GEOLOGIC MAPPING AND STRUCTURAL ANALYSIS OF THE CENTRAL YORK MOUNTAINS, SEWARD PENINSULA, ALASKA

New geologic mapping in part of the Teller B-5 and C-5 7.5’ quadrangles in the Bering Strait region of Alaska, sponsored by EDMAP, has led to the development of a new geologic framework for the area. The Seward Peninsula has a long history of deformation including Jurassic to Early Cretaceous shortening related to the Brooks Range orogeny and later periods of extension. Sainsbury (1969) defined the A.J. Collier thrust belt in the area. The two major faults in this proposed thrust belt are the Rapid River fault and the Mint River fault, each of which is approximately 10 to 15 km in length, dipping 25 to 35 degrees to the south. Our new geologic mapping and structural analysis have shown that both of these faults are actually normal faults. The stratigraphy of the area consists of thousands of feet of unmetamorphosed Early Ordovician carbonates south of the Mint River fault and the low-grade, mostly clastic rocks of the York Formation of uncertain age north of the fault which are intruded by gabbroic sills. The area is also intruded by Late Cretaceous mafic and felsic dikes and granitic plutons. New conodont data will help to constrain the ages of the carbonates. New ⁴⁰Ar/³⁹Ar geochronologic data will better constrain the timing of metamorphism of the York Formation and the timing of magmatism. Geochemical data for the dikes may shed light on their petrologic affinity.