The Geochemical and Isotopic Record of Wilson Cycles in Northwestern South America: From the Iapetus to the Caribbean

Isotopic and geochemical data delineate passive margin, rift and active margin cycles in northwestern South America since ~623 Ma, spanning from the Iapetus Wilson Cycle. Ultramafic and mafic rocks record rifting associated with the formation of the Iapetus Ocean during 623–531 Ma, while the initiation of subduction of the Iapetus and Rheic oceans is recorded by continental arc plutons that formed during 499–414 Ma, with alternating compressive and extensional stages. Muscovite 40Ar/39Ar dates suggest there may have been a phase of Carboniferous metamorphism, although this remains tentative. A Passive margin was modified by active margin magmatism that started at ~294 Ma and culminated with collisional tectonics that signaled the final stages of the amalgamation of western Pangaea. Early Pangaea fragmentation included back-arc rifting during 245–216 Ma, leading to a Pacific active margin that spanned from 213–115 Ma. Trench retreat accelerated during 144–115 Ma, forming a highly attenuated continental margin prior to the collision of the Caribbean Large Igneous Province at ~75 Ma.

In-situ laser ablation Lu–Hf geochronology of garnet across the Western Gneiss Region: Campaign-style dating of metamorphism

The development of in-situ laser ablation Lu–Hf geochronology of apatite, xenotime and garnet has opened avenues to quickly and directly date geological processes. We demonstrate the first use of campaign-style in-situ Lu–Hf geochronology of garnet across the high- to ultrahigh-pressure Western Gneiss Region in Norway. Mafic eclogites from this region have been the focus of much work, and were clearly formed during continental subduction during the Caledonian Orogen. However, abundant quartzofeldspathic and pelitic lithologies record a more complex history, with some preserving polymetamorphic age data, and most containing no indication of high-pressure mineral assemblages formed during subduction. Twenty metapelitic and felsic samples spanning 160 lateral kilometers across the Western Gneiss Region have been analysed using garnet Lu–Hf geochronology. The results reveal Caledonian ages for the majority of the garnets, suggesting some quartzofeldspathic and metapelitic lithologies were reactive and grew garnet during high- to ultrahigh-pressure metamorphism. However, two ultrahigh-pressure eclogite locations, Verpeneset and Fjørtoft, preserve both Caledonian and Neoproterozoic-aged garnets. Despite significant uncertainties on some of the Lu–Hf geochronologic ages, laser ablation Lu–Hf efficiently identifies the polymetamorphic history of parts of the Western Gneiss Region, illustrating the effectiveness of this novel analytical method for rapid mapping of metamorphic ages.Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycleSupplementary material:https://doi.org/10.6084/m9.figshare.c.5715453

The Revsegg and Kvitenut Allochthons, Scandinavian Caledonides: origins and evolutions in the Caledonian Wilson cycle

The Caledonian Orogen preserves the record of a complete Wilson cycle from rifting to continent-continent collision and orogenic collapse. The Revsegg Allochthon, the uppermost tectonostratigraphic unit of the Hardangervidda-Ryfylke Nappe Complex of the southern Scandinavian Caledonides, is an understudied example illustrating key temporal and tectonic stages in a Wilson cycle. It overlies 1600-1500 Ma gneisses of the Kvitenut Allochthon that were deformed, metamorphosed and juxtaposed onto the Dyrskard Allochthon at 1000 Ma. The Revsegg Allochthon consists of leucosome-bearing mica-schists with meta-sandstone, and amphibolite and granitoids lenses. The timing of sedimentation of metasedimentary rocks is constrained to the period 780 - 495 Ma, but its association with a 495 Ma bimodal mafic and felsic intrusive suite suggests concurrent sedimentation in a Cambrian extensional setting. The Revsegg Allochthon underwent peak metamorphism at 480-470 Ma, followed by several metamorphic stages from 460 to 440 Ma, probably at an active margin outboard of Baltica, as postulated for the eclogite-bearing Jæren Nappe to the southeast. The Revsegg Allochthon was thrust onto the Kvitenut-Dyrskard duplex from 437 Ma to 434 Ma during an early Scandian phase also recognized in the Seve Nappe. Syn-deformational pegmatites, emplaced at 428 Ma represent the final stage in the nappe stack development. Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycle

Structural style of the Languedoc Pyrenean thrust belt in relation with the inherited Mesozoic structures and with the rifting of the Gulf of Lion margin, southern France

The “Wilson cycle” involves reactivation of rifting structures during convergence-driven inversion, then thrust reactivation during post-orogenic dismantling and extension. Classic documented examples of the Wilson cycle, such as in the pyrenean orogen, are based on sequential sections normal to the orogen. However oblique convergence/divergence that involves strain partitioning, and arcuate segments of the orogen prevent simple tectonic restorations. Languedoc region (southern France) provides a case study of a complex poly-phased deformation involving a range of reactivated structures and cross-cutting relationships, acquired in response to different stress-regimes of varying orientations. We analyse and correlate the onshore-offshore structures of the Languedoc, based on reassessment of existing and newly acquired subsurface data. New results in the previously poorly documented coastal area point to the existence of unrecognized major structures that improves onshore-offshore correlations. Our results show i) the part played by the Mesozoic (early Jurassic, then mid-Cretaceous) extensional phases in the development and the localization of pyrenean-related contractional structures; ii) the control of the later Oligocene rifting of the Gulf of Lion. Restoration of the Pyrenean shortening and Oligocene rifting, constructed along sections of relevant orientation (i.e. close to perpendicular to each other) indicate minimum shortening of 26 km and extension of 14km, respectively, in the Languedoc foreland. Integration of the Pyrenean structural framework of Languedoc reveals a wide, NE-trending transfer zone linking the Iberian Pyrenees to Provence.

Evidence of salt rich Jurassic extension in the Tethyan-Pyrenean linkage zone. Pre-orogenic inheritance in the Corbières-Languedoc Transfer Zone (Corbières Arc, France)

A detailed field study of Jurassic tectono-stratigraphic architecture of the southwestern part of the Corbières-Languedoc Transfer Zone (CTZT) in the French Pyrenees demonstrates for the first time 3D variations in thickness and stratigraphic geometries on near-orthogonal syn-sedimentary structures linked to Jurassic extension with salt. Some of these structures were previously interpreted as compressional and Pyrenean in origin (Late Santonian-Eocene). Our study instead shows that these are extensional salt- related structures that were reactivated during Pyrenean compression and again during Oligo-Miocene extension. We propose that the structures evolved through a strong interaction between inherited crustal structures of the same orientation, and salt tectonics. Strong segmentation of the CLTZ supra-salt cover by oblique structures, is inherited from Jurassic and is linked to interaction between basement EW structures of the Pyrenean rift domain and NE-SW structures of the European Tethyan margin. We distinguish NE-SW trending stuctures (Cévenoles) as extensional forced folds and NE-SW trending salt ridges that developed above basement cutting faults NE-SW oriented. Salt ridges delimited the future NE-SW trending orogenic domains (retro-foreland, Frontal, Main Nappe). N110 trending Pyrenean structures are represented by the Treilles Fault, a major syn sedimentary fault that roots into Keuper evaporites. This study shows that the Corbières is a key area to better understand Pyreneo-provençal evolution along its whole Wilson cycle (rift to rift) and to better understand the processes that govern the formation of a salt-rich rift transfer zone in a strongly pre-structured crust, its positive inversion and the role of salt tectonics in different deformation phases.

Large meta-eclogite massifs within the Western Gneiss Region, Scandinavian Caledonides: Subducted ocean-continent transition?

<p>The northern part of the Western Gneiss Region (WGR) has distinctive belts of allochthonous metasediments and mafic rocks lying within tight infolds into the Baltica basement. They outcrop from the Grong-Olden Window to the Norwegian coast, possibly as far SW as Sørøyane, predominantly comprising metapelite and amphibolite with psammite, marble, calc-silicate, local large eclogite (>4km) lenses and ultramafites. These supracrustal lithotectonic units are attributed to the Blåhø Nappe, correlated with the Seve Nappe Complex (SNC) in its main outcrop in Sweden, which is considered to represent the pre-Caledonian continent-ocean transition (COT) of Baltica. They closely resemble the Lower Seve Nappe in northern Sweden where large amphibolite massifs with marbles are common, along with local eclogites. At least some have geochemical characteristics of spilitised extrusive MORB basalt in contrast to the better known, Neoproterozoic Baltoscandian Dyke Swarm in the SNC.</p><p>In the WGR near Molde a >10km long massif of such “amphibolite” at Tverrfjella commonly exhibits a relict high-P granulite precursor that has, in turn, overprinted eclogite. It encloses marble, scapolite-bearing calc-silicate, garnet peridotite (harzburgite) and Cu ores. Marble and meta-eclogite are intermixed which, along with its high Na spilitic character, suggests that the protolith was extrusive. Limited geochemical data suggest MORB composition. P-T estimates for eclogites in adjacent belts suggest UHP, possibly diamond-stable, conditions; in Sørøyane the well-known Ulsteinvik eclogite contains coesite. In the Molde area some of the mafic rocks and metasediments have partially melted. Eclogite metamorphism was Scandian in the Tverrfjell massif at 418 ± 11 Ma, with similar ages but tighter errors for adjacent belts and Ulsteinvik. These are significantly younger than ages for (U)HP metamorphism in the main SNC outcrop in Sweden, where early Ordovician subduction with a latest Ordovician granulite overprint is recorded. However, metapelites in other Blåhø-like supracrustal belts in the WGR do seem to record this earlier history as does one eclogite, consistent with the “double-dunk” hypothesis in this hinterland region. The protolith age of the metabasalts is unknown; analogy with the BDS suggests Neoproterozoic, but some zircon data from the WGR may suggest magmatic crystallisation during the Ordovician.<sup></sup>O-isotopes indicate that the marbles were Palaeozoic, rather than Proterozoic, carbonates. Overall, the available literature data show that some large mafic massifs in the WGR, with associated metasediments and peridotites, are allochthonous with respect to Baltica basement; they represent major additions of extrusive basalt to a far-distal COT or fully oceanic basin that have been subducted at least once during the Caledonian Wilson cycle. Isotopic data hint that at least some of their protoliths are unusually young. These supracrustal belts certainly merit closer attention.</p>