Abstract

A detailed study of the structure of the Mid-Atlantic Ridge median valley and rift mountains near lat 37°N (FAMOUS) was conducted using a deep-tow instrument package. The median valley may have either a very narrow inner floor (1 to 4 km) and well-developed terraces or a wide inner floor (10 to 14 km) and narrow or no terraces. The terraces appear to be non–steady-state features of the rift valley. The entire depth and gross morphology of the median valley may be accounted for by normal faulting, while volcanic relief contributes to the short-wavelength topography (<2 km). Most faults dip toward the valley axis an average of 50°, and the blocks are tilted back 2° to 3°. Fault dip is asymmetric about the valley axis. Active crustal extension in the inner floor and inner walls has the same sense of asymmetry as the local spreading rates, reaching a maximum of 18 percent. Thus, asymmetric spreading appears to be accomplished by asymmetric crustal extension on a fine scale as well as by asymmetric crustal accretion. Spreading is 17° oblique to the transform faults and shows no indication of readjusting to an orthogonal system, even on a fine scale. Eighty percent of the decay or transformation of median-valley relief into rift-mountain topography is accomplished by normal faults that dip away from the valley axis. Most of the outward-facing faulting occurs near the median-valley–rift-mountain boundary. Tilting of crustal blocks accounts for only 20 percent of the decay of median-valley relief. Most long-wavelength topography in the rift mountains has a faulted origin. As in the median valley, volcanic relief is short wavelength (<2 km) and appears to be fossil, originating in the median-valley inner floor. Bending of large faulted blocks toward nearby fracture zones suggests that spreading-center tectonics is affected by fracture-zone tectonics throughout the length of the rift in the FAMOUS area. Both the crustal accretion zone and transform fault zone are narrow, only 1 to 2 km wide, over short periods of time. In the course of millions of years, however, they apparently migrate over a zone 10 to 20 km wide.

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