Tectonic Amalgamation Research Articles

Reworking of ancient tectonic amalgamation belt beneath the central north of North China Craton revealed by dense seismic observations

The North China Craton (NCC) is one of the oldest cratons in the world, and its internal tectonic belt is often used to investigate the earth’s tectonic evolution events. During the late Mesozoic and Cenozoic, the western Pacific subduction zone caused the restructuring of NCC by damaging the craton beneath eastern NCC, resulting in the distinct lateral differences between western and eastern NCC, which ultimately formed the current NCC. Furthermore, the subsequent tectonic events activated the ancient tectonic weak zones, and their traces are imprinted in the deep earth. Here, we investigated the crust structures with a high-density seismic array beneath the splice position of the eastern margin of the Khondalite Belt and the northern part of the central orogenic belt in NCC. The array included 140 short-period seismographs spaced at 2–3 km intervals, which recorded teleseismic three-component waveforms over a one-month period. P-wave receiver functions calculated from 25 teleseismic events provided an image of the crustal structure. The weak Moho and Moho offset under the study area are visible in the migration image of receiver functions. The geological investigations and the rock outcrops were combined to establish the strong coupling relationship between the present surface fault-depression system and deep structures. The deep material circulation, which governs the surface extension of the basin-range structure, is controlled by the deep material circulation which is ultimately derived from the continuous subduction of the western Pacific. The study’s findings indicate that the ancient amalgamative belt might have transformed into a weak zone easily susceptible to modification by plate tectonic movements.

Local variations of metamorphic record from compositionally heterogeneous rocks (Cima di Gagnone, Central Alps): Inferences on exhumation processes of (U)HP–HT rocks

The record of metamorphic conditions is commonly highly heterogeneous in spatially close rocks of different composition and rheology. The well-studied Cima di Gagnone area in the Central Alps represents such an example, as relatively small, 1– to 100 s-meter scale, ultrahigh–pressure and high–temperature ultramafic lenses are enveloped within amphibolite–facies metasediments. We present new field observations, microstructural and petrological analyses, and thermodynamic modelling results on these metasediments, showing that these rocks generally experienced medium pressure and medium temperature conditions of 1.0–1.2 GPa and 640–700 °C, followed by a retrograde stage around 0.6–0.8 GPa and 600–675 °C. However, a few samples from the immediate proximity of the ultramafic lenses record significantly higher P–T conditions of 1.3–1.7 GPa and 750–850 °C, approaching the high pressure and high temperature conditions of the ultramafic bodies (1.5–3.1 GPa, 650–850 °C). Mineral/bulk chemistry changes during growth of new mineral phases hint to local melt/fluid interaction (i.e., metasomatism) between metasediments and ultramafics during the high temperature deformation. Preliminary U-Pb LA–ICP–MS dating suggests that zircon grains from the metasomatic reaction zone have been fully re–equilibrated during the early stage of Alpine exhumation (~36 Ma), while the large part of the metasediments records only pre–Alpine ages. We finally recast these new data into the regional P–T–t–D paths and discuss the consequences of these findings for understanding the exhumation processes of HP rocks. We found different local equilibria (ΔP up to 2 GPa; ΔT up to 160 °C) that cannot be explained simply by retrograde metamorphism or by tectonic amalgamation. Rather, our work suggests a significant role of deformation in producing variable pressure and temperature record within the rocks.

Along‐Margin Variations in Breakup Volcanism at the Eastern North American Margin

AbstractWe model the magnetic signature of rift‐related volcanism to understand the distribution and volume of magmatic activity that occurred during the breakup of Pangaea and early Atlantic opening at the Eastern North American Margin (ENAM). Along‐strike variations in the amplitude and character of the prominent East Coast Magnetic Anomaly (ECMA) suggest that the emplacement of the volcanic layers producing this anomaly similarly varied along the margin. We use three‐dimensional magnetic forward modeling constrained by seismic interpretations to identify along‐margin variations in volcanic thickness and width that can explain the observed amplitude and character of the ECMA. Our model results suggest that the ECMA is produced by a combination of both first‐order (~600–1,000 km) and second‐order (~50–100 km) magmatic segmentation. The first‐order magmatic segmentation could have resulted from preexisting variations in crustal thickness and rheology developed during the tectonic amalgamation of Pangaea. The second‐order magmatic segmentation developed during continental breakup and likely influenced the segmentation and transform fault spacing of the initial, and modern, Mid‐Atlantic Ridge. These variations in magmatism show how extension and thermal weakening was distributed at the ENAM during continental breakup and how this breakup magmatism was related to both previous and subsequent Wilson cycle stages.

The ~1.97 Ga dioritic block in the Hong’an Terrane, central China: syn-collisional alkaline magmatism at the northern margin of the Yangtze Block

It is uncertain whether the Yangtze Block was part of the Paleo- to Mesoproterozoic Columbia supercontinent and, if so, where it was located within the supercontinent. Our analysis of a dioritic intrusion near Tiepu village in the southern Hong’an Terrane of central China provides new constraints on the Paleoproterozoic tectonic evolution of the Yangtze Block. Three representative samples from the Tiepu diorite yield zircon U–Pb ages within analytical error of ~1970 Ma. The diorites have narrow ranges of SiO2 (53.5–57.7 wt%) and MgO (2.97–4.48 wt%). High contents of alkalis (K2O + Na2O = 6.70–10.2 wt%) and total rare-earth elements (∑REE = 570–803 ppm) suggest an affinity to intermediate alkaline rock. These rocks are generally characterized by arc-like trace-element patterns such as enrichment in large-ion lithophile elements and light REEs, and depletion in high-field-strength elements and heavy REEs. Zircons from the diorites display negative εHf(t) values (−12.5 to −3.1), and whole-rock samples show negative εNd(t) values (−6.53 to − 4.08). These isotopic characteristics are consistent with older crustal sources. Combining the new data and previous results from Paleoproterozoic rocks in the northern Yangtze Block, we propose a tectonic transition from pre- to post-collisional settings at ~2.0–1.95 Ga. The Tiepu alkaline diorite is interpreted as a syn-collisional magmatic intrusion that formed shortly after the cessation of oceanic plate subduction at ~2.0 Ga. The Tiepu alkaline diorite thus records the final tectonic amalgamation of the eastern and western Yangtze Block microcontinents during the assembly of Columbia.

Mid-Permian rifting in Central China: Record of geochronology, geochemistry and Sr–Nd–Hf isotopes of bimodal magmatism on NE Qinghai–Tibetan Plateau

Little has been known regarding ~200-Myr-long geologic history of Central China during the Late Paleozoic between two major continental collisional events marked by ~480–440-Ma North Qinling–North Qaidam and ~230-Ma Dabie–Sulu ultrahigh-pressure (UHP) belts, respectively. The Gonghe basin is juxtaposed with major Chinese continental blocks across several outstanding Tethyan sutures in Central China and may hold answers to many critical questions about the tectonic amalgamation of China. In this paper, geochronological (laser ablation–inductively coupled plasma–mass spectrometry, LA–ICP–MS), mineral chemistry, major and trace element, and Sr–Nd–Hf isotopic data are presented for newly discovered Mid-Permian magmatic rocks in the Gonghe basin on the northeastern Qinghai–Tibetan Plateau. The data indicate that magmatism was active at ~270Ma and shows a bimodal composition including gabbro, diabase and granite. Major oxides and trace elements are obviously correlated with SiO2 between the mafic and felsic rocks. They are both characterized by enrichments in light-rare-earth elements (LREEs) and large-ion-lithophile elements (LILEs), negative Eu anomalies and depletion in high-field-strength elements (HFSEs), together with similar Sr–Nd–Hf isotopic signature, suggesting that they likely shared the same magma chamber, and that the felsic rocks were produced by fractional crystallization of the mafic rocks. The high initial 87Sr/86Sr ratios (0.7061 to 0.7094) and low εNd(t) values (−6.4 to −3.7), as well as Nb–Ta deletions indicate that they were likely derived from enriched mantle metasomatized by subduction-related fluids during the Early Paleozoic. In view of absence of coeval arc-related magmatism in contrast to broad seamount mafic magmatism in the Gonghe basin, the bimodal magmatism is ascribed to an intracontinental rifting event that could have been caused by transtension in Central China since the Mid-Permian, which could have been responsible for the initiation of the Gonghe and Songpan–Ganzi basins.

Scales of deformation partitioning during exhumation in a continental subduction channel: A petrofabric study of eclogites and gneisses from NW Spain (Malpica‐Tui Allochthonous Complex)

AbstractDeformation partitioning is identified as the fingerprint of late Palaeozoic continental subduction that affected various lithologies whose field relationship, thermobarometric and petrofabric features are closely related. Different modes of deformation partitioning can be identified within medium temperature, high‐P eclogite lenses, between them and the host gneisses, and within the latter. Development of foliations and lineations with a coherent attitude in all these rocks and their related structural petrology demonstrate that eclogite enclosures and their country rocks underwent a common, pervasive deformational event. The published P–T stability fields of the eclogite phases that define the microscopic fabric are used to define the metamorphic conditions prevailing during the deformation event and relate it to the subduction process. The mineral equilibria of the gneisses (ortho‐ and paragneisses) fail to record the full range of those P–T conditions, but the field relationships show that eclogites were originally basic dykes emplaced in acid igneous rocks and demonstrate that the eclogites and gneisses shared a common tectonometamorphic evolution. Deformation partitioning within the latter occurred at variable scales and involved (1) meso macroscale preservation of virtually undeformed metagranite bodies, surrounded by (2) pervasively foliated and lineated gneisses, and (3) the simultaneous microscale operation in the latter of ductile and brittle–ductile mechanisms at conditions above 500°C and below 1.5 GPa. A subduction channel tectonic setting is proposed to explain the subduction of upper to mid‐crustal igneous rocks and exhumation subsequent to high‐P metamorphism. Its currently accessible dimensions, and its organization into several lithotectonic units mapped as nappes support tectonic amalgamation of units several km3 in volume. Maximum burial in the subduction channel likely reached depths shallower than the lithostatic pressure implied by geobarometric calculations, possibly conditioned by a sudden pressure drop during the initial retrogression stages accompanying exhumation.

Passive seismic studies show configuration of Paleoproterozoic subduction zones and their role in craton assembly in Western Australia

The Capricorn Orogen Passive source Array (COPA) is the passive source seismic component of a major Science and Industry Endowment Fund (SIEF) project, “The distal footprints of giant ore systems: Capricorn case study”. COPA focuses on the deep crustal and shallow lithospheric structure in the Capricorn Orogen with the aim to better understand the tectonic amalgamation of the Western Australian Craton. The objective of COPA is to produce 3D multiple scale seismic images across the orogeny, that together with other geological and geophysical datasets help constrain the timing and kinematic evolution of Capricorn Orogen’s fault zones, and their role in the metallogenic history.

Geocronologia U-Pb em zircão de rochas intrusivas e de embasamento na região do Vale do Jacurici, Cráton do São Francisco, Bahia

RESUMO O Complexo Jacurici, NE do Cráton do São Francisco, é constituído de vários corpos máficos-ultramáficos N-S mineralizados a Cr, possivelmente fragmentos de um único sill rompido durante deformação. Alguns trabalhos sugerem que está intruso no Bloco Serrinha, enquanto outros o inserem no Cinturão Salvador-Curaçá. O embasamento nessa região é dividido informalmente em paragnaisses e ortognaisses, o último supostamente mais jovem, considerando-se estar menos deformado. Petrografia revelou que alguns dos paragnaisses são álcali-feldspato granitos fortemente milonitizados. Os ortognaisses ocorrem no norte e consistem, ao menos em parte, de monzogranitos com deformação heterogênea, localmente de baixa temperatura. Datações U-Pb em zircão foram realizadas para cinco amostras representativas. Apenas três resultaram em boas idades concórdias. Uma rocha máfica produziu idade de 2102 ± 5 Ma e é petrograficamente similar aos metanoritos do Complexo Jacurici, sendo interpretada como registro dos primeiros pulsos do magmatismo máfico. Um monzogranito gerou idade de 2995 ± 15 Ma, sendo mais antigo do que o esperado, relacionado ao Bloco Serrinha. Álcali-feldspato granito produziu idade de 2081 ± 3 Ma. O Sienito Itiúba e os pegmatitos que cortam o Complexo Jacurici têm idades semelhantes. Considerando a falta de informações sobre a sequência supracrustal que hospeda as rochas alcalinas e máficas-ultramáficas intrusivas nas áreas de Ipueira e Medrado, é possível que parte do terreno pertença ao Cinturão Salvador-Curaçá. Sugerimos que o Complexo Jacurici possa ter sido intrudido após a colagem tectônica entre o Bloco Serrinha e a parte mais antiga do Cinturão Salvador-Curaçá e, portanto, poderia ser hospedado por ambos.

Tectonic amalgamation of the Gaoligong shear zone and Lancangjiang shear zone, southeast of Eastern Himalayan Syntaxis

The Gaoligong shear zone (GLGSZ) and Lancangjiang shear zone (LCJSZ) are two major large-scale shear zones southeast of Eastern Himalayan Syntaxis (EHS) and play important roles in the accommodation of the intracontinental deformation in Southeast Asia. The two shear zones, which are mainly composed of high-grade metamorphic rocks and have long been regarded as the Precambrian basement of this area, are gradually merged northward and become tectonically amalgamated from Fugong to Gongshan area. Structural and kinematic analyses reveal that the amalgamation area has experienced strongly partitioned dextral transpression. N–S trending tight to isoclinal folds, sub-vertical foliation and sub-horizontal lineation are ubiquitously developed both within the shear zones and the Carboniferous flysch sediments of Baoshan block in between. Micro- and macro-kinematic indicators such as CS structures, rotated porphyroclasts, asymmetric shear folds, sheath folds, domino and boudin structures suggest that the shear sense of the GLGSZ and LCJSZ is dextral in the amalgamation area. Comparative analyses of strata reveal that the Baoshan block is aligned with south Qiangtang block and the Lanping–Simao block was aligned with north Qiangtang block prior to the Cenozoic large scale tectonic extrusion and rotation. The amalgamation area, which has progressively switched positions from northeast of EHS to east and southeast of EHS, is just located at the neck of the large scale boudin of south Qiangtang–Baoshan block. A boudin model is proposed for the tectonic amalgamation of the GLGSZ and LCJSZ from Fugong to Gongshan area in response to the continued northward convergence between Indian plate and Eurasian plate since early Eocene, resistance of South China block, southeastward extrusion and clockwise rotation of Baoshan block around EHS.

An 800 million-year detrital zircon record of continental amalgamation: Tarim basin, NW China

Sedimentary provenance studies based on detrital zircon ages have proved to be a powerful tool for elucidating detailed patterns of erosion and sediment dispersal on and adjacent to the continents. This approach is commonly used to examine processes that occur over discrete intervals of geologic time, but it can also be used to address fundamental questions concerning the longer-term evolution of the continental crust. Central Asia, for example, consists of a Palaeozoic accretionary collage that contrasts with the cratonal cores of continents such as North America, which were fully consolidated during the Proterozoic. Detrital zircon provenance may thus provide important insights on the comparative evolution of these distinctly different styles of continental growth. The Tarim sedimentary basin is interpreted to be underlain by a micro-continental block that accreted to central Asia during the late Palaeozoic. Detrital zircon in pre-Permian strata of the northern Tarim basin includes Precambrian grains derived from Tarim basement, and Palaeozoic grains related to active tectonism along its margins. During this phase of tectonic amalgamation, overall sediment maturity increased up-section, consistent with similar observations from other cratons. However, in Tarim these trends reverse dramatically in Mesozoic and Cenozoic strata deposited after amalgamation of Tarim with central Asia. Palaeozoic zircon grains derived from the Tian Shan become dominant, Precambrian grains drop to trace abundances, and sandstone composition shift abruptly to immature litharenite. Moreover, a progressively increasing temporal gap occurs between zircon age and depositional age, reflecting the near absence of post-amalgamation magmatic or high-grade metamorphic rocks within the Tian Shan. Collectively, these observations support a model of Tarim and other central Asian basins as islands of long-lived tectonic stability, within an orogenic collage that has remained mobile for >250 million years.

Tectonic Amalgamation of Crustal Blocks along Gadag-Mandya Shear Zone in Dharwar Craton of Southern India

Abstract The crustal scale Shear Zone that can be traced from Gadag in the north to Mandya in the south in Dharwar Craton of southern India is considered as the boundary between two subcratonic blocks namely the Eastern Dharwar Craton (EDC) and the Western Dharwar Craton (WDC) in published literature. The present study on the Gadag-Mandya Shear Zone (GMSZ) in the Javanahalli-Hagalvadi sector has brought out a detailed account on the disposition, geometry and kinematics of the shear zone, and also the distinctive structural patterns of the two adjacent supracrustal belts, namely the Chitradurga schist belt (CSB) in the west and Javanahalli schist belt (JSB) in the east. The JSB has an overall N-S striking and gentle easterly dipping geometry, the structural features of which are indicative of a predominant non-coaxial deformation and westward transportation of the supracrustal assemblage. In contrast, deformation in the CSB, which is defined mainly by a flattening type of strain, has produced an overall verticality of the structures (dominant foliation, axial planes of regional folds). Difference in metamorphic grade is apparent from the mineral assemblages in the rocks of these two belts, while the JSB showing an amphibolite facies CSB is restricted to green schist facies mineral assemblages. The 2 to 3 km wide, N-S striking, prominent ductile shear zone, GMSZ, has affected both CSB and JSB rocks and also the syntectonically emplaced Bukkapatna granite. The geometric and kinematic evidences from both the belts and the GMSZ point to an E-W horizontal compressional regime. The gentle dipping, amphibolite grade JSB and its underlying gneisses acted as an allochthon which got amalgamated with the subvertical, green schist grade CSB by a top-W thrusting along the GMSZ. The subvertical CSB impeded further up-dip movement of the eastern block, resulting in the subsequent development of N-S trending sinistral strike slip shear along the contact of the two contrasting belts to accommodate the continued E-W compression. The linear Bukkapatna Granite, synkinematically emplaced along GMSZ shows evidences of both magmatic stage and solid state deformation in the shear zone.

Late Jurassic–Early Cretaceous Northern Qaidam Basin, NW China: Implications for the earliest Cretaceous intracontinental tectonism

Formation of Mesozoic western China, which was dominated by tectonic amalgamation along its southern margin and associated intracontinental tectonisms, holds a key for interpreting the succedent Cenozoic evolution. This paper presents new data including lithology, sedimentary facies, stratigraphic contact, seismic interpretation and paleo-structures within the Upper Jurassic–Lower Cretaceous strata in the northern Qaidam Basin, NW China. These data all account for a contractional tectonic deformation in the earliest Cretaceous. The South Qilian Shan, according to the sedimentary features and provenance analysis, reactivated and exhumated during the deformation, controlling the deposition of the Lower Cretaceous sequences. A simplified model for the Late Jurassic–Early Cretaceous paleogeography and tectonics of the northern Qaidam Basin is accordingly proposed. The results also support a ∼25° clockwise rotation of the Qaidam Basin since the Early Cretaceous and a more accurate Mesozoic evolution process for the basin. This earliest Cretaceous deformation, associated with the reactivation of the South Qilian Shan at the time, are part of the intracontinental tectonisms in central Asia during the Mesozoic, and probably driven by both the closure of the Mongol-Okhostk Ocean to the north and the collision of the Lhasa and the Qiangtang blocks to the south.

Incipient eclogite facies metamorphism in the Moldanubian granulites revealed by mineral inclusions in garnet

We investigated several mineral phases and their replacement products which occur as inclusions in garnets from felsic and mafic granulites of the Gföhl Unit in the Moldanubian Zone. The most important mineral inclusions, Ti-rich muscovite and omphacite, were used for the reconstruction of the metamorphic history of granulites. Some inclusions were transformed during high-temperature granulite facies metamorphism, partial melting and decompression to other phases, and so the original mineral can only be deduced from the inclusion morphology and reaction products. These inclusions have columnar shapes and consist of K-feldspar + kaolinite, albite + Fe-oxide, plagioclase + Fe-oxide, or albite + K-feldspar, respectively. The pseudomorphs with albite/plagioclase occur in a Ca-rich garnet that shows prograde zoning. Pressure–temperature ( PT) evolution, derived from mineral assemblages in granulite and based on the inclusions, suggests a prograde metamorphism from amphibolite through eclogite to granulite facies conditions with subsequent amphibolite facies overprint during exhumation. The estimated PT trajectory for the studied granulites, which also host lenses or boudins of eclogites and garnet peridotites, allows reconstruction of the complete clockwise metamorphic path that is consistent with subduction geotherm prior to the tectonic amalgamation within the continental collisional root.

Proximal provenance of the western Songpan–Ganzi turbidite complex (Late Triassic, eastern Tibetan plateau): Implications for the tectonic amalgamation of China

The Late Triassic Songpan–Ganzi turbidite complex on the eastern Tibetan plateau, which covers an area of ~ 2.2 × 10 5 km 2, is one of the largest flysch turbiditic basins on Earth. It is juxtaposed with major Chinese continental blocks across several outstanding Tethyan sutures, and is critical to elucidating the tectonic amalgamation of China. However, the provenance of the turbidites remains the subject of intense debate. Detrital modes and heavy-mineral spectra of sandstone samples from three turbidite profiles in the western Songpan–Ganzi complex were determined in an attempt to elucidate their provenance and the type of tectonic setting in which they were deposited. Upper Triassic turbidites in the western Songpan–Ganzi complex have an overwhelming derivation from an orogen source, as shown by the average framework compositions of the sandstones (Q 57F 21L 22, Qm 52F 21Lt 27, Qp 23Lvm 0Lsm 77). The compositional and textural immaturity of the sandstones suggests limited transport and nearby sources. Both light and heavy mineral and lithic fractions indicate dominant metamorphic source rocks and subordinate ophiolitic and sedimentary source rocks. The occurrence of C-type garnet, omphacite, rutile, and Si-rutile, and the high-silica content of phengites reveal that the source rocks underwent ultrahigh-pressure conditions. Therefore, we advocate that the western Songpan–Ganzi Late Triassic turbidites were derived from nearby central Qiangtang Triassic collisional orogen sources, rather than distant Dabie–Qinling sources eroded after North China–South China collision, as previously proposed. The analogy of the Songpan–Ganzi turbidites to the Bengal–Indus fans is not favored. The central Qiangtang metamorphic belt is unlikely to have occurred as an early Mesozoic mélange underthrust from the Jinsa suture, and is more likely to have been an in situ Triassic ultrahigh-pressure orogen. Our provenance interpretation implies that the Paleotethys, represented by the present Jinsa and Kunlun sutures, was not yet closed when central Qiangtang was being exhumed to supply the sediments of the Songpan–Ganzi basin. This challenges the conventional model for sequential amalgamation of China during the Phanerozoic.

A review of the geochronological constraints on the evolution of the Kaapvaal Craton, South Africa

Abstract All the published U–Pb geochronological data from zircon, titanite, sphene and monazite, and Pb–Pb evaporation data applicable to the Palaeoarchaean-to-Neoarchaean geology of the Kaapvaal Craton are employed to reconstruct the history and crustal architecture of the ca. 3600–2500 Ma cratonic basement in South Africa. Only data interpreted as representative of the age of crystallisation, or the 207 Pb / 206 Pb ages from zircon xenocrysts, have been considered in constructing graphs of cumulative probability versus age of rock formation. In this manner, it has been possible to identify the main magmatic events that have contributed to the assembly of the craton. For convenience of data handling, the Kaapvaal Craton has been subdivided into eastern, central, northern and western geographic domains. These domains are not intended as discrete geological terranes although each one has its own geological and chronological character, although consanguinity of events is also evident. The oldest rocks so far recognised are located in the Swaziland–Barberton areas where ages >3600 Ma have been recorded. The early stages of shield development are best exposed in the Barberton Mountain Land where it is now apparent that continent formation took place by magmatic accretion and tectonic amalgamation of small protocontinental blocks. At Barberton, several diachronous blocks, formed between 3600 and 3200 Ma, have been identified, each of which represents a cycle of arc-related magmatism and sedimentation. This phase of crustal development was followed by a period of Mesoarchaean cratonic magmatism, particularly prevalent between approximately 3100–3000 Ma, and marked by the development of a major, crescent-shaped, juvenile arc that was accreted onto the northern and western margins of the evolving Kaapvaal Shield. Cratonisation was accomplished by the emplacement of major granitoid batholiths, which thickened and stabilised the continental crust during the early stages of this cycle. Subsequent evolution of the craton, between 3000 and 2700 Ma, was associated with continent–arc collision, during which time the Witwatersrand Basin and its correlatives were deposited as foreland sequences, followed by episodic extension and rifting, when the Gaborone–Kanye and Ventersdorp sequences were developed.

The Late Archean history of the Wyoming province as recorded by granitic magmatism in the Wind River Range, Wyoming

The Wyoming province, a small, ca. 500 000 km 2 Archean craton, is the most southwestern of the Archean provinces in North America. It is composed primarily of Late Archean potassium-rich granitic rocks. In contrast to many other Archean provinces, rocks of tonalite-trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2.8 Ga. Field, petrologic, geochemical and isotopic study of the Late Archean granites exposed in the Wind River Range have allowed us to identify at least four periods of potassic calc-alkalic magmatism at ∼2.8, 2.67, 2.63 and 2.55 Ga. Granitic rocks of these ages appear to be widespread across the Wyoming province. The oldest calc-alkalic granites of the Wind River Range, emplaced at ca. 2.8 Ga, appear to be derived predominantly from pre-existing crust. However, Nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses. During at least two other periods of plutonism, at 2.67 and 2.63 Ga, generation of the Wind River Range batholiths involved the incorporation of substantial amounts of isotopically juvenile material, either from depleted mantle or young continental crust. The information presented below, as well as data available from elsewhere in the Wyoming province, is interpreted to suggest that the Wyoming province, unlike other Archean cratons, is not composed of a tectonic amalgamation of smaller, exotic terranes. Although the Wyoming province did experience crustal addition in Archean time, it was not by lateral accretion, but by incorporation of mantle-derived melts into large granitic batholiths.

Mesozoic Sedimentology, Stratigraphy, and Provenance of Western Mongolia as a Record of the Tectonic Amalgamation of Central Asia: ABSTRACT

Mesozoic Sedimentology, Stratigraphy, and Provenance of Western Mongolia as a Record of the Tectonic Amalgamation of Central Asia: ABSTRACT

Noyon Uul syncline, southern Mongolia: Lower Mesozoic sedimentary record of the tectonic amalgamation of central Asia

To explore the hypothesis that a regionally extensive early Mesozoic collisional foreland basin existed on strike to the east of documented Mesozoic foreland basins in western China, we conducted a reconnaissance study of a >5 km section of Upper Permian-Lower Jurassic(?) nonmarine strata exposed in the Noyon Uul syncline in southern Mongolia. This paper documents the depositional settings of these poorly known strata, their provenance, and their regional tectonic significance in the context of the Mesozoic amalgamation of central Asia. We interpret Noyon Uul strata to be composed of (1) a coarse-grained, braided fluvial facies, (2) a meandering fluvial facies, and (3) a poorly oxygenated lacustrine facies that represent three principal depositional environments. Sandstone compositions are dominantly volcanic (mean compositions = Qm 20 F 23 Lt 57 ; Qp 13 Lv 82 Ls+Lm 5 ), likely reflecting derivation from Carboniferous and older volcanic-arc sequences. Increased proportions of alkalic granite pebbles and cobbles above the first coarse-grained, braided fluvial unit and the first stratigraphic appearance of detrital K-feldspar in basal coarse-grained, braided fluvial strata suggest unroofing of Permian alkalic granites. The Noyon Uul deposits likely are syntectonic, as suggested by their thickness, environments of deposition, coarse-grained character, and accumulation rates. Overlap of the Noyon Uul syncline by relatively undeformed Upper Jurassic and Lower Cretaceous strata, together with the presence of a regional unconformity between Middle and Upper Jurassic rocks, requires a phase of Middle-Late Jurassic deformation. The erosional remnants of a Middle-Upper Jurassic north-verging thrust belt have been documented just 100 km south of Noyon Uul, suggesting that the Noyon Uul deposits may be foreland in origin. Specifically Triassic thrusting has not been recognized in southern Mongolia, but Permian-Jurassic contractile deformation associated with the collisional amalgamation of Asia is documented or reasonably inferred for extensive areas of China to the south and west. Although sedimentary attributes of the Noyon strata are most compatible with a foreland interpretation, it is possible that the Noyon deposits and their deformation may be related to strike-slip faulting. Neither the attributes of the Noyon deposits nor their ages are compatible with widespread Late Jurassic-Early Cretaceous rifting that occurred across southeastern Mongolia.

Late Paleozoic tectonic amalgamation of northwestern China: Sedimentary record of the northern Tarim, northwestern Turpan, and southern Junggar Basins

Sedimentary rocks contained in basins adjacent to the Tian Shan provide a long and complex record of the late Paleozoic continental amalgamation of northwestern China, complementing that provided by rocks preserved within the range. This record, which comprises dramatic changes in sedimentary facies, sediment dispersal patterns, sandstone provenance, and basin subsidence rates, broadly supports previous interpretations of a two-part evolution of the Tian Shan: Late Devonian to Early Carboniferous collision of the Tarim continental block with the Central Tian Shan, followed by collision of this combined block with island arcs in the north Tian Shan and Bogda Shan in Late Carboniferous–Early Permian times. The first collision resulted in widespread angular unconformities within the Tarim basin. Continued convergence following the collision created a long-lived flexural foredeep along the northern margin of the Tarim block, which received at least 2000 m of Lower Carboniferous through Lower Permian fluvial and marine sediment derived from the interior of Tarim. Subsequent Early Permian continental extension of the northern Tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks. Sandstone within this interval was derived from the paleo–Tian Shan, and is composed predominantly of lithic volcanic grains similar to the rhyolite. In contrast to the Tarim basin, calc-alkaline volcanic rocks and volcanogenic sedimentary rocks dominated Carboniferous and Permian sedimentation in the northern Turpan and northwestern Junggar basins. Volcanic arcs remained active in the North Tian Shan and Bogda Shan through the early Late Carboniferous, depositing a kilometers-thick interval of deep marine sediment-gravity flows in the northwestern Junggar basin. Major arc magmatism ceased in the Late Carboniferous in response to closure of the oceanic basin between the combined Tarim/Central Tian Shan block and the North Tian Shan/Bogda Shan arcs. Upper Carboniferous through Lower Permian rocks in the northwestern Junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth (on the northern side of the volcanic arcs), culminating in a 1000-m-thick marine regressive sequence. Middle to Upper Permian sandstones were derived from the uplifted paleo–Tian Shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range. Late Permian subsidence of the Junggar basin accommodated >5 km of nonmarine sediments; however, the cause of this subsidence and its relationship to regional tectonic events remain controversial.

Crustal velocity structure of the northern Yukon-Tanana upland, central Alaska: Results from TACT refraction/wide-angle reflection data

The Fairbanks North seismic refraction/ wide-angle reflection profile, collected by the U.S. Geological Survey Trans-Alaska Crustal Transect (TACT) project in 1987, crosses the complex region between the Yukon-Tanana and Ruby terranes in interior Alaska. This region is occupied by numerous small terranes elongated in a northeast-southwest direction. These seismic data reveal a crustal velocity structure that is divided into three upper-crustal and at least two middle- to lower-crustal domains. The upper-crustal domains are delineated by two steeply dipping low-velocity anomalies that are interpreted as signatures of the Victoria Creek fault, and the Beaver Creek fault or a fault buried by the Beaver Creek fault. This tripartite upper crust extends to 8-10 km depth where a subhorizontal interface undercuts the northern and central domains. Beneath the northern domain, this interface is interpreted as the southeastwardly dipping boundary between the Tozina and Ruby terranes. The continuation of this interface beneath the central domain suggests that it may represent the detachment or basal thrust for thin-skinned tectonic amalgamation of the terranes caught between the Yukon-Tanana and Ruby terranes. The lower crust and Moho reflection exhibit differences from north to south that define at least two lower-crustal domains, interpreted as the Yukon-Tanana and Ruby terranes. Finally, the crustal thickness along the profile is nearly uniform and ranges from 31 to 34 km. Our data suggest that after initial thin-skinned amalgamation of the various terranes, this region experienced thick-skinned tectonic reorganization via strike-slip faulting. This interpretation supports a model in which at least one strand of the Tintina fault exists in this important region of Alaska.