Peculiarities of roughness and thickness of oceanic crust in the Eurasian Basin, Arctic Ocean

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1 Geophys. J. Int. (21) 14, 16 Peculiarities of roughness and thickness of oceanic crust in the Eurasian Basin, Arctic Ocean Estella Weigelt and Wilfried Jokat Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, 2768 Bremerhaven, Germany. Accepted 2 December 4. Received 2 December 4; in original form 1999 November 19 INTRODUCTION The Gakkel Ridge, extending over 18 km through the Arctic Ocean, is an active mid-ocean ridge and forms the northern boundary of the Eurasian and American Plate (Fig. 1a). The Spitzbergen Fracture Zone (SFZ) connects it to the Mid- Atlantic Ridge system. From a ridge-transform junction at the northeastern continental margin of Greenland, the ridge crosses the Eurasian Basin towards the Siberian shelves, where its topography vanishes under a thick sedimentary cover (Sekretov 1998). Aeromagnetic investigations (Karasik 1974; Vogt et al. 1979) provide a well-defined pattern of seafloor spreading magnetic anomalies parallel to and symmetric about the Gakkel Ridge (Fig. 1b). The oldest identifiable anomaly is Chron 24, but a broad magnetic negative anomaly along the southwestern Lomonosov Ridge indicates that the formation of the Eurasian Basin started about 6 Ma. Spreading models reveal a slow SUMMARY The Gakkel Ridge, northern boundary of the American and Eurasian plates, has the slowest spreading rate of the global ridge system. Therefore, it provides an excellent opportunity to study any dependence of crustal fabric on spreading rate. Two parameters, crustal surface roughness and crustal thickness, at the super-slow-spreading Gakkel Ridge (<2 mm yr x1 full rate) are the subject of the following study. Seismic and gravity data acquired across the Gakkel Ridge and the adjacent Nansen and Amundsen basins during the ARCTIC 91 expedition are used. The surface of the basement, imaged along the seismic multichannel profiles, is very rough and varies in its topography from several hundreds of metres up to 1 m. Its RMS-roughness ranges from 4 m in the central Amundsen Basin to 84 m in the southern Eurasian Basin. These values agree reasonably well with RMS-roughness values derived by an empirical model from spreading rates. The gravity models reveal a 6 km thick oceanic crust (density of 29 kg m x3 )in the central part of the Amundsen Basin, increasing to 9 km towards the Gakkel Ridge. At the southwestern end of the Eurasian Basin, the oceanic crust is only 2 km thick and thickens towards the Gakkel Ridge. In our model the ridge is composed of a 2 km thick upper layer with a density of 26 kg m x3, underlain by an 8 km thick zone with a density of 29 kg m x3. The increase of crustal thickness does not confirm theoretical models for the relation between spreading rate and crustal thickness. The results indicate that the super-slow spreading rate of the Gakkel Ridge may have caused lateral variations in the crustal thickness of the Eurasian Basin. Key words: Arctic Ocean, crustal structure, Gakkel Ridge, gravity anomalies, mid-ocean ridges. spreading rate of 22 mm yr x1 at the initial opening, decreasing to the super-slow spreading velocity of 6 mm yr x1 full rate for the Gakkel Ridge between 3 and 2 Ma. The Gakkel Ridge is currently the slowest spreading segment of the world mid-ocean ridge system. Investigations at various ridges worldwide emphasize the influence of spreading rate on crustal genesis. For example, rough ridge topography and a pronounced median rift (e.g. Sclater & Francheteau 197; Hayes & Kane 1991; Malinverno 1991) seem to be typical characteristics of slow-spreading ridges (2 4 mm yr x1 full rate). Furthermore, few data indicate that the oceanic crust thins significantly beneath super-slow ridges (<2 mm yr x1 full rate, e.g. Bown & White 1994). However, all empirical models concerning spreading rate and its influence on crustal structure rely on investigations at ridges with a full-spreading rate of more than 2 mm yr x1, for example the Mid-Atlantic Ridge or the fast-spreading (>8 mm yr x1 ) Downloaded from by guest on 8 January 219 # 21 RAS

2 6 E. Weigelt and W. Jokat 86 N 9 W Greenland (a) Morris Jesup Rise 2 e Lomonosov f 3 b SFZ Makarov Basin line 3 a c 86 N 18 line 2 line 1 Yermak Plateau 79 N Ridge Eurasian Basin Amundsen Basin Gakkel Nansen Basin Svalbard East Pacific Rise. Therefore, the Gakkel Ridge provides an important opportunity to improve our knowledge of ridge systematics. Thermal models of mid-ocean ridges imply a faster cooling of magma and therefore an increasing viscosity of the melt below slow-spreading ridges (Sleep & Rosendahl 1979). This effect would lead to a lower basalt productivity and a thinner oceanic crust made rugged by faults. Presumably, a change between volcanic activity and normal faulting of crustal blocks due to tectonic strain during drifting supports the roughness of basement surface (Louden et al. 1996). These characteristics are supposed to be strongly developed along the super-slow Gakkel Ridge. Knowledge of the crustal structure in the Eurasian Basin is sparse due to its permanent ice cover, which limits access for research vessels. Since 198, US and British nuclear submarines have provided information about the bathymetry of the Gakkel Ridge and the adjacent basins (e.g. Dietz & Shumway 1961; Johnson & Heezen 1967). The data document a rough seafloor topography close to the Gakkel Ridge and a deep rift valley. In the older parts of the Amundsen and Nansen basins the basement structure is buried under sediments. Various seismic, magnetic, bathymetric and gravimetric experiments were carried out from drifting ice floes as the ARLIS II d line 4 Ridge 84 N 9 E N 9 W Lomonosov Amundsen Basin 6 86 N N 6 13 project (Kutschale 1966) or the FRAM I-IV expeditions (e.g. Duckworth et al. 1982; Jackson et al. 1982; Kristoffersen et al. 1982). Some of the seismic refraction measurements (Fig. 1a, line b) indicate the presence of anomalously thin crust (2 3 km) at the westernmost part of the Eurasian Basin (Jackson et al. 1982). Other investigations (Fig. 1a, lines a, d and f) yield a crustal thickness of 6 7 km (Kristoffersen et al. 1982; Duckworth et al. 1982). These results are too sparse in geographical distribution for any systematic match with global models to be perceived. This sparsity is mainly due to the unpredictable course of the drifting ice, and logistic reasons which restricted the expeditions to the southwestern Eurasian Basin. A large shooting distance and single-channel records also restricted the quality of the seismic data. Furthermore, the available submarine data from those early cruises are too sparse for any detailed investigation of the ridge s topography. This situation dramatically changed in 1993 through the initiation of the SCICEX programme, within which systematic submarine surveys across the ridge were conducted. In the 1998 and 1999 seasons a side-scan sonar system was added to provide a 3-D image of the ridge (Kurras et al. 1999). The studies on-board the US Navy nuclear submarine USS POGY along the Gakkel Ridge show a rough ridge morphology with axial depths of 46 1 m. Analyses of the gravity data indicate a Ridge Ridge Gakkel Nansen Basin Figure 1. (a) The study area in the Eurasian Basin of the Arctic Ocean with the ships track of the ARCTIC 91 expedition (thin line). Along lines 1 and 2 (thick line segments) seismic and gravity data were acquired during the cruise; along lines 3 and 4 (thick line segments) only gravity was measured. The thick broken lines show seismic refraction observations of the LOREX, and FRAM expeditions: (line a) from Kristoffersen et al. (1982); (line b) from Jackson et al. (1982); (line c) from Jackson et al. (1984); (line d) from Duckworth & Baggeroer (198); (line e) from Weber (1979); and (line f) from Duckworth & Baggeroer (198). (b) Pattern of the magnetic anomalies and the Chron numbers for the Amundsen and Nansen basins (modified after Kovacs et al. 198). The thick black lines show the locations of the seismic lines 1 and 2. Here, the ages of the magnetic anomalies are used to calculate the standard subsidence curves for oceanic crust of the Gakkel Ridge. (b) line 1 13 line N 9 E Downloaded from by guest on 8 January 219 # 21 RAS, GJI 14, 16

3 crustal thickness in the rift valley of less than 4 km (Coakley & Cochran 1998), which is supposed to be typical of super-slow spreading ridges (Bown & White 1994). However, the profiles extend only 4 km from the ridge axis, and therefore no information on the basement variations in the adjacent Amundsen and Nansen basins is available to be tied with the seismic refraction data acquired during the FRAM drift island expeditions. Information about off-axis parameters such as basement topography, sedimentary and crustal thickness were acquired during the ARCTIC 91 expedition (Fütterer 1992). Two research vessels, Polarstern and Oden, crossed the entire Eurasian Basin in a convoy, from the Barents Shelf across the Gakkel Ridge to the Lomonosov Ridge and back to Svalbard (Fig. 1a). Geological sampling, multichannel seismic reflection experiments, wide-angle and gravity recordings were carried out on-board Polarstern. The combination of seismic reflection and wide-angle data provides the first reliable information on the thickness of the entire sedimentary cover of the southwestern Eurasian Basin (Fig. 2; line 1 in Fig. 1a) (Jokat et al. 199b) and central Amundsen Basin (Fig. 3; line 2 in Fig. 1a) (Jokat et al. 199a). These data were used to map the topography of the basement surface and thus to establish a roughness study for the Gakkel Ridge. Furthermore, knowledge about the sedimentary thickness and seismic velocities provides important constraints on the crustal gravity models. In this contribution, the results of the gravity modelling and the roughness calculations for the oceanic basement will be presented along several transects in the Eurasian Basin. The results are compared with existing models for crustal thickness versus spreading rate. Oceanic crust in the Eurasian Basin 7 ACQUISITION OF SEISMIC AND GRAVITY DATA In total, 1 km of multichannel seismic reflection data were acquired during the ARCTIC 91 expedition across the Eurasian Basin. For the seismic investigations a cluster of eight airguns (each 3 l in volume) and a 3 m long streamer (12 channels) were used. In addition, wide-angle data were gathered using sonobuoys. The maximum signal range recorded by the buoys was 1 km. This set-up allowed us to image the whole sedimentary cover down to the basement with reasonable quality (Jokat et al. 1992). The resulting depth sections (Figs 2 and 3) allowed us to calculate the RMS-roughness of the basement surface and to compare the results with values for other ridges (Fig. 4). The gravity data recorded along the whole cruise track were acquired with a KSS31 gravimeter (Bodenseewerke) every 1 s. With an average ship speed of knots, the sample distance is 2 m. The ship s navigation system supplied GPS data for time, course, position and ship s speed over the ground to calculate the free-air gravity (Jokat 1992). The gravity data (Figs to 8) show high-frequency spikes resulting from the irregular movements of the ship during ice breaking. The noise along these transects from ice breaking is higher, since Polarstern operated without support from Oden. Nevertheless the records reveal very clearly the much longer wavelength anomalies of the important structures such as the ridge and basins. No de-spiking or further filtering was applied to the data: the records are shown here in their original quality. Low-pass filtering would result in a negative offset of the data because of the mainly negative acceleration during ice breaking. De-spiking has poor success because of the high number of spikes. Downloaded from by guest on 8 January 219 Figure 2. Depth transect along line 1 (Fig. 1a) from seismic reflection and refraction measurements. Black arrows mark the position and Chron number of the magnetic anomalies crossed (Fig. 1b). The references to determine the basement surface differences are marked by thick black lines. Here the subsidence depth of oceanic crust defined by Parson & Sclater (1977) was taken. The upper curve shows the residual roughness. The RMS-roughness values R for the basement surface are noted for the Amundsen and Nansen basins. # 21 RAS, GJI 14, 16

4 8 E. Weigelt and W. Jokat Figure 3. Depth transect along line 2 (Fig. 1a) from seismic reflection and refraction measurements. Black arrows mark the Chron number and position of the magnetic anomalies crossed (Fig. 1b). The references to determine the basement surface differences are marked by thick black lines. From Chron 21/2 to Chron 6 the regional basement trend is the subsidence depth of oceanic crust defined by Parson & Sclater (1977); from Chron 2 21/2 it is a linear function. The upper curve shows the residual roughness. The RMS-roughness values R of the basement surface are noted for the two segments. RMS-Roughness [m] Gakkel Ridge, this study Malinverno, Full Spreading Rate [mm/y] Figure 4. The relation between full-spreading rate and the RMSroughness of basement surface established by Malinverno (1991) (black line). The black circles show the database of Malinverno s empirical model with average values for ridges with the same spreading rate. The stars mark the included values for the RMS-roughness at the Gakkel Ridge resulting from this study. The grey shaded area marks the range of the full-spreading rate for the Gakkel Ridge. For this study four profiles acquired during the expedition were chosen (Fig. 1): a transect through the southwestern Eurasian Basin from the Morris Jesup Rise to the Yermak Plateau (line 1), a transect through the central Amundsen Basin from the rim of the Lomonosov Ridge towards the Gakkel Ridge (line 2), a transect almost parallel to the ridge axis along the Amundsen Basin (line 3), and a profile from the edge of the Barents Shelf through the Nansen Basin towards the Gakkel Ridge (line 4). Along lines 3 and 4 only gravity and bathymetric data are available. TOPOGRAPHIC ROUGHNESS OF THE OCEANIC BASEMENT The seismic reflection profiles across the Eurasian Basin show a rough basement surface with differences in elevation of up to 1 m. Analyses of four sonobuoys in the southwestern part (Fig. 1a; line 1) and 1 buoys in the central Amundsen Basin (Fig. 1a; line 2) provide seismic velocities ranging from 1.6 to 4. km s x1 for the sedimentary layers. No seismic velocities for the upper oceanic crustal layer were identified in the data (Jokat et al. 199a). Whereas in the southwestern part of the basin the oceanic crust is mostly exposed at the seafloor (Fig. 2), the basement in the central Amundsen Basin is buried under a thick sedimentary cover of 32 m at maximum (Fig. 3). For calculating the topographic roughness we determined the RMS deviation of the basement surface from a regional Downloaded from by guest on 8 January 219 # 21 RAS, GJI 14, 16

5 Oceanic crust in the Eurasian Basin 9 Free-air Gravity [mgal] N W Calculated Freeair Gravity Anomaly Observed Freeair Gravity Anomaly 82.6 N E Free-air Gravity [mgal] Line 1 Morris Jesup Rise Amundsen Basin Gakkel Ridge Nansen Basin Distance [km] Figure. Crustal model from free-air gravity data for line 1 (Fig. 1a). For calculating the density and depth values, seismic velocities for the sediments along the transect were taken from Jokat et al. (199b). The densities are annotated in kg m x3. The stars show thickness values based on observations by Kristoffersen et al. (1982) for the Gakkel Ridges centre (reference a: FRAM-I); from Jackson et al. (1982) for the Amundsen Basin (reference b: FRAM-I); and from Jackson et al. (1984) for the Nansen Basin towards the flank of the Yermak Plateau (reference c). depth trend (Malinverno 1991). R ¼½&ðh ðiþ 2 Þ=nŠ 1=2 for i ¼ 1, n; R ¼ roughness ½mŠ; h ¼ depth deviations ½mŠ : For a regional trend we take the theoretical subsidence depth of oceanic crust given by Parson & Sclater (1977). They defined the relation between age and depth of oceanic crust as dðtþ ¼2 þ 3t 1=2 for d ½mŠ; t ½MaŠ : For this study the age of the crust is derived from magnetic anomalies as reported by Vogt et al. (1979) for the Amundsen and Nansen basins. Along the transects, the roughness values were calculated with a spacing of 1 km. The observed roughness values are compared with calculated values derived from spreading rates as given by Malinverno (1991): R ¼ 1296l :39 for R ¼ roughness ½mŠ, l ¼ spreading rate ½mm yr 1 Š : 13 water oceanic crust b 29 oceanic crust a 33 mantle 32 mantle 33 mantle Malinverno s empirically determined relationship between roughness and spreading rate is based on 11 profiles across *** *** *** *** 29-1 a number of ridge systems with full-spreading rates between 14 (Southwest Indian Ridge) and 13 mm yr x1 (East Pacific Rise). Line 1: Southwestern Eurasian Basin The depth section through the southwestern Eurasian Basin (Fig. 2) shows strong basement elevations scarcely covered by sediments. Sediment pockets only a few hundred metres thick exist between the topographic highs. The topographic differences for the Amundsen Basin are determined along a 246 km long profile perpendicular to the axis of the Gakkel Ridge and extending from the northern ridge shoulders to Chron 13 (34 Ma) (Figs 1b and 2). The resulting RMS-roughness value for the basement topography amounts to 84 m. The RMS-roughness value for the Nansen Basin is determined for a 21 km long profile from the southern ridge shoulders to the corresponding southern magnetic anomaly Chron 13 (34 Ma) and amounts to 4 m (Figs 1b and 2). Spreading rate models based on magnetic anomaly interpretation imply a total opening rate of 6 12 mm yr x1 (34 Ma to recent) for the Amundsen Basin, and 6 14 mm yr x1 c c Yermak Plateau *** Downloaded from by guest on 8 January 219 # 21 RAS, GJI 14, 16

6 1 E. Weigelt and W. Jokat Free-air Gravity [mgal] N W Calculated Freeair Gravity Anomaly Observed Freeair Gravity Anomaly 86.2 N 9.4 E 1 - Free-air Gravity [mgal] Line2 ***** e Amundsen Basin 13 water 19-2 sediments 29 oceanic crust 33 mantle ***** Distance [km] (37 Ma to recent) for the Nansen Basin (Karasik 1974; Vogt et al. 1979). Applying the roughness spreading rate relation of Malinverno (1991), these opening rates would result in roughness values from 493 (corresponding to 6 mm yr x1 )to 312 m (corresponding to 14 mm yr x1 ). Line 2: Central Amundsen Basin The depth section across the central part of the Amundsen Basin runs obliquely to the Gakkel Ridge axis from the North Pole towards the Ridge (Fig. 1a). It reveals a similar rough basement surface (Fig. 3) to that observed on line 1. Here, the basement is covered by sediments with a maximum thickness of 32 m. A pronounced jump in basement depth of about 1 m occurs between Chrons 2 and 21 (km 47, Fig. 3). At this point, a significant change in the regional trend can be observed. This made it sensible to divide line 2 into two segments for RMS-roughness calculations (Fig. 3). The observed RMS-roughness for crust accreted between Chron 2 (46 Ma) and Chron 6 (2 Ma) amounts to 4 m. In this time range the full-spreading rates decrease from 13 to 6mmyr x1. The corresponding calculated roughness values (Malinverno 1991) are 32 and 493 m. For oceanic crust older than Chron 21 the general basement level is lowered abruptly (Fig. 3, km 4). This cannot be explained by sediment-corrected standard subsidence curves. Therefore a statistical approach with a linear regression line f Gakkel Ridge -----> ~ 1 km Figure 6. Crustal model from free-air gravity data for line 2 (Fig. 1a). For calculating the density and depth values, seismic velocities for the sediments along the transect were taken from Jokat et al. (199a,b). The stars show thickness values based on observations of the LOREX expedition after Weber (1979) for the northern Amundsen Basin (reference e), and from the FRAM-II expedition after Duckworth & Baggeroer (198) for the Amundsen Basin (reference f). (Malinverno 1991; Minshull 1999) was used to define the reference level for the crust created between Chron 24 (3 Ma) and Chron 2 (44 Ma). The resulting RMS-roughness value here is 4 m. The full-spreading rate in this area was between 13 and 19 mm yr x1 (Vogt et al. 1979) and therefore the calculated roughness values after Malinverno (1991) range from 32 to 26 m. Along both transects (Fig. 1; lines 1 and 2) the observed RMS-roughness for the crustal topography reveals high values, corresponding to the hypothesis of increasing roughness with decreasing spreading rate. To emphasize this, the new roughness values were plotted on a spreading rate versus roughness diagram (Malinverno 1991) (Fig. 4). Although the observations for the empirical equation are derived from ridges with a full-spreading rate faster than 14 mm yr x1, the theoretical curve predicts the roughness in our area of investigations. It should be noted that the full-spreading rates of the Gakkel Ridge range from 6 to 22 mm yr x1 (Fig. 4), and therefore the theoretical roughness values can range from 493 to 26 m. GRAVITY MODELLING OF THE CRUSTAL THICKNESS Gravity and bathymetry data were recorded along the whole cruise track of the ARCTIC 91 expedition (Fig. 1a). In general, the Nansen and Amundsen basins are characterized by a x2 to x4 mgal free-air anomaly (Figs to 8). Gravity increases to Downloaded from by guest on 8 January 219 # 21 RAS, GJI 14, 16

7 Oceanic crust in the Eurasian Basin 11 Free-air Gravity [mgal] N W Calculated Freeair Gravity Anomaly Observed Freeair Gravity Anomaly 86.2 N 9.4 E 1 - Free-air Gravity [mgal] Line2 ***** e Amundsen Basin 13 water 19-2 sediments 29 oceanic crust 33 mantle ***** Distance [km] 4 mgal at the shoulders of the Gakkel Ridge and decreases to x mgal in the central valley (Fig. ). 2-D crustal density models were constructed from these data by applying the LCT software (# by LCT, Inc., Houston). The basis for the 2-D modelling is the algorithm of Rasmussen & Pedersen (1979). Model parameters The thickness of the sediment layers and the topography of the basement surface result from the depth sections interpreted from the multichannel seismic reflection profiles (Jokat et al. 199a,b). Densities for the sediment layers were calculated according to the density velocity relation of Nafe & Drake (197) by applying the seismic velocities from the sonobuoy recordings. In our model the density for the sediments varies between 19 and 24 kg m x3. For the oceanic crust we mostly used a density of 29 kg m x3 according to similar studies along mid-ocean ridges (Miller & Christensen 199). In our model the mantle has a density of 33 kg m x3 (Christensen 1972). A low-density zone of 32 kg m x3 for the upper mantle is introduced below the central part of the ridge to model the strong negative gravity anomaly. Additional constraints on the thickness and structure of the oceanic crust were derived from results of seismic refraction investigations during the LOREX and FRAM expeditions (Weber 1979; Jackson et al. 1982; Kristoffersen et al. 1982), although their soundings were not along our transects. f Gakkel Ridge -----> ~ 1 km Figure 7. Crustal model from free-air gravity data for line 3 (Fig. 1a). The sedimentary thickness and density results are from our own seismic observations. The stars show the reference values for the thickness of the oceanic crust after this study (lines 1 and 2) and after Duckworth & Baggeroer (198) for the Amundsen Basin (reference f). Gravity model for the southwestern Eurasian Basin (line 1) Fig. shows the free-air gravity profile used for this study. The solid black line is the calculated gravity of the crustal model below. The free-air gravity anomaly increases from x3 mgal in the Amundsen and Nansen basins to + 4 mgal on the flanks of the Gakkel Ridge. In the central part of the Gakkel Ridge it decreases to x mgal. The thin sediments were modelled with densities of kg m x3. As indicated by the seismic reflection profiles (Jokat et al. 199b), only a little sediment is present in basement pockets in the central part of the transect. Therefore, the seafloor surface represents the basement surface in this part of the model. The wide-angle data from the ARCTIC 91 cruise did not provide any information on the composition and thickness of the oceanic crust. Therefore we included results of previous seismic refraction experiments about 1 km away from line 1 (Jackson et al. 1982, 1984; Kristoffersen et al. 1982). For the location of the profiles see Fig. 1(a). Based on results from Kristoffersen et al. (1982) the oceanic crust close to the Gakkel Ridge was divided into a 2. km thick upper part with a density of 26 kg m x3 and a lower part down to the Moho at 8 km depth with a density of 29 kg m x3. In a first modelling step, only the crustal thickness was varied until the differences between measured and calculated free-air Downloaded from by guest on 8 January 219 # 21 RAS, GJI 14, 16

8 12 E. Weigelt and W. Jokat Free-air Gravity [mgal] N 9.24 E Calculated Freeair Gravity Anomaly Observed Freeair Gravity Anomaly 83. N E 1 - Free-air Gravity [mgal] -1 Line km <----- Gakkel Ridge 4 gravity did not exceed mgal. The model (Fig. ) shows in general an increase of crustal thickness towards the Gakkel Ridge that is closely correlated with the shallowing of the basement. The model confirms in general the crustal thickness determined from the seismic studies mentioned above. The oceanic crust in the central part of the Amundsen and Nansen basins actually appears to be only 2 3 km thick (km and km 48, Fig. ). Towards the ridge a smooth increase of the crustal thickness from 3 km to 6 km from the basins towards the ridge s shoulders has been modelled (km 27 and 34, Fig. ). A low-density zone beneath the ridge is introduced to avoid a difference of 4 mgal between observed and modelled freeair gravity across the rift valley. Here, a density contrast of 1 kg m x3 between partial melt and surrounding mantle seems to be reasonable, as described by the studies of Talwani et al. (196). Otherwise, in the case of a constant density distribution in the mantle the modelling would demand either an increase of crustal thickness up to 1 km below the ridge s flanks or a decrease of density down to 2 kg m x3 for the complete crust off to km from the ridge centre. Both possibilities would differ significantly from other investigations in the Eurasian Basin (e.g. Kristoffersen et al. 1982; Coakley & Cochran 1998) and even from known structures of mid-ocean ridges in general (e.g. Bown & White 1994). 4 3 d ******* 3 2 Distance [km] 2 Nansen Basin water 23 sediments 29 oceanic crust 33 mantle Figure 8. Crustal model from free-air gravity data for line 4 (Fig. 1a). Values for the sedimentary thickness and density were chosen after Kristoffersen & Husebye 1984). Studies from Duckworth & Baggeroer (198) of the FRAM-IV expedition also give parameters for the sedimentary cover, as well as for the crustal depth, marked as stars in the Nansen Basin (reference d). Gravity model for the central Amundsen Basin (line 2) The free-air gravity anomaly increases from x3 to x4 mgal in the central Amundsen Basin to more than 1 mgal towards the Gakkel Ridge (Fig. 6). While the seafloor topography is almost flat along the whole transect, the free-air gravity shows some long-wavelength variations. Most of these can easily be explained by the basement topography as revealed by the seismic data along this line (Fig. 3; Jokat et al. 199a). The general trend in gravity, however, is caused by variations in crustal thickness. The only seismic control on the Moho depth of the Amundsen Basin is that available from the LOREX Expedition close to the Lomonosov Ridge (Weber 1979; Mair & Forsyth 1982). A depth of 12 km for the Moho about 8 km away from line 2 (Fig. 1a) was calculated from these seismic refraction data. In our gravity model we incorporated this information at the start of line 2, where the line is closest to the Lomonosov Ridge (reference e, Fig. 6). The final crustal model for the whole transect reveals a crust of 6 km thickness for the northern part of the Amundsen Basin (km 27, Fig. 6). While the general basement level shallows to the south the crust thickens up to 9 km towards the Gakkel Ridge (from km 4 southwards). This seems to be quite thick for oceanic crust, but a similar thickness for the oceanic crust close to our area of investigation was calculated Downloaded from by guest on 8 January 219 # 21 RAS, GJI 14, 16

9 from the seismic refraction data of the FRAM II expedition by Duckworth & Baggeroer (198) (reference f, Fig. 1). For the Amundsen Basin the authors suggested a 7 km thick oceanic crust about 28 km away from the centre of the Gakkel Ridge formed between Chron 2 and Chron 21. Even if we reduce the density of the oceanic crust from 29 to 28 kg m x3, a minimum depth of 8 km for the crust is still calculated. Taking into account the fact that the two measurements are almost 28 km apart, the results coincide well. All combinations of crustal thickness and reasonable crustal densities tested in this study result in a relative increase of 3 km for crustal thickness towards the centre of the Gakkel Ridge, correlating with the general rise of the basement topography. Gravity model for the Amundsen Basin parallel to the Gakkel Ridge (line 3) Line 3 almost parallels Chron 13 towards the Greenland continental margin (Fig. 1a). At the end of the transect the northernmost tip of the Morris Jesup Rise was reached. While the gravity field along the southwestern Amundsen Basin varies only from x1 to x2 mgal, the Morris Jesup Rise has a pronounced positive anomaly of 4 mgal at maximum. The northern edge of the plateau is marked by a large negative anomaly of x4 mgal (Fig. 7). The sediment layer was modelled with a density of 22 kg m x3, but has no large influence on the final results as the seismic data indicate only a 1 2 m thick cover of deposits (Weigelt 1998). Again the results of the FRAM II seismic refraction experiment (Duckworth & Baggeroer 198), situated 1 12 km away from line 3, were incorporated into the starting model (reference f, Fig. 7). At the start of the transect the oceanic crust is almost 1 km thick (Fig. 7). Its thickness decreases to 6 km in the centre of line 3. The Morris Jesup Rise has a 17 km thick crust, if we apply a uniform density of 29 kg m x3 because of the oceanic origin of the plateau (Feden et al. 1979; Vogt et al. 1979; Dawes 199) (Fig. 7). Gravity model for the central Nansen Basin (line 4) Line 4 represents the easternmost information gathered in the Nansen Basin during the ARCTIC 91 expedition (Figs 1a and 8). The gravity data show higher noise levels as Polarstern operated without a support vessel. The strong disturbances of the gravity data are caused by heavy ice breaking. Along the profile the free-air anomaly has a mean value of x2 mgal. Long-wavelength variations of 2 mgal are observed in the abyssal plain. Here, no control of the sedimentary thickness and basement topography is available from our seismic data (Weigelt 1998). For modelling, a sedimentary cover with a minimum thickness of 1. km deduced from previous experiments (Kristoffersen & Husebye 1984) was included. Furthermore, we assumed that the deposits thin towards the Gakkel Ridge, as observed in the Amundsen Basin (Fig. 3, line 2). Between km 22 and 3 we assumed a crustal thickness of 4 km with the Moho at 1 km depth, according to the results of the FRAM IV experiment (Duckworth & Baggeroer 198) about 17 km southwest of line 4 (reference d, Figs 1a and 8). For the final crustal model (Fig. 8), no major changes in crustal thickness are required for the central part of the Nansen Basin to fit the observed data. The wavelength of about Oceanic crust in the Eurasian Basin 13 2 mgal can be modelled by variations of the basement topography of several hundreds of metres only. So, the basement has a rough surface similar to that imaged by seismic studies for the central Amundsen Basin (Fig. 6) and the southwestern Eurasian Basin (Fig. ). Towards the Gakkel Ridge the crustal thickness increases from 4 km to more than 8 km (km 4, Fig. 8). This corresponds to an ascending basement level of more than 1 km. The model shows that the characteristics of the oceanic crust in the Nansen Basin are similar to those of the central Amundsen Basin (Fig. 3, line 2). Anonymous Soviet studies suppose a 4 km thick sedimentary cover for the Nansen Basin (Jackson & Oakey 199). If the sediment thickness were included in our gravity model, the oceanic crust would become extremely thin (less than 2 km). Such a result no longer satisfies the seismic refraction results of Duckworth & Baggeroer (198), which are the only and closest-situated constraints for modelling. DISCUSSION The combination of the seismic and gravity observations in this study indicates that the crustal structure of the Eurasian Basin is influenced by the extremely slow spreading rate of the Gakkel Ridge as expected. Variations in the basement topography of more than 1 m and the presence of 2 3 km thin crust are supposed to be typical of super-slow spreading ridges (e.g. Reid & Jackson 1981; Malinverno 1991; Coakley & Cochran 1998). The observed roughness along the transects in the Eurasian Basin reveal values of 4 84 m, which are significantly higher than the 1 3 m roughness at faster spreading ridges with a total opening rate of more than 2 mm yr x1. The new roughness data calculated for the Gakkel Ridge fit predictions based on the empirical equation for roughness versus spreading velocity introduced by Malinverno (1991). However, care is necessary because the empirical relation is exponential, and therefore a broad range of roughness values for the range of super-slow spreading rates is predicted. Some control can be exerted by comparing the spreading rates calculated from the roughness values with those defined from magnetic anomalies (Karasik 1974; Vogt et al. 1979). Whereas the results of the roughness study are consistent with published models, the gravity modelling on crustal thickness in the Eurasian Basin reveals surprising results. Our gravity models indicate a crustal thickness of almost 4 km in the rift valley (Fig. 2), confirming the results of Coakley & Cochran (1998). These authors suggested an oceanic crust less than 4 km thick if the crustal density is lower than 29 kg m x3 along the rift valley. However, their profiles, orientated perpendicular to the axis, are only 8 9 km long and cannot provide any information about the crustal fabric of the adjacent basins. Theoretical off-axis crustal thicknesses calculated for a full-spreading rate of 6 mm yr x1 range from 2. km (Reid & Jackson 1981) to 4. km (Bown & White 1994) (Fig. 9). The crustal thickness of 2 3 km modelled from our data for the centre of the southwestern Nansen and Amundsen basins (line 1) satisfies the predictions. Towards the Gakkel Ridge, however, the crustal thickness increases to 6 km (line 1). Although the spreading rate has increased to 13 mm yr x1 during the last 1 Ma, the theoretical models predict a crustal thickness of only 3.. km. A Downloaded from by guest on 8 January 219 # 21 RAS, GJI 14, 16

10 14 E. Weigelt and W. Jokat 12 1 Crustal Thickness [km] x 2 Su et al., 1994 Bown & White, 1994 Reid & Jackson, Full Spreading Rate [mm/y] Figure 9. Various empirical models of the relationship between spreading rate and crustal thickness: Bown & White (1994) (dotted), Reid & Jackson (1991) (dashed), Su et al. (1994) (bold line). The grey circles show the thickness values used for these studies, which are from ridges with spreading rates mostly faster than 2 mm yr x1. The stars mark the thickness values observed in this study for the Gakkel Ridge, and the triangles are thickness values for the Gakkel Ridge observed by Jackson et al. (1982), Kristoffersen et al. (1982), Duckworth & Baggeroer (198) and Duckworth et al. (1982). The cross shows the crustal thickness at the Southwest Indian Ridge (Muller et al. 199). correlation between the crustal thickness and shallowing basement level towards the ridge axis was also observed in gravity and seismic data acquired during the FRAM I experiment by Jackson et al. (1982). They interpreted this effect to be a result of a hot spot (Yermak Hot Spot), which caused the separation of the Morris Jesup Rise and Yermak Plateau. The crustal thickness of 9 km modelled in this study for the central Amundsen Basin (line 2) is much more than that predicted by theoretical models from the spreading rate. The crustal thickness of 7 km modelled along the southwestern Amundsen Basin (line 3) is in good agreement with the average for oceanic crust formed at ridges with a full-spreading rate of more than 2 mm yr x1. For the central Nansen Basin (line 4) the gravity models give crustal thickness values of 4 km, increasing to 8 km towards the flanks of the Gakkel Ridge, which is thicker than oceanic crust observed at other ridges (e.g. Bown & White 1994). The increase of crustal thickness towards the Gakkel Ridge modelled in this study is closely correlated with the rise of basement level. Even if in some parts the crustal thickening is not directly supported by seismic refraction, an increase of 3 km in crustal thickness is necessary to satisfy the recorded free-air gravity. Our observations conflict with those theoretical models of oceanic crustal formation that predict a thinner crust for a slow (<2 mm yr x1 ) spreading rate (e.g. Reid & Jackson 1981; Bown & White 1994). These models are based on the assumption that a thinner crust is the result of lower magma productivity, due to greater conductive cooling at slow-spreading ridges. Moreover, these models imply steady-state conditions in seafloor spreading for extremely low rates (<12 mm yr x1 ) (Coakley & Cochran 1998). In contrast, our gravity models show variations in crustal thickness from 3 to 9 km, which implies changing thermal and pressure conditions resulting in variable magma production below the Gakkel Ridge. One reason for such variable production, suggested by Louden et al. (1996), may be a pulsed supply of melt, which does not keep pace with the rate of plate separation. So the crust could be tectonically thinned and faulted after its accretion. This would also explain the wider median valley and rougher basement surface observed at slow-spreading ridges. Mutter & Karson (1992) also proposed a greater influence of mechanical deformation on crustal fabric for slow-spreading ridges. Further aspects could be mantle temperature and melt distribution additionally controlling lithospheric accretion (Small & Sandwell 1992). Earlier seismic refraction investigations indicated lateral variations in crustal structure in the southwestern Eurasian Basin (summarized in Kristoffersen 199), which are now confirmed by the ARCTIC 91 expedition for the central part of the basin. The results imply that the models for crustal thickness versus spreading rate cannot be transferred simply to the super-slow-spreading Gakkel Ridge. The currently poor geophysical and petrological database of the Eurasian Basin does not permit us to decide which parameters are responsible for the variations in crustal thickness. At present it is not known if the described observations are random or systematic for the Gakkel Ridge. The variations in crustal thickness derived in this study from the gravity data should therefore be treated as observations rather than as proven facts. CONCLUSIONS The crustal structure and the derived models of the relationship to spreading rate are based mostly on observations at ridges with a full-spreading rate greater than 2 mm yr x1. Therefore the investigations of the ARCTIC 91 expedition at the extremely slow-spreading Gakkel Ridge enlarge the spectrum of data to include slow spreading rates, where observations are still rare. The roughness values presented in this study are the first Downloaded from by guest on 8 January 219 # 21 RAS, GJI 14, 16

11 investigations of this kind for the Gakkel Ridge, and fit the predictions of global models. In contrast to this, the density models indicate surprising new results about crustal thickness. (1) A thin crust of almost 4 km was modelled for the rift valley along line 1, as expected for a super-slow spreading axis. This is in good agreement with other studies. (2) The gravity models suggest an increase of crustal thickness, if we approach the Gakkel Ridge from the Nansen and Amundsen basins, although the spreading rates remain almost constantly slow. This is in strong contrast to existing models for oceanic crust. The current database does not allow us to introduce a consistent model to explain our observations. (3) Our data confirm the presence of locally thin oceanic crust in the Amundsen and Nansen basins. However, this may be accompanied by density variations of the oceanic crust, which cannot be tested with the existing data set. The presented gravity models clearly indicate a lack of knowledge about the off-axis parameters of the super-slow Gakkel Ridge. While there is little doubt about the presence of thin crust along the rift valley of the Gakkel Ridge, no strong constraints exist on crustal thickness in the adjacent basins at all. Currently it is not clear whether theoretical thickness versus spreading-rate models are valid for the super-slow Gakkel Ridge, or whether the sparse geophysical and petrological data still provide a biased picture of the crustal fabric of the Gakkel Ridge. The gravity models have to be confirmed by seismic refraction data accompanied by petrological sampling, which hopefully will be acquired in the future. ACKNOWLEDGMENTS We are grateful for the excellent support of the captains and crews of the vessels Oden and Polarstern. Only the joint forces of the two icebreakers and the close co-operation during the measurements made these results possible. This is Alfred Wegener Institute contribution No. n12. REFERENCES Bown, J.W. & White, R.S., Variation of spreading rate of oceanic crustal thickness and geochemistry, Earth planet. Sci. Lett., 121, Christensen, N.I., The abundance of serpentinites in the oceanic crust, J. Geol., 8, Coakley, B.J. & Cochran, J.R., Gravity evidence of very thin crust at the Gakkel Ridge (Arctic Ocean), Earth planet. Sci. Lett., 162, Dawes, P.R., 199. The North Greenland Continental Margin, in The Arctic Ocean Region, vol. L, pp , eds Grantz, A., Johnson, L. & Sweeney, J.F., Geological Society of America, Boulder. Dietz, R.S. & Shumway, G., Arctic Basin geomorphology, Geol. Soc. Am. Bull., 72, Duckworth, G.L. & Baggeroer, A.B., 198. Inversion of refraction data from the Fram and Nansen basins of the Arctic Ocean, Tectonophysics, 114, 12. Duckworth, G.L., Baggeroer, A.B. & Jackson, H.R., Crustal structure measurements near FRAM II in the Pole Abyssal Plain, Tectonophysics, 89, Feden, R.H., Vogt, P.R. & Fleming, H.S., Magnetic and bathymetric evidence for the Yermak hot spot northwest of Svalbard in the Arctic basin, Earth planet. Sci. Lett., 44, Oceanic crust in the Eurasian Basin 1 Fütterer, D.K., ARCTICk91: The expedition ARK VIII/3 of RV Polarstern in 1991, Berichte zur Polarforschung, 17, Hayes, D.E. & Kane, K.A., The dependence of seafloor roughness on spreading rate, Geophys. Lett., 18, Jackson, H.R. & Oakey, G.N., 199. Sedimentary thickness map of the Arctic Ocean region, in The Arctic Ocean Region, vol. L, plate, eds Grantz, A., Johnson, L. & Sweeney, J.F., Geological Society of America, Boulder. Jackson, H.R., Reid, I. & Falconer, R.K.H., Crustal structure near the Arctic Mid-Ocean Ridge, J. geophys. Res., 87, Jackson, H.R., Johnson, G.L., Sundvor, E. & Myhre, A.M., The Yermak Plateau: Formed at a triple junction, J. geophys. Res., 89, Johnson, G.L. & Heezen, B.C., The Arctic-Mid-Ocean Ridge, Nature, 21, Jokat, W., Gravity measurements, in ARCTICk91: The Expedition ARK VIII/3 of RV Polarstern in 1991, ed. Fütterer, D.K., Berichte zur Polarforschung, 17, Jokat, W., Alvers, M., Buravtsev, V., Heesemann, B., Kristoffersen, Y. & Uenzelmann-Neben, G., Marine geophysics, in ARCTICk91: The Expedition ARK VIII/3 of RV Polarstern in 1991, ed. Fütterer, D.K., Berichte zur Polarforschung, 17, Jokat, W., Weigelt, E., Kristoffersen, Y., Rasmussen, T. & Schöne, T., 199a. New insights into the evolution of the Lomonosov Ridge and the Eurasian Basin, Geophys. J. Int., 122, Jokat, W., Weigelt, E., Kristoffersen, Y., Rasmussen, T. & Schöne, T., 199b. New geophysical results from the southwestern Eurasian Basin (Morris Jesup Rise, Gakkel Ridge, Yermak Plateau) and the Fram Strait, Geophys. J. Int., 123, Karasik, A.M., The Eurasia Basin of the Arctic Ocean from the point of view of plate tectonics; Problems in geology of polar areas of the earth, pp , Nauchno-Issledovateliskiy Institut Geologii Arktiki, Leningrad (in Russian). Kovacs, L.C., Bernero, C., Johnson, G.L., Pilger, R.H., Srivastava, S.P., Taylor, P.T., Vink, G.E. & Vogt, P.R., 198. Residual magnetic anomaly chart of the Arctic Ocean region, scale 1:6.. at Latitude 7uN (Projection polarstereographic), Naval Research Laboratory and Naval Ocean Research and Development Activity, The Geological Society of America, Boulder. Kristoffersen, Y., 199. Eurasian Basin, in The Arctic Ocean Region, vol. L, pp , eds Grantz, A., Johnson, L. & Sweeney, J.F., Geological Society of America, Boulder. Kristoffersen, Y. & Husebye, E.S., Multi-channel seismic reflection measurements in the Eurasian Basin, Arctic Ocean, from U.S. ice station, FRAM IV, Tectonophysics, 114, Kristoffersen, Y., Husebye, E.S., Bungum, H. & Gregersen, S., Seismic investigations of the Nansen Ridge during the FRAM I experiment, Tectonophysics, 82, Kurras, G., Edwards, M., Cochran, J. & Coakley, B., Tectonism and volcanism along the Gakkel Mid-Ocean Ridge (u-74ue): Initial proceedings and analysis of SCAMP sidescan data from SCICEX 98 and 99, EOS, Trans. Am. geophys. Un., 8, F998. Kutschale, H., Arctic Ocean geophysical studies; The southern half of the Siberia Basin, Geophysics, 21, Louden, K.E., Osler, J.C., Srivastava, S.P. & Keen, C.E., New constraints from an extinct spreading center in the Labrador Sea, Geology, 24, Mair, J.A. & Forsyth, D.A., Crustal structures of the Canada Basin near Alaska, the Lomonosov Ridge and adjoining basins near the North Pole, Tectonophysics, 89, Malinverno, A., Inverse square-root dependence of mid-oceanridge flank roughness on spreading rate, Nature, 32, 8 6. Miller, D.J. & Christensen, N.I., 199. Velocity behaviour of lower crustal and upper mantle rocks from the slow spreading Mid-Atlantic Ridge, south of the Kane fracture zone, Proc. Ocean Drill. Prog. Sci. Res., 13. Minshull, T.A., On the roughness of Mesozoic oceanic crust in the western North Atlantic, Geophys. J. Int., 136, Downloaded from by guest on 8 January 219 # 21 RAS, GJI 14, 16

12 16 E. Weigelt and W. Jokat Muller, M.R., Minshull, T.A. & White, R.S., 199. Crustal structure at the very slow-spreading Southwest Indian Ridge, Interridge News, 4, 3 6. Mutter, J.C. & Karson, J.A., Structural processes at slowspreading ridges, Science, 27, Nafe, J.E. & Drake, D.C., 197. Variation with depth in shallow and deep water marine sediments of porosity, density and the velocities of compressional and shear waves, Geophysics, 22, Parson, B. & Sclater, J.G., An analysis of the variation of ocean floor bathymetry and heat flow with age, J. geophys. Res., 82, Rasmussen, R. & Pedersen, L.B., End corrections in potential field modeling, Geophys. Prospect., 27, Reid, I. & Jackson, H.R., Oceanic spreading rate and crustal thickness, Mar. geophys. Res.,, Schöne, T. & Döscher, T., Bathymetric survey with HYDROSWEEP, in ARCTIC91: The Expedition ARK VIII/3 of RV Polarstern in 1991, ed. Fütterer, D.K., Berichte zur Polarforschung, 17, Sclater, J.G. & Francheteau, J., 197. The implications of terrestrial heat flow observations on current tectonic and geochemical models of the crust and upper mantle of the earth, Geophys. J. R. astr. Soc., 2, Sekretov, S.B., Southeastern Eurasian termination: structure and key episodes of tectonic history, in III Int. Conf. on Arctic Margins (ICAM), Abstracts, 16, Bundesonstact f. Geow. & Rohstoffe, Hannover. Sleep, N.H. & Rosendahl, B.R., Topography and tectonics of mid-oceanic ridge axes, J. geophys. Res., 84, Small, C. & Sandwell, D.T., An analysis of ridge axis gravity roughness and spreading rate, J. geophys. Res., 97, Su, W., Mutter, C.Z., Mutter, J.C. & Buck, R., Some theoretical predictions on the relationship among spreading rate, mantle temperature, and crustal thickness, J. geophys. Res., 99, Talwani, M., LePichon, X. & Ewing, M., 196. Crustal structure of mid-ocean ridges. 2. Computed model from gravity and seismic data, J. geophys. Res., 7, Vogt, P.R., Taylor, P.T., Kovacs, L.C. & Johnson, G.L., Detailed aeromagnetic investigations of the Arctic Basin, J. geophys. Res., 84, Weber, J.R., The Lomonosov Ridge Experiment, LOREX 79, EOS, Trans. Am. geophys. Un., 6 42, Weigelt, E., The crustal structure and sedimentary cover of the Eurasian Basin, Arctic Ocean: Results from seismic and gravity measurements, Berichte zur Polarforschung, 261 (PhD thesis). Downloaded from by guest on 8 January 219 # 21 RAS, GJI 14, 16