Necking of the lithosphere and the mechanics of slowly accreting plate boundaries

Abstract

Data on ridges with slow spreading rates (1-3 cm/yr), obtained through detailed studies with Deep-Tow instruments and manned submersibles (French-American Mid-Ocean Undersea Study), or where the active structures are not submarine (Afar triangle) yield a precise picture of the axial region. There is evidence for a thinned lithosphere to be present at the axis with a thickness of 4-5 km. The thermal structure and composition of this solid layer can be estimated from seismic data and thermal and petrologic models. Extensional tectonics prevails in a belt some l0 km wide and is expressed at the surface by normal faults and fissures. In most previous mechanical models the existence of the lithosphere at the axis is neglected, and the rise of viscous asthenospheric material in a narrow vertical cleft beneath the axis is considered the main cause for the steady state presence of an axial valley and the development of normal faults. In contrast with these models we suggest here that these features depend on the rheological behavior of the lithosphere at the axis: the lithosphere is continually thinned by tectonic strain but also thickened by cooling at the base and by volcanism at the surface. In the steady state this process can be viewed as a succession of tectonic 'neckings' in the central active part of the axial valley (10 km) followed by doming of the lithosphere over the whole width of the axial valley (30 km), in response to isostatic disequilibrium. Creep is likely to control the process at depth and because of the high temperature and large strains will be of the steady state type. The application of experimental flow laws for material constituting layer 3 and the uppermost mantle to this problem, where both temperature profile and strain rate can be estimated, allows an order of magnitude of the 'strength' of the lithosphere at a given strain rate to be calculated. With this strength, isostatic recovery in response to vertical shear stresses will occur at a distance of about 8-15 km from the axis. The resulting 'simple shear' strain progressively 'levels off' the mean topographic slope until it becomes horizontal in the rift mountains. Whereas for rifted ridges or slowly accreting plate boundaries the behavior of the lithosphere controls the mechanics of the axial region, where only small discontinuous transient magma chambers exist, we suggest that in the case of nonrifted ridges the behavior of the asthenosphere is more important, with axial crust in isostatic equilibrium over a large continuous permanent magma chamber. The two platesare separated by this liquid cut, and accretion occurs by cooling on each wall of the cut. 3. Tectonic phenomena (normal faulting and fissuring) and volcanic phenomena (lava accumulation)occurring near the axis are assumed to be less important than the magma chamber or the liquid-filled median dyke or cluster of dykes. Models have been suggested to explain the origin of a particular class of accreting plate boundaries: rift valleys [Heezen, 1960; Hill, 1960] which are associate...

Folding and necking across the scales: a review of theoretical and experimental results and their applications

“…Artemjev and Artyushkov (1971) were probably the first to suggest that rift systems are caused by crustal thinning due to a necking instability during lithospheric extension. It was subsequently shown that lithospheric necking for slow spreading rates (1-3 cm yr −1 ) is feasible for creep flow laws considered typical for the lithosphere (Tapponnier and Francheteau, 1978). Later, the stability analysis (described above for folding) has been applied to study necking instability during lithospheric extension (Fletcher and Hallet, 1983;Ricard and Froidevaux, 1986;, including lithospheric models with two competent layers (upper crust and upper mantle) separated by a weak lower crust .…”

Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observations

“…Figure 9 shows detailed bathymetry for a few ridge segments within this section. Slowspreading ridges such as the northern MAR are marked by a 1.5-3 km-deep, 30 -45 km-wide axial rift valley (Tapponnier & Francheteau 1978;Macdonald 1986). An inner valley floor (5-12 km wide) is bordered by valley walls in which large (hundreds of metres) inward-facing normal faults displace the crust upwards to form the crestal mountains.…”

The evolution of lithospheric deformation and crustal structure from continental margins to oceanic spreading centers

“…These offsets divide the ridge axis into individual spreading segments with lengths ranging from 10 to 100 km (e.g., Macdonald, 1986;Sempéré et al, 1993). The steady-state topography of a slow-spreading ridge is characterized by a prominent axial valley 10-0 km wide and 1-3 km deep (Macdonald and Atwater, 1978;Karson et at., 1987;Grindlay et at., 1992;Sempéré et al, 1993) and is consistent with a lithospheric necking model (e.g., Tapponnier and Francheteau, 1978;Phipps Morgan et al, 1987;Phipps Morgan and Chen, 1993;Shaw and Lin, 1996). Axial depth typically shallows toward the center of a ridge segment, with transform and NTOs intersecting the axis in topographic depressions at the segment ends.…”