EVOLUTION OF THE FOOTHILLS TERRANE, SIERRA NEVADA, CALIFORNIA: PROGRESS AND CONTINUING CHALLENGES

The Foothills terrane of the Sierra Nevada is a polydeformed ophiolitic lithostratigraphic assemblage that includes a pre-Jurassic ophiolitic basement; superimposed Early, Middle, and Late Jurassic volcanic arcs; coeval Jurassic siliciclastic, volcaniclastic and chert-argillite strata; and a cover of Late Jurassic, quartz-rich sandstones and shales. It lies west of the inferred Jurassic continental margin, and is separated from it by an Early Mesozoic subduction complex, and a large Paleozoic(?) peridotite body juxtaposed with Early Mesozoic blueschists. For over three decades, debate has focused on three broad issues: (1) Was the Foothills terrane assembled tectonically from unrelated volcanic arcs? (2) Is the terrane exotic to North America? (3) Does the observed deformation record Late Jurassic arrival and collision of this terrane with North America? The evidence acquired over the past 35 years suggests that the answer to each of these questions is “No.”

Construction of volcanic arcs began on a disrupted, pre-Jurassic ophiolitic basement. Late, Middle, and Early Jurassic volcanic and plutonic rocks of succeeding arcs were deposited on or intruded into fragments of the previous arc, suggesting that Jurassic magmatism continued above the same basement throughout the complex Jurassic deformation. Evidence for inheritance of Precambrian zircon is common in plutons within Early and Late Jurassic arcs, and in Middle Jurassic, Late Jurassic, and Early Cretaceous cross-cutting intrusions. The ages of the inherited zircon are consistent with a North American origin and suggest that all Jurassic magmatism occurred nearby. The Foothills terrane probably was in place adjacent to the North American by Middle Jurassic time. The evidence includes constraints on the age of deformation within and at the leading edge of the terrane, Middle Jurassic or older blueschists exposed inboard of the leading edge of the terrane, and widespread evidence for pre-Late Jurassic metamorphism. These observations suggest that the Late Jurassic deformation is a post-collisional phenomenon. Critical, unresolved issues include the origin and significance of the exposed blueschists, the nature and correlation of ultramafic basement rocks, and the kinematic histories of internal and terrane-bounding faults in the region.