Geochemical and geochronological records of tectonic changes along a flat-slab arc-transform junction: Circa 30 Ma to ca. 19 Ma Sonya Creek volcanic field, Wrangell Arc, Alaska

Abstract

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.).

Geochemical and Stratigraphic Analysis of the Chisana Formation, Wrangellia Terrane, Eastern Alaska: Insights Into Early Cretaceous Magmatism and Tectonics Along the Northern Cordilleran Margin

“…(b) Island arcs—Talkeetna arc, Alaska (Clift, Draut, et al, 2005); Japanese Honshu arc ( n = 181, GEOROC database); Izu‐Bonin arc ( n = 80; Tollstrup et al, 2010). (c) Continental arcs—Wrangell Arc, Alaska (Berkelhammer et al, 2019; Brueseke et al, 2019; Preece & Hart, 2004); High Cascades ( n = 874, GEOROC database). (d) Igneous rocks from the East African rift (Giordano et al, 2014) and Marianas backarc ( n = 61, Pearce et al, 2005).…”

“…Olson et al (2017) interpret these data to reflect interaction of subduction-derived melts produced in a volcanic arc, with Kahiltna sediment. Like the Wrangell Arc (Berkelhammer et al, 2019;Brueseke et al, 2019), Chisana magmas erupted through the Insular terranes and this is reflected by Devonian to Jurassic Nd depleted-mantle model ages from the White Mountain intrusive body (Snyder & Hart, 2007), and the continental lithosphere contribution implied by Chisana LILE abundances (Figure 7). Brueseke et al, 2019).…”

Detrital zircon geochronology and Hf isotope geochemistry of Mesozoic sedimentary basins in south-central Alaska: Insights into regional sediment transport, basin development, and tectonics along the NW Cordilleran margin

“…This estimate is generally consistent with recent studies suggesting ~85 km of offset since ca. 18 Ma (Berkelhammer et al, 2019), although some studies postulate that much of this offset could have occurred since ca. 6 Ma (Allen et al, 2018).…”

Detrital zircon geochronology of modern river sediment in south-central Alaska: Provenance, magmatic, and tectonic insights into the Mesozoic and Cenozoic development of the southern Alaska convergent margin

“…2); magmatism in this belt is interpreted to have begun ca. 30 Ma and has been relatively continuous since then (Richter et al, 1990;Preece and Hart, 2004;Berkelhammer et al, 2019;Brueseke et al, 2019). Zircons younger than 30 Ma, however, are relatively sparse in our detrital record from modern trunk rivers (Fig.…”

“…The convergent margin of southern Alaska includes several of the highest mountain ranges in North America, two active volcanic arc systems, and several ancient and active sedimentary basins, making it an ideal location to study the distribution of detrital zircon age signals in modern rivers across an active plate boundary. The upper plate of this convergent margin also has a punctuated Phanerozoic history of magmatism that is related to terrane accretion, spreading ridge subduction, and flat-slab subduction (Moll-Stalcup, 1994;Plafker and Berg, 1994;Bradley et al, 2003;Cole et al, 2006Cole et al, , 2007Trop and Ridgway, 2007;Finzel et al, 2011;Berkelhammer et al, 2019;Brueseke et al, 2019). In this study, we utilize detrital zircons from modern trunk rivers as recorders of the history of Mesozoic and Cenozoic felsic magmatism that occurred during the tectonic development of this margin.…”