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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.

Mid‐Cretaceous crustal extension played a fundamental role in the structural evolution of the Yukon‐Tanana terrane (YTT) in the northern Cordilleran interior. In the central portion of the YTT northwest of Delta Junction, Alaska, a mylonitic shear zone juxtaposes greenschist facies rocks in the upper plate against middle to upper amphibolite facies metamorphic rocks in the lower plate, a juxtaposition suggesting elimination of as much as 10 km of crustal section. The mylonites form a partial sheath enveloping a domal footwall structure and kinematic analysis of the mylonite zone yields a uniform transport direction of hanging wall to ESE. These relations suggest analogies to the metamorphic core complexes of the southern Cordillera. However, the YTT structures are entirely ductile, suggesting either a relatively deep erosional level or relatively high geothermal gradients during extension. In the study area remnants of an older preextensional thrusting event are preserved at the highest structural levels at the base of the Seventymile terrane and the leading edge of YTT in the Wickersham terrane. However, most areas display a regional, subhorizontal fabric that is superimposed on older fabrics, and in the study area this latest fabric is subparallel to the mylonitic sheath of the apparent extensional structure. Thus the conventional viewpoint that this latest fabric is related to thrusting needs to be reevaluated and this fabric may be entirely extensional in origin. Further evidence for extension is provided by clear similarities between YTT and characteristic features of other extensional terranes. Thus we suggest that the YTT is a deeply eroded view of highly extended continental crust. The tectonic mechanism for the extensional event and the magnitude of the extension is uncertain because of complications in regional timing relationships and in alternative interpretations of the reconstruction of the crustal section. Three end‐member models based on analogies with Neogene extensional systems are presented as working models to accommodate the alternative interpretations: (1) a Jurassic collision and Cretaceous extension model based on comparisons with the Neogene history of the Mediterranean region; (2) an Early to mid‐Cretaceous syncollisional model analogous to the Carpathian Mountains of eastern Europe; and (3) a syncollisional plateau uplift model with extension driven by gravity spreading.

Four large but poorly exposed rhyolitic calderas are present in the Yukon‐Tanana terrane (YTT) in east central Alaska. At least two are mid‐Cretaceous in age (∼93 Ma). Similar volcanic rocks, the South Fork Volcanics, occur northeast of the Tintina fault in Yukon Territory. Evidence for the calderas consists of thick deposits of de vitrified crystal‐ and lithic‐rich densely welded tuff, interpreted as caldera fill, associated with lava domes or shallow intrusive rocks. Coeval outflow sheets have been largely stripped by erosion. The calderas are preserved within a northeast trending depression extending across the axis of the elongate mid‐Cretaceous plutonic province. Trace element abundances in andesites and rhyolites associated with the caldera structures are similar to those of volcanic and plutonic rocks of subduction‐related magmatic arcs developed on continental crust and thus are suggestive of formation in such an environment. Late Cretaceous and early Tertiary igneous rocks in the YTT near the calderas are interpreted to have been emplaced in a more extensional setting when the subduction‐related magmatic front was farther ocean ward.