  The origin of asymmetric radio polarized emission and properties of magnetic
fields in the Virgo Cluster spiral NGC4254 are investigated with help of
"magnetic maps" presenting distributions of different magnetic field components
over the entire galaxy, free of Faraday rotation and projection effects. We
show that the dramatic variation of orientation of magnetic field vectors (from
0deg to more than 40deg) throughout the galaxy cannot arise from the dynamo
process alone, but must be dominated by the effects like density waves and
local gas flows. We determine within the galaxy the relation between the
strength of total magnetic field and the local star-formation rate (SFR) as a
power-law with an index of +0.18+-0.01. We find the opposite sense of relation
of magnetic field regularity with SFR (-0.32+-0.03) and suggest that it results
from efficient production of random field with rising turbulence in the regions
with actively forming stars. The distribution of Faraday rotation measures in
NGC4254 indicates a perturbed axisymmetrical mean-field dynamo mode or a
mixture of axisymmetrical and bisymmetrical ones with regular field directed
outwards the disk, which is contrary to most observed galaxies. The magnetic
fields within two outer magnetic arms (shifted downstream of a density wave)
are strong, up to 13muG in the regular field and 20muG in the total field. Our
modeling of cluster influence on different magnetic field components indicates
that within the outer magnetic arms the dynamo-induced magnetic fields are
modified by stretching and shearing forces rather than by cluster ram pressure.
Those forces, which are likely triggered by the galaxy's gravitational
interaction, produce an anisotropic component of the regular field and enhance
the polarized emission.(abridged)
