The relationship between magmatism and tectonism may be a difficult one to unravel. Lipman and Glazner (1991) summarize many important questions regarding tectonics and magmatism in the mid-Tertiary. Changes in plate velocities, plate boundary interactions, crustal density, pre-existing weaknesses (including thermal and mechanical), gravitational potential energy, and the thermal structure of the crust could affect the continental crust and lithosphere to promote or retard igneous activity.
Humphreys (1995) proposed a tectonic model to explain the migration of magmatism across the western United States. The patterns of magmatic migration are not easily explained with the standard Farallon flat slab model with a shallow subduction angle. As noted in the geologic background section, two waves of magmatism converged on southern Nevada between 54 Ma and 21 Ma (click here to view figure). The proposed models call for delamination or buckling of the subducting Farallon slab beneath North America (Figure 1).
Figure 1. Three models proposed by Humphreys (1995) to explain magmatism migration in western North America (figure from Humphreys, 1995). (a) Buckle model [this view is looking westward from underneath northeastern Canada], (b) parachute mode, and (c) drip model.
The buckle model has an east-west-trending buckle of Farallon slab descending beneath North America. The slab has torn free to the north and south to be able to descend. As the slab descends, the base of the continental lithosphere is exposed to the asthenosphere and magmatism is induced. This model would explain two bands of magmatism converging. However, the model requires a complicated plate geometry that (1) may not be necessary, and (2) may not be supported by geophysical evidence (i.e., tomography). The parachute model also allows for two bands of magmatism. Humphreys (1995) does not support this model because two 'flaps' of Farallon slab would have to sweep through asthenosphere and upper mantle as the parachute descended into the mantle. The drip model is possible if the Farallon slab material could become entrained in mantle flow.
Figure 2. Maps of extension and magmatism in Cordilleran (figure from Liu, 2001). (a) gray areas represent batholiths, black areas are metamorphic core complexes. (b) Patterned areas show intermediate-silicic rocks, numbers (in Ma) indicate magmatic front. (c) Gray areas represent basalt and bimodal volcanism, horizontal areas show major extension, small arrows show direction of magmatism migration.
Liu (2001) used thermal-rheological and thermomechanical model to investigate whether gravitational collapse of the thickened Cordilleran orogen could produce metamorphic core complexes, and also whether gravitational collapse could induced the mid-Tertiary flare-up (Figure 2). Numerical model results support gravitational collapse as a driving force for metamorphic core complexes, but Liu (2001) suggested that gravitational collapse did not induce the flare-up. The flare-up would require a magnitude of asthenospheric upwelling not produced in models of gravitational collapse (Figure 3). For several model runs with varied heat flux, less than 10 km of asthenospheric upwelling was produced. This upwelling would not be enough to induce partial melting from decompression. Liu (2001) provided the explanation that perhaps thermal weakening and rhelogical hardening due to intrusions and underplating of the crust could have produced the mid-Tertiary volcanism.
Figure 3. Plot of asthenospheric upwelling verses time (figure from Liu, 2001). Each curve is a model run labeled with the value of the initial steady state geotherm (mW/m2) for the model run.
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