Analysis of late Mesozoic and Cenozoic sedimentary basins, metamorphic rocks, and major faults in the eastern and central Alaska Range documents the progressive development of a suture zone that formed as a result of collision of an island-arc assemblage (the Wrangellia composite terrane) with the former North American continental margin. New basin-analysis, structural, and geochronologic data indicate the following stages in the development of the suture zone: (1) Deposition of 3-5 km of Upper Jurassic-Upper Cretaceous marine strata (the Kahiltna assemblage) recorded the initial collision of the island-arc assemblage with the continental margin. The Kahiltna assemblage exposed in the northern Talkeetna Mountains represents a Kimmeridgian-Valanginian backarc basin that was filled by northwestward-flowing submarine-fan systems that were transporting sediment derived from Mesozoic strata of the island-arc assemblage. The Kahiltna assemblage exposed in the southern Alaska Range represents a Valanginian-Cenomanian remnant ocean basin filled

Mesozoic and Cenozoic sedimentary strata exposed throughout southern Alaska contain a rich archive of information on the growth of collisional continental margins through the processes of terrane accretion, magmatism, accretionary prism development, and subduction of oceanic spreading ridges. Two major collisional events define the tectonic growth of southern Alaska: Mesozoic collision of the Wrangellia composite terrane and Cenozoic collision of the Yakutat terrane. The sedimentary record of these two collisional events can be summarized as follows. (1) Middle Jurassic volcaniclastic and sedimentary strata represent shallow-marine deposition in narrow subbasins adjacent to the volcanic edifice of the south-facing, intraoceanic Talkeetna arc. (2) Late Jurassic syndepositional regional shortening resulted in thick sections of conglomerate in proximal parts of both retroarc and forearc basins. In distal retroarc depocenters, fine-grained turbidite sedimentation commenced in a series of basins that presently extend for Ͼ2000 km along strike. This time interval also marked cessation of magmatism and rapid exhumation of the Talkeetna oceanic arc. We interpret these observations to reflect initial oblique collision, younging to the northwest, of the Wrangellia composite terrane with the continental margin of western North America. (3) During Early Cretaceous time, Jurassic retroarc basin strata were incorporated into an expanding north-verging thrust belt, and sediment was bypassed into more distal collisional retroarc basins located within the suture zone. Compositional data from these collisional basins show that the Wrangellia composite terrane was exhumed to deep stratigraphic levels. Detrital zircon ages from strata in these basins record some sediment derivation from source areas with North American continental margin affinity. Our data clearly show that the Wrangellia composite terrane and the former continental margin were in close proximity by this time. Accretion of this oceanic terrane and associated basinal deposits marked one of the largest additions of juvenile crust to western North America. The collision of the Wrangellia composite terrane also resulted in a change in subduction parameters that eventually prompted development of a new south-facing arc system, the Chisana arc. Construction of this arc was contemporaneous with renewed forearc basin subsidence and sedimentation. (4) Late Early Cretaceous to early Late Cretaceous time was characterized by regional deformation of retroarc collisional basin strata by south-verging thrust faults that are part of a regional thrust belt that

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Analysis of the Upper Jurassic NaknekFormation in the Talkeetna Mountains, Alaska, documents synorogenic sedimentation in a forearc basin along the outboard (southern) margin of the allochthonous Peninsular terrane during accretion to the western North American continental margin. New geochronologic, sedimentologic, and compositional data defi ne a two-part stratigraphy for the Naknek Formation. Microfossil, megafossil, and U-Pb clast ages document early Oxfordian to early Kimmeridgian deposition of the lower 690 m of the Naknek Formation and early Kimmeridgian to early Tithonian deposition of the upper 225 m of the Naknek Formation. Lithofacies and paleocurrent data from the lower Naknek Formation demonstrate initial deposition on a high-gradient, southward-dipping basin fl oor. Submarine mass fl ows deposited poorly sorted, cobble-boulder conglomerate in proximal fan-delta environments. Gravelly mass fl ows transformed downslope into sandy turbidity currents on a muddy prodelta slope. During early Kimmeridgian to early Tithonian time, fan-delta environments were replaced by lower gradient marine shelf environments characterized by deposition of cross-stratifi ed sandstone and bioturbated mudstone. Source-diagnostic clasts, feldspathic sandstone compositions, southwarddirected paleocurrent indicators, and U-Pb zircon ages of plutonic clasts (167.6 ± 0.3 Ma; 166.5 ± 0.2 Ma, 164-159 Ma, 156.2 ± 0.4 Ma)indicate that the Naknek Formation was derived primarily from volcanic and plutonic source terranes exposed along the northern basin margin in the southern Talkeetna Mountains. Geologic mapping documents the Little Oshetna fault, a newly identifi ed northward-dipping reverse fault that bounds the northern margin of the Naknek Formation in the Talkeetna Mountains. The concentration of boulder-rich mass-fl ow deposits in the footwall of the fault in combination with geochronologic and compositional data suggest that sedimentation was coeval with Late Jurassic shortening along the fault and exhumation of plutonic source terranes exposed in the hanging wall of the fault. From a regional perspective, coarse-grained forearc sedimentation and pluton exhumation along the outboard (southern) segment of the Peninsular terrane were coeval with crustal-scale shortening and synorogenic sedimentation in retroarc basins along the inboard (northern) margin of the Wrangellia terrane (Kahiltna, Nutzotin, and Wrangell Mountains basins). We interpret the regional and synchronous nature of Late Jurassic crustal-scale deformation and synorogenic sedimentation in south-central Alaska as refl ecting either initial collision of the Wrangellia and Peninsular terranes with the former continental margin of western North America or amalgamation of the two terranes prior to collision.

Upper Jurassic-Lower Cretaceous sedimentary strata of the Nutzotin basin, the Nutzotin Mountains sequence, crop out in the Nutzotin and Mentasta Mountains of the eastern Alaska Range. These strata represent one of the best-exposed and leastmetamorphosed examples of a basin that is interpreted to have formed during collision of an allochthonous volcanic arc (i.e., the Wrangellia terrane) with a continental margin. New stratigraphic, geologic mapping, and provenance data indicate that the Nutzotin basin formed as a retroarc foreland basin along the northern margin (present coordinates) of the Wrangellia terrane. Coeval with basin development along the northern margin, sedimentary basins and plutons located along the southern margin of the Wrangellia terrane were being incorporated into a regional fold-and-thrust belt. This fold-and-thrust belt, located south of the Nutzotin basin, exposed multiple structural levels of the Wrangellia terrane that were eroded and provided sediment that was transported northward and deposited in the Nutzotin basin. New sedimentologic and stratigraphic data from the ϳ3 km thick (minimum thickness) Nutzotin Mountains sequence define a three-part stratigraphy. The lower part consists of Upper Jurassic (Oxfordian to Tithonian) conglomerate with outsized limestone clasts (>10 m in diameter) and interbedded sandstone and shale that grade basinward into mainly black shale with minor micritic limestone and isolated lenses of conglomerate. The middle part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous (Valanginian) normal-graded sandstone and shale interbedded with massive tabular sandstone and lenticular conglomerate. The upper part of the stratigraphy consists of Upper Jurassic (Tithonian) to Lower Cretaceous

The Oligocene to Present Wrangell Volcanic Belt (WVB) extends for~500 km across south-central Alaska (USA) into Canada at a volcanic arc-transform junction.Previously, geochemistry documented mantle wedge and slab-edge melting in <12 Ma WVB volcanic rocks; new geochemistry shows that the same processes

Forearc basins are large sediment repositories that develop in the upper plate of convergent margins and are a direct response to subduction. These basins are part of the magmatic arc-forearc basin-accretionary prism "trinity" that defines the tectonic configuration of the upper plate along most subduction-related convergent margins. Many previous studies of forearc basins have explored the links between construction of magmatic arcs, exhumation of accretionary prisms, and sediment deposition in adjacent forearc basins. These studies provide an important framework for understanding firstorder tectonic processes recorded in forearc basins that are characterized by long-lived subduction of "normal" oceanic crust. Many convergent margins, however, are complicated by second-order subduction processes, such as flat-slab subduction of buoyant oceanic crust in the form of seamounts, spreading and aseismic ridges, and oceanic plateaus. These second-order processes can substantially modify the tectonic configuration of the upper plate both in time and space, and produce sedimentary basins that do not easily fit into the conventional magmatic arc-forearc basin-accretionary prism trinity.In this chapter, we discuss the modification of the southern Alaska forearc basin by Paleocene-Eocene subduction of a spreading ridge followed by OligoceneHolocene subduction of thick oceanic crust. This thick oceanic crust is currently being subducted beneath south-central Alaska and has an imaged maximum thickness of 30 km at the surface and 22 km at depth. Findings from southern Alaska suggest that forearc basins modified from flat-slab subduction processes may contain a sedimentary and volcanic stratigraphic record that differs substantially from typical forearc basins. Processes and sedimentary features that characterize modified forearc basins include the following: (1) flat-slab subduction of a buoyant, topographically elevated spreading ridge oriented subparallel to the margin prompts diachronous uplift of the forearc basin floor and exhumation of older marine forearc basin strata as the ridge is subducted. Passage of the spreading ridge leads to subsidence and renewed deposition of nonmarine sedimentary and volcanic strata that locally exceeds the thickness of the underlying marine strata. (2) Insertion of a slab window beneath the forearc basin during spreading ridge subduction produces local intrabasinal topographic highs with adjacent depocenters, as well as discrete volcanic centers within and adjacent to the forearc basin. (3) Flat-slab subduction of thick oceanic crust also results in surface uplift and exhumation of forearc basin sedimentary strata. However, the insertion of thick crust throughout the flat-slab region (i.e., lack of a slab window) inhibits subduction-related magmatism adjacent to the forearc basin. In the case of subduction of a >350-km-wide fragment of thick oceanic crust beneath south-central Alaska, exhumation of forearc basin strata located above the region of flat-slab subduction has prompted enhanced T...