overall consistently oriented NWeSE; 2) the fabrics are dominantly prolate; 3) there is a strong correlation between magma flow axes inferred from visible field flow indicators and K1; 4) there is little to no evidence of post-emplacement deformation; and 5) the K3 axes would be consistently oriented if an ambient tectonic strain field was recorded
Pre-existing conjugate fracture orientations Range of fractures that may be reactivated the majority of the Ardnamurchan inclined sheets (73%) contain anisotropy of magnetic susceptibility (AMS) fabrics suggestive of a lateral magma flow pattern
Summary
Discerning the emplacement mechanisms of sheet intrusions is key to understanding the transport, delivery and accommodation of magma in the upper crust. The propagation and geometry of sheet intrusions within the upper crust is controlled by the contemporaneous regional and local stress regimes, the magma dynamics and the behaviour and pre-existing structure of the host rock (Pollard, 1987; Rubin, 1995; Geshi, 2005; Gudmundsson and Phillip, 2006; Acocella and Neri, 2009; Siler and Karson, 2009; Schofield et al, 2010). Our data indicate that sub-horizontal to moderately plunging magnetic lineations, interpreted as parallel to the primary magma flow axes, dominate the magnetic fabrics of the Ardnamurchan cone sheets. This suggests that Ardnamurchan cone sheet emplacement was either more complicated than previous models. We provide a physical model that suggests geographical proximity is not necessarily indicative of magmatic cogenesis
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