Abstract

Subaerial plateau basalts, initially c. 1 km thick, overlie Cretaceous or older formations and are locally separated from pre-Tertiary rocks by (a) a quartzitic conglomerate and (b) a (younger) sequence of tuffaceous sediments and hyaloclastites. The plateau lavas are divisible into a Lower (typically microphyric, quartz tholeiitic) Series and an Upper (typically porphyritic, olivine tholeiitic) Series. N–S and NE–SW trending antithetic faults give rise to ocean-facing fault-line scarps and landward-facing dip slopes. Dolerite sills, abundant beneath the lava pile, occasionally cut the Lower Series lavas. While dykes trend in various directions, a very prominent NE–SW swarm (mainly quartz tholeiitic) traverses Hold with Hope, deflecting to more nearly N–S across Gauss Halvø.

A large volcanic centre (Myggbukta Complex) lies across the main dyke swarm in the vicinity of the deflection and may be genetically related to the swarm. The complex involves propylitized basaltic, rhyolitic and intermediate lavas and pyroclastic rocks, together with volcanogenic sediments, cut by a profusion of basic to acid intrusive sheets. Formation of this complex at a late stage in the volcanic history of the region was attended by subsidence and crustal down-sagging. The main dyke swarm, whose trend roughly parallels that of the spreading axes N and S of the Jan Mayen Fracture Zone, may reflect subordinate rifting to the W of the principal rift zones along which spreading was taking place.

30 km E of Myggbukta, the Kap Broer Ruys granophyre/felsite is a partially unroofed intrusion around which a broad metamorphic aureole has been superimposed on the Mesozoic and Tertiary sediments and lavas. The southern coastal region of Hold with Hope, connecting the Myggbukta and Kap Broer Ruys centres, may indicate a zone of crustal weakness which determined the initiation of the Jan Mayen Fracture Zone.

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