Using 3D gyrofluid simulations, we revisit the problem of Alfven-wave (AW)
collisions as building blocks of the Alfvenic cascade and their interplay with
magnetic reconnection at magnetohydrodynamic (MHD) scales. Depending on the
large-scale nonlinearity parameter ${\chi}_{0}$ (the ratio between AW linear
propagation time and nonlinear turnover time), different regimes are observed.
For strong nonlinearities (${\chi}_{0}\sim 1$ ), turbulence is consistent with a
dynamically aligned, critically balanced cascade -- fluctuations exhibit a
scale-dependent alignment $\mathrm{sin}{\theta}_{k}\propto {k}_{\perp}^{-1/4}$ , a
${k}_{\perp}^{-3/2}$ spectrum and ${k}_{\Vert}\propto {k}_{\perp}^{1/2}$ spectral anisotropy.
At weaker nonlinearities (small ${\chi}_{0}$ ), a spectral break marking the
transition between a large-scale weak regime and a small-scale
${k}_{\perp}^{-11/5}$ tearing-mediated range emerges, implying that dynamic
alignment occurs also for weak nonlinearities. At ${\chi}_{0}<1$ the alignment
angle ${\theta}_{{k}_{\perp}}$ shows a stronger scale dependence than in the
${\chi}_{0}\sim 1$ regime, i.e. $\mathrm{sin}{\theta}_{k}\propto {k}_{\perp}^{-1/2}$ at
${\chi}_{0}\sim 0.5$ , and $\mathrm{sin}{\theta}_{k}\propto {k}_{\perp}^{-1}$ at ${\chi}_{0}\sim 0.1$ .
Dynamic alignment in the weak regime also modifies the large-scale spectrum,
scaling roughly as ${k}_{\perp}^{-3/2}$ for ${\chi}_{0}\sim 0.5$ and as ${k}_{\perp}^{-1}$
for ${\chi}_{0}\sim 0.1$ . A phenomenological theory of dynamically aligned
turbulence at weak nonlinearities that can explain these spectra and the
transition to the tearing-mediated regime is provided; at small ${\chi}_{0}$ , the
strong scale dependence of the alignment angle combines with the increased
lifetime of turbulent eddies to allow tearing to onset and mediate the cascade
at scales that can be larger than those predicted for a critically balanced
cascade by several orders of magnitude. Such a transition to tearing-mediated
turbulence may even supplant the usual weak-to-strong transition