Solund Basin

The Solund Basin is a sedimentary basin containing at least 6 kilometres (3.7 mi) of mainly coarse conglomerates of Devonian age. It is the southernmost of a group of basins of similar age found along the southwest coast of Norway between Sognefjord and Nordfjord, developed in the hanging-wall of the Nordfjord-Sogn Detachment. It was formed as a result of extensional tectonics during the post-orogenic collapse of crust thickened during the Caledonian orogeny, towards the end of the Silurian period. It is named for Solund Municipality in Vestland county, Norway.

Tectonic setting

During the later part of the Silurian period, the western margin of the Baltic Plate was affected by the main collisional event of the Caledonian orogeny, known as the Scandian phase. This led to large-scale thrusting and the development of a mountain belt similar in scale to the Himalayas. Soon after the collision finished during the Early Devonian, the thickened crust began to extend. Initially the extension took place by reactivation of Caledonian thrust faults, known as Mode 1 extension. The uplift and exhumation led to a reduction of dip in these reactivated thrusts, making them progressively less mechanically viable. At this point Mode II extension took over, with development of large extensional shear zones that cross-cut the Caledonian thrust pile, such as the Nordfjord-Sogn Detachment.^[1]

Extent

The preserved part of the Solund Basin covers about 800 square kilometres (310 sq mi), including most of the islands of Ytre Sula, Steinsundøyna, Sula, Losna, many other smaller islands and the mainland peninsula on which the mountain of Lihesten lies in Hyllestad Municipality.^[2]

Basin fill

La litología dominante es el conglomerado, con algo de brecha y cantidades subordinadas de arenisca , organizado en ciclos algo irregulares de granoclasificación a escalas de decenas de metros hasta más de 100 m. ^{[ 3 ]} El conglomerado es típicamente muy grueso, compuesto por clastos del tamaño de cantos rodados a bloques. Se observa una fuerte fábrica deposicional con ejes largos de clastos que muestran una marcada orientación preferencial noroeste-sureste, a menudo con una imbricación bien desarrollada . ^{[ 2 ]}

Deslizamientos de tierra en Hersvik

En la zona cercana a Hersvikbygda , al norte de la isla de Sula, se encuentra una serie de grandes cuerpos lenticulares exóticos dentro de los conglomerados hacia la base de la secuencia. Estos cuerpos están compuestos por una amplia gama de litologías, incluyendo rocas volcánicas máficas y félsicas , granito , diorita , gabro y metasedimentos . Cuando se describieron por primera vez en 1926, se interpretaron como pequeñas láminas de cabalgamiento que implicaban el emplazamiento de láminas de las rocas alóctonas superiores subyacentes bajo la discordancia basal en la secuencia de conglomerados. Investigaciones posteriores no encontraron evidencia de contactos tectónicos en la base de las lentes, y un gran cuerpo de gabro brechado monomíctico se interpretó como un flujo de detritos. En la base de una de las lentes, un cuerpo de riolita se interpretó como un flujo de lava devónico contemporáneo, el único ejemplo de vulcanismo de esa edad descrito en Noruega. Una reevaluación de estos depósitos, incluyendo la datación del cuerpo de riolita como Silúrico , ha interpretado todos estos cuerpos como deslizamientos de tierra, probablemente derivados de rocas del Complejo Ofiolítico Solund-Stavfjord en el margen de la cuenca al norte. ^[⁴^]

También se han identificado depósitos de deslizamientos de tierra similares en el límite sureste de la cuenca cerca de Kråkevåg, en el nivel estratigráfico más alto conservado, lo que demuestra que dichos deslizamientos estuvieron activos durante toda la sedimentación de la parte conservada de la cuenca. ^{[ 4 ]}

Estructura

In the southeastern part of the basin, the conglomerate sequence dips 20–25° to the southeast. The main fold structure is the Lågøyfjorden anticline which runs southwest–northeast. To the northwest of the fold hinge the dip is towards the north or northwest. The southeastern margin of the basin is formed by the low-angle Solund Fault or Solund Detachment, part of the Nordfjord-Sogn Detachment. Close to this margin small down-to-the-northwest listric normal faults are developed within the conglomerates, which merge with the detachment itself. At the fault contact, the lowermost metre of the conglomerates is strongly sheared, with a foliation parallel to the fault and the clasts become elongated in a direction parallel to the lineation in the underlying mylonite. The conglomerate is also noticeably deformed with well-developed cleavage in zone about 2.5 km wide parallel to the Solund Fault.^[2] The conglomerates show evidence of very low-grade metamorphism with recognition of an assemblage of authigenic minerals that indicate temperatures in the range 230–330°C, consistent with maximum burial of up to 13 km.^[3] The mylonites immediately beneath the Solund Fault appear to be a highly deformed version of the mafic rocks of the upper allochthon. These mylonites contain a suite of minor structures that show slightly oblique top-down-to-the-northwest movement.

The basin is cut by a prominent series of nearly north–south trending faults that are interpreted to be of latest Paleozoic to Mesozoic age, associated with west–east extension in the North Sea rift that began during the Permo-Triassic.^[1]

References

1 2Fossen, H.; Khani, H.F.; Faleide, J.I.; Ksienzyk, A.; Dunlap, W.J. (2016). "Post-Caledonian extension in the West Norway –northern North Sea region: the role of structural inheritance"(PDF). In Childs, C.; Holdsworth, R.E.; Jackson, C.A.-L.; Manzocchi, T.; Walsh, J.J.; Yielding, G. (eds.). The Geometry and Growth of Normal Faults. Geological Society, Special Publications. Vol. 439. pp. 465–486. doi:10.1144/SP439.6. S2CID 132839546.
1 2 3Nilsen, T.H. (1968). "The Relationship of Sedimentation to Tectonics in the Solund Devonian District of southwestern Norway". Norsk Geologisk Undersøkelse. 259: 1–108.
1 2 Beinlich, A.; Austrheim, H.; Glodny, J.; Erambert, M.; Andersen, TB (2010). "Secuestro de CO2 y agotamiento extremo de Mg en clastos de peridotita serpentinizada de la cuenca de Solund del Devónico, suroeste de Noruega". Geochimica et Cosmochimica Acta . 74 (24): 6935– 6964. doi : 10.1016/j.gca.2010.07.027 .
1 2 Hartz, EH; Martin, MW; Andresen, A.; Andersen, TB (2002). "Rocas volcánicas en la cuenca de Solund del Devónico, Noruega occidental: grandes deslizamientos de riolitas del Silúrico (439 Ma)" . Journal of the Geological Society . 159 : 121–128 . doi : 10.1144/0016-764901-063 . S2CID 55652501 .

[Fossen_etal_2016-1] 1 2Fossen, H.; Khani, H.F.; Faleide, J.I.; Ksienzyk, A.; Dunlap, W.J. (2016). "Post-Caledonian extension in the West Norway –northern North Sea region: the role of structural inheritance"(PDF). In Childs, C.; Holdsworth, R.E.; Jackson, C.A.-L.; Manzocchi, T.; Walsh, J.J.; Yielding, G. (eds.). The Geometry and Growth of Normal Faults. Geological Society, Special Publications. Vol. 439. pp. 465–486. doi:10.1144/SP439.6. S2CID 132839546.

[Nilsen_1968-2] 1 2 3Nilsen, T.H. (1968). "The Relationship of Sedimentation to Tectonics in the Solund Devonian District of southwestern Norway". Norsk Geologisk Undersøkelse. 259: 1–108.

[Beinlich_etal_2010-3] 1 2 Beinlich, A.; Austrheim, H.; Glodny, J.; Erambert, M.; Andersen, TB (2010). "Secuestro de CO2 y agotamiento extremo de Mg en clastos de peridotita serpentinizada de la cuenca de Solund del Devónico, suroeste de Noruega". Geochimica et Cosmochimica Acta . 74 (24): 6935– 6964. doi : 10.1016/j.gca.2010.07.027 .

[Hartz_etal_2002-4] 1 2 Hartz, EH; Martin, MW; Andresen, A.; Andersen, TB (2002). "Rocas volcánicas en la cuenca de Solund del Devónico, Noruega occidental: grandes deslizamientos de riolitas del Silúrico (439 Ma)" . Journal of the Geological Society . 159 : 121–128 . doi : 10.1144/0016-764901-063 . S2CID 55652501 .

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