Gravitative Spreading Causes en-echelon Diking Along a Volcanic Rift Zone: Observations From Madeira and an Experimental Approach

Volcanic rift zones are typically defined by parallel-striking dike complexes and eruptive fissures resulting in morphological ridges. Here, we describe the unusual occurrence of a pronounced rift arm where dike orientation is clearly oblique to the morphological rift axis: Desertas Islands (Madeira Archipelago), surface expression of a 60 km long submarine ridge. The Desertas ridge intersects the easternmost long axis of Madeira Island forming an angle of 110° and is interpreted as an abandoned rift arm of Madeira (Geldmacher et al., 2000, Geochem. Geophys. Geosys. 1). The deeply eroded, wedge-shaped Desertas Islands consist largely of parallel, steeply dipping dike swarms and abundant cinder cones stacked one on another. Remarkably, the strike of the dike swarms (median = 138°) deviates significantly from that of the morphological ridge (162°) defining an en-echelon-type arrangement. We propose that the discrepancy between dike and ridge orientations reflects a particular scheme of volcano-spreading on weak substratum such as pre-volcanic sediments. At depth beneath the Desertas ridge, the regional stress field is such that magmatism produces ridge-parallel dikes resulting in the overall ridge morphology. At shallow levels, however, gravity-driven spreading and reinforcement of the Desertas Islands and eccentric Madeira Island adjust the principal stress axes' orientation in the Desertas ridge. This results in a sinistral stress component that influences dikes intruding near the surface, which became affected to rotate about 24° anticlockwise. We have studied this gravity effect experimentally by mounting analogue sand piles onto a sand and viscous PDMS substratum. Gravitative spreading of this setup produced en-echelon fractures that almost perfectly mimic the dike patterns observed on Madeira. Our results thus highlight the importance of substratum strength and volcano load on rifting of oceanic island volcanoes, and indicate that the main strike of dike swarms is not necessarily collinear with the associated rift axes. Superimposed gravity-induced stress fields may lead to complex near-surface adjustments of dike swarms, potentially variable with time.