The Alaska Range suture zone exposes Cretaceous to Quaternary marine
and nonmarine sedimentary and volcanic rocks sandwiched between oceanic
rocks of the accreted Wrangellia composite terrane to the south and older
continental terranes to the north. New U-Pb zircon ages, 40Ar/39Ar, ZHe, and
AFT cooling ages, geochemical compositions, and geological field observations
from these rocks provide improved constraints on the timing of Cretaceous to
Miocene magmatism, sedimentation, and deformation within the collisional
suture zone. Our results bear on the unclear displacement history of the seismically
active Denali fault, which bisects the suture zone. Newly identified
tuffs north of the Denali fault in sedimentary strata of the Cantwell Formation
yield ca. 72 to ca. 68 Ma U-Pb zircon ages. Lavas sampled south of the Denali
fault yield ca. 69 Ma 40Ar/39Ar ages and geochemical compositions typical of
arc assemblages, ranging from basalt-andesite-trachyte, relatively high-K, and
high concentrations of incompatible elements attributed to slab contribution
(e.g., high Cs, Ba, and Th). The Late Cretaceous lavas and bentonites, together
with regionally extensive coeval calc-alkaline plutons, record arc magmatism
during contractional deformation and metamorphism within the suture zone.
Latest Cretaceous volcanic and sedimentary strata are locally overlain by
Eocene Teklanika Formation volcanic rocks with geochemical compositions
transitional between arc and intraplate affinity. New detrital-zircon data from
the modern Teklanika River indicate peak Teklanika volcanism at ca. 57 Ma,
which is also reflected in zircon Pb loss in Cantwell Formation bentonites.
Teklanika Formation volcanism may reflect hypothesized slab break-off and
a Paleocene–Eocene period of a transform margin configuration. Mafic dike
swarms were emplaced along the Denali fault from ca. 38 to ca. 25 Ma based
on new 40Ar/39Ar ages. Diking along the Denali fault may have been localized
by strike-slip extension following a change in direction of the subducting
oceanic plate beneath southern Alaska from N-NE to NW at ca. 46–40 Ma.
Diking represents the last recorded episode of significant magmatism in the
central and eastern Alaska Range, including along the Denali fault. Two tectonic
models may explain emplacement of more primitive and less extensive Eocene–Oligocene magmas: delamination of the Late Cretaceous–Paleocene
arc root and/or thickened suture zone lithosphere, or a slab window created
during possible Paleocene slab break-off. Fluvial strata exposed just south
of the Denali fault in the central Alaska Range record synorogenic sedimentation
coeval with diking and inferred strike-slip displacement. Deposition
occurred ca. 29 Ma based on palynomorphs and the youngest detrital zircons.
U-Pb detrital-zircon geochronology and clast compositional data indicate the
fluvial strata were derived from sedimentary and igneous bedrock presently
exposed within the Alaska Range, including Cretaceous sources presently
exposed on the opposite (north) side of the fault. The provenance data may
indicate ~150 km or more of dextral offset of the ca. 29 Ma strata from inferred
sediment sources, but different amounts of slip are feasible.
Together, the dike swarms and fluvial strata are interpreted to record Oligocene
strike-slip movement along the Denali fault system, coeval with strike-slip
basin development along other segments of the fault. Diking and sedimentation
occurred just prior to the onset of rapid and persistent exhumation
ca. 25 Ma across the Alaska Range. This phase of reactivation of the suture
zone is interpreted to reflect the translation along and convergence of southern
Alaska across the Denali fault driven by highly coupled flat-slab subduction of
the Yakutat microplate, which continues to accrete to the southern margin of
Alaska. Furthermore, a change in Pacific plate direction and velocity at ca. 25 Ma
created a more convergent regime along the apex of the Denali fault curve, likely
contributing to the shutting off of near-fault extension- facilitated arc magmatism
along this section of the fault system and increased exhumation rates.
more »
« less
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
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.).
more »
« less
- Award ID(s):
- 1450730
- NSF-PAR ID:
- 10113436
- Date Published:
- Journal Name:
- Geosphere
- ISSN:
- 1553-040X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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
-
more » « less
Abstract The along‐strike variations of the velocity, thickness, and dip of subducting slabs and the volcano distribution have been observed globally. It is, however, unclear what controls the distribution of volcanoes and the associated magma generation. With the presence of nonuniform volcanism, the Aleutian‐Alaska subduction zone (AASZ) is an ideal place to investigate subduction segmentation and its relationship with volcanism. Using full‐wave ambient noise tomography, we present a high‐resolution 3‐D shear wave velocity model of the AASZ for the depths of 15–110 km. The velocity model reveals the distinct high‐velocity Pacific slab, the thicker, flatter, and more heterogeneous Yakutat slab, and the northeasterly dipping Wrangell slab. We observe low velocities within the uppermost mantle (at depth <60 km) below the Aleutian arc volcanoes, representing partial melt accumulation. The large crustal low‐velocity anomaly beneath the Wrangell volcanic field suggests a large magma reservoir, likely responsible for the clustering of volcanoes. The Denali volcanic gap is above an average‐velocity crust but an extremely fast mantle wedge, suggesting the lack of subsurface melt. This is in contrast with the lower‐velocity back‐arc mantle beneath the adjacent Buzzard Creek‐Jumbo Dome volcanoes to the east. The back‐arc low velocities associated with the Pacific, the eastern Yakutat, and the Wrangell slabs may reflect subduction‐driven mantle upwelling. The structural variation of the downgoing slabs and the overriding plate explains the change of volcanic activity along the AASZ. Our findings demonstrate the combined role of the subducting slab and the overriding plate in controlling the characteristics of arc magmatism.
-
more » « less
Abstract The Chisana Formation consists of Lower Cretaceous volcanic rocks that occur in the Nutzotin Mountains of eastern Alaska. New stratigraphic analysis indicates that the volcanic succession is >2 km thick at the Bonanza Creek type section. We present stratigraphic, geochemical, Sr‐Nd‐Pb isotope, and U‐Pb age data from samples collected from various stratigraphic levels of the Chisana Formation. We demonstrate that the Chisana Formation can be divided into a lower subaqueous unit, a middle transitional unit, and an upper subaerial unit. Chisana Formation lavas range from transitional to subalkaline basalts through andesites. Trace element geochemistry shows high field strength element depletions relative to large ion lithophile elements and hydrous mineral assemblages with calc‐alkaline to tholeiitic chemistries, all consistent with a magmatic arc origin. Chisana lavas yield geochemical compositions and isotope characteristics that overlap with magmas from volcanic suites formed within juvenile continental crust and immature island arcs. Volcanism occurred between ~131 and 117 Ma judging from previously reported lava ages and new U‐Pb ages of detrital zircons recovered from sandstones that conformably underlie the lowermost Chisana Formation lavas. Our results support existing tectonic models in which an east dipping subduction zone existed beneath Wrangellia during Early Cretaceous time. The upsection shift from marine to terrestrial depositional conditions in the Chisana Formation and the overlying ~117–93 Ma Beaver Lake Formation was coincident with regional shortening. Together, the geologic evidence for shortening and terrestrial deposition are interpreted to reflect accretion/suturing of Wrangellia against inboard terranes.
-
more » « less
Abstract 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 MaWVB volcanic rocks; new geochemistry shows that the same processes characterized ~18–30 MaWVB magmatism in Alaska. New40Ar/39Ar ages demonstrate thatWVB magmatism in Alaska initiated at ~30 Ma due to flat‐slab subduction of the Yakutat microplate and that the dextral Totschunda fault was active at this time. Our results, together with prior studies, show that AlaskanWVB magmatism occurred chiefly due to subduction and should be considered a volcanic arc (e.g. the Wrangell Arc). TheWVB provides a long‐term geological record of subduction, strike‐slip and magmatism. Slab‐edge upwelling, flat‐slab defocused fluid‐flux and faults acting as magma conduits are likely responsible for the exceptionally large volcanoes and high eruption rates of the Wrangell Arc.
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/GES02114.1
