Uplift deduced from remanent magnetization: Sudbury area since 1250 Ma ago

Abstract

Two contacts between Sudbury norite and northwest-trending diabase dikes and two contacts between the overlying micropegmatite and northwest dikes were investigated in order to estimate the depth of burial of the present erosion surface at the time of dike emplacement. A zone of hybrid paleomagnetic direction representing the vectorial sum of an older host component and an intrusion component of decreasing highest blocking temperature and intensity with distance from the intrusion was sought. Subtracting the calculated thermal effect of the intrusion from this highest blocking temperature yields the temperature of the host at the time of magma emplacement. Dividing this host temperature by an estimated paleogeothermal gradient yields the burial depth of the present erosion (or sampling) surface at the time of magma emplacement. Remanence direction in one of the dikes and norite contact zones is not typical for the Sudbury dike swarm of 1250 Ma age, and this contact is not further considered. An earlier published result for a norite-dike contact was reconsidered because of complicated dike geometry and included in this study. In one of the four usable contacts the hybrid zone is represented by three samples, in another by one sample, and in the remaining two only the contact zone width could be used. The final host temperature results are based on 4 individual calculations and show fair consistency with mean values of 287°C (s.d. 13°) and 267°C (s.d. 11°) calculated without and with a correction for viscosity of the host remanence respectively. Using a gradient of 26°C/km for 1250 Ma ago indicates a burial depth of 9.5 ± 2km at that time. The fair consistency encourages the use of the method to deduce quantitatively the history of vertical motions of Precambrian terranes, the detail obtained being dependent on the presence of hybrid zones and of intrusions of various ages.