NJG93-3/4-03
2013
Deep weathering, neotectonics and strandflat formation in Nordland, northern Norway
93
3, 4
pp. 189–213
978-82-92-39488-5

The strandflat area of Nordland is a region with frequent deep weathering and increased seismic activity. We argue that there is a causal relationship between the phenomena. We have carried out chemical and geophysical studies of the weathering and measured the tectonic strain using the GPS method. A parallel and shallow zone of increased seismicity along the coast largely reflecting extensional stress has previously been mapped. New GPS data and previous datasets such as DInSAR, repeated levelling and focal plane solutions indicate that the outer Ranafjorden area is under E–W extension and subsidence. In 1998–1999, a local seismic network detected earthquake clusters along N–S-trending fracture zones. An irregular relative subsidence pattern on the order of 1 mm yr-1 has previously been recorded on InSAR–PS data. During the period 1999–2008, GPS stations to the west of the earthquakes moved c. 1 mm yr-1 to the NW relative to GPS stations to the east. Deeply weathered bedrock occurs around Vestfjorden and southwards to Ranafjorden. Electrical resistivity profiling reveals weathered bedrock extending to 20–100 m depth over large areas.

Calculations of weathering indices and mass balance from XRF analysis show leaching of the major elements. We argue that the weathering is partly of Early Mesozoic age, since the Late Jurassic to Early Cretaceous faults in the Lofoten–Vesterålen area are little affected by the alteration. The weathered sites can be found up to an altitude of 500 m a.s.l. and were most likely exhumed during the Plio–Pleistocene. This observation is supported by previous marine-geological studies suggesting a Pleistocene erosion of ~500 m along the coast. The unloading of the crust thus most likely caused flexuring and accompanying fracture extension. The Nordland strandflat may consequently represent a deeply weathered and peneplaned surface of Triassic to Early Jurassic age that has been exhumed and levelled during Plio–Pleistocene erosion. The deeply weathered and fractured basement rocks could have facilitated effective glacial erosion during the Pleistocene. Glacial erosion combined with freezing/thawing and abrasion by ocean waves during non-glaciated periods would have assisted in the formation of a relatively wide strandflat along the Nordland coast as well as a rim of low land around islands and peninsulas.

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