THRUST-STRIKE SLIP INTERACTIONS DURING >125KM OF DEXTRAL OFFSET ALONG THE BORDER RANGES FAULT SYSTEM, SOUTHERN ALASKA

The latest Cretaceous-Eocene strike-slip rejuvenation of the Border Ranges fault during oblique convergence along the North American Cordillera margin produced a strike-slip system that can be traced as a continuous structure for over 700 km, from southeast Alaska to south-central Alaska. From Glacier Bay to the eastern Chugach Mountains the structure is marked by a 5-10 km wide band of mixed brittle and ductile structures characterized by steeply-dipping shear zones and faults. Rocks within the shear zone include both arc basement from inboard terranes and Mesozoic subduction assemblages from outboard of the fault, typically shuffled within the main shear zone trace. In the central Chugach Mountains, however, this distinct shear zone becomes progressively more cryptic as it is traced westward into a broad band of deformation dispersed from the northern Chugach Mountains, through the Matanuska Valley, and into the Castle Mountain fault system of the southern Talkeetna Mountains. Our recent work in this area suggests an important reason for this complexity is a mutual overprinting by thrust and strike-slip systems in a broad zone of transpression that linked the Border Ranges system to the Castle Mountain system in a large strike-slip duplex. In the central Chugach Mountains early strike-slip fault systems produced structures similar to the eastern Chugach Mountains but affected primarily subduction assemblages including low-grade mélange, greenschist facies schist, blueschist, and unmetamorphosed (Tertiary?) sedimentary rocks. The latter are significant because they contain unusual marble and volcanic clasts, with the closest likely source across strike occurring east of the Copper River, which suggests this part of the fault zone experienced a minimum of 125km of dextral slip. These early strike-slip systems were subsequently overridden by a brittle, south-directed thrust sheet that deformed the older steeply dipping fabrics and placed mafic plutonic rocks of the Jurassic Talkeetna arc (Peninsular terrane) atop the strike-slip zone. The late phase contraction along the structure was almost certainly coeval with continued strike-slip along the margin and these contractional structures probably obscure much the evidence for strike-slip in the central Chugach Mountains and upper Matanuska Valley.