HR: 0800h
AN: T41E-1344    [Abstracts]
TI: Slow to Ultraslow Seafloor Spreading in the Norway Basin Under Influence of the Iceland Hotspot
AU: * Breivik, A J
EM: a.j.breivik@geo.uio.no
AF: Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, Oslo, N-0316 Norway
AU: Mjelde, R
EM: rolf.mjelde@geo.uib.no
AF: Department of Earth Science, University of Bergen, Allegt. 41, Bergen, N-5007 Norway
AU: Faleide, J I
EM: j.i.faleide@geo.uio.no
AF: Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, Oslo, N-0316 Norway
AB: The Norway Basin was initiated by continental breakup between northern Europe and Greenland/Jan Mayen in the earliest Eocene (~54Ma). Being part of the North Atlantic Igneous Province, continental breakup and early seafloor spreading produced voluminous magmatism. An ocean bottom seismometer (OBS) profile acquired in the year 2000 from the Norwegian Moere margin to the extinct spreading axis of the Aegir Ridge, was used to estimate variations in magma productivity as the oceanic basin evolved. Due to low magnetic data coverage, a satellite derived gravity map proved suitable to reinterpret the East Jan Mayen Fracture Zone (EJMFZ) system, but none of the other proposed fracture zones within the Norway Basin could be identified along its ~500 km length. The revised EJMFZ trace was used to re-evaluate spreading direction in the Norway Basin, which is quite asymmetric as it is condensed mostly on the southwestern side. The magnetic track recorded along the OBS profile was used to identify magnetic seafloor spreading anomalies by forward modeling, and projected onto synthetic flow lines half spreading rates were derived along-profile. Maximum rate was above 3 cm/a between A24A and A24b, falling off to ~0.7 cm/a (ultra-slow) towards the mid-Oligocene (25-28 Ma) termination of seafloor spreading. Breakup magmatism created oceanic crust up to 10-11 km thick, tapering down to thin crust by C23 time (51.4 Ma), the increased melt potential was thus spent ~2.5 Ma after continental breakup. There is a conspicuous correlation between half spreading rate and oceanic crustal thickness. As this is not observed in a normal seafloor spreading environment at most rates observed here, both plate spreading and magma production should be governed by a common cause, presumably hot asthenosphere restricted to the continental rift zone. While Oceanic crust created during ultra-slow spreading is thin (4 km), crust created during slow spreading is also thinner than the world average (5 vs. 7.1 km), indicating a somewhat depleted mantle. A V-shaped pattern seen in the gravity field only around the northern part of the Aegir Ridge corresponds to increased crustal thickness in the OBS model, demonstrating the northeast migration of asthenosphere zones with increased melt production at a speed of 0.3-0.6 cm/a. The thin crust and lack of fracture zones in the Norway Basin is typical for ultra-slow seafloor spreading, even if much of it was created during slow spreading (~1.5 cm/a). This character may have developed not despite the closeness to the Iceland Hotspot, but because of it, as the mantle was partially depleted by high magma production both during early seafloor spreading and during the construction of the aseismic Iceland-Faeroe Ridge to the south. The southern part of the Norway Basin lacks V-shaped ridges and the Aegir Ridge has a deeper axial valley, indicating lower magma production than in the north, consistent with this.
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 3005 Marine magnetics and paleomagnetics (1550)
DE: 3025 Marine seismics (0935, 7294)
DE: 3035 Midocean ridge processes
DE: 8157 Plate motions: past (3040)
SC: Tectonophysics [T]
MN: Fall Meeting 2005