The structure of the eastern part of the Insch Mafic Intrusion Aberdeenshire

Abstract

Synopsis A ground magnetic survey and a programme of shallow drilling have enabled considerable revisions to be made to the position of the margin of the Insch mass at its eastern end and revealed that the distribution and attitude of the igneous rock units have been strongly influenced by shear zones trendin g ENE. New petrological information shows that, except in a restricted area, there is a lack of regular relationships between the cumulate igneous rock units and it is established that the major trends and discontinuities in the contoured magnetic map can be correlated with zones of shearing and mylonitization. Magnetic modelling indicates that the shear zones dip steeply (65–80°) to the south and that the mafic units extend no further than about 2 km in depth. Later faults on N or NW trends have also contributed to the disruption of the intrusion. These findings are discussed in relation to regional structure and it is suggested that such shearing is more important than has been recognized hitherto and may constitute a discrete and extensive episode of deformation about 470 m.y. in age.