  The early Universe might have undergone phase transitions at energy scales
much higher than the one corresponding to the Grand Unified Theories (GUT)
scales. At these higher energy scales, the transition at which gravity
separated from all other interactions; the so-called Planck era, more massive
strings called supermassive cosmic strings, could have been produced, with
energy of about 10^{19}GeV. The dynamics of strings formed with this energy
scale cannot be described by means of the weak-field approximation, as in the
standard procedure for ordinary GUT cosmic strings. As suggested by string
theories, at this extreme energies, gravity may be transmitted by some kind of
scalar field (usually called the {\it dilaton}) in addition to the tensor field
of Einstein's theory of gravity. It is then permissible to tackle the issue
regarding the dynamics of supermassive cosmic strings within this framework.
With this aim we obtain the gravitational field of a supermassive screwed
cosmic string in a scalar-tensor theory of gravity. We show that for the
supermassive configuration exact solutions of scalar-tensor screwed cosmic
strings can be found in connection with the Bogomol'nyi limit. We show that the
generalization of Bogomol'nyi arguments to the Brans-Dicke theory is possible
when torsion is present and we obtain an exact solution in this supermassive
regime, with the dilaton solution obtained by consistency with internal
constraints.
