Icelandia

ABSTRACT We propose a new, sunken continent beneath the North Atlantic Ocean that we name Icelandia. It may comprise blocks of full-thickness continental lithosphere or extended, magma-inflated continental layers that form hybrid continental-oceanic lithosphere. It underlies the Greenland-Iceland-Faroe Ridge and the Jan Mayen microplate complex, covering an area of ~600,000 km2. It is contiguous with the Faroe Plateau and known parts of the submarine continental rifted margin offshore Britain. If these are included in a “Greater Icelandia,” the entire area is ~1,000,000 km2 in size. The existence of Icelandia needs to be tested. Candidate approaches include magnetotelluric surveying in Iceland; ultralong, full-crust-penetrating reflection profiling along the length of the Greenland-Iceland-Faroe Ridge; dating zircons collected in Iceland; deep drilling; and reappraisal of the geology of Iceland. Some of these methods could be applied to other candidate sunken continents that are common in the oceans.

Ulungarat Basin: Record of a major Middle Devonian to Mississippian syn-rift to post-rift tectonic transition, eastern Brooks Range, Arctic Alaska

The Ulungarat Basin of Arctic Alaska is a unique exposed stratigraphic record of the mid-Paleozoic transition from the Romanzof orogeny to post-orogenic rifting and Ellesmerian passive margin subsidence. The Ulungarat Basin succession is composed of both syn-rift and post-rift deposits recording this mid-Paleozoic transition. The syn-rift deposits unconformably overlie highly deformed Romanzof orogenic basement on the mid-Paleozoic regional angular unconformity and are unconformably overlain by post-rift Endicott Group deposits of the Ellesmerian passive margin. Shallow marine strata of Eifelian age at the base of the Ulungarat Formation record onset of rifting and limit age of the Romanzof orogeny to late Early Devonian. Abrupt thickness and facies changes within the Ulungarat Formation and disconformably overlying syn-rift Mangaqtaaq Formation suggest active normal faulting during deposition. The Mangaqtaaq Formation records lacustrine deposition in a restricted down-faulted structural low. The unconformity between syn-rift deposits and overlying post-rift Endicott Group is interpreted to be the result of sediment bypass during deposition of the outboard allochthonous Endicott Group. Within Ulungarat Basin, transgressive post-rift Lower Mississippian Kekiktuk Conglomerate and Kayak Shale (Endicott Group) are older and thicker than equivalents to the north. North of Ulungarat Basin, deformed pre-Middle Devonian rocks were exposed to erosion at the mid-Paleozoic regional uncon­formity for ~50 m.y., supplying sediments to the rift basin and broader Arctic Alaska rifted margin beyond. Although Middle Devonian to Lower Mississip­pian chert- and quartz-pebble conglomerates and sandstones across Arctic Alaska share a common provenance from the eroding ancestral Romanzof highlands, they were deposited in different tectonic settings.

The role of inheritance in forming rifts and rifted margins and building collisional orogens: a Biscay-Pyrenean perspective

The aim of this paper is to provide a conceptual framework that integrates the role of inheritance in the study of rifts, rifted margins and collisional orogens based on the work done in the OROGEN project, which focuses on the Biscay-Pyrenean system. The Biscay-Pyrenean rift system resulted from a complex multistage rift evolution that developed over a complex lithosphere pre-structured by the Variscan orogenic cycle. There is a general agreement that the Pyrenean-Cantabrian orogen resulted from the reactivation of an increasingly mature rift system along-strike, ranging from a mature rifted margin in the west to an immature and segmented hyperextended rift in the east. However, different models have been proposed to explain the preceding syn-rift evolution and its influence on the subsequent reactivation. Results from the OROGEN project show a sequential reactivation of rift inherited decoupling horizons and identify the specific role of exhumed mantle, hyperextended and necking domains during reactivation. They also highlight the contrasting fate of segment centres vs. segment boundaries during convergence, explaining the non-cylindricity of internal parts of collisional orogens. Results from the OROGEN project also suggest that the role of inheritance is more important during the initial stages of subduction and collision, which may explain the complexity of internal parts of orogenic systems. In contrast, once tectonic systems get more mature, orogenic evolution becomes mostly controlled by first-order physical processes as described in the Coulomb Wedge theory for instance. This may account for the simpler and more continuous architecture of external parts of collisional orogens. It may also explain why most numerical models can reproduce mature orogenic and rift architectures with better accuracy compared to the initial stages of such systems. Thus, while inheritance may not explain steady-state processes, it is a prerequisite for comprehending the initial stages of tectonic systems. The new concepts developed from the OROGEN research are now ready to be tested at other orogenic systems that result from the reactivation of rifted margins, such as the Alps, the Colombian cordilleras and the Caribbean, Taiwan, Oman, Zagros or Timor.

Arctic Alaska Basin, Hanna Trough, and Beaufortian Rifted Margin Composite Tectono-Sedimentary Elements

The Arctic Alaska region includes three composite tectono-sedimentary elements (CTSEs): the (1) Arctic Alaska Basin (AAB), (2) Hanna Trough (HT), and (3) Beaufortian Rifted Margin (BRM) CTSEs. These CTSEs comprise Mississippian to Lower Cretaceous (Neocomian) strata beneath much of the Alaska North Slope, the Chukchi Sea and westernmost North Slope, and Beaufort Sea, respectively. These sedimentary successions rest on Devonian and older sedimentary and metasedimentary rocks, considered economic basement, and are overlain by Cretaceous to Cenozoic syn- and post-tectonic strata deposited in the foreland of the Chukotka and Brooks Range orogens and in the Amerasia Basin. (1) The Mississippian-Neocomian AAB CTSE includes two TSEs: (a) The Ellesmerian Platform TSE comprises mainly shelf strata of Mississippian to Middle Jurassic age and includes a relatively undeformed domain in the north and a fold-and-thrust domain in the south. (b) The Beaufortian Rift Shoulder TSE includes Middle Jurassic to Neocomian deposits related to rift-shoulder uplift. (2) The HT CTSE includes four TSEs: (a) The Ellesmerian Syn-Rift TSE comprises Late Devonian(?) to Middle Mississippian growth strata deposited in grabens and half grabens during intracontinental rifting. (b) The Ellesmerian-Beaufortian Sag-Basin TSE comprises Middle Mississippian to Upper Triassic strata deposited in a sag basin following cessation of rifting. (c) The Beaufortian Syn-Rift TSE comprises Jurassic to Neocomian graben-fill deposits related to rifting in the Amerasia and North Chukchi Basins. (d) The Beaufortian Rift-Shoulder TSE comprises Jurassic to Neocomian strata related to rifting and deposited outside rift basins. (3) The BRM CTSE includes two TSEs: (a) The Beaufortian Syn-Rift TSE comprises Middle Jurassic to Neocomian syn-rift strata deposited on attenuated continental crust associated with opening of the Amerasia Basin. (b) The Ellesmerian Platform TSE comprises mainly shelf strata of Mississippian to Middle Jurassic age that lie beneath Beaufortian syn-rift strata.The AAB, HT, and BRM CTSEs contain oil-prone source rocks in Triassic, Jurassic, and Cretaceous strata and proven reservoir rocks spanning Mississippian to Lower Cretaceous strata. A structurally high-standing area in the northern AAB CTSE, northern HT CTSE, and southernmost BRM CTSE lies in the oil window whereas all other areas lie in the gas window. Known hydrocarbon accumulations in the three CTSEs total more than 30 billion barrels of oil equivalent and yet-to-find estimates suggest a similar volume remains to be discovered.

The West Greenland Rifted Margin Composite Tectono-Sedimentary Element

The West Greenland Rifted Margin Composite Tectono-Sedimentary Element is composed of two distinct sedimentary accumulations: (1) pre-rift (pre-Cretaceous) and (2) syn-rift (Early Cretaceous-Danian), which represent individual tectono-sedimentary elements. The pre-rift tectono-sedimentary element includes the oldest sedimentary basins poorly known offshore, which mainly evolved in a cratonic platform setting and with initial rifting. In contrast, Cretaceous to lower Paleocene sedimentary basins of the syn-rift tectono-sedimentary element are known from several wells, outcrops and sea-bed samples, and basins are mapped in most parts of the margin. This element is the most prospective part of the margin, where hydrocarbon shows occur in some wells and seeps are known from outcrops in central West Greenland.

Highly magmatic breakup LIP centers – a revisit to the East Greenland VRM

Decompressional release of magma at continental triple rift breakup LIP centers, above mantle plume stems, result in highly magmatic settings. As a particularly well exposed example, it is proposed that the East Greenland coastal dyke swarm preserves a structural record of how dyke dilations versus tectonic extension increased upon approaching its Kangerlussuaq triple rift center. Such more magmatic breakup is reflected by how abruptly its volcanic rifted margin transitions into 100% dykes, and in this case up to 100 km farther inland than its geophysically determined continent-ocean boundary. Correspondingly high magma flux through an igneous Kap Edward Holm center sustained the lateral injection of up to 150 km-long dykes, evidenced by increased cut-off dyke thicknesses - below which there is an anomalously low abundance of thinner dykes - that conform to the cube root of their thermal arrest distance. Only the thickest and thereby longest dyke injections linked up with a more southerly located igneous Imilik center of an en echelon offset dyke swarm, the complex transition into which is also addressed. This highly magmatic central plumbing system is further compared to similar volcanic zones across Iceland and other post-Paleozoic breakup LIPs, in order to contextualize its importance.

Extension and early orogenic inversion along the basal detachment of a hyper-extended rifted margin: an example from the Central Pyrenees (France)

The North Pyrenean Zone (NPZ) inverts remnants of an Aptian-Cenomanian rifting during which subcontinental mantle was exhumed. These remnants contain a syn-rift HT-LP metamorphic domain, the Internal Metamorphic Zone (IMZ). New field and RSCM data and structural cross-sections constrain the structural and metamorphic relationships between the IMZ and the underlying low-grade NPZ. The IMZ is a tectonic nappe that overthrusts the European margin along the 3M Fault. Along this contact, the Tuc de Haurades peridotite is surrounded by tectonic breccia composed of ductilely deformed carbonate and sparse lherzolite clasts that passes upward into foliated marbles. Marbles contain top-to-south ductile shear, recording ongoing extensional deformation that marks the onset of HT metamorphism. During Early Cretaceous rifting, European Mesozoic sedimentary cover metamorphosed and its base brecciated as it slid basinward on Triassic salt onto exhumed mantle. As the exhumed mantle domain closed during early convergence, the detached metamorphosed cover was transported northward and thrust into the distal European margin, sampling lherzolite tectonic lenses. This triggered the first tectonic loading on the European plate. This study highlights the role of the IMZ in the early Pyrenean orogenic phase and gives new insights on the E-W diversity of structural setting of the NPZ peridotites.Table with RSCM temperatures and original and high quality photographs of the samples are available on the GSL Figshare portal https://doi.org/10.6084/m9.figshare.c.5539260.

Rheological inheritance controls the formation of segmented rifted margins in cratonic lithosphere

Observations from rifted margins reveal that significant structural and crustal variability develops through the process of continental extension and breakup. While a clear link exists between distinct margin structural domains and specific phases of rifting, the origin of strong segmentation along the length of margins remains relatively ambiguous and may reflect multiple competing factors. Given that rifting frequently initiates on heterogenous basements with a complex tectonic history, the role of structural inheritance and shear zone reactivation is frequently examined. However, the link between large-scale variations in lithospheric structure and rheology and 3-D rifted margin geometries remains relatively unconstrained. Here, we use 3-D thermo-mechanical simulations of continental rifting, constrained by observations from the Labrador Sea, to unravel the effects of inherited variable lithospheric properties on margin segmentation. The modelling results demonstrate that variations in the initial crustal and lithospheric thickness, composition, and rheology produce sharp gradients in rifted margin width, the timing of breakup and its magmatic budget, leading to strong margin segmentation.

Melt volume at Atlantic volcanic rifted margins controlled by depth-dependent extension and mantle temperature

Breakup volcanism along rifted passive margins is highly variable in time and space. The factors controlling magmatic activity during continental rifting and breakup are not resolved and controversial. Here we use numerical models to investigate melt generation at rifted margins with contrasting rifting styles corresponding to those observed in natural systems. Our results demonstrate a surprising correlation of enhanced magmatism with margin width. This relationship is explained by depth-dependent extension, during which the lithospheric mantle ruptures earlier than the crust, and is confirmed by a semi-analytical prediction of melt volume over margin width. The results presented here show that the effect of increased mantle temperature at wide volcanic margins is likely over-estimated, and demonstrate that the large volumes of magmatism at volcanic rifted margin can be explained by depth-dependent extension and very moderate excess mantle potential temperature in the order of 50–80 °C, significantly smaller than previously suggested.