Plate tectonics, a theory which revolutionized our
understanding of the
earth. The theory however, failed to fully explain the formation of the
Rocky Mountains, until the "flat subduction" model was developed.
Dickinson and snyder (1978) propsed that shallow-slab subduction could
have transmitted tectonic stresses into the foreland and cause the
Laramide-style block uplifts. Bird (1988) suggested that shear traction
of the shallowly subducting plate stripped away the mantle lithosphere
beneath the North American crust and transmitted shear stress capable of
causing the foreland uplifts.
The first image here is a sketch of an oceanic plate subducting beneath a
continental plate at a
collisional plate boundary. The oceanic plate typically sinks at a fairly
high angle (somewhat exaggerated here). A volcanic arc grows above the
subducting plate. The second shows the plate tectonic setting during the
growth of the Laramide orogeny. The angle of the subducting plate is
significantly flatter, moving the focus of melting and mountain building
muchy farther inland than is normally expected. (Source: USGS)
Click
on
either image to
see an animation of the subducting plate shifting from steep to shallow.
Diagram of the subduction regime (Livaccari 1993).
Source: Bird 1984.
After the formation of the Rocky Mountains, the flattened Farallon slab
began to detach from the North American lithosphere, and sink back into the
mantle. This detachment began at the edges of the slab and propagated until
the whole slab began to fall away. Thus the reverse migration of the
magmatic arc formation seen in the northern and southern United States began
about 60-50 Ma and spread to the central United States by 40-30 Ma, causing
the cessation of volcanic activity in the Rocky Mountains, and reactivation
of activity in the west. This also changed the stress regime of the region,
ultimately leading to extension in the Basin and Range area, and uplift of
the Colorado Plateau and the Rockies.
Source: Bird 1988.
Source: Bird 1998. Click to enlarge.
On to Tectonics: Uplift History
