THE REYKJANES RIDGE - STRUCTURE AND TECTONICS OF A HOT-SPOT-INFLUENCED, SLOW-SPREADING RIDGE, FROM MULTIBEAM BATHYMETRY, GRAVITY AND MAGNETIC INVESTIGATIONS

We report a comprehensive morphological, gravity and magnetic survey o f the oblique- and slow-spreading Reykjanes Ridge near the Iceland man tle plume. The survey extends from 57.9 degrees N to 62.1 degrees N an d from the spreading axis to between 30 km (3 Ma) and 100 km (10 Ma) o ff-axis; it includes 100 km of one arm of a diachronous 'V-shaped' or 'chevron' ridge. Observed isochrons are extremely linear and 28 degree s oblique to the spreading normal with no significant offsets. Along-a xis there are ubiquitous, en-echelon axial volcanic ridges (AVRs), sub -normal to the spreading direction, with average spacing of 14 km and overlap of about one third of their lengths. Relict AVRs occur off-axi s, but are most obvious where there has been least axial faulting, sug gesting that elsewhere they are rapidly eroded tectonically. AVRs main tain similar plan views but have reduced heights nearer Iceland. They are flanked by normal faults sub-parallel to the ridge axis, the inner most of which occur slightly closer to the axis towards Iceland, sugge sting a gradual reduction of the effective lithospheric thickness ther e. Generally, the amplitude of faulting decreases towards Iceland. We interpret this pattern of AVRs and faults as the response of the litho sphere to oblique spreading, as suggested by theory and physical model ling. An axial, 10-15 km wide zone of high acoustic backscatter marks the most recent volcanic activity. The zone's width is independent of the presence of a median valley, so axial volcanism is not primarily d elimited by median valley walls, but is probably controlled by the lat eral distance that the oblique AVRs can propagate into off-axis lithos phere. The mantle Bouguer anomaly (MBA) exhibits little mid- to short- wavelength variation above a few milliGals, and along-axis variations are small compared with other parts of the Mid-Atlantic Ridge. Neverth eless, there are small axial deeps and MBA highs spaced some 130 km al ong-axis that may represent subdued third-order segment boundaries. Th ey lack coherent off-axis traces and cannot be linked to Oligocene fra cture zones on the ridge flanks. The surveyed chevron ridge is morphol ogically discontinuous, comprising several parallel bands of closely s paced, elevated blocks. These reflect the surrounding tectonic fabric but have higher fault scarps. There is no evidence for off-axis volcan ism or greater abundance of seamounts on the chevron. Free-air gravity over it is greater than expected from the observed bathymetry, sugges ting compensation via regional rather than pointwise isostasy. Most of the observed variation along the ridge can be ascribed to varying dis tance from the mantle plume, reflecting changes in mantle temperature and consequently in crustal thickness and lithospheric strength. Howev er, a second-order variation is superimposed. In particular, between 5 9 degrees 30'N and 61 degrees 30'N there is a minimum of large-scale f aulting and crustal magnetisation, maximum density of seamounts, and m aximum axial free-air gravity high. To the north the scale of faulting increases slightly, seamounts are less common, and there is a relativ e axial free-air low. We interpret the 59 degrees 30'N to 61 degrees 3 0'N region as where the latest chevron ridge intersects the Reykjanes Ridge axis, and suggest that the morphological changes that culminate there reflect a local temperature high associated with a transient pul se of high plume output at its apex. (C) 1998 Elsevier Science B.V. Al l rights reserved.