Showing papers on "Terrane published in 2002"

Reconstruction of the tectonic evolution of the western Mediterranean since the Oligocene

Abstract: We present a tectonic synthesis and an animation of the tectonics of the western Mediterranean since the Oligocene. This work is based on data derived from different geological datasets, such as structural geology, the distribution of metamorphic rocks, magmatic activity, sedimentary patterns, palaeomagnetic data and geophysics. Reconstruction was performed using an interactive software package (PLATYPLUS), which enabled us to apply rotational motions to numerous microplates and continental terranes involved in the evolution of the western Mediterranean basins. Boundary conditions are provided by the relative motions of Africa and Iberia with respect to Europe, and the Adriatic plate is considered here as an African promontory. The reconstruction shows that during Alpine orogenesis, a very wide zone in the interface between Africa and Europe underwent extension. Extensional tectonics was governed by rollback of subduction zones triggered by gravitational instability of old and dense oceanic lithosphere. Back-arc extension occurred in the overriding plates as a result of slow convergence rates combined with rapid subduction rollback. This mechanism can account for the evolution of the majority of the post-Oligocene extensional systems in the western Mediterranean. Moreover, extension led to drifting and rotations of continental terranes towards the retreating slabs in excess of 100-800 km. These terranes - Corsica, Sardinia, the Balearic Islands, the Kabylies blocks, Calabria and the Rif-Betic - drifted as long as subduction rollback took place, and were eventually accreted to the adjacent continents. We conclude that large-scale horizontal motions associated with subduction rollback, back-arc extension and accretion of allochthonous terranes played a fundamental role during Alpine orogenesis.

Neoproterozoic to Paleozoic Geology of the Altai Orogen, NW China: New Zircon Age Data and Tectonic Evolution

Abstract: We present a synthesis and a new account of the geological and tectonic history of the terranes of the Chinese Paleozoic Altai orogen together with new, single zircon ages for granitic and rhyodacitic rocks. A central terrane consists of Neoproterozoic to Silurian, amphibolite facies, metasedimentary rocks, and abundant Devonian‐Carboniferous granites. The presence of Precambrian basement is indicated by Sinian fossils, our xenocryst ages, and published Nd mean crustal residence ages of granites. Felsic arc‐type lavas on the southern margin of the terrane have a mean 207Pb/206Pb zircon age of 505 Ma, reflecting the time of arc volcanism, and the presence of xenocysts with ages between 614 and 921 Ma suggests derivation by intracrustal melting. Accordingly, we suggest that a Cambro‐Ordovician continental magmatic arc was built on the southern margin of the central terrane by northward subduction. A low‐grade Ordovician Andean‐type arc with a continental basement is situated above a normal fault on...

Paleozoic evolution of pre-Variscan terranes: From Gondwana to the Variscan collision

Abstract: The well-known Variscan basement areas of Europe contain relic terranes with a pre-Variscan evolution testifying to their peri-Gondwanan origin (e.g., relics of Neoproterozoic volcanic arcs, and subsequent stages of accretionary wedges, backarc rifting, and spreading). The evolution of these terranes was guided by the diachronous subduction of the proto-Tethys oceanic ridge under different segments of the Gondwana margin. This subduction triggered the emplacement of magmatic bodies and the formation of backarc rifts, some of which became major oceanic realms (Rheic, paleoTethys). Consequently, the drifting of Avalonia was followed, after the Silurian and a short Ordovician orogenic event, by the drifting of Armorica and Alpine domains, accompanied by the opening of the paleo-Tethys. The slab rollback of the Rheic ocean is viewed as the major mechanism for the drifting of the European Variscan terranes. This, in turn, generated a large slab pull force responsible for the opening of major rift zones within the passive Eurasian margin. Therefore, the µrst Middle Devonian Variscan orogenic event is viewed as the result of a collision between terranes detached from Gondwana (grouped as the Hun superterrane) and terranes detached from Eurasia. Subsequently, the amalgamated terranes collided with Eurasia in a second Variscan orogenic event in Visean time, accompanied by large-scale lateral escape of major parts of the accreted margin. Final collision of Gondwana with Laurussia did not take place before Late Carboniferous time and was responsible for the Alleghanian orogeny.

Geology and Tectonic Evolution of the Archean North Pilbara Terrain,Pilbara Craton, Western Australia

Abstract: Results from a multidisciplinary geoscience program since 1994 are summarized for the North Pilbara terrain of the Pilbara Craton. Major findings include the recognition of three separate terranes with unique stratigraphy, geochronological, and structural histories; the ca. 3.72 to 2.85 Ga East Pilbara granite-greenstone terrane, the ca. 3.27 to 2.92 Ga West Pilbara granite-greenstone terrane, and the <3.29 Ga Kuranna terrane in the southeast. These are separated by two late, dominantly clastic sedimentary basins deposited within tectonically active zones; the ca. 3.01 to 2.93 Ga Mallina basin in the west and the undated Mosquito Creek basin in the east. The oldest supracrustal rocks are the ca. 3.51 to 3.50 Ga Coonterunah and ca. 3.49 to 3.31 Ga Warrawoona Groups in the East Pilbara granite-greenstone terrane, deposited on fragments of older sialic crust to 3.72 Ga. The Warrawoona Group is subdivided into three main (ultra)mafic-felsic volcanic cycles including from base to top, the Talga Talga (3.49–3.46 Ga), Salgash (3.46–3.43 Ga), and newly defined Kelly (3.43–3.31 Ga) Subgroups. These dominantly basaltic rocks include chert beds containing Earth’s oldest stromatolites and are interbedded with significant felsic volcanics erupted intermittently from 3.49 to 3.43 Ga during emplacement of sheeted sodic granitoid sills. Estimates of autochthonous stratigraphic thickness range from 9 to 18 km. Deformation involved extensional growth faulting, local folding, and tilting of greenstones away from synvolcanic granitoid domes. Rapid partial convective overturn of upper and middle crust occurred at 3.32 Ga during voluminous potassic felsic magmatism, followed by deposition of the Budjan Creek Formation at 3.31 Ga. Granitoid plutonism at ca. 3.29 Ga in the Kuranna terrane preceded deposition of ultramafic through felsic volcanics and chert in the West Pilbara granite-greenstone terrane (3.27–3.25 Ga Roebourne Group) and western margin of the East Pilbara granite-greenstone terrane (3.26–3.24 Ga Sulphur Springs Group). Geochemical and isotopic data suggest that volcanism resulted from plume-related rifting of the East Pilbara granite-greenstone terrane, which was accompanied by granitoid plutonism and deformation. Following this was ca. 100 m.y. of relative quiescence during which locally economic concentrations of banded iron-formation and siliciclastics of the Gorge Creek Group were deposited in the East Pilbara granite-greenstone terrane. Thereafter, geologic events are more consistent with microplate tectonics, commencing with deformation at 3.15 Ga followed by deposition of 3.13 to 3.11 Ga bimodal volcanics in the West Pilbara granite-greenstone terrane (Whundo Group), which have juvenile Nd isotope signatures and thus may represent either a rift or island-arc succession. Basaltic rocks and minor felsic tuff were deposited in the East Pilbara granite-greenstone terrane at 3.06 Ga and possibly in the West Pilbara granite-greenstone terrane (Regal Formation). At 3.02 Ga, the Whundo and Roebourne Groups share a common history of deposition of banded iron-formation and granitoid plutonism across the Sholl shear zone, suggesting accretion at, or immediately preceding, this time. This was followed by deposition in the Mallina basin of the volcanic Whim Creek Group at 3.01 Ga, possibly as an arc, and then the 2.97 to 2.93 Ga volcanic Bookingarra (west) and clastic De Grey (east) Groups during periods of intracontinental rifting interspersed with compression and granitoid intrusion. The geochemistry of 2.95 Ga high Mg diorites (sanukitoids) indicates a previous episode of subduction during either the Whundo or Whim Creek Groups or both. Final events include emplacement of ultramafic-mafic layered intrusions (2.925 Ga in the West Pilbara granite-greenstone terrane), local shearing and lode Au mineralization (2.92 Ga in the West Pilbara granite-greenstone terrane, 2.90 Ga in the Mosquito Creek basin, 2.89 Ga in the East Pilbara granite-greenstone terrane), and intrusion of fractionated, Sn-Ta-Li-bearing granites to 2.85 Ga (East Pilbara granite-greenstone terrane).

Nature of extensional accretionary orogens

Abstract: Extensional accretionary orogens form by creation and destruction of large arc/back arc basin systems, generated by extension and sediment infilling during prolonged slab retreat, but episodically thickened by basin inversion during short-lived (∼10 Ma), orogenic contraction events. They are characterised by widespread, syntectonic, silicic, and minor basaltic magmatism, regional low-P, variable-T metamorphism, and by the enigmatic development of rift basins throughout the peak orogenic history. These orogens have features associated with retreating subduction boundaries and contrast markedly with those formed by terrane accretion, such as the Canadian Cordillera. The Paleozoic Lachlan orogen example from eastern Australia shows that Silurian-Devonian synorogenic basalts and gabbros were intimately associated with rifting and granite emplacement, but they formed during a period of repeated orogenic contraction. Moreover, primitive basaltic compositions have oceanic affinities, indicating generation under lithosphere that was <30 km thick. Only in the final stages of orogeny (Middle Devonian), after at least three major crustal contraction events, did the lithosphere thicken to ∼80 km or more, leading to stabilization of the orogen. Extensional accretionary orogens grow by magmatic and sedimentary additions during extension, caused mainly by asthenospheric melting and rift basin formation/sedimentation, augmented by localized and repeated crustal thickening events. Orogenic contraction leaves an indelible structural imprint which may obliterate the prior-formed extensional structures. The orogen remains hot, despite repeated thickening events, because of ongoing extension, which promotes advective heat transfer into the crust by basalt injection and crustal melting. Rapid switching to contraction, possibly during intermittent arrival of buoyant oceanic plateaus, inverts the thermally softened basins and forms localized fold-thrust belts in which the penetrative foliations record the peak metamorphism. Their most diagnostic features are the presence of basaltic rocks and rift basins throughout the orogenic contraction history.

Organization of pre-Variscan basement areas at the north-Gondwanan margin

Abstract: Pre-Variscan basement elements of Central Europe appear in polymetamorphic domains juxtaposed through Variscan and/or Alpine tectonic events. Consequently, nomenclatures and zonations applied to Variscan and Alpine structures, respectively, cannot be valid for pre-Variscan structures. Comparing pre-Variscan relics hidden in the Variscan basement areas of Central Europe, the Alps included, large parallels between the evolution of basement areas of future Avalonia and its former peri-Gondwanan eastern prolongations (e.g. Cadomia, Intra-Alpine Terrane) become evident. Their plate-tectonic evolution from the Late Proterozoic to the Late Ordovician is interpreted as a continuous Gondwana-directed evolution. Cadomian basement, late Cadomian granitoids, late Proterozoic detrital sediments and active margin settings characterize the pre-Cambrian evolution of most of the Gondwana-derived microcontinental pieces. Also the Rheic ocean, separating Avalonia from Gondwana, should have had, at its early stages, a lateral continuation in the former eastern prolongation of peri-Gondwanan microcontinents (e.g. Cadomia, Intra-Alpine Terrane). Subduction of oceanic ridge (Proto-Tethys) triggered the break-off of Avalonia, whereas in the eastern prolongation, the presence of the ridge may have triggered the amalgamation of volcanic arcs and continental ribbons with Gondwana (Ordovician orogenic event). Renewed Gondwana-directed subduction led to the opening of Palaeo-Tethys.

Geochronology of the Karadere basement (NW Turkey) and implications for the geological evolution of the Istanbul zone

Abstract: Earlier geological work in the Istanbul zone, western Pontide tectonic belt, has revealed the presence of extensive basement outcrops exposed underneath Palaeozoic and Mesozoic to Tertiary cover sequences. The basement of suspected Neoproterozoic age plays an important role in understanding the crustal accretion process in NW Turkey. We report the first results of a detailed Pb–Pb and U–Pb zircon study complemented by Nd–Sr whole rock and mineral data from basement rocks exposed in the Karadere valley, Safranbolu area. Five samples were selected for this study, comprising three metagranitoids and two metasediments. Zircon geochronology indicates that the metagranitoids were formed during Late Proterozoic pan-African magmatic events between 590 and 560 Ma. The rocks are of tonalitic and granitic composition and have low Nb/Y ratios and Ti contents, consistent with those of arc rocks. A continental arc setting is supported by their Sr and Nd isotope data that indicate a contribution of a mantle source as well as crustal assimilation during magma genesis. The metasediments can clearly be distinguished from the metagranitoids by their higher 87Sr/86Sr ratios and lower eNd-values at 580 Ma, which supports the suggestion that the arc was underlain by mature continental crust. Zircons from the metasediments yield a range of Pb–Pb ages between 1,860 and 710 Ma. Thirty per cent of them fall between 890 and 710 Ma, possibly suggesting a derivation from Gondwana (Afro-Arabian) regions. A Sm–Nd garnet–whole rock analysis obtained on a metagranite gives an age of 559±8 Ma, which either reflects pre-metamorphic magmatic growth of garnet in a felsic melt or a syntectonic high-temperature metamorphic event. Uplift and cooling of the basement is further constrained by Rb–Sr biotite ages of 548–545 Ma. These lower Cambrian mineral ages demonstrate that the Istanbul zone was not thermally reactivated during the Hercynian, Cimmerian or Alpine orogeny, in contrast to its neighbouring tectonic zones, confirming its role as a suspect terrane in the modern western Pontide tectonic belt.

The Tectonic Evolution of Central and Northern Madagascar and Its Place in the Final Assembly of Gondwana

Abstract: Recent work in central and northern Madagascar has identified five tectonic units of the East African Orogen (EAO), a large collisional zone fundamental to the amalgamation of Gondwana. These five units are the Antongil block, the Antananarivo block, the Tsaratanana sheet, the Itremo sheet, and the Bemarivo belt. Geochronological, lithological, metamorphic, and geochemical characteristics of these units and their relationships to each other are used as a type area to compare and contrast with surrounding regions of Gondwana. The Antananarivo block of central Madagascar, part of a broad band of pre‐1000‐Ma continental crust that stretches from Yemen through Somalia and eastern Ethiopia into Madagascar, is sandwiched between two suture zones we interpret as marking strands of the Neoproterozoic Mozambique Ocean. The eastern suture connects the Al‐Mukalla terrane (Yemen), the Maydh greenstone belt (northern Somalia), the Betsimisaraka suture (east Madagascar), and the Palghat‐Cauvery shear zone syst...

How does the Alpine belt end between Spain and Morocco

Abstract: The Betic-Rif arcuate mountain belt (southern Spain, northern Morocco) has been interpreted as a symmetrical collisional orogen, partly collapsed through convective removal of its lithospheric mantle root, or else as resulting of the African plate subduction beneath Iberia, with further extension due either to slab break-off or to slab retreat. In both cases, the Betic-Rif orogen would show little continuity with the western Alps. However, it can be recognized in this belt a composite orocline which includes a deformed, exotic terrane, i.e. the Alboran Terrane, thrust through oceanic/transitional crust-floored units onto two distinct plates, i.e. the Iberian and African plates. During the Jurassic-Early Cretaceous, the yet undeformed Alboran Terrane was part of a larger, Alkapeca microcontinent bounded by two arms of the Tethyan-African oceanic domain, alike the Sesia-Margna Austroalpine block further to the northeast. Blueschist- and eclogite-facies metamorphism affected the Alkapeka northern margin and adjacent oceanic crust during the Late Cretaceous-Eocene interval. This testifies the occurrence of a SE-dipping subduction zone which is regarded as the SW projection of the western Alps subduction zone. During the late Eocene-Oligocene, the Alkapeca-Iberia collision triggered back-thrust tectonics, then NW-dipping subduction of the African margin beneath the Alboran Terrane. This Maghrebian-Apenninic subduction resulted in the Mediterranean basin opening, and drifting of the deformed Alkapeca fragments through slab roll back process and back-arc extension, as reported in several publications. In the Gibraltar area, the western tip of the Apenninic-Maghrebian subduction merges with that of the Alpine-Betic subduction zone, and their Neogene roll back resulted in the Alboran Terrane collage astride the Azores-Gibraltar transpressive plate boundary. Therefore, the Betic-Rif belt appears as an asymmetrical, subduction/collision orogen formed through a protracted evolution straightfully related to the Alpine-Apenninic mountain building.

Interaction of metamorphism, deformation and exhumation in large convergent orogens

Abstract: Coupled thermal-mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles. The models involve subduction of suborogenic mantle lithosphere, large amounts of convergence (≥ 450 km) at 1 cm yr−1, and a slope-dependent erosion rate. The model crust is layered with respect to thermal and rheological properties — the upper crust (0–20 km) follows a wet quartzite flow law, with heat production of 2.0 μW m−3, and the lower crust (20–35 km) follows a modified dry diabase flow law, with heat production of 0.75 μW m−3. After 45 Myr, the model orogens develop crustal thicknesses of the order of 60 km, with lower crustal temperatures in excess of 700 °C. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 1019 Pa.s at 700 °C in the upper crust and 900 °C in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T≥600 °C. The channel flow zone is characterised by a reversal in velocity direction on the pro-side of the system, and is driven by a depth-dependent pressure gradient that is facilitated by the development of a temperature-dependent low viscosity horizon in the mid-crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post-convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust-related exhumation on the flanks. Velocities in the pro-side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro-side flank of the plateau leads to ‘ductile extrusion’ of the channel flow zone. The exhumed channel displays apparent normal-sense offset at its upper boundary, reverse-sense offset at its lower boundary, and an ‘inverted’ metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P–T–t paths in both cases are loops where Pmax precedes Tmax, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen.

Microdiamonds in a megacrystic garnet websterite pod from Bardane on the island of Fjørtoft, western Norway: Evidence for diamond formation in mantle rocks during deep continental subduction

Abstract: We report the startling discovery of in situ microdiamonds in a mantle-derived peridotite lens from Bardane, Fjortoft, western Norway. Diamonds occur within multiphase solid-inclusion assemblages within spinels that are, in turn, inclusions within garnets. Euhedral inclusion-host morphologies of spinel and mineral chemistries all indicate diamond growth from infiltrating fluids at ultrahigh pressure ( P ) and moderate temperature. These results imply that the Bardane peridotite lens was present within a continental subduction zone at depths of ≥130 km. This paper not only documents the first discovery of in situ microdiamonds within the Caledonian ultrahigh-pressure terrane of western Norway, but also represents the first known global occurrence of subduction-related diamond formation within mantle rocks that have been incorporated into a major continental plate collision zone.

Carboniferous and Triassic eclogites in the western Dabie Mountains, east-central China: evidence for protracted convergence of the North and South China Blocks

Abstract: SHRIMP U–Pb dating and laser ablation ICP-MS trace element analyses of zircon from four eclogite samples from the north-western Dabie Mountains, central China, provide evidence for two eclogite facies metamorphic events Three samples from the Huwan shear zone yield indistinguishable late Carboniferous metamorphic ages of 312 ± 5, 307 ± 4 and 311 ± 17 Ma, with a mean age of 309 ± 3 Ma One sample from the Hong'an Group, 1 km south of the shear zone yields a late Triassic age of 232 ± 10 Ma, similar to the age of ultra-high pressure (UHP) metamorphism in the east Qinling–Dabie orogenic belt REE and other trace element compositions of the zircon from two of the Huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase, namely in the eclogite facies, an interpretation supported by the presence of garnet, omphacite and phengite inclusions Zircon also grew during later retrogression Zircon cores from the Huwan shear zone have Ordovician to Devonian (440–350 Ma) ages, flat to steep heavy-REE patterns, negative Eu anomalies, and in some cases plagioclase inclusions, indicative of derivation from North China Block igneous and low pressure metamorphic source rocks Cores from Hong'an Group zircon are Neoproterozoic (780–610 Ma), consistent with derivation from the South China Block In the western Dabie Mountains, the first stage of the collision between the North and South China Blocks took place in the Carboniferous along a suture north of the Huwan shear zone The major Triassic continent–continent collision occurred along a suture at the southern boundary of the shear zone The first collision produced local eclogite facies metamorphism in the Huwan shear zone The second produced widespread eclogite facies metamorphism throughout the Dabie Mountains–Sulu terrane and a lower grade overprint in the shear zone

Arc-ophiolite obduction in the Western Kunlun Range (China): implications for the Palaeozoic evolution of central Asia

Abstract: The nature of the ‘Kudi ophiolite’ in the Western Kunlun Range is hotly debated. Our new structural–geochemical data reveal that it is actually an arc ophiolite comprising: (1) arc- or ophiolite-derived turbidites of two generations containing Late Ordovician–Silurian and Late Devonian–Early Carboniferous radiolarians; (2) a central intra-oceanic (Yixieke) arc with basalt–andesite–tuff–agglomerate; (3) lower (Buziwan) oceanic crust containing dunite–harzburgite–gabbro. We propose the following tectonic evolution. South-dipping subduction in Late Cambrian to earliest Ordovician time generated the Yixieke arc on top of the Buziwan oceanic crust–mantle. This subduction led to emplacement of the arc northwards onto the North Kunlun terrane (Tarim block), creating an active continental margin with northward subduction below it. The Kudi ophiolite was thrust southeastward over the incoming Kudi continental (gneiss) terrane in mid-Ordovician–mid-Devonian time. During a tectonic hiatus in the Kudi region Late Devonian–Carboniferous subduction further west led to development of the Oytag arc, formerly regarded as an equivalent of the Kudi ophiolite. Lower Permian arc lavas and Upper Triassic granites in the Xiananqiao arc south of Kudi mark the resumption of north-dipping subduction before final collision with the incoming Qiangtang block. Comparison with the Lapeiquan ophiolite in the Eastern Kunlun assists regional correlation along this Palaeozoic orogen and constrains Cenozoic displacement of the Altyn Tagh fault.

Mesozoic and Cenozoic tectonics of the eastern and central Alaska Range: Progressive basin development and deformation in a suture zone

Abstract: Analysis of late Mesozoic and Cenozoic sedimentary basins, metamorphic rocks, and major faults in the eastern and central Alaska Range documents the progressive development of a suture zone that formed as a result of collision of an island-arc assemblage (the Wrangellia composite terrane) with the former North American continental margin. New basin-analysis, structural, and geochronologic data indicate the following stages in the development of the suture zone: (1) Deposition of 3–5 km of Upper Jurassic–Upper Cretaceous marine strata (the Kahiltna assemblage) recorded the initial collision of the island-arc assemblage with the continental margin. The Kahiltna assemblage exposed in the northern Talkeetna Mountains represents a Kimmeridgian–Valanginian backarc basin that was filled by northwestward-flowing submarine-fan systems that were transporting sediment derived from Mesozoic strata of the island-arc assemblage. The Kahiltna assemblage exposed in the southern Alaska Range represents a Valanginian–Cenomanian remnant ocean basin filled by west-southwestward–flowing submarine-fan systems that were transporting sediment derived from Paleozoic continental-margin strata uplifted in the along-strike suture zone. A belt of retrograde metamorphism and a regional anticlinorium developed along the continental margin from 115 to 106 Ma, roughly coeval with the end of widespread deposition in the Kahiltna sedimentary basins. (2) Metamorphism of submarine-fan deposits of the Kahiltna ba sin, located near the leading edge of the island-arc assemblage, occurred at ca. 74 Ma, as determined from a new U-Pb zircon age for a synkinematic sill. Coeval with metamorphism of deposits of the Kahiltna basin in the southern part of the suture zone was development of a thrust-top basin, the Cantwell basin, in the northern part of the suture zone. Geologic mapping and compositional data suggest that the 4 km of Upper Cretaceous nonmarine and marginal marine sedimentary strata in this basin record regional subaerial uplift of the suture zone. (3) Shortening and exhumation of the suture zone peaked from 65 to 60 Ma on the basis of metamorphic and geochronologic data. In the southern part of the suture zone, submarine-fan deposits of the Kahiltna basin, which had been metamorphosed to kyanite schists at ∼25 km depth and ∼650 °C, were exhumed and cooled through the biotite closure temperature (∼300 °C) by ca. 62 Ma. In the northern part of the suture zone, this time period was marked by shortening, uplift, and erosion of sedimentary strata of the Cantwell basin. (4) From 60 to 54 Ma, ∼3 km of volcanic strata were deposited over deformed sedimentary strata of the Cantwell basin, and several granitic plutons (the McKinley sequence) were emplaced along the suture zone. (5) Following igneous activity, strike-slip displacement occurred from ca. 54 to 24 Ma along the Denali fault system, which had developed in the existing suture zone. Late Eocene–Oligocene strike-slip displacement resulted in the formation of several small sedimentary basins along the Denali fault system. (6) Regional transpressive shortening characterized the suture zone from ca. 24 Ma to the present. Flexural subsidence, related to regional shortening, is represented by late Eocene to Holocene nonmarine deposits of the Tanana foreland basin. Regional subsidence resulted in Miocene coal seams up to 20 m thick and well-developed lacustrine deposits. Overlying the Miocene deposits are ∼1.2 km of Pliocene and Holocene conglomeratic deposits. Compositional and paleocurrent data from these younger deposits record regional Neogene uplift of the suture zone and recycling of detritus from older basins to the south that had become incorporated into the uplifted suture zone. Geologic mapping of major thrust faults along the northern and southern margins of the suture zone documents Paleozoic strata thrust over both Pliocene fluvial deposits and Quaternary glacial deposits of the Tanana basin. These mapping relationships provide evidence that regional shortening continues to the present in the eastern and central Alaska Range.

Caltepec fault zone: An Early Permian dextral transpressional boundary between the Proterozoic Oaxacan and Paleozoic Acatlán complexes, southern Mexico, and regional tectonic implications

Abstract: [1] The tectonic boundary between the Grenville-age Oaxacan and Paleozoic Acatlan crystalline complexes in southern Mexico, named the Caltepec fault zone (CFZ), is characterized for the first time as a dextral transpressional, NNW trending and ENE dipping ductile fault zone of Early Permian age. The complexes are welded by a syntectonic magmatic epidote-bearing granite along the entire length of the CFZ. From east to west, the 2–6 km wide CFZ consists of disrupted and retrograded banded gneisses of the Oaxacan complex, quartz-feldspar mylonite, and the syntectonic magmatic epidote-bearing Cozahuico granite (CZG) with huge xenoliths (up to several kilometers long and up to 600 m wide) of the Proterozoic gneisses, thrust westward over metasedimentary tectonites of the Acatlan complex. The CZG shows magmatic fabrics that represent a transition to solid-state deformation characterized by subvertical foliation, subhorizontal NNE and SSE dipping mineral stretching lineation and dextral kinematics. The megaxenoliths underwent partial melting developing banded migmatites with layers of epidote-bearing granitic neosome. The parallelism of fabrics in these anatexitic rocks and in the enclosing deformed granite suggests that ductile deformation, migmatization of xenolithic gneisses, and granite emplacement along the CFZ were coeval. The neosome yielded a U-Pb zircon concordant age of 275.6 ± 1 Ma probably dating the peak of the tectonothermal event. We interpret the CFZ as a major terrane boundary accommodating transpressional interaction between the Acatlan and Oaxaquia blocks, which were amalgamated in an oblique convergent setting by Early Permian time, as the leading edge of Gondwana impinged onto the southern margin of Laurentia along the Marathon-Ouachita suture to form Pangea.

Paleozoic–early Mesozoic gold deposits of the Xinjiang Autonomous Region, northwestern China

Abstract: The late Paleozoic–early Mesozoic tectonic evolution of Xinjiang Autonomous Region, northwestern China provided a favorable geological setting for the formation of lode gold deposits along the sutures between a number of the major Eastern Asia cratonic blocks. These sutures are now represented by the Altay Shan, Tian Shan, and Kunlun Shan ranges, with the former two separated by the Junggar basin and the latter two by the immense Tarim basin. In northernmost Xinjiang, final growth of the Altaid orogen, southward from the Angara craton, is now recorded in the remote mid- to late Paleozoic Altay Shan. Accreted Early to Middle Devonian oceanic rock sequences contain typically small, precious-metal bearing Fe–Cu–Zn VMS deposits (e.g. Ashele). Orogenic gold deposits are widespread along the major Irtysh (e.g. Duyolanasayi, Saidi, Taerde, Kabenbulake, Akexike, Shaerbulake) and Tuergen–Hongshanzui (e.g. Hongshanzui) fault systems, as well as in structurally displaced terrane slivers of the western Junggar (e.g. Hatu) and eastern Junggar areas. Geological and geochronological constraints indicate a generally Late Carboniferous to Early Permian episode of gold deposition, which was coeval with the final stages of Altaid magmatism and large-scale, right-lateral translation along older terrane-bounding faults. The Tian Shan, an exceptionally gold-rich mountain range to the west in the Central Asian republics, is only beginning to be recognized for its gold potential in Xinjiang. In this easternmost part to the range, northerly- and southerly-directed subduction/accretion of early to mid-Paleozoic and mid- to late Paleozoic oceanic terranes, respectively, to the Precambrian Yili block (central Tian Shan) was associated with 400 to 250 Ma arc magmatism and Carboniferous through Early Permian gold-forming hydrothermal events. The more significant resulting deposits in the terranes of the southern Tian Shan include the Sawayaerdun orogenic deposit along the Kyrgyzstan border and the epithermal and replacement deposits of the Kanggurtag belt to the east in the Chol Tagh range. Gold deposits of approximately the same age in the Yili block include the Axi hot springs/epithermal deposit near the Kazakhstan border and a series of small orogenic gold deposits south of Urumqi (e.g. Wangfeng). Gold-rich porphyry copper deposits (e.g. Tuwu) define important new exploration targets in the northern Tian Shan of Xinjiang. The northern foothills of the Kunlun Shan of southern Xinjiang host scattered, small placer gold deposits. Sources for the gold have not been identified, but are hypothesized to be orogenic gold veins beneath the icefields to the south. They are predicted to have formed in the Tianshuihai terrane during its early Mesozoic accretion to the amalgamated Tarim–Qaidam–Kunlun cratonic block.