Models of quartz overgrowth and vein formation: Deformation and episodic fluid flow in an ancient subduction zone

Abstract

Steady state models of overgrowth and vein formation are developed using kinetic data for quartz dissolution and precipitation and estimates of fluid advection, pore‐fluid and grain‐boundary diffusion. Application of these models to overgrowths and veins in the Kodiak accretionary complex suggests that the Kodiak Formation deformed continuously by a grain‐boundary diffusion‐limited mechanism, accompanied by episodic pore fluid diffusion of quartz from the matrix to vertical fluid‐filled fractures near the base of the accretionary wedge. These processes produced two types of syntectonic crystal textures within the Kodiak Formation: overgrowths containing displacement‐controlled fibers, and throughgoing veins composed of face‐controlled elongate blocky quartz crystals. Based on textural observations, displacement‐controlled quartz growth in overgrowths is rate‐limited by either diffusion along a cohesive interface or the rate of matrix strain. The magnitude of elongation recorded by displacement‐controlled crystal growth varies smoothly (elongation of 1 to 3) from the shallowest to the deepest structural levels of the Kodiak Formation, suggesting that the diffusional component of deformation in the accretionary wedge increases with depth. In contrast, face‐controlled quartz growth is largely restricted to Veins within the deepest level, where the cleavage is subhorizontal and deformation involves a component of simple shear, suggesting proximity to a decollement. The face‐controlled quartz veins represent mode I cracks which seal periodically and contain continuous planar solid inclusion bands, cracks which partially seal periodically and contain discontinuous solid inclusion bands, or cracks that remain open and contain euhedral quartz crystals with no solid inclusions. The initial crack aperture, inferred from spacing of inclusion bands, varies from 8 μm in crack seal features to much larger values in euhedral growth veins. Euhedral growth veins remain open throughout their development (105 to 106 years), and crack seal veins develop as a consequence of many crack‐seal events over a 103–106 year period. In both cases, textural evidence suggests that most transport of silica occurs by local pore‐fluid diffusion from matrix to vein. Wall rock inclusion bands suggest that “crack” events and “seal” events each occurred within periods of 102–104 years. A picture emerges of intermittent fluid flow upward from the decollement into a branching hierarchy of vertical fractures in the accretionary wedge during hydrofracturing events, followed by local transport and precipitation of silica causing sealing of the fractures at depth and propagation of pulses of fracture fluid upward.