Fracturing, fluid flow and shear zone development: Relationships between chemical and mechanical processes in Proterozoic mafic dykes from southwestern Montana, USA

Abstract

Fluids can play an important role in the localization of deformation in the deep crust, yet the specific mechanisms active during the complex interactions between metasomatism, metamorphism and deformation remain elusive. Precambrian metagabbronorite dykes in southwest Montana contain fractures filled with Hbl±Grt and discrete cm‐scale shear zones with well‐preserved strain gradients. This system offers an ideal opportunity to constrain the chemical and mechanical processes that facilitated strain localization. An early M1 assemblage of Grt1+Cpx1+Pl1+Qz developed at conditions of 0.51–0.85 GPa and 500–700°C and is preserved largely as a static replacement of relict igneous phases (Opx, Pgt, Pl) in coronitic textures. An M2 assemblage characterized by Grt2+Pl2±Cpx2+Hbl+Scp+Qz developed at 0.86–1.00 GPa and 660–730°C coincided with fluid flow and deformation associated with shear zone development. Microstructural observations in marginal protomylonite/mylonite and laminated ultramylonite suggest a shear zone evolution that involved (1) nucleation from pre‐existing fractures that were sites for major fluid infiltration, (2) initial widening coincident with grain‐size reduction by microfracturing, dislocation creep, and synkinematic metamorphic reaction by solution transfer, and (3) a switch in the dominant deformation mechanisms active in the ultramylonite from grain‐size insensitive mechanisms to grain‐size sensitive granular flow accommodated by fluid‐assisted diffusion. Throughout this evolution, the effective bulk compositions of the rock volumes responding to metamorphism changed through a combination of mechanical and metasomatic processes.

Embrittlement Within Viscous Shear Zones Across the Base of the Subduction Thrust Seismogenic Zone

“…Dissolution-precipitation creep and sliding along foliation surfaces (Figure 9) persists as the dominant mechanism within the plate boundary shear zone in metasediments and hydrated basalts over the entire range of P-T conditions recorded by the studied exposures. Microstructures indicating this deformation mechanism have also been observed in subduction thrust shear zones exhumed from temperatures <300°C (e.g., Meneghini & Moore, 2007;Palazzin et al, 2016), at comparable conditions to the studied shear zones (e.g., French & Condit, 2019;Platt et al, 2018), and also at warmer conditions in continental crust (e.g., Condit & Mahan, 2018;Stenvall et al, 2019). This mechanism likely accommodates plate-rate creep at displacement rates of a few cm per year and stresses of no more than a few 10s of MPa (e.g., Wassmann & Stöckhert, 2013;Fagereng & den Hartog, 2017).…”

Rheology of Metasedimentary Rocks at the Base of the Subduction Seismogenic Zone

“…Modal mineralogy was calculated using ImageJ to stack several WDS mapped elements with false RGB color scheme and then Adobe Photoshop to count pixels for each. These 2D modal abundances were then used in combination with phase compositions to calculate the bulk rock compositions of our schists using the methodology detailed in Condit and Mahan (2018). We also collected micro X-ray fluorescence (XRF) data to document silica concentrations (intensity) around vein margins.…”

Evidence for a Deep Hydrologic Cycle on Oceanic Transform Faults

“…In the second stage, deformation of the protolith by GBS (Figure 10a) causes fluids to be transported through a network of local dilatant zones (Figure 11b). This process, termed "creep cavitation," occurs when grain boundary slip cannot be fully accommodated by other deformation mechanisms, resulting in the opening and closing of intergranular pathways for fluid flow (Kassner & Hayes, 2003;Fusseis et al, 2009;Condit & Mahan, 2018;Figure 11b, inset). The tendency of GBS to promote fluid flow via creep cavitation may result from fluids being drawn into rapidly deforming zones, making them weaker and thus causing them to deform faster.…”