Understanding how microscopic structural dynamics relate to macroscopic mechanical response during yielding remains a central challenge in soft-matter physics. Here, we introduce rheo-echo x-ray photon correlation spectroscopy (Rheo-Echo-XPCS) with nonlinear acquisition synchronized to oscillatory shear, enabling direct measurement of irreversible nanoscale dynamics under controlled strain amplitude. Applying this approach to a carbon black colloidal fractal gel, we resolve time-periodic echoes in the vorticity-direction intensity autocorrelation function, whose decay encodes cycle-to-cycle nonaffine structural rearrangements spanning the linear viscoelastic regime, nonlinear deformation, and the yielding regime beyond ${\gamma }_y$. We find: (i) ballisticlike decorrelation with $\tau \propto q^{-1}$ at all strain amplitudes, where the decorrelation rate $v_{\tau }=1/\langle q\tau \rangle$ scales linearly with the loss tangent $tan\delta =G^{\prime \prime }/G^{\prime }$, establishing $tan\delta$ as a macroscopic measure of the rate of irreversible structural decorrelation; (ii) a continuous evolution of the relaxation function from compressed exponential ($\alpha \simeq 1.5$) at low strain—consistent with three-dimensional dipolar strain fields in the intact network—to stretched exponential ($\alpha \simeq 0.5$) at high strain, interpreted as a dimensional reduction from $d_f=3$ to $d_f=1$ as stress transmission shifts from bulk to quasi-one-dimensional filamentary backbones during network fragmentation.

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• Rheology
• Strain
• Colloidal gel
• X-ray photon correlation spectroscopy