Abstract Subeconomic molybdenum-bearing prophyry deposits and occurrences are scattered widely throughout mainland and southeastern Alaska. Most deposits in mainland Alaska are copper-molybdenum porphyries of probable calc-alkaline arc-related origins. These include Devonian examples in the Brooks Range, middle to Late Cretaceous examples in the northern Alaska Peninsula and northern Wrangell Mountains, and Miocene or younger examples in the Alaska peninsula and Aleutian Islands. Deposits in eastern Alaska and the northeastern Brooks Range are early Tertiary in age and may be related to a period or back-arc, extensional intraplate tectonics. Porphyry deposits in southeastern Alaska are primarily molybdenum bearing and can be roughly separated into two groups based upon their position relative to the Coast batholith. An outboard group of deposits is distributed between Prince of Wales Island and Glacier Bay and is mostly either Jurassic to Cretaceous or middle Eocene to early Oligocene in age. A secoud group of deposits lies within and along the western margin of the Coast batholith from north of Skagway to the latitude of Ketchikan. Isotopic age determinations suggest two periods of molybdenum mineralization during the Eocene-Paleocene and the Oligocene-Miocene. Alaska’s molybdenum-bearing porphyries, especially those of southeastern Alaska, span the classification spectrum from differentiated monzogranite-hosted, subduction-related, low-grade, low fluorine types to high silica rhyolite-alkalic-hosted, high-grade, high fluorine deposits that form in extensional tectonic settings. Endmember types of various ages are present in both southeastern Alaska groups, suggesting that multiple periods of botb lifting- and subduction-related porphyry molybdenum mineralization occurred. The giant Quartz Hill deposit is the only known potentially economic porphyry molybdenum deposit in Alaska. It is located 70 km east of Ketchikan and is hosted by an Oligocene felsic stock that has been emplaced into paragneiss and orthogneiss of the Coast batholith. Quartz Hill has characteristics of both rift-related and subduction-related types. Available data suggest that these unusual characteristics may be the product of the emplacement of a rift-generated porphyry into a subduction-related calc-alkaline arc. The Quartz Hill composite stock, the Travis stock to the southwest, and a smaller stock to the northeast define a 5- by 12-km area of igneous activity, localized along an N 30° E trend. The Quartz Hill composite stock has a surface exposure of 3 by 5 km and is located at the intersection of the southwestern extension of an N 30° E, major right lateral strike-slip fault and the N 55° W Stephens fault. The stock is composed predominantly of porphyritic granites and rhyolites. The intrusive sequence includes initial emplacement of an epizonal felsic magma with early-stage crystallization of biotite granite, followed by rapid crystallization of the Quartz Hill porphyry. Porphyritic rhyolite plugs and dikes intrude the Quartz Hill porphyry and are associated with the formation of semicontemporaneous breccia bodies. Associated breccia pipes suggest possible venting. Late emplacement of rhyolite dikes and small stocks represents the final stage of intrusive activity related to the Quartz Hill composite stock. Regional rhyolite and intermediate to mafic dike swarms cut all other rock types. The Quartz Hill molybdenum deposit occurs almost entirely within the Quartz Hill composite stock. The deposit is tabular to slightly convex upward and is exposed at the surface. Molybdenite is the only mineral of economic importance and occurs with or without quartz in randomly oriented veinlets forming a well-developed stockwork. Mineral inventory calculations reported by the U.S. Borax and Chemical Company in 1982 confirm approxmately 1,760 million short tons with an average grade of 0.127 percent MoS 2 using a 0.08 percent MoS 2 cutoff. The deposit is approximately 2,800 m long and 1,500 m wide and extends to depths of 370 to 500 m. Higher grade mineralization occurs in two zones that contain approximately 200 million short tons each with grades slightly greater than 0.20 percent MoS 2 . Pyrite concentrations average about 1 percent and occur with molybdenite in quartz veinlets and as disseminations in the stock. Concentrations of copper, lead, zinc, and tin are below average crustal abundances, and tungsten is about twice its average crustal abundance. No base metal halo has been recognized surrounding the deposit. The most important product of hydrothermal activity is the development of an intense quartz-molybdenite stocwork. Silicification and development of secondaty potassium feldspar occurs as vein selvages and irregular replacements. Chlorite is widespread and is best developed in more mafic rocks where it is observed replacing biotite. Sericite locally replaces plagioclase adjacent to quartz-pyrite veinlets. Moderate to intense argillic alteration is generally restricted to fault and shear zones. Weak argillic alteration is widespread, forming a clay dusting on feldspars. Calcite occurs as partial replacement after plagioclase, in veins with quartz, and as grains in the matrix. Although classic porphyry molybdenum alteration assemblages are recognnized at Quartz Hill, alteration zonation has not been clearly defined.

Abstract The Queen Charlotte Fault defines the Pacific–North America transform plate boundary in western Canada and southeastern Alaska for c. 900 km. The entire length of the fault is submerged along a continental margin dominated by Quaternary glacial processes, yet the geomorphology along the margin has never been systematically examined due to the absence of high-resolution seafloor mapping data. Hence the geological processes that influence the distribution, character and timing of mass transport events and their associated hazards remain poorly understood. Here we develop a classification of the first-order shape of the continental shelf, slope and rise to examine potential relationships between form and process dominance. We found that the margin can be split into six geomorphic groups that vary smoothly from north to south between two basic end-members. The northernmost group (west of Chichagof Island, Alaska) is characterized by concave-upwards slope profiles, gentle slope gradients (<6°) and relatively low along-strike variance, all features characteristic of sediment-dominated siliciclastic margins. Dendritic submarine canyon/channel networks and retrogressive failure complexes along relatively gentle slope gradients are observed throughout the region, suggesting that high rates of Quaternary sediment delivery and accumulation played a fundamental part in mass transport processes. Individual failures range in area from 0.02 to 70 km 2 and display scarp heights between 10 and 250 m. Transpression along the Queen Charlotte Fault increases southwards and the slope physiography is thus progressively more influenced by regional-scale tectonic deformation. The southernmost group (west of Haida Gwaii, British Columbia) defines the tectonically dominated end-member: the continental slope is characterized by steep gradients (>20°) along the flanks of broad, margin-parallel ridges and valleys. Mass transport features in the tectonically dominated areas are mostly observed along steep escarpments and the larger slides (up to 10 km 2 ) appear to be failures of consolidated material along the flanks of tectonic features. Overall, these observations highlight the role of first-order margin physiography on the distribution and type of submarine landslides expected to occur in particular morphological settings. The sediment-dominated end-member allows for the accumulation of under-consolidated Quaternary sediments and shows larger, more frequent slides; the rugged physiography of the tectonically dominated end-member leads to sediment bypass and the collapse of uplifted tectonic features. The maximum and average dimensions of slides are an order of magnitude smaller than those of slides observed along other (passive) glaciated margins. We propose that the general patterns observed in slide distribution are caused by the interplay between tectonic activity (long- and short-term) and sediment delivery. The recurrence (<100 years) of M > 7 earthquakes along the Queen Charlotte Fault may generate small, but frequent, failures of under-consolidated Quaternary sediments within the sediment-dominated regions. By contrast, the tectonically dominated regions are characterized by the bypass of Quaternary sediments to the continental rise and the less frequent collapse of steep, uplifted and consolidated sediments.

The Coast orogen of western coastal British Columbia and southeastern Alaska is one of the largest batholithic belts in the world. This paper addresses the structure and composition of the crust in the central part of this orogen, as well as the history of its development since the mid-Cretaceous. The core of the orogen consists of two belts of metamorphic and plutonic rocks: the western metamorphic and thick-skinned thrust belt comprising 105–90-Ma plutons and their metamorphic country rocks, and the Coast Plutonic Complex on the east, with large volumes of mainly Paleogene magmatic rocks and their high-temperature gneissic host rocks. These two belts are separated by the Coast shear zone, which forms the western boundary of a Paleogene magmatic arc. This shear zone is subvertical, up to 5 km wide, and has been seismically imaged to extend to and offset the Moho. Lithologic units west of the Coast shear zone record contractional deformation and crustal thickening by thrusting and magma emplacement in the mid-Cretaceous. To the east, the Coast Plutonic Complex records regional contraction that evolves to regional extension and coeval uplift and exhumation after ca. 65 Ma. Igneous activity in the Complex formed a Paleogene batholith and gave rise to high crustal temperatures, abundant migmatite and, as a result, considerable strain localization during deformation. In both belts, during each stage of the orogeny, crustal-scale deformation enabled and assisted magma transport and emplacement. In turn, the presence of magma, as well as its thermal effects in the crust, facilitated the deformation. After 50 Ma, the style of crustal evolution changed to one dominated by periods of extension oriented approximately perpendicular to the orogen. The extension resulted in tilting of large and small crustal blocks as well as intra-plate type magmatic activity across the orogen. Seismic-reflection and refraction studies show that the crust of this orogen is unusually thin, probably due to the periods of orogen-perpendicular stretching. Magmatic activity west of the Coast shear zone in the Late Oligocene and Miocene was related to one period of orogen-parallel transtension along the margin. Small-scale, mafic, mantle-derived volcanic activity continues in the region today. The change from convergence to translation and extension is related to a major plate reorganization in the Pacific that led to a change from subduction of an oceanic plate to northwestward translation of the Pacific plate along the northwest coast of North America. Although it has been proposed that this orogen is the site of major (up to 4000 km) pre-Eocene northward terrane translation, there is little evidence for such large-scale displacement or for the kind of discontinuity in the geological record that such displacement would entail.

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Abstract Glacial systems release sediment to the marine environment over a range of time periods, including short-term seasonal, tidal, and diurnal scales. Often, the sedimentary record providing the highest temporal resolutions of short-term processes is found in relatively inaccessible areas such as proximal to glacier termini or below floating ice sheets. To assess the importance of short-term glacial and oceanographic processes in creating glacimarine strata, it is necessary to evaluate strata production over similar time scales. Time-series coring, sediment traps, or bathymetric profiles may be difficult to perform in these harsh settings. Coupling the observations of sedimentary structures seen in core X-ray radiographs with sound chronologies allows for the evaluation of short-term glacimarine sedimentation. For seasonal time-scales, the highly particle-reactive radioisotope 234 Th (24 day half-life) can be used to measure the rates of strata production under both steady and non-steady sediment deposition. To create appropriate age-depth relationships, two approaches are used: a rigorous and exacting mathematical model developed from a steady-state transport-reaction equation; and the less exacting CIC (constant initial concentration) and CRS (constant rate of supply or constant flux) point transformations. These methods are used for developing chronologies of cores collected in 1995 in Icy Bay, Alaska at 4, 12, and 32 km from the tidewater terminus of the Guyot Glacier. Examples are given of the boundary conditions that must be satisfied to use each approach in developing age-depth relationships. For the two cores collected closest to the terminus, age-depth relationships can be generated using all three approaches, whereas the presence of bioturbation in the most ice-distal sample severely complicates matters. The high (∼0.5 cm d −1 ) but non-steady sedimentation rates at the ice-proximal station create a high-resolution sedimentary record. The controls on sedimentation at this location are evaluated by examining sedimentary structures (alternating light and dark laminae and beds) seen in digitized X-ray radiographs. The CRS method is used to convert depth to time in X-ray radiographs. The down-core pixel intensities of the grey-scale X-ray radiograph positives are used to create a time series of sedimentation reflected in proxy data (the bulk density of the sediments). By using Blackman-Tukey spectral analyses and relatively new wavelet techniques to evaluate periodic processes, sedimentation at this location can be related to seasonal variability in meltwater production, fortnightly tidal influences on particle settling rates, and episodic precipitation-induced sediment deposition.