  We present wide-field Herschel/PACS observations of Abell 1689, a massive
galaxy cluster at z=0.1832, from our Open Time Key Programme. We detect 39
spectroscopically confirmed 100micron-selected cluster members down to
1.5x10^10 Lsun. These galaxies are forming stars at rates in the range 1-10
Msun/yr, and appear to comprise two distinct populations: two-thirds are
unremarkable blue, late-type spirals found throughout the cluster; the
remainder are dusty red sequence galaxies whose star formation is heavily
obscured with A(Halpha)~2 mag, and are found only in the cluster outskirts. The
specific-SFRs of these dusty red galaxies are lower than the blue late-types,
suggesting that the former are in the process of being quenched, perhaps via
pre-processing, the unobscured star formation being terminated first. We also
detect an excess of 100micron-selected galaxies extending ~6 Mpc in length
along an axis that runs NE-SW through the cluster centre at >95% confidence.
Qualitatively this structure is consistent with previous reports of
substructure in X-ray, lensing, and near-infrared maps of this cluster, further
supporting the view that this cluster is a dynamically active, merging system.
