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In the year following the end of the 2009 eruption of Redoubt Volcano, Alaska, four significant swarms of low-frequency, low-magnitude (ML < 0.1) earthquakes occurred at shallow depths beneath the summit. Because swarms of low-frequency (LF) earthquakes preceded eruptions in 1989 and 2009, the posteruption swarms caused considerable concern and prompted the Alaska Volcano Observatory to raise the monitoring levels on three occasions. None of these swarms led to eruptions, however, and most observers (including us) initially concluded that the swarms had been caused by minor stress adjustments in the new lava dome or in the surrounding summit glaciers. New observations reveal that the shallow LF swarms were accompanied by 2 families of repeating earthquakes at depths between 3 km and 6 km below sea level, where the magma storage region is thought to be. These mid-crustal volcano-tectonic (VT) type earthquakes were identical to earthquakes recorded during the 2009 Redoubt eruption more than 6 months earlier. Focal mechanisms demonstrate that these earthquakes have thrust mechanisms inconsistent with the strike-slip nature of regional faulting. Based on these observations, we conclude that they are generated through processes occurring within the magma storage region. The concurrence of the repeating VT earthquakes with the shallow LF swarms indicates that the shallow LF earthquakes were also mag-matically driven. Our results emphasize that even brief episodes of low-amplitude earthquake activity, such as the LF swarms observed at Redoubt following the 2009 eruption, can be indicative of mag-matic activity. Perhaps more significant, however, is the demonstration that the conduit system at Redoubt remained active, intact, and capable of transporting heat and fluids to the surface months after the eruption was considered over. [ABSTRACT FROM AUTHOR]

The framework of Quaternary faults in Alaska remains poorly constrained. Recent studies in the Alaska Range north of the Denali fault add signifi cantly to the recognition of Quaternary deformation in this active orogen. Faults and folds active during the Quaternary occur over a length of ∼500 km along the northern fl ank of the Alaska Range, extending from Mount McKinley (Denali) eastward to the Tok River valley. These faults exist as a continuous system of active structures, but we divide the system into four regions based on east-west changes in structural style. At the western end, the Kantishna Hills have only two known faults but the highest rate of shallow crustal seismicity. The western northern foothills fold-thrust belt consists of a 50-km-wide zone of subparallel thrust and reverse faults. This broad zone of deformation narrows to the east in a transition zone where the range-bounding fault of the western northern foothills fold-thrust belt terminates and displacement occurs on thrust and/or reverse faults closer to the Denali fault. The eastern northern foothills foldthrust belt is characterized by ∼40-km-long thrust fault segments separated across leftsteps by NNE-trending left-lateral faults. Altogether, these faults accommodate much of the topographic growth of the northern fl ank of the Alaska Range. Recognition of this thrust fault system represents a signifi cant concern in addition to the Denali fault for infrastructure adjacent to and transecting the Alaska Range. Although additional work is required to characterize these faults suffi ciently for seismic hazard analysis, the regional extent and structural character should require the consideration of the northern Alaska Range thrust system in regional tectonic models. [ABSTRACT FROM AUTHOR]

Mountain ridges in the western Yakutat microplate are riddled with swarms of antislope scarps and troughs. These landforms were previously interpreted as gravity failures (sackungen), but if partly or wholly tectonic (fl exural slip or bending moment faults), they represent part of the strain budget from ongoing plate collision. To determine scarp origin, we mapped landforms, bedrock structure, and trenched scarps at Kushtaka Mountain and south of Martin Lake. The Kushtaka scarps paralleled west-dipping coal and sandstone beds in the Tertiary Kultieth Formation, and were spaced 35-60 m apart with heights of 1-4 m. Structural and paleosol relationships in a 6-m-long, 1.3-m-deep trench indicate the scarp was produced by Holocene creep on a normal fault underlying the scarp. The Kushtaka scarps thus represent toppling-style slip on bedding-plane faults in the eastern limb of a syncline, as the fold "unfolds" due to gravitational spreading. The Martin Lake scarps are more complex and include downslope-facing landslide scarps, upslope-facing fl exural toppling scarps, and an oblique-slip tectonic scarp. A 2-m-deep trench across the WNWtrending tectonic scarp exposed the underlying bedrock fault plane with slickensides raking only 17°-20°, indicating mainly leftlateral slip on a sinistral-normal fault. In contrast, swarms of ENE-trending antislope scarps showed normal-oblique (dextral) slip. The two scarp sets may form a conjugate pair that simultaneously accommodates left-lateral tectonic slip and NNE-directed gravitational spreading. Our results, plus those recently published by Li et al. (2010), show that most antislope scarps in the western Yakutat microplate are formed by normal slip on bedding-plane faults that dip into the mountain, and represent gravitational spreading expressed as toppling-style failure or "unfolding" of strata on fold limbs. Conversely, the sinistral-oblique slip fault at Martin Lake is one in a family of easttrending faults accommodating accretion of the Yakutat microplate into the cuspate syntaxis of the Alaskan plate margin. [ABSTRACT FROM AUTHOR]

Inconsistencies between regional seismicity and block tectonic models, the recent recognition of the Bering plate, and the availability of global positioning system data motivated a large-scale kinematic analysis of Alaska and western Canada. We provide a synoptic view of the neotectonics of the region through kinematic modeling of the longterm strain rate and velocity field constrained by geologic and geophysical observations. Our results provide evidence that a wide zone of diffuse deformation defines the boundaries between the North America, Pacific, and Bering plates, and that the relative motion between these plates may be the source for much of the modern deformation. [ABSTRACT FROM AUTHOR]

Present-day seafloor 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 fluvial input or shelf canyons. The sediment supply instead has been provided by glacial erosion in the still active 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 intensification carved cross-shelf sea valleys that connect the St. Elias Range to the Surveyor Fan. The direct deposition of newly increased terrigenous sediment flux 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 seismicreflection profiles. The change in morphology observed throughout the sequences allows us to characterize the influence 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 flux between glacial- interglacial cycles together with sea valley formation leads to a glacial shelf transport process not typical of a fluvial 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. [ABSTRACT FROM AUTHOR]

Ongoing oblique collision and flat-slab subduction of the Yakutat terrane has produced Earth's highest coastal mountain range, the Chugach-St. Elias orogen in Southeast Alaska. Massive glaciers and ice fi elds cause extensive erosion, but also limit access to rocks that allow detailed study of the orogen. Fission track and U/Pb dating of detrital zircon from glacial rivers reveal the exhumation history of the Chugach-St. Elias orogen. Orogenic development started ca. 30 Ma and pulses of exhumation occurred 20 ± 2 Ma and 11 ± 2 Ma. Differential exhumation occurred across major fault zones and the locus of exhumation shifted southward. The ca. 5 Ma cooling ages of detrital zircon in the Pliocene-Pleistocene Yakataga Formation reveal that source rock exhumation was likely associated with faulting along the ice-covered Fairweather fault-Contact fault system. [ABSTRACT FROM AUTHOR]

Apatite (U-Th)/He dating from the St. Elias orogen in southern Alaska illustrates a potential association between long-term denudation and glacier sliding. Cooling ages as young as 0.4 Ma (exhu mation ~4-5 mm/yr) are concentrated in a narrow band near the glacier equilibrium line altitude (ELA) front, where mean Quaternary ELA intersects the windward flank of the orogen. This band of denudation is not correlated with individual faults, structural trends, or known concentrations of precipitation, and we propose that it is produced by focused glacier sliding at or near the ELA front. This implies that long-term glacial erosion is a maximum at ELA, which corroborates model predictions that glaciers and climate can control the pattern of crust removal from orogens. Denudation rates do not covary with fluctuations in glacier size along the ELA front, suggesting that small glaciers are capable of keeping pace with incision by larger ones and tectonic rock uplift. Ice discharge may thus play a critical, but complex, role in excavating glaciated orogens. [ABSTRACT FROM AUTHOR]

The northern Cordilleran fold-and-thrust belt in Canada and Alaska is at the boundary between the broad continental margin mobile belt and the stable North American craton. The fold-and-thrust belt is marked by several significant changes in geometry: cratonward extensions in the central Yukon Territory and northeastern Alaska are separated by marginward re-entrants. These geometric features of the Cordilleran mobile belt are controlled by relations between lithospheric strength and compressional tectonic forces developed along the continental margin. Regional magnetic anomalies indicate deep thermal and compositional characteristics that contribute to variations in crustal strength. Our detailed analysis of one such anomaly, the North Slope deep magnetic high, helps to explain the geometry of the fold-and-thrust front in northern Alaska. This large magnetic anomaly is inferred to reflect voluminous mafic magmatism in an old (Devonian?) extensional domain. The presence of massive amounts of mafic material in the lower crust implies geochemical depletion of the underlying upper mantle, which serves to strengthen the lithosphere against thermal erosion by upper mantle convection. We infer that deep-source magnetic highs are an important indicator of strong lower crust and upper mantle. This stronger lithosphere forms buttresses that play an important role in the structural development of the northern Cordilleran fold-and-thrust belt. [ABSTRACT FROM AUTHOR]

Examines the tectonic regulation on greenhouse gas flux to the paleogene atmosphere from the Gulf of Alaska accretionary prism. Implication of methane loading in global warming; Occurrence of deformation, accretion and high heat flow following deposition; Effect of global warming on ocean waters.

Examines a natural experiment in river incision into bedrock produced by the 1912 eruption that inundated the Valley of Ten Thousand Smokes in Katmai National Park, Alaska. Influence of the rates and patterns of bedrock channel incision on landscape evolution and long-term interactions among climate, tectonics and erosion; Incision rate data.

Explores the active displacement partitioning and volcanic arc-parallel extension using Global Positioning System geodesy and kinematic analysis in the Aleutian Islands, Alaska. Evidence on a westward forearc migration; Details on partitioning in areas of impeded subduction; Implication for the exhumation of metamorphic rocks in ancient arc systems.

Studies the tectonic assembly of east-central Alaska through evidence from Cretaceous-Tertiary sandstones of the Kandik River terrane. Geochemistry; Paleotectonics; Petrography; Volcanic arc.

Presents the results of a study on the distribution of fluid pressure, the origin of cement and vein minerals that inhibit fluid flow, and the mechanisms for fluid migration and fluid-pressure generation within the northeastern part of Cook Inlet basin in Alaska. Tectonics of upper Cook Inlet basin; Lithology of tertiary and upper mesozoic sedimentary rocks.

Presents a study on the structural and kinematic evolution of tectonics in the Yukon-Tanana terrane in the northern North American Cordillera, Alaska. What the Yukon-Tanana upland consists up; Methodology used to conduct study; Factors that caused the kinematic evolution of tectonics in this area.

Discusses structural, metamorphic, and geochronological observations relating to the exhumation history of high pressure/low temperature metamorphic rocks in Seward Peninsula, Alaska. Crustal extension and unroofing of blueschists; Thinning of the crust; Partial erosional unroofing of metamorphic rocks.

Studies the crustal velocity structure of the northern Yukon-Tanana upland in central Alaska by analyzing the results of the US Geological Survey Trans-Alaska Crustal Transect (TACT) project in 1987. Description of the region; Division of the crustal velocity structure; Tectonic history.

The Kodiak batholith is one of the largest, most elongate intrusive bodies in the forearc Sanak-Baranof plutonic belt located in southern Alaska. This belt is interpreted to have formed during the subduction of an oceanic spreading center and the associated migration of a slab window. Individual plutons of the Kodiak batholith track the location and evolution of the underlying slab window. Six U/Pb zircon ages from the axis of the batholith exhibit a northeastward-decreasing age progression of 59.2 ± 0.2 Ma at the southwest end to 58.4 ± 0.2 Ma at the northeast tip. The trench-parallel rate of age progression is within error of the average slab-window migration rate for the entire Sanak-Baranof belt (19 cm/yr). Structural relationships, U/Pb ages, and a model of new gravity data indicate that magma from the Kodiak batholith ascended 5-10 km as a northeastward-younging series of 1-8-km-diameter viscoelastic diapirs. Individual plutons ascended by multiple emplacement mechanisms including downward flow, collapse of wall rock, stoping, and diking. Stokes flow xenolith calculations suggest ascent rates of 5-100 m/yr and an effective magmatic viscosity of &thkap;107-108 Pa s. Pre-existing structural or lithologic heterogeneities did not dominantly control the location of the main batholith. Instead, its location was determined by migration of the slab window at depth. [ABSTRACT FROM AUTHOR]

Late Neogene and younger deformation in Cook Inlet basin is caused by dextral transpression in the plate margin of south-central Alaska. Collision and subduction of the Yakutat microplate at the northeastern end of the Aleutian subduction zone is driving the accretionary complex of the Chugach and Kenai Mountains toward the Alaska Range on the opposite side of the basin. This deformation creates belts of fault-cored anticlines that are prolific traps of hydrocarbons and are also potential sources for damaging earthquakes. The faults dip steeply, extend into the Mesozoic basement beneath the Tertiary basin fill, and form conjugate flower structures at some localities. Comparing the geometry of the natural faults and folds with analog models created in a sandbox deformation apparatus suggests that some of the faults accommodate significant dextral as well as reverse-slip motion. We develop a tectonic model in which dextral shearing and horizontal shortening of the basin is driven by microplate collision with an additional component of thrust-type strain caused by plate subduction. This model predicts temporally fluctuating stress fields that are coupled to the recurrence intervals of large-magnitude subduction zone earthquakes. The maximum principal compressive stress is oriented east-southeast to east-northeast with nearly vertical least compressive stress when the basin's lithosphere is mostly decoupled from the underlying subduction megathrust. This stress tensor is compatible with principal stresses inferred from focal mechanisms of earthquakes that occur within the crust beneath Cook Inlet basin. Locking of the megathrust between great magnitude earthquakes may cause the maximum principal compressive stress to rotate toward the northwest. Moderate dipping faults that strike north to northeast may be optimally oriented for rupture in the ambient stress field, but steeply dipping faults within the cores of some anticlines are unfavorably oriented with respect to both modeled and observed stress fields, suggesting that elevated fluid pressure may be required to trigger fault rupture. [ABSTRACT FROM AUTHOR]