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The Sudbury Basin, the Southern Province, the Grenville Front, and the Penokean Orogeny

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The Sudbury Basin, the Southern Province, the Grenville Front, and the Penokean Orogeny The Sudbury Basin, the Southern Province, the Grenville Front, and the Penokean Orogeny STEPHAN J. BROCOUM* IAN W. D. DALZIEL** Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 ABSTRACT The Sudbury Basin in the Canadian Shield has been proposed as a meteorite impact site subsequently deformed by en- dogenic tectonism. Detailed study of the structural geology strengthens this hypothesis and strongly suggests that the deformation of the basin was coeval with major folding and flattening (2.0 to 1.6 b.y. ago) of the rocks of the eastern Southern province and the northwesternmost Gren- ville province. The structural geometry indicates that most of these rocks have a similar strain history. Finite-strain analysis of deformed concretions within the Sudbury Basin sug- gests that originally it was almost circular in outline. Key words: structural geology, deformation, finite-strain indicators, folds, impact features, meteor crater, orogeny, Precambrian, structural analysis, tectonic Figure 1. Geological setting of the Sudbury Basin. fabric. sublayer (Souch and Podolsky, 1969), 1972) or suevite breccia (Peredery, 1972) INTRODUCTION which apparently also intrudes the country has increased the number of workers who The Sudbury Basin, Ontario, is situated rock as radial and concentric dikes called believe that the Sudbury Basin was formed just north of Lake Huron near the intersec- offsets (Fig. 2; Naldrett and others, 1971). at least in part by the impact of a hyper- tion of the Superior, Southern, and Gren- The sublayer and offsets of the irruptive velocity bolide. ville provinces of the Canadian Shield (Fig. are, at present, the world's largest single Geologists who argue against the meteor- 1). The basin is 60 km long and 27 km source of nickel, as well as an important ite impact hypothesis interpret the breccia- wide. It is topographically and structurally source of copper, cobalt, iron, platinum, like Onaping Formation as an ash flow or defined by the Nickel Irruptive, which con- and 11 other elements. tuff (Stevenson, 1972). They emphasize the sists of a lower (outer) layer of augite- The origin of the Sudbury Basin has long facts that the Sudbury Basin is not circular norite, an upper (inner) layer of been a topic of debate, increasingly so since (Fig. 2), that it occurs on a domal structure granophyre, and a "transition zone" of Dietz (1964) proposed that it was formed and appears to be associated with a re- quartz gabbro between the two (Naldrett by meteorite impact. The presence of shat- gional positive gravity-magnetic anomaly at and Kellerud, 1967; Naldrett and others, ter cones in the surrounding country rocks the intersection of major fault systems in 1970, 1971); it is believed to have the shape (Dietz and Butler, 1964; Guy-Bray and the Canadian Shield (Card and Hutchinson, of an asymmetric lopolith (Popelar, 1972). others, 1966; Dietz, 1968, 1972), evidence 1972; Card and others, 1972a; Popelar, At the base of the augite-norite, there is a of shock metamorphism in the Onaping 1972; Fig. 1), and that the entire region discontinuous zone of inclusion and Formation at the base of the section within may represent a special metallogenic prov- sulfide-rich igneous intrusions known as the the basin and in the country rocks north of ince in which the Sudbury Irruptive is only the basin (French, 1967, 1968a, 1968b, one ore-bearing element (Card and Hutch- 1970; Fig. 2), and the interpretation of the inson, 1972). * Present address: Department of Geology, Texas Onaping as a "fall-back" breccia (Dietz, The present study was undertaken to in- Christian University, Fort Worth, Texas 76129. 3 * Also of: Department of Geology, Columbia Uni- 1964; French, 1967, 1968a, 1968b, 1970; vestigate the internal structure of the Sud- versity, New York, New York 10027. Guy-Bray, 1971; Dence and Guy-Bray, bury Basin and its relation to the apparently Geological Society of America Bulletin, v. 85, p. 1571-1580, 4 figs., October 1974 1571 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/10/1571/3418075/i0016-7606-85-10-1571.pdf by guest on 27 September 2021 Kl Olivine-Diabase Dikes (generalized) E Chelmsford Fm Onwatin Fm [ÔU Onaping Fm nn m Norite Nipissing Diabase E3 Creighton (CG) and Murray (MG) Granites • Mississagi Fm McKim, Ramsay Lake and Matinenda Fms Mafic and Felsic Volcanlcs Bedding, uninverted Bedding, overturned Bedding, facing direction unknown Layering First Folds; upright, overturned Second Folds, overturned Grenville Front, transitional Grenvilie Front, faulted Faults Figure 2. Generalized geologic map of the Sudbury Basin and eastern Southern province. After Card (1969), with modifications. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/10/1571/3418075/i0016-7606-85-10-1571.pdf by guest on 27 September 2021 THE SUDBURY BASIN 1573 complex structural geometry and tectonic larly demonstrated by concretions in the cannot be sustained. The paleo-flow indi- history of the surrounding rocks. Chelmsford Formation, which [with the ex- cators of Cantin and Walker were likely ro- ception of misleading locally variable tated toward and concentrated near the THE SUDBURY BASIN shapes due to coalescence of adjoining con- long axis of the basin during the The deeper structure of the Sudbury Ir- cretions and (or) preferential development northwest-southeast shortening of the ruptive, including possible feeders, is un- in certain beds] are always flattened in the basin. Rotation of linear and planar struc- known. The sedimentary rocks of the foliation surface and therefore must have tures during strain are well documented Whitewater Group that infill the basin form been originally spherical or nearly so. If (Flinn, 1962; Ramsay, 1967, p. 461-517). a doubly plunging synclinorium. The outer they were not originally nearly spherical, Removal of the effects of the strain would surface of the underlying irruptive plunges they would not always be flattened in the result in a much weaker preferred orienta- steeply southwestward at the northeast foliation surface (Ramsay, 1967, p. tion of the paleo-flow indicators but would closure and moderately northeastward at 204-205, and Figs. 5-20 to 5-22). The bed- still suggest a source of the Chelmsford the southwest closure (Speers, 1957). The ding and the foliation pass through them Formation from the northwest quadrant. northwest limb dips gently to moderately without deviation. Thus, these bodies can As mentioned above, all the rock units of southeastward. The more steeply dipping be used as finite-strain ellipsoids reflecting the Whitewater Group have gradational southeastern limb is locally thrust and over- the deformation undergone by the and conformable contacts and the same tec- turned northwestward (Naldrett and Chelmsford Formation. Finite strain tonic and metamorphic history. others, 1971; Card and Hutchinson, 1972; analysis (Brocoum and Dalziel, 1974) sug- Metamorphism was limited to lowest Fig. 2). gests that the Chelmsford was shortened greenschist facies and to local contact ef- The Whitewater Group consists., from approximately 30 percent of its original fects of the Irruptive (Card and Hutchin- bottom to top, of the Onaping Formation, a length normal to the cleavage, and ex- son, 1972; Rousell, 1972). tufflike breccia unit with little or no dis- tended 10 percent subhorizontally and 40 Finally, over-all fold vergence, the cernable mesoscopic stratification; the On- percent subvertically in the plane of cleav- asymmetry of the basin, and the decrease in watin Formation, a thinly bedded car- age (assuming no change in volume). The the amount of strain toward the northwest bonaceous slaty shale; and the Chelmsford axial ratio on a horizontal surface of the de- indicate compression from the south and Formation, a carbonaceous and arenaceous formed concretions varies between 1.7 to 1 east to account for the deformation that proximal turbidite (Rousell, 1972; Cantin and 1.8 to 1, somewhat less than that for must postdate the emplacement of the and Walker, 1972; Fig. 2). The rare un- the Nickel Irruptive (approximately 2.2 to 2.0-b.y.-old Irruptive (Gibbins and faulted contacts exposed between the units 1). This means that the Sudbury Basin (or at McNutt, 1972; Gibbins, 1973, personal are conformable and gradational. No rocks least the depositional area of the commun.) and predate the northwest- correlative with the Whitewater Group Chelmsford Formation) may have been al- southeast-trending olivine-diabase dike have been positively identified outside the most circular prior to deformation. swarm that cuts the basin and its structures basin. The Onaping Formation and the (Figs. 1, 2). The true age of the olivine- The most prevalent imposed structural granophyre layer of the South Range have a diabase dikes is still uncertain, but the most element in the Sudbury Basin is a slaty-type strong lineation within the foliation surface recent date is about 1.46 b.y. B.R, based cleavage which strikes east-northeast paral- (Fig. 3). This lineation is formed by elon- on the Rb-Sr whole-rock isochron method lel to the long axis of the basin and dips gated minerals and rock fragments. It sug- (Gates and Hurley, 1973). The Irruptive steeply but variably (Fig. 3). The grains are gests that the finite deformation ellipsoid in was formerly regarded as 1.7 b.y. old (Fair- flattened in the plane of this cleavage that is those rocks was triaxial with a steeply bairn and others, 1969), and the dike strongly developed in the southern two- northwesternly plunging axis of maximum swarm as 1.2 b.y. to 1.66 b.y. old (Van thirds of the basin but absent in the North finite extension. Finite-strain analysis Schmus, 1965; Fairbairn and others, 1969; Range Nickel Irruptive and the adjacent (Brocoum and Dalziel, 1974), using the Gates, 1971,1972). Onaping Formation (Fig. 3). Folds in the rock and mineral fragments in the Onaping Chelmsford Formation are open and up- Formation, suggests that it was shortened THE SOUTHERN PROVINCE right.
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