The northwest-trending Fairweather fault has undergone Cenozoic strike-slip displacement, with the most recent pulse of movement occurring in late Quaternary time. During the Lituya Bay earthquake (Ms = 7.9) of July 10, 1958, movement occurred probably along the entire 280 km onshore length of the Fairweather fault, with maximum measured displacements of 6.5 m dextral slip and 1 m dip slip near Crillon Lake. Three streams at Crillon Lake that flow on glacial till and bedrock have dextral displacements with maximum offset of roughly 55 m, and a lateral moraine of Finger Glacier southeast of Crillon Lake appears to have been offset at least 50 m in the same sense. Radiometric dating of wood from moraines near Crillon Lake and at Finger Glacier indicates that the offset stream drainages are probably not older than 940 ± 200 years BP and that the offset lateral moraine is about 1300 ± 200 years old. Data from the offset streams indicate that the minimum average displacement rate along this part of the Fairweather fault for the last millennium was at least 4.8 cm year−1 and was probably closer to 5.8 cm year−1.The deduced average offset rate is approximately equal to the full relative displacement of about 5.4 cm year−1 between the Pacific and North American plates in the Gulf of Alaska region that has been deduced from deep sea paleomagnetic data. These data indicate that the Fairweather fault is presently a transform boundary along which most, if not all, of the relative motion between the Pacific and North American plates is currently being taken up. Major valleys crossing the Fairweather fault that are probably older than Sangamon are systematically offset in a dextral sense an average of 5.5 km. These data suggest that the present high displacement rate on the Fairweather fault could not have begun more than 100 000 years ago and, together with data from other onshore faults, imply that before that time the relative motion between the plates was mainly on one or more submarine faults.

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.

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Field, petrology, and age data on southern Alaska plutonic rocks now enable the delineation of eight calc-alkaline.ptutonioobelts. .-.These belts of plutons or batholithic complexes are curvilinear to linear and trend parallel or subparallel to the continental margin. The belts represent the principal loci of emplacement for plutons of specific ages and although there is spatial or temporal overlap in some cases, they are more commonly spatially and temporally distinct. Intermediate lithologies such as quartz diorite, tonalite, and granodiorite dominate in most of the belts but granodiorite and granite characterize one. The belts are of Mesozoic or Cenozoic age and plutonism began in six of them at about 195, 175, 120, 75, 60, and 40 m.y. ago; age relations in two are poorly known. Recognition of the belts is important for future studies of regional geology, tectonism, and magmatism along the Pacific rim of southern Alaska.

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