  Chiral Random Matrix Theory has proven to describe the spectral properties of
low temperature QCD very well. However, at temperatures above the chiral
symmetry restoring transition it can not provide a global description. The
level-spacing distribution in the lower part of the spectrum of the Dirac
operator is Poisson-like. There the eigenmodes are localized in space-time and
separated from the rest of the spectrum by a so-called mobility edge. In
analogy to Anderson localization in condensed-matter systems with random
disorder this has been called the QCD-Anderson transition. Here, we study the
localization features of the low-lying eigenmodes of the massless overlap
operator on configurations generated with $N_f=2+1+1$ twisted mass Wilson sea
quarks and present results concerning the temperature dependence of the
mobility edge and the mechanism of the quark-mode localization. We have used
various methods to fix the spectral position of the delocalization transition
and verify that the mobility edge extrapolates to zero at a temperature within
the chiral transition region.
