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.

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This is a preliminary publication of the Alaska Division of Geological and Geophysical Surveys and as such has not received final editing and review. The author will appreciate candid comments o n the accuracy of the data, and welcome suggestions that will improve the report. Alaska Open-File Report 116 GENERAL GEOLOGY OF THE CENTRAL ALASXA RANGE BETWEEN THE NEXAM RIVER AND MOUNT DEBORAH

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The Cantwell Basin is a Paleogene intermontane basin located in south central Alaska. The Cantwell Formation which fills the basin is as much as 4000 m thick and consists of a lower sedimentary sequence of fluvial and alluvial fan origin and an upper sequence of calc‐alkaline volcanic rocks. The east trending Cantwell Basin developed as a southward thickening asymmetrical graben between the Hines Creek and Denali faults. Substantial dip‐slip displacement and local, dextral strike‐slip displacement on the Hines Creek fault during basin subsidence was followed by dip‐slip displacement in late Cenozoic time. Strike‐slip movement along the Denali fault, to the south, exerted primary control on basin subsidence and subsequent structural inversion. During the Paleocene, Kula plate motion was nearly perpendicular to the axis of the Cantwell Basin; however, Kula plate motion was oblique to southeastern Alaska and produced regional dextral slip on the Denali fault system. The Cantwell Basin appears to have developed as a pull‐apart basin in which northwest striking syndepositional normal faults linked the Denali fault with the western part of the Hines Creek fault. In early Eocene time an increased rate of plate convergence resulted in substantial displacement on the Denali fault. Concurrent counterclockwise rotation of southern Alaska produced a regional constraining bend in the Denali fault and resulted in strong compressional deformation of the Cantwell Basin.