1) Turbidite Deposit (Protolith of Metamorphic Rock)
At first, we were under the ocean near a break in the slope, which allowed for the formation of the turbidites. These were the protolith rocks for the metamorphic rocks we observed. I determined this because we saw an alternation of slate and a metasandstone within the Metamorphic deposit. This alternation is consistent with a turbidite deposit. Turbidites are typically off of the coast at a break in slope, where floods carrying lots of sediment (Metasandstone layers) can be deposited over mud (slate layers).
2) Metamorphic Rock
We then transferred from a passive to an active plate margin on the west coast, due to the break up of Pangaea, in the form of subduction. Because the plate margin had previously been passive, the oceanic plate was cold and dense allowing for a steep subduction angle as the oceanic plate sank into the mantle below the North American Plate. The stresses from the subduction took those turbidite deposits and began to metamorphose them. This also brought in plutons due to the addition of volatiles creating flux melting in the mantle. This would account for the low-grade metamorphic granite we saw in contact with this slightly more metamorphosed turbidite deposit.
3) Sierra Nevada Batholith (200-80Ma)
This subduction and flux melting continued creating more plutons in the subsurface, which would account for the Sierra Nevada Batholith. Through the addition of plutons coupled with surface volcanism created further uplift in this area, raising the local topography to aproximately 3,000 meters, similar to the modern day cascade arc.
4) Malakoff Diggins (55-45Ma)
There is a large gap in age of the Batholith and the age of the Malakoff Diggins. We decided that this must have meant that during this gap there was a time of incision or erosion, and that something had to have changed to initiate this deposition of the Malakoff Diggins. We hypothesized that during the formation of the Batholith, the subduction was at a steep angle due to the fact that this plate margin had been passive, so the initial subduction was subducting this cooler more dense plate. This focused the stress and pressure on this coastal area of California, creating a fast rate of mountain building. This rate was counteracted by the rate of erosion from rivers eroding the mountains. But this erosion was never able to transfer to deposition in this area as the mountain building rate remained fast keeping the gradient and river profile the same. Over time the subduction zone began subducting the younger, warmer, and more buoyant part of the oceanic plate creating a flat slab subduction. This transferred the stress out more inward, as well as removing the ability for plutons to create uplift by removing the mantle from the equation. It also moved the mountain building more inland, which moved beginning of the rivers further to the east, changing the river profile. The change in river profile meant that the rivers were now eroding the newly formed high points in the Nevada area to the east of the Sierra Nevada’s and depositing in the Sierra’s, where they had previously been incising. This led to the backfilling of the older river valleys. Creating the alluvium deposits we see today.
5) Ignimbrite and Andesite Peak (32-16Ma)
The farallon plate began to steepen (slab roll back), due to the fact that the plate had time to cool and become denser and less buoyant by the time it reached eastern Colorado. This denser plate began to pull down and steepen the angle of subduction, creating a westward propagation of volcanics due to the introduction of the mantle in this area again. This created volcanism along the Nevada Plano, re-establishing the cascade-like arc that was formed during the flattening of the slab. The first flows were ignimbrites, because the magma had more assimilation of the felsic crust as it made it’s way to the surface. These ignimbrite flows traveled down topography further infilling valleys. Later, there were more mafic flows, as the magma was able to travel up to the surface faster, incorporating less crustal material. This is because the magmatism that created the ignimbrites had weakened the crust, creating conduits for these younger magmas to reach the surface faster and assimilate less crustal material creating more mafic flows. These flows covered up topography even more, as they were hotter and less viscous lava flows and could therefor flow for much longer, completely filling paleovalleys creating a “blanket” over older topography.
6) 16 Ma to Present Day
To create the modern topography of this area, the high elevation Nevada Plano needs to be lowered to the modern elevation. To accomplish this, we have to go back in time to create the full story. First we must go back to the Cretaceous, when the western margin was subducting the Farallon plate creating massive uplift in modern day California and Nevada. This massive uplift led to gravitational collapse in the form of large listric normal faults. This is paradoxical in that one would expect to see only thrust faults from this time due to the compressional margin. Instead the uplift created lots of gravitational potential energy, which manifested itself in the formation of normal faults. These normal faults became listric due to the increase in lithostatic pressure as the fault moved deeper into the crust changing the stress field flattening the fault deeper in the crust. These listric faults did not create the displacement we see today during the Cretaceous however. It wasn’t until 25 Ma when the displacement begin. 25 Ma marks the initiation of the San Andreas plate boundary system. This was accomplished when the mid-ocean ridge between the Farallon plate and the Pacific plate began to be subducted allowing for the interaction between the Pacific plate and the North American plate to form a transform plate boundary. This switch in the plate boundary led to a change in the dominant regional stresses from E-W to NW-SE, allowing for E-W relaxation in the form of extension. As the San Andreas transform fault boundary grew, this E-W extensional stresses also grew. Around ~15 Ma the extensional stresses manifested itself through displacement along the previously formed listric normal faults throughout the Nevada and eastern California area. During this time, Nevada grew around 200%,lowering the Nevada Plano by 1500-2000 meters, creating the basin and range as we see it today.
Revision to panel 6
The papers we read on the isotopic data of the Sierra’s and the Basin and Range province in Nevada provided the most solid conclusions for paleotopogrophy. Crowley et al. 2008, determined that the relief in the Sierra’s remained the same from 16 Ma to present. This was interesting, as we had originally predicted that there would be some sort of uplift associated with the release of pressure due to the horst and graben formations, similar to the Sangre de Cristo’s and Jackson Hole. Horton, Poage, and Chamberlain also provided convincing evidence for a significant decrease in elevation in the Nevada area starting at 16 Ma, which is consistent with our horst and graben hypothesis due to rifting. What all of the papers seemed to agree on is that the Sierra Nevada’s have been around since the Eocene and probably before then. This is why I have the Sierra’s as a prominent feature, changing very little in elevation from the first panels to the last panel.