90-7 The Forearc Record of Late Jurassic to Mid-Cretaceous Tectonics of the Central North American Cordillera

Abstract:

High-resolution mantle tomography as well as increased recognition that changing relative plate motions influence the timing, locus, and style of upper-plate deformation have led to renewed debate about the Jurassic-Cretaceous development of the central segment of the North American Cordillera. Strata of the Upper Jurassic Galice and Mariposa Formations, the Upper Jurassic to Cretaceous Great Valley Group (GVG), and the Upper Cretaceous Hornbrook Formation and Ochoco Basin form an integrated record of tectonic processes along the margin. Middle to Late Jurassic Farallon slab rollback and associated extension during east-dipping subduction may have generated accommodation space for sedimentation and development of a fringing magmatic arc built on previously accreted terranes, or the Early Jurassic demise of the Mezcalera plate through double-sided subduction led to arc-continent collision and accretion of the western Sierran foothills and perhaps western Klamath terranes by Middle Jurassic time. Establishment of Farallon subduction beneath western North America or a return to a shallower angle of Farallon subduction by 160 Ma led to upper-plate contraction, rapid deposition of turbidites in the Galice and Mariposa basins, and syndepositional thrusting and metamorphism. Rapid northward drift of North America beginning 150 Ma may have led to sinistral transtension in the upper plate and another episode of slab roll-back and trench retreat that shifted magmatism westward. Early Cretaceous transtension along the margin included extensional growth faults in the Great Valley forearc that accommodated displacement of the GVG and underlying Coast Range Ophiolite and the forearc Klamath basement to the northwest, possibly shifting the Klamath block westward relative to the northern Sierran foothills. Rapid westward drift of North America beginning ca. 120 Ma resulted in nearly orthogonal convergence, development of an accretionary margin, and the Cretaceous magmatic flare up in the Sierra Nevada. Upper Cretaceous strata from the GVG, Hornbrook Formation, and Ochoco Basin may have formed a single marginal basin beginning ca. 85 Ma, with subsidence of the Klamath Mountains province, deep-water deposition in the Hornbrook and Ochoco systems, eastward migration of the Late Cretaceous GVG shoreline, an eastward shift in sediment sources, and uplift in the eastern Sierra Nevada and possibly Idaho arc and Lemhi basin in central Idaho. Collectively, these marginal basins provide a record of western North American tectonics from Jurassic through Cretaceous time.

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.