Earth and Planetary Science Letters 435 (2016) 115–123 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Multibeam investigation of the active North Atlantic plate boundary reorganization tip Richard Hey a,∗ , Fernando Martinez a , Ármann Höskuldsson b , Deborah E. Eason c , Jonathan Sleeper c , Sigvaldi Thordarson d , Ásdís Benediktsdóttir e , Sergey Merkuryev f,g a Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI, 96822, USA b Institute of Earth Sciences, University of Iceland, Reykjavik, Iceland c Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, HI, 96822, USA d Iceland GeoSurvey, Reykjavik, Iceland e University of Iceland, Reykjavik, Iceland f Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation of the Russian Academy of Sciences St.-Petersburg Filial, St. Petersburg, Russia g Saint Petersburg State University, Institute of Earth Sciences, St. Petersburg, Russia a r t i c l e i n f o a b s t r a c t Article history: Received 1 August 2015 Received in revised form 4 December 2015 Accepted 17 December 2015 Available online xxxx Editor: P. Shearer Keywords: plate tectonics plate boundaries seafloor spreading North Atlantic Reykjanes Ridge Iceland The previous orthogonal ridge/transform staircase geometry south of Iceland is being progressively changed to the present continuous oblique Reykjanes Ridge spreading geometry as North America– Eurasia transform faults are successively eliminated from north to south. This reorganization is commonly interpreted as a thermal phenomenon, caused by warmer Iceland plume mantle progressively interacting with the ridge, although other diachronous seafloor spreading reorganizations are thought to result from tectonic rift propagation. New marine geophysical data covering our reinterpretation of the reorganization tip near 57 ◦ N show successive transform eliminations at a propagation velocity of ∼110 km/Myr, ten times the spreading half rate, followed by abrupt reorganization slowing at the Modred transform as it was converted to a migrating non-transform offset. Neither the simple thermal model nor the simple propagating rift model appears adequate to explain the complicated plate boundary reorganization process.  2015 Elsevier B.V. All rights reserved. 1. Introduction There have been three distinct phases of seafloor spreading along the Mid-Atlantic Ridge (MAR) south of Iceland (e.g. Vogt, 1971; Vogt and Avery, 1974; White, 1997; Smallwood and White, 2002; Jones et al., 2002; Jones, 2003; Merkuryev et al., 2009), as shown by changes in plate boundary configuration in both gravity maps (Fig. 1; Sandwell et al., 2014) and magnetic anomaly maps (Fig. 2; Macnab et al., 1995; Merkuryev et al., 2009; Merkuryev and DeMets, 2014). Immediately after Greenland – Eurasia breakup ∼55 Ma (chron 24), an initial orthogonally-spreading (Smallwood and White, 2002) ridge system without transform faults was established, form- ing continuous magnetic and gravity lineaments nearly parallel to the continental margins (Figs. 1–2). This pattern changed ap- proximately synchronously ∼37 Ma (chron 17; Jones, 2003) into * Corresponding author. Tel.: +1 808 956 8972; fax: +1 808 956 3188. E-mail address: hey@soest.hawaii.edu (R. Hey). a more typical slow-spreading orthogonal ridge/transform pattern (Fig. 2). This transition to orthogonal spreading probably resulted from the nearly instantaneous response to a change in spread- ing direction from ∼125 ◦ to ∼100 ◦ , caused by the termination of spreading on the Greenland–North America Rift as those plates became coupled together (Vogt, 1971; Vogt and Avery, 1974; Vogt and Johnson, 1975; Jones, 2003), although other mechanisms have been suggested. Very shortly after that initial reorganization, at least by ∼34 Ma (between chrons 15 and 13; Fig. 2), the most recent major re- organization of this plate boundary initiated south of Iceland and continues at present. The orthogonal North America–Eurasia ridge/transform staircase geometry is progressively changing to the present non-segmented oblique spreading geometry on the Reyk- janes Ridge as transform faults are successively eliminated (Vogt, 1971; Vogt and Avery, 1974; White, 1997; Smallwood and White, 2002; Jones et al., 2002, 2014; Jones, 2003; Merkuryev et al., 2009). Various models have been proposed for this latest reorganiza- tion. Vogt and Johnson (1975) proposed that channeled subaxial http://dx.doi.org/10.1016/j.epsl.2015.12.019 0012-821X/ 2015 Elsevier B.V. All rights reserved.