The deep-water Amerasia Basin (AB) is the most ancient part of the Arctic Basin of the Arctic Ocean. The tectonic structure of the basin reflects all stages of its multistage development. It includes a number of large subsea basins and rises (Fig. 1) within the North and South Amerasia domains distinguished by tectonic criteria [1]. The North Amerasia domain includes a system of subsea rises, namely Alpha Ridge–Mendeleev Rise with adjoining basins—Alpha, Makarov, Nautilus, Podvodnikov, Stefansson, Toll, and the North Chukchi Basin, as well as the Chukchi Borderland and, supposedly, a crustal block including the Sever and Pearya spurs. The main feature of this domain is the coverage of almost all involved geostructures by a high-amplitude sign-alternating anomaly magnetic field associated with the High Arctic Large Igneous Province (HALIP). The South Amerasia domain is represented by the Canada Basin.

Fig. 1.
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New tectonic scheme for the Amerasia Basin. AMFZ–Alpha–Mendeleev Fault Zone, AchFZ—Alpha–Chukchi Fault Zone, MR—Mendeleev Rise, AR—Alpha Ridge, LR–Lomonosov Ridge, ChB—Chukchi Borderland.

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Fig. 1.
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(Contd.)

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The Chukchi Borderland is probably the only geostructure (geological structure) in the AB the origin of which is not fundamentally controversial. It is generally accepted that it is underlain by the continental crust up to ~35 km thick, which was affected by rifting. The cup and the rise of the same name, as well as the ridge and the Northwind Basin were formed within it [2]. The origin of the other geostructures listed above and the history of their tectonic development are the object of ongoing scientific disputes. The main reason for the diversity in the published models of the AB tectonic structure and evolution is the poor geological and geophysical knowledge due to severe ice conditions. In our opinion, most of the models are currently either not supported by the totality of the accumulated facts or are contemplative.

The purpose of this work was to create a new tectonic model of AB as appropriate as possible, both in scale and content for the entire volume of the accumulated geological and geophysical data. At first, these data were systematized by the authors in the form of a structured array, including both the initial factual information (MCS, DSS, potential fields, geological sampling, heat flow, bathymetry), and published interpretation materials (structural, tectonic, and evolutionary schemes). Further work included the following preliminary stages: verification of the zoning results of the latest digital models (DMs) of the potential fields; interpretation of the set of 68 depth and 170 time sections of MCS; construction of the representative 2D geological–geophysical models of the AB deep structure; updating the DM of sediment cover thickness [3] and its recalculation in the DM of basement surface using the seafloor relief DM IBCAO v.4.0; the updating of the Moho relief DM and the crustal thickness DM created on the basis of 3D-gravity modeling [4].

Our works [5, 6] describe in detail the methods for interpretation of geological and geophysical data, when determining the specific features of the tectonic structure of the Arctic Ocean, represent maps of potential fields, and give examples of the seismic sections and results of modeling. It should be noted only that the methodological techniques used are traditional for regional offshore works and that all constructions are based on seismic data. Two schemes were created at the main stage of the work: a scheme of the basement zoning on a scale of 1 : 10 000 000 (in preparation for publication) and the represented tectonic scheme of AB on a scale of 1 : 2 500 000 (further, the Scheme—Fig. 1 demonstrates a simplified version) with the corresponding legend. The scheme was created in GIS Arc Map by comprehensive interpretation of the results of the current study, taking into consideration the results of earlier works [5, 6].

The basis of the Scheme is the structural framework (shown in different colors and their shades), which reflects the nature and structure of the basement of the Amerasia Basin. The framework was developed by mapping the basement surface on the MCS sections. It was detailed according to the potential field data and the results of three-dimensional gravity modeling according to calculation of the Moho depths and the consolidated crust thickness (the modeling technique and results were described in [4], and information on the updated version of the models is being prepared for publication). First, the DM of the surface of the basin basement was created. The international DM of the sedimentary cover thickness in the Arctic Ocean was taken as the basis [3]. This model was detailed by the interpretation of the basement position in the MCS sections in the area where the largest volume of this data has been collected in recent years during domestic expeditions (Fig. 2a). Further, a clarified model of the basement was obtained by adding it to the DM of the seafloor relief (Fig. 2b). Due to the very poor and uneven study of the Arctic Ocean by MCS seismic surveys, this model describes only the regional features of the basement structure. At the same time, the local features of this surface are reflected in the potential fields, which have been studied much better. Therefore, we used the created DM of the basement to map the structures of rank II and rank III in the area of the Amerasia Basin (rank I structure) and the potential field data together with the MCS sections, to map the structures of ranks III and lower (local structures). Thus, the structural framework of the Scheme was created. This framework reflects the relative hypsometric position of the structural elements of the basement surface within the structures of all ranks.

Fig. 2.
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Basic digital models of deep crustal sections of the Amerasia Basin, laid in the basis of the tectonic scheme: (a) scheme of the data used to construct basement relief model; (b) basement relief; (c) Moho relief; (d) thickness of the consolidated crust. Legend: 1, using the DM of the sedimentary cover thickness [19]; 2, the area specified by the interpretation of the MCS sections in this study; 3, the DM in the Eurasian Basin (publication in preparation).

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The above-mentioned calculated digital models of the Moho relief (Fig. 2c) and the consolidated crust thickness (Fig. 2d), together with the results of 2D gravimagnetic modeling and the comprehensive data interpretation, were used to establish the origin of the structures of the study area. The basic colors, corresponding to the blocks of the normal and hyperextended continental crust, as well as to transitional and oceanic crust, are used for its representation in the Scheme. Three shades of each color indicate the relative hypsometric position (low, medium, high) of the structural elements (local geostructures) within the individual blocks, according to the structural framework created. Subsided elements (rifts, troughs, basins) correspond to the darkest shades; uplifted elements (horsts, rises, spurs, ridges) to the lightest shades; elements with an intermediate position in the block, to the medium shades. In addition, the Scheme demonstrates boundaries of the allocated regional geostructures of three ranks; axes of linear magnetic anomalies; discontinuities of two ranks (normal faults, thrusts/reverse faults, strike–slip faults, passive faults, faults with undefined kinematics); the HALIP location and the deformation area associated with the Beaufort Fold Belt [7] in the Canada Basin; the ocean floor relief (pseudoshaded relief); and the position of the shelf edge, ancient volcanoes, and deep wells. The Scheme is briefly described below; its main differences from the previously published tectonic schemes of AB are noted.

The South Amerasia domain is completely occupied by the Canada Basin. The basin involves the central part with the oceanic crust (MORB-zone) and the aborted center of spreading (Amerasia Ridge) and the flank parts with crust of the transitional type (ocean/continent transition (OCT) zones) [2]. In the southern part of the basin, the sedimentary cover is partly affected by deformations of the Beaufort Fold Belt [7], which formation started in the Upper Paleocene and ended in the Oligocene [8].

The central part of the Canada Basin extends fan-like from the Beaufort Sea region to ~79° N, from ~130 to ~500 km. It has two systems of discontinuities. The first system includes normal faults limiting ridges of mountains subparallel to the rift valley of the Amerasia Ridge. The second system is represented by transverse low-amplitude strike–slip faults (transform faults) and faults with undefined kinematics. Their arc shape in the Scheme corresponds to the model of the fan-shaped opening (sphenochasm) of this part of the basin relative to the rotation pole located near the Mackenzie River delta. This model is also confirmed by linear magnetic anomalies located symmetrically relative to the ridge axis. According to these anomalies, the opening time is considered by most researchers as the Neocomian (~142–126 Ma) [2, 9]. An important innovation in the Scheme is the hypothetical continuation of the Amerasia Ridge to the western part of the Nautilus Basin, based on the results of integrated analysis of the latest digital MSC data [10] and the potential field data. The question about the nature of the junction of the spreading center in the Canada Basin with the stretching center in the Stefansson Basin remains unresolved. Two variants are considered: the existence of the triple junction point of the oceanic ridges [11] in the northern part of the Canada Basin or the primary opening of the Stefansson Basin with the subsequent jump of the spreading center into the Nautilus Basin at the final stage of formation of the Canada Basin. In addition, the mysterious object in the basin, included in the Scheme in its central part, remains a system of rifts at 78°. The results of the interpretation of the MCS sections [10] show that this system appears to have been formed as a result of the most recent active tectonic movements in the basin, which is still difficult to relate to its spatial position away from the spreading Amerasian Ridge.

Flank OCT-zones of the Canada Basin are characterized by potential fields in the structure of which it is not possible to establish distinct local features and patterns; intensive local anomalies are absent. On the basis of a comprehensive interpretation of the geophysical data, with an emphasis on seismic data, most modern researchers suppose that the OCT-zones are underlain either by the hyperextended continental crust or by rocks of the serpentized mantle (ex., [2, 12]). However, the indicated properties of the potential fields, the specific deepening of the basement from the axis of the Amerasia Ridge towards the margins (except for the area of conjunction with the Chukchi Borderland), a low thickness of the consolidated crust (3–9 km)—all these features, in our opinion, indicate the possibility of the oceanic origin of the crust of these zones (as well as the MORB-zone), which was suggested by researchers in the 20th century and at the beginning of this century (ex. [13]). At the same time, there are no obvious stripe magnetic anomalies inherent to the oceanic crust in the area of the OCT zones. This may be due to the deep location of magnetic sources overlapped by a thick sedimentary cover. Taking these arguments into consideration, the Scheme shows the OCT-zones of the Canada Basin at this stage of the study as zones with a transitional crust, without specifying its nature.

In the North Amerasia domain, the central position is occupied by the Alpha Ridge (AR) and the Mendeleev Rise (MR), which form a single system of rises (AR-MR) with a length of more than 1500 km, with a width of ~200 to ~400 km. The AR is the least studied, mainly due to the most severe ice conditions in the region. Both rises have a complex tectonic structure. In the central parts, they include a large number of local rises and basins well-manifested in the seafloor relief. In the MCS sections, most of the local structures are limited by normal faults; i.e., they are horsts and grabens. Their orientation changes in space, which makes it possible to judge about the directions of ancient tension stresses, to identify zones of strike–slip faults and block-formed geostructures. The HALIP rock complex is widespread within the rises and adjacent basins of the domain considered. According to the combination of facts, the AR and MR have similar geophysical characteristics, indicating their syngenetic nature. In the Scheme, they are shown as blocks of the normal (moderately thinned) continental crust, the thickness of which varies from ~32 km in the central parts to ~20 km on the flanks of both geostructures.

The Alpha Ridge is characterized by a unified tectonic structure, expressed in the orderliness of linear structures within its blocks. It consists of crust blocks, including extended, differently oriented systems of horsts and grabens, discontinued by transverse displacements, including strike–slip faults. Isometric rises are also encountered; some of them are apparently ancient volcanoes. Two major blocks are distinguished within the ridge—the Western Plateau and the main (eastern) block of the ridge. In the latter, a special position is occupied by the Alpha Valley, which is framed by symmetrical horsts. The orientation of the horsts and grabens near the Alpha Valley is sublatitudinal. At the eastern boundary of the ridge, near the North American margin, both are abruptly terminated. The character of the conjugation zone of this margin and the AB geostructures between 40° W and 110° W has not been reliably established due to the absence of the MCS data here. The Scheme shows a band of the hyperextended continental crust in this zone, which includes the HALIP rock complex. Along the slope of the margin, the normal fault is conceptually shown, although a number of researchers suggest the presence of a large strike–slip fault here [2].

The Alpha Basin is located on the northern flank of the Alpha Ridge. The basin separates the Alpha Ridge from the Lomonosov Ridge. The structure of the basin inherits a tectonic style of the ridge of the same name. In the southern and western parts of the main block of the ridge, the strike of linear structures changes smoothly from latitudinal to NE–SW, up to submeridional. In the area of the Nautilus Spur, which is the southernmost part of the AR, horsts and grabens are aligned in the submeridional direction, similar to that of the structures of the adjacent northern part of the Canada Basin.

In the vicinity of the boundary of the main block AR and its Western Plateau, a significantly different orientation of the linear structures is observed—submeridional and sublatitudinal, respectively. The Alpha–Chukchi Fault Zone (AChFZ) [6], traced along the indicated boundary to the northern point of the Healy Spur of the Chukchi Borderland, is established here. Two variants of the AChFZ formation are considered: (1) as a strike–slip zone during the opening of the Makarov and Nautilus basins and left-lateral displacement of the Western Plateau block relative to the main block of the AR; (2) as an ancient, Paleozoic suture.

The more significant width and the degree of crustal thinning of the Makarov Basin compared to the Alpha Basin, located east of the continuation of this fault zone testifies in favor of the first option. The amplitude of the strike–slip fault within the AR along this zone hardly exceeded 100 km. The sublatitudinal system of the horsts and grabens of the western plateau of the AR is abruptly terminated at its western edge, at the boundary with the Podvodnikov Basin.

Mendeleev Rise, which has elongated, close to rhomboidal outlines, is not characterized by a unified tectonic style. Long local linear structures are practically not observed here; rises and basins close to isometric forms predominate. The rise itself can be divided into two parts: the northern part, which includes an area of small in size local rises and basins, and the southern part represented by larger blocks—the Sargo and Arliss plateaus. The tectonic structure of the MR is described in detail in [5], and on the Scheme, it is presented with minor clarifications concerning the position of tectonic faults. The MCS data clearly indicate the rift-related origin of the structure-forming surface of the MR, and, along with the anomaly magnetic field, indicate the presence of the cover of the HALIP rocks here [14].

The predominant orientations of the MR tectonic faults are submeridional for normal faults and NW–SE for transverse dislocations represented by normal faults and fault–slips. The thick Alpha–Mendeleev fault zone (AMFZ), with an assumed amplitude of the left-lateral strike–slip fault of up to 320 km, has been identified in the area of the AR and MR junction [6].

Let us briefly consider the structure of the basins adjacent to the AR–MR rises. Their sedimentary cover in the MCS sections is generally represented only by the post-rift complex in most cases. Only in some modern sections is it possible to record the structure of syn-rift deposits as well as SDR (seaward-dipping reflector) complexes [1]. This is explained by the high acoustic rigidity of rocks of the syn-rift complex associated with HALIP. Therefore, the structure-forming tectonic dislocations in the basins, except for their flank zones, are practically not manifested in the MCS sections. The inferred dislocations can be traced only on the basis of the potential fields. The above observations are valid for the Alpha, Makarov, Podvodnikov, Nautilus, Stefansson, and Toll basins and for the North Chukchi Basin, which indicates their quasi-synchronous formation as a result of rifting accompanied by intense magmatism.

Extended linear structures, oriented parallel to a strike of the Lomonosov Ridge and the AR slope, including the narrow Marvin Spur and the Marvin Basin, are established within the Alpha and Makarov basins. The Makarov Basin is underlain by a thin (7–12 km) crust. A distinct system of linear magnetic anomalies, typical of oceanic-type basins, is absent within its limits.

The Podvodnikov Basin differs from the aforementioned basins by its largest dimensions (~320 km in width) and the absence of linear structures subparallel to its sides in its central part: these structures have an oblique NW–SE orientation, similar to the strike of the AMFZ and transverse dislocations within the adjacent AR and MR blocks. In the northern part of the basin, in the area of the Toll Saddle, the thickness of the consolidated crust is ~20 km; in its southern part, it decreases to ~8 km. Along the border with the Lomonosov Ridge, narrow linear structures, the Geophysicists Trough and Geophysicists Spur, stretch out in the basin. This fact, along with the presence of listric normal faults on the sides of the Alpha, Makarov, and Podvodnikov basins, subparallel position of the flanks of the divided structures, indicate the formation of these basins as a result of rifting of the continental crust under conditions of transtension, while AR-MR rises were moving away from the Lomonosov Ridge. Furthermore, the signs of syn-rift volcanogenic–sedimentary deposits and SDR complexes in the flank parts of the basins [16] and coverage of their areas by the HALIP magnetic field—all together allow us to conclude that rifting was accompanied by abundant magmatism. This setting could have contributed to hyperextension of the continental crust, without disturbing its continuity (owing to the lower crust) and the initiation of spreading of the oceanic crust. These circumstances were indirectly established on the basis of the available geophysical data. Based on this conclusion, the Scheme demonstrates a hyperextended continental crust at the base of the Alpha, Makarov, and Podvodnikov basins.

Nautilus, Stefansson, and Toll basins were also formed according to the above-described scenario, i.e., in the place of the crustal areas, which were affected by the greatest thinning during continental rifting, but on the opposite flank of the AR–MR rises. This follows from the similar set of the established geological and geophysical characteristics. Stefansson Basin separates the southern flank of the AR and the block of the Pearya and Sever spurs. Groups of linear structures within these highs are identified as horsts and grabens; their strikes (NE–SW) are subparallel to the orientations of similar structures in the basin itself and on the adjacent flank of the AR. In the anomaly magnetic field of the southern part of the basin, there is a series of the intense extended (~250 km) stripe magnetic anomalies of both signs, which may indicate achievement of the spreading stage during the rifting [15], which is reflected in the Scheme.

Nautilus Basin, located between the Alpha Ridge and the Chukchi Borderland, can be divided into two parts. The eastern part is characterized by greater bottom depths and the minimal thickness of the consolidated crust (~8 km), and the western part, by the shallower bottom depths and the thicker crust (~14 km). Transverse dislocations, parallel to the strike of large fault zones, distinguished within the AR–MR rises, are outlined within the basin. Taking into account the above-mentioned facts, as well as the established continuation of the spreading Amerasia Ridge into the western part of Nautilus Basin, it is logical to assume that the western and eastern parts of this basin are underlain by oceanic and hyperexpanded crust, respectively. Nautilus Basin was formed as a result of the moving the Chukchi Borderland together with the MR away from the Alpha Ridge. At the time of opening of the basin, the AMFZ served as its western boundary. The AChFZ, located near its eastern boundary, probably, had a continuation here as a left-lateral transform fault at the later stage, with the start of spreading. During the aforementioned moving, a weakened zone appeared (or intensified) between the Mendeleev Rise and the borderland; this zone developed into the narrow Charlie Trough and the small, southward-expanding Toll Basin.

The North Chukchi Basin is located to the south of the MR and occupies an extensive area between the Chukchi Borderland and the De Long High. The thickness of the sedimentary cover within it is up to 22 km with a single structural style corresponding to post-rift sediments. The observed discontinues in its sequence were formed either at the stage of general submergence or during the minor Cenozoic activizations [16]. It is not possible to identify reliably ancient, structure-forming dislocations in the central part of the basin, as well as in other basins of the AB. Based on the combination of the available data, we assume that the basin formed under the conditions of hyper-extension of the continental crust. Considering the extreme thickness of the accumulated sediments in it, it may have formed in the place of the hypothetical dextral strike–slip (Podvodnikov–Mendeleev Fault Zone (PMFZ)) [6], which acted at the beginning of the phase of continental block disintegration.

The revealed features of the tectonic structure of the North Amerasia domain AB indicate that the evolution of its geostructures occurred under the conditions of continental rifting. The process began with the uplift of a vast region of the formerly unified ancient continental block (Arctida) and with the peak outpouring of magma with the formation of the cover of the HALIP platobasalts at the Barremian–Aptian boundary (~125 Ma ago) [11, 17]. The uplift was initiated by a mantle plume, the nature of which is debatable (e.g., [18, 19]). Further, in the Aptian–Albian, the lithosphere extended and Arctida subsequently disintegrated along the weakened zones into individual geostructures accompanied by abundant magmatism (e.g., [1]). As a result, the system of the AR–MR rises and adjacent basins of the domain considered was formed. To describe the formation history of this system, the model of conjugate volcanic passive margins is applicable [20]. According to this model, the rises evolve as central blocks (C-Blocks) surrounded by thinning (hyperextended) continental crust. Besides the aforementioned tectonic features, the presence of SDR-complexes [1] and high-velocity lower crustal bodies (HVLC bodies) also testify in favor of this model. The continuation of the ancient spreading center, the Amerasia Ridge, into the Nautilus Basin indicates that the formation of the MORB-zone of the Canada Basin occurred quasi-synchronously with the appearance of other basins of the North Amerasia domain AB, possibly, except for the Stefansson Basin. However, the estimates of the opening time of the MORB-zone (not later than ~126 Ma ago) [2, 9] contradict the assumed age of the formation of the HALIP platobasalts preceding the disintegration of Arctida (~125 Ma ago), which requires further study.