Crustal uplift in the southern Superior Province, Canada, revealed by paleomagnetism

Halls, H. C.; Zhang, Baoxing

doi:10.1016/s0040-1951(02)00634-0

Cited by 22 publications

(7 citation statements)

References 26 publications

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“…The soft, exponentially decreasing curves of TRM and SIRM of the dark minerals typify MD grains [ Dunlop et al , 2004]. Similar curves are shown by Halls and Zhang [2003, Figure 6] for a different core from dike TK49.…”

Section: Magnetic Properties Characterizationmentioning

confidence: 60%

“…Although whole rock susceptibility curves resembled those of the mafic minerals, the plagioclase, with its high SD Q n , would make a substantial contribution to the NRM. Halls and Zhang [2003, Figure 6] show that half the NRM of a core from TK49 has high‐coercivity AF demagnetization matching the feldspar separate from the same core. Thus plagioclase and mafic minerals (and in addition magnetite as a discrete phase) all potentially contribute to the NRM and must be considered in paleointensity work.…”

Section: Discussionmentioning

confidence: 91%

“…The first is to further test linear versus nonlinear Thellier behavior and reliability criteria such as pTRM checks using natural magnetic minerals with SD and MD behavior. The SD material is submicroscopic magnetite in cloudy feldspars extracted from Precambrian Matachewan diabase dikes [ Zhang and Halls , 1995; Halls and Zhang , 2003]. The MD material is coarse‐grained magnetite in mafic minerals (mainly clinopyroxene, with some biotite and amphibole) from the same rocks.…”

Section: Introductionmentioning

confidence: 99%

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Linear and nonlinear Thellier paleointensity behavior of natural minerals

2005

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We tested single‐domain (SD) and multidomain (MD) theories of the acquisition and thermal demagnetization of thermoremanent magnetization (TRM) and partial TRM (pTRM) in Thellier paleointensity determination, using as samples plagioclase and mafic minerals extracted from Matachewan diabase dikes. Susceptibility, hysteresis, and coercivity data indicate that the plagioclase contains mainly SD grains of magnetite while the mafic minerals (clinopyroxene, biotite, and actinolite) contain mainly MD magnetite. The extracted minerals were used in simulated Thellier experiments with laboratory TRM as a proxy for natural remanent magnetization (NRM). The plagioclase behaved ideally, as predicted by Néel SD theory, giving a linear NRM‐pTRM (Arai) plot with positive pTRM checks. The mafic minerals behaved nonideally, with strongly curved Arai plots sagging below the ideal SD line, in accord with observations for MD magnetites and with predictions that in heating to moderate temperatures T, more TRM is lost than can be regained as pTRM, causing convex‐down Arai plots. In experiments with pTRM parallel to NRM, all pTRM tail checks were zero, but pTRM checks increasingly deviated from the Arai curve toward the SD line as T increased. However, in repeat Thellier experiments, pTRMs at all T agreed with the original pTRMs. The negative pTRM checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original pTRMs. The inability of pTRM checks to reliably distinguish between MD behavior and irreversible alteration could lead to overestimates of paleointensity from straight‐line fits to the low‐T “unaltered” part of the Arai curve or underestimates from fits to high‐T data. Alternating field cleaning to 15 mT before each Thellier step made the Arai plot only marginally less curved. Low‐temperature demagnetization (LTD, zero‐field cycling through the 120 K Verwey transition) hardly changed the curvature of the Arai plot for pTRM parallel to NRM. For pTRM perpendicular to NRM, LTD led to an inflected Arai curve straddling the ideal SD line. Data of either form would not give reliable paleointensity values. No presently available pretreatment renders the paleointensity data of MD grains usable. SD and nearly SD grains which behave ideally in the Thellier experiment remain the only useful recorders of absolute paleointensity.

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Section: Magnetic Properties Characterizationmentioning

confidence: 60%

Section: Discussionmentioning

confidence: 91%

Section: Introductionmentioning

confidence: 99%

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Linear and nonlinear Thellier paleointensity behavior of natural minerals

2005

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“…Extensive paleomagnetic study of the Matachewan swarm [e.g., Schutts and Dunlop , 1981; Halls and Bates , 1990; Symons et al , 1994; Halls and Zhang , 1998] has provided important constraints on the tectonic history of the Superior province in the Proterozoic [e.g., Bates and Halls , 1991; Halls and Zhang , 2003]. Only a single geomagnetic reversal has been found; a structural model controlled by field mapping and the presence of clouded feldspar [ Poldervaart and Gilkey , 1954] suggests the reversal reflects different depths of the crust in which the dikes were intruded [ Bates and Halls , 1991; Halls , 1991].…”

Section: Background Geology and Samplingmentioning

confidence: 99%

Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?

Смирнов

Tarduno

2005

J. Geophys. Res.

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[1] Thellier paleointensity determinations from two dikes of the Early Proterozoic ($2.46 Ga) Matachewan dike swarm (Canada) yield field values of 2.14 ± 0.18 and 9.81 ± 1.65 mT. Corresponding values of virtual axial dipole moment are very low (0.54 ± 0.05 Â 10 22 and 2.49 ± 0.42 Â 10 22 Am 2 , respectively) when compared with the modern field. The characteristic remanent magnetization (ChRM) was isolated over a narrow range of high unblocking temperatures ($520-580°C). Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo single domain magnetite. Scanning electron microscopy (SEM) reveals that the magnetite is in the form of fine intergrowths with ilmenite, formed by oxyexsolution during cooling. The high-temperature oxidation defined in the SEM images could have continued at temperatures below the Curie point of magnetite. In this case, the ChRM would be a thermochemical remanent magnetization (TCRM) rather than a thermal remanent magnetization (TRM). Estimates of the TCRM/TRM ratio show that the Thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria. This uncertainty in TRM fidelity translates into a potential range of field values that spans that defined by the modern field ($8 Â 10 22 Am 2 ) and proposed low Precambrian levels ($2 Â 10 22 Am 2 ). Therefore understanding further how TCRM is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in Thellier studies aimed at defining the strength of the Precambrian field.Citation: Smirnov, A. V., and J. A. Tarduno (2005), Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?,

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“…They record regional rifting events and provide useful information for paleocontinental reconstruction (Halls, 1982; and for the rotation of cratonic blocks (Halls et al, 2000;Halls and Zhang, 2003). The NCC is composed of three Archean tectonic units: the Eastern Block (EB), the Western Block (WB), and the Central Orogenic Belt (COB) (Fig.1) Zhao et al, 2002;Kusky and Li, 2003).…”

Section: Introductionmentioning

confidence: 99%