The Loppa High: Compression Or Extension Feature during the Permian and Triassic?
Enrique Novoa and Tore Svånå
Statoilhydro, ASA, Harstad, Norway.
Traditionally, the Loppa High has been interpreted as an uplifted graben shoulder, related to a large tilted normal fault blocks of Carboniferous/Permian age to the west of this high. In this paper, an alternative model to this commonly accepted Industry and Statoilhydro interpretation is presented. Here, the internal geometry of growth sediments deposited on the flanks of the Loppa High has been used to demonstrate that the structure may have resulted from a phase of compression during Permian and Early-Mid Triassic, rather than extension. The most active phase of compression took place around the Permian / Triassic boundary. This is demonstrated via the use of 2D structural restorations and kinematic modelling. There are two end members of folding mechanisms in the upper crust: limb rotation and hinge migration. These folding mechanisms yield geometrically distinctive patterns of folded growth sediments that can be utilized to decipher the deformation history of folds and to constrain their interpretation. Growth sediments of Permian age deposited on the eastern flank of the Loppa High have been progressively restored showing growth by hinge migration rather than rotation, as would be predicted if related to an uplifted graben shoulder. Conversely, Late Jurassic - Early Cretaceous normal faults in the Bjørnøya Basin located to the west of the Loppa High, have been restored and indicate that the Permian and Triassic sediments have been folded, rather than normally faulted, suggesting that the west limb grew by limb rotation. It is therefore proposed, that the Loppa High may be a contraction structure related to the propagation of an east dipping thrust fault that is detaching in the Moho. This interpretation is also consistent with the timing of development of small and large salt domes in the Bjarmeland Platform during the Permian and Triassic, suggesting a phase of compression during this time. Later phases of deformation (e.g. Late Jurassic - Early Cretaceous extension) over the Loppa High are not discussed in this paper.
AAPG Search and Discover Article #90096©2009 AAPG 3-P Arctic Conference and Exhibition, Moscow, Russia