Modification of Continental Forearc Basins by Flat‐Slab Subduction Processes: A Case Study from Southern Alaska

Kenneth D Ridgway; Jeffrey M Trop; Emily S Finzel

doi:10.1002/9781444347166.ch16

Forearc basins are large sediment repositories that develop in the upper plate of convergent margins and are a direct response to subduction. These basins are part of the magmatic arc‐forearc basin‐accretionary prism “trinity” that defines the tectonic configuration of the upper plate along most subduction‐related convergent margins. Many previous studies of forearc basins have explored the links between construction of magmatic arcs, exhumation of accretionary prisms, and sediment deposition in adjacent forearc basins. These studies provide an important framework for understanding firstorder tectonic processes recorded in forearc basins that are characterized by long‐lived subduction of “normal” oceanic crust. Many convergent margins, however, are complicated by second‐order subduction processes, such as flat‐slab subduction of buoyant oceanic crust in the form of seamounts, spreading and aseismic ridges, and oceanic plateaus. These second‐order processes can substantially modify the tectonic configuration of the upper plate both in time and space, and produce sedimentary basins that do not easily fit into the conventional magmatic arc‐forearc basin‐accretionary prism trinity. In this chapter, we discuss the modification of the southern Alaska forearc basin by Paleocene‐Eocene subduction of a spreading ridge followed by Oligocene‐ Holocene subduction of thick oceanic crust. This thick oceanic crust is currently being subducted beneath south‐central Alaska and has an imaged maximum thickness of 30km at the surface and 22 km at depth. Findings from southern Alaska suggest that forearc basins modified from flat‐slab subduction processes may contain a sedimentary and volcanic stratigraphic record that differs substantially from typical forearc basins. Processes and sedimentary features that characterize modified forearc basins include the following: (1) flat‐slab subduction of a buoyant, topographically elevated spreading ridge oriented subparallel to the margin prompts diachronous uplift of the forearc basin floor and exhumation of older marine forearc basin strata as the ridge is subducted. Passage of the spreading ridge leads to subsidence and renewed deposition of nonmarine sedimentary and volcanic strata that locally exceeds the thickness of the underlying marine strata. (2) Insertion of a slab window beneath the forearc basin during spreading ridge subduction produces local intrabasinal topographic highs with adjacent depocenters, as well as discrete volcanic centers within and adjacent to the forearc basin. (3) Flat‐slab subduction of thick oceanic crust also results in surface uplift and exhumation of forearc basin sedimentary strata. However, the insertion of thick crust throughout the flat‐slab region (i.e., lack of a slab window) inhibits subduction‐related magmatism adjacent to the forearc basin. In the case of subduction of a >350‐km‐wide fragment of thick oceanic crust beneath south‐central Alaska, exhumation of forearc basin strata located above the region of flat‐slab subduction has prompted enhanced sediment delivery to active basins located along the perimeter of the flat‐slab region. These perimeter basins record an increase in subsidence and sediment accumulation rates coeval with flat‐slab subduction beneath the exhumed, inactive remnant forearc basin.

Modification of Continental Forearc Basins by Flat‐Slab Subduction Processes: A Case Study from Southern Alaska

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