Forced Folding and Basement-Detached Normal Faulting in the Haltenbanken Area, Offshore Norway

Martha O. Withjack, Kristian E. Meisling, Lee R. Russell

Triassic evaporites affected the structural development of the Haltenbanken area of offshore Norway during the Late Jurassic and Early Cretaceous by mechanically isolating Triassic and younger strata from older strata and basement. Many folds in the Haltenbanken area are forced folds above basement-involved normal faults. Forced folds formed, at least in part, because Triassic evaporites behaved ductilely, decoupling overlying strata from underlying faulted strata and basement. Seismic data show that these forced folds are asymmetric flexures that affect Lower Cretaceous, Jurassic, and Triassic strata. Strata beneath the Triassic evaporites are faulted. Some forced folds die out along strike into, and are cut by, basement-involved normal faults. Folding predominated above salt swells where decoupling was enhanced, whereas faulting occurred on the flanks of salt swells where salt thicknesses were reduced and decoupling was less effective. Many normal faults in the Haltenbanken area are basement-detached and flatten within the Triassic evaporites. Seismic data show that rollover anticlines and antithetic normal faults affect Lower Cretaceous, Jurassic, and Triassic strata within the hanging walls of these basement-detached normal faults. Strata beneath the evaporites are not affected by this deformation. Some basement-detached normal faults may be secondary structures associated with forced folding. Others, especially those with large displacements, may have formed in response to gravity sliding.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.