Structural and Stratigraphic Constraints on the Tectonics of the Chulitna Terrane

The Chulitna terrane is an enigmatic crustal block that lies in the suture zone between the Wrangellia composite terrane (WCT) and the Mesozoic margin of North America. New mapping of the Chulitna terrane reveals an ocean island complex (either aseismic ridge or seamounts) that accreted to North America ahead of the WCT. The terrane was deformed into a southeast vergent synclinorium with a series of out-of-syncline thrusts rooted in the basement. Basement of the complex consists of serpentinite, vesicular basaltic andesite, and lahars with blocks of andesite and chert. The cover rocks vary in facies from the southeast to northwest. Unconformably overlying the basement in the southeast is a poorly sorted red bed breccia, interpreted as slope colluvium. In the central part of the area the breccias transition to conglomerate with minor siltstone and sandstone, displaying cross-beds, suggestive of an alluvial origin. In the northwest the red beds are best described as a near shore terrestrial to marine facies, comprised of siltstone and sandstone interbedded with limestone. Underlying the red beds in the northwest are limestones and pillow basalts. A marine sandstone unit that thins to the southeast overlies the entire package. A cross section was constructed across the Chulitna terrane that depicts a regionally developed synclinorium with parasitic folds. In the southeast portion the syncline is cut by three northwest dipping thrust faults. The entire complex overthrusts Jurassic-Cretaceous argillite along a southeast vergent thrust, the Chulitna Fault. Restoration of these structures helps to place sedimentary facies in their relative positions prior to shortening. We suggest a model for the local depositional environments in the Triassic, with a volcanic basement edifice in the southeast. Early Triassic lavas flowed to the northwest toward a shoreline and interfingered with shallow water limestones. Subsequent red bed deposition involved colluvial transport on exposed basement slopes to the southeast with streams downslope carrying sediment to the northwest. Preliminary geochemical results suggest an aseismic or seamounts for the basalts in the northwest end of the terrane, suggesting that the Chulitna may be a fragment of thick oceanic crust accreted to the North American margin.