VOLCANO-TECTONIC EVENTS AND SEDIMENTATION SINCE LATE MIOCENE TIMES AT THE MOHNS RIDGE, NEAR 72-DEGREES-N, IN THE NORWEGIAN-GREENLAND SEA

The combined analysis of structural and magnetic data from the Mohns Ridge, near 72-degrees-N, in the Norwegian-Greenland Sea, shows the existence of different phases of seafloor spreading, from Anomaly 5A (12 Ma), which represents the limit of the magnetic survey, onwards. For the period comprised between Anomaly 5 (approximately 9 Ma) and Anomaly 3A (approximately 6-5 Ma), most particularly, a dramatic drecrease of spreading rate is observed, by as much as 50%. Between Anomaly 3A and 2A (approximately 3-2 Ma), an acceleration of spreading is observed, together with a global uplift of the acoustic basement. This latter period may correspond to a local enhancement of tectono-volcanic activity. The existence of ridges located at 25 and 50 km from the ridge axis, and parallel to it, suggests that both the beginning (5-4.5 Ma) and the end (3-2.5 Ma) of this phase were marked by intense volcanism. In the sedimentary basins of the survey area, two (at least) intra-sedimentary horizons appear, at, respectively, 140-160 ms and 340-360 ms two-way travel time beneath the sediments surface. Using an inferred value of sedimentation rate of approximately 50-70 m/Myr, the age of the first horizon (between approximately 3 and 2 Ma) corresponds to the beginning of mid-Pliocene glaciations in the Northern Hemisphere. The age of the second one (between approximately 6.5 and 4.5 Ma), to the Pliocene-Miocene boundary. Our data, and other data from other areas indicate that the three major sources of sediments (western Spitzbergen Margin, Bear Island Trough and Storfjord Trough) which supply the Mohns Ridge area near 72-degrees-N have been dramatically more active after approximately 5.5 Ma. In addition, local unconformities recorded in the single-channel seismic sections indicate that some tectonic event occurred off axis, little after the date corresponding to the deposition of the second horizon, S2. If one accepts that the Knipovich Ridge migrated eastward 6-5 Ma ago, it can be hypothesized that the tectonic release of the Lena Trough-Knipovich Ridge-Mohns Ridge system forced the sediment production to increase at each one of the three sediment sources after the Miocene-Pliocene transition.