  We demonstrate that in a class of the $U(1)'$ extension of the Standard Model
(SM), under which all the Standard Model matter fields are uncharged and the
additional neutral gauge boson $Z'$ couples to a set of heavy nonstandard
fermions, dark matter (DM) production mediated by $Z'$ can proceed through the
generation of generalized Chern-Simons (GCS) couplings. The origin of the GCS
terms is intimately connected to the cancellation of gauge anomalies. We show
that the DM production cross section triggered by GCS couplings is sufficient
even for an intermediate scale $Z'$ . A large range of DM and Z masses is then
allowed for reasonably high reheating temperature ($T_\rh \gtrsim
10^{10}~\text{GeV}$). This type of scenario opens up a new paradigm for unified
models. We also study the UV completion of such effective field theory
constructions, augmenting it by a heavy fermionic spectrum. The latter, when
integrated out, generates the GCS-like terms and provides a new portal to the
dark sector. The presence of a number of derivative couplings in the GCS-like
operators induces a high temperature dependence to the DM production rate. The
mechanism has novel consequences and leads to a new reheating dependence of the
relic abundance.
