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Geophys. J. Int. (2001) 145, 505–516 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, 27568 Bremerhaven, Germany. E-mail: [email protected] Accepted 2000 December 4. Received 2000 December 4; in original form 1999 November 19 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 (<20 mm yrx1 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 1000 m. Its RMS-roughness ranges from 450 m in the central Amundsen Basin to 584 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 5–6 km thick oceanic crust (density of 2900 kg mx3) 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 2–5 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 2600 kg mx3, underlain by an 8 km thick zone with a density of 2900 kg mx3. 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. INTRODUCTION The Gakkel Ridge, extending over 1800 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 MidAtlantic 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 60 Ma. Spreading models reveal a slow # 2001 RAS spreading rate of 22 mm yrx1 at the initial opening, decreasing to the super-slow spreading velocity of 6 mm yrx1 full rate for the Gakkel Ridge between 35 and 25 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 1970; Hayes & Kane 1991; Malinverno 1991) seem to be typical characteristics of slow-spreading ridges (20–40 mm yrx1 full rate). Furthermore, few data indicate that the oceanic crust thins significantly beneath super-slow ridges (<20 mm yrx1 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 20 mm yrx1, for example the Mid-Atlantic Ridge or the fast-spreading (>80 mm yrx1) 505 506 E. Weigelt and W. Jokat 86°N 180° en Ri dg Ba e ov ar ak sin M Ba sin 86°N 180° e ds dg 5 Am un Ri 21 90°E 18 13 6 16 56 sin ak 20 13 18 22 Ba 1000 ns en line 1 Na 13 e1 84°N 20 G 2000 Am un ds en 24 23 l Ri dge sin 3000 Ba en ns Na line 4 in Bas lin b 90°W Lo m on os ov Ba sin el kk n e3 lin line 2 86°N Ga Lo 3000 asia 2 20 00 Eur line f Morris Jesup Rise 84°N 90°E ke dg Ri os m on 86°N 90°W 5 6 e ov e 24 d a 6 13 24 c Greenland Yermak Plateau SFZ Svalbard 0° 79°N (a) 0° 79°N (b) 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 (1985); (line e) from Weber (1979); and (line f) from Duckworth & Baggeroer (1985). (b) Pattern of the magnetic anomalies and the Chron numbers for the Amundsen and Nansen basins (modified after Kovacs et al. 1985). 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. 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 1958, 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 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 4600–5100 m. Analyses of the gravity data indicate a # 2001 RAS, GJI 145, 505–516 Oceanic crust in the Eurasian Basin 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 40–50 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. 1995b) and central Amundsen Basin (Fig. 3; line 2 in Fig. 1a) (Jokat et al. 1995a). 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. 507 ACQUISITION OF SEISMIC AND GRAVITY DATA In total, 1500 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 300 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 15 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 10 s. With an average ship speed of 5 knots, the sample distance is 25 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 5 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. 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. # 2001 RAS, GJI 145, 505–516 E. Weigelt and W. Jokat 508 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 21/20 to Chron 6 the regional basement trend is the subsidence depth of oceanic crust defined by Parson & Sclater (1977); from Chron 25–21/20 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. 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. RMS-Roughness [m] 800 700 Gakkel Ridge, this study 600 Malinverno, 1991 500 400 300 TOPOGRAPHIC ROUGHNESS OF THE OCEANIC BASEMENT 200 100 0 0 20 40 60 80 100 120 140 160 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. The seismic reflection profiles across the Eurasian Basin show a rough basement surface with differences in elevation of up to 1000 m. Analyses of four sonobuoys in the southwestern part (Fig. 1a; line 1) and 10 buoys in the central Amundsen Basin (Fig. 1a; line 2) provide seismic velocities ranging from 1.6 to 4.5 km sx1 for the sedimentary layers. No seismic velocities for the upper oceanic crustal layer were identified in the data (Jokat et al. 1995a). 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 3200 m at maximum (Fig. 3). For calculating the topographic roughness we determined the RMS deviation of the basement surface from a regional # 2001 RAS, GJI 145, 505–516 Oceanic crust in the Eurasian Basin 85.10˚N -14.07˚W 82.60˚N 13.96˚E ------ Calculated Freeair Gravity Anomaly —— Observed Freeair Gravity Anomaly 100 150 100 50 50 0 0 -50 -50 -100 -100 -150 Free-air Gravity [mgal] Free-air Gravity [mGal] 150 509 -150 Line 1 Morris Jesup Rise Amundsen Basin Gakkel Ridge Yermak Plateau Nansen Basin 0 0 1030 water 2900 oceanic crust 10 *** a *** *** *** 2600 5 2900 oceanic crust b *** c 2900 15 10 15 3300 mantle 3200 mantle c 3300 mantle 20 *** 25 Depth [km] Depth [km] 5 20 25 50 100 150 200 250 300 350 400 450 500 550 600 650 Distance [km] Figure 5. 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. (1995b). The densities are annotated in kg mx3. 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 ¼ ½&ðhðiÞ 2 Þ=n1=2 for5 i ¼ 1, n; R ¼ roughness ½m; a number of ridge systems with full-spreading rates between 14 (Southwest Indian Ridge) and 153 mm yrx1 (East Pacific Rise). h ¼ depth deviations ½m : 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 ¼ 2500 þ 350t1=2 for5 d ½m; t ½Ma : 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 1296l0:539 for5 R ¼ roughness ½m, l ¼ spreading rate ½mm yr1 : Malinverno’s empirically determined relationship between roughness and spreading rate is based on 101 profiles across # 2001 RAS, GJI 145, 505–516 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 584 m. The RMS-roughness value for the Nansen Basin is determined for a 201 km long profile from the southern ridge shoulders to the corresponding southern magnetic anomaly Chron 13 (34 Ma) and amounts to 545 m (Figs 1b and 2). Spreading rate models based on magnetic anomaly interpretation imply a total opening rate of 6–12 mm yrx1 (34 Ma to recent) for the Amundsen Basin, and 6–14 mm yrx1 E. Weigelt and W. Jokat 100 100 Free-air Gravity [mGal] 89.76˚N -48.72˚W 86.25˚N 9.40˚E ------ Calculated Freeair Gravity Anomaly —— Observed Freeair Gravity Anomaly 50 50 0 0 -50 -50 -100 -100 Line2 Amundsen Basin Free-air Gravity [mgal] 510 Gakkel Ridge -----> ~ 100 km 0 0 1030 water 5 1900-2500 sediments 2900 oceanic crust 10 ***** 15 e 10 ***** f 15 Depth [km] Depth [km] 5 3300 mantle 20 20 25 25 0 50 100 150 200 250 300 350 400 450 500 Distance [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. (1995a,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 (1985) for the Amundsen Basin (reference f). (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 yrx1) to 312 m (corresponding to 14 mm yrx1). 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 3200 m. A pronounced jump in basement depth of about 1000 m occurs between Chrons 20 and 21 (km 470, 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 20 (46 Ma) and Chron 6 (20 Ma) amounts to 450 m. In this time range the full-spreading rates decrease from 13 to 6 mm yrx1. The corresponding calculated roughness values (Malinverno 1991) are 325 and 493 m. For oceanic crust older than Chron 21 the general basement level is lowered abruptly (Fig. 3, km 450). This cannot be explained by sediment-corrected standard subsidence curves. Therefore a statistical approach with a linear regression line (Malinverno 1991; Minshull 1999) was used to define the reference level for the crust created between Chron 24 (53 Ma) and Chron 20 (44 Ma). The resulting RMS-roughness value here is 540 m. The full-spreading rate in this area was between 13 and 19 mm yrx1 (Vogt et al. 1979) and therefore the calculated roughness values after Malinverno (1991) range from 325 to 265 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 yrx1, 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 yrx1 (Fig. 4), and therefore the theoretical roughness values can range from 493 to 265 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 x20 to x40 mgal free-air anomaly (Figs 5 to 8). Gravity increases to # 2001 RAS, GJI 145, 505–516 Oceanic crust in the Eurasian Basin 100 100 86.25˚N 9.40˚E ------ Calculated Freeair Gravity Anomaly —— Observed Freeair Gravity Anomaly 50 50 0 0 -50 -50 -100 -100 Line2 Amundsen Basin Free-air Gravity [mgal] 89.76˚N -48.72˚W Free-air Gravity [mGal] 511 Gakkel Ridge -----> ~ 100 km 0 0 1030 water 5 1900-2500 sediments 2900 oceanic crust 10 ***** 15 e 10 ***** f 15 Depth [km] Depth [km] 5 3300 mantle 20 20 25 25 0 50 100 150 200 250 300 350 400 450 500 Distance [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 (1985) for the Amundsen Basin (reference f). 40–50 mgal at the shoulders of the Gakkel Ridge and decreases to x50 mgal in the central valley (Fig. 5). 2-D crustal density models were constructed from these data by applying the LCT software (# 1987–1992 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. 1995a,b). Densities for the sediment layers were calculated according to the density–velocity relation of Nafe & Drake (1957) by applying the seismic velocities from the sonobuoy recordings. In our model the density for the sediments varies between 1900 and 2400 kg mx3. For the oceanic crust we mostly used a density of 2900 kg mx3 according to similar studies along mid-ocean ridges (Miller & Christensen 1995). In our model the mantle has a density of 3300 kg mx3 (Christensen 1972). A low-density zone of 3200 kg mx3 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. # 2001 RAS, GJI 145, 505–516 Gravity model for the southwestern Eurasian Basin (line 1) Fig. 5 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 x30 mgal in the Amundsen and Nansen basins to + 40–50 mgal on the flanks of the Gakkel Ridge. In the central part of the Gakkel Ridge it decreases to x50 mGal. The thin sediments were modelled with densities of 1900– 2200 kg mx3. As indicated by the seismic reflection profiles (Jokat et al. 1995b), 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 50–150 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 2.5 km thick upper part with a density of 2600 kg mx3 and a lower part down to the Moho at 8 km depth with a density of 2900 kg mx3. In a first modelling step, only the crustal thickness was varied until the differences between measured and calculated free-air E. Weigelt and W. Jokat 100 100 Free-air Gravity [mGal] 86.24˚N 59.24˚E 83.50˚N 29.64˚E ------ Calculated Freeair Gravity Anomaly —— Observed Freeair Gravity Anomaly 50 Free-air Gravity [mgal] 512 50 0 0 -50 -50 -100 -100 Line 4 60 km <----- Gakkel Ridge Nansen Basin 0 0 1030 water 5 2300 sediments 2900 oceanic crust d 10 ******* 10 15 15 Depth [km] Depth [km] 5 3300 mantle 20 20 25 25 500 450 400 350 300 250 200 150 100 50 0 Distance [km] 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 (1985) 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 did not exceed 5 mgal. The model (Fig. 5) 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 140–220 and km 480–550, Fig. 5). 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 270 and 340, Fig. 5). A low-density zone beneath the ridge is introduced to avoid a difference of 40 mgal between observed and modelled freeair gravity across the rift valley. Here, a density contrast of 100 kg mx3 between partial melt and surrounding mantle seems to be reasonable, as described by the studies of Talwani et al. (1965). Otherwise, in the case of a constant density distribution in the mantle the modelling would demand either an increase of crustal thickness up to 10 km below the ridge’s flanks or a decrease of density down to 2500 kg mx3 for the complete crust off to 50 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). Gravity model for the central Amundsen Basin (line 2) The free-air gravity anomaly increases from x30 to x40 mgal in the central Amundsen Basin to more than 10 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. 1995a). 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 80 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 5–6 km thickness for the northern part of the Amundsen Basin (km 0–270, Fig. 6). While the general basement level shallows to the south the crust thickens up to 9 km towards the Gakkel Ridge (from km 400 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 # 2001 RAS, GJI 145, 505–516 Oceanic crust in the Eurasian Basin from the seismic refraction data of the FRAM II expedition by Duckworth & Baggeroer (1985) (reference f, Fig. 1). For the Amundsen Basin the authors suggested a 7 km thick oceanic crust about 280 km away from the centre of the Gakkel Ridge formed between Chron 20 and Chron 21. Even if we reduce the density of the oceanic crust from 2900 to 2800 kg mx3, a minimum depth of 8 km for the crust is still calculated. Taking into account the fact that the two measurements are almost 280 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 x10 to x20 mgal, the Morris Jesup Rise has a pronounced positive anomaly of 40 mgal at maximum. The northern edge of the plateau is marked by a large negative anomaly of x40 mgal (Fig. 7). The sediment layer was modelled with a density of 2200 kg mx3, but has no large influence on the final results as the seismic data indicate only a 100–200 m thick cover of deposits (Weigelt 1998). Again the results of the FRAM II seismic refraction experiment (Duckworth & Baggeroer 1985), situated 100–120 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 10 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 2900 kg mx3 because of the oceanic origin of the plateau (Feden et al. 1979; Vogt et al. 1979; Dawes 1990) (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 x20 mgal. Long-wavelength variations of 20 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 1.5 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 220 and 350 we assumed a crustal thickness of 4–5 km with the Moho at 10 km depth, according to the results of the FRAM IV experiment (Duckworth & Baggeroer 1985) about 175 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 # 2001 RAS, GJI 145, 505–516 513 20 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. 5). Towards the Gakkel Ridge the crustal thickness increases from 4–5 km to more than 8 km (km 450–550, 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 4–5 km thick sedimentary cover for the Nansen Basin (Jackson & Oakey 1990). 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 (1985), 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 1000 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 450–584 m, which are significantly higher than the 100–300 m roughness at faster spreading ridges with a total opening rate of more than 20 mm yrx1. 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 2900 kg mx3 along the rift valley. However, their profiles, orientated perpendicular to the axis, are only 80–90 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 yrx1 range from 2.5 km (Reid & Jackson 1981) to 4.5 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 yrx1 during the last 10 Ma, the theoretical models predict a crustal thickness of only 3.5–5.5 km. A 514 E. Weigelt and W. Jokat 12 Crustal Thickness [km] 10 8 6 x 4 2 Su et al., 1994 Bown & White, 1994 Reid & Jackson, 1981 0 0 20 40 60 80 100 120 140 160 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 20 mm yrx1. 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 (1985) and Duckworth et al. (1982). The cross shows the crustal thickness at the Southwest Indian Ridge (Muller et al. 1995). 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 5–9 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 20 mm yrx1. For the central Nansen Basin (line 4) the gravity models give crustal thickness values of 4–5 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 (<20 mm yrx1) 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 yrx1) (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 1990), 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 20 mm yrx1. 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 # 2001 RAS, GJI 145, 505–516 Oceanic crust in the Eurasian Basin 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. 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