Mid‐Cretaceous extensional fragmentation of a Jurassic‐Early Cretaceous Compressional Orogen, Alaska

Several key geologic relations of interior and northern Alaska suggest that the hinterland of the Brooks Range thrust belt, which formed part of a major Middle Jurassic to earliest Cretaceous compressional orogen, underwent significant regional extension in the mid‐Cretaceous. These geologic relations include (1) simultaneous development of depositional basins in both foreland and hinterland of the Brooks Range fold and thrust belt, (2) regional normal faults along the “root zone” for Brooks Range allochthons, (3) anomalously thin (10–20 km) crust in the hinterland of the Brooks Range developed as a consequence of extension, (4) missing source regions for arc‐derived clastic rocks of the foreland, perhaps removed by rifting, (5) preservation of high pressure/low temperature metamorphic rocks in the hinterland, possibly resulting from extensional denudation and rapid uplift, (6) omission rather than duplication of structural section along many regional terrane boundaries in the hinterland, (7) shallow dipping metamorphic foliations, stretching lineations, and high‐temperature cooling histories consistent with normal sense displacement of overlying allochthonous rocks, (8) widespread crustal anatexis in parts of the hinterland, and (9) emplacement of rift‐related alkaline and associated igneous rocks in the Yukon‐Koyukuk basin. Consideration of the amount of crustal thinning, tectonic denudation, and throw on normal faults implied by the above relations indicates that mid‐Cretaceous events could have been responsible for at least 100% stretching (ß=2) across most of interior Alaska. Evaluation of timing constraints suggests that extensional tectonism in interior Alaska spanned an interval from about 130 to 90 Ma. Review of known geologic relations indicates that extension postdated an important episode of crustal shortening in the Brooks Range related to closure of an ocean basin (now represented by the Angayucham terrane) during the approach and collision of a Jurassic and older arc system with the continental margin. Extension in interior Alaska was coeval with the inception of rifting and with rift‐related magmatism in the Canadian Arctic and likely occurred during the opening of the Canada Basin. This important period of extensional deformation fragmented the various components of an earlier Jurassic to earliest Cretaceous compressional orogen in Alaska, broadening the cross‐sectional width of the Cordilleran orogen at this latitude and drastically modifiying paleogeography with the formation of large marine basins in the Cretaceous. As such, this episode of extension has important implications for our understanding of the paleogeography and tectonic setting of earlier crustal shortening and our understanding of the tectonic and thermal controls on subsequent Cretaceous sedimentation across Alaska. This extension also plays a critical role in larger‐scale plate tectonic problems such as the pre‐Cretaceous reconstruction of the Arctic and opening of the Canada Basin.