Denali Fault Research Articles

Orogen-scale inverted metamorphism during Cretaceous−Paleogene terminal suturing along the North American Cordillera, Alaska, USA

The northern North American Cordilleran margin has been active for >200 million years, as recorded by punctuated phases of crustal growth and deformation. Accretion of the exotic Wrangellia Composite Terrane (Insular Belt) is considered the largest addition of juvenile crust to the Cordilleran margin, though margin-parallel translation during the Cenozoic has obscured much of the accretionary history. Three zones of inverted metamorphism spatially correspond to the Insular−North American suture zone from north to south: (1) Clearwater Mountains; (2) Kluane Lake; and (3) Coast Mountains, each preserving kinematics indicative of thrusting of North American−derived rocks over Insular-derived assemblages. We performed in situ monazite petrochronology on samples collected across strike in both the Clearwater and Coast Mountain regions. New and recently published data from these three metamorphic belts indicate that thrust-sense deformation accompanied the formation of inverted metamorphic isograds from 72 to 56 Ma. We leverage recent estimates of Denali fault offset to reconstruct a >1000-km-long zone of inverted metamorphism and interpret it as the Insular−North America terminal suture.

Separation of Intrinsic and Scattering Seismic Wave Attenuation in the Crust of Central and South-Central Alaska

ABSTRACT Seismic hazard analysis is essential for evaluating the potential consequences and dangers linked to earthquakes, particularly in areas with regular seismic activity such as central and south-central Alaska. A detailed study of attenuation can help in better defining the wave behavior and so refine the ground-motion prediction. Here, we examined the scattering (Qs−1), intrinsic (Qi−1), and coda-wave (Qc−1) attenuation in central and south-central Alaska. To do so, we performed the multiple lapse time-window analysis (MLTWA) techniques and estimated the coda energy decay. We considered earthquakes that occurred between December 2014 and December 2020, with magnitudes between 2.0 and 6.5. We observed significant spatial variations in scattering loss (Qs−1) up to 3 Hz, which diminish at 6 and 12 Hz. The Wrangell block exhibits the most significant scattering loss at a frequency of 1.5 Hz. Another area of marked scattering loss was identified north of the Alaska Range (AR), where it was pronounced up to 6 Hz. The area around Anchorage registered the lowest intrinsic absorption across all the central frequencies, whereas the highest values were detected north of the AR, particularly at 3 and 6 Hz. The seismic albedo (B0) in central and south-central Alaska varies spatially and is mainly dominated by scattering loss up to 3 Hz. Both the Chugach mountains and Yakutat block (YB) area exhibit lower B0 values at all central frequencies showing the dominance of intrinsic absorption. Low values of Qc (high attenuation) are focused almost on all the frequencies along the Denali fault and YB, showing a strong influence of these structures on the attenuation. The results yield a comprehensive understanding of the unique attenuation characteristics of each region, underscoring the significance of investigating the behavior of seismic wave attenuation for seismic risk purposes.

Earthquake Relocations Delineate a Discrete Fault Network and Deformation Corridor Throughout Southeast Alaska and Southwest Yukon

AbstractDeformation in southeastern Alaska and southwest Yukon is governed by the subduction and translation of the Pacific‐Yakutat plates relative to the North American plate in the St. Elias region. Despite notable historical seismicity and major regional faults, studies of the region between the Fairweather and Denali faults are complicated by glacial coverage and the remote setting. In the last decade, significant improvements have been made to the density of regional broadband seismometer networks. We relocate more than 5,000 earthquakes between 2010 and 2021 in the region of southeastern Alaska and southwestern Yukon utilizing these improved seismic networks. With reductions in catalog uncertainty, particularly in depth, we quantify the thickness of the seismogenic layer in the crust throughout the region and locate seismicity on a shallow network of upper‐crustal faults. Relocated earthquakes, combined with an updated focal‐mechanism catalog, permit estimating and classifying motion of active faults. This includes mapping the Totschunda‐Fairweather “Connector” fault, which plays an important role in explaining regional deformation, and identifying new faults like the Kathleen Lake fault. We draw similarities between our seismic observations and simplified conceptual models of regional tectonics, which describe a dominant transpressional regime and localized slip partitioning. Our results support a hypothesis where current deformation is taking place on a well‐defined and evolved network of shallow faults in the corridor between the Totschunda‐Fairweather “Connector” and Denali faults.

The Crustal Magmatic Structure Beneath the Denali Volcanic Gap Imaged by a Dense Linear Seismic Array

AbstractThe 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.

Suture Reactivation, Slip Partitioning, and a Protracted Strike‐Slip Rate Gradient in the Denali Fault System, Southern Alaska, USA

AbstractActive strike‐slip fault systems commonly display along‐strike Quaternary slip rate gradients associated with fault bends and splay faults, which generate surface uplift by dip‐slip faulting or distributed “off fault” deformation. By analogy, the documentation of long‐term (107 yr) slip gradients on some continental strike‐slip fault systems implies long‐term coevolution of strike‐slip and dip‐slip fault systems. Here we leverage the observed ≥33 Myr right‐lateral slip gradient on the Denali fault, Alaska, USA to investigate the role of splay thrust systems in accommodating the slip gradient. We focus on the Broxson Gulch thrust system, which splays southwestward from the Denali fault in the eastern Alaska Range. Apatite and zircon (U‐Th)/He and fission‐track cooling ages from metasedimentary and metaplutonic rocks intersected by the thrust system record an along‐strike decrease in cooling ages commensurate with an increase in late Oligocene‐Neogene bedrock exhumation and shortening with proximity to the Denali fault. The dominant structure in the Broxson Gulch thrust system is the Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone–the main Late Cretaceous suture between western North America and outboard accreted arc terranes. After reactivation of the Valdez Creek shear zone at ca. 30 Ma, the thrust system grew by south‐vergent imbrication of the upper crust along thrust and reverse faults until at least 6 Ma. Incorporating results from the Broxson Gulch thrust system into the regional structural evolution of the Denali fault system reveals significant spatiotemporal heterogeneity in shortening adjacent to the Denali fault. Moreover, nearly all of the late Oligocene‐Neogene shortening south of the Denali fault was focused along reactivated terrane boundaries inherited from Mesozoic assembly of the North American Cordillera, and the spatial distribution of the inherited structures appears to control slip partitioning behavior of the Denali fault system across time scales ranging from 101 (historic seismicity) to 107 yr. The slip partitioning behavior of the Denali fault system highlights the mechanical importance of inherited structures leading to protracted shortening on splay thrust systems, which siphon slip from the master strike‐slip fault. We contend that the weakness of nearby reactivated terrane boundaries should be considered among other mechanisms commonly evoked to explain the partitioning behavior of continental strike‐slip fault systems (e.g., stress field rotation, obliquity angle, and strength of master strike‐slip fault).

Roadblocks and speed limits: Mantle-to-surface volatile flux through the lithospheric-scale Denali fault, Alaska

Abstract Helium and carbon isotopic data from 12 springs along a ~400 km segment of the Denali fault system (Alaska, USA), inform mantle-to-surface connections of this enigmatic structure. Warm springs on the main strand, west of the 2002 M7.9 Denali fault earthquake rupture, have 3He/4He as high as 2.4 RC/RA (air-corrected 3He/4He relative to air ratio) indicating ~30% mantle He. Corresponding δ13C values are −9.1‰ to −7.8‰ (relative to Vienna Peedee belemnite), suggesting that the CO2 at these western springs is partially mantle derived. At the eastern end of the 2002 rupture, Totschunda fault springs have 3He/4He of 0.65–0.99 RC/RA (~8%–12% mantle He), with δ13C values (~0‰) from carbonates. Results confirm the Denali fault system is a lithosphericscale feature tapping mantle volatiles. Springs along the 2002 rupture yield air-like 3He/4He of 1 R/RA and δ13C values from −9.2‰ to −3.4‰, interpreted as representing shallow groundwater circulation through shales and carbonates without mantle contributions. A thrust splay parallel to the rupture zone has air-like 3He/4He, whereas an along-strike high-angle normal splay yields 3He/4He of 1.3 RC/RA (~16% mantle He), implying that flow paths along the ruptured strand are disrupted in the upper 10 km of the fault zone. Because the Denali fault is a lithospheric-scale, transcurrent structure separating North America from independently moving southern Alaska, we suggest that it has characteristics of a transform boundary. The similarity of our results to helium isotope values at analogous tectonic settings suggest that without magmatism influence, there is a maximum mantle fluid flux through continental strike-slip faults.

Rapid active thrust faulting at the northern Alaska Range front

AbstractPlate convergence rates strongly influence seismicity and mountain building inboard of convergent margins, but the distribution and kinematics of structures accommodating farfield convergence can be elusive. In interior Alaska, Yakutat microplate convergence drives late Pleistocene–recent right slip on the Denali fault, but westward-decreasing slip rates leave substantial residual Yakutat motion unaccounted for. Here, we show that Northern Foothills thrust slip beneath the northern Alaska Range absorbs a modern 4.4 mm/yr geodetic velocity gradient equivalent to ~78% of the 5.6 mm/yr residual Yakutat convergence along the central Denali fault. Infrared-stimulated luminescence ages of strath terrace deposits (67–4 ka; six sites) quantify Totatlanika River bedrock incision across the 1947 Mw 7.1 thrust earthquake epicentral region. Incision rates increase abruptly from <1 mm/yr to 4.8–5.6 mm/yr above the blind thrust tip near the range front. Rapid slip at 6.7 mm/yr on a steep thrust ramp beneath the northern Alaska Range front accommodates the geodetic gradient, drives rock uplift at rates matching measured incision rates, and implies that large earthquakes like the 1947 event may recur with 500–1400 yr frequency. Results illuminate focused seismogenic strain inboard of a complex convergent margin and prompt reevaluation of Alaska’s neotectonic framework.

Basement structure of the Northern Foothills Fold and Thrust Belt and southern Fairbanks and Salcha, Alaska Seismic zones and their relation to large (M ≥ 6) earthquakes

We used seismic, gravity and aeromagnetic information to better define the basement structure of the southern Fairbanks and Salcha seismic zones and the Northern Foothills Fold and Thrust Belt (NFFTB), regions that accommodate deformation in interior Alaska north of the Denali fault. At least five earthquakes of M ≥ 6 occurred within this region between 1929 and 1947. Relocated epicenters of two foreshocks and the October 16, 1947 (Mw 7.2) mainshock suggest they occurred in a topographically low area covered with recent alluvial deposits located ∼20 km north of the northernmost mapped thrusts of the NFFTB, although the largest aftershock of the sequence occurred within the exposed NFFTB. The three largest events of the 1947 sequence had similar waveforms, indicating similar mechanisms. Relocations of the 1929 events, also with waveforms similar to the 1947 sequence, indicate they likely occurred along thrust faults associated with the exposed NFFTB. In contrast, the 1937 sequence (mainshock of Mw 7.3) involved strike-slip rupture along the Salcha seismic zone. We constructed and modeled three gravity/aeromagnetic profiles extending across the NFFTB to the northwestern edge of the Tanana Basin that support the presence of subsurface thrusting north to 64.4°N, coincident with the absence of the Totatlanika schist at this latitude. A third profile extending across the Salcha seismic zone suggests reactivation of high angle faults within the Yukon-Tanana crystalline complex. The extension of thrust faulting ∼20–30 km north of the exposed NFFTB has important implications for the seismic hazards of the Fairbanks region.

Variations in Lithospheric Thickness Across the Denali Fault and in Northern Alaska

AbstractWhile variations in crustal structure beneath the Denali fault in Alaska are well‐documented, the existence of fault‐correlated structures throughout the entire thickness of the continental lithosphere is not. A new model of shear‐wave velocity structure obtained through joint inversion of surface wave and converted body wave data shows a northward increase in lithospheric thickness and velocity occurring across the Denali fault system. In northern Alaska, a dramatic increase in lithospheric thickness at the southern margin of the Arctic‐Alaska terrane lies in the vicinity of the Kobuk fault system. These correlations support the view that transpressive deformation tends to localize at the margins of thicker, higher‐strength lithosphere.

Geothermal Exploration in the Burwash Landing Region, Canada, Using Three-Dimensional Inversion of Passive Electromagnetic Data

Sustainable development of Canada’s North requires an increased focus on renewable, zero-emission energy sources. Burwash Landing in Yukon is prospective for geothermal energy based on a high geothermal gradient, local occurrence of warm groundwater and proximity to the active, crustal-scale Denali fault. Uncertainties about the potential geothermal system include the nature and geometry of fluid pathways, and heat sources required to drive a hydrothermal system. In this study, we inverted three passive electromagnetic datasets—321 extremely low frequency electromagnetic, 33 audiomagnetotelluric and 51 magnetotelluric stations—to map the subsurface electrical structure to 8 km depth. Our new model reveals vertical conductive structures associated with the two main faults, Denali and Bock’s Creek, which we interpret to represent fluid-deposited graphite and hydrothermal alteration, respectively. Our model supports an interpreted releasing bend on the main Denali fault strand. This is associated with the deepest conductivity anomaly along the fault and potential for deeper penetration of fluids. Enigmatic conductive bodies from 1 to > 6 km depth are associated with intermediate to mafic intrusions. Fluids released from these bodies may advect heat and provide a possible heat source to mobilize hot fluids and sustain a geothermal system in the region.

A Travel‐Time Path Calibration Strategy for Back‐Projection of Large Earthquakes and Its Application and Validation Through the Segmented Super‐Shear Rupture Imaging of the 2002 Mw 7.9 Denali Earthquake

AbstractWe investigate the impact of source‐side 3D velocity structure on teleseismic travel‐time in back projection (BP) analysis of large earthquakes. We use travel‐time data of teleseismic events recorded by the Hi‐Net array to reveal how travel‐time errors vary with source location. In a source area of a few hundred km, where travel‐time error varies dominantly linearly, we propose a new interpolation scheme using earthquakes located around the mainshock rupture to calibrate the travel‐time error, and validate it by relocating inland M > 5.0 earthquakes in central Japan. We then apply it to image the rupture of the 2002 Denali earthquake. The calibrated BP result shows that most of the high‐frequency radiators are <15 km away from the surface rupture trace. The new result reveals that the rupture started on the Susitna Glacier Fault with a speed of ∼1.4 km/s, then propagated onto the Denali Fault and accelerated to a super‐shear speed approaching the crustal P‐wave velocity at approximately 30 km. The location of super‐shear transition and rupture speed in BP are highly consistent with that inferred from the timing and amplitude ratio of the super‐shear and trailing Rayleigh pulses observed on the near fault PS‐10 station. Subsequently, the rupture stagnated for ∼15 s before penetrating through the largest asperity, re‐accelerated to a speed of ∼5.2 km/s and continued on the last 60 km of the Denali fault and part of Totschunda Fault. This application shows the great potential of the new BP calibration strategy to refine the rupture imaging of other mega‐earthquakes.

Potency density tensor inversion of complex body waveforms with time-adaptive smoothing constraint

SUMMARY Large earthquakes are often accompanied by complex fault rupture, but it has been difficult to reliably estimate such a complex rupture process with conventional waveform analysis tools due to modelling errors originating from limited accuracy of the fault geometry. Recently, a potency density tensor inversion method has been developed to solve this problem; allowing any types of faulting mechanism on an assumed model plane, the method replaces the modelling error of fault orientation with that of fault location, which is insensitive in teleseismic waveforms with low spatial resolution. The method has successfully unveiled earthquake source processes with geometrically complex fault rupture. However, the method imposes the same intensity of smoothing constraint on all the basis slip components irrespective of possible changes of slip direction during the rupture. This leads to excessive smoothing to a slip component with large amplitude, which results in obscuring the rupture process. In this study, we propose a time-adaptive smoothing constraint that dynamically adjusts the smoothness intensity inversely proportional to the amplitude for each basis slip function. Through a numerical experiment assigning an input model involving a drastic change in the focal mechanism (reverse, strike-slip and normal faulting) during the rupture, we find that the time-adaptive smoothing constraint solves the problem of excessive smoothing to the dominant slip component, and the spatiotemporally non-uniform rupture episodes with different focal mechanisms are successfully reproduced. To evaluate the feasibility and effectiveness of the time-adaptive smoothing constraint, we apply the method to the teleseismic body waves of the 2002 Denali fault and the 2008 Wenchuan earthquakes, which involve complex fault ruptures with changing focal mechanisms. We find that the developed method well captures the focal mechanism transition in space and time from reverse to strike-slip faulting during the ruptures of the 2002 Denali fault and the 2008 Wenchuan earthquakes. Even though these source models are built using only the teleseismic P waveforms with simple model fault geometry that is represented by a horizontal rectangular plane, they well explain the complex observed waveforms and agree with characteristics of source processes obtained in previous studies using seismic and geodetic data as well as field surveys. The potency density tensor inversion method with time-adaptive smoothing constraint is a powerful tool to analyse earthquake rupture processes with complex fault geometries involving different faulting types.

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

AbstractOblique 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.

Neogene sedimentary record of the evolution of a translated strike-slip basin along the Denali fault system: Implications for timing of displacement, composite basin development, and regional tectonics of southern Alaska

Abstract Analysis of the late Miocene to Holocene McCallum sedimentary basin, located along the south side of the eastern Denali fault system, provides a better understanding of strike-slip basin evolution, timing of displacement on the Denali fault, and tectonics of the southern Alaska convergent margin. Analysis of the McCallum basin utilizing measured stratigraphic sections, lithofacies analyses, and 40Ar/39Ar tephra ages documented a 564-m-thick, two-member stratigraphy. Fine-grained, lacustrine-dominated environments characterized deposition of the lower member, and coarse-grained, stream-dominated alluvial-fan environments characterized deposition of the upper member. The 40Ar/39Ar dating of tephras indicated that the lower member was deposited from 6.1 to 5.0 Ma, and the upper member was deposited from 5.0 to 3.8 Ma. Our stratigraphic analysis of the McCallum basin illuminates the development of a composite strike-slip basin, with the deposition of the lower member occurring along a transtensional fault section, and deposition of the upper member occurring along a transpressional fault section. This change in depositional and tectonic settings is interpreted to reflect ~79–90 km of transport of the basin along the Denali fault system based on Pleistocene–Holocene slip rates. Previous studies of the timing of Cenozoic displacement on the Denali fault system utilizing sedimentary records emphasized a Paleogene component; our findings, however, also require a significant Neogene component. Neogene strike-slip displacement and basin development along the Denali fault system were broadly coeval with development of high topography and related clastic wedges across southern Alaska in response to flat slab subduction of the Yakutat microplate.

Limited dynamic triggering in the Utah region, USA

SUMMARYThe state of Utah, USA, experiences around 3800 catalogued earthquakes per year, highlighting that the region is seismically active and susceptible to earthquakes. Following the 2002 Denali Fault (M7.9) earthquake in Alaska, the region showed an elevated seismicity rate for 3 weeks following the passage of high amplitude surface waves, suggesting that the region may be particularly susceptible to dynamic triggering. With over 23 396 faults and each fault presenting a potential fault for triggering, we systematically search for dynamic triggering throughout the state of Utah caused by large, global earthquakes with M ≥ 7. Specifically, we analyse earthquake catalogues and all available waveform data to determine statistical increases of seismicity rate following the passage of seismic arrivals. While we find instances of dynamic triggering, our results show that these events occur sparsely in the region. In total, less than 20 per cent of the 273 main shocks that occur from 2000 to the end of 2017 show a statistical indication of dynamic triggering throughout the Utah region, highlighting that dynamic triggering is limited for stresses created by transient signals from global M ≥ 7 earthquakes, with the exception being the Denali Fault (M7.9), Alaska earthquake (i.e. an instance of significant triggering).

Why is Denali (6,194 m) so big? Caught inside the tectonic wake of a migrating restraining bend

AbstractTopography 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.

The role of preexisting upper plate strike-slip faults during long-lived (ca. 30 Myr) oblique flat slab subduction, southern Alaska

Upper plates of subduction zones commonly respond to flat slab subduction by structural reactivation, magmatic arc disruption, and foreland basin inversion. However, the role of active strike-slip faults in focusing convergent deformation and magmatism in response to oblique flat slab subduction remains less clear. Here, we present new detrital apatite fission-track (dAFT) ages from 12 modern catchments in the eastern Alaska Range, Alaska, USA, to reveal how the dextral Denali fault system has facilitated bedrock exhumation and topographic growth during ca. 30 Ma-to-present oblique flat slab subduction of the Yakutat oceanic plateau. Additionally, a 940 ka (40Ar/39Ar whole rock) basalt flow is spatially associated with Cenozoic structures, locally reset AFT ages and provides the first evidence for Quaternary volcanism along the southern flank of the eastern Alaska Range. We integrate our new data with other thermochronologic, geochronologic, and regional geologic datasets to show that (1) most high topography regions in southern Alaska have undergone rapid bedrock cooling and exhumation since ca. 30 Ma; (2) elevated terrain and young cooling are spatially associated with long-lived active strike-slip fault systems; (3) topographic growth associated with strike-slip fault deformation led to local inversion of basin systems and drainage reorganization; (4) the onset of oblique oceanic plateau subduction is coeval with a southward shift in arc magmatism from one region of active strike-slip faulting to another above the northeastern edge of the flat slab; and (5) Quaternary volcanism marks the revival of magmatism in the eastern Alaska Range above the geophysically imaged northeastern edge of the flat slab. Our analysis of the post-30 Ma geologic evolution of southern Alaska demonstrates that strike-slip fault systems that were active at the time of slab flattening evolved into transpression zones that focused bedrock cooling, rock exhumation, and topographic growth.

Deformation between the highly oblique Yakutat–North American plate boundary and the Eastern Denali fault

AbstractThis study investigates the spatial and temporal pattern of rock exhumation inboard of the highly oblique Yakutat–North American plate boundary. We aim to quantify how far deformation is transferred inboard of the Fairweather transform plate boundary and across the Eastern Denali fault. We present new detrital apatite and zircon fission track data from 27 modern drainages collected on both sides of the Eastern Denali fault and from the Alsek and Tatshenshini River catchments that drain the mountainous region between the Fairweather fault and the Eastern Denali fault. By integrating our data with published bedrock and detrital geochronology and thermochronology, we show that exhumation reaches much farther inboard (>100 km) of the Fairweather fault than farther north in the St. Elias syntaxial region (<30 km). This suggests that the entire corridor between the Fairweather and Eastern Denali faults exhumed since mid-Miocene time. The Eastern Denali fault appears to be the backstop, and late Cenozoic exhumation northeast of the fault is very limited.

Tectonic Underplating and Dismemberment of the Maclaren‐Kluane Schist Records Late Cretaceous Terrane Accretion Polarity and ~480 km of Post‐52 Ma Dextral Displacement on the Denali Fault

AbstractTerrane accretion introduces irregular geometry and allochthonous material to obliquely convergent margins, which create opportunities to quantify strike‐slip displacement along otherwise margin‐parallel fault systems. We present new bedrock geologic mapping and U‐Pb and40Ar/39Ar geochronology from the Alaska Range suture zone in the eastern Alaska Range, which confirm a long‐hypothesized correlation between the Maclaren Glacier metamorphic belt (Alaska, USA) and the Kluane metamorphic assemblage (Yukon Territory, Canada) across the right‐lateral Denali fault. The new data inform a palinspastic reconstruction showing that the dissected metamorphic belts and associated plutons record ~480 km of dextral displacement along the Denali fault since ca. 52 Ma. Before strike‐slip separation, the Maclaren‐Kluane schist formed by west‐vergent forearc underplating in the waning stage of the ca. 100–90 Ma arc built upon the Yukon‐Tanana terrane. The prograde structural and metamorphic evolution of the Maclaren‐Kluane schist records the final collision of the Wrangellia composite terrane at ca. 75–65 Ma along a set of east‐dipping thrust shear zones, which we infer to record the polarity of the Late Cretaceous plate boundary between the composite terrane and North America. Paleogene extension partially exhumed the schists to the upper crust and may be a consequence of regionally distributed strike‐slip faulting at that time. Localization of the modern Denali fault after ca. 52 Ma dismembered the schists and four neighboring belts of plutonic, metasedimentary, and volcanic rocks. The transition to Yakutat oblique flat slab subduction at ca. 30–25 Ma marks the onset of transpressional deformation in the Denali fault system, which reactivated Late Cretaceous collisional structures bounding the Maclaren schist. Neogene reactivation of the Totschunda fault reduced strike‐slip motion on the Denali fault east of the Denali‐Totschunda intersection and continues to transfer residual plate boundary slip onto the Denali fault west of the intersection. Key outcomes of our synthesis include: (a) Much of the ~480 km of displacement on the Denali fault accumulated after strike‐slip on the neighboring Tintina and Border Ranges fault systems had largely shut down; (b) The modern Denali fault system should not be grouped with strike‐slip faults credited with large‐scale margin‐parallel transport of Cordilleran terranes in the Cretaceous. Instead, a poorly understood proto‐Denali fault system may be a candidate for large‐scale Cretaceous translation; and (c) the longevity (≥33 Myr) of the highly localized Denali fault master strand (≤1 km wide) implies that it occupies a major mechanical boundary that penetrates the lithosphere.

Long distance plutonic relationships demonstrate 33 million years of strain partitioning along the Denali fault

AbstractWe identify two piercing point pairs along a ~500 km transect of the arcuate strike‐slip Denali fault to document long‐term slip partitioning. Geochemical and isotopic similarity between Foraker and Panorama‐Schist Creek‐Nenana Plutons suggest ~155 km of right‐lateral displacement on the western Denali fault since 37 Ma at a rate of ~4.2 mm/year. The eastern Denali fault Maclaren‐Cottonwood Terrane geochronology correlation establishes ~305 km of displacement on the eastern Denali fault since 33 Ma at a rate of ~9.2 mm/year. The ratio of Pleistocene‐Holocene slip rates between the western (5.3 mm/year) and eastern (12.9 mm/year) Denali fault is 0.41 and our new constraints yield a Late Eocene‐Holocene ratio of 0.46. Hence, we interpret that the overall arcuate geometry of the Denali fault master strand was established by 33 Ma. We infer that the persistent long‐wave geometric stability of the Denali fault and other highly slip partitioned fault systems are related to long‐term highly oblique transpressive environs.