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Title: A mid-Palaeozoic dipole low defined from new palaeointensity estimates from Russia and the UK
Author: Hawkins, L. M.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Variation in geomagnetic field behaviour occurs due to changes occurring deep within the Earth?s iYterior. BeÐause of this relatioYship, it is possidle to use palaeowagYetisw to constrain the evolution of the deep interior over time. A lack of reliable data for the Palaeozoic makes it difficult to constrain this relationship prior to ~320 Ma. In this PhD thesis, new palaeointensity data are presented, extending our insight into the strength of the geomagnetic field back by 80 Myrs (from ~ 335 Ma to ~415 Ma). The new palaeointensity measurements come from six different localities; five of these cover the period leading up to the onset of Permo-Carboniferous Reversed Superchron (PCRS; ~317 – 267 Ma), while the sixth quantifies the field strength afterwards. Current palaeointensity data prior to the most recent superchron, the Cretaceous Normal Superchron (CNS; ~125 – 84 Ma), suggests that the field was persistently weak during the Jurassic (up to ~80Myrs before the CNS) and this period is often referred to the ?MesozoiÐ Dipole Low? ;MDL?. New microwave palaeointensity data from the Siberian Traps, which was emplaced just after the PCRS (~250 Ma), is presented in this thesis and the reassessment of the palaeointensity data already published from this locality suggests that the field at this time was similar in strength to the Jurassic. This suggests that the MDL could have extended back to this time, persisting far longer than previously anticipated (up to ~125 Myrs before the CNS). The three other Siberian studies presented in this thesis provide field estimates covering most of the Devonian: the Minusa Basin (408 – 388 Ma), the Kola Peninsula (390 – 360 Ma) and the Viluy traps (376 – 364 Ma), all of which suggest the field was weak preceding the PCRS. Additional palaeointensity estimates from UK sites, the Strathmore region (415 – 410 Ma) and the Kinghorn (345 - 332 Ma), show that this period of weak field extends throughout the whole of the Devonian into the Early Carboniferous. An evaluation of the reliability of all of the available site data from 200 - 500 Ma indicates the reliability of this feature. The proposed name for this period weak field is the ?Mid-Palaeozoic Dipole Low? (MPDL). While this interval appears to be similar to the MDL based on the most recent estimates of the length and average field strength MDL, there are insufficient data to confirm whether they are a common feature prior to superchron onset. The implications of these new Palaeozoic field estimates are also discussed in terms of their relation to mantle convection processes and the average Phanerozoic field strength.
Supervisor: Biggin, Andy ; Duller, Rob ; Hill, Mimi Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral