An upper-mantle upwelling underneath Ireland revealed from non-linear tomography

Abstract

SUMMARY We present results of a non‐linear teleseismic P‐wave traveltime tomography in Ireland. Relative traveltime residuals are calculated from the data set of the Irish Seismological Lithospheric Experiment (ISLE 2002/3) and permanent stations onshore Ireland. At each of the 28 station sites, a local crustal 1‐D P‐wave velocity (vP) model is determined because the 3‐D crustal vP structure of Ireland is well known. These 1‐D models are used for a traveltime correction for crustal effects. The corrected residuals indicate two large‐scale traveltime anomalies of low amplitude: a negative traveltime anomaly in the western part of Ireland and a positive anomaly in the eastern part of the country. The subsequent traveltime tomography is calculated using an iterative non‐linear inversion using a variable parametrization and 3‐D ray tracing. Our inversion result contains a low P‐wave velocity (vP) zone underneath central Ireland, at 31–120 km depth. The amplitudes of the low velocity zone reach up to −2.5 per cent vP at 31–60 km depth, but decrease to −0.5 to −1 per cent vP at 120–150 km depth. Reconstruction tests confirm that large anomalies in our model are well resolved, although the resolution decreases towards the edges of the model. We infer that the low velocity zone underneath central Ireland is related to lithospheric thinning towards the north, which was determined by a previous S‐receiver function study. This anomaly is interpreted as presumably ancient or even recent mantle upwelling, related to the spreading of the Iceland plume head.

A shear wave velocity model of the European upper mantle from automated inversion of seismic shear and surface waveforms

“…The voluminous volcanism in northern Ireland is considered a part of the NAVP (White 1988; White & Lovell 1997; Arrowsmith et al 2005). The low velocities beneath Ireland—the western extremity of Europe—may be related to the opening of the North Atlantic, the Iceland Hotspot activity and episodic Cenozoic magmatism in the region (Rohrman & van der Beek 1996; Wawerzinek et al 2008; O’Donnell et al 2011; Polat et al 2012).…”

Integrating Gravity and Surface Elevation With Magnetic Data: Mapping the Curie Temperature Beneath the British Isles and Surrounding Areas

“…A more recent adjoint waveform propagation tomography model in Europe exhibits fast upper mantle shear wave speeds in the eastern margins of Scotland and England and southwestern Irish margin, whereas slow upper mantle is restricted to the S-W English peninsula (Zhu et al, 2015). Local seismic p-wave tomography studies are characterized by relatively moderate lateral variations in the seismic velocities across Ireland, with positive lithospheric anomalies in the western area (e.g., Wawerzinek et al, 2008;O'Donnell et al, 2011). In Great Britain, a local V p travel time tomography model suggests positive velocity anomalies in S-E England and S-E Scotland and negative anomalies in the Irish Sea, western Scotland, southern Wales margin and S-W English peninsula (Arrowsmith et al, 2005).…”

Caveats on tomographic images

“…VanDecar et al, 1995;Faccenna and Beker, 2010). For example, low relative wave speeds beneath the British Isles have been used to argue for hot, partially molten plume material with a temperature anomaly of 200°C (Arrowsmith et al, 2005;Wawerzinek et al, 2008). Low relative wave speeds below~200 km under the Siberian Craton (Fig.…”

“…These include proposals that the British Isles are underlain by hot plume material (e.g. Arrowsmith et al ., ; Wawerzinek et al ., ), that plumes underlie several islands in the Indian Ocean (Montelli et al ., ,b), that a plume feeding Hawaii is rooted variously to the NW, or the SE of the Big Island (Li et al ., ; Wolfe et al ., ), and that sodic alkaline‐to‐tholeiitic continental and oceanic mid‐plate magmatism requires thermal anomalies (Ritter et al ., ; Piromallo et al ., ). The assumption that low seismic wave speeds indicate hot material has led to proposals that sub‐lithospheric channels up to thousands of kilometres long connect volcanic regions thought to have similar geochemical signatures (e.g.…”

“…Arrowsmith et al (2005) suggested that absolute delay times in the UK are late with respect to the global mean, and interpreted low-velocity anomalies as evidence for elevated mantle temperatures beneath the region. Wawerzinek et al (2008) (Ritsema et al, 2011) and regional surfacewave models (measures of absolute wavespeed; e.g. Pilidou et al, 2004Pilidou et al, , 2005, and more recent catalogues of absolute travel-time delays (Amaru et al, 2008) to suggest that the background mean wave-speed in the UK/ Ireland is, in fact, fast compared with the global mean, precluding the need for a high-temperature, partialmelt hypothesis to explain wavespeed variations beneath the region.…”