An evaluation of the orogens inferred to traverse the southwest Barents Shelf

The Barents Shelf is an important piercing point for Arctic tectonic evolution because the Caledonian, Timanian, Grenville and earlier Precambrian orogens are all inferred to traverse the region. However, there is uncertainty over the crystalline basement architecture and the position of former continent boundaries as these are largely dependent on interpretation of geophysical data with non-unique solutions. Through the analysis of locally sourced Carboniferous sedimentary rocks that were deposited in small rift basins, this presentation gives examples of how sedimentary provenance signals can be used to constrain the tectonic evolution and architecture of the basement.

One example comes from the Loppa high, which is situated on the western margin of the Barents Shelf between Svalbard and northern Norway. The current consensus from geophysical data is that it is composed of high-grade metamorphic rocks, similar to parts of the Archaean-Palaeoproterozoic basement in the Fennoscandian Shield. Whether the crystalline basement comprising the high was originally part of Baltica or Laurentia and when it was metamorphosed remain uncertain because the trace of the Caledonian suture from north Norway to Svalbard is contentious. Moreover, the two wells that penetrate the high in its southwest margin encountered variably metamorphosed dolerites that are probably not representative of the high as a whole. U-Pb geochronology on detrital zircons and Pb isotope measurements on detrital feldspars from the Carboniferous sedimentary rocks adjacent to the basement high provide better constraints on its age and affinity. These data indicate that the zircons were not recycled from sedimentary or metasedimentary rocks. Zircons form essentially a single age peak that indicates the protolith formed between c. 1.95 and 2.05 Ga. Feldspar Pb isotopic signatures form isochrons indistinguishable from the zircon ages but are less radiogenic than is typical for Palaeoproterozoic crust. However, these patterns can be reconciled as a consequence of relatively recent Pb isotopic re-equilibration. This event may also have affected the zircons, which record variable but significant Late Devonian Pb loss.

These results and other examples are discussed in context of Caledonian, Timanian and Grenville orogens and Columbia Supercontinent palaeogeography.