Abstract: Field observations and strain measurements on the migmatite structures in the Cretaceous pelite unit and the fabric within the quartzite unit of the Isabella pendant, southern Sierra Nevada, California show that: (1) the migmatite had experienced similar magnitude of strain to that of the pebble conglomerates within the quartzite unit; (2) the Cretaceous pelite unit had undergone progressively increasing degrees of partial melting with the proximity of the Goat Ranch granodiorite; (3) and as a result, the loading framework of the Cretaceous pelite changed as a function of the distance away from the Goat Ranch pluton, e.g. from interconnected weak layer (IWL) structure to load-bearing framework (LBF) structure. This data presented here strongly support that formation of an interconnected network of melts in a partially molten rock is a critical point at which catastrophic rheology reduction can occur. Such a change would induce the collapse of an over-thickened mountain belts and lateral flow of deep crustal rocks, if relatively deep seated crustal rocks undergo progressively partial melting.