Off-axis structures of spreading zones according to results of experimental modeling

Abstract

The off-axis topography of spreading ridges is a result of tectonic and magmatic processes occurring in the axial zone and operating off the ridge axis during further evolution of the crust. The results of physical and numerical simulations have shown that differences in topography roughness, rift valley depth, frequency and amplitude of normal faults, and geometric stability of the rift axis are determined by (a) the rate of extension and accretion of the new crust, (b) the thickness of the brittle lithospheric layer, and (c) the temperature of the underlying asthenosphere. Under conditions of the fast spreading, the stationary axial magma chamber in the crust predetermines the existence of the thinner and weakened lithosphere. As a result, the axis jumps for a short distance and the axis geometry remains almost rectilinear. The destruction of the thin axial lithosphere with a low mechanical strength results in formation of frequent and low-amplitude normal faultings. All these factors lead to the formation of the characteristic poorly dissected topography of fast-spreading ridges. Without a stationary axial magmatic chamber in the crust of slow-spreading ridges and with a thick and strong lithosphere, a deeply dissected axial and off-axis topography arises. The axis jumps for a significant distance within the rift valley, giving rise to geometric instability of the axis and development of transform and nontransform offsets.