Nature of Thin Crust Across the Southwest Greenland Margin and its Bearing on the Location of the Ocean-Continent Boundary

Abstract

The detailed seismic refraction and a limited amount of deep reflection measurements across the southwest Greenland margin show presence of a wide zone of thin upper crust overlying a serpentinized mantle. The nature of the crust in this zone, however, remains in dispute. Interpretation that this is thinned continental crust is in conflict with the interpretation based on magnetic data which suggest it to be oceanic. The magnetic data from this region have, therefore, been re-examined here and it is shown that even though the magnetic anomalies are small in amplitudes and variable in shape over a short distance, they can reasonably be correlated with synthetic seafloor spreading anomalies. It is possible for these anomalies to be caused by injection of volcanic material through continental crust, but their overall continuity and linear character, their resemblance to anomalies formed in the southern Labrador Sea, their symmetry across the extinct ridge, and their correlation with seafloor spreading models, strongly argue for their formation by seafloor spreading. Such an interpretation is also consistent with the plate kinematic motions derived for the North American and Eurasian Plates, and shows that the Labrador Sea essentially started to form along a northwest continuation of the Mid-Atlantic Ridge at chron 33 time

The correlation of magnetic anomalies in the present model shows a drastic change in the half rate of spreading at chron 30 from 5.8 mm/y before to a mean value of 12.0 mm/y after. It is suggested that the decrease in amplitude of magnetic anomalies arises from fragmentation of the oceanic crust formed at such low spreading rates. The change in the rate of spreading correlates well with a change in basement topography, from rough during slow, to smooth during faster spreading and the occurrence of thin crust during slow spreading and a normal thickness during faster spreading. These changes in crustal properties are remarkably similar to those observed across the central Labrador Sea, where the half spreading rate changed from 10 mm/y to 3.5 mm/y before the cessation of spreading. Here a clear division in basement morphology and crustal thickness is observed between the crusts formed at these two rates. The crust formed at to mm/y half spreading rate exhibits smoothly undulating basement with slightly less than normal crustal thickness, while the crust formed at 3.5 mm/y half spreading rate show evidence of intense normal faulting, with many faults showing large offsets and extending to lower crust and Moho depths. Refraction results together with gravity modelling show the crust to be abnormally thin overlying serpentinized upper mantle. The similarities between the crustal structures formed at the central Labrador Sea and that formed across the SW Greenland margin support the suggestion that the thin crust across SW Greenland margin is oceanic and was formed during slow seafloor spreading. Furthermore, the magnetic modelling suggest that the oceancontinent boundary lies fairly close to the bottom of the continental slope in this region. Comparison of this crust with crust formed across several other continental margins show great similarities suggesting that they also were formed at slow spreading rates.

Geological Survey of Canada Contribution no. 37994

Geophysics, Geological Survey of Canada, 1 Observatory Crescent, Ottawa. Ont., K 1 A 0Y3.