Abstract
The nature and extent of the regional lithosphere-asthenosphere interaction beneath Ireland and Britain remains unclear. Although it has been established that ancient Caledonian signatures pervade the lithosphere, tertiary structure related to the Iceland plume has been inferred to dominate the asthenosphere. To address this apparent contradiction in the literature, we image the 3-D lithospheric and deeper upper-mantle structure beneath Ireland via non-linear, iterative joint teleseismic-gravity inversion using data from the ISLE (Irish Seismic Lithospheric Experiment), ISUME (Irish Seismic Upper Mantle Experiment) and GRACE (Gravity Recovery and Climate Experiment) experiments. The inversion combines teleseismic relative arrival time residuals with the GRACE long wavelength satellite derived gravity anomaly by assuming a depth-dependent quasilinear velocity-density relationship. We argue that anomalies imaged at lithospheric depths probably reflect compositional contrasts, either due to terrane accretion associated with Iapetus Ocean closure, frozen decompressional melt that was generated by plate stretching during the opening of the north Atlantic Ocean, frozen Iceland plume related magmatic intrusions, or a combination thereof. The continuation of the anomalous structure across the lithosphere-asthenosphere boundary is interpreted as possibly reflecting sub-lithospheric small-scale convection initiated by the lithospheric compositional contrasts. Our hypothesis thus reconciles the disparity which exists between lithospheric and asthenospheric structure beneath this region of the north Atlantic rifted margin.
Highlights
The tertiary formation of the north Atlantic has regularly been cited as the type example of continental breakup above a mantle thermal anomaly (e.g. White & McKenzie 1989)
Studies of the uppermost mantle (∼30–300 km), cite low-velocity anomalies imaged using P-wave teleseismic tomography (e.g. Arrowsmith et al 2005; Wawerzinek et al 2008) and forward and inverse modelling of gravity and wide-angle seismic data (Al-Kindi et al 2003) as evidence that hot, partially molten plume material from Iceland presently resides beneath lithospheric thin-spots in this region of the north Atlantic rifted margin
A much deeper 110–190 km lithosphere-asthenosphere boundary (LAB) beneath Ireland was estimated by Clark & Stuart (1981) in a surface-wave study, a result reasonably consistent with an estimated depth of 130–150 km derived from a thermal model of the European lithosphere that uses the 1200◦C isotherm as a proxy for LAB depth (Tesauro et al 2009)
Summary
The tertiary formation of the north Atlantic has regularly been cited as the type example of continental breakup above a mantle thermal anomaly (e.g. White & McKenzie 1989). Second-order anomaly sources which cannot so readily be explained may have to be considered We address these issues by using data from the ISLE (Irish Seismic Lithospheric Experiment), ISUME (Irish Seismic Upper Mantle Experiment) and GRACE (Gravity Recovery and Climate Experiment) experiments to perform a joint teleseismic-gravity inversion for Ireland’s lithospheric and uppermost mantle structure. By incorporating GRACE gravity data within a joint inversion scheme and drawing on increased seismic resolution relative to the previous tomographic study of Ireland’s uppermost mantle (Wawerzinek et al 2008), we seek to resolve the disparity that exists in the literature regarding a lithosphere pervaded by Paleozoic signatures overlying an asthenosphere purportedly dominated by tertiary structure
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