Proterozoic Plate Tectonics Research Articles

Secular changes in metamorphism and metamorphic cooling rates track the evolving plate-tectonic regime on Earth

When disturbed, dynamic emergent systems, such as tectonics on Earth, may transition from one stable state to another if the perturbation is sufficiently large. Here, we identify such state shifts through examination of statistically significant change points in the time series of metamorphic pressure–temperature and cooling rate data. Change points occur in the mid-Paleoproterozoic, the Mesoproterozoic, the early and late Paleozoic and the Cenozoic. To compare the timing of change points with mantle geodynamics, we interrogate the time series of calculated mantle potential temperature, which yields a statistically significant drop of >130°C in the mid-Paleoproterozoic, indicating a change in the mechanism of mantle cooling. We interpret changes in the mid-Paleoproterozoic to relate to the global emergence of stable subduction, orogenesis associated with the formation of the Nuna megacontinent, and slab breakoff, consistent with the operation of a distinctive style of Proterozoic plate tectonics. By contrast, changes from the dawn of the Cambrian onward may relate to the large volume of sediments supplied to trenches following multiple Cryogenian and Paleozoic glaciations. Sediments acted as a lubricant, allowing deeper subduction, transport of lower continental plates to mantle depths and faster metamorphic cooling rates, features characteristic of modern plate tectonics. Supplementary material: A dataset of metamorphic cooling rates is available at https://doi.org/10.6084/m9.figshare.c.5943418

The contribution of metamorphic petrology to understanding lithosphere evolution and geodynamics

In the early 1980s, evidence that crustal rocks had reached temperatures >1000 °C at normal lower crustal pressures while others had followed low thermal gradients to record pressures characteristic of mantle conditions began to appear in the literature, and the importance of melting in the tectonic evolution of orogens and metamorphic–metasomatic reworking of the lithospheric mantle was realized. In parallel, new developments in instrumentation, the expansion of in situ analysis of geological materials and increases in computing power opened up new fields of investigation. The robust quantification of pressure (P), temperature (T) and time (t) that followed these advances has provided reliable data to benchmark geodynamic models and to investigate secular change in the thermal state of the lithosphere as registered by metamorphism through time. As a result, the last 30 years have seen significant progress in our understanding of lithospheric evolution, particularly as it relates to Precambrian geodynamics.Eoarchean–Mesoarchean crust registers uniformly high T/P metamorphism that may reflect a stagnant lid regime. In contrast, two contrasting types of metamorphism, eclogite–high-pressure granulite metamorphism, with apparent thermal gradients of 350–750 °C/GPa, and granulite–ultrahigh temperature metamorphism, with apparent thermal gradients of 750–1500 °C/GPa, appeared in the Neoarchean rock record. The emergence of paired metamorphism is interpreted to register the onset of one-sided subduction, which introduced an asymmetric thermal structure at these developing convergent plate margins characterized by lower T/P in the subduction channel and higher T/P in the overriding plate. During the Paleoarchean to Paleoproterozoic the ambient mantle temperature was warmer than at present by ∼300–150 °C. Although the thermal history of Earth is only poorly constrained, it is likely that prior to ca. 3.0 Ga heating from radioactive decay would have exceeded surface heat loss, whereas since ca. 2.5 Ga secular cooling has dominated the thermal history of the Earth. The advent of paired metamorphism is consistent with other changes in the geological record during the Neoarchean that are best explained as the result of a transition from a stagnant lid to subduction and a global plate tectonics regime by ca. 2.5 Ga. This interpretation is supported by results from 2-D numerical experiments of oceanic subduction that demonstrate a change to one-sided subduction is plausible as upper mantle temperature declined to <200 °C warmer than at present during the late Neoarchean–Paleoproterozoic. This is the beginning of the Proterozoic plate tectonics regime.At 1.0 Ga the ambient mantle temperature was still ∼150–100 °C warmer than at present. Continued secular cooling caused a transition to cold subduction registered in the crustal record of metamorphism by the first appearance of blueschist and high to ultrahigh pressure metamorphism during the Neoproterozoic. Results of 2-D numerical experiments of continental collision demonstrate a transition from shallow to deep slab breakoff associated with stronger crust–mantle coupling that enabled continental subduction to mantle depths as upper mantle temperature declined to <100 °C warmer than at present during the late Proterozoic. This is the beginning of the modern plate tectonics regime.

Late Proterozoic plate tectonics and palaeogeography: a tale of two supercontinents, Rodinia and Pannotia

Abstract The plate tectonic and palaeogeographic history of the late Proterozoic is a tale of two supercontinents: Rodinia and Pannotia. Rodinia formed during the Grenville Event ( c . 1100 Ma) and remained intact until its collision with the Congo continent (800–750 Ma). This collision closed the southern part of the Mozambique Seaway, and triggered the break-up of Rodinia. The Panthalassic Ocean opened as the supercontinent of Rodinia split into a northern half (East Gondwana, Cathyasia and Cimmeria) and a southern half (Laurentia, Amazonia–NW Africa, Baltica, and Siberia). Over the next 150 Ma, North Rodinia rotated counter-clockwise over the North Pole, while South Rodinia rotated clockwise across the South Pole. In the latest Precambrian (650–550 Ma), the three Neoproterozoic continents – North Rodinia, South Rodinia and the Congo continents – collided during the Pan-Africa Event forming the second Neoproterozoic supercontinent, Pannotia (Greater Gondwanaland). Pan-African mountain building and the fall in sea level associated with the assembly of Pannotia may have triggered the extreme Ice House conditions that characterize the middle and late Neoproterozoic. Although the palaeogeographic maps presented here do not prohibit a Snowball Earth, the mapped extent of Neoproterozoic ice sheets favour a bipolar Ice House World with a broad expanse of ocean at the equator. Soon after it was assembled ( c . 560 Ma), Pannotia broke apart into the four principal Palaeozoic continents: Laurentia (North America), Baltica (northern Europe), Siberia and Gondwana. The amalgamation and subsequent break-up of Pannotia may have triggered the ‘Cambrian Explosion’. The first economically important accumulations of hydrocarbons are from Neoproterozoic sources. The two major source rocks of this age (Nepa of Siberia and Huqf of Oman) occur in association with massive Neoproterozoic evaporite deposits and in the warm equatorial–subtropical belt, within 30° of the equator.

Evolution of the Mazatzal province and the timing of the Mazatzal orogeny: Insights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks in southern New Mexico

Research Article| March 01, 2008 Evolution of the Mazatzal province and the timing of the Mazatzal orogeny: Insights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks in southern New Mexico Jeffrey M. Amato; Jeffrey M. Amato 1Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA Search for other works by this author on: GSW Google Scholar Andre O. Boullion; Andre O. Boullion 1Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA Search for other works by this author on: GSW Google Scholar Antonio M. Serna; Antonio M. Serna 1Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA Search for other works by this author on: GSW Google Scholar Amos E. Sanders; Amos E. Sanders 1Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA Search for other works by this author on: GSW Google Scholar G. Lang Farmer; G. Lang Farmer 2Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA Search for other works by this author on: GSW Google Scholar George E. Gehrels; George E. Gehrels 3Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Joseph L. Wooden Joseph L. Wooden 4U.S. Geological Survey, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2008) 120 (3-4): 328–346. https://doi.org/10.1130/B26200.1 Article history received: 13 Feb 2007 rev-recd: 06 Jul 2007 accepted: 19 Jul 2007 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeffrey M. Amato, Andre O. Boullion, Antonio M. Serna, Amos E. Sanders, G. Lang Farmer, George E. Gehrels, Joseph L. Wooden; Evolution of the Mazatzal province and the timing of the Mazatzal orogeny: Insights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks in southern New Mexico. GSA Bulletin 2008;; 120 (3-4): 328–346. doi: https://doi.org/10.1130/B26200.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract New U-Pb zircon ages, geochemistry, and Nd isotopic data are presented from three localities in the Paleoproterozoic Mazatzal province of southern New Mexico, United States. These data help in understanding the source regions and tectonic setting of magmatism from 1680 to 1620 Ma, the timing of the Mazatzal orogeny, the nature of postorogenic magmatism, Proterozoic plate tectonics, and provide a link between Mazatzal subblocks in Arizona and northern New Mexico. The data indicate a period from 1680 to 1650 Ma in which juvenile felsic granitoids were formed, and a later event between 1646 and 1633 Ma, when these rocks were deformed together with sedimentary rocks. No evidence of pre-1680 Ma rocks or inherited zircons was observed. The igneous rocks have ϵNd(t) from −1.2 to +4.3 with most between +2 and +4, suggesting a mantle source or derivation from similar-aged crust. Nd isotope and trace element concentrations are consistent with models for typical arc magmatism. Detrital zircon ages from metasedimentary rocks indicate that sedimentation occurred until at least 1646 Ma. Both local and Yavapai province sources contributed to the detritus. All of the samples older than ca. 1650 Ma are deformed, whereas undeformed porphyrob-lasts were found in the contact aureole of a previously dated 1633 Ma gabbro. Regionally, the Mazatzal orogeny occurred mainly between 1654 and 1643 Ma, during final accretion of a series of island arcs and intervening basins that may have amalgamated offshore. Rhyolite magmatism in the southern Mazatzal province was coeval with gabbro intrusions at 1633 Ma and this bimodal magmatism may have been related to extensional processes following arc accretion. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Metamorphic Conditions in Orogenic Belts: A Record of Secular Change

In general, Archean rocks exhibit rather ordinary moderate-P-high-T facies series metamorphism; neither blueschists nor any record of deep continental subduction and return are documented. However, the abundance and scale of ultrahigh-temperature (UHT) metamorphic belts from the Neoarchean to the Cambrian imply a significant change in geodynamics during the Neoarchean Era, after which transient sites of high heat flow were available at intervals throughout this period of Earth evolution. Many Neoproterozoic-Cambrian UHT metamorphic belts appear to have developed in settings analogous to modern backarcs that were closed and inverted during crustal aggregation and formation of the Gondwana supercontinent. If backarcs were the general setting for UHT metamorphism, then on a hotter Earth the cyclic formation of supercratons (in the Neoarchean Era) and supercontinents (in the Proterozoic Eon) required the destruction of oceans floored by thinner lithosphere that may have generated hotter backarcs than those associated with the current destruction of the Pacific Ocean on the modern Earth. The inherent weakness of the lithosphere in a hotter thermal regime inevitably localized magmatism and deformation at these sites contemporaneously with UHT metamorphism. Medium-temperature eclogites of crustal derivation and associated highpressure granulites are also first recognized in the Neoarchean, for example within the Belomorian Mobile Belt, and they occur at intervals throughout the Proterozoic, for example in the Orosirian Usagaran Orogen and in the Grenvillian belts of the Proto-Atlantic region, and Paleozoic, for example in the circum-North Atlantic Caledonides and European Variscides. Eclogite-high-pressure granulite (E-HPG) metamorphism is predominantly a Proterozoic-Paleozoic phenomenon—complementary to but sparser than UHT metamorphism to begin with, but extending further into the Paleozoic than does UHT metamorphism—that is inferred to record subduction-to-collision orogenesis. Blueschists appear in the Neoproterozoic Era, becoming common through the Phanerozoic Eon; they record the low thermal gradients associated with modern subduction. Lawsonite-bearing blueschists and eclogites, and ultrahigh-pressure (UHP) metamorphism characterized by coesite or diamond are predominantly Phanerozoic phenomena related to deep subduction within subductionto-collision orogens. In general, UHP metamorphic belts in subduction-to-collision orogens are not associated with a contemporary magmatic arc in the hanging wall; this suggests that deep subduction of continental crust may inhibit the generation of calc-alkaline magmas, a feature that may have enabled preservation during exhumation of the mineralogical evidence for extreme pressures. However, an enigma concerning UHP metamorphism is the first evidence of deep subduction of continental crust in the rock record. At issue is the recycling implied by the geochemistry of Archean and Proterozoic diamonds (since entrained from mantle to upper crust in younger magmatic events), which requires a supracrustal component. Are these diamonds evidence of deep subduction of continental crust and an early record of UHP metamorphism, or was some other mechanism (e.g., delamination of underthrust lithosphere, some form of slab break-off) responsible for taking a supracrustal component deep into the mantle source? The Archean and Proterozoic eons were characterized by higher but decreasing mean mantle temperatures and radioactive heat production (RHP), and a thinner thermal boundary layer (TBL) with a shorter residence time than modern Earth. Modeling the effect of increased RHP on the thermal evolution of crust instantaneously doubled in thickness predicts that metamorphic rocks in Archean collisional orogens should have experienced maximum temperatures several hundreds of degrees Celsius higher than those recorded by metamorphic rocks in modern collisional orogens. However, there is no evidence of this—the Archean record is dominated by rather ordinary P-T conditions and crustal melting at relatively low temperatures probably fluxed by water. I argue that a duality of metamorphic belts—reflecting a duality of thermal environments—is the characteristic metamorphic imprint of plate tectonics in the rock record, and it appears only since the Neoarchean Era. Based on the occurrence of both UHT and E-HPG metamorphic belts since the Archean-to-Proterozoic transition, I suggest this transition records the onset of a "Proterozoic plate tectonics regime," although the total ridge length and the number of plates undoubtedly were larger in the beginning than on modern Earth and the style of tectonics likely involved some differences. The Neoproterozoic transition to the "modern plate tectonics regime" registers a change to subduction of continental crust deeper into the mantle and its (partial) return from depths of up to 300 km, a change perhaps related to whole mantle convection as oceanic lithosphere became thicker with decreased thermal gradients. Although there was overlap in time and space between E-HPG and UHP metamorphism during the Paleozoic, since the Carboniferous UHP metamorphism has dominated, and extreme thermal metamorphism is rare.

Metamorphism, Plate Tectonics, and the Supercontinent Cycle

Granulite facies ultrahigh temperature metamorphism (G-UHTM) is documented in the rock record predominantly from Neoarchean to Cambrian; G-UHTM facies series rocks may be inferred at depth in younger, particularly Cenozoic orogenic systems. The first occurrence of G-UHTM in the rock record signifies a change in geodynamics that generated transient sites of very high heat flow. Many G-UHTM belts may have developed in settings analogous to modern continental backarcs. On a warmer Earth, the cyclic formation of supercontinents and their breakup, particularly by extroversion, which involved destruction of ocean basins floored by thinner lithosphere, may have generated hotter continental backarcs than those associated with the modern Pacific rim. Medium-temperature eclogite, high-pressure granulite metamorphism (E-HPGM), is also first recognized in the Neoarchean rock record and occurs at intervals throughout the Proterozoic and Paleozoic rock record. E-HPGM belts are complementary to G-UHTM belts and are generally inferred to record subduction-to-collision orogenesis. Blueschists become evident in the Neoproterozoic rock record; they record the low thermal gradients associated with modern subduction. Lawsonite blueschists and eclogites (high-pressure metamorphism, HPM) and ultrahigh pressure metamorphism (UHPM) characterized by coesite (±lawsonite) or diamond are predominantly Phanerozoic phenomena. HPM-UHPM registers the low thermal gradients and deep subduction of continental crust during the early stage of the collision process in Phanerozoic subduction-to-collision orogens. Although perhaps counterintuitive, many HPM-UHPM belts appear to have developed by closure of small ocean basins in the process of accretion of a continental terrane during a period of supercontinent introversion (Wilson cycle ocean basin opening and closing). A duality of metamorphic belts—reflecting a duality of thermal regimes—appears in the record only since the Neoarchean Era. A duality of thermal regimes is the hallmark of modern plate tectonics and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record. The occurrence of both G-UHTM and E-HPGM belts since the Neoarchean manifests the onset of a ‘Proterozoic plate tectonics regime’, although the style of tectonics likely involved differences. The ‘Proterozoic plate tectonics regime’ evolved during a Neoproterozoic transition to the ‘modern plate tectonics regime’ characterized by colder subduction and subduction of continental crust deep into the mantle and its (partial) return from depths of up to 300 km, as chronicled by the appearance of HPM-UHPM in the rock record. The age distribution of metamorphic belts that record extreme conditions of metamorphism is not uniform, and metamorphism occurs in periods that correspond to amalgamation of continental lithosphere into supercratons (e.g. Superia/Sclavia) or supercontinents (e.g. Nuna (Columbia), Rodinia, Gondwana, and Pangea).

Duality of thermal regimes is the distinctive characteristic of plate tectonics since the Neoarchean

Ultrahigh-temperature (UHT) granulite metamorphism is documented predominantly in the Neoarchean to Cambrian rock record, but UHT granulite metamorphism also may be inferred at depth in Cenozoic orogenic systems. The first occurrence of UHT granulite metamorphism in the record signifies a change in geodynamics that generated transient sites of very high heat flow. Many UHT granulite metamorphic belts may have developed in settings analogous to modern continental backarcs; on a warmer Earth, destruction of oceans floored by thinner lithosphere may have generated hotter backarcs than those associated with the modern Pacific ring of fire. Medium-temperature eclogite–high- pressure (EHP) granulite metamorphism is documented in the Neoarchean rock record and at intervals throughout the Proterozoic and Paleozoic record. EHP granulite metamorphic belts are complementary to UHT granulite metamorphic belts in that they are generally inferred to record subduction-to-collision orogenesis. Blueschists become evident in the Neoproterozoic rock record, but lawsonite blueschist–eclogite metamorphism (high pressure [HP]) and ultrahigh-pressure metamorphism (UHP) characterized by coesite or diamond are predominantly Phanerozoic phenomena. HP-UHP metamorphism registers the low thermal gradients and deep subduction of continental crust during the early stage of subduction-to-collision orogenesis. A duality of metamorphic belts—reflecting a duality of thermal regimes—appears in the record only since the Neoarchean Era. A duality of thermal regimes is the hallmark of modern plate tectonics, and the duality of metamorphic belts is the characteristic imprint of plate tectonics in the rock record. The occurrence of both UHT and EHP granulite metamorphism since the Neoarchean marks the onset of a “Proterozoic plate tectonics” regime, which evolved during a Neoproterozoic transition to the modern plate tectonics regime, characterized by colder subduction as chronicled by HP-UHP metamorphism.

Lateral constraint rather than escape along a terrane boundary near Skelleftehamn

Mylonitised greywackes with superposed pseudotachylites (fossil earthquakes) mark the eastern end of a W-E trending terrane boundary between the Skellefte supracrustal group of Svecokarelia to the north and the Bothnian gneisses of Svecofennia to the south. This region has been claimed to provide the clearest evidence that early Proterozoic plate tectonics (>1.85 Ga) involved the same processes as Phanerozoic continental suturing and convergence. Comparisons between the tectonic histories of greywackes in, and on either side, of this boundary demonstrate that the southern terrane already had major recumbent folds at upper amphibolite metamorphic facies before it met and converged on the northern terrane and eventually rose relative to it. These nappes were refolded by upright folds that are circumferential to a syn-Svecofennian granite south of the boundary but the first folds visible in the lower grade rocks to the north. The youngest Svecofennian structures in the region were kink bands that developed in the boundary while it was a steep seismic zone. These kink bands record lateral tectonic constraint rather than lateral escape. West of the study area, mylonites of the terrane boundary are intruded by sheets of post Svecofennian Skellefte granites and plutons of 1.75 Ga Revsund granites.

Chromites from the early Proterozoic Outokumpu-Jormua Ophiolite Belt: a comparison with chromites from Mesozoic ophiolites

Chromites from the ultramafic rocks of the early Proterozoic Outokumpu-Jormua Ophiolite Belt were found to possess textural and Cr (Cr + Al) and Mg (Mg + Fe 2+) ratios that resemble those described in Mesozoic ophiolite complexes. Chromites of cumulus texture were found at all locations along the Belt, whereas residual and podiform chromites were found only in the Outokumpu complex. The chromites in the Outokumpu complex originated in a supra-subduction zone environment, whereas those in the Jormua ophiolite complex originated in a mid-ocean ridge environment. The main difference between the early Proterozoic ophiolitic chromites and those of Mesozoic age is the relatively high Zn content in the former, which is a metasomatic feature related to the presence of large amounts of Cu-Co-Zn-Au ore. The data and observations presented provide evidence contributing to the concept of Proterozoic plate tectonics.

U-Pb dating and isotopic signature of the alkaline ring complexes of Bou Naga (Mauritania): its bearing on late proterozoic plate tectonics around the West African craton

In the West African fold belt of Mauritania, high-grade metamorphic series, similar to those of Amsaga (Reguibat shield-West African Craton), are exposed in a window. At Bou Naga-Mauritania (19° N, 13° 15′ W) in the South of this window, an alkaline ring complex has intruded the metamorphic country rocks. This complex consists of two geological formations: the Eastern formation is mainly composed of red rhyolite sills, whereas the Western formation is made up of several kinds of alkaline rocks both saturated and under-saturated which cross cut the earlier saturated units. Three U-Pb zircon age measurements have been made on the alkaline complex, and one on an orthogneiss from the metamorphic country rocks. The syenite and the alkaline granite of the Western block are 676 ± 8 and 687 ± 5 Ma old. The orthogneiss is Archaean with an age of 2709 ± 136 Ma, but the lower intercept of discordia on concordia, shows an age of 756 ± 25 Ma linked with the genesis of the alkaline complex. A major crustal contribution is recorded by Nd and O isotopes in the SiO 2-saturated rocks. These results provide evidence for the correlation of the metamorphic country rocks with the Reguibat Archaean basement and for an early Pan-African continental rifting phase in this area before the tectonometamorphic events in the Mauritanide belt. Furthermore, with regards with previous geodynamic works of the West African Craton, our results leads us to suggest a significant diachronism between late Proterozoic crustal evolution to the West and to the East of the West African Craton. This is a further evidence for modern-type plate tectonics at this time.

Evidence for early Proterozoic plate tectonics from seismic reflection profiles in the Baltic shield

Plate tectonics provides the linking framework for all tectonic and magmatic activity seen today, but it is not known when plate tectonics first developed on Earth. New deep seismic reflection and ...

Proterozoic plate tectonics, glaciation and iron-formations

Glacigenic diamictites and associated rocks are common in Early and Late Proterozoic sequences. Most of the preserved Proterozoic glacigenic rocks formed in marine basins in an extensional tectonic regime at or close to the rift-drift transition. The reasons for glaciation in this tectonic setting are not known but two possible contributing factors, given an initially cool climatic regime, are the formation of new seaways by continental fragmentation and uplift of the rift shoulders during early stages of breakup. The absence of such glacigenic deposits in the classical miogeoclines of the peri-Atlantic region may be due to an anomalously warm global climatic regime in the late Mesozoic. Iron-formations in the Archean and Early Proterozoic formed in many different environments; the critical factor was a dearth of siliciclastic deposition. Starved basins commonly developed during the transition from miogeocline to foreland basin in the Wilson cycle of ocean opening and closure, when drainage from the craton was replaced by that from a rising orogen. Early Proterozoic iron-rich sediments in the Great Lakes region, in southeast Wyoming and in the Northwest Territories of Canada are interpreted as the earliest deposits of such foreland basins. They are considered to be chemical precipitates from iron-rich sea water. Late Proterozoic iron-formations formed under an extensional regime, possibly in small Red Sea-type ocean basins where hydrothermal circulation produced metal-rich brines. These brines were displaced by movement of cold waters derived from sea-going glaciers, leading to dilution and precipitation of Fe and Si. Thus, in the Late Proterozoic, there was a critical association of continental rifting, glaciation and hydrothermal circulation. This combination of environmental parameters led to the production of the unique Late Proterozoic class of hydrothermal iron-formations.

Proterozoic plate tectonics in Canada with emphasis on evidence for a Late Proterozoic rifting event

Several Early Proterozoic fold belts in the Canadian shield have been interpreted in terms of plate tectonic processes. The most impressive evidence comes from those preserved on the margins of Archaean cratons such as the Wopmay orogen and the Penokean fold belt. There are striking similarities in the stratigraphic successions of many widely separated Early Proterozoic fold belts within the Canadian shield. Most of these are currently best interpreted in terms of some modified version of plate tectonics. Post-Grenville rocks contain several independent lines of evidence that support a major episode of continental fragmentation. Stratigraphic and sedimentological evidence from the Cordilleran region records a dramatic change from widespread development of essentially platformal marine sediments (L. Mackenzie Mountains Supergroup) through rift-related evaporites and clastics (U. Mackenzie Mountains Supergroup) to a largely resedimented sequence (Ekwi or Windermere Supergroup) that has been interpreted as a continental margin facies. A closely comparable succession of facies can be documented in other parts of the world but perhaps the closest analogue is found in S.E. Australia. It has been suggested that these two regions may formerly have been juxtaposed and represent the sundered fragments of a Late Proterozoic supercontinent. Relevant to these interpretations are geochemical data such as δ 34S values from Late Proterozoic evaporites and 87Sr/ 86Sr measurements from carbonates, and processes involved in precipitation of glacially-related iron-formations such as those of the Rapitan Group in the N. Cordillera. Preservation of Late Proterozoic ophiolites in North Africa provides additional evidence of plate tectonic processes at that time. These several lines of evidence all suggest a significant period of Late Proterozoic continental breakup which produced many of the continental margins that were and are the loci of subsequent Phanerozoic orogenic activity.

Plate Tectonics Model for the Musgrave Block—Amadeus Basin Complex of Central Australia

GEOLOGISTS studying plate tectonics have concentrated on both ends of the time scale—in regions of Phanerozoic or Archaean geology—so that little work has been done on Proterozoic orogenic belts. I believe that in the Musgrave Block–Amadeus Basin region of central Australia there is an example of Proterozoic plate tectonics or more specifically of a Proterozoic orogeny-plate tectonics correlation. If so this would be one of the oldest terrains to which plate tectonics concepts can be applied. Here I compare sections drawn for the Woodroffe and Ayers Rocks 1:250,000 geological maps (SA Mines Department and BMR 1967) with a modified schematic sequence for the collision of two continental blocks. The similarity, allowing for erosion to fairly deep levels, is striking (Figs 2 and 3). The location of the region and the broad structural features associated with it are shown in Fig. 1.