EVIDENCE FOR THE NORTHERN CONTINUATION OF EOCENE CRUSTAL EXTENSION, NORTHERN COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA

Rapid exhumation driven by post-orogenic extension characterizes the central Coast Mountains batholith (CMB) in British Columbia. Eocene crustal extension on the Eastside detachment exhumed the Central Gneiss Complex near Prince Rupert, but its northern continuation is uncertain. A new compilation of published hornblende and biotite K-Ar and 40Ar/39Ar ages may reflect continuation of the Eocene extensional system ~200 km northward on the eastside of the CMB, to the Stikine River. Ar-ages from two transects across the batholith, near Ketchikan (n = 141) and Wrangell (n = 98), are compared to a published transect near Prince Rupert (n = 115), where the geology is well known. On all 3 transects, patterns in age distribution and inferred cooling rates define 3 domains from SW to NE separated by known and proposed faults. Domain 1, on the SW flank of the batholith, terminates at the Coast shear zone (CSZ). Ar-ages become progressively younger to the northeast, mimicking documented magmatic migration. Northeast of the CSZ, domain 2 is characterized by nearly uniform ages (Hbl: 53 – 47 Ma, Bt: 50 – 43 Ma), higher inferred cooling rates and no ages > 60 Ma. In contrast, ages in domain 3 range from ca. 50 – 400 Ma with highly variable cooling rates. On the Prince Rupert transect, domain 2 represents the lower plate of the extensional system, and domain 3 is the upper plate. The similarity of the 3 transects suggests that the well documented crustal extension near Prince Rupert affected the two northern transects, particularly the Ketchikan transect. We propose that the continuation of the Eastside detachment forms the boundary between domains 2 and 3. If so, the detachment system would be ~450 km long, nearly doubling its known length and greatly enlarging the area of extension. A northward decrease in the width of domain 2 and an apparent decrease in metamorphic grade may signal a northward decrease in extension. Ongoing geochronologic, metamorphic and structural studies, and planned field work will test the proposed extent of the extensional system and constrain its origin.