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

The combined analysis of structural and magnetic data from the Mohns R idge, near 72-degrees-N, in the Norwegian-Greenland Sea, shows the exi stence of different phases of seafloor spreading, from Anomaly 5A (12 Ma), which represents the limit of the magnetic survey, onwards. For t he 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 pe riod may correspond to a local enhancement of tectono-volcanic activit y. 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-sedime ntary horizons appear, at, respectively, 140-160 ms and 340-360 ms two -way travel time beneath the sediments surface. Using an inferred valu e of sedimentation rate of approximately 50-70 m/Myr, the age of the f irst horizon (between approximately 3 and 2 Ma) corresponds to the beg inning of mid-Pliocene glaciations in the Northern Hemisphere. The age of the second one (between approximately 6.5 and 4.5 Ma), to the Plio cene-Miocene boundary. Our data, and other data from other areas indic ate that the three major sources of sediments (western Spitzbergen Mar gin, Bear Island Trough and Storfjord Trough) which supply the Mohns R idge area near 72-degrees-N have been dramatically more active after a pproximately 5.5 Ma. In addition, local unconformities recorded in the single-channel seismic sections indicate that some tectonic event occ urred off axis, little after the date corresponding to the deposition of the second horizon, S2. If one accepts that the Knipovich Ridge mig rated eastward 6-5 Ma ago, it can be hypothesized that the tectonic re lease of the Lena Trough-Knipovich Ridge-Mohns Ridge system forced the sediment production to increase at each one of the three sediment sou rces after the Miocene-Pliocene transition.