An experimental study of the temporal evolution of the shock-induced Richtmyer-Meshkov instability in the turbulent regime with three-dimensional random interfacial perturbations is carried out. The primary interest is the growth rate of the turbulent mixing layer that develops after an impulsive acceleration of the perturbed interface between two gases (air/SF6) by a weak Ma = 1.2 incident shock wave. Planar Mie scattering is used to visualize the flow, and image sequences are captured using a high-speed video camera. The analysis of the total mixing width has been extended to study the growth behaviors of the bubbles and spikes, separately. A novel definition of the bubble and spike widths is introduced using the mass and linear momentum conservation laws. For the planar incident shock wave the newly defined bubble and spike widths increase in time as hb.s ∝ tθ, with a growth exponent θ = 1/2 that does not depend on either the initial conditions or the physical properties of the gases composing the interface.