Gravitational Spreading Model
Instability of an over thickened crust (figure on right, Coney and Harms, 1984) formed during the Sevier-Laramide orogenies. The crust was gravitationally unstable and spread outward under its own weight. The origin of the spreading coming from a reduction of viscosity by a mantle derived heating event (Coney, 1987), thermal relaxation of the overthickened crust (Sonder, et el., 1987), or collapse and steepening of a previously shallow-dipping Laramide Benioff zone, which may have reduced the regional stress and possibly started extension (Coney and Harms, 1984).
The thicken crust is proposed to have spread laterally westward, away from the unmoveable Colorado Plateau. This reversed the earlier compression of the Laramide and Sevier Orogenies.
The simple diagram to the left shows the internal forces needed to cause the gravitational spreading. A thickened crust and elevation will initiate normal faulting and spreading (Jones, Class Notes).
The figure on the right, (Coney and Harms, 1984) shows Post-Laramide palinspastic and paleoisopach reconstruction and the crustal thickness associated with the reconstruction. The contours are crustal thickness (km). Stippled area is that of the presumed over thickened crust. They area it covers seems to coincide with the Cordillera and specifically with the metamorphic core complexes (see figure for comparison of areas).
Werneke and Axen (1988) "interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin."
From Werneke and Axen, 1988
Magmatic Underplating or Intrusion
Magmatic Intrusion development model (Rehrig and Reynolds, 1980).
- A) Three distinct layers of crust: 1) rigid plate, 2) zone of mylonitization, 3) zone of lateral dilation.
- B) Magma intrusion zone 2 stretches.
- C) Extension in zone 3 leads to tensional separation and intrusive dilation. Zone 2 develops mylonites (Arrows point to mylonitization, penetrative extension and flattening in zone 2).
- D) Differences in response to crustal extension between zones 1 and 2 create differential strain and fragmentation of rigid plate (Arrow points to zone 1 moving west and zone 2 moving east).
- E) Decollement forms (Arrow 2) and fault blocks rotate. Sedimentation occurs (Arrow 1).
- F) Denudation increases, crust is thinned considerably, and zone 1 is removed, zone 2 has deformational thinning and isostatic rebound occurs (Large arrows indicate uplift).
- G) End result.
If surface crustal thinning were to account for the amount of crustal thinning needed for extension, the should be 40-42 km thick crust under Arizona. Refraction data indicates SW-ward thinning to less than 30 km. There is a discrepancy zone where crustal thinning is not accounted for my normal crustal faulting. A Moho hinge can account for the removal of the lower crust.
Low angle normal faulting reaches deep into the crust. Shearing takes place and mylonites form.
Mylonites are pulled to the surface by normal faulting displacement, unroofing causes isostatic doming.
Subaerial denudation, and core complex emplacement.
Three stages of flexural uplift (Holt et al, 1986, after Wallace, et al, 1986):
Seismic refraction and Gravity studies (Holt et al, 1986; Wallace, et al, 1986) show a crustal root under the Catalina-Ricon core complex.
Figure (Holt, et al., 1996) shows flexural isostasy of 24-km-wide crustal root under the Catalina Mountains. Profiles are calculated from a 2D model for distributed loads on an elastic plate isostatically supported from below.
- Local crustal thickening or formation of a root before the mid-Tertiary,
- regional, low relief isostatic uplift over the local root in response to tectonic denudation, which resulted in the exposure of lower plate rocks at the surface,
- nearly complete local isostatic uplift after Basin and Range high angle faulting.
- A) Resulting flexure from elastic thickness 20 km, unbroken crust from load applied from below.
- B) Resulting flexure from elastic thickness 10 km, unbroken crust from load applied from below.
- C) Resulting flexure from elastic thickness 10 km, broken plate at dashed line.
The figure below (Spencer, 1984) shows a schematic diagram of warping and uplift of a detachment fault. In sequence B the antiformal warp has formed, and by C it has risen enough to denude and expose lower plate.