Geology topics

N.I. Christensen

Publications and source records attributed to N.I. Christensen.

7 recordsLinked to original sources

USGS2012

Crustal structure of accreted terranes in southern Alaska, Chugach Mountains and Copper River Basin, from seismic refraction results

Seismic refraction data were collected along a 320-km-long "transect' line in southern Alaska, crossing the Prince William, Chugach, Peninsular, and Wrangellia terranes, and along several shorter lines within individual terranes. Velocity structure in the upper crust (less than 9-km depth) differs among the four terranes. In contrast, layers in the middle crust (9- to 25-km depth) in some case extend across projected terrane boundaries. The top of a gently north dipping sequence of low- and high-velocity layers (5.7-7.8 km/s), more than 10 km thick, extends from near the surface in the southern Chugach terrane to more than 20-km depth beneath the southern Peninsular terrane. This sequence, truncated by the suture between the Prince William and Chugach terranes, is interpreted to be an underplated "terrane' made up of fragments of the Kula plate and its sedimentary overburden that were accreted during subduction in the late Mesozoic and/or early Tertiary, during or between times of accretion of the Prince William and Chugach terranes.

Alaska↗

USGS2012

Seismic reflection/refraction mapping of faulting and regional dips in the eastern Alaska Range

We present the results of a Trans‐Alaska Crustal Transect (TACT) investigation of the upper 2–5 km of the eastern Alaska Range in the vicinity of the Denali fault based on seismic reflection/refraction data, laboratory measurements of rock velocities, and structural mapping. The Denali fault is a major dextral slip structure mappable for more than 2000 km separating the Wrangellia and adjacent terranes to the south from the Yukon‐Tanana and adjacent terranes to the north. Geologic mapping suggests over 400 km of dextral slip has occurred on the fault, yet within the upper 1.5 km of the crust along the TACT corridor, basement rocks juxtaposed along the Denali fault reveal no significant seismic velocity differences, although the fault zone itself is associated with a minor lowering of velocity. The lack of seismic velocity contrast adjacent to the fault is in agreement with laboratory measurements of elastic wave velocities of samples from terranes bordering the fault. Laboratory measurements of elastic wave velocities of the metasedimentary mica‐quartz schists comprising the Yukon‐Tanana basement are highly anisotropic because of preferred orientation of mica and predict significant variations in velocity accompanying variations in foliation dip. Although other interpretations are possible, the northward shallowing of foliation dip of basement rocks in the Yukon‐Tanana terrane combined with the strong anisotropy associated with these highly foliated rocks can explain an observed northward increase in seismic velocity within this terrane. Seismic reflections from basement rocks within the Yukon‐Tanana terrane may originate from variations in anisotropy with depth and/or changes in composition reflecting different proportions of sandstone and shale in the protolith.

Alaska↗

USGS2001

Upper crustal structure in Puget Lowland, Washington: Results from the 1998 Seismic Hazards Investigation in Puget Sound

A new three-dimensional (3-D) model shows seismic velocities beneath the Puget Lowland to a depth of 11 km. The model is based on a tomographic inversion of nearly one million first-arrival travel times recorded during the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS), allowing higher-resolution mapping of subsurface structures than previously possible. The model allows us to refine the subsurface geometry of previously proposed faults (e.g., Seattle, Hood Canal, southern Whidbey Island, and Devils Mountain fault zones) as well as to identify structures (Tacoma, Lofall, and Sequim fault zones) that warrant additional study. The largest and most important of these newly identified structures lies along the northern boundary of the Tacoma basin; we informally refer to this structure here as the Tacoma fault zone. Although tomography cannot provide information on the recency of motion on any structure, Holocene earthquake activity on the Tacoma fault zone is suggested by seismicity along it and paleoseismic evidence for abrupt uplift of tidal marsh deposits to its north. The tomography reveals four large, west to northwest trending low-velocity basins (Tacoma, Seattle, Everett, and Port Townsend) separated by regions of higher velocity ridges that are coincident with fault-bounded uplifts of Eocene Crescent Formation basalt and pre-Tertiary basement. The shapes of the basins and uplifts are similar to those observed in gravity data; gravity anomalies calculated from the 3-D tomography model are in close agreement with the observed anomalies. In velocity cross sections the Tacoma and Seattle basins are asymmetric: the basin floor dips gently toward a steep boundary with the adjacent high-velocity uplift, locally with a velocity "overhang" that suggests a basin vergent thrust fault boundary. Crustal fault zones grow from minor folds into much larger structures along strike. Inferred structural relief across the Tacoma fault zone increases by several kilometers westward along the fault zone to Lynch Cove, where we interpret it as a zone of south vergent faulting overthrusting Tacoma basin. In contrast, structural relief along the Seattle fault zone decreases west of Seattle, which we interpret as evidence that the N-S directed compression is being accommodated by slip transfer between the Seattle and Tacoma fault zones. Together, the Tacoma and Seattle fault zones raise the Seattle uplift, one of a series of east-west trending, pop-up structures underlying Puget Lowland from the Black Hills to the San Juan Islands.

Washington↗

USGS1997

Deep seismic structure and tectonics of northern Alaska: Crustal-scale duplexing with deformation extending into the upper mantle

Seismic reflection and refraction and laboratory velocity data collected along a transect of northern Alaska (including the east edge of the Koyukuk basin, the Brooks Range, and the North Slope) yield a composite picture of the crustal and upper mantle structure of this Mesozoic and Cenozoic compressional orogen. The following observations are made: (1) Northern Alaska is underlain by nested tectonic wedges, most with northward vergence (i.e., with their tips pointed north). (2) High reflectivity throughout the crust above a basal decollement, which deepens southward from about 10 km depth beneath the northern front of the Brooks Range to about 30 km depth beneath the southern Brooks Range, is interpreted as structural complexity due to the presence of these tectonic wedges, or duplexes. (3) Low reflectivity throughout the crust below the decollement is interpreted as minimal deformation, which appears to involve chiefly bending of a relatively rigid plate consisting of the parautochthonous North Slope crust and a 10- to 15-km-thick section of mantle material. (4) This plate is interpreted as a southward verging tectonic wedge, with its tip in the lower crust or at the Moho beneath the southern Brooks Range. In this interpretation the middle and upper crust, or all of the crust, is detached in the southern Brooks Range by the tectonic wedge, or indentor: as a result, crust is uplifted and deformed above the wedge, and mantle is depressed and underthrust beneath this wedge. (5) Underthrusting has juxtaposed mantle of two different origins (and seismic velocities), giving rise to a prominent sub-Moho reflector.

Journal of Geophysical Research B: Solid Earth↗

USGS1995

Seismic images of the Brooks Range, Arctic Alaska, reveal crustal- scale duplexing

An integrated set of seismic reflection and refraction data collected across the Brooks Range, Arctic Alaska, in 1990, has yielded a composite image of this Mesozoic and Cenozoic fold-and-thrust belt that reveals duplexing to lower-crustal depths. Interpretations from this image are as follows. (1) Many terranes and subterranes that were amalgamated in the Late Jurassic to Early Cretaceous extend no deeper than the upper crust (3–10 km). (2) In contrast, crustal duplexing, extending to nearly 30 km depth above a south-dipping basal decollement, has produced latest Cretaceous to Cenozoic antiforms, including the Doonerak antiform in the central Brooks Range and anticlinoria near the northern range front. (3) The duplexing occurs in basement rocks of the North Slope subterrane, which core the antiforms. (4) North-dipping structures in the middle crust of the Yukon-Koyukuk basin and southern Brooks Range may postdate Mesozoic terrane amalgamation and predate or coincide with the duplexing. (5) The thickest crust, 50 km, occurs beneath the north-central Brooks Range, north of the root zone of the basal decollement. The position of the thickest crust may indicate that either the duplexed crust above the decollement was thrust onto and depressed the plate beneath the North Slope or the protracted tectonic history of the Brooks Range has left structures not simply explainable in terms of a single collisional event.

Geology↗

USGS1994

Composition of the crust beneath the Kenya rift

We infer the composition of the crust beneath and on the flanks of the Kenya rift based on a comparison of the KRISP-90 crustal velocity structure with laboratory measurements of compressional-wave velocities of rock samples from Kenya. The rock samples studied, which are representative of the major lithologies exposed in Kenya, include volcanic tuffs and flows (primarily basalts and phonolites), and felsic to intermediate composition gneisses. This comparison indicates that the upper crust (5-12 km depth) consists primarily of quartzo-feldspathic gneisses and schists similar to rocks exposed on the flanks of the rift, whereas the middle crust (12-22 km depth) consists of more mafic, hornblende-rich metamorphic rocks, probably intruded by mafic rocks beneath the rift axis. The lower crust on the flanks of the rift may consist of mafic granulite facies rocks. Along the rift axis, the lower crust varies in thickness from 9 km in the southern rift to only 2-3 km in the north, and has a seismic velocity substantially higher than the samples investigated in this study. The lower crust of the rift probably consists of a crust/mantle mix of high-grade metamorphic rocks, mafic intrusives, and an igneous mafic residuum accreted to the base of the crust during differentiation of a melt derived from the upper mantle.

Tectonophysics↗

USGS1993

Reconciling deep seismic refraction and reflection data from the Grenvillian-Appalachian boundary in western New England

The Grenvillian-Appalachian boundary is characterized by pervasive mylonitic deformation and retrograde alteration of a suite of imbricated allochthonous and parautochthonous gneisses that were thrust upon the Grenvillian continental margin during the lower Paleozoic. Seismic reflection profiling across this structural boundary zone reveals prominent dipping reflectors interpreted as overthrust basement slices (parautochthons) of the Green Mountain Anticlinorium. In contrast, a seismic refraction study of the Grenvillian-Appalachian boundary reveals a sub-horizontally layered seismic velocity model that is difficult to reconcile with the pronounced sub-vertical structures observed in the Green mountains. A suite of rock samples was collected from the Green Mountain Anticlinorium and measured at high pressures in the laboratory to determine the seismic properties of these allochthonous and parautochthonous gneisses. The laboratory-measured seismic velocities agree favorably with the modelled velocity structure across the Grenvillian-Appalachian boundary suggesting that the rock samples are reliable indicators of the rock mass as whole. Samples of the parautochthonous Grenvillian basement exposed in the Green Mountains have lower velocities, by about 0.5 km/s, than lithologically equivalent units exposed in the eastern Adirondack Highlands. Velocity reduction in the Green Mountain parautochthons can be accounted for by retrograde metamorphic alteration (hydration) of the paragneisses. Seismic anisotropies, ranging from 2 to 12%, in the mylonitized Green Mountain paragneisses may also contribute to the observation of lower seismic velocities, where the direction of ray propagation is normal to the foliation. The velocity properties of the Green Mountain paragneisses are thus insufficiently different from the mantling Appalachian allochthons to permit their resolution by the Ontario-New York-New England seismic refraction profile.

New York, Vermont↗