Abstract The Neoproterozoic depositional histories of the Timanian and Baltoscandian, orthogonal margins of Baltica show several important differences but also some similarities. The Timanian margin comprises mainly low-grade, terrigenous sedimentary successions with a distinctive, margin-parallel fault zone separating pericratonic and basinal domains. Magmatic rocks are comparatively rare on land, but are common in deep drillcores recovered from beneath the Pechora Basin. Conversion to an active margin occurred in latest Riphean time, ultimately leading to the accretionary and transpressional regime of the Vendian-age, Timanian Orogeny. Along the Baltoscandian margin, successions of low to high metamorphic grade are preserved in diverse Caledonian nappe complexes. Three main types of palaeobasin are distinguished, based largely on sedimentary facies and basin geometry. Magmatic rocks are more common than in the Timanides, ranging from mafic dyke swarms to the voluminous Seiland Igneous Province. This margin remained passive throughout the Neoproterozoic era. The Vendian-dated dyke swarms signify the onset of Iapetus/Ægir ocean opening at precisely the time when the orthogonal Timan margin was being deformed and telescoped during the Timanian Orogeny.

Abstract The absence of thick sediments on the margins of Galicia, Portugal, and North Biscay allows the underlying rift structure to be clearly seen on seismic reflection profiles. The rifted structure consists of a series of tilted, deformed, and rotated fault blocks bounded by listric normal faults, evidently produced by extension and subsidence. Comparison of the volume of a horizontal slice of the margin (sectional area) before and after extension suggests that the formation of the margin (rifting stage) results from two main tectonic processes acting simultaneously: extension by stretching of the lithosphere, and thinning of the crust by deep thermal processes. Simple models of margin formation dealing only with stretching processes may thus neglect the effect on heat flow and subsidence of the deep thermal processes.