Transtensional deformation was concentrated in a zone adjacent to the Tintina strike-slip fault system in Alaska during the early Tertiary. The deformation occurred along the Victoria Creek fault, the trace of the Tintina system that connects it with the Kaltag fault; together the Tintina and Kaltag fault systems girdle Alaska from east to west. Over an area of ∼25 by 70 km between the Victoria Creek and Tozitna faults, bimodal volcanics erupted; lacustrine and fluvial rocks were deposited; plutons were emplaced and deformed; and metamorphic rocks cooled, all at about the same time. Plutonic and volcanic rocks in this zone yield U-Pb zircon ages of ca. 60 Ma; 40 Ar/ 39 Ar cooling ages from those plutons and adjacent metamorphic rocks are also ca. 60 Ma. Although early Tertiary magmatism occurred over a broad area in central Alaska, metamorphism and ductile deformation accompanied that magmatism in this one zone only. Within the zone of deformation, pluton aureoles and metamorphic rocks display consistent NE-SW–stretching lineations parallel to the Victoria Creek fault, suggesting that deformation processes involved subhorizontal elongation of the package. The most deeply buried metamorphic rocks, kyanite-bearing metapelites, occur as lenses adjacent to the fault, which cuts the crust to the Moho (Beaudoin et al., 1997). Geochronologic data and field relationships suggest that the amount of early Tertiary exhumation was greatest adjacent to the Victoria Creek fault. The early Tertiary crustal-scale events that may have operated to produce transtension in this area are (1) increased heat flux and related bimodal within-plate magmatism, (2) movement on a releasing stepover within the Tintina fault system or on a regional scale involving both the Tintina and the Kobuk fault systems, and (3) oroclinal bending of the Tintina-Kaltag fault system with counterclockwise rotation of western Alaska.

We report 777 U-Pb SHRIMP detrital zircon ages from thirteen sandstones and metasandstones in interior Alaska. About sixty grains per sample were analyzed; typically, half to three-fourths of these were concordant within ± 10%. Farewell terrane . Two quartzites were collected from Ruby quadrangle and a third from Taylor Mountains quadrangle. All three are interpreted to represent a low stratigraphic level in the Nixon Fork platform succession; the samples from Ruby quadrangle are probably late Neoproterozoic, and the sample from Taylor Mountains quadrangle is probably Cambrian in age. The youngest detrital zircon in any of the three is 851 Ma. The two Ruby quadrangle samples area almost identical: one has a major age cluster at 1980–2087 and minor age clusters at 944–974 and 1366–1383 Ma; the other has a major age cluster at 1993–2095 Ma and minor age clusters at 912–946 and 1366–1395 Ma. The Taylor Mountains sample shows one dominant peak at 1914–2057 Ma. Notably absent are zircons in the range 1800–1900 Ma, which are typical of North American sources. The detrital zircon populations are consistent with paleontological evidence for a peri-Siberian position of the Farewell terrane during the early Paleozoic. Mystic subterrane of the Farewell terrane. Three graywackes from flysch of the Mystic subterrane, Talkeetna quadrangle, were sampled with the expectation that all three were Pennsylvanian. Asample from Pingston Creek is Triassic (as revealed by an interbedded ash dated at ca. 223 Ma) and is dominated by age clusters of 341–359 and 1804–1866 Ma, both consistent with a sediment source in the Yukon-Tanana terrane. Minor age clusters at 848–869 and 1992–2018 Ma could have been sourced in the older part of the Farewell terrane. Still other minor age clusters at 432–461, 620–657, 1509–1536, and 1627–1653 Ma are not readily linked to sources that are now nearby. A sample from Surprise Glacier is mid-Mississippian or younger. A dominant age cluster at 1855–1883 and a minor one at 361–367 Ma could have been sourced in the Yukon-Tanana terrane. Other age clusters at 335–336, 457–472, 510–583, and 1902–1930 have no obvious nearby source. A sample from Ripsnorter Creek is Silurian or younger. The dominant age cluster at 937–981 Ma and a minor one at 2047–2077 Ma could have been sourced in the Farewell terrane. Minor age clusters at 1885–1900 and 2719–2770 Ma could have been sourced in the Yukon-Tanana terrane. Other age clusters at 429–490, 524–555, 644–652, 1023–1057, 1131–1185, and 1436–1445 Ma have no obvious nearby source. The so-called Mystic subterrane is structurally complex and would appear to include more than one Phanerozoic turbidite succession; more mapping and detrital zircon geochronology are needed. Wickersham and Yukon-Tanana terranes. A grit from Wickersham terrane in Tanana quadrangle and a grit from Yukon-Tanana terrane in Talkeetna quadrangle have similar, exclusively Precambrian detrital zircon populations, supporting previous correlations. The Wickersham sample has major age clusters at 1776–1851 and 1930–1964 Ma, and the youngest grain is 1198 Ma. The Yukon-Tanana grit has a major age cluster at 1834–1867 Ma, and the youngest grain is 1789 Ma. A North American source has been previously proposed, and this seems likely based on detrital zircon data. Ruby terrane and Minook Complex. Detrital zircons from quartzites in the Ruby terrane show two quite different age patterns. A sample from the Bear Creek area of Tanana quadrangle has detrital zircon ages that are similar to those from the Wickersham and Yukon-Tanana grits. The dominant age clusters are 1823–1856 and 1887–1931 Ma. In contrast, a quartzite from nearby Senatis Mountain (Tanana quadrangle) yielded a completely different detrital zircon age spectrum, featuring a broad peak with no significant gaps from 1024 to 1499 Ma and a minor age cluster at 1671–1695 Ma. The youngest concordant zircon is 1024 ± 6 Ma. A quartzite from the Minook Complex, a sliver along the Victoria Creek strike-slip fault in Tanana quadrangle, is similar to the Senatis Mountain sample. Its detrital zircon population is dominated by grains between 1103 and 1499 Ma, with peaks within that range at 1161–1234 and 1410–1490 Ma; minorolderage clusters are at 1643–1676, 1765–1781, and 1840–1874 Ma. The youngest concordant grain is 1103 ± 6 Ma. Finally, a quartzite from Illinois Creek (Nulato quadrangle) at the extreme west end of the Ruby geanticline, previously assigned to the Ruby terrane, also has a detrital zircon age spectrum like that at Senatis Mountain. Mesoproterozoic zircons are predominant, with main age groups at 1329–1391 and 1439–1493 Ma and lesser ones at 1058–1072, 1184–1193, 1681–1692, and 1852–1879 Ma. The youngest concordant grain is 1058 ± 33 Ma. These barcodes are dominated by Mesoproterozoic zircons that are strikingly similar in age to detrital zircons in Neoproterozoic Sequence B in northwestern Canada (and easternmost Alaska, where it equates to the lower Tindir Group). Among other rocks, the Ruby geanticline thus might include a shortened, metamorphosed, and offset continuation of this ancient North American basin, which was sourced in the Grenville orogen. Rampart Group, Angayucham-Tozitna terrane. The Rampart Group is thought to have been deposited in an ocean basin that closed during the Brookian Orogeny. Detrital zircons from graywacke of the Rampart Group are dominated by an age cluster at 380–404 Ma, with lesser ones at 351–364, 426–440, 484–504, 909–920, 1001–1020, 1127–1128, 1211–1217, and 1912–1953 Ma. The youngest grain is 260 ± 1 Ma. The dominant 380–404 Ma age cluster can be reasonably linked to sources in Devonian plutons of the now-adjacent Brooks Range and Ruby terrane.

ABSTRACT Comprehensive understanding of the pre-Paleogene kinematic evolution of the North American Cordillera in the context of evolving global plate interactions must begin with an understanding of the complex Late Cretaceous–early Eocene structural geometry and evolution of the northwestern Cordillera of Alaska, United States, and Yukon, Canada. Here, I present a kinematic model of the region that shows how regional strike-slip fault systems, including plate-boundary transform faults, interacted with each other, and with north-striking oroclinal folds and fold-and-thrust belts, which formed progressively during coeval shortening between Eurasia and North America. These Late Cretaceous–early Eocene interactions are manifestations of the plate reorganizations in the Pacific and Atlantic-Arctic regions that took place at that time, and that led to rifting and seafloor spreading within the globe-encircling Eurasian–North American plate and to the formation of transform-dominant North American–Pacific (sensu lato) and possibly North American–Arctic plate boundaries.

The North American Cordillera in Canada and Alaska has been investigated through coincident and coordinated geological, geochemical, and geophysical studies along three corridors: (1) the Lithoprobe Southern Cordillera transect, (2) the ACCRETE and Lithoprobe Slave-Northern Cordillera Lithospheric Evolution (SNORCLE) transects, and (3) the Trans-Alaska Crustal Transect (TACT) program. Seismic-reflection and refraction experiments are integral to these studies and contribute to lithospheric-scale models that enable orogen-parallel comparisons to be made. Primary observations include three points: (1) Outward-verging, crustal-scale décollements are characteristic features of the orogen. The three trans-Cordillera transects exhibit decoupling zones that dip away from the Foreland belt to the lowermost crust or Moho. These inboard décollements above an indentor or cratonic backstop extend 500–600 km downdip in the Canadian Cordillera and 250 km downdip in the Alaskan Cordillera. The active subduction megathrusts form opposing décollements and generate structures in the overriding crust that mirror those above the facing intracrustal ramps. (2) Oblique convergence resulting in significant transpressional, transtensional, and orogen-parallel motion has yielded four major transcurrent fault systems that penetrate the entire crust and are associated with tectonic boundaries. (3) Beneath the entire Canadian Cordillera, the Moho remains remarkably flat and shallow despite the variety of ages, terrane compositions, and tectonomagmatic deformations spanned by the seismic corridors. These observations indicate that the Moho is an active, near-solidus, deformation zone that represents a young, re-equilibrated crust-mantle boundary. Beneath Alaska, crustal roots are observed over the subduction zone and at the indentor wedge, but the interior of the orogen also exhibits thin crust.

Abstract The Cordilleran orogen of western Canada and Alaska records tectonic processes than span over 1.8 billion years, from assembly of the Laurentian cratonic core of Ancestral North America in the Precambrian to sea-floor spreading, subduction, and geometrically linked transform faulting along the modern continental margin. The evolution of tectonic regimes, from Proterozoic intracratonic basin subsidence and Paleozoic rifting to construction of Mesozoic and younger intraoceanic and continent-margin arcs, has led to diverse metallogenetic styles. The northern Cordillera consists of four large-scale paleogeographic realms. The Ancestral North American (Laurentian) realm comprises 2.3 to 1.8 Ga cratonic basement, Paleoproterozoic through Triassic cover successions, and younger synorogenic clastic deposits. Terranes of the peri-Laurentian realm, although allochthonous, have a northwestern Laurentian heritage. They include continental fragments, arcs, accompanying accretionary complexes, and back-arc marginal ocean basins that developed off western (present coordinates) Ancestral North America, in a setting similar to the modern western Pacific basin. Terranes of the Arctic-northeastern Pacific realm include the following: pre-Devonian pericratonic and arc fragments that originated near the Baltican and Siberian margins of the Arctic basin and Late Devonian to early Jurassic arc, back-arc, and accretionary terranes that developed during transport into and within the northeastern paleo-Pacific basin. Some Arctic realm terranes may have impinged on the outer peri-Laurentian margin in the Devonian. However, main-stage accretion of the two realms to each other and to the Laurentian margin began in mid-Jurassic time and continued through the Cretaceous. Terranes of the Coastal realm occupy the western edge of the present continent; they include later Mesozoic to Cenozoic accretionary prisms and seamounts that were scraped off of Pacific oceanic plates during subduction beneath the margin of North America. Each realm carries its own metallogenetic signature. Proterozoic basins of Ancestral North America host polymetallic SEDEX, Cu-Au-U-Co-enriched breccias, MVT, and sedimentary copper deposits. Paleozoic syngenetic sulfides occur in continental rift and arc settings in Ancestral North America, the peri-Laurentian terranes, and in two of the older pericratonic Arctic terranes, Arctic Alaska, and Alexander. The early Mesozoic peri-Laurentian arcs of Stikinia and Quesnellia host prolific porphyry Cu-Au and Cu-Mo and related precious metal-enriched deposits. Superimposed postaccretionary magmatic arcs and compressional and extensional tectonic regimes have also given rise to important mineral deposit suites, particularly gold, but also porphyries. Very young (5 Ma) porphyry Cu deposits in northwestern Vancouver Island and sea-floor hotspring deposits along the modern Juan de Fuca Ridge off the southwest coast of British Columbia show that Cordilleran metallogeny continues.