Abstract This chapter describes the geology of northern Alaska, the largest geologic region of the state of Alaska. Lying entirely north of the Arctic Circle, this region covers an area of almost 400,000 km 2 and includes all or part of 36 1:250,000 scale quadrangles (Fig. 1). Northern Alaska is bordered to the west and north by the Chukchi and Beaufort seas, to the east by the Canadian border, and to the south by the Yukon Flats and Koyukuk basin. Geologically, it is notable because it encompasses the most extensive area of coherent stratigraphy in the state, and it contains the Brooks Range, the structural continuation in Alaska of the Rocky Mountain system. Northern Alaska also contains the largest oil field in North America at Prudhoe Bay, the world's second-largest zinclead- silver deposit (Red Dog), important copper-zinc resources, and about one-third of the potential coal resources of the United States (Kirschner, this volume; Magoon, this volume; Nokleberg and others, this volume, Chapter 10; Wahrhaftig and others, this volume).

The pre-Quaternary geology of the southern Chocolate and Cargo Muchacho mountains correlates with that exposed in San Gorgonio Pass between the Mission Creek and Banning branches of the San Andreas fault. Matching features include: (1) the southernmost exposures of the Triassic Mount Lowe Granodiorite on opposite sides of the San Andreas; (2) the presence in both areas of distinctive melanocratic quartz monzonite, dated as mid-Jurassic in the southern area; (3) the occurrence in both areas of kyanite schist formed by hydrothermal leaching of granitic and gneissic rocks; (4) the correlation of the Bear Canyon fanglomerate and associated late Miocene basalt flows in the southern area with the Coachella Fanglomerate and associated late Miocene andesite and basalt flows in the northern area; and (5) the correlation of the northern limit of the late Miocene to Pliocene marine Bouse Formation in the Yuma area east of the Salton Trough, with the northern limit of the late Miocene to Pliocene marine Imperial Formation adjacent to the Mission Creek fault in San Gorgonio Pass. These correlations require 185 ± 20 km of right slip on the Mission Creek-Coachella Valley segment of the San Andreas fault since late Miocene. An additional right slip of about 30 km has occurred in San Gorgonio Pass along the Banning-Coachella Valley segment of the San Andreas fault, as indicated by the displacement of the northern limit of the Imperial Formation. A total right slip of 90 ± 20 km is needed on the Banning fault to be consistent with other data presented by us, but the displacement cannot be demonstrated because basement terrane north of the Banning fault is unrelated to that south of the Banning fault. The Salton Creek fault, located east of the San Andreas fault between the Orocopia Mountains and the Chocolate Mountains, correlates with an unnamed fault in Soledad Pass, located west of the San Andreas at the west end of the San Gabriel Mountains. In both areas the correlative faults have volcanic necks along them, and separate Precambrian syenite and related rocks on the northwest from Triassic Mount Lowe Granodiorite overlain by late Oligocene to early Miocene volcanic rocks on the southeast. The displacement of these correlative faults is younger than about 10 Ma, since it postdates deposition of conglomerate within the middle to late Miocene Mint Canyon Formation in Soledad basin. The conglomerate contains clasts of unusual volcanic rocks whose only known source is in the northern Chocolate Mountains. This correlation requires 240 km of right slip on the San Andreas fault, including displacement contributed by the San Jacinto fault. The San Gabriel fault is an abandoned branch of the San Andreas fault located southwest of Soledad Pass. It probably connected to the Banning fault prior to development of the Coachella Valley segment of the San Andreas fault. The San Gabriel fault displaces middle Miocene and older rocks 60 km to the right. The addition of this displacement to the Soledad Pass-Salton Creek offset yields a total right slip of 300 km on the San Andreas fault in southern California since middle Miocene time. Our determination of the total offset on the San Andreas fault is consistent with that obtained by Matthews (1976) from the correlation of the Neenach Volcanic Formation, located east of the San Andreas fault in the central Transverse Ranges, with the Pinnacles Volcanic Formation, located west of the fault in the central Coast Ranges. However, there is a conflict between the evidence presented here for 240 km of right slip on the San Andreas fault between Salton Creek and Soledad Pass and the evidence presented by Frizzell and others (1986) and Matti (this volume) for 160 km of right slip on the San Andreas between the central San Bernardino Mountains and Liebre Mountain, based on their correlation of distinctive Triassic monzogranite in the two areas. This conflict needs to be resolved. Our data are consistent with the hypothesized origin of the San Andreas as a transform fault caused by crustal extension in the Gulf of California. It requires displacement to have started by about 10 Ma. Only about 60 km of right slip needs to have occurred prior to about 5 Ma; 240 Ma of right slip has probably occurred since then.