Relationships between melt-induced rheological transitions and finite strain: Observations from host rock pendants of the Tuolumne Intrusive Suite, Sierra Nevada, California

Abstract

Finite strain analysis in the northern (Piute Meadow pendant (PMP)) and eastern (Saddlebag Lake pendant (SLP)) host rock pendants of the Tuolumne Intrusive Suite (TIS) reveals plane strain geometries in the pendants and a steep strain gradient in the SLP. Average regional z-axis shortening in both pendants is ca. 43% and increases to ca. 70 and 85% in the SLP within ca. 100 m to the pluton contact. Microstructures in the SLP adjust to increasing temperatures and partial melting near the margin. Beyond the pluton margin, weakly elongate quartz grains with lobate grain boundaries, patchy to sweeping undulose extinction, deformation bands, and incipient subgrains are consistent with low-temperature dislocation creep. Towards the margin, quartz grain boundaries become increasingly irregular and display “chessboard” patterns indicating high-temperature dislocation creep. Grain size is markedly smaller near the margin and despite large finite strain grains do not show evidence for crystal-plastic deformation. Cuspate strain-free interstices are interpreted to mimic former anatectic melt. Outcrop-scale migmatization, coincidence with abruptly increasing finite strain, and microfracturing towards the margin implies that fracture-induced mechanical grain size reduction may have triggered the transition to melt-assisted granular flow, ultimately resulting in drastic rheological weakening.