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T.R. Bruns

Publications and source records attributed to T.R. Bruns.

15 recordsLinked to original sources
USGS1988

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

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.

Marine Geology↗
USGS1983

Structure maps and seismic stratigraphy of the Yakataga segment of the continental margin, northern Gulf of Alaska

Multichannel seismic-reflection data show the late Cenozoic structure, seismic stratigraphy, and geologic history of the Yakataga segment of the continental margin, between Icy Bay and Kayak Island, northern Gulf of Alaska. The structure of the Yakataga segment consists of broad folds and associated thrust faults beneath the continental shelf and slope, trending generally northeast in the eastern part of the s~gment to east-west in the western part. Anticlines are generally asymmetric and doubly plunging and are commonly bounded on the seaward side by high-angle thrust faults. The degree of deformation is less intense than is observed in adjacent onshore areas. The age of deformation decreases seaward and the deformation shows an overall southeastward migration with time. Structures of similar age define three structural zones; structural growth was roughly contemporaneous in each structural zone, although the local growth pattern is complex in detail. Deformation within each structural zone was followed by subsidence and burial by rapidly deposited marine sediment. Deformation appears to have been continuous during the late Cenozoic, rather than a series of discrete events, because subsidence of a particular structural zone is accompanied by initiation of growth on a younger, more seaward zone. Varying degrees of reactivation of the older structures within recent time have resulted in renewed uplift of these structures. Average sedimentation, uplift, and subsidence rates are all extremely high, and generally range from about 1 to 2 m/1,000 yr; these rates can locally be much higher. The average strike of structures in the Yakataga segment indicates northwest-southeastward compression and is consistent with the current convergence direction between the Pacific and North America plates. Observed shortening within the segment is much less than required by the late Cenozoic convergence rate (about 6 cm/yr), and the major deformation is taken up elsewhere, primarily onshore. Thus, the deformation of the Yakataga segment is caused by minor shortening of the continental margin between Cross Sound and Kayak Island, which together comprise the Yakutat block, as the margin moves northwestward with the Pacific plate and collides with the North America plate.

Alaska↗
USGS1982

Structure and petroleum potential of the continental margin between Cross Sound and Icy Bay, northern Gulf of Alaska

Major structural features of the Yakutat segment, the segment of the continental margin between Cross Sound and Icy Bay, northern Gulf of Alaska, are delineated by multichannel seismic reflection data. A large structural high is centered on Fairweather Ground and lies generally at the edge of the shelf from Cross Sound to west of the Alsek Valley. A basement uplift, the Dangerous River zone, along which the seismic acoustic basement shallows by up to two kilometers, extends north from the western edge of Fairweather Ground towards the mouth of the Dangerous River. The Dangerous River zone separates the Yakutat segment into two distinct subbasins. The eastern subbasin has a maximum sediment thickness of about 4 km, and the axis of the basin is near and parallel to the coast. Strata in this basin are largely of late Cenozoic age (Neogene and Quaternary) and approximately correlate with the onshore Yakataga Formation. The western subbasin has a maximum of at least 9 km of sediment, comprised of a thick (greater than 4.5 km) Paleogene section overlain by late Cenozoic strata. The Paleogene section is truncated along the Dangerous River zone by a combination of erosion, faulting, and onlap onto the acoustic basement. Within the western subbasin, the late Cenozoic basin axis is near and parallel to the coast, but the Paleogene basin axis appears to trend in a northwest direction diagonally across the shelf. Sedimentary strata throughout the Yakutat shelf show regional subsidence and only minor deformation except in the vicinity of the Fairweather Ground structural high, near and along the Dangerous River zone, and at the shoreline near Lituya Bay. Seismic data across the continental slope and adjacent deep ocean show truncation at the continental slope of Paleogene strata, the presence of a thick (to 6 km) undeformed or mildly deformed abyssal sedimentary section at the base of the slope that in part onlaps the slope, and a relatively narrow zone along the slope or at the base of the slope where faulting may have occurred. Observed deformation at the base of the slope is primarily related to the late Cenozoic uplift of Fairweather Ground, and to Quaternary folding perpendicular to the Pacific-North America relative convergence vector. No accretionary section or major deformation is observed along the continental slope. The absence of these features suggests that no major subduction of the Pacific plate beneath the Yakutat margin has occurred during the late Cenozoic. However, transform faulting along the base of the slope has occurred, because probable Oligocene oceanic basement is juxtaposed against Mesozoic and Paleogene sedimentary strata of the Yakutat slope. This juxtaposition most likely occurred during late Oligocene and Miocene time. During much of the late Cenozoic, and especially during Pliocene-Pleistocene time, the Yakutat segment has apparently been moving northward with the Pacific plate. Dredge samples from the continental slope recovered potential hydrocarbon source and reservoir rocks from the Paleogene sedimentary sequence. Most of the organic matter from these samples is immature to marginally mature. Lopatin calculations suggest that rocks beneath the shelf are likely to be thermally mature at a depth of 4 to 5 km and deeper. In general, the strata at these depths are largely of Paleogene age. Thus, the Paleogene strata may have significant resource potential if source and reservoir rocks similar to those dredged at the slope are present below the shelf. The Paleogene strata are contained primarily within the western subbasin; strata in the east subbasin appear to have little resource potential. Structural traps are apparently present in parts of the basin near and along the Dangerous River zone. These traps are in an updip position from potentially mature strata of the western subbasin, and may hold commercial accumulations of hydrocarbons, if sufficient hydrocarbon generation and migration has occurred

Open-File Report↗
USGS1981

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.

Geological Society of America Bulletin↗
USGS1978

Refraction studies between Icy Bay and Kayak Island, eastern Gulf of Alaska

Results of five seismic refraction lines shot by the U.S. Geological Survey in the Gulf of Alaska between Icy Bay and Kayak Island indicate the following: (1) The Continental Shelf is underlain by as much as 11 km of sedimentary rock of probable Tertiary age where refraction velocities range from 1.2 to 5.5 kilometers per second; (2) a section approximately 5 km thick, which has velocities of 4.1-5.5 km/s and which could represent the Orca Group (lower Tertiary), is present in the western part of the study area but not in the eastern part; and (3) consistent basement velocities of approximately 7.0 km/s could indicate oceanic crust underlying the continental margin.

Alaska↗