Structural inheritance in the Chukchi shelf, Alaska

Abstract

Interpretation of over 100,000 line-km of reprocessed 2D, time-migrated, seismic-reflection data from the U.S. Chukchi shelf documents at least four tectonic events in the last 400 m.y. that generated distinct inherited structures. A well stratified economic basement whose internal geometry clearly defines a pre-Mississippian, thin-skinned fold-and-thrust belt represents the earliest recognizable tectonism in the area, one that accommodated west-east (present-day coordinates) shortening, probably in the Devonian. (1) The first inversion event, and dominant crustal feature in the Chukchi shelf, is the Hanna Trough, a north-south, failed rift system that accommodated more than 10 km of Carboniferous-Jurassic syn-rift and post-rift (sag) strata. Carboniferous extensional collapse of the previous orogenic belt to form the Hanna Trough was influenced heterogeneously by the pre-Mississippian contractional fabrics. Many rift-phase normal faults detach along pre-Mississippian thrust faults in discrete negative inversion, creating distinctive synthetic growth strata that constrain the geometry of extension. Other normal faults cut across pre-Mississippian stratigraphy showing more typical rollover geometries in growth strata and dissection of contractional structures in the pre-rift strata in more distributed negative inversion. (2) After a period of Pennsylvanian to Early Jurassic subsidence, in the south-central Chukchi shelf several north-south, rift-phase normal faults were inverted in a second event in the Late Jurassic to Early Cretaceous, forming local, asymmetric, positive inversion structures due to west-east shortening likely associated with far-field effects of the Chukotkan orogeny. The Hanna Trough then was buried beneath >2 km of Cretaceous and Cenozoic foreland-basin deposits during which complex additional deformation occurred in the region. (3) The North Chukchi high resulted from Late Cretaceous-Paleogene deep thrusting, both south- and north-directed, and possible transpressional inversion across a complex older structural fabric. (4) Cenozoic east-west extension and strike-slip faulting in the northern Hanna Trough reactivated Carboniferous rift structures, in the west as dominantly discrete motion on the older faults, and in the east in more distributed fashion coalescing near older rifts and cutting Late Cretaceous-Paleogene contractional structures during a final negative inversion. Documentation of multi-phase structural inheritance in the Chukchi shelf is vital to unraveling the tectonic history and influences on petroleum systems in the area.