The vast bathymetric depression of the Podvodnikov Basin lies between the Lomonosov Ridge and Mendeleev - Alpha Ridges, bounded by the East-Siberian Sea shelf from the south and by the Makarov Basin - from the north. Sea depth in the Podvodnikov Basin varies from 800 m to 2700 m in the same direction. The Podvodnikov Basin has a benched structure and is considered to be a part of the terraced continental slope.
The seismic data divide the southern terrace of the Podvodnikov Basin into western and eastern parts, separated by the prominent Geofizikov Spur and its southern continuation as a deeper structure of the acoustic basement. Total thickness of the sedimentary cover here reaches 8 km.
The sedimentary cover of the Podvodnikov Basin is subdivided into six seismic-stratigraphic complexes, dated from late Permian to Holocene by correlation with ACEX well on the Lomonosov Ridge and exploration wells on the Alaska shelf. Integrated geophysical data interpretation and modeling classify the Basin’s crust as stretched continental, with thickness of the crystalline crust from 10 to 23 km.
KeywordsPodvodnikov Basin Geofizikov Spur Bathymetric depression Extension structure Sedimentary basin Extended continental crust
Pictures above clearly illustrate the difference between eastern an western sides of the basin. The eastern side of the southern terrace in contact with the Mendeleev Ridge can be classified as an ordinary slope with vertical drop decreasing from 600 m in the south to 200 m in the north. The eastern slope of the northern terrace is similar to its southern counterpart, except being taller – from 400–1000 m, with higher elevation in the center.
The western side, at the basin junction with the Lomonosov Ridge, can be best described as a combination of the contrasting morphological forms – the Geofizikov and Senchura Spurs and similar unnamed feature along the edge of the northern terrace (Fig. 1.8). These ranges, divided by linear depressions, are considered to be a splinter-blocks of the Amerasian slope of the Lomonosov Ridge, tectonically separated from it. Geophysical data suggest that these blocks, buried under the sediments, continue into the Podvodnikov Basin. Therefore, being a part of the Lomonosov Ridge, they also shape the structure of the Podvodnikov Basin, thus confirming their genetic ties – they both suffered a neo-tectonic subsidence during formation of the Podvodnikov Basin.
The second profile (Fig. 5.5) runs north from the East-Siberian Sea shelf through Podvodnikov and Makarov Basins close to DSS traverse “Transarktika-1989–91” 1500 km long. It convincingly illustrates terraced morphology of the Podvodnikov Basin.
In light of the presented information we consider the Podvodnikov Basin assembly of the inner slope, southern terrace, intermediate slope, northern terrace and outer slope as a part of the continental slope of the corresponding continental margin.
5.2 Potential Fields
2d — The De Long Uplift - strong linear and circular (following the uplift shape) gravity/magnetic anomalies related to mafic Aptian-Albian (occasionally, Paleozoic and Cenozoic) magmatizm and relatively shallow, intensively faulted pre-Cambrian (most probably Baikalian) basement;
2e —The Kolyma-Mendeleev Zone – wide north-east trending zone crossing East-Siberian Sea shelf form Kolyma Massif through the New Siberian depression to the Mendeleev Ridge, with weak gravity, and very weak magnetic, anomalies following trend.
2f — The East Siberian Uplift and the Wrangel Uplift – north-east and north-west trending anomalies of moderate strength, presumably related to folded and thrusted structures documented on the Wrangel Island;
2g — North-Chukchi Zone – clearly outlined by contrasting gravity and magnetic anomalies following southern (strong gravity and magnetic fields as well as seismic data) and northern (strong gravity, weak magnetic, anomalies) border of the North-Chukchi trough.
2l — Podvodnikov Basin – a complex assembly of Free Air gravity anomalies with gradually increasing amplitudes northward from 12 to 50 mGal and sub-linear magnetic anomalies trending north-west. These anomalies were interpreted as Cretaceous spreading-type linear anomalies (Gurevich and Mashenkova 2000), or as indicators of mafic magmatism related to continental rifting (Glebovsky et al. 1998) (numerical interpretation indicates mafic and ultra-mafic composition of the sources of the strong magnetic anomalies in the central parts of the region and seems to support the latter version); some positive magnetic anomalies correspond with basement uplifts expressed in seismic data, such as Geofizikov Spur.
5.3 Acoustic Basement
Geological and geophysical data identify the De Long Uplift (north-west of the East-Siberian Sea, Fig. 5.6, 2d) as fragment of an ancient platform with basement of Baikal-Caledonian fold belts and Paleozoic sedimentary (or volcano-sedimentary in reactivated terranes) cover. One of these Caledonian belts with numerous diorite-porfiry sills and dykes, basalt and basalt andesite lava flows outcrops on the Henrietta Island (Vinogradov et al. 2004) with K-Ar age between 310 and 450 Ma. There also are some signs of even older consolidation: fragments of methamorphic rocks (gneisses, schists and qaurtzites) in gravellites and sandstones, or un-deformed Cambrian and Ordovician deposits of the Bennet Island. Lacking direct evidence, the geology of the De Long Islands may represent, to a certain degree, the composition and structure of the entire Podvodnikov Basin basement.
5.4 Sedimentary Cover
As can be seen from the Fig. 5.12, the Podvodnikov Basin is reasonably well covered by the Russian MCS programs completed between 2011 and 2014. We also include in our compilation the MCS traverse PS-2008 across the Podvodnikov Basin acquired by RV Polastern in 2008 (Alfred Wegener Institute expedition) and discussed in (Jokat et al. 2013).
All approaches to developing and correlation of the stratigraphic models of the Podvodnikov Basin sedimentary cover are based on information from ACEX wells drilled on the Lomonosov Ridge (Moran et al. 2006; Backman et al. 2008). Using not only the ACEX well data, but also information from wells drilled on the Chuckhi Sea shelf, and Alaska shelf as it was comprehensively described in Chap. 2, we subdivided the Podvodnikov Basin sedimentary cover into six seismic-stratigraphic complexes (Fig. 5.13).
SSC-1 (Lower Miocene-Pleistocene, 18.2–20 Ma) — undisturbed hemipelagic sediments between seafloor and regional unconformity RU;
SSC-2 (Upper Paleocene - Lower Oligocene) - marine shallow formation between unconformities RU and post-Campanian pCU;
SSC-3 (Lower Brooks, Lower Cretaceous and, partially, Upper Cretaceous, K1a-K2, 90–80 Ma) – aleurites and sandstones deposited during the latest stages of the HALIP magmatizm;
SSC- 4 (Lower Cretaceous, K1h-br, 130—120 Ma) - associated with the initial stage of the HALIP magmatizm;
SSC-5 (Upper Jurassic – Lower Cretaceous, J3-K1b-v);
SSC-6 (Upper Ellesmere,? P3-J2).
Several MCS lines crossing the Podvodnikov Basin – East-Siberian Sea shelf transition zone (Fig. 5.14, 5.15) reveal large basins with 7–8 km of sediments accumulated inside. According to (Nikishin et al. 2014) these structures are related to the East-Siberian Sea rifting in Early Cretaceous. The same age had been postulated for extension of the Podvodnikov Basin as a whole.
MCS lines crossing the Podvodnikov Basin – the Lomonosov Ridge boundary and the Geofizikov Spur, also show Cretaceous syn-rift sedimentary sequences at the base of the Basin. The same normal-fault block tectonics with down-throw towards the basin exists at the Podvodnikov Basin-Mendeleev Ridge junction.
Strong continuous reflections are typical for the seismic signature of the Podvodnikov Basin. They can be traced from the eastern flank of the Lomonosov Ridge (with minimal thickness of SSC-2 interval between merging RU and pCU unconformities) to the Mendeleev Ridge (Fig. 5.16).
Total thickness of the Podvodnikov Basin sedimentary section grows from 1000–1500 m on the Mendeleev and Alpha ridges edge to 6000 m in its central parts and drops back to 1000 m at the crest of the Lomonosov Ridge.
The Geofizikov Spur and its southward continuation (as uplift of the acoustic basement buried under the sediments) divide the southern terrace of the Podvodnikov Basin into western and eastern parts. The acoustic basement of the western part, according to seismic, is moderately submerged continuation of the Lomonosov Ridge eastern slope with characteristic active normal faulting forming numerous rift half-grabens. In the eastern part the basement gradually plunges to 9–10 km b.s.l. and thickness of the sedimentary cover reaches 8 km.
Generally speaking, the structure of the Podvodnikov Basin southern terrace suggest two rifting episodes (Aptian –Albian and Late Cretaceous) leading to substantial stretching and thinning of the continental crust under the Basin. Several composite (seismic, gravity and magnetics) crustal models (Langinen et al. 2009; Lebedeva-Ivanova et al. 2011; Glebovsky et al. 2013; Jokat et al. 2013) support this hypothesis. They also demonstrate unity of crust under the East-Siberian Sea shelf and the Podvodnikov Basin, presenting the latter as a natural continuation of the former.
The syn-rift sedimentary formations of the Podvodnikov Basin are overlain by post-rift Paleocene-Pleistocene sequence. MCS lines 2011–2011 and 2011–53 clearly show the sedimentary wedge between Mid Eocene and Miocene deposits (≈ 44.4 Ma) thinning towards the Lomonosov Ridge eventually indicating non-depositional interval at the Ridge’s crest.
Unconformities RU and pCU merge on the Lomonosov Ridge. East into the Podvodnikov Basin the RU and pCU unconformities separate again. Thickness of Paleogene complex (≤ 300 m) is similar to that in the Lomonosov Ridge well (ACEX) where analysis of the core material indicates its neritic origin.
The most active accumulation of the Cretaceous bathial sediments (up to 2000 m) took place in the eastern and south-eastern parts of the Podvodnikov Basin and eastern parts of the Vilkitsky Trough, bordering the Basin from shelf side. Most of the Podvodnikov Basin domain can be considered as the Lomonosov Ridge submerged flank - the notion supported by (Jokat et al. 2013) analyzing the Polarstern MCS data.
Stratification of the Pre-Cenozoic sedimentary cover and correlation of the major seismic unconformities correspond with Popcorn well drilled on the Chukchi shelf. MCS sections show that three strong reflectors between the base of Cenozoic section (pCU) and acoustic basement (Fig. 5.14, 5.17) can be identified with Brooks (BU), Lower Cretaceous (LCU) and Jurassic (JU) unconformities, accordingly. The very basal sedimentary complex between JU and acoustic basement (TAB) is thought to contain Upper Paleozoic - Lower Mesozoic sediments.
The wide-angle sonobuoy refraction seismic in the Vilkitsky Trough - Podvodnikov Basin estimates the interval velocities of Upper Cretaceous (between pCU and BU) complex at 3.5–3.9 km/s, Pre-Cretaceous (between JU and TAB) – 4.1–4.4 km/s.
The latest seismic surveys made it possible to correlate the major seismic unconformities on the East-Siberian and Chukchi shelf with those established in the abyssal Arctic Basin. Fig. 5.18 presents the composite seismic profile connecting ACEX well on the Lomonosov Ridge with wells drilled on the Chukchi Sea shelf.
This correlation convincingly associates the onset of sedimentation in the Podvodnikov Basin to Late Permian in the center or Early Cretaceous periods in the west.
The regional MCS sub-meridian transect 2014–02—06 connect the East-Siberian Sea shelf, the Podvodnikov Basin and the Lomonosov Ridge (at junction with the Makarov Basin). Interpreted seismic section clearly demonstrates not only the terraced character of the Podvodnikov Basin, but also un-interrupted continuation of the Paleogene sedimentary complexes from the East-Siberian shelf into the Podvodnikov Basin (Fig. 5.19). The configuration of the acoustic basement confirms that shelf was subjected to continental rifting, presumably in Cretaceous.
The large-scale acoustic basement uplift separates the southern and northern terraces of the Podvodnikov Basin (MCS lines 2014–02-06, Fig. 5.19) while actual contact between the two coincides with high-amplitude normal fault. Similarity between TAB seismic signature in the northern terrace and the Mendeleev Ridge suggests that the northern terrace might be a downthrown segment of the Mendeleev Ridge (gravity and magnetic data (Fig. 5.6) also point out in the this direction). Alternatively, it can be just a saddle between the Lomonosov and Mendeleev Ridges.
As we noted before, the MCS line 2014–01 (Fig. 5.17) most vividly illustrates the continuity of pre-Cenozoic seismic unconformities. The south-eastern end of this line joins the northern end of the MCS line 5-AR (Fig. 5.20) on East-Siberian Sea shelf, thus allowing tracing the JU (Jurassic) unconformity from shelf to the deepwater part of the Podvodnikov Basin.
North Chukchi Basin has up to 18 km of sediments and the lowest sedimentary complex is dated as Upper-Permian(?) - Mid Jurassic. Detailed description of correlation and dating of the sedimentary complexes in the North Chukchi Basin is given in Chap. 2 (“Seismic stratigraphy of the sedimentary cover”).
Another important regional MCS traverse PS-2008 (Fig. 5.21) was acquired by RV “Polarstern” across the Podvodnikov Basin (for traverse location - see Fig. 5.12) in 2008 (Jokat et al. 2013; Weigelt et al. 2014). It was concluded (Jokat et al. 2013) that at least half of the Podvodnikov Basin resides on the stretched continental crust supporting considerably thick turbidite sequence no younger that Late Oligocene (Hegewald and Jokat 2013a, b) (this dating agrees with pre-Miocene dating (Poselov et al. 2014) of the regional unconformity terminating this sequence). Overall, the sedimentary section thickness varies from 1000 m at the top of the Lomonosov Ridge to 6000 m in the central part of the Podvodnikov Basin (incidentally, in (Jokat et al. 2013), it was named “Makarov Basin”).
Seismic program under the Russian expedition “Arktika-2014” delivered a large amount of new information which greatly improved our undrestanding of the Podvodnikov Basin geology in general, and its sedimentary cover, in particular. Some of the seismic cross-sections from this program are discussed below.
MCS line 2014–03 (Fig. 5.22), 650 km long, runs from the De Long Uplift through the southern Podvodnikov Basin to the western flank of the Mendeleev Ridge. Near the shelf edge, thickness of sedimentary cover inside the Vilkitsky Basin reaches 11 km. Apparent intrusion of igneous material (450–500 km) can be related to the Cretaceous magmatic activity, because the formations above the pCU unconformity are undisturbed. In contrast, the igneous intrusions visible at the foothills of the Mendeleev Ridge (630–650 km), penetrate the entire sedimentary section and expressed in the seafloor relief, placing this magmatic event in Pliocene – Pleistocene.
MCS line 2014–04 (Fig. 5.23) (250 km) obliquely crosses the shelf edge and terminates in the Podvodnikov Basin at depth ≈ 2500 m. The eye-catching trough, 10-km wide and 500 m deep, is interpreted as erosional channel carved into the sea bottom by high-energy turbidite flows, rather than tectonic structure. This conclusion is supported by continuous reflectors on both sides and under the bottom of the trough, as well as by presence of side mounds.
Main portion of the MCS line 2014–12 (Fig. 5.24) (≈350 km) covers the De Long Uplifts demonstrating sharply reduced thickness of sediments. Identification of the acoustic basement with confidence on this section is difficult without correlation with well established markers in proximity, which may come in the future.
MCS line 2014–13 (Fig. 5.25), slightly more than 300 km long, starts at the eastern flank of the Lomonosov Ridge and crosses southern section of the Podvodnikov Basin. The seismic section clearly demonstrates draping of the ragged acoustic basement by overlaying formations and gradual increase of Cenozoic sediments toward the center of the Podvodnikov Basin. The uplifted block of acoustic basement in 210–230 km interval may represent submersed continuation of the Geofizikov Spur, also recognizable on seismic lines north from, and parallel to, the present one.
MCS line 2014–14 (Fig. 5.26) (≈380 km), runs from the East-Siberian Sea shelf through the continental slope into the Podvodnikov Basin along the eastern flank of the southern Lomonosov Ridge. Here, at the shelf edge, another turbidite-related feature, similar to that of the line 2014–04, is present. The slope-basin transition is marked by dramatic increase of the sedimentary cover thickness.
5.5 Crustal Structure
The Podvodnikov Basin deep crustal investigations started after completion of the regional polar transect “Transarktika-89-91” (refraction deep seismic sounding or DSS, for short). It runs from the East-Siberian Sea shelf through the Podvodnikov and Makarov Basins to the near-polar segment of the Lomonosov Ridge foothills (Fig. 5.27). The obtained data were repeatedly interpreted and the results - published (Arctic Ridges: Results and Planning 1994; Verba 1996; Pavlenkin et al. 1996; Lebedeva-Ivanova et al. 2011).
In addition to the “Transarktika-89-91” DSS information, data from earlier geophysical programs (MCS line 90801, ice-borne seismic surveys “Transarktika-91” and SP-28-91) were also used. Integrated interpretation of all the above mentioned sources is presented on Fig. 5.27.
Total thickness of the sedimentary cover reaches its maximum of 7 km in the Vilkitsky Trough and then drops down in north-western direction to 4 km and even less, to few hundred meters.
Layer I (Vp = 1,7—3,8 km/s) — Meso-Cenozoic sedimentary complexes;
Layer II (Vp = 5,0—5,4 km/s) — older sedimentary (and possibly volcano-sedimentary in basins) formations;
Layers III and IV (Vp = 5,9—6,5 km/s and 6,7—7,3 km/s, accordingly) – crystalline crust.
Moho discontinuity is identified at depth of 20 km under the southern and northern parts of the Podvodnikov Basin plunging down to 30 in center, under the Arlis Gap. The authors associate the formation of the Podvodnikov basin to the Mesozoic stretching of the continental crust. The jointed Mendeleev—Alpha Ridges reside on the 25 km thick crystalline crust of layers III and IV. The abyssal Makarov Basin was suggested to have an oceanic crust of pre-spreading stage, 8–10 km thick, with possible inclusions of continental blocks split from the Lomonosov Ridge.
The total thickness of the continental crust in the Podvordnikov basin (300–950 km marks) varies from 15 to 19 km. Upper consolidated crust (3–5 km) is classified as felsic intrusive-methamorphic complex with average density 2.70–2.72 gcm3. Under the Mendeleev Ridge (950–1080 km interval) the crust looks like standard continental one, only with noticeably reduced lower layer. Further north-west, in the Makarov Basin (1080–1260 km), the crust acquired all characteristics of the oceanic type, with total thickness close to 8 km. Transition back to continental crust of the Lomonosov Ridge is visible at the very end (1260–1350 km) of the transect.
Ray modeling of DSS data from MCS lines (Arctic-2014-1 and Arctic-2014-2) acquired by expedition “Arktika-2014”, running from the De-Long Uplift across the entire Vilkitsky Trough into the Podvodnikov Basin, produced the detailed and reliable crustal velocity information (Fig. 5.29).
unconsolidated hemi-pelagic sediments (Pv =1.9–2.4 km/s) between the sea floor and pre-Miocene unconformity RU, with thickness varying from 1.5–2.5 km (shelf and basins) to almost 0 (De-Long Uplift);
shelf/neritic deposits below RU unconformity (Pv = 3.1–3.6 km/s) and thickness varying from 2–3 km (shelf and basins) to few hundred meters (Arctic-2014–02) or 0 (Arctic-2014-1) at the De-Long Uplift;
more consolidated and, perhaps, metamorphic formations (Pv =4.0–4.8 km/s (4,0—4,4 km/s in the Vilkitsky Trough - Podvodnikov Basin transition slope); thickness, in average in the range of 1.5–4 km, increases to 6.5 km in the Vilkitsky Trough depocenter (Arctic-2014-2), or drops to few hundred meters at the De-Long Uplift (Arctic-2014-1);
upper crust (P-wave velocity increasing from 5.8 to 6.4 km/s with depth); thickness drops from maximum around 22 km at the De Long Uplift to ≈5 km in the Podvodnikov Basin;
lower crust (P-wave velocity increases in northerly direction from 6.6–6,7 km/s at the De Long Archipelgo shelf to 6.8–6.9 km/s in the Podvodnikov Basin) with thickness growing from 8 to 10–11 km in the same direction.
upper mantle with Pv ≈ 8.0–8.1 km/s and Moho discontinuity fixed at ≈31 km depth under the De Long Uplift rising to ≈21 km in the Podvodnikov Basin, bringing the overall crustal thickness changing from 31 to 18–19 km, accordingly.
Total thickness of the sedimentary complexes, including possible metamorphic complexes, reaches ≈8 km in the shelf depocenter of the Vilkitsky Trough and ≈6.0 km – in its slope depocenter.
Presented information unambiguously shows the continuation of the stratified sedimentary complexes from the De Long shelf into the Vilkitsky-Podvodnikov depression, thus establishing structural and genetic commonality between them.
The crustal seismic-gravity model along the seismic MCS line 2014–01 (Fig. 5.30) was constructed using the subsurface geometry of crustal complexes from depth-converted interpretation of this line (Fig. 2.19) and their velocity-derived density.
The model stretches from the North-Chukchi Trough through the Podvodnikov Basin to the Geofizikov Spur.
2.0–2.55 g/cm3 for SSC1-SSC4 sedimentary complexes;
2,60—2,65 g/cm3 – for SSC5-SSC6 sedimentary complexes;
2,72—2,86 g/cm3 –for upper consolidated crust;
2,95—3,0 g/cm3 – for lower consolidated crust.
3,3 g/cm3 – for mantle.
The consolidated crust tends to increase thickness toward the Podvodnikov Basin from 10 to 13 km (upper crust – from 2 to 7 km, lower - 7 to 12 km). The steady increase of gravity field intensity towards the deepest part of the North-Chukchi Trough (0–170 km) can only be explained by rising Moho and thinning of the consolidated crust in this direction.
In the Podvodnikov Basin crustal thickness is reduced to 16–17 km (6–8 km - sedimentary cover, 4–6 km – upper crust, 5–7 km – lower). Densities are in the same range as for the North Chukchi Trough, the only difference is absence of lower sedimentary complexes. The local gravity minimum at the 630–680 km interval coincides. This seemingly conflicting situation was resolved by reducing density of the basement block to 2.65 g/cm3 (felsic intrusion?) and deepening Moho by 2 km.
At the Geofizikov Spur the total crustal thickness increases to 20 km along with sedimentary cover thinning to 3 km on average (around 1 km on uplifts, and up to 5 km in depressions). Consolidated crust is getting thicker mainly due to the upper crust almost doubling its thickness compare to adjacent parts of the Podvodnikov Basin - up to 9 km. Densities of the upper and lower crusts are laterally constant 2.70 and 2.90 g/cm3, accordingly.
The composite MCS profile 2014–02—2014-06 (location and seismic section (Fig. 5.19) most clearly illustrates the deeply rooted geological connections between the East Siberian Sea shelf and abyssal depressions of the Arctic ocean. The crustal seismic-gravity model of this important profile is shown on Fig. 5.31.
In the East-Siberian Sea shelf Cenozoic sedimentary complexes SSC1 and SSC2 with density 2.0–2.3 g/cm3 and 3 km thick on average, together with Mesozoic formation (2–6 km thick with density 2.50 g/cm3), filling basement depressions, reside on the continental crust 28–33 km thick. The upper crust is 6–17 km thick, lower - 17–20 km.
In the Podvodnikov Basin the crustal thickness drops to 17 km with slight increase towards the Geofizikov Spur. The sedimentary cover (2.00–2.55 g/cm3) shrinks in the same directions from 9 to 3 km. As in the previous model the crustal block with reduced density (2.65 g/cm3) is introduced into the upper crust at the depocenter of the Podvodnikov Basin. At the crest of the Geofizikov Spur there is only 1–1.5 km of sediments; underneath it the crustal thickness reaches 20–22 km.
In Makarov Basin sedimentary cover (1–4 km) consists of first three complexes with densities from 2.00 to 2.40 g/cm3, upper and lower crusts are 3–6 km and 3–4 km thick, accordingly. Total crustal thickness by modeling estimates (10–12 km) is in agreement with DSS data (“Ttransarktika 89–91”, Fig. 5.27). Transition to the Lomonosov Ridge domain is manifested by substantial increase of crustal thickness up to 21 km and strong (100 mGal) gravity anomaly marking sharp, 2.5 km high, rise of the ridge above the seafloor. The sediments thickness here is negligent (first hundred meters).
The modeling demonstrates good agreement between gravity and seismic data. Slight discrepancy (1–2 km) in configuration of upper - lower crusts and Moho (see solid lines on Fig. 5.32, 5.33) are well within the accuracy of the method. The uncertainty of boundaries interpolation over 250–300 km interval, is not supported by seismic rays, and averaged nature of grid–to profile interpolated gravity values, may also be the cause.
The Podvodnikov Basin resides on the continental crust, stretched and thinned during the Creataceous rifting process. (Nikishin et al. 2014, 2015; Langinen et al. 2009; Lebedeva-Ivanova et al. 2011; Kashubin et al. 2011, 2013; Glebovsky et al. 2013; Laverov et al. 2013; Jokat et al. 2013). Crustal thickness is between 19 and 25 km. The Basin subsidence and development supposedly took place between 118 and 56 Ma (Franke et al. 2004). The Geofizikov Spur separate the Podvodnikov Basin into western and eastern sub-basins distinguished mostly by stratigraphy of the basal sedimentary formations. The space between the Lomonosov Ridge and the Geofizikov Spur contains several half-grabens filled with syn-rift formations, presumably of Cretaceous age. In the eastern sub-basins seismic reflection data may suggest the presence of the older sedimentary complexes. The transitional slopes between the Lomonosov and Mendeleev Ridges are disturbed by multitude of north-south trending normal faults.
Total thickness of the Podvodnikov Basin sedimentary cover exceeds 6 km. The structure of the acoustic basement is radically different from the oceanic acoustic basement of the Amundsen basin.
Two strong seismic reflectors between the Post-Campanian unconformity pCU and the acoustic basement can be traced throughout the entire Podvodnikov Basin. These reflectors are correlated with Brooks and Jurassic unconformities identified at the base of Lower Brooks complex. Accordingly, the basal sedimentary complexes of the Podvodnikov Basin are dated as Pre- and Early Cretaceous.
Morphologically, the Podvodnikov Basin represents a complex terraced slope subjected to neo-tectonic normal faulting under extensional crustal regime causing stepped differential subsidence.
The Podvodnikov Basin can be classified as a part of the continental margin on the stretched continental crust. The stretching and thinning of the Podvodnikov Basin crust is presumed to be multiphase covering a certain period of time; possibility of the periodic changes of the tensional forces orientation also cannot be excluded.
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