Formation and elimination of segmentation and transform faults on the Reykjanes Ridge
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The objectives of R/V Neil Armstrong cruise AR35-04 (Fig. 1) were to survey the flanks of the Reykjanes Ridge and determine the timing, geometry and associated geophysical characteristics of the large-scale tectonic reorganizations that occurred there in the Paleogene and continue to the present (Fig. 2). The North Atlantic plate boundary between what is today the Bight Fracture Zone and Iceland, a distance of nearly 1000 km, was originally a linear orthogonally-spreading ridge that became abruptly fragmented in a stair-step fashion following a change in plate motion [Smallwood and White, 2002]. Its subsequent evolution involved the systematic and progressive removal of offsets from north to south to re-establish its original linear configuration [Hey et al., 2016; Martinez and Hey, 2017], even though this required the ridge to then spread obliquely, since the new spreading direction remained stable. These tectonic reorganizations took place within the region of influence of the Iceland “hotspot” which creates a strong gradient in mantle melting along the ridge, increasing crustal thicknesses by ~3-4 km and decreasing ridge axis depths by ~ 3000 m between the Bight Fracture Zone and Iceland [Louden et al., 2004]. A mantle gradient in melting properties (compositional and/or thermal) is presumably what results in the regional residual basement depth anomaly that extends throughout this region of the North Atlantic from the Greenland-Iceland-Faroe Ridge to south of the Bight Fracture Zone. This gradient in mantle properties with distance from the Iceland hotspot apparently had strong modulating effects on the tectonic reorganizations: the initial segment lengths and offsets appear in regional magnetic anomaly and satellite-derived gravity maps to be smaller toward Iceland and the segments evolved to re-establish the linear ridge configuration more quickly to the north [Hey et al., 2016]. As both kinematic and “hotspot” effects influence their development, the Reykjanes ridge flanks are key areas for investigating lithospheric and mantle controls on ridge segmentation, formation and elimination of transform faults and the mechanisms controlling their evolution.