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Modeling of crustal structures in southwest Georgia from magnetic dataHerbert, James Charles 08 1900 (has links)
No description available.
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Determination of crustal velocity structures from teleseismic p wavesJiang, Wei Ping 05 1900 (has links)
No description available.
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Geochronology and Petrogenesis of Hadean to Paleoarchean Mafic and Felsic Crust from the Northeastern Superior Province, CanadaSole, Christian 11 January 2021 (has links)
The first billion years of our planet’s history is almost devoid of geological records and this scarcity of Eoarchean/Hadean rocks and minerals greatly limits our understanding of how and when the first crust formed on Earth. The Nuvvuagittuq Greenstone Belt (NGB), located in the Hudson Bay terrane of the Northeastern Superior Province, may host the oldest preserved rocks on Earth. It is locally intruded in its southwestern corner by rare 3.76 Ga trondhjemite bands which impose a minimum age for the NGB, but its dominant lithology, a mafic cummingtonite-amphibolite called the Ujaraaluk unit, displays isotopic evidence suggesting it may represent a rare remnant of Hadean mafic crust as old as 4.3 Ga. However, this proposed Hadean age for the NGB has been heavily debated for more than a decade. As potentially the only remnant of crust formed within the first 500 million years of Earth’s history, the NGB could have important implications on our knowledge of the first terrestrial crust. In order to impose tighter geochronological constraints on the NGB, here we present U-Pb zircon and ¹⁴⁷Sm-¹⁴³Nd whole rock data for gneissic gabbro sills that intrude the Ujaraaluk unit as well as U-Pb data for zircons from intruding and surrounding granitoids. A new strategy of sampling for the gneissic gabbros targeted the most evolved plagioclase-rich zones and amphibole-rich cumulative rocks to better constrain their age of magmatic differentiation. The most evolved parts of the sills were also sampled because they are the most likely to contain igneous zircon or baddeleyite that could constrain their crystallization age. Zircons from two compositionally evolved gneissic gabbros yielded U-Pb ages between 2.7 and 2.6 Ga consistent with the timing of Neoarchean metamorphism in the region. A plagioclase-rich layer found within the gabbro sills yielded zircons defining a U-Pb Concordia upper intercept age of 2789 Ma, but their texture and Th/U ratios are more consistent with recrystallization of zircon subsequent to the breakdown of an older Zr-bearing phase and therefore do not constrain the age of emplacement of the sills. However, a ¹⁴⁷Sm-¹⁴³Nd isochron for the gneissic gabbros, including the newly identified plagioclase-rich evolved zone and hornblende-rich cumulative rock, yielded an isochron age of 4151 ± 290 (MSWD = 9, n = 6) interpreted as the timing of magmatic differentiation of the sills. This 4.1 Ga age thus strongly supports the previously proposed Hadean age for the NGB. New zircon U-Pb data reported here for plutonic trondhjemites found in the central and eastern parts surrounding the NGB suggests that the extent of the ~3.8 Ga Eoarchean felsic magmatism is greater than previously thought. New zircon trace element and oxygen isotope data for a series of granitoids surrounding and locally intruding the NGB previously dated at 3.76 Ga, 3.66 Ga, 3.51 Ga and 3.35 Ga provide a better understanding of the petrogenetic processes responsible for early felsic crust production. Zircons from the 3.76 Ga, 3.66 Ga and 3.35 Ga granitoids are characterized by rare earth element trends typical of unaltered igneous zircons. However, zircons from the 3.51 Ga magmatic event display unusual rare earth element patterns, with a striking positive Eu-anomaly, suggesting that they may have experienced some type of post-magmatic alteration. Zircon δ¹⁸O values appear to have slightly increased over time with the zircons from the oldest 3.76 Ga trondhjemites displaying mean δ¹⁸O values within the mantle zircon field and the younger Paleoarchean granitoids progressively deviating from the mantle zircon field to reach a mean zircon δ¹⁸O value of 6.58‰ at 3.35 Ga. This suggests that the 3.76 Ga trondhjemites were derived from an unaltered crustal source, whereas the ≤3.66 Ga granitoids were derived from a supracrustal source that had experienced some degree of low-temperature hydrothermal alteration. This trend of mantle-like zircon δ¹⁸O values preserved in the first evolved crust, which deviate towards higher zircon δ¹⁸O values in successive felsic magmatic events, has also been observed in other Hadean and Eoarchean terranes indicating that similar processes may have operated on a global scale during the production and evolution of early continental crust.
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Seismological studies of upper crustal structure of the southern Midland Valley of ScotlandDavidson, K. A. S. January 1986 (has links)
No description available.
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The free-air gravity anomaly edge effect and the mechanical properties of the lithosphereMarr, Catherine January 1995 (has links)
No description available.
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Crustal structures and tectonism in southeastern Alaska and western British Columbia from seismic refraction, seismic reflection, gravity, magnetic, and microearthquake measurementsJohnson, Stephen Hans 13 October 1971 (has links)
Seismic refraction measurements along two unreversed lines
indicate that the earth's crust is 26 km thick in southeastern Alaska
and 30 km thick along the Inside Passage of British Columbia. The
crust in southeastern Alaska, north of Dixon Entrance, consists of
a layer 9 km thick with a seismic velocity of 5.90 km/sec, a layer
7 km thick with a seismic velocity of 6.30 km/sec. and a layer 10 km
thick with a seismic velocity of 6.96 km/sec. The crust along the
Inside Passage of British Columbia, south of Dixon Entrance, consists
of a layer 13 km thick with a seismic velocity of 6.03 km/sec, a layer
5 km thick with a seismic velocity of 6.41 km/sec, and a layer 12 km
thick with a seismic velocity of 6.70 km/sec. The velocity of the
mantle below the M discontinuity is 7.86 km/sec in southeastern
Alaska and 8.11 km/sec in British Columbia.
A compilation of Bouguer gravity data along the Inside Passage
from northern Vancouver Island to northern southeastern Alaska
indicates near-zero anomalies between steep gradients offshore and
near the western margin of the Coast Mountains. A two-dimensional
gravity model, constrained by seismic refraction measurements,
suggests that the thickness of the crust is constant beneath the region
of near-zero gravity anomalies and indicates a step-like transition
between oceanic and continental structure.
Seismic reflection, gravity, and magnetic measurements,
obtained during a 1970 cruise of the R/V Yaquina, help to determine
upper crustal structures in Dixon Entrance. Gravity models, constructed to agree with these data and the measurements of previous
investigators, indicate sediment thicknesses of nearly 3 km east
of Learmonth Bank and west of Celestial Reef. Magnetic models
suggest large lateral changes in basement susceptibility. Either
highly metamorphosed rock or basaltic intrusions can account for
these changes in susceptibility. Folded sediments suggest post depositional
distortion due either to regional compression or to
major local intrusions. Several linear gravity features, observed
in northern Dixon Entrance, disappear north of Graham Island.
Either the structures responsible for the gravity features end or
thick layers of basalt, extending northward from Graham Island,
obscure the effect of the structures.
A single-station survey detected microearthquakes at nine
locations in western British Columbia and southeastern Alaska. The
majority of the observed distant microearthquakes probably originated
in the Queen Charlotte Islands fault zone. However, observed
nearby microearthquakes indicate a microearthquake seismicity of
several events per day along the mainland coast of British Columbia.
Temporary seismic arrays located at a site along the central
portion of Chatham Strait near the Chatham Strait fault and at a site
in Glacier Bay recorded few nearby microearthquakes. Arrivals at
the arrays permitted the location of distant microearthquakes, however,
with epicenters in the vicinity of northern Lynn Canal and along
the Fairweather fault. / Graduation date: 1972
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Sonobuoy refraction study of the crust in the Gorda BasinCook, Jeffrey A. 05 December 1980 (has links)
The Gorda Basin is a young oceanic plate which comes in direct
contact with the convergent margin of western North America. Two long
sonobuoy refraction profiles crossing the basin provide nearly continuous
data for computing the velocity structure of the crust and adjacent
continental slope. Time-term analysis utilizing multiple receivers
and overlapping profiles revealed a thick transition layer which averages
2.3 km but displays considerable lateral variation. The seismic
compressional velocity of this layer is 5.3 km/sec. Th average thickness
of Layer 3 is 3.4 km with a velocity of 6.9 km/sec. The base of
the crust is marked by the seismic Moho, the velocity below which is
8.1 km/sec. Refraction and reflection studies of sediment cover indicate
a thickening of turbidite deposits to the southeast from less than
100 meters to over 2.5 km along the continental margin.
Ophiolite studies indicate that the top of Layer 3 marks the upper
extent of amphibolite facies metamorphism of basaltic sheeted dikes.
Lateral depth variations of this seismic boundary in the Gorda Basin
may suggest the occurrence of isograd relief along the spreading center.
The Moho marks the boundary between mafic and ultramafic rocks near the
ridge but may represent the maximum depth of serpentinization in the
crust after it moves away from the spreading axis.
Thin crust (4-5 km) and deep bathymetry in the central portion of
the basin have resulted from crustal formation processes occurring at
ridge crest offsets and are coincident with recent seismicity in the
area. The Gorda ridge offsets and asymmetrical fan spreading of
magnetic anomalies are features observed in response to a regional
change in spreading directions and encroachment of the Pacific and North
American plates. The Gorda plate as a whole does not respond rigidly
to the resulting north-south compression.
Complex structures of the continental slope, revealed by seismic
reflection, limited the reduction of refraction data using plane layer
methods. A simplified seismic section was computed consisting of three
probable sediment layers with velocities of 1.8, 2.5 and 4.0 km/sec
overlying oceanic crust. The crust is observed to dip about two degrees
towards the continent at the base of the slope.
A model of subduction unique to the northern California margin is
one whereby young crust is subducted slowly and quickly reheated so
that no brittle portion remains at normal Benioff depths. Rapid sedimentation
rates balance the subduction of the crust at the margin, preventing
the formation of a deep trench. / Graduation date: 1981
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A comparison of seismic properties of young and mature oceanic crustBee, Michel 30 March 1984 (has links)
Seismic properties (P, S velocities and Poisson's ratio) of
young (0.75 m.y.) and mature (110 m.y.) oceanic crust are obtained
by studying explosive refraction data collected in the Pacific Ocean
using ocean bottom and downhole seismometers. A comparison of the
results for the two regions indicates that the upper crustal velocities
increase with age due to the cementation of cracks and fractures,
the upper mantle velocities increase with age due to cooling,
and the crust (mainly the lower crust) thickens with age. The Poisson's
ratios obtained in this study are too small to be consistent
with the presence of any serpentinization of the lower crust or
upper mantle which therefore precludes upper mantle serpentinization
as the cause for the thickening of the crust with age. When comparing
seismic structures of young and mature oceanic crust with
ophiolite models, we find close similarities between the Samail
ophiolite and young oceanic crust, and between the Bay of Islands
ophiolite and old oceanic crust. The 110 m.y. old crust of the
northwest Pacific Basin is characterized by high velocity gradients
in the upper crust, low velocity gradients in the lower crust, a
smooth 1 km-thick crust-mantle transition zone and the presence of a
minimum 14% anisotropy in the upper mantle compressional wave
velocities. Velocities are highest in an east-west direction. The
0.75 m.y. old crust at the intersection of the East Pacific Rise and
the Orozco fracture zone is characterized by a steady increase in
velocity with depth. A delay time analysis shows a trend to large
Layer 3 delay times in the Orozco fracture zone indicating a thicker
Layer 2 and/or low Layer 2 velocities.
An investigation of different model parameterizations for the
tau-zeta travel time inversion using a synthetic data set indicates
that the best velocity gradient solutions, based on the least deviation
of the solution from the true model, are obtained from models
in which the velocities of the layer bounds take on the values of
the observed velocities of the refracted waves. A trade-off curve
obtained from varying the number of layers in the model shows that a
model with as many layers as observed data points represents a satisfactory
compromise between model resolution and solution variance. / Graduation date: 1984
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A seismic refraction crustal study of the Southeastern United StatesKean, Allan Edwin 12 1900 (has links)
No description available.
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Utilisation de la déconvolution homomorphique pour obtenir l'absorption dans la croûte terrestreMercure, Stephan. January 1975 (has links)
No description available.
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