Lecture 17
Cenozoic Mountain Building and the Yellowstone Hotspot Trail
Focus Question:
What four tectonic processes shaped the western U.S. in Cenozoic time?
1.
The
Cenozoic Era is the most recent of the three Eras of the Phanerozoic Eon.
It began about 65 million years ago with the close of the Mesozoic and
the Cretaceous-Paleogene extinction event.
The Cenozoic is divided into 2 Geologic Periods, defined currently as the Paleogene Period (Paleocene, Eocene, and Oligocene Epochs), the Neogene Period
(Miocene, Pliocene, Pleistocene, Holocene Epochs), or also classified as the
Tertiary Period (Paleocene, Eocene, Oligocene, Miocene, Pliocene) and the Quaternary Period (Pleistocene, Holocene).
The most recent epoch is the Holocene Epoch, which includes the most
recent 11,700 years of Earth History.
2.
Today, we will examine tectonic processes that
began in the Paleogene and Neogene, and that are still actively shaping the
landscape. The view of the earth in
the early Paleogene can be seen in this Ron Blakey map here at this link
http://jan.ucc.nau.edu/~rcb7/50moll.jpg
Note the approaching impact of India with Eurasia, the lack of any
Indonesian archipelago, the shallowing of the Tethys area, and the general position of the continents.
3. A large mountain building event of the Paleogene was the Himalayan-Alpine Orogeny. The Himalayan uplift occurred beginning about 40 million years ago with the docking of India with Eurasia. At the same time, the African and Arabian Plates moved towards Eurasia, and the compression/collision here led to the closing of the Tethys, the rise of the Alps, Pyrenees, Appenines, and the Atlas Mountains. The Tethys pinched shut, and at one point (in the Miocene) actually dried out. Subsequent opening of the basin along the Gibraltar area allowed reflooding by the Atlantic Ocean, and the modern remnant of all of this is the Mediterranean Sea.
4. In the west, the
Laramide Orogeny continued from the Late Cretaceous. The mountain building
sequence of the Laramide Orogeny (see previous two lectures) becomes very
important in the early Paleogene.
1. subduction of an ocean floor
plate continues under North America, but at a shallow angle.
2. This shallow angle results in uplift and volcanics further to the east than
the uplift and volcanics of the Jurassic Period
3. The effect is the uplift of the
Cordilleran mountain chain, from Canada to Mexico, and as far eastward as the
Black Hills of South Dakota
4. Several basins developed as
uplift occurred-such as the Uinta Basin and the Green River Basin.
5.
A new rift developed in the continental interior: the Rio Grande Rift, in
what is today New Mexico. The rift
has low activity, but is not a
failed rift. Volcanics are exposed
in the rift, an the Rio Grande River is channeled through it today.
6.
Normal faulting and block faulting (think horsts and grabens again) resulted when the
continental crust was stretched thin during the Neogene, creating the Basin and
Range province (much of Nevada).
This area is marked by volcanos and graben-like basins, filled with eroded
sediment.
7.
The Yellowstone Hotspot trail:
A stationary deep plume of magma from the asthenosphere, called a
“hotspot” exists beneath the North American plate.
As the plate moves westward, the rising magma from this “hotspot” creates
a string of volcanos, much as we see with the Hawaiian Island chain in the ocean
lithosphere. In the case of North
America, this westward motion over the hotspot has resulted in a string of
volcanos, oldest in the west and getting younger in a line eastward, ending with
the current position of the plate over the hotspot.
The current position is Yellowstone National Park.
1. the hotspot trail begins
in the Neogene aged volcanics in southwestern Idaho, near the Oregon boundary.
The trail of inactive volcanos can be followed along the Snake River
Plain, until the most recent one is
found, which is the Yellowstone caldera. (A
caldera is the remnant of an exploded volcano).
2. the magma dome beneath
Yellowstone National Park is monitored carefully by the USGS, who measure the
rising “resurgent” dome activity.
3. The last volcanic
eruption in Yellowstone was about 600,000 years ago.
4. The evidence that the
area is still a potential area of volcanic activity includes the hydrothermal
activity (e.g., Old Faithful, the other geysers and mudpots) and the resurgent
rising magma domes within the caldera.
So we can see the four tectonic
processes shaping the western U.S. during the Cenozoic include 1) the Laramide
Orogeny, 2) the Rio Grande Rifting event, 3) the Basin & Range crustal thinning,
and 4) the Yellowstone Hotspot Trail of volcanic eruptions.
8.
Paleogene life:
The big story here is the radiation of the mammals into all the empty
niches available following the loss of the dinosaurs, marine reptiles, and
pterosaurs. The early Paleogene saw
small, burrowing and insect eating mammals radiate and expand in size to the
many different herbivores, insectivores, and scavengers of the period.
1.
No
mammalian carnivores until the Eocene
2.
Who were the carnivore/predators until the
mammalian carnivores appeared? The major
carnivorous predators of the continents in the Paleogene were large, predatory carnivorous birds such as Diatrymas, and of course,
also the
surviving reptiles of the time, such as crocodiles and snakes.
