  The low-mass members of OB associations, expected to be a major component of
their total population, are in most cases poorly studied because of the
difficulty of selecting these faint stars in crowded sky regions. Our knowledge
of many OB associations relies only on a relatively small number of massive
members. We study here the Sco OB1 association, with the aim of a better
characterization of its properties such as global size and shape, member
clusters and their morphology, age and formation history, and total mass. We
use deep optical and NIR photometry from the VPHAS+ and VVV surveys, over a
wide area (2.6 x 2.6 sq.deg.), complemented by Spitzer IR data, and Chandra and
XMM-Newton X-ray data. A new technique is developed to find clusters of
pre-main-sequence M-type stars using suitable color-color diagrams, which
complements existing selection techniques using narrow-band H-alpha photometry
or NIR and UV excesses, and X-ray data. We find a large population of
approximately 4000 candidate low-mass Sco OB1 members, net of field-star
contaminations, whose spatial properties correlate well with those of
H-alpha-emission, NIR-excess, UV-excess, and X-ray detected members, and
unresolved X-ray emission. The low-mass population is spread among several
interconnected subgroups: they coincide with the HII regions G345.45+1.50 and
IC4628, and the rich clusters NGC 6231 and Trumpler 24, with an additional
subcluster intermediate between these two. The total mass of Sco OB1 is
estimated to be 8500 Msun. Indication of a sequence of star-formation events is
found, from South (NGC 6231) to North (G345.45+1.50). We suggest that the
diluted appearance of Trumpler 24 indicates that the cluster is now dissolving
into the field, and that tidal stripping by NGC 6231 nearby contributes to the
process.
