Marine, mid-Cretaceous fore-arc strata on Cedros Island (Vargas and Pinos Formations) record the onset of extensional brittle deformation of the upper crust concomitant with initial uplift of blueschist-grade rock from great depth. The pre-extensional Vargas Formation is chiefly fine grained, and it contains facies interpreted as basin-plain, sand-rich-fan, and slope deposits that onlapped a Jurassic unconformity toward the north. The syn-extensional Pinos Formation is coarser grained than is the Vargas and records the initiation and filling of a northeast-southwest-trending, deep-marine, half-graben structure that formed by reactivation of a Jurassic fault zone located along the northern margin of the basin. The axis of the half graben acted as a submarine canyon that funneled gravelly sediment-gravity flows toward the south-west, whereas the shoulder of the half graben was draped by sandy turbidites and mud. A second episode of faulting on a fault parallel to, and synthetic with, the older half-graben fault produced a stress-transfer zone that steepened the intervening block, producing a catastrophic slope failure in the basin.
Several points of evidence suggest that the canyon-bounding fault zone mapped at the northern margin of the basin was syndepositionally active. Basin-floor scarps are indicated by the presence of large, angular blocks derived from the Jurassic strata; these blocks occur within Cretaceous conglomerate and olistostromes, and as clast-supported breccia. Asymmetric basin subsidence is recorded in monotonic lithofacies trends that occur laterally away from the inferred fault zone. These trends include a decrease in average grain size and bed thickness, and an increase in interbedded mudstone and in lateral continuity of individual beds. Abundant slope-failure features, including slide blocks as much as 100 m wide by 40 m thick, also point to frequently recurring intrabasinal seismicity.
The facies distribution and basin structures exposed on Cedros Island are analogous to aspects of the southern Viking Graben and the Ebro turbidite complex. Modern tectonic and physical analogues for the half graben are found at the Kuril margin and in the Aleutian are, where oblique convergence has produced fault-bounded submarine canyons of the same lateral and vertical scale as inferred for the Cretaceous basin on Cedros Island.
The temporal coincidence between brittle deformation in the upper plate with peak blueschist metamorphism in the lower plate of the convergent margin suggests that initial blueschist uplift began soon after deepest burial.