The geology of Northern Sotra : Precambrian gneisses west of the Bergen Arcs, Norway
Northern Sotra forms part of a hitherto poorly studied area of Precambrian
gneisses within the Western Gneiss Complex at the core of the Bergen Arcs.
The oldest rocks comprise gneisses of acidic to intermediate composition,
ortho-amphibolites and metasediments, which were variably migmatised
during the Svecofennian orogeny (c.1850Ma). These were deformed and
metamorphosed, and subsequently intruded by a suite of gabbros and dolerites,
and by Sveconorwegian granites (c.1000Ma).
The predominant deformation structures are Caledonian in age, and overprint,
and frequently obliterate, earlier structures. Two Caledonian
orogenic phases are recognised: the Finmarkian (Cambrian to L.Ordovician)
and Scandinavian (M. to L.Silurian).
During the Finmarkian deformation, overfolds developed in response to the
overthrusting of a thick nappe pile. Collapse of the nappe pile caused
intense flattening and the formation of thick protomylonite zones. Synto
post-tectonic, amphibolite facies metamorphism indicates a cover thickness
of approximately 20 -2Skm. Uplift to a near surface level occurred
by U.Ordovician to L.Silurian times.
The Scandinavian orogenic activity followed further nappe overthrusting,
and is restricted to major doming, accompanied by greenschist facies
metamorphism. The dome may be explained by fold superimposition or
gravity induced diapirism, and is responsible for the arcuate shape of
the Bergen Arcs.
Post-doming, brittle deformation occurred, developing faults, joints,
and cataclasite and pseudotachylite zones. Joint planes are the site
of basalt dyke intrusion and hydrothermal mineralisation.