Late Cretaceous-Cenozoic fracturing in Lofoten, North Norway: Tectonic significance, fracture mechanisms and controlling factors

摘要

The Lofoten Ridge is a fault-controlled basement horst residing between Mesozoic basins on the North Norwegian passive margin. This major horst and the adjacent offshore basin-bounding normal faults formed during stages of rifting from the Permian-Jurassic to the Early Cenozoic. Well-exposed, heterogeneous NW-SE striking brittle fracture sets have been studied onshore at Moskenes in the western Lofoten islands. This area provides an excellent frame for understanding onshore-offshore fault-fracture correlations and passive margin evolution as inferred from high-quality offshore seismic data. The fracture sets at Moskenes exhibit geometric and kinematic variability, i.e. systematic bisecting (conjugate) fractures, parallel extensional fractures, and anastomosing relay/stepping (shear) fractures. An incremental NW-SE oriented σ1 compressive stress axis and a NE-SW oriented σ3 was inferred from the attitudes of the conjugate fracture sets. The different fracture sets are interpreted to have formed in isolation and/or in a progression through time, as precursory parallel (Mode I) or conjugate fractures (Mode II) and/or more complex anastomosing shear fracture sets (Mode III) due to varying boundary stress conditions as controlling factors, and not by mechanical weaknesses in the host rock. Whether parallel, conjugate, or anastomosing fractures formed depended on the initial spacing of precursory fractures, the timing of development of complementary fractures, and on the evolving strain-stress conditions. In terms of a North Atlantic passive margin stress field, the NW-SE trend of the heterogeneous fractures in western Lofoten is roughly parallel to the Late Cretaceous-Early Cenozoic regional extension directions but oblique to that of the Permian-Jurassic. A switch of the extension direction from WNW-ESE in the Permian-Jurassic to NNW-SSE in the Late Cretaceous, may have initiated multiple reactivations and, e.g. right-lateral shearing along ENE-WSW striking master faults, producing a block-internal NW-SE σ1 in western Lofoten. Alternatively, the fractures formed by reactivation of transfer fault zones between NNE-SSW right-stepping normal faults. A third possibility is origin by Cenozoic ridge push forces, creating neotectonic fractures as a substitute to inversion structures and arc-shaped domes on the Mid-Norwegian margin farther south.