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Geological Society, London, Special Publications Present and past influence of the Iceland Plume on sedimentation Stephen M. Jones, Nicky White, Benjamin J. Clarke, Eleanor Rowley and Kerry Gallagher Geological Society, London, Special Publications 2002; v. 196; p. 13-25 doi:10.1144/GSL.SP.2002.196.01.02 Email alerting service click here to receive free email alerts when new articles cite this article Permission request click here to seek permission to re-use all or part of this article Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by on 31 May 2007 © 2002 Geological Society of London Present and past influence of the Iceland Plume on sedimentation S T E P H E N M. JONES 1, N I C K Y W H I T E 1, B E N J A M I N J. C L A R K E 1'2, E L E A N O R R O W L E Y I ' 3 & K E R R Y G A L L A G H E R 1'4 tBullard Laboratories, Madingley Rise, Madingley Road, Cambridge CB3 0EZ, UK (e-mail: jones @esc.cam, ac. uk) 2present address: Department of Geology and Geophysics, University of Edinburgh, King's Building, West Mains Road, Edinburgh EH9 3JW, UK 3present address: Shell Eygpt, 6 Hassan El-Sherley Street, PO Box 2681 El Horreya, Heliopolis, Cairo, Eygpt 4TH Huxley School of Environment, Earth Science, and Engineering, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AS, UK Abstract: The Cenozoic development of the North Atlantic province has been dramatically influenced by the behaviour of the Iceland Plume, whose striking dominance is manifest by long-wavelength free-air gravity anomalies and by oceanic bathymetric anomalies. Here, we use these anomalies to estimate the amplitude and wavelength of present-day dynamic uplift associated with this plume. Maximum dynamic support in the North Atlantic is 1.5-2 km at Iceland itself. Most of Greenland is currently experiencing dynamic support of 0.5-1 km, whereas the NW European shelf is generally supported by < 0.5 kin. The proto-Iceland Plume had an equally dramatic effect on the Early Cenozoic palaeogeography of the North Atlantic margins, as we illustrate with a study of phime-related uplift, denudation and sedimentation on the continental shelf encompassing Britain and Ireland. We infer that during Paleocene time a hot subvertical sheet of asthenosphere welled up beneath an axis running from the Faroes through the Irish Sea towards Lundy, generating a welt of magmatic underplating of the crust which is known to exist beneath this axis. Transient and permanent uplift associated with this magmatic injection caused regional denudation, and consequently large amounts of clastic sediment have been shed into surrounding basins during Cenozoic time. Mass balance calculations indicate agreement between the volume of denuded material and the volume of Cenozoic sediments deposited offshore in the northern North Sea Basin and the Rockall Trough. The volume of material denuded from Britain and Ireland is probably insufficient to account for the sediment in the Faroe-Shetland Basin and an excess of sediment has been supplied to the Porcupine Basin. We emphasize the value of combining observations from both oceanic and continental realms to elucidate the evolution of the Iceland Plume through space and time. The free-air gravity field over the northern hemisphere is dominated by a long-wavelength high centred on Iceland, stretching from the Azores to Siberia and from Baffin Island to Denmark (Fig. 1). Both theoretical considerations and observations from the world's oceans suggest that long-wavelength ( > 1000km) positive anomalies are generally associated with mantle upwelling and dynamic uplift (Sclater et al. 1975; McKenzie 1994). If these inferences also hold in the North Atlantic province, the areal extent of the gravity high suggests that the continental margins of NW Europe and eastern Greenland are dynamically supported at present. In the first part of this paper, we compare estimates of dynamic support derived from the bathymetry and gravity field of the North Atlantic oceanic realm to constrain the present magnitude of dynamic support of both oceanic crust and the adjacent continental margins. Next, we consider the region that was affected by the plume in the past. Temporal and spatial variation of plume-related uplift has played an important role in the evolution of both margins, especially in controlling the generation and distribution of clastic sediments. Here, we consider the specific example of sediment mass balance around Britain and Ireland. Geochemical evidence has proved that this region experienced magmatic underplating of the crust during From: DOR~, A.G., CARTWRIGHT,J.A., STOKER,M.S., TURNER,J.P. & WHITE,N. 2002. Exhumationof the North AtlanticMargin: Timing,Mechanismsand Implicationsfor PetroleumExploration. Geological Society, London, Special Publications, 196, 13-25. 0305-8719/02/$15.00 © The Geological Society of London 2002. 14 S.M. JONES E T A L . mGal -300-60-50-40-30-20-10 0 10 20 30 40 50 60 400 Fig. 1. Free-air gravity anomaly over part of the northern hemisphere, displayed using a Lambert azimuthal equal area projection centred on Iceland. The long-wavelength gravity high centred on Iceland, which extends from south of the Azores to Spitzbergen and from Baffin Island to Scandinavia, should be noted. Paleocene time, leading to permanent uplift, which drove denudation (Thompson 1974; Brodie & White 1995). Mass balance provides a means of comparing and verifying complementary measures of Early Cenozoic denudation, estimated from the onshore record of denudation and the offshore record of sediment accumulation. Two forms of surface uplift are associated with mantle plumes. Dynamic support always results when abnormally hot mantle is emplaced beneath the lithosphere. This support is transient and disappears when the thermal anomalies in the asthenosphere and lithosphere dissipate by convection and conduction. The North Atlantic region is dynamically supported today and it must also have been dynamically supported during Early Cenozoic time, as the volcanic record attests to abnormally hot mantle at that time. Permanent uplift may also occur if mantle thermal anomalies induce melting and the melt is injected into or just beneath the crust. Permanent uplift affected Britain and Ireland during Paleocene time, and the regions flanking the line of continental separation at the PaleoceneEocene boundary. Comparison of the two themes we present in this paper therefore demonstrates the relative spatial extent and magnitude of transient and permanent uplift. The term epeirogenic uplift refers to uplift that could be permanent and/or transient. In general, epeirogenic uplift refers to uplift that is not generated by horizontal plate motions, and the term need not imply any particular mechanism. However, the present and past epeirogenic uplift of the North Atlantic region we discuss in this study is generated by the Iceland Plume. Present-day dynamic support The North Atlantic Ocean is anomalously shallow in the vicinity of Iceland. Anomalous topography culminates at Iceland itself, which rises to c. 2 k m above sea-level, or c. 4.5km above the average depth of the global mid-ocean ridge system. Two methods can be employed to ICELAND PLUME PAST AND PRESENT investigate dynamic support of the region of oceanic crust round Iceland. The first method exploits the fact that the depth of oceanic crust away from the influence of mantle plumes varies with age in a well-understood manner, by comparing the present-day bathymetry around Iceland with this reference depth. The second method involves establishing a link between dynamic support and the long-wavelength freeair gravity anomaly. Analysis of gravity anomalies should always be treated with caution, as any gravity field can be explained by an infinite number of density distributions, each with different implications for dynamic support. In this section, we first establish that estimates of dynamic support derived from bathymetry and gravity are in general agreement over oceanic crust around Iceland. This result then gives us confidence in estimating dynamic support of the adjacent continental margins using gravity alone. -1 0 15 Estimates from bathymetry Figure 2 is a plot of anomalous topography in the North Atlantic, calculated by subtracting the well-known age-depth model of Parsons & Sclater (1977) from the bathymetry. Unfortunately, the anomalous topography in Fig. 2 cannot be interpreted solely in terms of presentday dynamic support because it also includes a component of permanent topography caused by spatial variations in the thickness of oceanic crust. However, the magnitude of this permanent topography can be estimated given determinations of oceanic crustal thickness from wideangle seismic experiments. In the oceans surrounding Iceland but away from the continental margins and the Greenland-IcelandFaroes Ridge, anomalously hot mantle has generated crust 7 - 1 0 k m thick (White 1997). Isostatic balancing shows that this variation of up to 3 km greater than the thickness of standard 1 2 Residual topography (km) Fig. 2. Anomalous topography of the North Atlantic Ocean, calculated by subtracting the age-depth cooling relationship of Parsons & Sclater (1977) from the ETOPO5 bathymetry grid. The age of oceanic lithosphere was taken from Mailer et a l. (1997); the Greenland-Iceland-Faroes Ridge was excluded from the calculation because its age is not well known. Anomalous topography is corrected for sediment loading of oceanic basement using the method of Le Douaran & Parsons (1982) and the sediment thickness map of Laske & Masters (1997). It should be noted that the anomalous topography displayed here contains both a component of present-day dynamic support and a permanent component caused by crustal thickness variations. 16 S.M. JONES ETAL. oceanic crust (7 km, White et al. 1992) generates permanent topography of 0-0.5 km. This value of 0.5 km is smaller than the total variation in anomalous topography across these regions, suggesting that most of the anomalous topography is generated by present-day dynamic support. However, close to the continental margins and the Greenland-Iceland-Faroes Ridge, the thickness of oceanic crust is 10-15 km, varying over distances of < 50 km (Barton & White 1997; Weir et al. 2001). In these regions, permanent topography probably accounts for most of the anomalous topography in Fig. 2. To summarize the discussion above, anomalous topography of those regions away from the continental margins and the GreenlandIceland-Faroes Ridge can be interpreted as an estimate of dynamic support with an error range of 0.5 km. However, short-wavelength variations in anomalous topography associated with continental margins and the Greenland-IcelandFaroes Ridge are most likely to reflect permanent topography associated with crustal thickness variations. Bearing in mind these caveats, three important results concerning present-day dynamic support can be gleaned from Fig. 2. First, the amplitude of dynamic support close to Iceland itself is 1 . 5 - 2 k m . Secondly, the amplitude of dynamic support is partially controlled by the active mid-ocean ridge system. This fact is clearly seen both to the north of Iceland, where the active Kolbeinsey Ridge is more anomalously elevated than the extinct Aegir Ridge, and to the south of Iceland, where a tongue of anomalous uplift extends along the Reykjanes Ridge. Thirdly, the continental margins of eastern Greenland and NW Europe are currently experiencing at least 0.5kin of dynamic support, implying that the adjacent continental shelves are also experiencing significant dynamic support at present. Estimates f r o m gravity The free-air gravity field over a convecting layer results from two competing effects: density variations within the layer and surface deformation induced by the convective circulation. In the case of upwelling plumes, hot mantle has a lower density than the surrounding mantle, which acts to reduce the gravity. However, an upwelling plume deforms the Earth's surface upwards, which acts to increase the gravity. The sign of the total free-air gravity anomaly depends on the relative magnitude of these effects. At Rayleigh numbers appropriate to Earth, the effect of surface deformation outweighs the effect of the reduction in density, so upwelling regions in the mantle are characterized by positive free-air gravity anomalies at the surface. The transfer function between the free-air gravity anomaly g and topography h is called the admittance and is given by Z(k) = g(k)/h(k). Admittance is a function of wavenumber k. At wavelengths shorter than c. 500 km, admittance is controlled by the mechanical properties of the lithosphere. Admittance decreases systematically with increasing wavelength (i.e. decreasing wavenumber), and the rate of decrease is dependent upon the effective elastic thickness of the lithosphere. McKenzie (1994) and McKenzie & Fairhead (1997) exploited this behaviour to estimate the effective elastic thickness of the lithosphere in the Pacific and Indian Oceans from the short-wavelength part of the gravity and topography fields. At wavelengths above c. 500km the admittance calculated from the observed gravity and topography diverges from that calculated from theoretical models assuming an elastic plate. At these long wavelengths, the flexural strength of the lithosphere plays no part in supporting topography. Instead, topography is supported by stresses exerted on the base of the lithosphere by the convecting mantle and by long-wavelength variations in the density structure of the lithosphere, which are isostatically compensated. Thus, dynamic support produces long-wavelength anomalous topography Ahconv that correlates with the long-wavelength free-air gravity anomaly. Isostatically compensated topography, such as the permanent anomalous topography that arises from crustal thickness variations in the North Atlantic, is not correlated with a measurable gravity anomaly. The behaviour of the admittance function within the wavelength band 5 0 0 - 3 0 0 0 k i n suggests a linear relationship between gravity and topography. If this simple relationship, observed in the Pacific and Indian Oceans, also holds in the North Atlantic then anomalous topography caused by dynamic convective support can be calculated from the longwavelength free-air gravity field using Ahconv = g/Z. The value of Z to be used can be constrained by observations. It has long been recognized that Z ~ 3 5 m G a l k m -1 is appropriate for Earth's oceans (Sclater et al. 1975). Another measure of the admittance to be used to calculate dynamic support can be obtained from numerical ICELAND PLUME PAST AND PRESENT convection experiments. McKenzie (1994) summarized several numerical models of axisymmetric plumes, including the model of Watson & McKenzie (1991) that successfully matched the observed gravity, topography and melt production of the Hawaiian Plume. These convection models are characterized by admittances in the range 34.4 + 2 . 2 m G a l k m - ~ . Hence, observed and theoretical values for the admittance between topography and gravity of the oceans at long wavelengths are in good agreement. The free-air gravity dataset used here is a compilation of point measurements over land, together with the satellite gravity dataset of Sandwell & Smith (1997) over the oceans, as described by McKenzie & Fairhead (1997). The short-wavelength part of the gravity field that is influenced by the flexural strength of the lithosphere was removed using a low-pass filter. The region of interest has dimensions comparable with the radius of Earth, so low-pass filtering was carded out using a spherical harmonic model of the gravity dataset. The long-wavelength gravity model was generated using spherical harmonic coefficients of degrees 0 < l --- m ~< 53 (equivalent to a low-pass filter of c. 750 km) with a suitable taper at the upper cut-off to prevent ringing. I -2 I I t 17 Figure 3 shows the dynamic component of anomalous topography Ahconv estimated from the free-air gravity field using two different values of Z. These estimates are simply a scaled version of the long-wavelength gravity field and resemble the original gravity field fairly closely (compare Figs 1 and 3). In the region surrounding Iceland, estimates of convective support from the gravity field agree reasonably well with independent estimates of convective support from topography (compare Figs 2 and 3). In particular, the dynamic support estimates from gravity reinforce the three observations derived from Fig. 2 and noted in the previous section. First, peak dynamic support is c. 1.8km at Iceland. Secondly, the active spreading axis exerts an important control on the long-wavelength gravity field and on the magnitude of dynamic support. Dynamic support is centred on the Mid-Atlantic Ridge, and to the north of Iceland the active Kolbeinsey Ridge is experiencing greater support than the extinct Aegir Ridge. Thirdly, the continental margins are currently experiencing significant dynamic support of 0.5-1 km. It is important to note that estimates of present-day dynamic support calculated directly from long-wavelength gravity anomalies are not always in agreement with estimates calculated from the bathymetry. Figure 3 suggests that I 0 p!iiiiiii#iiii~ Residual Topography (km) 2 Fig. 3. Estimates of present-day dynamic support in the North Atlantic region, calculated by dividing the longwavelength free-air gravity field by a constant admittance, as discussed in the text. Bold continuous lines indicate continent-ocean boundaries. (a) Dynamic support predicted using an admittance of Z = 35 mGalkm -1 (appropriate for subaqueous regions); (b) dynamic support predicted using an admittance of Z = 50 mGal km-1 (equivalent value for subaerial regions). K, Kangerlussuaq; S, Scoresby Sund. 18 S.M. JONES E T A L . significant dynamic support occurs from Iceland to the Azores. However, Fig. 2 indicates no measurable dynamic support just south of the Charlie Gibbs fracture zone. Thus, the amplitude of dynamic support calculated from gravity data is unlikely to be correct in detail and should be treated with some caution. Nevertheless, the agreement between dynamic support estimates based on bathymetry and gravity in the vicinity of Iceland suggests that Fig. 3 provides a reasonable estimate of dynamic support of the G r e e n l a n d - I c e l a n d - F a r o e s Ridge and the adjacent continental shelves. Free-air gravity anomalies may be used to estimate dynamic support of the continents in the same way as for the oceans. Whereas it is appropriate to employ an admittance of Z = 35 mGal km -1 to estimate dynamic support of regions covered by water, an equivalent airloaded value of Z = 50 reGal k m - 1 should be used for subaerial regions. Dynamic support estimates calculated using both values of admittance are shown in Fig. 3. Greenland is currently experiencing dynamic support of 0.5-1 km. Dynamic support is greatest on the east coast, notably in the region between Kangerlussuaq and Scoresby Sund, adjacent to the Greenland-Iceland-Faroes Ridge. In contrast, the magnitude of dynamic support beneath the NW European shelf seldom exceeds 0.5 km. Dynamic support increases westwards from Scandinavia across the Norwegian continental shelf. The North Sea and southern England do not appear to be dynamically supported. Cenozoic denudation of Britain and Ireland In this section, we focus on one aspect of the problem of determining the Early Cenozoic shape of the Iceland Plume, namely the relationship between Cenozoic uplift, denudation and sedimentation on the continental shelf surrounding Britain and Ireland. Our long-term goal is to use mass balance calculations in conjunction with estimates of permanent uplift caused by magmatic underplating to determine the behaviour of the Iceland Plume throughout Cenozoic time. We have chosen to carry out mass balance calculations for the British Isles for three important reasons. First, geochemical evidence collected from rocks of the British Cenozoic Igneous Province proves that the crust beneath much of Britain and Ireland has been thickened by a substantial amount of igneous material as we explain below, implying significant permanent uplift during Paleocene time. Secondly, a substantial body of information about the denudation of Britain and Ireland is available based on modelling of subsidence, vitrinite reflectance, apatite fission-track and sonic velocity datasets. Thirdly, the products of Cenozoic denudation have been carefully mapped in all of the surrounding sedimentary basins. Although detailed work has been carried out previously on both onshore denudation and offshore deposition, no attempts have yet been made to check these estimates by constructing a mass balance on a regional scale. The evidence for magmatic underplating of the crust beneath Britain and Ireland is not widely recognized, despite the fact that the consequences of such an igneous addition, in terms of uplift, denudation and sedimentation, can explain many features of the surface geology and thus have obvious implications for the hydrocarbon industry. For decades it has been recognized that the composition of flood basalts from the Hebrides can be explained only by fractional crystallization of up to 70% of their original liquid mass (Thompson 1974). The crystallized residuum must therefore remain at depth. From a surface processes point of view the argument ends here, because if such material is added anywhere within the upper half of the lithosphere its density will be less than that of the asthenosphere it displaces, and isostatic balancing shows that permanent uplift will result. The depth at which crystallization occurred, and at which the crystallized residuum remains, can be established using geobarometry techniques, which determine the pressure at which the major-element compositions of both flood basalts and the phenocrysts they contain are in equilibrium. Pressures of c. 1 GPa are estimated, equivalent to a depth of around 30 km, i.e. the depth of the Moho (Thompson 1974; Brodie & White 1995). This result is not surprising, as the fact that basaltic melt is less dense than the mantle lithosphere but roughly the same density as the lower crust means that the melt should rise through the mantle and pond at the base of the crust, giving rise to the concept of magmatic underplating. Wide-angle seismic experiments across North Atlantic volcanic continental margins have imaged high-velocity bodies at the base of the continental crust, which are usually interpreted as pods of igneous material underplated beneath the crust at the time of continental separation (Barton & White 1997). Preliminary results from modelling of a wideangle seismic line spanning the Irish Sea suggest that a high-velocity zone exists near the Moho, which probably represents the igneous material that we expect to see underplated beneath the onshore part of the ICELAND PLUME PAST AND PRESENT British C e n o z o i c Igneous Province (S. A1-Kindi, pers. comm.). However, as this discussion implies, seismic evidence alone can never directly reveal the age or nature of these high-velocity layers, so it is always necessary to interpret seismic evidence in conjunction with geochemical evidence. To conclude, it should be noted that crustal magmatic underplating is not a peculiar feature of the North Atlantic but is common to all continental flood basalt provinces (Cox 1993). 19 Mass balance calculations The mass balance calculation has three parts: the extent and amount of denudation of the sediment source area or catchment (Fig. 4); the mass of sediment accumulated through time in the sedimentary basins immediately adjacent to the British Isles (Fig. 5); and the mass of material lost from the system by solution and by escape to the deep ocean. #~f'}o 50 ° -10 ° .5o 0o -]0 .5 o 0o I 0.0 Denudation 0.5 1.0 1.5 2.0 (km) 2.5 Fig. 4. Estimates of Cenozoic denudation of Britain and Ireland based on modelling subsidence histories and apatite fission-track length distributions. +, locations of well sections in extensional sedimentary basins used for subsidence modelling. The amount of missing post-rift basin fill was predicted by fitting a theoretical subsidence curve to the remnant synrift stratigraphy, assuming the standard lithospheric stretching model. This technique has been described fully by Rowley & White (1998), and results have been tabulated by Hall (1995) and Rowley (1998). ©, locations of apatite samples. Apatite fission-track length distributions were modelled to find Mesozoic-Cenozoic thermal histories by K. Gallagher using the Laslett et al. (1987) annealing model for Durango apatite and the method described by Gallagher (1995); the results were reported by Rowley (1998). The amount of Cenozoic cooling was converted to a range of denudation estimates by Monte Carlo modelling using the range of geothermal gradients observed in the North Sea today (Rowley 1998). Data covering Ireland are from Allen et al. (2002). (a) Minimum denudation estimate found by contouring results derived from subsidence analysis, the lower bounds of the Rowley (1998) denudation estimates derived from modelling apatite fissiontrack length distributions, and the minimum denudation estimate for Ireland of Allen et al. (2002, fig. 10b). (b) Maximum denudation estimate found by contouring the modes of the Rowley (1998) denudation estimates and the maximum denudation estimate for Ireland of Allen et al. (2002, fig. 10a). These contour plots were generated by taking the mean of all estimates within blocks of dimension 1° longitude x 30t latitude (c. 50 km x 50 km) and gridding the resulting values using the continuous curvature spline method of Smith & Wessel (1990). It should be noted that a further set of Cenozoic denudation estimates based on modelling of vitrinite reflectance profiles from a subset of the wells used for subsidence analysis yields a denudation estimate that lies between the minimum and maximum estimates illustrated here (Rowley 1998). 20 S.M. JONES ETAL. 50 ° -10" 0.1 0.5 1.0 -5" 1.5 2.0 2.5 3.0 315 Solid Sediment Thickness (km) Fig. 5. Map of the NW European continental shelf showing the Cenozoic solid sediment thickness. This isopach map was constructed from a database of 2D and 3D seismic reflection surveys calibrated with well-log information. Solid thickness was calculated from the observed thickness by subtracting the pore-space volume predicted by the standard exponential relationship between fractional porosity q~ and depth z given by ~b= q~0exp(-z/A), where ~b0 = 0.6 is the depositional porosity and A = 2 km is the compaction length scale. The present-day canyon system at the southern end of the Porcupine Basin is shown. Contours on land show topography smoothed using a filter of width 50 km and used to define the drainage catchments supplying each offshore sediment sink. Segments of catchment boundaries that are offshore at present were positioned using present-day bedload transport patterns. Inset shows central igneous complexes, flood basalts and dykes of the North Atlantic Igneous Province, mostly emplaced during Paleocene time. The best-constrained element in the mass balance problem is the mass of Cenozoic sediment in the offshore basins. Here, the volume of solid sediment is used in place of the mass of sediment. The solid volume of the offshore sediment pile is calculated by removing the volume accounted for by porosity, predicted by the standard exponential porosity model. The principal source of error in calculating the solid volume is the depth conversion procedure. However, the error introduced by this uncertainty can be realistically quantified by using a range of velocity-depth functions, and the resulting error estimates are illustrated in Fig. 6. Parameterization of the porosity-depth relationship used in the compaction calculation is relatively well determined for the basins surrounding Britain and Ireland. For example, using parameters for the end-member lithologies of shale and sand determined by Sclater & Christie (1980) for the North Sea yields a variation in solid volume that is only 14% of the error range associated with the depth conversion calculation in that region. Figure 5 shows the solid sediment thickness accumulated around Britain and Ireland during Cenozoic time. During this time the main sediment sinks were the Porcupine Basin, the Rockall Trough, the Faroe-Shetland Basin and the North Sea Basin. The solid volume of Cenozoic deposits in southern England and offshore southern Ireland is negligible. The total Cenozoic denudation is shown in Fig. 4, based on three independent lines of evidence. The first line of evidence depends on our knowledge of extensional sedimentary basins. Many extensional basins around Britain and Ireland contain a fault-controlled syn-rift stratigraphy but the anticipated post-rift stratigraphy is partially or entirely absent (Brodie & White 1995). The amount of missing post-rift stratigraphy can be predicted, based on our knowledge of the kinematics of extensional ICELAND PLUME PAST AND PRESENT Solid volume (x 10 3 km 3) ~ o 0 0 • ~ 1 ~ 0 I , , , , I O Porcupine i-I Rockall i FaeroeShetland ~] • D N North Sea . . . . i ~ T .... Fig. 6. Summary of mass balance calculations for NW European continental shelf. Open bars represent the amount of solid sediment accumulated offshore, calculated by integrating Fig. 5 over each offshore depocentre. Error ranges reflect uncertainties in depth conversion. The solid sediment volume for the North Sea plotted here has been halved to account for the fact that Scandinavia has also supplied sediment to the North Sea. Filled bars represent volume of rock denuded from onshore catchments, calculated by integrating the minimum and maximum estimates given in Fig. 4 over the regions marked in Fig. 5. The Porcupine sediment sink is supplied by two catchments: SI, southern Ireland and the Irish Sea; WI, western Ireland. The other sediment sinks have one catchment each. basins (Rowley & White 1998). The other two methods of estimating denudation rely on thermal indicators within the sediment pile that retain a m e m o r y of their burial history. Reflectance of vitrinite increases with rising temperature by means of a non-reversible reaction, so vitrinite retains a memory of the highest temperature it has experienced. Denudation estimates can be obtained by comparing vitrinite reflectance profiles with a global reflectance dataset from non-inverted basins (Rowley 1998). Analysis of fission tracks in apatite also provides temperature histories that can be interpreted in terms of denudation through time. The most prominent feature of Fig. 4 is the peak in denudation centred on NW England. This region suffered denudation of 1-2.5 km during Cenozoic time. A peak in denudation centred on 21 NW England has also been suggested by previous studies based on apatite fission-track data alone (Lewis et al. 1992). The magnitude of Cenozoic denudation generally decreases towards the present offshore areas. Another important message from Fig. 4 is that the error range on the denudation estimate for any particular region is c. 1 km. It is difficult to quantify loss of mass from the system. Loss by solution depends strongly on the rock type being eroded. It would be difficult to include the amount of loss by solution in the mass balance calculations because of the uncertainty over the original lithologies being eroded and the variety of lithologies within each catchment. Here we apply no corrections to account for loss of mass from the system. When loss of mass from the system is neglected, the solid volume of sediment measured in offshore basins provides a lower bound on the denudation. If the solid sediment volume measured offshore is found to be greater than the onshore estimate of denudation then an additional, unidentified source of sediment to the offshore basin would be implied. On the other hand, if the offshore solid sediment volume is found to be significantly less than the onshore estimate of denudation then the difference between the two estimates may provide an estimate of the amount of material lost from the system. Mass balance results Figure 6 summarizes the result of balancing the volume of solid sediment accumulated offshore with direct estimates of denudation made onshore for the Porcupine, Rockall, F a r o e Shetland and northem North Sea systems. The drainage catchment matched with each offshore sediment sink is shown in Fig. 5. The boundaries of these catchments are based on the present-day topography and bedload transport patterns. Palaeotopographic reconstructions of Britain and Ireland during earliest Eocene time, at the time of maximum dynamic support by the Iceland Plume (see discussion later), suggest that the north-south drainage divide running through Scotland and England has remained relatively static since Paleocene time (Jones 2000). The positions of the west-east-oriented drainage divides are more likely to have altered position during Cenozoic time, and we consider the effect of such migration in the following discussion. In the Rockall Trough and northern North Sea systems, the volume of Cenozoic sediment accumulated offshore is the same as the denudation measured onshore, within error. 22 S.M. JONES ETAL. However, in the Faroe-Shetland system, the volume of sediment accumulated offshore is greater than the denudation measured onshore by at least 2 x 104km 3. This discrepancy implies that the Faroe-Shetland Basin has another source of Cenozoic sediment in addition to the catchment in the region of NW Scotland. The additional sediment source may be a catchment in the region of the Faroes or Greenland. Alternatively, the boundaries of the drainage catchment shown in Fig. 5 covering NW Scotland may be incorrect. The southern boundary of that catchment is not well constrained and it is possible that sediment was also sourced from western Scotland and channelled between the Outer Hebrides and the Scottish mainland towards the Faroe-Shetland Basin. A third possibility is that additional sediment was transported into the Faroe-Shetland Basin by currents running along the basin axis. Strong bottom currents flowing southwestwards along the Faroe-Shetland Trough were initiated in Oligocene time and persist to the present day (Davies et al. 2001). Lack of a significant difference between the two estimates of denudation in the Rockall and northern North Sea systems suggests that little mass has been lost from those systems. However, the total sediment supplied to the Porcupine Basin by both the catchment covering western Ireland and the catchment covering southern Ireland and the Irish Sea is greater than the material accumulated offshore by at least 4>< 104km 3. Thus, a significant mass of sediment has been lost from this system. The magnitude of this discrepancy shown in Fig. 6 is a lower bound because no account has been taken of sediment supplied to the western side of the Porcupine Basin by the Porcupine Bank and Ridge. Three effects have probably contributed to the loss of mass from the Porcupine system. First, Carboniferous limestone crops out over a large area of the Irish catchments, and erosion of this limestone produces negligible clastic sediment. Secondly, it is likely that most of the sediment supplied to the southern end of the Porcupine Basin has escaped directly to the deepsea sediment fan observed on the North Atlantic oceanic abyssal plain via the canyon system in the SE of the Porcupine Basin (Fig. 5). This canyon system has been active since Early Oligocene time (Jones 2000). Thirdly, a variety of evidence from both the continental shelf surrounding Britain and Ireland and adjacent oceanic crust suggests that the head of the Iceland Plume expanded rapidly during earliest Eocene time, causing dynamic uplift centred NW of Britain and Ireland (see discussion below). The resulting southeasterly tilt of Britain and Ireland probably caused most of the material eroded from southern Ireland and the Irish Sea during Eocene time to be shed southwards into the Bay of Biscay, rather than westwards into the Porcupine Basin. Cenozoic permanent and transient uplift In this section, we discuss evidence for temporal and spatial variation in the Iceland Plume throughout Cenozoic time. Paleocene p e r m a n e n t uplift The history of sediment flux into the basins surrounding Britain and Ireland during Paleocene time adds detail to the denudation history established using the mass balance. The rate of sediment flux into an offshore basin is related to the size and the rate of denudation of the corresponding drainage catchment. The lag time between an increase in denudation rate and the corresponding increase in sediment flux offshore is likely to be < 100ka (Reading 1991; Burgess & Hovius 1998). Therefore, sediment flux histories in the basins surrounding Britain and Ireland can be directly related to uplift of their sediment source regions. Calculation of the volumes of Paleocene and Eocene sediment sequences in the northern North Sea and FaroeShetland Basins has shown that sediment flux into these basins grew through Paleocene time to a maximum at 5 9 - 5 8 Ma and then decreased into Eocene time (Reynolds 1994; Clarke 2002). Thus we may infer that epeirogenic uplift of Scotland and northern England was initiated in Early Paleocene time and the rate of uplift peaked during mid-Late Paleocene time. In contrast, sediment flux into the Porcupine Basin remained low throughout Paleocene time, suggesting that uplift rates were lower away from the region of Scotland and northern England at this time (Jones 2000). The denudation history of Scotland inferred from the history of sediment flux around Scotland correlates with the history of onshore igneous activity, which also peaked at 59-58 Ma (White & Lovell 1997). As we have seen, the sediment flux data suggest that Paleocene epeirogenic uplift was confined to the region close to the present-day surface expression of onshore igneous activity. We therefore suggest that the Paleocene igneous activity, epeirogenic uplift and denudation were initiated when a subvertical sheet of unusually hot asthenosphere was injected beneath the present Faroes-Irish Sea-Lundy axis. The surface expression of ICELAND PLUME PAST AND PRESENT onshore igneous activity is concentrated in western Scotland and Ulster, but this igneous activity is offset from the locus of maximum denudation, which is centred on northern England and the Irish Sea. If the denudation shown in Fig. 4 is driven mainly by permanent uplift resulting from magmatic underplating, the greatest amount of melt must have been added to the crust beneath this region. Preliminary modelling of a wide-angle seismic line crossing Ireland, the Irish Sea and northern England has imaged a high-velocity pod near the Moho, which is thickest beneath the centre of the profile and thins towards western Ireland and towards the North Sea (S. A1-Kindi, pers. comm.). This pod may represent the layer of magmatic underplating beneath the crust that is predicted by both petrological and sedimentological evidence. P a l e o c e n e - E o c e n e dynamic support The sedimentary records in all basins surrounding the denuded area of Britain and Ireland indicate a major regression-transgression cycle, with maximum regression corresponding to the upper Flett Formation and its lateral equivalents, which were deposited during earliest Eocene time (e.g. Milton et al. 1990; Ebdon et al. 1995). It is now generally believed that this regressiontransgression cycle is related to a phase of transient dynamic uplift that peaked in earliest Eocene time (Nadin et al. 1997; Jones et al. 2001). The magnitude of dynamic support can be quantified within extensional sedimentary basins, where we can isolate epeirogenic vertical motions from tectonic vertical motions, which are ultimately caused by horizontal plate motions. Given the synrift subsidence history of an extensional basin, the post-rift subsidence history can be calculated using the wellestablished lithospheric stretching model. Postrift marker horizons with sedimentologically well-constrained water depths are then compared with the anticipated post-rift subsidence curve to reveal the magnitude of dynamic support through time. The most important of these marker horizons around Britain and Ireland are the earliest Eocene delta-top coals of the upper Flett Formation and its lateral equivalents. The results of this subsidence analysis show that peak dynamic support was c. 0.5 km in the Porcupine and Faroe-Shetland Basins, and dynamic support decreased in a southeasterly direction to zero across southern England (Nadin et al. 1997; Jones 2000; Jones et al. 2001). Peak dynamic support at the PaleoceneEocene boundary was coeval with voluminous 23 intrusive and extrusive igneous activity offshore NW of Britain and Ireland that was associated with break-up of Europe and Greenland above unusually hot asthenosphere. White (1997) collated oceanic crustal thickness measurements and showed that the hot asthenosphere of the Iceland Plume head extended at least 1000km along the continent-ocean boundaries to the north and south of the present GreenlandIceland-Faroes Ridge. Dynamic support estimates from subsidence analyses around Britain and Ireland suggest that the plume head extended a similar distance inboard of the NW European continental margin. We might expect that as maximum transient dynamic support of the basins surrounding Britain and Ireland occurred during earliest Eocene time, the acme of denudation and offshore Sediment flux should also have occurred at this time. However, as we discussed above, maximum sediment flux into the Faroe-Shetland and North Sea Basins actually occurred 3 - 4 Ma previously, during Late Paleocene time. Only in the Porcupine Basin, west of Ireland, was the peak sedimentation rate coeval with peak dynamic support in earliest Eocene time (Jones 2000). This discrepancy in timing of several million years between the two different measures of epeirogenic uplift is an important observation that has yet to be explained. One possible explanation is that there were two separate phases of mantle plume activity, the first an upwelling hot vertical sheet that led to magmatic underplating of the crust beneath the FaroesIrish Sea-Lundy axis, and the second the growth of a mushroom-shaped plume head beneath a region over 1000 km in radius. O l i g o c e n e - R e c e n t dynamic support Variations in crustal thickness and structure around Iceland record changes in the size of the thermal head of the Iceland Plume, measured at the Mid-Atlantic Ridge, through Cenozoic time. As we have already discussed, the radius of the plume head was > 1 0 0 0 k m at the time of continental separation between Europe and Greenland in earliest Eocene time. However, the distribution of normal-thickness oceanic crust south of Iceland suggests that during Late Eocene times, the part of the plume head beneath the Mid-Atlantic Ridge extended < 300 km from the present centre of Iceland (White 1997). The portion of the plume head beneath the ridge then increased to its present radius of c. 1000km between Oligocene time and the present (Figs 2 and 3). Studies of plate motion with respect to the hotspot reference frame suggest that the centre of 24 S.M. JONES ETAL. the Iceland Plume lay beneath Greenland during mid-Cenozoic time and has effectively moved eastwards towards Europe through time (Lawver & Mtiller 1994). We suggest that this relative motion between the Mid-Atlantic Ridge and a plume of relatively constant mass flux may account for the apparent increase in the size of the head of the Iceland Plume between Oligocene time and the present. This history of motion also agrees with our observation that Greenland is currently experiencing greater dynamic support than NW Europe (Fig. 3). The reason is that a greater volume of hot plume-head material accumulates beneath the plate that the plume stem is moving away from than accumulates beneath the plate that the plume stem is approaching (Ribe & Delattre 1998). As Rohrman & van der Beek (1996) have suggested, relative movement of the Iceland Plume towards Europe may also provide an explanation for the Miocene-Recent epeirogenic uplift and consequent increase in denudation that is known to have affected Scandinavia. Conclusions The Cenozoic evolution of the North Atlantic province has been dominated by interaction between the Iceland Plume convective system and sea-floor spreading between Europe and Greenland on a hierarchy of spatial and temporal scales. The continental sedimentary record seems to be influenced by the relative importance of dynamic support, which has varied through through time, and permanent uplift, which was driven by magmatic underplating of the crust. Further progress in understanding this relationship will depend upon measuring the distribution of Paleogene magmatic underplating and upon an improved quantitative understanding of Cenozoic denudation. Our conclusions concerning the present shape and size of the Iceland Plume and concerning Cenozoic mass balance are: (1) maximum dynamic support in the North Atlantic is 1.5-2 km at Iceland itself. (2) The magnitude of dynamic support in the North Atlantic is influenced by the location of the active spreading centres. (3) The continent-ocean boundary of NW Europe is currently experiencing dynamic uplift of c. 0.5km, which decreases to zero across Scandinavia and the North Sea. 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