Structural analysis of dyke emplacement directions as an aid to palaeomagnetic studies: an example from northern Chile

A major problem in palaeomagnetic studies of intrusive rocks lies in determ ining whether or not such rocks have been subjected to post-emplacement til ting. Structural analysis of dyke emplacement directions can be used to sho w the current attitude of the extension direction for the dyke swarm. If th e original extension direction, at the time of emplacement, can be deduced from geological evidence, this then provides a field test for post-emplacem ent tilting of the dyke swarm and its host rocks. In the example given from northern Chile, we were able to make a palaeomagnetic study of and structu rally analyse three successively younger dyke swarms that intrude similarly younging plutons. All three dyke swarms yielded extension directions close to horizontal but with markedly different azimuths. It is argued that the similarity in the plunge of the extension directions cannot be coincidental and that the dykes and their host plutons have not suffered significant po st-emplacement tilting. This simple technique should be widely applicable i n the assessment of post-emplacement tilting of dykes in palaeomagnetic stu dies.