Jurassic extension estimates for the North Sea ‘triple junction’ from flexural backstripping: implications for decompression melting models

Abstract

The Rattray volcanics lie at the ‘triple junction’ between the three arms of the North Sea rift system. The mildly undersaturated volcanics in this province were erupted during the early syn-rift phase of a Middle-Late Jurassic rifting event, and are thought to be the products of decompression-induced partial melting of the mantle, resulting from lithospheric attenuation during rifting. Situated as they are in one of the most intensively studied rift basins in the world, the Rattray volcanics provide an excellent opportunity to evaluate the relationship between extension and melting described by McKenzie and Bickle [1]. This study uses a modified version of the backstripping technique, which takes into account the flexural strength of the lithosphere, to constrain the stretching factor, β, for the Jurassic rift phase. Two-dimensional interpreted sections are flexurally backstripped to the base post-rift to give an initial estimate of β. β is then further constrained by forward modelling to the base post-rift basin geometry generated by backstripping. Forward modelling uses the flexural cantilever model, a combined simple-shear/pure-shear model for continental extension which takes into account the thermal, rheological and flexural isostatic consequences of extension. This is an approach that is different from that of previous studies [2–5], which assume local, rather than regional, isostatic compensation to allow the backstripping of individual wells. The results obtained in this new study give maximum Jurassic β-factors on each of the three rift arms close to the ‘triple junction’ of 1.2–1.35. This suggests a maximum Jurassic β-factor in the ‘triple junction’ itself of 1.35–1.75. This is considerably lower than previous estimates, and is due to the breakdown of the assumption used in these previous studies that Airy isostasy can approximate the response of a low flexural rigidity lithosphere in subsidence analysis. Whilst such a β-factor is insufficient to generate melt by decompression alone, an anomalously high mantle potential temperature (θ) and the presence of volatiles as well as a complex Mesozoic and Late Palaeozoic rifting history are likely to have played a role in the generation of melt.