THE BORDER RANGES FAULT SYSTEM IN GLACIER BAY NATIONAL-PARK, ALASKA -EVIDENCE FOR MAJOR EARLY CENOZOIC DEXTRAL STRIKE-SLIP MOTION

The Border Ranges fault system of southern Alaska, the fundamental bre ak between the arc basement and the forearc accretionary complex, is t he boundary between the Peninsular-Alexander-Wrangellia terrane and th e Chugach terrane. The fault system separates crystalline rocks of the Alexander terrane from metamorphic rocks of the Chugach terrane in Gl acier Bay National Park. Mylonitic rocks in the zone record abundant e vidence for dextral strike-slip motion along north-northwest-striking subvertical surfaces. Geochronologic data together with regional corre lations of Chugach terrane cocks involved in the deformation constrain this movement between latest Cretaceous add Early Eocene (similar to 50 Ma). These findings are in agreement with studies to the northwest and southeast along the Border Ranges fault system which show dextral strike-slip motion occurring between 58 and 50 Ma. Correlations betwee n Glacier Bay plutons and rocks of similar ages elsewhere along the Bo rder Ranges fault system suggest that as much as 700 km of dextral mot ion may have been accommodated by this structure. These observations a re consistent with oblique convergence of the Kula plate during early Cenozoic and forearc slivering above an ancient subduction zone follow ing late Mesozoic accretion of the Peninsular-Alexander-Wrangellia ter rane to North America.