Contact aureole rheology: New constraints from fieldwork in selected Cordilleran aureoles and from numerical modeling (California)

Detailed field work in spectacular exposures of the Tuolumne Intrusive Suite and its host rock pendants, including the Piute Meadow and Saddlebag Lake pendants suggests that voluminous Late Cretaceous magmatism occurred contemporaneously with partitioned regional contraction and dextral transpression. Preliminary field work in additional pendants, including the Benson Lake, May Lake pendants, and host rocks of the Soldier Lake and Green Lake plutons support these conclusions. These findings are compatible with previous studies which suggest that increasingly oblique convergence between the Farallon and North American plates resulted in partitioning of the resulting deformation. However, models that suggest decoupling of deformation into parallel reverse and strike-slip faulting cannot be supported. Insufficient magnitudes of arc-parallel strike-slip faulting are further supported by lacking strike-slip structures in the Piute Meadow pendant. This continually exposed pendant occupies a key position between shallow and deep water deposits of the Snow Lake and Saddlebag Lake pendants, respectively, which have been proposed to be tectonically juxtaposed along the speculative Mojave-Snow Lake fault. However, considering the Late Cretaceous plate tectonic framework, differential strike-slip displacement of more than ca. 200 km is unlikely to have occurred along the Continental margin of North America, even if large uncertainties are accounted for.; Finite strain analysis and microstructural observations along margin-perpendicular transects in the Saddlebag Lake pendant indicate that magma emplacement has a fundamental effect on crustal rheology. The observations show that dramatic rheologic effects are unlikely to occur in the dislocation creep regime. However, the onset of melt-assisted granular flow and diffusion creep coincides with a striking increase in finite regional strain, indicating spectacular rheological weakening. Strain analysis on magmatically folded dikes in the aureoles of the Mount Stuart batholith and the Tuolumne Intrusive suite combined with thermal modeling suggest that strain rates associated with this folding can range from 10-2 to 10-13 s -1. Numerical modeling of dislocation creep during spherical pluton expansion and associated host rock shortening yields maximum strain rates of 10-11 s-1, implying that melt-assisted granular flow and microfracturing are plausible deformation mechanisms that could accommodate extremely fast strain rates in pluton aureoles.