▪ Abstract  Cenozoic extension in the western United States presents a complex interrelation of extension, volcanism, and plate boundary tectonics that defeats simple notions of “active” or “passive” rifting. Forces driving extension can originate at plate boundaries, through basal traction, basal normal forces, or from buoyancy forces internal to the crust and lithospheric mantle. The latter two are most responsible for driving extension where it is observed in the Basin and Range. The complex evolution of the northern Basin and Range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust. In contrast, crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 Ma to drive extension in the southern Basin and Range. The central Basin and Range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western United States.

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The deformation of a thin viscous layer that has a moving •boundary is investigated for comparison with zones of deformation in the continental lithosphere. Exact analytical soluti6ns, for the case of a Newtonian fluid, and approximate solutions, for the case of fluid with power law rhedlogy, show that: When the imposed velocity vector is normal to the boundary (com. pressional or extensional regime) the deformation field decays away from the boundary with a characteristic length scale 1/3 to 1/10 the wavelength of the imposed boundary velocity distribution for n between 1 and 10, where n is the stress-strain exponent in the rheology; in contrast, when the imposed velocity vector is parallel to the boundary (transcurrent regime), the length scale of the deformation field is approximately 4 times smaller. In each case these length scales decrease approximately as n-x/2. The difference in length scales arises even in the absence of any buoyancy forces acting on thickened or thinned crust; such forces would modify the ratio of length scales, but not sufficiently to affect this result.

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We investigate the possibility that the onset and development of Cenozoic extension in western North America was governed by the potential energy contrast within, and mechanical properties of, lithosphere that was previously thickened during the Sevier and Laramide Orogenies. The strength of the lithosphere can be defined by its vertically averaged properties; to a first approximation, this strength is too great for geologically significant extension to occur unless the Moho temperature exceeds about 700~ (+ 100~ This means that there may be a delay between the end of compression and the beginning of extension, the length of which depends on the pre-thickening thermal structure and the compressional strain. Delays of up to 100 My may occur for the lowest initial Moho temperatures investigated (<450~ or extension may follow immediately on release of compression if the initial Moho temperature exceeds about 700~ The total extensional strain that is achieved depends on the potential-energy contrast between the thickened lithosphere and its surroundings. Partial melting of peridotite to produce basaltic magma is possible after small degrees of extension, but depends strongly on details of the initial temperature condition in the lower part of the lithosphere.The results of these calculations agree with observations of the Cenozoic extensional history of North America: late-Mesozoic/early-Tertiary compression in the Pacific Northwest was accompanied by extensive calc-alkaline magmatic activity and was followed almost immediately by extension; in the northern and southern Great Basin--which show respectively, little and no evidence of syn-compressional igneous activity--the gap between compression and extension was 20-40 Ma (N) to about 70 Ma (S).