Stress fields acting during lithosphere breakup above a melting mantle: A case example in West Greenland

We characterize and map the stress fields acting during plate breakup along the West Greenland volcanic margin. The determination of interpolated stress fields is based on an inversion of fault-slip data sets and magma-driven fractures, crosscutting mainly an exposed inner seaward-dipping basaltic wedge (i.e., SDRi: inner Seaward Dipping Reflectors). This SDRi is segmented along-strike, with differently oriented segments. Relative chronology of stress fields is inferred from published age results on oriented dykes. We identify two distinct tectonic episodes (P1 and P2) with a P1-P2 change over at similar to 54 Ma, i.e. during magnetic chron C24R P1 is syn-magmatic and purely extensional. It is associated with the major crustal stretching event affecting the margin. P1 probably acted as early as the Late Palaeocene. This stress field was first homogeneous with the minimum principal stress sigma 3 trending similar to N060E, defining a PM stage. During development of the SDRi, sigma 3 locally reoriented to become orthogonal to each margin segment and, thus, to the continentward-dipping detachment faults bounding the SDRi (P1B). P1 is coeval with lithosphere breakup and is associated with an extension orthogonal to the Labrador-Baffin axis, which is inherited from the Mesozoic. A regional and radical change of sigma 3 to a similar to NS trend takes place during P2, which follows on immediately from P1. P2 is also syn-magmatic. It is associated with only minor extension. sigma 3 runs parallel to the North American (NAM)/Greenland (GR) kinematic vector from C24R to C13. We establish therefore that the minimum horizontal stress sigma 3 for P1 and P2 is parallel to the relative displacement of Greenland related to NAM but not to its absolute displacement during the Tertiary. Taking into account those results as well as variations in magma chemistry from P1 to P2, we suggest that tectonic stresses at a volcanic margin could arise from the local dynamics of the melting mantle. (C) 2011 Elsevier B.V. All rights reserved.