DEEP SEISMIC-REFLECTION DATA ACROSS THE CONJUGATE MARGINS OF THE LABRADOR SEA

A deep seismic reflection transect of the conjugate margins of the Lab rador Sea is described, which represents one of the few data sets of t his kind. A characteristic reflectivity is ascribed to a 120 km wide r ibbon of very thin crust that may be either thinned continental crust, which has perhaps been intruded, or oceanic crust, perhaps modified b y the proximity of the continent. Most of the major changes in crustal thickness and in the subsidence and sedimentation patterns on the mar gins occur landward of these transitional zones, which are found on bo th margins. An interpretation of these regions as continental in origi n is compatible with other seismic observations on the west Greenland margin, but does not match the magnetic anomaly interpretation, which requires the transitional crust to be oceanic in origin. Models that s atisfy the gravity anomalies and the subsidence history have been used to assist in interpreting the seismic data. The subsidence models inc lude the effects of decompression melting during lithospheric extensio n and rifting, and we predict the thickness of igneous crust produced. However, the gravity models suggest that a lower crustal layer may ex tend farther inland below the Labrador shelf than is predicted by magm atic underplating. The present seismic results, combined with the othe r geophysical data, are consistent with a pure shear model of lithosph eric stretching, with faulting confined to the upper crust. Many of th e problems raised by this data set are similar to those identified in comparing the nonvolcanic margin of Iberia with the conjugate Grand Ba nks margin in the North Atlantic. If the transition zone results from stretching the continental lithosphere, then a large component of the very thin crust there must consist of igneous material formed by melti ng. Under these conditions a sharp, vertical ocean-continent boundary would be unlikely.