We investigate the age-chemical abundance structure of the outer Galactic disc at a galactocentric distance of r > 10 kpc as recently revealed by the SDSS/APOGEE survey .
Two sequences are present in the [ \alpha /Fe ] - [ Fe/H ] plane with systematically different stellar ages .
Surprisingly , the young sequence is less metal-rich , suggesting a recent dilution process by additional gas accretion .
As the stars with the lowest iron abundance in the younger sequence also show an enhancement in \alpha -element abundance , the gas accretion event must have involved a burst of star formation .
In order to explain these observations , we construct a chemical evolution model .
In this model we include a relatively short episode of gas accretion at late times on top of an underlying secular accretion over long timescales .
Our model is successful at reproducing the observed distribution of stars in the three dimensional space of [ \alpha /Fe ] - [ Fe/H ] -Age in the outer disc .
We find that a late-time accretion with a delay of 8.2 Gyr and a timescale of 0.7 Gyr best fits the observed data , in particular the presence of the young , metal-poor sequence .
Our best-fit model further implies that the amount of accreted gas in the late-time accretion event needs to be about three times the local gas reservoir in the outer disc at the time of accretion in order to sufficiently dilute the metal abundance .
Given this large fraction , we interpret the late-time accretion event as a minor merger presumably with a gas-rich dwarf galaxy with a mass M _ { * } < 10 ^ { 9 } M _ { \odot } and a gas fraction of \sim 75 per cent .