The fault zones are marked by a zone of sheared rocks characterized by mylonite. Some of the rock appears as mylonite. Closely associated with these gneisses are mylonite and meta-anorthosite and melagabbro. At depth, the angular breccias transit into a ductile shear texture and into mylonite zones. Other varieties of foliated rock include slates, phyllites, and mylonite. Metamorphism of the arc and preexisting Laurentia took place creating mylonite zones. Extensive faulting and mylonite marks its northern boundary with the other structural provinces. There is relatively little mylonite in the vicinity of Westborough. The zone has two 100 meter wide mylonite zones surrounding it. Anorthosite intrusions partly obscure it in places, but a mylonite zone marks it northern boundary. The Bulgan fault zone mylonite hosts azurite, malachite, chrysocolla and turquoise. Along detachment surfaces, mylonite forms due to shear.