Paul K. Sims

The Great Lakes tectonic zone — A major crustal structure in central North America

The Great Lakes tectonic zone is a major Precambrian crustal feature more than 1,200 km long extending eastward from Minnesota into Ontario, Canada. It is a zone of distinctive tectonism, affecting both Archean and early Proterozoic rocks, along the northern margin of the early Proterozoic Penokean fold belt adjacent to the Archean Superior province. The zone coincides with the boundary between two Archean crustal segments recognized in the region: a greenstone-granite terrane (∼2,700 m.y. old) to the north (Superior province) and an older (in part 3,500 m.y. old) gneiss terrane to the south. Tectonism along the zone began in the late Archean, during the joining together of the two terranes into a single continental mass, and culminated in the early Proterozoic, when steep or northward-facing overturned folds were formed in the supracrustal rocks, and intense cataclasis and a penetrative cleavage developed in subjacent basement rocks of the greenstone-granite terrane. The Proterozoic deformation took place under low to intermediate pressures. Movement occurred along the Great Lakes tectonic zone through much of the Precambrian time recorded in the region. In the early Proterozoic, crustal foundering, which was parallel to the zone and was diachronous, initiated the structural basins in which the early Proterozoic sequences of the Lake Superior and Lake Huron regions were deposited. Later, during the Penokean orogeny (∼1,850 to 1,900 m.y. ago), compression deformed the sequences in both regions. Still later, intermittent (∼1,850 to 1,100 m.y. ago) crustal extension provided sites for emplacement of abundant mafic igneous rocks. There is no definite evidence that any of the extensional events progressed to the stage of development of oceanic crust; probably the zone has been wholly intracratonal since its inception in late Archean time. During the Phanerozoic, minor differential movements occurred locally in the Great Lakes tectonic zone, as recorded by the thinning of Cretaceous strata and their subsequent tilting and by historic earthquakes in Minnesota.

Nd isotopes and the origin of 1.9-1.7 Ga Penokean continental crust of the Lake Superior region

Nd isotopic data on 26 samples demonstrate the origin of the Early Proterozoic crust of the Penokean orogen. The three major components are (1) the Marquette Range Supergroup, a predominantly miogeoclinal continental-margin sequence deposited before 1.85 Ga on Archean basement; (2) ca. 1.88 Ga felsic metavolcanic rocks of the Wisconsin magmatic terrane to the south, of evolved island-arc affinity; and (3) 1.87-1.76 Ga granitoids that intrude the metavolcanic rocks. Initial ϵ Nd values for the metavolcanic rocks of the magmatic terrane range between 0.0 and +2.4 and T DM ages between 1.9 and 2.2 b.y. The volcanic rocks primarily represent new crustal material that had only a limited Archean input, probably through mixing of subducted sediments into the magma source area. ϵ Nd (T) values for sedimentary rocks in the lower part of the Marquette Range Supergroup indicate an Archean source, most likely the 2.7 Ga Superior province to the north. On the other hand, the sedimentary rocks in the upper part of the Marquette Range Supergroup (graywackes of the upper Michigamme Formation) have initial ϵ Nd values between -0.8 and +1.5, indicative of an Early Proterozoic source. These graywackes probably are foredeep deposits derived from the volcanic rocks of the Wisconsin magmatic terrane to the south. Thus, the time of deposition of the upper Michigamme Formation dates the final convergence of the Wisconsin magmatic terrane with the continental margin. The granitoids of the Wisconsin magmatic terrane have a wide range of ϵ Nd (T) values, from -4.5 to +4.0. They represent mixtures of variable amounts of new crustal material and recycled Archean detritus. The more negative ϵ Nd (T) values occur in samples close to the northern boundary of the Wisconsin magmatic terrane, which is marked by a mylonitic shear zone (the Niagara fault zone). We suggest that the miogeoclinal lower Marquette Range Supergroup rocks having an Archean Nd signature became involved in magma genesis close to the collisional boundary, whereas increasingly lesser amounts of this older material were available for mixing farther away from the suture. The 1.9-1.7 Ga Penokean events involved major growth of new crust from the mantle.

Boundary between two Precambrian W terranes in Minnesota and its geologic significance

Minnesota lies astride two Precambrian W (lower Precambrian) terranes that differ in age, rock assemblages, metamorphic grade, and structural style. In northern Minnesota, greenstone-granite complexes 2,700 to 2,750 m.y. old, which are typical of the majority of the Canadian Shield, are exposed. These rocks trend northeastward, dip steeply, and typically produce narrow curvilinear aeromagnetic and gravity anomalies. In southwestern Minnesota, much older (3,550 m.y.) granulite-facies granitic and mafic gneisses, which are moderately flat-lying and produce relatively broad magnetic and gravity anomalies, are exposed through a window in the Phanerozoic cover in the Minnesota River valley. Similar gneiss, some of which has been dated radiometrically, is exposed sporadically in central Minnesota and is considered part of the same terrane as the gneiss in the Minnesota River valley. Judged from the outcrop pattern and available geophysical data, the boundary between the two terranes trends diagonally across central Minnesota, from approximately latitude 45°30′N at the western boundary to the vicinity of Duluth, on Lake Superior. We postulate that the volcanic and sedimentary rocks in the greenstone terrane accumulated adjacent to the pre-existing gneiss terrane, which 2,700 m.y. ago was part of a sialic protocontinent of moderate size. There is no geologic or geochemical evidence that these rocks were deposited on a sialic crust. The tectonic environment that existed 2,700 m.y. ago, when the greenstone-granite complexes were formed, is not known; there is no compelling evidence that they were formed in a plate tectonic regime.

Early Precambrian tectonic-igneous evolution in the Vermilion district, northeastern Minnesota

The Vermilion district and adjacent areas are part of the greenstone-granite terrane of northern Minnesota and consist of a thick succession of subaqueous volcanic rocks, derivative sedimentary rocks, and intrusive granitic rocks, which formed during the interval 2,750 to 2,700 m.y. ago. The volcanic-sedimentary succession now constitutes an eastward-trending, upright, tight anticlinorium between flanking granitic batholiths. The folding was broadly contemporaneous with emplacement of the oldest recognized plutonic rocks, which range in composition from granite to tonalite, and is attributed to compression caused by the relative upwelling and convergence of the buoyant granitic bodies. Metamorphism of the supracrustal rocks to greenschist and, locally, middle-amphibolite facies accompanied the folding. Later, anorogenic monzonite–quartz monzonite and other syenitic rocks intruded the supracrustal rocks, apparently as diapirs. Regional strike-slip faulting followed emplacement of the anorogenic plutons. The principal fault, named the Vermilion fault system, transects several greenstone-granite complexes; within the district, stratigraphic offsets indicate a right-lateral strike-slip separation of 17 to 19 km along the fault. The strike-slip faulting marked the transition from an unstable crust dominated by vertical tectonic movements to a more stable crust capable of sustaining regional fractures.

The Dunbar Gneiss-granitoid dome: Implications for early Proterozoic tectonic evolution of northern Wisconsin

The Dunbar dome in northeastern Wisconsin is a critical structural feature in the early Proterozoic Penokean orogen. It provides exposures of gneisses (Dunbar Gneiss) that structurally underlie the voluminous metavolcanic rocks of northeastern Wisconsin, and exposures of abundant granitoid rocks ranging from tonalite to granite. The granitoid rocks cut both the gneisses in the core and the supracrustal (cover) metavolcanic rocks and were emplaced essentially along the core-cover boundary. The Dunbar Gneiss is calc-alkaline and was derived from volcanic and intrusive rocks of intermediate composition. The various intrusive rocks have calcic, calc-alkaline, and alkali to alkali-calcic compositions, and they progress with time to more SiO 2 and K 2 -rich compositions. U-Pb zircon ages indicate that accumulation of the layered rocks in the core and cover, deformation and metamorphism, and intrusion of the granitoid rocks spanned a relatively short time, ∼1865–1835 Ma. We interpret the dome as being a large-scale, fold-interference structure resulting from polydeformation modified by diapirism. Northeast-oriented folds (F 3 ) and a related mylonitic foliation (S 3 ), nearly confined to the dome, are superposed on northwest-oriented folds (F 2 ) that developed during regional deformation. In the core-cover boundary, these structures are obliterated by a zone of intense deformation—a mylonitic foliation and a steeply plunging stretching lineation—as much as 500 m wide, which we interpret as resulting from diapirism. Metamorphic zoning is concentric: amphibolite facies in inner parts of the mantle and greenschist facies in the outer part of the mantle. The Wisconsin magmatic terrane, as represented by the rocks in the Dunbar dome, differs from the epicratonic, early Proterozoic sedimentary-volcanic sequence (Marquette Range Supergroup) in Michigan, to the north, in stratigraphy, structure, and volume and composition of igneous rocks. Whereas the basalts in northern Michigan are compositionally similar to continental rift basalts, the volcanic rocks in the Dunbar dome have over-all island-arc compositional affinities. The over-all calc-alkaline compositions of the intrusive rocks are similar to those in magmatic arcs formed at convergent plate-margin settings. Accordingly, we interpret the Wisconsin magmatic terrane as an oceanic-arc complex that was sutured to the North American continent during development of the Penokean orogen. Similar interpretations based on broad regional observations have been proposed previously.

Middle Proterozoic uplift events in the Dunbar dome of northeastern Wisconsin, USA

Isotopic ages of granitic and metamorphic rocks exposed in the Dunbar structural dome of northeastern Wisconsin identify a protracted series of tectonic and “hydrothermal” events that culminated in major regional uplift during Middle Proterozoic (Keweenawan; ca 1,100 Ma) continental rifting and volcanism. The major rock-forming events and the structural development of the dome occurred during the interval 1,862+/−4 Ma to 1,836+/−6 Ma. Whole-rock Rb-Sr ages are partly reset in response to a widely recognized but cryptic event in Wisconsin and Michigan at about 1,630 Ma. The scale and systematic character of the whole-rock resetting strongly suggests the presence of a fluid phase derived in situ from water dissolved in the silicates or externally from a subthrust plate of low-grade metamorphic rocks. The regional nature of the 1,630-Ma disturbance possibly indicates that it is related to a major tectonic event such as an active plate margin far to the south. Rb-Sr biotite ages for the Dunbar dome (this study), the southern complex of the Marquette district (Van Schmus and Woolsey 1975) and the Felch trough area (Aldrich and others 1965) provide a remarkably coherent pattern that reflects multiple episodes of differential uplift. Younger events superimposed on a regional 1,630-Ma imprint are recorded at 1,330 Ma and 1,140 Ma. The 1,330 Ma disturbance could reflect stabilization following intrusion of the Wolf River batholith at 1,485 Ma. The 1,140-Ma uplift event occurred during Keweenawan rifting and volcanism as a result of stresses imposed on a mosaic of fault-bounded blocks with possible subcrustal influence. The remarkably small variance in the 1,140-Ma biotite age peak argues for rapid uplift and cooling, and hence rapid erosion. Detritus from the uplift probably was being shed into nearby tectonic basins most of which did not survive subsequent uplift and erosion.

Precambrian Folding in the Idaho Springs-Central City Area, Front Range, Colorado

Metasedimentary gneisses in the Idaho Springs-Central City area of the Front Range have been deformed twice in Precambrian time. The older and major deformation was plastic folding; it was accompanied by intrusion of a series of plutons and sheets, recrystallization of the meta-sedimentary rocks, and metamorphism of the earlier members of the igneous series. It produced a major fold system consisting mainly of open, but disharmonic asymmetric and upright anticlines and synclines whose axes trend sinuously north-northeast and are spaced 1-2 miles apart. Over most of the area the fold axes are nearly horizontal or plunge at low angles, but in the southern part the axes plunge steeply northeastward. Small folds and a well-developed mineral alignment characteristically parallel the major fold axes ( B o ); small-scale folds, boudinage and sparse mineral alignment are present in the A o direction. The younger deformation was dominantly cataclastic and restricted chiefly to a 2-mile-wide zone in the southeast part of the area. Within this zone small folds were developed locally in the relatively incompetent rock masses, and intense granulation was developed locally in the more competent units. Cataclastic products are pervasively distributed, however, through all the rocks in the zone. The younger folds are mainly terrace, monoclinal, and chevron types; the largest has a breadth of about 400 feet. These folds trend N. 55° E., are remarkably straight, and plunge at various angles, largely depending upon their position on the older, larger folds. They consistently are strongly asymmetric and show their northwest limbs raised structurally. Associated with these folds are two lineations, one (B y ) parallel to the fold axes and one (A y ) oriented at about 80° to the fold axes. The younger deformation is a manifestation of extensive Precambrian shearing defined by a zone of intense cataclasis that extends both northeast and southwest of this region.

New Mexico structural zone—an analogue of the Colorado mineral belt

Abstract Updated aeromagnetic maps of New Mexico together with current knowledge of the basement geology in the northern part of the state (Sangre de Cristo and Sandia–Manzano Mountains)—where basement rocks were exposed in Precambrian-cored uplifts—indicate that the northeast-trending Proterozoic shear zones that controlled localization of ore deposits in the Colorado mineral belt extend laterally into New Mexico. The shear zones in New Mexico coincide spatially with known epigenetic precious- and base-metal ore deposits; thus, the mineralized belts in the two states share a common inherited basement tectonic setting. Reactivation of the basement structures in Late Cretaceous–Eocene and Mid-Tertiary times provided zones of weakness for emplacement of magmas and conduits for ore-forming solutions. Ore deposits in the Colorado mineral belt are of both Late Cretaceous–Eocene and Mid-Tertiary age; those in New Mexico are predominantly Mid-Tertiary in age, but include Late Cretaceous porphyry-copper deposits in southwestern New Mexico. The mineralized belt in New Mexico, named the New Mexico structural zone, is 250-km wide. The northwest boundary is the Jemez subzone (or the approximately equivalent Globe belt), and the southeastern boundary was approximately marked by the Santa Rita belt. Three groups (subzones) of mineral deposits characterize the structural zone: (1) Mid-Tertiary porphyry molybdenite and alkaline-precious-metal deposits, in the northeast segment of the Jemez zone; (2) Mid-Tertiary epithermal precious-metal deposits in the Tijeras (intermediate) zone; and (3) Late Cretaceous porphyry-copper deposits in the Santa Rita zone. The structural zone was inferred to extend from New Mexico into adjacent Arizona. The structural zone provides favorable sites for exploration, particularly those parts of the Jemez subzone covered by Neogene volcanic and sedimentary rocks.

Signs from the Precambrian: The Geologic Framework of Rocky Mountain Region Derived From Aeromagnetic Data

Recently compiled aeromagnetic data greatly enhance our understanding of the Precambrian basement from the Rocky Mountain region by providing a means to (1) extrapolate known geology exposed in generally widely separated uplifts into broad covered areas, and (2) delineate large-scale structural features that are not readily discernable solely from outcrop mapping. In the Wyoming Province, Archean granite and gneiss terranes generate semi-circular bands of magnetic highs and lows, respectively, primarily reflecting Late Archean magmatic and deformation events that modified the older craton. In contrast, the subdued magnetic signature of the Paleoproterozoic crystalline basement of the Rocky Mountain region does not allow straightforward distinction of the Yavapai, Matzatzal and Mojave provinces. This is not the case for the Mesoproterozoic (∼1.4 Ga) iron-rich granites. Although variable in magnetic expression where exposed and drilled, most are associated with highs. Many of these plutons intruded shear zones and therefore produce long, linear magnetic highs, particularly conspicuous in Arizona. A spectacular, high-amplitude magnetic potential high defines broad region of thick (∼>10 km) magnetite-rich granite, perhaps underlain by coeval mafic crust. In the east, this high corresponds to the Western Granite-Rhyolite Province. Based on the continuity of the regional magnetic high, we extend the western limit of the province from its current position in New Mexico to southeastern California. This implies that the province lay at the edge of the North American margin at the time of the late Proterozoic break-up of Rodinia and may be present in one of the conjugate rifted pieces.