Evaluating Forced Versus Plume‐Induced Subduction Initiation for the Talkeetna‐Bonanza Arc System: Implications for Permo‐Triassic Paleogeography of the North American Cordillera

The mechanisms by which new subduction zones form remain highly debated. Here we use Permo‐Triassic strata to evaluate potential mechanisms for initiation of the Talkeetna‐Bonanza Arc of the northern North American Cordillera. New mapping and analysis of Permian and Upper Triassic strata on the Alaska Peninsula suggest a period of uplift followed by extension and rapid subsidence. Extension and subsidence beginning in the late Carnian‐early Norian coincided with syn‐initiation slab rollback and the formation of supra‐subduction zone crust 50 km trenchward of our study area. We then integrate our findings with a synthesis of Permo‐Triassic stratigraphic relationships across the Insular terrane. We find that unconformity development occurred at the plate scale, where deformation and erosion was concentrated inboard of a Paleozoic arc sequence that is unconformably overlain by Upper Triassic volcanic and sedimentary strata. As subduction initiated in the Late Triassic (ca. 235 Ma), the upper plate experienced rifting, bimodal volcanism, and the eruption of flood basalts interpreted by some workers as plume‐derived. Two subduction initiation mechanisms may be consistent with this geologic record: (a) forced initiation involving a subduction polarity reversal linked to convergence between fringing arc terranes and western Laurentia during the greater Sonoma Orogeny; or (b) plume‐induced subduction initiation which invokes no specific paleogeographic configuration. Forced initiation is consistent with pre‐initiation unconformity development but does not account for the eruption of plume‐derived flood basalts. Plume‐induced subduction initiation explains the petrogenesis of the flood basalts but does not account for regional Permo‐Triassic unconformity development and Late Triassic rifting.