  Stellar associations can be discerned as overdensities of sources not only in
the physical space but also in the velocity space. The common motion of their
members, gradually eroded by the galactic tidal field, is partially reminiscent
of the initial kinematic structure. Using recent data from Gaia EDR3, combined
with radial velocities from GALAH and APOGEE, we traced back the present
positions of stars belonging to Upper Scorpius, a subgroup of
Scorpius-Centaurus, the nearest OB association. About one half of the subgroup
(the "clustered" population) appears composed of many smaller entities, which
were in a more compact configuration in the past. The presence of a kinematic
duality is reflected into an age spread between this younger clustered
population and an older diffuse population, in turn confirmed by a different
fraction $f_D$ of disc-bearing stars ($f_D = 0.24\pm0.02$ vs $f_D = 0.10\pm
0.01$). Star formation in Upper Scorpius appears to have lasted more than 10
Myr and proceeded in small groups that, after a few Myr, dissolve in the field
of the older population but retain for some time memory of their initial
structure. The difference of ages inferred through isochrones and kinematics,
in this regard, could provide a powerful tool to quantify the timescale of gas
removal.
