Saunders, A.D., Larsen, H.C., and Wise, S.W., Jr. (Eds.), 1998 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 152 463 39. CRUSTAL STRUCTURE ALONG THE LEG 152 DRILLING TRANSECT 1 Hans Christian Larsen, 2 Trine Dahl-Jensen, 2 and John R. Hopper 2 ABSTRACT Seismic data, drilling data, and gravity data define a 40-km-wide continent-ocean transition (COT) below the outer half of the 70- to 80-km-wide shelf along the Leg 152 drilling transect. The seismic data include a detailed grid of shallow high-reso- lution seismic data; a regional grid of conventional multichannel seismic data (including sonobuoy refraction data); and a deep crustal, vertical incidence multichannel seismic profile. Below the outer shelf, the top of the Precambrian crust off southeast Greenland dips through a normally faulted, seaward-facing flexure zone below the landward feather edge of the seaward-dip- ping reflector sequence (SDRS), which is subaerially erupted basaltic lavas. Associated with the flexure zone, the continental crust thins from approximately 28 km thick to almost zero over a maximum distance of 40 km. Variations in present-day crustal thickness are much less because of accretion of thick igneous crust during breakup. Most of the thinning of the continental crust takes place within only 25 km from the mid-shelf flexure zone. Within this zone, landward-dipping normal faults are present in the upper continental crust, indicating that thinning was accommodated by fault-related crustal extension. Lower crustal thin- ning appears to be of a different nature and may be related to “intrusive” underplating. The seismically defined COT is consis- tent with a distinct development from initially continental to oceanic volcanism observed in the boreholes. The new igneous SDRS crust seems to be 20-22 km thick, including at least 5 km of lava in the upper crust. Deep reflections located below the COT at 30-35 km depth may represent a pillow of thick (10 km or more) underplated material or intra-mantle reflections from a residual mantle. INTRODUCTION The southeast Greenland margin is characterized by a wide, mar- gin-parallel zone with a seaward-dipping reflector sequence (SDRS; Larsen and Jakobsdóttir, 1988; Larsen, 1990). The SDRS is present below the middle to outer shelf and in the adjacent deep-water area (Figs. 1, 2). Larsen and Jakobsdóttir (1988) interpreted the main part of southeast Greenland SDRS as subaerially erupted lavas that ema- nated from a fairly narrow, Icelandic-type spreading center. This im- plies that the SDRS crust represents Icelandic-type oceanic (igneous) crust (Pálmason, 1986) with the continent-ocean transition (COT) lo- cated below the inner part of the SDRS. Testing these interpretations by borehole data was one of the major tectonic objectives of Ocean Drilling Program (ODP) Leg 152. The Leg 152 drilling transect is the southernmost of two southeast Greenland margin transects studied by ODP and Deep Sea Drilling Project (DSDP), named the EG63 and EG66 transects, respectively (Fig. 1). Drilling during Leg 152 penetrated the feather edge of the SDRS and sampled the central part of the SDRS (Figs. 3, 4). The re- covered rocks were exclusively subaerially erupted and mainly basal- tic lavas. The SDRS comprises a lowermost (innermost) continental- ly contaminated lava sequence followed by picrites and tholeiites with much reduced or no continental contamination (Larsen, Saun- ders, Clift, et al., 1994; Fitton et al., Chaps. 28 and 29, this volume; Saunders et al., this volume). This evidence is in strong support of the interpretation of the SDRS as representing Icelandic-type crust, as well as for a location of the COT directly below the feather edge of the SDRS (middle outer shelf). However, because there are strong variations along the margin in the degree of Neogene shelf-edge pro- gradation and in the width of the shelf, the position of the COT can vary from below the shelf slope to the inner middle shelf (see also Larsen, 1990). The goal of the present paper is to provide a more detailed model for the crustal structure and crustal deformation across the COT along the EG63 transect. The model is supported by (1) Leg 152 drilling da- ta; (2) high-resolution, conventional, and deep crustal multichannel seismic (MCS) data; and (3) gravity data. Acquisition of wide-angle seismic data for deep crustal velocity control is planned for 1996. SETTING OF THE EG63 TRANSECT In the northeast Atlantic Ocean, the oldest identified pair of sea- floor spreading anomalies are C24n-C24r (Talwani and Eldholm, 1977; Srivastava and Tapscott, 1986; Larsen, 1988). Anomaly C24n (53 Ma; Cande and Kent, 1992) is developed off southeast Greenland as a double-peaked anomaly. Along the Leg 152 transect it is partly overlapped by the most seaward and youngest part of the SDRS; hence, it provides a minimum age for the SDRS (Fig. 2; Larsen and Jakobsdóttir, 1988). However, the SDRS extends seaward to chrons C22n-C21n (49-47 Ma) and may extend into even younger anoma- lies in the north near the Iceland-Greenland Ridge. Weak and semi- linear magnetic anomalies are present over the main SDRS along the Leg 152 transect, and they may represent either low amplitude anom- alies older than C24n (e.g., C25n-C27n, 56-61 Ma) or short magnet- ic events within C24r (the cryptochrons of Cande and Kent, 1992; see also the discussion in the “Summary and Principal Results” chapter of Larsen, Saunders, Clift, et al., 1994; and Larsen and Saunders, this volume). Lavas from the Middle Series at Site 917, which are among the oldest part of the SDRS along this transect, have yielded 40 Ar- 39 Ar ages of 61-62 Ma (Sinton et al., 1994; Sinton and Duncan, this volume; Werner et al., this volume). The crustal structures landward of the East Greenland SDRS show considerable variation from south to north. A three-fold divi- sion of the margin structure along strike was defined by Larsen (1990). The transect studied in this paper is located within the south- ernmost segment of these three margin segments, which stretch from the southern tip of Greenland to the northern part of the Iceland- Greenland Ridge in the north, and consists almost entirely of high- 1 Saunders, A.D., Larsen, H.C., and Wise, S.W., Jr. (Eds.), 1998. Proc. ODP, Sci. Results,152: College Station, TX (Ocean Drilling Program). 2 Danish Lithosphere Centre (DLC), Øster Voldgade 10, 1350 K, Copenhagen, Den- mark. larsenhc@dlc.ggu.min.dk