The Healy quadrangle Is underlain by a wide variety of sedimentary, volcanic, and plutonic rocks, ranging In age from Precambrlan and (or) early Paleozoic to Recent. There are fifty five map units on the geologic map. All the pre-Cenozolc rocks are Intensely deformed, mainly by overthrusting and folding, and most of them underwent at least one period of low-to medium-grade regional metamorphlsm. This deformation 1s the result of the middle Cretaceous collision and subsequent obductlon of the northward-moving Talkeetna superterrane with and onto the Yukon-Tanana and Nixon Fork terranes of the ancient North American continent. Late Cenozoic deformation, the result of continued northward plate motions, has modified but not substantially altered the geology of the quadrangle.
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.
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