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Abstract

The surface expressions of most Holocene rift-zone volcanic systems in Iceland are 40–150 km long a nd 5–20 km wide swarms of tension fractures, normal faults and basalt volcanoes each of which extends from a central volcano (a composite volcano or a caldera). Below the Holocene surface, the swarms are mainly composed of subvertical dykes and normal faults except within the central volcanoes where the main tectonic elements are inclined sheets. The inclined sheets are mostly 0.5 m thick, whereas the regional dykes are commonly 3–6 m thick and occasionallyas thick as 50–60 m. The two principal ways by which rift-zone volcanic systems become loaded are (1) the magmatic overpressure induced by dykes, and (2) the plate ‘pull’ associated with extension in the direction of the spreading vector (1058). This paper shows that both loading conditions give rise to mechanical interaction between volcanic systems in general, and their central volcanoes in particular. Here we show that the magmatic overpressure ofa regional dyke may reach tens of mega-pascals. We model the effects of simultaneous dyke injections, each dyke with an overpressure of 10 MPa, in the echelon systems on the Reykjanes Peninsula. The results indicate north-trending zonesof high shear stress between the nearby ends of the volcanic systems, favouring strike–slip faulting. Geometrically similar shear-stress zones develop between the volcanic systems on the peninsula when acted on by a plate pull of 5 MPa in a direction parallel with the spreading vector. The results agree with the observation that there are many north-trending strike–slip faults on the Reykjanes Peninsula. When the same plate pull is applied to a cluster of eight central volcanoes in Central Iceland, zones of high tensile stress develop between many of the volcanoes. These highly stresses zones encourage mechanical interaction between the volcanoes, such as simultaneous dyke emplacement and seismogenic faulting, as is supported by observational data.

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