Terry R. Bruns

Model for the origin of the Yakutat block, an accreting terrane in the northern Gulf of Alaska

A composite oceanic and continental terrane, the Yakutat block, is currently colliding with and accreting to North America in the northern Gulf of Alaska. Marine geophysical data indicate that the block has moved with the Pacific plate during Pliocene and Quaternary time, but are indeterminant about its previous movement history. A model that explains some observed geologic and structural features of the block is that it originated as a result of subduction of the Kula-Farallon spreading center beneath North America. In this model, the spreading center that formed the basaltic basement of the Yakutat block was offset from a spreading center to the south by a left-lateral transform fault 400–500 km long. Subduction of the northern spreading center about 45 m.y. ago sliced off part of the North America continental margin, attached it to the Kula plate, and initiated northward movement of the composite terrane as the Yakutat block. During Eocene and Oligocene time, migration of the southern spreading center toward North America truncated the Yakutat block on the south and left a fossil fracture zone that is now seen as the Transition fault. The southern spreading center subducted about 25 m.y. ago, and the Yakutat block has since traveled with the Pacific plate to southern Alaska.

Structural development of an accretionary prism by thrust and strike-slip faulting: Shumagin region, Aleutian Trench

A combination of SEABEAM swath bathymetric data, closely spaced single-channel seismic reflection profiles, and multichannel seismic reflection profiles from the Shumagin Islands region indicates that the large-scale deformation of the accretionary prism in this segment of the Aleutian Trench occurs progressively through several stages: (1) folding along axes perpendicular to the plate-convergence direction in the region, (2) thrust faulting in the direction of plate convergence and (3) oblique strike-slip faulting along conjugate west-northwest-trending right-lateral and north-northeast-trending left-lateral faults. Strike-slip faults occur in distinct domains, with right-lateral oblique faults present west of a major transverse fault at long 161°W. and left-lateral oblique faults prevalent east of 161°W. The structural development of the modern Aleutian Trench fore arc in this region is probably affected by pre-Eocene inherited structures that are still active in the present-day stress regime. A major domain boundary fault located at long 161°W. is probably part of a strike-slip fault system that also bounds Sanak Basin farther landward on the Shumagin shelf. This fault zone is nearly coincident with the mapped position of the western edge of the Shumagin seismic gap. These transverse structures in the Shumagin fore arc probably influence the spatial distribution of stress buildup related to subduction, and hence the locations of rupture zones of great interplate earthquakes, along this segment of the Aleutian Trench.

Submarine valleys in the northeastern Gulf of Alaska: Characteristics and probable origin

Abstract The continental shelf of the northeastern Gulf of Alaska Between Prince William Sound and Cross Sound is cut by at least eight major valleys. From west to east, these are Hinchinbrook Seavalley, Egg Island Trough, Kayak Trough, Bering Trough, Pamplona Troughs, Yakutat Valley, Alsek Valley and Yakobi Valley. Evidence common to most of these troughs or valleys indicating that the present morphology is due to glacial processes includes: (1) a pre-Holocene subbottom erosional surface incised into the underlying lithified strata of the shelf; (2) U-shaped cross sections, both at the sea floor and at the pre-Holocene erosional surface; (3) concave longitudinal sections, commonly shoaling at the seaward end; (4) till-like sediments collected from the walls or outer shelf adjacent to the troughs; and (5) seismic stratigraphy that can be correlated with bottom samples indicative of glacially derived strata. Depressions with tens of meters of relief are present on the pre-Holocene subbottom erosional surface beneath most of these valleys. These depressions have been partially filled by a seaward-thinning wedge of Holocene glacial flour (clayey silt) that is filling the valleys and blanketing the inner shelf at rates as high as 15 mm/yr (based on 210 Pb measurements). Although glaciation played a dominant role in the modern morphology of these sea valleys, structural features, including structurally controlled topographic highs on the shelf (e.g. Tarr Bank, Kayak Island, Pamplona Spur and Fairweather Ground) influenced the flow directions of the glacial lobes.

Transverse tectonic boundaries near Kodiak Island, Alaska.

Transverse tectonic boundaries exist at the northeast and southwest ends of the Kodiak islands, so that the Aleutian arc-trench system is longitudinally segmented in this area. Evidence for the transverse boundaries includes alignments of such geologic features as offset volcanic lineations, terminations of structural trends, and boundaries of discrete zones of earthquake aftershock sequences. The boundaries appear to be broad zones of disruption that began to form during the late Miocene or Pliocene. Although oceanic fracture zones and seamount chains intersect the continental margin near the boundaries, subduction of these features probably did not cause the tectonic boundaries. The fracture zones and seamount chains have swept northeastward along the margin, at least since the late Pliocene, because of the direction of convergence of the Pacific and North American plates.

Late Cenozoic offsets on the offshore connection between the Fairweather and Queen Charlotte faults off southeast Alaska

Abstract Closely spaced seismic reflection profiles permit us to extend the onshore Fairweather fault across the continental margin off southeast Alaska to the Queen Charlotte fault. Evidence for Quaternary movement along this major plate boundary includes 20–40-m high sea-floor scarps, offset subsurface seismic reflectors, sea-floor bathymetry that indicates as much as 70 km of right-lateral offset in the last 1 m.y., 5–6 cm/yr of movement on the onshore Fairweather fault, and recorded seismicity along the fault. The Fairweather-Queen Charlotte fault system, similar in dimension to the San Andreas fault system, is a dextral transform fault that truncates the continental margin of North America. Much of the late Cenozoic motion between the Pacific and North American plates has been taken up along the fault system. The Fairweather-Queen Charlotte fault evolved to its present form after passage of the Yakutat block, the youngest of the allochthonous terranes that form southern Alaska.

Development of slope valleys in the glacimarine environment of a complex subduction zone, Northern Gulf of Alaska

Abstract Morphological, seismic-reflection, and sedimentological evidence indicates that glacial ice tongues cut large sea valleys into the Gulf of Alaska continental shelf during the Pleistocene. During the Holocene, glacially-derived sediments from the Copper River and other meltwater streams have been prograding seaward across the shelf, covering the glacial and glacimarine upper Yakataga diamicts that blanketed the shelf and accumulated on the upper slope seaward of the sea valleys during the Pleistocene. GLORIA imagery near Middleton Island provides a new perspective on the glacimarine depositional environment on the continental slope in a collision zone between the Pacific and North American plates. Southwest of Middleton Island, along the subduction margin, sinuous valleys funnel sediment around shelf-edge-parallel, subduction-created, anticlinal ridges that have deflected and locally trapped glacimarine sediment. The slope south and southeast of Middleton Island where oblique convergence occurs, is incised by dendritic, erosional gulley systems, contains no compressional ridges, and thus, the apparently active sediment pathways to the trench are unrestricted. However, below the sea valley mouths, apparently both glacial and glacimarine sediments blanket the upper slope, covering any dendritic gulley systems that may have formed during or since the Pleistocene low stands of sea level.

Bering Trough: a Product of the Bering Glacier, Gulf of Alaska

The Gulf of Alaska area has experienced glaciation since Miocene time [Lagoe et al., 1993]. Large broad seavalleys that are incised in the continental shelf (Fig. 1) indicate that the extensive glaciers, which presently border the northeastern shore of the Gulf of Alaska, had lobes of ice that extended completely across the shelf [Carlson et al., 1982]. This chapter will concentrate on one of the glacially-carved seavalleys, the 25-km-wide Bering Trough (Fig. +1), that extends about 60 km across the shelf just seaward of the Bering Glacier, which recently re-advanced about 9 km between September, 1993 and September, 1994 [Molnia et al., 1994].

Structure of Shumagin Continental Margin, Western Gulf of Alaska: ABSTRACT

The Shumagin margin is characterized by five major structural features or trends: (1) Shumagin basin, containing about 2.5 km of late Miocene and younger strata above acoustic basement; (2) Sanak basin, containing as much as 8 km of dominantly late Cenozoic strata in two subbasins separated by a basement high; (3) cenozoic shelf-edge and upper-slope sedimentary wedges that are 3-4 km thick and possibly as thick as 6 km; (4) a midslope structural trend, Unimak ridge, that is characterized by numerous surface and subsurface structural highs; and (5) a 30-km wide accretionary complex at the base of the slope. A thin (less than 1-2 km) sediment cover of Miocene and younger age covers the continental shelf areas outside of Shumagin and Sanak basins. The tectonic history of the margin includes: (1)j late Cretaceous or early Tertiary removal of the seaward part of the Cretaceous Alaska Peninsula margin along the Border Ranges fault and accretion of the Shumagin Formation against the truncated margin; (2) Miocene uplift and erosion of the shelf; (3) middle or late Miocene uplift of Unimak ridge; and (4) late Miocene and younger subsidence and infilling of Sanak and Shumagin basins, and subduction-accretion along the Aleutian Trench.

Structural Deformation in Northern Gulf of Alaska: Transition from Transform to Convergent Plate Motion: ABSTRACT

Multichannel seismic reflection data reveal the late Cenozoic structure along the continental margin in the northern Gulf of Alaska, where transform motion along the Queen Charlotte-Fairweather fault system gives way to convergent motion along the Aleutian Trench. The active trace of the Fairweather fault system lies generally near the outer shelf and upper slope but, south of Sitka, broad folds and associated faults in late Cenozoic strata seaward of the active trace may indicate additional fault splays beneath the continental slope. The intensity of deformation in these strata decreases to the north, and slope deposits seaward of the Fairweather fault are undeformed between Sitka and Cross Sound. Between Cross Sound and Icy Bay (the Yakutat segment), Eocene and younger helf strata are relatively undeformed along the continental slope. Late Cenozoic abyssal strata, which partly onlap the continental slope, are relatively undeformed except for local recent deformation seaward of Fairweather Ground. The observed structure along the Yakutat segment of the continental margin is more readily explained by strike-slip motion between the Yakutat segment and the Pacific plate than by oblique subduction of the Pacific plate as deduced from plate tectonic models. Between Icy Bay and Kayak Island (the Yakataga segment), northeast-trending faults and folds that deform Cenozoic strata beneath the shelf and slope suggest relatively continuous late Cenozoic convergence between the Yakataga and Yakutat segments of the continental margin. Thus, the Yakutat segment may ha e been coupled to the Pacific plate during much of the late Cenozoic. End_of_Article - Last_Page 907------------

Subduction-Related Structure Along Eastern Aleutian Trench: ABSTRACT

Along the eastern Aleutian Trench between Kayak and Sanak Islands, the trench, magmatic arc, and active Benioff zone define a modern convergent margin 13,000 km long. The structures developed during the past 10 to 15 m.y. show the deformation associated with subduction. Fore-arc basins have been formed on pre-Neogene rock that was presumably accreted, uplifted, and then locally depressed. Structures in the basins are compressional near the shelf edge and extensional farther toward the arc. The trench lower slope is underlain by rocks that are coeval with those in the fore-arc basin, but are deformed as a result of subduction. Despite a uniform convergence history, the margin has marked structural variations along strike that reflect local non-tectonic influences on struct ral style. The structure could be influenced by a variety of geologic variables such as sediment volumes on the slope, in the trench, and on the ocean crust, and perhaps by the morphology of the igneous ocean crust. Although we have not yet successfully reconciled all the effects of local variation on structural style, a comparative study of these styles and local features can help define kinematic processes associated with plate convergence. End_of_Article - Last_Page 1004------------