DEEP STRUCTURE AND ORIGIN OF THE NORTHUMBERLAND AND STAINMORE TROUGHS

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Abstract
SUMMARY: The Lower Carboniferous trough and block structures of northern England were discovered and delineated by geological and gravity observations. New information on the depth to the underlying basement comes from seismic refraction studies using quarry blasts, and from observations of the LISPB shots of 1974 along a north-south line of temporary stations from the Northumberland Trough to the Craven Basin (the NERL line). In agreement with earlier gravity interpretations, this new seismic evidence shows that the Northumberland Trough is about 3–4 km deep beneath north-eastern England, and that the Stainmore Trough is about 2.5 km deep across the Pennines. Seismic data from NERL and from the Rookhope array station indicate a 6.5 km s −1 layer at about 12 km depth and the Moho at about 28 km depth. To within the accuracy attained, the Moho appears to be at the same depth beneath both blocks and troughs. The regional and differential subsidence are both primarily attributed to stretching of the lithosphere (including crust) in a north-south direction. The lithospheric tension was probably mainly caused by the slabpull force (or trench suction?) resulting from subduction of the attached oceanic part of the plate which formed the closing Hercynian marginal sea to the south, supplemented in the continental crust by tension related to the crustal thickness. It is suggested that the ductile lower part of the lithosphere, including the lower continental crust, was stretched to a much greater extent than the strong and brittle uppermost layer as a result of resistance to subduction at shallow depth. The regional subsidence was partly the isostatic response to thinning of the lower ductile part of the continental crust; subordinate thermal subsidence probably contributed, but this was cut short by a new thermal event at the end of the Carboniferous. Differential subsidence is attributed to faulting in the upper continental crust in response to the tension. This allowed the buoyant granites to remain relatively stable as the intervening regions subsided, with the Moho shallowing regionally but not developing strong local variations in depth.

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