A properly scaled thermo-mechanical experimental model of slow seafloor spreading is developed to investigate the mechanism of lithosphere accretion. The melt of a specially fabricated hydrocarbon compositional system is cooled from above in a tank. The crystallizing upper layer (the lithosphere) possesses semiplastic-semibrittle properties. Horizontal tension of the layer is produced by a paddle drawn at a constant rate. Spreading in the model is very unstable, asymmetric, and involves regular jumps of the spreading center. Scaling up the experimental results, to match seafloor spreading, gives the length of the jumps of the order of 10 km and the period of the order of 105--106 years. The jumps normally occur in the wall of a rift valley resulting in quasiplastic stretching of the lithosphere at this place and then, in the formation of two intersecting major normal fault zones crossing the plate from some shallow depth to the base. The wedge separated by these zones rises up symmetrically prior to the cessation of movement along one zone, with motion only continuing along the other. From this moment the deformation becomes asymmetrical. The material crystallized at the base of the lithosphere is dragged along the active zone to the upper horizons of the plate and to the surface. This process leads to both the nonisostatic uplifting of the central part of the pulled block and subsidence near the inlet on the surface of the fault (detachment) zone which forms the center of this asymmetric lithospheric accretion.

At a specific stage, under the action of both horizontal tension and vertical nonisostatic forces the strength of the pulling block is exceeded within the rift valley. The result is the next jump of the spreading center to the point of breakage and a repetition of the whole cycle (the jump can also occur at other places weakened during previous cycles). Normally, the asymmetry of the spreading alternates at a new center. The dimensions of the valley, as well as the overall seafloor reflied, are strongly dependent on the spreading rate V; the lower the V value, the rougher the relief. The jumps of the spreading centers are not synchronous along the plate boundary. Moreover, they can be of different types, with varying distances and in opposite directions on adjacent segments of the spreading axis, leading to the occurrence of transfer or accommodation zones. ¿ American Geophysical Union 1994