Abstract This paper synthesizes the framework and geological evolution of the Arctic Alaska–Chukotka microplate (AACM), from its origin as part of the continental platform fringing Baltica and Laurentia to its southward motion during the formation of the Amerasia Basin (Arctic Ocean) and its progressive modification as part of the dynamic northern palaeo-Pacific margin. A synthesis of the available data refines the crustal identity, limits and history of the AACM and, together with regional geological constraints, provides a tectonic framework to aid in its pre-Cretaceous restoration. Recently published seismic reflection data and interpretations, integrated with regional geological constraints, provide the basis for a new crustal transect (the Circum-Arctic Lithosphere Evolution (‘CALE’) Transect C) linking the Amerasia Basin and the Pacific margin along two paths that span 5100 km from the Lomonosov Ridge (near the North Pole), across the Amerasia Basin, Chukchi Sea and Bering Sea, and ending at the subducting Pacific plate margin in the Aleutian Islands. We propose a new plate tectonic model in which the AACM originated as part of a re-entrant in the palaeo-Pacific margin and moved to its present position during slab-related magmatism and the southward retreat of palaeo-Pacific subduction, largely coeval with the rifting and formation of the Amerasia Basin in its wake. Supplementary material: Supplementary material Plate 1 (herein referred to as Sup. Pl. 1) comprises Plate 1 and its included figures, which are an integral part of this paper. Plate 1 contains regional reflection-seismic-based cross sections and supporting material that collectively constitute CALE Transects C1 and C2 and form an important part of our contribution. Plate 1 is referred to in the text as Sup. Pl. 1, Transects C1 and C2 as Plate 1A and 1B, and plate figures as fig. P1.1, fig. P1.2, etc.). Supplementary material 2 contains previously unpublished geochronologic data on detrital zircon suites and igneous rocks. Supplementary material are available at https://doi.org/10.6084/m9.figshare.c.3826813

Abstract In the study area of the eastern Gulf of Guinea continental margin slope, offshore Nigeria, turbidite depositional systems are confined to slope-valley and slope-basin bathymetric lows bounded by densely faulted, structurally complex zones. Each depositional system consists of three architectural segments (in order): (1) upper slope, small-scale channel elements converging downslope, (2) single channel and nested channel elements with linear to sinuous map patterns, grading further downslope (3) slope-basin lobe and sheet elements. Incised channels and constructional levees indicate transport by turbulent flow. Comparison of the mapped seismic amplitude patterns of different sequences suggests switching of the inferred sand-prone turbidite systems from one slope valley to another through time. This is interpreted to reflect both the lateral shifting of the fluvial sediment supply on the shelf, and the local tectonic modification of slope-valley geometry.

Abstract Gravity-flow depositional systems of a portion of the Niger delta slope are imaged and documented in seismic amplitude extraction and coherency displays from a large 3-D seismic volume. The displays are extracted immediately above regionally significant sequence boundaries. Interpretation of the 3-D seismic reflection volume demonstrates the strength of combining seismic sequence stratigraphic analysis methodology with the turbidity system architectural element framework. Each of the gravity-flow systems consists of confined-flow channel-form elements grading down slope into less confined flow, lobe-to sheet-form elements. The Niger slope depositional systems are confined to slope-valley and slope-basin bathymetric lows. The valleys are bounded by densely faulted, structurally complex zones clearly imaged on coherency processed 3-D time slices but poorly imaged on 2-D profiles where they have been previously interpreted as zones of shale diapirism. Restoration of the valley systems suggests that they had very low relief paleo-bathymetric profiles similar to those observed on the present sea floor. Lower slope basins are “piggy-back” basins above toe thrusts linked to deep-seated gravity-driven, extensional failure of the slope. Each gravity-flow depositional system consists of three architectural segments: (1) upper slope, small-scale channel elements converging down slope into, (2) singlechannel and nested-channel elements with linear to sinuous map patterns, grading farther down slope into, (3) slope basin lobe and sheet elements. Three types of channel elements are observed: (1) erosional, (2) erosional-depositional, and (3) depositional. The depositional channel elements have geometries strongly suggestive of channel-levee-overbank complexes, indicating that transport must include turbidity-flow processes. In one case, the gradation from mapped channel to lobe to fan elements occurs over less than 1 km (0.65 mi). Within the sheets, lateral accretion of depositional elements suggests compensation sedimentation composed of amalgamated depositional events. Mapped amplitude patterns of the sequences suggest switching of the inferred sand-prone depositional system from one slope valley to another through time. This is interpreted to reflect both the lateral shifting of the fluvial sediment supply on the shelf, and the local tectonic modification of valley system bathymetry. The 3-D seismic reflection volume resolves the depositional systems formed by gravity-flow processes through all mapped sequences of the study area. These depositional systems occur within the stratigraphic interval immediately above seismic sequence boundaries and consist of depositional elements known to occur in other modern and ancient turbidite systems.

Abstract Triassic evaporites strongly influenced the structural development of the Haltenbanken area of off-shore Norway during Late Jurassic and Early Cretaceous time by mechanically decoupling Triassic and younger strata from older strata and basement. Many folds in the Haltenbanken area are forced folds above basement-involved normal faults. Seismic data show that they are asymmetric flexures affecting Triassic, Jurassic, and Lower Cretaceous strata above the evaporites. They commonly are cut by, or die out along strike into, basement-involved normal faults. Extensional forced folds formed, at least in part, because Triassic evaporites behaved in a ductile manner, decoupling overlying strata from underlying faulted strata and basement. Many normal faults in the Haltenbanken area are basement-detached faults that flatten within the Triassic evaporites. Seismic data show that rollover anticlines and secondary normal faults affect Triassic, Jurassic, and Lower Cretaceous strata within the hanging walls of Thèse basement-detached normal faults. Strata beneath the Triassic evaporites are unaffected by this deformation.