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.).
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Tectonic drivers of the Wrangell block: Insights on fore-arc sliver processes from 3-D geodynamic models of Alaska: TECTONIC DRIVERS OF THE WRANGELL BLOCK
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Abstract. Carbonate rocks are highly reactive and can have higher ratios of chemical weathering to total denudation relative to most other rock types. Their chemical reactivity affects the first-order morphology of carbonate-dominated landscapes and their climate sensitivity to weathering.However, there have been few efforts to quantify the partitioning ofdenudation into mechanical erosion and chemical weathering in carbonatelandscapes such that their sensitivity to changing climatic and tectonicconditions remains elusive. Here, we compile bedrock and catchment-averagedcosmogenic calcite–36Cl denudation rates and compare them to weathering rates derived from stream water chemistry from the same regions. Local bedrock denudation and weathering rates are comparable, ∼20–40 mm ka−1, whereas catchment-averaged denudation rates are ∼2.7 times higher. The discrepancy between bedrock and catchment-averaged denudation is 5 times lower compared to silicate-rich rocks, illustrating that elevated weathering rates make denudation more spatially uniform in carbonate-dominated landscapes. Catchment-averaged denudation rates correlate well with topographic relief and hillslope gradients, and moderate correlations with runoff can be explained by concurrent increases in weathering rates. Comparing denudation rates with weathering rates shows that mechanical erosion processes contribute ∼50 % of denudation in southern France and ∼70 % in Greece and Israel. Our results indicate that the partitioning between largely slope-independent chemical weathering and slope-dependent mechanical erosion varies based on climate and tectonics and impacts the landscape morphology. This leads us to propose a conceptual model whereby in humid, slowly uplifting regions, carbonates are associated with low-lying, flat topography because slope-independent chemical weathering dominates denudation. In contrast, in arid climates with rapid rock uplift rates, carbonate rocks form steep mountains that facilitate rapid, slope-dependent mechanical erosion required to compensate for inefficient chemical weathering and runoff loss to groundwater systems. This result suggests that carbonates represent an end member for interactions between climate, tectonics, and lithology.more » « less
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Abstract The Wrangell Arc in Alaska (USA) and adjacent volcanic fields in the Yukon provide a long-term record of interrelations between flat-slab subduction of the Yakutat microplate, strike-slip translation along the Denali–Totschunda–Duke River fault system, and magmatism focused within and proximal to a Cretaceous suture zone. Detrital zircon (DZ) U-Pb (n = 2640) and volcanic lithic (DARL) 40Ar/39Ar dates (n = 2771) from 30 modern river sediment samples document the spatial-temporal evolution of Wrangell Arc magmatism, which includes construction of some of the largest Quaternary volcanoes on Earth. Mismatches in DZ and DARL date distributions highlight the impact of variables such as mineral fertility and downstream mixing/dilution on resulting provenance signatures. Geochronologic data document the initiation of Wrangell Arc magmatism at ca. 30–17 Ma along both sides of the Totschunda fault on the north flank of the Wrangell–St. Elias Mountains in Alaska, followed by southeastward progression of magmatism at ca. 17–10 Ma along the Duke River fault in the Yukon. This spatial-temporal evolution is attributable to dextral translation along intra-arc, strike-slip faults and a change in the geometry of the subducting slab (slab curling/steepening). Magmatism then progressed generally westward outboard of the Totschunda and Duke River faults at ca. 13–6 Ma along the southern flank of the Wrangell–St. Elias Mountains in Alaska and then northwestward from ca. 6 Ma to present in the western Wrangell Mountains. The 13 Ma to present spatial-temporal evolution is consistent with dextral translation along intra-arc, strike-slip faults and previously documented changes in plate boundary conditions, which include an increase in plate convergence rate and angle at ca. 6 Ma. Voluminous magmatism is attributed to shallow subduction-related flux melting and slab edge melting that is driven by asthenospheric upwelling along the lateral edge of the Yakutat flat slab. Magmatism was persistently focused within or adjacent to a remnant suture zone, which indicates that upper plate crustal heterogeneities influenced arc magmatism. Rivers sampled also yield subordinate Paleozoic–Mesozoic DZ and DARL age populations that reflect earlier episodes of magmatism within underlying accreted terranes and match magmatic flare-ups documented along the Cordilleran margin.more » « less
