Lithosphere-asthenosphere interaction beneath Ireland from joint inversion of teleseismic P-wave delay times and GRACE gravity

The nature and extent of the regional lithosphere–asthenosphere interaction beneath Ireland and Britain remains unclear. Although it has been established that ancient Caledonian signa- tures 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 Exper- iment), 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 as- sociated 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 mag- matic 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.