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Earth and Planetary Science Letters 224 (2004) 193 – 211 www.elsevier.com/locate/epsl Regional anomalies of sediment thickness, basement depth and isostatic crustal thickness in the North Atlantic Ocean Keith E. Louden a,*, Brian E. Tucholke b,1, Gordon N. Oakey c,2 a Department of Oceanography, Dalhousie University, Halifax, NS, Canada B3H 4J1 Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA c Geological Survey of Canada-Atlantic Region, P.O. Box 1006, Bedford Institute of Oceanography, Dartmouth, NS, Canada B3Y 4A2 b Received 3 November 2003; received in revised form 30 April 2004; accepted 5 May 2004 Abstract We calculate the anomalous basement topography for the North Atlantic Ocean from 30j to 70jN latitude and from 0j to 70jW longitude at a resolution of roughly 6 6 km, using grids of total sediment thickness and observed and predicted sea-floor bathymetry to correct for the effects of isostatic sediment loading and lithospheric age. Plotting this residual topography for various plate reconstructions during opening of the North Atlantic, we delineate consistent patterns of basement highs related to variations in hotspot-related volcanism. In addition to Iceland and the Azores, we recognize three centers of excess volcanism at the midAtlantic ridge: the Milne Seamounts and Azores-Biscay Rise ( f 75 – 40 Ma), the Southeast Newfoundland Ridge and MadeiraTore Rise ( f 130 – 110 Ma), and the East and West Thulean Rises ( f 60 – 50 Ma). The duration of volcanic activity ranges from 8 to 10 m.y. (Thulean Rises) to 60 m.y. (Iceland) and thus it appears that both long- and short-lived hotspots coexist, even in relatively close proximity. In contrast, during the period 110 – 60 Ma we observe little excess volcanism during either continental breakup or seafloor spreading. We estimate isostatic crustal thickness from the anomalous basement depths, after first removing dynamic effects created by mantle flow. Maximum thicknesses of volcanic features, from 30 km beneath the Greenland – Iceland – Faeroe ridge to f 15 km beneath the Azores-Biscay Rise, are broadly consistent with seismic data and predictions of decompression melting. Widths of volcanic features indicate that thickening primarily occurs within 100 – 200 km of hotspots except along continental margins that rifted at the time of the hotspot activity (i.e. East Greenland and the HattonRockall Bank). We observe conjugate structures south of Greenland and Edoras Bank, where excess volcanism appears to have extended beyond the margin proper and into oceanic crust. Similar conjugate features appear in the Labrador Sea south of Davis Strait. Finally, we identify anomalous oceanic regions adjacent to some continental margins, where unusually low values of predicted crustal thickness suggest either additional variations in plate properties or non-isostatic effects within the mantle. D 2004 Elsevier B.V. All rights reserved. Keywords: North Atlantic; hot spots; plate reconstructions; sediment thickness; basement depth * Corresponding author. Tel.: +1-902-494-3452. E-mail addresses: [email protected] (K.E. Louden), [email protected] (B.E. Tucholke), [email protected] (G.N. Oakey). 1 Tel.: +1-508-289-2494. 2 Tel.: +1-902-426-3549. 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.05.002 194 K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 1. Introduction In this study, we present and interpret images of anomalous basement topography for the North Atlantic Ocean from 30j to 70jN latitude and from 70jW to 0j longitude. A digital grid of sediment thickness [1] is presented and together with the seafloor bathymetry [2] is used to correct for the load of the sediment cover. This region contains a wide variety of geological features that include the Iceland and Azores hotspots, volcanic and non-volcanic rifted continental margins with deep sedimentary basins, active and extinct spreading centers with a range of morphologies, fracture zones, aseismic plateaus, and seamounts. These features are characterized by anomalous topography, i.e. by basement depth that indicates departure from standard oceanic crustal thickness and underlying mantle thermal structure. Seafloor spreading magnetic anomalies and poles of plate rotations are well determined in the region of this study. Thus we are able to reconstruct positions of basement anomalies at the time of their formation and identify conjugate structures that were created by episodes of anomalous ridge-crest volcanism. Long-lasting volcanism that began after f 60 Ma at the Iceland and Azores hotspots is apparent in this study, but we also find evidence for three more localized and shorter-lived pulses of volcanism. These pulses formed conjugate structures comprising the Milne Seamounts and Azores-Biscay Rise (at f 75– 40 Ma), the Southeast Newfoundland Ridge and Madeira-Tore Rise ( f 130 – 110 Ma), and the East and West Thulean Rises ( f 60 – 50 Ma). It is notable that the period 110 –60 Ma is characterized mostly by normal volcanic activity along the spreading centre and by formation of non-volcanic rifted margins, in contrast to the formation of volcanic margins that followed. We find that simple isostatic thickening of the crust can explain basement depth anomalies close to centers of hotspot volcanism (i.e., within 100 – 200 km), although this does not hold true along volcanic margins where the crustal thickening extends much greater distances along the strike of the margins. In contrast, longer-wavelength depth anomalies in the ocean basins cannot be explained by crustal thickness variations; rather, they require variations in lithospheric thermal structure and/or dynamic topography in response to deeper mantle convection. 2. Sediment thickness A consistent digital grid of sediment thickness has been produced for the North Atlantic region from 30jN to 70jN latitude and 70jW to 2jE longitude [1]. The compilation was constructed from digitized contours extracted from 21 published maps of basement depth and sediment thickness [3 –23]; these maps were originally produced from seismic and well data and they have varying degrees of resolution. Conversion of basement-depth maps to sediment thickness was made when required using ETOPO-5 digital bathymetry [26], with a minimum sediment thickness set at 1 m. Contours from individual map sheets were digitized and interpolated using a minimum curvature algorithm [24,25]. These data were then gridded with a cell resolution of 0.05 0.05 degrees, (a grid of roughly 6 km). Some sub-areas with closely spaced contours were gridded initially at a higher resolution and then regridded before merging with other map areas. Fig. 1. Location of datasets used to construct the digital sediment thickness map (after Oakey and Stark [1]). Numbers are keyed to publications [3 – 23] in the references. Areas where overlapping maps were merged are indicated by ‘‘M’’. K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 Each grid file was compared with the original contours and control points were added so that interpolated values did not exceed the contour values of the original map. Where overlaps existed between separate maps, the overlaps were trimmed to produce smooth transitions from map to map. The overlapping segments then were superimposed and recontoured by hand to avoid any discrepancies between contours. Generally, the manipulation needed to join map sheets was minimal and well within 195 original interpretation errors. No major discrepancies were found between overlapping maps. Finally, the overlapping regions were digitized, interpolated, gridded and merged into the master file with the adjacent grids. Data coverage and references are shown in Fig. 1 and a color display of the gridded sediment thickness data is shown in Fig. 2. Sediment distribution is controlled by the following general factors, as previously described by Tucholke and Fry [5]: (a) the Fig. 2. Map of sediment thickness with values indicated by the color scale bar, plotted using GMT software [82]. See text for explanation of the gridding technique. Thin lines are 2000 and 4000 m isobaths from gridded bathymetry of Smith and Sandwell [2]. Features discussed in text are: Erik drift (ED), Feni drift (FD), Gardar drift (GD), LS (Labrador Sea), NR (Southeast Newfoundland Ridge), RR (Reykjanes Ridge). 196 K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 tectonic history and age of the crust, (b) the nature and location of sediment sources, (c) structural trends in the basement, and (d) processes delivering sediments to depocenters. The greatest variations in sediment thickness occur on the continental margins, as a result of varying amounts of tectonic extension and subsidence (see [28] for a more detailed description of sediment thickness variations offshore eastern Canada). Near-zero thicknesses occur over unrifted basement blocks, while sediments more than 10 km thick commonly fill adjacent rift basins. Conjugate margins typically show asymmetric sediment distributions; thicker sediments occur in basins of the eastern margins of Labrador, Newfoundland and Nova Scotia than in the conjugate margins of West Greenland, NW Europe and Morocco, respectively. On oceanic crust, sediment thickness generally increases with crustal age. The youngest crust along the mid-ocean spreading center is devoid of seismically detectable sediment cover; whereas uppermost Paleocene and Eocene oceanic crust along southeast Greenland and Rockall has sediment thicknesses of 1.5– 2.0 km, and the oldest oceanic crust adjacent to Nova Scotia, Morocco and Newfoundland (Upper Jurassic to Cretaceous) has thicknesses of 3– 6 km. The general pattern of increasing sediment thickness with increasing crustal age can be modified substantially by the availability of sediments. For example, increased thickness relative to crustal age in the northernmost basins is caused by significant sediment input from glaciation, pro-glacial outwash, and resulting gravity flows to the deep basin (e.g. Labrador Sea). These glacial effects are more pronounced in the western than in the eastern North Atlantic. Off eastern Newfoundland and Nova Scotia, sediments over ocean crust are significantly thicker than over conjugate ocean crust in the eastern North Atlantic. Deep currents also affect sediment distribution on oceanic crust by depositing often thick drifts downstream from sources of sediment supply. A prominent example is on the east flank of the Reykjanes Ridge, where sediments are thick relative to conjugate crust on the west flank. This thick sediment drift (Gardar drift) was deposited by westward-intensified, contourfollowing bottom currents emanating from the Norwegian –Greenland Sea [27]. Other examples are the Erik drift off the southern tip of Greenland, the Feni drift in Rockall Trough, and thick drift deposits on the Southeast Newfoundland Ridge (Fig. 2). At smaller scales, sediment thickness is strongly controlled by the original structural configuration of the underlying basement. Structural lows (e.g. fracture zones) become filled while structural highs (e.g. seamounts) retain little sediment cover. Sediment thickness in such areas generally is a direct reflection of the underlying crustal topography. 3. Basement depth anomaly The major purpose of this paper is to map variations in basement depth and compare them to a model basement that has no sediment and no variation in crust and mantle structure. To do this, we correct the basement depth by removing the sediment load with a simple model of isostatic adjustment used by Crough [29]. Because Crough’s formulation is in reflection time, we first convert the sediment-thickness map to travel-time using the relationship H(t) = Vot + 0.5kt2, where t is the one-way travel-time beneath the seafloor, Vo = 1.63 km/s, and k = 1.14 km/s2, based on average North Atlantic values [8]. The sediment velocity and isostatic correction factors are primarily constrained by sonobuoy and borehole data for sediment thickness < 2000 m [29]. Adjustment of basement depth from the isostatic model amounts to adding 600 m to seafloor depth for each second of two-way reflection time through the sediment. Because sediment thicknesses are substantially greater than 2000 m on many of the continental margins, we compare Crough’s [29] borehole constraints and model against results based on seismic refraction data over regions of thick sediment cover off the Labrador [30] and Nova Scotian [31] margins (Fig. 3). This comparison shows agreement for the thinner sediments but progressively larger adjustment factors (650 – 750 m/s reflection time) for thicker sediments. This variation is consistent with a value of 675 m determined for the COST B-2 borehole [29], and it probably is an effect of velocities increasing proportionally faster than densities in the deeper sediments. We compared a grid of basement depths derived using the 600 m correction factor against basement depths (5 – 7 km) determined in the refrac- K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 tion profile offshore Nova Scotia[31]. There is generally good agreement, although the gridded values are systematically deeper byf 100 ms. Because the gridded values are already deeper than the refraction depths, we decided that use of a higher correction factor was not warranted. Thus, all sediment corrections were made using a value of 600 m per second reflection time. These corrections were added to a grid of water depths from Smith and Sandwell[2] to determine adjusted basement depth. We next corrected for expected variations in unloaded basement depth predicted by a model of conductive cooling of the underlying lithosphere as a function of its age, again assuming local isostasy. We first produced a grid of basement age using Mueller et al.’s [32] magnetic isochrons and the time scale of Cande and Kent [33]. This grid was also used to define the limits of oceanic crust. Near the continental boundaries this map is only approximate because it does not consider complex transitional regions that are known to occur between oceanic and continental crust, especially at non-volcanic margins (e.g.[34]). Model depths, shown inFig. 4, were calculated from the age grid using the lithospheric thermal model GDH1 of Stein and Stein [35], with an assumed zero age depth of 2600 m. This model was developed to match observed worldwide depth and heat flow, in particular basement depths that are shallower and heat flow that is higher on old seafloor than predicted from previous models (e.g. the plate model of Parsons and Sclater[36] or the half-space model of Davis and Lister [37]). This approach does not consider the effects of other thermal processes (e.g. mantle plumes), and thus it minimizes basement anomalies on old seafloor that might result from them. By removing basement depth predicted by the GDH1 model from sediment-corrected basement depth, we produced a plot of anomalous basement depth as shown inFig. 5. As expected, there are significant regions of elevated basement topography, most notably in the areas of the Iceland and Azores hot-spots. Regions of excess basement depth are 70°N 13 21 13 5 25 34 21 5 13 21 60°N 21 34 25 25 34 50°N 25 13 40°N M 30°N 70°W -5600 25 M1 6 M0 60°W -5000 -4500 34 21 21 5 5 13 25 34 34 M1 M 6 0 M2 5 Fig. 3. Depth corrections for removal of sediment load above basement, assuming local isostasy; corrections are added to seafloor depth to obtain unloaded basement depth. Data for ODP and COSTB2 boreholes from Crough[29]. Corrections are compared with results derived from seismic refraction profiles off the Labrador (LAB-R) [30] and Nova Scotia (NS-R) [31] margins. The data values fall between correction factors of 600 and 750 m/s of twoway travel time. 197 50°W 40°W -3500 30°W -2500 20°W 10°W 0° G DH1 B athymetry (m) Fig. 4. Theoretical depth of unsedimented ocean crust derived using the plate model of Stein and Stein [35] with a zero-age depth of 2600 m. Ages are determined from the indicated magnetic isochrons of Mueller et al. [32] and the time scale of Cande and Kent [33]. Plots for Figs. 4 – 8 and 10, 11 used GMT software [82]. 198 K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 Fig. 5. Residual basement depth anomaly relative to the GDH1 plate model [35]. Dashed lines are magnetic isochrons shown in Fig. 4, and thin lines are 2000 m and 4000 m isobaths as shown in Fig. 2. Features discussed in text are ABR (Azores-Biscay Rise), AGFZ (Azores-Gibraltar Fracture Zone), ATJ (Azores Triple Junction), CGFZ (Charlie-Gibbs Fracture Zone), CI (Canary Islands), CP (Cruiser Plateau), CRS (Corner Rise Seamounts), DS (Davis Strait), EG (East Greenland Margin), ETR/WTR (East/West Thulean Rise), FIR (Faeroe – Iceland Ridge), GIR (Greenland – Iceland Ridge), GMS (Great Meteor Seamount), HRB (Hatton-Rockall Bank), KT (King’s Trough), LS (Labrador Sea), MI (Madeira Island), MS (Milne seamounts), MTR (Madeira-Tore Rise), NAFZ (Newfoundland-Azores Fracture Zone), NES (New England Seamounts), NR (Southeast Newfoundland Ridge), RR (Reykjanes Ridge), SAP (Sohm Abyssal Plain), and SGH (South Greenland High). much more limited and they appear primarily beneath the Sohm and Biscay abyssal plains and offshore Portugal. If we used other thermal models, they would affect regional anomalies primarily at crustal ages older than chron 25 (56 Ma) (i.e. the regions of excess basement depth). For example, use of Parsons and Sclater’s [36] plate model would reduce the negative anomalies in the Sohm Abyssal Plain by about 500 m, although a substantial region of excess depth would remain. This model would also increase the regions of elevated topography in the oldest crust between the Charlie-Gibbs (CGFZ) K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 and Newfoundland-Azores-Gibraltar (N-AGFZ) fracture zones, although to a lesser degree. However, our discussion of basement topography in the following section is primarily qualitative and deals with relative crustal depths, so it matters little which thermal model is used. 4. Plate reconstructions Our intent is to discuss basement depth anomalies that formed during the opening of the North Atlantic Ocean. To do this, we plot the anomalies in Figs. 6 –8 at reconstructed positions for the magnetic isochrons used to define the age grid, back to chron M0. A small region north of Iceland is omitted in the reconstructions to avoid complexities related to an extinct spreading center (Aegir Ridge) 199 within the Norwegian basin. The map of present basement anomalies (Fig. 5) was first split into 6 plates (North America, Greenland, Eurasia/Rockall, Porcupine, Iberia and Africa). The separate plates were then rotated using poles of rotation relative to the North American plate (Table 1 and [38 –43]) and they were joined at the relevant isochron. This result is simplified because the depth data for the rotated plates are not regridded after rotation. Instead, the rotated depth data are re-scaled to achieve a best fit of the actual rotated and the re-projected positions of isochrons and coastlines. Most rotations are east –west, rather than north – south, so distortion of the original Mercator grid is not significant for the large regional scale of this study. The following sections describe primary features of the depth anomalies in the reconstructions beginning with chron M0. Fig. 6. Basement depth anomaly reconstructed at chrons M0 and 34. Thin lines are magnetic isochrons (shown in Fig. 4) and isobaths (shown in Fig. 2). Abbreviations for features discussed in text (additional to those given in Fig. 5) are BTJ (Biscay Triple Junction), IAP (Iberia Abyssal Plain), NGFZ (Newfoundland-Gibraltar Fracture Zone), NB (Newfoundland Basin), NS (Newfoundland Seamounts), TAP (Tagus Abyssal Plain) and TS (Tore Seamount). 200 K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 Table 1 Poles of rotation relative to North America Greenland Eurasia Rockall Porcupine Iberia Africa Chron 5 13 21 25 34 M0 M16 M25 References Lat Long Deg Lat Long Deg Lat Long Deg Lat Long Deg Lat Long Deg 0 0 0 68 137 2.5 68 137 2.5 68 137 2.5 79.08 77.95 2.41 0 0 0 68 129 7.78 68 129 7.78 48.06 143.5 7.2 76.41 7.12 9.81 62.8 91.95 2.61 67.12 137.28 10.94 62.55 142.01 10.25 73.38 129.05 11.04 74.51 4.83 15.32 24.48 137.25 3.12 63.25 143.89 14.15 57.15 148.19 13.22 72.72 135.5 14.29 80.6 0.5 18.07 65.3 122.45 11 76.23 148.8 21.83 72.82 152.76 20.6 87.18 57.43 24.67 76.55 20.73 29.6 67.5 118.48 13.78 69.67 154.26 23.17 69.67 154.26 23.17 68.88 15 50.62 66.09 20.18 54.45 0 0 0 0 0 0 0 0 0 66.9 12.93 60.45 66.24 18.33 59.71 0 0 0 0 0 0 0 0 0 66.9 12.93 60.45 66.7 15.85 64.9 [40] 4.1. Chrons M0 (120 Ma—early Aptian) and 34 (83 Ma—early Campanian) (Fig. 6) Major elevations are associated with the Southeast Newfoundland Ridge (NR) and Madeira-Tore Rise (MTR) at the junction of the Mid-Atlantic Ridge (MAR) with the Newfoundland-Gibraltar fracture zone (NGFZ). These depth anomalies are attributed to a period of enhanced volcanism that began ca. chron M4 and died out shortly following chron M0, which coincides with the formation of the magnetic J-anomaly [44]. Oceanic character of the NR [45] and a high velocity, locally compensated crustal root beneath Josephine Seamount at the northern end of the MTR [46] are consistent with this interpretation. Anomalous elevations are also associated with the Newfoundland Seamounts (NS) in the Newfoundland Basin (NB). They are paired with the conjugate Tore Seamounts (TS) in the Iberia Abyssal Plain (IAP) (Fig. 6, left), and thus they appear to have been formed by a ridge-centered hotspot. This volcanism appears to have followed breakup by at least 20 m.y., based on an age date (97.7 F 1.5 Ma) for one of the Newfoundland seamounts [62], and waned by the time of chron 34 (Fig. 6, right). There is significant asymmetry in basement depth between the large negative anomaly in the Sohm Abyssal Plain (SAP) [47] and more normal depths in the Canary Basin. It is possible that this asymmetry is not an original feature of the basin but was 5 – 34 M0 [41] [42] 5 – 34 M0 [41] [42] 5 – 34 M0 – M25 [41] [42] 5 – 34 M0 – M25 [43] [44] created (or enhanced) by regional uplift on the African plate, possibly associated with more recent volcanism of the Canary Island (CI) [48] and Madeira [49] hotspots. To the north of the NR and MTR, asymmetric depths also appear in the opening NewfoundlandIberia rift, with basement f 1 km deeper off Iberia. The Newfoundland Basin was affected by early postrift magmatism [83], but it is not yet clear if the magnitude of this event was sufficient to account for the asymmetry. Thus it is possible that the depth offset may have existed from the time of continental separation. By chron 34, crust forming along the MAR is relatively symmetric in depth and it is not disturbed by large anomalies. There is only a slight shallowing of basement depth at the Biscay triple junction (BTJ) and northwards. 4.2. Chrons 25 (56 Ma—late Paleocene) and 21 (47 Ma—middle Eocene) (Fig. 7) During this period there was a general increase in volcanic activity along the MAR. Elevated basement in Davis Strait (DS) increases in depth rapidly toward the central Labrador Sea (LS). Elevated basement is coincident with the volcanic margins of East Greenland (EG) [50] and Hatton-Rockall Bank (HRB) [51,52]. The elevated basement extends south of Greenland (SGH) and Hatton Bank, with a few isolated highs continuing to the Charlie-Gibbs fracture zone (CGFZ). Seismic studies have shown basement K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 volcanics immediately south of Greenland [53] and Edoras Bank [54]; these features were originally contiguous with one another and with the SGH. They most likely were created by an early stage of volcanism associated with the Iceland plume, in which plume magmatism spread as a convective sheet or was channeled along the rifting plate boundary [53,55]. South of the CGFZ, elevated basement is associated with the East and West Thulean rises and with a rejuvenated Newfoundland-Milne (or a separate Milne) hotspot. The Thulean rises (TR) were formed over a short time interval that ended by the time of chron 21, and they appear to be spatially separated from elevated basement associated with Iceland. The Milne hotspot (MS) may have been active at the MAR axis shortly following chron 34, and there is a suggestion that it migrated south along the MAR, forming a V-shaped trail [56]. Subsequently, the Milne hotspot appears to have migrated back to the north along the MAR, forming elevated topography at the ridge axis. The appearance of elevated topography at the Azores triple junction (ATJ) first occurred immediately following Chron 21, and it later developed a more robust Azores high. To the south, the Great Meteor hotspot that earlier had formed the New England—Corner seamount chain (NES and CRS) migrated across the MAR at f 76 Ma and formed the Cruiser Plateau (CP) as an off-axis feature on the east side of the MAR [57]. Volcanism associated with this hotspont also may have migrated northward from f 75 to f 30 Ma to form the elevated areas immediately north of the Hayes FZ (i.e. Plato and Atlantis seamounts). 4.3. Chrons 13(33 Ma—early Oligocene) and 5 (10 Ma—late Miocene) (Fig. 8) By chron 13, increasing elevation along the Reykjanes Ridge (RR) near Iceland is apparent. To the south, most of the region of elevated basement is flat. It was not until after chron 13 that the elevated, Vshaped topography of the Reykjanes Ridge formed. There was little anomalous basement elevation from the Charlie-Gibbs fracture zone (CGFZ) to f 100 km northward, but the area became shallower by chron 5 as elevated topography developed on the Reykjanes Ridge. 201 Anomalous basement elevation is reduced south of Charlie-Gibbs fracture zone. Volcanism stopped quickly at the Thulean rises, and the influence of the Milne hotspot declined by the time of chron 5. For a period around chron 13, shallow topography was created east of the MAR around the Kings Trough (KT) and Azores-Biscay Rise (ABR) plate boundary [58]. From chron 13 to chron 5, there was a significant increase in basement elevation associated with Azores hotspot (ATJ), but it was asymmetrically displaced toward the east side of the MAR along the AzoresGibraltar plate boundary. This was followed by a relative decrease in anomalous elevation from chron 5 to present along the MAR [59] (Fig. 5), while volcanism continued on the Azores Islands to the east (e.g. [60]). South of the Azores, elevated topography was restricted primarily to the Africa plate and was associated with the Great Meteor hotspot. There was southward relative migration of the Great Meteor hotspot as it passed beneath Cruiser plateau (CP) and formed the Great Meteor seamount (GMS) [57]. 5. Implications 5.1. Variations in hotspot activity with time The above analysis of residual basement topography indicates that all of the significant positive depth anomalies (i.e. excess topography) can be systematically reconstructed to specific periods and locations of excess volcanism (hotspots). South of the Azores, hotspots were not fixed at the MAR axis, so magnetic-anomaly ages of basement do not constrain the timing of volcanism, except during the period when the Great Meteor hotspot crossed the MAR ( f 75 – 30 Ma) to form the elevated topography on the African plate [57]. Since that time, all volcanism in this region has been located on the African plate. It is noteworthy that the Azores and GMS hotspots may have interacted during the period f 47– 33 Ma (Figs. 7 and 8); this could complicate interpretation of isotopic anomalies in ocean crust that formed during this time along the MAR between the Hayes and Azores fracture zones (e.g. [61]). From the Azores northwards, all of the excess volcanism appears to have been centered on or closely 202 K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 Fig. 7. Basement depth anomaly reconstructed at chrons 25 and 21. Thin lines are magnetic isochrons (shown in Fig. 4) and isobaths (shown in Fig. 2). Abbreviations for features discussed in text (additional to those given in Fig. 5) are HFZ (Hayes Fracture Zone), and TR (Thulean Rise). K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 203 Fig. 8. Basement depth anomaly reconstructed at chrons 13 and 5. Thin lines are magnetic isochrons (shown in Fig. 4) and isobaths (shown in Fig. 2). Labels are as given in Fig. 5. 204 K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 adjacent to the MAR axis at five distinct locations. There is a significant range in duration of volcanism at these sites (Table 2), from 60 m.y. (Iceland) to as little as 8 –10 m.y. (Thulean Rise). The present period of excess ridge-crest volcanism associated with Iceland, and to a lesser extent the Azores, is quite distinct from earlier periods of seafloor spreading. In particular, from the end of volcanic activity associated with formation of the Newfoundland Rise and Madeira-Tore Rise ( f 120 –110 Ma) to the start of robust volcanism associated with Milne seamounts and the Azores-Biscay Rise ( f 80 – 75 Ma) and eventually Iceland (60 Ma), there is little evidence from basement topography for excess volcanism throughout the entire region. One exception, as noted earlier, is the Newfoundland and possibly Tore seamounts, which seem to have formed at about 98 Ma. We also note the lack of significant volcanism during continental breakup over roughly the same period [62]. 5.2. Variations in crustal thickness Plots of basement depth anomaly versus distance along isochrons are given in Fig. 9 relative to a fixed position for the Charlie-Gibbs fracture zone (CGFZ). Along the present MAR, we observe elevated basement zones with f 1500 km half-widths centered on the Azores and Iceland hotspots. These features have often been described in previous work, generally relating the elevations to geochemical anomalies (e.g. [63]). The CGFZ is clearly identified with the boundary between these two large-scale anomalies. Note also that the maximum amplitude of the Azores signature is reduced relative to Iceland, in part be- Table 2 Duration of hotspot activity Hotspot Start Stop Duration Reference Iceland Azores Milne A-BR + KT Newfoundland Ridge Madeira-Tore Rise Thulean Rise West Thulean Rise East 60 33 – 48 76 79 129 138 59 54 Active Active 53 – 36 52 – 31 109 118 49 46 60 33 – 48 23 – 40 27 – 48 20 20 10 8 *[51] *[59] * *[54] * * * * * This paper. Fig. 9. Basement depth anomaly from Fig. 5 plotted along isochrons 0, 5, 13, 21, 25, 34 and M0. Depths west and east of the MAR are shown in black and gray, respectively. Plots are aligned relative to the Charlie-Gibbs Fracture Zone (CGFZ). Positions of the Newfoundland-Azores and Azores-Gibraltar fracture zones are shown by vertical black and gray lines, respectively. Mean depths (horizontal lines) are shown for the segments delineated by the fracture zones. Labels of features are as given in Fig. 5. cause the present Azores hotspot has shifted east of the MAR. Along isochrons older than chron 5, wavelengths of basement anomalies (300 – 500 km) related to various hotspots, are much shorter than those along the present spreading axis. This agrees with existing models in which anomalous mantle flow caused by a hotspot is directed for significant distances along the spreading center [64,65]. The K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 basement anomaly that is present along the older isochrons is caused by thickened crust produced by excess decompression melting at elevated asthenospheric temperatures [66]. Studies from Iceland and the Reykjanes Ridge show that the extent of primary excess crustal thickness is much more limited (i.e., a half wavelength of 100 –200 km) than the excess elevation along the MAR [67], and therefore the elevated temperatures are similarly narrow [68]. Another prominent feature in Fig. 9 is the apparent asymmetry between average depths on opposite sides of the MAR, with east-flank depths consistently shallower than depths to the west (except for the Newfoundland basin as previously mentioned). There is some evidence that this long-wavelength feature may be a dynamic effect of mantle flow. In Fig. 10 we show a calculation of dynamic topography [69] for our study area, derived from the seismic tomographic model of Grand [70]. An obvious feature of the dynamic topography is uplift associated with the Iceland plume and the African plate. To remove these effects, we subtract the dynamic topography from our Fig. 10. Dynamic basement topography [69] predicted by a geodynamic model of mantle flow derived from the seismic tomography model of Grand [70]. 205 observed anomalous basement depths and then convert the remaining anomalous basement topography to crustal thickness variations assuming local isostatic balance (Fig. 11). In this case, qm qw tc ¼ Dh þ ta qm qc where tc is the isostatic crustal thickness; Dh is the observed anomalous basement depth; qw (1.03 Mg/m3) and qm (3.3 Mg/m3) are the water and mantle densities; and ta (7 km) is the average oceanic crustal thickness [71]. For the average crustal density (qc) we use a value of 2.95 Mg/m3, assuming that most of the excess crust is produced by underplating of mafic melt with typical velocities of 6.8– 7.4 km/s and densities of 2.9 –3.0 Mg/m3 (e.g. [72,73]). Profiles across features of interest, compared with observed values of crustal thickness from seismic models, are shown in Fig. 12. The largest predicted thicknesses of 27 – 29 km (Fig. 12, top left) are associated with the Greenland – Iceland – Faeroe Ridge, consistent with observed values of f 30 km [72,74]. Excess volcanic thicknesses of 17 –18.5 km observed along the conjugate margins of East Greenland [74,84] and Hatton-Rockall Bank [75,76] also agree with our predicted thicknesses under the continental slope (Fig. 12, bottom). Further landward, the greater predicted crustal thicknesses along profiles 12 and 13 probably are related to increasing contributions from continental crust across the ocean – continent transitions. As shown in Fig. 11, thick volcanic crust (15 – 20 km) also is predicted south of Davis Strait and south of Greenland where it forms the SGH. For oceanic crust in the Iceland and Irminger basins, our predicted crustal thicknesses of f 12 km are close to observed values of 9.5– 11 km [74 – 76] (Fig. 12, bottom). However, for the Reykjanes Ridge predicted values of f 13 –15 km are significantly greater than observed values of 7.5 –10 km [67,77]. This probably indicates that dynamic uplift related to shallow-mantle variations in the Iceland plume has not been completely removed by the theoretical calculation. Volcanic features between the Charlie-Gibbs F.Z. and the Azores have predicted maximum thicknesses on the order of 20 km. The Milne Seamounts (MS) and Azores-Biscay Rise (ABR) are somewhat thicker 206 K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 Fig. 11. Adjusted residual basement depth and isostatic crustal thickness computed from the residual depth anomaly (Fig. 5), with the dynamic topography of Fig. 10 removed. Conversion from residual depth to crustal thickness assumes an average crustal density of 2.95 Mg/m3. See text for details. Extracted data for the numbered profiles are given in Fig. 12. Note that profiles 12 and 13 approximately connect margin conjugates but do not follow flow lines. and thinner than this, f 23 and 15– 16 km, respectively, and the Southeast Newfoundland Ridge is also thicker at f 26 km. The seismically determined thickness for the Madeira-Tore Rise (MTR) (16 km) [46] is only 4 km less than our prediction, but the ABR seismic thickness (9 km) [56] is much lower ( f 7 km) than predicted. The latter seismic value is complicated by the unusual interpretation of a lowvelocity zone (6.4 km/s) beneath the ridge, and this model is not as well constrained as for the other profiles. Predicted crustal thicknesses at the Azores range from about 25 –35 km. K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 207 Fig. 12. Relative isostatic crustal thickness variations for the profiles identified in Fig. 11. The thick grey bars give observed crustal thickness determined from refraction profiles for the Iceland Ridge [72,74], Madeira-Tore Rise [46], Azores-Biscay Rise [56], Reykjanes Ridge [67,77], Hatton-Rockall Bank and adjacent basin [75,76], and the East Greenland margin and adjacent basin [74,84]. The widths of the above features vary from 250 to 300 km (Azores, MTR and NR) to 150 km (ABR, MS and TR). These values suggest less robust volcanism and lower mantle temperatures than at Iceland. For instance, a simple passive melting model [66] would predict temperatures f 50 – 100 jC lower for the Azores and f 100 –150 jC lower for the ABR and MS relative to Iceland. Previous studies of Schilling [78] and Ito and Lin [65] also predict a 50 – 100 jC difference in mantle temperature between the Iceland and Azores plumes. More complex dynamic models of upwelling are also consistent with such narrow plumes of excesss melt [79]. Removal of the dynamic topography produces a much more symmetric pattern of depths between the oldest ocean basins off Nova Scotia and Morocco (cf. Figs. 5 and 11). In contrast, however, there is an increased discrepancy between the residual depths of basins off Newfoundland and Iberia. In addition, there are significant regions of ocean basins, primarily those adjacent to some continents (e.g. France, Iberia, NW Africa, Nova Scotia) where the predicted isostatic crustal thickness is very small or even negative. Although the crust may be thin adjacent to some nonvolcanic margins (e.g. [34]), these particular values are certainly not valid, and they most likely indicate either non-isostatic effects, unmodeled variations in the mantle temperature or flow field, or variations from the assumed densities of crust and mantle. For instance, the low values in the Sohm Abyssal Plain off Nova Scotia could indicate edge-driven convection at the continent –ocean boundary [80] that eventually could con- 208 K.E. Louden et al. / Earth and Planetary Science Letters 224 (2004) 193–211 tribute to possible subduction [81]. In the future, with constraints provided separately from seismic crustal thickness, anomalous basement depth, mantle tomography, and heat flow, it may be possible to better quantify such effects. 6. Conclusions A digital map of total sediment thickness for the North Atlantic Ocean, from 30j to 70jN latitude and from 0j to 70jW longitude, is described which clearly depicts the basic characteristics of sediment distribution in the region. This grid in combination with grids of observed and predicted seafloor bathymetry allows us to calculate a grid of basement-depth anomalies corrected for the effects of isostatic sediment loading and lithospheric age. Grids in GMT format [82] of sediment thickness, basement depth anomaly and isostatic crustal thickness (see below) are available at ftp tp://ftp.phys.ocean.dal.ca/pub/users/klouden/ EPSL7162. Using maps of basement depth anomalies plotted for plate reconstructions representing stages of opening of the North Atlantic, we delineate consistent patterns in basement depth that are related to hotspot volcanism. In addition to Iceland and the Azores, we recognize three centers of excess volcanism at the Mid-Atlantic ridge: the Milne Seamount and AzoresBiscay Rise (75 –40 Ma), the Southeast Newfoundland Ridge and Madeira-Tore Rise (130 – 110 Ma), and the East and West Thulean Rises (60 –50 Ma). The duration of volcanic activity ranges from 8 to 10 m.y. (Thulean Rises) to as much as 60 m.y. (Iceland). Thus it is possible for both long- and short-lived hotspots to coexist, even in relatively close proximity. We note that there was little excess volcanism north of the Azores during the period from approximately 110 to 60 Ma. Finally, we note that all hotspots north of the Newfoundland-Gibraltar F. Z. have been located at or near the spreading center, while those to the south are not. From the anomalous basement depths, we estimate isostatic crustal thickness after removing a model of dynamic effects created by mantle flow. Maximum predicted thicknesses of volcanic features, from 30 km beneath the Greenland –Iceland –Faeroe Ridge to f 15 km beneath the Azores-Biscay Rise, are broadly consistent with seismic models and with calculations of decompression melting. Widths of volcanic features indicate that thickening primarily occurs within 100 –200 km of hotspots except along continental margins (i.e. East Greenland and the Hatton-Rockall Bank) that rifted at the time of the hotspot activity. We identify conjugate structures south of Greenland and Edoras Bank, where excess volcanism appears to have extended beyond the margin proper and into oceanic crust. Similar features appear south of Davis Strait in the Labrador Sea. Finally, we locate oceanic regions adjacent to some continental margins where unusually low values of predicted crustal thickness suggest that there are additional variations in plate properties or nonisostatic effects within the mantle that were not modeled. It is hoped that in the future combined constraints from measurement of crustal thickness, mantle velocity, and heat flow will help to better understand these remaining anomalous regions. Acknowledgements K.E.L. acknowledges support from the Natural Sciences and Engineering Research Council (NSERC) of Canada and from the Southampton Oceanography Centre during a sabbatical visit in 2003. 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