Timothy M. Kusky

Paleoproterozoic tectonic evolution of the North China Craton

The Archean North China Craton consists of two major blocks, separated by the Central Orogenic Belt. The age of collision of the two blocks along the Central Orogenic Belt is controversial. Some models suggest that the Archean blocks collided at 1.8 Ga, during the Luliang Orogeny (1.7– 1.9 Ga). In this model, high-pressure granulite facies metamorphism accompanied collision at 1.8 Ga. Other models have suggested that the Eastern and Western Blocks collided at 2.5 Ga, soon after 2.6 – 2.5 Ga ophiolitic and arc rocks throughout the orogen were formed. We synthesize the geology, geochronology, and tectonics of the Neoarchean through Mesoproterozoic evolution of the North China Craton. We suggest that the Eastern and Western Blocks collided at 2.5 Ga during an arc/continent collision, forming a foreland basin on the Eastern Block, a granulite facies belt on the western block, and a wide orogen between the two blocks. This collision was followed rapidly by post-orogenic extension and rifting that formed mafic dike swarms and extensional basins along the Central Orogenic Belt, and led to the development of a major ocean along the north margin of the craton. An arc terrane developed in this ocean, and collided with the north margin of the craton by 2.3 Ga, forming a 1400 km long orogen known as the Inner Mongolia – Northern Hebei Orogen. A 1600 km long granulite-facies terrain formed on the southern margin of this orogen, representing a 200 km wide uplifted plateau formed by crustal thickening. The orogen was converted to an Andean-style convergent margin between 2.20 and 1.85 Ga, recorded by belts of plutonic rocks, accreted metasedimentary rocks, and a possible back-arc basin. A pulse of convergent deformation is recorded at 1.9– 1.85 Ga across the northern margin of the craton, perhaps related to a collision outboard of the Inner Mongolia – Northern Hebei Orogen, and closure of the back arc basin. This event caused widespread deposition of conglomerate and sandstone of the basel Changcheng Series in a foreland basin along the north margin of the craton. At 1.85 Ga the tectonics of the North China Craton became extensional, and a series of aulacogens and rifts propagated across the craton, along with the intrusion of mafic dike swarms. The northern granulite facies belt underwent retrograde metamorphism, and was uplifted during extensional faulting. High pressure granulites are now found in the areas where rocks were metamorphosed to granulite facies and exhumed two times, at 2.5 and 1.8 Ga, exposing rocks that were once at lower crustal levels. Rifting led to the development of a major ocean along the southwest margin of the craton, where oceanic records continue until 1.5 Ga. q 2003 Elsevier Ltd. All rights reserved.

Accretionary orogens through Earth history

Abstract Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero.

Geochemistry of Neoarchean (ca. 2.55-2.50 Ga) volcanic and ophiolitic rocks in the Wutaishan greenstone belt, central orogenic belt, North China craton : Implications for geodynamic setting and continental growth

Geological investigation of the Neoarchean (2.55–2.50 Ga) Wutaishan greenstone belt in the central orogenic belt of the North China craton has provided new information on the geodynamic origin of this belt and its mineral deposits. Structural, geochronological, and geochemical characteristics of the Wutaishan greenstone belt suggest that it formed in a forearc tectonic environment at ca. 2.55 Ga and accreted to the Eastern continental block at ca. 2.50 Ga. A ridge subduction model is proposed to explain several unique geological features of the Wutaishan greenstone belt, such as the generation of dunites and chromitite-hosting harzburgites with U-shaped rare earth element (REE) patterns, formation of volcanogenic massive sulphides (VMS) and banded iron formations (BIF), extrusion of mafic to felsic volcanic rocks, and intrusion of tonalite-trondhjemite-granodiorite plutons (TTG). Anomalously high geothermal gradients in the subarc mantle-wedge beneath the Wutaishan forearc may have increased its buoyancy, resulting in its accretion to the continental crust. We propose that ridge subduction also played an important role in the growth of Archean continental crust. In this model, the origin of Archean TTG is genetically linked to eclogites through partial melting of accreted and/or underplated oceanic plateaus and normal oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of an anomalously hot asthenospheric mantle window resulting from ridge subduction. TTG suites intruding Archean accretionary complexes formed the nuclei of intra-oceanic island arcs; subsequent juxtaposition of these arcs resulted in the lateral growth of Archean continental crust.

Growth of granite–greenstone terranes at convergent margins, and stabilization of Archean cratons

Abstract Archean granite–greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events. Most Archean granite greenstone terranes appear to have acquired their first-order structural and metamorphic characteristics at convergent plate margins, where large accretionary wedges similar in aspect to the Chugach, Makran, and Altaids grew through offscraping and accretion of oceanic plateaux, oceanic crustal fragments, juvenile island arcs, rifted continental margins, and pelagic and terrigenous sediments. Buoyant slabs of parts of Archean oceanic lithosphere may have been underplated beneath these orogens, forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere. Back-stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges. Dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas (sanukitoid suite) in the mantle wedge, whereas other magmas (tonalite, trondhjemite, granodiorite or TTG suite) appear to have been generated by melting of hot young subducted slabs. Eventual collision of these juvenile orogens with other continental blocks formed anatectic granites, then thickened the crust beyond its ability to support its own mass, which initiated gravitational collapse and decompressional release of syn- to late-tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs, and aided in the cratonization of the granite–greenstone terranes. Deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid- and upper-crust, forming ore deposits and plutons. Most preserved granite–greenstone terranes have been tectonically stable since the Archean, and form the cratonic interiors of many continents.

Remnants of an Archean oceanic plateau, Belingwe greenstone belt, Zimbabwe

Stratigraphic and structural data from the Archean Zimbabwe craton suggest that a major detachment surface exists within the Belingwe greenstone belt. The surface separates ultramafic and mafic magmatic rocks of the upper greenstones in the hanging wall from an ancient gneiss complex, older volcanic-sedimentary rocks, and a shallow-water sedimentary sequence in the footwall. Rocks dated at ca. 2.7 Ga above the detachment surface form the proposed Mberengwa allochthon. The regionally extensive upper greenstone succession represents tectonically emplaced allochthonous sheets, not indigenous magmas erupted within autochthonous continental rifts. Magmatic rocks of the Mberengwa allochthon resemble oceanic plateaus preserved in younger mountain belts. Comparison of the Zimbabwe craton with the Proterozoic Birrimian terranes of west Africa leads us to suggest that Precambrian continental growth may have been characterized by intense structural imbrication related to the difficulty of subduction of buoyant oceanic lithosphere.

Geologic signature of early Tertiary ridge subduction in Alaska

A mid-Paleocene to early Eocene encounter between an oceanic spreading center and a subduction zone produced a wide range of geologic features in Alaska. The most striking effects are seen in the accretionary prism (Chugach-Prince William terrane), where 61 to 50 Ma near-trench granitic to gabbroic plutons were intruded into accreted trench sediments that had been deposited only a few million years earlier. This short time interval also saw the genesis of ophiolites, some of which contain syngenetic massive sulfide deposits; the rapid burial of these ophiolites beneath trench turbidites, followed immediately by obduction; anomalous high-T, low-P, near-trench metamorphism; intense ductile deformation; motion on transverse strike-slip and normal faults; gold mineralization; and uplift of the accretionary prism above sea level. The magmatic arc experienced a brief flare-up followed by quiescence. In the Alaskan interior, 100 to 600 km landward of the paleotrench, several Paleocene to Eocene sedimentary basins underwent episodes of extensional subsidence, accompanied by bimodal volcanism. Even as far as 1000 km inboard of the paleotrench, the ancestral Brooks Range and its foreland basin experienced a pulse of uplift that followed about 40 million years of quiescence. All of these events-but most especially those in the accretionary prism-can be attributed with varying degrees of confidence to the subduction of an oceanic spreading center. In this model, the ophiolites and allied ore deposits were produced at the soon-to-be subducted ridge. Near-trench magmatism, metamorphism, deformation, and gold mineralization took place in the accretionary prism above a slab window, where hot asthenosphere welled up into the gap between the two subducted, but still diverging, plates. Deformation took place as the critically tapered accretionary prism adjusted its shape to changes in the bathymetry of the incoming plate, changes in the convergence direction before and after ridge subduction, and changes in the strength of the prism as it was heated and then cooled. In this model, events in the Alaskan interior would have taken place above more distal, deeper parts of the slab window. Extensional (or transtensional) basin subsidence was driven by the two subducting plates that each exerted different tractions on the upper plate. The magmatic lull along the arc presumably marks a time when hydrated lithosphere was not being subducted beneath the arc axis. The absence of a subducting slab also may explain uplift of the Brooks Range and North Slope: Geodynamic models predict that long-wavelength uplift of this magnitude will take place far inboard from Andean-type margins when a subducting slab is absent. Precise correlations between events in the accretionary prism and the Alaskan interior are hampered, however, by palinspastic problems. During and since the early Tertiary, margin-parallel strike-slip faulting has offset the near-trench plutonic belt-i.e., the very basis for locating the triple junction and slab window-from its backstop, by an amount that remains controversial. Near-trench magmatism began at 61 Ma at Sanak Island in the west but not until 51 Ma at Baranof Island, 2200 km to the east. A west-to-east age progression suggests migration of a trench-ridge-trench triple junction, which we term the Sanak-Baranof triple junction. Most workers have held that the subducted ridge separated the Kula and Farallon plates. As a possible alternative, we suggest that the ridge may have separated the Kula plate from another oceanic plate to the east, which we have termed the Resurrection plate.

Evolution of the East African and related orogens, and the assembly of Gondwana

Neoproterozoic closure of the Mozambique Ocean collapsed an accretionary collage of arc and microcontinental terranes and sutured East and West Gondwana along the length of the East African Orogen ( Fig. 1). This special issue of Precambrian Research brings together articles on aspects of the East African Orogen’s tectonic history to provide a better understanding of this ancient mountain belt and its relationships to the evolution of crust, climate, and life at the end of Precambrian time. The formation of Gondwana at the end of the Precambrian and the dawn of the Phanerozoic represents one of the most fundamental problems being studied inEarth Sciences today. It links many different fields, and there are currently numerous and rapid changes in our understanding of events related to the assembly of Gondwana. One of the most fundamental and most poorly understood aspects of the formation of Gondwana is the timing and geometry of closure of the oceanic basins which separated the continental fragments that amassed to form the Late Neoproterozoic supercontinent. It appears that the final collision between East and West Gondwana most likely followed the closure of the Mozambique Ocean, forming the East African Orogen. The East African Orogen encompasses the Arabian–Nubian Shield in the north and the Mozambique Belt in the south ( Fig. 1). These and several other orogenic belts are commonly referred to as Pan-African belts, recognizing that many distinct belts in Africa and other continents experienced deformation, metamorphism, and magmatic activity spanning the period of 800–450 Ma. Pan-African tectonothermal activity in the Mozambique Belt was broadly contemporaneous with magmatism, metamorphism and deformation in the Arabian–Nubian Shield. The difference in lithology and metamorphic grade between the two belts has been attributed to the difference in the level of exposure, with the Mozambican rocks interpreted as lower crustal equivalents of the juvenile rocks in the Arabian–Nubian Shield. Recent geochronologic data indicate the presence of two major Pan-African tectonic events in East Africa. The East African Orogeny (800–650 Ma; Stern, 1994) represents a distinct series of events within the Pan-African of central Gondwana, responsible for the assembly of greater Gondwana. Collectively, paleomagnetic and age data indicate that another later event at 550 Ma (Kuunga Orogeny) may represent the final suturing of the Australian and Antarctic segments of the Gondwana continent ( Meert and van der Voo, 1996 ). Three main topics are the focus of ongoing studies by the contributors to this volume. The first is to better understand the timing of Gondwana’s amalgamation, a topic that is central to the debate-relating global-scale tectonics to biologic and climatic change. The second is to constrain the configuration of cratons within Rodinia and mechanisms by which Gondwana formed. The third concerns the very nature of the East African Orogen, the youngest collision zone between East and West Gondwana: what are its continental and oceanic constituents? When did it form? What is the geometry of the major collision zones that bound its accreted terranes? All of these issues are central to a current, much-debated hypothesis of Neoproterozoic geology: Can the dramatic biologic, climatic, and geologic events that mark Earth’s transition into the Cambrian be linked to the distribution of continents to the breakup and reassembly of a supercontinent?

Geochemical and petrological evidence for a suprasubduction zone origin of Neoarchean (ca. 2.5 Ga) peridotites, central orogenic belt, North China craton

The 2.55–2.50 Ga Zunhua and Wutaishan belts within the central orogenic belt of the North China craton contain variably metamorphosed and deformed tectonic blocks of peridotites and amphibolites that occur in a sheared metasedimentary matrix. In the Zunhua belt, dunites comprise podiform chromitites with high and uniform Cr-numbers (88). Peridotites and associated picritic amphibolites are characterized by light rare earth element (LREE)–enriched patterns and negative high field strength element (HFSE: Nb, Zr, and Ti) anomalies. They have positive initial ϵ Hf values (+7.9 to +10.4), which are consistent with an extremely depleted mantle composition. Mass-balance calculations indicate that the composition of the 2.55 Ga mantle beneath the Zunhua belt was enriched in SiO 2 and FeO T compared to modern abyssal peridotites. These geochemical signatures are consistent with a suprasubduction zone geodynamic setting. Metasomatism of the subarc mantle by slab-derived hydrous melts and/or fluids at ca. 2.55 Ga is likely to have been the cause of the subduction zone geochemical signatures in peridotites of the Zunhua belt. In the Wutaishan belt, chromitite-hosting harzburgites and dunites display U-shaped rare earth element (REE) patterns and have high Mg-numbers (91.1–94.5). These geochemical characteristics are similar to those of Phanerozoic forearc peridotites. The dunites might have formed by dissolution of orthopyroxene in reactive melt channels, similar to those in modern ophiolites. However, they differ in detail, and they might be residues of Archean komatiites. Following the initiation of an intra-oceanic subduction zone, they were trapped as a forearc mantle wedge between the subducting slab and magmatic arc. Slab-derived hydrous melts infiltrating through the mantle wedge metasomatized the depleted mantle residue, resulting in U-shaped rare earth element (REE) patterns.

Archean Podiform Chromitites and Mantle Tectonites in Ophiolitic Mélange, North China Craton: A Record of Early Oceanic Mantle Processes

We report 2.5 billion-year-old oceanic mantle podiform chromitite and mantle tectonite in ophiolitic mélange in the North China craton. Tectonic blocks of peridotite, wehrlite, pyroxenite, harzburgitic tectonite, dunite, podiform chromitite, layered gabbro, sheeted dikes, and pillow lava are embedded in a strongly deformed metasedimentary and metavolcanic matrix. The blocks are traceable along strike into the relatively complete ca. 2.505 Ga Dongwanzi ophiolite. Textures in the ultramafic blocks provide a window into igneous and structural processes active in Archean suboceanic mantle. Chromitites in dunitic envelopes preserve igneous nodular, orbicular, antinodular, banded, massive, and disseminated textures. Dunite envelopes are common features of podiform chromitites, forming almost exclusively in the upper mantle or the crust-mantle transition zone of suprasubduction zone (harzburgite-type) ophiolites of younger geological ages. Nodular and orbicular chromite textures are known only from ophiolites and are interpreted to form during partial melting of flowing upper mantle, conditions needed to keep chromite suspended and growing concentrically into the magma. Minor orthopyroxene porphyroclasts with asymmetrical recrystallized tails and kink-banded olivine inclusions in chromite grains record plastic deformation and high-temperature shearing, before or during growth of the chromite. We attribute this deformation to flow in the Archean oceanic mantle. Later deformation is related to dismemberment of the ophiolite and incorporation into a mélange during collision of the Eastern and Western blocks of the North China craton. This collision formed the 1600km-long ophiolite-rich Central Orogenic Figure 1. Distribution of Archean cratons, areas underlain by Precambrian crust, and Phanerozoic crust and podiform chromite deposits (modified after Kusky and Polat, 1999). Filled patterns show locations of major ophiolitic podiform chromite deposits in relation to crust and accretionary orogens of different ages. Squares—Archean; triangles—Proterozoic; circles—Phanerozoic. Zunhua podiform chromitites are associated with the 2.505 Ga Dongwanzi and related ophiolites of North China craton. Figure 2. Tectonic map of North China craton showing division of North China craton into Eastern and Western blocks, separated by the Central Orogenic belt. (Modified after Li et al., 2000a, 2000b; Kusky et al., 2001). *Corresponding author: kusky@eas.slu.edu. 4 JULY 2002, GSA TODAY GSA TODAY, JULY 2002 5 belt, and the 2.5–2.4 Ga Qinglong foreland basin and fold-thrust belt on the Eastern block, and provides an important record of the operation of plate tectonics

The Columbia connection in North China

Abstract The Archaean and Proterozoic geology, structure and metamorphism of the North China Craton (NCC) reveal that the amalgamated Eastern and Western blocks of the craton collided as a single entity with the Columbia supercontinent at 1.93 Ga, and that the Northern Hebei orogen correlates with the Transamazonian-Eburnian belts of Africa and South America, and the Svecofennian of Baltica. This metamorphic belt preserves evidence for extreme crustal metamorphism with diagnostic ultrahigh-temperature (UHT) assemblages such as sapphirine+quartz, spinel+quartz, high alumina orthopyroxene+sillimanite+quartz and high temperature perthites which record temperatures exceeding 1000 °C and pressure above 12 kbar. The metamorphic P–T trajectory is characterized by initial isobaric cooling followed by steep isothermal decompression, defining an overall anticlockwise exhumation history. Electron probe monazite geochronology and precise SHRIMP zircon dating of sapphirine-bearing granulites constrain the timing of the UHT event at c. 1.92 Ga, suggesting that the UHT metamorphism coincided with collisional orogenesis as the North China Craton joined the Columbia supercontinent amalgam along the Northern Hebei orogen during the Palaeoproterozoic. Fluid inclusion studies in the UHT rocks provide evidence for the involvement of synmetamorphic pure CO2, linking the thermal anomaly and fluid flux to underplated mafic magmas during asthenospheric upwelling. Evidence for probable-plume related mafic magmatism is also provided by the extensive mafic dyke swarms cutting the region and elsewhere within the NCC, with geochemical characters testifying to emplacement within rifts that opened up during the extensional collapse of the orogen subsequent to the collisional event. Recognition of the Palaeoproterozoic Columbia suture in the Northern Hebei orogen represents a major paradigm shift, as one popular group of models for the NCC suggests that the Palaeoproterozoic suture resides in an older orogen, the Central Orogenic belt. However, either model for the location of the suture is able to explain metamorphic P–T–t data for crustal thickening at 1.85 Ga, whereas only the model for a Late Archaean collision in the Central Orogenic belt and a Palaeoproterozoic collision in the Northern Hebei orogen can explain the structural, sedimentological, geochronological, and petrological data. The Central Orogenic belt contains several hundred fragments of a c. 2.505 Ga ophiolite suite, a contemporaneous 2.5–2.4 Ga foreland basin deposited on 2.7–2.5 Ga passive margin sediments on the Eastern Block, and contains rare evidence for c. 2.5 Ga granulite facies metamorphism that was largely overprinted by 1.92–1.85 Ga high-grade assemblages. East-directed 2.5 Ga fold-thrust structures are overprinted by 1.92–1.85 Ga south-directed thrusts associated with large-scale thickening of the craton, succeeded by strike-slip shear zones that slice the orogen into numerous fault-bounded terranes that preserve different levels of exhumation.