DATING REGIONAL EVENTS WITHIN A BRITTLE FAULT IN THE SOUTHERN ALASKAN CHUGACH ACCRETIONARY COMPLEX

Unraveling the history of ductile-to-brittle faulting is complex but necessary to understand how mineralogical processes influence the mechanical behavior of faults. We examined a pervasively cataclasized 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 fault strength. We characterized microstructures and compositions, and dated whole rock and mineral separates by ⁴⁰Ar/³⁹Ar thermochronology to constrain the timing and conditions of accretion, uplift, and deformation recorded in this fault zone. The ⁴⁰Ar/³⁹Ar results are not straightforward, and indicate influence from regional heating and uplift events, but can be interpreted within the context of geological relationships.

The 50-m-wide fault lies south of the Talkeetna volcanic arc, at the contact between southern Alaska’s McHugh complex mélange and Valdez Group flysch. The oldest ⁴⁰Ar/³⁹Ar fault-rock ages of ca. 160 Ma indicate deposition of volcanic material directly into the subduction complex. These rocks contain CM that was examined by Raman spectroscopy and indicates precipitation at ~300˚C, near ductile–brittle transition temperatures. Ca. 115 Ma ⁴⁰Ar/³⁹Ar plateaus are coincident with previously suggested uplift and dike intrusion. Our new detrital-zircon results from flysch directly adjacent to the fault overlap prior detrital-zircon studies that constrain onset of Valdez Group deposition to ca. 80 Ma. ⁴⁰Ar/³⁹Ar plateaus of ca. 60 Ma from fault rocks and from oceanic crust incorporated into the McHugh Complex overlap ages of regional igneous intrusions that have been associated with subduction of an actively spreading ridge. The fault is overlain by minimally sheared conglomerate that is younger than 58 Ma. Given the regional constraints, we suggest that this is a brittle fault that was active between 80 and 60 Ma. Because 45˚-dipping foliations and lineations indicate it was a transpressional fault, we suggest this is the eastward continuation of the Eagle River fault. Dating material from the fault zone has provided a link between phases of fault-zone evolution and documented tectonic events from southern Alaska subduction complex.