The altitude dependence of magnetic remanence in the slowly-cooled Precambrian plutonic terrain of West Greenland

Abstract

To evaluate the change in magnetic remanence with altitude through a slowly-cooled Precambrian basement terrain three vertical sections have been sampled in West Greenland. The study employs the principle that higher structural levels passed through their blocking temperatures earlier than lower levels and therefore record earlier pole positions, and it utilises dolerites and diorites intruded late in, or after, the tectonic history to minimise anisotropy effects. In the amphibolite facies terrain at Qa´qatoqaq (1400 m) highly-stable magnetite-held remanences move on demagnetisation progressively along small circles interpreted to record younger to older apparent polar wander (a.p.w.) motions during cooling through the blocking temperature ranges. Although the raw data show no systematic variation with altitude, when account is taken of the blocking temperature spectra as defined by thermal demagnetisation there is a systematic change in palaeofield direction in the same sense as that recorded by the demagnetisation trends. Granulite facies terrain at Igdlu´nguit qulaˆt (600 m) again shows systematic variation with altitude when the sites are divided into those with a remanence dominated by hemo-ilmenite and those dominated by magnetite. A third section at Praestefjeldet (250 m) yields a palaeofield reversal and a high blocking temperature component. The age evidence is evaluated to suggest that the a.p.w. path defined by 5 mean palaeopoles between 318°E, 1°N and 247°E, 38°N represents up to 50 Ma of palaeofield motion recorded by the uplift and cooling of this basement terrain at crustal depths of the order of 10 km. The calculated rate of a.p.w. motion is 1–2°/Ma and the rate of crustal uplift 10–20 m/Ma, these rates are respectively up to an order higher, and at least an order lower, than Phanerozoic rates. The collective data from Greenland agree closely with post-“Hudsonian” poles from the Laurentian Shield and represent part of a very widespread uplift event following this mobile episode. They show that altitude sections can yield a systematic record of the magnitude and direction of Precambrian a.p.w. motions provided that the blocking temperature spectra are taken into account.