  The formation of a thick brane in five-dimen\-sional space-time is
investigated when warp geometries of $AdS_5$ type are induced by scalar matter
dynamics and triggered by a thin-brane defect. The scalar matter is taken to
consist of two fields with $O(2)$ symmetric self interaction and with manifest
$O(2)$ symmetry breaking by terms quadratic in fields. One of them serves as a
thick brane formation mode around a kink background and another one is of a
Higgs-field type which may develop a classical background as well. Scalar
matter interacts with gravity in the minimal form and gravity effects on
(quasi)localized scalar fluctuations are calculated with usage of gauge
invariant variables suitable for perturbation expansion. The calculations are
performed in the vicinity of the critical point of spontaneous breaking of the
combined parity symmetry where a non-trivial v.e.v. of the Higgs-type scalar
field is generated. The nonperturbative discontinuous gravitational effects in
the mass spectrum of light localized scalar states are studied in the presence
of a thin-brane defect. The thin brane with negative tension happens to be the
most curious case when the singular barriers form a potential well with two
infinitely tall walls and the discrete spectrum of localized states arises
completely isolated from the bulk.
