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Long-lived continental magmatic arcs may migrate large (hundreds of kilometers) trench perpendicular distances as convergent margin configurations and slab geometry vary over time; however, many arc-magmatic belts are spatially localized over tens of millions of years.We document, by compiling published crystallization geochronology data for southern Alaska (6,485 total bedrock and single-grain detrital ages combined), that since ca. 100 Ma, arc magmatism has been localized along the Alaska Range suture zone (in places within a 10km × 5km swath) and at times over 500km inboard. However, since ca. 100 Ma, incoming subducting slab characteristics and convergent margin configurations varied greatly and include both normal oceanic plate and oceanic plateau subduction, plate vector changes, oroclinal bending and reconfiguration of trench shape, terrane accretion, long-distance terrane translation, and a Paleocene slab break off/slab window event. Therefore, it is inferred that inherited upper-plate lithospheric thickness and thermal variations must control in part the geometry of the subducting slab below a mobile southern Alaskan margin through hydrodynamic mantle wedge “suction” forces. Additionally, crustal thickness heterogeneity may focus magma ascent through melt ponding along Moho offsets, and upper-plate lithospheric-scale strike-slip faults may be acting as passive and active conduits for arc magmatism. 

Southern Alaska is a collage of accreted terranes. The deformation history of accreted terranes and the geometric history of their bounding faults reflect both inherited features and associated convergent margin events. We employ S-to-P receiver functions on multiple dense (<20km spacing) arrays of broadband seismometers across southern Alaska to investigate signals of dynamic tectonic activity. An inboard-dipping (∼15∘) boundary is imaged aligning with the trace of the Border Ranges Fault, which is interpreted as an unrotated inboard-dipping paleo-subduction (Mesozoic) interface. This observation, along with previous seismic imaging of the Border Ranges Fault and the next outboard terrane-bounding fault, the Contact Fault, buttresses a known history of convergent tectonics that varies along the margin. Large (>10 km) crustal thickness offsets imaged across both the Denali Fault system and the Eureka Creek Fault support a Mesozoic-to-Present inboard-dipping (east and northward) subduction polarity in the region. Additionally, our imaging reveals a significant velocity increase with depth at ∼25km beneath the Copper River Basin, which we interpret as the top of a region of active underplating and/or intrusion of basaltic magmatism. This feature may be related to the generation of a newWrangell Volcanic Field volcano, resulting from the underlying tear in the subducting slab. 

Changes in magmatism and sedimentation along the late Neoproterozoic-early Paleozoic Ross orogenic belt in Antarctica have been linked to the cessation of convergence along the Mozambique belt during the assembly of East-West Gondwana. However, these interpretations are non-unique and are based, in part, on limited thermochronological data sets spread out along large sectors of the East Antarctic margin. We report new 40Ar/39Ar hornblende, muscovite, and biotite age data for plutonic (n = 13) and metasedimentary (n = 3) samples from the Shackleton–Liv Glacier sector of the Queen Maud Mountains in Antarctica. Cumulative 40Ar/39Ar age data show polymodal age peaks (510 Ma, 491 Ma, 475 Ma) that lag peaks in U-Pb igneous crystallization ages, suggesting igneous and metamorphic cooling following magmatism within the region. The 40Ar/39Ar ages are similar to ages in other sectors of the Ross orogen, but younger than detrital mineral 40Ar/39Ar cooling ages indicative of older magmatism and cooling of unexposed inboard areas along the margin. Detrital zircon trace element abundances suggest that the widespread onset of magmatism in outboard localities of the orogen correlates with a ~560–530 Ma decrease in crustal thickness. The timing of crustal thinning recorded by zircon in magmas overlaps with other evidence for the timing of crustal extension, suggesting that the regional onset of magmatism with subsequent igneous and metamorphic cooling probably reflects slab rollback that coincided with possible global plate motion changes induced during the final assembly of Gondwana. 

Abstract Terrane accretion forms lithospheric-scale fault systems that commonly experience long and complex slip histories. Unraveling the evolution of these suture zone fault systems yields valuable information regarding the relative importance of various upper crustal structures and their linkage through the lithosphere. We present new bedrock geologic mapping and geochronology data documenting the geologic evolution of reactivated shortening structures and adjacent metamorphic rocks in the Alaska Range suture zone at the inboard margin of the Wrangellia composite terrane in the eastern Alaska Range, Alaska, USA. Detrital zircon uranium-lead (U-Pb) age spectra from metamorphic rocks in our study area reveal two distinct metasedimentary belts. The Maclaren schist occupies the inboard (northern) belt, which was derived from terranes along the western margin of North America during the mid- to Late Cretaceous. In contrast, the Clearwater metasediments occupy the outboard (southern) belt, which was derived from arcs built on the Wrangellia composite terrane during the Late Jurassic to Early Cretaceous. A newly discovered locality of Alaska-type zoned ultramafic bodies within the Clearwater metasediments provides an additional link to the Wrangellia composite terrane. The Maclaren and Clearwater metasedimentary belts are presently juxtaposed by the newly identified Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone, the Late Cretaceous plate boundary that initially brought them together. 40Ar/39Ar mica ages reveal independent post-collisional thermal histories of hanging wall and footwall rocks until reactivation localized on the Valdez Creek fault after ca. 32 Ma. Slip on the Valdez Creek fault expanded into a thrust system that progressed southward to the Broxson Gulch fault at the southern margin of the suture zone and eventually into the Wrangellia terrane. Detrital zircon U-Pb age spectra and clast assemblages from fault-bounded Cenozoic gravel deposits indicate that the thrust system was active during the Oligocene and into the Pliocene, likely as a far-field result of ongoing flat-slab subduction and accretion of the Yakutat microplate. The Valdez Creek fault was the primary reactivated structure in the suture zone, likely due to its linkage with the reactivated boundary zone between the Wrangellia composite terrane and North America in the lithospheric mantle. 

The Ross orogenic belt in Antarctica is one of several Neoproterozoic-early Palaeozoic orogens that crisscrossed Gondwana and are associated with Gondwana’s assembly. We present new age data from the Queen Maud Mountains, Ross orogen, from areas that hitherto have lacked precise ages from the local plutonic rocks. The zircon U-Pb igneous crystallization ages (n = 7) and a hornblende 40Ar/39Ar cooling age (n = 1) constrain plutonism to primarily lie within the Cambrian to Ordovician. Cumulative zircon U-Pb crystallization age data yield polymodal age distributions (516 Ma, 506–502 Ma, and 488 Ma age peaks) that are similar to other areas of the Queen Maud-Horlick Mountains, consistent with regional magmatic flare-ups along the Pacific-Gondwana margin during these times. The ages of deformed plutons constrain deformation to the Cambrian (Series 2) to Ordovician (Lower), with some regions indicating a transition to post-tectonic magmatism and cooling at ~509-470 Ma. Collectively, the data indicate that the Queen Maud-Horlick Mountains share a similar petrotectonic history with other regions of the Pacific-Gondwana margin, providing new evidence that this tectonostratigraphic province is part of and not exotic to the larger igneous-sedimentary successions developed in the peri-Gondwana realm under a broadly convergent margin setting. 

The Nutzotin basin of eastern Alaska consists of Upper Jurassic through Lower Cretaceous siliciclastic sedimentary and volcanic rocks that depositionally overlie the inboard margin of Wrangellia, an accreted oceanic plateau. We present igneous geochronologic data from volcanic rocks and detrital geochronologic and paleontological data from nonmarine sedimentary strata that provide constraints on the timing of deposition and sediment provenance. We also report geochronologic data from a dike injected into the Totschunda fault zone, which provides constraints on the timing of intra–suture zone basinal deformation. The Beaver Lake formation is an important sedimentary succession in the northwestern Cordillera because it provides an exceptionally rare stratigraphic record of the transition from marine to nonmarine depositional conditions along the inboard margin of the Insular terranes during mid-Cretaceous time. Conglomerate, volcanic-lithic sandstone, and carbonaceous mudstone/shale accumulated in fluvial channel-bar complexes and vegetated overbank areas, as evidenced by lithofacies data, the terrestrial nature of recovered kerogen and palynomorph assemblages, and terrestrial macrofossil remains of ferns and conifers. Sediment was eroded mainly from proximal sources of upper Jurassic to lower Cretaceous igneous rocks, given the dominance of detrital zircon and amphibole grains of that age, plus conglomerate with chiefly volcanic and plutonic clasts. Deposition was occurring by ca. 117 Ma and ceased by ca. 98 Ma, judging from palynomorphs, the youngest detrital ages, and ages of crosscutting intrusions and underlying lavas of the Chisana Formation. Following deposition, the basin fill was deformed, partly eroded, and displaced laterally by dextral displacement along the Totschunda fault, which bisects the Nutzotin basin. The Totschunda fault initiated by ca. 114 Ma, as constrained by the injection of an alkali feldspar syenite dike into the Totschunda fault zone. These results support previous interpretations that upper Jurassic to lower Cretaceous strata in the Nutzotin basin accumulated along the inboard margin of Wrangellia in a marine basin that was deformed during mid-Cretaceous time. The shift to terrestrial sedimentation overlapped with crustal-scale intrabasinal deformation of Wrangellia, based on previous studies along the Lost Creek fault and our new data from the Totschunda fault. Together, the geologic evidence for shortening and terrestrial deposition is interpreted to reflect accretion/suturing of the Insular terranes against inboard terranes. Our results also constrain the age of previously reported dinosaur footprints to ca. 117 Ma to ca. 98 Ma, which represent the only dinosaur fossils reported from eastern Alaska. 

The Sonya Creek volcanic field (SCVF) contains the oldest in situ volcanic products in the ca. 30 Ma–modern Wrangell Arc (WA) in south-central Alaska, which commenced due to Yakutat microplate subduction initiation. The WA occurs within a transition zone between Aleutian subduction to the west and dextral strike-slip tectonics along the Queen Charlotte–Fairweather and Denali–Duke River fault systems to the east. New 40Ar/39Ar geochronology of bedrock shows that SCVF magmatism occurred from ca. 30–19 Ma. New field mapping, physical volcanology, and major- and trace-element geochemistry, coupled with the 40Ar/39Ar ages and prior reconnaissance work, allows for the reconstruction of SCVF magmatic evolution. Initial SCVF magmatism that commenced at ca. 30 Ma records hydrous, subduction-related, calc-alkaline magmatism and also an adakite-like component that we interpret to represent slab-edge melting of the Yakutat slab. A minor westward shift of volcanism within the SCVF at ca. 25 Ma was accompanied by continued subduction-related magmatism without the adakite-like component (i.e., mantle-wedge melting), represented by ca. 25–20 Ma basaltic-andesite to dacite domes and associated diorites. These eruptions were coeval with another westward shift to anhydrous, transitional-tholeiitic, basaltic-andesite to rhyolite lavas and tuffs of the ca. 23–19 Ma Sonya Creek shield volcano; we attribute these eruptions to intra-arc extension. SCVF activity was also marked by a small southward shift in volcanism at ca. 21 Ma, characterized by hydrous calc-alkaline lavas. SCVF geochemical compositions closely overlap those from the <13 Ma WA, and no alkaline lavas that characterize the ca. 18–10 Ma eastern Wrangell volcanic belt exposed in Yukon Territory are observed. Calc-alkaline, transitional-tholeiitic, and adakite-like SCVF volcanism from ca. 30–19 Ma reflects subduction of oceanic lithosphere of the Yakutat microplate beneath North America. We suggest that the increase in magmatic flux and adakitic eruptions at ca. 25 Ma, align with a recently documented change in Pacific plate direction and velocity at this time and regional deformation events in southern Alaska. By ca. 18 Ma, SCVF activity ceased, and the locus of WA magmatism shifted to the south and east. The change in relative plate motions would be expected to transfer stress to strike-slip faults above the inboard margin of the subducting Yakutat slab, a scenario consistent with increased transtensional-related melting recorded by the ca. 23–19 Ma transitional-tholeiitic Sonya Creek shield volcano between the Denali and Totschunda faults. Moreover, we infer the Totschunda fault accommodated more than ~85 km of horizontal offset since ca. 18 Ma, based on reconstructing the initial alignment of the early WA (i.e., 30–18 Ma SCVF) and temporally and chemically similar intrusions that crop out to the west on the opposite side of the Totschunda fault. Our results from the SCVF quantify spatial-temporal changes in deformation and magmatism that may typify arc-transform junctions over similar time scales (>10 m.y.).