THE NORTHERN MID-ATLANTIC AND REYKJANES RIDGES - SPREADING CENTER MORPHOLOGY BETWEEN 55-DEGREES-50'N AND 63-DEGREES-00'N

SeaMARC II and Hydrosweep acoustic imagery and bathymetry reveal that the northern Mid-Atlantic Ridge (MAR) and Reykjanes Ridge between 55 degrees 50'N and 63 degrees 00'N are segmented seafloor spreading centers that exhibit structural morphologies that vary with distance from Iceland. The northern 100 km of the MAR (55 degrees 50'N-56 degrees 47'N) is oriented 005 degrees (perpendicular to the spreading azimuth) and contains four third-order spreading segments separated along strike by right- and left-stepping axial discontinuities. The boundary between MAR and Reykjanes Ridge occurs at 56 degrees 47'N, where the Bight transform fault offsets the neovolcanic axis 15 km right laterally across a single linear fault oriented 092 degrees. This is the only transform fault between 55 degrees 50'N and 63 degrees 00'N and is inferred to have generated the Bight Fracture Zone. The Reykjanes Ridge lies between the Fight transform and Iceland's Reykjanes Peninsula and is oriented similar to 30 degrees oblique to the spreading-orthogonal direction. Axial faults and volcanic systems on the Reykjanes Ridge are arranged in a right-stepping en echelon pattern and display strikes intermediate between the spreading-orthogonal direction and the strike of the ridge, which is compatible with experimental models of oblique extension. Spreading segments on the northern MAR and Reykjanes Ridge are defined structurally and bathymetrically. At the smallest scale, the volcanic axis is segmented into individual volcanic systems 4-45 km long, which are separated along strike by intervolcano gaps or offsets. These features are analogous to fourth-order spreading segments documented elsewhere on the MAR. Axial volcanic systems are superimposed on intermediate-wavelength (13-65 km) axial topographic highs that constitute third- or second-order spreading segments. Segment boundaries occur at axial depth maxima, which correspond to offsets in the volcanic axis. Four primary characteristics of the spreading axis (axial valley depth, axial boundary fault throw, relief along the neovolcanic axis, and degree of inter-segment structural discontinuity) decrease toward the north. This variation is consistent with higher crustal temperatures near Iceland, due to either thicker crust or elevated mantle temperatures