Abstract
Slab rollback, lithospheric body forces, or evolution of plate boundary conditions are strongly debated as possible lithospheric driving mechanisms for Cenozoic extension in southwestern North America. By incorporating paleo-topography, lithospheric structure, and paleo-boundary conditions, we develop a complete geodynamic model that quantifies lithospheric deviatoric stresses and predicts extension and shear history since Late Eocene. We show that lithospheric body forces together with influence of change-over from subduction to transtensional boundary conditions from Late Eocene to Early Miocene were the primary driving factors controlling direction and magnitude of extensional deviatoric stresses that produced topographic collapse. After paleo-highlands collapsed, influence of Pacific-North America plate motion and associated deformation style along the plate boundary became increasingly important from Middle Miocene to present. Smaller-scale convection stress effects from slab rollback and associated mantle flow played only a minor role. However, slab rollback guided deformation rate through introduction of melts and fluids that impacted rheology.
Highlights
Slab rollback, lithospheric body forces, or evolution of plate boundary conditions are strongly debated as possible lithospheric driving mechanisms for Cenozoic extension in southwestern North America
We have produced a timedependent kinematic solution that preserves the land-based observations[11], but that includes the accommodation of the Farallon- and Pacific-North America relative plate motions[6,7] from Late Eocene to present, with the influence of the change-over from subduction boundary conditions to the progressive development of the San Andreas Fault System (SAFS) with northward migration of the Mendocino Triple Junction (MTJ) (Supplementary Movie 1 and Supplementary Data 1)
Through blind test benchmarking we show that if paleotopography and crustal structure (GPE), plate motions, and scalar values of strain rates (E) are known (Fig. 4a, b), it is possible to recover the true depth-integrated effective viscosity and deviatoric stresses within the lithosphere (Fig. 4c) (Methods)
Summary
Lithospheric body forces, or evolution of plate boundary conditions are strongly debated as possible lithospheric driving mechanisms for Cenozoic extension in southwestern North America. Wide-spread volcanism, and the development of the San Andreas Fault System (SAFS)[1,2,3,4,5] followed the earlier shallow- to flat-subduction of the east dipping Farallon slab During this plate margin transition[6,7] high elevations of orogenic plateaus underwent profound extension and shear that resulted in the present-day Basin and Range Province[1,6,8,9,10,11,12] (Fig. 1a, b). There is controversy about how much of the paleohighland (e.g., Nevadaplano) was associated with dynamic topography[15,20] versus static support (crustal root)[10,23,24,25] Another controversy is whether the collapse was controlled by boundary conditions changing along the plate margin or whether it required significant lithospheric weakening. Bahadori et al.[8] have used displacement histories on land in southwestern North America from the model of McQuarrie and Wernicke[11] to develop a model of finite strain history, crustal thicknesses, and paleo-elevations since Late
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