The possible initial conditions just before the uplift of the Sierra in the Late Cenozoic are:STEP 1
- Last major crustal accretion event seen at the surface [Jones and Phinney, 1998]
- A crustal root of ~60 km predicted based on the fact that the event produced a high heat flow regime that would in turn depress the gabbro-eclogite transition which signals the seismic crust mantle transition [Wernicke et al., 1996]
- A large eclogitic root would have required an extraordinarily thick crust to produce any topography [C. Jones Personal Communication, 1999]
- Leads to question of Where is the root now? and What type of topographic expression has it had through time?
STEP 2
- Root may have been modified by shallow subduction[Jones and Phinney, 1998]
- Should show signs of deformation from subduction such as an east dipping Moho [Jones and Phinney, 1998]
- Temporary end of volcanism related to subduction [Saltus and Lachenbruch, 1991]
- Topographic expression could be supressed due to crustal rigidity [Chase and Wallace, 1988] inferred from low surface heat flow and the abnormal depth (~38 km) of the brittle-ductile transition zone in the northern Sierra [Wong and Savage, 1983].
- Shallow subduction may be responsible for low heat flow values observed in southern Sierra [Saltus and Lachenbruch, 1991]
- Crust and possibly mantle lithosphere thickening to the east
STEP 3
- Dip of subducting Farallon slab steepens and melt generation resumes but it is relatively minor compared to earlier accretion event [Chase and Wallace, 1988].
- Crust to the east has thickened possibly due to flat slab subduction or other mechanisms
- 33 MA is documented age of younger magmatism from surface outcrops [Saltus and Lachenbruch, 1991]
STEP 4
- Cessation of magmatism with the end of subduction
- slow subduction of Vancouver plate gradually becomes transform boundary and the plate is broken [Sonder and Jones, 1999]
- crustal weld is formed between oceanic plate and north America [Jones et al, 1994]
- Plate Boundary Forces are no longer strong enough to overcome potential energy of thickened crust [Sonder and Jones, 1999]
- northern Basin and Range province undergoes extension to the northwest as a result of increased bouyancy frolm destabilized mantle lithosphere after Laramide flat slab subduction. This in turn drives more extension in the Central Basin and Range directly adjacent to the Sierra's Eastern foothills. This begins to push the Sierra to the west [Sonder and Jones, 1999]
- Strong Plate becomes broken due to Basin and Range extension which could allow uplift of Bouyant Sierra batholith [Chase and Wallace, 1988]
- Chase and Wallace plotted profiles of paleotopography at 10 MA, current topography, and the possible location of the paleotopographic surface on todays topography. See that figure here. It clearly shows How the Sierra have been uplifted and rotated as a block north of the southern Sierra but southern Profile indicates a more block style uplift with less rotation.
- Click here, to see a more detailed petrologic cross section of the Sierra Nevada around this time done by L. Farmer. It shows evolution of Sierra as the "Slab Window" opens with the end of subduction and disintegration of the Vancouver Slab.
From Farmer's cross section we can assume a lower bound of crustal thickness to be ~30 km with an eclogitic root of another 60 km acting as a weight to the overlying crust. The elevation of the range is believed to be between 1.5 to 2 km which indicates a large amount of erosion has taken place since the Mesozoic. Heat flow is probably low from insulating effects of the subducting slab and relatively recent opening of the "Slab Window" [Crough and Thompson, 1977]. If Farmer's model is believed, then we should expect the range to be in isostatic equilibrium with gravity variations east to west based on compositional gradation within the batholith.
CARTOON | MODEL | CONCLUSIONS | REFERENCES | HOME
