Unraveling 1.5 Ga of brittle deformation history in the Laxemar‐Simpevarp area, southeast Sweden: A contribution to the Swedish site investigation study for the disposal of highly radioactive nuclear waste

Abstract

The Swedish Nuclear Fuel and Waste Management Company (SKB) is undertaking site investigation at two locations in Sweden, Forsmark and Laxemar‐Simpevarp, with the aim of identifying a suitable area for the construction of a deep repository for the disposal of highly radioactive nuclear waste. Fault slip data from outcrops and oriented drill cores were used to compute paleostress states and to unravel the sites' brittle deformation history. Results from the Laxemar‐Simpevarp area show that its suggested brittle history results from multiple reactivation of fracture and fault sets caused by the many orogenic episodes that affected the region during at least 1.5 Ga of geological evolution in the brittle deformational regime. Two compressional, approximately NW/NNW‐SE/SSE and NNE‐SSW oriented shortening events generated sets of conjugate, steep strike‐slip fractures. These sets formed during the late stages of the Svecokarelian and possibly also of the Gothian orogeny, soon after the region entered the brittle deformation domain. The Mesoproterozoic Sveconorwegian orogeny generated fractures and faults that are assigned to a third set of conjugate strike‐slip faults, which constrain an approximately E‐W σ1. The Caledonian shortening, oriented approximately NW‐SE to E‐W, reactivated the latter but also formed a new, similarly oriented set of subvertical strike‐slip fractures. Permian transtension was oriented NW‐SW and caused a prominent set of moderately dipping NW‐SE trending normal faults in the Precambrian basement of the study area. Two other approximately NW‐SW and NW‐SE oriented shortening events are recorded in Ordovician limestones and can be tentatively linked to the far‐field effects of the Laramide and Alpine orogenies.