James A. Chalmers

Neogene uplift and tectonics around the North Atlantic: overview

Abstract There appear to have been at least two significant episodes of uplift around the North Atlantic during the Cenozoic, and in many places it is not easy to separate the two. Effects related to emplacement of the Iceland plume probably caused one episode, mostly in the Palaeogene. The second episode took place in the late Cenozoic, and comprised uplift of basin margins as well as accelerated subsidence of basin centres adjacent to the uplifted landmasses. Cenozoic uplift of Scandinavia and of the British Isles has been suggested since at least the beginning of the 20th century. However, it is only recently being recognised in the literature that a major Neogene tectonic event has affected nearly every continental margin in the area (including western and eastern Greenland) and far into the European craton. Pre-Cenozoic rocks are generally exposed onshore and the pre-Quaternary sediments offshore are generally of Neogene age. Between the two, inclined Palaeogene and older beds are truncated by erosional unconformities along many coastlines. This structural configuration is in accordance with a Neogene uplift of the continents. A variety of methods have been used to investigate uplift, erosion and redeposition: studies of maximum burial, fission tracks, geomorphology, sediment supply and structural relations. These methods each investigates only one aspect of the phenomenon, and a thorough understanding of the processes of uplift and erosion can only be achieved if results from these methods are integrated. The main mechanisms suggested in the literature for the large-scale, late Cenozoic events are: emplacement of magma in and at the base of the crust leading to isostatic uplift, flow of asthenospheric material into active diapirs, isostacy associated with glacial erosion, phase changes in the lithosphere due to pressure relief and regional compression of the lithosphere. It is premature to judge between these mechanisms because of the insufficient regional analyses carried out so far. A general model must be constrained by observations from all affected areas, it must separate the effects of Palaeogene uplift from those of Neogene uplift that reach beyond the passive margins, and also include the subsidence patterns observed adjacent to the landmasses.

Labrador Sea: the extent of continental and oceanic crust and the timing of the onset of seafloor spreading

Abstract Regional reflection seismic profiles across the Labrador Sea originally acquired in 1977 have been reprocessed and reinterpreted. Zones of different structural style have been identified. The seismic interpretations have been used to constrain magnetic modelling and oceanic crust has been confirmed from magnetic anomaly 27N and seaward. However, all attempts to model the area landward of magnetic anomaly 27N as a series of remanent magnetizations of alternating polarity have failed. Interpretations which fit the magnetic and seismic data consist of a zone of block-faulted and subsided continental crust on both the Greenland and Canadian sides, which is separated from oceanic crust by zones of continental crust intruded by and in places overlain by magnetized igneous material. It is concluded that seafloor spreading started in the Labrador Sea in the Palaeocene (Chron 27N) and that large areas under deep water formerly thought to be underlain by oceanic crust should now be considered to be continental.

New insight into the structure of the Nuussuaq Basin, central West Greenland

Abstract Interpretation of seismic and magnetic data, forward modelling of gravity profiles and a reappraisal of all available data on faults onshore provides the first revision in 30 years of our understanding of the structure of the Nuussuaq Basin, central West Greenland. In the western part of the area Mesozoic sediments at least 6 km and possibly up to 10 km thick occur in an early rift basin dominated by NS faults. Recently discovered oil in surface seeps and in shallow boreholes occurs almost exclusively in the early rift basin. In the east, sediments are thinner, and faults trend both NS and WNW-SSE, the latter parallel to shear zones in the adjoining basement area. The eastern area may be part of a Late Cretaceous thermal subsidence basin. Renewed faulting involving both reactivation of older faults and generation of new faults took place in latest Cretaceous-early Paleocene time, and was followed by extensive erosion and phases of incision and infilling of valley systems. Renewed subsidence occurred immediately prior to the eruption of extensive middle Paleocene and Eocene continental flood basalts. The final phase of faulting took place in connection with sea-floor spreading in Baffin Bay and the Labrador Sea during the Eocene. Movement of North America relative to Greenland was transferred from the Labrador Sea to Baffin Bay along a strike-slip fault system in continental crust, the Ungava transform fracture zone. A splay of this system gave rise to a prominent SW-NE fault in the western part of the basin.

Separation of Palaeogene and Neogene uplift on Nuussuaq, West Greenland

The geological record exposed on Nuussuaq, central West Greenland, shows that uplift in the Palaeocene, probably caused by impact of the Iceland plume head, was followed by kilometre-scale subsidence. Analysis of apatite fission-track and vitrinite-reflectance data from borehole samples down to 3 km depth reveals that the samples cooled from maximum palaeotemperatures between 40 and 30 Ma followed by two further cooling episodes beginning in the intervals 11–10 and 7–2 Ma. When the first cooling episode began, the samples from the neighbouring Gro-3 and Gane-1 boreholes were buried 1500–2000 m deeper than at the present day, and the palaeogeothermal gradient was 40–48 °C km−1. It is not clear whether this cooling involved exhumation or if it was due solely to reduction in heat flow and a drop in surface temperature. The two later episodes definitely involved exhumation because by then the palaeogeothermal gradient had declined to a value close to the assumed present value of 30 °C km−1, which agrees with estimates from offshore wells. The most recent cooling episode corresponds to the incision of the present-day relief (c. 1100 m) below the summits around the two boreholes. We conclude that the present-day high mountains of West Greenland were not uplifted during the Palaeogene, but are erosional remnants of a landmass uplifted during the Neogene.

Offshore evidence for Neogene uplift in central West Greenland

Abstract Multi-channel seismic lines off southern and central West Greenland show a >3-km-thick sedimentary section of mid-Eocene and younger age that dips seaward and is truncated either at the seabed or by an erosional unconformity a short distance below the seabed. This pattern indicates that there has been uplift and erosion of the section and probably of the nearby landmass. The timing of the uplift is not well constrained by borehole data, but certainly took place after the early Eocene, probably during the Neogene and possibly as late as the onset of glaciation in West Greenland in the early Pliocene. The uplift took place substantially later than the cessation of magmatism in the early Eocene and the abrupt slowing or cessation of sea-floor spreading in the Labrador Sea between Chrons 20 and 13 (middle–late Eocene). This means that, whatever the cause of the uplift, it is unlikely to be directly related to processes either of magmatic emplacement or sea-floor spreading.

The continental margin off southern Greenland: along-strike transition from an amagmatic to a volcanic margin

The southern part of the continental margin off southern West Greenland is an amagmatic margin that may have taken at least 30 and possibly more than 60 million years to form during the Cretaceous at an average extension rate of between 8.7 and 4.4 mm a -1. To its northwest and east are volcanic continental margins formed in the Early Tertiary when sea-floor spreading started above the hot North Atlantic plume head. The survival of the amagmatic margin means that plume head material could never have been present under it and therefore the plume head could not have had the circularly symmetric shape commonly depicted in the literature.

Episodic uplift and exhumation along North Atlantic passive margins: implications for hydrocarbon prospectivity

Abstract We present observations that demonstrate that the elevated passive margins around the North Atlantic were formed by episodic, post-rift uplift movements that are manifest in the high-lying peneplains that characterize the coastal mountains, in the unconformities in the adjacent sedimentary basins and in accelerated subsidence in the basin centres. Results from West Greenland show that subsidence of the rifted margin took place for c . 25 Myr after rifting and breakup in the Paleocene, as predicted by classical rift theory, but that this development was reversed by a series of uplift movements (starting at c . 35, 10 and 5 Ma) that remain unexplained. East Greenland and Scandinavia seem to have had a similar evolution of post-rift subsidence followed by uplift starting at c . 35 Ma. There was no notable fall in sea-level at this time, so the subsiding basins must have been inverted by tectonic forces. We speculate that the forces causing this phase were related to the plate boundary reorganization in the North Atlantic around Chron 13 time. One feature that these areas have in common is that uplift took place along the edges of cratons where the thickness of the crust and lithosphere changes substantially over a short distance. It may be that the lateral contrasts in the properties of the stretched and unstretched lithosphere make the margins of the cratons unstable long after rifting. These vertical movements have profound influence on hydrocarbon systems, not only in frontier areas such as West and East Greenland, where Mesozoic basins are deeply truncated and exposed onshore, but also for the understanding of near-shore hydrocarbon deposits in mature areas such as the North Sea Basin, where low-angular unconformities may represent episodes of deposition and removal of significant sedimentary sections.

An AVO Study of a Possible New Hydrocarbon Play, Offshore Central West Greenland

Since 1992, extensive oil seeps have been discovered in Cretaceous sediment and Paleogene basalt onshore central West Greenland. Offshore, the basalts are buried under younger sediments. Interpretation of seismic data offshore has shown the presence of a closed structure at top basalt level. Within the sediments above the closed structure can be seen bright reflections that have strong AVO (amplitude variation with offset) anomalies. These features may indicate the presence of hydrocarbons that have migrated through the basalts from deeper source rocks and been trapped in the sediments above the basalts.

Labrador Sea, Davis Strait, and Baffin Bay

The Labrador Sea is a small oceanic basin about 900 km wide between Green land and North America that opens to the southeast into the North Atlantic Ocean. To the north, it shallows and passes into the Davis Strait, a 300 km wide seaway leading into Baffin Bay. These seaways are flanked by typical Arctic continental shelves with banks <200 m deep separated by glacially eroded channels. Water depths in the Labrador Sea reach over 3500 m, are 500–2000 m in the Davis Strait, and reach over 2500 m in Baffin Bay.

Mountains of southernmost Norway: uplifted Miocene peneplains and re-exposed Mesozoic surfaces

The origin of the Norwegian mountains (the Scandes) is a key controversy in modern geoscience. Are they remnants from the Caledonian Orogeny, modified shoulders of late Mesozoic rifts, or are they evidence of Neogene uplifts? Our synthesis of geological data, landscape analysis and new thermochronological data from Norway south of c. 60°N, combined with previously published data from southern Sweden, reveals a four-stage history: (1) Middle Triassic and Middle Jurassic exhumation produced a weathered basement surface with a hilly relief; (2) after late Mesozoic rifting, Upper Jurassic–Oligocene sediments accumulated across most of the area; (3) early Miocene uplift and erosion to the base level of the adjacent ocean led to formation of a peneplain that extended across sedimentary basins and Caledonian rocks; the subhorizontal Hardangervidda plateau represents this peneplain; (4) early Pliocene uplift raised Hardangervidda to its present elevation of c. 1200 m above sea-level and led to re-exposure of the tilted, Mesozoic surface at lower elevations. The Southern Scandes are thus, like other elevated passive continental margins around the world, the product of post-breakup uplift. Identification of the mechanisms driving these uplifts awaits geodynamic modelling constrained by observations such as those presented in this study. Supplementary material: Table S1: AFTA data, sample details and associated thermal history interpretations; southernmost Norway. S2: analytical details and thermal history reconstructions for all apatite fission-track analysis (AFTA) samples as well as analytical data for one vitrinite reflectance sample, together with background information on the AFTA technique, are available at https://doi.org/10.6084/m9.figshare.c.4085996