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The CAGE20-4 expedition on RV Helmer Hanssen targeted areas on the outer margins of the Møre and Vøring basins, with emphasis on IODP drilling proposal P944 sites. High-resolution 2D and 3D P-Cable seismic studies focused in particular on the structure of igneous deposits and how tectonic and igneous events have affected the Paleogene and global climate through gas production from organic-rich sediments and release into the atmosphere via hydrothermal venting. This expedition was part of a collaboration between CAGE and the Center for Earth Evolution and Dynamics (CEED) at the University of Oslo. The cruise may be known as: CAGE20_4

The overall goal of this expedition was the acquisition of geophysical data on the Vøring Plateau and the southernmost part of the Lofoten Basin to improve the understanding of the subsurface structures and geological processes at four sites (U1571-U1574) where scientific drilling was recently successfully completed as a part of the the International Ocean Discovery Program (IODP) Expedition 396 The cruise may be known as: CAGE22_5

Atolls are faithful recorders helping us understand eustatic variations, the evolution of carbonate production through time, and changes in magmatic hotspots activity. Several early Cretaceous Mid-Pacific atolls were previously investigated through ocean drilling, but due to the low quality of vintage seismic data available, few spatial constraints exist on their stratigraphic evolution and large-scale diagenesis. Here, we present results from an integrated core-log-seismic study at Resolution Guyot and comparison with modern and ancient analogues. We identify six seismic-stratigraphic units: (1) platform initiation with aggradation and backstepping through the Hauterivian which ended by platform emersion; (2) reflooding of the platform with progradation and aggradation through the Barremian till the early-Aptian when ocean anoxic event 1a resulted in incipient drowning; (3) platform backstepping till the mid-Aptian when the platform shifted to progradation and aggradation till the mid-Albian; (4) platform emersion; (5) reflooding with backstepping ending at the latest-Albian by platform emersion; and (6) final drowning. The stratigraphic surfaces bounding these units are coeval with some of the Cretaceous eustatic events, which suggest an eustatic control on the evolution of this atoll and confirm that several previously reported sea-level variations in the early Cretaceous are driven by eustasy. Changes in subsidence and carbonate production rates and suspected later magmatism have also impacted the stratigraphic evolution. The suspected later magmatism could lead to environmental perturbations and potentially platform demise. Contrary to previous studies, we identify two emersion events during the mid- and late-Albian which resulted in intensive meteoric dissolution and karstification. The platform margin syndepositional fractures interacted with the subaerial exposure events by focusing the dissolution which formed vertically stacked flank-margin fracture-cave system. The study gives a unique insight into the interplay between eustasy, subsidence, and volcanic activity(ies) on long-term evolution of early Cretaceous shallow-marine carbonates. It also documents the impact and distribution of hypogenic and epigenic fluid-flow in atolls serving as an analogue for isolated carbonate platforms.

A key aspect on the evolution of rifted terranes and the prospectivity of the overlying sedimentary basins is heat. Temperature determines the deformation regime of crustal and mantle rocks and, thus, the style of rifting and geometry of rift basins. The generation of hydrocarbons from organic-rich rocks and reservoir conditions depend primarily on temperature. In this study, we model the thermal–burial history of the southern Vøring Basin (Mid-Norway Margin) along a regional transect (2-D), integrating basin- and lithospheric-scale processes. A model that accounts for the main extensional pulses that shaped the Mid-Norway Margin is in good general agreement with the present–past geothermal gradients inferred from borehole temperature and maturity data and the surface heat flux measurements in the proximal and intermediate margin. This supports a near steady-state, post-rift margin setting, following the break-up in the early Eocene. Significant discrepancies are, however, observed in the distal margin, where the borehole temperatures suggest (much) higher thermal gradients than model predicted and implied by the average surface heat flux. We speculate that the higher thermal gradients may result from deep-seated (mantle dynamics) thermal anomalies and/or recurrent hydrothermalism during periods of greater tectonic stress (regional compression and glacial loading rebound) and test the implications for the maturity of the Vøring Basin. The modelling results show, for example, that, depending on the thermal model assumptions, the depth and age of the optimal mid-Late Cretaceous source-rock horizons may vary by more than 2 km and 10 Ma, respectively.

Highlights: • The Giant Gjallar Vent is still active in terms of fluid migration and faulting. • The Base Pleistocene Unconformity acts as a seal to upward fluid migration. • Seal bypass in at least one location leads to a new phase of fluid venting. The Giant Gjallar Vent (GGV), located in the Voring Basin off mid-Norway, is one of the largest (~ 5 × 3 km) vent systems in the North Atlantic. The vent represents a reactivated former hydrothermal system that formed at about 56 Ma. It is fed by two pipes of 440 m and 480 m diameter that extend from the Lower Eocene section up to the Base Pleistocene Unconformity (BPU). Previous studies based on 3D seismic data differ in their interpretations of the present activity of the GGV, describing the system as buried and as reactivated in the Upper Pliocene. We present a new interpretation of the GGV’s reactivation, using high-resolution 2D seismic and Parasound data. Despite the absence of geochemical and hydroacoustic indications for fluid escape into the water column, the GGV appears to be active because of various seismic anomalies which we interpret to indicate the presence of free gas in the subsurface. The anomalies are confined to the Kai Formation beneath the BPU and the overlying Naust Formation, which are interpreted to act as a seal to upward fluid migration. The seal is breached by focused fluid migration at one location where an up to 100 m wide chimney-like anomaly extends from the BPU up to the seafloor. We propose that further overpressure build-up in response to sediment loading and continued gas ascent beneath the BPU will eventually lead to large-scale seal bypass, starting a new phase of venting at the GGV.

Polygonal faults, mainly oriented N50, N110 and N170, are abundant in the upper part of the mud-dominated Kai Formation (upper Miocene–lower Pliocene) of the Voring Basin. A second, less-developed tier of polygonal faults, oriented N20, N80 and N140, exists at the base of the overlying Naust Formation (upper Pliocene–Present). The faults abruptly terminate upward below a thick interval of debris flows. We propose a dynamic model in which: (1) the development of polygonal faults discontinues temporarily as a result of a change in regional sedimentation, leading to inactive polygonal faults; (2) rapid emplacement of debris flows in the late Pleistocene creates a new interval of polygonal faults in the lower part of the Naust Formation immediately beneath the debris flow and some faults penetrate into the underlying Kai Formation; (3) some polygonal faults within the Kai Formation are reactivated and propagated upward into the base of the Naust Formation. The high interconnectivity between faulted layers allows the fluids to reach shallower depths, forming well-expressed pipes and pockmarks on the sea floor. The model of cessation/reactivation of polygonal faults constrains the sealing capacity of sedimentary cover over the reservoirs and helps to reconstruct the fluid migration history through the sedimentary column.