  We present high spatial-resolution (~2kpc) Atacama Large
Millimeter/submillimeter Array (ALMA) observations of [CII] 158um and
dust-continuum emission from a galaxy at z=3.7978 selected by its strong HI
absorption (a damped Lya absorber, DLA) against a background QSO. Our ALMA
images reveal a pair of star-forming galaxies separated by ~6kpc (projected)
undergoing a major merger. Between these galaxies is a third emission component
with highly elevated (2x) [CII] 158um emission relative to the dust continuum,
which is likely to arise from stripped gas associated with the merger. This
merger of two otherwise-normal galaxies is not accompanied by enhanced
star-formation, contrary to mergers detected in most luminosity-selected
samples. The DLA associated with the merger exhibits extreme kinematics, with a
velocity width for the low-ionization metal lines of Dv90~470km/s that spans
the velocity spread revealed in the [CII] 158um emission. We propose that DLAs
with high Dv90 values are a signpost of major mergers in normal galaxies at
high redshifts, and use the distribution of the velocity widths of metal lines
in high-z DLAs to provide a rough estimate the fraction of z>3 galaxies that
are undergoing a major merger.
