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Hawley, B.W., 1992

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Bibliographic Reference

Hawley, B.W., 1992, Structural, metamorphic, and geochemical study of the Seldovia Bay fault, Alaska; a relict Cretaceous subduction zone: University of Utah, Salt Lake City, Ph.D. dissertation, 131 p., illust.

Abstract

The southwestern Kenai Peninsula straddles the Border Ranges Fault System (BRFS). The BRFS is the suture between continental rocks of the Peninsula terrane, on the north, and accretionary sequences of the Chugach terrane on the south. Local mapping on the peninsula has shown that this suture has had a two-stage history. The first stage is characterized by ductile deformation on the northwest-dipping Seldovia Bay fault. The second stage is characterized by high-angle brittle faulting. Locally, the two stages are spatially separated by the Kachemak terrane; elsewhere along the BRFS the two are superimposed. This area therefore represents a unique location to study the ductile first-stage deformation related to subduction of the McHugh Complex along the Seldovia Bay fault. There is a noticeable strain gradient in both the McHugh Complex, the lower plate, and the Kachemak terrane, the upper plate, of the Seldovia Bay fault. Axial ratios of the finite strain ellipsoid range from near 1:1 at a distance of 2 km from the fault, to greater than 10:1 within the fault. Strain ellipsoids are predominantly oblate, with flattening perpendicular to cleavage, and long axis downdip. On a grain scale, deformation and metamorphism progress simultaneously. The process starts with microfracturing along grain boundaries and alteration of the matrix and feldspar grains. At higher strain the lithic clasts become highly altered along the previously existing microfractures, and the feldspar grains break apart along the previously altered crystal lattice. Directly adjacent to the fault, the lithic clasts become indistinguishable from the matrix, the feldspar grains are extensively altered, and quartz grains start to become fractured. The mylonites of the main fault are a fairly homogeneous assemblage of primarily phyllosilicates with minor very-fine-grained clusters of recrystallized quartz. Geochemical data show no measurable change in bulk chemistry associated with the main fault zone. In these rocks, this could be explained by one of two scenarios: (1) The fluids that must have been present to produce the hydrated phyllosilicates were already in equilibrium with the McHugh complex. This situation could easily result from dewatering of the accretionary wedge during subduction. (2) Although the main fault may have acted as a fluid conduit, there was probably sufficient permeability away from the fault to allow enough fluid through the Kachemak terrane and McHugh Complex to accommodate fairly uniform alteration.

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