CRUSTAL VELOCITY STRUCTURE OF THE NORTHERN YUKON TANANA UPLAND, CENTRAL ALASKA - RESULTS FROM TACT REFRACTION WIDE-ANGLE REFLECTION DATA

The Fairbanks North seismic refraction/wide-angle reflection profile, collected by the U.S. Geological Survey Trans-Alaska Crustal Transect (TACT) project in 1987, crosses the complex region between the Yukon-T anana and Ruby terranes in interior Alaska. This region is occupied by numerous small terranes elongated in a northeast-southwest direction. These seismic data reveal a crustal velocity structure that is divide d into three upper-crystal and at least two middle- to lower-crustal d omains. The upper-crustal domains are delineated by two steeply dippin g low-velocity anomalies that are interpreted as signatures of the Vic toria Creek fault, and the Beaver Creek fault or a fault buried by the Beaver Creek fault. This tripartite upper crust extends to 8-10 km de pth where a subhorizontal interface undercuts the northern and central domains. Beneath the northern domain, this interface is interpreted a s the southeastwardly dipping boundary between the Tozitna and Ruby te rranes. The continuation of this interface beneath the central domain suggests that it may represent the detachment or basal thrust for thin -skinned tectonic amalgamation of the terranes caught between the Yuko n-Tanana and Ruby terranes. The lower crust and Moho reflection exhibi t differences from north to south that define at least two lower-cryst al domains, interpreted as the Yukon-Tanana and Ruby terranes. Finally , the crustal thickness along the profile is nearly uniform and ranges from 31 to 34 km. Our data suggest that after initial thin-skinned am algamation of the various terranes, this region experienced thick-skin ned tectonic reorganization via strike-slip faulting. This interpretat ion supports a model in which at least one strand of the Tintina fault exists in this important region of Alaska.