The Cenozoic Denali Fault System and the Cretaceous accretionary development of southern Alaska

Abstract

The juxtaposition of disparate geologic terranes in southern Alaska has been previously interpreted to be mainly the result of several hundred kilometers of right lateral offset along the Denali fault system in Cenozoic time. Recent geologic investigations in the Healy quadrangle strongly suggest that Cenozoic horizontal displacements of such magnitude along the Denali fault system do not exist. In the Healy quadrangle, isograds and metamorphic facies boundaries of an early Late Cretaceous metamorphic belt trend across the Cenozoic McKinley strand of the Denali system without significant horizontal offsets. The present geologic makeup of most of southern Alaska is primarily the result of the Talkeetna superterrane, consisting of the previously assembled Peninsular terrane and Wrangellia, colliding with and subsequently being thrust upon the Yukon‐Tanana and Nixon Fork terranes of the ancient North American continent in about middle Cretaceous time. The leading edge of the Talkeetna superterrane faces a wide, complexly deformed zone that contains numerous northwestward thrust miniterranes tectonically intermixed with Jurassic and Cretaceous flysch. The flysch is interpreted to have been deposited mostly in the narrowing and subsequently collapsed oceanic basin between the converging continental blocks. The postcollisional Denali fault system developed in Cenozoic time across the already accreted continental margin, in eastern Alaska along an older, Cretaceous suture.

Aftershock Sequence of the Mw 7.9 Denali Fault, Alaska, Earthquake of 3 November 2002 from Regional Seismic Network Data

“…The Wrangellia and Peninsula terranes include metamorphosed and deformed, upper Paleozoic, volcanic and sedimentary rocks that were originally part of an island arc. The Yukon-Tanana terrane includes intensely metamorphosed and ductilely deformed, Devonian and older, sedimentary and subordinate volcanic rocks as well as Devonian metaplutonic rocks (Csejtey et al, 1982;Redfield and Fitzgerald, 1993).…”

Late Cenozoic Uplift of Denali and Its Relation to Relative Plate Motion and Fault Morphology

“…Plutonic rocks of south-central Alaska were emplaced in the early and middle Jurassic (176 to 154 Ma), the late Cretaceous and early Tertiary (83 to 56 Ma), and the middle Tertiary (38 to 28 Ma) (1 0). Most (6,9). Elsewhere along the similar to that of Durango apatite, an age American, Farallon, Kula, and Pacific DFS, esiimates of post-Mesozoic dextral standard in which tracks are effectively plates can help constrain the geologic hisoffset vary from -400 to -200 km (7, 8).…”

Bedrock geologic map of the Chulitna region, southcentral Alaska

“…The Kahiltna assemblage (Reed and Nelson, 1980, Csejtey and others, 1978, 1982, 1992Nokleberg and others, 1994) is found in the northwest and eastern map areas, and may underlie the intervening older rocks, although a recent study suggests that Kahiltna-age rocks were deposited into two separate basins on either side of the Chulitna area, which formed a syndepositional structural barrier (Eastham and others, 2000). The Kahiltna assemblage consists of a monotonous, highly deformed flysch succession of fine-grained lithic sandstone, argillite, highly siliceous argillite, and rare conglomerate and limestone of prehnitepumpellyite metamorphic grade.…”