A MICROSTRUCTURAL AND GEOCHEMICAL STUDY OF A SHEAR ZONE WITHIN THE CRETACEOUS CHUGACH ACCRETIONARY PRISM, SOUTHCENTRAL ALASKA

Recent glacier retreat in southern Alaska has revealed a previously unmapped fault contact between mélange and flysch of the Cretaceous Chugach accretionary prism. We are studying this fault in order to address the still-debated growth of the Chugach terrane, and to analyze the evolution of fault-rock fabrics and geochemistry in a fluid-rich subduction complex. The damage zone is ~50 m wide, with E–W foliation dipping ~35˚N and lineations plunging ~45˚NW, consistent with dip slip. Black fault rock is interlayered with and injected into the mélange, which contains <5 mm quartz-rich clasts surrounded by ultrafine groundmass. XRD and EDS mapping indicate the ultrafine material is quartz, feldspar, calcite, dolomite, pyrite, and kaolinite. Preliminary XRF of the black fault rock resulted in ~20% Loss on Ignition, likely from carbonate and graphite. These often form due to solution transfer of material at convergent plate margins, with graphite signifying fast slip rate. Quantifying the fluid budget during faulting can help assess fluid pressure, which plays a crucial role in shear zone development. SEM imaging of the black fault rock reveals micron-scale layering wrapping around quartz clasts, vein-like injections, and lack of crystallinity, typical of fault gouge. Oriented samples were examined under EBSD to characterize the microstructures and quantify CPO. Within the fault zone, quartz-rich clasts preserve low-grade deformation microstructures—pressure solution; deformation lamellae; grain bulges; sweeping undulose extinction—characteristic of 300–400°C at the brittle–ductile transition. CPO is weak in most quartz clasts, except for one population with moderate CPO. Brittle overprint is expressed by the black fault rock and fractures cross-cutting the stretched quartz phacoids. In contrast, the mélange was metamorphosed to greenschist facies (400–450°C) during subduction. We therefore associate the fault-rock fabrics with progressive down-temperature deformation as the fault was exhumed. An Eocene conglomerate locally bounds the fault; we will apply Ar/Ar thermochronology to stretched/rotated K-feldspar and metamorphic muscovite to more precisely define the age of faulting. Our observations will provide constraints on the timing and nature of deformation within a major subduction complex.