Palaeointensity and palaeomagnetic study of Cretaceous and Palaeocene rocks from Western Antarctica

Abstract

SUMMARY A combined palaeodirectional and palaeointensity study of a representative collection of plutonic rocks from the Antarctic Peninsula batholith from the western part of the Antarctic Peninsula, near the Ukrainian Antarctic base 'Academik Vernadsky' were carried out. Petrographically, the collection includes gabbros, diorites and quartz diorites, tonalities, granodiorites and granites. The ages of igneous complex emplacement vary from 50 to 117 Ma with most of the rocks belonging to the Cretaceous Normal Superchron. The characteristic remanent magnetizations were isolated by stepwise thermal demagnetization over the temperature interval 440–590°C and their intensities amount to 95 per cent of the NRM. The geographic positions of palaeopoles do not contradict the ‘key poles’ of the Antarctic Peninsula between 90 and 60 Ma. A significant part of the collection was subjected to Coe‐modified Thellier palaeointensity experiments with the pTRM checks, which yielded seven reliable palaeointensity determinations for seven different locations. The obtained VDMs are relatively low for all sites, being on average about half of the present day VDM. The analysis of available palaeointensity data for the Cretaceous, Miocene and Middle Jurassic indicates the existence of strong correlations between the mean VDM and VDM scatter versus the rate of reversals. However, due to the shortage of data, the correlations are not significant at the 5 per cent significance level.

More stable yet bimodal geodynamo during the Cretaceous superchron?

“…Conversely, other studies report low geomagnetic intensities at times within the CNS [Prévot et al, 1990;Pick and Tauxe, 1993;Shcherbakova et al, 2008]. These diverging views may be partially reconciled by a high variability of the dipole moment during the CNS [Granot et al, 2012;Shcherbakova et al, 2012], unless the variability reflects experimental difficulties in obtaining reliable palaeointensity data [e.g., Ingham et al, 2014]. Another way to quantify geomagnetic secular variation is through the dispersion in palaeomagnetic directions or poles.…”

A new set of qualitative reliability criteria to aid inferences on palaeomagnetic dipole moment variations through geological time

“…In the few cases where AGE was not met, this was either because the associated palaeomagnetic direction was anomalous and therefore suspected to reflect a remagnetization (Donadini et al, 2011;PINT REF 670) or where the age constraints were unusually weak (dating based on petrological similarities to distant intrusive units; e.g., Shcherbakova et al, 2012;PINT REF 673). The addition of reliable dates in the future could lead to such values being increased.…”

“…In another case (Donadini et al, 2011;PINT REF 670), a microscope analysis was performed, but this revealed the existence of some secondary iron-oxide grains leading to failure of this criterion. Finally, the study of Shcherbakova et al (2012;PINT REF 673) only provided microscopy information for one locality, but argued for a thermal origin of remanence for all localities on the basis of strong similarities in the measured shapes of continuous thermal demagnetization and remagnetization curves. This is not as strong a constraint as dedicated microscopy analyses, but nevertheless was judged sufficient to fulfill the TRM criterion.…”

“…In most cases, this reflected the inclusion of pTRM tail checks or the use of the domain state independent Wilson technique (Muxworthy, 2010). However, in other cases where the Thelliertype methods were used without specific checks, the criterion was met in some cases by the Arai plot of some estimates being linear over a demonstrably large (≥70%) fraction of the NRM unblocking range (Morales et al, 2003;Calvo-Rathert et al, 2011;Laj et al, 2011;Shcherbakova et al, 2011Shcherbakova et al, , 2012PINT REFs 676, 680, 693, 681, 673, respectively). Whenever the MD criterion was fulfilled by Thellier results with large fraction values or by Wilson results, these comprised at least half of the total number of sample estimates for that site mean or else produced measurements within the range of the other sample estimates for that site mean.…”

Palaeointensity determinations and magnetic properties on Eocene rocks from Izu-Bonin-Mariana forearc (IODP Exp. 352)

“…In the time range span by these rocks, only two studies of palaeointensity determinations are available. One is from submarine basaltic glass (SBG; Tauxe 2006) from DSDP Site 322 measured with the IZZI protocol that combines the Aitken (in-field, zero-field heatings) and the Coe (zero-field, infield heatings) methods (Yu & Tauxe 2005); the other one is from plutonic rocks from Western Antarctica (Shcherbakova et al 2012) measured with the Thellier-Coe (Coe 1967) method. The Virtual Dipole Moment (VDM) values given by these studies are between 2 and 5 × 10 22 Am 2 , which is much lower than the present-day value of ∼8 × 10 22 Am 2 .…”