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In contrast to continent/continent collision, arc-continent collision generates very short-lived orogeny because the buoyancy-driven impedance of the subduction of continental lithosphere, accompanied by arc/suprasubduction-zone ophiolite obduction, is relieved by subduction polarity reversal (flip). This tectonic principle is illustrated by the early Ordovician Grampian Orogeny in the British and Irish Caledonides, in which a wealth of detailed sedimentologic, heavy mineral, and geochronologic data pin the Orogeny to a very short Arenig/Llanvirn event. The Orogeny, from the initial subduction of continental margin sediments to the end of postflip shortening, lasted $\\approx18$ million years (my). The collisional shortening, prograde-metamorphic phase of the Orogeny lasted 8 my, extensional collapse and exhumation of midcrustal rocks lasted 1.5 my, and postflip shortening lasted 4.5 my. Strain rates were a typical plate-boundary-zone 10-15. Metamorphism, to the second sillimanite isograd, with extensive partial melting, occurred within a few my after initial collision, indicating that conductive models for metamorphic heat transfer in Barrovian terrains are incorrect and must be replaced by advective models in which large volumes of mafic/ultramafic magma are emplaced, syn-tectonically, below and into evolving nappe stacks. Arc/continent collision generates fast and very short orogeny, regional metamorphism, and exhumation.

The Turunç Unit, which represents one of the tectonic slices within the Lycian Nappes in southwestern Anatolia, preserves the remnants derived from the northern branch of Neotethys. The unit includes basalts intercalated with pelagic limestones of middle Carnian age (early Late Triassic) based on the characteristic radiolarian assemblage of the Tetraporobrachia haeckeli Zone. The Turunç lavas reflect trace element signatures resembling those from subduction zones, displaying selective enrichment of Th and light rare earth elements over high-field strength elements and heavy rare earth elements. Considering the overall geochemical characteristics of the Turunç basalts and given that they are found to be associated with no continent-derived detritus, the Turunç lavas appear to represent fragments of a Late Triassic island arc formed on the Neotethyan oceanic lithosphere. This result is of particular importance, since it reflects the oldest subduction age obtained from the entire Neotethyan realm to date. It may further indicate that the Neotethyan oceanic lithosphere had already been formed by the early Late Triassic, thus suggesting a pre–early Late Triassic oceanization of the northern branch of Neotethys. On the basis of this, we also suggest that the initial rifting leading to the opening of the northern branch of Neotethys should have taken place during the Middle Triassic or earlier.

Despite the presence of a deep history of continental evolution from Archean to Recent and a wealth of outcrop, existing studies of detrital zircon geochronology in Morocco are limited, focused mostly in the Cambrian and Neoproterozoic Pan-African exposures in the Anti-Atlas belt. We broaden the use of this tool by surveying the detrital zircon geochronology of three stratigraphic units from three distinct Mesozoic terranes: the Tisiren unit of the Maghrebian flysch domain, the Ketama unit of the Intrarif, and the Bou Rached sandstones of the Middle Atlas Mountains. Samples were analyzed for zircon U-Pb ages using laser-ablation inductively coupled plasma mass spectrometry, and zircon fission-track (ZFT) ages were analyzed using the emerging technique of scanning electron microscope high-density fission-track analysis. In total, 259 U-Pb ages are presented, along with 60 ZFT ages. U-Pb ages reveal dominant Pan-African (Cryogenian-Ediacaran) and West African cratonic (Paleoproterozoic) signatures that are typical of northwest Africa. All samples also include significant Mesoproterozoic U-Pb populations that are hitherto unknown in Morocco and scarce in North Africa. Proposed scenarios for the provenance of these zircons include delivery to Morocco from the Amazonian craton or the African Volta basin during Ordovician glaciation and/or delivery from Avalonian terranes during the Variscan orogeny. The Bou Rached sandstones contain a significant Ordovician-Devonian U-Pb population not found in the other samples and are of an age without a viable source currently dated in Morocco. An Avalonian source is likewise suggested for these Ordovician-Devonian ages. The Ketama and Tisiren units have comparable U-Pb ages, but their preliminary ZFT cooling age distributions differ, lending support to the idea that different source regions are required for the two domains.

Most natural fold systems are not sinusoidal in profile. A widely held view is that such irregularity derives solely from inherited initial geometrical perturbations. Although, undoubtedly, initial perturbations can contribute to irregularity, we explore a different (but complementary) view in which the irregular geometry results from some material or system softening process. This arises because the buckling response of a layer (or layers) embedded in a weaker matrix is controlled in a sensitive manner by the nature of the reaction forces exerted by the deforming matrix on the layer. In many theoretical treatments of the folding problem, this reaction force is assumed to be a linear function of some measure of the deformation or deformation rate. This paper is concerned with the influence of nonlinear reaction forces such as arise from nonlinear elasticity or viscosity. Localized folds arising from nonlinearity form in a fundamentally different way than the Biot wavelength selection process. As a particular example of nonlinear behaviour, we examine the influence of axial plane structures made up of layers of different mineralogy formed by chemical differentiation processes accompanying the deformation; they are referred to as metamorphic layering. The alternating mineralogical composition in the metamorphic layers means that the embedding matrix exerts a reaction force on the folded layers that varies not only with the deflection or the velocity of deflection of the layer, but also in a periodic manner along the length of the folded layers. The influence of this spatially periodic reaction force on the development of localized and chaotic folding is explored numerically.

Three series of volcanic rocks accumulated during the Cambrian to Silurian in the metasediment-dominated Variscan basement of Sardinia. They provide a record of the changing geodynamic setting of the North Gondwana margin between Upper Cambrian and earliest Silurian. A continuous Upper Cambrian-Lower Ordovician succession of felsic submarine and subaerial rocks, dominantly transitional alkaline in character (ca. 492-480 Ma), is present throughout the Variscan nappes. Trace element data, together with Nd isotope data that point to a depleted mantle source, indicate an ensialic environment. A Middle Ordovician (ca. 465 Ma) calc-alkaline bimodal suite, restricted to the external Variscan nappes, overlies the Sardic Unconformity. Negative εNd1 values (-3.03 to -5.75) indicate that the suite is a product of arc volcanism from a variably enriched mantle. A Late Ordovician-Early Silurian (ca. 440 Ma) volcano-sedimentary cycle consists of an alkalic mafic suite in a post-Caradocian transgressive sequence. Feeder dykes cut the pre-Sardic sequence. The alkali basalts are enriched in Nb-Ta and have Zr/Nb ratios in the range 4.20-30.90 (typical of a rift environment) and positive εNd1 values that indicate a depleted mantle source. Trachyandesite lavas have trace element contents characteristic of within-plate basalt differentiates, with evidence of minor crustal contamination.

The Kalak Nappe Complex (KNC) within the northernmost Arctic Norwegian Caledonides has traditionally been interpreted as representing the tectonically shortened margin of Baltica, consisting of a Precambrian basement and a late Precambrian to Cambrian cover deposited on the margin of the Iapetus Ocean. However, new geochronology indicates a distinctly different scenario. Detrital zircon U-Pb dating, together with the magmatic and deformation history, shows that the KNC metasediments, previously considered as a single stratigraphic sequence, belong to at least two distinct successions. Metasediments of the Svaerholt Succession, within the lower (Kolvik and Olderfjord) nappes of the KNC and affected by the late Grenvillian Rigolet deformation phase, were deposited between ca. 980 and 1030 Ma, constrained by intrusive granites and the youngest detrital zircons. The Soroy Succession occurs within the Soroy-Seiland Nappe, the Havvatnet Imbricate Stack, and Veines Nappe (the upper nappes of the KNC). It was affected by the Porsanger Orogeny at or before ca. 840 Ma and contains detrital zircons as young as 910±15 Ma, and it was thus deposited between ca. 840 and 910 Ma. The Eidvageid Paragneiss, classically interpreted as the basement to the KNC metasediments, has a similar detrital zircon population to the Soroy Succession and may be correlative to it. It cannot represent the basement to the entire KNC and was affected by metamorphic events at ca. 710, 670, 560, and 520 Ma. The detrital zircon populations of both KNC successions are consistent with a Laurentia-Baltica affinity, with age peaks corresponding to Labradorian/Gothian, Pinwarian, and Grenville/Sveconorwegian events. The detrital age populations of the Soroy Succession bear strong similarity to those from the Moine Supergroup of Scotland, while the Svaerholt Succession is comparable with the Krummedal supracrustal sequence in Greenland. The provenance data are consistent with episodic amalgamation of two terranes, exotic to the Baltoscandian margin of Iapetus, which developed in successor basins to the Grenville Orogen along the indented Rodinia margin.

The age of high-elevation planation surfaces in Corsica is constrained using new apatite (U-Th)/He data, field observations, and published work (zircon fission track, apatite fission track [AFT] data and landform/stratigraphical analysis). Thermal modeling results based on AFT and (U-Th)/He data, and the Eocene sediments uncomformably overlapping the Variscan crystalline basement indicate that present-day elevated planation surfaces in Corsica are the remnants of an erosion surface formed on the basement between ∼120 and ∼60 Ma. During the Alpine collision in the Paleocene-Eocene, the Variscan crystalline basement was buried beneath a westward-thinning wedge of flysch, and the eastern portion was overridden by the Alpine nappes. Resetting of the apatite fission track thermochronometer suggests an overburden thickness of >4 km covering Variscan Corsica. Protected by soft sediment, the planation surface was preserved. In the latest Oligocene to Miocene times, the surface was re-exposed and offset by reactivated faults, with individual basement blocks differentially uplifted in several phases to elevations of, in some cases, >2 km. Currently the planation surface remnants occur at different altitudes and with variable tilt. This Corsican example demonstrates that under favorable conditions, paleolandforms typical of tectonically inactive areas can survive in tectonically active settings such as at collisional plate margins. The results of some samples also reveal some discrepancies in thermal histories modeled from combined AFT and (U-Th)/He data. In some cases, models could not find a cooling path that fit both data sets, while in other instances, the modeled cooling paths suggest isothermal holding at temperature levels just below the apatite partial annealing zone followed by final late Neogene cooling. This result appears to be an artifact of the modeling algorithm as it is in conflict with independent geological constraints. Caution should be used when cross-validating the AFT and (U-Th)/He systems both in the case extremely old terrains and in the case of rocks with a relatively simple, young cooling history.

The South Fork slide (SFS) is exposed over an area of 800 km2 in northwest Wyoming, and its evolution is connected in space and time to the adjacent and overlying 48.87 Ma Heart Mountain slide (HMS). The youngest rocks deformed as part of the SFS are the Eocene Willwood Formation sands and mudstones, which have a U/Pb detrital zircon youngest depositional age (TuffZirc calculation) of 51.80 +1.04/−1.08 Ma as well as a spectrum of older zircons that were eroded from the Sevier Highlands to the west and the Archean Beartooth uplift to the north (n = 379 U-Pb zircon ages). The SFS is overlain by allochthonous Paleozoic carbonate and Eocene volcanic rocks of the HMS. The SFS deformation involves shallowly plunging thin-skinned folds that trend northeast-southwest, where the slip surface could have been the top of the Jurassic Gypsum Springs Formation. Detailed mapping reveals the absence of any cleavage or joints and a plethora of minor structures unreported in any thin-skinned belt as well as the fact that nearly a third of the exposed allochthonous sediments are overturned. Calcite in Sundance Formation limestones is mechanically twinned, and the resultant strain analysis reveals that the predetachment Sevier layer-parallel shortening strain axes (31/31 samples; n = 1231 twins) are now in a random orientation. Synemplacement calcite veins (320°, 90°; n = 2) record a horizontal shortening strain parallel to the veins. Neither calcite strain data set has any record of a strain overprint (low negative expected values). Rare slip indicators in the basal Sundance suggest northwest-southeast motion. Poorly constrained cross sections and palinspastic restorations indicate >25 km of shortening (and uncertain transport distances), and the above evidence suggests that the SFS formed as an Eocene landslide slightly earlier than the HMS. The SFS, then, represents the largest single allochthon (terrestrial or marine) that maintained its internal stratigraphy and did not disaggregate while in motion.

The Western Gneiss Region, western Norway, consists of Paleoproterozoic crust of Baltica ancestry (Baltican Basement), partly subducted to high- and ultrahigh-pressure (HP-UHP) conditions during the Scandian Orogeny between 415 and 395 Ma. The dominant felsic gneisses carry little evidence for the HP-UHP history but were affected by amphibolite-facies reworking during exhumation. Laser ablation-ICP-MS and secondary ion mass spectrometry (SIMS) zircon U-Pb data collected in augen gneiss samples constrain the magmatic and metamorphic geochronology in this crust. Five samples from the eclogite-bearing HP-UHP basement near Molde yield intrusion ages ranging from 1644±6 to 1594±10 Ma. Two samples of the structurally underlying eclogite-free basement yield ages of 1685±18 and 1644±13 Ma, and a sample from the infolded Middle Allochthon Risberget Nappe yields an equivalent age of 1676±18 Ma. Two samples of the eclogite-bearing basement contain low Th/U neocrystallized zircon with an age of 950±26 Ma. This zircon provides the northernmost direct evidence for at least amphibolite-facies Sveco-norwegian metamorphism in unquestionable Baltica crust, close to the known \"Sveconorwegian boundary\" in the Western Gneiss Region. The Western Gneiss Region (1686-1594 Ma magmatism), the Eastern Segment of the Sveconorwegian Orogen (1795-1640 Ma magmatism), and the Idefjorden terrane hosting the type Gothian active margin magmatism (1659-1520 Ma) probably represent three distinct Proterozoic growth zones of Baltica into which Sveconorwegian reworking propagated. Samples of the eclogite-bearing basement lack Scandian neocrystallized zircon but do show partial recrystallization of zircon. Paired cathodoluminescence and electron backscatter diffraction images indicate that zircon crystals underwent crystal-plastic deformation during the Scandian subduction-exhumation cycle. They illustrate a relationship between crystal-plastic deformation by dislocation creep, fading of oscillatory growth zoning, and loss of radiogenic lead.

The postcollisional tectonic development of northeast Mozambique and subsequent cooling from high-temperature metamorphism is delineated with an extensive new set of U-Pb titanite, 40Ar/39Ar hornblende, and 40Ar/39Ar mica analyses. The complex data suggest a polyphase metamorphic history from the late Neoproterozoic to the Ordovician within the East African-Antarctic Orogen (EAAO), with marked differences between the major constituent blocks. In all the data sets, samples from the basement south of the Lurio Belt show generally younger ages than those from the north, resulting from a late metamorphic event and slow cooling between ca. 520 and 440 Ma. The ages north and south of the Lúrio Belt are consistently offset by ca. 30-70 Ma, a difference that is maintained and even appears to increase during cooling from very high temperatures to ca. 350°C. Based on the first-order assumption that all the ages are cooling ages, cooling rates in the south are estimated at ca. 7°-8°C/Ma, while those north of the Lurio Belt are faster at ca. 16°C/Ma. The data are consistent with previous geochronological, petrographic, and field data and suggest a late high-temperature/low-pressure metamorphic event that affected only the basement rocks south of the Lurio Belt and portions of the latter. This late metamorphism and subsequent delayed, slower cooling agree well with a model of elevated heat flow following lithosphere delamination in the southern part of the orogen, which also explains the observed widespread granitoid magmatism, migmatization, and renewed deformation in the southern basement.