DEFORMATION MECHANISM-INDUCED CHANGES IN HOST ROCK RHEOLOGY IN A CONTACT AUREOLE: THE TUOLUMNE INTRUSIVE SUITE, CALIFORNIA

Previous work on the rheology of crust effected by magmatism suggests that magma emplacement and/or partial melting is commonly associated with strain localization and rheological softening. For example, most field and experimental studies conclude that the presence of magma in the crust, whether by magma emplacement or by in-situ melt generation, yields substantial weakening which may result in tectonic surges. On the other hand, it is also likely that melt-induced weakening may be followed by post-crystallization hardening of the crust. However, the quantitative impact of magmatism on rheology is difficult to determine in natural settings. Contact aureoles are ideal settings to examine rheological changes for the following reasons: (1) magma emplacement produces a pronounced temperature gradient; (2) prograde metamorphic reactions record temperature changes if suitable rock types are present; (3) finite strains commonly increase towards pluton margins; and (4) gradients in rheologically critical parameters (temperature, stress, fluids, strain rate) induce transitions in deformation mechanisms.

Our recent field work and microstructural observations in aureoles around the Tuolumne Intrusive Suite (TIS), Sierra Nevada, reveal a striking spatial coincidence of abruptly increasing finite regional strain and a transition in dominant deformation mechanisms. Strain analyses at the northern tip and eastern margin of the TIS show a dramatic increase of north-northeast to south-southwest directed regional strain from ca. 43% to ca. 80% shortening towards the eastern pluton margin. The abrupt change in finite strain corresponds with a transition from dislocation creep to superplasticity aided by melt and high diffusion rates. Additionally, we observed increasingly more microfracturing and prograde metamorphic phase transitions towards the margin. Our results document the critical importance of magma emplacement on the rheology of pluton aureoles and of the lower crust. Furthermore, this study implies that temperature-induced rheological changes, which normally are observed along vertical crustal cross sections, may be condensed into much shorter length scales in contact aureoles.