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The relationship between rock-slope failure and glacier retreat is complex, and paraglacial failures often lack clearly identified triggers. To better understand the role of glacier retreat in rock-slope failures, we analysed the processes that led to the October 1967 Grewingk landslide in Kachemak Bay State Park on the Kenai Peninsula, Southcentral Alaska. The rock material collapsed onto the glacier toe and into its pro-glacial lake and produced a tsunami wave that swept the outwash plain. On the day of the failure, rainfall and snowmelt were well within normal ranges, and seismic records show no significant shaking. Three years prior to the 1967 failure, the slope withstood the second largest earthquake ever recorded (Great Alaskan earthquake, MW 9.2). We reassessed the volume of the failure by differencing pre- and post-digital terrain models and found a value of 20–24 × 106 m3, which is four times smaller than a previous estimate. The back analysis of the Grewingk landslide is based on remote sensing data and field measurements including aerial satellite image analysis, detailed surveying and understanding of the structural geology, a kinematic analysis, and runout modelling. Our research provides an example of a major paraglacial failure that lacks an obvious trigger and points to several geological factors and changing environmental conditions that likely promote such failures. This study further indicates that the Grewingk landslide, pre-conditioned by the geometry of faults and joints, may have reached a critical stability state due to internal processes and the potential combined effects of seismic activity and glacier retreat prior to the collapse. [ABSTRACT FROM AUTHOR]

Effective tsunami risk reduction requires an understanding of how at-risk populations are specifically vulnerable to tsunami threats. Vulnerability assessments primarily have been based on single hazard zones, even though a coastal community may be threatened by multiple tsunami sources that vary locally in terms of inundation extents and wave arrival times. We use the Alaskan coastal communities of Cordova, Kodiak, Seward, Valdez, and Whittier (USA), as a case study to explore population vulnerability to multiple tsunami threats. We use anisotropic pedestrian evacuation models to assess variations in population exposure as a function of travel time out of hazard zones associated with tectonic and landslide-related tsunamis (based on scenarios similar to the 1964 M 9.2 Good Friday earthquake and tsunami disaster). Results demonstrate that there are thousands of residents, employees, and business customers in tsunami hazard zones associated with tectonically generated waves, but that at-risk individuals will likely have sufficient time to evacuate to high ground before waves are estimated to arrive 30-60 min after generation. Tsunami hazard zones associated with submarine landslides initiated by a subduction zone earthquake are smaller and contain fewer people, but many at-risk individuals may not have enough time to evacuate as waves are estimated to arrive in 1-2 min and evacuations may need to occur during earthquake ground shaking. For all hazard zones, employees and customers at businesses far outnumber residents at their homes and evacuation travel times are highest on docks and along waterfronts. Results suggest that population vulnerability studies related to tsunami hazards should recognize non-residential populations and differences in wave arrival times if emergency managers are to develop realistic preparedness and outreach efforts. [ABSTRACT FROM AUTHOR]

The Lower Cretaceous Fortress Mountain Formation occupies a spatial and temporal niche between syntectonic deposits at the Brooks Range orogenic front and post-tectonic strata in the Colville foreland basin. The formation includes basin-floor fan, marine-slope and fan-delta facies that define a clinoform depositional profile. Texture and composition of clasts in the formation suggest progressive burial of a tectonic wedge-front that included older turbidites and mélange. These new interpretations, based entirely on outcrop study, suggest that the Fortress Mountain Formation spans the boundary between orogenic wedge and foredeep, with proximal strata onlapping the tectonic wedge-front and distal strata downlapping the floor of the foreland basin. Our reconstruction suggests that clinoform amplitude reflects the structural relief generated by tectonic wedge development and load-induced flexural subsidence of the foreland basin. [ABSTRACT FROM AUTHOR]

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]

Abstract: A tectonic process commonly overlooked in the growth and formation of continental crust is ridge subduction. This process can stabilize a forearc accretionary complex through high grade metamorphism, anatexis, pluton emplacement, and underplating of mafic rock. An excellent example of the process is in the Chugach metamorphic complex (CMC), southern Alaska, formed as a result of late Cretaceous to Paleogene subduction of the Kula–Farallon (or Kula–Resurrection) ridge. Ridge subduction subjected the trench-fill turbidites of the accretionary prism to greenschist to amphibolite grade metamorphism under high-temperature/low-pressure conditions, converting them to a gneissic basement in the core of the CMC, and regional near-trench plutonism. In addition to high grade metamorphism and near-trench plutonism, the CMC experienced three distinct phases of deformation associated with triple-junction position in the process of becoming stabilized continental crust: 1) accretionary prism-related compression and metamorphism, 2) eastward migration of the triple-junction and associated ridge subduction producing orogen-parallel extension, peak metamorphism, and mafic underplating, and 3) dextral transpression following the migrating triple junction. This combination of progressive deformation and tectonic phases produces continental crust, effectively becoming a stable tectonic backstop by the time of Neogene Yakutat terrane collision, the present-day locus of deformation. [Copyright &y& Elsevier]

We tested the hypothesis that afterslip should be accompanied by tremor using observations of seismic and aseismic deformation surrounding the 2002 M 7.9 Denali fault, Alaska, earthquake (DFE). Afterslip happens more frequently than spontaneous slow slip and has been observed in a wider range of tectonic environments, and thus the existence or absence of tremor accompanying afterslip may provide new clues about tremor generation. We also searched for precursory tremor, as a proxy for posited accelerating slip leading to rupture. Our search yielded no tremor during the five days prior to the DFE or in several intervals in the three months after. This negative result and an array of other observations all may be explained by rupture penetrating below the presumed locked zone into the frictional transition zone. While not unique, such an explanation corroborates previous models of megathrust and transform earthquake ruptures that extend well into the transition zone. [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]

Abstract: The Coast Mountains of British Columbia record an increase in magmatic activity, acceleration in exhumation rate, and a change from transpression to extension between ∼60 and 52 Ma. Structural analysis of fabrics in three mid-crustal plutons and country-rocks leads to conclusions about pluton emplacement mechanisms and strain partitioning during changing tectonic conditions. The Quottoon and Kitlope plutons (∼60 Ma) have steep foliations and lineations consistent with partitioned transpressional deformation. The Chief Matthew’s pluton (∼57–55 Ma) intruded during the formation of a sub-horizontal transposition foliation, and has radially distributed lineations consistent with sub-vertical flattening during extension. The change in orientation of the foliation represents an almost orthogonal rotation of the shortening direction from sub-horizontal to sub-vertical. The Chief Mathew’s pluton is interpreted to intrude initially into gently dipping fractures perpendicular to the steeply dipping foliation. These melt-filled fractures acted as conduits for melt, triggering horizontal flow, and eventually formed kilometer-scale sills. The steeply dipping fabrics of the Coast shear-zone provided pathways for melt to ascended from lower to middle-crustal depths. Partitioning deformation into three-dimensional domains of flattening, simple shear, and constrictional strain created the space for plutons. This pattern of deformation reflects the interaction of regional deformation with magma emplacement. [Copyright &y& Elsevier]

significant number of volcano-tectonic (VT) earthquake swarms, some of which are accompanied by ground deformation and/or volcanic gas emissions, do not culminate in an eruption. These swarms are often thought to represent stalled intrusions of magma into the mid- or shallow-level crust. Real-time assessment of the likelihood that a VT swarm will culminate in an eruption is one of the key challenges of volcano monitoring, and retrospective analysis of non-eruptive swarms provides an important framework for future assessments. Here we explore models for a non-eruptive VT earthquake swarm located beneath Iliamna Volcano, Alaska, in May 1996-June 1997 through calculation and inversion of fault-plane solutions for swarm and background periods, and through Coulomb stress modeling of faulting types and hypocenter locations observed during the swarm. Through a comparison of models of deep and shallow intrusions to swarm observations, we aim to test the hypothesis that the 1996-97 swarm represented a shallow intrusion, or 'failed' eruption. Observations of the 1996-97 swarm are found to be consistent with several scenarios including both shallow and deep intrusion, most likely involving a relatively small volume of intruded magma and/or a low degree of magma pressurization corresponding to a relatively low likelihood of eruption. [ABSTRACT FROM AUTHOR]

Abstract: On September 17th, 2006, Fourpeaked volcano had a widely-observed phreatic eruption. At the time, Fourpeaked was an unmonitored volcano with no known Holocene activity, based on limited field work. Airborne gas sampling began within days of the eruption and a modest seismic network was installed in stages. Vigorous steaming continued for months; however, there were no further eruptions similar in scale to the September 17 event. This eruption was followed by several months of sustained seismicity punctuated by vigorous swarms, and SO2 emissions exceeding a thousand tons/day. Based on observations during and after the phreatic eruption, and assuming no recent pre-historical eruptive activity at Fourpeaked, we propose that the activity was caused by a minor injection of new magma at or near 5km depth beneath Fourpeaked, which remained active over several months as this magma equilibrated into the crust. By early 2007 declining seismicity and SO2 emission signaled the end of unrest. Because the Fourpeaked seismic network was installed in stages and the seismicity was punctuated by discrete swarms, we use Fourpeaked to illustrate quantitatively the efficacy and shortcomings of rapid response seismic networks for tracking volcanic earthquakes. [Copyright &y& Elsevier]

A numerical model of the wave dynamics in Passage Canal, Alaska during the Mw 9.2 megathrust earthquake is presented. During the earthquake, several types of waves were identified at the city of Whittier, located at the head of Passage Canal. The first wave is thought to have been a seiche, while the other two waves were probably triggered by submarine landslides. We model the seiche wave, landslide-generated tsunami, and tectonic tsunami in Passage Canal and compute inundation by each type of wave during the 1964 event. Modeled results are compared with eyewitness reports and an observed inundation line. Results of the numerical experiments let us identify where the submarine landslides might have occurred during the 1964 event. We identify regions at the head and along the northern shore of Passage Canal, where landslides triggered a wave that caused most of the damage in Whittier. An explanation of the fact that the 1964 tectonic tsunami in Whittier was unnoticed is presented as well. The simulated inundation by the seiche, landslide-generated tsunami, and tectonic tsunami can help to mitigate tsunami hazards and prepare Whittier for a potential tsunami. [ABSTRACT FROM AUTHOR]

This study summarizes the structure of the western Brooks Range fold and thrust belt of Arctic Alaska. The fold and thrust exposes a series of NE-SW-trending, NW-transported and regionally folded thrust sheets. Emplacement of thrust sheets results from ophiolite obduction and telescoping of the ancient continental passive margin of Arctic Alaska in the Cretaceous. Integration of stratigraphic, structural and geophysical data suggests that the structure of the fold and thrust belt is controlled by inheritance of NW-SE-trending Paleozoic structures of the pre-existing continental margin. [ABSTRACT FROM AUTHOR]

Models of postseismic deformation following the 2002 M 7.9 Denali Fault, Alaska earthquake provide insight into the rheologic structure of the Alaskan lithosphere and the physical processes activated following a large earthquake. We model coseismic GPS displacements and 4 yr of postseismic GPS position time-series with a coupled model of afterslip on the fault in the lithosphere and distributed viscous flow in the asthenosphere. Afterslip is assumed to be governed by a simplified version of a laboratory-derived rate-strengthening friction law that is characterized with a single parameter, , where σ is the effective normal stress on the fault and is a dimensionless empirical parameter. Afterslip is driven by coseismic stress changes on the fault generated by the main shock. The lithosphere is modelled as an elastic plate overlying a linear, Maxwell, viscoelastic asthenosphere. We devise a scheme to simultaneously estimate the distributions of coseismic slip and afterslip, friction parameters, lithosphere thickness and asthenosphere viscosity. The postseismic GPS time-series are best reproduced with a 45–85 km thick elastic lithosphere overlying an asthenosphere of viscosity Pa s. The 45–85 km elastic lithosphere thickness is greater than or equal to the average crustal thickness in the region suggesting that distributed postseismic flow occurs largely within the mantle while postseismic deformation in the crust is confined largely to slip on a discrete fault penetrating the entire crust and perhaps into the uppermost mantle. For typical laboratory values of of order 10−2 at temperatures corresponding to the inferred depth of afterslip, the estimated effective normal stress on the fault is ∼50 MPa, which is about an order of magnitude lower than effective normal stresses at hydrostatic pore pressure. Previous studies showed that models with linear Newtonian rheology cannot reproduce the observed GPS time-series but that models incorporating non-linear or biviscous flow of the mantle do fit the data. We show that a model with afterslip governed by a non-linear fault zone rheology coupled to Newtonian mantle flow is sufficient to reproduce the GPS time-series. [ABSTRACT FROM AUTHOR]

Abstract: The massive unmetamorphosed 212 Ma Taylor Mountain batholith underlies ∼648 km2 of the Yukon–Tanana terrane (YTT) in east-central Alaska, and intrudes klippen thrust sheets of the Fortymile River assemblage and the Chicken Metamorphic Complex. Structural evidence indicates that the batholith is neither tilted nor rotated about a vertical axis. Paleomagnetic analysis of 295 specimens from 24 sites in the batholith and cross-cutting mafic dikes isolates a stable primary characteristic remanent magnetization in magnetite with unblocking temperatures of 500 to 585 °C that places the batholith''s paleopole at 61.4°N, 102.2°E (N =17, α 95 =5.2°) on the North American apparent polar wander path (APWP) at 215±5 Ma (1σ). The Taylor Mountain paleopole alone and all YTT paleopoles together show that the YTT has been a para-autochthonous part of the North American margin since the Late Triassic. Further, the Intermontane Belt (IMB) paleopoles are distinct from the North American and YTT paleopoles prior to the mid Eocene and provide two APWP segments. One IMB segment includes the Quesnel, Cache Creek, Eastern Coast Belt, and northern Stikine terranes, and the second more-speculative IMB segment includes the central and southwestern Stikine and Methow terranes. This analysis supports tectonic models for the northern Cordillera in which the IMB terranes were obducted onto North America during the Late Cretaceous to mid Eocene, co-eval with the Laramide orogeny. [Copyright &y& Elsevier]

Abstract: Between the western and the eastern parts of the southern Alaskan Sanak–Baranof belt there exist variations in pluton spacing, geochronology, geochemistry and metamorphism. All such characteristics can be explained by increase in heat flux towards the eastern parts of the belt. Despite such differences, the initiation of magmatism propagated eastward at a constant rate of 19 cm/year throughout the entire belt. The average spacing of magmatic centers changes from 165±88 km in the west to 67±41 km in the east. The duration of magmatism as measured by U/Pb zircon and monazite ages increases from 1–2 Ma in the west to 4–5 Ma in the east. The duration of Ar–Ar biotite cooling ages also increases to the east. In agreement with the extended cooling times, large regions of greenschist and amphibolite facies metamorphism are present in the eastern segments, whereas outside of narrow pluton aureoles high-temperature metamorphism is not present in the western Sanak–Baranof belt. Finally, intrusions with adakitic characteristics such as high Sr/Y ratios are present in the east, but not in the west suggesting that some degree of slab melting occurred in the eastern Sanak–Baranof belt. We suggest that the above east–west thermal and magmatic variations can be explained by a decreasing rate of subduction and a persistent slab window in the eastern Sanak–Baranof belt caused by the oblique subduction of a segmented spreading ridge along a curved continental margin, and that this model is compatible with the Resurrection plate hypothesis. [Copyright &y& Elsevier]

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]

Abstract: The region of Alaska and adjacent northwest Canada is tectonically active and is subjected to multiple tectonic processes including plate subduction and terrane accretion. These tectonic processes and the forces originating thereof are responsible for high seismicity in the region and deformation of the crust. In the present-day tectonic setting, the Yakutat terrane is obliquely colliding with Alaska along the Aleutian Trench. Also, flat subduction due to under thrusting of a thickened crust, probably of oceanic affinity, is contributing to the tectonic evolution of this region in a basal traction collision style. This study uses the 2D, planform, thin-viscous-sheet model to investigate the effect of the Yakutat terrane colliding with Alaska and adjacent northwest Canada. Along with the obliquity and velocity of convergence, the lateral strength heterogeneities in the crust are considered in this investigation. The results of the numerical model are constrained with the observed topography and stress orientation in Alaska. It is shown that the Alaska–Yakutat collision is producing asymmetric deformation of the crust with respect to the normal to the collision boundary and that lateral strength heterogeneities contribute significantly to the deformation of the crust. Also, the influence of this collision can be observed up to a distance of ∼700km inland from the collision boundary. [Copyright &y& Elsevier]

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]

Volcanic eruptions are usually preceded by measurable signals of growing unrest, the most evident of which are the increase in seismicity and ground deformation. It is also important to identify precursors of a possible renewal of the volcanic activity and to distinguish between an eruptive activity characterized by an intrusion (with the related destructive power) and a migration of magma stored in the main conduits. The 2005–2006 eruption at Mt. Augustine (Alaska, USA) is a good example of a massive migration of magmatic fluids from depth (about 1 km b.s.l.) under the effect of gas overpressure. The movements, recorded by High Rate GPS (HRGPS) data (15 s of sampling and processing rate) from the stations deployed on the volcano, define the dimensions and the characteristics of the shallow plumbing system. In this study, we propose a model of the different stages preceding the effusive phase (the ‘precursory phase’), where gas overpressure in the body of the volcano opens the terminal conduit. [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]

Surface and subsurface data from northern Yukon document a northward facies transition from shelf carbonates to basinal graptolitic shales and cherts from Late Cambrian to Early Devonian time. Parts of this north-facing continental margin were deformed during separate orogenic events of Early Devonian and Early Carboniferous ages. The first event, the Romanzof Orogeny, is identified in exposures across northwestern Yukon, in adjacent northeastern Alaska, and locally in the subsurface of the Alaska North Slope. It resulted in tight folds, north-directed thrust faults, and intrusion by Late Devonian posttectonic granitic plutons. Notwithstanding the thrust-fault orientations, southward diminution of deformation intensity combined with facies variations suggest that tectonic transport was generally southward. Evidence for an Early Carboniferous event is preserved in the northern Richardson Mountains and locally in the subsurface of the Mackenzie Delta region. It consists of detached open folds and minor thrust faults. Geological and geophysical data from northern Yukon document the location and orientation of the Early Carboniferous deformation front, and define a regional tectonic transport direction toward the south or southeast. This event is a distal foreland element of the Ellesmerian Orogeny (sensu stricto) of the Canadian Arctic Islands and is distinct from the Romanzof event in age, intensity, and extent. Endicott and Lisburne group strata, deposited on a southwest-facing subsiding shelf, overstep rocks deformed by the Romanzof event even as Ellesmerian deformation encroached from the north. [ABSTRACT FROM AUTHOR]

Sediments deposited in three fjord systems latitudinally spanning the northeast Gulf of Alaska coastline have distinctive rock magnetic characteristics that are derived from glacially eroded accreted terranes. Ideal conditions exist in coastal Alaska to preserve primary rock magnetic properties within the sediment record because of the dominance of glacial erosion and rapid sediment transportation and burial. In temperate glacially dominated marine settings, terrigenous sediment is not diluted by biogenic material and diagenetic alteration is minimal. The juxtaposition of temperate glaciers on the northwest-trending accreted terranes of the southern Alaska continental margin provides magnetic provenance tracers of sediment derived from this region that can aid in the interpretation of glacial, paleoclimate, and tectonic proxy records from marine sediment. Holocene sediment deposited in Glacier Bay is eroded from the Craig subterrane of the Alexander terrane; it is characterized by high-volume magnetic susceptibility and coarse-grained, pseudosingle-domain (PSD) and multidomain (MD) magnetite. In Yakutat and Disenchantment bays, farther to the north, glaciers also erode the Yakutat and Chugach terranes. There, sediment contains slightly finer-grained PSD and MD magnetite, which is chemically distinct from that of Glacier Bay. In the northern-most sector, sediment is eroded from the Chugach terrrane in Harriman and College fiords, adjacent to Prince William Sound. These sediments have low magnetic susceptibility and contain micron-size pyrrhotite. [ABSTRACT FROM AUTHOR]

The Brooks Range is a 950-km-long north-vergent fold and thrust belt, which was formed during Mesozoic convergence of the continental Arctic Alaska terrane and the oceanic Angayucham terrane and was further shortened and uplifted in Tertiary time. The Arctic Alaska terrane consists of parautochthonous rocks and the Endicott Mountains and De Long Mountains subterranes. The Endicott Mountains allochthon of the western Brooks Range is the setting for many sulfide and barite occurrences, such as the supergiant Red Dog zinc-lead mine. Mineralization is sediment hosted and most commonly is present in black shale and carbonate turbidites of the Mississippian Kuna Formation. The reconstructed Kuna basin is a 200 by +600 km feature that represents the culmination of a remarkable chain of events that includes three fluvial-deltaic and two or more orogenic cycles, Middle Devonian to Mississippian episodes of extension and igneous activity, and the emergence of a seaward Lower Proterozoic landmass that may have constituted a barrier to marine circulation. Mississippian extension and related horst-and-graben architecture in the western Brooks Range is manifested in part by strong facies variability between coeval units of allochthons and structural plates. Shallow marine to possibly nonmarine arkose, platform to shelf carbonate, slope-to-basin shale, chert and carbonate turbidites, and submarine volcanic rocks are all represented in Mississippian time. The structural setting of Mississippian sedimentation, volcanism, and mineralization in the Kuna basin may be comparable to documented Devono-Mississippian extensional sags or half-grabens in the subsurface north of the Brooks Range. Climate, terrestrial ecosystems, multiple fluvial-deltaic aquifers, and structural architecture affected the liberation, movement, and redeposition of metals in ways that are incompletely understood. [ABSTRACT FROM AUTHOR]

We calculated seismic moment rates from crustal earthquake information for the upper Cook Inlet region, including Anchorage, Alaska, for the 30 yr prior to and 36 yr following the 1964 Great Alaska earthquake. Our results suggest over a factor of 1000 decrease in seismic moment rate (in units of dyne centimeters per year) following the 1964 mainshock. We used geologic information on structures within the Cook Inlet basin to estimate a regional geologic moment rate, assuming the structures extend to 30 km depth and have near-vertical dips. The geologic moment rates could underestimate the tree rates by up to 70% since it is difficult determine the amount of horizontal offset that has occurred along many structures within the basin. Nevertheless, the geologic moment rate is only 3-7 times lower than the pre-1964 seismic moment rate, suggesting the 1964 mainshock has significantly slowed regional crustal deformation. If we compare the geologic moment rate to the post-1964 seismic moment rate, the moment rate deficit over the past 36 yr is equivalent to a moment magnitude 6.6-7.0 earthquake. These observed differences in moment rates highlight the difficulty in using seismicity in the decades following a large megathmst earthquake to adequately characterize long-term crustal deformation. [ABSTRACT FROM AUTHOR]

Reprocessing and interpretation of seismic profiles from the Gulf of Alaska in the vicinity of the St. Elias Mountains indicate that over 3000 km3 of sediment were deposited on the ocean shelf since the end of the last glacial epoch, approximately 10,000 years ago. This estimate of the sediment volume does not include material moved out of the region by long shore currents, transportation from the shelf to deeper waters, or material trapped in fiords and basins on and near land. Thus, an estimated sedimentation rate offshore of 7.9 mm/year and the average erosion rate of 5.1 mm/year from the St. Elias Mountains, derived from the estimated sedimentation rate, represent minimum estimates. We infer that these estimated rates are not typical of the approximately 5 m.y. history of mountain building in the area, but rather, they represent the result of efficient erosion by glaciers during this interglacial period. [Copyright &y& Elsevier]

U–Pb geochronologic studies have been conducted on 60 detrital zircon grains from Permian(?) and Triassic metasandstones of the Taku terrane in central southeast Alaska. The resulting ages are mainly in the range 349–387 Ma, with five additional grains that yield probable ages ranging from ~906 to ~2643 Ma. These ages are similar to the ages of detrital zircons in Carboniferous and older rocks of the Yukon–Tanana terrane, which lies directly east of the Taku terrane. In contrast, these ages are different from the ages of detrital zircon grains in the Alexander terrane to the west. The data are accordingly consistent with models in which the Taku terrane is a western component of the Stikine and Yukon–Tanana terranes, and that this crustal fragment is separated by a fundamental tectonic boundary from rocks of the Alexander and Wrangellia terranes to the west.Des études géochronologiques ont été effectuées sur soixante grains détritiques de zircon provenant de métagrès (Permien(?) et Trias) du terrane de Taku dans le centre sud-est de l'Alaska. Les âges déterminés sont principalement dans la fourchette 349–387 Ma et cinq grains additionnels ont donné des âges probables allant de ~906 à ~2643 Ma. Ces âges sont similaires aux âges des zircons détritiques dans des roches datant du Carbonifère et dans des roches plus anciennes du terrane de Yukon–Tanana, lequel est situé directement à l'est du terrane de Taku. Par contre, ces âges diffèrent des âges de grains détritiques de zircon dans le terrane d'Alexander, plus à l'ouest. Les données concordent donc avec des modèles dans lesquels le terrane de Taku est une composante occidentale des terranes de Stikine et de Yukon–Tanana et que ce fragment de la croûte est séparé des roches des terranes d'Alexander et de Wrangellia, plus à l'ouest, par une frontière tectonique fondamentale.[Traduit par la Rédaction] [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 which discussed the kinematics of basin development during the transition from terrane accretion to tectonics in Alaska. History of tectonics of the Pacific margin; Age of volcanic rocks in Cantwell, Alaska; Overview of volcanic stratigraphy.

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.

Central Interior Alaska is one of the most seismically active regions in North America, exhibiting a high concentration of intraplate earthquakes approximately 700 km away from the southern Alaska subduction zone. Seismological evidence suggests that intraplate seismicity in the region is not uniformly distributed, but concentrated in several discrete seismic zones, including the Nenana basin and the adjacent Tanana basin. Although the location and magnitude of the seismic activity in both basins are well defined by a network of seismic stations in the region, the tectonic controls on these intraplate earthquakes and the heterogeneous nature of Alaska's continental interior remain poorly understood. We investigated the crustal structure of the Nenana and Tanana basins using available seismic reflection, aeromagnetic and gravity anomaly data, supplemented by geophysical well logs and outcrop data. We developed nine new two-dimensional forward models to delineate internal geometries and the crustal structure of Alaska's interior. The results of our study demonstrates a strong crustal heterogeneity beneath both basins. The Tanana basin is a relatively shallow (up to 2 km) asymmetrical foreland basin with its southern, deeper side controlled by the northern foothills of the Central Alaska Range. Northeast-trending left lateral strike-slip faults within the Tanana basin are interpreted as a zone of clockwise crustal block rotation. The Nenana basin has a fundamentally different geometry. It is a deep (up to 8 km), narrow transtensional pull-apart basin that is deforming along the left-lateral Minto Fault. This study identifies two distinct modes of current tectonic deformation in Central Interior Alaska and provides a basis for modeling the interplay between intraplate stress fields and major structural features that potentially influence the generation of intraplate earthquakes in the region. [ABSTRACT FROM AUTHOR]

The article offers information on the geologic map of Alaska. It includes data on Albers Conical Equal Area, Geodetic model and horizontal datum name. It cites the difference between the faults theme and the geologic theme and that the faults coverage was in a different projection. It mentions a change in geology in the northern Brooks Range to estimate in geology in Bennison. It also notes the modification and simplification made on some polygon shapes.

Southeastern Alaska is home to one of the most actively deforming continental margins in the world, and temperate glaciers covering the area contribute to high rates of erosion and offshore sedimentation, making the Neogene sediment covering the offshore continental shelf an ideal location to study interactions between climate and tectonics. We present a high-resolution two-dimensional P-wave velocity model of the glaciomarine sediment and underlying basement in the area and compare to coincident seismic reflection profiles in order to determine lithologic boundaries and correlate with stratigraphic units observed onshore and in offshore boreholes. Data used in this study includes ~115 km of multichannel seismic reflection and coincident ocean-bottom seismometer wide-angle reflection and refraction data. In Yakutat Bay, sediment velocities are high (~ 4 km/s) compared to velocities farther offshore (< 3 km/s), possibly indicating greater compaction of the sediments in the bay. In the central portion of the shelf, three distinct refractions from sedimentary layers are observed and are interpreted as the Yakataga, Poul Creek, and Kulthieth Formations. Northern profiles show only two refractions from sedimentary layers. Basement refractions are observed in the north at close offsets, evidence of shallow basement that could be indicative of uplift in the bay. Our velocity profile used in conjunction with seismic reflection data shows transitions in dominant depositional processes over time in the central portion of the block and helps to provide a regional framework of the geometries of the sedimentary packages and their spatial relationship to major tectonic features. [ABSTRACT FROM AUTHOR]

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]

Focuses on a report which was published in the periodical 'Geophysical Research Letters,' where Greg Anderson and Chen Ji described their modelling of stress transfer during earthquakes that occurred in Alaska on October 23, 2002 and November 3, 2002. Measurement of the stress transferred to the hypocenter of the Denali fault system; Similarity of the Denali fault mainshock to the san Francisco earthquake of 1906.