Radial viscous fingering of hot asthenosphere within the Icelandic plume beneath the North Atlantic Ocean
Introduction
It is generally agreed that a substantial convective upwelling or plume centered beneath Iceland has had a significant effect on the stratigraphic evolution of the North Atlantic Ocean (White and McKenzie, 1989; Jones et al., 2012). This plume developed during Early Cenozoic times and its inception is usually linked with the appearance of basaltic magmatism at 64 Ma. It is bisected by a mid-oceanic ridge which provides a helpful window into the detailed temporal evolution of this globally significant feature (Parnell-Turner et al., 2014). Fluctuations in plume activity over the last 50 Ma are recorded in the pattern of diachronous V-shaped ridges that are imaged in the oceanic basins on either side of the Reykjanes Ridge. During the Neogene period, regional bathymetric changes associated with these fluctuations appear to have moderated overflow of Northern Component Water, the ancient precursor of North Atlantic Deep Water (Poore et al., 2011).
The present-day planform of the Icelandic plume is determined from a combination of three different sets of observations (Fig. 1). The simplest and most striking manifestation is the pattern of long wavelength (700–2500 km) free-air gravity anomalies. A positive anomaly of 30–50 mGal is centered on Iceland. Together, other anomalies form an irregular planform that reaches from Baffin Island to western Scandinavia, and from the Charlie-Gibbs fracture zone to Svalbard. The inference that this pattern of long wavelength anomalies is a manifestation of mantle convective upwelling is strengthened by the existence of significant residual depth anomalies throughout adjacent oceanic basins. Hoggard et al. (2017) built a database of seismic reflection and wide-angle profiles that they used to accurately calculate water-loaded depths to oceanic basement as a function of plate age. In this way, residual depth anomalies are determined that build upon previous analyses (White, 1997; Marquart and Schmeling, 2004). These combined results show that oceanic crust surrounding Iceland is considerably shallower than expected (Fig. 1a). For example, residual depth anomalies of up to 2 km are recorded adjacent to Iceland. This regional shallowing dies out gradually with increasing distance from Iceland. The match between residual depth measurements and long wavelength gravity anomalies is reasonable, although a notable exception is observed north of Greenland. The relationship between the gravity field and residual depth measurements suggests that the water-loaded admittance is mGal km−1, in agreement with global studies (Crosby and McKenzie, 2009).
Finally, the presence of a mantle convective anomaly is corroborated by earthquake tomographic models which suggest that an extensive and irregular patch of low shear wave velocity lies beneath the lithospheric plates (Bijwaard and Spakman, 1999; Ritsema et al., 2011). The most striking of these studies is that of Rickers et al. (2013) who use full-waveform tomography to build a high resolution shear wave velocity model of the North Atlantic region from the surface to a depth of 1300 km (Fig. 1b). A significant negative velocity anomaly of with respect to their reference model is centered beneath Iceland, in agreement with earlier studies. One notable feature of their model is the existence of narrow, slow velocity fingers that protrude beneath the fringing continental margins. Two prominent fingers reach beneath the British Isles and western Norway. In both cases, the associated negative shear wave velocity anomalies are >2% and sit within a km thick horizontal layer immediately beneath the lithospheric plate (Fig. 2). Rickers et al. (2013) show that there is a reasonable match between the loci of these fingers and long wavelength gravity anomalies. Significantly, both fingers also coincide with crustal isostatic anomalies and with the general pattern of Neogene vertical movements observed across the northwest shelf of Europe (Anell et al., 2009; Davis et al., 2012). In the southern North Sea, the fast (i.e. cooler) region between these fingers has a water-loaded subsidence anomaly of ∼500 m that grew in Neogene times and represents a significant departure from the expected thermal subsidence trajectory (Fig. 2b–d; Kooi et al., 1991). This region probably subsided as a result of small-scale convective downwelling between the two warm fingers.
Here, we combine these different geologic and geophysical observations to investigate the causes and consequences of radial fingering within the asthenospheric mantle. In a series of contributions pioneered by Weeraratne et al. (2003), it has been suggested that some combination of rectilinear viscous fingering instabilities, small-scale convection, and shear-driven upwelling may play a role in explaining the observed pattern of seismic velocity anomalies beneath the southern portion of the East Pacific Rise (Weeraratne et al., 2007; Harmon et al., 2011; Ballmer et al., 2013). Although there are significant geometric and mechanical differences, our analysis evidently builds upon these previous contributions and upon the analysis of Morgan et al. (2013).
Our approach is divided into three steps. First, we present the physical characteristics of the Icelandic plume, such as its size, shape and vigor. By combining the correlation between shear wave velocity anomalies and the pattern of regional Neogene epeirogeny with a global empirical relationship between shear wave velocity and temperature, we estimate how viscosity within the plume head spatially varies. Secondly, we compare these observations of plume behavior beneath Iceland and elsewhere with published laboratory experiments that investigate the development of radial miscible viscous fingering. Thirdly, the development of radial fingering is discussed using a suite of theoretical and heuristic arguments. We conclude by exploring the implication of our hypothesis for a small selection of well-known plumes.
Section snippets
Physical characteristics of Icelandic plume
The temperature structure of the Icelandic plume can be estimated in a variety of related ways. In the North Atlantic Ocean, a mid-oceanic spreading center transects this plume and provides the most straightforward method for determining this structure. Within the region of influence, the average thickness of oceanic crust increases from 7 to 14 km and the seabed is anomalously shallow by up to 2 km (Fig. 1; White, 1997). Both of these observations are consistent with an average temperature
Saffman–Taylor instability
When a less viscous fluid displaces a more viscous fluid, the boundary between the two fluids can become unstable and promote viscous fingering (Saffman and Taylor, 1958). A considerable amount of experimental and theoretical work has been carried out on this fingering process for a variety of geometries under different dynamic conditions. The general aim is to predict conditions under which fingering occurs and to estimate the number of fingers that develop (Homsy, 1987). Here, the relevant
Discussion
Fig. 5 captures the idealized geometry of the Icelandic plume. On Iceland itself, the putative conduit is located at Vatnajökull in southeast Iceland and has a diameter of ∼100 km (Fig. 1; Shorttle et al., 2010). We have shown that , and km for the Icelandic plume. By combining this scaling analysis with the results of laboratory experiments on radial miscible viscous fingering, we suggest that a Saffman–Taylor instability will manifest itself at the distal
Conclusions
We use a combination of geophysical and geologic observations from the North Atlantic Ocean to confirm that the Icelandic plume has an irregular planform. Sub-plate and physiographic evidence shows that about five radial fingers of hot asthenosphere protrude beneath adjacent continental margins. A quantitative comparison with appropriately scaled laboratory experiments suggests that these fingers are generated by the classic Saffman–Taylor instability. This manifestation of viscous fingering
Correspondence
Correspondence and requests for materials should be addressed to N.J. White.
Author contributions
This project was conceived and managed by NJW. CMS processed data with guidance from NJW. DP provided mathematical analysis and insight in consultation with NJW. The paper was written by NJW with contributions from CMS and DP.
Competing financial interests
The authors declare no competing financial interests.
Acknowledgments
CMS is supported by Shell Exploration and by the British Geological Survey. We thank M. Hoggard, C. Richardson, and A. Woods for their help. M. Ballmer provided a constructive review. This paper is dedicated to Maeve White. Figures were prepared using Generic Mapping Tools. Cambridge Earth Sciences contribution number esc. 3904.
References (50)
- et al.
Cenozoic uplift and subsidence in the North Atlantic region: geological evidence revisited
Tectonophysics
(2009) - et al.
Implications of grain size evolution on the seismic structure of the oceanic upper mantle
Earth Planet. Sci. Lett.
(2009) - et al.
Tomographic evidence for a narrow whole mantle plume below Iceland
Earth Planet. Sci. Lett.
(1999) - et al.
Mantle heterogeneity and off axis volcanism on young Pacific lithosphere
Earth Planet. Sci. Lett.
(2011) - et al.
Lithospheric dynamics and the rapid Pliocene-Quaternary subsidence phase in the southern North Sea basin
Tectonophysics
(1991) - et al.
New observational and experimental evidence for a plume-fed asthenosphere boundary layer in mantle convection
Earth Planet. Sci. Lett.
(2013) - et al.
The thermal structure of the lithosphere from shear wave velocities
Earth Planet. Sci. Lett.
(2006) - et al.
The dynamical origin of Hawaiian volcanism
Earth Planet. Sci. Lett.
(1999) - et al.
The Iceland–Jan Mayen plume system and its impact on mantle dynamics in the North Atlantic region: evidence from full-waveform inversion
Earth Planet. Sci. Lett.
(2013) - et al.
A plume model of transient diachronous uplift at the Earth's surface
Earth Planet. Sci. Lett.
(2008)
Geodynamics of the Yellowstone hotspot and mantle plume: seismic and GPS imaging, kinematics, and mantle flow
J. Volcanol. Geotherm. Res.
Formation of Intraplate Seamount Chains by Viscous Fingering Instabilities in the Asthenosphere Using Low Reynolds Number Miscible Fluids with a Moving Surface Boundary
Non-hotspot volcano chains produced by migration of shear-driven upwelling toward the East Pacific Rise
Geology
The Cenozoic uplift and earthquake belt of mainland Britain as a response to an underlying hot, low-density upper mantle
J. Geol. Soc.
Growth of radial viscous fingers in a Hele–Shaw cell
J. Fluid Mech.
Understanding the Evolution of Miscible Viscous Fingering Patterns
Interface evolution during radial miscible viscous fingering
Phys. Rev. E
Anomalous heat flow and geoid across the Cape Verde Rise: evidence for dynamic support from a thermal plume in the mantle
Geophys. J. R. Astron. Soc.
An analysis of young ocean depth, gravity and global residual topography
Geophys. J. Int.
Crustal structure of the British Isles and its epeirogenic consequences
Geophys. J. Int.
Two-phase flow in Hele–Shaw models
AIChE J.
Rheology of the Upper Mantle and the Mantle Wedge: A View from the Experimentalists
Oceanic residual depth measurements, the plate cooling model, and global dynamic topography
J. Geophys. Res., Solid Earth
Viscous fingering with a single fluid
Can. J. Phys.
Sampling the Cape Verde mantle plume: evolution of melt compositions on Santo Antão, Cape Verde Islands
J. Petrol.
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