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 d egrees 50'N and 63 degrees 00'N are segmented seafloor spreading cente rs that exhibit structural morphologies that vary with distance from I celand. The northern 100 km of the MAR (55 degrees 50'N-56 degrees 47' N) is oriented 005 degrees (perpendicular to the spreading azimuth) an d contains four third-order spreading segments separated along strike by right- and left-stepping axial discontinuities. The boundary betwee n MAR and Reykjanes Ridge occurs at 56 degrees 47'N, where the Bight t ransform fault offsets the neovolcanic axis 15 km right laterally acro ss a single linear fault oriented 092 degrees. This is the only transf orm 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 be tween the Fight transform and Iceland's Reykjanes Peninsula and is ori ented similar to 30 degrees oblique to the spreading-orthogonal direct ion. Axial faults and volcanic systems on the Reykjanes Ridge are arra nged in a right-stepping en echelon pattern and display strikes interm ediate between the spreading-orthogonal direction and the strike of th e ridge, which is compatible with experimental models of oblique exten sion. Spreading segments on the northern MAR and Reykjanes Ridge are d efined structurally and bathymetrically. At the smallest scale, the vo lcanic axis is segmented into individual volcanic systems 4-45 km long , which are separated along strike by intervolcano gaps or offsets. Th ese features are analogous to fourth-order spreading segments document ed elsewhere on the MAR. Axial volcanic systems are superimposed on in termediate-wavelength (13-65 km) axial topographic highs that constitu te third- or second-order spreading segments. Segment boundaries occur at axial depth maxima, which correspond to offsets in the volcanic ax is. Four primary characteristics of the spreading axis (axial valley d epth, axial boundary fault throw, relief along the neovolcanic axis, a nd degree of inter-segment structural discontinuity) decrease toward t he north. This variation is consistent with higher crustal temperature s near Iceland, due to either thicker crust or elevated mantle tempera tures