Geophysical investigations of the Reykjanes Ridge and Kolbeinsey Ridge seafloor spreading centers

Abstract

I used a suite of marine geophysical tools to study the structure and tectonics of the slow-spreading Kolbeinsey, Reykjanes and northern Mid-Atlantic Ridges. Including Iceland, these ridges constitute a continuous spreading center system more than 2055 km long, and vary in their structural expression and obliquity to the spreading direction. The northern Mid-Atlantic Ridge (MAR) and Reykjanes Ridge between 55°50'N and 63°00'N exhibit systematic along-strike variation in axial valley depth, axial boundary fault throw, relief along the neovolcanic axis, and degree of inter-segment structural discontinuity. The orthogonal northern MAR is separated from the Reykjanes Ridge by the Bight transform fault (56°47'N), a right-stepping linear fault 15 km wide. The volcanic axis of Reykjanes Ridge contains individual volcanic systems 4-45km long (fourth-order segments), superimposed on intermediate-wavelength (13-65km) axial topographic highs that constitute third- or second-order spreading segments. The modem Kolbeinsey Ridge axis contains three first-order segments oriented orthogonally to the spreading direction. These segments are separated by large right-stepping nontransform offsets, the Spar (69.0°N) and Eggvin (70.4°N) discontinuities. The northern KR segment is a robust volcanic edifice 125km long and more than 1000m high. Shallow crust extends east from the northern KR axis to Jan Mayen Island, and I suggest the Jan Mayen hotspot is located beneath the northern KR rather than near Jan Mayen Island. A tectonic reconstruction based on aeromagnetic data indicates that the axial structure of the KR changed from continuous to segmented after anomaly 4. The subsequent structural evolution of the ridge involved ridge propagation, along-strike migration of axial discontinuities, asymmetric spreading, and lateral migration of segment axes that occurred via ultrafast propagation or synchronous ridge jumps. Two of the three original discontinuities still exist and contain active north-directed propagators. An ephemeral, catalytic change in plate motion is inferred to have triggered the axial reorientation at anomaly 4 time, which initiated the subsequent phase of ridge propagation and migration of nontransform offsets.