Geological constraints on the origin of the mantle root beneath the Canadian shield

1990 ◽  
Vol 331 (1620) ◽  
pp. 523-532 ◽  
Keyword(s):  
Seismic Anisotropy ◽  
Sedimentary Cover ◽  
Oceanic Lithosphere ◽  
Systematic Difference ◽  
Canadian Shield ◽  
Cratonic Mantle ◽  
Crustal Reworking ◽  
The Mean ◽  
Geological Constraints ◽  
Dyke Swarms

Cratonic North America is composed of a cluster of Archaean microcontinents centred on the Canadian shield, and juvenile Proterozoic crust that lies mainly buried beneath the sedimentary cover of the western and southern interior platforms. The shield is underlain by an anomalous low-temperature mantle root that is absent beneath the platform. As there appears to be no systematic difference in crustal thickness or density between the shield and the platform, the long-lived arching of the shield implies an intrinsic buoyancy imparted by the mantle root that more than offsets its colder temperature. Isotopic and seismic anisotropy data indicate an Archaean age for the mantle root, close to the time of formation of the overlying crust. The preferential development of the mantle root beneath Archaean crust is consistent with an origin by imbrication of partly subducted slabs of highly depleted oceanic lithosphere, assuming that buoyant subduction was more common in the Archaean. Formation of the mantle root was not dependent on collisional orogenesis, as has been suggested, but the Archaean cratonic mantle was sufficiently buoyant and refractory to survive later tectonic thickening. The mantle root persists beneath Archaean crust that was transected by mafic dyke swarms and subjected to short-lived episodes of post-orogenic crustal melting, but the root is reduced at mantle plume initiation sites. The partitioning of Archaean and Proterozoic crust between the shield and the platform, respectively, causes the shield to misrepresent Precambrian crust as a whole. Studies of the shield falsely conclude that a high percentage of Precambrian crust formed in the Archaean, and that the Proterozoic was characterized by epicontinental volcanism and sedimentation, and crustal ‘reworking’. Furthermore, the isotopic ratios of detritus eroded from the craton may tend to overestimate the mean age of continental crust.

PALAEOMAGNETISM OF DIABASE DYKES OF THE CANADIAN SHIELD

1965 ◽  
Vol 2 (4) ◽  
pp. 278-298 ◽  
Author(s):  
W. F. Fahrig ◽  
E. H. Gaucher ◽  
A. Larochelle
Keyword(s):  
Remanent Magnetization ◽  
Lake Superior ◽  
Canadian Shield ◽  
Igneous Rocks ◽  
Palaeomagnetic Data ◽  
The World ◽  
Oriented Samples ◽  
The Mean ◽  
The Individual ◽  
Dyke Swarms

Some areas of the Canadian Shield are profusely intruded by swarms of subparallel basaltic dykes. These typically have a diabasic texture and formerly were considered to be strictly the intrusive equivalents of tholeiitic flood basalt.About 650 oriented samples were collected from about 25 dyke swarms and preliminary palaeomagnetic data from six of them are presented here. The Mackenzie swarm has a K–Ar age of 1 295 million years, occurs throughout the western Canadian Shield, and is the most extensive swarm of basic dykes known anywhere in the world. The other swarms are the Molson dykes (1 445 m.y.) in northeastern Manitoba, the Marathon dykes (1 810 m.y.) just north of Lake Superior, the Sudbury dykes (1 285 m.y.) of southeastern Ontario, the Matachewan dykes (2 485 m.y.) of western Quebec and northeastern Ontario, and the Abitibi dykes (1 230 m.y.), which occur from Sault Ste. Marie, Ontario, to Lake Mistassini, Quebec.The palaeomagnetic virtual pole positions of six swarms were derived from the mean of their measured remanent magnetization directions. These directions of magnetization were determined from the samples after they were magnetically washed in an a-c. field of 80 or 130 oersteds. The reasons for the dispersion within the individual swarms are discussed. The pole positions of the Sudbury and Mackenzie dykes are almost identical and the two swarms are the same age within the limits of analytical uncertainty. However, basalt of the Sudbury swarm is alkalic and more undersaturated than basalt of the Mackenzie dykes.No evidence was found to contradict the usual assumption that unaltered basic igneous rocks of this type acquire and generally retain a stable magnetization which was parallel with the earth's magnetic field at the time of the dyke intrusion.

scholarly journals Exploring the shallow structure of the San Ramón thrust fault in Santiago, Chile (~33.5° S), using active seismic and electric methods

Solid Earth ◽  
2014 ◽  
Vol 5 (2) ◽  
pp. 837-849 ◽  
Author(s):  
D. Díaz ◽  
A. Maksymowicz ◽  
G. Vargas ◽  
E. Vera ◽  
E. Contreras-Reyes ◽  
...  
Keyword(s):  
Seismic Anisotropy ◽  
Average Length ◽  
Sedimentary Cover ◽  
Hanging Wall ◽  
Thrust Fault ◽  
Fault System ◽  
Surface Rupture ◽  
Thrust Tectonics ◽  
Rock Substratum ◽  
Fault Scarps

Abstract. The crustal-scale west-vergent San Ramón thrust fault system, which lies at the foot of the main Andean Cordillera in central Chile, is a geologically active structure with manifestations of late Quaternary complex surface rupture on fault segments along the eastern border of the city of Santiago. From the comparison of geophysical and geological observations, we assessed the subsurface structural pattern that affects the sedimentary cover and rock-substratum topography across fault scarps, which is critical for evaluating structural models and associated seismic hazard along the related faults. We performed seismic profiles with an average length of 250 m, using an array of 24 geophones (Geode), with 25 shots per profile, to produce high-resolution seismic tomography to aid in interpreting impedance changes associated with the deformed sedimentary cover. The recorded travel-time refractions and reflections were jointly inverted by using a 2-D tomographic approach, which resulted in variations across the scarp axis in both the velocities and the reflections that are interpreted as the sedimentary cover-rock substratum topography. Seismic anisotropy observed from tomographic profiles is consistent with sediment deformation triggered by west-vergent thrust tectonics along the fault. Electrical soundings crossing two fault scarps were used to construct subsurface resistivity tomographic profiles, which reveal systematic differences between lower resistivity values in the hanging wall with respect to the footwall of the geological structure, and clearly show well-defined east-dipping resistivity boundaries. These boundaries can be interpreted in terms of structurally driven fluid content change between the hanging wall and the footwall of the San Ramón fault. The overall results are consistent with a west-vergent thrust structure dipping ~55° E in the subsurface beneath the piedmont sediments, with local complexities likely associated with variations in fault surface rupture propagation, fault splays and fault segment transfer zones.

Some new measurements of heat flow in the Superior Province of the Canadian Shield

1987 ◽  
Vol 24 (7) ◽  
pp. 1486-1489 ◽  
Author(s):  
Malcolm Drury ◽  
Alan Taylor
Keyword(s):  
Standard Deviation ◽  
Heat Flow ◽  
Canadian Shield ◽  
Superior Province ◽  
Flow Measurements ◽  
Radiogenic Heat ◽  
Radiogenic Heat Production ◽  
Climatic Variations ◽  
The Mean ◽  
A Site

Borehole heat-flow measurements are reported from six new sites in the Superior Province of the Canadian Shield. Values adjusted for glaciation effects, but not for Holocene climatic variations, range from 42 to 56 mW/m2. When these new values are combined with 21 previously published borehole values the mean is 42 mW/m2 with a standard deviation of 11 mW/m2. The data for a site on the Lac du Bonnet batholith suggest that the batholith has a thin veneer, less than 3 km, of rock of high radiogenic heat production at the surface.

scholarly journals The thermal conductivity of air

1934 ◽  
Vol 144 (853) ◽  
pp. 494-495 ◽  
Keyword(s):  
Thermal Conductivity ◽  
Temperature Gradient ◽  
Parallel Plate ◽  
Systematic Difference ◽  
Hot Wire ◽  
Plate Method ◽  
Metal Plates ◽  
Wire Method ◽  
The Mean ◽  
Plate Apparatus

When Miss Nelson and the writer prepared in 1929 an article for the 'International Critical Tables' on the thermal conductivity of gases, we found that the value for air had been measured by 19 observers and that the mean departure from the mean was 7%. Further the values obtained by the hot wire method by Weber, Gregory and Archer, and Schneider were higher than the value (5.40 X 10 -5 cal. cm. -1 sec. -1 deg. -1 ) which Hercus and the writer had found by a parallel plate method, and higher than 13 of the 14 determinations (including hot wire ones) made previous to 1918. In view of these facts it was desirable to repeat the parallel plate method and to obtain evidence as to whether or not there was a systematic difference between the two methods mentioned. The hot wire method, as used by the experimenters named, has the practical advantages measured, and it is convenient and simple. As carried out in the experiments referred to in which fine wires were used it has the disadvantages that the elimination of the convection effects is not attained with certainly, the temperature gradient in the gas is large (which introduces both theoretical and practical difficulties) and there is a temperature discontinuity at the surface of the wire which has to be determined. Hercus and Sutherland have nearly completed in this laboratory a measurement of the thermal conductivity of air with a parallel plate apparatus. This method has the inherent advantages that there are no convection currents in the horizontal lamina of gas used, that the temperature gradient may be made small, and the temperature disments now in progress radiation is eliminated by using the metal plates at two different separations and considerable improvements have been made us compared with the experiment of Hercus and in their measurement.

Solubility of magnetite-hematite assemblages in slab-derived saline fluids

2020 ◽  
Author(s):  
Carla Tiraboschi ◽  
Carmen Sanchez-Valle
Keyword(s):  
Subduction Zones ◽  
Microprobe Analysis ◽  
Mantle Wedge ◽  
Sedimentary Cover ◽  
Low Pressure ◽  
Oceanic Lithosphere ◽  
Major Elements ◽  
Piston Cylinder ◽  
Water Saturated ◽  
Subducting Slab

<p>In subduction zones, aqueous fluids derived from devolatilization processes of the oceanic lithosphere and its sedimentary cover, are major vectors of mass transfer from the slab to the mantle wedge and contribute to the recycling of elements and to their geochemical cycles. In this setting, assessing the mobility of redox sensitive elements, such as iron, can provide useful insights on the oxygen fugacity conditions of slab-derived fluid. However, the amount of iron mobilized by deep aqueous fluids and melts, is still poorly constrained.</p><p>We experimentally investigate the solubility of magnetite-hematite assemblages in water-saturated haplogranitic liquids, which represent the felsic melt produced by subducted eclogites. Experiments were conducted at 1 GPa and temperature ranging from 700 to 900 °C employing a piston cylinder apparatus. Single gold capsules were loaded with natural hematite, magnetite and synthetic haplogranite (Na<sub>0.56</sub>K<sub>0.38</sub>Al<sub>0.95</sub>Si<sub>5.19</sub>O<sub>12.2</sub>). Two sets of experiments were conducted: one with H<sub>2</sub>O-only fluids and the second one adding a 1.5 m H<sub>2</sub>O–NaCl solution. The capsule was kept frozen during welding to ensure no water loss. After quench, the presence of H<sub>2</sub>O in the quenched haplogranite glass was checked by Raman spectroscopy, while major elements were determined by microprobe analysis.</p><p>Preliminary results indicate that a significant amount of Fe is released from magnetite and hematite in hydrous melts, even at relatively low-pressure conditions. At 1 GPa the FeO<sub>tot</sub> quenched in the haplogranite glass ranges from 0.60 wt% at 700 °C, to 1.87 wt% at 900 °C. In the presence of NaCl, we observed an increase in the amount of iron quenched in the glass (e.g., at 800 °C from 1.04 wt% to 1.56 wt% of FeO<sub>tot</sub>). Our results suggest that hydrous melts can effectively mobilize iron even at low-pressure conditions and represent a valid agent for the cycling of iron from the subducting slab to the mantle wedge.</p>

Sedimentary Cover of the Canadian Shield through Mesozoic Time Reflected by Nd Isotopic and Geochemical Results for the Sverdrup Basin, Arctic Canada

2004 ◽  
Vol 112 (1) ◽  
pp. 39-57 ◽  
Author(s):  
P. J. Patchett ◽  
A. F. Embry ◽  
G. M. Ross ◽  
B. Beauchamp ◽  
J. C. Harrison ◽  
...  
Keyword(s):  
Sedimentary Cover ◽  
Canadian Shield ◽  
Arctic Canada ◽  
Sverdrup Basin

scholarly journals Seismic Anisotropy at the Hikurangi Subduction Margin

2021 ◽  
Author(s):  
◽  
Thomas Wilson
Keyword(s):  
Stress Field ◽  
Delay Time ◽  
Seismic Anisotropy ◽  
The West ◽  
Study Region ◽  
East Region ◽  
West Region ◽  
Gravitational Stress ◽  
Significant Difference ◽  
The Mean

<p>To determine the stress state in the southern North Island of New Zealand, we use shear wave splitting analysis to measure seismic anisotropy and infer the orientation of the maximum horizontal stress directions (Shmax) in the crust. We use data recorded by 44 temporary seismometers deployed as part of the Seismic Array Hikurangi Experiment, and from six permanent stations from the national GeoNet network. Using 425 local earthquake events recorded across the 50 stations we made 13,807 measurements of the two splitting parameters, φ (fast direction) and δt (delay time). These measurements are compared to SHmax directions obtained from previous focal mechanism studies (SfocalHmax), and stresses due to the weight of topography (SgravHmax). Generally there is good agreement between the alignment of SfocalHmax, SgravHmax, and the mean φ measured at each station. We also find a∼ 90◦ change in the trend of φ in the Wairarapa region for stations across the Wairarapa Fault trace. Based on the variation of φ, we divide the study region into three regions (West, Basin, and East), whose bounds approximately coincide with the Wairarapa and Dry Creek faults. We find the average φ of the West region average agrees with previous anisotropy studies, which were undertaken within the bounds of the West region on the Tararua array. Also, we use our delay time measurements to estimate a 3.7±1.2% strength of anisotropy in the overriding Australian Plate, which agrees with the 4% crustal anisotropy measured previously. There is close alignment of the region average φ of the West and East regions, which also agrees with the deep splitting measurements previously obtained. There is no significant difference between the mean φ and Sgravhmax for the West and Basin regions; however, we find a difference of 31± 19.5◦ for the East region. We argue that this difference is due to tectonic loading stresses being sufficiently large in the East region to cause the total stress field to deviate from the gravitational stress field.</p>

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