TSK 11 Göttingen 2006 Doman et al. Deformation mechanisms in the eastern Sudbury Igneous Complex, Canada: Evidence for meteorite impact into an active orogen Vortrag Daniel Doman1 Ulrich Riller1 Kai Hofmann2 The 1.85Ga Sudbury Igneous Complex (SIC) in central Ontario is now widely considered to be the erosional rem- nant of a deformed paleo-horizontal im- pact melt sheet, about 2.5 km in thick- ness. Deformed impact melt breccias of the Onaping Formation and post- impact metasedimentary rocks overlie the layered SIC, which in turn rests on shocked Archean basement and Pa- leoproterozoic cover rocks. The main mass of the Igneous Complex is sub- divided from top to bottom into gra- nophyre, quartz-gabbro and norite lay- ers. Previous workers considered non- cylindrical folding and NW-directed re- verse faulting as the main structural processes that formed the asymmetric, syn-formal geometry of the SIC ap- parent in map view and seismic sec- tion. Structural studies support this model in the southern part of the im- pact structure, where greenschist-facies metamorphic tectonites of the South Range Shear Zone (SRSZ) accomplished structural uplift of the southern SIC by NW-directed reverse shearing. How- ever, little evidence for pervasive duc- tile strain has been reported from the weakly metamorphosed eastern part of the SIC, the East Range, which is char- acterised by steep basal dips and max- 1 Humboldt-Universität zu Berlin, Museum für Naturkunde, Invalidenstrasse 43, D-10115 Berlin 2 Freie Universität Berlin, Insti- tut für Geologische Wissenschaften, Malteser- strasse 74–100, D-12249 Berlin imal curvature in plan view. The ob- jective of this study is to assess the structural inventory of the East Range in terms of post-emplacement deforma- tion mechanisms. Our interpretation is based on published and newly acquired structural data. Planar mineral shape fabrics of cumu- late plagioclase and pyroxene are devel- oped in the intermediate quartz-gabbro and lower norite layers of the south- ern East Range SIC. Microstructures show little intracrystalline deformation in quartz. Euhedral cumulate plagio- clase retains an angular outline indicat- ing magmatic mineral fabric develop- ment. This magmatic foliation is con- cordant to SIC contacts or large-scale discontinuities in their vicinity (Fig. 1). Magmatic fabrics are observed rarely in the northern portion of the East Range. Here, tectonic foliations and S–C fab- rics are developed sporadically at, and concordant to, brittle structures strik- ing N–S. A weak tectonic foliation de- fined by chlorite that replaces magmatic minerals is developed in the upper gra- nophyric SIC of the NE-lobe that con- nects the SIC’s North and East Ranges via a 105° arc. This foliation grades into a shape-preferred orientation of pri- mary, i.e., magmatic, mafic minerals ob- served in the lower granophyre and un- derlying layers of the SIC. Mineral fab- rics observed in the NE-lobe SIC are concordant to metamorphic foliations developed in the overlying Onaping For- mation breccias. Both foliations strike parallel to the NE-Lobe’s acute bisec- trix and, thus, display an axial-planar geometry typical for fabrics formed in the core of a buckle fold (Fig. 1). Brit- tle structures including centimetre-scale shear-fractures to kilometre-scale fault- zones are observed in the eastern 1 Doman et al. TSK 11 Göttingen 2006 Figure 1: Strike and dip directions of magmatic and tectonic foliations in the East Range SIC and overlying Onaping Formation. 2 TSK 11 Göttingen 2006 Doman et al. SIC and its host rocks. Large- scale faults striking N–S cut the NE- lobe’s eastern limb causing variable magnitudes of strike separation of SIC contacts. Centimetre- to metre- scale, brittle faults and chlorite-filled brittle-ductile shear-zones occur perva- sively in the eastern SIC, often caus- ing centimetre-scale offset of mark- ers. Microstructures from first-order fault-zones indicate deformation at, and below, greenschist-facies metamorphic conditions. The concordance of magmatic and tec- tonic mineral shape fabrics in the NE- lobe indicates progressive deformation of the SIC during cooling from the magmatic state to lower greenschist- facies metamorphic conditions. Syn- magmatic deformation of the SIC sug- gests that it was emplaced during on- going orogenic deformation. Further- more, maximum principal stress direc- tions inferred from inversion of fault- slip data collected in the Onaping For- mation are orthogonal to metamorphic foliation surfaces at the same locali- ties. This points to a similar defor- mation regime in the Onaping Forma- tion during ductile and brittle deforma- tion. The concordance of magmatic, metamorphic and brittle fabrics is ex- plained best by a single progressive de- formation event that was active while the SIC cooled and solidified. The lack of pervasive ductile deformation fab- rics in the East Range SIC can be ex- plained by rapid cooling of the impact melt sheet (within 100–500 ka) with re- spect to natural tectonic strain rates. While the geometry of mineral fabrics in the study area is compatible with large-scale, non-cylindrical folding, the low levels of ductile deformation suggest that shape-change of the eastern SIC has been accomplished mainly by dis- continuous deformation. This deforma- tion mechanism may have accomplished bulk NW-SE shortening that was ac- commodated by reverse shearing within the SRSZ, resulting in large strike sepa- rations of SIC contacts observed in the western part of the impact structure. By contrast, the eastern SIC may have accomplished such shortening by brittle- ductile, non-cylindrical folding at the eastern terminus of the SRSZ. The com- plex post-impact deformation pattern of the central Sudbury Structure results from impact into an active orogen. References Cowan EJ (1999) Magnetic constraints on the initial geometry of the Sudbury Igneous Complex: a folded sheet or a basin-shaped igneous body? Tectonophysics 307, 135-162 3