INTRODUCTION

Scientific drilling in the northeast Atlantic during the past three decades has been crucial for establishing the presence of voluminous basaltic constructions along more than 2000 km of the ocean's conjugate margins (Fig. 1). These volcanic rocks were erupted in a short period during the early Tertiary continental breakup between Greenland and Eurasia, forming so-called rifted volcanic margins (Eldholm and Grue, 1994). The results from the northeast Atlantic drilling legs have been utilized for interpretation of voluminous volcanic breakup complexes along many of the world's rifted margins (Coffin and Eldholm, 1994). Voluminous volcanism is now regarded as a common response to continental breakup, having potentially important implications for the paleoenvironment and for crustal deformation and growth (Eldholm et al., in press).

Several drilling legs have been devoted to studies of the northeast Atlantic volcanic margins. Initially, the acoustic basement on the VÝring Plateau was found to represent basaltic volcanics in Deep Sea Drilling Project (DSDP) Sites 338, 342, and 343 (Talwani, Udintsev, et al., 1976). Similarly, basalts were recovered in the shallow-basement penetrating DSDP Sites 552-555 on the Rockall Margin, suggesting that the intrabasement seaward-dipping reflectors (SDR) and the Outer High represented subaerial and shallow-marine volcanic constructions, respectively (Roberts, Schnitker, et al., 1984). Later, more than 900 m of subaerial lavas were drilled at Ocean Drilling Program (ODP) Site 642 near the apex of the SDR on the VÝring Margin (Eldholm, Thiede, Taylor, et al., 1987; 1989). Recently, seven ODP sites were drilled to as much as 779 m into basaltic basement on the Southeast Greenland Margin, recovering dominantly subaerially emplaced lava flows (Larsen, Saunders, Clift, et al., 1994; Duncan, Larsen, Allan, et al., 1996).

Seismic reflection data are essential for extrapolating the detailed geological information provided by the boreholes in two and three dimensions. Unfortunately, seismic reflection data are often of poor quality in volcanic terrains. This is related to large acoustic impedance contrasts and rough structures, both between the sediments and underlying lavas and within the basaltic lava piles because of large changes of physical properties within individual lava flows (Planke and Eldholm, 1994). Recently, fairly high-quality reflection images have been obtained within and below the volcanic pile in different volcanic provinces (Symonds et al., 1998; Alvestad, 1997). These improved intrabasement reflectivity images have led to the development of the concept of seismic volcanostratigraphy (Fig. 2).

The aim of this study was to reassess and reinterpret seismic reflection data in the vicinity of boreholes penetrating breakup volcanic rocks in the northeast Atlantic. Special emphasis was on the constraints that the well data provide for interpretation of characteristic seismic facies units identified in volcanic terrains. An understanding of the cause and significance of the variations in reflection patterns is important for understanding the nature and severity of tectonic and volcanic processes that occurred during continental breakup and the early phase of sea-floor spreading. The study focus was on using the recent drilling and seismic data from the Southeast Greenland Margin, both locally and along a conjugate margin transect. Furthermore, these results were compared with a conjugate margin transect to the north crossing the southern Jan Mayen Ridge and the central MÝre Margin (Fig. 1). The northern and southern transects are located at a similar distance from the Iceland plume trail but show distinctly different seismic volcanic reflection characteristics.

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