Deformation microstructures, mechanisms, and history of a shear zone within the Chugach accretionary complex in the Nelchina area, South-Central Alaska

Abstract

Ductile-to-brittle fault zones reveal mineralogical processes that are thought to be responsible for the mechanical behavior of faults. I examined a pervasively deformed zone within the Jurassic to Cretaceous accretionary complex of southern Alaska that preserves hydrothermal alteration, dissolution precipitation, carbonaceous material (CM), clay minerals, and intracrystalline plasticity, all of which influence the strength of a fault. I characterized microstructures by SEM and EBSD, determined compositions by XRD, XRF, and Raman spectroscopy for one carbon-rich sample, and dated whole rock, rotated K-feldspar, and metamorphic muscovite by ⁴⁰Ar/³⁹Ar thermochronology to constrain the timing and conditions of accretion, uplift, and deformation recorded by this fault zone. I interpret the specific mineralogy and complex network of deformation microstructures as a result of multiple deformation events. Highest-temperature deformation recorded within the shear zone is lower greenschist facies (400-450°C). Quartz-rich clasts preserve deformation lamellae, grain bulges, sweeping undulose extinction, pressure solution, and brittle fractures characteristic of low grade (300-400°C) at the brittle-ductile transition. Brittle overprint is expressed by fractures cross-cutting the stretched quartz phacoids, and black fault rock that has entrained stretched quartz grains. Raman spectroscopy places precipitation of the CM at ~300˚C. I therefore associate the fault-rock fabrics with progressive down-temperature deformation as the fault was exhumed. I suggest that pressure solution and mineral alteration in all fault-zone samples, as well as quartz and phyllosilicate preferred orientation in a subset of the samples, indicate aseismic slip. Growth of clay and precipitation of CM reduced the friction coefficient, lowering the frictional strength and influencing the dynamic behavior of this fault zone. Constraining the relative timing of the different slip behaviors is hard to determine. It is possible they were active at the same time, especially with the increase of width and complexity at the deeper part of the fault. What is preferentially preserved in the rock record is the latest stage of slip. Pseudotachylite structures generated during earthquakes, however, are rarely preserved due to their susceptibility to alteration. In my field area, consequent exhumation and cooling lead to progressive down-temperature brittle deformation and strong hydrothermal alteration, which could have eradicated any evidence for frictional melting. Using ⁴⁰Ar/³⁹Ar thermochronometry alongside regional and local age constraints, I was able to provide some insight on timing of fault-zone and local tectonic activity. The fault lies between the McHugh Complex and Valdez Group, the two main components of the Jurassic to Cretaceous Chugach accretionary prism whose development and disruption is still debated. I interpret that fault activity lasted from ca. 120 Ma to ca. 60 Ma., and was followed by two stages of accelerated uplift and cooling during ca. 40 Ma and ca. 20 Ma. The cease of major fault activity after ca. 60 Ma, the lack of pervasive strike-slip motion indicators, and the presence of undeformed Eocene dikes as well as Eocene sediments deposited on top of both the McHugh Complex and Valdez Group, suggest they were deposited in proximity and were in place in Southern Alaska at the start of the Eocene epoch.