THE BLAIRGOWRIE MAGNETIC ANOMALY AND ITS INTERPRETATION USING SIMPLEX OPTIMIZATION

The large magnetic anomaly which runs along the Highland Boundary Fault of Scotland immediately to the NW of Blairgowrie has been studied by both a ground-based magnetic survey and a palaeomagnetic investigation. A newly developed optimisation program, making use of a simplex algorithm, was used to model the long wavelength component of this effectively two-dimensional anomaly. The consequent model consists of a vertical, rectangular body, 3 km wide and 13 km deep, whose top surface is 2 km below ground level, with a magnetisation directed vertically downwards. The composition of this body is most likely to be that of an ultra-basic, metamorphic complex which has been brought up to its present position between two of the many near-vertical faults in the area. The medium wavelength structure of the anomaly was modelled using the common technique of trial and error, and can be interpreted in terms of a pair of Devonian andesitic lavas and a small extension of the main ultrabasic block. Subsequent palaeomagnetic remanence measurements confirmed that the magnetisation of the two lavas recorded a Devonian polarity reversal. The magnetisation of the ultrabasics is directed vertically downwards, exactly as deduced from the earlier modelling work.