WESTWARD CONTINENTAL SUBDUCTION BENEATH THE EAST FLANK OF THE SIERRAN ARC

Cordilleran geologist have long sought to identify locations where the upper-crustal shortening manifested in fold-thrust belts might "root" into geometrically required shortening of deeper levels of the crust. Because many Cordilleran fold-thrust belts are east-vergent and thus apparently rooted towards the west, attention commonly focused on the east margin of the continental margin (Sierran) arc as a likely root zone, in part based on the inference that it marks a thermally induced lithospheric strength boundary. Until recently, however, little specific supporting evidence derived from the arc or its immediate eastern flank had been presented. Ducea (2001) proposed that petrologic, geochemical, and crustal-structure characteristics of the central Sierran arc could be explained by melting of substantial amounts of continental crust material that had been underthrust beneath the eastern flank of the arc.

I postulate that the East Sierran thrust system (ESTS) in Eastern California may constitute an additional expression of rooting of some back arc fold/thrust belt contraction beneath the east flank of the Sierran arc. The ESTS is an east-vergent contractional belt that has several characteristics pertinent to this hypothesis, including the observations that the ESTS (1) tracks the east margin of the Jurassic batholith for 150+ km; (2) experienced repeated episodes of moderate-magnitude arc-normal contraction in Middle and Late Jurassic time—and possibly in Late Triassic and Cretaceous time as well; (3) has transverse distribution of deformational intensity characterized by strongest deformation against the Jurassic batholith, decreasing eastward; and (4) experienced at least one major, synchronous shortening episode along its full length (at 150 Ma) that cannot be readily ascribed to batholithic space-making processes. These characteristics are consistent with an underthrusting model that envisions the arc serving as a buttress against which the shallower levels of the batholith’s eastern wall rocks (the greenschist-grade rocks composing the ESTS) were episodically compressed and shortened as their substrate—linked to fold-thrust belts located farther east—moved relatively westward and downward into the Sierran melt zone.