THE CRUSTAL STRUCTURE OF THE AMUNDSEN BASIN IN THE ARCTIC OCEAN DERIVED FROM SEISMIC REFRACTION DATA

From 2007 to 2012, three expeditions (LOMROG I through III) were carried out to acquire marine refraction seismic data in the Amundsen Basin and on the Lomonosov Ridge in the Arctic Ocean. Previous geophysical investigations indicate that the area is characterized by irregular magmatic seafloor generation as evidenced by alternating peridotite and basalt dredge samples along the ultra-slow spreading Gakkel Ridge. A small number of older seismic profiles on the Lomonosov Ridge image the breakup of the Eurasia Basin. The position of the continent-ocean boundary at the Lomonosov Ridge remains unclear as the character of the first weak magnetic anomaly (interpreted as Chron C25) is discussed controversially.

The data of the LOMROG expeditions consist of 1028 km of seismic reflection data supplemented by sonobuoys to obtain information on the velocity structure. A total of 18 sonobuoys were deployed along nine seismic lines on the Lomonosov Ridge, another 72 buoys were used on 25 lines in the Amundsen Basin. The length of these lines varied between 6 and 135 km. The seismic energy produced by the airgun cluster (volume between 605 and 1040 cu. in.) could be recorded to maximum offsets of 25 km. This was sufficient to obtain velocity information down to the Moho on a few lines in the Amundsen Basin.

P-wave velocity models of the sediments and the underlying crust are obtained by forward modeling of the travel times using raytracing software (RAYINVR). The coincident seismic reflection data constrain the structure within the sedimentary column including the basement geometry. The main objectives of the experiment are to (i) map the crustal character and thickness in the Amundsen Basin, (ii) in particular check for the presence of exhumed mantle in the basin, and (iii) to determine the nature of the crust close to the disputed weak magnetic anomaly (C25?) close to the Lomonosov Ridge.