163 Preliminary Report


INTRODUCTION

Strategy

The Southeast Greenland margin is a type example of a volcanic-rifted margin. The margin is characterized by a broad seaward-dipping reflector sequence (SDRS) composed of basalt that onlaps continental (mainly Precambrian) crust to the west and terminates eastward in oceanic crust of early Tertiary age (Figs. 1, 2). In the Northeast Atlantic, seafloor-spreading Anomalies 24n-24r are the oldest identified pair of anomalies (Talwani and Eldholm, 1977; Srivastava and Tapscott, 1986; Larsen, 1988). Anomaly 24n is developed off Southeast Greenland as a double-peaked anomaly, reflecting the three short positive events C24n.1 through C24n.3 and relatively high spreading rates during this interval (approximately 3 cm/yr half-rate; Larsen, 1980).

The minimum age of the Southeast Greenland SDRS is constrained by the seaward occurrence of well-developed seafloor-spreading anomalies (Fig. 1; Larsen and Jakobsdottir, 1988). In the north, close to the Iceland-Greenland Ridge, the SDRS extends seaward to Chrons C22n-C21n (49-47 Ma). However, most of the Southeast Greenland SDRS is found landward of and is older than C24n.1 (53 Ma; Cande and Kent, 1992). Weak and semilinear magnetic anomalies are present over the main SDRS and may represent either low-amplitude anomalies older than C24n (e.g., C25n-C27n, 56-61 Ma) or short magnetic events within C24r (the chryptochrons of Cande and Kent, 1992; see also discussion in "Summary" chapter, Larsen, Saunders, Clift, et al., 1994).

Evidence for significant magmatism and tectonism during continental breakup is not restricted to the offshore areas. A coast-parallel dike swarm and associated seaward flexuring of the crust one present from 63°N along the east Greenland coast and northward. Within this zone, gabbroic and syenitic intrusions are present (Fig. 1; Myers, 1980; Myers et al., 1993) and locally associated with basaltic lavas overlying thin sediments (for review see Larsen, 1980; Nielsen et al., 1981; Brooks and Nielsen, 1982). Farther north, a much more extensive and thick flood basalt province is preserved (Fig. 1; Larsen et al., 1989). Comprehensive studies of the onshore region are being conducted in parallel with Ocean Drilling Program (ODP) drilling and will be augmented in 1996 with a program of deep crustal seismic imaging that includes the region (Larsen et al, in press). In particular, ODP drilling and field geological studies will aim at correlating the on- and offshore parts of the crustal flexure zone and the volcanic stratigraphy within the two areas.

Drilling was positioned along two margin transects, located distal (Legs 152 and 163) and proximal (Leg 163) to the Iceland plume center. The two transects were named EG63 and EG66, respectively, in reference to their approximate latitudes. At each transect, drilling was targeted at the prerift crust, the breakup unconformity and earliest volcanism, the transition from initial continental volcanism to ocean crust volcanism, and, most seaward, a reference hole in steady-state spreading crust. This drilling strategy was designed (Larsen et al., 1991) with two primary objectives: (1) the investigation of the development with time along each transect would tell us about the progressive weakening of the continental crust and the associated magmatic development and (2) the study of magmatic development and the magma source at different offsets from the Iceland plume would enable us to evaluate possible radial zonation in the original plume structure. Additional reference points for the second objective are provided by the earlier Deep Sea Drilling Project (DSDP) Leg 81 drilling at the Hatton Bank margin (most distal southern offset; Joron et al., 1984) and ODP Leg 104 drilling at the Vøring margin (intermediate northern offset; Viereck et al., 1988; see also Larsen, Saunders, Clift et al., 1994).

Geophysical Database

Legs 152 and 163 were based on a large set of seismic data over the Southeast Greenland margin (Fig. 3). The database comprises three different sets of seismic data: (1) regional to detailed grids of shallow, high-resolution multichannel seismic (MCS) data (Larsen et al., 1994); (2) a regional grid of deep 7-s (two way traveltime, TWT) MCS data; and (3) deep 14-s (TWT) MCS data (Larsen, in press). In addition, aeromagnetic and regional marine gravity data exist (for a more extensive review and references see Larsen, 1990; Larsen, Saunders, Clift, et al., 1994).

Leg 152 Results and Implications for Leg 163

A number of important observations made during Leg 152 drilling into the Southeast Greenland SDRS significantly affected the detailed planning of Leg 163. These include the following: (1) highly tilted to subvertical prerift sediments occur below the inner part of the SDRS; (2) an early, continentally hosted and contaminated basaltic to andesitic volcanism of 61-62 Ma age (Sinton et al; 1994) overlies these sediments; (3) the upper limit of these lower lavas is a sharp transition possibly a hiatus into picritic to tholeiitic lavas followed by basalts of a rather uniform composition that resemble depleted tholeiites from Iceland and appear to make up the main part of the SDRS; and (4) all recovered igneous units were erupted subaerially. Thus, Leg 152 confirmed that the SDRS is a wedge of predominantly basaltic material extruded subaerially in accord with the model for crustal accretion in Iceland (Pàlmason, 1986) and with the interpretation of seismic data (Mutter et al., 1982; Larsen and Jakobsdottir, 1988).

The Leg 152 findings imply the presence of a rapid transition from continental to oceanic crust below the inner part of the SDRS. During formation of this continent to ocean transition, pre-rift sediments were deposited in a basin of unknown width and, later, in a zone close to the final line of breakup, subjected to faulting and crustal extension, uplift and erosion prior to volcanism (see also Larsen, Saunders, Clift et al., 1994).

The Leg 152 data are deficient in a number of aspects. These include a lack of suitable material for age determination of the oceanic succession (i.e., the main part of the SDRS), noncontinuous sampling of the transition from initial picritic to depleted tholeiitic volcanism within the oceanic succession, and nonrecovery of the oldest part of the continental volcanic succession. In addition, the prerift sediments were poorly sampled because of their subvertical orientation, and they have been too strongly metamorphosed to yield any age-diagnostic fossils or palynomorphs.

Leg 163 was planned to overcome these deficiencies within the southern EG63 transect, as well as to sample the breakup and early seafloor spreading volcanism in a more proximal position to the proposed Iceland "hot spot" track along the northern EG66 transect. The faint signature of the Iceland plume in the Leg 152 rocks suggests that a stronger plume imprint could be present at this location closer to the former plume axis, which, if true, would indicate a radial zonation within the original plume structure.

Integration of observations from drilling, field geology, and geophysics on crustal structure and deformation, timing of volcanism and the involvement of Iceland plume material in the breakup process eventually will enable a critical review of current models of plume structure and the impact of mantle plumes on the process of continental breakup (e.g., Mutter et al. 1988; White and McKenzie, 1989; Richards et al., 1989; Campbell and Griffiths, 1990; Coffin and Eldholm, 1992; Kent et al., 1992; Holbrook and Keleman, 1993).

Drilling Plan

In order to meet the main objectives, a total of six first-priority sites was planned for the two transects. Three sites were planned for the innermost part of the EG63 transect. Two sites were targeted to increase the sampling of the prerift crust and oldest volcanic cover, and one site was to deepen Site 915 in order to provide stratigraphic overlap with Site 917 (Figs. 2, 3). Three sites were also planned for the northern EG66 transect. The two sites within the innermost part had objectives roughly similar to the inner sites of the EG63 transect, though less ambitious in terms of stratigraphic coverage. The additional seaward site was planned in SDRS-type oceanic crust of Anomaly 22 age (i.e., in steady-state accreting Icelandic-type oceanic crust).

Changes in the Drilling Plan Imposed by Drilling Problems and Weather

A drilling accident and damage to the ship sustained during extreme storm conditions reduced the scientific drilling operations at Leg 163 to less than one-half of the planned program. Recoil from a break in the drill pipe on 10 September 1995 damaged the top-drive assembly after only one day of drilling at the first, shallow-water site. A port call to Reykjavik, Iceland, was made in order to make the necessary repairs. Permission to drill in water depths shallower than 300 m was temporarily withdrawn by ODP/TAMU, pending review of safety procedures and the delivery of supplementary drilling hardware. Operations therefore resumed at the deeper water sites along the southern EG63 transect on 16 September. Drilling progressed, though with interruptions, because of heavy seas and icebergs drifting across the drill sites, until 29 September.

Extreme hurricane conditions built rapidly through the night of 29 September. At many times the north-northeast wind exceeded 100 kt, and it remained at hurricane force for at least 26 hr. By the morning of 30 September, the ship was being battered by short-period, 60-70-ft-high waves, and she was unable to maintain position without risking severe damage. The ship's bridge took water through a broken window, which caused both radars to fail and threatened the computers for the dynamic-positioning system. Numerous thrusters were mechanically damaged or became inoperable because of flooding. In spite of reduced maneuverability, the ship was able to maintain heading in the wind and sea while it was being forced south at a speed of up to 4 kt. While drifting in this manner, there was an increased potential of colliding with icebergs. When the storm abated to gale force on 1 October , the ship was turned to the south and the transit to Halifax, Nova Scotia, for repair was started.

At this point, major repairs and a thorough examination of the ship's structure and systems were needed. This ruled out the possibility of further drilling operations during Leg 163. As a result of these untimely events, only three of the planned six sites were drilled.


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