STRUCTURE OF ARCHEAN CRUST AND PASSIVE MARGIN OF SOUTHWEST GREENLAND FROM SEISMIC WIDE-ANGLE DATA

To investigate the structure and composition of the Archean crust beneath the shelf of southwest Greenland parallel to the passive margin, we conducted an onshore/offshore seismic wide-angle experiment in 1989. One of our goals was to obtain a velocity/depth profile along the thinned continental or transitional crust to place constraints on the magmatic rifting history in the northern Labrador Sea. Seismic recording instruments at four locations along the coast recorded energy from an air gun array up to 400 km distance. Due to the dense shot spacing of 100 to 150 m we were able to identify continuous reflected and refracted phases from the upper to lower crust. Results from a one-dimensional extremal inversion indicated a high-velocity zone (HVZ) with P wave velocities of more than 7.2 km/s at the bottom of the transitional crust. Combined two-dimensional travel time forward modeling and inversion produced a velocity/depth model that contains midcrustal and lower crustal discontinuities between 12 and 25 km depth and a Moho dipping northward at 30 to 42 km depth. An average crustal velocity of about 6.6 km/s indicates a high portion of mafic components in the crust. The lowermost crust consists of a HVZ with a maximum thickness of 8 km and temperature-corrected velocities of 7.5 to 7.85 km/s. The HVZ may be explained by an underplated igneous complex often observed under passive margins worldwide. Metamorphism to garnet-pyroxene-granulite during postrift isobaric cooling of the 30- to 40-km-deep, thinned continental crust could explain the velocities that are higher than on other margins. A large volume of magmatic material must have been added to the crust but did not produce extrusions. Magmatic underplating of the thinned continental or transitional crust of SW Greenland is likely associated with a possible hot spot magmatism in the Davis Strait and Baffin Bay region. Magmatism associated with continental rifting was found to reach farther south along the northern and central part of the Archean block than previously thought.