The AMEX expedition in 2008: Consequences for tectonic models of the Amerasia Basin

The tectonic evolution of the older part of the Arctic Ocean, the Amerasia Basin is still not sufficiently understood, despite of a good coverage of aerogeophysical data. In contrast, to the Eurasia Basin no seafloor spreading anomalies can be identified for most parts of the Amerasia Basin. The critical information for any progress are dateable basement samples from the various ridges and plateaus, sufficient marine seismic data and a sound understanding of the geology in Siberia and Alaska. This contribution reports from new geophysical data along the East Siberian margin. The objective of the RV Polarstern cruise in 2008 was to gather new information on the deeper structure of the Mendeleev Ridge at its junction with the Siberian shelf, and the Makarov/Podvodnikov Basin, which is situated between the Mendeleev and Lomonosov ridges. Favourable ice conditions allow to gather an almost continuous 1000 km-long seismic transect along 81°N with a 300 m streamer and an 32 ltr airgun array as well as several seismic profiles across the East Siberian margin. The seismic data imaged the entire sedimentary column down to the acoustic basement, and provide the following simple insights into the tectonic evolution: The Amerasia margin of the Lomonosov Ridge along our transect is very wide. It shows all characteristics of a rifted continental margin. Most likely rotated basement blocks are successively covered by sediments, which are not affected by any tectonic faulting. Thus, most of the Makarov/Podvodnikov Basin along the transect is underlain by stretched continental crust from the Lomonosov Ridge margin. The Marakov-facing margin of the Mendeleev Ridge has a completely different appearance. It looks like that the ridge has more or less the same age as the small part of oceanic crust formed underneath the Makarov/ Podvodnikov Basin, and was covered by sediments afterwards. At approximately 1000 m below the seafloor the Lomonosov Ridge hosts a prominent reflector, which has been attributed with an age of 10-20 Myr. Taking the entire basin geometry into account as well as the drilling results from the ACEX campaign, it is more likely that this reflector has an age of at least 55 Myrs. Underneath this unconformity several kilometers of sediments are observed. This has several significant consequences for any geodynamic models: a) the Laptev Sea stratigraphy from which the younger age of the unconformity was deduced is in error b) The Makarov/Podvodnikov Basin is definitely of Cretaceous age. c) Most likely the oceanic Mendeleev Ridge was formed contemporaneously with the initial formation of oceanic crust in the Makarov/Podvodnikov Basin. A likely age is 100 Myrs for the age of the oceanic crust. d) The rifting and/or formation of the seafloor spreading stopped with the eruption of the Mendeleev Ridge. Another explanation is that the stress regime changed from perpendicular to the Lomonosov Ridge margin to strike slip. e) Thus, the early tectonic evolution of the East Siberian margin between the Lomonosov Ridge and the Chukchi Plateau was similar to the Cenozoic Laptev Sea. Aerogravity data, however, show that this model is only true for the Makarov/ Podvodnikov Basin. The Lomonosov Ridge margin significantly changes its tectonic style towards the Canadian margin