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Analogue modelling of parasitic folds in Grangesberg, Bergslagen, Sweden

Eklof S.

Geology, Geophysics and Environment

2012

Vol. 38, no. 4

466--468

The host rocks to the Grangesberg apatite-iron oxide ore boudinage show folds with opposite vergenee on either side of the boudin and a strong Hneation in the necks (Nilsson et al. 2012). This study aims to, by means of analogue modeling, investigate if the competence contrast between the stiff ore bodies and the host rocks could contribute in creating this deformation pattern. The Grangesberg apatite-iron oxide deposit is located in the Bergslagen province, an area in central Sweden that is known for its numerous mineralisations of various kinds. From the 16th century up until 1989, when the largest mine in Grangesberg close due to falling metal prices, the Grangesberg ore field produced 150 Mt iron ore, to be compared with the 420 Mt iron ore that the entire Bergslagen province produced up until 1992. The Bergslagen province is located in the south-western part of the Svekokarehan orogen in the Fennoscandian shield. The dominating rocks were formed at ca. 1.9 Ga in a back-arc setting inboard an active continental margin which were later multiple deformed and metamorphosed (Stephens et al. 2009). The host rocks to the Grangesberg deposit are 1.90-1.87 Ga regionally metamorphosed and locally hydrothermally altered, manifested as felsic to intermediate volcanic rocks rich in phyllosilicates (Jonsson et al. 2011). The ore bodies occurs as NW-trending sheet like boudins with a moderate to steep easterly dip. Both the ore and the host rock have been affected by at least two fold phases (Fl, F2) and show well developed L- and S-structures. Locally, the stretching hneation associated with D2 is strong (Nilsson et al. 2012). The earlier structures have later been refolded by large scale open F3 buckle folds. Close to the ore bodies, the less competent metavolcanic rocks show F2 domical folds with opposite vergenee on either side of the lenses and with a strong hneation at the boudin necks. A granitoid has been thrust over the host rocks near the ore bodies (Nilsson et al. 2012). To investigate if the competence contrast between the ore bodies and the host rock could contribute to the formation of the opposite verging folds and the lineation at the boudin necks seen in Grangesberg, a series of analogue models were run at the Hans Ramberg Tectonic Laboratory at Uppsala University. Analogue modelling is a way to test natural events, but over much shorter time scales. To make analogue models comparable with natural examples, scaling is needed: a faster sequence of events gives room for softer materials to be used as rock analogues. In the Hans Ramberg Tectonic Laboratory, a large centrifuge was used to simulate the force of gravity (Koyi 1997). For this experiment, acrylic glass and modeling clay was used as rock analogues. To mimic the high competence of the ore bodies, a wedge of acrylic glass was used. On the side facing the "host rock" two elongated convex downwards bulges represented the ore bodies. These convex bulges were separated by a narrow neck zone. The wedge was inclined 60°C to simulate the 50-70-C dip of the ore bodies. As the less competent host rock, plastilina, a soft oil-based clay, was used. Forty 1 mm thick layers of yellow, brown and white plastilina were put together in random order to create a 40 mm thick model. The top most layer was white, and on that a black marker with circles and squares was imprinted. The plastilina was then put in an acrylic glass box with the wedge and run in the centrifuge. During centrifuging, the rigid wedge indented and shortened the plastilina layers. When completed, the model had experienced 800 rpm for a total of 12 minutes, which resulted in 32.5% shortening. After shortening, the surface circles closest to the wedge had deformed into ellipses and the surface squares closest to the wedge had become parallelograms. The long axis of the ellipses and parallelograms showed opposite vergence on the sides of the rigid bulges of the wedge. When cutting sections perpendicular to the shortening direction, the model showed patterns between the two boudins resembling those of stretching lineation. Sectioning the model horizontally revealed S- & Z-folds around the indenting boudins. During centrifuging, the wedge indented the plastilina, resembling the possible deformation during thrusting of the deep granite. These results show that the competence of the ore bodies could be sufficient to create opposite verging folds and stretching lineation between the ore bodies.