The fine-scale structure of upper continental lithosphere from seismic waveform methods: insights into Phanerozoic crustal formation processes

Abstract

SUMMARY Studies of the fine-scale structure of the continental lithosphere using seismic coda, associated with reflections from within the crust and the uppermost mantle, have almost solely been based on the compressional wavefield, observed at near vertical to wide-angle incidence. Here the results from a full waveform analysis of both P- and S-wave coda, obtained during a crustal scale wide-angle refraction experiment, from southwest Ireland are presented. The basic foundation for the study is a 2-D model for the P- and S-wave structure of the crust and its chemical composition, based on traveltime inversion of first arrivals and reflections for the Irish Caledonides. This sector of the Caledonian orogenic belt straddles the boundary between the Laurentian and Avalonian tectonic plates, along the Iapetus Suture Zone. The excellent quality and relatively broad frequency range (e.g. 1–20 Hz) of the P- and S-wave coda permits finer structure in the upper 40 km of lithosphere to be resolved than would be possible using just primary seismic phases. A series of 1-D waveform calculations are performed to systematically build a model for the fine-scale structure of the 32-km-thick crust and the uppermost mantle lithosphere. Petrophysical and geochemical data from felsic lower-crustal xenoliths from Central Ireland place limits on the permitted velocity fluctuations. Additional constraints are imposed by vertical P-wave reflections observed on contiguous offshore seismic reflection profiles. The results indicate that the uppermost mantle to a depth of ca. 40 km consists of a vertically stacked sequence of alternating high (Vp∼ 8.0 km s−1) and low (Vp∼ 7.25 km s−1) velocity layers. Each layer is about 500 m thick with a Vp/Vs ratio ∼1.73. These layers are interpreted as a series of lens-shaped mafic sill complexes, intruded into the subcrustal lithosphere. The fine structure within the lower crust consists of similar seismic velocity fluctuations (Vp∼ 6.4 and 7.1 km s−1) but on a scale of 200–300 m thickness. Velocity variations within the lower crust are consistent with the petrophysical properties of metapelitic, orthogneissic and psammitic lithologies, from lower-crustal xenoliths in Central Ireland, and with similar exhumed sections of lower crust in southern Europe. These results support tectonic theories for the origin of the crust by tectonic accretion of predominantly sedimentary and volcaniclastic material derived from oceanic, island-arc and continental margin sources during the Caledonian orogenic cycle. A conceptual model for crustal accretion and growth involves partial melting of the accreted sedimentary wedge, producing granites and a chemically differentiated crust during the late Caledonian (latest Silurian/early Devonian). The heat source that drove melting of the accreted sediments may have been provided by the interpreted mafic sills intruded into the uppermost mantle during oblique late Caledonian (sinistral) collision of the Laurentian and Avalonian plates.