Earth-Science Reviews xxx (xxxx) xxxx Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Reykjanes Ridge evolution: Effects of plate kinematics, small-scale upper mantle convection and a regional mantle gradient Fernando Martineza,⁎, Richard Heya, Ármann Höskuldssonb 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 Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland ARTICLE INFO ABSTRACT Keywords: The Reykjanes Ridge is a key setting to study plate boundary processes overlaid on a regional mantle melting Slow-spreading ridges anomaly. The ridge originated as one arm of a ridge-ridge-ridge triple junction that separated Greenland, Small-scale mantle convection Eurasia, and North America. It initially formed a linear axis, spreading orthogonally at slow rates without Ridge segmentation transform faults or orthogonal crustal segmentation. Stable spreading continued in this configuration for ∼18 Mantle melting anomaly Myr until Labrador Sea spreading ceased, terminating the triple junction by joining Greenland to North America Mantle gradient and causing a rapid ∼30° change in opening direction across the ridge. The ridge abruptly became segmented Transform faults ff Ridge-hot spot interaction and o set by a series of transform faults that appear to decrease in length and spacing toward Iceland. Without further changes in opening direction, the ridge promptly began to reassemble its original linear configuration systematically and diachronously from north to south, even though this required the ridge to spread obliquely as it became linear again. Prominent V-shaped crustal ridges spread outward from the axis as the ridge became linear. This reconfiguration is presently nearly complete from Iceland to the Bight transform fault, a distance of nearly 1000 km. Both mantle plume and plate boundary processes have been proposed to control the tectonic reconfigurations and crustal accretion characteristics of the Reykjanes Ridge. Here we review the ridge char- acteristics and tectonic evolution and various models proposed to influence them. 1. Introduction eliminate the just-formed offsets progressively from north to south to re-establish its original linear configuration (Hey et al., 2016; Martinez The Reykjanes Ridge is a slow-spreading (∼20 mm/yr full rate) and Hey, 2017), even though this required it to spread obliquely by mid-ocean ridge located in the North Atlantic between the Bight ∼30°. As it became linear again, prominent southward-pointing V- Fracture Zone and Iceland (Fig. 1). It originated at about anomaly 24 shaped crustal ridges and troughs developed outward from the axis forming the Greenland-Eurasia plate boundary, which was part of a onto the ridge flanks. Since the Reykjanes Ridge intersects the Iceland triple junction that partitioned opening between North America, “hot spot”, its changing tectonic configurations and crustal structure Greenland and Eurasia (Johnson et al., 1973; Kristoffersen and Talwani, have been interpreted to directly reflect mantle plume flow and tem- 1977; Laughton, 1971; Vogt et al., 1969; Vogt and Avery, 1974). Since perature variations and it is often viewed as a window into mantle about anomaly 18 it became the North America-Eurasia plate boundary plume dynamics (e.g., Ito, 2001; Ito et al., 1999; Jones et al., 2002; when the Labrador Sea arm of the triple junction failed (Smallwood and Jones, 2003; Parnell-Turner et al., 2013, 2014, 2017; Vogt, 1971; White White, 2002). Over the course of its evolution it has exhibited a re- et al., 1995; White, 1997). However, the fundamental processes of markable series of changes in tectonic and crustal structure as well as forming and eliminating ridge segmentation and transform faults, de- displaying prominent gradients in depth and crustal thickness between veloping migrating segmentation discontinuities (Carbotte et al., 2015; the Bight Fracture Zone and Iceland (Smallwood and White, 2002). The Grindlay et al., 1991; Hey, 1977; Macdonald et al., 1988; Okino et al., Reykjanes Ridge initially had a linear geometry and spread stably in 1998; Tucholke et al., 1997; Wang et al., 2015), and developing large this configuration for ∼18 Myr. When Labrador Sea spreading failed systematic changes in crustal thickness and axial depth with changing the direction of spreading across the Reykjanes Ridge changed by ∼30° mantle volatile content (Martinez et al., 2006; Taylor and Martinez, and the ridge abruptly became segmented and offset by transform faults 2003) are exhibited at many normal mid-ocean and back-arc ridges and in a stair-step geometry (Vogt et al., 1969). It then promptly began to do not require “mantle plume” effects. These observations support an ⁎ Corresponding author. E-mail address: [email protected] (F. Martinez). https://doi.org/10.1016/j.earscirev.2019.102956 Received 27 November 2018; Received in revised form 9 September 2019; Accepted 9 September 2019 0012-8252/ © 2019 Elsevier B.V. All rights reserved. Please cite this article as: Fernando Martinez, Richard Hey and Ármann Höskuldsson, Earth-Science Reviews, https://doi.org/10.1016/j.earscirev.2019.102956 F. Martinez, et al. Earth-Science Reviews xxx (xxxx) xxxx Fig. 1. Maps of the Reykjanes Ridge and flanking North Atlantic basin. A) Topography grid from the compilation of Becker et al. (2009), version SRTM15. B) Satellite-derived free air gravity anomalies (Sandwell et al., 2014) grid version 24.1. C) Compiled magnetics grid (Macnab et al., 1995) with additional ship data from RRS Charles Darwin (CD87) (Searle et al., 1998), R/V Knorr (KN189-4) (Hey et al., 2010) and R/V Langseth (MGL1309) (Martinez and Hey, 2017). Fine crosses show axis location. In C red and blue lines are seafloor spreading flowlines from the rotation poles of Smallwood and White (2002). Black lines are the rotated position of the current Reykjanes Ridge axis at the stage pole chrons numbered at the bottom showing the nearly parallel align- ment with the early magnetic isochrons. This geometric alignment indicates that the current linear but obliquely spreading Reykjanes Ridge reassembled the same geometry it had during its early linear but orthogonally-spreading stage following the intervening stair-step segmented phase of spreading that formed the prominent fracture zones evident in the gravity data (B). (For in- terpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) but amplified in magnitude and directed by underlying mantle gra- dients. Here we focus on sampled horizontal gradients in chemistry, particularly volatile content (Nichols et al., 2002), and possibly tem- perature (Cochran and Talwani, 1978; Delorey et al., 2007) because these properties affect bulk mantle melting and viscosity and are especially important in controlling small-scale upper mantle convection (Choblet and Parmentier, 2001). Small-scale upper mantle convective instabilities within the “damp” melting regime are generally recognized as a primary cause of crustal segmentation at slow-spreading ridges (Lin and Phipps Morgan, 1992; Lin et al., 1990; Forsyth, 1992; Phipps Morgan and Parmentier, 1995) and the anomalous properties of the regional Iceland hot spot (increased volatile content and mildly ele- vated temperature) are predicted to enhance such instabilities (Braun et al., 2000). Because the Reykjanes Ridge has experienced both abrupt kinematic changes as well as progressive and systematic changes in plate boundary configuration while overlying a regional mantle melting anomaly that diminishes with distance from Iceland it presents a type- setting to examine mechanical (plate-controlled) and magmatic (mantle) drivers of plate boundary evolution. We draw on several key properties of mid-ocean ridges to explain its evolution: (1) Large and abrupt changes in plate motion are observed to induce abrupt plate boundary segmentation such that the new segments develop quickly and are typically oriented orthogonal to the new re- lative plate opening direction (Goodliffe et al., 1997; Hey et al., 1988; Menard and Atwater, 1968; Okino et al., 1998). It is possible that the abrupt reconfigurations are effected by ridge rotation (Menard and Atwater, 1968) or propagation (Hey, 1977; Hey et al., 1988) but that these mechanisms are not always resolved. These reconfigurations ap- pear to be mechanically controlled in the sense that they represent responses of lithospheric plate boundary zones to stresses imposed by kinematic plate motion changes. (2) Where ridge offsets are small, crustal segmentation on slow-spreading ridges appears to often migrate along axis producing broad V-shaped wakes forming crustal thickness variations bounded by non-transform discontinuities (e.g., Gente et al., 1995; Sempéré et al., 1993; Wang et al., 2015). Further, (3) ridge segments can evolve by asymmetric spreading to form new offset seg- ments from a previous long linear axis and offset segments can merge to form new longer unsegmented axes (Sempéré et al., 1993; Schouten and White, 1980; Tucholke et al., 1997). These observed changes in seg- alternative hypothesis: that dynamic plate boundary processes super- mentation, even when relative plate motion is steady, suggest a pri- imposed on a rather passive regional mantle anomaly may explain the marily magmatic (mantle) control on the evolution