Tectonic influences on the morphometry of the Sudbury impact structure: Implications for terrestrial cratering and modeling

Abstract

Abstract-Impact structures developed on active terrestrial planets (Earth and Venus) are susceptible to pre-impact tectonic influences on their formation. This means that we cannot expect them to conform to ideal cratering models, which are commonly based on the response of a homogeneous target devoid of pre-existing flaws. In the case of the 1.85 Ga Sudbury impact structure of Ontario, Canada, considerable influence has been exerted on modification stage processes by late Archean to early Proterozoic basement faults. Two trends are dominant: 1) the NNW-striking Onaping Fault System, which is parallel to the 2.47 Ga Matachewan dyke swarm, and 2) the ENE-striking Murray Fault System, which acted as a major Paleoproterozoic suture zone that contributed to the development of the Huronian sedimentary basin between 2.45-2.2 Ga. Sudbury has also been affected by syn-to post-impact regional deformation and metamorphism: the 1.9-1.8 Ga Penokean orogeny, which involved NNW-directed reverse faulting, uplift, and transpression at mainly greenschist facies grade, and the 1.16-0.99 Ga Grenville orogeny, which overprinted the SE sector of the impact structure to yield a polydeformed upper amphibolite facies terrain. The pre-, syn-, and post-impact tectonics of the region have rendered the Sudbury structure a complicated feature. Careful reconstruction is required before its original morphometry can be established. This is likely to be true for many impact structures developed on active terrestrial planets.Based on extensive field work, combined with remote sensing and geophysical data, four ring systems have been identified at Sudbury. The inner three rings broadly correlate with pseudotachylyte (friction melt) -rich fault systems. The first ring has a diameter of ~90 km and defines what is interpreted to be the remains of the central uplift. The second ring delimits the collapsed transient cavity diameter at ~130 km and broadly corresponds to the original melt sheet diameter. The third ring has a diameter of ~180 km. The fourth ring defines the suggested apparent crater diameter at ~260 km. This approximates the final rim diameter, given that erosion in the North Range is <6 km and the ring faults are steeply dipping. Impact damage beyond Ring 4 may occur, but has not yet been identified in the field. One or more rings within the central uplift (Ring 1) may also exist. This form and concentric structure indicates that Sudbury is a peak ring or, more probably, a multi-ring basin. These parameters provide the foundation for modeling the formation of this relatively large terrestrial impact structure.

Observations and interpretations at Vredefort, Sudbury, and Chicxulub: Towards an empirical model of terrestrial impact basin formation

“…While such arbitrary scaling biases the interpretation of the Sudbury impact structure towards that of a multi-ring basin, it also reduces considerably the potential genetic significance of the so-called "rings". Spray et al (2004), however, also noted that pre-impact Matachewan dikes apparently lose their preimpact magnetic signature at the 130 km diameter "ring" (Fig. 6), which also marks the termination of the radial Foy offset dike of the SIC, and that the outcrop of the concentric Hess offset dike roughly occurs at and follows the 90 km diameter "ring".…”

“…It is now widely accepted that the SIC represents the differentiated impact-melt sheet within the impact structure (Grieve et al 1991;Dickin et al 1999;Therriault et al 2002). Estimates of the apparent crater diameter are mostly in the ~150-200 km range (Stöffler et al 1994;Grieve et al 1991), but some recent estimates range up to 260 km (Tuchscherer and Spray 2002;Spray et al 2004). Turtle et al (2005) estimated the original final crater diameter at Sudbury to have been ~130-180 km.…”

“…Thus, although there is a large proprietary database on Sudbury held by industry, it is much more spatially limited than at Vredefort. Away from the immediate SIC-footwall contact, systematic geologic mapping covers only ~30% of the area at scales of 1:30,000 or larger (Spray et al 2004) and was conducted largely at a time when impact-related structures at Sudbury were not a focus (e.g., Card 1968Card , 1978Card and Meyn 1969). The structural situation is further complicated by the fact that the Sudbury structure has been extensively tectonized following the impact (Riller 2005;Klimczak et al 2007).…”

“…In addition to the concentration close to the SIC, there are claims of zones several kilometers wide of increased breccia development north of the SIC at radial distances of ~25, 40, and 80 km from the North Range of the SIC (Spray 1997;Thompson and Spray 1994;Rousell et al 2003). Spray et al (2004) suggested the presence of four rings in the North Range footwall (Fig. 6), at diameters of 90 km (structural uplift), 130 km (transient cavity), 180 km, and 260 km (estimated original final crater diameter), respectively, on the basis of fieldwork, geophysics, and remote sensing, with the three inner rings corresponding roughly to the zones of increased Sudbury Breccia development (Thompson and Spray 1994).…”

“…At Vredefort, McCarthy et al (1986) noted a series of concentric anticlinal and synclinal structures that extend out to a diameter of some 300 km from the central uplifted core of basement rocks. At Sudbury, evidence has been presented for annular zones characterized by an apparent increase in the occurrence of pseudotachylitic breccia and there are claims of rings of lineaments in Landsat imagery north of the Sudbury Igneous Complex (SIC), which have been equated with various attributes related to transient cavity formation and subsequent modification (Spray et al 2004). At Chicxulub, there are ring structures in the potential field geophysical data that have been related to crater structure.…”