- Email: [email protected]
Crustal evolution of an ore district illustrated – 4D-animation from the Skellefte district, Sweden
Computers & Geosciences 48 (2012) 157–161
Contents lists available at SciVerse ScienceDirect
Computers & Geosciences journal homepage: www.elsevier.com/locate/cageo
Crustal evolution of an ore district illustrated – 4D-animation from the Skellefte district, Sweden Pietari Skytta¨ 1 Division of Geosciences and Environmental Engineering, Lule˚ a University of Technology, SE-97187 Lule˚ a, Sweden
a r t i c l e i n f o
abstract
Article history: Received 6 March 2012 Received in revised form 4 May 2012 Accepted 24 May 2012 Available online 5 June 2012
This paper exemplifies how the dynamic crustal evolution within geologically complex areas, such as the Palaeoproterozoic VMS-hosting Skellefte district in northern Sweden, may be illustrated by 4D-animations. Furthermore, the paper gives a brief description on how the animation was constructed. The modeled steps through the geological evolution include crustal extension accommodated by normal faults, synchronous volcanism and VMS deposition, block rotations and erosion post-dating the volcanism, deposition of post-volcanic sediments and, finally, inversion of the early normal faults and the related development of upright folds. Specific emphasis is put on illustrating how the originally stratabound VMS sheets were transposed into contrasting geometries depending on erosional features and variations in the tectonic overprint. Summing up, 4D-animations provide a powerful tool for sharing the scientific results and for use in educational purposes. & 2012 Elsevier Ltd. All rights reserved.
Keywords: 4D-animation 3D-modeling Structural geology VMS
1. Introduction Traditionally, results from geological investigations have been presented by geological maps, cross-sections and block drawings, or a combination of these. To further illustrate the crustal evolution through time, progression of the geological events has been shown by still figures capturing the successive steps through the geological events (e.g. Bauer et al., 2011). However, even at their best, the visualizations remain static and fail to shown the dynamic aspect of the geological evolution. To overcome the problem, a pilot investigation of creating a schematic 4D-animation over the geometrically complex Skellefte district, Sweden, was carried out. Since the district hosts around 85 deposits of polymetallic minerals and has recently been the target of intense geological and geophysical modeling activities (Bauer et al., 2011; Skytta¨ et al., 2010; 2011; 2012 and references therein), it is considered as an ideal target for 4D-modeling. This paper describes how a 4D-animation explaining the principal crustal events within the ore-bearing Skellefte district was constructed. By showing the animation (Fig. 1; Video 1) it is demonstrated that 4D-modeling may be successfully used in understanding the crustal evolution. Most importantly, the animations may be used in sharing the scientific results for a wider audience and in educational purposes.
E-mail address: [email protected] Present adress: Boliden Mineral AB, SE-93681 Boliden, Sweden.
1
0098-3004/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cageo.2012.05.029
Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.cageo.2012.05.029.
2. Geological background The majority of the Volcanogenic Massive Sulfide (VMS) deposits of the Skellefte district (Fig. 2) occur in the stratigraphically upper part of the 1.89–1.88 Ga Skellefte Group, close to the contact with the overlying metasedimentary 1.88–1.87 Ga Varg¨ fors Group (Allen et al., 1996; Billstrom and Weihed, 1996; Montelius et al., 2007; Skytta¨ et al., 2011). Early orogenic intrusive rocks were synchronous with the Skellefte Group volcanism at 1.89–1.88 Ga (Fig. 2; e.g. Gonza´lez-Rolda´n, 2010) and another main period of intrusive magmatism occurred at 1.82–1.78 Ga, late- to post-tectonically with respect to the main deformation (Weihed et al., 2002). Two sets of syn-extensional faults, WNW-ESE striking normal faults and N–S to NE–SW striking transfer faults controlled the emplacement of early orogenic intrusive rocks and led to segmentation of the upper parts of the crust, resulting in development of fault-bound half-grabens with contrasting stratigraphies and structural geometries, including opposing polarities between the neighboring compartments (Fig. 3; Bauer et al., 2011; Skytta¨ et al., 2012). Subsequently, the Skellefte Group volcanic rocks were uplifted and eroded (Allen et al., 1996). Weak to moderate coaxial deformation with a strong strain partitioning into inverted normal faults, transposition of sedimentary strata against the faults, and development of
158
P. Skytt¨ a / Computers & Geosciences 48 (2012) 157–161
A layer-cake stratigraphy acting as the basement for the Skellefte Group volcanism was constructed in 2D MOVE. One listric normal fault was drawn to accommodate crustal extension which was generated by using the ‘‘simple shear’’ algorithm of ‘‘move along fault’’ workflow by 2D MOVE. Skellefte Group volcanic rocks and VMS sheets were deposited into the developed half-grabens (Fig. 5a). 2D-modeling was continued by developing several listric faults as splays of the original normal fault.
Consequently, several half-grabens were developed. Horizontal ore sheets were placed against the normal faults and along the erosion level. Due to progressive extension and deposition of new volcanic layers, the ore sheets got buried and tilted away from horizontal. After volcanism, an early orogenic intrusion was emplaced at depth below the volcanic rocks, and subsequently uplifted towards the erosion level during a break in crustal extension (Fig. 5b). This caused block-rotations and erosion of the volcanic rocks, followed by deposition of the Vargfors Group sediments and some minor uplift of the early orogenic intrusive rocks. After a total of approximately 35% of extension, the crust got subjected to compressional deformation. The compression was accommodated by the normal faults which were inverted and now displayed reverse kinematics (Fig. 5c). Like during the extension, ‘‘simple shear’’ algorithm of ‘‘move along fault’’ workflow by 2D MOVE was applied, but this time with an opposite movement sense. The movement caused the strata in the vicinity of the faults to transpose into steeper orientations, and the related drag caused generation of asymmetric synclines. During the progressive shortening, existing folds were tightened and new upright folds generated. Furthermore, break-back faults were formed in particular close to the margin of the early orogenic intrusive rocks. Finally, late to post-orogenic granites intruded the crust at around 1.8 Ga and were later eroded together with their host rocks.
Fig. 1. This paper describes a 4D-animation illustrating the geological evolution of the central part of the Skellefte district, Sweden. A progression from crustal extension to compression is showed, with special emphasis to tectonic transposition of the VMS ore deposits. The animation is available as ‘‘Video 1’’ though the Journal website.
Fig. 3. The inferred post-extensional geometry of the segmented Vargfors syncline. The fault-bound blocks from left to right correspond to compartments III–VI in Bauer et al. (2011). The vertical transfer faults separating the fault block are removed and the blocks are horizontally translated for clarity.
upright folds with WNW-ESE striking axial surfaces characterized the deformation in the upper crustal domain shown in Video 1 (Fig. 4; Bauer et al., 2011).
3. Methods Modeling behind the animation was performed as forward modeling with MOVETM software package from Midland Valley Exploration Ltd. The initial modeling was performed in two dimensions by 2D MOVE and the 2D-sections were later transferred over to 3D MOVE where a total of 39 successive 3D-models through the geological evolution were constructed. Screenshots from all the stages were taken and the screenshots were subsequently animated using iMovie software from Apple. 3.1. 2D-modeling
Fig. 2. Inset: Generalized Fennoscandian Shield geology. Main map: Geological overview of the Skellefte district, as loosely defined by the occurrence of the Skellefte Group metavolcanic rocks, and their immediate vicinity. The approximate location of the animated volume is shown by the rectangle. Key (1) Late- to post-tectonic granites, 1.82–1.78 Ga. (2) Metasedimentary rocks, 1.87 Ga. (3) Skellefte Group metavolcanic rocks, 1.89–1.88 Ga. (4) Mafic intrusions 1.96–1.86 Ga. ¨ (5) Metagranitoids, 1.96–1.86 Ga. (6) Active mines: Kr¼Kristineberg, Ma ¼ Maurliden, Re¼ Renstrom. (7) Axial trace of the main deformation event folds, plunge indicated by arrow. (8) Major high-strain zones. Map drawn after Bergman Weihed (2001), Kathol et al. (2005) and Skytta¨ et al. (2012).
P. Skytt¨ a / Computers & Geosciences 48 (2012) 157–161
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Fig. 4. Schematic cross-sections through the Vargfors syncline showing the structural development from crustal extension to basin inversion (a), with development of new compressional faults in structurally more complex parts of the syncline (b; Bauer et al., 2011).
3.2. 3D-modeling After each of the 2D-modeling steps was completed, the 2Dsections were transferred into 3D MOVE. The sections were duplicated and the duplicates were translated orthogonally away from the originals so that corresponding lines in the two sections could be joined into surfaces (Fig. 5d). After creating one complete set of surfaces, they were copied, rotated 1801 around a vertical axis, and laterally translated so that the edges of the new surfaces were in contact with the old ones. Thereafter, a vertical plane was placed to separate the original and the copied set of surfaces. As a result, a slice of crust characterized by two half-grabens with opposing polarities, and separated by a transfer fault was created (Fig. 5e). Continued extension with opposing directions between the neighboring blocks was accommodated by strike-slip shearing along the transfer fault (Fig. 5f). After the shift to crustal shortening, the movement sense along the transfer faults was reversed (Fig. 5g) Until the uplift of the early orogenic intrusive rocks, the modeling was done solely in 2D MOVE while 3D MOVE was only used to generate 3D-surfaces. To simulate differential block-rotations and erosion between the blocks, one of the blocks was rotated around the short axis of the block (by 2D MOVE, Fig. 5h), whereas the other block was rotated around its long axis (parallel to the transfer fault, Fig. 5i) by 3D MOVE. After the block rotations, the same guidelines as described in section ‘‘2D-modeling’’ were followed but, due to the increased complexity of the geometrical relationships between the modeled objects, mainly 3D MOVE was used. Besides the MOVE software package, at least Kine3D and GeoSec for gOcad from Paradigm could be used in 4D-modeling.
4. Animation The animation has a total length of 7 min 8 s. It starts with an overview through the evolution of the investigated slice of the Skellefte district (Fig. 2) and proceeds to a section displaying the distribution of the VMS ore lenses during the phase of extension and volcanism. The next part highlights the blockrotations around variably oriented axes, effects of which to the angular relationships between the Skellefte and Vargfors Group rocks are well visible in the following part viewed from directly above. Basin inversion is shown from two perspectives, after which the focus is on showing the behavior of a major fault
during inversion. Finally, three different cases of VMS ore distribution and transposition are shown (Fig. 5j–l) and reviewed in a ‘‘round-tour’’ through the final model.
5. Concluding remarks The greatest advantage of 4D-animations is their capability of illustrating the dynamic evolution of the crust. In specific, the relationship between different events such as volcanism, emplacement of intrusive rocks, erosion and deformation, and their combined effects for different geological units and geographical areas may be illustrated. For this reason, the animations may be used in summarizing and generalizing the main outcomes of large geological investigations. Furthermore, geometrically complex objects and their evolution may be showed in an understandable way, e.g. the complex angular relationships between the ores and the overlying metasedimentary rocks in the presented model. Therefore, 4D-modeling is applicable also within VMS exploration where it helps in understanding the general systematics of the ore-forming events and their subsequent deformation. A major issue in modeling is the validation of the models. Since the presented model is more a conceptual overview than a fully geometrically constrained model, it may not be validated by standard 3D-restoration tools. In contrast, the validation needs to make use of evaluating the probability of alternative structural/tectonic models to solve the crustal evolution of the studied area. Applied to the present model, this could involve constraining the mechanisms and timing of the uplift of the early orogenic intrusive rocks, the geographical extent of the erosional event intermediate between the crustal extension and shortening, and coupling between the high-strain zones, within the upper and lower parts of the crust. General restrictions in using the animations are that they are somewhat time-consuming to construct when complex evolution is presented. Furthermore, strain gradients along, e.g. inverted faults may not be applied automatically with the used software but have to be accomplished manually by modeling along successive sections. For this reason, generation of curvilinear fold hinges, typical for instance for the central Skellefte district, is restricted. One further issue is the scaling of the model; how to simplify enough without losing too much of the important details—a common question for the most geological investigations.
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P. Skytt¨ a / Computers & Geosciences 48 (2012) 157–161
Fig. 5. Screenshots highlighting specific model building and modeling issues. (a) Initial 2D-modeling; configuration of the crust after development of normal faults, some ¨ intrusive complex. (c) Transposition of strata against the inverted normal faults during extension and ore-formation. (b) Emplacement and uplift of the early orogenic Jorn crustal shortening. (d) Construction of 3D-surfaces from corresponding lines in the original and in the duplicated sections. (e) Two crustal compartments with opposing normal fault dip directions, separated by a vertical transfer fault. (f) and (g) Strike-slip shearing along transfer faults accommodated the opposing movements between the neighboring blocks: (f) extensional stage, (g) compressional stage. (h) Block-rotation around a rotation axis perpendicular to the transfer-fault. (i) Block-rotation around a rotation axis parallel with the transfer-fault. (j) Block-rotations and related erosion caused contrasting shapes for the three shown VMS-lenses. (k) A gently dipping ore lens transected by an inverted normal fault (lila surface on the right) and a younger break-back fault (red surface dipping left). (l) Two steeply dipping ore lenses transposed into sub-parallelism with an inverted normal fault.
P. Skytt¨ a / Computers & Geosciences 48 (2012) 157–161
Acknowledgments Thanks to Tobias Bauer for the use of Figs. 3 and 4, and to Midland Valley Exploration Ltd. for the use of MOVETM software under the Academic Software Initiative (ASI). This work is part of ‘‘VINNOVA 4D-modeling of the Skellefte district’’ funded by VINNOVA and Boliden Mineral AB, and ‘‘PROMINE’’ project partially funded by the European Commission under the 7th Framework Program. Comments by Stuart Clark and an anonymous journal reviewer helped to improve the manuscript. References ˚ 1996. Setting of Zn–Cu–Au–Ag massive Allen, R.L., Weihed, P., Svenson, S.-A., sulfide deposits in the evolution and facies architecture of a 1.9 Ga marine volcanic arc, Skellefte District, Sweden. Economic Geology 91, 1022–1053. ¨ P., Allen, R.L., Weihed, P., 2011. Syn-extensional faulting Bauer, T.E., Skytta, controlling structural inversion – Insights from the Palaeoproterozoic Vargfors syncline, Skellefte mining district, Sweden. Precambrian Research 191, 166–183. Bergman Weihed, J., 2001. Palaeoproterozoic deformation zones in the Skellefte and the Arvidsjaur areas, northern Sweden. In: Weihed, P. (Ed.), Economic ¨ Geology Research 1. Sveriges Geologiska Undersokning C, vol. 833, pp. 46–68. ¨ Billstrom, K., Weihed., P., 1996. Age and provenance of host rocks and ores of the Palaeoproterozoic Skellefte District, northern Sweden. Economic Geology 91, 1054–1072.
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Gonza´lez-Rolda´n, M.J., 2010. Mineralogy, Petrology and Geochemistry of synvolcanic intrusions in the Skellefte mining district, Northern Sweden, Unpublished. Ph.D. Dissertation, University of Huelva, Spain, 273 pp. ¨ Kathol, B., Weihed, P., Antal Lundin, I., Bark, G., Bergman Weihed, J., Bergstrom, U., ¨ ¨ Billstrom, K., Bjork, L., Claesson, L., Daniels, J., Eliasson, T., Frumerie, M., Kero, ¨ ¨ L., Kumpulainen, R.A., Lundstrom, H., Lundstrom, I., Mellqvist, C., Petersson, J., ¨ ¨ ¨ ¨ Skiold, T., Strang, T., Stølen, L.-K., Soderman, J., Triumf, C.-A., Wikstrom, A., ˚ ¨ ¨ Wikstrom, T., Areback, H., 2005. Regional geological and geophysical maps of the Skellefte district and surrounding areas. Bedrock map. Sveriges geologiska ¨ undersokning Ba 57, 1. ˚ Weihed, P., 2007. Facies architecture of the Montelius, C., Allen, R.L., Svenson, S.-A., Palaeoproterozoic VMS-bearing Maurliden volcanic centre, Skellefte district, Sweden. GFF 129, 177–196. ˚ Bauer, T., 2010. Magnetic fabrics as ¨ P., Hermansson, T., Elming, S-A., Skytta, constraints on the kinematic history of a pre-tectonic granitoid intrusion, Kristineberg, northern Sweden. Journal of Structural Geology 32, 1125–1136. ¨ P., Hermansson, T., Andersson, J., Whitehouse, M., Weihed, P., 2011. New Skytta, zircon data supporting models of short-lived igneous activity at 1.89 Ga in the western Skellefte District, central Fennoscandian Shield. Solid Earth 2, 205–217. ¨ P., Bauer, T., Tavakoli, S., Hermansson, T., Andersson, J., Weihed, P., 2012. Skytta, Pre-1.87 Ga development of crustal domains overprinted by 1.87 Ga transpression in the Palaeoproterozoic Skellefte district, Sweden. Precambrian Research 206–207, 109–136. ¨ Weihed, P., Billstrom, K., Persson, P.-O., Bergman Weihed, J., 2002. Relationship between 1.90–1.85 Ga accretionary processes and 1.82–1.80 Ga oblique subduction at the Karelian craton margin, Fennoscandian Shield. GFF 124, 163–180.
Contents lists available at SciVerse ScienceDirect
Computers & Geosciences journal homepage: www.elsevier.com/locate/cageo
Crustal evolution of an ore district illustrated – 4D-animation from the Skellefte district, Sweden Pietari Skytta¨ 1 Division of Geosciences and Environmental Engineering, Lule˚ a University of Technology, SE-97187 Lule˚ a, Sweden
a r t i c l e i n f o
abstract
Article history: Received 6 March 2012 Received in revised form 4 May 2012 Accepted 24 May 2012 Available online 5 June 2012
This paper exemplifies how the dynamic crustal evolution within geologically complex areas, such as the Palaeoproterozoic VMS-hosting Skellefte district in northern Sweden, may be illustrated by 4D-animations. Furthermore, the paper gives a brief description on how the animation was constructed. The modeled steps through the geological evolution include crustal extension accommodated by normal faults, synchronous volcanism and VMS deposition, block rotations and erosion post-dating the volcanism, deposition of post-volcanic sediments and, finally, inversion of the early normal faults and the related development of upright folds. Specific emphasis is put on illustrating how the originally stratabound VMS sheets were transposed into contrasting geometries depending on erosional features and variations in the tectonic overprint. Summing up, 4D-animations provide a powerful tool for sharing the scientific results and for use in educational purposes. & 2012 Elsevier Ltd. All rights reserved.
Keywords: 4D-animation 3D-modeling Structural geology VMS
1. Introduction Traditionally, results from geological investigations have been presented by geological maps, cross-sections and block drawings, or a combination of these. To further illustrate the crustal evolution through time, progression of the geological events has been shown by still figures capturing the successive steps through the geological events (e.g. Bauer et al., 2011). However, even at their best, the visualizations remain static and fail to shown the dynamic aspect of the geological evolution. To overcome the problem, a pilot investigation of creating a schematic 4D-animation over the geometrically complex Skellefte district, Sweden, was carried out. Since the district hosts around 85 deposits of polymetallic minerals and has recently been the target of intense geological and geophysical modeling activities (Bauer et al., 2011; Skytta¨ et al., 2010; 2011; 2012 and references therein), it is considered as an ideal target for 4D-modeling. This paper describes how a 4D-animation explaining the principal crustal events within the ore-bearing Skellefte district was constructed. By showing the animation (Fig. 1; Video 1) it is demonstrated that 4D-modeling may be successfully used in understanding the crustal evolution. Most importantly, the animations may be used in sharing the scientific results for a wider audience and in educational purposes.
E-mail address: [email protected] Present adress: Boliden Mineral AB, SE-93681 Boliden, Sweden.
1
0098-3004/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cageo.2012.05.029
Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.cageo.2012.05.029.
2. Geological background The majority of the Volcanogenic Massive Sulfide (VMS) deposits of the Skellefte district (Fig. 2) occur in the stratigraphically upper part of the 1.89–1.88 Ga Skellefte Group, close to the contact with the overlying metasedimentary 1.88–1.87 Ga Varg¨ fors Group (Allen et al., 1996; Billstrom and Weihed, 1996; Montelius et al., 2007; Skytta¨ et al., 2011). Early orogenic intrusive rocks were synchronous with the Skellefte Group volcanism at 1.89–1.88 Ga (Fig. 2; e.g. Gonza´lez-Rolda´n, 2010) and another main period of intrusive magmatism occurred at 1.82–1.78 Ga, late- to post-tectonically with respect to the main deformation (Weihed et al., 2002). Two sets of syn-extensional faults, WNW-ESE striking normal faults and N–S to NE–SW striking transfer faults controlled the emplacement of early orogenic intrusive rocks and led to segmentation of the upper parts of the crust, resulting in development of fault-bound half-grabens with contrasting stratigraphies and structural geometries, including opposing polarities between the neighboring compartments (Fig. 3; Bauer et al., 2011; Skytta¨ et al., 2012). Subsequently, the Skellefte Group volcanic rocks were uplifted and eroded (Allen et al., 1996). Weak to moderate coaxial deformation with a strong strain partitioning into inverted normal faults, transposition of sedimentary strata against the faults, and development of
158
P. Skytt¨ a / Computers & Geosciences 48 (2012) 157–161
A layer-cake stratigraphy acting as the basement for the Skellefte Group volcanism was constructed in 2D MOVE. One listric normal fault was drawn to accommodate crustal extension which was generated by using the ‘‘simple shear’’ algorithm of ‘‘move along fault’’ workflow by 2D MOVE. Skellefte Group volcanic rocks and VMS sheets were deposited into the developed half-grabens (Fig. 5a). 2D-modeling was continued by developing several listric faults as splays of the original normal fault.
Consequently, several half-grabens were developed. Horizontal ore sheets were placed against the normal faults and along the erosion level. Due to progressive extension and deposition of new volcanic layers, the ore sheets got buried and tilted away from horizontal. After volcanism, an early orogenic intrusion was emplaced at depth below the volcanic rocks, and subsequently uplifted towards the erosion level during a break in crustal extension (Fig. 5b). This caused block-rotations and erosion of the volcanic rocks, followed by deposition of the Vargfors Group sediments and some minor uplift of the early orogenic intrusive rocks. After a total of approximately 35% of extension, the crust got subjected to compressional deformation. The compression was accommodated by the normal faults which were inverted and now displayed reverse kinematics (Fig. 5c). Like during the extension, ‘‘simple shear’’ algorithm of ‘‘move along fault’’ workflow by 2D MOVE was applied, but this time with an opposite movement sense. The movement caused the strata in the vicinity of the faults to transpose into steeper orientations, and the related drag caused generation of asymmetric synclines. During the progressive shortening, existing folds were tightened and new upright folds generated. Furthermore, break-back faults were formed in particular close to the margin of the early orogenic intrusive rocks. Finally, late to post-orogenic granites intruded the crust at around 1.8 Ga and were later eroded together with their host rocks.
Fig. 1. This paper describes a 4D-animation illustrating the geological evolution of the central part of the Skellefte district, Sweden. A progression from crustal extension to compression is showed, with special emphasis to tectonic transposition of the VMS ore deposits. The animation is available as ‘‘Video 1’’ though the Journal website.
Fig. 3. The inferred post-extensional geometry of the segmented Vargfors syncline. The fault-bound blocks from left to right correspond to compartments III–VI in Bauer et al. (2011). The vertical transfer faults separating the fault block are removed and the blocks are horizontally translated for clarity.
upright folds with WNW-ESE striking axial surfaces characterized the deformation in the upper crustal domain shown in Video 1 (Fig. 4; Bauer et al., 2011).
3. Methods Modeling behind the animation was performed as forward modeling with MOVETM software package from Midland Valley Exploration Ltd. The initial modeling was performed in two dimensions by 2D MOVE and the 2D-sections were later transferred over to 3D MOVE where a total of 39 successive 3D-models through the geological evolution were constructed. Screenshots from all the stages were taken and the screenshots were subsequently animated using iMovie software from Apple. 3.1. 2D-modeling
Fig. 2. Inset: Generalized Fennoscandian Shield geology. Main map: Geological overview of the Skellefte district, as loosely defined by the occurrence of the Skellefte Group metavolcanic rocks, and their immediate vicinity. The approximate location of the animated volume is shown by the rectangle. Key (1) Late- to post-tectonic granites, 1.82–1.78 Ga. (2) Metasedimentary rocks, 1.87 Ga. (3) Skellefte Group metavolcanic rocks, 1.89–1.88 Ga. (4) Mafic intrusions 1.96–1.86 Ga. ¨ (5) Metagranitoids, 1.96–1.86 Ga. (6) Active mines: Kr¼Kristineberg, Ma ¼ Maurliden, Re¼ Renstrom. (7) Axial trace of the main deformation event folds, plunge indicated by arrow. (8) Major high-strain zones. Map drawn after Bergman Weihed (2001), Kathol et al. (2005) and Skytta¨ et al. (2012).
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Fig. 4. Schematic cross-sections through the Vargfors syncline showing the structural development from crustal extension to basin inversion (a), with development of new compressional faults in structurally more complex parts of the syncline (b; Bauer et al., 2011).
3.2. 3D-modeling After each of the 2D-modeling steps was completed, the 2Dsections were transferred into 3D MOVE. The sections were duplicated and the duplicates were translated orthogonally away from the originals so that corresponding lines in the two sections could be joined into surfaces (Fig. 5d). After creating one complete set of surfaces, they were copied, rotated 1801 around a vertical axis, and laterally translated so that the edges of the new surfaces were in contact with the old ones. Thereafter, a vertical plane was placed to separate the original and the copied set of surfaces. As a result, a slice of crust characterized by two half-grabens with opposing polarities, and separated by a transfer fault was created (Fig. 5e). Continued extension with opposing directions between the neighboring blocks was accommodated by strike-slip shearing along the transfer fault (Fig. 5f). After the shift to crustal shortening, the movement sense along the transfer faults was reversed (Fig. 5g) Until the uplift of the early orogenic intrusive rocks, the modeling was done solely in 2D MOVE while 3D MOVE was only used to generate 3D-surfaces. To simulate differential block-rotations and erosion between the blocks, one of the blocks was rotated around the short axis of the block (by 2D MOVE, Fig. 5h), whereas the other block was rotated around its long axis (parallel to the transfer fault, Fig. 5i) by 3D MOVE. After the block rotations, the same guidelines as described in section ‘‘2D-modeling’’ were followed but, due to the increased complexity of the geometrical relationships between the modeled objects, mainly 3D MOVE was used. Besides the MOVE software package, at least Kine3D and GeoSec for gOcad from Paradigm could be used in 4D-modeling.
4. Animation The animation has a total length of 7 min 8 s. It starts with an overview through the evolution of the investigated slice of the Skellefte district (Fig. 2) and proceeds to a section displaying the distribution of the VMS ore lenses during the phase of extension and volcanism. The next part highlights the blockrotations around variably oriented axes, effects of which to the angular relationships between the Skellefte and Vargfors Group rocks are well visible in the following part viewed from directly above. Basin inversion is shown from two perspectives, after which the focus is on showing the behavior of a major fault
during inversion. Finally, three different cases of VMS ore distribution and transposition are shown (Fig. 5j–l) and reviewed in a ‘‘round-tour’’ through the final model.
5. Concluding remarks The greatest advantage of 4D-animations is their capability of illustrating the dynamic evolution of the crust. In specific, the relationship between different events such as volcanism, emplacement of intrusive rocks, erosion and deformation, and their combined effects for different geological units and geographical areas may be illustrated. For this reason, the animations may be used in summarizing and generalizing the main outcomes of large geological investigations. Furthermore, geometrically complex objects and their evolution may be showed in an understandable way, e.g. the complex angular relationships between the ores and the overlying metasedimentary rocks in the presented model. Therefore, 4D-modeling is applicable also within VMS exploration where it helps in understanding the general systematics of the ore-forming events and their subsequent deformation. A major issue in modeling is the validation of the models. Since the presented model is more a conceptual overview than a fully geometrically constrained model, it may not be validated by standard 3D-restoration tools. In contrast, the validation needs to make use of evaluating the probability of alternative structural/tectonic models to solve the crustal evolution of the studied area. Applied to the present model, this could involve constraining the mechanisms and timing of the uplift of the early orogenic intrusive rocks, the geographical extent of the erosional event intermediate between the crustal extension and shortening, and coupling between the high-strain zones, within the upper and lower parts of the crust. General restrictions in using the animations are that they are somewhat time-consuming to construct when complex evolution is presented. Furthermore, strain gradients along, e.g. inverted faults may not be applied automatically with the used software but have to be accomplished manually by modeling along successive sections. For this reason, generation of curvilinear fold hinges, typical for instance for the central Skellefte district, is restricted. One further issue is the scaling of the model; how to simplify enough without losing too much of the important details—a common question for the most geological investigations.
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P. Skytt¨ a / Computers & Geosciences 48 (2012) 157–161
Fig. 5. Screenshots highlighting specific model building and modeling issues. (a) Initial 2D-modeling; configuration of the crust after development of normal faults, some ¨ intrusive complex. (c) Transposition of strata against the inverted normal faults during extension and ore-formation. (b) Emplacement and uplift of the early orogenic Jorn crustal shortening. (d) Construction of 3D-surfaces from corresponding lines in the original and in the duplicated sections. (e) Two crustal compartments with opposing normal fault dip directions, separated by a vertical transfer fault. (f) and (g) Strike-slip shearing along transfer faults accommodated the opposing movements between the neighboring blocks: (f) extensional stage, (g) compressional stage. (h) Block-rotation around a rotation axis perpendicular to the transfer-fault. (i) Block-rotation around a rotation axis parallel with the transfer-fault. (j) Block-rotations and related erosion caused contrasting shapes for the three shown VMS-lenses. (k) A gently dipping ore lens transected by an inverted normal fault (lila surface on the right) and a younger break-back fault (red surface dipping left). (l) Two steeply dipping ore lenses transposed into sub-parallelism with an inverted normal fault.
P. Skytt¨ a / Computers & Geosciences 48 (2012) 157–161
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