The Kahiltna assemblage of southern Alaska crops out in an 800-km-long belt that forms the core of much of the rugged Alaska Range. New sedimentologic, provenance, and geologic mapping data suggest that the Kahiltna assemblage exposed in the western Alaska Range represents a late Early Cretaceous to Late Cretaceous marine basin that formed in response to oblique collision between a composite island-arc terrane and the Mesozoic continental margin of North America. The Kahiltna assemblage in the study area crops out in two belts located north and south of the Denali fault system. Measured stratigraphic sections show that the Kahiltna assemblage in the southern outcrop belt has a minimum thickness of 5560 m and consists of eight siliciclastic lithofacies that represent tabular and weakly channelized mixed sand-mud submarine-fan systems that developed in a base-of-slope environment of deposition. Our analysis of the Kahiltna assemblage located north of the Denali fault indicates the presence of similar lithofacies along with additional strata that we interpret to represent outer-shelf and/or upper-slope (slope apron) depositional environments. Geologic mapping for this study identified the depositional basement of both outcrop belts as Upper Triassic to Lower Jurassic marine-volcanic and volcaniclastic strata that form the upper part of the Mystic subterrane. Detrital zircon data constrain the depositional age of most of the Kahiltna assemblage in the study area to Early Cretaceous time (Aptian or Albian) or later and suggest a significantly younger timing of basin development than previously recognized. Compositional data indicate that sandstone and conglomerate of the Kahiltna assemblage were derived from both Mesozoic continental margin and composite island-arc terrane sources. Modal sandstone compositions (n = 41) are consistent with a mixed arc and recycled orogen provenance (Q 23 F 9 L 68 ; Qm 11 F 9 Lt 80 ). Detrital zircons from sandstone collected in the lower part of the Kahiltna assemblage yield Precambrian (32%), Paleozoic (12%), and Mesozoic (56%) U-Pb ages. Concordant ages are consistent with the age distributions of Proterozoic, Devonian, Mississippian, and Triassic-Jurassic plutonic rocks of the former continental margin that formed the northern boundary of the Kahiltna basin. Plutons of the Talkeetna and Chisana arcs, part of the composite island-arc terrane located south of the basin, also probably contributed to the abundance of detrital zircons with ages between 200 and 163 Ma and between 124 and 106 Ma, respectively. Our new findings indicate that by Early Cretaceous time, the North American continental margin and composite island-arc terrane were in close enough proximity for both to contribute sediment to the Kahiltna basin. Stratigraphic, structural, and geochronologic relationships presented here, combined with previous regional studies of Mesozoic strata in the suture zone, suggest that the Kahiltna assemblage is the product of oblique island-arc terrane collision. Oblique collision resulted in the juxtaposition of continental margin and oceanic strata within thrust sheets along the closing suture zone. Dominantly west- and southwest-directed submarine-fan systems transported detritus axially away from the closing suture zone and into the along-strike marine basin represented by most of the Kahiltna assemblage exposed in the western Alaska Range. Comparisons with along-strike uplifted Mesozoic marine basins suggest westward time-transgressive closure of a suture zone that extends from British Columbia to southwestern Alaska.

Mesozoic strata of the northwestern Talkeetna Mountains are located in a regional suture zone between the allochthonous Wrangellia composite terrane and the former Mesozoic continental margin of North America (i.e., the Yukon-Tanana terrane). New geologic mapping, measured stratigraphic sections, and provenance data define a distinct three-part stratigraphy for these strata. The lowermost unit is greater than 290 m thick and consists of Upper Triassic–Lower Jurassic mafic lavas, fossiliferous limestone, and a volcaniclastic unit that collectively we informally refer to as the Honolulu Pass formation. The uppermost 75 m of the Honolulu Pass formation represent a condensed stratigraphic interval that records limited sedimentation over a period of up to ca. 25 m.y. during Early Jurassic time. The contact between the Honolulu Pass formation and the overlying Upper Jurassic–Lower Cretaceous clastic marine strata of the Kahiltna assemblage represents a ca. 20 m.y. depositional hiatus that spans the Middle Jurassic and part of Late Jurassic time. The Kahiltna assemblage may to be up to 3000 m thick and contains detrital zircons that have a robust U-Pb peak probability age of 119.2 Ma (i.e., minimum crystallization age/maximum depositional age). These data suggest that the upper age of the Kahiltna assemblage may be a minimum of 10–15 m.y. younger than the previously reported upper age of Valanginian. Sandstone composition (Q-43% F-30% L-27%—Lv-71% Lm-18% Ls-11%) and U-Pb detrital zircon ages suggest that the Kahiltna assemblage received igneous detritus mainly from the active Chisana arc, remnant Chitina and Talkeetna arcs, and Permian–Triassic plutons (Alexander terrane) of the Wrangellia composite terrane. Other sources of detritus for the Kahiltna assemblage were Upper Triassic–Lower Jurassic plutons of the Taylor Mountains batholith and Devonian–Mississippian plutons; both of these source areas are part of the Yukon-Tanana terrane. The Kahiltna assemblage is overlain by previously unrecognized nonmarine strata informally referred to here as the Caribou Pass formation. This unit is at least 250 m thick and has been tentatively assigned an Albian–Cenomanian-to-younger age based on limited palynomorphs and fossil leaves. Sandstone composition (Q-65% F-9% L-26%—Lv-28% Lm-52% Ls-20%) from this unit suggests a quartz-rich metamorphic source terrane that we interpret as having been the Yukon-Tanana terrane. Collectively, provenance data indicate that there was a fundamental shift from mainly arc-related sediment derivation from sources located south of the study area during Jurassic–Early Cretaceous (Aptian) time (Kahiltna assemblage) to mainly continental margin-derived sediment from sources located north and east of the study area by Albian–Cenomanian time (Caribou Pass formation). We interpret the three-part stratigraphy defined for the northwestern Talkeetna Mountains to represent pre- (the Honolulu Pass formation), syn- (the Kahiltna assemblage), and post- (the Caribou Pass formation) collision of the Wrangellia composite terrane with the Mesozoic continental margin. A similar Mesozoic stratigraphy appears to exist in other parts of south-central and southwestern Alaska along the suture zone based on previous regional mapping studies. New geologic mapping utilizing the three-part stratigraphy interprets the northwestern Talkeetna Mountains as consisting of two northwest-verging thrust sheets. Our structural interpretation is that of more localized thrust-fault imbrication of the three-part stratigraphy in contrast to previous interpretations of nappe emplacement or terrane translation that require large-scale displacements.

Volcanic and granitic rocks of the Jack River igneous field were erupted and emplaced in the suture zone between the accreted Wrangellia composite terrane and the former margin of southern Alaska. The volcanic rocks unconformably overlie Jurassic-Cretaceous shale and sandstone of the Kahiltna assemblage and include 100–300 m of basalt, basaltic andesite, and andesite lava flows overlain by a rhyolite unit that consists of over 900 m of lava flows and pyroclastic deposits. Seven basaltic and rhyolite lava samples yield 40 Ar/ 39 Ar ages ranging from 56.0 ± 0.3 to 49.5 ± 0.3 Ma. Two granitic samples yield 40 Ar/ 39 Ar ages of 54.6 ± 0.4 and 62.7 ± 0.4 Ma. These age dates indicate that the onset of Jack River magmatism at ca. 62.7 Ma coincided with the terminal phase of terrane accretion and continued after accretion to at least 49.5 Ma. The volcanic rocks range between tholeiitic and high-K calc-alkaline series and show a bimodal distribution with respect to silica (dacite is absent). The Jack River basalts are tholeiitic, have rare earth element and high field strength element ratios that are in the range between Pacific enriched mid-ocean-ridge basalts and Hawaiian ocean-island basalts (e.g., La/Yb = 5.0–8.4; Nb/Zr = 0.07–0.11), and have a within-plate geochemistry (e.g., Ti/V >50; high Zr/Y). All of the Jack River volcanic rocks exhibit some degree of enrichment in large ion lithophile and/or fluid mobile elements (e.g., Cs, Ba, Th, U, K, and Pb), although the basalts have low ratios between large ion lithophile and high field strength elements (e.g., Ba/Nb as low as 32.7 and Pb/Nb of 0.28–0.35). The granitic rocks (granites to granodiorites) are strongly depleted in the heavy rare earth elements, and most samples exhibit characteristics of adakites (e.g., Al 2 O 3 >15 weight %, Yb = 0.6–1.2 ppm, Y = 5.5–12.5 ppm, and Sr/Y = 20.4–66.2). The Jack River basalts were derived from partial melts of a mantle source that was more enriched than depleted mid-ocean-ridge basalt mantle and that ranged toward an enriched mantle (EM-I-type) composition.The basalts then evolved by assimilation and fractional crystallization to form intermediate magmas. Rhyolite magmas were formed later as anatectic melts of upper crustal argillaceous rocks (Kahiltna assemblage), resulting in the bimodal volcanism. The granitic adakite magmas may have formed by melting of garnet-bearing metamorphosed sedimentary rocks (meta-Kahiltna assemblage) that formed lower crustal rocks in the suture zone. Although the Jack River igneous rocks do exhibit some arc-like geochemical characteristics (e.g., elevated large ion lithophile elements), they differ from calc-alkaline arc rocks in that (1) they are a bimodal volcanic suite; (2) the rhyolites are not comagmatic with the basaltic and intermediate rocks; (3) the basalts and andesites have higher TiO 2 (>1.5 weight %) than is typical for arc basalts and andesites; (4) the basalts do not exhibit depletion of high field strength elements (e.g., Ta and Nb) with respect to large ion lithophile elements; (5) the basalts have an intraplate geochemical affinity; and (6) adakites are present. These characteristics show that the geochemistry of postcollisional suture zone magmatism can be transitional between calc-alkaline arc and intraplate magmatism. The Jack River volcanic field is deformed into a broad, northeast-trending syncline, which is crosscut by small-scale brittle faults that include northwest- and west-trending normal-slip and oblique-slip faults, and a southeast-dipping reverse fault that places Kahiltna assemblage rocks over the Jack River volcanic rocks. The pattern of Jack River deformation is consistent with right-lateral simple shear along the Denali fault system and indicates an episode of post-49.5 Ma strike-slip along the McKinley strand of the Denali fault. The Jack River rocks, therefore, record the magmatic response to terrane accretion and the kinematics of margin-parallel transport of an accreted terrane assemblage after it was sutured to the continental margin.