Paleomagnetism of the Mealy Mountain Anorthosite Suite and of the Shabogamo Gabbro, Labrador, Canada

1974 ◽  
Vol 11 (1) ◽  
pp. 18-29 ◽  
Author(s):  
W. F. Fahrig ◽  
K. W. Christie ◽  
E. J. Schwarz
Keyword(s):  
Pole Position ◽  
Canadian Shield ◽  
The South ◽  
Clockwise Rotation ◽  
End Points ◽  
Stable Component ◽  
Sample Area ◽  
Initial Magnetization ◽  
East Component ◽  
Middle Proterozoic

Two stable, ancient, N.R.M. components were isolated by a.f. demagnetization of samples from 29 sites in the Helikian (Middle Proterozoic) anorthosite and adamellite suites of the Mealy Mountains. The directions of the two components are chiefly to the northwest, 20° upward (northwest component) and to the east, 65 °downward (east component). The northwest direction is thought to be the primary igneous T.R.M. and no reversely magnetized sites with this direction were encountered in the Mealy suite. The 11 sites from which the northwesterly component was isolated yielded a mean pole at 179 °W, 8 °N, α95 = 12°. This is some distance away from poles derived from rocks of comparable age from elsewhere in the Canadian Shield and may reflect a 50° clockwise rotation of this section of the Grenville Orogen relative to the Superior after the initial magnetization of the Mealy rocks. Evidence of a second stable component was found in 14 sites (east component); 11 of these are reversely magnetized and three are normally magnetized. The 14 sites with easterly (and reversed easterly) magnetization yielded a combined pole for this component at 182 °W, 38 °S, α95 = 9°. This pole position suggests that the east component was acquired after the rotation of this part of the Grenville Orogen. The cores from 3 sites exhibited two clear end-points during a.f. demagnetization and thus contain both the northwest and the east component.The Helikian Shabogamo Gabbro at the south end of the Labrador Trough was sampled at 13 sites on a line extending 80 km north from the Grenville Front. After a.f. cleaning these sites yielded a pole at 171 °W, 10 °N, α95 = 12°. This pole lies about 20° northwest of poles for rocks of similar age in the Superior Orogen so it is suggested that the rocks of the sample area, though lying a bit north of the line generally accepted as the 'Grenville Front', have been rotated clockwise (about 20°) since intrusion of the gabbro.

Paleomagnetic evidence for the extent of Mackenzie igneous events

1969 ◽  
Vol 6 (4) ◽  
pp. 679-688 ◽  
Author(s):  
W. F. Fahrig ◽  
D. L. Jones
Keyword(s):  
Volcanic Rocks ◽  
Pole Position ◽  
Canadian Shield ◽  
Paleomagnetic Data ◽  
Paleomagnetic Pole ◽  
Great Slave Lake ◽  
The Mean ◽  
Coppermine River ◽  
Age Determinations ◽  
Middle Proterozoic

North-northwesterly striking Mackenzie diabase dikes of middle-Proterozoic (Helikian) age are profuse in the western part of the Canadian Shield. Published paleomagnetic data on dikes of this trend in Mackenzie District, on the Muskox Intrusion, the Coppermine River volcanic rocks, and the Sudbury dikes suggest that they are all products of closely related igneous events. This paper presents paleomagnetic data that suggest that the intrusion of extensive diabase sheets in the East Arm of Great Slave Lake, and of dikes as far to the northeast as Melville Peninsula and as far to the southeast as Manitoba, were also parts of these events. The mean paleomagnetic pole position for the Mackenzie dikes and for related intrusive and extrusive rocks is [Formula: see text], 171 °W with [Formula: see text]. Radioactive age determinations, some of which are unpublished, indicate an age of about 1200 m.y. for the formation of these rocks. It is suggested that for convenience all of these apparently related intrusive and extrusive igneous episodes be referred to as Mackenzie igneous events.

scholarly journals Paleomagnetic Rotation Study of Woodlark-Australia plate motions in the Woodlark Rift, SE Papua New Guinea

2021 ◽  
Author(s):  
◽  
Elizabeth Ann Cairns
Keyword(s):  
Papua New Guinea ◽  
Vertical Axis ◽  
The South ◽  
Clockwise Rotation ◽  
Plate Motions ◽  
Continental Extension ◽  
Woodlark Basin ◽  
Australian Plate ◽  
Anticlockwise Rotation ◽  
Oceanic Spreading

<p>The Woodlark Rift in SE Papua New Guinea is a continental rift to the west of active oceanic spreading in the Woodlark Basin, which separates the Australian Plate to the south from the relatively anticlockwise rotating Woodlark Plate to the north. During Pliocene to Recent times the Woodlark Rift has been the setting for rapid exhumation of the world’s youngest UHP rocks (Baldwin et al., 2004, 2008; Gordon et al, 2012; Little et al., 2011), and is currently one of few places on the globe where active continental breakup is occurring ahead of a propagating oceanic spreading centre. While the Woodlark Basin contains a record of oceanic spreading since ˜6 Ma (Taylor et al., 1999), and GPS data describe present-day crustal motions (Wallace et al., manuscript in review), the Neogene temporal and kinematic evolution of continental extension in the Woodlark Rift is less well constrained. We compare Characteristic magnetization directions for six formations, Early Miocene (˜20 Ma) to Late Pliocene (3 ± 0.5), with contemporaneous expected field directions corresponding to Australian Plate paleomagnetic pole locations. We interpret declination anomalies (at 95% confidence) to estimate finite vertical-axis rotations of crustal blocks with respect to a fixed Australian Plate. Temporal and spatial relationships between declination anomalies for six formation mean directions, across four paleomagnetic localities, provide new evidence to constrain aspects of the Miocene to Recent history of the Woodlark Rift.  We obtained 250 oriented core samples from Miocene to Pliocene aged rocks at four localities in the Woodlark Rift. Components of Characteristic Remanent Magnetization (ChRM) have been determined from step-wise thermal and alternating field demagnetization profiles of >300 individual specimens. A total of 157 ChRM components contribute to the calculation of representative paleomagnetic directions for six formations, which have undergone vertical-axis rotations with respect to the Australian Plate associated with development of the Woodlark Rift.  Pliocene volcanic rocks at two key localities near the northern extent of the rift record that: 1) The Amphlett Islands has experienced 10.1 ± 7.6° of anticlockwise rotation since 3 ± 0.5 Ma; 2) NW Normanby Island has undergone a 16.3 ± 9.5° clockwise rotation during the same time interval. Sedimentary rocks at Cape Vogel Peninsula on the northern coast of the mainland Papuan Peninsula, record variable anticlockwise finite rotations of 28.4 ± 10.9° and 12.4 ± 5.5° for Early and Middle Miocene rocks respectively, in contrast to a younger clockwise rotation of 6.5 ± 11.2° for Late Miocene rocks. At the Suau Coast locality, on the south eastern coast of the Papuan Peninsula, Late Miocene dikes record 22.7 ± 13.3° of anticlockwise rotation.  At the Amphlett Islands and NW Normanby localities paleomagnetic data are consistent with current GPS plate motions, suggesting the current kinematics in the rift were established by at least ˜3 Ma. The Amphlett Islands result is consistent with the rate of Pliocene sea floor spreading in the Woodlark Basin, suggesting that locality can be considered as fully on the Woodlark Plate. The clockwise rotation indicated at NW Normanby Island may record development of an incipient dextral transfer fault within an active part of the Woodlark Rift.  Time-varying declination anomalies from the Cape Vogel Peninsula suggest that rifting began there by ˜15 Ma, 7 Ma earlier than previously inferred based on stratigraphic evidence. Furthermore, paleomagnetic data from the south coast of the Papuan Peninsula suggests that early rifting extended further south, and has since contracted to where continental extension is currently accommodated north of the Papuan Peninsula.</p>

Paleomagnetism of the 1.1 Ga Lackner Lake Complex and tectonics of the Kapuskasing Structural Zone

1989 ◽  
Vol 26 (9) ◽  
pp. 1778-1783 ◽  
Author(s):  
D. T. A. Symons
Keyword(s):  
Fault Zone ◽  
Canadian Shield ◽  
Superior Province ◽  
Thermal Demagnetization ◽  
Abitibi Subprovince ◽  
Reversed Polarity ◽  
Middle Proterozoic

The Middle Proterozoic Lackner Lake Complex is a circular alkalic syenite–carbonatite stock with a diameter of about 5.5 km. It intrudes granulite-rank Archean gneisses in the Kapuskasing Structural Zone of the Wawa Subprovince in the Superior Province of the Canadian Shield. It adjoins the Ivanhoe Lake fault zone, which forms the boundary with the Abitibi Subprovince and is the probable locus of maximum motion between the subprovinces. Specimens from 18 sites in the complex were analyzed paleomagnetically by alternating-field and thermal demagnetization and by saturation isothermal remanence tests. Large, recent viscous remanence components required removal before a stable remanence with a mean direction of 305.4°, 64.1 °(α95 = 5.2°) was isolated. Its pole of 53.7°N, 156.5°W (dp = 6.7°, dm = 8.3°) indicates emplacement at 1108 ± 10 Ma during a brief normal interval in a predominantly reversed-polarity time. This study indicates that there has been no postintrusion tilting of the Kapuskasing Structural Zone and that postintrusion uplift by unroofing did not exceed about 8 km.

The structural evolution of the Falkland Plateau

2020 ◽  
Author(s):  
Roxana Mihaela Stanca ◽  
Douglas Paton ◽  
Estelle Mortimer ◽  
David Hodgson ◽  
Dave McCarthy
Keyword(s):  
Seismic Reflection ◽  
Normal Faults ◽  
Falkland Islands ◽  
The South ◽  
Clockwise Rotation ◽  
Transform Faults ◽  
Stress Regime ◽  
Seismic Reflection Data ◽  
Western Margin ◽  
Structural Framework

&lt;p&gt;The palaeogeographic reconstruction of the Falkland Plateau transform margin in a Gondwana pre-break-up configuration has been the subject of debate for years. This is mainly due to the uncertainty in the position of the Falkland Islands microplate. The islands were an extension of the south-east coast of South Africa, being either i) part of a rigid Falkland Plateau fixed to the South American plate or ii) undergoing a vertical-axis clockwise rotation of between 80&amp;#176; to 120&amp;#176; along the transform faults generated during the initial stages of fragmentation of south-western Gondwana. The absence of documented evidence of this rotation within the sedimentary infill of the basins surrounding the Falkland Islands represents an ongoing issue. Furthermore, a structural framework of the eastern continental shelf of the islands that takes into account the most recent seismic reflection surveys has not been published yet.&lt;/p&gt;&lt;p&gt;This study presents an updated description of the structural configuration of the Falkland Plateau Basin, focusing on the Volunteer and Fitzroy sub-basins. This structural framework, based on extensive 2D and 3D seismic reflection data and aided by seismic attribute mapping, provides new insights into the evolution of the Falkland Islands microplate and the Falkland Plateau Basin.&lt;/p&gt;&lt;p&gt;Three main structural trends were identified across this section of the Falkland Plateau. WNW-ESE trending half-grabens were mapped north-west of the Volunteer sub-basin; these correlate laterally with linear gravity anomalies following the same trend north of the Falkland Islands. NNE-SSW to N-S normal faults are predominant west of the Volunteer sub-basin and are believed to control the western margin of the Falkland Plateau Basin. Locally, the NNE-SSW trend is subdued by NNW-SSE striking en-&amp;#233;chelon normal faults suggestive of left-lateral movement along a NNE-SSW direction. A similar trend is interpreted in the southern part of the Fitzroy sub-basin, supporting sinistral wrenching along the western margin of the Falkland Plateau Basin.&lt;/p&gt;&lt;p&gt;These results suggest intra-plate deformation that is consistent with a clockwise rotation of the Falkland Islands microplate along the transform faults that accommodated the initial fragmentation of Gondwana. The interpreted fault network allows us to understand the temporal variation in the orientation of the minimum horizontal stress across the Falkland Islands microplate. By comparing this variation with the regional stress regime in south-western Gondwana, the timing and mechanism of the rotation of the islands can be better constrained.&lt;/p&gt;

Age of the Firesand River carbonatite complex from paleomagnetism

1989 ◽  
Vol 26 (11) ◽  
pp. 2401-2405 ◽  
Author(s):  
D. T. A. Symons
Keyword(s):  
Remanent Magnetization ◽  
Pole Position ◽  
Blocking Temperature ◽  
Isothermal Remanent Magnetization ◽  
Carbonatite Complex ◽  
Polar Wander ◽  
Partial Contact ◽  
Apparent Polar Wander Path ◽  
Temperature Spectra ◽  
Middle Proterozoic

The 2.3 km diameter Firesand River complex intrudes Archean volcanics and granites of the Wawa Subprovince in the Superior Province about 8 km east of Wawa, Ontario. It has given differing Middle Proterozoic K–Ar biotite ages of 1018 ± 50 and 1097 Ma. Alternating-field and thermal step demagnetization of specimens from three calcific carbonatite sites, five ferruginous dolomitic carbonatite sites, and one lamprophyre dike site isolated a stable mean direction of 290°, 33 °(α95 = 12°). Isothermal remanent magnetization tests indicate the remanence is held by single-to pseudosingle-domain magnetite and hematite in the carbonatite. The dike remanence is Keweenawan in age, thereby confirming its genetic relationship to the complex, and it gives a positive partial contact test with its host rock, indicating no postintrusive remagnetization. The blocking-temperature spectra indicate that postintrusive uplift has not exceeded about 4 km. The pole position for the complex is 183°E, 27°N (dp = 8°, dm = 13°). This pole lies directly on the well-dated Keweenawan apparent polar wander path, giving an age of 1090 ± 10 Ma, in agreement with the older K–Ar age. It also confirms geologic and aeromagnetic evidence that the complex has not been tectonically tilted since emplacement.

scholarly journals Spatiotemporal Structure Features of a Pine Stand on the South Slope of the Eastern Sayan Mountain

2021 ◽  
pp. 34-47
Author(s):  
Natalya F. Ovchinnikova ◽  
Keyword(s):  
Density Ratio ◽  
Stand Density ◽  
Stand Dynamics ◽  
The South ◽  
Horizontal Structure ◽  
Spatiotemporal Structure ◽  
Trunk Diameter ◽  
Sample Area ◽  
South Slope ◽  
Eastern Sayan

Due to relatively long development and long life of forest-forming species most of the conclusions on stand dynamics are based on the data collected by indirect methods of comparative research and analysis of cenosis forming theoretically temporal and spatial succession sequences and need to be verified and clarified. The most reliable results on the stand dynamics can be received on stationary objects such as permanent sample areas. The article presents the analysis results of long-term monitoring (1968–2012) of a pine (Pinus sylvestris L.) stand in low-mountain Eastern Sayan on the south slope with a steepness of 7–8º. The intermittent test of morphometric indicators of trees mapped on the permanent sample area was used. It is shown that the sum of tree trunk cross-section areas at breast height (1.3 m) varied by no more than 10 % with timber reserves on the lower, central and upper equal sites of the sample area during the entire observation period. At the same time heterogeneity of ecological conditions, taking place even on a small section of a smooth slope, influences the plantation self-thinning intensity. In the 42-year-old stand density ratio at lower, central and upper sites was 1:1.5:2.1. Stand density difference along the slope had been decreasing gradually over time, although it remained around 30 % at the time of the last observation. Over the years of observation, 53, 47, and 32 % of pines on the lower, central, and upper sites, respectively, have survived. The calculated average age of the died trees showed that in all groups of natural diameter classes, there was an earlier die-off of pines at the bottom of the slope with an increase in dying age up to the slope. Trees with larger trunk diameter (first telling) died later. All this led to a rearrangement of the horizontal structure of the plantation, a decrease in the average diameter and average volume of trees up to the slope. Typical for mountain forests clinal tree distribution, which affects their morphological indicators, is important to consider when studying and modeling forest ecosystems, as well as when implementing forest management measures. For citation: Ovchinnikova N.F. Spatiotemporal Structure Features of a Pine Stand on the South Slope of the Eastern Sayan Mountains. Lesnoy Zhurnal [Russian Forestry Journal], 2021, no. 5, pp. 34–47. DOI: 10.37482/0536-1036-2021-5-34-47

New Paleomagnetic Results from the Upper Belt – Purcell Supergroup of Alberta

1975 ◽  
Vol 12 (1) ◽  
pp. 52-61 ◽  
Author(s):  
M. E. Evans ◽  
D. K. Bingham ◽  
E. W. McMurry
Keyword(s):  
Pole Position ◽  
Canadian Shield ◽  
Considerable Potential ◽  
Paleomagnetic Pole ◽  
Belt Supergroup ◽  
Order Of Magnitude ◽  
Global Time ◽  
Geomagnetic Polarity ◽  
Apparent Conflict

Stable remanent directions have been obtained from fifteen sites located in the Purcell, Shepard and Kintla Formations of the upper Belt – Purcell Supergroup. The geomagnetic polarity sequence revealed by these and other studies suggests that the frequency of reversals was probably an order of magnitude lower than that observed over the last few million years. The polarity transitions have considerable potential as regional, and possibly, global, time lines for geological correlation.The paleomagnetic pole obtained (142 °W, 17 °S, dp = 4°, dm = 7°) agrees well with other results from the Belt Supergroup. It falls in a cluster of poles derived from various geological provinces of the Canadian Shield, which may imply that the shield was an integral unit as early as 1400 m.y. ago. However, the constancy of the pole position obtained from Belt rocks thought by some workers to be as young as 1100 m.y. conflicts with the large northward polar excursion (the Logan Loop) derived from other provinces. Possible explanations of this apparent conflict are discussed.

Heat flow, thermal regime, and elastic thickness of the lithosphere in the Trans-Hudson Orogen

2005 ◽  
Vol 42 (4) ◽  
pp. 517-532 ◽  
Author(s):  
J C Mareschal ◽  
C Jaupart ◽  
F Rolandone ◽  
C Gariépy ◽  
C MR Fowler ◽  
...  
Keyword(s):  
Heat Flow ◽  
Heat Production ◽  
Large Fraction ◽  
Flow Region ◽  
Canadian Shield ◽  
Poor Correlation ◽  
Cold Spot ◽  
The South ◽  
Effective Elastic Thickness ◽  
Lynn Lake

Heat flow studies on the exposed part of the Trans-Hudson Orogen (THO) in northern Manitoba and Saskatchewan allow constraints on crustal composition and lithosphere structure. The average of all heat flow values in the THO is the same as in other geological provinces of the Canadian Shield. However, where juvenile crust is exposed, heat flow is on average lower than in the Superior and Grenville provinces (37 vs. 41 mW m–2). Heat flow increases towards the surrounding Archean provinces, Rae–Hearne to the west, Sask to the south, and Superior to the east. There are strong differences in heat flow within and between the belts of the THO. The poor correlation between heat flow and heat production in the rocks exposed at the surface implies that these differences involve a large fraction of the crustal column. One new heat flow determination confirms the existence of a ``cold spot'' around the town of Lynn Lake in the northern part of the THO. Heat flow data in the Kisseynew and Glennie domains remain sparse, but they indicate that this low heat flow region extends as far south as the Flin Flon – Snow Lake Belt. The Lynn Lake Belt is underlain by poorly radiogenic rocks, possibly Kisseynew-type crust with oceanic basement. Northward increase in heat flow along the Thompson Belt is consistent with the view that the belt is thrust over Kisseynew-type basement only in the south. Heat flow increases southward in the Paleozoic basin because of higher heat production in basement rocks, probably from the Sask craton. We used the low heat flow regions to obtain an upper bound of 15 mW m–2 for the mantle heat flow in the THO. The effective elastic thickness of the lithosphere can be determined from the coherence between the topography and the Bouguer gravity. The effective elastic thickness is high (>40 km) thoughout the Canadian Shield and is highest in the central part of the shield, in particular in the Lynn Lake region. There seems to be a negative correlation between elastic thickness and heat flow in the central and western Canadian Shield. This indicates that, even in stable continents, the elastic thickness is largely controlled by the lithospheric temperatures that depend strongly on crustal heat generation and hence crustal structure.

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