Atypical post-breakup magmatism along the Jan Mayen Fracture Zone: Do we need a mantle plume?

Laurent Gernigon¹, Odleiv Olesen¹, Carmen Gaina¹ & Susann Wienecke²

¹ Geological Survey of Norway (NGU), Trondheim, Laurent.Gernigon@ngu.no ; Odleiv.Olesen@ngu.no ; Carmen.Gaina@ngu.no

² StatoilHydro, Trondheim, suw@statoilhydro.com

1. Introduction

The Norwegian-Greenland Sea began to open when Norway and Greenland separated in the Early Tertiary (Figure 1). This major tectonic event was accompanied by a significant and well-known magmatic event (see Meyer et al., 2007 for a recent review). Large uncertainties still remain about the actual amount of melt formed during the breakup (Gernigon et al., 2004; see also Norwegian Volcanic Margin page). Some authors have argued that the “voluminous” breakup magmatism may reflect compositional heterogeneities and/or plate-driven dynamic processes in the upper mantle and not necessarily excess mantle temperature associated with a deep mantle plume (van Wijk et al., 2001; Foulger, 2007; Gernigon et al., 2006; Korenaga, 2004; see also Decompressional Melting During Extension of Continental Lithosphere).

Figure 1: Bathymetric map and main physiographic features of the Norwegian-Greenland Sea. The Jan Mayen Fault Zone (JMFZ), which separates the Lofoten Basin from the Norway Basin in the south consists of three distinct segments, the western (WJMFZ), eastern (EJMFZ) and central Jan Mayen Fracture zones (CJMFZ), respectively. The JMFZ is almost sub-parallel to the Greenland-Iceland-Faroes Ridge (GIFR). Click here or on Figure for enlargement.

Recent contributions have asked challenging questions about the timing, variability and origin of the atypical magmatic events that affected the Norwegian-Greenland Sea after the breakup phase (Breivik et al., 2006; Greenhalgh & Kusznir, 2007; Olesen et al., 2007; Breivik et al., 2008). These contributions concur that a clear understanding of the tectonic and magmatic history of the Norwegian oceanic domain is essential when dealing with breakup, spreading rate evolution, intraplate magmatism and the influence of deep but controversial sub-lithospheric processes that may or may not involve an Icelandic mantle plume.

The main objective of the present work is to re-examine the geophysical and tectonic setting of the Norwegian-Greenland Sea in the vicinity of the Jan Mayen Fracture Zone (JMFZ), west of the Vøring Marginal High, that significantly affected breakup magmatism (Figure 1). Changes in plate stress configuration, seen in the fabric of the oceanic crust, clearly influence the location of atypical magmatic features.

2. The Vøring Spur (VS): an intriguing oceanic feature in the Norwegian Greenland Sea

The Vøring Spur (VS) is a remarkable bathymetric high located along the trend of the JMFZ (Figures 1, 2 & 3). A recent high-resolution magnetic survey (Gernigon et al., 2009) suggests that it is an atypical oceanic feature (Figure 2B).

Figure 2: A) Bathymetry near the Vøring Spur. B) New JAS-05 aeromagnetic survey between the Vøring Marginal High and Jan Mayen island (Gernigon et al., 2009).

Figure 3: Profile AB across the Vøring Spur. This section illustrates the seismic and structural characteristics of the VS located between the EJMFZ and the prolongation of the WJMFZ (section NPD modified after Gernigon et al., 2009). Location is shown in Figure 2. Click here or on figure for enlargement.

The VS displays a negative gravity anomaly. 3D isostatic modelling suggests the presence of a deep crustal root and local compensation (Figure 4). The thick oceanic crust of the VS (16-17 km) is surprising, as typical oceanic crust usually does not exceed 7-10 km in thickness. The thickening of the crust is similar to a lower crustal body modelled beneath the Vøring Marginal High (e.g., Mjelde et al., 2007, Breivik et al., 2008).

Figure 4: Bouguer anomaly map of the eastern Norwegian Greenland Sea combined with depth contours of the Moho estimated using the ASEP algorithm of Wienecke et al. (2007). The Vøring Spur (VS) is characterised by an apparent Bouguer low and thick oceanic crust, contrasting with surrounding oceanic regions. SDRS: seaward dipping reflector basaltic sequences emplaced along the continent-ocean transition.

The existence of thick oceanic crust below the VS is evidence that anomalous melt production still persisted after the breakup of the Mid-Norwegian volcanic margin. Two explanations for this have been published recently:

• late- and post-rift underplating that accumulated in the Miocene under the pre-existing crust, as favoured by Breivik et al. (2008);
• anomalous melt accumulation (so-called "overcrusting"), emplaced beneath the VS during ridge accretion, as favoured in Gernigon et al. (2009) (Figure 5).

Figure 5: 3D crustal structure of the Vøring Spur. The oceanic root, observed beneath the VS, is interpreted as an oceanic, syn-rift, mafic feature formed during the Eocene. To avoid confusion with the Late Miocene underplating hypothesis we use the term "overcrusting" to signify anomalous but syn-spreading magmatic production.

3. Tectonic model for the origin and evolution of the VS

We propose that overcrusting could have occurred along the JMFZ crust in the Eocene. Breivik et al. (2008) suggested that partially molten mantle from the lowest part of the melt column was produced under the Aegir Ridge and captured by asthenospheric flow from a plume under Iceland, before surfacing northeast of the EJMFZ in the Miocene. This model requires that the asthenosphere retain a molten component for a long time (10-15 Ma), but the reason for such a temporal delay in extracting the melt is unclear. Most of the sedimentary sequences near the VS are not affected by significant intrusions (Figure 3). This is inconsistent with a major Late Miocene magmatic event.

We propose instead that the VS is a volcanic edifice formed during the Eocene and we suggest genetic kinematic control of the JMFZ on the melt production and distribution since breakup time. Also, magmatism is still active and anomalous along the trend of the JMFZ as shown by the stratovolcano Beerenberg on Jan Mayen island (Figure 2).

Plate reconstruction (Figure 6) shows that during the Eocene the VS was located between the Vøring Marginal High and the Traill igneous complex, located now offshore Greenland (e.g. Olesen et al., 2007). Gernigon et al. (2009) propose that the Traill igneous complex were genetically associated with the Vøring Marginal High convex volcanic rift system in a kind of triple junction initiated during the early stage of oceanic spreading. The distribution of the anomalous magmatism, observed at the VS, is explained by a leaky transform or oblique crack developing along the JMFZ.

Lithospheric weakening and thinning may have subsequently resulted in local upwelling and decompression melting of the upper mantle under the VS. Melt developed preferentially near the oceanic transform, which also created a major pathway for magma to reach the surface.

The VS or the Traill igneous complex could have behaved, at some stage, as a leaky transform axis, similar to the Terceira "rift" in the Azores Plateau (Searle, 1980; Vogt & Jung, 2004). Like the VS, its origin is still uncertain and a deep mantle plume influence is debatable (Bonatti, 1990; Yang et al., 2006).

Figure 6: a) Plate reconstruction of Greenland and Norway at C21 (~47 Ma ). This figure illustrates a triple junction between three magnetic (and magmatic) branches: (1) and (2) coincide with the location of SDR wedges along theVøring Marginal High and (3) with the Greenlandic part of the Traill igneous complex. b) A leaky transform model (e.g., Searle, 1980) can be proposed for both and the Traill igneous complex lying in the trend of the VS could have formed obliquely along the trend of the pre-existing EJMFZ. c) The Azores Plateau can be used as a modern analogue to the Jan Mayen spreading system. The situation is similar; a triple junction and volcanic traps formed along the spreading ridge and seem to be influenced by the pre-existing oceanic fracture zones. A third branch (the Terceira Rift) propagates in transtension and/or as a slowly extending rift from the spreading ridge toward the adjacent oceanic fracture zones.

4. Anomalous post-breakup magmatism along the Jan Mayen Fracture Zone: do we need a mantle plume?

Whether a plume influenced the opening of North Atlantic is still a matter of debate. Here we show that simple tectonic plate dynamics or mantle composition heterogeneities may have enhanced melt production and produced the VS.

Complex lithospheric stresses along the JMFZ can explain outbursts of magmatic activity observed along the oceanic fracture zone. Brittle weakening of the lithosphere and development of a leaky transform could explain enhanced magmatism along the JMFZ. Many authors have shown that plate boundaries such as transform faults can channel magma to the surface and they also document the prevalence of "coincidental" relationships between supposed hotspot features and pre-existing weakness zones (Beutel, 2005; see also South Atlantic page).

Behn et al. (2007) have also shown that a rheology that incorporates brittle weakening of the lithosphere along the fracture zone can explain regions of enhanced mantle upwelling and elevated temperatures under a transform. Huang et al. (2003) also provided numerical evidence that small-scale convection can develop beneath the oceanic transform itself.

5. Summary

1. The VS is an anomalous oceanic high lying north of the EJMFZ. The structure and the low Bouguer gravity signature of the VS can be explained by thick oceanic crust, which locally reaches 15 km. We propose that this thick oceanic crust formed by overcrusting, syn-accretion–during the Eocene–and not necessarily during the Miocene as previously proposed.
2. The large melt production that initiated along the Vøring Marginal High during breakup continued episodically along the trend of the JMFZ. The local increase of magma production along the JMFZ suggests that the oceanic transform acted, and still acts, as a long-lived magmatic pathway for melts in the lithosphere.
3. Plate reconstruction suggests that a triple junction, similar to the Azores Plateau system, could have initiated slightly after breakup between the Vøring Marginal High, the VS and the Traill igneous complex, now located offshore Greenland. Volcanic activity may have increased locally along a leaky transform acting as the third branch of the junction, slightly oblique to the pre-existing EJMFZ.
4. This webpage illustrates the importance of oceanic fracture zones in melt production. Processes involved along the JMFZ might provide clues to help understand the evolution of further oceanic controversial features, such as the Greenland-Iceland-Faeroes Ridge, or simply to understand better the processes involved during the breakup of the Mid-Norwegian volcanic margin. Nevertheless, more work needs to be carried out and future data acquisition is required to solve the complex magmato-tectonic puzzle of the Norwegian-Greenland Sea and the meaning of the Icelandic “anomaly”.

Acknowledgments

This paper summarises part of the recent paper Gernigon, L., Olesen, O., Ebbing, J., Wienecke, S., Gaina, C., Mogaard, J.O., Sand, M. & Myklebust, R. 2009: Geophysical insights and early spreading history in the vicinity of the Jan Mayen Fracture Zone, Norwegian-Greenland Sea. Tectonophysics, 468 185-205. The Norwegian Petroleum Directorate kindly provided seismic, high-resolution bathymetric data from the Jan Mayen-Vøring area. The grid rotation program developed by M. Smethurst was used for the plate reconstruction. We acknowledge the invitation of Gillian Foulger to present a summary of our paper in this webpage.

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