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.
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Cretaceous to Miocene magmatism, sedimentation, and exhumation within the Alaska Range suture zone: A polyphase reactivated terrane boundary
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.
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- NSF-PAR ID:
- 10099654
- Date Published:
- Journal Name:
- Geosphere
- Volume:
- v.15
- Issue:
- 3
- ISSN:
- 1553-040X
- Page Range / eLocation ID:
- 800-835
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/GES02014.1
