  Neutron scattering is used to study magnetic field induced ordering in the
quasi-1D quantum spin-tube compound Sul--Cu$_2$Cl$_4$ that in zero field has a
non-magnetic spin-liquid ground state. The experiments reveal an incommensurate
chiral high-field phase stabilized by a geometric frustration of the magnetic
interactions. The measured critical exponents $\beta\approx0.235$ and
$\nu\approx0.34$ at $H_c\approx3.7$ T point to an unusual sub-critical scaling
regime and may reflect the chiral nature of the quantum critical point.
