Presented at the Geological Society of America National Meeting, Boston, 2001
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
The 1850 Ma Sudbury Igneous Complex has been variously interpreted as a lopolith, a caldera collapse structure, an intrusive emplaced along an impact-induced ring fracture, and most recently as a differentiated impact melt sheet. The complex is differentiated, with norite along its outer (lower) circumference overlain by transitional rocks and finally granophyre in the inner (upper) portion. Along its southern margin, the complex is north-facing, but is in intrusive contact with south-facing metavolcanic and metasedimentary rocks of the early Proterozoic Huronian Supergroup. Both the igneous complex and the adjoining Proterozoic rocks are nearly vertical. This back-to-back facing relationship requires that the Huronian rocks have been overturned before the emplacement of the igneous complex.
In some of the older models, the Huronian rocks might have been only moderately overturned before emplacement of the igneous complex. However, the melt sheet model requires that the Huronian rocks were nearly flat-lying, in other words, overturned through nearly 180 degrees. Although early Proterozoic tectonism predating the Sudbury Igneous Complex has been claimed by many authors, such an extreme degree of overturning seems incompatible with the structural styles in rocks farther from the igneous complex.
Recent reinterpretations of the Sudbury Basin have suggested that major breccia zones are actually superfaults created during terrace collapse following the impact. Terrace collapse along listric normal faults would make the back-to-back facing relationships still harder to explain, and also is hard to reconcile with the stratigraphically lowest Huronian rocks being exposed adjacent to the igneous complex. A modified superfault model, however, fits the observed structure very well. I propose that the Sudbury impact crater was actually a peak-ring crater. In a peak-ring crater, rebound of the crater floor results in a very high central uplift that collapses and spreads outward by overturning and thrust-faulting the uplifted rocks. In such a collapsed central uplift, the rocks would be overturned, then overlain by an upright melt sheet. Later Penokean tectonism (2000-1800 Ma ago) tilted the rocks to their present vertical orientation. In this model the breccia zones are still superfaults, but some of the inner superfaults are thrust faults rather than normal faults. This model also would thrust lowermost Huronian rocks upward and outward, accounting for their presence immediately adjacent to the igneous complex. A thrust superfault would override fallback breccia and probably impact melt, resulting in a thicker fault breccia zone and incorporation of impact melt in the breccia. In addition, overthrusting of impact melt could provide a mechanism for injecting impact melt into dikes and sills, accounting for the so-called "offset" intrusions at Sudbury.
In recent years there have been radical reinterpretations of both the Sudbury breccias and the igneous complex. The breccias were generally interpreted as injection breccias until Spray (1997) reinterpreted some of the major breccia zones as superfaults. Superfaults are faults where single displacements of kilometers occur, and are principally related to the collapses of calderas and impact craters. With displacements several orders of magnitude greater than other faulting events, superfaults can be expected to produce large amounts of frictional melt.
The igneous complex, once widely regarded as a post-impact funnel-shaped intrusion, was reinterpreted by Grieve and others (1991) as impact melt. With its temperature unconstrained by normal magmatic processes, the impact melt could have been superheated and had negligible viscosity. Therefore the top surface of the igneous complex must have been horizontal immediately after impact. Also, the lower contact of the Sudbury Igneous Complex reflects post-impact topography, possibly modified by erosion and melting during and after emplacement of the melt sheet
This reinterpretation creates a serious problem in interpreting the structure of the Huronian rocks because it requires that the basal Huronian rocks be completely overturned when the impact melt was deposited. However, this problem can be resolved in terms of the collapse of a central peak ring. Scott and Benn (2001) interpreted some of the Sudbury breccias and intrusions of impact melt as the result of the collapse of the inner margin of a peak ring. This study, in contrast, interprets structure along the southern margin of the igneous complex as the result of the collapse, overturn and overthrusting of the outer margin of the peak ring. However, Scott and Benn's proposed mechanism for offset dike emplacement remains viable.
|In older models of the Sudbury Igneous Complex, the complex was interpreted as a funnel-shaped intrusion, either endogenic or emplaced along an impact-induced fracture. The intrusion then underwent gravitational differentiation (white arrow). These models required no more than moderate pre-emplacement overturn of the lower Huronian rocks (Dutch, 1976, p. 52-67).|
|More recent models of the Sudbury Igneous Complex as an impact melt sheet present a profound structural problem. The highly fluid and superheated melt sheet would have been horizontal, requiring complete overturn of the lowest Huronian rocks. This model requires either a thrust contact between the lowest Huronian rocks and the upright units farther south, or a large isoclinal fold.|
Cross-Sections of the South Range of the Sudbury Igneous Complex and Adjacent Huronian Rocks
|The cross sections at left are constructed along the section lines shown above, with no vertical exaggeration. The Sudbury Igneous Complex is steeply or vertically north-dipping and faces north. The immediately adjacent rocks of the lowermost Huronian Supergroup are vertical and south-facing, and merge southward into tightly folded but generally upright rocks.There appears to be a real tendency for the northernmost folds in the Huronian rocks to be of smaller wavelength than those farther away. This may be a buttress effect as the rocks were folded against the rigid crystalline rocks of the Sudbury Igneous Complex and the Archean Superior Province. The folds in the Mississagi Formation south of the Sudbury Basin are typically a few km or less in wavelength. Since the Mississagi Formation is roughly a kilometer thick, the folds are almost certainly disharmonic at depth.Although units like the Pecors, Espanola and Serpent are relatively thin, they frequently display large outcrop areas. These outcrop patterns suggest that the Archean basement beneath the Huronian rocks is subhorizontal in many places with an overall gentle dip to the south.|
|The morphological progression of crater type with increasing diameter is well known (Melosh, 1989). Small impacts produce simple craters. Larger simple craters have oversteepened walls that collapse.|
|Still larger craters evolve central peaks as the compressed material in the crater floor rebounds. Centripetal motion of wall and floor material results in listric normal faulting and the formation of terraces on the inner rim.|
|Still larger impacts like the lunar basin Schroedinger evolve peak rings. In peak-ring craters a very large central uplift collapses to create a ring of peaks rather than a central peak. The diameters of each type of crater scale inversely proportional to a planet's surface gravity. The greater the surface gravity, the smaller the crater for a given morphological type. On earth, the transition from simple to central peak crater occurs at 2-5 km in diameter, and the transition to peak-ring craters at about 20 km.|
Morgan and others (2000) conducted geophysical investigations and modeling of the Chicxulub impact in Yucatan, an impact basin approximately as large as Sudbury. Seismic profiles showed a clearly defined buried peak ring underlain by an inward-dipping thrust fault. Their hydrocode modeling of the Chicxulub impact produced results consistent with the seismic data.The collapse of the central peak overturns and overthrusts peak material onto the still incoming floor and wall material. The figures below show crater collapse at 0, 60, 130 and 220 seconds after maximum development of the transient cavity.
|2500-2300 Ma |
Deposition of Huronian Supergroup rocks
|2300-2000 Ma |
Blezardian deformation, intrusion of Creighton and Murray Plutons. The 2300-Ma Creighton Pluton was forcefully emplaced (Dutch, 1976, 1979). Since foliation in the pluton is now generally vertical, the pluton was probably emplaced in much its present orientation, implying that the basal Huronian metavolcanic rocks were vertical at that time (Dutch, 1976, p. 52-67). Riller and Schwerdtner (1997) also argue for a vertical orientation for the greenstones.
|1850 Ma |
Maximum extent of transient cavity
|1850 Ma |
Inward collapse of crater walls and outward collapse of peak ring results in complete overturn of lowest Huronian rocks. Impact melt fills crater and fallback breccia is deposited atop melt. The result is an upward-facing differentiated melt sheet with overlying breccia, resting on a completely inverted Huronian succession.
|1850 Ma and after |
The impact occurs during the Penokean Orogeny. The orogeny folds the Huronian supergroup and the Sudbury Igneous Complex. The south side of the complex is tilted steeply northward, returning the lowest Huronian rocks about to their original Blezardian orientation. Lithoprobe studies (Milkereit and others, 1992) suggest the presence of deep thrusts as shown, as well as continuation of the northern portion of the igneous complex beneath the southern portion. Post-orogenic erosion strips rocks to the present level of exposure (red line).
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