The geology of North Atlantic transform plate boundaries and their aseismic extensions

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Abstract
By the late 1960s the morphotectonic character (Menard and Dietz, 1952; Menard, 1955, Heezen and others, 1964a,b) and kinematic significance (Wilson, 1965; Sykes, 1963, 1965, 1967; Stover, 1966; McKenzie and Parker, 1967; Morgan, 1968; LePichon, 1968) of fracture zones had been established and it was generally accepted that fracture zones are comprised of two principal tectonic elements; a seismically active zone of strike-slip tectonism linking two offset ridge axes called the transform fault or ridge-transform-ridge (RTR) plate boundary, and two aseismic limbs, continuous with each end of the transform fault, that represent the fossil trace of the transform. With this basic tectonic framework developed, investigators turned their attention to elucidating the details about how RTR plate boundaries behave in time and space and how this fundamental plate boundary conditions the accretion of oceanic lithosphere. There are important kinematic and geometric variables that are likely to condition and govern the structural and petrologic processes operating along a RTR plate boundary. These variables will be reviewed before we summarize the salient properties of the slowly-slipping RTR plate boundaries found in the North Atlantic. Kinematic considerations of the plate tectonic hypothesis mandate that strike-slip motion must occur along an oceanic transform fault at a rate equal to the rate of plate separation of the associated ridge segments. The total opening rates of the ridge segments comprising the mid-oceanic ridge system vary from less than 1.7 cm/yr (Arctic Ridge) to approximately 18 cm/yr southern

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