Sverre Planke

Release of methane from a volcanic basin as a mechanism for initial Eocene global warming

A 200,000-yr interval of extreme global warming marked the start of the Eocene epoch about 55 million years ago. Negative carbon- and oxygen-isotope excursions in marine and terrestrial sediments show that this event was linked to a massive and rapid (∼10,000 yr) input of isotopically depleted carbon. It has been suggested previously that extensive melting of gas hydrates buried in marine sediments may represent the carbon source and has caused the global climate change. Large-scale hydrate melting, however, requires a hitherto unknown triggering mechanism. Here we present evidence for the presence of thousands of hydrothermal vent complexes identified on seismic reflection profiles from the Vøring and Møre basins in the Norwegian Sea. We propose that intrusion of voluminous mantle-derived melts in carbon-rich sedimentary strata in the northeast Atlantic may have caused an explosive release of methane—transported to the ocean or atmosphere through the vent complexes—close to the Palaeocene/Eocene boundary. Similar volcanic and metamorphic processes may explain climate events associated with other large igneous provinces such as the Siberian Traps (∼250 million years ago) and the Karoo Igneous Province (∼183 million years ago).

Seismic characteristics and distribution of volcanic intrusions and hydrothermal vent complexes in the Vøring and Møre basins

A voluminous magmatic complex was emplaced in the Voring and More basins during Paleocene/Eocene continental rifting and break-up in the NE Atlantic. This intrusive event has had a significant impact on deformation, source-rock maturation and fluid flow in the basins. Intrusive complexes and associated hydrothermal vent complexes have been mapped on a regional 2D seismic dataset ( c .150 000 km) and on one large 3D survey. The extent of the sill complex is at least 80 000 km 2 , with an estimated total volume of 0.9 to 2.8 × 10 4 km 3 . The sheet intrusions are saucer-shaped in undeformed basin segments. The widths of the saucers become larger with increasing emplacement depth. More varied intrusion geometries are found in structured basin segments. Some 734 hydrothermal vent complexes have been identified, although it is estimated that 2–3000 vent complexes are present in the basins. The vent complexes are located above sills and were formed as a direct consequence of the intrusive event by explosive eruption of gases, liquids and sediments, forming up to 11 km wide craters at the seafloor. The largest vent complexes are found in basin segments with deep sills (3–9km palaeodepth). Mounds and seismic seep anomalies located above the hydrothermal vent complexes suggest that the vent complexes have been re-used for vertical fluid migration long after their formation. The intrusive event mainly took place just prior to, or during, the initial phase of massive break-up volcanism (55.0–55.8Ma). There is also evidence for a minor Upper Paleocene volcanic event documented by the presence of 20 vent complexes terminating in the Upper Paleocene sequence and the local presence of extrusive volcanic rocks within the Paleocene sequence.

NE Atlantic continental rifting and volcanic margin formation

Abstract Deep seismic data from the Hatton-Rockall region, the mid-Norway margin and the SW Barents Sea provide images of the crustal structure that make it possible to estimate the relative amounts of crustal thinning for the Late Jurassic-Cretaceous and Maastrichtian-Paleocene NE Atlantic rift episodes. In addition, plate reconstructions illustrate the relative movements between Eurasia and Greenland back to Mid-Jurassic time. The NE Atlantic rift system developed as a result of a series of rift episodes from the Caledonian orogeny to early Tertiary time. The Late Palaeozoic rifting is poorly constrained, particularly with respect to timing. However, rifted basin geometries, inferred to be of this age, are observed at depth in seismic data on the flanks of the younger rift structures. Intra-continental rifting in Late Jurassic-Cretaceous times caused c. 50–70 km of crustal extension and subsequent Cretaceous basin subsidence from the Rockall Trough-North Sea areas in the south, to the SW Barents Sea in the north. In late Early to early Late Cretaceous times, new rifting occurred in the Rockall Trough and Labrador Sea associated with the northward propagation of North Atlantic sea-floor spreading. When sea-floor spreading was approached in the Labrador Sea the Rockall rift apparently became extinct. The final NE Atlantic rift episode was initiated near the Campanian-Maastrichtian boundary, lasted until continental separation near the Paleocene-Eocene transition, and caused c. 140 km extension. The late syn-rift and the earliest sea-floor spreading periods were affected by widespread igneous activity across a c. 300 km wide zone along the rifted plate boundary. The deep seismic data provide lower-crustal structural geometries that represent boundary conditions for a better mapping and understanding of the extensional thinning of the crust. The crustal geometries question extension estimates previously made from basin subsidence analysis, and aid in the definition of bodies of magmatic underplating beneath the outer volcanic margins.

Hydrothermal vent complexes associated with sill intrusions in sedimentary basins

Abstract Subvolcanic intrusions in sedimentary basins cause strong thermal perturbations and frequently cause extensive hydrothermal activity. Hydrothermal vent complexes emanating from the tips of transgressive sills are observed in seismic profiles from the Northeast Atlantic margin, and geometrically similar complexes occur in the Stormberg Group within the Late Carboniferous-Middle Jurassic Karoo Basin in South Africa. Distinct features include inward-dipping sedimentary strata surrounding a central vent complex, comprising multiple sandstone dykes, pipes, and hydrothermal breccias. Theoretical arguments reveal that the extent of fluid-pressure build-up depends largely on a single dimensionless number (Ve) that reflects the relative rates of heat and fluid transport. For Ve >> 1, ‘explosive’ release of fluids from the area near the upper sill surface triggers hydrothermal venting shortly after sill emplacement. In the Karoo Basin, the formation of shallow (< 1 km) sandstone-hosted vents was initially associated with extensive brecciation, followed by emplacement of sandstone dykes and pipes in the central parts of the vent complexes. High fluid fluxes towards the surface were sustained by boiling of aqueous fluids near the sill. Both the sill bodies and the hydrothermal vent complexes represent major perturbations of the permeability structure of the sedimentary basin, and are likely to have long time-scale effects on its hydrogeological evolution.

Structure and evolution of hydrothermal vent complexes in the Karoo Basin, South Africa

The Karoo large igneous province, formed at c. 183 Ma, is characterized by the presence of voluminous basaltic intrusive complexes within the Karoo Basin, extrusive lava sequences and hydrothermal vent complexes. These last are pipe-like structures, up to several hundred metres in diameter, piercing the horizontally stratified sediments of the basin. Detailed mapping of two sediment-dominated hydrothermal vent complexes shows that they are composed of sediment breccias and sandstone. The breccias cut and intrude tilted host rocks, and are composed of mudstone and sandstone fragments with rare dolerite boulders. Sandstone clasts in the breccias are locally cemented by zeolite, which represents the only hydrothermal mineral in the vent complexes. Our data document that the hydrothermal vent complexes were formed by one or a few phreatic events, leading to the collapse of the surrounding sedimentary strata. We propose a model in which hydrothermal vent complexes originate in contact metamorphic aureoles around sill intrusions. Heating and expansion of host rock pore fluids resulted in rapid pore pressure build-up and phreatic eruptions. The hydrothermal vent complexes represent conduits for gases and fluids produced in contact metamorphic aureoles, slightly predating the onset of the main phase of flood volcanism.

Formation of saucer-shaped sills

Abstract We have developed a coupled model for sill emplacement in sedimentary basins. The intruded sedimentary strata are approximated as an elastic material modelled using a discrete element method. A non-viscous fluid is used to approximate the intruding magmatic sill. The model has been used to study quasi-static sill emplacement in simple basin geometries. The simulations show that saucer-shaped sill complexes are formed in the simplest basin configurations defined as having homogeneous infill and initial isotropic stress conditions. Anisotropic stress fields are formed around the sill tips during the emplacement due to uplift of the overburden. The introduction of this stress asymmetry leads to the formation of transgressive sill segments when the length of the horizontal segment exceeds two to three times the overburden thickness. New field and seismic observations corroborate the results obtained from the modelling. Recent fieldwork in undeformed parts of the Karoo Basin, South Africa, shows that saucer-shaped sills are common in the middle and upper parts of the basin. Similar saucer shaped sill complexes are also mapped on new two- and three-dimensional seismic data offshore of Mid-Norway and on the NW Australian shelf, whereas planar and segmented sheet intrusions are more common in structured and deep basin provinces.

Seismic response and construction of seaward dipping wedges of flood basalts: Vøring volcanic margin

Geological and geophysical data from the >900 m of volcanic basement drilled at Ocean Drilling Program Site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins. The main part of the drilled section, corresponding to a prominent seaward dipping reflector sequence, consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments. The basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity, pore aspect ratio distribution and alteration. Stacks of laterally continuous basalt flows appear to have thin-layer transverse isotropic properties for typical wavelengths in multichannel seismic data. Vertical seismic profiling and average sonic log velocities are similar, 3.77 km/s and 3.88 km/s, respectively, while comparable refraction velocities are 10–20% higher. Synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces, though the most continuous reflectors are related to thick flows. Seismic models based on the characteristic velocity and density basalt flow distributions and Site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows. The base of the dipping reflector sequence, reflector K, correlates with flows in the lower part of upper series basalts, while the transition from basaltic to underlying dacitic/andesitic lavas correlates with a locally defined reflector. A model for the breakup related volcanism includes (1) prebreakup dacitic/andesitic volcanism, (2) early breakup basaltic volcanism infilling the prebreakup relief, (3) main breakup stage with intense, focused volcanism, large subsidence and lava pounding, and (4) late breakup volcanism during a period of decreased subsidence and local off-axis activity.

Seismic volcanostratigraphy of the Norwegian margin : constraints on tectonomagmatic break-up processes

Voluminous volcanism characterized Early Tertiary continental break-up on the mid-Norwegian continental margin. The distribution of the associated extrusive rocks derived from seismic volcanostratigraphy and potential field data interpretation allows us to divide the Møre, Vøring and Lofoten–Vesterålen margins into five segments. The central Møre Margin and the northern Vøring Margin show combinations of volcanic seismic facies units that are characteristic for typical rifted volcanic margins. The Lofoten–Vesterålen Margin, the southern Vøring Margin and the area near the Jan Mayen Fracture Zone show volcanic seismic facies units that are related to small-volume, submarine volcanism. The distribution of subaerial and submarine deposits indicates variations of subsidence along the margin. Vertical movements on the mid-Norwegian margin were primarily controlled by the amount of magmatic crustal thickening, because both the amount of dynamic uplift by the Icelandic mantle plume and the amount of subsidence due to crustal stretching were fairly constant along the margin. Thus, subaerial deposits indicate a large amount of magmatic crustal thickening and an associated reduction in isostatic subsidence, whereas submarine deposits indicate little magmatic thickening and earlier subsidence. From the distribution of volcanic seismic facies units we infer two main reasons for the different amounts of crustal thickening: (1) a general northward decrease of magmatism due to increasing distance from the hot spot and (2) subdued volcanism near the Jan Mayen Fracture Zone as a result of lateral lithospheric heat transport and cooling of the magmatic source region. Furthermore, we interpret small lateral variations in the distribution of volcanic seismic facies units, such as two sets of Inner Seaward Dipping Reflectors on the central Vøring Margin, as indications of crustal fragmentation.

Crustal structure off Norway, 62° to 70° north

Abstract Extensive geophysical surveys have been undertaken on the volcanic passive continental margin offshore Norway between 62° and 70°N during the last 25 years. Three main margin segments have been identified, the Lofoten-Vesteralen Margin, the Voring Margin and the More Margin. The main features of the margins are prominent marginal highs, including seaward dipping reflector sequences and an up to 22 km thick volcanic and transitional crust, prominent escarpments (the Voring Plateau Escarpment and the Faeroe-Shetland Escarpment), and up to 12 km deep post-Jurassic sedimentary basins east of the escarpments. Velocity-depth solutions from about 250 sonobuoys, expanding spread profiles and refraction profiles have been compiled and contoured. Isovelocity horizon contour maps and velocity transects outline a crust which broadly thickens from an oceanic crust with a normal oceanic-type velocity structure to a ca. 35 km thick continental crust with a continental velocity structure, beneath the Norwegian coast. Anomalous features include local crustal thickening below the More and Vering marginal highs, and high-velocity bodies in the lower crust in the extension of the Precambrian Lofoten-Vesteralen archipelago. The free-air SEASAT-derived gravity anomalies show a good correlation with the high-velocity bodies, and show prominent NE-trending highs from the Rockall Plateau/Porcupine Plateau region, over the More, Voring and Lofoten-Vesteralen margins, to the southwestern Barents Sea.

Extension, crustal structure and magmatism at the outer Vøring Basin, Norwegian margin

Regional analysis of new 2D and 3D multichannel seismic data has improved interpretation of the crustal configuration and structural style along the Norwegian margin. Five domains with different structural styles and evolutions are defined along the outer Vøring Basin: (1) the Nyk High–Naglfar Dome; (2) the north Gjallar Ridge; (3) the Gleipne saddle; (4) the south Gjallar Ridge; (5) the Rån ridge. Early Campanian–Early Paleocene and Early to mid-Cretaceous extensional events are evidenced. Timing of deformation and structural styles observed along each segment reflect a lateral variation of the rifted system, probably affected by magma-tectonic processes. Correlation with the deep structures of the outer Vøring Basin shows that the shallow structure in that basin is directly controlled by a deep-seated, strong, high-amplitude reflection (the T reflection), marking the top of a high-velocity body (Vp >7 km s−1). The relation between the lower-crustal architecture and the subsurface basin structures has implications for the margin evolution and for the nature of the high-velocity body.