Here's one abstract:
The primary fragmentation processes in the thermal decomposition of polymers were studied in detail on a series of structurally related polyureas by direct pyrolysis with a mass spectrometer. Our results indicate that polyureas I–III undergo a quantitative depolycondensation process analogous to that observed for N-monosubstituted polyurethanes. The thermal decomposition of polyureas IV–VI proceeds by intramolecular hydrogen transfer processes that occur at higher temperatures with respect to depolycondensation. Polycarboxypiperazine VI is decomposed by a single-stage decomposition mechanism that leads to fragments with amino end groups and carbon oxide.
I took a look at the article. Depolycondensation is accurate: most of the polyureas examined decomposed to the "parent" isocyanates and amines. The temperature range examined was 150°C to 350°C.
If you're interested in what happens at higher temperatures, I would guess a decomposition to mixtures of CO, ammonia, water, and possibly methane and/or hydrogen. But I didn't find any references that would support me after a quick look