Analogue Modelling of Ductile Deformation at Competent Lenses in Grängesberg, Bergslagen, Sweden
The Grängesberg Mining District (GMD) is located in the western part of the Bergslagen province in south central Sweden, and is the only known apatite-iron oxide mineralization south of Norrbotten. The Grängesberg deposit is the largest of the mineralizations along GMD and consists of a line of steeply to moderately dipping 20-100 m wide lenses extending to a depth of 1.7 km. It is hosted by felsic volcanic rocks that are phyllosilicate altered in the vicinity of the ore. Field relationships indicate that a competent granitoid was structurally emplaced on top of the ore during D2, and that the less competent phyllosilicate-rich host rocks accommodated large parts of the strain. These D2 structures described from the area around the ore bodies in Grängesberg include stretching lineation at the tapering edges of the lenses, asymmetric folds with opposite vergence along strike, sheath folds and possibly fold interference patterns. These structures are proposed to have formed in response to the competence contrast and the reverse movements on a large scale. The hypothesis is that the competence contrast between the competent ore and granitoid and the less competent host rocks could explain the formation and location of the key D2-structures in Grängesberg.
To test the hypothesis, four analogue tectonic models were run in the centrifuge at the Hans Ramberg Tectonic Laboratory (HRTL), Uppsala University. The competent bodies were represented by an acrylic glass wedge with two lenses corresponding to the ore lenses, with an inclination of either 60° or 45° mimicking the approximate dip of the ore. Plasticine was used as the less competent host rocks. On the top surface, circles and squares were printed and used as strain markers. The models were run at c. 300-400 G until penetrative deformation and shortening ranging from 32.5 to 39.2% was reached. During centrifuging, the wedge indented the plasticine, resembling the possible deformation during thrusting of the deep granitoid. The models were then cut to reveal the structures formed, and one model was digitalized using the software Move.
After shortening, the strain markers had deformed to show a change in direction of strain around the lenses. Vertical sections perpendicular to the shortening direction showed that stretching lineation developed between the lenses. Horizontal sections revealed asymmetric non-cylindrical folds with opposite vergence along strike and fold interference patterns. These results show that the observed field relationships were reproducible with the model setup, and that the competence contrast between the ore bodies, the granitoid and the host rocks could be the controlling factor for localization of shear and sites of stretching in the area.
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