Lindsay L. Worthington

The Yakutat terrane: Dramatic change in crustal thickness across the Transition fault, Alaska

We present new constraints on the crustal structure of the Yakutat terrane and evidence of the role of the Transition fault in southern Alaska. The Yakutat terrane south of Yakutat Bay includes crystalline crust that is 24–27 km thick overlain by sedimentary units that are 4.5–7.5 km thick. The Yakutat terrane crustal thickness and velocity structure are consistent with an oceanic plateau origin. The southern edge of the Yakutat terrane is bounded by the Transition fault, which is imaged as a near-vertical fault zone ∼1 km wide. The Transition fault is coincident with a dramatic change in Moho depth from 32 km for Yakutat oceanic plateau crust to 11.5 km for Pacific Ocean crust occurring over a horizontal distance of 0–5 km. There is no evidence for underthrusting of the Pacific Ocean crust beneath the Yakutat terrane at the Transition fault. We argue that the Yakutat terrane formed on the Kula or Farallon plate and was later juxtaposed next to younger Pacific Ocean crust by the Transition fault.

Structure of the actively deforming fold-thrust belt of the St. Elias orogen with implications for glacial exhumation and three-dimensional tectonic processes

Previous studies in the Yakataga fold-thrust belt of the St. Elias orogen in southern Alaska have demonstrated high exhumation rates associated with alpine glaciation; however, these studies were conducted with only a rudimentary treatment of the actual structures responsible for the deformation that produced long-term uplift. We present results of detailed geologic mapping in two corridors across the onshore fold-thrust system: the Duktoth River transect just west of Cape Yakataga and the Icy Bay transect in the Mount St. Elias region. In the Duktoth transect, we recognize older, approximately east-west–trending structures that are overprinted by open, northwest-trending fold systems, which we correlate to a system of northeast-trending, out-of-sequence, probably active thrusts. These younger structures overprint a fold-thrust stack that is characterized by variable structural complexity related to detachment folding along coal-bearing horizons and duplexing within Eocene strata. In the Icy Bay transect, we recognize a similar structural style, but a different kinematic history that is constrained by an angular unconformity at the base of the syntectonic Yakataga Formation. At high structural levels, near the suture, structures show a consistent northwest trend, but fold-thrust systems rotate to east-west to northeast trends in successively younger structures within the Yakataga Formation. We present balanced cross sections for each of these transects where we project the top of basement from offshore seismic data and assume a subsurface structure with duplex systems similar to, but simplified from, structures observed in the onshore transects. These sections can account for 150–200 km of shortening within the fold-thrust system, which is Our section restorations also provide a simple explanation for the observed elongate bullseye pattern of low-temperature cooling ages in the thrust belt as a consequence of exhumation above the growing duplex and/or antiformal stack. Comparison with analog model studies suggests that structural feedbacks between erosion and development of decollement horizons in coal-bearing strata led to this structural style. Although previous studies based on thermochronology suggested an active backthrust at the northern edge of the thrust belt, section restorations indicate that a backthrust is allowable but not required by available data. The Yakataga fold-thrust belt has been treated as a dominantly 2D system, yet our work indicates that 3D processes are prominent. In the Duktoth transect, we interpret a group of northeast-trending thrusts as younger, out-of-sequence structures formed in response to the rapid destruction of the orogenic wedge by glacial erosion and deposition immediately offshore. We infer that these northeast-trending thrusts transfer slip downdip into a duplex system that forms the antiformal stack modeled in cross-section restorations, and we infer that these structures represent thrusting stepping back from the active thrust front attempting to rebuild an orogenic wedge that is being destroyed as rapidly as, or more rapidly than, it is being rebuilt. In the Icy Bay transect, we use the relative chronology provided by an angular unconformity beneath the syntectonic Yakataga Formation to infer that early, northwest-trending fold-thrust systems were formed along the Fairweather transform as transpressional structures. Continued strike slip carried these structures into the tectonic corner between the Fairweather and Yakataga segments of the orogen, producing a counterclockwise rotation of the shortening axis until the rocks reached their present position.

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska.

Significance In coastal Alaska and the St. Elias orogen, over the past 1.2 million years, mass flux leaving the mountains due to glacial erosion exceeds the plate tectonic input. This finding underscores the power of climate in driving erosion rates, potential feedback mechanisms linking climate, erosion, and tectonics, and the complex nature of climate−tectonic coupling in transient responses toward longer-term dynamic equilibration of landscapes with ever-changing environments. Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8–1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2–0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50–80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

Tectonic and climatic influence on the evolution of the Surveyor Fan and Channel system, Gulf of Alaska

Present-day seafl oor morphology and sediment distribution in the deep-water Surveyor Fan, Gulf of Alaska, is dominated by the >700-km-long Surveyor Channel, an anomaly in a system with no major fl uvial input or shelf canyons. The sediment supply instead has been provided by glacial erosion in the stillactive Chugach‐St. Elias orogen, and glacial transport across the shelf. Glaciation has periodically increased in the St. Elias Range since the Miocene, but began dominating erosion and spurred enhanced exhumation since the mid-Pleistocene transition, at ~1 Ma. Ice associated with this glacial intensifi cation carved cross-shelf sea valleys that connect the St. Elias Range to the Surveyor Fan. The direct deposition of newly increased terrigenous sediment fl ux into the fan triggered the formation of the Surveyor Channel and its growth across the Surveyor Fan. Through the formation of the Surveyor Channel, climate events created three major differentiable sequences across the Surveyor Fan that we mapped using seismicrefl ection profi les. The change in morphology observed throughout the sequences allows us to characterize the infl uence that a glaciated orogen can have in shaping margin processes and the sediment pathway from source to sink. We show that the large variation in sediment fl ux between glacial- interglacial cycles together with sea valley formation leads to a glacial shelf transport process not typical of a fl uvial system. This glacial shelf transport along with the channel terminus in the Aleutian Trench makes the Surveyor Fan and Channel morphologically one of the most unique systems in the world.

Structural relationships in the eastern syntaxis of the St. Elias orogen, Alaska

The eastern syntaxis in the St. Elias orogen (Alaska, USA) is one of the most complex and least understood regions within the southern Alaska coastal mountain belt. The syntaxis contains many features unique to the orogen that are essential to understanding the structural architecture and tectonic history of the collision between North America and the allochthonous Yakutat microplate. The eastern syntaxis contains the transition from transpressional structures associated with the Queen Charlotte–Fairweather fault system in the east to the Yakataga fold-and-thrust belt (YFTB) to the west. Throughout the eastern syntaxis, a prominent unconformity at the base of the synorogenic Yakataga Formation records an erosional event related to the development of the YFTB. Strain accumulations in the eastern YFTB predate the deposition of the Yakataga Formation, extending estimates for the early development of the St. Elias orogen. Structural and stratigraphic relationships in the eastern syntaxis suggest that forethrusts associated with the transpressional system shut down and were overprinted by fold-and-thrust structures in the Early to latest Miocene. Basement in the eastern syntaxis consists of the Yakutat Group, part of the Chugach accretionary complex, which is carried by numerous low-angle thrust faults in the eastern syntaxis. Exposures of basement and fault patterns within the syntaxis have implications for tectonic reconstructions of the Yakutat microplate and the geodynamics of the orogen.

Seismic images of the Transition fault and the unstable Yakutat-Pacific-North American triple junction

In southern Alaska, the Pacific plate and Yakutat terrane subduct beneath the North American plate along the Aleutian Trench and Pamplona zone, respectively, and are sliding past each other at minimal rates along the Transition fault. As the deformation front of the Pamplona zone stepped eastward during the Pliocene–Pleistocene, the Pacific–North American–Yakutat triple junction became unstable. Four recent seismic images reveal that the Transition fault changes from a single strike-slip boundary east of the deformation front to three strands that step increasingly seaward between the deformation front and the Aleutian Trench. The southern two strands deform the Pacific crust, and the outermost of these became increasingly convergent sometime since 1 Ma, as demonstrated by young growth strata. We propose that this internal deformation of the Pacific plate is an attempt to re-attain stability, which can only be reached by creating a tectonic boundary collinear with the Pamplona zone. The plate reorganization will result in initiation of subduction such that a portion of former Pacific crust will become accreted to the North American plate. Such accretion events caused by triple-junction instability may be an important mechanism for transferring oceanic crust to continental margins.

Late Quaternary glacial dynamics and sedimentation variability in the Bering Trough, Gulf of Alaska

Ice dynamics, tectonic setting, and sediment supply are the key parameters controlling the architecture of high-latitude margins and the formation of trough mouth fans (TMFs). Current understanding of these archives of paleo–ice streams is based on studies of ice sheets adjacent to stable, passive margins, while the behavior of active, convergent glacier-influenced margins remains relatively unconstrained. We integrate high-resolution seismic data and chronology from Integrated Ocean Drilling Program Expedition 341 cores in southeast Alaska across the actively converging Yakutat terrane margin to examine the late Quaternary evolution of the Bering Glacier, the largest outlet glacier of the poorly understood Cordilleran Ice Sheet (CIS). We interpret at least eight glacial advances to the shelf break since the end of the mid-Pleistocene transition, showing a more dynamic CIS than hitherto realized. During the past ∼130 k.y., the temperate, meltwater-charged Bering Glacier delivered ∼925 km 3 of sediment to the shelf and slope, providing one of the highest rates of sustained sediment accumulation (5–10 m/k.y.) ever reported globally. Rapid formation of a TMF, reaching ∼600 m thick in ∼130 k.y., emphasizes the extreme sediment flux that can be produced by wet-based glacial systems, and its critical role in the development of high-latitude margin stratigraphy. TMF formation despite initially steep, tectonically controlled slopes in this active setting reflects an autogenic shift in the evolution of the Bering Trough, suggesting that major transitions between sedimentary regimes need not reflect some externally driven change in climate variability.

Rapid sedimentation and overpressure in shallow sediments of the Bering Trough, offshore southern Alaska

Pore pressures in sediments at convergent margins play an important role in driving chemical fluxes and controlling deformation styles and localization. In the Bering Trough offshore Southern Alaska, extreme sedimentation rates over the last 140 kyr as a result of glacial advance/retreats on the continental shelf have resulted in elevated pore fluid pressures in slope sediments overlying the Pamplona Zone fold and thrust belt, the accretionary wedge resulting from subduction of the Yakutat microplate beneath the North American Plate. Based on laboratory experiments and downhole logs acquired at Integrated Ocean Drilling Program Site U1421, we predict that the overpressure in the slope sediments may be as high as 92% of the lithostatic stress. Results of one-dimensional numerical modeling accounting for changes in sedimentation rate over the last 130 kyr predicted overpressures that are consistent with our estimates, suggesting that the overpressure is a direct result of the rapid sedimentation experienced on the Bering shelf and slope. Comparisons with other convergent margins indicate that such rapid sedimentation and high overpressure is anomalous in sediments overlying accretionary wedges. We hypothesize that the shallow overpressure on the Bering shelf/slope has fundamentally altered the deformation style within the Pamplona Zone by suppressing development of faults and may inhibit seismicity by focusing faulting elsewhere or causing deformation on existing faults to be aseismic. These consequences are probably long-lived as it may take several million years for the excess pressure to dissipate.