MODIFICATION OF CONTINENTAL FOREARC BASINS BY SPREADING RIDGE SUBDUCTION AND FLAT-SLAB SUBDUCTION PROCESSES: AN EXAMPLE FROM SOUTHERN ALASKA

The Cenozoic forearc basin of southern Alaska was impacted by Paleocene–Eocene spreading ridge subduction and Oligocene–Recent flat-slab subduction.

Spreading ridge subduction significantly modified the >800-km-long Late Cretaceous marine forearc basin system. Important modifications include an abrupt change from marine to nonmarine depositional systems across a regional unconformity, reactivation of intrabasinal fault systems, intrusion of forearc strata by mantle-depleted igneous rocks attributable to slab-window magmatism, and localized accumulation of thick siliciclastic-volcaniclastic wedges adjacent to slab-window volcanic centers. These features are much different than those associated with the typical model for forearc basins, where most of the siliciclastic strata is derived from igneous rocks produced by arc magmatism and deposited in submarine environments.

Flat-slab subduction also had a profound effect on Miocene-Recent forearc basin development. To the west, the Cook Inlet forearc basin contains Neogene strata >7,500 m thick; this active basin marks the western perimeter of the flat-slab region. To the east, directly above the flat-slab region, the Matanuska forearc basin has been exhumed and is a zone of erosion and sediment bypass. Sediment eroded from the exhumed region was transported westward to the active Cook Inlet basin. Further to the east, along the eastern perimeter of flat-slab subduction, slab-edge volcanism and transtensional basin development resulted in overprinting of Late Cretaceous marine forearc basin strata by Miocene–Recent strike-slip faults, volcanic fields, and alluvial-lacustrine volcaniclastic strata. These along-strike changes in basin dynamics point to the significant modification that occurs along convergent margins affected by flat-slab subduction.

Spreading ridge and flat-slab subduction are common along modern convergent margins, but are more difficult to identify in the geologic record. Characteristics that distinguish modified forearc basins from typical forearc settings include an abrupt change from marine to nonmarine deposition, abrupt along-strike changes in stratigraphy, regional and local unconformities, and intrabasinal igneous rocks or lack of coeval arc rocks.