  The spreading of the X-line out of the reconnection plane under a strong
guide field is investigated using large-scale three-dimensional (3D)
particle-in-cell (PIC) simulations in asymmetric magnetic reconnection. A
simulation with a thick, ion-scale equilibrium current sheet (CS) reveals that
the X-line spreads at the ambient ion/electron drift speeds, significantly
slower than the Alfv\'en speed based on the guide field $V_{Ag}$. Additional
simulations with a thinner, sub-ion-scale CS show that the X-line spreads at
$V_{Ag}$ (Alfv\'enic spreading), much higher than the ambient species drifts.
An Alfv\'enic signal consistent with kinetic Alfv\'en waves develops and
propagates, leading to CS thinning and extending, which then results in
reconnection onset. The continuous onset of reconnection in the signal
propagation direction manifests as Alfv\'enic X-line spreading. The strong
dependence on the CS thickness of the spreading speeds, and the X-line
orientation are consistent with the collisionless tearing instability. Our
simulations indicate that when the collisionless tearing growth is sufficiently
strong in a thinner CS such that $\gamma/\Omega_{ci}\gtrsim\mathcal{O}(1)$,
Alfv\'enic X-line spreading can take place. Our results compare favorably with
a number of numerical simulations and recent magnetopause observations. A key
implications is that the magnetopause CS is typically too thick for Alfv\'enic
X-line spreading to effectively take place.
