Frauke Klingelhöfer

Seismic study of the transform-rifted margin in Davis Strait between Baffin Island (Canada) and Greenland: What happens when a plume meets a transform

[ 1] The Davis Strait transform margin was studied using a 630-km-long wide-angle reflection/ refraction seismic transect extending from SE Baffin Island to Greenland. Dense airgun shots were recorded by 28 ocean bottom seismometers deployed along the line. A P wave velocity model was developed from forward and inverse modeling of the wide-angle data and incorporation of coincident deep multichannel reflection seismic data. Off Baffin Island in the Saglek Basin, 7 to 11-km-thick two-layered continental crust (5.8 - 6.6 km/s) is observed. Off Greenland, continental crust is divided into three layers (5.4 - 6.8 km/s) with a maximum thickness of 20 km. Farther offshore Greenland the crust thins to 7 - 12 km and the lower crust disappears. Between the continental blocks a 140-km-wide zone with oceanic crust ( layer 2 is 5.4 - 6.2 km/s and layer 3 is 6.7 - 7.0 km/s) is located. The western half of this zone is interpreted to be part of a volcanic margin with seaward dipping reflectors; the eastern part is associated with the Ungava fault zone (UFZ), the major transform fault in Davis Strait. The UFZ thus acted as leaky transform fault during phases of transtension. Southward flow of material from the Iceland plume created a 4 to 8-km-thick underplated layer (7.4 km/s) beneath the thinned portions of the continental crust and beneath previously emplaced oceanic crust. Plume related Paleogene volcanism is indicated by an up to 4-km thick layer (4.3 - 5.8 km/s) with basalts and interbedded sediments that can be traced from SE Baffin Island 400 km toward the east.

geophysical and geochemical constraints on crustal accretion at the very‐slow spreading mohns ridge

The composition of upper mantle and lower crustal material at very-slow spreading centers cannot be reliably determined by seismic studies alone. Since the range of P-wave velocities for serpentinized peridotites and gabbros overlap, additional information provided by the major and rare earth element (REE) content of the basalts is useful to constrain interpretations of seismic data. Refraction seismic data from the very-slow spreading (16 mm/a, full rate) Mohns Ridge in the Norwegian-Greenland Sea yields a highly variable thin crust of 4.0 ± 0.5 km thickness. Analysis of S-waves suggests that Layer 3 is composed primarily of gabbro containing at most a small percentage (< 20%) of mantle material. The Na8 content of Mohns Ridge basalts suggests a magmatic crustal thickness of 4–5 km. Inversion of the REE concentrations yields a melt thickness of ∼5 km. This agreement between seismic and geochemical data suggests that neither large quantities of mantle material are found in the lower crust nor is a large volume of basaltic magma frozen in the upper mantle.