Broadband Surface Wave Tomography of Ireland, Britain and Other Regions

Over the last decades, seismic surface-wave studies have produced increasingly detailed images of the Earth s structure at a regional scale. In this study, we have tuned well-established techniques and when required implemented new ones in order to investigate regions in which important debates are still ongoing, regarding the structure and the evolution of the Earth beneath them. Several studies suggested that the Paleogene uplift of parts of Britain and Ireland was caused by a lateral branch of the Iceland mantle plume, which played a fundamental role in the evolution of the North Atlantic Ocean over the past 60 M.y. Alternatively, among competing hypothesis, it was suggested that the uplift could be due to the far-field stress associated with the Alpine and Pyrenees Orogenies, with reactivation of old Variscan and Caledonian faults across Ireland and Britain. A major part of this study is aimed at gaining new insights into the seismic structure of the British Isles, which can help us answer these open questions. Teleseismic earthquakes and ambient noise, recorded at densely spaced seismic stations in the region, were used to determine the surface-wave dispersion across the British Isles and construct detailed images of the seismic structure beneath the area. The measurements, obtained using independent surface-wave analysis techniques (cross-correlation of teleseismic surface waves, multimode waveform fitting, and ambient noise interferometry), were applied to produce the first 3D shear-velocity model of the lithosphere and the asthenosphere of the entire region including Ireland, Britain, and the Irish Sea. The application of different methodologies yielded complementary frequency bands of the measurements, sensitive to different depths, from the shallow crust to the deep upper mantle. Abundant, newly available data was used to image the region with higher resolution than previously. The highly uneven station coverage resulted in a considerably irregular distribution of the measurements in the area; this, and the effects of errors on the measurements, required the development of a new, multi-resolution tomographic scheme. This scheme allows us to maximize the information extracted from the data and reach an optimal target resolution of the model at each knot, minimizing the effects of uneven data sampling and of the propagation of systematic errors.The multi-resolution phase-velocity maps, obtained at densely spaced periods, were inverted, point by point, for shear-velocity structure in order to produce a 3D, shear-velocity model of the lithosphere and asthenosphere. The optimal resolution tomography offers important new insights into the structure and evolution of the British Isles. A robust, low-velocity anomaly beneath the Irish Sea and its surroundings persists in the models from ~60 to at least 140 km depth, indicating an anomalously thin lithosphere. The area that exhibits the low velocity anomaly corresponds to where uplift and volcanism are evidenced by geological data. Our results also show a striking correlation with proposed underplating thickness and denudation, gravity, and thermochronological measurements, and rule out the once common assumption of a constant lithospheric thickness across Britain and Ireland. At lithospheric depths, a clear contrast in seismic velocities between Ireland and Britain could possibly explain why the seismicity is nearly absent in Ireland, while modest but clearly higher in Britain. The higher velocities beneath most of Ireland indicate thicker lithosphere and colder geotherms, likely resulting in a higher-strength lithosphere, resisting deformation. In the lithospheric mantle, the model displays an elongated high-velocity anomaly stretching W-E approximately along the Iapetus Suture Zone in Ireland, which may be the expression of a remnant of the Caledonian Iapetus slab beneath the suture or, alternatively, fragments of thick continental lithosphere incorporated into the Irish landmass in the course of the Caledonian Orogeny. Another part of this study was on using surface-wave analysis to investigate the lithosphere-asthenosphere system beneath the Tristan da Cunha Hotspot, with the goal of understanding the enigmatic intraplate volcanism in the region. Surface-wave analysis was applied in a challenging setting, as this work involved the use of data recorded by ocean-bottom seismometers, which required data-processing and measurement approaches substantially different from those tuned for land-based arrays of stations. We constrained a region-average, shear-velocity structure, using two-station, cross-correlation measurements across the area, and inferred the temperature of the lithosphere and asthenosphere beneath the area by means of petrological modeling. Seismic inversion and petrological modeling show a lithospheric thickness of only 65 70 km, confirming the previous estimates obtained from receiver functions. Our observations are consistent with a hot plume from the deep mantle, but the excess temperature estimated is much smaller than that reported for some other major hotspots, in particular, Hawaii