dc.description.abstract | 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. | en |