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 µnal breakup to at least 7 m.y. after breakup ca. 56 Ma. The predominantly maµc 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, µeld classiµcation, and relative sequence of emplacement of 1410 dikes within this region were studied in the µeld and by photogrammetry along 6 margin transects.Statistical treatment of the µeld 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 deµne 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 deµned 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. The seaward ×exure of the crust is interpreted to re×ect initial tectonic extension by rotation of fault blocks followed by monoclinal ×exing due to loading by the

Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

“…The concept that magmatic intrusions can significantly weaken the continental lithosphere, leading ultimately to its rupture, is now widely accepted in hypotheses of rift evolution (Klausen and Larsen, 2002;Buck, 2004;Buck, 2006;Bialas et al, 2010). The best exposed example of magma-rich breakup is the East Greenland continental margin, where glacier-cleared exposure reveals a regional pattern of diking, faulting and warping in a breakup zone initiated over a mantle plume (Myers, 1980;Klausen and Larsen, 2002;Hanghoj et al, 2003). The dikes along the western Afar margin share many of the same characteristics of the East Greenland dike swarm.…”

“…Later dikes from western Afar are similar to those from east Greenland that are typically less mafic and have no extrusive equivalent within the flood basalt sequence. Statistical treatment of more than 1400 dikes (Klausen and Larsen, 2002) documents a progressive shift in orientation from predominantly subvertical inland to predominantly landward dipping (as low as 40°) offshore. This expresses the progressive seaward rotation of crustal units during the evolution of the margin (Morton and Black, 1975).…”

“…The presence of buoyant melt can facilitate the intrusion of dikes into thick continental lithosphere at comparatively small extensional stresses, and this heating can significantly reduce the strength of the plate, further facilitating extension (e.g., Fialko and Rubin, 1999;Buck, 2004;Bialas et al, 2010). Thus, the initial phase of rifting above a mantle plume may be marked by a pulse of widespread dike intrusion (e.g., Renne et al, 1996;Fialko and Rubin, 1999;Klausen and Larsen, 2002). During the later stages of continental rifting, extension occurs principally by dike intrusion into a thinned lithosphere (Ebinger and Casey, 2001;Keranen et al, 2004;Rooney et al, 2005;Daly et al, 2008;Keir et al, 2009;Bastow et al, 2010;Ebinger et al, 2010;Wright et al, 2012), before a final stage of plate stretching and associated decompression melting characterizing the final stages of continentocean transition (e.g., Bastow and Keir, 2011).…”

Crustal accretion of thick mafic crust in Iceland: implications for volcanic rifted margins

“…Later dikes cut the dipping ones, demonstrating that the rotation and intrusion were broadly synchronous. Near the coast the dike density increases to nearly 100% in a sheeted dike complex marking the continent-ocean transition (Karson and Brooks 1999;Klausen and Larsen 2002;Nielsen 1978;Nielsen and Brooks 1981). Offshore, lavas and SDRs overlie mafic crust ϳ30 km thick.…”

Transtension y transpresion del Jurasico Medio-Superior al Cretacico Inferior durante la construccion del arco magmatico en Chile central: evidencia a partir de enjambres de diques maficos.

“…13). According to recent results in mafic dike swarms in Greenland (Klausen and Larsen, 2002) and in the Deccan Traps (Ray et al, 2007), differences in dike thickness can be con-trolled by the depth of magma reservoirs, but also by differences in magma viscosity (Wada, 1994), where deeper reservoirs or also more viscous magmas can produce thicker dikes. Since major elements geochemistry and therefore magma viscosity is not dramatically different between NE and NW-striking dikes, we suggest that differences in dike thickness are controlled by injection of magmas from reservoirs located at different depths.…”