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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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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

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

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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