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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Large-scale, crustal-block vertical extrusion between the Hines Creek and Denali faults coeval with slip localization on the Denali fault since ca. 45 Ma, Hayes Range, Alaska, USA
Oblique convergence along strike-slip faults can lead to both distributed and localized deformation. How focused transpressive deformation is both localized and maintained along sub-vertical wrench structures to create high topography and deep exhumation warrants further investigation. The high peak region of the Hayes Range, central Alaska, USA, is bound by two lithospheric scale vertical faults: the Denali fault to the south and Hines Creek fault to the north. The high topography area has peaks over 4000 m and locally has experienced more than 14 km of Neogene exhumation, yet the mountain range is located on the convex side of the Denali fault Mount Hayes restraining bend, where slip partitioning alone cannot account for this zone of extreme exhumation. Through the application of U-Pb zircon, 40Ar/39Ar (hornblende, muscovite, biotite, and K-feldspar), apatite fission-track, and (U-Th)/He geo-thermochronology, we test whether these two parallel, reactivated suture zone structures are working in tandem to vertically extrude the Between the Hines Creek and Denali faults block on the convex side of the Mount Hayes restraining bend. We document that since at least 45 Ma, the Denali fault has been bent and localized in a narrow fault zone (<160 m) with a significant dip-slip component, the Mount Hayes restraining bend has been fixed to the north side of the Denali fault, and that the Between the Hines Creek and Denali faults block has been undergoing vertical extrusion as a relatively coherent block along the displacement “free faces” of two lithospheric scale suture zone faults. A bent Denali fault by ca. 45 Ma supports the long-standing Alaska orocline hypothesis that has Alaska bent by ca. 44 Ma. Southern Alaska is currently converging at ~4 mm/yr to the north against the Denali fault and driving vertical extrusion of the Between the Hines Creek and Denali faults block and deformation north of the Hines Creek fault. We apply insights ascertained from the Between the Hines Creek and Denali faults block to another region in southern Alaska, the Fairweather Range, where extreme topography and persistent exhumation is also located between two sub-parallel faults, and propose that this region has likely undergone vertical extrusion along the free faces of those faults.
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- NSF-PAR ID:
- 10321106
- Date Published:
- Journal Name:
- Geosphere
- ISSN:
- 1553-040X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Topography along strike‐slip fault restraining bends is theoretically self‐limited by erosion, block translation and the expected abandonment of fault bends. However, Denali (6,194 m) and Foraker (5,304 m) are located within a restraining bend of the dextral Denali Fault system. We reveal the role of bend evolution in mountain building with physical experiments scaled to simulate the Alaska Mount McKinley restraining bend (MMRB). Despite the natural complexity of the MMRB, first‐order patterns (of strike‐slip separation rates, uplift and lateral bend migration) from the geologic data align with patterns from scaled experiments. Thermochronology, seismicity, and slip rate data show that the persistence of a single Denali Fault strand through the ~6 Ma MMRB is facilitated by simultaneous advection of crust through the bend and migration of the eastern vertex of the bend.
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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
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Scaled physical experiments allow us to directly observe deformational processes that take place on time and length scales that are impossible to observe in the Earth’s crust. Successful evaluation of advection and uplift of material within a restraining bend along a strike-slip fault zone depends on capturing the evolution of strain in three dimensions. Consequently, we require deformation within the horizontal plane as well as vertical motions. While 3D digital image correlation systems can provide this information, their high costs have prompted us to develop techniques that require only two DSLR cameras and a few Matlab® toolboxes, which are available to researchers at many institutions. Matlab® plug-ins can perform particle image velocimetry (PIV), a technique used in many analog modeling studies to map the incremental displacements fields. For tracking material advection throughout experiments more suitable Matlab® plug-ins perform particle tracking velocimetry (PTV), which tracks the complete two-dimensional displacement path of individual particles. To capture uplift the Matlab® Computer Vision ToolboxTM, uses pairs of photos to capture the evolving topography of the experiment. The stereovision approach eliminates the need to stop the experiment to perform 3D laser scans, which can be problematic when working with materials that have time dependent rheology. We demonstrate how the combination of PIV, PTV, and stereovision analysis of experiments that simulate the Mount McKinley restraining bend reveal the evolution of the fault system and three-dimensional advection of material through the bend.more » « less
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Abstract The crustal structure in south‐central Alaska has been influenced by terrane accretion, flat slab subduction, and a modern strike‐slip fault system. Within the active subduction system, the presence of the Denali Volcanic Gap (DVG), a ∼400 km region separating the active volcanism of the Aleutian Arc to the west and the Wrangell volcanoes to the east, remains enigmatic. To better understand the regional tectonics and the nature of the volcanic gap, we deployed a month‐long north‐south linear geophone array of 306 stations with an interstation distance of 1 km across the Alaska Range. By calculating multi‐component noise cross‐correlation and jointly inverting Rayleigh wave phase velocity and ellipticity across the array, we construct a 2‐D shear wave velocity model along the transect down to ∼16 km depth. In the shallow crust, we observe low‐velocity structures associated with sedimentary basins and image the Denali fault as a narrow localized low‐velocity anomaly extending to at least 12 km depth. About 12 km, below the fold and thrust fault system in the northern flank of the Alaska Range, we observe a prominent low‐velocity zone with more than 15% velocity reduction. Our velocity model is consistent with known geological features and reveals a previously unknown low‐velocity zone that we interpret as a magmatic feature. Based on this feature's spatial relationship to the Buzzard Creek and Jumbo Dome volcanoes and the location above the subducting Pacific Plate, we interpret the low‐velocity zone as a previously unknown subduction‐related crustal magma reservoir located beneath the DVG.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/GES02466.1
