East Greenland coast-parallel dike swarm and its role in continental breakup

Abstract

The East Greenland coast-parallel dike swarm is exposed along ∼250 km of the outermost coast south of the Iceland hotspot track manifest by the Iceland-Greenland Ridge. The dike swarm and associated plutonic centers were emplaced over a period extending from a few million years before final breakup to at least 7 m.y. after breakup ca. 56 Ma. The predominantly mafic dikes are hosted in Precambrian high-grade basement, but locally cut through early Tertiary plutonic complexes. Seaward, continental crust is replaced by 20-30-km-thick Icelandic-type igneous crust, including thick sequences of subaerially erupted lavas that dip oceanward. The orientations of dikes, dike thickness, dike density, field classification, and relative sequence of emplacement of 1410 dikes within this region were studied in the field and by photogrammetry along 6 margin transects. Statistical treatment of the field data documents a shift in orientation of dikes from predominantly subvertical inland to predominantly landward dipping (as low as 40°) along the outer coast. The earliest dikes, largely intruded prior to breakup, have dip directions at right angles to the bedding of locally preserved lavas and sediments, implying subvertical intrusion of dikes followed by strong seaward rotation of the crust during breakup. Consistent with this, younger generations of dikes emplaced after breakup show steeper to almost vertical dips. The strikes of the earliest dikes suggest that these were intruded in an en echelon pattern and slightly oblique to the margin. These dikes are relatively thick (average 6-8 m) and may have been intruded subvertically from a deep source, possibly at the base of the crust. The other dikes define more curved, coast-parallel trajectories extending out from some of the major plutonic complexes and form partly overlapping dike swarm segments. Together with observed systematic decrease in dike density and increasing underrepresentation of thin dikes away from major igneous complexes, this suggests that with time crustal plutonic complexes became important reservoirs for lateral magma propagation along the margin. In cross section, the density of dikes increases seaward and reaches ∼50% along the outer coast. The average dike thickness decreases from a maximum of 8 m inland to 3-4 m along the coast. A hinge line is located ∼15 km inland and is defined by landward rapidly decreasing dike density, maximum dike thickness, and, in general, subvertical orientation of dikes, except for a small population of early dikes that seems to have intruded along preexisting and landward-dipping faults. The parallel increase seaward in dike density and decreasing dike thickness suggests an ophiolitic sheeted dike complex (i.e., continent-ocean boundary) to be present offshore 20-40 km from the inland hinge line.