A Subspace tear is a dangerous and turbulent subspace anomaly which can be created by an isolytic blast, a result of the unpredictable nature of subspace weaopons.
In 2375, the Son'a created a subspace tear in the Briar Patch after firing an isolytic weapon at the Enterprise-E. The tear threatened to engulf the Enterprise, so Commander Will Riker ordered the ship's warp core to be programmed for a breach and ejected. The ejected warp core acted as a magnet, pulling the tear towards it before it breached, sealing the tear and allowing the Enterprise to escape. (Star Trek: Insurrection)