Regional aeromagnetic anomalies in Ellsworth Land: Crustal structure and Mesozoic microplate boundaries within West Antarctica

The structural relationship between West Antarctic crustal blocks has remained uncertain due to a lack of information concerning sub‐ice crustal structure. A regional aeromagnetic reconnaissance of Ellsworth Land between 72 and 80°S and 65 and 100°W, completed as part of an integrated geophysical and geological investigation, provides the opportunity to study the ice‐covered area where several continental crustal blocks meet: Antarctic Peninsula, Thurston Island, Ellsworth‐Whitmore mountains and Haag Nunataks. The anomaly map is dominated by two major positive magnetic areas. Firstly, the Pacific Margin Anomaly (PMA) continues as a coastline‐parallel feature 100 km wide through Antarctic Peninsula and Thurston Island. The broad curvilinear anomaly with magnetic maxima of 200–600 nT is related to a mafic batholith and related intrusions and structures of the Pacific margin, mostly of Mesozoic age. Second, a zone of high intensity anomalies (200–1000 nT) of variable wavelengths (20–150 km) define an extensive area of highly magnetic Precambrian basement which is exposed at Haag Nunataks but is for the most part buried beneath ice and non‐magnetic cover. The geophysical data suggest that some of the crustal blocks have common structural features, their boundaries are more complex than originally thought and aeromagnetic provinces do not entirely correspond to the topographically defined boundaries of the crustal blocks. The Antarctic Peninsula and Thurston Island blocks are thought to have remained juxtaposed since the emplacement of the intrusive body related to the PMA. The Haag Nunataks Precambrian basement probably extends beneath the south and west margins of the Weddell Sea embayment and beneath the Ellsworth Mountains. The origin of long wavelength lineations beneath the Weddell Sea embayment is speculative. If they are related to Haag Nunataks Precambrian basement then the data offer insight into the trend of the Palaeozoic Ellsworth Mountain fold belt and support rotation models. Alternatively, if the anomalies are imaging break‐up related rifting trends, the lineations may provide extension and movement directions