The deep seismic structure of northern England and adjacent marine areas from the caledonian suture seismic project

Abstract

This thesis describes the interpretation of the deep seismic structure from the wide-angle reflection/refraction Caledonian Suture Seismic experiment which extends from the Mid North Sea High across Northern England, the Irish Sea to southwest Ireland. A new method of displaying the large number of travel times in the form of contoured composite plots for a particular phase has been developed. The interpretation methods used include analysis of the wide-angle reflection travel times, time-term analysis, gravity modelling, and, ray tracing and synthetic seismogram modelling. A mid-crustal gradient which returns PcP occurs between depths of 15.0-18.5 km beneath the line and is overlain by an upper crust with an average velocity of 6.16-6.20 km/s excluding sediments. This mid-crustal gradient was not recognised previously in the Irish Sea. A lower-crustal boundary recognised from the PmP phase occurs at 25.0 km depth beneath the Irish Sea and at 30 km beneath the North Sea. The average crustal velocity is 6.49-6.54 km/s excluding sediments and for the lower crust is 6.75-6.77 km/s. The sub-Moho Pn velocity is estimated to be 8.19 km/s beneath the Irish Sea, 8.32 km/s beneath Northern England and 8.02 km/s beneath the North Sea. The PmP and Pn boundaries diverge beneath the Irish Sea and define a transitional Moho over a 5 km depth range with an average velocity of 7.8-7.9 km/s. The PcP and Pn boundaries correspond approximately with the top and base respectively of the reflective deep crust observed beneath the Irish Sea on BIRPS WINCH. The transitional Moho beneath the Irish Sea has a reflective character similar to the lower crust. The coincident PmP and Pn boundaries beneath the North Sea define an abrupt increase in velocity from 6.75 to 8.0 km/s which approximately corresponds to the prominent reflections beneath the non reflective lower crust observed on the BIRPS NEC line at 10.5 to 11.0 s .The upper crust of velocity 6.15-6.20 km/s appears to lie below the suture beneath Northern England and the North Sea along the line but may also occur above it further west. The lower crust and deeper structures lie below the suture and represent crust of the Southern Caledonides. The anomalous structure of the Moho and upper mantle beneath the Irish Sea found from CSSP and BIRPS may be related to the post-Caledonian formation of the Irish Sea basins. The reflective lower crust and transitional Moho may result from crustal thinning produced by ductile stretching.